Synthesis and Photovoltaic Properties of Solution Processable Small ...

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Synthesis and Photovoltaic Properties of Solution Processable Small Molecules Containing 2-Pyran-4-ylidenemalononitrile and Oligothiophene Moieties Zaifang Li, Jianing Pei, Yaowen Li, Bin Xu, Meng Deng, Zhaoyang Liu, Hui Li, Hongguang Lu, Qiang Li, and Wenjing Tian* State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: May 11, 2010; ReVised Manuscript ReceiVed: September 17, 2010

A series of solution processable small molecules (4TPM, 6TPM, and 8TPM) were synthesized with 2-pyran4-ylidenemalononitrile (PM) as the electron-accepting unit and oligothiophene with different numbers as the electron-donating unit. Differential scanning calorimetry (DSC) measurement indicated that melting point and crystal temperature of molecules increased with the increase of thiophene number. UV-vis absorption demonstrated that the combination of PM with oligothiophene resulted in an enhanced intramolecular charge transfer (ICT) transition, which led to an extension of the absorption of the molecules. Cyclic voltammetry investigation displayed that the highest occupied molecular orbital (HOMO) energy levels of the three molecules were relatively low, which promised good air stability and high open circuit voltage (Voc) for photovoltaic application. Theoretical calculations revealed that the variation laws of HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels are well consistent with cyclic voltammetry measurement. The bulk heterojunction (BHJ) photovoltaic devices with the structure of ITO/PEDOT-PSS/small molecules-PCBM/ LiF/Al were fabricated, with the three molecules as donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. The device based on 6TPM/PCBM (30:70 w/w) successfully achieved a maximum power conversion efficiency (PCE) of 1.15% under the illumination of AM 1.5, 100 mW/cm2. Introduction Photovoltaic devices based on organic semiconductors are evolving into a promising cost-effective alternative to siliconbased solar cells due to their low-cost fabrication through solution processing, lightweight nature, as well as excellent compatibility with flexible substrates.1,2 Up to date, the highest power conversion efficiency (PCE) of the bulk heterojunction (BHJ) polymer photovoltaic devices has been up to 7.73%.3 Generally, polymers possess the following advantages: strong absorption ability, admirable solution processability, good filmforming ability, and tunable energy levels. However, the purification of polymers is still one of the most difficult problems. On the other hand, a polymer is often a mixture of molecules with different molecular weight. The impurity and higher molecular weight dispersity would significantly decrease the carrier mobility of polymers and then result in a relatively lower fill factor (FF) and PCE for photovoltaic applications.4,5 In contrast with polymers, solution processable small molecules are of high purity, well-defined molecular structures, and definite molecular weights. Until now, some profound progresses have been achieved in the synthesis of new solution processable small molecules and corresponding photovoltaic applications.6-16 In order to improve the matching between the solar spectrum and the absorption spectrum of organic conjugated molecules, molecules of donor-acceptor (D-A) structure were specifically designed and synthesized.7,8,14-16 The intramolecular charge transfer (ICT) from the electron-donating unit to the electronaccepting unit inside D-A molecules can efficiently extend the absorption spectrum of molecules toward a good matching with the solar spectrum. Moreover, the incorporation of varied * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +86-431-85193421.

electron-donating and electron-accepting units can adjust the ICT strength of the D-A molecules, and thus, the optical band gap, electrochemical properties, and energy levels would be finetuned. Through this method, organic conjugated molecules could be achieved with low band gap, good air stability, and high open circuit voltage (Voc).21 In 2009, Walker et al.successfully fabricated a bulk-heterojunction photovoltaic device demonstrating the highest PCE of 4.4%. For this device, DPP-OT-3,6bis(5-(benzofuran-2-yl) thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione(DPP(TBFu)2),aD-Asmallmolecule, acted as the donor; meanwhile, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) acted as the acceptor.16 It is well-known that 2-pyran-4-ylidenemalononitrile (PM) is a strong electron-withdrawing unit, which can increase electron affinity and extend the absorption spectrum when combined with strong electron-donating units. In recent years, small molecules consisting of a PM unit have been synthesized and applied for photovoltaic applications. However, these small molecules (such as DADP7 and TPA-DCM-TPA8) could only harvest photons of wavelength below 650 nm. The corresponding photon energy among this range is relatively small compared with the whole solar spectrum. The narrow absorption band (covering from 350 to 650 nm) was mainly because of the weak intramolecular charge transfer from the electron-donating unit (triphenylamine) to the electron-accepting units (PM or DCM). Furthermore, the triphenylamine unit with three-dimensional structure also decreases the intermolecular stacking and thus leads to a narrow absorption band. Compared with the triphenylamine unit, oligothiophene possesses stronger ICT strength and intermolecular interactions because of its stronger electrondonating ability and better coplanarity, when combined with a PM unit. This could efficiently extend the absorption spectrum of the molecule. Furthermore, thiophene-based small molecules

10.1021/jp1042848  2010 American Chemical Society Published on Web 10/05/2010

Properties of Solution Processable Small Molecules also exhibit high carrier mobilities, good air stability, and excellent film-forming properties. All of these advantages are beneficial for photovoltaic applications.14,15,17,25,26 Herein, we synthesized a series of solution processable small molecules (4TPM, 6TPM, 8TPM) consisting of a PM unit and oligothiophenes (corresponding molecular structures are shown in Scheme 2). The symmetrical combination of a PM unit with alkylated thiophenevinyl assured longer conjugated length and better coplanarity, which would enhance the intermolecular interactions and further reduce the band gap of the molecules.18,19,27 The introduction of different oligothiophene units would lead to different ICT strength and thus different absorption band gaps.28,29 In addition, we also fabricated BHJ photovoltaic devices using small molecules as the donor and PC61BM as the acceptor. Under the illumination of AM 1.5, 100 mW/cm2, measurements show that our device based on 6TPM/PCBM (3:7 w/w) has the PCE of 1.15%, Jsc of 4.27 mA/cm2, Voc ) 0.80 V, and FF ) 0.30. These results indicated that molecules consisting of PM and oligothiophene units are promising photovoltaic materials. Experimental Section Materials. All reagents and chemicals were purchased from commercial sources (Aldrich, Across, Fluka) and used without further purification unless stated otherwise. All solvents were distilled over appropriate drying agents prior to use and were purged with nitrogen. Compounds 4, 7, and 9 were synthesized according to literature procedure.20-22 Synthesis of Small Molecules. Synthesis of 4TPM. Compound 5 (500 mg, 0.58 mmol), compound 6 (295 mg, 1.40 mmol), and 20 mg of (PPh3)4Pd(0) (2 mol % with respect to the compound 5) were dissolved in a mixture of toluene (6 mL) and 2 M K2CO3 aqueous (4 mL, 3/2 volume ratio). The solution was stirred under N2 atmosphere and refluxed with vigorous stirring at 85 °C for 48 h. The resulting solution was then poured into water and was extracted with dichloromethane. The extract was washed with brine and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified with column chromatography on silica gel with dichloromethane/ petroleumether (1:1) as the eluant to a gold yellow solid with a yield of 367 mg (0.43 mmol, 75.0%). 1H NMR (300 MHz, CDCl3, TMS): δ(ppm) 7.614 (d, 2H, J ) 15.6 Hz, -vinylic), 7.388 (d, 2H, -Th), 7.225 (d, 2H, -Th), 7.109 (t, 2H, -Th), 6.614 (s, 2H, -PM), 6.474 (d, 2H, J ) 15.3 Hz, -vinylic), 2.716 (t, 8H, -CH2), 1.545 (m, 8H, -CH2), 1.316 (m, 16H, -CH2), 1.257 (m, 8H, -CH2), 0.879 (m, 12H, -CH3). 13C NMR (75 MHz, CDCl3, TMS): δ(ppm) 158.090, 155.319, 146.964, 140.278, 135.606, 134.272, 133.099, 128.402, 127.627, 125.708, 125.528, 125.403, 116.344, 115.450, 105.519, 58.902, 31.707, 31.611, 31.484, 30.446, 29.502, 29.448, 27.870, 27.445, 22.593, 14.020, 13.969. MALDI-TOF MS: calcd for C52H64N2OS4 861.34; found 861.40. Synthesis of 6TPM. Compound 5 (500 mg, 0.58 mmol), compound 8 (638 mg, 1.40 mmol), and 20 mg of (PPh3)4Pd(0) (2 mol % with respect to the compound 5) were dissolved in a mixture of toluene (8 mL) and DMF (2 mL, 4/1 volume ratio). The solution was stirred under Ar atmosphere and refluxed with vigorous stirring at 120 °C for 24 h. The resulting solution was then poured into water and was extracted with dichloromethane. The extract was washed with brine and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified with column chromatography on silica gel with dichloromethane/petroleumether (1:1) as the eluant to a dark green shining powder with a yield of 80.3%. 1H NMR (500

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18271 MHz, CDCl3, TMS): δ(ppm) 7.608 (d, 2H, J ) 15.6 Hz, -vinylic), 7.269 (s, 2H, -Th), 7.224 (d, 2H, -Th), 7.167 (d, 2H, -Th), 7.136 (d, 2H, -Th), 7.054 (m, 2H, -Th), 6.618 (s, 2H, -PM), 6.473 (d, 2H, J ) 15.3 Hz, -vinylic), 2.730 (m, 8H, -CH2), 1.573 (m, 8H, -CH2), 1.445 (m, 8H, -CH2), 1.332 (m, 16H, -CH2), 0.921 (m, 6H, -CH3), 0.859 (m, 6H, -CH3). 13C NMR (75 MHz, CDCl3, TMS): δ(ppm) 140.273, 138.264, 136.727, 134.399, 134.067, 133.136, 128.196, 127.946, 124.904, 124.590, 124.101, 123.966, 116.411, 115.416, 106.503, 58.901, 31.675, 31.610, 31.460, 30.334, 29.500, 29.499, 27.965, 27.721, 22.619, 22.585, 14.015, 13.969. MALDI-TOF MS: calcd for C60H68N2OS6 1025.58; found 1025.70. Synthesis of 8TPM. Compound 5 (500 mg, 0.58 mmol), compound 10 (577 mg, 1.4 mmol), and 20 mg of (PPh3)4Pd(0) (2 mol % with respect to the compound 5) were dissolved in a mixture of toluene (8 mL) and DMF (2 mL, 4/1 volume ratio). The solution was stirred under Ar atmosphere and refluxed with vigorous stirring at 120 °C for 24 h. The resulting solution was then poured into water and was extracted with chloroform. The extract was washed with brine and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified with column chromatography on silica gel with dichloromethane/petroleumether (1:1) as the eluant to a black shining powder with a yield of 72.6%. 1H NMR (500 MHz, CDCl3, TMS): δ(ppm) 7.601 (d, 2H, J ) 15.6 Hz, -vinylic), 7.252 (m, 2H, -Th), 7.202(m, 2H, -Th), 7.134 (m, 8H, -Th), 7.042 (m, 2H, -Th), 6.605 (s, 2H, -PM), 6.460 (d, 2H, J ) 15.3 Hz, -vinylic), 2.733 (m, 8H, -CH2), 1.587 (m, 8H, -CH2), 1.460 (m, 8H, -CH2), 1.323 (m, 16H, -CH2), 0.927 (m, 6H, -CH3), 0.864 (m, 6H, -CH3).13C NMR (75 MHz, CDCl3, TMS): δ(ppm) 158.021, 155.199, 147.211, 147.119, 140.325, 137.928, 136.925, 136.888, 135.445, 134.542, 134.048, 133.028, 128.167, 127.923, 127.294, 124.717, 124.571, 124.419, 124.035, 123.883, 116.482, 106.558, 58.964, 31.696, 31.638, 31.493, 30.334, 29.688, 29.537, 29.482, 28.033, 27.746, 22646, 14.050, 13.992. MALDI-TOF MS: calcd for C68H72N2OS8 1189.83; found 1189.20. Instruments and Measurements. Differential scanning calorimetry (DSC) was performed under nitrogen flushing at a heating rate of 20 °C/min with a NETZSCH (DSC-204) instrument. 1H NMR and 13C NMR spectra were measured using a Bruker AVANCE-500 NMR spectrometer and a Varian Mercury-300 NMR, respectively. The time-of-flight mass spectra were recorded with a Kratos MALDI-TOF mass system. UV-visible absorption spectra were measured using a Shimadzu UV-3100 spectrophotometer. The photoluminescenece spectra of spin-cast films and solution were measured with a RF-5301PC spectrofluorophotometer. Electrochemical measurements of these derivatives were performed with a Bioanalytical Systems BAS 100 B/W electrochemical workstation. Atomic force microscopy (AFM) images of blend films were carried out using a Nanoscope IIIa Dimension 3100. Fabrication and Characterization of PhotoWoltaic DeWices. For device fabrication, the ITO glass was precleaned and modified by a thin layer of PEDOT/PSS, which was spincast from a PEDOT/PSS aqueous solution (H. C. Starck) on the ITO substrate, and the thickness of the PEDOT/PSS layer is about 50 nm. The active layer contained a blend of small molecules as the electron donor and PCBM as the electron acceptor, which was prepared from a chloroform solution with the optimized weight ratios (50:50, 30:70, and 50:50) for 4TPM/PCBM, 6TPM/PCBM, and 8TPM/PCBM, respectively. After spin-coating the blend from solution at 2500 rpm, the devices were completed by evaporating a 0.6 nm

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SCHEME 1: Synthetic Routes of Compounds

LiF layer protected by 100 nm of Al at a base pressure of 5 × 10-4 Pa. The effective photovoltaic area as defined by the geometrical overlap between the bottom ITO electrode and the top cathode was 4 or 5 mm2. The thickness of the photoactive layers were 80, 130, and 80 nm, measured by the Ambios Technology XP-2. The current-voltage (J-V) characterization of PV devices in the dark and under whitelight illumination from a SCIENCETECH 500 W solar simulator (AM 1.5 100 mW/cm2) was measured on a computer-controlled Keithley 2400 Source Meter measurement system. All the measurements were performed under ambient atmosphere at room temperature. Results and Discussion Material Synthesis and Structural Characterization. The general synthetic routes toward all compounds are outlined in Scheme 1. In the first step, 3,4-dihexylthiophene (compound 1) was synthesized from 3,4-dibromothiophene by nickel-catalyzed cross-coupling with bromohexylmagnesium reagent.19 Bromination of compound 1 by N-bromosuccinimide (NBS) gave compound 2 in high yield. Critical to the synthetic strategy was the selective halogen-metal exchange followed by conversion to the aldehyde. The monoaldehyde of 2,5-dibromo-3,4-dihexylthiophene (compound 3) was prepared by a modified procedure with 1.1 equiv of n-Buli followed by DMF. Compound 5 was prepared through Knoevenagel condensation of 2-(2,6-dimethypyran-4-yliden-

e)malomonitrile (compound 4) with compound 3. The structure of compound 5 was confirmed by 1H NMR and 13C NMR. In 1H NMR spectroscopy of compound 5, the coupling constant (J ) 15.5 Hz) of olefinic protons indicates that the Knoevenagel reaction afforded the pure all-trans isomers. Compounds 6, 8, and 10 were prepared by a modified procedure with 1.1 equiv of n-Buli followed by 2-isopropyl4,4,5,5-tetramethyl-1,3,2-dioxaborolane, tributylchlorostannane, and trimethylchlorostannane, respectively. The synthetic routes and structures of the final small molecules are shown in Scheme 2. The final products were prepared by the wellknown palladium-catalyzed Suzuki and Still coupling reaction between varied compounds (6, 8, and 10) and the functionalized dibromo aromatic compound (5).231H NMR and 13C NMR spectroscopy were used to characterize the structure of these compounds, which clearly indicated well-defined 4TPM, 6TPM, and 8TPM have been obtained. The two legible double peaks that appear at ∼6.400 and 7.600 ppm with the coupling constant (J ∼ 16 Hz) are due to the alltrans double bond, which further confirms the regular structure. All the small molecules exhibited excellent solubility in common organic solvents such as chloroform, tetrahydrofuran, dichloromethane, and chlorobenzene. Molecular weights were determined by Kratos MALDI-TOF. Thermal Properties. Figure 1 shows the DSC curves of the small molecules. When the small molecules were heated, the

Properties of Solution Processable Small Molecules

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SCHEME 2: Synthetic Routes and Structures of Small Molecules

endothermic peaks due to melting were observed at 159, 175, and 191 °C, respectively. Cooling scans exhibited oppositely exothermic peaks due to crystallization at 135, 144, and 153 °C, respectively. Apparently, both the melting point and crystal temperature increased with the increase of the thiophene numbers due to the incremental molecular weight. Optical Properties. The normalized UV-vis absorption spectra of 4TPM, 6TPM, and 8TPM in dilute chloroform solution (concentration 10-5 M) are shown in Figure 2a, and the main optical properties are listed in Table 1. 4TPM with the weak electron-donating unit showed three absorption

Figure 1. Differential scanning calorimetry (DSC) measurements of 4TPM, 6TPM, and 8TPM with a scan rate of 10 °C min-1.

Figure 2. Normalized absorption spectra of the small molecules (a) in chloroform solutions with the concentration of 10-5 mol/L; (b) films spin-coated from a 10 mg/mL chloroform solution.

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TABLE 1: Optical and Electrochemical Data of 4TPM, 6TPM, and 8TPM in solutiona

in filmb

molecule

λmax abs[nm] (εmax[M-1 cm-1])

λedge [nm]

λmax abs [nm]

λedge [nm]

4TPM 6TPM 8TPM

477 (57300) 504 (75640) 517 (51450)

564 604 624

492 560 550

619 658 681

onset (V)/ EOx HOMO [eV]

onset ERed (V)/ LUMO [eV]

electrochem Eg,ec [eV]

opticalc Eg,opt [eV]c

0.90/-5.59 0.75/-5.44 0.62/-5.31

-1.21/-3.48 -1.25/-3.44 -1.19/-3.50

2.11 2.00 1.81

2.00 1.88 1.82

a 1 × 10-5 M in anhydrous chloroform. b Spin-coated from a 10 mg/mL chloroform solution. c The optical band gap (Eg,opt) was obtained from the absorption edge.

bands at 310, 382, and 477 nm, which can be assigned to the intrinsic absorption of the thiophene unit, π-π* transition of the molecules backbone, and ICT transition between oligothiophene and PM. Similarly, the absorption spectra of 6TPM and 8TPM in dilute solutions showed also three bands near 310, 400, and 510 nm due to the intrinsic absorption of the thiophene unit, π-π* transition, and the ICT transition. The solution absorption spectrum of 8TPM, with an absorption maximum (λmax) at 517 nm and the absorption edge (λedge) at 624 nm, is broadened compared with those of 4TPM (λmax ) 477 nm, λedge ) 564 nm) and 6TPM (λmax ) 504 nm, λedge ) 604 nm), which can be explained by much stronger ICT degree in 8TPM than that in 4TPM and 6TPM. Among the three small molecules, there is a D-A-D structure, where D is the electron-donating unit and A is the electron-accepting unit. The stronger electron-donating ability D possesses, the higher electronic delocalization degree and the stronger ICT degree the small molecule has. Since the order of the electrondonating abilities of the three D is quaterthiophene > terthiophene > bithiophene, the strongest electron-donating ability of quaterthiophene compared to bithiophene and terthiophene improves the effective conjugation length along the molecule backbone, resulting in an increase in the ICT degree and electronic delocalization.24 Moreover, the relatively high absorption coefficients could be calculated from the Beer’s law equation with the same dilute concentration of the molecules in chloroform (absorption coefficients are listed in Table 1). Figure 2b shows the optical absorption spectra of thin films of these small molecules. Compared to their absorption spectra in solution, all small molecules exhibited a pronounced broadening and red-shift absorption spectra. It should be noted that the absorption peak of 6TPM in thin film exhibits a 56 nm red shift and 8TPM shows a 33 nm red shift while 4TPM only shows a 15 nm red shift, which is attributed to the better coplanarity and the stronger intermolecular interactions in the solid state.23 The optical band gap (Eg,opt) of these small molecules derived from the absorption edge of the thin film spectra is in the range of 2.00-1.82 eV (Table 2). As expected, 8TPM with the strongest intramolecular charge transfer interaction has the lowest optical band gap of 1.82 eV, which is lower than that of 4TPM (2.00 eV) and 6TPM (1.88 eV). Electrochemical Properties. Figure 3 shows the cyclic voltammetry (CV) diagrams of the small molecules using TBAPF6 as supporting electrolyte in methylene dichloride solution with platinum button working electrodes, a platinum wire counter electrode, and an Ag/AgNO3 reference electrode under N2 atmosphere. Ferrocene was used as the internal standard. The redox potential of Fc/Fc+, which has an absolute energy level of -4.8 eV relative to the vacuum level for calibration, is located at 0.11 V in 0.1 M TBAPF6/ methylene dichloride solution.30 So the evaluation of the

TABLE 2: Characteristic Current-Voltage Parameters from Device Testing at Standard AM 1.5G Conditions and Blend Films Roughness of AFM Measurement small small molecule/ Jsc [mA/ molecule PCBM (w/w ratio) Voc [v]a cm2]a 4TPM 4TPM 4TPM 6TPM 6TPM 6TPM 8TPM 8TPM 8TPM

70:30 50:50 30:70 70:30 50:50 30:70 70:30 50:50 30:70

0.70 0.78 0.70 0.96 0.90 0.90 0.78 0.76 0.76

0.08 0.18 0.12 2.64 3.93 4.27 2.82 4.08 3.46

FFa

rms [nm]b

PCE [%]a

0.28 0.23 0.25 0.30 0.30 0.30 0.30 0.31 0.31

0.369 0.419 1.510 -

0.016 0.03 0.02 0.76 1.06 1.15 0.66 0.97 0.81

a Photovoltaic properties of small molecule/PCBM-based devices spin-coated from a chloroform solution for 4TPM/PCBM (70:30 w/w, 50:50 w/w, and 30:70 w/w), 6TPM (70:30 w/w, 50:50 w/w, and 30:70 w/w), and 8TPM (70:30 w/w, 50:50 w/w, and 30:70 w/w). b Root mean-square (rms) roughness from AFM measurement.

Figure 3. Cyclic volatammetry curves of 4TPM, 6TPM, and 8TPM solutions on platinum electrode in 0.1 mol/L n-Bu4NPF6 in CH2Cl2 solution, at a scan rate of 100 mV/s.

highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels as well as the electrochemical band gap (Eg, ec) could be done according to the following equations: onset HOMO(eV) ) -e(EOx + 4.69)(eV)

onset LUMO(eV) ) -e(ERed + 4.69)(eV)

Eg,ec ) -(HOMO - LUMO)(eV) onset onset and ERed are the measured potentials relative to where EOx + Ag/Ag . The results of the electrochemical measurements and calculated energy levels of the small molecules are listed in Table 1.


Figure 4. Band diagram for acceptor PCBM and donors 4TPM, 6TPM, and 8TPM. Dashed lines indicate the thresholds for air stability (5.2 eV) and effective charge transfer PCBM (4.0 eV).

Table 1 shows the calculated HOMO energy level (-5.59 eV) and LUMO energy level (-3.48 eV) of 4TPM. The LUMO energy levels of 6TPM and 8TPM are -3.44 and -3.50 eV, which are very similar to that of 4TPM and the reported PMcontaining small molecules.7,8 Therefore, the substitution of varied thiophene units with different electron-donating ability has almost no effect on the reduction potential of the small molecules, and the relatively low LUMO energy levels of the three small molecules should result from the stronger reduction of PM-based acceptor unit. It is clear that the HOMO levels of 4TPM (-5.59 eV), 6TPM (-5.44 eV), and 8TPM (-5.31 eV) are gradually increased with the enhancement of electron-donating abilities of quaterthiophene > terthiophene > bithiophene. We know that the HOMO energy level of an electron-donor material is very important for high performance photovoltaic device. First, an electron-donor material should have good air stability with HOMO energy level being below the air oxidation threshold (ca. -5.2 eV).32 Second, the open circuit potential (Voc) value for the photovoltaic device is determined by the difference between the HOMO energy level of the donor and LUMO energy level of the acceptor; the relatively low HOMO level of an electron-donor material can allow a high open circuit potential (Voc) value for the photovoltaic device.29 The energy band structure of the three small molecules and PCBM is presented in Figure 4. The first dashed line indicates the threshold for air stability, and the second dashed line represents the threshold value for an effective charge transfer from the small molecules to PCBM (-4.3 eV).30 Both the HOMO energy levels and LUMO energy levels of the small molecules are in the ideal range. Furthermore, the electrochemical band gap Eg,ec (listed in Table 1) is in good agreement with the optical band gap Eg,opt for the three small molecules. Theoretical Calculation. The geometry and electronic properties of 4TPM, 6TPM, and 8TPM have been investigated by means of theoretical calculation with the Gaussian 03 program package at a hybrid density functional theory (DFT) level.39 Figure 5 presents the geometry and the HOMO and the LUMO. H atoms were used in place of the n-hexyl groups to limit computation time. Electron density of the HOMO distributes not only on the thiophene-donor moieties but also on the PM-acceptor unit, while that of the LUMO mainly delocalizes on the PM-acceptor unit and the adjacent thiophene unit, indicating a charge-transfer nature of HOMOfLUMO from the electron-donating unit of the oligothiophene to the PM-acceptor unit. From Figure 5, we can see that, with an increase in the number of thiophene units, the HOMO energy level increased

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Figure 5. Molecular orbital surfaces of the HOMO and LUMO of 4TPM, 6TPM, and 8TPM obtained at B3 LYP/6-31G* level.

Figure 6. Current-voltage characteristics of small molecule photovoltaic cells based on 4TPM, 6TPM, and 8TPM in the dark and under illumination of AM 1.5, 100 mW/cm2. (The inserted figure shows the corresponding logarithmic J-V characteristics in the dark.)

from -5.47 to -5.31 eV, -5.17 eV for 4TPM, 6TPM, and 8TPM, respectively, whereas the LUMO energy level remained relatively unchanged due to its highly localized state around the PM segment. Meanwhile, the HOMO and LUMO energy level variation laws from molecular orbital distribution calculations are well consistent with those from electrochemical tests. Photovoltaic Performance. To demonstrate the potential of the small molecules as electron donors in an organic solar cell, photovoltaic devices with a structure of indium tin oxide ITO/ PEDOT-PSS/small molecules-PCBM/LiF/Al were fabricated by spin-coating from chloroform solutions at a constant concentration of 20 mg/mL comprising a mixture of a small molecule and PCBM in varied blend ratios and tested under simulated 100 mW cm-2 AM 1.5G illumination. The optimized weight ratios of molecules to PCBM for 4TPM, 6TPM, and 8TPM are 50:50, 30:70, and 50:50, respectively. Device current density/voltage (J-V) optimized characteristics are shown in Figure 6, and the parameters of devices with different blend ratios listed in Table 2. From the J-V curves, a significant increase in short-circuit current (Jsc) is clearly observed from 4TPM to 6TPM. A Voc as high as 0.90 V was observed in devices based on 6TPM. Combined with its high Jsc of 4.27 mA/cm2 and fill factor (FF) of 0.30, a highest PCE of 1.15% was achieved in the 6TPM system. Otherwise, the photovoltaic device based on 4TPM/PCBM exhibits a Voc of 0.78 V, a Jsc of 0.18 mA/cm2, a FF of 0.23, and a PCE of 0.03%; a Voc of 0.8 V, Jsc of 4.08 mA/cm2, FF of 0.30, and a PCE of 0.97% for the device based on 8TPM/PCBM is observed. The low Jsc of the device based on 4TPM/PCBM may be caused by the weak ICT interaction inside 4TPM and the weak intermolecular interactions in the solid state, which lead to the

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Figure 7. Absorption spectra of the blend films containing small molecules and PCBM spin-coated on ITO substrate from chloroform solutions at 2500 rpm. The thickness of the active layer is about 80, 130, and 80 nm for 4TPM/PCBM, 6TPM/PCBM, and 8TPM/PCBM, respectively.

Figure 8. J-V curve in the dark of an ITO/PEDOT/small molecule/ Au device in log axis for estimating the hole mobility of 4TPM, 6TPM, and 8TPM. The thicknesses of the small molecules are around 60, 80, and 100 nm, respectively. The solid line from 0.01 to 0.50 V means log J is fitted linearly dependent on log V with a slope of 1. For the solid line from 0.5 to 1.5 V, log J is fitted linearly dependent on log V with a slope of 2 (SCLC area).

low absorption of the solar spectrum. Compared with 4TPM, the ICT inside 6TPM and 8TPM enhanced because of the strong electron-donating ability of terthiophene and quaterthiophene, and the intermolecular interactions in the solid state become strong because of the better coplanarity, which caused a large red shift of the absorption spectra of 6TPM and 8TPM.

Li et al. Furthermore, we measured the absorption spectra of the active layers with the same thickness as used in devices (Figure 7). From the spectra, it can be seen that, although the absorption intensity of 4TPM/PCBM (50:50) is stronger than that of 6TPM/ PCBM (30:70) and 8TPM/PCBM (50:50), the absorption range is narrower. The absorption intensity of 6TPM/PCBM (30:70) is a little stronger than 8TPM/PCBM (50:50) while the absorption range is almost the same, which could be one of the reasons for a little higher Jsc of 6TPM/PCBM (30:70) than that of 8TPM/PCBM (50:50). Considering that charge carrier mobility of photovoltaic materials is one of the important factors that influences the Jsc of photovoltaic devices,38 we measured the hole mobility of the three small molecules by space charge limited current (SCLC) method.31 The hole only devices with a structure of ITO/ PEDOT-PSS/small molecule/Au were fabricated and their current-voltage curves in the dark are shown in Figure 8. The hole mobilities were measured to be 3.254 × 10-6 cm2 v-1s-1, 5.862 × 10-5 cm2 v-1s-1, and 6.193 × 10-5 cm2 v-1s-1 for 4TPM, 6TPM, and 8TPM, respectively. Obviously, the hole mobility of 4TPM is more than one order lower than that of 6TPM and 8TPM, which is consistent with the relatively low Jsc of the photovoltaic device based on 4TPM/PCBM (0.18 mA/ cm2). Furthermore, the similar Jsc obtained from the 6TPM/ PCBM device (4.27 mA/cm2) and the 8TPM/PCBM device (4.08 mA/cm2) may be caused by the nearly same hole mobilities of 6TPM and 8TPM. To gain further insight into what might affect the Jsc of the photovoltaic device, we analyzed the morphology of 4TPM/ PCBM, 6TPM/PCBM, and 8TPM/PCBM blend films. Figure 9 shows the AFM height images of 4TPM/PCBM, 6TPM/ PCBM, and 8TPM/PCBM blend films with the weight ratio 50: 50, 30:70, and 50:50, respectively. It is clearly evidenced by AFM that the 4TPM/PCBM, 6TPM/PCBM, and 8TPM/PCBM blend films exhibit relatively flat surfaces with root-mean-square (rms) values of 0.369, 0.419, and 1.510 nm, respectively (Figure 7 and Table 2). The high Jsc obtained from the 6TPM/PCBM device (4.27 mA/cm2) and the 8TPM/PCBM device (4.08 mA/ cm2) may be attributed to the significant phase separation with the formation of large PCBM-rich domains appears. The PCBMrich domains could improve the charge transportation and carrier collection effectively, which results in a decrease of recombination loses and an increase of Jsc.35,37 Contrarily, the relatively low Jsc (0.18 mA/cm2) obtained from the 4TPM/PCBM device could be caused by the smooth surface with the root-meansquare (rms) of 0.369 nm and the formation of more homogeneous morphology without obvious PCBM-rich domains in the

Figure 9. Topography image (size 2 µm × 2 µm) obtained by tapping-mode AFM showing the morphology of the blend films spin-coated from chloroform for 4TPM, 6TPM, and 8TPM. (a) 4TPM/PCBM (50:50 w/w); (b) 6TPM/PCBM (30:70 w/w); (c) 8TPM/PCBM (50:50 w/w).


J. Phys. Chem. C, Vol. 114, No. 42, 2010 18277 has the shortest chain in molecular structure and a largest bandgap compared with 6TPM and 8TPM. The results indicate that it is an effective way to improve PCE by adjusting the numbers of oligothiophene and the molecules containing PM and the oligothiophene unit are promising donor materials for photovoltaic device. Conclusions

Figure 10. External quantum efficiency (EQE) curves for devices using 50:50, 30:70, and 50:50 blend of 4TPM/PCBM, 6TPM/PCBM, and 8TPM/PCBM, respectively.

4TPM/PCBM blend film, which increases the recombination loses of excitons. Figure 10 shows the external quantum efficiencies (EQE) spectra of devices based on 4TPM/PCBM, 6TPM/PCBM, and 8TPM/PCBM. It can be seen that the response range of photovoltaic devices based on 6TPM/PCBM and 8TPM/PCBM were much broader and stronger than that of devices based on 4TPM/PCBM. So the devices based on 6TPM/PCBM and 8TPM/PCBM show much higher Jsc than the device based on 4TPM/PCBM. Besides, in the range from 500 to 700 nm, the device based on 6TPM/PCBM exhibits better EQE than the device based on 8TPM/PCBM, which is also consistent with the higher Jsc of the photovoltaic device based on 6TPM/PCBM than that of 8TPM/PCBM. When comparing the device based on 8TPM/PCBM, the higher Voc of the device based on 6TPM/PCBM could be explained by the lower lying HOMO energy level of 6TPM (-5.44 eV) than that of 8TPM (-5.31 eV), because Voc is related to the difference between the LUMO energy level of the acceptor and the HOMO energy level of the donor within the active layer. However, it is strange that the device based on 4TPM exhibits the lowest Voc among the three devices though the HOMO energy level of 4TPM (-5.59 eV) was the lowest among the three small molecules. Therefore, there must be some other factors affecting the Voc of these devices. An analysis of the properties of photovoltaic device using equivalent-circuit methods by Kippelen and others33,34 reveals that Voc is proportional to the logarithm of the ratio of the photocurrent density Jsc. Hence, the lower Voc of 4TPM should be caused by its relatively lower Jsc (0.18 mA/cm2) compared with that of 6TPM (4.27 mA/cm2) and 8TPM (4.08 mA/cm2). As for FF, the device based on 6TPM/PCBM and 8TPM/ PCBM exhibited higher FF than the device based on 4TPM/ PCBM. The FF of the photovoltaic devices is strongly influenced by the mobility of the photovoltaic materials.36 The higher hole mobility of 6TPM and 8TPM might contribute to a more balanced charge transport in the 6TPM/PCBM or 8TPM/PCBM composite layer and more efficient carrier collection at the interface between the active layer and the respective electrode, which is favorable for achieving high FF.35,36 As for PCE, the photovoltaic device based on 4TPM/PCBM exhibited an extremely low PCE of 0.03% while the remarkably improving PCE of 1.15 and 0.97% were obtained from the photovoltaic devices based on 6TPM/PCBM and 8TPM/PCBM. This could be mainly caused by its low hole mobility and weak optical absorption of 4TPM in the solar spectrum, because it

We have designed and synthesized three novel D-A conjugated small molecules consisting of PM as an acceptor coupled to oligothiphene with different thiophene units. DSC measurement indicated that these new small molecules exhibited high melting point and crystallinity. Optical property investigations clearly indicated that these new small molecules exhibited strong π-π stacking and long wavelength ICT absorption bands. Both the electrochemical properties and molecular orbital distribution calculations revealed that the HOMO and LUMO energy levels of the resulting small molecules can be fine-tuned by increasing the conjugated length of oligothiophene. The photovoltaic devices based on the small molecules show the PCE in the range of 0.03-1.15%, which indicate that it is an effective way to improve the PCE of photovoltaic devices by adjusting the electron-donating abilities for the type of D-A molecules. Acknowledgment. This work was supported by the State Key Development Program for Basic Research of China (Grant No. 2009CB623605), the National Natural Science Foundation of China (Grant No. 20874035), Program for New Century Excellent Talents in Universities of China Ministry of Education, the 111 Project (Grant No. B06009), and the Project of Jilin Province (20080305). Supporting Information Available: Experimental synthesis details of compounds 1, 2, 3, 5, 6, 8, and 10, photoluminescence spectra of the small molecules in chloroform solution and film, and current-voltage characteristics of small molecule photovoltaic cells based on 4TPM, 6TPM, and 8TPM under light and dark conditions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324–1338. (b) Bungaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954–985. (2) Brabec, C. J. Sol. Energy Mater. Sol. Cells 2004, 83, 273–292. (3) Chen, H.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G Nat. Photonics 2009, 3, 649–653. (4) Salzman, R. F.; Xue, J. G.; Rand, B. P.; Alexander, A.; Thompson, M. E.; Forrest, S. R. Org. Electron. 2005, 6, 242–246. (5) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Frechet, J. M. J. Appl. Phys. Lett. 2005, 86, 122110. (6) Roncali, J.; Leriche, P.; Cravino, A. AdV. Mater. 2007, 19, 2045– 2060. (7) He, C.; He, Q. G.; Yang, X. D.; Wu, G. L.; Yang, C. H.; Bai, F. L.; Shuai, Z. G.; Wang, L. X.; Li, Y. F. J. Phys. Chem. C 2007, 111, 8661–8666. (8) Xue, L. L.; He, J. T.; Gu, X.; Yang, Z. F.; Xu, B.; Tian, W. J. J. Phys. Chem. C 2009, 113, 12911–12917. (9) He, C.; He, Q. G.; Yi, Y. P.; Wu, G. L.; Yang, C.; Bai, F. L.; Shuai, Z. G.; Li, Y. F. J. Mater. Chem. 2008, 18, 4085–4090. (10) Kroneneberg, N. M.; Deppish, M.; Wu¨rthner, F.; Ledemann, H. W. A.; Deing, K.; Meerholz, K. Chem. Commun. 2008, 6489–6491. (11) Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.; Roncali, J. Chem. Commun. 2009, 1673–1675. (12) Valentini, L.; Bagnis, D.; Marrocchi, A.; Seri, M.; Taticchi, A.; Kenny, J. M. Chem. Mater. 2008, 20, 32–34. (13) Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 17640–17641. (14) Tamayo, A. B.; Walker, B.; Nguyen, T. Q. J. Phys. Chem. C 2008, 112, 11545–11551.

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