Dye-Sensitized Solar Cells Based on Organic Sensitizers with

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J. Phys. Chem. C 2009, 113, 7469–7479

7469

Dye-Sensitized Solar Cells Based on Organic Sensitizers with Different Conjugated Linkers: Furan, Bifuran, Thiophene, Bithiophene, Selenophene, and Biselenophene Renzhi Li,†,‡ Xueju Lv,† Dong Shi,† Difei Zhou,†,‡ Yueming Cheng,† Guangliang Zhang,*,† and Peng Wang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Graduate School, Chinese Academy of Sciences, Beijing 100039, China ReceiVed: February 2, 2009; ReVised Manuscript ReceiVed: March 5, 2009

Six organic dyes with different conjugated linkers such as furan, bifuran, thiophene, bithiophene, selenophene, and biselenophene have been prepared in combination with the dihexyloxy-substituted triphenylamine donor and the cyanoacrylic acid acceptor. In conjunction with an acetonitrile-based electrolyte and a solvent-free ionic liquid electrolyte, these dyes exhibit 6.88-7.77% and 6.39-7.00% efficiencies, respectively. We have demonstrated that furan and selenophene can be employed as building blocks of sensitizers in stable solar cells for the first time. We have also studied the influence of heteroatoms on photocurrents and photovoltages with the aid of quantum calculations and transient photoelectrical decay measurements. Temperature-dependent electrical impedance experiments have shown that a relatively low external quantum efficiency of the dye with biselenophene linker is not related to the charge collection yield in the case of an acetonitrile electrolyte. 1. Introduction A large amount of research and development effort has been made to the mesoscopic dye-sensitized solar cell (DSC) ever since the seminal demonstration of its feasibility as a costeffective photovoltaic technology.1 The foremost feature of DSC consists in a wide band gap nanocrystalline film grafted with a quasi-monolayer of dye molecules and submerged in a redox electrolyte. This elegant architecture can synchronously address two critical issues of employing organic materials for the photovoltaic applications: (i) efficient charge generation from the Frenkel excitons; (ii) long-lived electron-hole separation up to the millisecond time domain. The latter attribute can often confer an almost quantitative charge collection for several micrometer-thick active layers, even if the electron mobilities in nanostructured semiconducting films are significantly lower than those in the bulk crystalline materials. Benefited from systematic device engineering and continuous material innovation, a state of the art DSC with a ruthenium sensitizer has achieved a validated efficiency of 11.1%,2 measured under the air mass 1.5 global (AM1.5G) conditions. In view of the limited ruthenium resource and the heavy-metal toxicity, metal-free organic dyes have received surging research interest in recent years.3 Apart from a high molar extiction coefficient and a low-cost in contrast to a ruthenium dye, the flexibility in molecular tailoring of an organic sensitizer provides a large area to explore. Meanwhile, we have lately witnessed rapid progress of organic dyes, reaching close to 10.0% efficiencies in combination with a volatile acetonitrile-based electrolyte.4 Note that the usage of any volatile electrolyte to achieve a high efficiency has baffled the large-scale application, and especially the integration of DSC with a plastic matrix. In a previous paper,5 we have reported that there can be a negligible * To whom correspondence should be ciac.jl.cn and [email protected]. † Changchun Institute of Applied Chemistry. ‡ Graduate School.

addressed,

zhgl_jl@

variation of photocurrents for organic dye-sensitized solar cells based on a volatile electrolyte and a solvent-free ionic liquid electrolyte. This is a very distinctive and attractive merit of organic dyes considering the photocurrent gap always observed for ruthenium sensitizers with different electrolytes.6 On the basis of a previous study7 on some donor-πconjugated linker-acceptor (D-π-A) dyes, we have identified that dihexyloxy-substituted triphenylamine is a good donor in the construction of efficient organic sensitizers, embodied by the C206 sensitizer with a thienothiophene linker and a cyanoacrylic acid acceptor. Here by employing the same donor and acceptor, we prepare six D-π-A dyes possessing different π-conjugated linkers, i.e., furan, bifuran, thiophene, bithiophene, selenophene, and biselenophene. Molecular structures of C209, C210, C213, C214, C215, and C216 are presented in Scheme 1. The effect of π-conjugated linkers on light absorption, energylevel, photocurrent, and photovoltage will be detailed in terms of quantum calculation, transient photoelectrical decay measurement, and electrical impedance analysis. 2. Experimental Section 2.1. Materials. All solvents and reagents, unless otherwise stated, were of analytical grade and used as received. nButyllithium, and tetra-n-butylammonium hexafluorophosphate (TBAPF6) were purchased from Aldrich. Guanidinium thiocyanate (GNCS), tert-butylpyridine (TBY), 2-cyanoacetic acid, and 3R,7R-dihydroxy-5β-cholic acid (Cheno) were purchased from Fluka. 1-Ethyl-3-methylimidazolium tetracyanoborate (EMITCB) and 400-nm-sized TiO2 scattering paste were received as gifts from Dyesol. The synthesis of 1,3-dimethylimidazolium iodide (DMII) and 1-ethyl-3-methylimidazolium iodide (EMII) was described in our previous paper.8 NButylbenzimidazole (NBB) was synthesized according to the literature method9 and distilled before use. N,N-Bis(4-hexyloxyphenyl)-4-bromoaniline (1),7 2-tributylstannylfuran,10 2-tributylstannylthiophene,11 2-tributylstannylselenophene,12 5-bromo-

10.1021/jp900972v CCC: $40.75  2009 American Chemical Society Published on Web 04/02/2009

7470 J. Phys. Chem. C, Vol. 113, No. 17, 2009

Li et al.

SCHEME 1: Synthetic Route of the C209, C210, C213, C214, C215, and C216 Sensitizers

furan-2-carbaldehyde13 and 5-bromothiophene-2-carbaldehyde14 were synthesized according to the corresponding literature methods. 2.2. Synthesis. 2.2.1. The General Synthesis of 2. A mixture of compound 1 (8.2 mmol), 2-tributylstannylfuran (or 2-tributylstannylthiophene or 2-tributylstannylselenophene, 12.3 mmol), Pd(PPh3)2Cl2 (0.41 mmol), and dry THF (50 mL) was refluxed under Ar for 20 h. After removal of solvent under a reduced pressure, the residue was loaded onto a silica gel column for purification with dichloromethane/hexane (1/5, v/v) as eluent. 2-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}furan (2a). Yield: 79%. 1H NMR (300 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.92 (t, J ) 6.6 Hz, 4H), 6.53 (d, J ) 1.8 Hz, 1H), 6.68 (d, J ) 3.0 Hz, 1H), 6.77 (d, J ) 9.0 Hz, 2H), 6.93 (d, J ) 9.0 Hz, 4H), 7.01 (d, J ) 9.0 Hz, 4H), 7.47 (d, J ) 9.0 Hz, 2H), 7.64 (m, 1H). 2 - {4 - [N,N - Bis(4 - hexyloxyphenyl)amino]phenyl}thiophene (2b). Yield: 66%. 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.93 (t, J ) 6.4 Hz, 4H), 6.76 (d, J ) 8.8 Hz, 2H), 6.90 (d, J ) 8.8 Hz, 4H), 7.00 (d, J ) 8.8 Hz, 4H), 7.07 (m, 1H), 7.30 (d, J ) 4.4 Hz, 1H), 7.41 (d, J ) 6.0 Hz, 1H), 7.44 (d, J ) 8.8 Hz, 2H). 2-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}selenophene (2c). Yield: 50%. 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.4 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.93 (t, J ) 4.4 Hz, 4H), 6.73 (d, J ) 5.6 Hz, 2H), 6.89 (d, J ) 6.0 Hz, 4H), 7.01 (d, J ) 6.0 Hz, 4H), 7.27 (m, 1H), 7.40 (d, J ) 5.6 Hz, 2H), 7.43 (d, J ) 2.4 Hz, 1H), 8.00 (d, J ) 3.6 Hz, 1H). 2.2.2. The General Synthesis of 3. Compound 2 (1 mmol) and DMF (3 mmol) were dissolved in chloroform (10 mL). After the solution was cooled to 0 °C, phosphorus oxychloride (2 mmol) was added dropwise. The reaction was kept at room

temperature for 4 h under Ar. Subsequently, water (20 mL) was added into the reaction mixture, followed by neutralizing with sodium acetate. The mixture was extracted with dichloromethane, and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed with a rotoevaporator, and the residue was purified on a silica gel column with ethyl acetate/petroleum ether (1/5, v/v) as eluent. 5-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}-2-furaldehyde (3a). Yield: 79%. 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.76 (d, J ) 8.8 Hz, 2H), 6.93 (d, J ) 8.8 Hz, 4H), 7.02 (d, J ) 3.6 Hz, 1H), 7.08 (d, J ) 8.8 Hz, 4H), 7.60 (d, J ) 3.6 Hz, 1H), 7.65 (d, J ) 8.8 Hz, 2H), 9.50 (s, 1H). 5-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}thiophene-2carbaldehyde (3b). Yield: 82%. 1H NMR (400 MHz, DMSOd6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.6 Hz, 4H), 6.73 (d, J ) 8.8 Hz, 2H), 6.93 (d, J ) 8.8 Hz, 4H), 7.07 (d, J ) 8.8 Hz, 4H), 7.54 (d, J ) 3.6 Hz, 1H), 7.60 (d, J ) 8.8 Hz, 2H), 7.97 (d, J ) 3.6 Hz, 1H), 9.83 (s, 1H). 5-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}selenophene2-carbaldehyde (3c). Yield: 83%. 1H NMR (400 MHz, DMSOd6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.8 Hz, 4H), 6.71 (d, J ) 8.8 Hz, 2H), 6.92 (d, J ) 8.8 Hz, 4H), 7.06 (d, J ) 8.8 Hz, 4H), 7.54 (d, J ) 8.8 Hz, 2H), 7.66 (d, J ) 4.0 Hz, 1H), 8.18 (d, J ) 4.0 Hz, 1H), 9.70 (s, 1H). 2.2.3. Synthesis of 4. The general synthesis of 4a and 4b is as follows: Compound 2a (or 2b, 3.2 mmol) was dissolved in dry THF (15 mL), and the solution was cooled to -78 °C. After addition of a hexane solution of n-butyllithium (1.6 M, 2.4 mL, 3.8 mmol), the reaction was stirred under Ar at 0 °C for 30 min. A solution of chlorotributylstannane (1.1 mL, 4.1 mmol)

Dye-Sensitized Solar Cells in dry THF (5 mL) was added, and the mixture was stirred for further 40 min. The reaction mixture was quenched with aqueous NH4Cl and extracted with ether. The combined ether layers were dried over anhydrous sodium sulfate and concentrated in vacuum to give the crude 2-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-5-tributylstannylfuran or 2-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-5-tributylstannylthiophene. The unpurified stannane (2.5 mmol), 5-bromofuran-2-carbaldehyde or 5-bromothiophene-2-carbaldehyde (5.0 mmol), and Pd(PPh3)2Cl2 (0.26 mmol) in 30 mL of dry THF was refluxed for 24 h under Ar. The reaction mixture was concentrated in vacuum. The residue was loaded onto a silica gel column for purification using ethyl acetate/petroleum ether (1/5, v/v) as eluent. 5′-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}-2,2′-bifuran5-carbaldehyde (4a). Yield: 50%. 1H NMR (400 MHz, DMSOd6) δ: 0.88 (t, J ) 7.2 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.78 (d, J ) 8.8 Hz, 2H), 6.91 (d, J ) 8.8 Hz, 4H), 6.94 (d, J ) 3.6 Hz, 1H), 7.02 (d, J ) 3.6 Hz, 1H), 7.03 (d, J ) 8.8 Hz, 4H), 7.14 (d, J ) 3.6 Hz, 1H), 7.60 (d, J ) 8.8 Hz, 2H), 7.66 (d, J ) 3.6 Hz, 1H), 9.57 (s, 1H). 5′-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}-2,2bithiophene-5-carbaldehyde (4b). Yield: 40%. 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.31 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.75 (d, J ) 8.8 Hz, 2H), 6.91 (d, J ) 8.8 Hz, 4H), 7.03 (d, J ) 8.8 Hz, 4H), 7.38 (d, J ) 4.0 Hz, 1H), 7.50 (d, J ) 8.8 Hz, 2H), 7.51 (d, J ) 4.0 Hz, 1H), 7.57 (d, J ) 4.0 Hz, 1H), 7.98 (d, J ) 4.0 Hz, 1H), 9.87 (s, 1H). 5-{4-[N,N-Bis(4-hexyloxyphenyl)amino]phenyl}-2,2′-biselenophene-5′-carbaldehyde (4c). A solution of N-bromosuccinimide (0.38 g, 2.1 mmol) in DMF (5 mL) was added dropwise to a stirred solution of 2c (1.1 g, 1.92 mmol) in chloroform (10 mL) at 0 °C. After the solution was stirred for 10 min at 0 °C, water (50 mL) was added and the aqueous layer was extracted with chloroform. The organic layer was dried over anhydrous sodium sulfate, and the solvent was removed with a rotoevaporator. The residue was purified on a silica gel column with dichloromethane/hexane (1/3, v/v) as eluent to obtain 2-{4-[N,Nbis(4-hexyloxyphenyl)amino]phenyl}-5-bromoselenophene (1.2 g, 92.3%). A mixture of 2-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-5-bromoselenophene, 2-tributylstannylselenophene (2.0 g, 4.76 mmol), Pd(PPh3)2Cl2 (0.13 g, 0.184 mmol), and dry THF (20 mL) was refluxed for 20 h under Ar. The solvent was removed with a rotoevaporator, and the residue was purified on a silica gel column with chloroform/hexane (1/5, v/v) as eluent to give compound 5. Yield: 60%. 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.93 (t, J ) 6.8 Hz, 4H), 6.72 (d, J ) 8.8 Hz, 2H), 6.90 (d, J ) 8.8 Hz, 4H), 7.02 (d, J ) 8.8 Hz, 4H), 7.24 (m, 1H), 7.31 (m, 2H), 7.37 (d, J ) 4.0 Hz, 1H), 7.39 (d, J ) 8.8 Hz, 2H), 8.06 (d, J ) 6.4 Hz, 1H). Compound 5 (0.4 g, 0.57 mmol) and DMF (0.22 mL, 2.9 mmol) were dissolved in chloroform (10 mL). After the solution was cooled to 0 °C, phosphorus oxychloride (0.1 mL, 2 mmol) was added dropwise. The reaction was kept at room temperature for 4 h under Ar. Subsequently, water (10 mL) was added to the reaction mixture, followed by neutralizing with sodium acetate. The mixture was extracted with dichloromethane, and the organic phase was dried over anhydrous sodium sulfate. The solvent was removed with a rotoevaporator, and the residue was purified on a silica gel column with ethyl acetate/petroleum ether (1/5, v/v) as eluent. Yield: 93%. 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 7.2 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.94 (t,

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7471 J ) 6.4 Hz, 4H), 6.72 (d, J ) 8.8 Hz, 2H), 6.91 (d, J ) 8.8 Hz, 4H), 7.04 (d, J ) 8.8 Hz, 4H), 7.44 (d, J ) 8.8 Hz, 2H), 7.48 (d, J ) 4.0 Hz, 1H), 7.52 (d, J ) 4.0 Hz, 1H), 7.64 (d, J ) 4.0 Hz, 1H), 8.15 (d, J ) 4.0 Hz, 1H), 9.75 (s, 1H). 2.2.4. The General Synthesis of Sensitizers. Carbaldehyde 3 (or 4, 1 mmol), cyanoacetic acid (3 mmol), and piperidine (7 mmol) were added to dry chloroform (20 mL). The mixture was refluxed for 10 h, and water (20 mL) was added. The solution was acidified with 20% aqueous HCl and extracted with chloroform. The organic phase was dried over anhydrous sodium sulfate. The solvent was removed with a rotoevaporator, and the residue was purified on a silica gel column with chloroform as eluent. The crude product was dissolved in chloroform and washed with a 2 M HCl aqueous solution and water. The removal of solvent under a reduced pressure gave the title compound. 2-Cyano-3-{5-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}furan-2-yl}acrylic Acid (C209). Yield: 88%. IR (KBr) ν/cm-1: 1684 (COOH), 2218 (CN). 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.31 (m, 8H), 1.42 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.75 (d, J ) 8.8 Hz, 2H), 6.92 (d, J ) 8.8 Hz, 4H), 7.08 (d, J ) 8.8 Hz, 4H), 7.11 (d, J ) 3.6 Hz, 1H), 7.51 (d, J ) 3.6 Hz, 1H), 7.70 (d, J ) 8.8 Hz, 2H), 7.98 (s, 1H), 13.48 (s, 1H). 13C NMR (100 MHz, DMSO-d6) δ: 13.86, 22.05, 25.18, 28.66, 30.98, 67.60, 94.90, 108.02, 115.54, 116.78, 117.41, 119.01, 126.32, 127.53, 137.23, 138.64, 146.69, 149.91, 155.99, 159.80, 164.18. ESI-MS m/z calcd for (C38H42N2O5): 606.7. Found: 607.6. Anal. Calcd for C38H42N2O5: C, 75.2; H, 7.0; N, 4.6. Found: C, 75.3; H, 7.1; N, 4.6. 2-Cyano-3-{5′-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}2,2′-bifuran-5-yl}acrylic Acid (C210). Yield: 91%. IR (KBr) ν/cm-1: 1687 (COOH), 2218 (CN). 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.31 (m, 8H), 1.39 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.76 (d, J ) 8.8 Hz, 2H), 6.91 (d, J ) 8.8 Hz, 4H), 6.97 (d, J)3.6 Hz, 1H), 7.04 (d, J ) 8.8 Hz, 4H), 7.05 (d, J)3.6 Hz, 1H), 7.09 (d, J ) 3.6 Hz, 1H), 7.55 (d, J ) 3.6 Hz, 1H), 7.61 (d, J ) 8.8 Hz, 2H), 8.03 (s, 1H), 13.60 (s, 1H). 13C NMR (100 MHz, DMSOd6) δ: 13.84, 22.03, 25.17, 28.67, 30.97, 67.58, 96.59, 106.46, 108.87, 113.00, 115.45, 116.30, 118.34, 120.52, 125.04, 126.24, 127.06, 136.96, 139.20, 142.64, 147.21, 148.62, 150.23, 155.58, 163.91. ESI-MS m/z calcd for (C42H44N2O6): 672.3. Found: 673.1. Anal. Calcd for C42H44N2O6: C, 75.0; H, 6.6; N, 4.2. Found: C, 75.4; H, 6.7; N, 4.2. 2-Cyano-3-{5-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}thiophene-2-yl}acrylic Acid (C213). Yield: 90%. IR (KBr) ν/cm-1: 1682 (COOH), 2217 (CN). 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.95 (t, J ) 6.6 Hz, 4H), 6.75 (d, J ) 8.8 Hz, 2H), 6.93 (d, J ) 9.2 Hz, 4H), 7.08 (d, J ) 9.2 Hz, 4H), 7.56 (d, J ) 4.0 Hz, 1H), 7.58 (d, J ) 8.8 Hz, 2H), 7.96 (d, J ) 3.6 Hz, 1H), 8.44 (s, 1H), 13.58 (s, 1H). 13C NMR (100 MHz, DMSO-d6) δ: 13.87, 22.03, 25.16, 28.64, 30.95, 67.61, 96.55, 115.56, 116.67, 117.95, 122.99, 123.23, 127.30, 127.47, 132.83, 138.83, 141.89, 146.55, 149.81, 153.99, 155.90, 163.82. ESIMS m/z calcd for (C38H42N2O4S): 622.8. Found: 621.6. Anal. Calcd for C38H42N2O4S: C, 73.3; H, 6.8; N, 4.5. Found: C, 73.1; H, 6.8; N, 4.6. 2-Cyano-3-{5′-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}2,2′-bifuran-5-yl}acrylic Acid (C214). Yield: 85%. IR (KBr) ν/cm-1: 1680 (COOH), 2214 (CN). 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 7.2 Hz, 6H), 1.30 (m, 8H), 1.41 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.73 (d, J ) 8.8 Hz, 2H), 6.91 (d, J ) 8.8 Hz, 4H), 7.08 (d, J ) 8.8 Hz, 4H),

7472 J. Phys. Chem. C, Vol. 113, No. 17, 2009 7.40 (d, J ) 4.0 Hz, 1H), 7.51 (d, J ) 8.8 Hz, 2H), 7.54 (d, J ) 4.0 Hz, 1H), 7.57 (d, J ) 4.0 Hz, 1H), 7.94 (d, J ) 4.0 Hz, 1H), 8.44 (s, 1H), 13.30 (s, 1H). 13C NMR (100 MHz, DMSOd6) δ: 13.84, 22.02, 25.16, 28.66, 30.96, 67.59, 98.24, 115.47, 116.73, 118.53, 123.47, 123.93, 124.36, 126.37, 127.08, 128.24, 132,35, 133.53, 139.21, 141.18, 145.71, 145.82, 145.91, 148.65, 155.58, 163.63. ESI-MS m/z calcd for (C42H44N2O4S2): 704.9. Found: 704.4. Anal. Calcd for C42H44N2O4S2: C, 71.6; H, 6.3; N, 4.0. Found: C, 71.2; H, 6.3; N, 4.1. 2-Cyano-3-{5-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}selenophene-2-yl}acrylic Acid (C215). Yield: 80%. IR (KBr) ν/cm-1: 1686 (COOH), 2213 (CN). 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.31 (m, 8H), 1.42 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.70 (d, J ) 8.8 Hz, 2H), 6.91 (d, J ) 8.8 Hz, 4H), 7.07 (d, J ) 8.8 Hz, 4H), 7.56 (d, J ) 8.8 Hz, 2H), 7.65 (d, J ) 3.6 Hz, 1H), 8.16 (d, J ) 3.6 Hz, 1H), 8.46 (s, 1H), 13.58 (s, 1H). 13C NMR (100 MHz, DMSO-d6) δ: 13.86, 22.04, 25.17, 28.65, 30.97, 67.61, 96.80, 115.53, 117.11, 117.83, 124.49, 125.25, 127.47, 127.68, 136.97, 138.75, 145.68, 149.35, 149.91, 155.93, 161.47, 163.86. ESIMS m/z calcd for (C38H42N2O4Se): 669.7. Found: 670.0. Anal. Calcd for C38H42N2O4Se: C, 68.2; H, 6.3; N, 4.2. Found: C, 68.4; H, 6.4; N, 4.0. 2-Cyano-3-{5′-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}2,2′-biselenophene-5-yl}acrylic Acid (C216). Yield: 79%. IR (KBr) ν/cm-1: 1682 (COOH), 2213 (CN). 1H NMR (400 MHz, DMSO-d6) δ: 0.88 (t, J ) 6.8 Hz, 6H), 1.31 (m, 8H), 1.39 (m, 4H), 1.70 (m, 4H), 3.94 (t, J ) 6.4 Hz, 4H), 6.72 (d, J ) 8.8 Hz, 2H), 6.92 (d, J ) 8.8 Hz, 4H), 7.05 (d, J ) 8.8 Hz, 4H), 7.46 (d, J ) 8.8 Hz, 2H), 7.49 (d, J ) 4.0 Hz, 1H), 7.52 (d, J ) 4.0 Hz, 1H), 7.70 (d, J ) 4.0 Hz, 1H), 8.14 (d, J ) 4.0 Hz, 1H), 8.49 (s, 1H). 13C NMR (100 MHz, DMSO-d6) δ: 13.86, 22.04, 25.18, 28.68, 30.98, 67.60, 97.55, 115.47, 117.04, 118.48, 125.29, 126.10, 126.78, 127.11, 131.69, 137.96, 139.16, 145.14, 148.79, 149.22, 153.00, 154.37, 155.63, 163.67. ESI-MS m/z calcd for (C42H44N2O4Se2): 798.7. Found: 798.3. Anal. Calcd for C42H44N2O4Se2: C, 63.2; H, 5.6; N, 3.5. Found: C, 63.5; H, 5.5; N, 3.6. 2.3. UV-vis, Photoluminescence, and Voltammetric Measurements. Electronic absorption spectra were measured on a UNICO WFZ UV-2802PC/PCS spectrometer. Emission spectra were recorded with a PerkinElmer LS55 luminescence spectrometer. The emitted light was detected with a Hamamatsu R928 red-sensitive photomultiplier. A computer-controlled CHI660C electrochemical workstation was used for square-wave voltammetric measurements with a three-electrode electrochemical cell. 2.4. Computation. All calculations were performed in the Gaussian03W program package. Without any symmetrical constraints, the geometrical structures of all sensitizers were optimized employing the density functional theory (DFT) method combined with Becke’s three-parameter hybrid functional15 and Lee-Yang-Parr’s gradient corrected correlation functional.16 In particular, a 6-31G(d) basis set was applied for all atoms.17,18 Incorporating the optimized model in the ZINDO/S method, we calculated the lowest 40 singlet-singlet electronic transitions. Subsequently, the ZINDO/S results were entered into the SWizard program (http://www.sg-chem.net/ swizard/) to compute the absorption profile as a sum of the Gaussian band.19 2.5. Device Fabrication. A screen-printed double layer film of interconnected TiO2 particles was used as the mesoporous negative electrode. A 7 µm thick transparent layer of 20 nm sized titania particles were first printed on the fluorine-doped

Li et al. SnO2 (FTO) conducting glass electrode and further coated by a 5 µm thick scattering layer of 400 nm sized titania particles. The film thickness was measured by a benchtop Ambios XP-1 stylus profilometer. The detailed preparation procedures of TiO2 nanocrystals, pastes for screen-printing, and nanostructured TiO2 film have been reported in a previous paper.20 A cycloidal TiO2 electrode (∼0.28 cm2) was stained by immersing it into a dye solution containing C209, C210, C213, C214, C215, or C216 sensitizer (300 µM) and Cheno (2 mM) in acetonitrile for 5 h. After being washed with acetonitrile and dried by air flow, the sensitized titania electrode was assembled with a thermally platinized FTO electrode. The electrodes were separated by a 35 µm thick Bynel (DuPont) hot-melt gasket and sealed up by heating. The internal space was filled with a liquid electrolyte using a vacuum backfilling system. The electrolyte-injecting hole on the counter electrode glass substrate, made with a sandblasting drill, was sealed with a Bynel sheet and a thin glass cover by heating. Two electrolytes were used for device evaluation. EL01: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M TBY, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (v/v, 85/15).6b EL02: DMII/EMII/EMITCB/ I2/NBB/GNCS (molar ratio, 12/12/16/1.67/3.33/0.67), where the iodide and triiodide concentrations are 3.182 and 0.238 M, respectively.21 2.6. Photovoltaic Characterization. A LS1000 solar simulator (Solar Light Com. Inc., USA) was used to give an irradiance of 100 mW cm-2 (the equivalent of one sun at AM1.5G) at the surface of a testing cell. The current-voltage characteristics were obtained by applying external potential bias to the cell and measuring the dark current and photocurrent with a Keithley model 2602 digital source meter. This process was fully automated using Labview 8.0. A similar data acquisition system was used to control the incident photon-to-collected electron conversion efficiency (IPCE) measurement. Under full computer control, light from a 1000 W xenon lamp was focused through a monochromator onto the photovoltaic cell under test. A computer-controlled monochromator (Omni λ300) was incremented through the spectral range (300-900 nm) to generate a photocurrent action spectra with a sampling interval of 10 nm and a current sampling time of 2 s. IPCE is defined by IPCE(λ) ) hcJsc/eφλ, where h is Planck’s constant, c is the speed of light in a vacuum, e is the electronic charge, λ is the wavelength (m), Jsc is the short-circuit photocurrent density (A m-2), and φ is the incident radiative flux (W m-2). Photovoltaic performance was measured by using a metal mask with an aperture area of 0.158 cm2. A homemade heating-cooling system was used for temperature-dependent J-V measurements. 2.7. Transient Photoelectrical Measurements. In the transient photoelectrical decay experiment, steady-state light was supplied with a homemade white-light-emitting diode (LED) array, and a green LED array controlled with a fast solid-state switch was used to generate a perturbing pulse with a width of 200 ms. The pulsed green light and steady-state white light were both incident on the photoanode side of a testing cell. The green pulse was carefully controlled by the driving potential of diodes to keep the modulated photovoltage below 5 mV. We used green light as a probe to generate a photovoltage perturbation near the open-circuit photovoltage (Voc) of the cell under white light and measured the voltage decay process thereafter. Normally, the transient signals follow a monoexponential decay; thus the recombination rate constant, kr, can be extracted from the slope of the semilogarithmic plot. The capacitance (Cµ) of the TiO2/ electrolyte interface at the Voc is calculated by Cµ ) ∆Q/∆V, where ∆V is the peak of the photovoltage transient and ∆Q is

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Figure 1. Normalized electronic absorption and emission spectra of the C209, C210, C213, C214, C215, and C216 sensitizers dissolved in chloroform.

the number of electrons injected during the green light flash. The latter is obtained by integrating a short-circuit photocurrent transient generated from an identical green light pulse. A homemade heating-cooling system was used for temperaturedependent measurements. 2.8. Electrical Impedance Measurements. Electrical impedance experiments were carried out in the dark with an IM6ex electrochemical workstation, with a frequency range from 50 mHz to 100 kHz and a potential modulation of 10 mV. The obtained impedance spectra were fitted with the Z-view software (v2.80, Scribner Associates Inc.) in terms of appropriate equivalent circuits. A homemade heating-cooling system was used for temperature-dependent measurements. 3. Results and Discussion

Figure 2. Square-wave voltammograms of a Pt ultramicroelectrode in DMF solution containing the C209, C210, C213, C214, C215 or C216 sensitizer. Supporting electrolyte: 0.1 M TBAPF6. The LUMO and HOMO were estimated vs vacuum: ELUMO/HOMO ) -4.88 - Fφredox.

In comparison with crystal-Si solar cells, a relatively low efficiency of even the “champion” DSC2 can be mainly ascribed to a low photocurrent, because of a narrow spectral response over a broad solar wavelength range. At the present time, this issue has become more serious for cells made from metal-free organic sensitizers.3-5,7 Therefore, we first evaluate the electronic absorption of these D-π-A dyes not only dissolved in solution but also anchored on the mesoporous titania film, with a special concern on the solvent effect. In view of the low solubility of these molecules in ethanol, we prefer to select chloroform as a good solvent for testing their molar extinction coefficients. The normalized electronic absorption and emission spectra of the C209, C210, C213, C214, C215, and C216 dyes in chloroform are presented in Figure 1. The origins of these electronic absorptions are detailed by calculating the singlet electronic transitions with the ZINDO/S method in the Gaussian03W program suite. Calculations show that these visible bands originate intrinsically from the intramolecular charge-transfer transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The bathochromic absorption and emission as well as the molar extinction coefficient enhancement (Table 1) can be easily perceived for a dye (C210, C214, or C216) with two five-membered aromatic heterocycles relative to its counterpart (C209, C213, or C215) with one resulting from the extension of π-conjugation length. Here we remark that the molar extinction coefficient of C214 with the bithiophene conjugated linker3r is significantly lower than those of its analogues C206 with thienothiophene (42 × 103 M-1 cm-1) and C211 with bisthienothiophene (47 × 103 M-1 cm-1),7,3p indicating the merit of employing a planar fused unit in further dye design. Here

the shifts of absorption peaks by using one more conjugated unit are 16, 9, and 24 nm for the furan, thiophene, and selenophene series, respectively. This tendency also holds for the absorptions measured with the THF solvent as listed in Table 1, with a smallest bathochromic effect of only 7 nm for the thiophene series. Additionally, the solution absorption peaks not only shift to a lower energy but also show an enhanced molar extinction coefficient with the decreased electronegativity of heteroatoms. As shown in Figure S1 of the Supporting Information, DFT calculations virtually indicate that furan has a negligible torsion angle with respect to phenyl and another furan. It can be imaged that if the planarity of conjugated units was better for the thiophene and selenophene series, the electronegative effect could be more evident in the spectral red-shifting and the absorptivity enhancement. Moreover, the absorptions of all these dyes both as a dissolved state and as an anchored state are hypsochromic with THF in contrast to chloroform, due to the effect of dielectric locales. The influence of dielectrics on light harvesting can also be felt in the measurements of photocurrent action spectra as discussed below. Note that the aggregation does not make a contribution to the spectroscopic observation on these dyes, because we have not probed any change of normalized spectra of film stained with highly diluted dye molecules coadsorbed with Cheno. Thus, we can unambiguously conclude that the carboxylate-titanium assembly is a weaker acceptor in contrast to the carboxylic acid, by comparing the electronic absorptions in solution with those on film without altering the dielectric. While a good light absorption is an essential requirement for photovoltaic cells, the following charge generation from the

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TABLE 1: Detailed Spectral Parametersa of C209, C210, C213, C214, C215, and C216 solution

film

max λabs /nm

max λpl /nm

max λabs /nm

sensitizer

εmax/103 M-1 cm-1

CHCl3

THF

CHCl3

THF

air

CHCl3

THF

C209 C210 C213 C214 C215 C216

27.1 28.4 27.5 30.5 31.5 35.1

506 522 514 523 525 549

469 484 482 489 495 520

689 716 715 723 720 772

626 665 660 677 689 709

435 456 453 465 445 457

468 478 458 480 464 476

450 451 459 461 451 465

max Absorption maximum wavelength, λabs ; maximum molar extinction coefficient in chloroform, εmax; photoluminescence maximum max wavelength, λpl . a

closely bound electron-hole pair (Frenkel exciton) confined in a single organic molecule22 also determines device operation. This process is triggered at the interface between a donor component and an acceptor component (dye and titania in DSC), featuring a favorite energy alignment, although a hot electron injection is also possible.23 Here we measured the HOMO and LUMO of these six dyes precisely in a nitrogen-filled glovebox by means of the ultramicroelectrode square-wave voltammograms shown in Figure 2. The downhill energy offset by the measured LUMO (-3.35 eV for C209; -3.41 eV for C210; -3.42 eV for C213; -3.48 eV for C214; -3.43 eV for C215; -3.53 eV for C216 vs vacuum), relative to the conduction band edge (-4.00 eV vs vacuum) of TiO2,24 provides a driving force for charge generation. Also, our DFT calculations show that LUMO (Figure S2 in Supporting Information) of these dyes is mainly populated on the cyanoacrylic acid anchoring moiety. These thermodynamic and kinetic features suggest a possibility of ultrafast interfacial electron injection from the excited dyes to the titania conduction band, before the occurrence of other radiative or nonradiative deactivations of excitons. Moreover, in order to reach a current collector, photoinjected electrons in the titania must traverse a several micrometer thick mesoporous film within the millisecond time domain; however, in the absence of electrolytes, the oxidized sensitizer can recapture electrons within submilisecond time domain.25 The uphill energy offset by the HOMO (-5.09 eV for C209; -5.03 eV for C210; -5.09 eV for C213; -5.05 eV for C214; -5.09 eV for C215; -5.06 eV for C216 vs vacuum) with respect to that (-4.60 eV vs vacuum) of iodide3p could supply a negative Gibbs energy change for dye regeneration. Moreover, HOMO (Figure S2 in Supporting Information) of C209, C210, C213, C214, C215, and C216 is mainly populated over the triphenylamine moiety, which is spatially convenient for the electron donor to approach. These together may ensure efficient dye regeneration at a moderate rate compared to the ultrafast charge generation at the titania/dye interface, avoiding the geminate charge recombination between oxidized dye molecules and photoinjected electrons in the nanocrystalline titania film. We further compared the measured electronic energy levels of these six dyes as well as the calculation data. As depicted in Figure S2 of the Supporting Information, adding one more furan, thiophene, or selenophene unit not only lifts the HOMO level but also drives down the LUMO, narrowing the HOMO and LUMO gap. This pattern is the same as that of dyes with thienothiophene and bisthienothiophene conjugated linkers3p but is different from that of dyes with electron-rich 3,4-ethylenedioxythiophene and 2,2′-bis(3,4-ethylenedioxythiophene) units,3q where only the uplifted HOMO is noted along with the extension of conjugation length. It is possible to realize a rational control on the energy-level alignment and thereby energy gap, attaining a panchromatic organic sensitizer.

Photocurrent action spectra of 12 cells, made from these 6 sensitizers in combination with an acetonitrile electrolyte (EL01) and a solvent-free ionic liquid electrolyte (EL02), are presented in parts A, C, and E of Figure 3, where incident photon-tocollected electron conversion efficiencies (IPCEs) are plotted versus wavelength. Interestingly, all ionic liquid cells exhibit a feature of bathochromic photocurrent response in comparison to acetonitrile cells, even if there is no difference in the dyecoated titania film. In order to clarify the origin of this observation, we fabricate dummy cells based on the C216 dye along with a transparent nanocrystalline titania film and measured the absorption of a cell filled with EL01 or EL02. As shown in Figure S3 of the Supporting Information, the C216coated film immersed in EL02 can harvest more low-energy photons than that of EL01, probably because of the dielectric variation of these two electrolytes. Compared to the cells based on dyes (C209, C213, and C215) with one heterocycle linker, the red-shifted photocurrent action spectra of cells made from dyes (C210, C214, and C216) having two conjugated heterocycles are well in agreement with the absorption measurements in the preceding discussion. The C216 dye with a biselenophene spacer has a lower IPCE maximum than C215 possessing a selenophene linker, while there is no evident decrease of the high plateaus of IPCEs for the furan and thiophene series. Moreover, a close look on the relative IPCE heights (Figure S4 in Supporting Information) of the furan (C209), thiophene (C213), and selenophene (C215) dyes reveals a gradual decrease of photocurrent conversion efficiencies with the decrease of heteroatom electronegativity. We deduce that this could be closely related to dye-dependent electron injection yields, probably stemming from a shortened exciton lifetime which will be measured in further experiments and/or a depressed LUMO. The latter point has been partially solidified with the observation on the improvement of IPCEs for thiophene and selenophene based cells, upon removing the TBY base from the electrolyte EL01. This probably indicates a large exciton binding energy for organic dyes. Charge collection yield is not considered as the origin of low IPCEs, because we do not observe the height difference in IPCEs for the corresponding cells with EL01 and EL02. Actually, our previous work6b has shown that the ionic liquid electrolyte EL02 with a low fluidity and a high triiodide concentration endows a shorter diffusion length of electrons in the nanocrystalline film than an acetonitrile electrolyte EL01. Owing to the red-shifted photocurrent response, it is reasonable to expect higher short-circuit photocurrent densities (Jsc) for ionic liquid cells, from the overlap integral of IPCE curves with the standard AM1.5G solar emission spectrum. However, this is not in accord with the J-V data (parts B, D, and F of Figure 3) measured under an irradiance of 100 mW cm-2 AM1.5G sunlight, where ionic liquid cells counterintuitively

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Figure 3. Photocurrent action spectra of (A) C209 and C210, (C) C213 and C214, and (E) C215 and C216. J-V characteristic of cells with (B) C209 and C210, (D) C213 and C214, and (F) C215 and C216 measured under an irradiance of 100 mW cm-2 AM1.5G sunlight and in the dark. EL01: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M TBY, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (v/v, 85/15). EL02: DMII/EMII/EMITCB/I2/NBB/GNCS (molar ratio, 12/12/16/1.67/3.33/0.67).

show similar or lower photocurrents in contrast to the corresponding acetonitrile cells. Thus, we measured photocurrents of these two types of cells by varying incident light intensities. This experiment has shown that different from acetonitrile cells, there is a slight nonlinear dependence of photocurrent on light intensities in the case of EL02. Note that IPCEs were measured under weak monochromatic lights. Furthermore, the open-circuit photovoltages of ionic liquid cells are lower than those of acetonitrile, in the range of 37-67 mV for these six dyes, which causes a relatively lower power conversion efficiency (η). This Voc variation has also been observed in our previous work on both a ruthenium dye6b and some organic sensitizers.3q,4b,5 This can be rationalized in terms of the following reasons: (i) the calculated equilibrium potential of EL01 is 17 mV positive-shifted in contrast to EL02; (ii) a small chemical capacitance at the titania/electrolyte interface can be probed if EL01 is used; (iii) a high concentration of triiodide in EL02 can cause a fast charge recombination at the titania/interface interface, augmenting the reverse saturation current. In addition, the tendency of a gradually reduced Voc for the one and two heterocycle series can be felt along with the decrease of heteroatom electronegativity. It is also obvious to note that Voc becomes low along with extending the π-conjugation. Overall, the enhanced Jsc due to better light

TABLE 2: Detailed Photovoltaic Parameters Measured at an Irradiance of 100 mW cm-2 AM1.5G Sunlight sensitizer

electrolyte

Voc/mV

Jsc/mA cm-2

FF

η/%

C209

EL01 EL02 EL01 EL02 EL01 EL02 EL01 EL02 EL01 EL02 EL01 EL02

793.6 746.9 769.1 702.8 775 720.3 744.3 688.2 755.4 718.7 720.4 663.7

12.08 12.02 13.84 13.86 11.88 11.82 14.32 14.16 12.84 12.21 14.57 14.01

0.748 0.742 0.683 0.705 0.747 0.751 0.720 0.716 0.737 0.732 0.740 0.710

7.17 6.66 7.27 6.89 6.88 6.39 7.67 7.00 7.15 6.42 7.77 6.60

C210 C213 C214 C215 C216

harvesting is partially canceled by the loss in Voc. Detailed device parameters of these six dyes are listed in Table 2, showing 6.88-7.77% efficiencies for EL01 and 6.39-7.00% for EL02. In order to gain some insight on the origins of the dye-Voc relationship explicitly depicted in Figure S4B of the Supporting Information, we first take a look at chemical capacitance at the titania/electrolyte interface, measured by the transient photocurrent and photovoltage decay techniques.26 As presented in Figure 4A, the capacitances of cells with different dyes all

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Li et al.

Figure 4. Plots of (A) chemical capacitance and (B) charge recombination rate constant vs open-circuit photovoltage for cells based on the C209, C213, C215, and C216 dyes in conjunction with electrolyte EL01. EL01: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M TBY, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (v/v, 85/15).

Figure 5. J-V characteristics of cells based on the C216 dye along with electrolytes EL01 (A) and EL02 (B) measured in the dark and under an irradiance of 100 mW cm-2 AM1.5G sunlight at various temperatures. EL01: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M TBY, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (v/v, 85/15). EL02: DMII/EMII/EMITCB/I2/NBB/GNCS (molar ratio, 12/12/16/1.67/3.33/ 0.67).

Figure 6. Plots of temperature-dependent (A) open-circuit photovoltage and (B) short-circuit photocurrent for cells based on the C216 dye along with electrolytes EL01 and EL02.

increase exponentially along with the increase of Voc, which was generated by applying a gradually enhanced light intensity. It is known that the chemical capacitance is closely related to density of surface states below the conduction band edge. At a given Voc, a cell made from the C215 dye exhibits the highest Cµ among the three single-heterocycle-linker dyes, while the lowest value is obtained for C209. This is probably related to the planarity (C209 > C213 > C215) of dye molecules calculated by the DFT method and shown in Figure S1 of the Supporting Information. It is interesting to note that the torsion angle of the five-membered heterocycles with respect to the phenyl group increases with the heteroatom size, i.e., C209 < C213 < C215. Apparently, if the dye molecule is planar, one can expect an ordered close-packing on the titania nanocrystals. Introduction of one more selenophene has resulted in a higher chemical capacitance owing to a worse planarity of the C216 dye molecules, which can also be perceived from Figure S1 of the Supporting Information. As the same electrolyte and titania film

are used for the evaluation of these four dyes, we can derive from the capacitance measurements that the density of surfaces states increases in the order of C209 < C213 < C215 < C216. A poor close packing of dye molecules also results in a fast charge recombination at the titania/electrolyte interface as shown in Figure 4B. The shortening of electron lifetime with the extension of π-conjugation length can also be felt by the comparison of C209 and C210 or C213 and C214 as presented in Figure S5 of the Supporting Information. Whether the enhancement of charge recombination is also related to the possible interactions of heterocycles of metal-free organic dyes with triiodide and/or iodine is still an open question and needs to be clarified in further studies. At the moment, our studies on several ruthenium dyes with heterocycle building blocks of furan, thiophene, and selenophene do not seem to support this assumption. Keeping in mind that the electron diffusion length is always significantly larger than the film thickness if an acetonitrile

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Figure 7. Plots of chemical capacitance vs open-circuit photovoltage for cells based on the C216 dye along with electrolytes EL01 (A) and EL02 (B).

Figure 8. Plots of charge recombination rate constant vs extracted charge density for cells based on the C216 dye along with electrolytes EL01 (A) and EL02 (B).

Figure 9. Plots of (A and D) electron diffusion coefficient, (B and E) electron lifetime, and (C and F) electron diffusion length vs DOS for cells based on the C216 dye along with electrolytes EL01 and EL02.

electrolyte is used, we may not rush to correlate these recombination measurements with the aforementioned IPCE plateaus. On the other side, we have found that the charge recombination rate constant increases in the order of C209 < C213 < C215 < C216, which intimately follows the tendency

of reverse saturation currents, derived from the fitting of the dark J-V curves (Figure S4 of the Supporting Information) by the diode equation. This may explain the Voc trend observed for these four dyes, because the open-circuit voltage increases logarithmically with decreasing saturation current.

7478 J. Phys. Chem. C, Vol. 113, No. 17, 2009 We further measured the J-V characteristics (Figure 5) at different temperatures for two cells with the C216 dye in conjunction with EL01 and EL02, respectively. Obviously, both cells exhibit a progressive enhancement of dark current along with the increase of temperature. As presented in Figure 6A, the open-circuit photovoltage decreases linearly along with temperature, consistent with previous publications.27 At a given Voc (Figure 7), the apparently large Cµ at a high temperature is directly related to more deep surface states because the calculated equilibrium potential of EL01 (or EL02) at 50 °C has an only 3.34 mV (or 4.59 mV) negative shift with respect to that at 10 °C. Note that in both electrolytes, the imidazolium cations and the additivies (TBY or NBB) can have a strong interaction with the unsaturated titanium species of the mesoporous titania film and function as trap filling reagents. It seems that the filling effect becomes weak with the increase of temperature, resulting in more deep electron-trapping states below the conduction band edge. Obviously, the augmented deep electron-trapping states with temperature have a significant effect on open-circuit photovoltage. As shown in Figure 8, at a given extracted charge density, the charge recombination becomes fast with increasing temperature for both cells. We imagine that this could be mainly ascribed to the enhanced diffusion coefficient of triiodide ions and electrons trapped in the titania film. As the temperature increases, a gradual enhancement of photocurrents is observed in Figures 5 and 6B for the two C216 based cells with either EL01 or EL02. This is distinguished from the result by Dittrich et al.,28 showing a temperatureindependent photocurrent. We resorted to the electrical impedance measurements29 to diagnose if this current enhancement is related to a better charge collection. As reported by others30 and also depicted in parts A and D of Figure 9, the chemical diffusion coefficient of an electron (Dn) is improved with temperature, indicating a fast thermally activated detrapping of electrons to the conduction band for transport. However, with the increase of temperature electron lifetime (τn) is shortened in a more evident fashion as shown in parts B and E of Figure 9, implying a bimolecular recombination mechanism, where the enhanced triiodide mobility also plays a vital role. Overall, we have found from parts C and F of Figure 9 that the electron diffusion length (Ln) becomes short along with the increase of temperature, although it may be still long enough for quantitative electron collection in the temperature range discussed here. Moreover, there is no observable variation in the light absorption of C216 stained titania film at various temperatures (Figure S6 of the Supporting Information). Thereby we suspect that the current enhancement is caused by an improved charge generation yield at a high temperature owing to more low-energy electron-accepting states. This needs to be clarified in our further pump-probe measurements. We exposed solvent-free cells based on C209, C210, C213, C214, C215, and C216, to the 1000 h accelerated testing at 60 °C, in a solar simulator with a light intensity of 100 mW cm-2. As presented in Figures S7-S12 of the Supporting Information, all of these cells exhibit excellent stabilities, maintaining over 90% of their initial efficiencies after 1000 h of accelerated aging. Apart from the well-known stability of thiophene units, this is the first time to see such a good photochemical stability of furan and selenophene as building blocks of active components in dye-sensitized solar cells. We believe that the iodide/triiodide electrolyte deactivates the photochemical reaction of these heterocycles.

Li et al. 4. Conclusions In summary, we have synthesized several organic D-π-A dyes by employing different π-conjugated linkers such as furan, bifuran, thiophene, bithiophene, selenophene, and biselenophene in combination with the hydrophobic triphenylamine donor and the hydrophilic cyanoacrylic acid acceptor. These dyes can reach 6.88-7.77% and 6.39-7.00% efficiencies for an acetonitrilebased electrolyte and a solvent-free ionic liquid electrolyte, respectively. For the first time we have proved that furan and selenophene can be employed as stable units in dye-sensitized solar cells, enlarging the selection range of building blocks for further dye design. We have found that there is a trade-off between enhanced photocurrent and reduced photovoltage, along with the use of electropositive heteroatoms as well as more conjugated units. Preliminary electrical measurements have indicated that the slight decrease of external quantum efficiencies may not be related to charge collection rather than charge generation. The latter issue will be addressed in our further photophysical studies. Acknowledgment. The National Key Scientific Program (No. 2007CB936700), the CAS Knowledge Innovation Program (No. KGCX2-YW-326), and the CAS 100-Talent Program have supported this work. We are grateful to Dyesol for supplying EMITCB and 400 nm sized scattering paste and to DuPont Packaging and Industrial Polymers for supplying the Bynel film. Supporting Information Available: Calculation details, stability data, and additional spectroscopic and recombination measurements. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature (London) 1991, 353, 737. (2) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (3) For example, see:(a) Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783. (b) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai, X.; Lian, T.; Yanagida, S. Chem. Mater. 2004, 16, 1806. (c) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (d) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. ReV. 2004, 248, 1363. (e) Thomas, K. R. J.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C. Chem. Commun. 2005, 4098. (f) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Chem. Commun. 2006, 2245. (g) Li, S.-L.; Jiang, K.-J.; Shao, K.-F.; Yang, L.-M. Chem. Commun. 2006, 2792. (h) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am. Chem. Soc. 2006, 128, 14256. (i) Kim, S.; Lee, J. W.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; De Angellis, F.; Di Censo, D.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 16701. (j) Wang, Z.-S.; Cui, Y.; Hara, K.; Dan-oh, Y.; Kasada, C.; Shinpo, A. AdV. Mater. 2007, 19, 1138. (k) Edvinsson, T.; Li, C.; Pschirer, N.; Scho¨neboom, J.; Eickemeyer, F.; Sens, R.; Boschloo, G.; Herrmann, A.; Mu¨llen, K.; Hagfeldt, A. J. Phys. Chem. C 2007, 111, 15137. (l) Wang, M.; Xu, M.; Shi, D.; Li, R.; Gao, F.; Zhang, G.; Yi, Z.; HumphryBaker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. AdV. Mater. 2008, 20, 4460. (m) Shi, D.; Pootrakuchote, N.; Yi, Z.; Xu, M.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 17478. (n) Zhou, G.; Pschirer, N.; Scho¨neboom, J. C.; Eickemeyer, F.; Baumgarten, M.; Mu¨llen, K. Chem. Mater. 2008, 20, 1808. (o) Lin, J. T.; Chen, P.-C.; Yen, Y.-S.; Hsu, Y.-C.; Chou, H.-H.; Yeh, M.-C. P. Org. Lett. 2009, 11, 97. (p) Zhang, G.; Bai, Y.; Li, R.; Shi, D.; Wenger, S.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. Energy EnViron. Sci. 2009, 2, 92. (q) Xu, M.; Wenger, S.; Bara, H.; Shi, D.; Li, R.; Zhou, Y.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. J. Phys. Chem. C 2009, 113, 2966. (r) Yum, J.-H.; Hagberg, D. P.; Moon, S.-J.; Karlsoon, K. M.; Marinado, T.; Sun, L.; Hagfeldt, A.; Nazeeruddin, M. K.; Gra¨tzel, M. Angew. Chem., Int. Ed. 2009, 48, 1576. (4) (a) Ito, S.; Miura, H.; Uchida, S.; Takata, M.; Sumioka, K.; Liska, P.; Comte, P.; Pe´chy, P.; Gra¨tzel, M. Chem. Commun. 2008, 5194. (b) Zhang, G.; Bala, H.; Cheng, Y.; Shi, D.; Lv, X.; Yu, Q.; Wang, P. Chem. Commun., DOI 10.1039/b822325d. (5) Qin, H.; Wenger, S.; Xu, M.; Gao, F.; Jing, X.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2008, 130, 9202.

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