Synthesis of a Low-Band-Gap Small Molecule Based on

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Synthesis of a Low-Band-Gap Small Molecule Based on Acenaphthoquinoxaline for Efficient Bulk Heterojunction Solar Cells )

J. A. Mikroyannidis,*,† A. N. Kabanakis,† Anil Kumar,‡ S. S. Sharma,‡ Y. K. Vijay,‡ and G. D. Sharma*,§, Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece, ‡ Physics Department, Thin Film & Membrane Science Laboratory, University of Rajasthan, Jaipur (Raj.) 302004, India, §Physics Department, Molecular Electronic and Optoelectronic Device Laboratory JNV University, Jodhpur (Raj.) 342005, India, and Jaipur Engineering College, Kukas, Jaipur (Raj.), India )

†

Received April 28, 2010. Revised Manuscript Received June 23, 2010 A novel small molecule (SM) with a low-band-gap based on acenaphthoquinoxaline was synthesized and characterized. It was soluble in polar solvents such as N,N-dimethylformamide and dimethylacetamide. SM showed broad absorption curves in both solution and thin films with a long-wavelength maximum at 642 nm. The thin film absorption onset was located at 783 nm, which corresponds to an optical band gap of 1.59 eV. SM was blended with PCBM to study the donor-acceptor interactions in the blended film morphology and the photovoltaic response of the bulk heterojunction (BHJ) devices. The cyclic voltammetry measurements of the materials revealed that the HOMO and LUMO levels of SM are well aligned with those of PCBM, allowing efficient photoinduced charge transfer and suitable open circuit voltage, leading to overall power conversion efficiencies (PCEs) of approximately 2.21 and 3.23% for devices with the as-cast and thermally annealed blended layer, respectively. The increase in the PCE with the thermally annealed blend is mainly attributed to the improvement in incident photon to current efficiency (IPCE) and short circuit photocurrent (Jsc). Thermal annealing leads to an increase in both the crystallinity of the blend and hole mobility, which improves the PCE.

Introduction Harvesting energy directly from sunlight using photovoltaic technology is considered to be one of the most important ways to address growing global energy needs. Organic solar cells (OSCs) are a promising alternative for clean, renewable energy because of their potential advantage in being fabricated onto large-area, lightweight flexible substrates by solution processing at a lower cost.1 Certain reviews for advanced materials and processes of polymer solar cells have recently been reported.2 Bulk-heterojunction (BHJ) polymer photovoltaic cells,3 whose absorber material is a blend of a polymer-based electron donor (D) and a fullerene derivative acceptor (A) such as [6,6]-phenylC61 butyric acid methyl ester (PCBM), offering a promising potential for commercial applications because of their compatibility with low-cost manufacturing processes. Several donor materials have been developed for BHJ photovoltaic cells, including *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) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15. (b) G€unes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. (c) Ma, C. Q.; Fonrodona, M.; Schikora, M. C.; Wienk, M. M.; Janssen, R. A. J.; Bauerie, P. Adv. Funct. Mater. 2008, 18, 3323. (d) Tamayo, A. B.; Dang, X. D.; Walker, B.; Seo, J.; Kent, T.; Nguyen., T. O. Appl. Phys. Lett. 2009, 94, 103301. (e) Zhang, J.; Yang, Y.; He, C.; He, Y. J.; Zhao, G. J.; Li, Y. F. Macromolecules 2009, 42, 7619. (f) Thompson, B. C.; Frechet, J. M. Angew. Chem., Int. Ed. 2008, 47, 58. (g) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45. (h) Krebs, F. C.; Nielsen, T. D.; Fyenbo, J.; Wadstrøm, M.; Pedersen, M. S. Energy Environ. Sci. 2010, 3, 512. (i) Krebs, F. C.; Gevorgyan, S. A.; Gholamkhass, B.; Holdcroft, S.; Schlenker, C.; Thompson, M. E.; Thompson, B. C.; Olson, D.; Ginley, D. S.; Shaheen, S. E.; et al. Sol. Energy Mater. Sol. Cells 2009, 93, 1968. (2) (a) Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 394. (b) Helgesen, M.; Sondergaard, R.; Krebs, F. C. J. Mater. Chem. 2010, 20, 36. (3) Spanggaard, H.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2004, 83, 125. (4) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222.

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poly(3-hexylthiophene),4-9 poly[2-methoxyl-5-(30 ,70 -dimethyloctyloxy)-p-phenylenevinylene],10,11 and poly[2-methoxyl-5-(20 ethylhexyoxyl)-1,4-phenylene vinylene],12 and the achievable power-conversion efficiency (PCE) has approached 6%.5 The key attributes of the donor material contributing to high PCE have been identified to include a high hole mobility, a low band gap (which increases light absorption), and good miscibility with the acceptor material to maximize the BHJ area. BHJ OSCs constitute a promising technology because they are easy to fabricate by solution processing and are predicted to yield a PCE of up to 10-15% if suitable low-band-gap donor material is discovered.13 Although considerable research effort has been expanded to develop such low-band-gap donor materials, the highest reported PCEs of OSC based on a conjugated polymer donor and PCBM acceptors are in the range of 5-6.5%.14 Recently, Liang et al. have reported a PCE of about 7.4% for BHJ solar cells based on a benzodithiphene polymer and a PC71BM blend.15 (5) Kim, K.; Liu, J.; Namboothiry, M. A. G.; Carroll, D. L. Appl. Phys. Lett. 2007, 90, 163511. (6) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Appl. Phys. Lett. 2005, 86, 063520. (7) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. Adv. Funct. Mater. 2007, 17, 1636. (8) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15, 1617. (9) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (10) van Duren, J. K. J.; Yang, X. N.; Loos, J.; Bulle-Lieuwma, C. W. T.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J. Adv. Funct. Mater. 2004, 14, 425. (11) Tuladhar, S. M.; Poplavskyy, D.; Choulis, S. A.; Durrant, J. R.; Bradley, D. D. C.; Nelson, J. Adv. Funct. Mater. 2005, 15, 1171. (12) Gupta, D.; Kabra, D.; Kolishetti, N.; Ramakrishnan, S.; Narayan, K. S. Adv. Funct. Mater. 2007, 17, 226. (13) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec., C. J. Adv. Mater. 2006, 18, 789.

Published on Web 07/12/2010

DOI: 10.1021/la1016966

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The majority of the research relating to BHJ solar cells has focused on polymeric donor materials because they generally have better film-forming properties than nonpolymeric materials. However, the structural characteristics, including molecular weight, polydispersity, and regioregularity, and the purity of the polymer greatly affect its functional properties and the performance and stability of the resulting devices.16 In addition to the conjugated polymers, OSCs based on conjugated small-molecule donors and the PCBM acceptor have attracted growing interest because small molecules have well-defined molecular structure, definite molecular weight, and high purity without batch-to-batch variations. As a result, OSCs fabricated by vacuum-deposited small-molecule heterostructures have reached a PCE as high as 5.6%.17 Within the class of organic semiconductors, quinoxaline-based polymers and small molecules are active materials for OSCs.18,19 The quinoxaline unit has an attractive structure from the viewpoint of controlling the morphology because it has the advantage of introducing the substituent groups easily onto the two and three positions of quinoxaline. Quinoxaline is an n-type building block, and the electron-deficient N-heterocycle has been utilized to construct some donor polymers with typical donor-acceptor (D-A) structure. Specifically, an alternating copolymer derived from fluorene and 5,8-dithienylquinoxaline possessed an Eg of 1.94 eV, a very low lying HOMO of -6.3 eV, and a photovoltaic device with a PCE of 3.7%.19 Moreover, a random quinoxalinecontaining polyfluorene with 30% 5,8-dithienylquinoxaline had a slightly larger Eg of 2.14 eV, and its device showed a lower PCE of 1.18%.20 In addition, an alternating copolymer, derived from 2,7carbazole and 5,8-dithienylquinoxaline, possessed an Eg of 2.02 eV and the device had a PCE of 1.8%.18 Finally, quinoxaline moieties have recently been used as acceptors in polymers with the D-A structure.21-23 Acenaphthylene is a fused aromatic segment that has been inserted into materials for photovoltaic applications. In particular, several dithiophene and fluorene copolymers containing acenaphthylene thieno[3,4-b]pyrazine moieties have been synthesized. These copolymers displayed an optical band gap of 1.33-1.62 eV, and their BHJ solar cells with PCBM as an acceptor had a PCE of 0.13-0.70%.24 (14) (a) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 826. (b) Kim, Y.; Cook, S.; Tualdhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C. S.; Ree., M. Nat. Mater. 2006, 5, 197. (c) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (d) Park, S. H.; Roy, A.; Beaupre, S.; Coates, S.; Cho., N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 2, 297. (e) Chen, H. Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649. (f) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.; Son, H.; Li, G.; Yu, L. J. Am. Chem. Soc. 2006, 131, 56. (15) Linag, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. doi: 10.1002/adma.200903528. (16) Jorgensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008, 92, 686. (17) Chan, M. Y.; Lai, S. L.; Fung, M. K.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 2007, 90, 023504/1. (18) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2008, 130, 732. (19) Gadisa, A.; Mammo, W.; Andersson, L. M.; Admassie, S; Zhang, F.; Andersson, M. R.; Ingan€as, O. Adv. Funct. Mater. 2007, 17, 3836. (20) Sun, M.; Wang, L.; Xia, Y.; Du, B.; Liu, R.; Cao, Y. Acta Polym. Sin. 2007, 952. (21) Lee, W. Y.; Cheng, K. F.; Wang, T. F.; Chen, W. C.; Tsai, F. Y. Thin Solid Films 2010, 518, 2119. (22) Chen, C. H.; Hsieh, C. H.; Dubosc, M.; Cheng, Y. J.; Hsu, C. S. Macromolecules 2010, 43(2), 697. (23) Zhang, X.; Steckler, T. T.; Dasari, R. R.; Ohira, S.; Potscarage, W. J.; Tiwari, S. P.; Coppee, S.; Ellinger, S.; Barlow, S.; Bredas, J. L.; Kippelen, B.; Reynolds, J. R.; Marder, S. R. J. Mater. Chem. 2010, 20, 123. (24) Mondal, R.; Miyaki, N.; Becerril, H. A.; Norton, J. E.; Parmer, J.; Mayer, A. C.; Tang, M. L.; Bredas, J. L.; McGehee, M. D.; Bao, Z. Chem. Mater. 2009, 21, 3618.

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Recently, we have synthesized a series of low-band-gap polymers and small molecules containing cyanovinylene 4-nitrophenyl segments that have been used to fabricate BHJ solar cells.25 In the present investigation, we successfully synthesized a broadly absorbing small molecule (SM) that carries a central acenaphthoquinoxaline unit, intermediate thiophene, and terminal cyanovinylene 4-nitrophenyl on both sides. The fused central unit of SM can enhance the π-π stacking and the intermolecular interactions, thus leading to higher charge-carrier mobilities.26 In addition, planarization of a part of SM may reduce its band gap. Finally, the presence of the terminal cyanovinylene 4-nitrophenyls in SM is expected to extend its absorption into the nearinfrared spectrum region.25 The photovoltaic properties of the BHJ devices based on SM/PCBM blends were investigated, and the overall PCEs for these devices were 2.21 and 3.23% on the basis of the as-cast and thermally annealed blend.

Experimental Section Reagents and Solvents. 4-Nitrobenzyl cyanide was synthesized from the nitration of benzyl cyanide with concentrated nitric and sulfuric acid.27 It was recrystallized from ethanol. 2-Tributylstannylthiophene was prepared from the reaction of thiophene with n-BuLi in hexane and subsequently with tributylchlorostannane according to the literature.28 1,2-Phenylenediamine was sublimed under vacuum and then recrystallized from toluene. 1,2-Acenaphthoquinone was recrystallized from toluene. N,NDimethylformamide (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. Acenaphtho[1,2-b]quinoxaline (1). 1,2-Acenaphthoquinone (0.91 g, 5.00 mmol) was dissolved by heating in a mixture of glacial acetic acid (20 mL) and acetonitrile (20 mL). 1,2-Phenylenediamine (0.54 g, 5.00 mmol) was added portionwise to the stirred solution, and the resulting mixture was refluxed for 2 h under N2. Compound 1 precipitated during heating and was filtered off, washed with water, and dried. The crude product was recrystallized from acetonitrile (0.98 g, 76%). FT-IR (KBr, cm-1): 3048, 1614, 1480, 1432 (aromatic); 758 (quinoxaline). 1H NMR (CDCl3) ppm: 8.13-8.11 (m, 4H, H5, H6 of acenaphthylene and H6, H7 of quinoxaline); 8.02 (m, 2H, H3, H8 of acenaphthylene); 7.86-7.77 (m, 4H, H4, H7 of acenaphthylene and H5, H8 of quinoxaline). Anal. Calcd for C18H10N2: C, 85.02; H, 3.96; N, 11.02. Found: C, 84.95; H, 4.08; N, 11.10. 5,8-Dibromo-acenaphtho[1,2-b]quinoxaline (2). A flask was charged with a solution of 1 (0.30 g, 1.17 mmol) in chloroform (25 mL). Bromine (0.37 g, 2.34 mmol) dissolved in chloroform (15 mL) was added dropwise to the stirred solution at room temperature. The mixture was subsequently refluxed for 2 h. Compound 2 precipitated during the heating. It was filtered off, washed with water, and dried. The crude product was recrystallized from acetonitrile (0.36 g, 75%). FT-IR (KBr, cm-1): 3056, 1606, 1494, 1448 (aromatic); 1104 (C-Br); 754 (quinoxaline). 1H NMR (DMSO-d6) ppm: 8.13-8.10 (m, 4H, H5, H6 of acenaphthylene and H6, H7 of quinoxaline); 8.02 (m, 2H, H3, H8 of acenaphthylene); 7.85 (m, 2H, H4, H7 of acenaphthylene).

(25) (a) Mikroyannidis, J. A.; Stylianakis, M. M.; Balraju, P.; Suresh, P.; Sharma, G. D. ACS Appl. Mater. Interfaces 2009, 1, 1711. (b) Mikroyannidis, J. A.; Stylianakis, M. M.; Suresh, P.; Balraju, P.; Sharma, G. D. Org. Electron. 2009, 10, 1320. (c) Mikroyannidis, J. A.; Sharma, S. S.; Vijay, Y. K.; Sharma, G. D. ACS Appl. Mater. Interfaces 2010, 2, 270. (26) Zhang, X. N.; Johnson, J. P.; Kampf, J. W.; Matzger, A. J. Chem. Mater. 2006, 18, 3470. (27) Robertson, G. R. In Organic Syntheses; Gilman, H., Blatt, A. H.,Eds.; J. Wiley & Sons: New York, 1941; Vol. 1, p 396.Robertson, G. R. In Organic Syntheses; J. Wiley & Sons: New York, 1922; Vol. 2, p 57. (28) Xia, Y.; Luo, J.; Deng, X.; Li, X.; Li, D.; Zhu, X.; Yang, W.; Cao, Y. Macromol. Chem. Phys. 2006, 207, 511.

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Article Scheme 1. Synthesis of SM

Anal. Calcd for C18H8Br2N2: C, 52.46; H, 1.96; N, 6.80. Found: C, 52.13; H, 1.87; N, 6.94.

8,11-Bis(thiophene-2-yl)-acenaphtho[1,2-b]quinoxaline (3). A flask was charged with a mixture of 2 (0.10 g, 0.24 mmol), 2-tributylstannylthiophene (0.22 g, 0.58 mmol), dioxane (25 mL), and PdCl2(PPh3)2 (0.0050 g, 3% mol). The flask with the reagents was evacuated several times and filled with N2. The mixture was refluxed for 12 h under N2 and cooled to room temperature. Then the solution was filtered, and the filtrate was concentrated under reduced pressure. The residue was triturated with ethanol and filtered to give 3 (0.09 g, 88%). FT-IR (KBr, cm-1): 3052, 1614, 1482, 1434 (aromatic); 760 (quinoxaline). 1H NMR (DMSO-d6) ppm: 8.12-8.10 (m, 4H, H5, H6 of acenaphthylene and H6, H7 of quinoxaline); 8.01 (m, 2H, H3, H8 of acenaphthylene); 7.86 (m, 2H, H4, H7 of acenaphthylene); 7.09 (s, 4H, H3, H4 of thiophene); 7.20 (s, 2H, H5 of thiophene). Anal. Calcd for C26H14N2S2: C, 74.61; H, 3.37; N, 6.69. Found: C, 74.27; H, 3.26; N, 6.58.

8,11-Bis(5-formylthiophene-2-yl)-acenaphtho[1,2-b]quinoxaline (4). A flask was charged with a solution of 3 (0.37 g, 0.89 mmol) in 1,2-dichloroethane (30 mL). DMF (0.22 g, 3.01 mmol) and POCl3 (0.45 g, 2.93 mmol) were added dropwise, and the mixture was refluxed for 17 h. After cooling to room temperature and adding dichloromethane (15 mL) and a saturated aqueous solution of CH3COONa (30 mL), the mixture was stirred for 2 h at room temperature. The organic phase was then washed with water and dried over Na2SO4. Solvent removal and column chromatography (silica gel/dichloromethane) gave 4 (0.32 g, 76%). FT-IR (KBr, cm-1): 3052, 1570, 1434 (aromatic); 1724 (formyl); 760 (quinoxaline). 1H NMR (CDCl3) ppm: 10.03 (s, 2H, formyl); 8.13-8.10 (m, 4H, H5, H6 of acenaphthylene and H6, H7 of quinoxaline); 8.01 (m, 2H, H3, H8 of acenaphthylene); 7.86 (m, 2H, H4, H7 of acenaphthylene); 7.09 (s, 4H, H3, H4 of thiophene). Anal. Calcd for C28H14N2O2 S2: C, 70.87; H, 2.97; N, 5.90. Found: C, 70.46; H, 3.12; N, 6.03. Compound SM. A flask was charged with a solution of 4 (0.42 g, 0.89 mmol) and 4-nitrobenzyl cyanide (0.29 g, 1.78 mmol) in ethanol (40 mL). Sodium hydroxide (0.20 g, 5.00 mmol) dissolved in ethanol (10 mL) was added portionwise to the stirred solution. The mixture was stirred for 1 h at room temperature under N2 and then was concentrated under reduced pressure. The concentrate was cooled in a refrigerator to precipitate a dark-green solid. It was filtered, washed thoroughly with water, and dried to afford SM (0.54 g, 80%). FT-IR (KBr, cm-1): 2168 (cyano); 1584, 1344 (nitro); 1522, 1436 (aromatic); 758 (quinoxaline). 1H NMR (DMSO-d6) ppm: 8.18 (m, 4H, phenylene ortho to nitro); 8.138.10 (m, 4H, H5, H6 of acenaphthylene and H6, H7 of quinoxaline); Langmuir 2010, 26(15), 12909–12916

8.02 (m, 2H, H3, H8 of acenaphthylene); 7.85 (m, 2H, H4, H7 of acenaphthylene); 7.70 (s, 2H, olefinic); 7.50 (m, 4H, phenylene meta to nitro); 7.10 (s, 4H, H3, H4 of thiophene). Anal. Calcd for C44H22N6O4S2: C, 69.28; H, 2.91; N, 11.02. Found: C, 68.95; H, 2.81; N, 11.10. 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 DMF. Elemental analyses were carried out with a Carlo Erba model EA1108 analyzer. The electrochemical properties of SM were studied by cyclic voltammetry (CV) (CH Instruments). A platinum (Pt) disk was coated with SM and immersed in a 0.1 M Bu4NPF6 acetonitrile solution. A cyclic voltammagram was recorded using Pt as the working electrode and Ag/Agþ as the reference electrode at a scan rate of 20 mV/s. X-ray diffraction experiments were performed on a Bruker D8 advanced model diffractometer with Cu KR radiation (λ = 1.542 A˚) at a generator voltage of 40 kV and a current of 40 mA. Atomic force microscopy (AFM) studies were performed using Digital Instruments nanoscope in trapping mode. Device Fabrication and Characterization. The organic photovoltaic devices were fabricated on patterned indium tin oxide (ITO)-coated glass substrates, which had been cleaned by successive ultrasonic treatment in acetone and isopropyl alcohol and then dried at 80 °C for 30 min. A 50-nm-thick PEDOT/PSS layer was spin coated onto the ITO-coated glass from a PEDOT/ PSS aqueous solution at 2000 rpm and then baked at 100 °C for 30 min under ambient conditions. The active layer of the SM and PCBM blend was spin coated, using DMF as a solvent, onto the PEDOT/PSS layer. An aluminum top electrode was deposited in vacuum onto the active layer at a pressure of 10-5 Pa. The area of the device was ∼10 mm2, and it was defined by the mask depositing Al. Prethermal annealing of the active layer was carried out at 100 °C for 2 min on a hot plate before the deposition of the Al electrode. Current-voltage (J-V) characteristics of the devices were measured using a computer-controlled Keithley 238 source meter in the dark and under an illumination intensity of about 100 mW/cm2 under ambient conditions. A 100 W halogen lamp without a solar simulator was used as a light source. The J-V characteristics measurements under illumination were carried out in a dark chamber (wooden box) with a window slit of 2 cm2 area for illumination. DOI: 10.1021/la1016966

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Figure 1. 1H NMR spectrum of SM in DMSO-d6 solution. The incident photon to current efficiency (IPCE) of the devices was measured by illuminating the device with a halogen lamp using a monochromator and measuring the resulting photocurrent with a Keithley electrometer under short circuit conditions. The IPCE was determined from the following expression IPCEð%Þ ¼

1240Jsc λPin

where Jsc is the short circuit photocurrent and λ and Pin are the wavelength and illumination intensity of the incident light, respectively.

Results and Discussion Synthesis and Characterization. Scheme 1 outlines the fivestep reaction sequence for the synthesis of SM. In particular, 1,2acenaphthoquinone was condensed with 1,2-diphenyldiamine to afford quinoxaline derivative 1. The bromination of the latter gave dibromo compound 2. This was coupled with 2-tributylstannylthiophene in dioxane by utilizing PdCl2(PPh3)2 as a catalyst to yield 3. The formylation of 3 was carried out by means of DMF and POCl3 to give 4. Finally, the condensation of 4 with 4-nitrobenzyl cyanide in ethanol in the presence of sodium hydroxide afforded SM. The presence of the fused acenaphthoquinoxaline segment as well as the absence of aliphatic chains in the SM molecule renders it soluble only in polar solvents such as DMF and dimethylacetamide. A literature survey revealed that 2 has been previously synthesized by an alternative synthesis route, specifically, by the condensation of 1,2-acenaphthoquinone with 3,6-dibromo-1,2-phenylenediamine.29 Moreover, the 4-hexylthiophene derivative of 3 has been synthesized and electrochemicaly polymerized for electrochromic applications.29 SM and intermediate compounds 1-4 were characterized by FT-IR and 1H NMR spectroscopy. SM showed characteristic absorption bands at 2168 cm-1 (cyano) and 1584 and 1344 cm-1 (nitro). The 1H NMR spectrum of SM (Figure 1) displayed upfield signals at 8.18 ppm (phenylene ortho to nitro) and 8.13-7.85 ppm (acenaphthylene and quinoxaline). Finally, the olefinic protons of SM gave a singlet at a higher shift (7.70 ppm) than that of the unsubstituted olefinic protons because of the electron-withdrawing cyano group. Photophysical Properties. Figure 2 presents the UV-vis absorption spectra of SM in both DMF solution and a thin film. The latter was prepared on a quartz substrate from a DMF solution of SM by spin casting at relatively high temperature. The (29) Udum, Y. A.; Durmus, A.; Gunbas, G. E.; Toppare, L. Org. Electron. 2008, 9, 501.

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Figure 2. Normalized UV-vis absorption spectra of SM in DMF solution and a thin film. (The spectra were normalized with respect to the long-wavelength absorption maximum.) Table 1. Photophysical Properties of SM compound

SM

λa,max in solution (nm)a 642 642 λa,max in a thin film (nm)a 1.59 Egopt (eV)b HOMO (eV) -5.10 LUMO (eV) -3.45 c el 1.65 Eg (eV) a λa,max is the absorption maxima from the UV-vis spectra in DMF solution or a thin film. b Egopt is the optical band gap determined from the absorption onset in a thin film. c Egel is the electrochemical band gap determined from cyclic voltammetry.

photophysical characteristics of SM are summarized in Table 1. Both absorption spectra were broad and extended approximately up to 800 nm, which is desirable for photovoltaic applications. The presence of the cyanovinylene 4-nitrophenyl moieties is responsible for the broadening of the absorption curve, as has been well established in our previous publications.25 Both absorption spectra displayed a long-wavelength absorption maximum (λa,max) at 642 nm. The thin film absorption onset was located at 783 nm, corresponding to an optical band gap of 1.59 eV that conforms to our previous data.25 The thin film absorption spectrum was broader than that in solution owing to the higher intermolecular interactions in the solid state. The fused and planar aromatic structure of acenaphthoquinoxaline provides a flat π-electron-rich face that can promote π-π stacking between the molecules of SM. These enhanced interactions can improve the crystallinity as well as the absorption coefficients in the solid state. The absorption spectrum of the pure SM compound changes considerably after thermal annealing, as shown in Figure 3. In particular, a significant increase in the absorption intensity at 642 nm occurs after thermal annealing. This type of increase in the optical absorption after thermal annealing has been observed for conjugated polymers30 and small molecules.31 This feature is attributed to the aggregation/interchain interactions and the increase in the crystallinity of the material, which enhances the intensity of π-π* electronic transitions. To investigate the crystallinity of SM in the solid state, the XRD pattern was recorded (Figure 4). The sharp peak at 2θ = 24.2° (30) (a) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schillnsky, P.; Waldauf, C.; Brabec, C. J. Adv. Funct. Mater. 2005, 15, 1193. (b) Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V. Nanotechnology 2004, 15, 1317. (31) Walker, B.; Tamayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q. Adv. Funct. Mater. 2009, 19, 3063.

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Figure 5. Cyclic voltammogram of an SM film on a Pt electrode at a scan rate of 20 mV/s. Figure 3. UV-vis absorption spectrum of the thermally annealed SM thin film.

Figure 4. X-ray diffraction patterns of the as-cast and thermally annealed SM films.

reveals a short π-π distance of 5.6 A between the acenaphthoquinoxaline central unit, indicating that this unit has a planar conformation in the solid state. The intensity of this peak increases for the thermally annealed film, which implies a higher degree of crystallinity after thermal annealing at 100 °C. The electrochemical properties of the SM were studied by cyclic voltammetry. Figure 5 shows the cyclic voltammogram of a compound film on the Pt working electrode in an acetronitrile solution of 0.1 mol/L with a scan rate of 100 mV/s. The compound exhibited a reversible reduction/reoxidation (n doping/dedoping) process indicating the stability of the n-doping state before n doping. The onset of oxidation and the reduction potential were 0.59 V and -1.06 V vs Ag/Agþ, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the compound were estimated according to the following equations32 EHOMO ¼ - eðEonset ox þ 4:71Þ eV E LUMO ¼ - eðEonset red þ 4:71Þ eV where the unit of potential is V versus Ag/Agþ. The calculated HOMO and LUMO energy levels of the material are listed in Table 1. The LUMO level of SM is 0.65 eV higher than that of PCBM (-4.1 eV). This favors efficient exciton charge separation at the interface of SM and PCBM in OSCs with the compound as (32) Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J. Mater. Chem. 2003, 13, 800.

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Figure 6. Current-voltage characteristics of an ITO/PEDOT/ PSS/SM/Au device in the dark and under illumination.

the donor and PCBM as the acceptor.13 The HOMO-LUMO gap obtained from the electrochemistry for SM is 1.65 eV, which is slightly higher than the optical band gap (1.59 eV) estimated from the onset of the absorption edge in the film, which is a common phenomenon reported in the literature.33-35 The deeper HOMO level of SM is beneficial for a higher open circuit voltage (Voc) of OSCs with SM as the donor material because Voc is usually proportional to the difference between the LUMO level of the acceptor and the HOMO level of the donor. Electrical and Photovoltaic Properties of SM. Figure 6 shows the J-V characteristics of the device based on a pure SM thin film sandwiched between ITO/PEDOT/PSS and Al electrodes in the dark as well as under an illumination intensity of 100 mW/cm2 at room temperature. The J-V characteristics of the device in the dark show a rectification effect when a positive potential is applied to a (PEDOT/PSS)-coated ITO electrode with respect to an Al electrode. Because the work function of PEDOT/ PSS (-5.2 eV) is very close to the HOMO level of SM (-5.1 eV), this electrode behaves as a nearly ohmic contact for hole injection (33) Johansson, T.; Mammo, W.; Svensson, M.; Andersson, M. R.; Inganas, O. J. Mater. Chem. 2003, 13, 1316. (34) Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. J. Am. Chem. Soc. 2006, 128, 4911. (35) Shang, H.; Fan, H.; Shi, Q.; Li, S.; Li, Y.; Zhan, X. Sol. Energy. Mater. Sol. Cells 2010, 94, 457.

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Figure 7. Current-voltage characteristics from the ITO/PEDOT/ PSS/SM/Au device for the as-cast and annealed device plotted in ln [Jd3/(Vappl - Vbi)2] vs [(Vappl - Vbi)0.5].

from the electrode into the HOMO level of SM. However, the LUMO level of SM (-3.45 eV) is very far from the work function of Al (-4.2 eV) and forms a Schottky barrier for electron injection from Al into the LUMO level of SM. Therefore, the rectification effect is due to the formation of the Schottky barrier at the AlSM interface. The charge carrier mobility of materials used in OSCs is also an important factor that influences the performance of the devices. The hole mobility of SM was measured using space charge limited current (SCLC)36 with device structure ITO/PEDOT/PSS /SM/ Au. The J-V curve of the device was plotted as ln [Jd3/(Vappl Vbi)2] versus [(Vappl - Vbi)/d ]0.5 and is shown in Figure 7. The hole mobilities of SM calculated from the intercept of the corresponding line are 1.2  10-5 and 3.6  10-5 cm2/V s for the as-cast and thermally annealed SM film, respectively. These values are higher than the hole mobilities measured for certain small molecules (i.e., 10-6 cm2/V s for triphenylamine derivatives37 and 10-6-10-5 cm2/V s for oligothiophene and diketopyrrolopyrrole31,38 derivatives, which have been determined by the SCLC model). The hole mobility of SM is slightly lower than that of the widely used donor poly(3-hexylthiophene) (P3HT), which has a typical value of hole mobility of (1.4-3.0)  10-4 cm2/V s.39 The π-electron delocalization, the planar aromatic structure of the central acenaphthoquinoxaline unit, and the π-π* stacking observed from the XRD data are believed to be directly responsible for the high SCLC mobility of SM. The increase in the hole mobility upon thermal annealing is attributed to the ordering in the structure of SM. The photovoltaic parameters (i.e., short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) are 0.40 mA/cm2, 0.77 V, 0.42, and 0.13%, respectively). The value of PCE for the Schottky barrier device using SM is very low, but the hole mobility is quite reasonable. Therefore, SM can be used as an electron donor for BHJ OSCs using PCBM as an acceptor. (36) (a) Chirvase, D.; Chiguvare, Z.; Knipper, M.; Parisi, J.; Dyakonov, V.; Hummelen, J. C. J. Appl. Phys. 2003, 93, 3376. (b) Martens, H. C. F.; Brom, H. B.; Blow, P. W. M. Phys. Rev. B 1999, 60, R8489. (c) Liang, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L. P. J. Am. Chem. Soc. 2009, 131, 7792. (37) He, C.; He, Q. G.; Yang, X. D.; Wu, G. L.; Yang, C. H.; Bai, F. L.; Shuai, Z. G.; Wangand, L. X.; Li., Y. F. J. Phys. Chem. C 2007, 111, 8661. (38) Tamayo, A. B.; Walker, B.; Nguyen, T. Q. J. Phys. Chem. C 2008, 112, 11545. (39) Mahailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Adv. Funct. Mater. 2006, 16, 699.

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Figure 8. Current-voltage characteristics of OSCs based on the SM/PCBM blend in the dark, under an illumination intensity of 100 mW/cm2.

Figure 9. IPCE spectra of the photovoltaic devices based on the as-cast and thermally annealed blends.

Photovoltaic Studies for the BHJ Device. In the BHJ organic solar cell, the performance is mainly governed by (i) the band gap of the donor material, (ii) the position of the HOMO and LUMO levels of the donor and acceptor materials used in the active layer, and (iii) the mobilities of the charge carriers in the donor and acceptor phase. It can be seen from Table 1 that the band gap of SM is 1.60 eV and the difference between the LUMO levels of SM and PCBM is about 0.65 eV. This difference is greater than the exciton binding energy, which is a prerequisite for efficient photoinduced charge separation in the BHJ active layer. In OSCs, the low mobility of the charge carrier results in charge accumulation and insufficient charge collection, and the difference between the electron and hole mobilities decreases the fill factor and PCE because of charge recombination.40 Consequently, electron and hole mobilities of acceptor and donor materials play an important role in the photovoltaic performance of the BHJ device. We have measured the hole and electron mobilities in the BHJ active layer (40) (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, 052104/1.

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using ITO/PEDOT/PSS/SM/PCBM/Au hole-only and ITO/PEDOT/PSS/SM/PCBM/Al electron-only devices, respectively, for the as-cast and annealed BHJ active layer according to the SCLC model, as explained earlier. For the as-cast BHJ layer, the hole and electron mobilities are 4.5  10-6 and 4.8  10-4 cm2/V s, respectively. However, for the thermally annealed BHJ layer, the values of hole and electron mobilities are about 2.4  10-5 and 5.2  10-4 cm2/V s, respectively. The electron mobility does not change to a large extent after thermal annealing. However, the hole mobility changes by a factor of 5.4. The increase in the mobility after thermal annealing was attributed to the enhanced crystalline nature of SM, as confirmed by the XRD measurement, which increases the number of percolation pathways for hole transport in the SM phase. Therefore, the carrier mobilities in the thermally annealed BHJ OCSs are more balanced and may be one of the factors increasing the PCE of the devices on the basis of the thermally annealed active layer. We have studied the photovoltaic response of BHJ OSCs based on the SM/PCBM blend with different ratios of SM and PCBM Table 2. Photovoltaic Parameters of OSCs Based on the SM/PCBM Blenda device

Jsc (mA/cm2)

Voc (V)

FF

η (%)

as cast 5.20 0.87 0.49 2.21 annealed 7.13 0.84 0.54 3.23 a Jsc is the short circuit current, Voc is the open circuit voltage, FF is the fill factor, and η is the power conversion efficiency.

and found that the blend with a 1:1 wt ratio gives the best performance. Figures 8 and 9 show the C-V characteristics in the dark and under illumination and the IPCE spectra, respectively. The photovoltaic parameters of the device are summarized in Table 2. Both devices have a relatively large Voc (0.84 and 0.87 V for the annealed and as-cast blends) because of the low-lying HOMO level of SM (-5.1 eV), resulting in larger differences relative to the LUMO level of PCBM. The Voc is slightly lower for the device based on the thermally annealed SM/PCBM blend. However, Jsc substantially increases from 5.2 to 7.16 mA/cm2 when a thermally annealed BHJ layer is used in the device, which increases the overall PCE from 2.21 to 3.23%. The increase in Jsc may be attributed to the improved crystallinity of the SM phase in the blend. This increases both the donor-acceptor (D-A) surface area and hole mobility in the blend and reduces the recombination due to the more balanced charge transport. The FF also increases for the device based on the annealed BHJ active layer. The shape of the IPCE spectra (Figure 9) of the device resembles the absorption spectra of the blended film, which confirms the formation of dispersive D-A interfaces throughout the volume of the active layer. The increase in the IPCE in the longer-wavelength region after thermal annealing may be attributed to the greater excitons generation by the SM owing to the increase in the crystallinity of SM. The IPCE values of the devices based on the as-cast and thermally annealed blend are 47% and 63%, respectively, at or near the absorption maxima. The increase in the IPCE and Jsc are related to the increase in absorption as well

Figure 10. AFM images of the as-cast and thermally annealed SM/PCBM blended films: (a, b) heights images and (c, d) rms images. Langmuir 2010, 26(15), 12909–12916

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as the hole mobility in the blend due to the enhancement in the crystallinity of SM induced by thermal annealing. The IPCE spectra of the devices are similar to the absorption spectra of the corresponding blend used in the devices. This indicates that the Jsc is mainly attributed to the exciton generated because of the photons absorbed in the blend. The Jsc is directly related to the external quantum efficiency (EQE), which is the product of light absorption, exciton diffusion, charge transfer, and charge collection efficiency.41 Hence, the increase in the IPCE and Jsc upon thermal annealing is ascribed to the improved light absorption by the photoactive layer, the change in the crystallinity of the donor SM in the blend, and the more balanced charge transport due to the increase in hole mobility. We have also investigated the morphologies of a pure SM film and blends with PCBM to get information about the average surface roughness of the film using atomic force microscopy (AFM). SM is soluble in DMF and forms smooth films with an average surface roughness 1.5 nm. Upon thermal annealing of the pure SM film at 100 °C, the average surface roughness increases to ∼2.1 nm. Figure 10 shows the topographic and rms AFM images of the as-cast and thermally annealed blended films. It can be seen from these images that thermal annealing results in significant changes in the surface morphology. The surface roughness has been increased from 1.8 to 3.4 nm when the film is annealed at 100 °C. The surface domain size has also been increased upon thermal annealing. This indicates that both the increased domain size and average surface roughness result in an (41) (a) Zhao, Y.; Xie, Z.; Qu, Y.; Geng, Y.; Wang, L. Appl. Phys. Lett. 2007, 90, 043504. (b) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693.

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improvement in the crystallinity of the blend, which is responsible for the enhancement in the PCE due to the more efficient photoinduced charge at the D-A interfaces.

Conclusions SM was successfully synthesized by a five-step reaction sequence. It was soluble in polar solvents such as N,N-dimethylformamide and dimethylacetamide. The long-wavelength absorption maximum in both the solution and the thin film appeared at 642 nm. The thin film absorption onset was at 783 nm, which corresponds to an optical band gap of 1.59 eV. OSCs have been fabricated by solution spin coating with SM as the donor and PCBM as the acceptor. Overall PCEs of 2.21 and 3.23% have been achieved for the devices based on the as-cast and thermally annealed SM/PCBM blends, respectively. The improvement in PCE with the device based on a thermally annealed blended active layer has been explained in terms of an increase in the surface roughness and crystallinity of the active layer upon thermal annealing. The enhancement in the crystallinity also increases the absorption of the active layer and charge transport in the device, which is responsible for the improvement in the PCE of the device based on the thermally annealed active layer. These results indicate that a higher PCE comparable to the solution-processed polymer-based BHJ devices could be achieved for BHJ based on small molecules, which have the advantage of mondisperse structure. Acknowledgment. We are thankful to scientists at IUAC, New Delhi, for helping to record the XRD and AFM data.

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