A Wide Band Gap Naphthalene Semiconductor for Thin‐Film ...

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A Wide Band Gap Naphthalene Semiconductor for Thin-Film Transistors

Lijia Yan, Filip Popescu, M. Rajeswara Rao, Hong Meng,* and Dmitrii F. Perepichka*

DOI: 10.1002/aelm.201600556

π-conjugation in aromatic molecules has served as one of the key design cri-teria of high-mobility organic semicon-ductors (OSCs).[10–13] A number of high mobility OSCs have been developed based on π-extended acenes,[12,14,15] oligothio-phene, and their hybrid derivatives.[16–18] Extended π-conjugation lowers charge carrier reorganization energy, improves the electronic coupling between the mole-cules, and facilitates the charge injection by bringing the highest occupied mole-cular orbital (HOMO) and lowest unoc-cupied molecular orbital (LUMO) of the semiconductor closer to the Fermi level on an electrode. The latter is a result of the lowered HOMO–LUMO gap (HLG), one of the hallmarks of π-conjugation. In fact, low HLG (3 eV) is commonly con-sidered an essential factor for semicon-ducting behavior in organic materials.[10]

However, lowering an HLG also renders the semiconductor intrinsically less stable, which is detrimental for many applica-tions.[5] Furthermore, absorption of ambient light by low HLG materials, while benefiting photovoltaic and some other appli-cations, is not desired for OFET electronics, as it can perturb the electric response of a device and cause photooxidation of the active materials.

On the one hand, there exists no correlation between the HLG and conductivity in organic semiconductors. A number of anthracene derivatives[19–22] show hole mobility (2–10 cm2 V–1 s–1) similar or exceeding that of pentacene and its derivatives. Electron mobility of up to 6 cm2 V–1 s–1 was demonstrated for some dialkylnaphthalenediimides,[23,24] which is at least on par with its perylenediimide homologs, despite the smaller HLG. Nonetheless, to the best of our knowledge, all of these OSCs have band gaps (Eg) within the above “<3.0 eV” semiconducting region.

One application specifically requiring semiconductors with Eg ≥ 3.3 eV is that of transparent electronics.[25–27] Until now transparent FETs have been mainly fabricated with oxide semi-conductors (Zn, SnOx, In2O3)[28–31] and carbon nanotubes.[32] Most OSCs are not well suited as active layers in these applica-tions due to their relatively low band gap (<3 eV). While trans-lucent OFETs can be prepared with ultrathin layers of low band gap semiconductors,[33] most of these devices have a residual pale color due to the absorption of photons in the visible range. More importantly, the visible light affects their current response by inducing photoconductivity. To our knowledge,

This study reports a new simple organic semiconductor 2,6-bis(4-meth-oxyphenyl)naphthalene (BOPNA) with unprecedentedly large band gap of 3.35 eV and an apparent hole mobility of ≈1 cm2 V–1 s–1 measured in thin-film organic field-effect transistors in a saturation regime. This large band gap leads to complete optical transparency in the visible range (>370 nm), very high stability and independence of the device current with illumination condi-tions; this is in contrasts to the behavior of common organic semiconductors that have the band gap in the visible or near-IR range of the spectrum. Crystal structure of BOPNA reveals a highly isotropic electronic coupling in two directions of a herringbone-packing plane. A uniform, nearly single crystal-line morphology can be achieved in highly crystalline BOPNA films through a rapid thermal annealing. Temperature-dependent studies reveal increase of the hole mobility as the temperature is reduced from +25 to –25 °C, sug-gesting a band-like transport, followed by a nearly temperature-independent mobility down to at least –150 °C.

L. Yan, Prof. H. MengKey Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM)Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM)Nanjing Tech University (Nanjing Tech)30 South Puzhu Road, Nanjing 211816, ChinaE-mail: [email protected]. Yan, F. Popescu, Dr. M. R. Rao, Prof. D. F. PerepichkaDepartment of ChemistryMcGill University801 Sherbrooke Street West, H3A 0B8 Montreal, CanadaE-mail: [email protected]

Organic Semiconductors

1. Introduction

Organic field-effect transistors (OFETs) have attracted consider-able attention due to their low cost and easy processing, excel-lent compatibility with flexible and biological substrates, and chemical tunability of properties, enabling wide application in electronics, including large-area flexible displays, smart cards and sensors, radio-frequency identification tags, biomedical devices, etc.[1–5] Since the early days of OFETs,[6–8] the progress in the field has been largely driven by the design of new semi-conducting molecules (and polymers) as well as improving crystallinity/molecular packing[9] in their films. Extending

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2,7-diphenybenzothieno[3,2-b]benzothiophene[34] is currently the largest band gap OSC (Eg = 3.0 V) with high field-effect mobility (μ = 0.9 cm2 V–1 s–1) that can be used to prepare trans-parent OFETs with absorption cutoff at ≈412 nm.

In this paper, we report 2,6-bis(4-methoxypheny)naphthalene (BOPNA), the first organic semiconductor with a band gap in the UV region of the spectrum (3.35 eV, 370 nm cutoff) and dis-playing hole mobility of up to ≈1 cm2 V–1 s–1 in thin film OFETs. We present a detailed analysis of photophysical, electrical, and structural properties of BOPNA, showing its high thermal sta-bility, facile morphology reorganization upon annealing at low temperatures, and band-like transport established through the temperature-dependent mobility measurements. We note that a structurally relevant naphthalene-derived semiconductor, 2,6-di(octylthienyl)naphthalene, with μ+ = 0.14 cm2 V–1 s–1 has been reported by Oikawa et al.[35] While its solid state optical properties have not been reported, the solution absorption of this compound is significantly redshifted (35 nm, 0.39 eV) com-paring to BOPNA.

2. Results and Discussion

BOPNA was synthesized on a multigram scale by a Suzuki coupling of commercially available 2,6-dibromonaphthalene with 4-methoxyphenylboronic acid (Scheme 1) in 90%. The material was purified by gradient sublimation (in vacuum) two times. Ther-mogravimetric analysis of BOPNA under N2 reveals an excellent thermal stability, showing a clean sublimation at 350–430 °C (Figure S1b, Supporting Information). Differential scanning cal-orimetry (DSC) confirms the stability of the compound up to at least 375 °C, with reversible melting at 290 °C (crystallization at 258 °C) and a second (possibly liquid crystal) phase transition at 370 °C (Figure S1a, Supporting Information). The compound also shows an outstanding photo oxidative stability, with no signs of degradation upon ambient light/air exposure in either films or solutions. Photolysis in air-saturated CHCl3 solutions revealed approximately ten times slower decomposition compared to the corresponding anthracene derivative 2,6-bis(p-methoxyphenyl)anthracene BOPAnt (Figure S2, Supporting Information).

Photophysical properties of BOPNA were studied in solution and in thin films (Figure 1a and Figure S5, Supporting Informa-tion). In solution, BOPNA shows two UV absorption bands at λmax = 273 and 318 nm; the latter is assigned primarily to the HOMO–LUMO transition, according to the time-dependent density functional theory (TD-DFT) calculations (Figure S4, Sup-porting Information). The broad appearance of these bands is most likely due to torsional disorder around the phenyl–naphtha-lene bond in solution. In the solid state, the vibronically struc-tured absorption shows a slight blueshift (λmax 269 nm) and a sig-nificant decrease of the relative intensity of the HOMO–LUMO transition, indicating the formation of H-aggregate structures

(see the discussion of X-ray structure below).[36] The optical band gap of 3.35 eV (372 nm), estimated from the absorption onset, indicates complete optical transparency of the film in the visible region (390–700 nm). In solution BOPNA reveals a deep-blue photoluminescence (PL) with λPL

max = 372 nm (PL quantum yield, PLQY = 32 ± 5% in chloroform). The PL maximum in the films (435 nm) is significantly redshifted relative to that in solu-tion (by 0.48 eV) and also exhibits a large Stokes shift (0.57 eV). This could likely be attributed to pronounced exciton delocaliza-tion in the close-packed BOPNA solids.[37] The external PLQY in vacuum deposited thin films measured in an integrating sphere (16 ± 5%) is lower than that in solution, likely due to quenching on surface defects. Indeed, an enhanced PLQY of 41 ± 5% was measured for vacuum sublimed crystals (≈0.1 mm in size).

The electrochemical properties of BOPNA were investi-gated by cyclic voltammetry on the vacuum deposited thin film (Figure 1b). The oxidation potential of BOPNA estimated from an onset of the first oxidation wave is 0.79 V versus ferrocene, from which the HOMO energy of –5.59 eV was deduced. DFT calculations of BOPNA in the gas phase, using B3LYP func-tional and 6–13G(d) basis set predicts the HOMO at –5.20 eV and LUMO at –1.12 eV (HOMO–LUMO gap of 4.09 eV). The HOMO is evenly distributed through the diphenylnaphthalene π-system, while the LUMO is primarily localized on the naph-thalene core, likely due to electron-donating effect of methoxy groups (Figure S3, Supporting Information).


Scheme 1. Synthesis of BOPNA.

200 300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0 UV in solution UV in film PL in solution PL in film

Abs

orba

nce

/ a.u

.

λ /nm

0.0 0.5 1.0 1.5 2.0

BOPNA

I/µΑ

V vs Fc/Fc+

(a)

(b)

Figure 1. a) UV–vis excitation and PL spectra of BOPNA in solution (dashed line) and in thin film (solid line). b) Cyclic voltammetry of BOPNA thin film on glasssy carbon electrode in 0.2 m Bu4NPF6/CH2Cl2 solution (the irreversible appearance is due to dissolution of the film upon oxidation).




Single crystals of BOPNA were obtained by vacuum sublima-tion. The results of X-ray crystallographic analysis are shown in Figure 2 and crystal structure determination data are also given in the Supporting Information. BOPNA crystallizes in the (common for acenes) orthorhombic space group Pbca. The phenyl substituents are slightly twisted out of the naphthalene plane, with torsion angle of ≈15o. The molecules are arranged in a typical herringbone packing motif, which provides only non-cofacial π…π interactions (the shortest C…C distance 3.53 Å).

In the crystal, the molecules participate in two distinct closest-neighbor contacts with nearly identical intermolecular distances (shown by green and purple arrows in Figure 2b). Although the contacting molecules are not parallel (herringbone packing), the π–π interactions are strong as they involve both the naphtha-lene and phenyl groups. Within the herringbone-packed plane (ab) of π–π interacting molecules, there is almost no shift of the molecules versus each other along their long axis (Figure 2b,c), which results in efficient orbital overlap and H-aggregation-like features in the optical spectra (see above).[38] The corresponding electronic coupling (approximated as HOMO splitting)[39] is quite high and, remarkably, is nearly identical for the two orthogonal contacts, t1 = t2 = 0.406 eV. This means that the same hole mobility is expected in both directions, implying an iso-tropic 2D charge transport. For comparison, electronic coupling in pentacene calculated using the same approach is similar (t1 = 0.412 eV, t2 = 0.340 eV) but anisotropic and pentacene show mobility anisotropy μ/μ⊥ ≈ 2.5.[40] We note that 2D elec-tron coupling is an essential requisite for efficient, band-like transport in molecular semiconductors at room temperature.

This could likely be attributed to the reduced impact of thermal fluctuations and defects in 2D versus 1D conducting channels.[41,42]

To investigate the charge transport prop-erties of BOPNA, bottom-gate top-contact OFETs were fabricated by thermal evapo-ration of the semiconductor onto octade-cyltrichlorosilane (ODTS) treated Si/SiO2 wafers. The electrical characteristics were measured for devices after annealing at various temperatures for 15 min in air (Figure S7, Supporting Information). OFET figures of merit (μ, Vth, Ion/off), measured in the saturation regime on the reverse scans of the transfer curves at Vds = –60 V (Figure 3a), are listed in Table 1. We note that the mobility is lower in the linear regime and is field dependent (Figure S8, Supporting Information). Upon increasing the annealing temperature, very little change of the current is seen up to 150 °C; the mobility improves a little and maximizes for films annealed at 120 °C (Figure 3c). However, high annealing temperature (180 °C) leads to a sharp drop of the current and decrease of the meas-ured mobility. Optical properties of the films remain unchanged up to 180 °C (Figure S6, Supporting Information) and the observed differences are attributed to the changing morphology (see below).

The highest mobility was consistently measured for OFETs with the longest channel length (l = 98 μm). Plotting 1/μ versus 1/l (Figure 3d) reveals an effect of contact resistance; an extrap-olated “intrinsic” mobility is almost twice higher than the meas-ured one.

On the other hand, the threshold voltage (Vth) measured for all BOPNA devices (–45 to –52 V) is rather large, which can be attributed to significant hole trapping[43] explained by the very low HOMO energy. Dynamic formation[44–47] of interfa-cial traps also explains a strong hysteresis of the transfer char-acteristics between the forward and reverse gate voltage (VG) sweeps. This leads to a substantial ambiguity in the calculated charge mobility that differs by up to six to seven times when determined on the forward and reverse sweeps (Figure S10 and Table S1, Supporting Information). This hysteresis is mini-mized but not completely eliminated upon successive sweeps, which leads to the apparent increase of μ and VT on the second scan in the forward (but not in the reverse) sweep (Figure S11, Supporting Information). Suspecting that this dynamic trap-ping could be due to diffusion of impurities at the dielectric/semiconductor interface, we have replaced the ODTS with a more hydrophobic fluoropolymer CYTOP (prepared by spin-coating). Comparing the transfer characteristics of the OFETs fabricated with ODTS and CYTOP-treated wafers (Figure S11, Supporting Information), the hysteresis decreases dramatically for the CYTOP-treated substrates, and the threshold voltage is reduced by 10–15 V. The hole mobility calculated for the for-ward and backwards sweeps of CYTOP-treated devices is nearly identical (Table S1 and Figure S11b, Supporting Information),


19.7 Å

15.0 o -15.8 o

3.53Å

(a)

(b)

t1t2

0

a

b

(c) 0a

cFigure 2. a) Molecular structure and b,c) crystal packing of BOPNA. Unit cell: a = 7.474, b = 6.209, c = 37.299, α = β = γ = 90o.




albeit approximately one order of magnitude lower than that for ODTS-treated devices, presumably due to high roughness of the CYTOP surface (rms = 1.18 nm; Figure S9, Supporting Information). The reduced trapping at the CYTOP/semicon-ductor interface has been noted before[48–50] and most likely originates from its highly hydrophobic nature (suppressing water/ions diffusion and interactions).

To obtain a deeper insight into the transport behavior of BOPNA, we have carried out temperature-dependent OFET measurements. Cooling the devices below room temperature (RT) significantly reduces the hysteresis between the forward and reverse transfer sweeps (Figure S12, Supporting Informa-tion), further suggesting that the above-mentioned dynamic traps (whose diffusion/reorientation is suppressed at low tem-perature) are responsible for the observed behavior. The hole mobility is increased by ≈80% as the temperature is lowered from RT to –25 °C, and remains constant upon further cooling. A lack of temperature activation suggests a band transport mechanism, as was shown in time-of-flight mobility meas-urements on pure naphthalene crystals.[51] The temperature-dependent region of this transport model is characterized by an inverse power dependence of mobility μ ∼ T–n.[51] The loga-rithmic plot of mobility versus temperature (Figure 4) illus-trates a similar non-Arrhenius dependence of the field-effect mobility in BOPNA (n ≈ 3.3) between 25 and –25 °C. This is very different from the behavior of most polycrystalline organic semiconductors. Although a nearly temperature-inde-pendent mobility at low temperatures has been noted in many studies,[52–54] most of them show temperature-activated mobility near RT, in line with the hopping or “multiple thermal release”


-60 -50 -40 -30 -20 -10 010-12

10-11

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-I D1/

2 / A1/

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RT 50oC 80oC 100oC 120oC 150oC 180oC

VG / V

-I D /

A

0.0000

0.0004

0.0008

0.0012

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0.0020

0.0024

0 -10 -20 -30 -40 -50

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I D /

A

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VG

-60V

50 100 150 2000.0

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µ /c

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-1

T / oC0.00 0.01 0.02 0.03

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38 µm

58 µm

78 µmµ−1

/ V

s c

m-2

L-1 / µm-1

98 µm

(a) (b)

(c) (d)

Figure 3. a) Transfer plots of BOPNA OFETs at different annealing temperatures. b) Output characteristics of nonannealed BOPNA films. c) Hole mobility of BOPNA OFETs as a function of annealing temperature. d) Reciprocal mobility of BOPNA OFETs as a function of reciprocal channel length.

Table 1. OFET performance of BOPNA at different annealing temperatures.

Tannl. [°C]

μ+ave

[cm2 V–1 s–1]μ+

max [cm2 V−1 s–1]

Ion/Ioff Vth [–V]

30 (RT) 0.72 ± 0.15 0.98 6.0 × 104 48.3 ± 3.7

50 0.76 ± 0.16 1.10 5.9 × 104 50.4 ± 0.9

80 0.78 ± 0.13 1.04 3.3 × 104 50.0 ± 1.5

100 0.83 ± 0.15 1.10 1.0×105 49.7 ± 0.8

120 1.00 ± 0.21 1.40 1.1 × 105 50.6 ± 1.5

150 0.85 ± 0.09 1.00 2.4 × 105 50.0 ± 0.8

180 0.31 ± 0.07 0.44 3.5 × 105 46.8 ± 1.9




mechanisms. It is not clear whether reduced phonon scattering alone could account for such substantial increase of the observed charge mobility. No phase transition was observed in DSC in the RT to –50 °C range (Figure S1a, Supporting Information), but we cannot exclude external effects, e.g., reduced trapping at low temperature (as mentioned above).[46,47]

In order to understand the observed band-like transport properties and the effect of annealing, we investigated the evolution of the morphology of BOPNA films on ODTS-treated Si/SiO2 wafers, using X-ray diffraction (XRD) and atomic force microscopy (AFM). In the entire range of annealing tempera-tures, the crystallographic structure of the films remains unchanged and the intensity of diffraction peaks increases only slightly as the temperature advances from 50 to 180 °C (Figure 5a). The peaks observed in the out-of-plane XRD pattern can be indexed to (0 0 l) planes that correspond well to the BOPNA single crystal (Figure S15, Supporting Infor-mation). The exclusive observation of (0 0 l) reflections (from (0 0 2) to (0 0 12)) suggests a highly uniform orientation of the BOPNA crystalline domains on the surface. The (0 0 2) reflection at 2θ = 4.8° corresponds to a d spacing of 18.53 Å. This is close to the molecular length (19.7 Å; Figure 2a) and cor-responds to 1/2 of the unit cell vector c meas-ured from single crystals. Thus, the mole-cular packing in the films closely resembles that in single crystals, and the molecules are oriented almost vertically (75°) with respect to the surface.

The AFM images show a remarkable improvement of morphology as the films are annealed (Figure 5). While the surface of RT (nonannealed) films shows multiple crystallites <0.5 μm in size, these undergo a progressive ripening as the temperature

increases from 50 to 180 °C. The boundaries disappear at ≈120 °C and the grain size continues increasing to tens of μm. On the other hand, annealing at higher temperatures also leads to the development of macroscopic cracks in the film due to different thermal expansion of BOPNA and SiO2. The cracks are very visible in films annealed at 180 °C (Figure 5b and Figure S17, Supporting Information). These changes of mor-phology explain the enhanced charge mobility of OFET devices upon annealing to 120 °C, as well as a sharp drop in perfor-mance at 180 °C. We also emphasize that the observed morpho-logical change takes place at temperatures significantly lower than BOPNA melting point (290 °C).

As mentioned in the introduction, one advantage of using wide band-gap semiconductors in OFET devices is their high photostability. This minimizes photooxidative degrada-tion and also eliminates the unwanted photosensitivity of the


5.5 5.0

-2.0

-1.5Forward sweep Backward Sweep

ln (

µ )

ln(T )

Figure 4. Double logarithmic plot of mobility versus temperature upon heating from –150 to +25 °C on ODTS-treated, top-contact BOPNA OFETs. Lines used as visual guides for the two transport regimes.

Figure 5. a) XRD patterns and b) tapping mode AFM images of BOPNA thin films after annealing at different temperatures. Note the systematic absences of the odd (00l) peaks due to Pbca space group.




electronics built with such OFETs. To demonstrate the latter effect, we have compared the current response of the OFETs fabricated with BOPNA and its larger cousin BOPAnt (Eg = 2.7 eV; Figure S4d, Supporting Information), upon light expo-sure. Figure 6 shows time-dependent drain current of turned-on OFETs, under flashing white light. While the current of BOPNA OFET remains unperturbed by the lighting conditions (a continued decrease of the current might be due to gate bias stress and possibly also resistive heating of the device), BOPAnt OFET shows 25% spikes of the current increase upon lighting. This difference is readily explained by the smaller band gap of BOPAnt which, as all other reported OSCs, absorbs a portion of the visible light and thus is photosensitive. An excellent photo-stability of current response of BOPNA (and photomodulation of the current in BOPAnt) is also observed in the output char-acteristics of the OFETs (Figure S18, Supporting Information).

3. Conclusion

In summary, we have synthesized new naphthalene semicon-ductor BOPNA with a wide band gap of 3.35 eV, high thermal/photostability, and complete optical transparency (absorption cutoff 370 nm). The OFET devices fabricated with vacuum-deposited BOPNA thin films exhibited p-type charge mobility of up to ≈1 cm2 V–1 s–1 measured in the saturated regime. Good linear characteristics in the low voltage range of the output characteristics (before saturation) suggests an efficient charge injection of Au contact, despite the low HOMO of the material (–5.6 eV). The major limitation brought by the low HOMO of the BOPNA is charge trapping at SiO2 interface, which leads to a large threshold voltage and hysteresis in the OFET transfer characteristics. Indeed, replacing ODTS mon-olayer on SiO2 with a more hydrophobic CYTOP dramatically reduces the hysteresis and lowers the threshold voltage, and thus even better efficiency could be expected in polymer dielec-tric top gate devices. Due to small molecular size, BOPNA films can be easily annealed after the deposition to eliminate grain boundaries. The temperature-dependent device measurements reveal an increase of mobility by ≈80% when the temperature is reduced from +25 to –25 °C, which is followed by nearly

temperature-independent behavior down to –150 °C. Overall, our work shows that it is possible to design efficient thin-film OFET with large band-gap (>>3 eV) semiconductors. A unique characteristic of such devices is their excellent photostability and independence of the transistor characteristics of the illumi-nation conditions; this is important for a wide-range practical application of low-cost organic electronics.

4. Experimental SectionNMR spectra were recorded on a 500 MHz Bruker AVIII instrument. UV–vis measurements were performed using a JASCO-670 spectrophotometer and fluorescence spectra were acquired using a Cary Eclipse (Varian) fluorometer. Absolute quantum yields of solution/solid/single crystal were measured in the integrating sphere of FluoroLog-3 fluorometer (Horiba Jobin Yvon Inc). Electrochemical experiments were performed at room temperature using CHI-770 electrochemical workstation. Cyclic voltammetry measurements were performed in 0.2 m TBAPF6 solution in CH2Cl2 using GCE working electrode, Ag/AgCl reference electrode, and Pt wire as a counter electrode. Scan rates were 100 mV s–1 and ferrocene was added to the electrolyte solution at the end of each experiment, as an internal standard. DFT calculations of the molecular geometry and electronic structure were performed for isolated (gas phase) molecule, using the B3LYP functional and the 6–31G(d) basis set as implemented in Gaussian 09.[55] TD-DFT single point calculations were performed by mixing ten states, and the UV absorption spectrum was produced using 0.20 eV line broadening. Electronic coupling in the crystal was calculated for molecular pairs in the geometry obtained from X-ray crystallographic analysis, via single point calculations (B3LYP/6-31G(d)), and expressed as an orbital splitting (HOMO–HOMO-1).[39]

2,6-dibromonaphthalene and 4-methoxyphenylboronic acid were purchased from Energy Chemical and used without further purification.

Synthesis of 2,6-bis(4-methoxyphenyl)naphthalene: 2,6-dibromonaphthalene (5.68 g, 20.0 mmol), 4-methoxyphenylboronic acid (9.18 g, 60.0 mmol), tetrakis(triphenylphosphine) Pd(PPh3)4 (0.46 g, 0.40 mmol), Aliquat 336 (2 mL), 2 m Na2CO3 aqueous solution (60 mL), and toluene (100 mL) were added in a 350 mL high-pressure reaction bottle, degassed by purging with nitrogen for 15 min and heated at 105 °C for 24 h. After cooling, the formed precipitate was collected by filtration and washed sequentially with methanol, diluted (5%) aqueous hydrochloric acid solution and acetone. The crude vacuum-dried product (6.12 g, 90%) was purified via two zone sublimations under vacuum to afford pure crystalline BOPNA used in device fabrication: HRMS(+ESI) m/z calcd. for C24H20O2[M+H+]− 341.1463, found 341.1529. Anal. Calculated for C24H20O2 (Mw: 340.1463): C, 84.68%; H, 5.92%. Found: C, 84.99%; H, 5.52%. 1H NMR (500 MHz, Cl2C6D4 at 100 °C) δ(ppm): 7.79 (s, 2H), 7.70 (d, J = 8.5 Hz, 2H), 7.54 (d, J = 8.5Hz, 2H), 7.45 (d, J = 5 Hz, 4H), 6.82 (d, J = 5Hz, 4H), 3.58 (s, 6H) (Figure S15, Supporting Information). 13C NMR (500 MHz, Cl2C6D4 at 100 °C) δ(ppm): 159.4, 137.8, 133.4, 132.8, 128.2, 127.9, 125.4, 124.4, 114.4, 54.8.

Device Fabrication and Measurement: Top contact OFETs were constructed on heavily doped n-type silicon wafers with 200 nm of thermal grown silicon dioxide, which serve as a gate electrode and a gate dielectric, respectively. ODTS-treated substrates were obtained by immersing in 0.1 m ODTS solution in toluene at room temperature for 12 h. CYTOP-treated substrates were obtained by a spin-coated process using CYTOP solution (CYTOP: solvent = 1:40, 3000 rpm, 30 s), and baking at 160 °C under nitrogen for 24 h. The layer of semiconductor film (60 nm) was deposited on the ODTS-treated substrates through a shadow mask under high vacuum < 2.0 × 10−6 Torr. Then, the gold source/drain electrodes were deposited on the semiconductor films. Devices were fabricated with channel lengths (L) of 38, 58, 78, and 98 μm and fixed width/length ratio W/L = 10. Annealing was performing by heating the final devices on a hot plate in air atmosphere for 15 min.


Figure 6. Time domain output current signals under dark and LED flash mode. The distance from LED to device is ≈10 cm; the luminous flux is ≈240 Lm.





The OFET measurements were carried out at room temperature in air using a Keithley 4200-SCS semiconductor parameter analyzer and Lakeshore probe station with CuBe contact probes. The carrier mobility was calculated form the data in the saturation regime according to the equation ISD = μCi (W/2L)(VG − VT)2, where ISD is the drain current, μ is the carrier mobility, Ci is the capacitance per unit area of the gate dielectric layer, W and L are the channel width and length, respectively, and VG and VT are the gate voltage and threshold voltage, respectively.

Variable temperature measurements were carried out under vacuum (≤10−5 Torr) in the –150…25 °C range, in both cooling and heating regimes, using liquid nitrogen cryostat and LakeShore 332 temperature controller.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThe work was supported by the NSERC of Canada (Discovery Grant), NSFC (51373075), the National Basic Research Program of China (973 Program, Grant Nos. 2015CB856500 and 2015CB932200), the National Research Foundation for the Doctoral Program (20133221110004), Shenzhen Key Laboratory of Shenzhen Science and Technology Plan (ZDSYS20140509094114164), Guangdong Academician Workstation (2013B090400016), Guangdong Key Research Project (2015B090914002 and 2014B090914003), and Guangdong Talents Project and Natural Science Foundation of Guangdong Province (2014A030313800).

Keywordsnaphthalene, organic field-effect transistors, organic semiconductors, wide band gap semiconductors

Received: December 22, 2016Revised: February 20, 2017

Published online:

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