Accepted Manuscript

Anchoring stability and photovoltaic properties of new D(-π-A)2 dyes for dye-sensitized solar cell applications

Roberto Grisorio, Luisa De Marco, Giovanni Allegretta, Roberto Giannuzzi, GianPaolo Suranna, Michele Manca, Piero Mastrorilli, Giuseppe Gigli

PII: S0143-7208(13)00062-4

DOI: 10.1016/j.dyepig.2013.02.012

Reference: DYPI 3859

To appear in: Dyes and Pigments

Received Date: 11 January 2013

Revised Date: 19 February 2013

Accepted Date: 20 February 2013

Please cite this article as: Grisorio R, De Marco L, Allegretta G, Giannuzzi R, Suranna GP, Manca M,Mastrorilli P, Gigli G, Anchoring stability and photovoltaic properties of new D(-π-A)2 dyes for dye-sensitized solar cell applications, Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.02.012.

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Anchoring stability and photovoltaic properties of new D(-ππππ-A)2 dyes

for dye-sensitized solar cell applications

Roberto Grisorio,a Luisa De Marco,b Giovanni Allegretta,a Roberto Giannuzzi,b Gian Paolo

Suranna,a,* Michele Manca,b Piero Mastrorilli,a,c Giuseppe Giglib,d,e

aDICATECh - Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e di Chimica,

Via Orabona, 4 I-70125 Bari, Italy. E-mail: surannag@poliba.it

bCenter for Biomolecular Nanotechnology (CBN) Fondazione Istituto Italiano di Tecnologia (IIT),

Via Barsanti 1, Arnesano, 73010, Italy.

cIstituto di Chimica dei Composti Organometallici, Area di Ricerca CNR di Firenze, Via Madonna

del Piano 10, 50019 Sesto Fiorentino (Firenze) Italy.

dNNL - Istituto Nanoscienze, CNR c/o distretto tecnologico Lecce, via Arnesano 16, 73100 Lecce,

Italy.

eDepartment of Mathematics and Physics “E. De Giorgi”, University of Salento, Campus

Universitario, via Monteroni, 73100 Lecce, Italy.

Abstract

This study deals with the synthesis and characterization of two new di-anchoring dyes for

applications in dye-sensitized solar cells. The materials were designed with a branched D(-π-A)2

structure containing i) a rigid alkyl-functionalized carbazole core as the donor part, ii ) one (DYE1)

or two (DYE2) thiophene units as the π-bridge and iii ) a cyano-acrylic moiety as acceptor and

anchoring part. Electrochemical impedance spectroscopy indicated that the injected electron

lifetime are higher in the case of DYE2, probably due to the length of the π-spacer that, in

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combination with the alkyl chain on the carbazole unit, hampers the charge recombination with the

electrolyte. Stability tests on TiO2-sensitized films revealed that the di-anchoring remarkably slows

down the desorption process, which conversely is evident for classic reference dyes. The highest

power conversion efficiency reaches 5.01 % in the case of DYE2 with a photovoltage of 0.70 V and

a photocurrent of 10.52 mA cm–2, substantially deriving from a broader absorption with respect to

DYE1, as also confirmed by IPCE measurements. These results support the efforts aimed at the

structural engineering of D(-π-A)2 dyes to design new, more efficient and stable organic sensitizers.

Keywords: DSSC, di-anchoring dye, anchoring stability.

1. Introduction

The concerted optimization of the four components (i.e. semiconducting oxide, dye, electrolyte,

counter-electrode) of a dye-sensitized solar cell (DSSC) can rightly be included amongst the

intriguing quests, that chemistry faces today as the multidisciplinary interface science devoted to

unravelling the behaviour of complex matter [1,2]. The continuing contributions from a significant

part of the whole scientific community ultimately aimed at allowing a further decisive efficiency

and long-term stability improvement needed to fulfill the promises of this technology. The

performances of DSSC are now beyond the second generation of solar cells, but the result of the

huge research would qualify them as third generation devices, which are expected to deliver electric

power in a larger scale at a lower price per Watt [3].

In this wider framework, the goal of obtaining more efficient and robust dyes is being carried out

since the first seminal report by Grätzel [4]. While this two-decade long race has not yielded

breakthrough improvement in terms of efficiency with respect to the well-known paradigmatic

ruthenium dyes N3 and N719 [5], it has nevertheless allowed a considerable understanding of the

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principles underlying the structure-property relationships of sensitizers, that led to unquestionable

durability improvement. Thousands of dyes have been tested to date, making their classification a

quite difficult task. A very coarse subdivision can, however, be made between metal complexes [6]

and synthetic fully-organic dyes [7]. There are several reasons that indicate how organic dyes might

have a considerable edge over noble metal complexes, among which their predicted lower

production cost, lower environmental impact and higher molar extinction coefficients. The degrees

of freedom provided by organic synthesis has granted enormous versatility to this class of

sensitizers, although a generally accepted design rule relies on the D-π-A concept. In this motif, an

electron rich group (D) is linked, through a π-conjugated spacer (π), to an electron acceptor group

(A) carrying an anchoring group, most typically a carboxyl group functionality allowing a strong

binding to the TiO2 mesoporous layer. While a convergence has been reached on the use of cyano-

acrylic acid, as acceptor, much of the current research is devoted to the choice of the suitable

donor/π-bridge to increase the DSSC performance.

In the search for novel dye design that could lead to amelioration of the D-π-A concept, it was

recently highlighted that the presence of only one anchoring group in the common organic dyes

could represent a limitation with respect to the Ru-based dyes carrying up to four anchoring groups

[8,9,10,11]. The multi-branched multi-anchoring dye idea was consequently proposed over the

traditional D-π-A architectural design. This concept was exploited for the design of cruciform [12],

spiro- [13] and H-shaped [14] architectures and has also been implemented for the preparation of

dyes for p-type DSSC devices [15,16,17]. The interest toward this design approach is justified by

several reasons. Primarily, the more extended π-system due to the presence of two π-bridges should

lead to an extension of the absorption to longer wavelengths, to a broadening of the absorption

profile and to an enhanced molar extinction coefficient with respect to those of D-π-A sensitizers,

all factors contributing to an improved light harvesting. Moreover, the presence of a further

electron-withdrawing unit (A) could decrease the HOMO-LUMO gap of the organic dye and,

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hence, prove beneficial for its photo-stability. Last but not least, less complex synthetic strategies

are required for the preparation of symmetrically branched organic molecules, with evident

reduction of the prospected production costs of the material.

However, it should be pointed out that the adoption of a di-anchoring architecture D(-π-A)2 does not

necessarily imply a performance enhancement if, for instance, the structural features of the dye do

not efficiently shield the TiO2 surface from the oxidized form of the redox shuttle [18], thereby

allowing unwanted recombinations, detrimental to the overall performances [19].

Our contribution in this field aims at gaining a deeper insight into D(-π-A)2 structures studying the

effect of the π-bridge length on dyes obtained binding an N-octyl-3,6-carbazole core [20] as donor

to cyano-acrylic acid groups as acceptors, through either thiophene or bithiophene spacers (Figure

1).

Figure 1. Structures of the synthesized molecules DYE1 and DYE2.

2. Results and discussion

2.1. Synthesis and characterization

The synthetic sequence for the obtainment of DYE1 and DYE2 is reported in Scheme 1. The

carbazole core was straightforwardly prepared from 3,6-dibromo-N-octyl-carbazole (1) that was

converted into the corresponding boronic ester (2) by lithiation with n-BuLi and subsequent

reaction with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. Concerning the thiophene-

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based π-conjugated precursors needed for the subsequent step, the commercially available 5-

bromothiophene-2-carbaldeyde was reacted with tributyl(thiophen-2-yl)stannane in a Pd-catalyzed

Stille coupling to obtain the bithiophene derivative 4. Bromination of 4 with NBS permitted to

obtain the aldehyde derivative 5 needed for the following step. The electron-donating carbazole unit

was then bound to the two thiophene-based π-conjugated systems by a Suzuki-Miyaura coupling.

The reaction between 2 and two equivalents of 5-bromothiophene-2-carbaldeyde yielded the

corresponding dialdehyde 3. The synthesis of the target molecule DYE1 was achieved by the

Knoevenagel condensation of the dialdehyde 3 with cyano-acetic acid in the presence of ammonium

acetate. For the preparation of the second target molecule, a similar approach was followed: the

reaction between 2 and two equivalents of 5 allowed the obtainment of the dialdehyde 6 that, after

Knoevenagel condensation with cyano-acetic acid yielded DYE2. The two target molecules were

sufficiently soluble in THF, DMF, ethanol and DMSO to permit their characterization in solution as

well as their application in DSSC devices. The structures of DYE1-2 were confirmed (see

experimental part) by elemental analysis, 1H-NMR, ESI-HRMS as well as by IR analyses.

N

Br Br

C 8H1 7

N

BB

C8H 17

O

OO

O

S C HOB r

NC8H 17

S

S

C HO

CH O

SB rS

C HO NC 8H17

S

S S

S C HO

C H O

n-B uLi

BO

OO

S S n(n -Bu)3

SS

CH O

P d(P Ph3)4

N CC H 2C OO H

A cON H 4/A cOH

Pd(PP h3)4

NB S NC C H2C OO H

A cON H4 /A cOHP d(P P h3)4

12

3

2

5 64

DYE1

DYE22

Scheme 1. Synthetic sequence followed for the preparation of DYE1 and DYE2.

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2.2. Optical characterization

The photophysical properties of DYE1 and DYE2 were investigated by UV-Vis and

photoluminescence (PL) spectroscopy. The UV-Vis absorption spectra of the two compounds

measured in THF, are shown in Figures 2 and 3. The presence of a bithiophene π-bridge between

the donor and the acceptor moieties in DYE2 leads to a substantial red-shift of both the absorption

maximum (470 nm) as well as of the absorption onset (557 nm) with respect to those exhibited by

the monothiophene bridged DYE1 (453 and 520 nm, respectively). The broader absorption profile

of DYE2 in the visible region was also accompanied by a higher molar extinction coefficient

(54000 M–1cm–1 at 470 nm) compared to DYE1 (33900 M–1cm–1 at 453 nm). The length of the π-

spacer also has a strong influence on the solution (THF) PL behaviour of the synthesised dyes: the

λem of DYE1 was recorded at 559 nm while that of DYE2 was considerably red shifted, falling at

610 nm.

When organic dyes are adsorbed onto the TiO2 layer, it is very likely that the free –COOH group is converted

into a titanium carboxylate (–COOTi) form, corresponding to a deprotonation of the carboxylic acid

functionality [21]. Most D-π-A organic dyes show a large absorption blue-shift upon binding onto TiO2,

which is detrimental in terms of light harvesting. Generally, in donor-acceptor architectures, the carboxylic

acid deprotonation leads to a blue-shift of the absorption maximum deriving from the attenuation of the

intramolecular charge transfer, due to the lower electron acceptor character of the carboxylate moiety with

respect to the carboxylic acid. To deepen this aspect, the UV-Vis spectra in THF of the dyes were also

recorded in the presence of an excess of a base (triethylamine) and of an acid (formic acid), as shown in

Figures 2 and 3. The absorption maxima for DYE1 and DYE2 in the presence of triethylamine

(THF/NEt3 = 4/1 v/v) were found at 430 nm and 432 nm respectively, remarkably blue-shifted with

respect to those recorded in THF, as a consequence of the complete deprotonation of the carboxylic

acid functionality [22]. Conversely, a remarkable broadening of the absorption spectra in the

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presence of formic acid (THF/HCOOH = 4/1 v/v) was observed for DYE1 and DYE2 with red-

shifted absorption maxima at 468 nm and 479 nm respectively.

350 400 450 500 550 600 650 7000,00

0,25

0,50

0,75

1,00

Nor

mal

ized

Abs

orba

nce

Wavelength (nm)

THF THF/HCOOH (4/1 v/v) THF/NEt

3 (4/1 v/v)

on TiO2

Figure 2. Normalized absorption spectra of DYE1 in THF, THF/NEt3 (4/1 v/v), THF/HCOOH (4/1

v/v) and on TiO2 film.

350 400 450 500 550 600 650 7000,00

0,25

0,50

0,75

1,00

Nor

mal

ized

Abs

orba

nce

Wavelength (nm)

THF THF/HCOOH (4/1 v/v) THF/NEt

3 (4/1 v/v)

on TiO2

Figure 3. Normalized absorption spectra of DYE2 in THF, THF/NEt3 (4/1 v/v), THF/HCOOH (4/1

v/v) and on TiO2 film.

Furthermore, when DYE1 and DYE2 are adsorbed to the TiO2 surface, their UV-vis spectra are

further blue-shifted compared to those in THF/NEt3, although in this case their spectral responses

are broadened due to intermolecular interactions between dye molecules on TiO2 film. The

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absorption maxima for DYE1 and DYE2 adsorbed onto 5 µm-thick TiO2 mesoporous film were, in

fact, found at 397 nm and 424 nm, respectively, blue-shifted by 56 and 46 nm with respect to those

recorded in THF. On the bases of the previously described solvatochromic behaviour, this

observation is indicative of the deprotonation of both carboxylic acid groups on TiO2 surface

[18,23].

To corroborate this assumption and since the anchoring modes of the dyes are related with the

interfacial electron injection, FT-IR measurements of the dyes absorbed on the TiO2 surface were

carried out. Figure 4 shows the IR spectra in the region 4000–400 cm–1 of DYE1- and DYE2-

sensitized TiO2 films and of bare TiO2.

4000 3500 3000 2500 2000 1500 1000

4000 3500 3000 2500 2000 1500 1000

Tra

nsm

ittan

ce (

%)

Wavenumber [cm -1]

DYE1 DYE2 DYE1-sensitized TiO

2

DYE2-sensitized TiO2

Figure 4. FT-IR spectra of DYE1 and DYE2 in KBr and adsorbed on TiO2 film.

The IR spectra of the sensitized films display characteristic narrow absorption bands at 2930 cm–1

and 2850 cm–1, which correspond to the antisymmetric and symmetric C–H stretching vibrations,

respectively, of the –CH2– moieties of the hydrocarbon chain; moreover, the band associated to the

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C≡N stretching of the cyano-acrylic acid group was clearly observed at 2216 cm–1. The two

characteristic bands centred at ~1590 and ~1385 cm–1, respectively, are associated with the COO–

antisymmetric and symmetric stretching vibrations of carboxylate groups coordinated with surface

titanium atoms; the lack of clear evidence for the C=O stretching band of free carboxyl groups at

~1710–1720 cm–1 and the difference in wavenumbers between the aforementioned carboxylate

bands (∆υ = ~ 205 cm–1) confirm that the dye acts as bidentate since both its carboxyl moieties are

bound to the TiO2 surface. These data confirm the absence of free carboxylic acid groups as well as

the bidentate adsorption of both dyes onto the TiO2 surface [9,18, 24].

2.3. Electrochemical properties

Cyclic voltammetry (CV) of sensitizers for DSSC applications provide crucial information on the

feasibility of the electron injection from the excited states of the dye to the TiO2 conduction band

and on the possibility of dye regeneration by means of the electrolyte redox couple. As shown in

Figure 5, the two dyes exhibited only one reversible oxidation event. The CV wave did not undergo

substantial modification after repeated scans, revealing that both DYE1 and DYE2 are

electrochemically stable under oxidative conditions. It is worth noting that the presence of a

thiophene unit in the π-bridge of DYE2 lowered its oxidation potential (corresponding to the onset

of the anodic event) of ~0.2 V with respect to DYE1. The evaluation of the HOMO energy levels

(see experimental part) led to values of –5.4 and –5.2 eV for DYE1 and DYE2, respectively. The

obtained values indicate that an efficient regeneration of the oxidised dyes by the iodide/triiodide

couple is thermodynamically feasible, lowering the possibility of a geminated recombination

between the oxidized dye and the photo-injected electrons in the TiO2-based anode. Noteworthy,

the relatively deep HOMO energy values of both dyes lead to a prediction of a high stability

towards oxidation, which constitutes an advantage for the prospected applications of these

materials.

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-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6C

urre

nt

Voltage vs Ag/Ag + (V)

DYE1 DYE2

0.5 µA

Figure 5. Anodic CV curves of the dyes in THF solutions.

Since no cathodic behaviour of both DYE1 and DYE2 could be recorded by CV, their excited state

oxidation potentials (corresponding to the LUMO energy levels) were calculated by adding the

energy gap (estimated from the onset of the absorption spectra recorded in THF solutions) to

HOMO energy values. It can be noted how the obtained LUMO of both dyes (–2.9 eV) lays above

the TiO2 conduction band edge (~ –4.0 eV) warranting the necessary driving force for an efficient

electron transfer process.

2.4. Theoretical investigations

A key step for the accurate theoretical description of the electronic and thermodynamic properties

of an organic semiconductor is a reliable evaluation of its molecular geometry. In the specific case

of di-branched dyes, the identification of an accurate and reliable molecular modelling method for

the prediction of the equilibrium molecular geometry is mandatory. Hence, since the two thienyl

moieties bound to the carbazole unit can adopt a different orientation with respect to each other,

molecular geometries were optimized in vacuo, constraining the dihedral angles of the C–C bonds

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connecting the thienyl units to carbazole to fixed values and subsequently finding the minimum

energy structures. To minimize the computational load, the optimization was performed using the

semi-empirical (PM6) method. Since, due to the molecular structure, the length of the alkyl

substituent on the carbazole nitrogen atom is not expected to have an influence on the conformation

or on the HOMO and LUMO energy, it was chosen to replace the n-octyl with n-propyl groups.

Moreover, in the case of DYE2, the geometry optimization was carried out starting from a structure

in which the thiophene units of the bithiophene moiety were placed in the anti-orientation, as

proposed for similar compounds. These calculations [24] demonstrated that in the most stable

conformers the thienyl units are tilted towards the same side (as shown in Figure 1) although the

low torsional barrier (~2.9 kcal/mol) does not allow the freezing of the conformational equilibrium.

Starting from these conformations, the geometry of both dyes were fully optimized in vacuo until

convergence by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level. The

molecular orbital energy of the dyes as well as their electron density distribution and their

optimized geometry were estimated by DFT calculations at the B3LYP/6-31G(d,p) level including

solvent effects (THF) using non-equilibrium implementation of the conductor-like polarizable

continuum model (C-PCM).

In the case of DYE1, the electron density of the HOMO state is distributed along the entire

molecule with a higher contribution of the electron-donor carbazole moiety, as evident from Figure

6. The presence of a further thiophene unit in the π-bridge of DYE2 spacing the electron-donor

from the electron acceptor leads to a decrease of the HOMO electron density on the cyano-acrylic

units that, in principle, could reduce the probability of the aforementioned charge recombination of

the oxidised dye with the TiO2 electron. Conversely, the LUMO electron density distributions of

both dyes are prevalently localized around the cyano-acrylic units, thereby favouring an efficient

electron transfer from the excited state of the dyes to the TiO2 conduction band edge. Although a

straightforward correspondence between experimental and theoretical HOMO-LUMO energy levels

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is not often observed for organic dyes, in the case of DYE1 and DYE2, the calculated and

electrochemical HOMO-LUMO energies are in rather good agreement (Table 1).

Figure 6. Electronic density distributions of the HOMO–1, HOMO, LUMO and LUMO+1 orbitals

of DYE1 and DYE2.

The calculation of the vertical S0 → Sn excitation energies at the TDDFT level of theory provides

more insight into the optical behaviour of the two materials. For a reliable description of the

transitions with non-negligible charge transfer character, such as those expected for our D(-π-A)2

structure, a judicious choice of the proper functional is crucial. In fact, use of a conventional

exchange-correlation functional such as a pure and hybrid functional would remarkably

underestimate the energy of the excited states with significant charge transfer character due to the

self-interaction error [21]. The Coulomb attenuating method CAM-B3LYP functional [22] was

therefore chosen to calculate both the vertical excitations energy and the oscillator strengths. This

method considers long range corrections, which are appropriate for the description of charge-

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transfer type excitations. Also, before describing the results, it should be highlighted that, although

the HOMO→LUMO energy transition is generally used as a measure of the excitation energy, this

might not be dominated by the HOMO→LUMO transition and, analogously, the charge injection

from the excited state of the dye into the semiconductor cannot always be described as a simple

transfer from the LUMO of the dye into the semiconductor’s conduction band.

Table 1. Theoretical HOMO and LUMO energy values in THF as determined at the C-PCM-

B3LYP/6-31g(d,p) level of theory for DYE1 and. Excited-state transition energies (wavelengths),

their electronic configurations and oscillator strength (f) determined in THF by TD-C-PCM-CAM-

B3LYP/6-31g(d,p) using the B3LYP/6-31g(d,p) geometries for DYE1 and DYE2.

As shown in Table 1, the TDDFT calculation gave two main transitions in the visible region for

both dyes, which are in good agreement with the experimental data, since DYE2 showed red-shifted

vertical excitation energies with higher oscillator strengths with respect to those exhibited by

DYE1. Furthermore, according to these results, the S0→S1 transition at 423 nm and 457 nm for

DYE1 and DYE2, respectively, not only correspond to the promotion of an electron from the

HOMO to the LUMO but is also associated with the promotion of an electron from the HOMO–1 to

the LUMO+1. These latter two molecular orbitals are also involved in the description of the S0→S2

transition at 390 nm and 438 nm for DYE1 and DYE2, respectively. Nevertheless, it is noteworthy

that while HOMO–1 electronic density is almost fully spread along the entire molecule both in

dye HOMO LUMO nm f configuration

DYE1 –5.54 –2.72 423

390

1.28

1.01

H→L (0.59); H–1→L+1 (0.30)

H→L+1 (0.52); H–1→L (0.36)

DYE2 –5.28 –2.84 457

438

1.77

1.50

H→L (0.51); H–1→L+1 (0.40)

H→L+1 (0.48); H–1→L (0.44)

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DYE1 and in DYE2, the LUMO+1 electronic density is predominantly concentrated around the

cyano-acrylic groups. This electronic localization of the LUMO+1 should warrant an efficient

electron transfer also for the transitions involving this excited state. From the results reported up to

this point, it is apparent how the “branched” architecture approach positively affects the light

harvesting efficiency (by increasing the number of aromatic units present in the related molecules)

without depressing the dye key features: the electron injection ability, the feasibility of the oxidised

dye reduction by the electrolyte and their photo-stability.

2.5. Photovoltaic performances and stability

Having established the potential of the new sensitizers, these have been implemented in

photoelectrochemical solar cells devices and their performances investigated both in terms of J-V

characteristics and of electrochemical impedance spectroscopy (EIS) analysis. Two different

typologies of photoanodes were fabricated to this purpose: a first batch of 7 µm-thick transparent

films made of 20 nm-sized TiO2 nanocrystals, and a second batch of 17 µm-thick photoelectrodes

comprising a 12 µm-thick transparent layer and a 5 µm-thick scattering layer constituted by 300

nm-sized light reflecting titania particles. The photovoltaic characteristics of these devices

(measured under standard illumination conditions, namely AM 1.5G at 100 mW cm–2) are

summarized in Table 2 and compared with similar devices employing D5 and N719 as reference

organic and ruthenium-based dyes; the J-V plots are reported in Figure 7A. The DYE1- and DYE2-

based devices made with the 7 µm-thick layer exhibited a short circuit photocurrent (Jsc) of 7.38 and

8.41 mA cm–2, open circuit voltages (Voc) of 0.723 and 0.715 V and fill factors (ff) of 0.69 and 0.68,

respectively, resulting in an efficiency (η) of 3.68 % and 4.09 % respectively. Not much higher

performances are obtained for the N719-based device (Jsc = 9.27 mA cm–2, Voc = 0.820 V, ff = 0.67,

η = 5.09 %) and for the D5-based device ( Jsc = 9.11 mA cm–2, Voc = 0.74 V, ff= 0.72, η = 4.85 %)

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fabricated and measured under the same conditions. Subsequently, the implementation of thicker

photoelectrodes with improved light harvesting capabilities led, as expected, to a significant

enhancement of the short circuit photocurrent and, hence, of the DSSC performances.

D5 17 µm DYE2 17 µm DYE1 17 µm D5 7 µm DYE2 7 µm DYE1 7 µm

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,80

1

2

3

4

5

6

7

8

9

10

11

12

Pho

tocu

rren

t (m

A/c

m2 )

Bias Voltage (V)

A

350 400 450 500 550 600 650 700 7500

10

20

30

40

50

60

70

IPC

E (

%)

Wavelength (nm)

DYE1 DYE2 D5

B

Figure 7. A) J-V curve of DSSCs prepared with transparent photoelectrodes 7 µm thick and opaque

double-layered photoanodes 17 µm-thick. B) IPCE spectra of DSSCs prepared with double-layered

17 µm-thick photoelectrodes.

Devices made with the 17 µm-thick photoanodes yielded, in fact, remarkably higher photocurrents

(9.36 and 10.52 mA cm–2 for DYE1 and DYE2, respectively) but substantially similar Voc (0.71 and

0.70 V) and ff values (0.67 and 0.68) corresponding overall conversion efficiencies of 4.47 % and

5.01 %, respectively. However, the device parameters of the reference dyes showed different

degrees of improvement for N719 and D5 upon increasing of the photoelectrode thickness.

Whereas, in the case of the 7 µm-thick photoelectrode, the ruthenium complex showed only 1.2 and

1.1 times higher photocurrent than those of the DYE1 and DYE2, in the case of thicker

photoelectrodes, the Jsc produced by N719 were found to be 1.5 and 1.3 times higher with respect to

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DYE1 and DYE2 respectively. It is reasonable to suppose that, due to their much higher extinction

coefficients, DYE1 and DYE2 tend to saturate incoming light within a relatively low thickness. As

a consequence, the gap in photocurrent respect to the ruthenium dye increases with the electrode

thickness. By contrast, the photocurrent produced by D5 using thicker photoelectrodes (10.06 mA

cm–2) was even lower than that of DYE2, probably due to the better light-harvesting efficiency of

the latter, supported by theoretical calculations (vide supra).

Table 2. Solar cell performance values. aTiO2 thickness = 7 µm. bTiO2 thickness = 17 µm. cuptake

performed in the presence of CDCA (10 mM).

Spectra of the monochromatic incident photon-to-current conversion efficiency (IPCE) for the

DSSCs based on the two here presented dyes are reported in Figure 7B. The IPCE spectrum of the

DYE2-sensitized cell is red shifted compared to that of DYE1, which is in good agreement with the

absorption behavior of the dyes absorbed on TiO2 film. The onset of the IPCE spectrum of DYE2

falls at 700 nm and a high conversion efficiency (60 %) was observed in the range 400-550 nm. In

contrast, the DYE1-based cell showed a blue-shifted onset of the IPCE spectrum (at 620 nm) and

exhibited a good plateau (>60 %) in a range comprised between 400 and 500 nm. Therefore, the

relatively lower photocurrent produced by DYE1, leading to its relatively lower efficiency, is

Dye Jsc (mA cm–2) Voc (V) ff η (%)

DYE1 7.38a ; 9.36b 0.723a ; 0.712b 0.69a ; 0.67b 3.68a ; 4.47b

DYE2 8.41a ; 10.52b 0.715a ; 0.701b 0.68a ; 0.68b 4.09a ; 5.01b

DYE1c 7.89b 0.703b 0.66b 3.66b

DYE2c 8.87b 0.700b 0.68b 4.22b

D5 9.11a ; 10.06b 0.740a ; 0.732b 0.72a ; 0.72b 4.85a ; 5.34b

N719 9.27a ; 13.91b 0.820a ; 0.804b 0.67a ; 0.70b 5.09a ; 7.83b

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clearly attributable to its narrow and blue-shifted spectral response compared to the one exhibited

by DYE2.These results pinpoint the bithiophene spacer present in DYE2 between the donor and the

acceptor moieties as an excellent π-conjugation system to harvest a larger fraction of the solar

spectrum. In fact, DYE2-based device gave a better higher short-circuit current density when

compared to analogous DYE1 which contains just one thienyl unit.

The electron lifetime in a DSSC is a fundamental parameter to assess the recombination dynamics

at the TiO2–dye–electrolyte interface [25]. If all the devices are fabricated and tested in the same

conditions, only the differences in the molecular structure of the sensitizers can influence the

electron lifetime. It has been calculated by an electrochemical impedance spectroscopy (EIS)

investigation carried out on the devices under 1 sun illumination. Figure 8A reports the Nyquist

plots obtained for devices based on DYE1, DYE2 and D5 at the VOC, the range explored being

from 100 kHz to 10 mHz. Figure 8 shows the plot of the apparent electron lifetime in DYE1, DYE2

and D5-based devices as obtained from EIS measurements performed under 1 sun illumination. EIS

spectra were analyzed through the well-known equivalent circuit [25]. By this approach, a

transmission line model is applied to the nanocrystalline TiO2 film. The electron lifetime was

calculated by the equation:

τ = Rct/Cµ

in which Rct and Cµ are the charge transport resistance at the TiO2/dye/electrolyte interface and the

chemical capacitance respectively. The apparent electron lifetimes reported in Figure 8B have been

plotted as a function of the corrected potential, as described in the literature [25]. The potential

correction is mandatory in order to account for the losses due to the total series resistance Rs, which

leads to a potential drop that is not associated with the displacement of the Fermi level.

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10 20 30 40 50 60 70 80 90 100 1100

5

10

15

20

25

30

DYE 1 DYE 2 D5

Z''

[ ΩΩ ΩΩ]

Z' [ΩΩΩΩ]

A

0,55 0,60 0,65 0,70 0,75 0,80 0,85

0,002

0,004

0,006

0,008

DYE1 DYE2 D5

App

aren

t e- L

ifetim

e (s

)

Corrected potential (V)

B

Figure 8. (A) Nyquist plots obtained from EIS measurements at VOC under 1 sun illumination for

devices based on DYE1, DYE2, and D5. (B) Apparent electron lifetime as a function of the

corrected potential for DYE1, DYE2, and D5.

It is evident how the apparent electron lifetimes of DYE1 and DYE2 are remarkably shorter than

those of the organic reference D5, which suggests that injected electrons from DYE1 and DYE2

undergo an easier recombination than those injected from D5. Such recombination can occur with

the oxidized form of the redox species present in the electrolyte as well as with the oxidized form of

the dye generated after the electron transfer (geminate recombination). This drawback can either be

ascribed to the absence of a bulky triarylamine group that, in D5 keeps the electrolyte from the TiO2

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surface, or to the non-optimal HOMO and HOMO–1 electronic distributions of DYE1 and DYE2,

which, being involved in the main electronic transitions, could favor the geminate recombination.

Furthermore, an estimate of the dye loading for DYE1 (5.68 × 10–8 mol cm–2), DYE2 (5.63 × 10–8

mol cm–2) and D5 (6.73 × 10–8 mol cm–2) revealed that the amount of adsorbed DYE1 and DYE2 is

lower compared to that of D5. The double anchoring structure reasonably hampers the access to

adsorption sites with respect to the monobranched sensitizer, providing an explanation for the lower

efficiencies, ascribable to the increased probability of detrimental charge recombination processes.

Nevertheless, DYE2-sensitised photoelectrodes are characterised by a slightly longer apparent

electron lifetime with respect to those of DYE1, supporting a more favorable behavior in terms of

recombination dynamics, which might be associated with the presence of the bithiophene spacer in

DYE2.

Since the recombination of injected electrons from the semiconductor to the oxidized species of the

redox electrolyte might be circumvented by the use of suitable additives promoting a complete

coverage of the TiO2 surface, two devices were prepared using chenodeoxycholic acid (CDCA, 10

mM) as coadsorbant. The photovoltaic characteristics of the devices are reported in Table 2.

Unfortunately, no efficiency improvement could be observed, suggesting that the geometry of the

di-branched molecules hampers the efficient intercalation of the CDCA at the TiO2 surface. As a

consequence the additive competes with the dye molecules for the anchoring sites, slightly

degrading the performances.

Beside the quantum conversion efficiency, another fundamental issue to be addressed to qualify a

sensitizer as promising refers to the device stability. The 20-years durability is in fact a key

requirement for the commercialization of dye sensitized solar cells. Although many factors affect

the lifetime of DSSCs the anchoring stability of the adsorbed sensitizers is an issue that should be

regarded as critical. In particular, it is well known that the presence of even small traces of water

impairs long term stability of the cells since it facilitates dye desorption [26]. Hence, a robust and

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indissoluble anchoring on the TiO2 surface is also an indispensable feature of any “good” sensitizer.

To investigate this aspect, we also aimed at assessing the anchoring strength of DYE1 and DYE2 in

comparison with the reference molecules D5 and N719. Dye-sensitized nanocrystalline TiO2 films

were thus immersed in an acetonitrile solution containing a 5%w of water. The stability of dyes

adsorbed on the photoanodes was evaluated by carrying out UV-Vis absorption measurements at

different time intervals, as shown in Figure 9. The D5-sensitized photoelectrode exhibited a

remarkable drop of the optical density as a consequence of the water promoted carboxylate group

cleavage from the TiO2 surface bond. Remarkably, no substantial changes in the absorbance spectra

were observed for DYE1- and DYE2-sensitized films.

350 400 450 500 550 600 650 700 7500,0

0,5

1,0

1,5

2,0

2,5

Abs

orba

nce

Wavelength (nm)

DYE1 t=0 h t=4 h t=14 h

350 400 450 500 550 600 650 700 7500,0

0,5

1,0

1,5

2,0

2,5

Abs

orba

nce

Wavelength (nm)

DYE2 t=0 h t=4 h t=14 h

350 400 450 500 550 600 650 700 7500,0

0,5

1,0

1,5

2,0

Abs

orba

nce

Wavelength (nm)

D5 t=0 h t=4 h t=14 h

350 400 450 500 550 600 650 700 750

0,2

0,4

0,6

0,8

1,0

1,2

Abs

orba

nce

Wavelength (nm)

N719 t=0 h t=4 h t=14 h

Figure 9. Absorption spectra of D5-, DYE1-, DYE2- and N719- sensitized nanocrystalline TiO2

films before and after 4 hours and 14 hours of desorbing treatment.

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This difference in behavior is clearly correlated to the exceptional robustness of their di-anchoring

mode as well as to their superior hydrophobicity imparted by the long pending n-octyl chain.

Figure 10 shows a picture of D5, DYE1, DYE2 and N719 adsorbed on TiO2 films before the

treatment in acetonitrile/water solution, then after 4 hours and 14 hours of immersion in solution.

As evident, the purplish color imparted by the N719 almost completely vanished after the treatment,

and this observation is accompanied by the almost complete disappearance of its UV-Vis peak as

the result of the detachment of dye molecules from the semiconductor surface. The desorption is

much more contained for the organic dyes, being lowest for DYE1 and DYE2 as evidenced by the

UV-vis measurements described above.

Figure 10. Picture of D5-, DYE1-, DYE2- and N719-sensitized nanocrystalline TiO2 films before

the treatment with the acetonitrile/water solution, after 4 hours and 14 hours of desorbing treatment.

3. Conclusions

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We have designed and synthesized new D(-π-A)2 organic molecules DYE1 and DYE2,

incorporating a thiophene or bithiophene unit as the π-bridge between the carbazole donor and the

cyano-acrylic acceptor. The absorption, energy levels and frontier orbitals of DYE1 and DYE2

were studied in detail. We found that the D(-π-A)2 organic dyes show an additional strong transition

in comparison to the common D-π-A dyes, resulting in a broadening of the absorption range.

Electrochemical impedance spectroscopy showed that the injected electron lifetime were increased

for DYE2 devices, that can be attributed to the introduction of a further thiophene unit in the π-

bridge with respect to DYE1. The highest power conversion efficiency reaches 5.01% in the case of

DYE2 with a photocurrent of 10.52 mA cm–2 and a photovoltage of 0.70 V, substantially deriving

from a broader absorption with respect to DYE1, as also confirmed by IPCE measurements.

Stability tests aimed at assessing the anchoring strength of DYE1 and DYE2 revealed their

exceptional robustness, deriving from the di-anchoring architecture, in comparison with the

common reference dyes. These results confirm and encourage the use of a dianchoring architecture

for the development of robust and efficient organic dyes and stimulate more research in the field

aimed at their optimization for practical application.

4. Experimental section

4.1. General remarks

Reactants were purchased from commercial sources and used without further purification. All

manipulations were carried out under inert nitrogen atmosphere using standard Schlenk techniques.

All solvents used were carefully dried and freshly distilled according to standard laboratory

practice. Flash chromatography was performed using a silica gel of 230–400 mesh.1H-NMR and

13C1H-NMR spectra were recorded on a Bruker Avance 700 MHz. The high resolution

electrospray mass spectrometry analyses were performed using a Bruker micro TOF QII mass

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spectrometer equipped with an electrospray ion source operated in negative ion mode. The sample

solutions (CH2Cl2/CH3OH) were introduced by continuous infusion at a flow rate of 180 µL min−1

with the aid of a syringe pump. The instrument was operated with end-plate offset and capillary

voltages set to 500 V and 3500 V, respectively. The nebulizer pressure was 0.8 bar (N2), and the

drying gas (N2) flow rate was 7.0 L min−1. The capillary exit and skimmer 1 voltages were 90 V and

30 V, respectively. The drying gas temperature was set at 180°C. The calibration was carried out

with sodium formiate. Elemental analyses were obtained on a EuroVector CHNS EA3000

elemental analyser. FT-IR measurements were recorded on a JASCO FT/IR 4200 instrument. UV-

Vis spectra were recorded on a Jasco V-670 instrument and fluorescence spectra were obtained on a

Varian Cary Eclipse spectrofluorimeter. Cyclic voltammetry was carried out on a Metrohm Autolab

PGSTAT 302-N potentiostat. Measurements were carried at 25 °C in THF solution (~10–4 M)

containing tetrabutylammonium tetrafluoroborate (0.025 M) as supporting electrolyte with a scan

rate of 100 mVs–1. The potentials were measured versus Ag/Ag+ as the quasi-reference electrode.

Subsequently to each experiment, the potential of the Ag/Ag+ electrode was calibrated against the

ferrocene/ferrocenium (Fc/Fc+) redox couple. The HOMO energy values were estimated from the

onset potentials (Eoxonset) of the first oxidation event. After calibration of the measurements against

Fc/Fc+, the oxidation potential of which is assumed at + 4.8 eV below the vacuum level, the HOMO

energy level were calculated according to the following equations:

EHOMO (eV) = – [Eoxonset – E1/2(Fc/Fc+) + 4.8]

where E1/2(Fc/Fc+) is the half-wave potential of the Fc/Fc+ couple against the Ag/Ag+ electrode. All

theoretical calculations were carried out with the Gaussian09 program package. Analyses of the

ground-state structures for the molecules were carried out using density functional theory (DFT)

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and the B3LYP function [27] was used in conjunction with the 6-31G(d,p) basis set. Time-

dependent DFT (TDDFT) calculations performed to assess the excited-state transition energies.

4.2. Fabrication of DSSC and photovoltaic measurements

Fluorine-doped tin oxide (FTO, 10 ohm/sq., provided by SolaronixS.A.) glass plates were first

cleaned in a detergent solution using an ultrasonic bath for 15 min, and then rinsed with water and

ethanol. For the fabrication of photoelectrodes, two different titanium nanoxide pastes were

deposited by doctor blading: 18NR-T purchased by Dyesol, which gave transparent TiO2 films 7

µm-thick, and D/SP paste provided by Solaronix, which was used as active-opaque overlayer for 17

µm-thick double-layered photoanodes. The doctor blading procedure was repeated to obtain the

desired film thickness, finally measured using a profilometer (Tencor Alpha-Step 500 Surface

Profiler). The active area of photoanodes was 0.16 cm2. The electrodes coated with the TiO2 pastes

were gradually heated under an air flow and sintered at 450 °C for 30 min. The substrate

temperature was then allowed to slowly decrease. Once cooled down at about 80°C, the electrodes

were immersed into 0.2 mM solutions of dyes DYE1 and DYE2 in EtOH and kept for 12 h in dark

at room temperature. The reference photoanodes were prepared by dyeing into a solution 0.2 mM of

(bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N′-bis (4-carboxylato-4′-carboxylic acid-2, 2-

bipyridine) ruthenium(II) (N719, provided by Solaronix S.A.) in a mixture of acetonitrile and tert-

butyl alcohol (v/v = 1:1) at room temperature for 14 h. 3-[5-(4-(Diphenylamino)styryl)-thiophen-2-

yl]-2-cyano-acrylic acid (D5) and the corresponding photoanode were prepared according to a

literature procedure [28]. The counter electrodes were prepared by sputtering a 50 nm Pt layer on a

hole drilled cleaned FTO plate. The two plates were faced and assembled by means of a gasket of

50µm-thick Surlyn® foil (Dyesol Ltd) interposed between them. The redox electrolyte (0.1 M LiI,

0.05 M I2, 0.6 M 1-methyl-3-propylimidazolium iodide, and 0.5 M tert-butylpyridine in dry

acetonitrile) was vacuum-injected into the space between the electrodes. Photocurrent-voltage I-V

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measurements were performed using a Keithley unit (Model 2400 Source Meter). A Newport AM

1.5 Solar Simulator (Model 91160A equipped with a 300W Xenon Arc Lamp) serving as a light

source. The light intensity (or radiant power) was calibrated to 100 mW/cm2 using as reference a Si

solar cell. The incident photon-to-current conversion efficiency (IPCE) was measurement by a DC

method. IPCE measurements were carried out with a computerized setup consisting of a xenon arc

lamp (140 W, Newport, 67005) coupled to a monochromator (Cornerstore 260 Oriel 74125). Light

intensity was measured by a calibrated UV silicon photodetector (Oriel 71675) and the short circuit

currents of the DSSCs were measured by using an optical power/energy meter, dual channel

(Newport 2936-C). To evaluate the dye loading, transparent 5-µm thick photoelectrodes (1 cm × 1

cm in size) were stained with DYE1, DYE2 and D5, following the same procedures described

above. Subsequently, the substrates were thoroughly rinsed with the corresponding solvent, in order

to remove the fraction of dye that was not chemisorbed onto the TiO2 surface, and were eventually

placed in a KOH solution in DMF (0.05 M) for 48 h in order to achieve a complete desorption, as

testified by substrate discoloration. The evaluation of the dye concentration in the solvent, obtained

by UV-Vis measurements, allowed the calculation of the amount of the adsorbed molecules,

expressed in terms of moles of dye anchored per projected unit area of the photoelectrode.

Electrochemical impedance spectroscopy (EIS) was performed by an AUTOLAB PGSTAT 302N

(Eco Chemie B.V.) in a frequency range between 300 kHz–30 mHz. The impedance measurements

were carried out at different voltage biases under 1 sun illumination. The resulting impedance

spectra were fitted with ZView software (Scribner Associates).

4.3. Synthesis

4.3.1. 9-Octyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (2)

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A solution of n-BuLi (1.6 M in hexanes, 8.2 mL, 13.17 mmol) was added dropwise to a solution of

1 [29] (1.92 g, 4.39 mmol) in THF (80 mL) kept at –80 °C. The obtained mixture was vigorously

stirred for 1 hour at –80 °C before the addition of 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-

dioxaborolane (2.45 g, 13.17 mmol) in one portion. The resulting solution was then allowed to

reach room temperature and to react overnight. After removing the solvent under vacuum, CH2Cl2

(50 mL) was added and the obtained solution was washed with water (3 × 50 mL) and dried over

Na2SO4. The solvent was removed under vacuum and the obtained crude product was purified by

flash column chromatography (SiO2, petroleum ether 40-60 °C/CH2Cl2 = 1/1 v/v) to give 2 (1.42 g,

61%) as a white solid. 1H-NMR (700 MHz, CDCl3): δ 8.68 (s, 2H), 7.87-7.56 (m, 4H), 4.36 (t, J =

7.0 Hz, 2H), 1.95-1.90 (m, 2H), 1.44-1.39 (m, 2H), 1.38-1.33 (m, 2H), 1.32-1.23 (m, 6H), 0.88 (t, J

= 7.0 Hz, 3H) ppm. 13C1H NMR (176 MHz, CDCl3): δ 144.6, 141.2, 132.1, 128.4, 124.3, 111.4,

83.9, 43.6, 31.7, 29.3, 29.1, 29.0, 27.2, 25.4, 22.6, 14.0 ppm.

4.3.2. 5,5'-(9-Octyl-9H-carbazole-3,6-diyl)dithiophene-2-carbaldehyde (3)

A mixture of 5-bromo-thiophene-2-carbaldehyde (0.52 g, 2.72 mmol), 2 (0.72 g, 1.35 mmol) and

Pd(PPh3)4 (23 mg, 0.02 mmol) in toluene (16 mL) and a 2.0 M K2CO3 aqueous solution (8 mL) was

refluxed overnight. After cooling the mixture down to room temperature, CH2Cl2 (40 mL) was

added and the obtained mixture was washed with water (3 × 40 mL). After solvent removal, the

crude product was purified by flash chromatography (SiO2, CH2Cl2) to afford 3 (0.55 g, 81%) as a

yellow solid.1H-NMR (700 MHz, CDCl3): δ 9.92 (s, 2H), 8.47 (d, J = 1.4 Hz, 2H), 7.85 (dd, J = 8.5,

1.4 Hz, 2H), 7.81 (d, J = 3.9 Hz, 2H), 7.51 (d, J = 3.9 Hz, 2H), 7.49 (d, J = 8.5 Hz, 2H), 4.36 (t, J =

7.0 Hz, 2H), 1.95-1.90 (m, 2H), 1.44-1.39 (m, 2H), 1.38-1.33 (m, 2H), 1.32-1.23 (m, 6H), 0.88 (t, J

= 7.0 Hz, 3H) ppm. 13C1H-NMR (176 MHz, CDCl3): δ 182.6, 155.7, 141.7, 141.6, 137.7, 125.2,

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124.9, 123.3, 123.2, 118.7, 109.8, 43.6, 31.7, 29.3, 29.1, 29.0, 27.2, 22.6, 14.0 ppm. Elemental

analysis for C30H29NO2S2: calcd C, 72.11; H, 5.85; N, 2.80; found C, 72.00; H, 5.80, N, 2.77.

4.3.3. 2,2'-Bithiophene-5-carbaldehyde (4)

A mixture of 5-bromo-thiophene-2-carbaldehyde (0.52 g, 2.72 mmol), tributyl(thiophen-2-

yl)stannane (1.02 g, 2.72 mmol) and Pd(PPh3)4 (23 mg, 0.02 mmol) in toluene (20 mL) was

refluxed overnight. After cooling the mixture down to room temperature, CH2Cl2 (40 mL) was

added and the obtained mixture was washed with water (3 × 40 mL). After solvent removal, the

crude product was purified by flash chromatography (SiO2, CH2Cl2) to afford 4 (%) as a pale

yellow solid. 1H-NMR (700 MHz, CDCl3): δ 9.70 (s, 1H), 7.55 (d, J = 4.0 Hz, 1H), 7.30-7.10 (m,

3H), 6.97 (d, J = 3.9 Hz, 1H) ppm. 13C1H-NMR (176 MHz, CDCl3): δ 182.7, 155.7, 141.9, 136.7,

135.1, 131.5, 126.7, 123.2, 113.8 ppm.

4.3.4. 5'-Bromo-2,2'-bithiophene-5-carbaldehyde (5)

N-bromosuccinimide (1.78 g, 10.0 mmol) was portionwise added to a solution of 4 (1.94 g, 10.0

mmol) in DMF (60 mL) kept at 0 °C. The system was then allowed to reach room temperature and

to react overnight. The reaction mixture was poured into water (500mL) and the resulting

precipitate was filtered, washed with water and purified by flash chromatography (SiO2, CH2Cl2) to

give 6 in 86% yield as a pale yellow solid. 1H-NMR (700 MHz, CDCl3): δ 9.89 (s, 1H), 7.68 (d, J =

4.0 Hz, 1H), 7.20 (d, J = 4.0 Hz, 1H), 7.13 (d, J = 3.9 Hz, 1H), 7.06 (d, J = 3.9 Hz, 1H) ppm.

13C1H-NMR (176 MHz, CDCl3): δ 182.4, 145.8, 142.1, 137.5, 137.1, 131.2, 126.2, 124.4, 114.2

ppm.

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4.3.5. 5',5''-(9-Octyl-9H-carbazole-3,6-diyl)di-2,2'-bithiophene-5-carbaldehyde (6)

A mixture of 5 (0.74 g, 2.72 mmol), 2 (0.72 g, 1.35 mmol) and Pd(PPh3)4 (23 mg, 0.02 mmol) in

toluene (16 mL) and a 2.0 M K2CO3 aqueous solution (8 mL) was refluxed overnight. After cooling

the mixture down to room temperature, CH2Cl2 (40 mL) was added and the obtained mixture was

washed with water (3 × 40 mL). After solvent removal, the crude product was purified by flash

chromatography (SiO2, CH2Cl2) to afford 6 (0.57 g, 64%) as an orange solid. 1H-NMR (700 MHz,

CDCl3): δ 9.90 (s, 2H), 8.39 (s, 2H), 7.78 (d, J = 8.8 Hz, 2H), 7.72 (d, J = 3.9 Hz, 2H), 7.46 (d, J =

8.8 Hz, 2H), 7.41 (d, J = 3.6 Hz, 2H), 7.36 (d, J = 3.6 Hz, 2H), 7.31 (d, J = 3.9 Hz, 2H), 4.36 (t, J =

7.0 Hz, 2H), 1.95-1.90 (m, 2H), 1.44-1.39 (m, 2H), 1.38-1.33 (m, 2H), 1.32-1.23 (m, 6H), 0.88 (t, J

= 7.0 Hz, 3H) ppm.13C1H NMR (176 MHz, CDCl3): δ 182.4, 147.6, 141.3, 140.9, 137.4, 134.0,

127.3, 125.2, 124.5, 123.6, 123.3, 123.2, 118.0, 109.6, 43.5, 31.8, 29.3, 29.1, 29.0, 27.3, 22.6, 14.0

ppm. Elemental analysis for C38H33NO2S4: calcd C, 68.74; H, 5.01; N, 2.11; found C, 68.71; H,

4.95, N, 2.00.

4.3.6. 3,3'-(5,5'-(9-Octyl-9H-carbazole-3,6-diyl)bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid)

(DYE1).

A mixture of 3 (150 mg, 0.30 mmol), cyanoacetic acid (76 mg, 0.90 mmol) and ammonium acetate

(8 mg, 0.10 mmol) in acetic acid (10 mL) was refluxed overnight. After cooling to room

temperature, the reaction mixture was poured into water (100 mL) obtaining a precipitate that was

filtered and repeatedly washed with water. The obtained crude product was purified by flash

column chromatography (SiO2, CH2Cl2/ethanol = 7/1 v/v) to afford DYE1 (130 mg, 68%) as a red

powder. 1H-NMR (700 MHz, CDCl3): δ 8.78 (s, 2H), 8.40 (s, 2H), 7.97 (d, J = 4.0 Hz, 2H), 7.90 (d,

J = 8.7 Hz, 2H), 7.80 (d, J = 4.0 Hz, 2H), 7.47 (d, J = 8.7 Hz, 2H), 4.35 (t, J = 6.9 Hz, 2H), 1.95-

1.90 (m, 2H), 1.44-1.39 (m, 2H), 1.38-1.33 (m, 2H), 1.32-1.23 (m, 6H), 0.88 (t, J = 7.0 Hz, 3H)

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ppm. IR (KBr): ν 3435, 3067, 2924, 2853, 2215, 1698, 1573, 1424, 1257, 1212, 794 cm–1. ESI-

HRMS for C36H31N3O4S2: found 632.1894 m/z (M–H)–; calcd 633.1756 m/z. Elemental analysis for

C36H31N3O4S2: calcd C, 68.22; H, 4.93; N, 6.63; found C, 68.28; H, 4.99, N, 6.75.

4.3.7. 3,3'-(5',5''-(9-Octyl-9H-carbazole-3,6-diyl)bis(2,2'-bithiophene-5',5-diyl))bis(2-cyanoacrylic

acid) (DYE2)

A mixture of 6 (200 mg, 0.30 mmol), cyanoacetic acid (76 mg, 0.90 mmol) and ammonium acetate

(8 mg, 0.10 mmol) in acetic acid (10 mL) was refluxed overnight. After cooling to room

temperature, the reaction mixture was poured into water (100 mL) obtaining a precipitate, that was

filtered and repeatedly washed with water. The obtained crude product was purified by flash

column chromatography (SiO2, CH2Cl2/ethanol = 6/1 v/v) to afford DYE2 (160 mg, 67%) as a dark

red powder. 1H-NMR (700 MHz, CDCl3): δ 8.77 (s, br, 2H), 8.44 (s, br, 2H), 7.96 (s, br, 2H), 7.89

(d, J = 8.4 Hz, 2H), 7.72-7.66 (m, 6H), 7.61 (s, br, 2H), 4.35 (t, J = 6.9 Hz, 2H), 1.95-1.90 (m, 2H),

1.44-1.39 (m, 2H), 1.38-1.33 (m, 2H), 1.32-1.23 (m, 6H), 0.88 (t, J = 7.0 Hz, 3H) ppm. IR (KBr): ν

3439, 3067, 2924, 2853, 2216, 1683, 1576, 1438, 1052, 792 cm–1. ESI-HRMS for C44H35N3O4S4:

found 796.1594 m/z (M–H)–; calcd 797.1510 m/z. Elemental analysis for C44H35N3O4S4: calc C,

66.22; H, 4.42; N, 5.27; found C, 66.28; H, 4.49, N, 5.15.

5. Acknowledgements

The authors gratefully acknowledge for funding: Regione Puglia (APQ-Reti di Laboratorio, Project

PHOEBUS, cod. 31-FE1.20001); MIUR (FIRB project RBPR05JH2P: Rete Nazionale di Ricerca

sulle Nanoscienze ItalNanoNet) and Italian CNR (project EFOR-Energia da Fonti Rinnovabili,

Iniziativa CNR per il Mezzogiorno L. 191/2009 art. 2 comma 44). Dr. Claudia Barolo (University

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of Turin, Dipartimento di Chimica Generale e Chimica Organica) is gratefully acknowledged for

useful discussions.

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HIGHLIGHTS

Di-anchoring dyes with thienyl (DYE1) or bithienyl (DYE2) as π-bridge were prepared.

The DSSC device based on DYE2 achieved an efficiency of 5.01%.

The two dyes showed better anchoring stability with respect to standard sensitizers.

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HIGHLIGHTS

Two di-anchoring organic dyes, bearing a thienyl (DYE1) or a bithienyl (DYE2) as the π-

conjugated bridge, were applied in dye-sensitized solar cells.

The device based on DYE2 achieved an efficiency of 5.01%.

The two materials exhibited a better anchoring stability with respect to standard DSSC reference

dyes.