Molecular Design of Organic Dyes with Double Electron Acceptor for

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Molecular Design of Organic Dyes with Double Electron Acceptor for Dye-Sensitized Solar Cell Sung Soo Park,*,† Yong Sun Won,† Young Cheol Choi,‡ and Jae Hong Kim*,‡ Samsung Electro-Mechanics Co. Ltd., Corporate R&D Institute, Suwon, 443-743, Korea, and School of Display and Chemical Engineering, Yeungnam UniVersity, Gyeongsan, 712-749, Korea ReceiVed March 10, 2009. ReVised Manuscript ReceiVed May 10, 2009

We designed and synthesized novel organic dyes of double electron acceptor type based on phenothiazine framework, as photosensitizers for the dye-sensitized solar cell (DSC). The density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were used to estimate the photovoltaic properties of the dyes in the design stage. The molecular structure having two electron acceptors on both sides of phenothiazine moiety provided the efficient electron extraction paths from electron donor part, which was demonstrated by the analysis of the electronic structures on donor and acceptor, and the excitations between HOMOs and LUMOs. In accordance, the measurements of photovoltaic properties of the DSCs prepared in the laboratory scale showed that the organic dyes of double electron acceptor type gave about 20% higher performace than their counterparts of single electron acceptor type. The effect of the kind of electron acceptor (C, cyanocrylic acid and R, rhodanine-acetic acid) on the performance of DSC was studied as well. We found that the organic dyes with R as acceptor gave much lower efficiencies, compared to those with C, for both single and double electron acceptor type. It was attributed to the electronic decoupling between anchoring and TiO2 conduction band accompanying the lack of π-conjugation of carboxylic group as anchoring on R, despite that the dyes with R had relatively broad and intense absorption spectra in the visible region.

1. Introduction The improvement of solar energy-to-elctricity conversion efficiency has continued to be an important research area of dye-sensitized solar cell (DSC).1-5 A typical DSC is constructed with dye-absorbed wide band gap oxide semiconductor electrode such as TiO2 or ZnO, electrolyte containing I-/I3- redox couples, and Pt-coated counter electrode. The mechanism of DSC is based on the injection of electrons from the photoexcited dye into the conduction band of nanocrystalline TiO2 or ZnO. The oxidized dye is reduced by the hole injection into either the hole counter or electrolyte. The electronic structures, such as HOMO, LUMO, and HOMO-LUMO gap, of dye molecule in DSC are deeply related to the electron transfer by photoexcitation and redox potential.6,7 The photosensitizer (dye) is thus one of the critical components in DSC to improve the solar energy-to-electricity conversion efficiency.7 The most widely used photosensitizer for the DSC application is cis-di(thiocyanato)bis(4,4-dicarboxy-2,2-bipyridine) ruthenium(II), coded as * To whom correspondence should be addressed. E-mail: sung.s.park@ samsung.com (S.S. Park), [email protected] (J.H. Kim). Phone: 82-31210-5621 (S.S. Park), 82-53-810-2521. † Samsung Electro-Mechanics Co. Ltd. ‡ Yeungnam University. (1) Tributsch, H. Photochem. Photobiol. 1971, 14, 95. (2) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (3) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gratzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (4) Baxter, J. B.; Aydil, E. S. Appl. Phys. Lett. 2005, 86, 053114. (5) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Valchopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (6) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (7) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Yoshihara, T.; Murai, M.; Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.; Arakawa, H. AdV. Funct. Mater. 2005, 15, 246.

N3 or N719 dye, depending on whether it contains four or two protons.5,8 High performance and good stability of DSC based on Ru dyes had been widely addressed in the literatures.9 However, the Ru dyes are facing the problem of manufacturing costs and environmental issues. Organic dyes, because of their many advantages, such as high molar extinction coefficients, convenience of customized molecular design for desired photophysical and photochemical properties, inexpensiveness with no transition metals contained, and environment-friendliness,10 are suitable as photosensitizers for DSC. The efficiency of DSC based on metal-free organic dyes is known to be much lower than that of Ru dyes generally,11 but a very high solar energy-to-electricity conversion efficiency of up to 8% in full sunlight has been achieved by Ito et al. using an indoline dye.12 This result suggests that smartly designed metal-free organic dyes are also highly competitive candidates for photosensitizers of DSCs with their advantages mentioned above. The performance of DSC based on metalfree organic dyes has been remarkably improved recently by (8) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.; Vlachopoulos, N.; Shklover, V.; Fischer, C.-H.; Gratzel, M. Inorg. Chem. 1999, 38, 6298. (9) Gratzel, M. Inorg. Chem. 2005, 44, 6841. (10) (a) Hara, K.; Tachibana, Y.; Ohga, Y.; Shinpo, A.; Sugab, S.; Sayamaa, K.; Sugiharaa, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 77, 89. (b) Hara, K.; Wang, Z.-S.; Sato, T.; Furube, A.; Katoh, R.; Sugihara, H.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S. J. Phys. Chem. B 2005, 109, 15476. (c) Kim, D.; Lee, J. K.; Kang, S. O.; Ko, J. Tetrahedron 2007, 63, 1913. (11) (a) Wang, Z.-S.; Li, F.-Y.; Huang, C.-H. Chem. Commun. 2000, 2063. (b) Wang, Z.-S.; Li, F.-Y.; Huang, C.-H. J. Phys. Chem. B 2001, 105, 9210. (12) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; P’echy, P.; Takata, M.; Miura, H.; Uchida, S.; Gratzel, M. AdV. Mater. 2006, 18, 1202.

10.1021/ef900207y CCC: $40.75  2009 American Chemical Society Published on Web 06/25/2009

Organic Dyes with Double Electron Acceptor

Energy & Fuels, Vol. 23, 2009 3733

several groups.12-15 These researchers applied to tune the HOMO and LUMO energy levels of the chromophores, where the donor,13 linker,14 or acceptor15 moieties were alternated independently. In this paper, we for the first time suggest the novel organic dye with double electron acceptor, which is a new strategy to design an efficient photosensitizer for DSC. To verify the strategy, we synthesized organic dyes whose geometries, electronic structures and optical properties were derived from preceding density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The DSCs were then prepared with the metal-free organic dyes of double electron acceptor type on the phenothiazine framework, as well as of single electron acceptor type for comparison. The photovoltaic properties were measured to identify the effects of the number of electron acceptor (single or double) and the kind of electron acceptor (C, cyanocrylic acid or R, rhodanineacetic acid) on the performance of DSCs. 2. Experimental and Computational Details 2.1. Synthesis of Organic Dyes. Figure 1 shows synthetic procedure for the organic dyes with single or double electron acceptor. In the general procedure, chemicals were purchased from Aldrich and used without purification. N-(2-Ethyl-hexyl)-phenothiazine (1). A 100 mL 3-neck flask was charged with phenothiazine (10.00 g, 31.10 mmol), NaH (1.00 g, 41.70 mmol), and 40 mL of DMF. The resulting mixture was stirred for 30 min. 2-Ethylhexylbromide (8.06 g, 41.70 mmol) was then added and the mixture was stirred overnight at room temperature. The reaction mixture was quenched with water and extracted three (13) (a) Mosurkal, R.; Hea, J.; Yanga, K.; Samuelson, L. A.; Kumara, J. J. Photochem. Photobiol. A 2004, 168, 91. (b) Justin Thomas, K. R.; Lin, J. T.; Hsu, Y. C.; Ho, K. C. Chem. Commun 2005, 4098. (c) Tan, S.; Zhai, J.; Fang, H.; Jiu, T.; Ge, J.; Li, Y.; Jiang, L.; Zhu, D. Chem.sEur. J. 2005, 11, 6272. (d) Yanagida, S.; Senadeera, G. K. R.; Nakamura, K.; Kitamura, T.; Wada, Y. J. Photochem. Photobiol. A 2004, 166, 75. (e) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Chem. Commun. 2006, 2245. (f) Horiuchi, T.; Miura, H.; Uchida, S. Chem. Commun 2003, 3036. (g) Shibano, Y.; Umeyama, T.; Matano, Y.; Imahori, H. Org. Lett. 2007, 9, 1971. (h) Edvinsson, T.; Li, C.; Pschirer, N.; Scho¨neboom, J.; Eickemeyer, F.; Sens, R.; Boschloo, G.; Herrman, A.; Mu¨llen, K.; Hagfeldt, A. J. Phys. Chem. C 2007, 111, 15137. (i) Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.; Tsukagoshi, S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 1173. (j) Rao, T. N.; Bahadur, L. J. Electrochem. Soc. 1997, 144, 179. (k) Nasr, C.; Liu, D.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1996, 100, 11054. (l) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 4490. (m) Sayama, K.; Sugino, M.; Sugihara, H.; Abe, Y.; Arakawa, H. Chem. Lett. 1998, 753. (n) Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2005, 17, 813. (14) (a) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J. H.; Fantacci, S.; De Angelis, F.; Di Censo, D.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 16701. (b) Li, S. L.; Jiang, K. J.; Shao, K. F.; Yang, L. M. Chem. Commun 2006, 2792. (c) Velusamy, M.; Thomas, K. R. J.; Lin, J. T.; Hsu, Y. C.; Ho, K. C. Org. Lett. 2005, 7, 1899. (d) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Andersson, N. A.; Ai, X.; Lian, T.; Yanagida, S. Chem. Mater. 2004, 16, 1806. (e) Jung, I.; Lee, J. K.; Song, K. H.; Song, K.; Kang, S. O.; Ko, J. J. Org. Chem. 2007, 72, 3652. (f) Liang, M.; Xu, W.; Cai, F.; Chen, P.; Peng, B.; Chen, J.; Li, Z. J. Phys. Chem. 2007, 111, 4465. (g) Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783. (15) (a) Otaka, H.; Kira, M.; Yano, K.; Ito, S.; Mitekura, H.; Kawato, T.; Matsui, F. J. Photochem. Photobio. A 2004, 164, 67–73. (b) Hara, K.; Sayama, K.; Ohga, Y.; Shinpo, A.; Suga, S.; Arakawa, H. Chem. Commun. 2001, 569–570. (c) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (d) Wang, Z. S.; Cui, Y.; Hara, K.; Dan-oh, Y.; Kasada, C.; Shinpo, A. AdV. Mater. 2007, 19, 1138. (e) Horiuchi, T.; Miura, H.; Uchida, S. Chem. Commun. 2003, 3036. (f) Horiuchi, T.; Miura, H.; Uchida, S. H. J. Photochem. Photobiol. A 2004, 164, 29. (g) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (h) Tian, H.; Yang, X.; Chen, R.; Pan, Y.; Li, L.; Hagfeldt, A.; Sun, L. Chem. Commun. 2007, 3741.

Figure 1. Synthetic procedures for organic dyes: (a) ethylhexyl-bromide, DMF, NaH, (b) 1,2-dichloroethane, POCl3, DMF, (c) CH3CN, cyano acetic acid, piperidine, and (d) AcOH, AcONH4, rhodanin-3-acetic acid.

times with chloroform. The combined organic fractions were washed with brine and dried over MgSO4. The solvent was removed under reduced pressure and the residue was purified by column chromatography using silica gel and n-hexane/ethylacetate (9/1; v/v) as the eluent to give N-(2-ethyl-hexyl)-phenothiazine (1) as a yellow viscous liquid. Isolated yield ) 90%. 1H NMR (300 MHz, DMSOd6): δ (ppm) 7.18-7.12(m, 4H), 7.01(d, 2H, J ) 8.1), 6.92(t, 2H), 3.74(d, 2H, J ) 6.6), 1.79(m, 1H), 1.38-1.17(m, 8H), 0.82 -0.75(m, 6H). N-(2-Ethyl-hexyl)-phenothiazine-3-carbaldehyde (2) and N-(2-Ethylhexyl)-phenothiazine-3,7-dicarbaldehyde (3). To a solution of N-(2ethyl-hexyl)-phenothiazine (1) (5.00 g, 16.00 mmol) and dry DMF (5.87 g, 80.00 mmol) in 1,2-dichloroethane (DCE) (50 mL), phosphorus oxychloride (12.30 g, 80.00 mmol) was added slowly at 0 °C in an ice water bath. Then the bath was heated to reflux and maintained for overnight. The reaction mixture was quenched with water and extracted three times with chloroform. The combined organic fractions were washed with brine and dried over MgSO4. The solvent was removed under reduced pressure and the residue was purified by column chromatography using silica gel and n-hexane/ethylacetate (8/2; v/v) as the eluent to give N-(2-ethylhexyl)-phenothiazine-3-carbaldehyde (2) and N-(2-ethyl-hexyl)phenothiazine-3,7-dicarbaldehyde (3). Isolated yield ) 15.1%. 1H NMR (300 MHz, DMSO-d6): δ (ppm) 9.81(s, 2H), 7.76(d, 2H, J ) 8.7), 7.67(s, 2H), 7.30(d, 2H, J ) 8.7), 3.95(d, 2H. J ) 4.2), 1.80(m, 1H), 1.39-1.19(m, 8H), 0.83-0.75(m, 6H). PR6C1 and PR6C2. An acetonitrile solution of N-(2-ethylhexyl)-phenothiazine-3-carbaldehyde (2) or N-(2-ethyl-hexyl)-phenothiazine-3,7-dicarbaldehyde (3) was refluxed in the presence of piperidine and cynaoacetic acid for 3 h to produce PR6C1 and PR6C2, respectively. After removal of the solvent, the residue was purified by column chromatography using silica gel and dichloromethane/methanol (9/1; v/v) as the eluent to give PR6C1 and PR6C2. PR6R1: Dark-red solid. Yield ) 83.1%. 1H NMR (300

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MHz, DMSO-d6): δ(ppm) 8.70(s, 1H), 8.22(s, 1H), 8.10(d, 1H, J ) 9), 7.88(d, 1H, J ) 7.2), 7.78(d, 1H, J ) 9.3), 7.65(d, 1H, J ) 8.7), 7.54(t, 1H), 7.31(t, 1H), 4.33(d, 2H, J ) 7.2), 1.97(m, 1H), 1.28 -1.16(m, 8H), 0.85 -0.73(m, 6H). PR6R2: Dark-red solid. Yield ) 70.0%. 1H NMR(300 MHz, CDCl3): δ (ppm) 8.10(s, 2H), 7.93(d, 2H, J ) 8.7), 7.73(s, 2H), 7.19(d, 2H, J ) 8.4), 3.88(d, 2H, J ) 7.2), 1.82(m, 1H), 1.39 -1.22(m, 8H), 0.85 -0.80(m, 6H). PR6R1 and PR6R2. N-(2-Ethyl-hexyl)-phenothiazine-3-carbaldehyde (2) or N-(2-ethyl-hexyl)-phenothiazine-3,7-dicarbaldehyde (3) and rhodanine-3-acetic acid were added into glacial acetic acid and refluxed for 3 h in the presence of ammonium acetate. After it was cooled to room temperature, the mixture was poured into ice water. The precipitate was filtered and washed by distilled water to separate crude products. The products were purified by column chromatography using silica gel and dichloromethane/methanol (10/ 1; v/v) as the eluent to give PR6R1 and PR6R2. PR6R1: Darkbrown solid. Yield ) 82%. 1H NMR (300 MHz, DMSO-d6): δ (ppm) 7.68(s, 1H), 7.46(d, 1H, J ) 8.7), 7.42(s, 1H), 7.25 -7.17(m, 3H), 7.10(d, 1H J ) 7.8), 6.99(t, 1H), 4.40(s, 2H), 3.84(d, 2H J ) 7.2). 1.81(m, 1H), 1.38 -1.05(m, 8H), 0.84 -0.76(m, 6H). PR6R2: Dark-red solid. Yield ) 80%. 1H NMR (300 MHz, DMSO-d6): δ (ppm) 13.47(s, 2H), 7.80(s, 2H), 7.53(d, 2H, J ) 8.4), 7.50(s, 2H), 7.30(d, 2H, J ) 8.7), 4.73(s, 4H), 3.93(d, 2H, J ) 7.2), 1.83(m, 1H), 1.37 -1.22(m, 8H), 0.86 -0.77(m, 6H). 2.2. Computation. All species considered in this work have been subjected to quantum chemical analysis by density functional theory (DFT) using the B3LYP hybrid density functional method in conjunction with the d-polarized 6-31G* basis set implemented in the Gaussian 03 program.16-18 Each unit reported here was fully optimized without symmetry constraints, taking into account all electrons. The structures resulting from geometry optimization have been classified as local minima on their respective potential energy surfaces according to the frequencies. The frequency analysis has been performed at the B3LYP/6-31G* level, as necessitated by the considerable sizes of the systems analyzed in this work. A variety of DFT, ab initio and hybrid procedures was employed to assess the ground-state geometry of the dye isomers. For the simulation of absorption spectra of organic dyes, we performed time-dependent (TD)-DFT excited state calculations in dichloromethane solution using a 6-31G* basis set. The 30 lowest singlet-singlet excitation energies were considered in those calculations. Solvation effects were included by means of the polarizable continuum model.19 2.3. Fabrication of DSCs. The paste for the transparent nanocrystalline TiO2 layer (15 nm size) was coated on the FTO glass plates by screen printing, and then sintered for 30 min at 450 °C. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.02; Gaussian, Inc.:Wallingford, CT, 2004. (17) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (18) Ditchfield, R.; Herhe, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (19) (a) Barone, V.; Cossi, M. J. Phys Chem. A 1998, 102, 1995. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669. (20) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; DeAngelis, F.; DiCenso, Md, D.; Nazeeruddin, K.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 16701. (21) Hagberg, D.; Yum, J.-H.; Lee, H.; Angelis, F. D.; Marinado, T.; Karlsson, K. M.; Humphry-Baker, R.; Sun, L.; Hagfedt, A.; Gratzel, M.; Nazeeruddin, M. K. J. Am. Chem. Soc. 2008, 130, 6259.

Park et al.

Figure 2. Molecular structures of four synthesized organic dyes (R ) ethylhexyl).

Figure 3. Bent molecular structure of phenothiazine moiety with single or double electron acceptor at the B3LYP/6-31G(d) level of theory: (a) top (upper) and front (bottom) view of PR6C2 and (b) top (upper) and front (bottom) view of PR6R2 (R ) ethylhexyl group).

The TiO2 films were treated with 1 mM TiCl4 aqueous solution and sintered at 450 °C for 30 min. After the mixture was cooled down to 100 °C, the TiO2 electrodes were immersed into the dye solutions, and they were kept at room temperature for 24 h. Organic dyes used in this study (see Figure 2) were synthesized with the known methods as shown in section 2.1. The dye-adsorbed TiO2 electrode and Pt counter electrode were assembled into a sealed sandwich-type cell by heating with a hot melt of Sealing film (SX1170, 60 µm thickness, Solaronix), which served as a spacer between the electrodes. A drop of the electrolyte solution was placed on a drilled hole in the counter electrode of the assembled cell and was driven into the cell by vacuum backfilling. Finally, the hole was sealed using additional cover glass.

3. Results and Discussion 3.1. Molecular Design of Organic Photosensitizers for DSC. Figure 2 shows the molecular structures of the PR6C1, PR6C2, PR6R1, and PR6R2 organic dyes. PR6C1 and PR6C2 were designed and synthesized with phenothiazine as electron donor and one and two C moieties as electron acceptor, respectively, whereas PR6R1 and PR6R2 were synthesized with phenothiazine as electron donor and one and two R moieties as electron acceptor, respectively. As shown in Figure 3, the phenothiazine itself is bent along the N-S axis. As confirmed by X-ray structure analysis,23 the nitrogen atom in the phenothiazine moiety induces a nonplanar geometry similar to the sp3-hybridized, pyramidal nitrogen. The calculated molecular orbitals and electronic structures of PR6C1, PR6C2, PR6R1, and PR6R2 with isodensity (0.02) plots of the HOMO, LUMO, and LUMO+1 are illustrated in Figure 4 and Figure 5. The HOMOs of PR6C1 and PR6C2 are delocalized on the entire molecule including donor and acceptor, whereas those of PR6R1 and PR6R2 contribute less (22) Sommeling, P. M.; O’Regan, B. C.; Haswell, R. R.; Smit, H. J. P.; Bakker, N. J.; Smits, J. J. T.; Kroon, J. M.; van Roosmalen, J. A. M. J. Phys. Chem. B 2006, 110, 19191. (23) McDowell, J. J. H. Acta Crystallogr. 1976, B32, 5.

Organic Dyes with Double Electron Acceptor

Energy & Fuels, Vol. 23, 2009 3735 Table 1. Absorption Properties of Organic Dyes on the Observed and Calculated λmax (in nm) and Molar Extinction Coefficient (ε) in Dichloromethane Solutions calcda

exptl dye PR6C1 PR6C2 PR6R1 PR6R2

-1

ε (M

-1

cm )

18 122 22 548 20 752 34 723

b

λmax

462 (433) 469 (459) 477 (463) 484 (475)

ε (M

-1

cm-1)

13 640 23 350 18 940 39 280

λmaxb 485 507 516 534

a All geometries are obtained at the B3LYP/6-31G(d) in gas phase. Solvation effects were considered by means of PCM in dichloromethane. Excitation energies are computed with TD-B3LYP/6-31G(d)// B3LYP/6-31G(d) level of theory. b Values of parentheses are λmax in film state.

Figure 4. Frontier molecular orbitals (HOMO, LUMO, and LUMO+1) of organic dyes.

Figure 5. Comparison between the molecular orbital energy levels of four organic dyes at PCM-B3LYP/6-31G(d)//B3LYP/6-31G(d) in CH2Cl2 solution.

to the acceptor part. The LUMOs of these molecules are concentrated on the acceptor side of phenothiazine. The LUMO+1 of PR6C1 is centered on the phenothiazine moiety, which differs from that of PR6R1. In contrast, the LUMO+1s of PR6C2 and PR6R2, which are almost degenerate with LUMOs, are delocalized by π*-orbital throughout the entire backbone. It is noted that the LUMOs of PR6C1 and PR6C2 are stretched out to the anchoring part of carboxylic group, whereas the LUMOs of PR6R1 and PR6R2 are not. The HOMOs are less sensitive to the substituents for the anchoring part than the LUMOs. The LUMO and LUMO+1 states stretched out to the both anchoring parts in PR6C2 are likely to ensure high electronic coupling between the excited states of dye and TiO2 conduction band, as well as to provide two doubly folded paths for the electron transfer due to the

degeneration of LUMO and LUMO+1. In other words, a high photocurrent density in DSC devices is possibly achieved with PR6C2. We calculated the lowest singlet-singlet excitation energies of PR6C1, PR6C2, PR6R1, and PR6R2 at 2.56, 2.44, 2.37, and 2.28 eV, respectively. The lowest energy transition corresponds to HOMO f LUMO excitation for PR6C1, PR6C1, PR6R1, and PR6R2. The second lowest energy transition was HOMO-1 f LUMO excitation at 3.44 eV (+0.88 eV) for PR6C1 and HOMO-2 f LUMO transition at 2.91 eV (+0.54 eV) for PR6R1. For PR6C2 and PR6R2, they were virtually degenerate HOMO f LUMO+1 excitations at 2.89 (+0.46) and 2.60 (+0.32) eV, respectively. The smaller energy difference between the lowest and the second lowest energy transitions in PR6C2 and PR6R2 demonstrates more favorable photosensitivity, compared to their counterparts, PR6C1 and PR6R1. The calculated electronic structures in Figure 5 were compared to the absorption spectroscopic data. Although the computationally obtained TD-DFT absorption spectra underestimated the experimental value in general as shown in Table 1, analogous spectroscopic behaviors were reproduced qualitatively. 3.2. Photovoltaic and Photochemical Properties of Organic Sensitizers. We synthesized a series of heterocyclic chromophores containing phenothiazine unit as photosensitizers for the DSC to characterize the photovoltaic properties of the organic dyes designed as shown in Figure 2. As expected, a systematic redshift in the absorption maximum with increasing the number of acceptor was observed in Figure 6. The dye couples, PR6C1/PR6C2 and PR6R1/PR6R2, were designed to have different number of electron acceptor to investigate the relationship between the dye adsorption ability on TiO2 surface and photovoltaic performance of DSC. The UV-vis absorption maximum (λmax) of PR6C1 and PR6R1 of single electron acceptor type was measured at 462 and 477 nm, respectively, in CH2Cl2 solution, as shown in Figure 6a and Table 1. The λmax of PR6C2 and PR6R2 of double electron acceptor type shows red shift of each 7 nm from that of PR6C1 and PR6R1 of single electron acceptor type in CH2Cl2 solution to be 469 and 484 nm. It agrees well with TD-DFT and molecular orbital calculation results in Figure 5 and Table 1. Interestingly, the couple of PR6R1/PR6R2 had longer wavelength absorption compared to those of PR6C1/PR6C2 chromophore. The absorption properties of organic dyes on TiO2 film state are also shown in Figure 6b. The overall blue shift of the absorption spectra on TiO2 film was observed from those obtained in CH2Cl2 solution. The degrees of the shift of PR6C1, PR6C2, PR6R1, and PR6R2 were 29, 10, 14, and 9 nm, respectively. It has been known that the blue shift is due to aggregation via molecular stacking and formation of the deprotonated form (i.e., the carboxylate anion) of the dyes adsorbed on TiO2 surface.10b,c,20,21

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Figure 7. Photocurrent density-photovoltage curve for DSCs based on N3, PR6C1, and PR6C2 under AM 1.5 G radiation (100mW/cm2).

on N3 dye in the same measurement condition. The PR6C2 and PR6R2 of double electron acceptor type showed better photovoltaic performances compared to their counterparts, PR6C1 and PR6R1, consistent with the arguments given by quantum chemical analysis in section 3.1. 4. Conclusion

Figure 6. UV-vis absorption spectra of organic dyes in (a) CH2Cl2 solutions and (b) TiO2 thin film state. Table 2. Photocurrent-Photovoltage Characteristics of DSCsa dye N3 PR6C1 PR6C2 PR6R1 PR6R2

bV

oc

(mV)

675 675 675 425 525

cJ

sc

(mA/cm2) 15.05 12.52 14.96 1.57 3.94

dFF

(%)

69.3 67.6 68.2 63.5 62.8

eη

(%)

7.3 5.6 6.8 0.4 1.3

a Dye-sensitized 20 µm nanofilms were measured under 100 mW cm-2 of simulated AM 1.5 G solar light. b Open circuit voltage. c Short circuit current. d Fill factor. e Power conversion efficiency.

The DSCs were prepared according to the procedure presented in section 2.3. The surface treatment of TiO2 electrode with TiCl4 aqueous solution before dye adsorption is well-known to improve photovoltaic properties of the DSC substantially.22 We found that the same was valid for the DSCs prepared with organic dyes as well. The TiCl4 treatment is likely to increase the necking between TiO2 particles, and thus facilitating the percolation of photoinjected electrons from one particle to another and lowering the probability of unwanted recombination. The photovoltaic properties of the DSCs based on PR6C1, PR6C2, PR6R1, and PR6R2 were measured and compared to those of N3 dye, as shown in Table 2 and Figure 7. High efficiency of the 6.8% value was attained using PR6C2 containing double electron acceptor under AM 1.5 irradiation. The efficiency approaches the 7.3% value for the DSC based

We have designed and synthesized novel organics dyes with double electron acceptor moieties on the phenothiazine framework and demonstrated the effect of the number and the kind of electron acceptor on controlling electron transfer and photovoltaic properties of the DSC. The organic dyes of double electron acceptor type as photosensitizers for DSC turned out more efficient compared to those of single electron acceptor type due to the increase of their electron extraction paths from electron doner and the higher molar extinction coefficients. The intense electron-withdrawing property and abundant electronic coupling with TiO2 of cyanoacrylic acid moiety in PR6C1 and PR6C2 drives more efficient electron injection from the LUMO state of the dye to the TiO2 conduction band, although rhodanine-containing organic dyes, PR6R1 and PR6R2, have relatively higher molar extinction coefficients. The electronic structures based on DFT and TD-DFT calculations well explained the better performance of organic dyes with double electron acceptor and substitution effect. Further structural modification to render the more LUMO electron density transferred from the acceptor to the anchoring group is needed to improve the DSC performance for organic dyes with phenothiazine framework. The DSC based on PR6C2 showed the most efficient phototo-electricity conversion efficiency compared to other dyes, which is the maximum η value of 6.8% (Voc ) 676 mV, Jsc ) 14.96 mA/cm2, FF ) 0.68%) under simulated AM 1.5 irradiation (100 mW/cm2). In comparison, the 7.3% value was obtained for N3 dye under the same conditions. Our results suggest that the dyes with double electron acceptor moieties are promising for getting higher solar-toelectricity conversion efficiencies in DSC. Supporting Information Available: Optimized geometries of four organic dyes at the B3LYP/6-31G(d) level of theory. This material is available free of charge via the Internet at http://pubs.acs.org. EF900207Y