Synthesis and photovoltaic properties of solution-processable star-shaped small molecules with triphenylamine as the core and alkyl cyanoacetate or 3-ethylrhodanine as the end-group

Abstract

Two novel star-shaped donor–acceptor small molecules, TPA–TBT–CN and TPA–TBT–R, with triphenylamine (TPA) as the core, benzothiadiazole (BT) as the arm, and alkyl cyanoacetate or 3-ethylrhodanine as the end-group are synthesized for application as donor materials in OSCs. The two small molecule films show broad absorption bands from 300 nm to 850 nm, narrow optical band gaps (1.5–1.7 eV), deep HOMO energy levels (−5.0 to −5.1 eV) and moderate hole mobilities. OSCs based on blends of the two donors and PC₇₀BM acceptors exhibit power conversion efficiencies of 1.34% and 1.79%, respectively. Notably, TPA–TBT–R with 3-ethylrhodanine as the end-group displays a broader solar spectral coverage, a lower HOMO level, a higher hole mobility and higher photovoltaic properties. Our results indicate that 3-ethylrhodanine as the acceptor and end-group is a promising linker in constructing donor materials for high efficiency OSCs.

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Introduction

Organic solar cells (OSCs) have attracted great interest due to their advantages of low-cost, light weight, processability and high mechanical flexibility.¹ OSCs typically consist of a phase-separated blend film of electron donor and electron acceptor materials as the active-layer placed between a modified tin-doped indium oxide (ITO) anode and low work function metal cathode.^1c Ideal donor materials should have the following two key characteristics: a narrow band gap (E_g) and a deep lying highest occupied molecular orbital (HOMO) level, which could increase the open-circuit voltage (V_oc) of the devices.² Donor–acceptor (D–A) molecular structures have become the most efficient approach to realize low band gap materials.³ To date, polymer and small molecule bulk heterojunctions (BHJs) have reached a power conversion efficiency (PCE) of up to 9.2%,⁴ and 8.9%,⁵ respectively. However, polymeric materials usually bear several defects, including difficult purification, broad molecular-weight distributions, and batch-to-batch variation. In this regard, solution-processed small molecules exhibit their merits, such as well-defined molecular structures, easier purification, less batch-to-batch variation in properties, intrinsic mono-dispersity, and reproducible device performance.^1b,6 To date, a variety of small molecule donors have been reported to fabricate small-molecule organic solar cells, including branched oligothiophenes,⁷ linear D–A molecules,^6b,8 porphyrins and phthalocyanines,⁹ and triphenylamine (TPA)-containing molecules.^8f,10 Chen et al. synthesized a series of linear broad absorption A–D–A molecules, with efficiencies reaching 5–8%.^5,8b,11 Among those excellent molecular backbones, TPA-based 3D propeller organic small molecules, possessing good solution processability, enhanced hole transport, well-defined film-forming properties and electron-donating capabilities, have been widely investigated for applications in solution-processed organic light-emitting diodes (OLEDs) and OSCs in recent years.¹²

In this contribution, we synthesized two new TPA-containing star-shaped molecules, TPA–TBT–CN and TPA–TBT–R (Scheme 1). The star-shaped molecules, with three-dimensional spatial structures, show good solution processability. In addition, a D–A structure is introduced into these TPA-containing molecules to improve the light absorption and electron-withdrawing (alkyl cyanoacetate and 3-ethylrhodanine) groups are used as the end-capping unit to further decrease the material band gap. The solution-processed bulk-heterojunction OSCs based on blends of TPA–TBT–CN or TPA–TBT–R as the donor and PC₇₀BM as the acceptor reached relatively high efficiencies of 1.34% or 1.79%, respectively, under AM 1.5G, 100 mW cm⁻² illumination.

Scheme 1 Chemical structures of TPA–TBT–CN and TPA–TBT–R.

Results and discussion

Synthesis and thermal stability of the molecules

The synthetic routes of TPA–TBT–CN and TPA–TBT–R are shown in Scheme 2. Monomers 1, 4 and 5 were synthesized according to the literature.^10i,13 Compound 3 was prepared from 1 and 4,7-dibromo-1,2,3-benzothiadiazole by the Stille reaction with a yield of 91%. Compound 6 was prepared from monomers 1 and 5 by the Suzuki reaction in a yield of 75%. Finally, TPA–TBT–CN and TPA–TBT–R were obtained according to methods in the literature.^5,11a The two star-shaped compounds are soluble in common organic solvents such as CHCl₃, toluene, and chlorobenzene. The thermal stability of the compounds was investigated by thermogravimetric analysis (TGA). The temperatures with 5% weight loss are 352 and 383 °C for TPA–TBT–CN and TPA–TBT–R, respectively, as shown in Fig. 1. This indicates that both materials have excellent thermal stabilities, which are good enough for application in organic solar cells.

Scheme 2 Synthesis routes of TPA–TBT–CN and TPA–TBT–R.

Fig. 1 TGA plots of TPA–TBT–CN and TPA–TBT–R.

Optical characterization

Fig. 2 shows the UV-vis absorption spectra of TPA–TBT–CN and TPA–TBT–R in chloroform solution and solid films. The optical absorption maxima (λ_max) are summarized in Table 1. Benefiting from their D–A molecular structures, the two molecules in chloroform solution display two strong absorption bands in the wavelength range from 300 to 650 nm. There is no obvious difference in the solution absorption spectra for the two materials, which indicates the similarity of the electron withdrawing functions of the 3-ethylrhodanine unit and the cyanoacetate group. The absorption at 300–400 nm is ascribed to the π–π* transition of the molecules, and the visible absorption peak located at 420–650 nm can be assigned to the intramolecular charge transfer transition between the TPA donor moiety and the acceptor unit.¹⁴ In comparison to their absorptions in solution, the absorption bands of the thin films are red-shifted ∼60–80 nm, which indicates effective aggregation of the molecules in the film states. Notably, the film absorption spectrum of TPA–TBT–R displays obvious broadening relative to that of TPA–TBT–CN, thus indicating a better molecular packing of TPA–TBT–R due to strong intermolecular interactions, which is favorable to improve the optical absorption of devices. The optical band gaps calculated from the offset of the film absorptions are 1.72 and 1.61 eV for TPA–TBT–CN and TPA–TBT–R, respectively. Considering that the 3-ethylrhodanine unit has stronger molar absorption than the cyanoacetate group,^11b the material TPA–TBT–R probably exhibits higher photovoltaic performance for organic solar cells.

Fig. 2 Normalized optical absorption spectra of TPA–TBT–CN and TPA–TBT–R in CHCl₃ and thin films on quartz.

Table 1 Optical and physical data of TPA–TBT–CN and TPA–TBT–R in CHCl₃ solution and in neat films

Compound	λmax (nm)	λonset (nm)	E^optgap (eV)
a CHCl3 was used as the solvent.b As-cast from CB (chlorobenzene) on quartz.
TPA–TBT–CN^a	520	630	1.97
TPA–TBT–R^a	530	630	1.97
TPA–TBT–CN^b	554	720	1.72
TPA–TBT–R^b	560	768	1.61

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Electrochemical properties and electronic energy levels

Cyclic voltammetry (CV) in CH₂Cl₂ with tetrabutylammonium hexafluorophosphate as supporting electrolyte was used to evaluate the electrochemical characteristics of TPA–TBT–CN and TPA–TBT–R. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the onset of the oxidation and reduction waves, respectively (Fig. 3 and Table 2). Compared to TPA–TBT–CN, the band gap of TPA–TBT–R decreased ca. 0.2 eV. The band gaps calculated here are consistent with those calculated from optical absorption. The two molecules show similar energy levels of HOMO and LUMO, which are −5.1 and −3.4 eV for TPA–TBT–CN and −5.0 and −3.5 eV for TPA–TBT–R, which match well with the acceptor material of PC₇₀BM. According to CV and the optical absorption results, TPA–TBT–CN and TPA–TBT–R should have relatively high open circuit voltages in OSCs when blended with PC₇₀BM.¹⁵

Fig. 3 Electrochemical voltammograms of TPA–TBT–CN (a) and TPA–TBT–R (b).

Table 2 Electrochemical data of TPA–TBT–CN and TPA–TBT–R

Compound	HOMO^a (eV)	LUMO^a (eV)	Egap^a (eV)
a HOMO = −e(4.8 + Eox), LUMO = −e(4.8 + Ered) and Egap = LUMO − HOMO.
TPA–TBT–CN	−5.1	−3.4	1.7
TPA–TBT–R	−5.0	−3.5	1.5

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Photovoltaic properties

The hole mobility of the two pristine small molecules were measured by the space charge limited current method with a device structure of ITO/PEDOT:PSS/active layer/MoO₃/Al. The hole mobilities were estimated to be 0.98 × 10⁻⁴ cm² V⁻¹ s⁻¹ for TPA–TBT–CN and 1.08 × 10⁻⁴ cm² V⁻¹ s⁻¹ for TPA–TBT–R. These mobility values are similar to that of a recently reported small molecule material in highly efficient OSCs.^11b

Bulk-heterojunction OSCs were fabricated by using the two molecules as the donor materials and PC₇₀BM as the acceptor material.¹⁶ The device structure was ITO/PEDOT/TPA materials:PC₇₀BM/Ca/Al. The photoactive layers are composed of blends of the star molecule and PC₇₀BM. For the donor–acceptor weight in the photoactive layers, the most efficient photovoltaic cells were obtained from OSC systems using TPA–TBT–CN or TPA–TBT–R with PC₇₀BM that were optimized at a weight ratio of 1 : 2. The photovoltaic performance data of the OSCs including the open circuit voltage (V_oc), short circuit current (J_sc), fill factor (FF) and power conversion efficiency (PCE) values are summarized in Table 3 and the corresponding current–density versus voltage (J–V) curves of these devices are shown in Fig. 4. All the devices based on TPA–TBT–CN or TPA–TBT–R yielded relatively high V_oc values. The V_oc value of TPA–TBT–R is as high as 0.87 V, which is 0.07 V higher than that of TPA–TBT–CN due to its lower-lying HOMO level. The J_sc values for TPA–TBT–R and TPA–TBT–CN are 6.42 mA cm⁻² and 5.41 mA cm⁻², respectively. The improved J_sc of TPA–TBT–R relative to TPA–TBT–CN can be attributed to the better solar spectral coverage and the higher hole mobility. Though both materials have relatively high V_oc and moderate J_sc, the devices show relatively low power conversion efficiencies, 1.79% and 1.34% for TPA–TBT–R and TPA–TBT–CN, respectively, due to the poor fill factors (32% and 31%) in the devices. Compared with TPA–TBT–CN, the device based on TPA–TBT–R displays better photovoltaic performance, which strongly demonstrates that 3-ethylrhodanine is a promising candidate for an active material as the acceptor and end-group. The poor FF is the core limiting factor, which implies that the electron transport ability is still limited and the film morphology is not ideally controlled. We believe that the FF value could be enhanced by introducing co-planar molecules, such as a carbazole instead of the TPA unit, and adjusting the alkyl in the molecule, which should improve the carrier mobility, the film morphology and the efficiency of the device.

Table 3 Photovoltaic performances of the OSCs under AM 1.5, 100 mW cm⁻² illumination

Compound	Voc (V)	Jsc (mA cm^−2)	FF	PCE (%)
TPA–TBT–CN	0.80	5.41	0.31	1.34
TPA–TBT–R	0.87	6.42	0.32	1.79

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Fig. 4 Current density–voltage characteristics of the OSC devices.

Fig. 5 shows the external quantum efficiency (EQE) spectra of the devices based on TPA–TBT–R and TPA–TBT–CN. From the curves, the two molecules display broad EQE spectra covering from 300 nm to 800 nm. It is noteworthy that the EQE value of the TPA–TBT–R based device is much higher than that of the TPA–TBT–CN based device. This result is also consistent with their absorption spectra in thin films, thus showing the contribution of the CT-band to the EQE and J_sc. The factors above determine that the TPA–TBT–R based OSC has higher photocurrent.

Fig. 5 The EQE curves of the corresponding OSCs.

Conclusions

In conclusion, we have designed and synthesized a class of solution processable star-shaped small molecules, TPA–TBT–CN and TPA–TBT–R, using TPA as the core and alkyl cyanoacetate/3-ethylrhodanine as the end-group for application in BHJ OSCs. TPA–TBT–CN and TPA–TBT–R show broad and strong absorption features in the range of 300–800 nm in thin films, and appropriate energy levels that are in good match with PC₇₀BM, as well as hole mobility. The solution processed bulk heterojunction OSCs based on TPA–TBT–CN:PC₇₀BM and TPA–TBT–R:PC₇₀BM (1 : 2, w/w) afforded PCEs of 1.34% and 1.79%, respectively. Notably, TPA–TBT–R with 3-ethylrhodanine as the end-group displays better solar spectral coverage, a lower HOMO level, higher hole mobility and higher photovoltaic properties. Our results indicate that 3-ethylrhodanine as the acceptor and end-group is a promising linker in constructing donor materials for high efficiency OSCs.

Experimental section

Materials

All chemicals were purchased from Aldrich, Aladdin or TCI Chemical Co. Toluene and tetrahydrofuran (THF) were dried over Na and distilled under nitrogen atmosphere. CHCl₃ was dried over CaH₂. All reactions were carried out under nitrogen atmosphere with the use of standard Schlenk techniques. Deuterated solvents were purchased from J&K and used as received. All reactants and reagents are commercially available and used as received unless otherwise noted.

Synthesis of compounds

The synthetic routes of the compounds are shown in Scheme 2. The detailed synthetic processes are as follows.

4,4′,4′′-Tris(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)triphenylamine (1). Tris(4-bromophenyl)amine (5 g, 10.4 mmol) was dissolved in 250 mL dry THF under an argon atmosphere. At −78 °C, butyllithium (25 mL, 62.5 mmol) (2.5 M solution in hexane) was added by syringe. The mixture was stirred at −78 °C for 1 h, then warmed up to 0 °C for 15 min, and cooled to −78 °C again. 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (11.6 g, 62.5 mmol) was added rapidly to the solution, and the resulting mixture was warmed to room temperature and stirred overnight. The reaction mixture was poured into water and extracted with dichloromethane. The organic extract was dried over magnesium sulfate, and the solvent was removed with a rotary evaporator. The crude product was further purified by column chromatography on a silica gel column using a mixture of petroleum ether and EA (15 : 1) as eluent to afford 1 as a white solid (3.9 g, 60% yield). ¹H NMR (400 MHz, CDCl₃), δ(ppm): 7.68 (d, J = 8 Hz, 6H), 7.07 (d, J = 8 Hz, 6H), 1.31 (s, 36H).

Tributyl(4-hexylthiophen-2-yl)stannane (2). Under an argon atmosphere, 3-hexylthiophene (5 g, 30 mmol) was dissolved in 250 mL dry THF. Butyllithium (12 mL of 2.5 M solution in hexane, 30 mmol) was added by syringe at −78 °C. The mixture was stirred at −78 °C for 1 h, then warmed up to 0 °C for 15 min, and cooled again to −78 °C, then chlorotributyltin (9 mL, 33 mmol) was added into the solution. The reaction mixture was poured into water and extracted with petroleum ether. The organic extract was dried over magnesium sulfate, and the solvent was removed with a rotary evaporator. The crude product was not further purified.

4,7-Bis-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (3). 4,7-Dibromo-2,1,3-benzothiadiazole (0.58 g, 2 mmol), compound 2 (2.7 g, 6 mmol) and Pd(PPh₃)₄ (23 mg, 0.02 mmol) were dissolved in toluene under an argon atmosphere. The solution was stirred at 120 °C for 12 h. Finally, the reaction mixture was cooled to room temperature and added to a saturated aqueous solution of KF. The mixture was stirred for 30 min, and then washed successively with water. The aqueous phase was extracted with CHCl₃. The organic phase was dried over MgSO₄. The solvent was removed under vacuum, and the residue was purified by column chromatography on silica gel using a mixture of petroleum ether and CHCl₃ (8 : 1) as eluent to afford monomer 3 as a red solid (0.85 g, 91% yield). ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.99 (d, J = 4 Hz, 2H), 7.79 (s, 2H), 7.04 (s, 2H), 2.71 (t, J = 8 Hz, 4H), 1.73 (m, 4H), 1.39 (m, 12H), 0.93 (m, 6H).

3-Hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[1,2,5]thiadiazol-4-yl)thiophene-2-carbaldehyde (4). POCl₃ (0.5 mL, 5.5 mmol) was added to a solution of compound 3 and anhydrous DMF (0.87 mL, 5.5 mmol) in anhydrous 1,2-dichloroethane (30 mL) at 0 °C under N₂ atmosphere. The solution was warmed to room temperature and then refluxed for 12 h. At room temperature, the mixture was poured into an aqueous solution of sodium acetate (1 M, 200 mL) and stirred for 2 h to complete the hydrolysis. After separation of the organic phase by decantation, the aqueous phase was extracted with CHCl₃. The organic phases were gathered, dried over MgSO₄, and evaporated in vacuo. The residue was purified by column chromatography on silica gel (eluent: CHCl₃–petroleum ether = 1/3) to afford a red solid (2.1 g, 84% yield). ¹H NMR (400 MHz, CDCl₃), δ(ppm): 10.09 (s, 1H), 8.03 (s, 1H), 8.02 (s, 1H), 7.93 (d, J = 4 Hz, 2H), 7.83 (d, J = 8Hz, 2H), 7.08 (s, 1H), 3.02 (t, J = 8 Hz, 2H), 2.69 (t, J = 8 Hz, 2H), 1.73 (m, 4H), 1.36 (m, 12H), 0.89 (m, 6H).

5-(7-(5-Bromo-4-hexylthiophen-2-yl)benzo[1,2,5]thiadiazol-4-yl)-3-hexylthiophene-2-carbaldehyde (5). Under exclusion of light, NBS (0.15 g, 0.82 mmol) was added to a stirred solution of compound 4 (0.34 g, 0.68 mmol) in CHCl₃ (20 mL). The reaction was left to stir overnight at room temperature before being poured into H₂O. The aqueous phase was extracted with CHCl₃ 3 times. The organic phases were gathered, dried over MgSO₄, and evaporated in vacuo. The residue was purified by column chromatography on silica gel (eluent: CHCl₃–petroleum ether = 1/3) to afford a red solid (0.27 g, 67% yield). ¹H NMR (400 MHz, CDCl₃), δ(ppm): 10.10 (s, 1H), 8.06 (s, 1H), 7.95 (d, J = 8 Hz, 1H), 7.82 (s, 1H), 7.80 (d, J = 8 Hz, 1H), 3.03 (t, J = 8 Hz, 2H), 2.65 (t, J = 8 Hz, 2H), 1.77 (m, 2H), 1.66 (m, 2H), 1.35 (m, 12), 0.89 (m, 6H).

Monomer (6). Under an argon atmosphere, compound 5 (1.24 g, 2.15 mmol), compound 1 (312 mg, 0.5 mmol), K₂CO₃ (2.76 g, 20 mmol) and Pd(PPh₃)₄ (57 mg, 0.05 mmol) were dissolved in toluene–H₂O (30/10 mL). The solution was stirred at 85 °C for 72 h. After separation of the organic phase by decantation, the aqueous phase was extracted with CHCl₃. The organic phases were gathered, dried over MgSO₄, and evaporated in vacuo. The residue was purified by column chromatography on silica gel (eluent: CHCl₃–petroleum ether = 1/2) to afford a black solid (600 mg, 70% yield). ¹H NMR (400 MHz, CDCl₃), δ(ppm): 10.08 (s, 3H), 8.06 (s, 3H), 8.02 (s, 3H), 7.9 (d, J = 8 Hz, 3H), 7.81 (d, J = 8 Hz, 3H), 7.46 (d, J = 16 Hz, 6H), 7.25 (d, J = 8 Hz, 6H), 2.99 (t, J = 8 Hz, 6H), 2.78 (t, J = 8 Hz, 6H), 1.74 (m, 12H), 1.35 (m, 36H), 0.87 (m, 18H). ¹³C NMR (100 MHz, CDCl₃), δ(ppm): 182.08, 153.47, 152.44, 152.37, 147.54, 146.63, 140.39, 139.77, 137.27, 136.35, 130.37, 130.07, 129.13, 127.80, 124.44, 124.21, 123.90, 31.70, 31.61, 31.48, 31.02, 29.30, 29.08, 28.70, 22.66, 22.59, 14.16, 14.08.

TPA–TBT–CN. Monomer 6 (0.346 g, 0.2 mmol) was dissolved in a solution of dry CHCl₃ (20 mL), three drops of triethylamine and then octyl cyanoacetate (0.8 g, 4 mmol) were added, and the resulting solution was stirred for 24 h under nitrogen at room temperature. The reaction mixture was then extracted with CHCl₃, washed with water, and dried over Mg₂SO₄. After removal of the solvent, it was purified by column chromatography on silica gel using a mixture of petroleum ether and CHCl₃ (1 : 2) as eluent to afford 7 as a black solid (340 mg, 75% yield). ¹H NMR (400 MHz, CDCl₃), δ(ppm): 8.47 (s, 3H), 8.19 (s, 3H), 8.09 (s, 3H), 7.99 (d, J = 8 Hz, 3H), 7.84 (d, J = 8 Hz, 3H), 7.5 (d, J = 8 Hz, 6H), 7.28 (d, J = 8 Hz, 6H), 4.33 (t, J = 4 Hz, 6H), 2.89 (t, J = 8 Hz, 6H), 2.81 (t, J = 8 Hz, 6H), 1.76 (m, 18H), 1.36 (m, 66H), 0.92 (m, 27H). ¹³C NMR (100 MHz, CDCl₃), δ(ppm): 163.51, 155.80, 152.25, 147.49, 146.51, 140.55, 136.24, 129.99, 127.73, 124.16, 116.35, 96.63, 66.49, 31.81, 31.59, 31.32, 31.02, 29.38, 29.24, 29.21, 29.09, 28.63, 26.87, 22.70, 22.67, 22.62, 14.19, 14.11, 14.08. Elemental analysis: anal. calcd (%) for C₁₃₂H₁₅₆N₁₀O₆S₉: C, 69.93; H, 6.94; N, 6.18; S, 12.73. Found: C, 69.81; H, 7.16; N, 6.40; S, 13.11.

TPA–TBT–R. Compound 6 (173 mg, 0.1 mmol) was dissolved in a solution of dry CHCl₃ (10 mL), three drops of piperidine and then 3-ethylrhodanine (0.32 g, 2 mmol) were added, and the resulting solution was refluxed and stirred for 24 h under nitrogen. The reaction mixture was then extracted with CHCl₃, washed with water, and dried over Mg₂SO₄. After removal of the solvent, it was purified by chromatography on a silica gel column using a mixture of petroleum ether and CHCl₃ (1 : 1) as eluent to afford 8 as a black solid (160 mg, 74% yield). ¹H NMR (400 MHz, CDCl₃), δ(ppm): 7.93 (s, 3H), 7.87 (s, 3H), 7.84 (s, 3H), 7.66 (d, J = 8 Hz, 3H), 7.59 (d, J = 8 Hz, 3H), 7.41 (d, J = 8 Hz, 6H), 7.19 (d, J = 8 Hz, 6H), 4.19 (m, 6H), 2.76 (m, 12H), 1.7 (m, 12H), 1.35 (m, 45H), 0.93 (m, 18H). Elemental analysis: anal. calcd (%) for C₁₁₄H₁₂₀N₁₀O₃S₁₅: C, 63.41; H, 5.60; N, 6.49; S, 22.28. Found: C, 63.48; H, 6.0; N, 6.84; S, 22.18.

Fabrication and characterization of the organic solar cells

The organic solar cells (OSCs) were fabricated in the configuration of the traditional sandwich structure with an ITO positive electrode and a metal negative electrode. The ITO glass was cleaned in an ultrasonic bath of acetone and isopropanol, and treated with UVO (ultraviolet ozone cleaner). Then a thin layer (40 nm) of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)) (Baytron PVP Al 4083, Germany) was spin-coated onto the ITO glass. Subsequently, the photosensitive layer was prepared by spin-coating the blend solution of the star molecules and PC₇₀BM (1 : 2 w/w) on top of the PEDOT:PSS layer and baking at 140 °C for 20 min. The concentration of the blend solution was 6 mg mL⁻¹ in chlorobenzene. Finally, a layer of 20 nm Ca and 100 nm Al was vacuum evaporated onto the photoactive layer under a shadow mask in a vacuum of ca. 10⁻⁴ Pa. The active area of the devices is 6 mm². The current–voltage (J–V) measurement of the devices was conducted on a computer-controlled Keithley 2400 Source Measure Unit. A xenon lamp coupled with AM 1.5 solar spectrum filters was used as the light source, and the optical power at the sample was ca. 100 mW cm⁻².

Instrumentation

¹H NMR and ¹³C NMR spectra were recorded on a Bruker-400 MHz. TGA was carried out on a Perkin-Elmer Pyris Diamond TG instrument under purified nitrogen gas flow with a heating rate of 10 °C min⁻¹. The cyclic voltammetry (CV) measurements were performed using a Metrohm instrument model PGSTA302N in a standard three-electrode, one compartment configuration equipped with a Ag/AgCl electrode, Pt wire and glassy carbon electrode, as the pseudo reference, counter electrode and working electrode, respectively. The glassy carbon electrodes were polished with alumina prior to use. The CV experiments were performed in anhydrous dichloromethane solution with 0.1 M tetrabutyl electrolyte at a scan rate of 100 mV s⁻¹ unless otherwise stated. Solution CV measurements were carried out with sample concentrations of 1 mg mL⁻¹ in CH₂Cl₂. Ferrocene was used as the internal standard, assuming Fc/Fc⁺ to be −4.8 eV versus vacuum. Absorption spectra were recorded with a Perkin Elmer (model Lambda 950) UV-visible spectrophotometer. The film on quartz used for UV measurements was prepared by spin-coating with CHCl₃ solution.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51003080, 51273208), the Youth Science Plan for Light of the Morning Sun of Wuhan City (201271031385), the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology) (2012-09), the Natural Science Foundation of Hubei Province (2012FFB04705), the Hundred Talent Program of the Chinese Academy of Sciences, the Starting Research Fund of Team Talent (Y10801RA01) in NIMTE and the Ningbo Natural Science Foundation of China (2012A610114).

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