Bulk Heterojunction Photovoltaics Using Broadly Absorbing Small

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Bulk Heterojunction Photovoltaics Using Broadly Absorbing Small Molecules Based on 2-Styryl-5-phenylazo-pyrrole J. A. Mikroyannidis,*,† A. N. Kabanakis,† P. Balraju,‡,§ and G. D. Sharma*,‡, †

Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece, Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur (Raj.) 342005, India, §Molecular Electronics Laboratory JNCASR, Bangalore, India, and Jaipur Engineering College, Kukas, Jaipur (Raj.), India )

‡

Received August 10, 2010. Revised Manuscript Received October 2, 2010 Three new soluble small molecules (B, B6, and A) with a low band gap based on 2-styryl-5-phenylazo-pyrrole were synthesized. Molecules B and B6 contained pyrrole and N-hexylpyrrole, respectively, as the central unit, which was connected to N,N-dimethylphenyl-4-azo on one side of the pyrrole molecule. Molecule A contained N-hexylpyrrole as the central unit, which was connected to anthracenyl-9-azo on one side of the pyrrole molecule. The other side of the pyrrole molecule was connected to cyanovinylene 4-nitrophenyl for all molecules. The long-wavelength absorption maximum of the molecules was located at 601-637 nm, and their optical band gap was 1.62-1.67 eV. The photovoltaic properties have been investigated using blends of B, B6, or A with PCBM, and it was found that the device based on A:PCBM had a higher power conversion efficiency (PCE) (2.06%) than the devices based on B:PCBM (1.33%) and B6: PCBM (1.36%). This has been attributed to the higher hole mobility, the lower band gap of A relative to that of B or B6, and the higher energy difference between the LUMO of A and PCBM. The effect of solvent annealing and thermalsolvent annealing on the photovoltaic response of the device based on the A:PCBM blend has been investigated, and it was found that the devices based on solvent-treated and subsequent thermally annealed blends have PCEs of 2.56 and 2.83%, respectively. The increase in the PCE has been attributed to the enhanced crystallinity of the blend and the improvement in the charge transport due to a reduction in the difference between the electron and hole mobility in the blend.

Introduction Conjugated polymer-based bulk-heterojunction (BHJ) solar cells have received considerable attention as potential alternative renewable energy sources1 because of their suitability for low-cost, fast solution processing techniques and their compatibility with flexible substrates. Significant effort is being put forth to develop conjugated polymers that meet various criteria for high-performance photovoltaic (PV) applications.2 Currently, state-of-the-art polymer-based BHJ solar cells have reached power conversion efficiencies (PCE) of 6%.3a,b A PCE of 8.13% has been reported for BHJ solar cells, which is the current world record.3c However, these numbers are still significantly lower than the critical efficiencies of 10-12% that many believe are required for commercial utility.4 *Corresponding authors. (J.A.M.) Tel: þ30 2610 997115. Fax: þ30 2610 997118. E-mail: [email protected]. (G.D.S.) Tel: 91-02912720857. Fax: 91-0291-2720856. E-mail: [email protected]. (1) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (b) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (c) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. (d) Cheng, Y.; Yang, S.; Hsu, C. Chem. Rev. 2009, 109, 5868. (2) (a) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954. (b) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; Boer, B. D. Polym. Rev. 2008, 48, 531. (c) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J. Am. Chem. Soc. 2009, 131, 7792. (3) (a) Hou, J.; Chen, H.-Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.; Li, G. J. Am. Chem. Soc. 2009, 131, 15586. (b) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photon. 2009, 3, 297.(c) www.solarmer.com. (4) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. L. Adv. Mater. 2006, 18, 789. (5) (a) Scherf, U.; Gutacker, A.; Koenen, N. Acc. Chem. Res. 2008, 41, 1086. (b) Taranekar, P.; Qiao, Q.; Jiang, H.; Ghiviriga, I.; Schanze, K. S.; Reynolds, J. R. J. Am. Chem. Soc. 2007, 129, 8958. (c) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130, 3619. (d) Kose, M. E.; Mitchell, W. J.; Kopidakis, N.; Chang, C. H.; Shaheen, S. E.; Kim, K.; Rumbles, G. J. Am. Chem. Soc. 2007, 129, 14257.

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The strong motivation for approaching such efficiencies is driving various research efforts in the area of organic-based BHJ solar cells.1-5 In such BHJ devices, when blended with PCBM, the processable high-molecular-weight polymeric semiconductor should exhibit a band gap of between 1.2 and 1.9 eV with a HOMO level lower than -5.2 eV to allow air stability and a LUMO level near -3.8 to -4.0 eV to allow electron transfer to PCBM while maximizing the open circuit voltage. Furthermore, the polymer should show a hole mobility higher than 10-3 cm2 V-1 s-1.6 Certain publications concerning processes for the production of flexible large-area polymers and generally organic solar cells have been recently reported.7 Molecular crystalline semiconductors as alternatives to conjugated polymers offer several intrinsic advantages in solution-processable BHJ solar cells. Because of their monodisperse nature with well-defined chemical structures, together with no end-group contaminants or batch-to-batch variations, molecular organic semiconductors can be reproducibly prepared, functionalized, and purified. Even though the fact that the highest PCEs to date for small-molecule-based solar cells remain lower than their polymer-based analogs, the considerations above make molecular crystalline semiconductors attractive as active materials in BHJ solar cells.8 Theoretical and experimental advances in (6) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (b) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323. (c) Li, Y. F.; Zou, Y. P. Adv. Mater. 2008, 20, 2952. (7) (a) Krebs, F. C.; Fyenbo, J.; Jorgensen, M. J. Mater. Chem. 2010, 20, 8994. (b) Krebs, F. C.; Tromholt, T.; Jorgensen, M. Nanoscale 2010, 2, 873. (c) Krebs, F. C.; Nielsen, T. D.; Fyenbo, J.; Wadstrom, M.; Pedersen, M. S. Energy Environ. Sci. 2010, 3, 512. (d) Helgesen, M.; Sondergaard, R.; Krebs, F. C. J. Mater. Chem. 2010, 20, 36. (8) (a) Mataga, N.; Chosrowjan, H.; Taniguchi, S. J. Photochem. Photobiol. 2005, 6, 37. (b) Hou, J.; Park, M.; Zhang, S.; Yao, Y.; Chen, L.; Li, J.; Yang, Y. Macromolecules 2008, 41, 6012.

Published on Web 10/25/2010

DOI: 10.1021/la103168t

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organic solar cells (OSCs) have demonstrated that the donoracceptor (D-A)-type small molecules have great potential as one class of promising materials in solution-processed OSCs.9 Recently, the PCE of 2.33 and 4.4% have been reported in OSCs combined with electron-poor diketopyrrole (DPP)containing low-dimensional oligothiophene as the donor with PCBM or C71-PCBM as the acceptor.10 However, azo dyes were widely studied as charge generation materials because of their technological advantages such as high sensitivity, wide spectral range (450-800 nm), and excellent stability.11 Various azo dyes have been used as sensitizers for dye-sensitized solar cells (DSSCs).12,13 Nevertheless, the utilization of azo compounds for BHJ solar cells is limited. A series of 2,2-dipyrrole monomers separated by aza spacers have recently been synthesized using modified Schiff and azo-coupling reactions. These 2,2-dipyrroles linked with conjugated aza spacers have been evaluated as precursors of narrow-band-gap conducting polymers.14 Finally, anthracene-containing compounds have been used for BHJ solar cells. In particular, a diarylanthracene bearing two dihexyloxy-substituted benzene rings has been synthesized and used as a donor for BHJ solar cells with PCBM.15 A variety of anthracene-containing copolymers and small molecules have recently been used for PV applications.16-21 Very recently, we have synthesized an alternating phenylenevinylene copolymer with 2,5-bisazopyrrole segments along the backbone and the corresponding BF2-azopyrrole complex. The latter has been used as an n-type organic semiconductor for BHJ solar cells with a PCE of up to 2.32%.22 Moreover, we have synthesized a symmetrical bisazopyrrole (A) and the corresponding BF2-azopyrrole complex (B) and used them for BHJ solar cells. The PCE values were 2.70 and 3.15% for the devices based on the A:PCBM and B:PCBM blends, respectively.23 Finally, we have synthesized symmetrical 2,5-bisazopyrrole-containing anthracene terminal units on both sides and symmetrical 1,4-bisazophenylenebearing pyrrole terminal units on both sides. The solution-processed BHJ PV devices fabricated from these materials as donors, (9) (a) Tamayo, A. B.; Walker, B.; Nguyen, T. Q. J. Phys. Chem. C 2008, 112, 11545. (b) Tamayo, A. B.; Dang, X. D.; Walker, B.; Seo, J.; Kent, T.; Nguyen, T. Q. Appl. Phys. Lett. 2009, 94, 103301. (10) (a) Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 17640. (b) Zhang, J.; Yang, Y.; He, C.; He, Y.; Zhao, G.; Li, Y. Macromolecules 2009, 42, 7619. (c) Roquet, S.; Cravino, A.; Leriche, P.; Aleveque, O.; Frere, P.; Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459. (d) Ma, C.-Q.; Fonrodona, M.; Schikora, M. C.; Wienk, M. M.; Janssen, R. A. J.; Bauerle, P. Adv. Funct. Mater. 2008, 18, 3323. (e) Roncali, J. Acc. Chem. Res. 2009, 42, 1719. (11) Law, K. Y. Chem. Rev. 1993, 93, 449. (12) Millington, K. R.; Fincher, K. W.; King, A. L. Sol. Energy Mater. Sol. Cells 2007, 91, 1618. € (13) Dincalp, H.; Yanuz, S.; Hakli, O.; Zafer, C.; Ozsoy, C.; Durucasu, I.; Icli, S. J. Photochem. Photobiol., A 2010, 210, 8. (14) Pozo-Gonzalo, C.; Pomposo, J. A.; Rodriguez, J.; Schmidt, E. Y.; Vasiltsov, A. M.; Zorina, N. Y.; Ivanov, A. V.; Trofimov, B. A.; Mikhaleva, A. I.; Zaitsev, A. B. Synth. Met. 2007, 157, 60. (15) Valentini, L.; Bagnis, D.; Marrocchi, A.; Seri, M.; Taticchi, A.; Kenny, J. M. Chem. Mater. 2008, 20, 32. (16) Sharma, G. D.; Balraju, P.; Mikroyannidis, J. A.; Stylianakis, M. M. Sol. Energy Mater. Sol. Cells 2009, 93, 2025. (17) Vellis, P. D.; Mikroyannidis, J. A.; Bagnis, D.; Valentini, L. J. Appl. Polym. Sci. 2009, 113, 1173. (18) Marrocchi, A.; Silvestri, F.; Seri, M.; Facchetti, A.; Tatticchia, A.; Marks, T. J. Chem. Commun. 2009, 1380. (19) Gunes, S.; Wild, A.; Cevik, E.; Pivrikas, A.; Schubert, U. S.; Egbe, D. A. M. Sol. Energy Mater. Sol. Cells 2010, 94, 484. (20) Egbe, D. A. M.; Turk, S.; Rathgeber, S.; Kuhnlenz, F.; Jadhav, R.; Wild, A.; Birckner, E.; Adam, G.; Pivrikas, A.; Cimrova, V.; Knor, G.; Sariciftci, N. S.; Hoppe, H. Macromolecules 2010, 43, 1261. (21) Mikroyannidis, J. A.; Vellis, P. D.; Yang, S. H.; Hsu, C. S. J. Appl. Polym. Sci. 2010, 115, 731. (22) Mikroyannidis, J. A.; Stylianakis, M. M.; Sharma, G. D.; Balraju, P.; Roy, M. S. J. Phys. Chem. C 2009, 113, 7904. (23) Mikroyannidis, J. A.; Kabanakis, A.; Tsagkournos, D.; Balraju, P.; Sharma, G. D. J. Mater. Chem. 2010, 20, 6464.

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blended with PCBM as the acceptor, produced a PCE of up to 2.42%.24 The present investigation describes the synthesis of three new small molecules (SMs) (B, B6, and A) with a low band gap based on 2-styryl-5-phenylazo-pyrrole. They were successfully synthesized from inexpensive starting materials by a three-step reaction sequence. The last step in their synthesis included the condensation of a substituted 2-formyl-5-azo-pyrrole with 4-nitrobenzylcyanide. These small molecules contained hexyl chain attached to the nitrogen of the central pyrrole and/or N,N-dimethyl terminal units that enhanced their solubility in common organic solvents. They are broadly absorbing materials because they combine the structural characteristics of the azo compounds and cyanovinylene 4-nitrophenyl. All SMs have the donor-acceptor (D-A) architecture. Specifically, they contained the azo-substituted pyrrole as the D unit and the cyanovinylene 4-nitrophenyl as the A unit. D-A materials usually possess relatively low-lying LUMO levels and absorb towards long wavelength from intramolecular charge transfer. We have investigated the possibility of using these SMs as electron donors along with PCBM as an electron acceptor for BHJ PV devices. The PCEs for the devices based on the as-cast B: PCBM, B6:PCBM, and A:PCBM blends are 1.33, 1.36, and 2.05%, respectively. We have investigated the effect of solvent treatment and subsequent thermal annealing of the A:PCBM blend on the PV response of the device and have found that the PCE is about 2.56 and 2.83%, respectively. The improvement of the PCE has been attributed to the increases in surface roughness and crystallinity of the blend, leading to better charge separation and collection efficiency.

Experimental Section Reagents and Solvents. 9-Nitroanthracene was prepared from the nitration of anthracene by means of concentrated nitric acid in glacial acetic acid.24 It was purified by recrystallization from methanol. 9-Aminoanthracene was prepared from the reduction of 9-nitroanthracene using hydrazine monohydrate in ethanol in the presence of a catalytic amount of Pd/C.25 N-Hexylpyrrole was prepared from the reaction of pyrrole with bromohexane in CH2Cl2-H2O in the presence of NaOH and (C4H9)4NBr as a phase-transfer catalyst.26 4-Nitrobenzylcyanide was synthesized from the nitration of benzyl cyanide with concentrated nitric and sulfuric acid.27 It was recrystallized from ethanol. N,N-Dimethylformamide (DMF) and tetrahydrofuran (THF) were dried by distillation over CaH2. All other reagents and solvents were commercially purchased and were used as supplied.

Synthesis of Compounds. 1H-Pyrrole-2-carbaldehyde (2a) and 1-Hexyl-1H-pyrrole-2-carbaldehyde (2b). Com-

pounds 2a and 2b were prepared according to a reported method.28

5-((4-(Dimethylamino)phenyl )diazenyl )-1H-pyrrole-2carbaldehyde (3a). A flask was charged with a suspension of N, N-dimethyl-p-phenylenediamine (0.2681 g, 1.96 mmol) in water (8 mL). Hydrochloric acid (2 mL) was added to the suspension. The mixture was cooled and kept at 0-5 °C in an ice bath and diazotized by adding a solution of NaNO2 (0.1500 g, 2.17 mmol) in water (5 mL) followed by stirring for 0.5 h at 0-5 °C. The (24) Mikroyannidis, J. A.; Tsagkournos, D. V.; Sharma, S. S.; Kumar, A.; Vijay, Y. K.; Sharma, G. D. Sol. Energy Mater. Sol. Cells 2010, 94, 2318. (25) Adams, H.; Bawa, R. A.; McMillan, K. G.; Jones, S. Tetrahedron: Asymmetry 2007, 18, 1003. (26) Brockmann, T.; Tour, J. M. J. Am. Chem. Soc. 1995, 117, 4437. (27) (a) Robertson, G. R. Organic Syntheses; Wiley & Sons: New York, 1941; Collect. Vol. 1, p 396.(b) Robertson, G. R. Organic Syntheses; Wiley & Sons: New York, 1922; Collect. Vol. 2, p. 57. (28) Hunt, J. T.; Mitt, T.; Borzilleri, R.; Gullo-Brown, J.; Fargnoli, J.; Fink, B. J. Med. Chem. 2004, 47, 4054.

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Mikroyannidis et al. solution of the diazonium salt thus prepared was immediately used for the next coupling reaction. The solution of the above diazonium salt was slowly added to a solution of 2a (0.1872 g, 1.96 mmol) in ethanol (20 mL) at 0-5 °C. The resulting mixture was stirred at room temperature for 10 h and then concentrated under reduced pressure. The precipitate was filtered, washed with dilute hydrochloric acid (0.1 M) and water, and dried to afford 3a. It was purified by column chromatography via elution with a mixture of dichloromethane and hexane (1:1). Yield 60% (0.2871 g). FT-IR (KBr, cm-1): 3320 (N-H stretching); 2916 (C-H stretching of aliphatic); 1654 (carbonyl). 1 H NMR (CDCl3) ppm: 10.80 (br, 1H, NH); 9.50 (s, 1H, formyl); 7.16 (m, 2H, phenylene ortho to azo); 7.01 (m, 1H, H3 of pyrrole); 6.77 (m, 2H, phenylene meta to azo); 6.34 (m, 1H, H4 of pyrrole); 2.96 (s, 6H, N(CH3)2). Anal. Calcd for C13H14N4O: C, 64.45; H, 5.82; N, 23.13. Found: C, 64.22; H, 5.63; N, 23.07.

5-((4-(Dimethylamino)phenyl )diazenyl )-1-hexyl-1H-pyrrole2-carbaldehyde (3b). Compound 3b was prepared in 65% yield by reacting the diazonium salt derived from N,N-dimethyl-p-phenylenediamine with 2b according to the procedure described for 3a. FT-IR (KBr, cm-1): 2926, 2856 (C-H stretching of aliphatic); 1660 (carbonyl). 1 H NMR (CDCl3) ppm: 9.50 (s, 1H, formyl); 7.16 (m, 2H, phenylene ortho to azo); 7.01 (m, 1H, H3 of pyrrole); 6.77 (m, 2H, phenylene meta to azo); 6.34 (m, 1H, H4 of pyrrole); 4.30 (m, 2H, NCH2); 2.96 (s, 6H, N(CH3)2); 1.81 (m, 2H, NCH2CH2); 1.31 (m, 6H, N(CH2)2(CH2)3); 0.87 (t, J = 6.6 Hz, 3H, N(CH2)5CH3). Anal. Calcd for C19H26N4O: C, 69.91; H, 8.03; N, 17.16. Found: C, 69.75; H, 8.16; N, 17.04. Small Molecule B. A flask was charged with a solution of 3a (0.2871 g, 1.85 mmol) and 4-nitrobenzylcyanide (0.1920 g, 1.85 mmol) in ethanol (20 mL). Sodium hydroxide (0.20 g, 5.00 mmol) dissolved in ethanol (10 mL) was added to this solution. The reaction mixture was stirred for 1 h at room temperature under N2 and then was concentrated under reduced pressure. Water was added to the concentrate, and B precipitated as a dark-green solid. It was recrystallized from ethanol/water (0.3630 g, 51%). FT-IR (KBr, cm-1): 3360 (N-H stretching); 2920 (C-H stretching of aliphatic); 2170 (cyano); 1518, 1346 (nitro). 1 H NMR (CDCl3) ppm: 8.33 (m, 2H, phenylene ortho to nitro); 8.00 (br, 1H, NH); 7.90 (s, 1H, vinylene); 7.52 (m, 2H, phenylene meta to nitro); 7.16 (m, 2H, phenylene ortho to azo); 6.77 (m, 2H, phenylene meta to azo); 6.18 (m, 2H, pyrrole); 2.96 (s, 6H, N(CH3)2). Anal. Calcd for C21H18N6O2: C, 65.27; H, 4.70; N, 21.75. Found: C, 65.03; H, 4.42; N, 21.34. Small Molecule B6. B6 was prepared as a dark-green solid in 60% yield from the reaction of 3b with 4-nitrobenzylcyanide in ethanol in the presence of sodium hydroxide according to the procedure described for B. FT-IR (KBr, cm-1): 2926, 2856 (C-H stretching of aliphatic); 2174 (cyano); 1514, 1346 (nitro). 1 H NMR (CDCl3) ppm: 8.33 (m, 2H, phenylene ortho to nitro); 7.90 (s, 1H, vinylene); 7.52 (m, 2H, phenylene meta to nitro); 7.16 (m, 2H, phenylene ortho to azo); 6.77 (m, 2H, phenylene meta to azo); 6.18 (m, 2H, pyrrole), 4.30 (m, 2H, NCH2); 2.96 (s, 6H, N(CH3)2); 1.81 (m, 2H, NCH2CH2); 1.31 (m, 6H, N(CH2)2(CH2)3); 0.87 (t, J = 6.6 Hz, 3H, N(CH2)5CH3). Anal. Calcd for C27H30N6O2: C, 68.91; H, 6.43; N, 17.86. Found: C, 68.53; H, 6.38; N, 17.52

5-(Anthracen-10-yldiazenyl )-1-hexyl-1H-pyrrole-2-carbaldehyde (4). A flask was charged with a suspension of 9-aminoanthracene (0.7170 g, 3.71 mmol) in water (10 mL). Hydrochloric acid (2 mL) was added to the suspension. The mixture was cooled and kept at 0-5 °C in an ice bath and diazotized by adding a solution of NaNO2 (0.2800 g, 4.06 mmol) Langmuir 2010, 26(22), 17739–17748

Article in water (5 mL) followed by stirring for 0.5 h at 0-5 °C. The solution of the diazonium salt thus prepared was immediately used for the next coupling reaction. The solution of the above diazonium salt was slowly added to a solution of 2b (0.6651 g, 3.71 mmol) in ethanol (20 mL) at 0-5 °C. The resulting mixture was stirred at room temperature for 10 h and then concentrated under reduced pressure. Water (20 mL) containing hydrochloric acid (1 mL) was added to the concentrate. The mixture was extracted with dichloromethane. The organic layer was dried (Na2SO4) and concentrated under reduced pressure to afford 4. It was purified by column chromatography via elution with a mixture of dichloromethane and hexane (1:1). Yield 70% (1.00 g). FT-IR (KBr, cm-1): 2930, 2856 (C-H stretching of aliphatic); 1666 (carbonyl). 1 H NMR (CDCl3) ppm: 9.50 (s, 1H, formyl); 8.40 (s, 1H, H10 of anthracene); 7.98 (m, 4H, H1, H4, H5, H8 of anthracene); 7.44 (m, 4H, H2, H3, H6, H7 of anthracene); 7.01 (m, 1H, H3 of pyrrole); 6.34 (m, 1H, H4 of pyrrole); 4.30 (m, 2H, NCH2); 1.81 (m, 2H, NCH2CH2); 1.31 (m, 6H, N(CH2)2(CH2)3); 0.87 (t, J = 6.6 Hz, 3H, N(CH2)5CH3). Anal. Calcd for C25H25N3O: C, 78.30; H, 6.57; N, 10.96. Found: C, 77.94; H, 6.68; N, 10.70. Small Molecule A. A was prepared as a dark-green solid in 63% yield from the reaction of 4 with 4-nitrobenzylcyanide in ethanol in the presence of sodium hydroxide according to the procedure described for B. FT-IR (KBr, cm-1): 2926, 2854 (C-H stretching of aliphatic); 2174 (cyano); 1584, 1344 (nitro). 1 H NMR (CDCl3) ppm: 8.40 (s, 1H, H10 of anthracene); 8.33 (m, 2H, phenylene ortho to nitro); 7.98 (m, 4H, H1, H4, H5, H8 of anthracene); 7.90 (s, 1H, vinylene); 7.52 (m, 2H, phenylene meta to nitro); 7.44 (m, 4H, H2, H3, H6, H7 of anthracene); 6.18 (m, 2H, pyrrole); 4.30 (m, 2H, NCH2); 1.81 (m, 2H, NCH2CH2); 1.31 (m, 6H, N(CH2)2(CH2)3); 0.87 (t, J = 6.6 Hz, 3H, N(CH2)5CH3). Anal. Calcd for C33H29N5O2: C, 75.12; H, 5.54; N, 13.27. Found: C, 74.89; H, 5.33; N, 13.07. Characterization Methods. IR spectra were recorded on a Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets. 1H NMR (400 MHz) spectra were obtained using a Bruker spectrometer. Chemical shifts (δ values) are given in parts per million with tetramethylsilane as an internal standard. UV-vis spectra were recorded on a Beckman DU-640 spectrometer with spectrograde THF. Elemental analyses were carried out with a Carlo Erba model EA1108 analyzer. The photoluminescence spectra of the materials in the thin film were recorded with a 4500 model Hitachi spectrophotometer. The XRD patterns of the blend thin films were recorded with an XRD system using Cu KR as the radiation source having a wavelength of λ = 1.5405 A˚. The atomic force microscopy (AFM) images were recorded with a Digital Instruments nanoscope. The electrochemical properties of both SMs were examined using cyclic voltammetry (CV) (EDCA electrochemistry system). The SMs were coated onto a glassy carbon electrode, which was used as the working electrode, and immersed in a 0.1 mol/L Bu4NPF6 acetonitrile solution used as the supporting electrolyte. Cyclic voltammograms were recorded using Ag/Agþ as the reference electrode at a scan rate of 100 mV/s. Device Fabrication and Characterization. The organic PV devices were prepared in a glovebox on indium tin oxide (ITO)coated glass substrates. The substrates were cleaned in an ultrasonic bath with water and acetone and then dried under ambient conditions. A thin film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (about 70 nm) was deposited by the spin-coating technique at 3000 rpm for 60 s and then baked for 30 min at 80 °C. The thickness of the PEDOT:PSS layer is about 80 nm. In the glovebox, a mixture of B or B6 or A:PCBM (1:1 ratio) was dissolved in THF and spin coated on the top of the ITO/PEDOT:PSS film at 2500 rpm for 60 s. The thickness of the active layer was in the range of 70-80 nm. Finally, an aluminum DOI: 10.1021/la103168t

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Mikroyannidis et al. Scheme 1. Synthesis of Molecules B and B6

Scheme 2. Synthesis of Molecule A

(Al) electrode was thermally evaporated through a shadow mask under high vacuum. The effective area of the device was 10 mm2. The solvent treatment (annealing) of the SM:PCBM blend film was carried out before thermal annealing and metal electrode deposition. The blend was transferred to a glass jar filled with THF and remained for 20 min. The solvent treatment was controlled by the slow evaporation rate of the solvent, which was carried out by adding a small amount of solvent to the glass jar to keep the film wet until it had completely solidified. The subsequent thermal annealing of the solvent-treated active layers was carried out at 120 °C for 2 min on a hot plate before the deposition of the Al electrode. For the electrical and photoelectrical characterization of the pristine SM, we have also fabricated the devices based on pristine SM of the ITO/PEDOT:PSS/B or B6 or A/Al structure. For the hole mobility measurement, we have fabricated devices with a structure of ITO/PEDOT:PSS/B or B6 or A/Au. We have also fabricated separate hole- and electron-only devices with ITO/PEDOT:PSS/A:PCBM/Au and Al/A:PCBM/ Al structures, respectively, to measure the hole and electron mobility of the BHJ active layer. The current-voltage (J-V) measurements of the devices in the dark and under illumination were measured by a semiconductor parameter analyzer (Keithley 4200-SCS). A xenon light source (Oriel) was used to give a stimulated irradiance of 100 mW/cm2 (equivalent to AM 1.5 irradiation) at the surface of the device. The photoaction spectrum of the devices was measured using a monochromator (Spex 500 M), and the resulting photocurrent was measured with a Keithley electrometer (model 6514), which is interfaced to a computer via LABVIEW software.

Results and Discussion Synthesis and Characterization. Scheme 1 outlines the three-step synthesis of molecules B and B6. In particular, pyrrole (1a) and N-hexylpyrrole (1b) were formylated28 by DMF and phosphorus oxychloride to afford aldehydes 2a and 2b, respectively. These aldehydes reacted with the diazonium salt, which 17742 DOI: 10.1021/la103168t

was prepared from the reaction of N,N-dimethyl-p-phenylenediamine with NaNO2/aqueous HCl, to give compounds 3a and 3b, respectively. The electron-donating N,N-dimethyl groups of the diazonium salt favor this coupling reaction. Finally, 3a and 3b were condensed with 4-nitrobenzylcyanide in ethanol in the presence of sodium hydroxide to yield target molecules B and B6. The third small molecule, A, was synthesized according to Scheme 2. Specifically, N-hexylpyrrole-2-carbaldehyde (2b) was coupled with the diazonium salt, which was derived from 9-aminoanthracene, to afford compound 4. The latter was condensed with 4-nitrobenzylcyanide to give A. All molecules were soluble in common organic solvents such as dichloromethane, chloroform, and THF. Molecule B displayed lower solubility in these solvents than did the other molecules because they lacked the solubilizing N-hexyl chain. The FT-IR and 1H NMR spectra of the molecules were consistent with their chemical structures. The IR spectra showed common absorption bands at around 2930 and 2850 (C-H stretching of aliphatic i.e., N-hexyl and/or N,N-dimethyl), 2170 (cyano), and 1520 and 1340 cm-1 (nitro). Figure 1 presents a typical 1H NMR spectrum of B6. It shows upfield signals at 8.33 (phenylene ortho to nitro), 7.90 (cyanovinylene), and 7.52 ppm (phenylene meta to nitro). The other phenylene resonated at 7.16-6.77 ppm, and the pyrrole resonated at 6.18 ppm. Finally, the aliphatic moieties gave peaks at 4.30-0.87 ppm. Photophysical Properties. Figure 2 presents the UV-vis absorption spectra of molecules in THF solution (10-5 M) and a thin film. Table 1 summarizes the photophysical and electrochemical characteristics of molecules. The absorption spectra were broad and extended from 300 up to approximately 700 nm in solution and 750 nm in the thin film. The long-wavelength absorption maximum (λa,max) was at 601-637 nm. These λab,max values could be attributed to an intramolecular charge transfer (ICT) between the electron-donating azo-substituted pyrrole and the Langmuir 2010, 26(22), 17739–17748

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Article Table 1. Optical and Electrochemical Properties of SMs material

B

B6

A

632 631 601 λa,max in solution (nm)a 633 630 637 λa,max in thin film (nm)a thin film absorption onset (nm) 743 752 766 1.67 1.65 1.62 Egopt (eV)b 0.60 0.58 0.40 Eox onset (V) red -1.15 -1.17 -1.28 Eonset (V) HOMO (eV) -5.30 -5.28 -5.10 LUMO (eV) -3.55 -3.53 -3.42 1.75 1.75 1.68 Egel (eV)c a λa,max: Absorption maxima from the UV-vis spectra in THF solution or in a thin film. b Egopt: Optical band gap determined from the absorption onset in a thin film. c Egel: Electrochemical band gap determined from cyclic voltammetry.

1

Figure 1. H NMR spectrum in CDCl3 solution of molecule B6. The solvent peak is denoted by an asterisk.

segment. Molecules B and B6 were almost equivalent, with respect to their photophysical properties, even though B6 carried an Nhexyl chain and B lacked it. Generally, the optical properties of these molecules were dominated by the presence of the cyanovinylene 4-nitrophenyl segment. This segment is capable of broadening and red shifting the absorption curves of the molecules, as has been well established in our previous publications.29 Electrochemical Properties. Electrochemical cyclic voltammetry has been widely employed to investigate the redox behavior of the organic semiconducting polymers and monomers and to estimate their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The onset oxidation (Eox) and reduction (Ered) potentials of the SMs are summarized in Table 1. The HOMO and LUMO energy levels as well as the electrochemical energy band gap (Egel) were estimated according to following equations30 HOMO ¼ - qðEox þ 4:7Þ eV LUMO ¼ - qðE red þ 4:7Þ eV and Eg el ¼ qðEox - E red Þ eV

Figure 2. UV-vis absorption spectra of SMs in THF solution (top) and a thin film (bottom).

electron-deficient cyanovinylene 4-nitrophenyl. The thin film absorption onset was located at 743, 752, and 766 nm, corresponding to optical band gaps (Egopt) of 1.67, 1.65, and 1.62 eV for B, B6, and A respectively. Molecule A displayed the lowest Egopt value among the synthesized molecules because of the anthracene Langmuir 2010, 26(22), 17739–17748

where the unit of the potential is V versus Ag/Agþ. The values of the HOMO, LUMO, and Egel are also summarized in Table 1. The Egel of these SMs is slightly higher than the corresponding optical band gap, which is probably due to the exciton binding energy of the organic materials.31 The LUMO levels of B, B6, and A are -3.55, -3.53, and -3.42 eV, respectively, and are higher than that of PCBM (-4.0 eV). This guarantees the effective exciton charge separation at the interface donor (B, B6, or A) and acceptor (PCBM) in the organic BHJ solar cells.4 In addition, the deeper HOMO levels of SMs (-5.30, -5.28, and -5.10 eV for B, B6, and A, respectively) are beneficial for the higher open circuit voltage (Voc) of the OSCs because Voc is usually proportional to the difference between the LUMO level of the acceptor and the HOMO level of the donor, and the deeper the HOMO level of the donor, the larger the difference will be. Electrical and Photovoltaic Properties. We have investigated the electrical and PV properties of single-layer devices based on the pristine SMs (i.e., B, B6, or A). Figure 3 shows the J-V (29) Mikroyannidis, J. A.; Kabanakis, A. N.; Balraju, P.; Sharma, G. D. Macromolecules 2010, 43, 5544. (30) Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J. Mater. Chem. 2003, 13, 1377. (31) Zhu, Y.; Chempion, R. D.; Jeneke, S. A. Macromolecules 2006, 39, 8712.

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Figure 3. Current-voltage (J-V) characteristics of pristine SM thin films in (a) the dark and (b) under an illumination intensity of 100 mW/ cm2, sandwiched between PEDOT:PSS/ITO and Al electrodes.

characteristics of the devices based on SM thin films sandwiched between ITO/PEDOT:PSS and Al electrodes in the dark and under an illumination intensity of 100 mW/cm2 at room temperature. The J-V characteristics of all devices in the dark show the rectification effect when a positive potential is applied to the ITO/PEDOT:PSS electrode with respect to the Al electrode. Because the HOMO level of PEDOT:PSS is very close to the HOMO levels of all SMs (Table 1), this electrode behaves as an ohmic contact for hole injection from the PEDOT:PSS-coated ITO electrode into the HOMO of SMs. However, the LUMO level of all SMs (Table 1) is very far from the work function of Al (-4.2 eV) and forms the Schottky barrier for electron injection from Al into the LUMO level of SMs. Therefore, the rectification effect is due to the formation of the Schottky barrier at the Al-SM layer interface, and all SMs behave as p-type organic semiconductors. The charge carrier mobility of the organic semiconductors used as photoactive material thin films in organic PV devices is also an important factor that influences the short circuit current (Jsc) and PCE of the devices. The hole mobility of B, B6, and A was measured using the space charge limited current (SCLC) method.32 The J-V characteristics of the devices having structure ITO/PEDOT:PSS/B or B6 or A/Au, in dark at room temperature, were plotted as ln [Jd3/ (Vapp -Vbi)] vs [(Vapp -Vbi)/d]1/2 (where Vapp and Vbi are the applied voltage and built in voltage, respectively) and are shown in Figure 4. The hole mobilities of B, B6 and A estimated from the intercepts of the corresponding straight lines in lower voltage region are 2.3 x10-6 2.8 x10-6 and 6.7 x10-6 cm2/(V s), respectively. The PV parameters, i.e., short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) were estimated from the J-V characteristics under illumination intensity of 100 mW/cm2 (Figure 3b) and listed in Table 2. It can be seen from this Table that all PV parameters of the device based on B and B6 are almost the same, but the Jsc and FF for the device based on A is higher than that for the devices based on B and B6. The Jsc depends on the number of excitons generated in the photoactive layer and their dissociation into free charge carriers at the D-A interfaces present in the layer and their transportation to the external circuit. The number of exciton generation mainly depends on the band gap and optical absorp(32) (a) Chirvase, D.; Chiguvare, Z.; Knipper, M.; Parisi, J.; Dyakonov, V.; Hummelen, J. C. J. Appl. Phys. 2003, 93, 3376. (b) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, C. Phys. Rev. B 1998, 58, R13411.

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Figure 4. Current -voltage characteristics of ITO/PEDOT:PSS/ B or B6 or A/Au devices, in the form of ln[Jd3/(Vapp- Vbi)2] vs [(Vapp - Vbi)/d]1/2. Table 2. Photovoltaic Parameters of the ITO/PEDOT:PSS/B or B6 or A/Al Devices material

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

B B6 A

0.088 0.094 0.13

0.70 0.69 0.71

0.30 0.30 0.33

0.018 0.019 0.030

tion spectra of the photoactive material employed in the device. The band gap of B and B6 is almost similar; however the band gap of A is slightly lower than that of B and B6. Additionally, the optical absorption of A is broader than that of B and B6 (Figure 2). The lower band gap and broader absorption spectra of A as compared to B and B6 imply that the number of excitons generated in the device based on A is higher than that for B or B6. Moreover, the hole mobility of A is higher than that for B and B6, thus causing a more efficient hole transport in the device based on A, as compared to the devices based on B and B6. Therefore, the higher value of Jsc and PCE for the device based on the A is due to the lower band gap, broader optical absorption spectra and higher hole mobility. The overall PCE for the PV devices based on the Langmuir 2010, 26(22), 17739–17748

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Figure 6. IPCE spectra of the BHJ photovoltaic devices based on the as-cast B:PCBM, B6:PCBM, and A:PCBM blends.

Figure 5. Current-voltage characteristics of the devices based on the as-cast B:PCBM, B6:PCBM, and A:PCBM blend thin films at an illumination intensity of 100 mW/cm2. Table 3. Photovoltaic Parameters of the ITO/PEDOT:PSS/B or B6 or A:PCBM/Al Devices material

Jsc (mA/cm2)

Voc (V)

B:PCBMa 3.89 0.98 3.94 0.98 B6:PCBMa 5.10 0.92 A:PCBMa 5.86 0.91 A:PCBMb 6.56 0.88 A:PCBMc a As-cast blend. b Solvent-treated blend. solvent-treated blend.

FF

PCE (%)

0.35 1.33 0.36 1.36 0.44 2.06 0.48 2.56 0.49 2.83 c Thermally annealed

the device based on the A:PCBM blend as compared to that for the other two devices. The higher light-harvesting property of the A:PCBM blend leads to higher exciton generation in the blend, resulting in a higher photocurrent in the device. The values of the incident photon to current efficiency (IPCE) have been estimated using the following expression IPCEð%Þ ¼ 1240J sc =λPin

pristine B, B6 and A is very low. However, based on the position of LUMO and HOMO of these SMs with respect to the PCBM, these materials can be used as donor components along with PCBM as acceptor for BHJ PV devices. BHJ Photovoltaic Devices Based on SMs. In order to achieve the optimum device performance, BHJ devices using B, B6 or A:PCBM were fabricated, and their weight ratios were varied from 1:1 to 1:4 and it is found the blend with weight ratio 1:2 gives the optimum PV response. Figure 5 shows the J-V characteristics of the ITO/PEDOT:PSS/B or B6 or A:PCBM (1:2)/Al devices based on the as-cast active BHJ layers, and Table 3 lists the corresponding PV parameters (Jsc, Voc, FF, and PCE) of the devices under an illumination intensity of AM 1.5, 100 mW/ cm2. The PCE values for the devices based on as-cast blends B: PCBM, B6:PCBM, and A:PCBM are 1.33, 1.36, and 2.06%, respectively. It can be seen from Table 3 that the Voc for the BHJ PV device based on B is higher than that for A. The Voc strongly depends on the energy difference between the HOMO of the donor and the LUMO of the acceptor.33 The HOMO of B is deeper than that of A, which results in a higher value of Voc. However, the order of the Jsc values is A:PCBM > B6:PCBM > B:PCBM. The higher value of PCE for the A:PCBM-based BHJ device may be due to the higher values of Jsc and FF. As can be seen from the absorption spectra of the donor materials, the absorption band in the longer-wavelength region is broader for A. The broader absorption results in improved light harvesting for

where Jsc (mA/cm2) is the photocurrent under short circuit conditions and Pin (mW/cm2) and λ (nm) are the illumination intensity and wavelength of the monochromatic light, respectively. The IPCE spectra of the devices (Figure 6) shows two peaks, one near 400 nm and another broad band in the longerwavelength region (520-650 nm). The IPCE spectra resemble the absorption spectra of SMs, which indicates that the excitons produced by the absorbed photons in the photoactive layer are dissociated into free charge carriers at the donor-acceptor interfaces formed between the SMs and PCBM throughout the whole volume of the photoactive layer and subsequently collected by the electrodes The peak at around 400 nm corresponds to photocurrent generation due to the exciton generation in the blend (light absorption by the PCBM), and the broad band observed in the longer-wavelength region can be ascribed to exciton generation in the donor material (B, B6, or A) in the blend. The value of IPCE that corresponds to the shorterwavelength region is almost the same for all devices, but the value of IPCE in the longer-wavelength band is higher for the device based on A:PCBM than for other devices. Photocurrent generation in the BHJ PV device is determined by the product of the absorbed photons within the solar spectrum and the incident quantum efficiency (IQE).34 It can be seen from Table 1 and Figure 2 that the optical band gap of A is lower than that for B and B6. Moreover, the absorption band of A in the longer-wavelength region is broader than that of B and B6, indicating that the number of photons absorbed by the A:PCBM blend is higher than that for B or B6:PCBM blends, leading to a larger number of generated excitons. Therefore, the larger number of generated excitons may be one of the reasons for higher values of the photocurrent or IPCE value for the device based on A:PCBM than for other blends. The IQE is determined by three

(33) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromhertz, T.; Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 374.

(34) (a) Lloyd, M. T.; Anthony, J. E.; Malliaras, G. G. Mater. Today 2007, 10, 34. (b) Schillinsky, R.; Waldauf, C.; Brabec, C. J. Appl. Phys. Lett. 2002, 81, 3885. (c) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924.

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Figure 7. Photoluminescence (PL) spectra of B, B:PCBM, B6: PCBM, and A:PCBM thin films.

processes: the diffusion of photogenerated excitons toward the D/A interface, exciton dissociation, and charge separation at the interfaces and the collection of the charge carriers by the electrodes. The diffusion of the exciton toward the D/A interface in the photoactive blend layer depends upon the nanoscale morphology of the blend. We have observed that the AFM images of all blends are similar, indicating that the diffusion of the photogenerated excitons in all devices is the same. The driving force for exciton dissociation into the free charge carriers is the energy difference between the LUMOs of the acceptor and donor components used in the active layer of the device. This difference should be higher than the binding energy of the exciton (0.34 eV) for efficient photoinduced charge separation and to avoid the undesired process of back charge transfer.35 The photoluminescence (PL) measurement was performed to examine the photoinduced charge transfer efficiency from the donor to PCBM. Figure 7 compares the PL spectra of B, B: PCBM, B6:PCBM, and A:PCBM blends in the thin film. All SMs show strong PL with an emission maximum in the range of 730740 nm (shown only for B in Figure 7). The intensity of the PL emission of all pure SMs is almost the same. The PL intensity of B:PCBM, B6:PCBM, and A:PCBM is quenched (with respect to SM used in the blend), indicating that photoinduced charge transfer takes place effectively from B, B6, or A to PCBM in the respective devices. The higher PL quenching in A:PCBM as compared to that in B:PCBM and B6:PCBM indicates that more efficient charge transfer occurs in the former as compared to that in other devices. The driving force for the photoinduced charge transfer in the BHJ photovoltaic device is the energy difference between the LUMOs of the acceptor and donor components used in the photoactive layer.4 The HOMO and LUMO levels for PCBM acceptor are -6.2 and -3.95 eV, respectively. It can be seen from Table 1 that the LUMO levels of A, B, and B6 are -3.42, -3.53, and -3.55 eV, respectively. The energy difference between the LUMO (acceptor) and LUMO (donor) is higher for the A:PCBM blend as compared to that for the B:PCBM and B6: PCBM blends. This also supports our conclusion that the degree of photoinduced charge transfer is faster in the device based on A:PCBM than that for other devices. (35) (a) Sun, S.; Fan, Z.; Wang, Y.; Haliburton, J. J. Mater. Sci. 2005, 40, 1429. (b) Brabec, C. J.; Winder, C.; Sariciftci, N. S.; Hummelen, J. C.; Dhanabalan, A.; vanHal, P. A.; Janssen, R. A. J. Adv. Funct. Mater. 2002, 12, 709.

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The charge collection efficiency in the BHJ PV device depends upon the charge carrier mobility, the percolated path of electrons and holes in the active layer, and also the position of the HOMO and LUMO energy levels of the donor and acceptor materials, respectively, relative to the work function of the anode and cathode. The hole mobility for the A is higher than that for B and B6, resulting in a better collection of charge carriers by the electrodes. Consequently, the higher the light-harvesting ability of the A:PCBM blend, the higher efficient photoinduced charge transfer and collection of charge carriers, leading to an improvement in Jsc, IPCE, and PCE for the device based on the A:PCBM blend as compared to devices based on B:PCBM and B6:PCBM blends. We have investigated the effect of solvent treatment (annealing) and subsequent thermal annealing of a solvent-treated film on the PV response of BHJ devices based on the A:PCBM blend. The J-V characteristics under illumination and the IPCE spectra of the devices are shown in Figure 8. The overall PCEs for the devices based on solvent-treated and thermally annealed solventtreated A:PCBM blends are 2.56 and 2.83%, respectively. It can be seen from the IPCE spectra of the device that the values of IPCE that correspond to the lower-wavelength peak (belonging to the PCBM phase) are more or less the same, but the IPCE value has been increased upon both solvent treatment and thermal annealing of the solvent-treated blend. This indicates that A plays an important role in the enhancement in the IPCE and Jsc and consequently the PCE of the device. The increase in the IPCE and Jsc of the devices based on both solvent-treated and thermally annealed solvent-treated blends is attributed to the increase in the crystallinity of the blend and the hole mobility in the A phase upon both solvent treatment and subsequent thermal annealing of solvent-treated blends. This can be ascribed to the effective formation of phase-separated structure induced by combined solvent treatment and subsequent thermal annealing, which leads to better connectivity between the donor and acceptor phases. The increase in the IPCE (Figure 8b) in the longer-wavelength region after the solvent treatment and subsequent thermal annealing may be attributed to the morphology change occurring in the A phase of the blend. The IPCE values of the devices fabricated from the as-cast, solvent-treated, and subsequent thermally annealed blends are around 45, 61, and 69%, respectively. The maximum Jsc is given by the following expression Z Jsc ¼

λ2 λ1

qIPCEðλÞ Nph ðλÞ dλ

where Nph(λ) is the photon flux intensity in the spectrum at any wavelength (λ), q is the electronic charge, and λ1 and λ2 are the wavelengths that correspond to the lower and upper limits of the IPCE spectra, which are 350 and 800 nm, respectively. After the IPCE values are integrated, the theoretical values of Jsc are about 4.72, 5.65, and 6.66 mA/cm2 for the as-cast, solvent-treated, and subsequent thermally annealed solvent-treated blends, respectively, which is consistent with the experimental values. Because the self-assembly of electroactive molecules has a strong effect on their optical and electronic properties,36 we have examined the morphology of the as-cast, solvent-treated, and subsequent thermally annealed solvent-treated A:PCBM blends using atomic force microscopy (AFM) images of the films (Figure 9). We have estimated from the AFM images rms surface roughness values of 3.4, 4.2, and 5.4 nm for the as-cast, (36) Mishra, A.; Ma, C. Q.; Bauerle, P. Chem. Rev. 2009, 109, 1141.

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Figure 8. (a) Current-voltage characteristics and (b) IPCE spectra of the devices based on solvent-treated and thermally annealed solventtreated A:PCBM blend thin films.

Figure 9. AFM topography of A:PCBM blend thin films. The image size is 3 μm  3 μm.

solvent-treated, and simultaneous thermally annealed solventtreated A:PCBM, respectively. The increase in the roughness of the blend films after annealing is due to the fact that the A molecules are self-organized into ordered structures after solvent and thermal annealing.37 The higher surface roughness of the blends supports the increase in the phase separation of A:PCBM after solvent treatment and subsequent thermal annealing. The increased phase separation leads to an increase in the D/A interfacial area for efficient charge separation, resulting in higher Jsc and PCE for BHJ PV devices based on the solvent-treated and thermally annealed solvent-treated blend films. The XRD pattern (Figure 10) of the thin films was also used to determine the difference in the crystallinity of the A:PCBM blend films (as-cast, solvent-treated, and thermally annealed solventtreated). It can be seen from this figure that the diffraction peak of the A:PCBM blend film is centered at 2θ = 15.1°, which corresponds to an interplanar distance of 13.4 A˚. Because most of the fullerene acceptors, such as PCBM, do not show any diffraction peak in the range of 2θ from 5 to 25°, we assume that this diffraction peak observed in the XRD pattern corresponds to A. Both solvent treatment and simultaneous thermal annealing (37) (a) Zhao, Y.; Xie, Z.; Qu, Y.; Geng, Y.; Wang, L. Appl. Phys. Lett. 2007, 90, 043504. (b) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (c) Zhao, Y.; Guo, X.; Xie, Z.; Qu, Y.; Geng, Y.; Wang, L. J. Appl. Polym. Sci. 2009, 111, 1799. (d) Chen, L. M.; Hong, Z.; Li, G.; Yang, Y. Adv. Mater. 2009, 21, 1434.

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Figure 10. XRD patterns of the as-cast, solvent-treated, and thermally annealed solvent-treated A:PCBM blend thin films.

lead to increases in the peak intensity, indicating a higher degree of crystallinity. The increase in the crystalline nature of the A:PCBM blend after annealing is mainly caused by the A molecules’ self-organization into an ordered structure.38 The increase in the crystallinity of A in the A:PCBM blend upon solvent treatment and thermal annealing leads to an improvement in (38) (a) Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R. Org. Lett. 2010, 12, 660.(b) Aich, R. B.; Zou, Y.; Leclerc, M.; Tao, Y. Org. Electron. 2010 doi:10.1016 j.orgel.2010.03.004.

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the light-harvesting property by extending the conjugation length.1b,39 The enhancement of the surface roughness may increase the heterojunction area and may also reduce the charge transport distance for carriers while providing a nanoscaled texture that further enhances internal light absorption.1b,40 This rough film morphology and increased crystallinity combined with the good solar spectral coverage of the annealed blend film can account for the enhancement in the PV performance. Low charge carrier mobilities result in charge accumulation and inefficient charge collection, and unbalanced charge carrier mobilities decrease the FF and PCE of BHJ PV devices by promoting charge recombination.41 The charge mobility is also strongly dependent on the molecular packing and film morphology.36 To get information about charge transport in the BHJ devices based on the as-cast and annealed blends, we have fabricated hole-only devices with ITO/PEDOT:PSS/A: PCBM/Au structure for the as-cast, solvent-treated, and subsequent thermally annealed blend, and the J-V characteristics in the dark were measured. The zero-field hole mobility was extracted using the space charge limited current (SCLC) model.42 The hole mobilities for the as-cast, solvent-treated, and simultaneous thermal and solvent-treated films are 5.2  10-6, 1.2  10-5, (39) (a) Jo, J.; Kim, S. S.; Na, S. I.; Yu, B. K.; Kim, D. Y. Adv. Funct. Mater. 2009, 19, 866. (b) Walker, B.; Tamayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garica, A.; Tantiwiwat, M.; Nguyen, T. Q. Adv. Funct. Mater. 2009, 19, 3063. (40) (a) Chang, Y. T.; Hsu, S. L.; Chen, G. Y.; Su, M. H.; Singh, T. A.; Diau, E. W. G.; Wei, K. H. Adv. Funct. Mater. 2008, 18, 2356. (b) Liu, Y.; Wan, X.; Yin, B.; Zhou, J.; Long, G.; Yin, S.; Chen, Y. J. Mater. Chem. 2010, 20, 2464. (41) (a) Peumans, P.; Forrest, S .R. Chem. Phys. Lett. 2004, 398, 27. (b) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006, 88, 052104. (42) Mihailetchi, V. D.; Xie, H. X.; deBoer, B.; Koster, L .J. A.; Blom, P. W. M. Adv. Funct. Mater. 2006, 16, 699.

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and 3.5  10-5 cm2/(V s), respectively. The increase in the hole mobility for an annealed blended film may be due to an enhancement in the crystallinity of A and the surface roughness of the blend. These factors improve the charge transport in the device leading to an increase in the Jsc, FF, and PCE of the device based on annealed blends. The increase in the hole mobility in the blend upon both solvent treatment and annealing treatment also indicates more balanced charge transport in the devices based on annealed blends.

Conclusions Three new SMs (B, B6, and A) with a low band gap based on 2-styryl-5-phenylazo-pyrrole were synthesized. They were soluble in common organic solvents, showed broad absorption curves, and had an optical band gap of 1.62-1.67 eV. We have used these SMs as electron donors along with PCBM as the electron acceptor. The PCEs of the BHJ devices based on as-cast B:PCBM, B6:PCBM, and A:PCBM are 1.33, 1.36, and 2.05%, respectively. The higher PCE value for the device based on the A:PCBM blend as compared to the other blends has been attributed to the higher hole mobility, lower band gap of A, and the higher energy difference between the LUMOs of A and PCBM. The BHJ devices with solvent-treated and subsequent thermally annealed A:PCBM blends have PCEs of about 2.56 and 2.83%, respectively. The improved PCE has been interpreted in terms of more balanced charge transport due to the increase in the surface roughness and crystallinity of the blend. The increase in the hole mobility is also one of the reasons for the increase in the PCE of the devices based on the solvent-treated and subsequent thermally annealed blends.

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