Designing π-conjugated polymers for organic electronics

Accepted Manuscript

Title: Designing �-Conjugated Polymers for OrganicElectronics

Author: Xin Guo Martin Baumgarten KlausMullen<ce:footnote id="fn1"><ce:note-paraid="npar0005">Tel.: +49 6131 379152.</ce:note-para></ce:footnote>

PII: S0079-6700(13)00119-6DOI: http://dx.doi.org/doi:10.1016/j.progpolymsci.2013.09.005Reference: JPPS 836

To appear in: Progress in Polymer Science

Received date: 23-1-2013Revised date: 10-9-2013Accepted date: 17-9-2013

Please cite this article as: Guo X, Baumgarten M, Mullen K, Designing �-Conjugated Polymers for Organic Electronics, Progress in Polymer Science (2013),http://dx.doi.org/10.1016/j.progpolymsci.2013.09.005

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http://dx.doi.org/doi:10.1016/j.progpolymsci.2013.09.005

http://dx.doi.org/10.1016/j.progpolymsci.2013.09.005

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Designing π-Conjugated Polymers for Organic Electronics

Xin Guo, Martin Baumgarten*, and Klaus Müllen*

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany

Abstract

Conjugated polymers have attracted an increasing amount of attention in recent years for

various organic electronic devices because of their potential advantages over inorganic and

small-molecule organic semiconductors. Chemists can design and synthesize a variety of

conjugated polymers with different architectures and functional moieties to meet the

requirements of these organic devices. This review concentrates on five conjugated polymer

systems with 1D and 2D topological structures, and on one polymer designing approach. This

includes i) conjugated polyphenylenes (polyfluorenes, polycarbazoles, and various stepladder

polymers), ii) other polycyclic aromatic hydrocarbons (PAHs) as substructures of conjugated

polymers, iii) thiophene and fused thiophene containing conjugated polymers, iv) conjugated

macrocycles, v) graphene nanoribbons, and finally vi) a design approach, the alternating

donor-acceptor (D-A) copolymers. By summarizing the performances of the different classes

of conjugated polymers in devices such as organic light-emitting diodes (OLEDs), organic

field-effect transistors, (OFETs), and polymer solar cells (PSCs), the correlation of polymer

structure and device property, as well as the remaining challenges, will be highlighted for

each class separately. Finally, we summarize the current progress for conjugated polymers

and propose future research opportunities to improve their performance in this exciting

research field.

Keywords:

Conjugated polymers, organic electronics, organic light-emitting diodes (OLEDs), organic

field-effect transistors (OFETs), organic photovoltaics (OPV), polymer solar cells (PSCs).

______________

Corresponding authors: Klaus Müllen, email: [email protected] Tel: +49 6131 379 150;

Martin Baumgarten, email: [email protected] Tel: +49 6131 379 152

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Table of Contents

1. Introduction

2. One-dimensional (1D) conjugated polymers

2.1 Conjugated polyphenylenes

2.1.1 Linear and ladder-type polyphenylenes

2.1.2 Stepladder polyphenylenes with bridging atoms

2.2 Polycyclic aromatic hydrocarbons (PAHs)-based conjugated polymers

2.3 Thiophene-containing conjugated polymers

2.3.1 Polythiophenes and their derivatives

2.3.3 Bridged bithiophene-containing polymers

2.3.2 Thienoacene-containing conjugated polymers

2.3.4 Benzodithiophene-containing conjugated polymers

2.3.5 Naphthodithiophene-containing conjugated polymers

2.3.6 Other fused thiophene-containing conjugated polymers

2.4 Donor-acceptor polymers

2.4.1 Donors and acceptors

2.4.2 One-dimensional D-A polymers

3. Two-dimensional (2D) conjugated polymers

3.1 Conjugated macrocycles

3.1.1 Carbazole and fluorene-based macrocycles

3.1.2 PAHs-based macrocycles

3.1.3 Thiophene-based macrocycles

3.2 Graphene nanoribbons (GNRs) from polyphenylene precursors

3.2.1 Solution synthesis of GNRs using polyphenylene precursors

3.2.2 Surface-mediated synthesis of GNRs using polyphenylene precursors

3.3 Two-dimensional donor-acceptor polymers

3.3.1 2D mD-sA polymers

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3.3.2 Benzo[1,2-b:4,5-b’]dithiophene-containing 2D D-A polymers

4. Conclusions and outlook

5. Acknowledgments

6. References

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1. Introduction

Conjugated polymers as semiconducting materials have attracted broad academic

and industrial interest for various optoelectronic devices. In particular, their

applications in polymer light-emitting diodes (PLEDs), polymer solar cells (PSCs),

and organic field-effect transistors (OFETs) offer opportunities for the resolution of

energy issues as well as the development of display and information technologies [1].

In comparison with inorganic materials and small-molecule organic semiconductors,

conjugated polymers provide several advantages including low cost, light weight, and

good flexibility. More importantly, soluble polymer semiconducting materials can be

readily processed and easily printed, removing the conventional photolithography for

patterning, which is a critical issue for the realization of large-scale roll-to-roll

processing of printed electronics [2].

Since poly(p-phenylenevinylene) (PPV)-based PLEDs were reported in 1990 [3], a

large library of polymer semiconductors have been synthesized and investigated.

Organic chemist have developed tremendous amounts of building blocks, such as

fluorene, carbazole, thiophene and its fused derivatives, benzothiadiazole and its

derivatives, rylene diimides, and diketopyrrolopyrrole (see structures in Scheme 1.1),

which have been employed to design various polymers according to individual

demands for specific applications. One particular approach was the application of

alternating donor (D) and acceptor (A) units to steer the HOMO and LUMO levels, as

well as the band gap of the resulting copolymers (so-called D-A polymers), which is

an efficient strategy for tailoring the properties of conjugated polymers for

applications in OFETs and PSCs [4]. Based on this strategy, current p-type and n-type

mobilities have reached 8.2 and 3.0 cm2/V s [5, 6], respectively, and power

conversion efficiencies (PCEs) of around 9% have been achieved for PSCs [7, 8]. A

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world record for a tandem bulk hetero junction (BHJ) organic solar cell reached 12%

very recently [9].

Scheme 1.1

To design conjugated polymers for different applications several general principles

should be kept in mind, including 1) side chains to enhance the solubility and

processibility, 2) high molecular weights, 3) band gap and absorption behavior, 4)

HOMO and LUMO energy levels, and 5) suited morphology with low barriers. These

factors are dependent on each other and must be comprehensively considered for

specific applications. For instance, side chains play a significant role for improving

the solubility and the obtainable molecular weight of conjugated polymers, but also

influence their intermolecular interactions through changes in morphology and

thereby the charge carrier mobility. Tuning the energy band gap for obtaining the

desired absorptions will usually change the HOMO and LUMO energy levels, and

thus change the emitting color in an OLED or influence the open-circuit voltage (Voc)

in a PSC. Therefore it is necessary to fully account for these designing principles and

balance the guiding concepts in pursuit of ideal polymers for specific applications.

The extension of dimensionality strongly affects the optoelectronic properties of

conjugated polymers as well. Most polymer systems usually possess conventional

one-dimensional (1D) structures with continuous π-conjugations along the polymer

backbone. Extending the conjugation in the lateral direction of polymer backbones

can lead to different supramolecular structures in the solid state and to

multidirectional charge transport [10]. In particular, graphene, which is considered as

an ultimately sized two-dimensional (2D) polymer, possesses extraordinary electronic,

thermal, and mechanical properties making it a promising candidate for many

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practical applications [11]. Other 2D polymers containing D-A structure also

exhibited promising performances in the PSCs [12].

In this review, we try to summarize the broad scope of publications that have

reported on various 1D and 2D conjugated polymers exhibiting high performances, or

at least desirable potentials in PLEDs, PSCs, and OFETs from a viewpoint of material

oriented chemists. The operational mechanisms of these organic electronic devices

have been outlined many times before, and the synthetic details of all described

polymers, as well as device fabrication, can be found in the original publications, not

needed to be described here in detail. This review is organized into two sections,

namely, 1D conjugated polymers (section 2) and 2D conjugated polymers (section 3).

In this vein four sorts of 1D conjugated polymers, specifically, i) conjugated

polyphenylenes (section 2.1), ii) conjugated polymers from polycyclic aromatic

hydrocarbons (PAHs) (section 2.2), iii) thiophene and fused thiophene containing

conjugated polymers (section 2.3), and iv) 1D D-A conjugated polymers (section 2.4)

will be discussed in section 2, respectively. Particularly, fluorene- and carbazole-

based (sections 2.1.2.1 and 2.2.2.3) conjugated polymers and their derivatives have

become vitally significant for applications in organic electronics in the past decade,

while D-A polymers (including 1D and 2D cases (sections 2.4 and 3.3)) have

achieved extremely high performances in both PSCs and OFETs applications in

recent years, and represent a popular research topic. Section 3 includes three parts,

namely, i) conjugated macrocycles (section 3.1), ii) graphene nanoribbons (section

3.2), and iii) 2D D-A conjugated polymers (section 3.3). Among them, graphene

nanoribbons will be especially highlighted because currently their synthesis and

applications are one of the most intriguing research areas. Finally, section 4 will

present a brief summary and outlook on conjugated polymers to conclude. We hope

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that after reading this review researchers will have a clear grasp of the advancements

of conjugated polymers, past and present, to gain inspirations towards designing new

conjugated polymers for applications in organic electronics.

2. One-dimensional (1D) conjugated polymers

Most conjugated polymers possess 1D π-conjugated backbones. Early research

focused on poly-para-phenylenes (PPPs) and polyphenylenevinylenes (PPVs), and

then extended to various polyphenylene derivatives such as ladder-type poly-para-

phenylenes (LPPP) and shorter bridged so-called stepladder polymers, like

polyfluorenes (PFs), polycarbazoles (PCz), and polyindenofluorenes (PIFs). Among

these polymers, PFs and PCz have attracted the highest attentions and have been

widely studied as active materials in PLEDs and PSCs. A variety of PAHs have also

been used as repeating units of conjugated polymers and tested as blue emitters.

Thiophenes are another important class of building blocks, which were used to make

conjugated polymers for OFETs and PSCs. All these building blocks can be used as

donor units combined with electron-deficient acceptor units for the construction of

alternating D-A copolymers, resulting in the rapid evolution of high-performance

charge transporting and photovoltaic polymer materials.

This section is divided into four parts to describe the four classes of 1D conjugated

polymers mentioned above. The D-A polymers containing fluorene, carbazole,

indenofluorene and some other PAHs as donor units will be discussed in section 2.1

(conjugated polyphenylenes) and section 2.2 (PAHs-based conjugated polymers)

rather than in section 2.4 (D-A polymers).

2.1 Conjugated polyphenylenes

In the last two decades, conjugated polyphenylenes have formed one of the most

extensively studied classes of conducting polymers and have been used in various

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organic electronic devices [13]. Poly-para-phenylene (PPP, 1 Scheme 2.1.1), the

simplest but most important conjugated polymer, and their derivatives have been

developed as early as in the 1960s and used for many significant applications [14, 15].

Unfortunately, unsubstituted PPP is insoluble and intractable, limiting its

characterization, reproducibility, and thereby its proper applications. Introducing

solubilizing substituents onto the main chain can improve the solubility of the PPP,

which, however, leads to increasing torsion between neighboring repeating units and

thereby interrupts conjugation to quite an extent [16, 17].

In order to increase the conjugation and bathochromically shift the fluorescence into

the visible region, several methods were applied including the use of

phenylenvinylenes in the polymer backbone and the fully or partially planarized

neighboring phenylene units via carbon or heteroatoms (such as Si and N) bridges.

This resulted in the tremendous development of linear polyphenylenes derivatives

such as PPVs, LPPPs, stepladder polyphenylenes and their heteroatoms bridged

analogues (see structures in Schemes 2.1.1 and 2.1.2). These methods can restrict the

rotation between the repeating units, improve the solubility and the stability of the

polymers, and adjust the optical and electrochemical properties of polyphenylenes. In

this part, the development of these conjugated polyphenylenes will be reviewed and

their optoelectronic properties will be described. In particular, emphasis will be put on

PFs and PCz, both of which have become vitally important conjugated polymers for

applications in organic electronics in the past decade.

2.1.1 Linear and ladder-type polyphenylenes

PPP 1 has good thermal stability and large band gap of about 3.5 eV and therefore

can be used as active component of blue PLEDs [18]. Nevertheless, it is undefined

and not solution processable. As mentioned above, side chains can be easily

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introduced into the phenylenes (2) to improve solubility, however, emissions shift into

the UV region. To obtain visible emitting color, the neighboring phenylene units in

PPP could be planarized via a methine bridge while still carrying solubilizing side

chains. In this vein, fully ladderized poly-para-phenylene)s 3 (LPPPs) were

developed as demonstrated in Scheme 2.1.1 [19, 20]. It turned out that this fully

planar alignment of polymer backbone led to a strong enhancement of the conjugation,

which was indicated by bathochromically shifted optical absorptions. The

photoluminescence of 3 was obviously red-shifted compared to that of non-bridged

PPP, with maximum emission at 450 nm in diluted toluene, but both

photoluminescence in the solid state and electroluminescence shifted to 600 nm [21],

partially explained by excimer formation and ketone-defects [22]. To overcome the

yellow-green emissions, a stepladder copolymer with dialkylated phenylenes

incorporated into LPPP main chains was synthesized, which suppressed the excimer

formation and displayed blue emission in the PL and EL spectra owing to the

distorted structure of the copolymer [21, 23]. Another LPPP derivative 4 containing

spiro-fluorene at bridged carbons was also developed, where no low energy emission

was observed in PL spectra of thin films even after annealing at 110 °C for 24 h in air

[24].

Quite a different class of linear conjugated polymers derived from phenylenes is

polyphenylenevinylenes (PPVs), which are very renowned semiconducting polymers

and widely applied in PLED and PSCs. Unsubstituted PPV 5 (Scheme 2.1.1) was

initially prepared by way of a solution-processable precursor and first used as an

electroluminescent polymer in a large-area LED with an emission in the green-yellow

region in 1990 [3]. Later on, the studies on PPV were focused on improving the

solubility by introducing side chains into the backbone (typically via a Gilch method

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[25]), which made PPV-based polymers able not only to be solution-processable but

also to tune the light-emitting color covering the whole visible region.

Scheme 2.1.1

The most important soluble PPV is poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-

phenylene vinylene] 6 (MEH-PPV). The development of the PSCs was propelled in

1992 by photophysical studies on a bilayer of 6 with C60, which revealed an ultrafast,

reversible, metastable photo-induced electron transfer from 6 to C60 in solid films

[26]. Moreover, it was also the first conjugated polymer applied to fabricate a flexible

“plastic” LED emitting orange light with an external quantum efficiency of about 1%

[27].

Most PPV derivatives have low electron affinities and are commonly used as donor

materials in PSC devices. By introducing electron-withdrawing cyano groups on the

vinylenes, polymer 7 (CN-PPV) exhibited a larger electron affinity and narrower band

gap leading to red emission and utilization of high work-function aluminum as a

cathode of LEDs. High internal efficiencies of up to 4% were achieved based on such

devices with 7 as the emitting layer [28]. This polymer was used for the first time as a

polymer electron acceptor in a two-layer PSC fabricated by a lamination device

processing technique, providing an IPCEmax of 29% and a PCE of 1.9% [29].

2.1.2 Stepladder polyphenylenes with bridging atoms

There was a strong need for stable and pure blue-light emitting polymers being

solution-processable in order to prepare large-area LEDs. The linear polyphenylenes

discussed above regardless of PPP, LPPP, or PPV, could not match such requirements.

Thereby, further research interests were raised to design and develop so-called

stepladder polymers, namely, partially ladderized PPPs, where two or more adjacent

phenylene units in the polymer backbone were planarized by additional methine

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bridges (Scheme 2.1.2) or other bridgehead atoms such as N, Si, and P (Schemes

2.1.17-20).

Scheme 2.1.2

Among them, carbon bridged stepladder polyphenylenes have attracted substantial

research interest because of their relative easy of synthesis, fewer defects, and

excellent photophysical properties as comparison with the fully ladderized polymers.

More importantly, the absorption and fluorescence properties of these stepladder

polyphenylenes can be well tuned by varying the size of the bridged unit. For

examples, the emission maximum of polyfluorenes (8, PFs, Scheme 2.1.2) is around

420 nm, while those for polyindenofluorenes (9, PIFs), ladderized

polytetraphenylenes (10), and ladderized polypentaphenylenes (11) are gradually red-

shifted from 430 nm to around 440–445 nm, as shown in Figure 1 [13, 30].

Figure 1.

Before considering the important exchange of the methine bridges by other

heteroatoms we will first discuss PFs, a most widely studied class of conjugated

polymers.

2.1.2.1 Polyfluorenes (PFs)

Polyfluorenes are the most shining star in conjugated polymers because of their use

in various polymer optoelectronic devices, such as LEDs, OPVs, FETs, chemical and

biosensors, lasers, memories, and light-emitting electrochemical cells (LECs) [31a].

PF comprising coplanar fluorene as repeating unit can be regarded as that each

adjacent pair of phenylene groups in the PPP is flattened by methylene bridges [31b].

The 9-position carbon atom of fluorene unit adopts a sp3 hybrid, where various

functional substituents can be introduced at this position. Furthermore, the ease of

functionalization of the 2,7-postions of fluorene allowed facile preparation of

polymerizable monomers, which led to a large amount of PF homo and copolymers to

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meet the requirements of organic devices. During the past ten years, a tremendous

amount of fluorene-based conjugated polymers were published. Hence only extremely

important and highly representative examples will be presented here and other

reported fluorene-based polymers can be found in a recent review [31a].

2.1.2.1.1 PFs as blue-light emitting polymers for PLEDs

PFs are regarded as the most promising candidates for blue PLEDs because of

structural features that lead to facile functionalization as discussed above, and

promising properties, such as excellent chemical stability, great thermal stability,

good film-forming property, and high fluorescence quantum yield in solid state (ΦPL =

0.55) [32]. However, PFs with solubilizing alkyl substituents at the 9-position have

suffered from low EL efficiency, poor color stability, and insensitive emission to

human eyes. The three problems will be separately discussed below, combined with

their suggested solutions to achieve better results.

1) Reason of low EL efficiency and solutions

Poly(9,9-dialkylfluorene), like poly(9,9-di-n-octylfluorene) (12, PFO, Scheme 2.1.3),

has a large band gap with low HOMO and high LUMO energy levels of −5.80 and

−2.12 eV, respectively [33]. Particularly, a big difference between the HOMO level of

PFO and the work function of PEDOT modified ITO (-5.2 eV) makes it difficult for

hole injection from the anode. On the other hand, PFO exhibits good hole transport

[34], but its electron affinity is very low, which results in inefficient combination of

charge carriers and therefore low efficiency of exciton formation. The unbalanced

injection and transport of charge carriers also can cause their combination in the

region close to electrodes, thereby leading to the quenching of the excitons. Therefore,

electron-rich and electron-poor groups were introduced into PFs, in order to improve

the injection and transport of charge carriers, and thus the EL efficiency.

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Scheme 2.1.3

Triphenylamine and its derivatives are typical electron-rich compounds, which have

been used to enhance the hole injection and transport of PFs by incorporating them

either as side chains (13) [35], as end-groups (14, 15) [36], or in the main chains (16,

17) [37, 38]. Among them, the triarylamine end-capped PFs had a higher HOMO

level of -5.48 eV and emitted in the deep-blue region with brightnesses of up to 1600

cd/m2 and EL efficiencies higher than 1.1 cd/A at 8.5 V. The EL efficiency increased

by one order of magnitude compared to the non-end-capped PDAF without disturbing

the electronic and chemical structure of the backbone. In addition, carbazole is also

used to improve the hole injection and transport by being introduced into PFs [39-41].

Although the problem of electron injection can be overcome by using low work-

function metals such as calcium as cathodes, another layer of aluminum on top of

calcium must be used because of the reactivity of such metals. Thereby, the electron-

deficient units were used to improve the electron accepting ability of PDAFs. A PF

polymer (18) containing electron-withdrawing aryloxadiazole groups at the 9-position

of fluorene was developed, which had a LUMO level of -2.47 eV and showed an EQE

of 0.52% from an EL device, two times higher than PDAF under same device

processing conditions [42]. Such electron-withdrawing units have also been employed

as comonomers inserted into the main chains of PDAF (19) [43], as pendant groups of

PDAF (20) [44], and as end-capping groups (21) [45]. The other electron-deficient

units introduced to PDAFs for improving electron injection include cyano groups,

quinoxaline, thiazolothiazole etc. [46-49].

Another approach was to develop a PF copolymer bearing both electron-rich and

electron-poor groups such as copolymer 22. The HOMO and LUMO levels of this

polymer are -5.30 and -2.47 eV, respectively, suggesting that the electron and hole

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injections were simultaneously improved. The EL device with this polymer as an

active layer exhibited a much improved EL efficiency (1.21%) compared to PFO 12

(0.2%) [50].

2) Reason of poor color stability and solutions

PDAFs suffer from the tendency to generate an undesired long-wavelength emission

band around 530 nm during device operation, and upon heat treatment or

photoirradiation, which result in red-shifted emission spectra and reduced device

efficiency. It was initially believed to arise from the formation of excimers [51-59].

However, this point was argued [60] and subsequently a new point of view was

established, claiming that the long-wavelength emission arose from fluorenone defects,

probably formed by oxidation of incomplete alkylated bridgeheads [61-63]. A

possible mechanism for the formation of fluorenone defects is provided in Figure 2

according to the most accepted approach [64-68].

Figure 2.

To avoid these defects, careful purification of the monomers and attempt to decrease

polydispersity of polymer were described [69, 70]. Crosslinking PDAFs derivatives

containing functional end-groups was found to suppress the long wavelength emission

as well [71, 72]. However, the most widely used method to avoid the appearance of

long wavelength emission is to incorporate bulky groups such as dendrimers, aryl

substituents, and spiro-linked structures into PFs to reduce defect formation or to

suppress exciton migration to defect sites.

Attachment of bulky dendrimers at bridgeheads has been examined as a means to

suppress interchain interactions. The suppression of the long wavelength emission

could be achieved by attaching polyphenylene dendrimers as 23 in Scheme 2.1.4. The

emission from 23, however, is still not completely stable over time, possibly because

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of the susceptibility of the benzylic linkages to oxidation [73]. By directly attaching

the dendrons to the bridgehead, polymer 24 was developed, which was exceptionally

thermally stable and displayed stable blue EL [74]. Other dendrimers such as Fréchet

dendrons (25 and 26) and carbazole-based dendrons (27) have also been introduced

into PFs as side chains [75-77]. Suppression of the long-wavelength emission could

be observed for these polymers but was related to the number of the dendrimer

generation.

Scheme 2.1.4

Stable blue emission is also observed from the polymers (28-30, Scheme 2.1.5) with

alkyl or alkoxyl phenyl groups attached at the 9-position of fluorene [73, 78, 79].

Additionally, above-mentioned polymers 13, 18, and 22 (Scheme 2.1.3) containing

triphenylamine and oxadiazole substituents at bridgeheads could not only improve the

injection and transport, but also prevent ketone defects, as in 28-30 through their aryl

groups inhibiting the green emission. Another effective approach to obtain stable PL

and EL emissions is the application of spiro-linked structures. An alternating

copolymer 31 has been found to give a more stable emission than a homopolymer 12,

but green emission was still observed upon heating in air at 150 oC [80]. By contrast

spirobifluorene homopolymers 32 and 33 produced a more stable blue light without

green emissions even with annealing at 200 oC [81, 82]. Other spiro-linkages have

also been introduced at the 9-position of fluorene for stabilizing the emission.

Polymers, both 34 and 35, displayed stable blue PL and EL upon annealing [83, 84].

Scheme 2.1.5

It should be noted that both aryl-substituted and dendronized PFs can stabilize the

blue emission. Bulky dendrimers in 24, on the other hand, can increase the lifetime of

triplet excited states, which may reduce the efficiency of LEDs because of increasing

singlet–triplet quenching [85, 86]. Thus the small-size aryl substituents are

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recommended rather than bulky dendrimers for suppressing the long wavelength

emission.

Copolymerization with other emitting or bulky units is another method used to

obtain stable blue emission from PFs. A copolymer 36 (Scheme 2.1.6) containing

15% anthracene has been reported to produce a stable blue emission, which remains

stable even upon annealing up to 200 oC for three days [87]. PF copolymers

containing 3,6-carbazole (37) and more twisted units through seven-membered ring

formation with O or S bridging as in 38 and 39, have also been made for suppression

of long-wavelength green emission bands [88, 89]. Hyperbranched comonomers with

strong steric effects were copolymerized with fluorene to produce network polymers

such as 40 and 41, which have high fluorescence quantum yields in both solution and

solid-state, and display stable blue EL [90, 91]. In addition, attachment of bulky

substituents at the chain ends such as polyhedral siloxanes (42), has also been found

to significantly reduce the long wavelength emission compared to PDAFs without

end-capped groups [92].

Scheme 2.1.6

3) Reason of insensitive emission to human eyes and solutions

A human eyes sensitivity curve is shown in Figure 3, defined by the Commission

Internationate del’Eclairage (CIE), and describes the visual light characteristics of

human eyes. The human eye is most sensitive to a color at around 555 nm, in the

yellow-green region of the optical spectrum. The sensitivity decreases rapidly towards

the blue and red regions. In the blue region, the human eye is most sensitive around

440-450 nm [93, 94], while PFs typically emit at 420 nm. Thus for better eye catching

visibility, the emission color of PFs need a moderate shift to longer wavelength. On

the other hand, in order to ensure a high color purity, the CIE coordinates should be in

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a pure blue region as well, apart from the emission wavelength within the appropriate

range.

Figure 3

One solution is to prepare new conjugation extended polyphenylenes with chemical

structures intermediate between those of PF and LPPP, for example ladderized

polytetraphenylene and polypentaphenylene (10 and 11, Scheme 2.1.2), which will be

discussed in the part of 2.1.2.2. Another solution is to tune the emission color of PFs

to the pure blue region by incorporating a lower energy unit compared with PF, which

traps the excitons and becomes the emitter. This is a so-called “host-guest” or “host-

dopant” system, where PF serves as a host, but the low energy unit as a guest or

dopant (hereinafter referred to as dopant). In fact, this strategy can not only improve

the color purity by tuning the emission color of PFs, but also the EL efficiency on

account of the energy transfer from PF host to dopant units. However, it is difficult to

seek out a proper blue dopant since it must possess a suitable band gap, ensuring pure

blue emission after incorporating into PFs, and good spectroscopic overlap with the

PF host.

A few fluorene-based copolymers with the CIE coordinates in the pure blue region

have been reported. A PF copolymer (43, Scheme 2.1.7) that contains bis(2,2-

diphenylvinyl)fluorene attached orthogonally to the 9-positions of fluorene was

reported to show a maximum external quantum efficiency of 1.06% with color

coordinates of (0.15, 0.17) [96]. Its analogue with triphenylamine replacing 2,2-

diphenyl (44) exhibited a high EL performance with a LE of 2.87 cd/A and CIE

coordinates of (0.15, 0.18) , but this was realized by blending 44 in a host polymer

and the maximum emission shifted to 461 nm that was close to green-blue region [97].

Covalent attaching a fluorescent dye 4-dimethylamino-1,8-naphthalimide (ΦPL=0.84)

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to the side chains of PF, an efficient blue light-emitting polymer 45 was developed.

The single-layer device exhibited the maximum luminance efficiency (LE) of 6.85

cd/A with the CIE coordinates of (0.15, 0.19) [98]. By incorporating 1 mol% 2-

hexylbenzotriazole (HBT) into the main chain of PFO (46) a fluorene-based

copolymer with the CIE coordinates of (0.15, 0.17), a LE of 2.69 cd/A, and good EL

spectroscopic stability was developed [99]. Copolymer 47 consisting of alternating

dialkylfluorene and substituted meta-phenylene units carrying hole (carbazole and

triphenylamine) as well as electron (oxadiazole) transporting units in the side chain

showed a maximum efficiency of 2.50 cd/A and CIE coordinates of (0.16, 0.080 [100].

Scheme 2.1.7

By incorporating 7,7,15,15-tetraoctyldinaphtho-s-indacene (NSI) into the backbone

of PFO (12), a series of blue light-emitting copolymers (48) were developed. The

insertion of the NSI unit into the PFO backbone led to an increase of local effective

conjugation length, forming low-energy fluorene–NSI–fluorene (FNF) segments

which served as exciton trapping sites. Energy transfer could occur from the high-

energy PFO segments to the low-energy FNF segments, which caused these

copolymers to show red-shifted emissions compared with PFO, with a high efficiency,

good color stability, and high color purity. The best device performance with a

maximum EL at 448 nm, an LE of 3.43 cd/A, a maximum brightness of 6539 cd /m2,

and CIE coordinates of (0.15, 0.16) was achieved [101].

2.1.2.1.2 PFs as host materials for RGB and white PLEDs

PFs are excellent host materials owing to their high PL efficiency, large band gap,

and good charge carrier transporting properties. They were widely used as a host to

realize highly efficient RGB and white PLEDs through either physical blending or

chemical doping. In this review, we focus on the chemical variations, namely, the

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host/dopant system in which, as previously mentioned, efficient and fast Förster

energy transfer from larger-band-gap PF segments to a narrow-band-gap dopant may

occur leading to highly efficient emission solely or predominantly from the latter [101,

102]. Meanwhile, the trapping of excitons formed on PF segments at low-energy sites

resulted in an improved color stability. Blue light-emitting polymers with PF as a host

have been discussed above, and thus green, red, and white light-emitting polymers

with PF as a host will be further presented next. The EL performances of typical PF-

based polymers are listed in Table 1.

Table 1

Copolymer 49 (Scheme 2.1.8) containing benzothiadiazole (BTZ) in the PF

backbone is a representative green light-emitting polymer. A device comprised of this

polymer has a low turn-on voltage of 2.25 V and exhibits a peak efficiency of 10.5

Cd/A at 6600 Cd/m2 at 4.85 V with very good stability [103]. By tuning the content of

BTZ unit in the PF backbone, an LE as high as 10.9 cd/A and a maximum brightness

of 22800 cd/cm2 was obtained [104]. Attaching a naphthalimide derivative as the

pendant of PF, polymer 50 was synthesized and a maximum LE of 7.45 cd/A with

CIE coordinates of (0.26, 0.58) were observed based on a single-layer device [105].

Apart from incorporating electron-withdrawing units into PFs for realization of green

emission, copolymerization with some electron donor units such as thienothiophene

(51) [106], bisthiophene (52) [107], and perylene (53) [102] were proven to shift the

emission of PFs to the green visible region.

Scheme 2.1.8

Red light-emitting polymers could be obtained by incorporating small-molecule red

emitters into PFs. Polymer 54 (Scheme 2.1.9) is a typical red light-emitting material,

which exhibited a LE of 1.5 cd/A with CIE coordinates of (0.67, 0.33) [103].

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Attaching two alkyl chains at the thiophene units of 54 improved the fluorescence

quantum yield of polymer 55 in the solid state, but the EL efficiency and CIE

coordinates (1.45 cd/A and (0.66, 034), respectively) were similar to those of the

parent polymer [108, 109]. Benzoselenadiazole (56a) and naphthoselenadiazole (56b)

have also been used as low-band-gap units to realize red emissions, leading to a red-

shift of 50 nm compared with analogues containing benzothiadiazole [110-112]. For

example, polymer 56b emitted at 657 nm and the external quantum efficiency based

on a single-layer reached 3.1% with CIE coordinates of (0.64, 033) [111]. Perylene

dyes, a class of typical red light-emitting chromophores, could be attached to PF

chains either as end-capping groups at the chain termini (57), as pendant side groups

(58), or as comonomers in the main chain (59), to achieve red emissions. The best EL

efficiency from these polymers is up to 1.6 cd/A [102].

Scheme 2.1.9

Another class of red light-emitting small molecules introduced into PFs is iridium-

based complexes with triplet emissions. In such polymers, the singlet energy is

transferred from the PF sections to the iridium complexes where finally red

phosphorescence is emitted. The first reported red electrophosphorescent PF-based

material was polymer 60, from which an EL efficiency of 2.8 cd/A was achieved

[113]. Then many such materials were reported and recently a maximum LE of 8.3

cd/A with CIE coordinates of (0.63, 0.35) was realized based on a polymer 61 [114-

116].

White EL from polymers can be realized using several strategies such as the

multilayer-device system [117-119], the single-layer polymer-blend system [120-124],

and a single-polymer system [125]. However, the first two have some disadvantages,

for instance, the interfacial mixing of different layers for the former and the intrinsic

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phase separation for the latter, which limit their applications. These problems are

avoidable in a single-polymer system. An important approach for designing a single-

polymer emitting white light is based on the color principles to introduce two or three

different emission components (for instance blue and orange, or blue, green, and red)

into a single polymer which can produce balanced emissions from every component

by controlling partial energy transfer and charge trapping. PF is usually used as a host

polymer serving for a blue emission and one or two dopant units are introduced as

long-wavelength emission components.

The first single white polymer 62a (Scheme 2.1.10) was reported in 2004, exhibiting

a LE of 5.3 cd/A with CIE coordinates of (0.25, 0.35) [125]. Although the color

coordinate was not quite close to a pure white-light point of (0.33, 0.33), this

propelled a number of studies on single-component white light-emitting polymers.

Successively, many similar two-color white polymers such as 62b and 63 were

reported with improved EL efficiency and more pure color coordinates [126-131].

Other ways such as introducing iridium complexes as long-wavelength emission units

[132, 133] and constructing polymers with star-like architectures [134, 135] have also

been used to design single white polymers. In particular, a star-like single polymer 64

using orange emissive species as the central core and PFs as the outer arms has been

made and exhibited a high current efficiency of 18.0 cd/A and a power efficiency of

8.38 lm/W with CIE coordinates of (0.33, 0.35) [135].

Scheme 2.1.10

The three-color white single polymers have been prepared by realizing simultaneous

singlet emissions from blue, green, and red components (65, 66 and 67, Scheme

2.1.11) [136-142] and mixing singlet and triplet emissions in PF polymers (68 and 69)

[143-145]. It was found that the EL efficiency is higher when moving a dopant unit

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from polymer backbone to side chains. For example, polymer 66 with both green and

red dopants at side chains showed a higher current efficiency of 7.3 cd/A compared to

1.6 cd/A for polymer 65 with green dopant at side chains and red dopants in main

chains [136].

Scheme 2.1.11

All of the single white light-emitting polymers discussed above, regardless of

consisting of two- or three-color and singlet or mixed singlet and triplet emitters, were

achieved based on a mechanism of partial energy transfer and charge trapping on

dopant in the EL process. Another strategy to realize white EL from a single polymer

was proposed based on a new mechanism of electron trapping on the host rather than

charge trapping on the dopant. Several polymers (70, 71 and 72, Scheme 2.1.12)

containing phosphonate-functionalized PFs as the blue light-emitting host have been

designed according to such a mechanism. The single polymers containing

phosphonate groups show white EL with CIE coordinates (0.34, 0.35) at 8V when the

dopant content is up to a centesimal level, which is two orders of magnitude higher

than for those without phosphonate groups. Moreover, the EL devices based on these

phosphonate-substituted polymers can be fabricated using high-work-function and air-

stable Al as cathode without inserting an electron-injection layer [146].

Scheme 2.1.12

2.1.2.1.3 PFs as donor materials for polymer solar cells

PF-based polymers have also been widely applied in the PSCs. However, the

homopolyfluorenes are inappropriate for the solar-electricity conversion owing to

their weak absorptions in the visible region of the solar spectrum. By copolymerizing

fluorene units with other aromatic building blocks, in particular electron-deficient

units, the absorptions of the resulting polymers could be remarkably red shifted. The

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fluorene units in such copolymers are responsible for improving the solubility via

attaching solubilizing groups at the 9-position and maintaining the conjugation in the

whole copolymer backbone. They are in most cases used as donor materials blended

with fullerene derivatives such as PCBM to fabricate bulk-heterojunction solar cells.

The performances of such PF-based PSCs are summarized in Table 2.

Table 2

One of the most important fluorene-based polymers for solar cells is an alternating

copolymer 73a (Scheme 2.1.13), whose films showed obvious red-shift with the

maximum absorption wavelength at approximately 545 nm compared with PDAFs.

This could be attributed to the neighboring BTZ, thiophene, and fluorene moieties by

lowering the band-gap and thereby largely improving the light absorption in the

visible range. The PSC with a configuration of ITO/PEDOT:PSS/73a:PCBM/LiF/Al

showed 2.2% PCE [147]. Polymers 73a-d were synthesized by introducing different

alkyl chains to investigate the influence of substituents on the packing of the polymers

and the morphology of the active layer, which will lead to different photovoltaic

performances [148]. Among them, the polymer 73c with octyls exhibited the best

PCE of 2.6% [149, 150].

Scheme 2.1.13

Many additional PF copolymers such as 74-78 with other acceptor units to replace

BTZ in 73 have successively been reported [151-161]. A typical alternative of BTZ is

thienopyrazine, which was incorporated into PF main chains to prepare polymers 74a-

c. Within this series of polymers, the polymer 74b provided a best PCE of 2.2% when

blended with PCBM [157]. Other acceptor units flanked with two thiophenes were

introduced in the PF backbone, including thiadiazoloquinoxaline (TQ) [154],

quinoxaline [155-157], pyrazino[2,3-g]quinoxaline [158], diketopyrrolopyrrole (DPP)

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[159, 160] and so forth. The corresponding copolymers 75-78 exhibited narrow band

gaps and broad absorption bands. Among them, polymer 76 yielded a PCE of 3.7%

based on a blend device with PCBM under standard AM 1.5 solar illumination [156].

Polymer 78 containing DPP as an acceptor showed a moderate PCE lower than 1%,

but DPP has emerged as a promising acceptor to synthesize D-A polymers for high-

performance PSCs and OFETs. This will be discussed in detail in Section 1.4.

Recently, polymers 79a-b were reported with a new acceptor containing

thienopyrazine moiety. A PCE of 1.4% was achieved from 79b/PCBM-based solar

cells [162].

In addition to using electron-withdrawing units several other methods have also been

attempted to decrease the band gap and tune the absorption behavior of the fluorene-

based polymers, such as incorporating silole moieties (80) [163, 164], selenium-

containing heterocycles [110], rigid ribbon-type building blocks (pentacene and

anthradithiophene) [165], and heavy metal platinum [166] into the PF main chains.

All of these polymers possess narrower band gap and red shifted absorption compared

to PF homopolymers, but the PCE of solar cells were generally lower than 3%. In

view of the high performance (PCE higher than 7%) of other polymers for example

benzodithophene-based D-A copolymers, the research on fluorene-based polymers for

PSC have shown a downward trend in recent years.

2.1.2.1.4 PFs as semiconducting polymers for FETs

PFs are typical hole transporting materials and their charge mobilities are highly

sensitive to phase morphology [61]. It has been demonstrated that relatively high hole

mobility of up to 8.5×10−3 cm2/V s can be achieved when PFO (12) was oriented in

the nematic phase [167]. Another PF derivative 81 (Scheme 2.1.14) with alkylidene

substitution has increased conjugation lengths and better coplanarity compared to

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PDAFs. This polymer gave a FET mobility of 2×10−3 cm2/V s and on/off ratios of

104-106 at its nematic mesophase [168].

Scheme 2.1.14

Besides the PF homopolymers, many PF copolymers have been developed for

application in polymer FETs. Some of them have been discussed above as light-

emitting and photovoltaic materials. The classical green copolymer derived from 9,9-

dioctylfluorene and BTZ (49, Scheme 2.1.8) has higher electron-transport ability than

PFO, and exhibits ambipolar properties with an electron mobility of 10−3 cm2/V s and

a hole mobility of 2×10−3 cm2/V s [169, 170]. Polymers 73c and 75a that were widely

investigated as PSC materials have also been reported as hole conductors with

mobilities of 1.1×10−3 cm2/V s and 0.03 cm2/V s, respectively [171, 172]. The above-

mentioned polymers 79a-b showed even higher hole mobilities of up to 0.2 cm2/V s

in OFETs [162].

The most promising fluorene-based polymers for OFETs are fluorene and

bithiophene copolymers, such as 82a-b [173]. Polymer 82a possesses excellent

thermotropic liquid crystallinity and a hole mobility of up to 0.01–0.02 with

alignment of polymer chains in the LC phase [173, 174]. Inkjet printing of this

polymer after annealing led to a transistor with a charge mobility of 0.02 cm2/V s with

an on/off ratio of 105 [175]. By contrast, polymer 82b provided a relatively lower hole

mobility of 8.4×10−5 cm2/V s, probably owing to its unaligned polymer chains [176].

Replacing thiophene rings in 82a by selenophene moieties, polymer 83 was prepared

and presented better FETs performance. Solution-processable polymer FETs based on

83 with a thermotropic liquid-crystalline phase exhibited a hole mobility of 0.012

cm2/V s and a low threshold voltage of -4 V [177]. Another polymer 84 containing a

fused thieno[3,2-b]thiophene in the PF backbone yielded better FET mobility of

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1.1×10−3 cm2/V s by comparison with 82a (4.0×10−4 cm2/V s) under the same

processing conditions. This improvement could be attributed to its higher crystallinity

and a more ordered morphology [178].

2.1.2.1.5 Alcohol/water-soluble PFs

Alcohol/water-soluble PFs have become an important class of conjugated polymers

for sensor applications partially due to their ease of accessibility and relatively high

fluorescence quantum yield. These polymers typically have a conjugated PF main

chain providing good conductivity for charge transport and polar pendant groups

endowing the polymers with good solubility in highly polar solvents such as methanol,

ethanol, and even water. More importantly, these alcohol/water-soluble polymers can

be deposited as electron injection and transporting layers (EIL) on top or below

another active layer casted from unpolar solvents (principle of orthogonal solvents)

for fabricating fully solution-processed devices such as LECs, PLEDs and solar cells

[31a]. In such devices fabricated using alcohol/water-soluble polymers as an EIL,

high-work-function metals (such as Al, Ag, or Au) can be employed as the cathode

because of their good environmental stability and the simplicity for device processing.

Typical polar groups attached to PF side chains include dimethylammonium and their

quaternary salts (85-88) [179-182], diethanolammonium (89 and 90) [183-185], and

phosphonate (91) [186]. Several representative examples of alcohol/water-soluble PFs

are shown in Scheme 2.1.15.

Scheme 2.1.15

Ammonium-functionalized polyfluorene 85 and its quaternary salt analogue 86 as

EIL materials for highly efficient PLEDs were first reported in 2004 [180]. The

unique electron injection property of these alcohol/water soluble conjugated polymers

was demonstrated, leading subsequently to rapid development of EIL materials. A

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cationic water-soluble polyfluorene 87 containing ion-transporting side groups and

alkali or alkaline earth metal ions was synthesized. For PLEDs with polymer 87 as

EIL, a high LE of up to 15.4 cd/A was found for green fluorescence using Al cathodes.

In particular, when the Ca containing derivative of 87 was used, an extremely high

external quantum efficiency of 4.8% was obtained, approaching the theoretical limit

of about 5% [181]. Although these polyelectrolytes as EIL can improve electron

injection from high-work-function metals, they sometimes encounter problems with

counter ions migration into the emissive layer, thereby reducing the long-term

stability of such devices [187]. In addition, it has also been demonstrated that the

electron injection ability of these polyelectrolytes is strongly influenced by the nature

of their counter ions (88) [188]. Therefore, neutral PF-based organic surfactants were

developed as EIL materials.

The neutral organic surfactants, diethanolammonium-functionalized PFs 89 and 90,

were synthesized and applied as EIL materials to improve the device performance of

PLEDs [183, 184]. Highly efficient phosphorescent PLEDs using 89 as the EIL

exhibited a LE as high as 43.0 cd/A for green emission and 14.2 cd/A for blue

emission with a stable Al cathode. These efficiencies were much higher than those

made from commonly used CsF/Al cathode [183]. For Li2CO3-doped polymer 90 as

the ETL, white light-emitting PLEDs with a maximum LE of 36.1 cd/A and PE of

23.4 lm/W were achieved [185].

Phosphonate-functionalized PFs (91) are another class of important neutral

polyelectrolytes. Not only they serve as EIL materials [101, 116], but also as EL

polymers themselves as well as candidates for chemo sensor applications [186, 189,

190]. As chemosensory materials, these polymers are highly sensitive and selective

for Fe3+ with an emission quenching up to 210-fold upon addition of Fe3+. The PLEDs

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constructed with 91 as emitting layer and Al as cathode display a maximum LE of 4.0

cd/A, which is attributed to the efficient electron injection from Al to the active layer

due to the phosphonate groups. Additionally, the above mentioned single-component

white EL polymers 70-72 (Scheme 2.1.12) based on a mechanism of electron trapping

on the host were realized with this phosphonate-functionalized PF as the blue light-

emitting host material [146]. Similar water/alcohol soluble light-emitting polymers

with green, yellow, and red emissions (92) have also been obtained by doping

different narrow band gap units in the ammonium-functionalized PFs main chains

[191-193]. The alcohol/water-soluble PFs as conjugated polyelectrolytes for other

applications such as electrochromic color-changing displays, photodetectors, and

chemical and biological sensors can be found in a relevant review [194].

2.1.2.2 Other carbon bridged stepladder polyphenylenes

Besides PFs and PIFs, ladderized polytetra- and polypenta-phenylenes (Scheme

2.1.16) are important carbon bridged polyphenylenes. Their alkyl-substituted

homopolymers were initially developed based on the consideration of

bathochromically shifting the emission of PF to the pure blue region because they

possess chemical structures intermediate between PF and LPPP. Successively, many

derivatives based on these ladderized multi-phenylene monomers have also been

prepared and thoroughly investigated. In particular, copolymers derived from

indenofluorene (IF) attracted a lot of research interest.

The alkyl-substituted PIFs 93a-b (Scheme 2.1.16) were reported for the first time in

2000 [93]. Their absorption and emission bands are slightly red-shifted compared

with those of PFs, with the PL emission maxima in solution occurring at around 430

nm. The color stability of PIFs, however, was similar to that of PFs, demonstrating

that 93a-b were green emitters with solid-state luminescence (PL and EL) at around

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560 nm, although 93b was slightly more stable with blue emission to start but rapidly

undergoing a red shift [55]. This red-shift was ascribed to aggregation, and the green

emission located at 560 nm observed from 93a was attributed to more efficient

exciton migration to defect sites [195]. To circumvent the green emission problem,

several approaches have been adopted. Stable blue EL has been obtained from a

copolymer with anthracene 94, presumably because of exciton confinement on the

anthracenes [196]. The polymer 95 bearing diaryl side chains was also synthesized.

The emission from 95 was blue in both solution and the thin film without green

emission being observed [197]. Another polymer 96 containing spiro-linked

substituents at both carbon bridges also served for blue emission [198].

Scheme 2.1.16

Apart from endeavoring to achieve stable blue emission from IF homopolymers,

several IF-based copolymers have also been made and studied for organic electronics.

Bisthiophene and terthiophene have been used for copolymerization with IF to afford

polymers 97a-b, both of which exhibited green emissions peaked at 529 nm for 97a

and 554 nm for 97b. The FETs of the two polymers showed mobilities of 1.5×10−5

cm2/V s for 97a and 1.1×10−4 cm2/V s for 97b, respectively. The higher value of

terthiophene-containing polymer was attributed to highly regular dense packing

resulting in well-defined fibrillar morphology. By contrast, such dense π-stacked

structures were not observed for 97a [199, 200].

The IF was used to replace the fluorene in some of the above-mentioned fluorene-

based low-band-gap copolymers. For the polymers applied in the PSCs, the IF unit

was regarded as a donor component to deepen HOMO level and thus achieve higher

Voc, while for those used in the PLEDs it was expected to function as a blue light-

emitting host to realize long-wavelength and white emissions. By replacing fluorene

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in polymer 73c (Scheme 2.1.13) with IF, polymers 98a was synthesized. The

absorption band of this polymer became more intense and was red-shifted relative to

that of the fluorene-based analogue. A Voc of 0.77 V, a Jsc of 5.50 mA/cm2 and a PCE

of 1.70% were obtained from the 98a/PC71BM-based solar cells [201]. The FETs of

the polymers with hexyl and dodecyl at carbon bridges exhibited hole mobilities of up

to 0.011 cm2/V s [202]. Further replacing BTZ in 98a with thienopyrazine, polymer

98b was synthesized demonstrating a band gap of 1.6 eV that was lower than that of

98a (1.9 eV). However, a PSC of this polymer as a donor and PC71BM as an acceptor

yielded a PCE of only 0.45% [201]. Polymer 98c was prepared as an EL material with

the blue-green emission and low turn-on voltages [203]. White EL from an IF-based

polymer 99 was realized, in which IF functioned as a host material to replace fluorene

in polymer 67 (Scheme 2.1.10). By controlling the content of green and red dopants, a

white EL device was achieved with a maximum brightness of 4088 cd/m2 at 8 V and

CIE coordinates of (0.34, 0.32) [204].

Another important IF derivative is polymer 100, which was developed as the first

air-stable ambipolar polymer. The FETs based on this polymer exhibited balanced

electron and hole mobilities of 2×10−4 cm2/V s with on/off ratios of ~104 under

ambient conditions [205].

The emission from PIFs is still in the deep blue region and therefore

polytetraphenylene 101 was synthesized as a pure blue light-emitting polymer. The

maximum emission occurred at 442 nm in both solution and thin film that was more

close to human eye sensitivity than those from PFs and PIFs. However, the EL

spectrum exhibited an extra band at 510 nm, which did not appear to be a result of

defects from oxidation, as heating 101 in air produced a band at 575 nm, but was

interpreted to arise from an interaction between the polymer and the metal cathode

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[206]. When copolymerizing with dithienobenzothiadiazole (DTBT), this copolymer

102 showed promising performances in solar cells. A PCE of 3.67% with a Voc of

1.06 V was realized using a 102/PC61BM blend, while a higher PCE of 4.5% was

obtained when using PC71BM as the acceptor [202].

Continuing to extend the size of the ladderized multi-phenylene monomers with

carbon bridges, ladderized pentaphenylene and its corresponding polymer 103 were

designed and synthesized. The emission from this polymer was pure blue with an

emission maximum at 445 nm and a very small Stokes shift. The EL device provided

a best LE of 0.5 cd/A with CIE coordinates of (0.17, 0.09). An analysis of the

sensitivity curve proved that polymer 103 had its maximum at a region where the

human eye is 3 times as sensitive as for PIFs and 6 times as sensitive as for PFs,

generating a sufficiently pure blue emission spectrum optimized to the human eye.

Although the blue emission from 103 was much more stable than that from alkyl-

substituted LPPP, a long wavelength emission band still appeared rapidly upon

heating in air [94]. To replace the alkyl chains at all bridgehead positions in 103 with

aryl substituents, polymer 104 was prepared, which gave remarkably stable blue

emission in PLEDs and was not subject to oxidative degradation-induced defect

formation even after prolonged heating at 200 oC in air [207].

2.1.2.3 Polycarbazoles (PCz) and other N-containing bridged stepladder

polyphenylenes

The incorporation of nitrogen atoms instead of methylene bridges into a

polyphenylene changes the chemical and electronic behavior of the polymers [13].

This can be tracked back to varying reactivity of the positions around the benzene

rings and the increased HOMO energy levels resulting in electron-rich materials. The

simplest nitrogen-bridged polyphenylenes, namely polycarbazoles (PCz), are one of

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the most widely studied organic semiconductors on account of their photoconductive

properties and ability to form charge transfer complexes arising from the electron-

donating character of the carbazole moiety [31b, 208]. Other stepladder

polyphenylenes containing nitrogen bridges, for example poly(indolo[3,2-

b]carbazole)s [209] and poly(diindenocarbazole)s [206], have also been developed

and studied as active materials in PLEDs and PSC.

Early studies on PCz were centered on non-conjugated poly(N-vinylcarbazole)

(PVK) that could be used as photoconductive polymers, and serve as host materials in

PLEDs because of their high energy singlet excited state with absence of low energy

triplet state. They also serve as hole transporting material blended with TiO2 in

DSSCs [210]. Then conjugated PCz was synthesized and investigated widely. PCz

can be synthesized by two ways. The first is to connect the carbazole units at the 3,6-

positions to form poly(3,6-carbazole)s (105, Scheme 2.1.17), and the second is to link

them via the inactive 2,7-positions to yield poly(2,7-carbazole)s (106). While in

carbazole itself electrophilic substitution occurs at the 3,6-positions rather than at the

2,7-positions as in fluorene. Poly(3,6-carbazole)s and their derivatives can thus be

easily obtained using 3,6-disubstituted carbazole as monomers. However, their

applications in organic electronics are limited owing to the difficulties of obtaining

high molecular weights and because of poor conjugation within the kinked backbone.

Despite this, 3,6-carbazole-based polymers are still used as blue emitters themselves

and as host materials because of their large band gap and good hole transporting

ability [211-213]. An OFET based on n-butyl substituted poly(3,6-carbazole) showed

a hole mobility of 10-3 cm2/V s [214]. On the other hand, poly(3,6-carbazole)

possesses a high triplet energy level, which renders them interesting as hosts for blue

or green phosphorescent emissions with high EL efficiencies [215]: for example,

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green electrophosphorescent polymers with poly(3,6-carbazole) as host were recently

designed and synthesized, showing high efficiency of 33.9 cd/A [216].

Scheme 2.1.17

Poly(2,7-carbazole)s cannot be prepared by routes starting from carbazole owing to

the weak reactivity of 2,7-positions as mentioned above. Following a route suggested

by Cadogan for the reduction of nitrobiphenyls in order to synthesize 2,7-

functionalized carbazole, poly(2,7-carbazole)s and their derivatives were readily

obtained by various polycondensations and have attracted a lot of research interests

[217, 218]. These polymers were initially applied in blue PLEDs. Poly(N-alkyl-2,7-

carbazole)s 106 produced stable blue emission with maximum wavelength at 437 nm

in the solid state, without green emission appearing after annealing or upon operating

an LED [218, 219]. However, the EL efficiencies for devices using these polymers

were modest [220].

In recent years, poly(2,7-carbazole)s have been regarded as a promising class of

polymers for photovoltaic applications thanks to good hole transporting properties,

great versatility to fine tune the bandgap, low HOMO energy levels that provide good

air-stability and a high VOC in BHJ solar cells [210]. The OPV data of these polymers

are presented in Table 2. The first solar cell using poly(n-alkyl-2,7-carbazole) as

active material was reported in 2006 [221]. Long and branched alkyl chains were

introduced (107, Scheme 2.1.18) to improve the solubility and processability of the

PCzs. In first attempts towards solar cells, both perylenetetracarboxydiimide (PDI)

and PCBM were compared as acceptors. Thereby it was confirmed that the 107/PDI

(1:1) blend devices afforded a 3 times higher PCE than those of 107/PCBM-based

ones under the same conditions. The device with 107/PDI (1:4) blend gave an overall

PCE of 0.63% [221]. Another small-molecule acceptor based on 2-vinyl-4,5-

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dicyanoimidazole was also applied to blend with 107 for BHJ solar cells, yielding a

bit higher PCE of 0.75% [222].

Scheme 2.1.18

To improve the light-harvesting ability, oligothiophenes were introduced into the

poly(2,7-carbazole)s main chains (108a-e). This method could tune the band gap of

the polymers depending on the addition of thiophene units. The best device

performance was achieved from 108e with a PCE of 0.8% [223].

All the above-mentioned poly(2,7-carbazole)s-based polymers exhibited modest

efficiencies, typically lower than 1%. The breakthrough in terms of PCE of poly(2,7-

carbazole)s was obtained by incorporating strong acceptors into the polymer chains.

Polymers 109a-f with different acceptor units were therefore synthesized and

provided promising device results [224]. Polymer 109c had a band gap of 1.88 eV, a

HOMO level of −5.5 eV, and exhibited a hole mobility of 1×10-3 cm2/V s and a high

PCE of 3.6% [225]. Carefully optimizing the processing condition of device

fabrication allowed an improvement of the PCE up to 4.6% [226]. The 109c-based

device efficiency was further improved to 6.1% when blending with PC71BM and

inserting a hole-blocking layer of TiOx under aluminum cathode [227]. By inserting

an alcohol/water-soluble PF-based polyelectrolyte (85, Scheme 2.1.15) as an

interlayer between the active layer and Al cathode, the efficiency of 109c:PC71BM

based devices was pushed further to 6.79% [228]. Quite recently, a similar PCE of

6.77% with a FF of 70.7%, based on an inverted PSC with 109c:PC70BM blend as

active layer, the MoO3-Al composite film as the cathode buffer layer, and the

MoO3/Al as the anode, was achieved [229].

As alternatives to BTZ in 109c, as described for the PF copolymers, TQ and DPP

have also been introduced into poly(2,7-carbazole) to develop polymers 110 and 111

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[230-232]. TQ-based polymers usually showed low Voc and thereby modest PCE. For

example, the devices with 110c:PCBM (1:1) as the active layer exhibited a PCE of

0.61% with a low Voc of 0.41 V [230]. DPP-containing polymer 111a delivered a PCE

of 1.6% based on the 111a:PCBM BHJ solar cells, but the performance reached 2.3%

by varying the alkyl side chains on both the carbazole and the DPP units (111b) [159,

232]. Another 2,7-linked carbazole-based conjugated copolymer (112) containing

ladderized pentaphenylene with diketone bridge as an acceptor was developed,

showing potential for photovoltaic devices due to the efficient energy and charge

transfer between donor and acceptor units in the solid state [233].

Apart from PCz, other nitrogen-containing bridged stepladder polymers such as

indolo[3,2-b]carbazole-based polymers 113 and 114 with all-nitrogen bridges, and

polymers 115-117 with both carbon and nitrogen bridges (Scheme 2.1.19) were

developed for optoelectronic applications [206, 209, 234]. Poly(diindenocarbazole)

115 displayed blue emission without an extra band at 510 nm in the EL spectrum

compared with polymer 101 (Scheme 2.1.14) containing all carbon bridges. The EL

efficiency, however, from unoptimized devices was as low as 0.1 cd/A [206].

Comparing 116 and 117, it was found that the emission was red-shifted and the

HOMO energy level was higher for the latter because of the larger number of

electron-donating nitrogens. When applying them in solar cells using blends of the

polymers with a fullerene, polymer 117 gave the best device result with a PCE of

0.74% [234]. Indolo[3,2-b]carbazole-containing homopolymers were obtained by

both meta- and para-linked ways and they exhibited obvious differences in charge

carriers transport and conductivity [209, 235]. Indolo[3,2-b]carbazole exhibited

stronger electron-donating ability and higher hole mobility than carbazole [236] and

thus were applied as a donor unit for copolymerization with acceptors. Polymers

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118a-e and their analogues with meta-coupled indolo[3,2-b]carbazole in the main

chains were thus developed for PSC applications [237-239]. Within this series of

polymers, 118c exhibited a best photovoltaic performance with a PCE as high as 3.6%,

a VOC of 0.69 V, JSC of 9.17 mA/cm2 and a FF of 0.57, applying a device structure of

ITO/PEDOT:PSS/118c:PCBM/LiF/Al [238].

Scheme 2.1.19

2.1.2.4 Other heteroatom bridged stepladder polyphenylenes

Besides nitrogen, other heteroatoms including silicon, germanium, sulfur, and

phosphorus have been used to replace the methine bridges in stepladder

polyphenylenes. Dibenzosilole (or silafluorene) containing a silicon atom at the

bridge position is an analogue of fluorene. Silicon has similar electronic properties to

carbon, but can not undergo oxidation to form a ketone. Therefore, it was anticipated

that dibenzosilole-based homopolymers would not display an additional green

emission band as observed from the PF homopolymer. To verify this, poly(2,7-

dibenzosilole) with alkyl side chains (119, Scheme 2.1.20) has been reported to show

similar EL emission to PF with an maximum wavelength at 425 nm but no

degradation visible upon heating in air at 200 oC for 4 h, whereas its PF analogue

produced a strong green emission under the same conditions [240]. Like carbazole,

dibenzosilole can also be polymerized at 3,6-postions to form polymer 120, which has

a large band gap of 4.0 eV, and thus can serve as a potential host for long-wavelength

singlet and triplet emissions [241].

Scheme 2.1.20

Dibenzosilole was incorporated into the PF backbone to prepare blue light-emitting

copolymers [240, 242]. The 3,6-dibenzosilole introduced into the PFO main chains

(121) can not only inhibit the additional emission bands but also enhance the color

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purity and luminous efficiency of the copolymers. The device with the configuration

of ITO/PEDOT:PSS/PVK/121/Ba/Al yielded an EQE of 3.34% and a LE of 2.02 cd/A

with the CIE coordinates of (0.16, 0.07) [242]. The copolymer (122a) containing 3,6-

dimethoxy-2,7-dibenzosilole in the PFO main chains exhibited an obvious steric

hindrance effect which lowered the degree of polymerization and in addition resulted

in a blue-shift of the PL [243]. To replace the silicon in polymer 122a by a

germanium atom, polymer 122b was designed and synthesized. A single-layer EL

device of 122b exhibited an efficient blue-light emission with a maximum brightness

of 2630 cd/m2 at 7.8 V. This polymer was also employed as a host material for

phosphorescence with high brightness and efficiency [244].

Similar to fluorene and carbazole, dibenzosilole has also been copolymerized with

electron-accepting DTBT to produce polymer 123a for PSC. From a film of this

polymer two broad absorption peaks at 391 nm to 560 nm were measured providing

an optical band gap of 1.85 eV. A 123a:PCBM blend device displayed a PCE of 1.6%

under AM 1.5 (90 mW/cm2) illumination [245]. A higher PCE of 5.4% with a similar

device structure was achieved for this polymer with higher molecular weight [246]. A

structurally similar polymer 123b with germanium in place of silicon was reported to

display a PCE of 2.8% from a blend device with PC71BM [247].

Another analogue of fluorene is dibenzothiophene that contains sulfur at the

bridgehead. The sulfur atom can be easily oxidized to a sulfonyl group as an electron-

withdrawing building block. Both of them were used to prepare sulfur bridged

stepladder polymers upon copolymerizing with fluorene (124 and 125) [248-250].

The introduction of sulfur-containing moieties into PFs improves the color purity,

lowers the LUMO energy, and balances the charge injection and transport [248, 249].

The sulfone-based polymer 125b as the active layer enabled fabrication of PLEDs

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emitting saturated blue light with a LE of 3.1 cd/A and an EQE of 3.9% [250].

Reports on phosphorus-bridged polyphenylene are relatively rare. One example is

polymer 126a containing a repeating unit of dibenzophosphole that is a phosphorus-

bridged analogue of fluorene, whose fluorescence turned green with a maximum

emission at 516 nm in the solid state, being a candidate for PLEDs [251]. The

introduction of this phosphorus-bridged building block into PFO main chains

produced copolymer 126b, which displayed red-shifted absorption and emission

bands along with a reduced band gap because of the oxidation of the phosphorus

centre [252].

2.2 Polycyclic aromatic hydrocarbons-based conjugated polymers

Polycyclic aromatic hydrocarbons (PAHs) define a unique class of compounds that

consist of fused aromatic rings that do not contain heteroatoms or carry substituents.

PAHs have attracted enormous research interest due to their unique electronic and

optoelectronic properties as well as the potential applications in organic electronics

[253]. Some examples of PAHs are shown in Figure 4. Among them, several well-

known PAHs with simple structures such as phenanthrene, triphenylene, and pyrene

have been utilized to prepare conjugated polymers. Other unconventional PAHs

containing methylene bridges have also been developed as building blocks to

construct novel polymer semiconductors. The optoelectronic properties of these

PAHs-based conjugated polymers were carefully investigated and some of them were

also applied in PLEDs and PSCs.

Figure 4

Phenanthrene is one of the simplest PAHs and can be polymerized at different

positions to form two kinds of polymers, poly(2,7-phenanthrylene)s 127 and poly(3,6-

phenathrylene)s 128 (Scheme 2.2.1). The introduction of alkyl or aryl substituents at

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the 9,10-positions of the phenanthrene rendered these molecules soluble and

processable. In case of alkyl substitution, however, strong aggregation was indicated

by a large bathochromic shift of the fluorescence of the film compared to their

solution property. This aggregation could then be efficiently suppressed by aryl

substitution. The arylated polymers displayed deep blue EL and good color stability

when tested in PLEDs [254].

Scheme 2.2.1

Triphenylene, a small discotic PAH, has also been used as a repeating unit to

synthesize homo-and co-polymers (129 and 130) with a variety of surrounding alkyl

and aryl substituents for PLEDs [255, 256]. Because of the twisted phenyl rings

around the triphenylene core, π-π stacking in the polymers was prevented, resulting in

almost identical PL spectra in both solutions and films. All polymers exhibited narrow

emission in the range of 430-450 nm, where the human eyes are most sensitive for the

blue range. The best performance in terms of turn-on voltage (4.6V) and LE (0.73

cd/A) was obtained by blending with a hole-transporting material.

Pyrene, a well-known PAH with unique properties, has inspired researchers from

many scientific areas, making pyrene the chromophore of choice in fundamental and

applied photochemical studies [257]. For preparing pyrene-based conjugated

polymers, there are three ways to link the pyrene into the polymer main chains, as

shown in Figure 5. The 1-, 3-, 6-, and 8-positions of pyrene are reactive for the

electrophilic substitution, while the 2- and 7-substituted pyrenes are not directly

accessible from pyrene itself. Taking this into account, 2,7-linked conjugated

polypyrenylene 131 bearing four aryl groups was synthesized by introducing

substituents prior to the annulation of pyrene. This polymer exhibits an intriguing blue

fluorescence and a remarkable long-wavelength tail [258]. For another 2,7-linked

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poly(tetraalkoxypyrenylene) 132, it was very important to verify that the fluorescence

maxima in solution and in the film were almost identical, such that the alkoxy groups

seem to hinder aggregation induced red shifting of the fluorescence. In the LED

devices, however, a strong red shift appeared [259].

Figure 5

Pyrene was introduced via 1,6-linkages into the backbones of PPV and

polyfluorenevinylene to produce copolymers 133a-b [260, 261]. For both polymers

red-shifted emissions in thin films and significant decrease of PL efficiencies were

found, while blue-green fluorescence were observed in solution, indicating the

formation of excimers/aggregates in the solid state. This result suggested that the

introduction of double bond at the 1,6-positions of pyrene into the polymer main

chains is not effective on avoiding aggregation.

A novel 1,3-linked polypyrenylene 134 was reported with stable blue emission. No

aggregation was observed in thin films due to the large dihedral angle of ca. 70o

between the neighboring pyrene units providing a highly twisted structure of the

polymer backbone. OLED devices with the setup ITO/PEDOT:PSS/134/CsF/Al

exhibited bright blue turquoise EL with a maximum at 465 nm and a profile similar to

the solid state PL [257,262]. Modest LE of 0.3 cd/A and luminance of 300 cd/m2 were

obtained [262].

Some PAHs have been fused by five- or six-membered rings to form new aromatic

units carrying solubilizing alkyl chains for preparing soluble conjugated polymers.

For example, a new fused building block, cyclopenta[def]phenanthrene, can be

considered either as a phenanthrene fused with a five-membered ring at the bay

position or as a fluorene bridged by a double bond. Homopolymer 135a (Scheme

2.2.2) exhibited stable and efficient blue electroluminescence without additional

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emission bands relative to PDAFs. Its monolayer PLEDs gave a maximum LE of 0.70

cd/A at 180 mA/cm2 with CIE coordinates of (0.17, 0.12) [263, 264]. To incorporate

BT and DTBT into the backbone of 135a, copolymers 135b and 135c were

synthesized [265, 266]. The introduction of BT units induced a red shift of the

emission to the green region with maximum peaks between 510–535 nm owing to an

energy transfer process and an increased electron affinity of polymer 135b. EL

devices based on a copolymer with a BT content of 30% yielded the highest LE of

1.25 cd/A [265]. Copolymer 135c has a low band gap of 2.0 eV and emits red light in

diluted solution, with an emission maximum at 621 nm. A PSC fabricated from

135c/PC71BM blend afforded a PCE of 1.0% [266].

Scheme 2.2.2

Another novel building block, dinaphtho-s-indacene, a fused naphthalene-phenylene-

naphthalene with two methylene bridges, and its conjugated polymers (136 and 137)

have been developed as EL materials [267]. These two polymers were designed by

replacing two benzene units in indenofluorene with naphthalenes, which caused an

extension of conjugation perpendicular to rather than along the polymer backbone.

This provided slightly red-shifted emission making the polymer more sensitive to the

human eye, while the steric hindrance between the adjacent repeating units favors the

amorphous character, and thereby enhances their spectral stabilities in the solid state.

Both polymers exhibited more stable PL spectra than that of PFO while annealing

upon 200 oC in an inert atmosphere. The EL devices with the configuration of

ITO/PEDOT:PSS/136/Ca/Al turned on at 3.7 V, and emitted at a maximum of 461 nm

with a maximum LE of 1.40 cd/A, a maximum brightness of 2036 cd/m2, and CIE

coordinates of (0.19, 0.26). Meanwhile, the emission color of the devices was

independent on the driving voltage and remains unchanged during continuous

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operation [267]. By incorporating alkylated dinaphtho-s-indacene into the backbone

of PFO (48, Scheme 2.1.7), higher LE (3.43 cd/A) and brightness (6539 cd /m2) as

well as purer blue emissions were achieved, as discussed above [101].

Stepladder and ladder polymers 138 and 139 were prepared containing planarized

9,10-diphenylanthracene within the main chain. The idea underlying the design of

these two polymers was to planarize 9,10-anthrylene and phenylene units, whose

alternating connection otherwise induces a strong twist around the aryl-aryl bond

limiting conjugation. Upon bridging the aryl units with methylene carbons and

attaching solubilizing substitutents paved the way to these new polymers. This

planarization led to a strong bathochromically shifted fluorescence with yellow

emission for the stepladder polymer 138 (λmaxem = 584 nm) and even red emission for

the ladderized polymer 139 (λmaxem = 693 nm). Upon irradiation with visible light in

air, however, both polymers underwent photo-oxygenation with colorless

endoperoxides being formed. The peroxide formation proved to be reversibly, since

the oxygen could be removed upon thermal treatment [268].

A new fused unit, 5,10-dihydroindeno[2,1-a]indene, has also been reported and the

corresponding homopolymer and D-A copolymers were synthesized for PLED and

PSC applications. Polymer 140a can be seen as an analogue of PPV, in which the

vinylene group in the main chain was cyclized using two five-membered rings,

therefore reducing the oxidation ability of the vinylene groups. As a result, polymer

140a exhibited higher stability than PPV as demonstrated by their UV-vis, and PL,

and EL spectra under the same conditions. The copolymers made from this new

building block and BTZ as well as DTBT (140b and 140c) were introduced as the

active layer in a PSC. The best performance was observed by blending DTBT-based

polymer 140c with PCBM where a PCE of 1.88% was achieved [269].

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2.3 Thiophene-containing conjugated polymers

The thiophene ring has become one of the most widely used building blocks for

construction of conjugated polymers. These thiophene-containing polymers have

attracted comprehensive research interest as active components in organic electronic

devices. Thiophene chemistry has been well established and developed for a long time,

allowing tuning of the electronic properties in a wide range via enormous and

attractive variations in structure. In particular, the polymers containing thiophene

moieties present intriguing electronic, optical, and redox properties as well as unique

self-assembling abilities on solid surfaces or in the bulk. Moreover, the high

polarizability of sulfur atoms in thiophenes leads to a stabilization of the conjugated

chains and to excellent charge carrier transport, which are crucial assets for

optoelectronic applications [270].

Because of the strong electron-donating nature, thiophene and its benzo- and thieno-

fused derivatives have been widely utilized as donor units to construct D-A polymers

for applications in organic electronics. Through homopolymerization or

copolymerization with other electron-rich units, the resulting conjugated polymers

possess tunable optical and electronic properties owing to the enlargement of the π-

conjugation systems. In majority, these thiophene containing polymers exhibit hole

transport characteristics, and have been applied as p-type semiconductors for OFETs

and OPV. In this section, the focus will be on the one-dimensional thiophene and

fused thiophene-based conjugated polymers without additional electron-deficient

units, while such D-A polymers containing thiophene moieties will be separately

discussed in the next section.

First, we briefly summarize several representative polythiophene derivatives

(Scheme 2.3.1) taking their importance into account, although they have been

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mentioned in many reviews. Then we will discuss thiophene-based polymers

containing various fused-thiophene building blocks including multi-thienothiophene

(Scheme 2.3.2), bridged dithiophene (Scheme 2.3.3), benzodithiophene (Scheme

2.3.4), naphthodithiophene (Scheme 2.3.5), and other complex fused thiophene

(Scheme 2.3.6).

2.3.1 Polythiophenes and their derivatives

The early research on polythiophenes without substituents in the 1980s was carried

out for FETs applications, showing mobilities of only 10-5 cm2/V s [271]. In order to

improve the solubility and film-forming property, a variety of side chains were

introduced into polythiophene backbones, leading to the emergence of a large number

of polythiophene derivatives and related studies. One of them is poly(3-

hexylthiophene) (P3HT, 141, Scheme 2.3.1), a key polymer material applied in

OFETs and OPV. The regioregularity of P3HT plays an important role in the

molecular arrangement and subsequent device performances [272]. Regioregular

P3HT (head-to-tail coupling) exhibits mobilities from 0.05 to 0.2 cm2/V s by adopting

an edge-on orientation [272, 273]. The performance of P3HT in OPV is also

constantly enhanced by various optimization strategies, and a record PCEs of 6.5%

has been achieved when using proper acceptors [274].

Scheme 2.3.1

To improve the stability of polythiophene-based FETs, new polymers containing

longer but reduced number of chains were synthesized. Solution-processable

regioregular polythiophene 142a with half of the thiophene rings substituted by alkyl

chains afforded excellent hole mobility of 0.14 cm2/V s with on/off ratio higher than

107 under ambient conditions [275]. A selenophene-containing analogue 142b

exhibited a lower mobility of 0.02 cm2/V s [276]. In fact, the selenophene-based

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polymers were initially developed as alternatives to polythiophenes for OPV

applications, however studies found that this kind of polymers showed generally

ambipolar charge transport [277]. For examples, polymers 143 and 144 provided

electron and hole mobilities at the level of 10-2 cm2/V s [277, 278]. Introducing big

aromatic units such as naphthalene (145) can increase the torsion of the polythiophene

backbones, resulting in lower HOMO levels and thus better stability, but this

approach usually sacrifices the charge carrier mobility [279].

Recent developments in polythiophenes concentrated on 3,4-disubstituted

polyalkylthiophenes. Polymer 146a-c had comparable transistor mobilities of 0.17

cm2/V s and greater environmental stability than regioregular mono-substituted

poly(3-alkylthiophenes), although they did not undergo more distinct π-π stacking as

evidenced from X-ray diffraction studies. The PCEs of these polymers in

polymer:fullerene BHJ solar cells reached 4.2% [280]. Further studies on these

polymers in comparison with P3HT demonstrated that the degree of polymer

backbone twisting induced by 3,4-disubstituents increased the ionization potential and

enhanced the Voc while retaining the Isc. This molecular design provides a simple

method to tune the degree of backbone twisting in polymer backbones for the

optimization of organic electronic devices [281].

2.3.2 Thienoacene-containing conjugated polymers

Fused aromatic units like thieno[3,2-b]thiophene or even larger oligothienoacenes

introduced into the polythiophene backbones can lower the HOMO level and increase

the stability of the polymers. This is due to the larger resonance stabilization energy

of the extened conjugated units compared to the single thiophene ring improving the

delocalization of electrons [282]. Thieno[3,2-b]thiophene-based polythiophenes 147a-

e (Scheme 2.3.2) were thus synthesized and show HOMO levels of 0.3 eV lower than

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that for P3HT. Transistors based on these polymers exhibited mobilities of 0.2-0.7

cm2/V s, whereas 147c can provide a value higher than 1.0 cm2/V even using high

work function metals such as Pt as the source and drain electrodes [282-284]. By

tuning the dip-coating speed, a monolayer and subsequent microstructure of 147c on

the surface can be precisely controlled. At a low dip-coating speed, the polymer

chains are uniaxially oriented, yielding pronounced structural anisotropy and high

charge carrier mobilities of 1.3 cm2/V s in the alignment direction [285].

Other thienoacenes (148-150, Scheme 2.3.2) with further extended conjugation have

also been incorporated into polythiophene backbones to develop p-type

semiconductors. For the trithienoacene polymers, 148c with alkyl chains only at

thiophene rings exhibited an excellent hole mobility of 0.3 cm2/V s with a very high

current on/off ratio of over 107 [286], remarkably higher than those of the other two

polymers (1.7×10-3 cm2/V s for 148a [287] and 0.05-0.06 cm2/V s for 148b [288]).

Polymer 148b has also been investigated in a PSC device by blending with PC71BM,

affording a best PCE of 3.2% [288]. Comparison of tetrathienoacene polymers 149a-c

indicate that 149b with bithiophene as a comonomer affords higher mobility of 0.33

cm2/V s [289] than 149a and 149c (10-2 cm2/V s for both) [290], whereas for

pentathienoacene polymer 150 a lower mobility of only 2.3×10-3 cm2/V s was found

[287]. The difference in the field-effect behavior of these thienoacene-based polymers

could be attributed to the influence of C2 symmetry of different thienoacenes on the

lamellar spacing [290].

Scheme 2.3.2

2.3.3 Bridged bithiophene-containing polymers

Bridged bithiophenes are a class of very promising fused aromatic building blocks

with structural similarity to the aforementioned bridged biphenyl (fluorene, carbazole,

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and dibenzosilole). They exhibit a completely planar structure, indicating good π-

conjugation across the fused rings. Typical bridged bithiophenes include dithieno[3,2-

b:2’,3’-d]pyrrole (DTP), dithieno[3,2-b:2’,3’-d]silole (DTS), and cyclopenta[2,1-

b:3,4-b’]dithiophene (CPDT). Unlike sulfur-bridged bithiophene (trithienoacene,

corresponding polymers 148 in Scheme 2.3.2), the nitrogen-, silicon-, and carbon-

bridged bithiophene can be introduced with various solubilizing groups at the bridge

atoms and they do not interfere with the conjugation of the backbone. Many p-type

polymer semiconductors derived from these building blocks have been developed in

recent years and several examples are presented in Scheme 2.3.3.

Scheme 2.3.3

DTP was copolymerized with several alkyl substituted thiophene and bithiophene

comonomers to yield polymers 151a-f [291]. The incorporation of soluble substituted

thiophenes and planar DTP units resulted in low band gap polymers with high

conductivity. Optical characterization revealed that the band gaps of 151a-f were

between 1.74 and 2.00 eV, lower than for regioregular P3HT. Their HOMO energy

levels were estimated from CV characterization between −4.68 and −4.96 eV. Among

these polymers, thin films of 151c and 151d just drop-casted exhibited poorly defined,

randomly ordered lamellar structures, which improved significantly after thermal

annealing. Interestingly, the mobilities of the less ordered samples were much higher

than those observed after annealing. The highest performance was obtained from

151d with a maximum mobility of up to 0.21 cm2/V s. These results suggest that the

presence of highly ordered microcrystalline structures in thin films of organic

semiconductors is not always necessary for decent performance of organic transistors

[280], but this finding rather seems like an exception and cannot be generalized.

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DTS has been used as a repeating unit to prepare homopolymer and mono- and

bithiophene-containing copolymers (152a-c) with good hole mobility, solution

processibility, and air stability [292]. FETs fabricated on HMDS-passivated SiO2/Si

substrates under ambient conditions gave hole mobilities of 0.08 cm2/V s, low turn-on

voltages, and current on/off ratios 106. CPDT and fluorene alternating copolymers

153a and 153b have also been investigated as semiconductor layers in polymer FETs.

However, they only exhibited hole mobilities at a level of 10-6~10−7 cm2/V s because

of being amorphous in the solid state [293], while pentacene-containing polymer 153c

gave mobility between 10-3~10−4 cm2/V s. The impact of regioregularity and direction

of conjugation extension on thin-film order and device performance has also been

demonstrated for 153c [294]. In fact, CPDT is a very promising electron donor when

combining with acceptors in a D-A polymer system, which will be discussed in the

next section.

2.3.4 Benzodithiophene-containing conjugated polymers

Benzodithiophenes (BDT) are another class of important building blocks containing

fused-ring thiophenes. There are several structural isomers which possess different

linkages and thereby varying geometries (Scheme 2.3.4). The influence of the degree

of curvature along BDT-based polymer backbones on the solubility, the electronic

levels, the film morphology, and the charge carrier mobility in OFETs has been

investigated in depth, e.g. by comparing the five copolymers (154a-e) containing

different BDT isomers and alkyl substituted bithiophene as comonomers [295]. It has

been shown that the optical gap increased with increasing curvature of the polymers,

probably because of a reduced effective conjugation length. The order in bulk is

owered by increasing the curvature, but the π-stacking distance remained unaffected.

The more highly curved polymers possess reduced order in the film, whereas the less

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curved ones have too low solubility for processing. As a result, FETs measurements

revealed that polymer 154c with an intermediate degree of curvature possessed good

solubility combined with high order and thus highest mobility of 0.13 cm2/V s in this

series of polymers. These findings may serve as a guideline for the rational design of

other semiconducting polymers.

Scheme 2.3.4

Further studies focused on the benzo[2,1-b;3,4-b’]dithiophene-containing homo- and

copolymers 155 and 156a-c. After rationally optimizing the molecular structure by

regulating the positions at which alkyl chains were attached, the best polymer 156c

(same as 154c) was sought out displaying an easy way of solution processing, low

hysteresis behavior, and high charge-carrier mobility without any long-term annealing.

The curvature of the monomer unit guarantees an optimum compromise between

solubility and aggregation tendency towards formation of highly ordered films. Top-

gate devices based on 156c on a polyethylene terephthalate (PET) film exhibited a

mobility as high as 0.5 cm2/V s [296].

Benzo[1,2-b:4,5-b’]dithiophene carrying solubilizing alkyl chains at the central

benzene ring is the most widely used BDT isomer for the synthesis of conjugated

polymers [297]. A homopolymer 157 with dodecyl-substituted benzo[1,2-b:4,5-

b’]dithiophene as a repeating unit exhibited a modest mobility of 0.012 cm2/V s [298],

while a hexyl-substituted bithiophene-containing copolymer 158 afforded higher

mobilities of 0.15-0.25 cm2/V s with current on/off ratios of 105-106 in the top-gate

OFETs, which was achieved without thermal post-treatment. Investigations of the

durability carried out over 30 days proved no significant changes in the transistor

characteristics of 158 [299].

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Recently, alkyl chains at the central benzene ring of benzo[1,2-b:4,5-b’]dithiophene

were replaced by alkoxy, thioalkoxy, and phenylethynyl side chains to synthesize

BDT-based polymers (159a-c and 160a-c) for PSC applications [300, 301]. The

change from alkoxy groups to thioalkoxy groups lowered the HOMO energy level of

the conjugated polymers from -5.31 to -5.41 eV, and consequently enhanced the Voc.

As a result, polymers 159a-c displayed Voc of 0.99, 0.91, and 0.83 V, respectively.

The polymer 159b-based PSC presented a PCE of 4.0%, which is one of the highest

efficiencies reported from a homopolymer-based PSC without thermal/solvent

annealing or incorporated additives. However, the hole mobilities of these polymers

were low with values of 1.3×10−5 cm2/V s for 159b and 1.1×10−6 cm2/V s for 159a

[300]. For phenylethynyl-substituted polymers 160a-c, the spacing between alkyl

substituents increased by introduction of thiophene and bithiophene units in the

backbone of the polymer was systematically studied. As the spacing between the side

chains increased, an increase of the weight ratio of polymer to fullerene acceptor was

required for achieving the highest PCE of BHJ solar cells. However, these polymers

exhibited moderate PCEs lower than 2% [301].

2.3.5 Naphthodithiophene-containing conjugated polymers

Naphthodithiophene (NDT), a very recently developed building block, is expected to

provide a highly rigid planar backbone when introduced into the polythiophene

system. An advantage of the structure of NDT over benzothienobenzothiophene

(BTBT, 161, Scheme 2.3.5) with two benzene rings fused at the end of thieno[3,2-

b]thiophene is to avoid twisting between the adjacent thiophene rings, which would

destroy the π-stacking and thereby reduce the charge carrier mobility. The BTBT

copolymer 161 gave a highly twisted backbone (λmax < 400 nm) and no OFET

response, despite the fact that BTBT has been successfully applied as a small-

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molecule semiconductor in OFETs. In sharp contrast, the naphtho[1,2-b:5,6-

b’]dithiophene cpolymers 162a-e exhibited high field-effect mobilities (i.e. >0.5

cm2/V s) as a result of the strong π-stacking [302].

Scheme 2.3.5

Similar to BDT, NDT also occurs in several isomers (163 and 164) with different

structural motifs extending linearly, in bent fashion, or appearing disc shaped. The

symmetry of the different isomers and the related curvature of the polymer chains

have impact on their HOMO levels and charge carrier mobility. NDTs with linear and

angular shapes (163a-d) have comprehensively been studied [303]. The angular-

NDT-based polymers 163a-b displayed lower HOMO levels and larger band gaps

than their linear counterparts (163c-d). The polymers with angular NDTs, on the other

hand, gave the highly ordered structures with a very close π-stacking distance of 3.6 Å,

whereas those with linear NDTs had a very weak or no π-stacking order, which

resulted in higher mobilities observed from the corresponding polymers 163a-b than

163c-d. As a result, the polymer163a bearing naphtho[1,2-b:5,6-b’]dithiophene, an

angular-shaped NDT, exhibited the highest mobility of ~0.8 cm2/V s among the four

polymers with isomeric NDTs.

Recently a disc shaped NDT isomer, naphtho[2,1-b:3,4-b’]dithiophene, has also

been copolymerized with thiophene and bithiophene to produce p-type polymers

164a-b recently [304]. Relatively, more highly ordered intermolecular structures were

found for 164b than for 164a because the bithiophene unit in the former polymer

provided crystallinity with increasing planarity and enough space for interdigitation of

the long alkyl side chains for high order. FETs measurements exhibited charge carrier

mobilities of 0.01 and 0.076 cm2/V s with current on/off ratios of 105 and 106 for 164a

and 164b, respectively, which was in agreement with observation of the structural

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ordering.

2.3.6 Other fused thiophene-containing conjugated polymers

Apart from those alluded to the previous discussion, some more complex fused-

thiophene building blocks have also been reported to serve as repeating units for

synthesis of linear polythiophenes. Herein, we will introduce several recent examples,

as depicted in Scheme 2.3.6.

Scheme 2.3.6

Indacenodithiophene (IDT) has further extended conjugation compared to NDT,

consisting of five fused rings along the backbone. It has been reported to homo- and

co-polymerize with several thiophene-based comonomers to produce polymers 165a-e

[305, 306]. Among them, polymer 165a exhibited amorphous thin-film microstructure,

as revealed by X-ray diffraction. FETs gave a mobility of 0.2 cm2/V s with the current

on/off ratio of 106 and excellent ambient stability [305]. Polymers 165b-e were

applied in PSC devices. The maximum PCE based on 165c/PC71BM system reached

3.3% [306].

Polymer 166 contains a larger fused π-system, tetrathiahexacene as a repeating unit.

The strong interchain aggregation led to a highly ordered bulk material with short π-π

distance, demonstrating the efficiency of introducing larger sized monomers. In an

FET, low contact resistance and low hysteresis were measured but a mobility of only

10-3 cm2/V s was obtained [307]. Another new heteroheptacene with the inclusion of

carbazole and thiophene units was synthesized and introduced into the polythiophene

backbone (167). The incorporation of heteroatoms in the fused-ring system lead to

small optical band gaps of these polymers. Similar to 166, this polymer exhibited a

low mobility of 10-4 cm2/V s in an FET [308]. A homopolymer derived from this

heteroheptacene bearing aryl side chains displayed a lower FET performance [309].

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Although both examples did not show exciting charge carrier mobilities, the results

supported the potential of conjugated polymers containing large fused building blocks.

Two isomers of S- and N-containing heteroacenes, namely, dithienocarbazoles (DTC)

were recently developed and copolymerized with alkyl substituted bithiophene to

produce 168a-c and 169a-b [310, 311]. Both polymers were quite soluble owing to

the introduction of additional alkyl substituents at the nitrogen atom. For polymers

168a-c, spun-casted films were characterized by a lamellar layered structure oriented

normal to the substrate. Bottom-gate, top-contact OTFT devices based on these

polymers demonstrated that the mobility increased with an increase of the alkyl chain

length in bithiophene units, with values of 0.0064, 0.022, and 0.035 cm2/V s for each

polymer, respectively [310]. The isomeric polymers 169a-b formed ordered thin films

in which the polymer backbones dominantly adopted an edge-on orientation to the

substrate with a lamellar spacing of ~24 Å and a π-stacking distance of ~3.7 Å upon

thermal annealing. The OTFT devices with mobilities of up to 0.39 cm2/V s have been

demonstrated with 169b. This value is the highest among the conjugated polymers

containing heteroacenes larger than four fused rings [311]. Especially the different

directions of the solubilizing side chains in 168 (all to one side) and 169 (alternating

up and down of the plane of the backbone) seem to control the face-on vs. edge-on

alignment in 168 and 169, respectively. Thus a further investigation on the impact of

the molecular shape but also the backbone curvature of the DTC-based polymers on

the structural property and charge carrier mobility should be quite worthwhile, as

described above for BDT- and NDT-containing polymers. For instance a further

improvement of the promising OFET characteristics of 169b, can be anticipated if the

π-stacking distance could be further reduced, by slight changes of the R groups or the

annealing conditions.

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New thiophene-based building blocks have also been designed by fusing thiophene

rings to form disc-shaped molecules. Benzo[2,1-b:-3,4-b’:5,6-b’’]-trithiophene is one

of several benzotrithiophene (BTT, read more about BTT in next section) isomers and

has been used as a repeating unit for copolymerization with thiophene and thieno[3,2-

b]thiophene (170a-b) [312]. The planar BTT core induced strong aggregation effects

in alternating copolymers. The choice of the comonomer was found to play a crucial

role in determining the backbone conformation, the interchain interactions, and the

polymer solubility. A best hole mobility of 0.24 cm2/V s was achieved from 170a.

The asymmetry of this BTT isomer, however, leads to a regiorandom polymerization

as described, making this issue of gaining access to regioregular copolymers worth

exploring.

All the above-discussed thiophene and fused thiophene polymers without electron-

withdrawing units have up to date mainly been applied as p-type semiconductors in

OFETs. In principle, it is impossible for these polymers to afford n-type transport

thanks to their exclusive electron-donating nature. On the other hand, although some

of such polymers were also used as active components in BHJ solar cells, the obtained

PCEs were usually not high in comparison with D-A polymers. Actually, the D-A

polymer approach provides a very broad potential for the thiophene-containing

moieties as donor units, which will consequently be outlined in the following section.

2.4 Donor-acceptor polymers

Integration of electron donors and electron acceptors in one polymer system (D-A

polymers) has proved to be an efficient strategy enabling us to tailor the properties of

conjugated polymers for the desired applications in OFETs and PSCs. Concerning

OFETs, the intra- and intermolecular interactions between donor and acceptor units

within a D-A polymer can lead to self-assembly into ordered structures and strong π-

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stacking of polymer chains, both of which favor charge carrier transport [4]. The

HOMO and LUMO energy levels and hence band gaps can be well tuned in a D-A

polymer system via selection of different donor and acceptor units because the

HOMO is mainly located at the donor unit but the LUMO mainly at the acceptor unit

[313]. This knowledge can be utilized to control the Voc and Jsc so as to obtain high

PCEs in PSC devices. Such strategy offers a great opportunity for all aforementioned

electron-rich building blocks to act as donor units in combination with appropriate

acceptors yielding well-designed D-A polymers. Some of the donors applied like

fluorene, indenofluorene, carbazole, indolocarbazole have been discussed before and

exhibited promising performances in PSCs. Here many more high-performance D-A

polymers composed of novel donating and accepting building blocks with cutting

edge high-performance will be comprehensively reviewed. Those D-A polymers with

further extended 2D structures will be discussed separately in section 3.3.

Many review articles have been published recently that cover developments in the

PSC [1, 313-316] or OFET [2, 317, 318] fields, or describe one or several classes of

D-A polymers [319-322]. This section focuses on the donor and acceptor building

blocks which are the most often applied constructing units for D-A polymers. Many

new donor or acceptor units and corresponding polymers have been reported in the

last two years, which will be covered here to present the latest developments in this

exciting research area.

2.4.1 Donors and acceptors

As the name suggests, D-A polymers are made up of electron donors and electron

acceptors. The electron pushing/pulling ability (or strength) and molecular geometry

of the donor and acceptor as well as their interactions determine the basic optical

behavior and electronic properties of a D-A polymer. It is thus appropriate to

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rationally design electron donor and acceptor units before combining them to develop

high-performance D-A polymers. In this part we attempt to collect and classify the

reported donor and acceptor building blocks and then describe their corresponding D-

A polymers according to our classification given in the next section.

2.4.1.1 Donors

Donor, herein, refers to an electron-rich unit which determines the HOMO energy of

the D-A polymer system. A large number of donor units, which are derived from

benzene and thiophene have been reported in the past decade. Here these donors are

classified into two types according to the presence or absence of the bridging atoms,

and listed in two Donor Boxes A and B, as respectively depicted in Figures 6 and 7.

Figure 6

Figure 7

The electron-donating ability is determined by the HOMO level, however, because

of the different standards and methods as well as the systematic errors, the values

from different publications are often not comparable. Basically, there are several

empirical principles to judge the strength of donors. First, since the electron-donating

ability of thiophene is stronger than that of benzene, the donors consisting only of

thiophene moieties are stronger than those built up only of benzene moieties. Thus,

bridged bithiophenes D11-D15 are usually regarded as strong donors, whereas

bridged biphenyls and terphenyls D1-D10 are weak donors. Those examples

containing both benzene and thiophene can be considered as medium donors.

Secondly, nitrogen bridging enhances the donor strength compared to other bridging

atoms thanks to the lone pair of electrons of the nitrogen atom. In addition, the

symmetry of donor units affects the electron-donating ability, for example, the C2

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symmetrical D38 possesses higher HOMO level than the other benzotrithiophene

isomers according to DFT calculations and CV experiments (see details below).

Most of the donors used for construction of D-A polymers are planar and possess

symmetric geometries, which is particularly beneficial for intra- and intermolecular

ordering of the polymer chains in the solid state. All the donors listed here and their

D-A polymers will be mentioned in the following chapter. Emphasis, however, will

be put to those examples with high device performance.

2.4.1.2 Acceptors

Acceptor, herein, refers to an electron-poor unit determining the LUMO level of an

alternating D-A copolymer. Most acceptor units possess at least one or more strong

electron-withdrawing groups, like an imine nitrogen (-C=N) or a carboxyl unit (-C=O).

Herin, we sort them into five types according to their chemical structure, namely,

thiazole-, thiadiazole-, pyrazine-, annulated amide or imide-, and carboxyl-containing

donors, as illustrated in Figure 8. Some examples with other electron-poor groups are

listed as well.

Figure 8

The electron-accepting abilities of these acceptors are described by their LUMO

level which is experimentally accessible through CV characterizations or photo

electron spectroscopy (PES), theoretically through DFT or ab initio calculations. One

can also visually estimate the acceptor strength according to the absorption edges of

D-A polymers when copolymerizing different acceptors with the same donor.

Empirically, strong acceptors contain thiadiazole or two thiadiazole rings such as in

A6 and its derivatives A20, A21, and A22, and further annulated rings such as in A40,

A42, and A45. Many D-A polymers with excellent OPV and OFET performances are

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derived from these acceptors. Those containing thiazole or only one amide or imide

ring such as A1, A3, and A34 are usually considered as relatively weak acceptors.

It should be mentioned here that acceptors A28 [323] and A58 [324] were reported

as donor units, however, in view of the existence of imine nitrogens containing a free

electron pair (pyrazine ring in A28 and pyridine ring in A58) they can also be

considered as weak acceptors. Actually, the definition of donors and acceptors in

copolymers depends on their combinations and individual HOMO and LUMO levels,

and is thus relative, e.g. a strong acceptor combined with a weak acceptor let the latter

become the electron-donating moiety, for donors vice versa. Furthermore it is clear

that some acceptors mentioned in Figure 8 also contain mixtures of two accepting

components themselves, exhibiting some interesting properties, which will be

separately discussed in Section 2.4.2.3. As for the extended donor moieties mentioned,

most of the acceptors are coplanar and even symmetric, which is critically important

for the properties of the copolymers. All the acceptors listed here and their D-A

polymers will be mentioned in the following section, however, emphasis is focused on

representatives examples with high performances.

2.4.2 One-dimensional D-A polymers

Based on the donors and acceptors listed above, a large library of D-A polymers has

been developed for successful applications in organic electronics. In this section, we

review typical examples of D-A polymers and their performances in OPV and OFET

devices, which are summarized in Table 4. Every donor and acceptor will be referred

by an alias as assigned in Figures 6-8 in the following discussions. It must be noted in

advance that benzothiadiazole (BT, A6) and diketopyrrolopyrrole (DPP, A42) are the

two most common acceptors and both of them are often combined with other donors

by surrounding two thiophenes, namely, 4,7-dithienobenzothiadiazole (DTBT) and

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dithienyl-DPP. For ease of discussion, they will be addressed by the two acronyms

“A6TT” and “A42TT”.

Table 4

2.4.2.1 D-A polymers based on donors D1-D45

D-A polymers based on the donors derived from bridged biphenyl and terphenyl

usually have low HOMO levels thanks to the electron-deficient nature and aromaticity,

which generally leads to high Voc in their OPV devices. Detailed discussions

regarding D1-D7-based D-A polymers can be found in Section 2.1. Donor D8 was

prepared by different routes [325-327] and a polymer with D8 as a repeating unit was

also synthesized using hydrosilylation reactions [328], but D-A polymers based on

this donor remain elusive. Monomer D9 has also been reported [329], however, to the

best of our knowledge polymers using this unit have not yet appeared in the literature.

Molecule D10, actually, is not known so far but could be a target in the near future.

Donors D11-D15 based on bridged bithiophene have emerged as a class of important

building block for constructing D-A polymers, since they are highly planar and the

five membered thiophene rings cause less steric hindrance with their neighboring

acceptor units. Among these five donors, cyclopenta[2,1-b:3,4-b’]dithiophene (CPDT,

D11), dithieno[3,2-b:2’,3’-d]pyrrole (DTP, D12), and dithieno[3,2-b:2’,3’-d]silole

(DTS, D13) have become most popular and have been copolymerized with many

acceptors such as A6, A6TT, A36, A42TT etc. Scheme 2.4.1 presents several

representative D-A polymers based on D11-D13 copolymerized with the same

acceptor A6.

Scheme 2.4.1

CPDT-BT D-A polymer (171) has become one of the most successful examples

achieving excellent performances in both OFET and OPV applications. Polymer 171a

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initially exhibited a hole mobility of 0.17 cm2/V s in FET devices [330] , but the

optimization of the molecular weight of polymer and the processing conditions of

devices improved continuously the hole mobilities to 1.4 and 3.3 cm2/V s [331, 332].

Recently, an ultrahigh mobility of 5.5 cm2/V s based on 171a has been realized by

reaching high molecular order and pronounced alignment in single fibers (Figure 9)

within a short OFET channel via solvent vapor enhanced drop casting [333]. With

respect to OPV applications, polymer 171b with different side chains from 171a

displayed a broad absorption spectrum with a band gap of 1.46 eV and exhibited a

preliminary PCE of 3.2% when blending with PC71BM [334, 335]. By adjusting the

thin-film nanosphase segregation in blends 171b:PC71BM blends using processing

additives such as alkanedithiols and 1,8-dihalo-octanes, PCEs were significantly

enhanced up to 5.5% [336, 337]. Donor D11 was also copolymerized with various

acceptors such as A6TT and A1 [338], A2 [339], A24 [340], A36 [341], A56 [342,

343], and A42TT [344], but the PCEs from these polymers were not higher than that

of 171b.

Figure 9

DTP (D12) contains an electron-rich nitrogen atom at the bridging position,

increasing the electron-donating ability compared to D11 so that the D-A polymers

using D12 as a donor (172a-c) displayed a broader absorption range, narrower band

gap (1.41-1.43 eV), and higher HOMO energy levels (between -4.81 and -4.89 eV)

compared to 171. The narrow band gap is beneficial for producing high Isc (12.3

mA/cm2 for 172c) in OPV devices but high HOMO levels led to low Voc (0.55 V for

172c). The PCEs based on these polymers (1.1-2.8%) were thus not higher than those

of 171b containing D11. It was demonstrated that the PCEs of these polymers were

also strongly dependant on the length of alkyl chains [345]. D-A polymers based on

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D12 containing other acceptors such as A6TT [346], A42TT [347], and A36 [348,

349] exhibited similar properties as 172, namely, narrow band gaps (high Isc) and high

HOMO levels (low Voc), and subsequently moderate PCEs (lower than 3%.) It can

thereby be concluded that strong donors are unfavorable for the construction of D-A

polymers with high performances in OPV applications but they are useful for

preparing D-A polymers with high charge carrier mobility [347] and with narrow

band gap absorbing light in the NIR optical range [350].

DTS (D13) is another appealing donor based on bridged bithiophene. The longer C-

Si bond than C-C bond in D11 was expected to release some strain from the planar

heterocycle and thereby to allow stronger π-stacking interactions to occur. Polymer

173 is the first polymer which showed similar absorption behavior and band gap to

171 but higher hole mobility and an excellent PCE of 5.1% (173:PC71BM = 1:1)

without further optimization [351]. Many D13-containing D-A polymers have

appeared to be effective in improving charge mobility and increasing the Voc in OPV

devices and therefore exhibiting outstanding performances compared to D11- and

D12-containing polymers in PSC applications [351-354]. For example, D13 combined

with A36 as an acceptor had better hole transporting ability and lower HOMO lever (-

5.57 eV) in comparison with the analogues containing D11 as a donor, and afforded a

PCE of 6.2% with simultaneously high Isc (10.95 mA/cm2), Voc (0.9 V), and FF (63%)

from a blend device with PC71BM. By changing the processing solvent and adding

3% DIO as an additive, a PCE as high as 7.3% was achieved [352].

It should be pointed out that because of the popularity of D11-D13 several polymers

with the same or very similar structures were separately reported by different research

groups. The published device properties (mobilities in FETs and PCEs in PSCs),

however, varied to quite an extent. These differences could be attributed to the

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synthetic methods, varying molecular weights and polydispersities of the polymers,

and the device processing conditions. Therefore, the direct comparison of these

donors with the same acceptor in copolymers carried out under the same conditions

for synthesis and device fabrication was particularly important and should be

encouraged to be done also for other polymers of high interest [355, 356].

D14 has a sulfur atom at the bridging position, not providing access to attach

solubilizing groups at this position. Alternatively, the alkyl chains can be placed at the

β-positions of two outer thiophene rings. As discussed in Section 2.3 it has been found

that copolymers of the alkyl substituted D14 with a non-alkylated bithiophene have

lower hole mobilities than those of unsubstituted D14 and dialkylated bithiophene. In

this regard, D-A polymers using alkylated D14 as a donor and alkoxylphenyl-

substituted A24 [357], alkoxyl-substituted BT derivative A13 [358], and A42TT [358,

359] as acceptors, respectively, provided relative modest mobilities (10-6~10-2 cm2/V

s) and PCEs (0.3~3%) in PSC devices. An unsubstituted D14 combined with the

alkyl substituted A42TT in a copolymer was reported to exhibit more promising

ambipolar FET performance with hole mobilities up to 0.23 cm2/Vs and electron

mobilities of 0.015 cm2/Vs, as well as a high PCE of 5.1% upon blending with

PC71BM in a PSC device [360]. An analogue with selenophenes replacing two

surrounding thiophenes at both sides of DPP also yielded decent performance in

OFETs (0.13 cm2/V s for holes and 0.01 cm2/V s for electrons) and OPVs (PCE of

4.05%) [360].

D15 with a germanium bridge was recently developed. Compared with D13, the

longer C-Ge bond in D15 can further lower the steric hindrance between neighboring

polymer chains and thus improve the intermolecular π-π stacking. D15 with 2-

ethylhexyl chains attached at the germanium atom was copolymerized with n-octyl

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substituted A36, delivering a best PCE of 7.3% as blending with PC71BM and using

5% DIO as processing additive, higher than that of the D13 corresponding copolymer

analogue (6.6%) under identical device processing conditions [361]. This promising

polymer was also applied in an inverted PSC, displaying a best PCE of 8.5% and a

certified PCE of 7.4%, which allows for the realization of a high-efficiency PSC with

large-scale roll-to-roll processing [362]. Recently, D15 was also copolymerized with

fluorinated A6TT (F at thiophenes rather than at BT) producing a polymer with a

significant enhancement in OPV performance in comparison with an analogue

without fluorine substituents [363].

Donors D16-D20 can be prepared by bridging thiophene-phenylene-thiophene (Th-

Ph-Th) with C and other heteroatoms. The combination of benzene and thiophene in

these donors makes their electron-donating abilities be medium as compared to the

aforementioned oligophenyl- and bithiophene-based donors. The rigidity and

coplanarity of these ladder-type molecules can lead to strong π-π interactions of the

final polymers which improve the charge carrier mobility even upon combination

with acceptors.

The C-bridged donor D16, namely, indacenodithiophene (IDT), with n-hexylphenyls

groups attached at the bridge positions was initially copolymerized with A6

containing additional thiophene or bithiophene moieties in the main chains,

demonstrating a PCE of 4.4% [364]. A copolymer of n-hexadecyl substituted D16 and

A6 was applied in OFETs, exhibiting a hole mobility of up to 1 cm2/V s with good

ambient stability [305]. Subsequent studies demonstrated that the substituents at the

bridging centers strongly affected the OFET and OPV performances of these IDT-BT

copolymers. A very long linear chain is most beneficial for OFETs, whereas 2-

ethylhexyl side chains can lead to very desirable phase separation in the polymer and

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extremely good OPV performance with a best PCE of 5.5% [365]. The donor D16

with n-hexylphenyl side chains has also been combined with many acceptors such as

dithienyl A1, A2, A6, and A56 [366], A24 bearing phenyl groups [367], A31 [367],

A32 [368], and A33 [368], and most of them exhibited excellent PCEs of around 6%.

Particularly, a D16-based D-A polymer with octoxylphenyl substituted A24 as an

acceptor and two additional thiophenes in the backbone was recently reported to give

a PCE as high as 7.51% upon carefully optimizing the polymer/PC71BM ratios [369].

Other heteroatom bridged (thienyl-phenylenyl-thienyl)s D17-D20 and their D-A

polymers with various acceptors were successively reported in last two years [370-

375]. All of them are promising as polymer semiconductors applied in OFETs and

OPV but the performances are not better than those containing D16. This result is

similar to the aforementioned bridged bithiophene donors, in which C-bridged D11

polymers revealed again the best performance.

More coplanar π-conjugated bridged heteroacenes including D21-D29 were

synthesized and applied to design D-A polymers [308, 309, 376-385]. The

heteropentacenes D21 and D22 represent two dithienocarbazole isomers, which led to

distinct backbone curvature of the resulting D-A polymers. As a result, copolymers of

D21 combined with A36, A42TT, and A45 as acceptors demonstrated low OFET

mobilities in the level of 10−3 cm2/V s, whereas the corresponding D22 containing

copolymers had better performances, particularly, a hole mobility as high as 1.36

cm2/V s was achieved from the D22-A42TT polymer [377]. Hexacyclic donors D23

and D24 with the above acceptors were also synthesized and applied in OFETs and

OPV [378, 379]. By carefully choosing the acceptor, a PCE of 5.52% was reached for

the D24-A12 polymer [379]. A heteroheptacene D25 with the inclusion of carbazole

and thiophene units, and its D-A polymers were reported separately by several

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research groups, but their performances in OFETs and OPV were moderate [308, 309,

380]. A structurally similar analogue D26 with a silicon instead of a nitrogen bridge

exhibited a slightly better PCE of 4.2% when copolymerized with A6 [381]. A best

performance of this kind of heptacyclcic donors in OPV devices was achieved from a

carbon-bridged D27 polymer with A6 as an acceptor as well, providing a PCE of as

high as 7.0% [382]. Two more heptacyclic donors D28 and D29 were reported quite

recently [383-385]. The corresponding D-A polymers exhibited modest field-effect

mobilities between 10−4~10−2 cm2/V s, but good performance in OPV devices. A PCE

of 6.6% was obtained for the D28-A36 polymer by optimizing the device processing

conditions [383]. A polymer derived from D29 and A12 exhibited a PCE of 7.0%

with a large Voc of 0.95 V without any additives or thermal annealing processes. The

higher PCE compared with the D16-A12 copolymer was attributed to the improved

charge carrier mobility and higher absorption coefficient [385].

By fusing benzene and thiophene to design donor units, one can obtain several

isomers of a class of molecules with same numbers of benzene and thiophene. This

characteristic was typically reflected by comparing the isomers of benzodithiophenes

(BDT, D33-D35), benzotrithiophenes (BTT, D36-D38), and naphthodithiophenes

(NDT, D39-D41). As for thienothiophenes, there exists also three isomers (D30-D32)

but they are employed as donors in only a few cases. Particularly D31 derivatives

containing electron-withdrawing substituents (A48-A50 and A55) are a class of

important acceptors for synthesizing high-performance OPV polymers, which are

summarized in a recent review [316].

Among all BDT isomers, benzo[1,2-b:4,5-b’]dithiophene (D33) has attracted the

widest interest for synthesizing D-A polymers with excellent PCEs even exceeding

6.5% [12, 386-389]. Further details of these polymers and their applications in OPV

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devices can be found in related reviews [390, 391]. For D33, on the other hand, two

additional aromatic substituents can be introduced at the central benzene, leaving the

planarity of the whole unit intact. This has been utilized to construct D-A polymers

with 2D architectures and recently a PCE as high as 8.8% has been achieved based on

such a 2D D-A polymer [8] (see also Section 3.3). Two other BDT isomers D34 and

D35 possess a similar coplanarity and electron-donating ability as D33 but different

topology such that their polymers demonstrated deeper HOMO energy levels and

higher Voc [160, 392, 393]. Higher PCEs, however, were not achieved.

BTTs have emerged as a class of attractive donor units for the design of D-A

polymers in recent years. The coplanarity and extended π-conjugation of the BTT

skeleton should promote intermolecular π-stacking, which would induce strong

aggregation and enhanced packing in the solid state of the BTT-containing polymers.

As outlined above these features are particularly desirable for improving charge

carrier transport in OFET devices [4]. There are seven BTT isomers as presented in

Figure 10. Among them, BTT1 [394, 395] , BTT2 (D36) [396, 397], and BTT3 (D37)

[398-401] have been synthesized and widely studied, particularly, D37-containing D-

A polymers thereby demonstrated promising potential for PSCs. Another isomer

BTT7 (D38) has also been synthesized recently with higher HOMO level than for the

other isomers as estimated by DFT calculations [402]. Its copolymers with A6 and

A6TT as acceptors (174 and 175, Scheme 2.4.2) exhibited a pronounced difference in

their supramolecular organization and only 175 promising OFET performance. No

charge carrier transport was found for 174 even though a good π-stacking distance of

only 0.35 nm in the bulk was evidenced. In stark contrast, polymer 175 exhibited a

hole mobility of 0.04 cm2/V s, in agreement with the well-ordered film and the

organization into lamellar structures with a π-stacking distance of 0.37 nm. This

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variation in device behavior was attributed to the evidently different degree of

curvature in the polymer backbone induced by the additional two thiophene units in

175 [4].

Figure 10

Scheme 2.4.2

Three NDI isomers have been applied to synthesize D-A polymers which also

exhibited promising PSC performances. D39 with different side chains was

copolymerized with alkyl substituted A6TT to investigate the influence of the size of

the side chains on the photovoltaic properties. The overall efficiency of these OPV

devices varied significantly from 1.20% to 3.36%, depending on the different side

chains [347]. When replacing A6 with A12, the PCE was improved up to 5.6% [403].

This donor was also combined with dialkylthienyl A15 thereby exhibiting a PCE of

6.2% with a high Jsc of 14.16 mA/cm2 [404]. Two other isomers of NDT, D40 and

D41, possess linear extension leading to small π-π stacking distance. Polymers

combining D40 and dialkylthienyl carrying acceptors A6 and A21 presented a very

close π-π stacking of 3.5 Å. Particularly, the latter exhibited a hole mobility of 0.54

cm2/V s and a PCE of 4.9% [405]. D41 has been combined with A36 and A42TT in

corresponding copolymers yielding PCEs of 4.0% and 5.4%, respectively [406, 407].

For copolymer D41-A42TT in an inverted PSC the PCE reached up to 6.9 % [407].

Additionally, larger fused donating units with extended π-conjugation have emerged

in developing narrow band gap polymers for the PSCs. A polymer containing

tetrathienoanthracene D42 with A50 as an acceptor was reported to result in a high

PCE of up to 5.6% by using 2% DIO as an additive during device fabrication [408].

Two anthradithiophene isomers D43 and D44 having different linear and angular

shapes were both copolymerized with A42TT. The polymer based on D43 with

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additional ethinylene units in the backbone exhibited hole mobilities on the order of

0.1 cm2/V s, while the bent isomer containing polymer (D44) gave a lower FET

mobility but a promising PCE of 4.24% in an OPV device when using a processing

additive [409, 410]. Phenothiazine D45, that is a tricyclic nitrogen-sulfur heterocycle,

can also serve as a donor unit. When copolymerized with NDI (A40) and PDI (A41),

the polymers exhibited n-channel field-effect behavior with electron mobilities of

0.05 cm2/V s [411]. D45 and its derivative with an oxidized sulfur group (S,S-

dioxides) were also combined with A6TT and the resulting polymers were

investigated in OFET and OPV devices, but the performances in both cases were only

moderate [412].

2.4.2.2 D-A polymers with acceptors A1-A60

As depicted in Figure 8, we classify the reported acceptors units into six types.

Among these acceptors, A6, A36, and A42 have received the greatest attention.

Almost all donors have been subjected to copolymerization with these acceptors and

most of the resulting polymers have already been discussed above. Especially,

polymers containing A42 have reached record values for both p- and n-type mobilities,

as well as very high PCEs in PSCs. Therefore, we shall now concentrate on the latest

developments of A42-based polymers and another two more polymers based on

tetracarboxydiimides (A40 and A41) and isoindigos (A45-A47). The other acceptors

are listed in Table 3 together with corresponding publications, in which the

preparation and performance in electronic devices of the corresponding D-A polymers

can be found in detail.

Table 3

The tetracarboxydiimides (A40 and A41) have strong electron-withdrawing ability,

which can be employed to impart n-type characteristics to organic/polymer

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semiconductors [452]. Most of these polymers exhibited dominant electron

transporting property and were tested as alternatives to fullerene for fabricating all-

polymer solar cells (all-PSCs).

Many D-A polymers with A40 as an acceptor and oligothiophenes as donors have

been reported [450, 451]. Among them, polymer 176 (Scheme 2.4.3) provided

excellent OFET performance with an electron mobility as high as 0.45-0.85 cm2/V s

under ambient conditions [452]. A new polymer 177 with biselenophene replacing

bithiophene in 176 was reported to give an electron mobility of 0.07 cm2/V s. It was

also applied as the acceptor component blended with P3HT in an all-PSC, yielding a

PCE of 0.9% [453]. A D-A block copolymer 178 composed of P3HT and poly(A40)

segments was recently synthesized and blended with P3HT to make the all-PSCs with

a PCE of 1.28% [454]. A slightly higher PCE of 1.63% for an all-PSC was obtained

from 179 after optimizing the morphology with 1,8-diiodooctane as a solvent additive

also providing a decent FF of 0.66 [455].

Scheme 2.4.3

Polymer 180 containing A41 exhibited a lower electron mobility of 2×10-3 cm2/V s

and worse stability compared to the analogue polymer 176 with A40 as the acceptor

[456]. Another polymer 181 with D14 as the donor resulted in a relatively high

electron mobility of 0.013 cm2/V s. This polymer was also tested as an acceptor

material to fabricate all-PSCs, giving an average PCE over 1% [457]. More donor

units have been combined with A41 to make polymers 182a-f applied in all-PSCs. By

optimizing the solvent mixtures and controlling the phase separation, a high PCE of

2.23% from the all-PSCs with 182f as the acceptor material was achieved [458]. Up to

now, the reported PCEs from the all-PSCs are still not comparable to those of

polymer/fullerene PSCs. More investigations are needed to precisely control the

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energy levels of the donor and acceptor components as well as carefully optimize

their mixing conditions and the morphology.

Diketopyrrolopyrrole (DPP, A42) has emerged as extremely attractive acceptor to to

construct D-A polymers for both OFETs and PSCs in recent years. The coplanarity of

the DPP unit as well as its ability to readily form hydrogen bonds with neighboring

units in the polymer backbone, result in strong intermolecular π-π stacking. Moreover,

solubilizing and functional substituents can be easily attached to the nitrogen atoms of

the DPP unit for improving the solubility and tuning the microstructure of the

polymer [5].

Numerous DPP-based D-A polymers have been reported as active materials in

OFETs and PSCs. Many DPP-containing polymers are listed in related reviews [321,

322] but record p- and n-type mobilities and very high PCEs were reached only in

2012 (Scheme 2.4.4).

Scheme 2.4.4

An important breakthrough with a record hole mobility was recently achieved for

polymers 183a and 183b. In the two polymers, the highly π-extended (E)-2-(2-

(thiophen-2-yl)vinyl)thiophene was used as a donor unit, which improves coplanarity

and promotes intermolecular π-π stacking. The studies demonstrated that the lengths

of the branched alkyl chains significantly influence the film-forming ability,

intermolecular interaction forces, and charge-carrier transport characteristics. The

polymer 183b with longer side chains exhibited more uniform thin films and closer π-

π stacking distances compared to those of 183a. Therefore, the polymer 183b

exhibited an unprecedented high mobility of up to 8.2 cm2/V s with a current on/off

ratio of 105-107, and a good environmental stability. For 183a still a high mobility of

2.0-4.5 cm2/V s was measured with a current on/off ratio of 105-107. FETs fabricated

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from these polymers can well compete with those of organic small-molecule

semiconductors and amorphous silicon semiconductors [5]. An analogue of 183b,

polymer 184 with selenophene replacing thiophene at both sides of the vinylene unit,

was reported soon after. Similar to the thiophene-vinylene-thiophene unit in 183b, the

selenophene-vinylene-selenophene group in 184 can also serve to extend the

conjugation length, leading to enhanced intermolecular interactions. Furthermore, the

lone pair of selenophene atoms is more mobile than that of thiophene, because it

participates less in aromaticity, which leads to enhancement of the chain interaction

between neighboring chains and to the carrier transport phenomena. As a result, a

highly ordered structure and a remarkably high carrier mobility of 4.97 cm2/V s with a

current on/off ratio of 1.6×107 was found for polymer 184 [459].

There are some interesting reports on the modification of side chains attached to the

nitrogen centers of the DPP unit. Polymer 185 has an alternating conjugated backbone

between DPP and bithiophene but one of the DPP comonomers is functionalized with

triethylene glycol (TEG) side chains. This was found to induce spontaneous chain

crystallization while providing maximum solubility and allowing the synthesis of

high-molecular-weight polymers. Carefully engineered top-gate OFETs based on the

polymer 185 exhibited a maximum electron mobility of 3 cm2/V s [6]. In another case

186, one of the DPP comonomers was protected by thermally labile tert-

butoxycarbonyl (t-BOC) groups which can be thermally cleaved leading to the

formation of a hydrogen-bonded network within the DPP-based polymers to enhance

intermolecular charge-carrier hopping. Upon a solution-shearing process, the polymer

186 presented p-channel dominant characteristics, with hole and electron mobilities of

0.0132 and 0.0026 cm2/V s, respectively. After the thermal cleavage of t-BOC

groups at 200 °C, the dominant polarity of charge carriers changed from positive to

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negative, resulting in superior electron mobility of 0.0460 cm2/V s compared with the

hole mobility of 0.0043 cm2/V s [460]. Inspiration was also taken from the initial

discovery which demonstrated that siloxane-terminated solubilizing groups as side

chains in an isoindigo-containing polymer could enhance charge transport by inducing

a denser π-π spacing and a larger crystalline coherence length [461]. Thus a DPP-

based polymer 187 bearing siloxane-terminated solubilizing groups was also reported

quite recently [462]. By optimizing the solution-processing techniques, high hole and

electron mobilities of up to 3.97 and 2.20 cm2/V s with relatively well-balanced

polarities were achieved.

In addition to the numerous variations of the comonomer unit, the thienyl units

introduced at both sides of the DPP unit were also replaced with other aromatic

groups to further optimize the electrical properties of DPP-containing copolymers. A

typical substitute is thieno[3,2-b]thiophene that was directly linked with DPP units to

design polymers 188 and 189 reaching a maximum hole mobility of 1.95 cm2/V s.

PSCs comprising the polymer 189 and PC71BM gave a PCE of 5.4% [463]. Other

electronic moieties such as furan and selenophene were applied to replace thiophene

in DPP copolymers [464-466]. More examples can be found in the recent review on

DPP containing polymers [322].

Isoindigo (A45) is a structural isomer of the famous pigment indigo and has been

developed as an electron acceptor for designing D-A polymers recently [467]. Its

homopolymer presented an n-type character, capable of serving as an acceptor

material to fabricate all-PSCs. when blended with P3HT it yielded a PCE of 0.5%

[468]. After copolymerization with donor units, many A45 containing D-A polymers

were synthesized for OFET and PSC devices. Polymers 190 and 191 displayed similar

photophysical and electrochemical properties but completely different OFET

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performance. The latter exhibited an air-stable hole mobility as high as 0.79 cm2/V s

with a current on/off ratio of 107 [469]. For the application in solar cells, polymer 192

afforded a PCE of 6.3% when blended with PC71BM, which is a record reported for

isoindigo polymers [470]. Other donor units such as DTS (D13) [471] and BDT (D33)

[472] were also combined with A45, producing polymers 193-195, which exhibited

acceptable PCEs higher than 4%.

A promising approach was the investigation of the influence of the branching

position of the alkyl side chains on the FET performance by comparing isoindigo-

based polymers 191, 196-198 [473]. The highest mobility was achieved from 197

with an impressive value of 3.62 cm2/V s. As described for the DPP-based polymers,

the introduction of siloxane-terminated side chains can remarkably improve the

charge transport. This strategy was employed for the isoindigo-based polymer 199

[461]. The solution-processed FETs based on this polymer gave a maximum hole

mobility of 2.48 cm2/V s, which was much higher than that (0.57 cm2/V s) obtained

from a reference polymer with branched alkyl chains. The result was supported by the

polymer packing where 199 exhibited a closer π-π stacking distance of 3.58 Å,

compared to the analogue (3.76 Å).

Isoindigo was structurally modified to design new acceptors. Fluorination of the

isoindigo unit gave a new acceptor A46 which was expected to lower the LUMO level

of the corresponding polymer and increase the electron mobility. As a result, the

LUMO level of polymer 200 was reported at -3.88 eV which was 0.18 eV lower than

that of the polymer without fluorine. FET devices applying 200 and fabricated under

ambient conditions provided improved electron mobility of 0.43 cm2/V s by

comparison with the non-fluorinated polymer, while maintaining high hole mobility

up to 1.85 cm2/V s [474]. Substituting the benzene in isoindigo with thiophene

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produces another new acceptor, namely thienoisoindigo (A47). It has been

copolymerized with many donor units (201) as displayed in Scheme 2.4.5. The FET

performances from these polymers, however, were moderate compared with the

isoindigo-based analogues, reaching hole mobilities between 10-4~10-2 cm2/V s [475,

476].

Scheme 2.4.5

2.4.2.3 D-A polymers containing dual acceptors

We note that apart from the alternating copolymers between one donor and one

acceptor several D-A copolymers containing different acceptor units have also been

reported. Such polymers usually exhibit ambipolar or electron-dominant transporting

characteristics owing to the introduction of two acceptors. A typical example is the

copolymer formed from A42TT and A6. Two polymers (202a and 202b Scheme

2.4.6) with the A42TT-A6 alternating conjugated backbone and minor difference in

the length of the alkyl chains were reported independently by two research groups

[477, 478]. Polymer 202a afforded a HOMO level of -5.2 eV and a LUMO level as

low as -4.0 eV, which was ideal for the formation of electron and hole accumulation

layers. The mobilities reached 0.35 cm2/V s for holes and 0.40 cm2/V s for electrons,

respectively, after annealing at 200 °C [477]. Polymer 202b presented an ambipolar

transporting property as well, with balanced mobilities of ~0.1 cm2/V s for both

charges [478].

Scheme 2.4.6

Further work reported polymer 203 with one more BT unit inserted into the

backbone [479]. The consecutive BT groups cause a strong twist in the polymer main

chains, which led to an amorphous microstructure. This polymer exhibited n-channel

FET characteristics with electron mobility of 10-3 cm2/V s, one order of magnitude

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higher than the hole mobility. Another modification for 202 is to replace the

thiophene units with selenophenes. The resulting polymer 204 exhibited a more red-

shifted absorption than 202a, and high hole and electron mobilities of 0.46 and 0.84

cm2/V s for spin-coated polymer films annealed at 200 °C [466].

Another A42TT and A6-containing polymer 205 was studied in relation to polymers

206a and 206b with a stronger acceptor A20 replacing A6 [480]. Compared with the

A42TT-A6 polymers, the integration of A20 typically resulted in polymers having

very narrow band gaps of around 0.65 eV. These polymers exhibited excellent

transistor performance, with mobilities above 0.1 cm2/V s, even exceeding 1 cm2/V s

for 206b. The two strong acceptors in the polymers 206a and 206b rendered them

strongly ambipolar.

Polymers 207 and 208 containing two acceptors A6 and A7 in the backbone were

synthesized by testing dipotassium bis(trifluoroborate) derivative of A7 as a monomer

in this modified Suzuki polycondensations reactions [481]. The polymer 207

composed of all electron-accepting units gave an electron mobility of 0.02 cm2/V s.

The good electron-transport properties of the polymers let them be tested as

replacements of PCBM in preparing all-PSCs with Voc larger than 1.2 V [482]. The

new acceptor A47 (thiophene substituted isoindigo) was also copolymerized with A6.

The resulting polymer 209 exhibited a narrow optical band gap in the film of about

0.92 eV which is roughly 0.3 eV lower than that of DPP-BT copolymer (202).The

FETs exhibited excellent ambipolar behavior, with both hole and electron mobilities

recorded over 0.1 cm2/V s [483].

Polymers containing dual acceptors were also used as donor materials in PSCs. A

polymer 210, which contained alkoxylated A6 and quinacridone A52 in the main

chains, possessed an optical band gap of 1.92 eV and a LUMO level of -3.32 eV.

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When blending with PC71BM in a 1:1 weight ratio, a PSC afforded the PCE of 2.5%

[484]. Another polymer, 211, containing A49 and A50 as acceptors and BDT (D33)

as a donor was applied in an inverted PSC, yielding a PCE of 7.0%. Further

modifications on the fabrication of the devices could improve the performance of

PSCs up to an ultimately high PCE value of 7.9% [485].

In this section, we have reviewed the broad scope of the reported donor and acceptor

units and typical D-A polymers derived thereof. By summarizing the performances of

these polymers in the OFETs and PSCs, we could make an attempt at highlighting

some guidelines to design D-A polymers for two kinds of applications. General

requirements for the D-A polymers include (1) sufficient solubility for ease of

processing, (2) high molecular weights, (3) proper positioning of solubilizing side

chains to not hinder the conjugation but also for tuning the intermolecular interactions

and microstructure in the film, and (4) careful selection of donors and acceptors

strength provided by their HOMO and LUMO energy levels to control the absorption

behavior and band gap. Optimizations on the active layer morphology and the device

processing conditions are equally important for improving the performances of D-A

polymers, which should be considered in more depth from the perspective of device

physics.

In addition, two topics are worthy of being emphasized again. First, we have

proposed a concept of “dual-acceptor polymers” and discussed some examples. This

concept can be employed to design ambipolar or electron-transporting polymers, as

well as the acceptor materials to replace fullerene derivatives for realizing all-polymer

solar cells. It can be predicted that more and more polymers of this kind will be

reported in the near future by combining different acceptors, in particular, A6, A15,

A21, A36, A40, A41, A42, and A45, as depicted in Figure 8.

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Secondly, special attention should be put on inverted PSCs for the device setup. The

PCEs of state-of-the-art PSCs around 8-9% discussed above were all obtained from

such a device structure [7, 8, 485]. The inverted PSCs have several advantages over

the conventional ones: (1) avoid the use of the corrosive and hygroscopic hole-

transporting PEDOT:PSS, facilitating better long-term ambient stability [7]; (2) use

air-stable metals such as Ag or Au as top electrodes, which are more resistant towards

degradation [485]; (3) take advantage of the vertical phase separation and

concentration gradient in the active layer [7]. These inverted PSCs are currently

attracting more attention and will be compared in more detail for their efficiencies and

durability with the standard ones for appealing polymers.

3. Two-dimensional (2D) conjugated polymers

In the previous sections, linear conjugated polymers for organic electronics have

been presented. These linear polymer systems could be widely extended to higher

dimensionalities and sophisticated topologies. So far, however, there is no commonly

accepted definition for two-dimensional (2D) polymers, although the term has been

widely used [10]. Several approaches have been applied to realize 2D polymers, such

as covalent synthesis, H-bonded networks, and supramolecular self-assembly leading

to layered structures [10]. In the part of 2D conjugated polymers, the focus is limited

those attained by covalent chemistry.

Various molecular architectures having such 2D conjugated structures and

sophisticated topologies have emerged and currently represent a quickly spreading

field of research [270]. The increase of dimensionality in π-conjugated systems can

lead to different superstructures in the solid state and to multidirectional charge

transport. Conjugated macrocycles with fully π-conjugated and shape-persistent

nature are able to self-assemble into 2D supramolecular structures thanks to π-

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stacking interactions and show unique optical and electronic behaviors [486].

Graphene, an individual atomically thick sheet of graphite, is a most desired 2D

polymer. The extraordinary electronic, thermal, and mechanical properties of

graphene tend to be considered highly useful for manifold applications [11].

D-A polymers, on the other hand, with 2D extension, i.e. additional conjugation at

the direction perpendicular to the conjugated polymer backbone have also been

proposed (see Figure 18) [12]. Such polymers were synthesized and investigated as

OPV materials and quite recently an extremely high PCE of near to 9% has been

achieved [8]. This section is divided up into three parts to describe aforesaid three

classes of macromolecular systems with 2D conjugated structures, namely i)

conjugated macrocycles, ii) grapheme, and iii) extended D-A polymers.

3.1 Conjugated macrocycles

Macrocycles with rigid, shape-persistent, non-collapsible, and fully π-conjugated

backbones have attracted great interest in the past two decades, owing to their unique

properties and their role as building blocks of discotic liquid crystals, columnar

nanotubes, guest-host complexes, porous surface networks, and three dimensional

nanostructures [257]. Moreover, macrocycles as discrete molecular entities possess

unique optical and electronic properties with high potential for applications in organic

electronics [486]. A wide variety of building blocks, including benzene, thiophene,

pyridine, acetylene, and even large-size PAHs and porphyrins have been employed to

synthesize shape-persistent conjugated macrocycles [486]. Herein some macrocycles

composed of carbazole, fluorene, PAHs, and thiophene, with interesting self-

assembled structures and potential applications in OLEDs and OFETs will be

discussed.

3.1.1 Carbazole and fluorene-based macrocycles

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While 2,7-fluorene- and 2,7-carbazole-containing conjugated polymers have been

widely studied and applied in organic electronics the corresponding macrocycles

without any end group-defects should also be of great interest as perfect polymer

models. Furthermore they are capable of forming ordered columnar structures with

porous cavities. Such macrocycles can be synthesized both by choice of proper

geometries for ring closure and by template-assisted reactions. Often meta-phenylene

units were applied for macrocycles since they facilitate the ring formation, but they

also interrupt the conjugation. For instance, 3,6-linked carbazole was used to

synthesize square carbazole-ethynylene macrocycles with a diameter of 1.25 nm,

which can form nanofibrils [254]. Thus, a new approach towards cyclic conjugated

oligomers was developed using even para-phenylene units.

Scheme 3.1.1

A monodisperse fully conjugated 2,7-carbazole-based macrocyclic dodecamer 212

(Scheme 3.1.1) has been synthesized using a porphyrin template-assisted method

[487]. The 2D-WAXS studies on extruded filaments of 212 demonstrated that the

molecules self-assemble into hexagonal arrays of columns with a packing parameter

of 4.7 nm between the columns. Within the columns the stacking distance was found

to be 0.4 nm, and a correlation between every four molecules along the column was

found, i.e. each macrocycle was rotated by ~22.5 degrees towards each other in

agreement with the molecular structure where every third carbazole unit carried a

hydroxyalkyl chain. Additionally, mono-layers of the macrocycles could be visualized

(Figure 11) by scanning tunnelling microscopy (STM) on highly oriented pyrolitic

graphite (HOPG), which provided a well-ordered hexagonal packing of ‘‘face-on’’

structures.

Figure 11

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Another 2,7-carbazole-based macrocycle 213 containing diethynyls in between

triscarbazoles was achieved by elaborate functional design via the same porphyrin

template, where the sterically demanding solubilizing alkyl side chains pointed to the

outside of the giant ring. A 2D supramolecular structure of the host–guest complexes

was obtained by physisorption of 213 on HOPG followed by gas-phase deposition of

hexabenzocoronene (HBC) molecules under high vacuum leading to an inclusion of

the HBCs in the hollow macrocycles. The formation of such a host–guest complex of

213 with HBC was visible by STM, as depicted in Figure 12. Assuming that the guest

molecule would also be able to form columnar stacks, this new type of self

organization enabled columnar assemblies consisting of conjugated macrocycles with

discotic pillars inside their channels. These supramolecular structures have a high

potential for electronic devices, such as light amplification using cascade energy

transfer [488].

Figure 12

Similar to 2,7-carbazole-based macrocycle 212, a monodisperse, completely π-

conjugated, cyclododeca-2,7-fluorene macrocycle 214 has been synthesized using the

same method. Since oligo- and polyfluorenes represent efficient blue-emitting

materials for OLEDs, such a monodisperse macrocycle serves as an attractive model

compound in order to gain a better understanding of related linear architectures, e.g.,

to study the impact of end groups on the optoelectronic properties of linear

polyfluorenes. STM measurements indicate that macrocycle 214 self-organizes on

HOPG, forming a well-defined hexagonal pattern [489].

Scheme 3.1.2

In addition, several smaller carbazole-based macrocycles 215-217 were synthesized

using 1,8-linked carbazole as a building block [490, 491]. They were designed to form

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a cavity similar to that of a porphyrin combined with the ability to bind metal ions.

Since porphyrins are by far the most intensively studied nitrogen-containing

macrocycles, due to their biological importance as well as their ability to function as

efficient metal complexing ligands, porphyrin-related macrocycles have attracted

tremendous research interests. They could be applied in broad areas, ranging from

catalysis, nonlinear optics and photodynamic therapy to organic electronics [490].

Therefore, a large amount of research has been devoted to porphyrins, expanded

porphyrins, and porphyrin nanorings [492-496]. Related systems such as the

triangular porphyrinoid (218a-e), not only demonstrated the closest distance between

three cavities in plane, but also showed high catalytic activity towards oxygen

reduction after complexation with three cobalt (II) ions [493, 494]. More details on

modified and expanded porphyrins can be found in recent reviews [486, 497].

Scheme 3.1.3

3.1.2 PAHs-based macrocycles

Some larger-sized PAHs and their nitrogen-containing derivatives could also be

used as building blocks to construct macrocycles. Phenanthrene, one of the simplest

PAHs, which can be polymerized at 3,6-positions to form deep blue EL polymers as

discussed above, was also employed to synthesize a trimeric macrocycle by the same

way of linkage. A stiff phenanthrene-based macrocycle 219 (Scheme 3.1.4) formed

well-oriented columnar superstructures with a striking thermal stability. Additionally

it demonstrated strong π-stacking, resulting in a pronounced intermolecular

interaction. Therefore, mesophase formation with a large transition window ranging

from 148 to 500 oC was found [498].

Triphenanthroline macrocycles 220a-c were reported with 3 different alkoxyl groups,

varying from octyloxy to hexadecyloxy, in order to compare the discotic self-

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assembly behavior [499]. The variation of the length of the alkyl chains led to severe

changes of column formation with a π-stacking distance of 0.37 nm for 220a. As

anticipated the inter-columnar distances increased from 3.32 over 3.75 up to 4.22 nm

for 220a-c reflecting nicely the distance dependence between columns on the length

of the alkoxy side chains with overall hexagonal arrangement of the macrocycles. The

interior is slightly larger than the one in standard porphyrin or phthalocyanine-based

macrocycles, reaching opposite N-N distances of 0.55 nm. Thus a number of rather

large alkali (Na+, K+) and transition metal ions (Ag+, Pb2+, Cd2+, Zn2+, and Cu2+)

could be incorporated into the cycle. The replacement of sodium in the macrocycle

upon addition of silver triflate was also proven by optical absorption, leading to a red

shift from 444 nm to 474 nm for the silver macrocycle.

Scheme 3.1.4

Triphenylene can be applied for the buildup of triangular-shaped macrocycles by

covalently linking three triphenylene units through the 7- and 10-positions. A set of

such molecules 221a-d was synthesized, indicating pronounced self-assembly

behaviors and unique optoelectronic properties [500, 501]. The self-assembly of 221a

produced oligomers in solution and fibers in the solid state. The 1:3 charge transfer

(CT) complex between 221a and 2,4,7-trinitrofluorenone resulted in a microball

structure [500]. With long alkoxy side chains, macrocycle 221b exhibited strong self-

assembly and liquid crystallinity as studied by means of 2D-WAXS techniques. The

highly organized columnar superstructures adopted by this fully planarized discotic

molecule could be applied as charge-carrier pathways in OFET devices. Compounds

221b-d provided deep-blue emission in solution and a ten-fold increase in the

fluorescence quantum yield with respect to parent triphenylene as a result of

conjugation along the cyclic backbone. The macrocycles were tested successfully as

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active materials in OLEDs with promising emissive properties and device stabilities.

A two-layer, solution-processed OLED using 221d as an active layer and TPBI as an

electron-transport layer exhibited an onset voltage of 3.9 V, a maximum luminance of

200 cd/m2 at 6.5 V, and a narrow deep blue emission similar to PL with CIE

coordinates of (0.16, 0.07). These results demonstrated the great potential of this new

class of triphenylene-based macrocycles for solution-processed, small-molecule

OLED applications [501].

A pyrene-based macrocycle 222 was synthesized and investigated in solution by

comparison with a linear pyrene hexamer. This macrocycle displayed blue emission

with a maximum at 439 nm, which is slightly red-shifted compared to the linear

hexamer, suggesting additional intermolecular interactions in the macrocycle [257].

A triphenylamine derivative, dimethylmethylene-bridged triphenylamine

(heterotriangulene), has been recently used to construct macrocycles [502]. A

tribrominated heterotriangulene could self-organize to form surface-supported

molecular-thin 2D polymer films via a surface-mediated synthesis (read more in next

section: graphene nanoribbons). Depending on activation temperature, 2D porous

metal-coordination or covalent networks were obtained. A heterotriangulene

macrocycle 223 was prepared using traditional solution-based Yamamoto coupling

and the results compared to those materials obtained via surface-mediated synthesis

[503]. The macroscopic quantities obtained from solution chemistry allowed single-

crystal X-ray analysis and a characterization of optical and electrochemical properties

of this macrocycle. Due to high symmetry with few predicted active vibrational

modes, this heterotriangulene is a promising candidate for hole-transport applications

in optoelectronic devices with low power loss.

3.1.3 Thiophene-based macrocycles

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Several kinds of macrocyclic oligothiophenes and their π-extended derivatives

exhibiting unusual electronic properties have been reported, and some of them have

also been used as semiconductor materials for OFETs. Octathio[8]circulene 224a, a

so-called “sulflower” and its selenium-containing derivative 224b were reported

(Scheme 3.1.5), demonstrating coplanar structures with a columnar stacking. OFETs

based on 224a-b exhibited hole mobilities of about 9×10−3 and 1×10−3 cm2/V s,

respectively [504, 505]. Recently, neutral meso-substituted

tetrathia[22]annulene[2,1,2,1] aromatic macrocycles 225a-c were prepared, which

displayed porphyrin-like Soret and Q bands in the absorption spectra. A high hole

mobility of up to 0.63 cm2/V s was achieved from highly crystalline thin films of

these macrocycles [506]. A benzothiophene-cornered rectangular thiophene-

ethynylene macrocycle 226a with diameter of up to 2 nm was synthesized and tested

in OFETs. A film fabricated by spin-coating exhibited a hole mobility with a

maximum value of 7.3×10−3 cm2/V s [507]. In fact, there was reported an enormous

number of macrocyclic thiophenes which have been synthesized and characterized as

interesting candidates for various future perspectives and applications in organic and

molecular electronics also with respect to host-guest interaction, aggregation, and

self-assembly on surfaces. For example, the first fully conjugated oligothiophene

macrocycles 226b were found to form well-ordered 2D monolayers at the

solution/HOPG interface or two-component donor/acceptor complexes with C60

presenting a 3D supramolecular assembly [508]. Further information regarding

macrocyclic oligothiophenes can be found in related reviews [270, 486].

Scheme 3.1.5

3.2 Graphene nanoribbons from polyphenylene precursors

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Graphene, an individual sheet of graphite forming a 2D honeycomb lattice, is

composed of a hexagonal network of sp2-hybridized carbon atoms. Thus, it holds

potentials for a manifold of applications in catalysis, sensing, energy storage, as well

as optoelectronic devices grounded on its extraordinary electronic, thermal, and

mechanical properties [11]. These properties of graphenes are strongly dependent on

three factors: size, shape and edge structures. Therefore we start with the discussion

of the implication of the size. Here it is necessary to clarify the correlation between

size and desired applications of graphene-type molecules because there are several

terminologies such as graphene molecules, graphene nanoribbons, nanographenes,

and graphene sheets, which sometimes are confusing. As defined recently (Figure 13)

[11], a length of 100 nm can be considered as a cut-off point between graphene and

nanographene. Graphene should have sizes exceeding 100 nm in both longitudinal

and lateral directions, while graphene fragments with length smaller than 100 nm are

defined as nanographenes. The small graphene segments that are smaller than 5 nm

can be called extended PAHs instead of graphene molecules. The most interesting

graphene-type molecules are graphene nanoribbons (GNRs), which are defined as

graphene strips with a width of 10 to 50 nm while maintaining an aspect ratio of

larger than 10. Obviously, these graphene species with different sizes should exhibit

distinct optical and electronic properties, in particular, GNRs present ribbon length

dependent band gaps and charge carrier mobilities [509, 510]. Concerning the shape,

a graphene sheet can be regarded as an infinitely large PAH, maintaining 2D planar

geometry. However, specific to the graphene molecules and GNRs, various types of

derivatives which appear with linear, triangular, hexagonal, zigzag, and chevron (V-

shaped) geometries have been prepared [253, 511-513] and showed different

optoelectronic behaviors as well.

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Figure 13

Finally one has to consider the edge structures. There are two types of achiral edges

in GNRs, namely, zigzag and armchair edges, defined by the orientation of the

hexagons relative to the ribbon extension, as presented in Figure 14. The edge

structure and the row number of carbon atoms normal to the ribbon axis determine the

electronic structure and ribbon properties [514]. Thus, producing GNRs with defined

edge structures is of great interest for many chemists and materials scientists.

Unfortunately, GNRs fabricated to date usually suffer from disordered edges that

make the band gap poorly defined, which in turn results in a dramatical degradation of

carrier mobility [515]. In addition, GNRs with pure zigzag edges remain rare relative

to those with armchair ones. They have been predicated to possess metallic features

and to be useful for spin electronics [516].

Figure 14

Therefore, to obtain graphenes in particular GNRs with well-defined size, shape, and

edge structures is the most important challenge for the application in electronic

devices. Toward this end, two types of protocols have been explored for the synthesis

of graphene and GNRs. One is top-down exfoliating graphite towards graphene; the

other is a bottom-up building up of graphene from molecular building blocks. The

top-down approaches for graphene synthesis comprise of mechanical exfoliation of

highly oriented pyrolyzed graphite (HOPG) [518], liquid-phase exfoliation of graphite

intercalation compounds [519], and chemical oxidation/exfoliation of graphite

followed by reduction of graphene oxide (GO) [520, 521]. Those examples for GNRs

synthesis typically include the cutting or etching of graphene or graphite precursors

into narrow graphene strips [515, 522, 523], and the longitudinal unzipping of carbon

nanotubes to corresponding GNRs [524, 525]. Two typical bottom-up methods

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include epitaxial growth on metallic substrates by means of chemical vapor deposition

(CVD) for production of large-area graphene film [526, 527] and organic synthesis

based on precursor molecules for preparation of nanographenes and GNRs [512, 513,

528].

Although the top-down methods mentioned above were widely applied for synthesis

of graphene and GNRs, they suffer from drawbacks such as uncontrollable sizes and

irregular edge structures. In stark contrast, the bottom-up chemical approaches offer a

great opportunity to create structurally defined graphenes. Nanographenes and GNRs

of various sizes and shapes have been obtained by atomically precise syntheses.

Initially, some extended PAHs were synthesized via intramolecular

cyclodehydrogenation as small nanographene molecules with sizes between 1 and 5

nm and various molecular geometries [529-531]. The limited conjugation length of

graphene molecules lies outside the scope of this review, and for a more detailed

discussion on synthetic strategies, as well as issues of solubility and partial

cyclodehydrogenation the reader is referred to more specialized literature [11, 253].

Thus, we herein highlight the bottom-up synthesis of 2D nanographenes using 1D

linear polyphenylenes as precursors both in solution and on surfaces.

3.2.1 Solution synthesis of GNRs using polyphenylene precursors

As noted above, a graphene sheet can be considered as an infinite PAH built from

benzene, therefore the bottom-up protocols were considered to include high-

molecular-weight, well soluble polyphenylene precursors which later on undergo

cyclodehydrogenation [512, 513, 528, 532, 533].

Scheme 3.2.1

Thus soluble branched polyphenylene precursor 227 can be converted to a GNR 228

by intramolecular oxidative cyclodehydrogenation [532]. High-resolution TEM

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images of GNR 228 disclosed two different solid-state domains: an ordered layer

structure with a layer distance of ca. 3.8 Å, and another disordered one due to the

existence of “kinks” in the obtained graphene-type polymers and/or random stacking

of GNRs. Standard spectroscopic characterizations in solution, however, were not

possible. Thus GNRs 230 with larger number of side chains even though extended

linearly, to provide better solubility were developed [513]. The polyphenylene

precursor 229, which was prepared by Suzuki–Miyaura coupling reaction between

diboronic ester of hexaphenylbenzene and 1,4-diiodo-2,3,5,6-tetraphenylbenzene,

with Mn of 14 kg/mol and low polydispersity of 1.2, was soluble in common organic

solvents. Subsequent intramolecular oxidative cyclodehydrogenation with FeCl3

provided GNRs 230, which was still soluble owing to the presence of branched alkyl

chains. Comparison of the MALDI-TOF spectra of the precursor 229 and product 230

demonstrated that the intramolecular cyclodehydrogenation proceeded smoothly.

These soluble nanoribbons were fully characterized by UV-Vis absorption (λmax = 485

nm) and STM techniques [513]. The latter showed that the nanoribbons 230 had

lengths of up to 12 nm (Figure 15).

Figure 15

Another series of five monodisperse GNRs 232a-e were prepared from precursor

231 by the bottom-up solution synthesis. The obtained GNRs contain increasing

number of carbon atoms ranging from 132 (232a, n=1) to 372 (232e, n=5) along with

the increase of the ribbon length. The smallest representative 232a was still

sufficiently soluble exhibiting a maximum absorption at 644 nm [528]. STM images

(Figure 16) of self-assembled monolayers of this homologue revealed the formation

of extended lamellar structures when adsorbed on HOPG, thus rendering it an

attractive candidate for applications in organic electronic devices. Unfortunately, the

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larger homologues of this series of GNRs were poorly soluble, hampering further

investigations.

Figure 16

In order to obtain longer ribbons, high-molecular-weight polyphenylene precursors

with good solubility were required. However, the rigidity of polyphenylene

backbones normally induces a strong aggregation resulting in poor solubility of the

precursors. To overcome this obstacle, polyphenylene precursors 233a-b with a

flexible kinked backbone were designed and synthesized [512]. Benefiting from such

a molecular design, the precursor with solubilizing dodecyl chains had a molecular

weight of up to 20 kg/mol as detected by MALDI-TOF MS. The GPC analysis with

PS as a standard indicated a relatively low Mn of 9.9 kg/mol, however, with a PDI of

1.4. This made a graphene nanoribbon 234b achievable after an oxidative

cyclodehydrogenation with FeCl3. These GNRs had good solubility that allowed for

further structural characterizations and solution processing, an important requirement

for large-scale preparation of electronic devices.

The aforementioned GNRs fabricated by bottom-up chemical approaches are limited

to those with lateral dimensions smaller than 1 nm, with absorption only up to 670 nm

and calculated band gap larger than 1.6 eV. Quite recently, a structurally defined and

laterally extended GNR 236 with an unprecedented width of close to 2 nm was

synthesized, exhibiting a broad absorption extending into the NIR region with an

optical band gap as low as 1.12 eV [533]. It should be noted that the precursor 235

was prepared via a Yamamoto polycondensation using one dihalogenated monomer.

This reaction was believed to yield high-molecular-weight polymers as compared to a

commonly used Suzuki-Miyaura cross-coupling because of the intrinsic sensitivity of

the latter to stoichiometry. As a result, MALDI-TOF MS characterization of the

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precursor 235 indicated the presence of a regular pattern with molecular weight up to

35 −40 kg/mol. On this basis, the number of repeating units in 235 was 21−24, which

corresponded to approximately 30 nm of the resulting GNRs.

Despite the great achievements in the bottom-up organic synthesis of graphenes

one clearly faces a trade-off between ribbon size and solubility of the GNRs. The

solubility issue also hampers the solution processing of the graphenes for electronic

devices. To obtain even larger, but still soluble GNRs with different aspect ratios

remains a challenge for materials chemistry. Another issue for the solution synthesis

of graphenes concerns the efficiency of the cyclization during the final step under

Scholl reaction conditions, which often suffers from intrinsic problems of incomplete

dehydrogenation and side reactions such as chlorination. Nevertheless, the power of

these synthetic protocols to create structurally defined graphenes, and the encouraging

results achieved to date should undoubtedly promote more research.

3.2.2 Surface-mediated synthesis of GNRs using polyphenylene precursors

Different from the cyclodehydrogenation reactions for graphene synthesis in solution,

the surface-mediated synthesis of graphenes is carried out under ultrahigh vacuum

conditions by first depositing halogen substituted oligophenylene monomers onto a

suitable metallic surface, then finishing the thermal-assisted polycondensation on the

surface, and eventually producing the desired GNRs with shapes and edge structures

dependant upon the selected monomers. Several representative examples have been

reported based on such a surface synthesis for graphenes [511].

A linear-type nanoribbon 239 with armchair edges was prepared from a monomer of

dibrominated bianthryl 237 using the surface-mediated synthesis [511]. The steps for

fabricating such a nanoribbon are illustrated in Figure 17. First, an assembled

monolayer of monomer 237 is deposited onto Ag(111) or Au(111) surfaces; then a

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dehalogenation occurs at 200 oC forming surface-stabilized biradical species, which

couple in a kind of Ullmann reaction to yield the linear polyanthrylene precursor 238.

Finally, the sample is heated up to 400 oC, enabling intramolecular

cyclodehydrogenation of the precursor polymer chains to produce GNR 239. STM

images before (Figure 17b) and after (Figure 17c) cyclodehydrogenation reveal the

formation of atomically precise GNRs with fully hydrogen-terminated armchair edges.

A transfer method of GNR 239 from the gold surface to silicon dioxide substrate has

also been demonstrated by a polydimethylsiloxane (PDMS) stamp transfer process

with subsequent etching of the gold layer in KI solution.

Figure 17

An advantage of this method is that the topology of the GNRs produced is

determined by the halogen substituted monomers, GNRs with different architectures

could thus be prepared using different oligophenylene precursors. On this basis,

tetraphenyl-substituted dibromotriphenylene 240 was further selected as a monomer

to produce a chevron-type GNR 242 with a periodicity of 1.70 nm and pure armchair

edge structures. The temperatures for surface polymerization to yield precursor

polymer 241 and for intramolecular cyclodehydrogenation are both higher than those

in the above case, as shown in Figure 17. Most of the GNRs exhibited a length of 20–

30 nm and in a few cases up to 100 nm [511]. This approach can be expanded to

binary-monomer systems, allowing for heteromolecular coupling to form GNRs 245

with more complex shapes, as e.g, a threefold GNR junction was clearly visible in the

STM image (Figure 17e) [511].

Besides rigid graphene nanoribbons, porous graphene networks with periodical order

can be built up using the bottom-up surface-mediated synthesis. By thermally

annealing precursor monomers, such as hexaiodo-cyclohexa-m-phenylene and

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tribrominated dimethylmethylene-bridged triphenylamine, regularly porous 2D

polymer networks were established [502, 534]. For the first case, the domain size of

the porous graphene is about 50×50 nm with pore spacing of 7.4 Å, while the second

one shows pore spacing from 2.08 to 1.74 nm.

The bottom-up synthesis in solution and on surface has successfully produced GNRs

with defined size, shape, and edge structures. These chemical strategies are, in our

opinion, the most promising way towards the controlled synthesis of graphene-type

materials. Further research should focus on their structural characterization and the

understanding of their electronic properties, in particular, charge transport through

single GNRs. Theoretical research has also demonstrated that GNRs with properly

designed edge structures fulfill the requirements in terms of electronic level alignment

with common acceptors (PC60BM), solar light harvesting, and singlet-triplet

exchange energy to be used as low band gap semiconductors for OPVs [535].

Therefore, it can be expected that the synthesis and application of nanographenes and

graphene nanoribbons will be one of the more popular research directions in the

future.

3.3 Two dimensional donor-acceptor polymers

Besides linear D-A polymers as discussed above, a concept of two-dimensional (2D)

D-A polymers with extended conjugation at the direction perpendicular to the

polymer backbone has been proposed to design narrow-band gap polymers for PSC

applications [12]. Their performances in the PSCs are listed in Table 5. So far there

exist two types of 2D D-A polymers which have been synthesized and investigated.

One is the so-called main-chain donor and side-chain acceptor (mD–sA) polymers, a

concept corresponding to main-chain donor and main-chain acceptor (mD–mA)

polymers. In the mD-sA polymers, a conjugated backbone serving as an electron-rich

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donor component is grafted with a conjugated pendant chain containing an electron-

withdrawing acceptor at the terminus. The other polymer still has both donor and

acceptor in the polymer main chains, similar to the linear D-A polymers, but the

donor or acceptor part contains aromatic substituents remaining in conjugation with

the backbone, forming a 2D extended architecture. Structural differences between

linear D-A polymers and both types of 2D D-A polymers are schematically illustrated

in Figure 18.

Table 5

Figure 18

3.3.1 2D mD–sA polymers

Compared with the traditional 1D mD–mA copolymers, 2D mD–sA polymers have

several interesting features, such as high hole transporting properties of the main

chains, isotropic charge transport and internal charge transfer (ICT) (from main chain

to side chain). The first two mD–sA polymers (246a-b, Scheme 3.3.1) were reported

in 2009, in which a fluorene-triarylamine copolymer backbone was attached with

styrylthiophene π-bridged malononitrile or diethylthiobarbituric acid as electron-

accepting pendant groups [12]. Both polymers exhibited two obvious absorption

peaks, with the π-π* transition of main chains at 385 nm and the strong ICT of side

chains at 600 nm. Their HOMO levels were similar to that of the fluorene-

triarylamine copolymer backbone, while LUMO levels were lowered down to −3.43

and −3.50 eV, respectively, owing to the strong electron affinity of the side chains.

Theoretical studies also indicate that the HOMO and LUMO energy levels were

separately controlled by main-chain donor and side-chain acceptor in this type of D-A

polymers [373, 536]. PSC devices exhibited PCEs of 4.7% and 4.4% for 246a-b,

respectively, when blending with PC71BM [12].

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Scheme 3.3.1

Afterwards, studies on 2D mD–sA polymers focused on the replacement of either

the fluorene segment by other hole transporting units attached with solubilizing alkyl

chains or the triphenylene segment by other electron donors. Dibenzosilole, polymers

247a-b were reported to exhibit similar band gaps as 246a-b due to the same polymer

main chains but lower PCEs of 2.5% and 3.2%, respectively [537]. A series of 2,7-

carbazole-based 2D mD-sA polymers 248-250 have also been synthesized [538, 539].

These polymers have similar optical and electrochemical properties as 246a-b,

including two absorption bands stemming from the π-π* transition of the main chains,

a strong ICT between side and main chains, and identical HOMO levels because of

the identical polymer backbones. Among them, polymers 250a-b possess broader

absorption bands and stronger absorption intensities than the other ones because of

extended length of conjugation along the main chains. Another feature of these

polymers is that their hole mobilities strongly rely on the length of alkyl chains linked

at the nitrogen atom of carbazole. The hole mobilities of 2-ethylhexyl-substituted

polymers 248a-b were measured to be 2.4×10−4 and 1.1×10−3 cm2/V s, respectively,

higher than those of polymers 249 and 250 with long branched side chains. A PCE of

4.2% was obtained from blended 248a:PC71BM devices. Other hole transporting

units used to replace fluorene include cyclopentadithiophene [540], dithienopyrrole

[541], and phenothiazine [541], but their performances in PSC are moderate with

PCEs below 1.5%. Thus, polymers 251a-c exhibited broad absorption bands ranging

from 400 to 900 nm, and narrow optical band gaps of 1.34 eV, however their solar

cells yielded PCEs of only about 1.2% [540].

There are also some reports on replacing the triphenylene with other electron-

donating units in the mD-sA polymers. Polymers 252 and 253 were synthesized

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containing thiophene in the polymer backbone as an alternative to triphenylene,

aiming at improving the planarity of the polymer main chain [373, 542]. For both

series of polymers, those with malononitrile as acceptor side chains showed better

performance than other counterparts, regardless of the fluorene or carbazole in the

main chains. The device based on 253a/PC71BM demonstrated the highest PCE of

2.2% with a high Voc of 1.03 V in these polymers.

For all 2D mD-sA polymers reported to date, only a few examples (246a, 246b,

247b, and 248a) displayed relatively high PCEs between 3-5%. Most of these

polymers provided moderate PCEs with low FF values, probably due to the poor

packing of the polymer chains. Although this strategy allows for the design of new D-

A polymer architectures showing some interesting properties, more efforts have to be

made to improve the performances in PSC when compared with conventional D-A

polymers.

3.3.2 Benzo[1,2-b:4,5-b’]dithiophene-containing 2D D-A polymers

Another type of 2D D-A polymers maintain the donor and acceptor moieties in the

polymer main chains, but forms a 2D structure by attaching conjugated side chains

with the polymer main chains. This design was realized by using benzo[1,2-b:4,5-

b’]dithiophene (BDT) as a donor unit, whose central benzene can be substituted with

two additional aromatic groups bearing solubilizing alkyl chains, leading to 2D

conjugated polymers with good solubility. Unlike the insulating alkyl chains, these

aromatic substituents can potentially assist the charge transport. Moreover, the

attached aromatic side chains afford a more electron-rich donor to lower the HOMO

levels, which could be expected to provide a higher Voc. Typical BDT-based 2D D-A

polymers are shown in Scheme 3.3.2.

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Polymer 254 was the first BDT-based 2D D-A polymer with dialkyl substituted

thiophene attached to the central benzene of BDT [543]. This polymer exhibited three

absorption bands in the range 300–700 nm both in chloroform and in the thin film.

PCEs up to 5.66% were obtained from the 254-based device, with a Voc of 0.92 V, a

Jsc of 10.7 mA/cm2, and an FF of 57.5% The high Voc and Jsc were attributed to the

low HOMO level of the polymer, the higher IQE (above 80%), and the EQE response

value beyond 50% in a wide spectral range, respectively. By substituting the BT unit

in polymer 254 with fluoro groups as acceptors and changing the alkyl chains

attached at the side-chain thiophenes, copolymers 255a-b were synthesized [415].

They provided the desired deep HOMO and LUMO energy levels providing high Voc

and Jsc. The solar cells prepared from these two polymers were fabricated using

PC71BM as the acceptor, achieving PCEs of 6.21% and 5.64% for 255a-b,

respectively. Besides BT, several other electron-deficient units, such as

thiazolothiazole [544], naphtho[1,2-c:5,6-c]bis[1,2,5]-thiadiazole [427], benzotriazole

[447], and 5,5’-bibenzo[c]-[1,2,5]thiadiazole [545], surrounded by two thiophenes

have also been used as acceptors in a copolymerization with alkylthienyl-substituted

BDT to make 2D D-A polymers (256-258). Most of them exhibited excellent PCEs of

5~6% with high Voc and Jsc. A recently reported polymer 258c had a hole mobility of

6×10−3 cm2/V s and was applied in a phototransistor, but its performance in PSC was

not mentioned.

Bithienyl substituted BDT as a donor with extended conjugation along the side

chains has also been reported. By using BT and 5-hexylthieno[3,4-c]pyrrole-4,6-dione

without additional thiophenes as acceptors, polymers 259a-b were synthesized [546].

The use of bithienyl substituents allowed six alkyl substituents per BDT repeating unit,

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resulting in relatively high molecular weight of the polymers but their performances

in PSC were modest.

Scheme 3.3.2

The advantage of BDT-based D-A polymers over 1D D-A polymers was

demonstrated by a comparative study on alkylthienyl-substituted BDT and alkoxyl--

substituted BDT, respectively, as a donor. Polymers 260a-b exhibited better thermal

stability, red-shifted absorption spectra, lower HOMO and LUMO energy levels,

higher hole mobility, and improved photovoltaic properties, in comparison with their

corresponding alkoxy-substituted analogues. PCEs as high as 6.21% and 7.59% were

achieved from 260a-b, respectively [391]. Another polymer 260c containing a bulky

sulfonyl side group at the thieno[3,4-b]thiophene as an acceptor unit was synthesized

[448]. The PCE of 260c was as high as 7.81%, indicating nearly a 25% increase

relative to an alkoxy-substituted analogue.

Apart from thienyl-based conjugated side chains, decylphenylethynyl substituents

were also introduced at the benzene ring of BDT donors combined with several

acceptors to produce 2D D-A polymers 261a-d [547]. These polymers were

comparatively investigated by experimental and theoretical methods but the

photovoltaic properties were not discussed.

Quite recently, a series of low band gap polymers 262a-d having a backbone based

on BDT and DPP units have been synthesized. These polymers exhibited low energy

band gap (1.4−1.5 eV), deep HOMO energy levels, and high hole mobility. Single

junction PSC devices exhibited PCEs of 3−6% by morphology optimization. Higher

efficiencies exceeding 8% were realized in tandem PSCs [8].

Additionally, two D-A polymers containing tris(thienylenevinylene) side chains with

extended length of conjugation have been reported recently, yielding significant

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improvement of PCEs in comparison to the analogues without conjugated side chains.

An Isc of 22.6 mA/cm2 is the record for OPV devices so far. These polymers hold

promise to establish a new class of 2D D-A materials [548].

4. Conclusions and outlook

Herein, several categories of conjugated polymers have been highlighted, as

classified by molecular topology. 1D and 2D conjugated polymers including five

material systems and a molecular design strategy for D-A polymers have been

outlined. Without getting into significant synthetic details, we focused on their

performances in electronics, such as in PLEDs, OFETs, and PSCs.

Among these material systems, conjugated polyphenylenes are a large family with

PFs and PCz as the most important members. PFs have been widely studied as

versatile semiconductors for various optoelectronic applications, in particular as light-

emitting materials for red-green-blue (RGB) basic-color and white

electroluminescence. Earlier carbazole-based polymers were used as light-emitting

and host materials, but have also shown promising performance in PSCs. Other

stepladder conjugated polymers containing carbon or heteroatom bridges have been

applied in optoelectronic devices, but after first consideration, did not become the

leading materials for further applications. PAH-based polymers were investigated as

semiconductors, enriching the materials basis for PLEDs and PSCs. The thiophene-

containing polymers have attracted extensive research interest for organic electronic

devices. We first discussed the thiophene- and fused thiophene-based polymers

without additional electron-withdrawing units, which were mainly applied as p-type

semiconductors in the OFETs. Fully π-conjugated and shape-persistent macrocycles

exhibit self-assembled supramolecular structures and potential applications in OLEDs

and OFETs. Graphene is a unique case of a 2D polymer, and is considered promising

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as a candidate for organic electronics. The bottom-up approach in solution and on

surfaces for the synthesis of graphene nanoribbons with defined size, shape, and edge

structures was discussed. The chemical strategies suffer from obstacles that need to be

circumvented in the future but are regarded as the most promising way towards the

graphene materials with defined structure.

The D-A strategy has appeared to be most promising for obtaining high-performance

conjugated polymers for OFETs and PSCs. All aforementioned electron-rich building

blocks can serve as donor units for designing D-A polymers when combining with

appropriate acceptor units. We have listed the known donor and acceptor units and

separately discussed the performances of typical D-A polymers with 1D and 2D

architectures. Several important guidelines for designing D-A polymers could then be

proposed. To date the highest FET mobilities and PCEs of PSCs are both obtained

from D-A copolymers.

The D-A polymers should take the leading role in the research of conjugated

polymers for organic electronics. Thiophene-containing building blocks will probably

attract more research interest than pure hydrocarbons. In addition, new donor and

acceptor units will be continuously designed and synthesized. Unprecedented

structure types may become rare in view of the large number of already existing

donors and acceptors (see Figures 6, 7, and 8). Access to new building blocks will

include the modification of already existing ones for example, by attaching fluoro or

cyano groups to acceptors. Recent reports on A25 (derived from A24) [433, 434] and

A46 (derived from A45) [474] point in this direction. More research will probably

focus on the elaborate selection of existing donor and acceptor units and their proper

combination in polymers. Thereby, the optimization of molecular weights and

polydispersities as well as more rigorous purification of established polymers will

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demand a greater part to further improve their performances. It can also be predicted

that the dual-acceptor polymers will receive more attention owing to their potential in

producing ambipolar or electron-transporting polymers, and as a possible replacement

of fullerene acceptors. Another popular research direction comprises the synthesis and

application of nanographenes and graphene nanoribbons. Chemically synthesizing

graphene nanoribbons with large lateral dimensions and defined structures will remain

an ongoing target of materials chemists. The focus, thereby, will be on the structural

characterization, understanding of electronic properties, charge transport, and

applications in electronic devices of GNRs. It is believed that further research into

conjugated polymers for organic electronics will continue to furnish excellent

achievements in succession to the rapid progress in the past decade.

This review has laid an emphasis on the design of conjugated polymers for

optimizing their performance in electronic devices. It did not focus on the detailed

methods of synthesis. Even if a satisfying laboratory-scale synthesis of such a

polymer has been established, there remain critical issues, such as costs, upscaling, or

the avoidance of chlorinated solvents for processing. However, there are many factors

influencing the performance of devices beyond the molecular structure of a

conjugated polymer as active component. While these are beyond the scope of the

present text, the formation of supramolecular order (and the related defects) or of

phase-separated morphologies together with the role of the interfaces should be

carefully considered. Organic electronics is thus a multidisciplinary research field

requiring the interaction between synthetic chemists, on the one hand, and physicists

and device engineers, on the other hand. This review is intended to summarize current

research progress and is hoped to inspire future developments in the research of π-

conjugated polymers for organic electronics.

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5. Acknowledgments

We thank Dr. Brenton Hammer for language polishing. This work was financially

supported by the European Community EC-ITN-SUPERIOR (GA-2009-238177), the

Nano Sci-E SENSORS, and the ERC Advanced Grant NANOGRAPH (AdG-2010-

267160). X.G. gratefully acknowledges the Alexander von Humboldt Stiftung for

granting a research fellowship. Portions of this review are repeated from our previous

publications (ref [13, 31b]).

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Figure 1. Normalized fluorescence spectra of stepladder polyphenylenes 8, 9, 10,

11 and LPPP 3 in dilute toluene solution (10 μg/mL) at room

temperature. [30], Copyright 2007. Reproduced from Wiley-VCH

Verlag GmbH Co.

Figure 2. Proposed mechanism for formation of fluorenone defects in PDAFs.

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Figure 3. Curve of human eyes sensitivity to light according to its color [95].

Figure 4. Important examples of PAHs.

Figure 5. The numbered positions of pyrene and different options to extend pyrene

into polymers.

Figure 6. Donor Box A: Heteroacene donors containing bridging atoms.

Figure 7. Donor Box B: Heteroacene donors without bridging atoms.

Figure 8. Acceptors classified by the chemical structure.

Figure 9. Analysis of single 171a fiber on HMDS: a) SEM and b) SAED (arrow

indicates the fiber axis), c) schematic illustration of the polymer

organization in the fiber fabricated via a method of solvent vapor

enhanced drop casting. [333], Copyright 2012. Adapted from with

permission from Wiley-VCH Verlag GmbH Co.

Figure 10. Molecular structures of seven benzotrithiophene isomers and their

calculated frontier orbitals (DFT-B3LYP 6-31G*). The HOMO (lower)

and LUMO (upper) orbital energies are given in eV.

Figure 11. STM images of 212 as a) a monolayer with hexagonal packing, b)

individual macrocycles, and c) a single macrocycle on an HOPG surface.

[487], Copyright 2006. Adapted from with permission from Wiley-VCH

Verlag GmbH Co.

Figure 12. STM images of 213 as a monolayer with hexagonal packing a) before

and b) after deposition of HBC molecules on an HOPG surface. [488],

Copyright 2009. Adapted from with permission from Wiley-VCH Verlag

GmbH Co.

Figure 13. Schematic illustration of graphene terminology defined according to

their size scale. [11], Copyright 2012. Adapted from with permission

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from Wiley-VCH Verlag GmbH Co.

Figure 14. Armchair (top) and zigzag (bottom) edges in monolayer GNRs. [517],

Copyright 2009. Reproduced from the American Institute of Physics.

Figure 15. STM images of 230 at the solid/liquid interface on HOPG. [513],

Copyright 2008. Adapted with permission from the American Chemical

Society.

Figure 16. STM images of 232a on HOPG and corresponding model of molecular

arrangement of the aromatic core. The values of intracolumnar (0.4 nm)

and intercolumnar (5 nm) spacing are obtained from STM measurements.

[528], Copyright 2009. Adapted with permission from the American

Chemical Society.

Figure 17. GNRs synthesized using polyphenylene precursors on surface and their

STM images. a) Overview STM image of straight GNR 239 after

cyclodehydrogenation at 400 oC. Inset: a higher-resolution STM image.

b) STM image taken after surface-assisted C–C coupling at 200 oC

before the final cyclodehydrogenation showing a chain of the

polyanthrylene precursor 238, and DFT-based simulation of the STM

image with partially overlaid model of the polymer. c) High-resolution

STM image with partly overlaid molecular model (blue) and a DFT-

based STM simulation (greyscale) of GNR 239. d) Overview STM

image of chevron-tpye GNR 242 fabricated on a Au(111) surface. Inset:

a higher-resolution STM image and DFT-based simulation of the STM

image (greyscale) with partly overlaid molecular model of the ribbon. e)

STM image of coexisting straight and chevron-type GNR 245

sequentially grown on Ag(111). Inset: model of the colligated and

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dehydrogenated molecules forming the threefold junction overlaid on the

STM image. [511], Copyright 2010. Images (a-e) reproduced with

permission from the Nature publishing group.

Figure 18. Schematic illustrations of the architectures of 1D and 2D D-A polymers.

Table 1. EL performances of typical PF-based polymers (Vonset: onset voltage; Bmax: maximum brightness; LEmax: maximum luminous efficiency; PEmax: maximum power efficiency; EQE: external quantum efficiency; CIE1931: CIE color coordinates created by the CIE in 1931.).

Polymer Vonset (V)

Bmax (cd/m2)

LEmax (cd/A)

PEmax (lm/W)

EQE (%)

CIE1931 (x, y) Ref.

13 ~4.0 - 0.84 0.45 - 0.19, 0.18 [35] 14 3.5 1600 1.1 0.4 - 0.15, 0.08 [36] 18 5.3 2770 0.25 0.08 0.52 - [42] 22 4.4 4080 0.63 0.19 1.21 0.19, 0.14 [50] 24 6-7 380 - 0.06 - 0.19, 0.24 [74] 28 3.7 820 0.03 - - 0.14, 0.16 [78] 33 - - - - 0.017 0.16, 0.07 [81] 34 6.0 1600 0.19 0.06 0.20 0.17, 0.12 [83] 43 4.6 3137 1.42 - 1.06 0.15, 0.17 [96] 45 3.5 ~9000 6.85 5.38 3.54 0.15, 0.19 [98]

Blue

48 3.3 6539 3.43 - 2.42 0.15, 0.16 [101]

49 2.25 10000 10.5 - - 0.40, 0.58 [103]50 4.8 21595 7.43 2.96 - 0.26, 0.58 [105]

Green

53 12 - 0.9 - 0.6 0.36, 0.56 [102]

55 10.6 ~850 1.45 - 2.54 0.66, 0.34 [109]56b - 2104 0.91 - 3.1% 0.64, 0.33 [111]59 8 - 1.6 - 0.5 0.64, 0.34 [102]60 4.9 4321 2.8 - 1.59 - [113]

Red

61 ~3.5 ~10000 8.3 7.5 - 0.63, 0.35 [116]

62a - 11100 5.3 2.8 - 0.25, 0.35 [125]63 3.5 18480 12.8 8.5 5.4 0.31, 0.36 [129]64 5.8 12040 18.0 8.38 6.36 0.33, 0.35 [135]66 4.0 12710 7.3 4.17 - 0.31, 0.32 [136]67 7.2 6994 6.2 - 3.84 0.35, 0.34 [140]69 2.8 ~1000 8.2 7.2 3.7 0.35, 0.38 [143]

White

71 3.1 6770 2.96 2.33 1.37 0.34, 0.35 [146]

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Table 2. Typical PF- and PCz-based polymers for the PSCs (JSC: short-circuit current density; Voc: open-circuit voltage; FF: fill factor; PCE: power conversion efficiency.).

Polymer:acceptor (w:w) Jsc (mA/cm2) Voc (V) FF PCE (%) Ref. 73a:PCBM (1:4) 4.66 1.04 0.46 2.2 [147]74b:PCBM (1:3) 8.88 0.59 0.42 2.2 [155]

75a:C70 derivative (1:4) 3.4 0.58 0.35 0.7 [154]76:PCBM (1:3) ~6 ~1 0.63 3.7 [156]77:PCBM (1:3) 6.5 0.81 0.44 2.3 [158]78: PCBM (1:2) 2.0 0.78 0.50 0.78 [160]79b:PCBM (1:4) 5.35 0.52 0.50 1.38 [162]

107:PDI (1:4) 0.26 0.71 0.37 0.63 [221]108e: PCBM (1:4) 1.56 0.8 0.55 0.8 [223]109c: PCBM (1:4) 6.92 0.89 0.63 3.6 [225]109c: PCBM (1:2) 10.22 0.89 0.51 4.6 [226]109c: PCBM (1:4) 10.6 0.88 0.66 6.1 [227]109c: PCBM (1:4) Alcohol/water-soluble

polymer as the cathode interlayer

12.7 0.9 0.59 6.8 [228]

109c: PCBM (1:4) MoO3-Al composite film as

the cathode buffer layer

10.88 0.88 0.71 6.8 [229]

110c:PCBM (1:1) 5.16 0.41 0.29 0.61 [230]111b:PCBM (1:2) 5.35 0.76 0.56 2.3 [159]

Table 3. The acceptors that are not discussed in the main text and the corresponding references.

Acceptor A1-A5 A7 A8 A9&A10 A11&A13 A12 Ref [320] [387] [224, 413] [414] [415] [416]

Acceptor A14 A15 A16 A17 A18 A19

Ref [387, 416] [224, 404, 417, 418] [419, 420] [421] [422] [423]


Ref [350, 424, 425] [426, 427] [172, 330,

428, 429] [430] [155, 224, 357, 431, 432] [433, 434]


Ref [153, 154, 161, 201, 237] [224] [323, 404] [162, 435] [435] [367]

Acceptor A32&A33 A34 A35 A37 A38 A39 Ref [368] [436] [437, 438] [439] [382] [440-442]

Acceptor A43 A44 A48-A50 A51 A52 A53 Ref [443] [444] [316] [445] [281] [446]

Acceptor A54 A55 A56 A57 A58 A59&A60Ref [447] [448] [343, 366] [205] [203, 324] [449]

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Table 4. OFET and PSC performances of selected D-A polymers.

PSCs OFETs

Polymer Jsc (mA/cm2

)

Voc (V) FF

PCE

(%)

μh, max (cm2/V

s) Ion/Ioff

μe, max (cm2/V

s) Ion/Ioff

Ref.

171a - - - - 5.5 106 - - [333]

171b 16.2 0.62 0.55 5.5 - - - - [336

]

172c 11.9 0.54 0.44 2.8 - - - - [345

]

173 12.7 0.68 0.55 5.1 - - - - [351

] 175 - - - - 0.04 ~104 - - [4]

176 - - - - - - 0.45-0.85 106 [452

] Replacement of PCBM 177

3.79 0.53 0.44 0.88

- - 0.07 105 [453]

Replacement of PCBM 178

4.57 0.56 0.50 1.28

- - - - [454]


3.63 0.68 0.66 1.63

- - - - [455]

180 - - - - - - 0.002 - [456]


4.2 0.63 0.39 1

- - 0.013 104 [457]

Replacement of PCBM

3.31 0.74 0.47 1.15

182f

Solvent optimization results in a PCE of 2.23%

- - 10-4-10-3 - [458]

183b - - - - 4.0-8.2 105-107 - - [5]

184 - - - - 4.97 1.6×107 - - [459

] 185 - - - - 0.01 - 3 104 [6]

186 - - - - 0.0043 2.6×104 0.046 6.0×1

05 [460

]

187 - - - - 3.97 104 2.20 10 [462]

189 15.0 0.58 0.61 5.4 1.42 105 0.063 103-

104 [463

]

191 - - - - 0.79 107 - - [469]

192 13.7 0.70 0.69 6.3 - - - - [470

]

195a 7.87 0.79 0.68 4.22 - - - - [472

] 197 - - - - 3.62 106 - - [473

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]

199 - - - - 2.48 106 - - [461]

200 - - - - 1.85 106-107 0.43 105-

106 [474

]

202a - - - - 0.35 - 0.40 - [477]

202b - - - - 0.1 - 0.09 - [478]

204 - - - - 0.46 ~104 0.84 ~104 [466]

206b - - - - 1.17 - 1.32 - [480]


0.95 1.1 - 0.31

- - 0.02 - [481]

[482]

209 - - - - 0.16 - 0.14 - [483]

210 5.6 0.79 0.56 2.5 - - - - [484

]

211 17.2 0.68 0.67 7.9 - - - - [485

] Table 5. PSC performances of 2D D-A polymers.

Polymer:acceptor (w:w) Jsc (mA/cm2) Voc (V) FF PCE (%) Ref.

246a:PCBM (1:4) 9.62 0.99 0.50 4.7 [12] 247b:PCBM (1:4) 6.22 0.90 0.45 3.2 [547] 248a:PCBM (1:4) 8.94 0.91 0.51 4.2 [539] 254:PCBM (1:2) 10.7 0.92 0.58 5.7 [543] 255a:PCBM (1:2) 12.1 0.86 0.60 6.2 [415] 257:PCBM (1:1) 11.7 0.80 0.61 6.0 [427] 260a:PCBM (1:2) 14.6 0.68 0.63 6.2 [391]

260b:PCBM (1:1.5) 17.5 0.74 0.59 7.6 [391] 260c:PCBM (1:1) 16.4 0.69 0.66 7.8 [448] 262a:PCBM (1:2) 14.0 0.73 0.65 6.6 [8] 262a:PCBM (1:2)

Inverted tandem solar cell

~8.5 ~1.6 0.60 8.8 [8]

Designing π-conjugated polymers for organic electronics

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