Triarylamino and Tricyanovinyl End-Capped Oligothiophenes with

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Triarylamino and Tricyanovinyl End-Capped Oligothiophenes with Reduced Optical Gap for Photovoltaic Applications Ping Fang Xia,† Xin Jiang Feng,† Jianping Lu,*,‡ Raluca Movileanu,‡ Ye Tao,*,‡ Jean-Marc Baribeau,‡ and Man Shing Wong*,† Department of Chemistry and Centre for AdVanced Luminescence Materials, Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong SAR, China, and Institute for Microstructural Sciences (IMS), National Research Council of Canada (NRC), 1200 Montreal Road, Ottawa ON K1A 0R6, Canada ReceiVed: June 24, 2008; ReVised Manuscript ReceiVed: August 7, 2008

Novel low optical gap, p-type semiconducting oligothiophenes asymmetrically end-capped with triarylamino and tricyanovinyl groups, PhN-OFOT(n)-TCN (n ) 2, 3), have been synthesized and characterized for photovoltaic applications. With an incorporation of a tricyanovinyl accepting group to the triarylamine, the optical energy gap greatly reduces to 1.46 eV and the LUMO level lowers to 3.9 eV. The initial studies of the bilayer heterojunction PV cells based on the newly developed tricyanovinyl-substituted chromophores as a donor material and C60 as an acceptor material showed a PCE up to 1.33% with a large open-circuit voltage of 0.82 V in the annealed devices which makes this class of materials promising for further development. Our findings also suggest for the first time that the tricyanovinyl group is highly efficient to lower the LUMO level and reduce the optical energy gap of a p-type semiconducting photosensitizer. It is interesting to find that PhN-OFOT(2)-TCN showed better device performance in bilayer solar cells than PhN-OFOT(3)-TCN although the latter has a slightly narrower optical gap. Since the LUMO energy level of PhN-OFOT(3)TCN (ca. 3.9 eV vs vacuum) is too close to that of C60 (4.0 eV), the yield of photoinduced charge carriers was low, leading to a low power conversion efficiency. Our findings highlight the importance of a large LUMO level offset between electron donors and acceptors to the achievement of high performance organic solar cells. Introduction There has been considerable effort in the development of efficient organic/polymeric photovoltaic (PV) devices to harvest solar energy in the past few years, as they can offer advantages of low-cost and ease of fabrication.1 Recently, there has been substantial progress in the development of the bulk heterojunction and the multilayer heterojunction PV cells through the use of the new materials, fabrication techniques, and device structures with which the power conversion efficiencies (PCE) greater than 5% has been achieved.2,3 To further improve the efficiency of a heterojunction PV cell, the development of new p-type organic/polymer semiconductors that possess a broad and narrow absorption bandgap to harvest more solar radiation as well as a high charge mobility to transport the photogenerated charges to the electrodes is crucial. In addition, proper control of the positions of the frontier molecular orbitals of the donor material is indispensible to achieve effective exciton dissociation at the heterojunction and a large open-circuit voltage (Voc) of a solar cell.1c-f As a result, the donor-acceptor type molecular/ polymer system is considered to be one of the most promising candidates for photovoltaic applications, as the molecular/ physical properties of this p-type semiconductor such as the optical energy gap as well as the HOMO and LUMO levels can be easily modulated through the extension of π-conjugated system and the incorporation of an appropriate electron-donating moiety or/and an electron-accepting moiety into the system. * Corresponding authors. E-mail: (M.S.W.) [email protected]; (J.L.) [email protected]; (Y.T.) [email protected]. † Hong Kong Baptist University. ‡ National Research Council of Canada.

Recently, there have been considerable activities in exploring donor-acceptor type polymeric materials for PV cell applications; on the other hand, there is relatively less attention on the molecular system so far.4 Furthermore, tricyanovinyl-substituted chromophores have rarely been investigated for heterojunction photovoltaic cell applications except being studied for secondorder nonlinear optics5 and n-type thin-film transistors.6 Our recent success in the development of novel low bandgap oligothiophenes7 asymmetrically end-capped with diphenylaminofluorenyl and dicyanovinyl groups motivated us to further reduce their bandgaps by attaching a stronger electronwithdrawing tricyanovinyl group to oligothiophenes for a better match the solar spectrum. We report herein the synthesis and properties of novel holetransporting and low optical gap oligothiophenes asymmetrically incorporated with an electron-donating triarylamino group and an electron accepting tricyanovinyl group, PhN-OFOT(n)-TCN, where n ) 2, 3 for photovoltaic applications. Cyclic voltammetric study showed that the LUMO energy levels of oligothiophene were significantly lowered by the very strong electronwithdrawing tricyanovinyl group, while the HOMO energy levels were barely modified. As a result, the electrochemical energy gaps were greatly reduced to 1.29 eV for PhN-OFOT(2)TCN and 1.25 eV for PhN-OFOT(3)-TCN. The bilayer heterojunction PV cells fabricated from these tricyanovinylbased materials with a device structure of ITO/PEDOT-PSS/ PhN-OFOT(n)-TCN/C60/BCP/Ag showed a PCE up to 1.33% with Voc ) 0.82 V, short circuit current (Jsc) ) 4.03 mA/cm2 and fill factor (FF) ) 0.44 in the annealed devices.

10.1021/jp805555c CCC: $40.75  2008 American Chemical Society Published on Web 09/25/2008

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SCHEME 1: Syntheses of Triarylamino and Tricyanovinyl-Disubstituted Oligothiophenes

Experimental Method Synthesis. 2a. To a 100 mL two-necked flask containing the solution of 2,2-bithiophene, 1a (1.66 g, 10 mmol), in 40 mL of dry THF was added 1.6 M n-BuLi (6.6 mL, 10.6 mmol) dropwise at -78 °C under nitrogen while maintaining good stirring. After the mixture was stirred for 40 min, iodine (2.54 g, 10 mmol) was added and then further stirred for another 2 h at room temperature. A solution of sodium sulfite was added to decolorize the reaction mixture which was then extracted with dichloromethane (3 × 50 mL). The combined organic layer was washed with water, dried over anhydrous Na2SO4, filtered, and evaporated to dryness. The crude product was filtered through a short silica-gel column using petroleum ether/dichloromethane (5:1) as eluent, affording the desired product (2.78 g, 95%) which was used in next step without further purification. 1H NMR (400 MHz, CDCl3, δ) 7.22 (dd, J ) 1.2, 5.2 Hz, 1H), 7.15 (d, J ) 3.6 Hz, 1H), 7.11 (dd, J ) 1.2, 4.0 Hz, 1H), 7.00 (m, 1H), 6.84 (d, J ) 4.0 Hz, 1H). 13C NMR (100 MHz, CDCl3, δ) 143.2. 137.6, 136.2, 127.8, 125.1, 124.8, 124.1, 71.8. MS (FAB) m/z: 292 (M+). 4a. A mixture of 9,9-bis(n-butyl)-2-diphenylamino-7-fluorenylboronic acid 3 (1.76 g, 3.6 mmol), 2a (0.88 g, 3.0 mmol), Pd(PPh3)4 (105 mg, 0.09 mmol), THF (40 mL), and 2 M K2CO3 (7 mL) was stirred at 80 °C overnight under nitrogen. After being cooled to room temperature, the reaction mixture was poured into cold water and extracted with chloroform (3 × 80 mL). The combined organic layer was dried with anhydrous Na2SO4, filtered, and evaporated to dryness. The crude product was then purified by silica-gel column chromatography with petroleum ether/dichloromethane as gradient eluent, affording a light-yellow solid (1.23 g, 67%). Mp ) 171 °C. 1H NMR (400 MHz, CDC3, δ) 7.64-7.55 (m, 4H), 7.31-7.24 (m, 7H), 7.21-7.16 (m, 6H), 7.07-7.03 (m, 4H); 1.97-1.90 (m, 4H), 1.17-1.09 (m, 4H), 0.75 (t, J ) 7.2 Hz, 6H), 0.72-0.65 (m, 4H). 13C NMR (100 MHz, CDCl3, δ) 152.2, 151.4, 147.8, 147.2, 143.9, 140.6, 137.5, 136.1, 135.6, 132.0, 129.1, 127.8, 124.5, 124.1, 123.8, 123.4, 123.3, 123.2, 122.5, 120.4, 119.5, 119.4, 119.1, 55.0, 39.9, 26.0, 22.9, 13.9. HRMS (MALDI-TOF) calcd for C41H39NS2: 609.2518; Found: 609.2526.

PhN-OFOT(2)-TCN. To a dry 100 mL two-necked flask containing 4a (463 mg, 0.76 mmol) in 15 mL of dry THF was added n-BuLi (0.5 mL, 0.8 mmol) at -78 °C under N2. The solution was stirred at -78 to 0 °C for 2 h, recooled to -78 °C, and treated with one portion of solution of tetracyanoethylene (122 mg, 0.95 mmol) in 2 mL of THF. The reaction mixture was stirred at -78 °C to room temperature for 2 h. After the reaction mixture was stirred for another 2 h at room temperature (the solution changes from yellow to dark green), THF was subsequently removed by rotary evaporation and the residue was dissolved in CHCl3, neutralized with 0.1 M HCl, washed with H2O, dried over Na2SO4, filtered, and evaporated to dryness. The residue was carried out silica gel flash column chromatograph using a mixture of petroleum ether/toluene as eluent, affording the desired product and 4a of 0.15 g (33%). Recrystallization of the product from CHCl3/Et2O gave the pure product as a dark olive-green solid. (0.27 g, 74%). Mp ) 422 °C. 1H NMR (400 MHz, CDCl3, δ) 7.98 (d, J ) 4.4 Hz, 1H), 7.63-7.52 (m, 5H), 7.40 (d, J ) 4.0 Hz, 1H), 7.37 (d, J ) 4.4 Hz, 1H), 7.23-7.20 (m, 4H), 7.13-7.10 (m, 5H), 7.02 (t, J ) 6.8 Hz, 3H) 1.96-1.83 (m, 4H), 1.13-1.04 (m, 4H), 0.70 (t, J ) 7.2 Hz, 6H), 0.68-0.61 (m, 4H). 13C NMR (100 MHz, CDCl3, δ) 152.6, 152.5, 151.8, 151.2, 147.9, 147.7, 142.5, 141.7, 134.9, 132.7, 131.8, 131.4, 130.5, 130.2, 129.2, 125.1, 124.9, 124.6, 124.1, 123.1, 122.8, 120.7, 120.0, 119.7, 118.7, 112.8, 112.7, 112.3, 80.4, 55.1, 39.9, 26.0, 22.9, 13.9. HRMS (MALDITOF) calcd. for C46H38N4S2: 710.2532; Found 710.2525. Anal. Calcd for C46H38N4S2: C 77.71, H 5.39, N 7.88; found: C 77.57, H 5.34, N 7.66. 2b. The above procedure for 2a was followed using 1b (1.24 g, 5.0 mmol), n-BuLi (3.2 mL, 5.1 mmol), and iodine (1.27 g, 5.0 mmol). The product was purified by a silica-gel column chromatography using petroleum ether/dichloromethane (v/v ) 5:1) as eluent and then recrystallized from CHCl3 affording a yellow solid (1.68 g, 90%). Mp ) 141 °C. 1H NMR (400 MHz, CDCl3, δ) 7.27 (d, J ) 5.2 Hz, 1H), 7.17-7.15 (m, 2H), 7.05 (d, J ) 3.6 Hz, 1H), 7.03-7.00 (m, 2H), 6.83 (d, J ) 3.6 Hz,

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Figure 1. Absorption spectra of (a) PhN-OFOT(2)-TC N in different solvents, (b) PhN-OFOT(3)-TCN in different solvents, and (c) thin films of PhN-OFOT(n)-TCN.

Figure 2. CV traces of (a) PhN-OFOT(2)-TCN and (b) PhN-OFOT(3)-TCN obtained at a scan rate of 100 mV/s. The potential scale is referenced to SCE.

TABLE 1: Summaries of Physical Measurements of PhN-OFOT(n)-TCN

PhN-OFOT(2)-TCN PhN-OFOT(3)-TCN

λabsmax,a nm (
max 104, M-1 cm-1)

optical gap,b eV

Eoxd1/2,c V

Ered1/2 or Eredp,c V

HOMO,c eV

LUMO,c eV

energy gap,d eV

Tm,e °C

Tdec,f°C

629 (2.91) 643 (3.14)

1.48 1.46

0.38, 0.89 0.35 0.69

-0.91, -1.50 -0.90,-1.59

-5.18 -5.15

-3.89 -3.90

1.29 1.25

236 249

422 440

a Measured in CHCl3. b Estimated from the absorption edge of thin film c Determined by CV method using a platinum disk electrode as a working electrode, platinum wire as a counter electrode, and SCE as a reference electrode with an agar salt bridge connecting to the oligomer solution, and ferrocene was used as an external standard, E1/2 (Fc/Fc+) ) 0.46 V vs SCE and calculated with reference to ferrocene (4.8 eV vs vacuum). d Energy gap ) HOMO - LUMO. e Determined by differential scanning calorimeter from remelt after cooling with a heating rate of 10 °C/min under N2. f Determined by thermal gravimetric analyzer with a heating rate of 10 °C/min under N2.

1H). 13C NMR (100 MHz, CDCl3, δ) 143.0, 137.7, 136.8, 136.7, 134.9, 127.9, 124.9, 124.7, 124.3, 123.9, 71.9. MS (FAB) m/z: 374 (M+). 4b. The above procedure for 4a was followed using 3 (1.63 g, 3.33 mmol), 2b (1.04 g, 2.78 mmol), Pd(PPh3)4 (97 mg, 0.084

mmol), THF (40 mL), and 2 M K2CO3 (6 mL). The pure product was separated by a silica-column chromatography using petroleum ether/CH2Cl2 as gradient eluent affording a yellow solid (1.32 g, 69%). Mp ) 177 °C. 1H NMR (400 MHz, CDCl3, δ) 7.58-7.51 (m, 4H), 7.26-7.22 (m, 4H), 7.20-7.12 (m, 9H),

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Figure 3. Current density-voltage curves of (a) PhN-OFOT(2)-TCN- and (b) PhN-OFOT(3)-TCN-based devices under AM 1.5 simulated solar illumination of 100 mW/cm2.

Figure 4. UV-vis absorption spectra of (a) PhN-OFOT(2)-TCN and (b) PhN-OFOT(3)-TCN thin films (20 nm) before and after thermal annealing.

TABLE 2: Summary of Device Performance for PhN-OFOT(n)-TCN (20 nm)/C60 (30 nm)-Based PV Cells donor material PhN-OFOT(2)-TCN PhN-OFOT(3)-TCN

Rshun,a kΩ · cm2 12.0 9.8 10.1 8.8

Rse,bΩ · cm2

Voc, c (V)

Jsc, c (mA/cm2)

FF,c

PCE, c (%)

53 34.8 315 43.4

0.79d

1.96d

0.28d

0.82e 0.73d 0.79e

4.03e 1.63d 3.49e

0.40e 0.22d 0.39e

0.43d 1.33e 0.26d 1.07e

a Shunt resistance. b Series resistance. c Under simulated AM 1.5 solar illumination at an irradiation intensity of 100 mW/cm2. as-fabricated devices. e For devices annealed at 100 °C for 20 min.

7.08 (d, J ) 3.6 Hz, 1H), 7.06 (d, J ) 3.6 Hz, 1H), 7.03-6.98 (m, 4H), 1.93-1.85 (m, 4H), 1.12-1.05 (m, 4H), 0.70 (t, J ) 7.2 Hz, 6H), 0.68-0.64 (m, 4H). 13C NMR (100 MHz, CDCl3, δ) 152.2, 151.4, 147.9, 147.3, 144.0, 140.7, 137.1, 136.3, 136.0, 135.7, 135.6, 131.9, 129.1, 127.8, 124.55, 124.52, 124.4, 124.3, 124.0, 123.9, 123.6, 123.3, 122.5, 120.4, 119.53, 119.50, 119.1, 55.0, 39.9, 26.0, 22.9, 13.9. HRMS (MALDI-TOF) calcd for C45H41NS3: 691.2396 Found.691.2418. PhN-OFOT(3)-TCN. The above procedure for PhN-OFOT(2)TCN was followed using 4b (0.54 g, 0.78 mmol), n-BuLi (0.5 mL, 0.8 mmol), and tetracyanoethylene (122 mg, 0.95 mmol). After the typical procedure workup, separation by column chromatograph, and recrystallization, pure product was obtained as a dark olive-green solid (0.26 g, 71%) with 0.22 g (41% recovery) of 4b. Mp ) 440 °C. 1H NMR (400 MHz, CDCl3, δ) 7.96 (d, J ) 4.4 Hz, 1H),7.61 (d, J ) 8.0 Hz, 1H), 7.57-7.52 (m, 3H), 7.46 (d, J ) 4.0 Hz, 1H), 7.34-7.31 (m, 2H), 7.29-7.24 (m, 5H), 7.22 (d, J ) 4.0 Hz, 1H), 7.15-7.13 (m, 5H), 7.04-7.01 (m, 3H), 1.93-1.84 (m, 4H), 1.11-1.07 (m, 4H), 0.72 (t, J ) 7.2 Hz, 6H), 0.69-0.60 (m, 4H). 13C NMR (100 MHz, CDCl3, δ) 152.3, 152.0, 151.6, 147.8, 147.6, 146.9,

d

For

143.1, 141.53, 141.50, 135.2, 134.1, 132.4, 131.9, 131.3, 131.2, 130.0, 129.2, 126.6, 125.0, 124.8, 124.0, 123.8, 123.3, 122.7, 120.5, 119.7, 119.6, 118.9, 112.7, 112.6, 112.2, 80.6, 55.1, 39.9, 26.0, 23.0, 13.9. HRMS (MALDI-TOF) calcd for C50H40N4S3: 792.2410; Found 792.2405. Anal. Calcd for C50H40N4S3: C 75.72, H 5.08, N 7.06, found: C 75.40, H 4.95, N 6.93. Physical Measurements. The melting point was determined by differential scanning calorimeter (Perkin-Elmer Pyris Diamond DSC) from remelt after cooling with a heating rate of 10 °C/min under N2. The decomposition temperature was determined by thermal gravimetric analyzer (Perkin-Elmer TGA 6) with a heating rate of 10 °C/min under N2. The AFM images were acquired in a tapping mode with the samples kept in air, using a DI EnviroScope Veeco instrument. A silicon probe tip (spring constant 10-130 N/m) was used for imaging and tuned for a drive frequency of ∼320 kHz. The scanning parameters were as follows: before annealing, scanning rate, 1.2 Hz; data scale, 5 nm; after annealing, scanning rate, 1.1 Hz; data scale, 50 nm. PV Device Fabrication and Testing. ITO-coated glass substrates (15 Ω/0) were patterned by conventional wet-etching

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Figure 5. AFM images of the PhN-OFOT(3)-TCN thin film (a) before and (b) after thermal annealing at 100 °C for 20 min.

process using an acid mixture of HCl (6 N) and HNO3 (0.6 N) as the etchant. The active area of each solar cell device was 2.5 × 5 mm2. After patterning, the substrates were rinsed in deionized water and then ultrasonicated sequentially in acetone (20 °C) and 2-propanol (65 °C). Immediately prior to device fabrication, the ITO substrate was treated in a UV-ozone oven for 15 min. Subsequently, a poly(3,4-ethylenedioxythiophene)poly(styrene sulfonate) (PEDOT-PSS) thin film (50 nm) was spin-coated at 5000 rpm from its aqueous solution onto the treated substrate and then baked at 135 °C under nitrogen for 30 min. PhN-OFOT(n)-TCN (20 nm), C60 (30 nm), and BCP (8 nm) were sequentially vacuum-deposited at 2 × 10-7 Torr. The device fabrication was completed by the thermal evaporation of Ag cathode (60 nm). Postproduction device annealing

was carried out at 100 °C for 20 min under nitrogen. The solar cells with no protective encapsulation were subsequently tested in air under air mass (AM) 1.5 simulated solar illumination (100 mW/cm2, Sciencetech Inc., Model SF150). Current-voltage (I-V) characteristics were recorded using a computer-controlled Keithley 2400 source meter. Results and Discussion The synthesis of triarylamino and tricyanovinyl asymmetrically disubstituted bi- and ter-thiophenes is outlined in Scheme 1. Iodination of bi- or ter-thiophene was achieved by deprotonation of 1a or 1b with n-BuLi at -78 °C and subsequent treatment with iodine, affording monoiodinated product 2a or

End-Capped Oligothiophenes

Figure 6. XRD pattern of the annealed PhN-OFOT(3)-TCN film.

2b in an excellent yield (90-95%), respectively. The palladiumcatalyzed Suzuki cross-coupling of 2a or 2b with 7-diphenylamino-9, 9-di-n-butylfluorenyl-2-boronic acid,8 3, using Pd(PPh3)4 as a catalyst afforded the desired triarylamino-substituted oligothiophene, 4a or 4b, in 67% or 69% yield, respectively. Lithiation of 4a or 4b at low temperature followed by the reaction with tetracyanoethylene afforded PhN-OFOT(2)-TCN or PhN-OFOT(3)-TCN in good yield (71-74%), respectively. All the newly synthesized asymmetrically disubstituted oligothiophenes were fully characterized with 1H NMR, 13C NMR, MALDI-TOF HRMS, and elemental analysis and found to be in good agreement with their structures. The absorption spectra of PhN-OFOT(n)-TCN (n ) 2 or 3) as shown in Figure 1a and 1b show a broad and structureless absorption peaked (λabsmax) in chloroform at around 629 and 643 nm, respectively, corresponding to the intramolecular charge-transfer (ICT) transition. There are also several absorption bands at shorter wavelengths which correspond to other higher energy transitions such as a π-π* transition. It is worth mentioning that upon replacing the dicyanovinyl with the tricyanovinyl group, there is a dramatic red-shift of the intramolecular charge transfer (ICT) band by ∼120 nm with a reduction in the optical bandgap of 0.42 eV (from 2.07 to 1.65 eV for n ) 2, and from 2.02 to 1.60 eV for n ) 3) measured in chloroform, indicating much stronger withdrawing strength of tricyanovinyl group. In addition, these donor-acceptor oligomers exhibit an inverted solvatochromic effect in which the absorption maximum initially red-shifts when changing from nonpolar solvent, toluene (i.e., PhN-OFOT(3)TCN: λabsmax ) 616 nm), to polar solvent, chloroform (λabsmax ) 643 nm), and then blue-shifts when changing to a more polar solvent, DMSO (λabsmax ) 613 nm) (Figure 1a and 1b). This behavior indicates that highly polar solvents can stabilize the ground state of these highly polar donor-acceptor oligomers to a certain extent which leads to a widening of the optical gap.9 Because of the very strong charge-transfer character of PhNOFOT(n)-TCN, there is no detectable fluorescence emission when measured in common organic solvents. Figure 1c shows the absorption spectra of PhN-OFOT(n)-TCN (n ) 2, 3) thin films (20 nm) vacuum-deposited on quartz slides which showed pronounced peak broadening and red-shifts of ∼25 nm of λabsmax as compared to those of the solution spectra. This observation is an indication for the planarization of π-conjugated aryl core and the presence of interchain interactions in the solid state through π-π stacking, which could enhance charge carrier mobilities favorable for applications in various optoelectronic devices including organic solar cells.10 The thermal properties of PhN-OFOT(n)-TCNs were analyzed by DSC and TGA. The two oligomers exhibit no distinct glass transition except a sharp

J. Phys. Chem. C, Vol. 112, No. 42, 2008 16719 melting transition at 236 and 249 °C, respectively. They also show a high thermal stability with decomposition temperatures >422 °C under nitrogen. (Table 1). Cyclic voltammetry (CV) was carried out in a three-electrode cell setup with 0.1 M of Bu4NPF6 as a supporting electrolyte in CH2Cl2 to examine the electrochemical properties of these molecules. All the potentials reported are referenced to Fc/Fc+ standard, and the results are tabulated in Table 1. PhNOFOT(n)-TCNs exhibit a low first either quasi-reversible or reversible one-electron cathodic wave with E1/2red1 at -0.91 V (Figure 2) which corresponds to the formation of the radical anion on the tricyanovinyl moiety. Such facile first reduction of tricyanovinyl moiety indicates a substantially lower LUMO level (∼3.90 eV) and a much stronger withdrawing nature of tricyanovinyl group as compared to those of dicyanovinyl group (∼3.30 eV). Following the first redox couple, there is an irreversible one-electron wave with Epred2 at -1.50 V for PhNOFOT(2)-TCN but an reversible one-electron wave with E1/ 2red2 at -1.59 V for PhN-OFOT(3)-TCN which correspond to the formation of radical dianion on the tricyanovinyl moiety.5b In addition, these oligomers also exhibit two reversible oneelectron anodic waves corresponding to the arylamine oxidation with E1/2oxd1 at 0.35-0.38 V and the oxidation of fluorenyldithiophene core with E1/2oxd2 at 0.69-0.89 V, respectively (Figure 2). It is worth mentioning that the presence of tricyanovinyl accepting group does not affect the anodic oxidation behavior of PhN-OFOT(2)-TCN to a great extent as compared to that of the symmetrically diphenylamino-substituted analogues (OF(2)DTP-NPh: E1/2oxd1 ) 0.32 V and E1/2oxd2 ) 0.62 V).6b,11 As a result, the incorporation of tricyanovinyl functionality to the triarylamine-based molecule does not reduce the high first ionization energy of this molecule (HOMO ) ∼5.20 eV), which is essential to achieve a large Voc of a PV device. To probe the potential of these hole-transporting lightabsorbing oligomers in photovoltaic cells, the bilayer heterojunction PV cells with device structure of ITO/PEDOTPSS/PhN-OFOT(n)-TCN (20 nm)/C60 (30 nm)/BCP (8 nm)/ Ag (60 nm) were fabricated by the spin-casting of PEDOTPSS layer and then vacuum deposition of the active layers, in which BCP (bathocuproine) was used as an excitonblocking layer (EBL) to ensure the realization of highefficiency small-molecule-based solar cells.12 Figure 3 shows current-voltage characteristics of the PV cells in the dark and under the AM 1.5 simulated solar illumination at irradiation intensity of 100 mW/cm2 before and after being annealed at 100 °C for 20 min. These PV cells gave a relatively large Voc in the range of 0.73-0.79 V in the asfabricated devices, which are attributed to the high oxidation potential of these oligomers. Importantly, the annealed devices exhibit dramatic enhancement in device performance (see Table 2) with enhanced Voc, 2-fold increase in Jsc and FF, and more than a 3-fold increase in PCE up to 1.33% (Figure 3). In contrast, performance enhancement after annealing was not observed in the CuPc-based standard device under our experimental conditions. It is important to find that thermal annealing of these oligothiophene thin films at 100 °C (a temperature well below their melting points) resulted in a reduced and slightly red-shift absorption as shown in Figure 4. This suggested that the significant enhancement in device performance should not be due to an increased absorption. A more detailed study of the dark I-V curves revealed that the device series resistance was greatly reduced after thermal annealing, suggesting high charge mobility and hence much better fill factors. AFM study

16720 J. Phys. Chem. C, Vol. 112, No. 42, 2008 clearly showed the formation of nanocrystalline structures in the PhN-OFOT(3)-TCN thin film after annealing at 100 °C for 20 min. As shown in Figure 5, after 20 min of annealing at 100 °C, the surface root-mean-square (rms) roughness of the PhN-OFOT(3)-TCN film increased from 0.21 to 7.88 nm. XRD analysis on the same sample further confirmed the formation of nanocrystallites in the annealed film, as evidenced by a detection of a diffraction peak at 2θ ) 4.4°. (Figure 6). This Bragg angle corresponds to a periodicity of about 2 nm, close to the molecular size of PhNOFOT(3)-TCN. This diffraction peak is not observed before annealing. Here, we would like to point out that the broad feature at 21° is from the quartz substrate. Since the diffraction peak at 4.4° is found both in ω/2θ and GIXRD (grazing incidence X-ray diffraction) scans, we concluded that the crystallites were randomly orientated in the film. In addition, this phenomenon was observed with the PhNOFOT(2)-TCN sample. The difference in the LUMO energy levels between PhNOFOT(n)-TCN (ca. 3.9 eV vs vacuum) and C60 (4.0 eV) is too small to offset the exciton binding energy, estimated to be around 0.4-0.5 eV.13 As a result, the exciton dissociation process may not be efficient, and this situation may be worse for the PhN-OFOT(3)-TCN-based device since it has a lower LUMO energy level. Therefore, the power conversion efficiency of the PhN-OFOT(3)-TCN-based device is even smaller than the PhN-OFOT(2)-TCN-based one. This result was consistent with a recent report that an excess thermal energy of the initially formed polaron pairs between electron donor and acceptor is required to overcome the Coulombic binding energy.14 Conclusions In conclusion, we have reported the synthesis and characterization of triarylamino and tricyanovinyl asymmetrically disubstituted oligothiophenes as a new class of holetransporting and low energy-gap chromophores for photovoltaic applications. With an incorporation of a tricyanovinyl accepting group to the triarylamine, the optical energy gap greatly reduces to 1.46 eV. The initial studies of the bilayer heterojunction PV cells based on the newly developed tricyanovinyl-substituted chromophores as a donor material and C60 as an acceptor material showed a PCE up to 1.33% in the annealed devices. Our findings also suggest that the tricyanovinyl group is highly efficient to lower the LUMO level and reduce the optical energy gap of a p-type semiconducting photosensitizer. More importantly, our observation confirmed that a large LUMO level offset is required to achieve efficient charge dissociation.

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