New amorphous small molecules—Synthesis, characterization and their application in bulk heterojunction solar cells

Solar Energy Materials & Solar Cells 95 (2011) 2272–2280

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

New amorphous small molecules—Synthesis, characterization and their application in bulk heterojunction solar cells Zaifang Li, Qingfeng Dong, Bin Xu, Hui Li, Shanpeng Wen, Jianing Pei, Shiyu Yao, Hongguang Lu, Pengfei Li, Wenjing Tian n State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 January 2011 Received in revised form 17 March 2011 Accepted 22 March 2011 Available online 14 April 2011

We successfully synthesized a series of novel solution processible small molecules (2TAPM, 4TAPM and 2BTAPM) consisting of electron-accepting unit (2-pyran-4-ylidenemalononitrile) (PM) and electrondonating unit (Triphenylamine and different thiophene units). Differential scanning calorimetry (DSC) measurement indicates that these small molecules are amorphous. UV–vis absorption spectra show that the combination of PM with moieties having gradually increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT) transition, leading to an extension of the absorption spectral range and a reduction of the band gap of the molecules. Both cyclic voltammetry measurement and theoretical calculations show that the highest occupied molecular orbital (HOMO) energy levels of the molecules could be fine-tuned by changing the electron-donating ability of the electron-donating moieties. The bulk heterojunction (BHJ) photovoltaic devices with a structure of ITO/PEDOT:PSS/small molecules:PC71BM/LiF/Al were fabricated by using the small molecules as donors and (6,6)-phenyl C71-butyric acid methyl ester (PC71BM) as acceptor. Power conversion efficiencies of 1.76% and 2.47% were achieved for the photovoltaic devices based on 2TAPM:PC71BM and 4TAPM:PC71BM under simulated air mass 1.5 global irradiation (100 mW/cm2), respectively. & 2011 Elsevier B.V. All rights reserved.

Keywords: Solution process Amorphous small molecules Organic solar cells

1. Introduction Photovoltaic devices based on organic semiconductors are evolving into a promising cost-effective alternative to the silicon-based solar cells due to their low-cost fabrication through solution processing, light weight, as well as excellent compatibility with flexible substrates [1]. According to theoretical models [2], devices based on these materials are predicted to reach a power conversion efficiency (PCE) close to 10%. Up to date, the highest PCE of the bulk heterojunction (BHJ) polymer photovoltaic devices has been up to 7.73% [3]. It can be seen that there is still a gap between the practical efficiency and the theoretical one. Therefore, it is critically important to design and synthesize new donor or acceptor materials for achieving higher PCE. Generally, organic polymers possess the advantages of strong absorption ability, admirable solution processability, good filmforming ability [4] and tunable energy levels. However, the purification of polymers is one of the most difficult problems. For example, it is very difficult to separate the Pd-catalyst from the polymer [5]. As usual, a polymer is a mixture of molecules with different molecular weights. The impurity and relatively

n

Corresponding author. Tel.: þ86 431 85166368; fax: þ 86 431 85193421. E-mail addresses: [email protected], [email protected] (W. Tian).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.03.040

high dispersity of molecular weight would significantly decrease the charge carrier mobility of polymers and further lead to relatively low fill factor (FF) and PCE in the resulting photovoltaic devices [6]. In contrast to polymers, small molecules have attracted more and more attention for photovoltaic applications due to their high purity, high charge carrier mobilities (15 cm2/V s) [7], solution processability, well-defined molecular structures and definite molecular weights. Recently, profound progress has been achieved in the synthesis of new solution processable small molecules and corresponding photovoltaic applications [8–17]. Although the highest PCE of solution processed bulk-heterojunction photovoltaic devices based on small molecules has reached 4.4% [17], the mismatch between the absorption spectrum of small molecules and the solar spectrum is still one of the primary problems for improving PCE of photovoltaic device. Therefore, more attention is being directed to the design and synthesis of small molecules with donor–acceptor (D–A) structure [8,9,15–22]. The intramolecular charge transfer (ICT) from the donor moiety to the acceptor moiety inside a D–A molecule can efficiently extend the absorption spectrum of the molecule for better matching the solar spectrum. Moreover, the incorporation of electron-withdrawing moiety with different electron-donating moieties will bring different ICT degrees to the conjugated molecules and thus provide a means to tune their energy levels [27,23].

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It has been demonstrated that conjugated small molecules (such as oligothiophene) are usually apt to crystallize and, thus, are difficult to form a uniform film by solution process [25,26]. One effective way to increase the film forming ability of small molecules is to construct amorphous small molecules by introducing triphenylamine(TPA) into the structure [8–10]. The resulting amorphous small molecules possess isotropic optical properties and excellent film-forming ability, which increases the absorption cross section for the incident light as well as the charge transport efficiency through the active layer [27–29]. Furthermore, TPA possesses a high oxidation potential, and thus a high Voc could be expected. Due to the advantages of TPA based small molecules, a series of amorphous small molecules have been synthesized and applied to the organic bulk heterojunction solar cell as donor materials in recent years [8–10,18,21,27–29]. Up to now, the highest PCE for this kind of small molecules has reached 2.39% [21]. In this paper, we report three new solution processable D–A small molecules (2TAPM, 4TAPM and 2BTAPM) containing of 2-pyran-4-ylidenemalononitrile(PM) as electron-accepting moiety, triphenylamine and different thiophene units as electrondonating moieties. 2-pyran-4-ylidenemalononitrile is a strong electron-accepting group, which can increase the electron affinity and reduce the band gap of the conjugated system when combined with strong electron-donating moieties [8,9,23]. Triphenylamine was introduced into the molecule in order to increase the electron-donating ability, high-dimensionality and hydrotropy [30–32]. And thiophene units as well as the increased oligothiophene length can provide stronger light absorption and promote better p–p stacking/aggregation of small molecules leading to a small energy band gap and a broader absorption spectrum [28]. DSC measurement indicates that these small molecules are

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amorphous. The normalized UV–vis absorption spectra showed that 4TAPM exhibited higher molar absorption coefficient and optical density (70,900 M  1 cm  1 and 3.60  10  3 nm  1, respectively) than that of 2TAPM (57,600 M  1 cm  1 and 2.18  10  3 nm  1). Both cyclic voltammetry measurement and theoretical calculations show that the highest occupied molecular orbital (HOMO) energy levels of the molecules could be finetuned by changing the electron-donating ability of the electrondonating moieties. The bulk heterojunction photovoltaic devices were fabricated by using small molecules (2TAPM or 4TAPM) as donor and PC71BM as acceptor. A high short-circuit current density of 7.86 mA/cm2 and PCE of 2.47% were achieved for 4TAPM:PC71BM under simulated air mass 1.5 global (AM 1.5 G) irradiation (100 mW/cm2). This PCE of 2.47% is among the top values for the solution-processed amorphous small moleculebased organic solar cells reported so far.

2. Experimental 2.1. 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 agent(s) prior to use and were purged with nitrogen. Compound 2, 5 and 7 were synthesized according to literature procedure.[23,33,34] The synthesis routes and molecule structures (2TAPM, 4TAPM and 2BTAPM) are shown in Schemes 1 and 2, respectively.

Scheme 1. Synthetic routes of compounds.

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Scheme 2. Synthetic routes and structures of small molecules.

2.2. Synthesis of compounds 2.2.1. Synthesis of compounds 1 n-Butyllithium (4 mL of a 2.5 M solution in hexane, 51.65 mmol) was added dropwise into a solution of 2-bromothiophene (1.63, 10 mmol) in tetrahydrofuran (THF, 50 mL) at 78 1C under a dry argon atmosphere. The mixture was stirred at 78 1C for 1 h, and then 2-isopropoxy-4,4,5,5-tetramethyl 1,3,2-dioxaborolane (2.47 mL, 12 mmol) was injected promptly into the flask, and the new solution was warmed to room temperature for 12 h. The mixture was poured into water and was extracted with ether. The extract was washed with brine and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was recrystallized from ethanol and gave a white crystal 1.26 g (6.24 mmol, 62.4%). 1H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.657 (d, 1H,  Th), 7.635 (d, 1H, Th), 7.194 (t, 1H, Th), 1.241 (m, 12H, –CH3).

2.2.2. Synthesis of compound 3 A 2.0 g (14 mmol) sample of 3,4-(ethylenedioxy)thiophene in 150 mL of dry THF was treated dropwise with 6.4 mL (16.2 mmol) of 2.5 M n-butyllithiumat  78 1C under argon. After the solution was stirred for 1 h, 5.95 g (18.8 mmol) of tributylstannyl chloride was added to the solution, and the new solution was warmed to room temperature. The solvent was removed by rotary evaporation after the solution was stirred for 12 h. The residue was dissolved in hexanes and filtered. The filtrate was dried in vacuum to afford 6.0 g of 2-(tributylstannyl)  3,4-(ethylenedioxy) thiophene as a yellow liquid. The compound was used for the next reaction as obtained, with no further purifications. 1 H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.302 (d, 1H, Th), 7.187 (m, 2H,  Th), 7.071 (d, 1H, Th), 7.008 (t, 1H,  Th), 1.595 (m, 6H,  CH2), 1.373 (m, 6H,  CH2), 1.128 (m, 6H, CH2), 0.916 (m, 9H,  CH3).

2.2.3. Synthesis of compound 4 DMF (0.83 mL, 10 mmol) was put into a 50 mL flask, kept in ice-water, and then phosphorous oxychloride (0.84 mL, 9.2 mmol) was added dropwise to the stirred DMF. After 20 min, 4-bromo-N,Ndiphenylbenzenamine (2.59 g, 8 mmol) was added into the mixture with stirring at 60 1C for 2 h. After cooling, the solution was poured into cold water. The resulting mixture was neutralized to pH¼7 with 2 M NaOH aqueous solution and extracted with chloroform. The extract was washed with plenty of water and NaCl successively. The organic extracts were dried over anhydrous MgSO4, evaporated and purified with column chromatography on silica gel with ethyl acetate:petroleumether (1:10) as the eluant to give yellow solid 2.38 g (6.8 mmol, yeild 85%). 1H NMR (500 MHz, CDCl3, TMS): d (ppm) 9.826 (s, 1H,  CHO), 7.694 (d, 2H, Ph), 7.435 (m, 2H,  Ph), 7.347 (t, 2H, Ph), 7.172 (m, 3H,  Ph), 7.035 (m, 4H,  Ph). 13C NMR (75 MHz, DMSO, TMS): d (ppm) 190.388, 132.718, 132.063, 131.280, 129.827, 129.281, 127.130, 126.211, 125.173, 124.408, 123.944, 123.271, 119.869.

2.2.4. Synthesis of compound 6 A mixture of 4-((4-bromophenyl)(phenyl)amino)benzaldehyde (2.23 g, 6.35 mmol), 2-(2,6-dimethylpyran-4-ylidene)-malononitrile (4) (0.50 g, 2.89 mmol), piperidine (20 drops), and acetonitrile (20 mL) were refluxed under N2 for 24 h. The reaction mixture was cooled to room temperature and poured into water and extracted with chloroform. The combined organic extractions were washed three times with water, dried over anhydrous MgSO4, evaporated under vacuum and purified with column chromatography on silica gel with dichloromethane:petroleumether (3:1) as the eluant to dark red solid 2.02 g (2.40 mmol, 83.1%). 1H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.405 (m, 10H,  TPA), 7.325 (d, 2H,  vinylic), 7.323 (s, 2H,  TPA), 7.140 (t, 6H,  TPA),7.026 (m, 8H, TPA), 6.643 (s, 2H,  PM), 6.614 (d, 2H, J¼15.9 Hz,  vinylic), 13C NMR (75 MHz, CDCl3, TMS): d (ppm)

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158.589, 156.741, 149.432, 146.296, 145.866, 137.243, 132.529, 129.666, 129.048, 128.025, 126.538, 125.554, 124.633, 121.976, 116.632, 116.099, 115.518, 106.433, 58.540. 2.2.5. Synthesis of compound 8 4-(N,N-Diphenylamino)benzaldehyde (14.00 g, 51.28 mmol), potassium iodide (11.43 g, 68.85 mmol), and acetic acid (210 mL) were heated to 85 1C, and the solution was allowed to cool. Then the potassium iodate (10.97 g, 51.26 mmol) was added and the reaction mixture was heated at 85 1C for 5 h. The solution was allowed to cool to room temperature and poured into icewater under stirring. The precipitated yellow solid was collected by filtration, and the collected solid was poured into 5% NaHSO3 (100 mL) to clear I2 and KIO3. The final filtrated yellow solid was pure compound 5. 1H NMR (500 MHz, CDCl3) 9.85 (s, 1H,  CHO), 7.71 (d, 2H,  Ph), 7.63 (d, 4H,  Ph), 7.05 (d, 2H,  Ph), 6.89 (d, 4H,  Ph). 13C NMR (75 MHz, DMSO, TMS): d (ppm) 190.381, 145.677, 138.836, 138.417, 131.340, 130.275, 127.600, 126.013, 120.276. 2.2.6. Synthesis of compound 9 The synthetic procedure for compound 9 was similar to that for compound 6, and yielded a dark red solid of 2.88 g (2.43 mmol, 81.0%). 1H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.609 (m, 4H,  Ph), 7.580 (m, 4H,  Ph), 7.430 (m, 6H,  Ph, vinylic), 7.048 (m, 4H,  Ph), 6.886 (m, 4H, Ph), 6.857 (m, 4H, Ph), 6.629 (d, 4H, PM,  vinylic). 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 158.401, 155.674, 148.728, 146.120, 138.626, 136.984, 129.080, 128.791, 126.854, 125.616, 122.706, 116.649, 115.383, 106.687, 57.138. 2.3. Synthesis of final compounds 2.3.1. Synthesis of compound 2TAPM Compound 1 (231 mg, 1.10 mmol), 6 (420 mg, 0.50 mmol) and (PPh3)4Pd(0) (11.6 mg, 2 mol% with respect to compound 6) were dissolved in a mixture of toluene (6 mL) and aqueous 2 M K2CO3 (3/2 volume ratio). The solution was stirred under an Ar atmosphere and refluxed with vigorous stirring 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 (2:3) as the eluant to dark red solid 305 mg (0.36 mmol ,72.0%). 1H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.538, (d, 4H, Ph), 7.441, (t, 6H, Ph), 7.330 (m, 4H,  Ph), 7.267 (s, 2H,  Ph), 7.120 (m, 18H, Ph), 6.640 (s, 3H,  PM,  vinylic), 6.586 (s, 1H,  vinylic). 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 158.634, 155.712, 148.037, 146.432, 145.946, 143.788, 137.347, 130.164, 129.607, 129.035, 128.027, 127.806, 126.906, 125.624, 125.272, 124.815, 124.406, 122.001, 121.921, 115.891, 115.545, 106.323, 58.397. MALDI-TOF MS: calcd. for C56H38N4OS2 847.06, found 846.90. 2.3.2. Synthesis of compound 4TAPM The synthetic process of 4TPAM was similar with that of 2TAPM. After evaporation of the solvent, the residue was purified with column chromatography on silica gel with dichloromethane:petroleumether (1:1) as the eluant to dark red solid 324 mg (0.32 mmol, 64.0%). 1H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.558 (d, 4H, Ph), 7.434 (m, 6H,  Ph), 7.334 (m, 4H,  Ph and vinylic), 7.215 (d, 2H,  Ph), 7.169 (t, 8H,  Ph), 7.131 (t, 6H, Ph), 7.089 (d, 4H,  Ph), 7.024 (t, 2H,  Ph), 6.637 (s, 2H,  PM), 6.612 (d, 2H, J¼ 16 Hz,  vinylic). 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 158.579, 155.757, 149.271, 145.660, 143.684, 137.231,

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130.391, 129.612, 129.063, 128.071, 127.007, 125.616, 125.419, 124.597, 122.773, 122.302, 116.125, 115.566, 106.479, 58.389. MALDI-TOF MS: calcd for C64H42N4OS4 1011.30, found 1011.5. 2.3.3. Synthesis of compound 2BTAPM Compound 3 (501 mg, 1.10 mmol), 6 (420 mg, 0.50 mmol) and (PPh3)4Pd(0) (11.6 mg, 2 mol% with respect to the compound 6) were dissolved in a mixture of toluene (6 mL) and DMF (3 mL). The solution was stirred under an Ar atmosphere and refluxed with vigorous stirring 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 (3:2) as the eluant to dark red solid 253 mg (0.25 mmol, 50.0%). 1H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.517 (d, 4H, Ph), 7.434 (m, 6H, Ph), 7.334 (t, 4H,  Ph and  vinylic), 7.215 (d, 2H, Ph), 7.169 (t, 8H,  Ph), 7.131 (t, 6H,  Ph), 7.089 (d, 4H,  Ph), 7.024 (t, 2H, Ph), 6.637 (s, 2H,  PM), 6.612 (d, 2H, J¼16 Hz,  vinylic). 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 158.742, 155.771, 149.743, 146.574, 146.260, 142.677, 137.502, 137.417, 136.561, 129.909, 129.703, 129.079, 128.169, 127.839, 126.728, 125.781, 125.260, 124.697, 124.656, 124.379, 123.645, 123.370, 122.231, 116.205, 115.463, 106.409, 58.893. MALDI-TOF MS: calcd for C64H42N4OS4 1011.30, found 1010.9. 2.4. Instruments and measurements Differential scanning calorimetry (DSC) experiments were performed under nitrogen flushing at a heating rate of 10 1C/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-offlight mass spectra were recorded with a Kratos MALDI-TOF mass system. UV–visible absorption spectra were measured using a Shimadzu UV-3100 spectrophotometer. Electrochemical measurements of these derivatives were performed with a Bioanalytical Systems BAS 100 B/W electrochemical workstation. Atomic force microscopy (AFM) images of the blend films were carried out using a Nanoscope IIIa Dimension 3100. 2.5. Fabrication and characterization of photovoltaic devices For device fabrication, the ITO glass was precleaned and modified by a thin layer of PEDOT:PSS, which was spin-cast from a PEDOT:PSS aqueous solution (H.C. Starck), and the thickness of the PEDOT:PSS layer is about 50 nm. The active layer contained a blend of small molecules as electron donor and PC71BM as electron acceptor, which was prepared from chlorobenzene solution with different weight ratios (1:1, 1:2, 1:3 and 1:4) for 2TAPM/PC71BM, and 4TAPM/PC71BM, respectively. After spincoating the blend from solution at 1000 rpm, the devices were completed by evaporating a 0.6 nm LiF layer and a 100 nm Al layer at a base pressure of 5  10  4 Pa. The effective photovoltaic area is 5 mm2 as defined by the geometrical overlap between the bottom ITO electrode and the top cathode. The current–voltage (J–V) characteristics were recorded using a Keithley 2400 Source Meter in the dark and under simulated AM 1.5 G irradiation (100 mW/cm2) by a SCIENCETECH SS-0.5K solar simulator. The spectral response was recorded by an SR830 lock-in amplifier under short circuit condition when devices were illuminated with a monochromatic light from a Xeon lamp through a spectrometer. Films thickness was measured by a Veeco DEKTAK 150 surface profilometer. AFM images were measured by a S II Nanonavi

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probe station 300hv. All fabrication steps and characterizations were performed in an ambient environment.

0.8 2TAPM 4TAPM 2BTAPM

3. Results and discussion

The general synthetic routes toward all compounds and the final small molecules were outlined in Schemes 1 and 2. The compounds 6 and 9 were prepared through Knoevenagel condensation of compound 5 with 4 and 8, respectively. The structures of the compounds were confirmed by 1H NMR, 13C NMR spectra, and the data were included in Section 2. In 1H NMR spectroscopy of monomers, the coupling constant (J15.5 Hz) of olefinic protons indicates that the Knoevenagel reaction afforded the pure all-trans isomers. The final products (shown in Scheme 1) were prepared by the well-known palladium-catalyzed Suzuki and still coupling reactions between varied compounds (6, 9) and compounds 1 and 3. 1H NMR and 13C NMR spectra were used to characterize the structure of these molecules, which clearly indicate that welldefined 2TAPM, 4TAPM and 2BTAPM had been obtained. The two legible double peaks that appear at  6.400 and 7.300 ppm with a coupling constant of J16 Hz are due to all-trans double bond, which further confirms the regular structure. Molecular weights were determined by Kratos MALDI-TOF. These results are consistent with the proposed structure and molecule weights. Among these molecules, 2TAPM and 4TAPM exhibited excellent solubility in common organic solvents such as chloroform, tetrahydrofuran, dichloromethane, and chlorobenzene. However, 2BTAPM showed poor solubility in these solvents for its rigidity. 3.2. Thermal properties

3.3. Optical properties The normalized UV–vis absorption spectra of 2TAPM, 4TAPM and 2BTAPM in dilute chloroform solution (concentration

2TAPM 136°C

4TAPM

endothermic

0.2

300

400 500 Wavelength (nm)

600

700

0.35 0.30

2TAPM 4TAPM 2BTAPM

0.25 0.20 0.15 0.10 0.05

Differential scanning calorimetry (DSC) was performed to investigate the thermal properties of 2TAPM, 4TAPM and 2BTAPM. Fig. 1 shows the DSC curves of small molecules purified readily by recrystallization. When the small molecules were heated, the endothermic peak due to the glass transition temperatures (Tg) of 2TAPM, 4TAPM and 2BTAPM were observed at 136, 165 and 167 1C, respectively, demonstrating that these small molecules are amorphous. The relatively high Tg indicates that these small molecules possess good thermal stability.

165°C

2BTAPM

90

0.4

0.0

Absorption (a.u.)

3.1. Material synthesis and structural characterization

Absorption (a.u.)

0.6

150 120 Temperature (°C)

167°C

180

Fig. 1. Differential scanning calorimetry (DSC) measurement of 2TAPM, 4TAPM and 2BTAPM, scan rate 10 1C min  1.

0.00 300

400

500 600 Wavelength (nm)

700

Fig. 2. Normalized absorption spectra of the small molecules (a) in chloroform solutions with a concentration of 10  5 mol/L; (b) films spin-coated from a 10 mg/mL chloroform solution for 2TAPM and 4TAPM, 5 mg/mL from a chloroform solution for 2BTAPM.

10  5 M) are shown in Fig. 2a, and the main optical properties are listed in Table 1. 2TAPM with the weak electron-donating unit shows two absorption peaks at 347 and 499 nm which can be assigned to the p–p* absorption of the molecule and ICT transition from electron-donating units (thiophene and TPA) to electron-accepting unit (PM). As for 4TAPM and 2BTAPM, the absorption spectra in dilute solutions exhibit two absorption bands at 364, 505 nm and 380, 502 nm, respectively, which also due to the p–p* transition and the ICT transition of the molecules. Both 4TAPM and 2BTAPM exhibit extended absorption edges as compared to that of 2TAPM in dilute solution, which could be caused by the increased electron-donating abilities of the electron-donating moiety with the increase of thiophene units and the enhancement of the ICT transition of the molecules. Moreover, relatively high absorption coefficients could be calculated from Beer’s law equation with the same dilute concentration of the molecules in chloroform (absorption coefficients are listed in Table 1). 4TAPM exhibits the highest molar absorption coefficient of 70,900 M  1 cm  1. Fig. 2b shows the optical absorption spectra of the small molecules in thin films. The absorption spectra in thin films are generally similar in shape to those in dilute solution. There are some small red shifts of the absorption peaks and large extent

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Table 1 Optical and electrochemical data of 2TAPM, 4TAPM and 2BTAPM. Molecules

In solutiona

e (M  1 cm  1)

2TAPM 4TAPM 2BTAPM a b c

57,600 70,900 43,700

In filmb

labs max (nm)

347,499 364,505 380,502

ledge (nm)

574 584 577

Optical density (10  3 nm  1)

2.18 3.60 1.60

labs max (nm)

351,510 369,510 406,528

ledge (nm)

624 622 647

Eonset Ox (V)

Eonset Red (V)

Electrochem.

Opticalc

HOMO (eV)

LUMO (eV)

Eg,ec (eV)

Eg,opt (eV)c

0.55/  5.24 0.49/  5.18 0.48/  5.17

 1.23/  3.46  1.23/  3.46  1.22/  3.47

1.78 1.72 1.67

1.99 1.99 1.92

1  10  5 M in anhydrous chloroform. 2TAPM and 4TAPM were spin-coated from a 10 mg/mL chloroform solution, 2BTAPM was spin-coated from a 5 mg/mL chloroform solution. The optical band gap (Eg,opt) was obtained from film absorption edge.

12 9 2TAPM 6 Current (uA)

4TAPM 3 2BTAPM

0 -3 -6 -9 -12 -15 -2.0

-1.6

-1.2

-0.8 -0.4 0.4 0.0 Potential (V) vs Ag/Ag+

0.8

1.2

Fig. 3. Cyclic volatammetry curves of 2TAPM, 4TAPM, and 2BTAPM solutions on platinum electrode in 0.1 mol/L n-Bu4NPF6 in CH2Cl2 solution, at a scan rate of 100 mV/s.

broadening of the absorption edge in films as compared to solution (For 2TAPM, 2 and 50 nm; for 4TAPM, 5 and 38 nm; and for 2BTAPM, 26 and 70 nm, respectively), which may be caused by the existence of some aggregation or p–p stacking of the molecules [15,24]. Both the red shift and the broadening of absorption edge of 2BTAPM are larger than those of 2TAPM and 4TAPM, which could be attributed to the enhanced p–p stacking in solid state with the increase of conjugation length [35,36]. As the absorption edges of the three small molecules are tuned from 622 to 647 nm, the optical band gaps (Eg, opt) of the three small molecules derived from the absorption edge of the thin film spectra are in the range of 1.99–1.92 eV. 3.4. Electrochemical properties Fig. 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 the N2 atmosphere. Ferrocene was used as the internal standard. The redox potential of Fc/Fcþ 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]. The results of the electrochemical measurements and calculated energy levels of these small molecules are listed in Table 1. From Table 1, it can be seen that 2TAPM shows a HOMO energy level of  5.24 eV and a LUMO energy level of  3.46 eV.

Fig. 4. Molecular orbital surfaces of the HOMO and LUMO of 2TAPM, 4TAPM, and 2BTAPM obtained at B3 LYP/6-31G* level.

The LUMO energy levels of 4TAPM and 2BTAPM are  3.46 and  3.47 eV, which are very similar to that of 2TAPM and the reported PM-containing small molecules [7,8]. Therefore, the substitution of varied thiophene units with different electrondonating ability has almost no effect on the reduction potential of the small molecules. Furthermore, the relatively low LUMO energy levels of the three small molecules should be resulted from the stronger reduction of PM-based acceptor unit. It is clear that the HOMO levels of 2TAPM (  5.24 eV), 4TAPM (  5.18 eV) and 2BTAPM (  5.17 eV) gradually increase with the decrease of their oxidation potentials from 0.55, 0.49 to 0.48 eV, which should be caused by the enhanced electron-donating abilities of donor units in these D–A small molecules.[28] 3.5. Theoretical calculation The geometry and electronic properties of 2TAPM, 4TAPM and 2BTAPM were also investigated by means of theoretical calculation with the Gaussian 03 program package at a hybrid density functional theory (DFT) level. Fig. 4 presents the geometry and the HOMO and LUMO. Electron density of the HOMO distributes not only on the donor moieties (thiophene and TPA) but also on the PM-acceptor unit, while that of the LUMO mainly delocalizes on the PM-acceptor unit and the adjacent benzene ring unit, indicating a charge-transfer nature of HOMO-LUMO from the electrondonating units of the thiophene and TPA to the PM-acceptor unit. From Fig. 5 we can see that with an increase in the electrondonating abilities of donor units, the HOMO energy level increases from  5.14 to  5.09 and  5.06 eV for 2TAPM, 4TAPM and 2BTAPM, respectively, whereas the LUMO energy levels remain relatively unchanged due to their highly localized state around

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Table 2 Characteristic current–voltage parameters from devices spin-coated from chlorobenzene solutions and tested at standard AM 1.5 G conditions.

0 Current density (mA cm-2)

2TAPM:PC71BM -1 -2

Small molecule

Small molecule: PC71BM

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE(%)

2TAPM

1:1 1:2 1:3 1:4

0.81 0.85 0.89 0.93

2.40 5.20 5.40 5.50

29.3 33.0 33.9 34.4

0.57 1.46 1.63 1.76

4TAPM

1:1 1:2 1:3 1:4

0.89 0.93 0.89 0.89

0.18 6.36 7.86 6.16

18.7 35.3 35.3 34.5

0.03 2.09 2.47 1.89

-3 -4

1:1 AM1.5 1:2 AM1.5 1:3 AM1.5 1:4 AM1.5

-5 -6 -0.1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Voltage (V) 1

Current density (mA cm-2)

0 -1

4TAPM:PC71BM

-2 -3 -4 -5 -6

1:1 AM1.5 1:2 AM1.5 1:3 AM1.5 1:4 AM1.5

-7 -8 -9 -0.1 0.0

0.1

0.2

0.3

0.4 0.5 0.6 Voltage (V)

0.7

0.8

0.9

1.0

Fig. 5. Current–voltage characteristics of photovoltaic cells based on 2TAPM and 4TAPM under 100 mW/cm2 irradiation of AM 1.5 G, and in dark.

the PM segment. Meanwhile, the HOMO and LUMO variation laws from molecular orbital distribution calculations are well consistent with those from electrochemical test.

3.6. Photovoltaic performance To demonstrate the application potential of these small molecules as an electron donor in organic solar cells, photovoltaic devices were fabricated by spin-coating at a constant concentration of 30 mg/mL comprising a mixture of small molecule (2TAPM or 4TAPM) and PC71BM. The photovoltaic performance of 2BTAPM was not investigated because of its poor solubility. We fabricated the photovoltaic devices with different weight ratios (1:1, 1:2, 1:3 and 1:4) for obtaining optimized PCE. Furthermore, chlorobenzene (CB) was chosen as the solvent due to its slow evaporation and thus tends to produce pinhole-free uniform films. The photovoltaic parameters of the photovoltaic devices are summarized in Table 2 and the corresponding current–voltage (I–V) characteristics with various blend weight ratios are presented in Fig. 5, respectively. The optimized photovoltaic devices with a structure of indium tin oxide (ITO)/PEDOT:PSS/small molecules:PC71BM/LiF/Al exhibit a Voc of 0.93 V, a short-circuit current (Jsc) of 5.50 mA/cm2, a fill factor (FF) of 34.4%, and a PCE of 1.76% for the device based on 2TAPM:PC71BM and 0.89 V, 7.86 mA/cm2, 35.3%, 2.47% for the device based on 4TAPM:PC71BM, respectively.

Fig. 6. Height images (size 5 mm  5 mm) and phase images (size 0.5 mm  0.5 mm) obtained by tapping-mode AFM showing the morphology of the blend films spincoated from chlorobenzene solutions: (a) height images: 2TAPM:PC71BM (1:4 w/w); (b) height images: 4TAPM:PC71BM (1:3 w/w); (c) phase images: 2TAPM:PC71BM (1:4 w/w); and (d) phase images: 4TAPM:PC71BM (1:3 w/w).

The photovoltaic devices based on 4TAPM:PC71BM (7.87 mA/cm2) have higher Jsc than the devices based on 2TAPM:PC71BM (5.50 mA/ cm2), which could be explained by the higher absorption coefficient of 4TAPM (70,900 M  1 cm  1) than that of 2TAPM (57,600 M  1 cm  1) and the optical densities of 2TAPM (2.18  10  3 nm  1) and 4TAPM (3.60  10  3 nm  1). To gain further insight into what might affect the Jsc of the photovoltaic device, we analyzed the morphology of 2TAPM:PC71BM and 4TAPM:PC71BM blend films. Fig. 6 shows the AFM height and phase images of these blend films with weight ratios of 1:4 and 1:3 (small molecule: PC71BM). It is clearly shown by AFM that both 2TAPM:PC71BM and 4TAPM:PC71BM blend films exhibit relatively smooth surfaces with root-mean-square (rms) roughness of 0.36 and 0.44 nm, respectively, which demonstrates that these two small molecules possess excellent film-forming ability and good compatibility with PC71BM. From the phase images of Fig. 6c and d, we can see that 2TAPM:PC71BM (1:4) blend film

Z. Li et al. / Solar Energy Materials & Solar Cells 95 (2011) 2272–2280

60

4TAPM:PC71BM 2TAPM:PC71BM

EQE (%)

50 40 30 20 10 0 350

400

450

500 550 600 Wavelength (nm)

650

700

Fig. 7. External quantum efficiency (EQE) curve for device using 1:4 and 1:3 blend of 2TAPM:PC71BM and 4TAPM:PC71BM, respectively.

exhibited the relatively large phase separation size as the existence of a large amount of PC71BM, which is disadvantageous for charge transport and carrier collection [37,38]. As for 4TAPM:PC71BM (1:3) blend film, the phase separation size minished effectively with the decreasing of PC71BM content, which is beneficial for the charge transport, collection and thus decrease its recombination [37,38]. Therefore, the higher Jsc obtained from the 4TAPM:PC71BM device (7.86 mA/cm2) may be attributed to its more ideal phase separation size than that of 2TAPM:PC71BM device (5.50 mA/cm2). The higher Voc of the device based on 2TAPM:PC71BM (Voc ¼0.93 V) comparing with the device based on 4TAPM:PC71BM (Voc ¼0.89 V) could be explained by the lower lying HOMO energy level of 2TAPM (  5.24 eV) as compared to that of 4TAPM (  5.18 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. The device based on 4TAPM:PC71BM exhibited higher FF value of 35.3% than that of device based on 2TAPM:PC71BM (34.4%). This could be caused by the reduction of exciton recombination in 4TAPM:PC71BM active layer, which benefits from its better morphology than that of 2TAPM:PC71BM (Fig. 6) [37,38]. As for PCE, the photovoltaic device based on 2TAPM:PC71BM exhibited a PCE of 1.76% while while a remarkably improved PCE of 2.47% obtained from the photovoltaic device based on 4TAPM:PC71BM. This could be mainly caused by the higher absorption coefficient and optical density of 4TAPM than those of 2TAPM. Furthermore, the improved morphology could be another reasons for increased PCE. Fig. 7 shows the external quantum efficiency (EQE) spectra of the devices based on 2TAPM:PC71BM and 4TAPM:PC71BM blend films with weight ratio of 1:4 and 1:3, respectively. It can be seen that both devices show efficient photoconversion efficiency in the range of 350–700 nm. Although the spectral ranges of these photovoltaic devices are almost equal, the maximum values are close to 42.1% at 487 nm for the 2TAPM:PC71BM device and 56.4% at 477 nm for the 4TAPM:PC71BM device. So the device based on 4TAPM:PC71BM shows higher Jsc than the device based on 2TAPM:PC71BM.

4. Conclusions A series of novel solution processable small molecules (2TAPM, 4TAPM and 2BTAPM) were designed and synthesized according to

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well-known palladium-catalyzed Suzuki and still coupling reactions. DSC measurements indicate that these small molecules are amorphous. UV–vis absorption spectra show that combining PM with moieties having gradually increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT) transition. Both cyclic voltammetry measurement and theoretical calculations demonstrate that the highest occupied molecular orbital (HOMO) energy levels of the molecules could be finetuned by tuning the electron-donating ability of the electrondonating moieties. Meanwhile, the morphologies of the blend films containing small molecules and PC71BM demonstrated these small molecules possess the excellent film-forming abililties and compatibilities with PC71BM. The photovoltaic devices based on these small molecules showed the PCE in the range of 1.76–2.47%, respectively, which indicates that 2TAPM and 4TAPM are promising donor materials for photovoltaic devices.

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), the 111 Project (Grant no. B06009), and the Project of Jilin Province (20080305).

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