Effect of Additives on the Photovoltaic Performance of Coumarin

Rui Su , Mohamed R. Elmorsy , Mira Abed , Ashraful Islam , Meghan Lord , Ahmed A. ..... Roberto Grisorio , Luisa De Marco , Rita Agosta , Rosabianca I...
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Langmuir 2004, 20, 4205-4210

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Effect of Additives on the Photovoltaic Performance of Coumarin-Dye-Sensitized Nanocrystalline TiO2 Solar Cells Kohjiro Hara,*,† Yasufumi Dan-oh,‡ Chiaki Kasada,‡ Yasuyo Ohga,‡ Akira Shinpo,‡ Sadaharu Suga,‡ Kazuhiro Sayama,† and Hironori Arakawa*,† Photoreaction Control Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan and Hayashibara Biochemical Laboratories, Incorporated, 564-176 Fujita, Okayama 701-0221, Japan Received September 20, 2003. In Final Form: February 28, 2004 The effects of deoxycholic acid (DCA) and 4-tert-butylpyridine (TBP) as additives on the photovoltaic performance of coumarin-dye-sensitized nanocrystalline TiO2 solar cells were investigated. DCA coadsorption improved both the photocurrent and photovoltage of the solar cells, even though it decreased the amount of dye adsorbed on the TiO2 electrode. The improved photocurrent may arise from suppression of the deactivation of the excited state via quenching processes between dye molecules or a more negative LUMO level of the dye in the presence of DCA, resulting in a high electron-injection yield from the dye into TiO2. The increased photovoltage is probably due to suppression of recombination between the injected electrons and I3- ions on the TiO2 surface (dark current). The addition of TBP to the electrolyte also markedly improved the photovoltage and fill factor of the solar cell, and consequently, the total conversion efficiency increased from 3.6% to 7.5%. FT-IR spectroscopy indicated that a large amount of TBP was adsorbed on the dye-coated TiO2 films in the presence of Li cations. This result suggests that TBP, like DCA, suppressed the dark current on the TiO2 surface, which resulted in the improved photovoltage.

Introduction Dye-sensitized solar cells (DSSCs) based on nanocrystalline oxide semiconductors have been intensively studied and developed over the past decade because these unconventional solar cells exhibit high performance and have the potential for low-cost production.1-5 The main components of a DSSC are a nanocrystalline oxide semiconductor electrode, a photosensitizer, a redox electrolyte, and a counter electrode. In addition, several kinds of additives have been found to improve the photovoltaic performance of DSSCs. For example, Kay and Gra¨tzel found that when they employed cholic acid (CA) derivatives as coadsorbates in DSSCs based on porphyrin-derived photosensitizers, both the photocurrent and the photovoltage of the solar cells were improved.6 Furthermore, CA derivatives have been used in DSSCs based on porphyrins,7 phthalocyanines,8,9 naphthalocyanines,10 trithiocyanato 4,4′4′′-tricarboxy-2,2′:6′,2′′-terpyridine ruthenium(II) (the black dye),11,12 and a Ru phenanthroline * To whom correspondence should be addressed. Phone: +8129-861-4494. Fax: +81-29-861-6771. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Hayashibara Biochemical Laboratories, Inc. (1) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382-6390. (2) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49-68. (3) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269-277. (4) Gra¨tzel, M. Nature 2001, 414, 338-344. (5) Gra¨tzel, M. J. Photochem. Photobiol. C: Photochem. Rev. 2003, 4, 145-153. (6) Kay, A.; Gra¨tzel, M. J. Phys. Chem. 1993, 97, 6272-6277. (7) Odobel, F.; Blart, E.; Lagre´e, M.; Villieras, M.; Boujtita, H.; Murr, N. E.; Caramori, S.; Bignozzi, C. A. J. Mater. Chem. 2003, 13, 502-510. (8) Nazeeruddin, Md. K.; Humphry-Baker, R.; Gra¨tzel, M.; Murrer, B. A. Chem. Commun. 1998, 719-720. (9) He, J.; Benko¨, G.; Korodi, F.; Polı´vka, T.; Lomoth, R.; A° kermark, B.; Sun, L.; Hagfeldt, A.; Sundstro¨m, V. J. Am. Chem. Soc. 2002, 124, 4922-4932. (10) Li, X.; Long, N. J.; Clifford, J. N.; Campbell, C. J.; Durrant, J. R. New J. Chem. 2002, 26, 1076-1080.

complex13 to improve solar cell performance. In addition, Wang et al. found that using hexadecylmalonic acid (HDMA) as a coadsorbate for a DSSC based on a Ru bipyridil complex (Z907 dye) improved both the photocurrent and photovoltage of the cell.14 The improved photocurrent due to coadsorbates can be attributed to a positive shift of the conduction band edge of TiO2 in the presence of acid6 or to suppression of quenching processes due to energy transfer;9 both of these effects result in increases in electron-injection yields. The improved photovoltage is believed to be caused by suppression of recombination between the injected electrons and I3- ions (dark current).6,14 In addition, pyridine compounds, such as 4-tert-butylpyridine (TBP), have been also used as additives in organic electrolytes,1,9,11,15,16 or in dye coating solutions,10 or as reagents for treating the dye-coated TiO2 electrode.17 TBP remarkably improves the photovoltage of the solar cell, and the improvement is attributable to suppression of the dark current.1,17 Recently, we reported the successful design of novel coumarin dyes for use as photosensitizers in DSSCs.18-20 We constructed highly efficient DSSCs based on a new (11) Nazeeruddin, M. K.; Pe´chy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613-1624. (12) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. J. Phys. Chem. B 2002, 106, 12693-12704. (13) Hara, K.; Sugihara, H.; Tachibana, Y.; Islam, A.; Yanagida, M.; Sayama, K.; Arakawa, H.; Fujihashi, G.; Horiguchi, T.; Kinoshita, T. Langmuir 2001, 17, 5992-5999. (14) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; HumphryBaker, R.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 14336-14341. (15) Barbe´, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am. Ceram. Soc., 1997, 80, 3157-3171. (16) Kusama, H.; Konishi, Y.; Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 80, 167-179. (17) Huang, S. Y.; Schlichtho¨rl, G.; Nozik, A. J.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. 1997, 101, 2576-2582. (18) Hara, K.; Tachibana, Y.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 77, 89-103.

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Hara et al.

Chart 1. Coumarin Dyes NKX-2586 and NKX-2677

coumarin dye with thiophene moieties (NKX-2677). We achieved solar energy-to-electricity conversion efficiencies of up to 7.7% under simulated AM 1.5 irradiation (100 mW cm-2).20 In this paper, we report the effects of deoxycholic acid (DCA) as a coadsorbate and TBP as an additive in the electrolyte on the photovoltaic performance of DSSCs based on coumarin dyes. Our results indicate that these additives played important roles in increasing solar cell performance. To improve the performance of DSSCs based on organic dyes further, a better understanding of the effects of additives is important. Experimental Section 1. Synthesis of Dyes. The molecular structures of the coumarin dyes used in this work (NKX-2586 and NKX-2677) are shown in Chart 1. The syntheses of these dyes have been reported previously.18,20 2. Preparation and Characterization of Dye-Coated TiO2 Electrodes. TiO2 nanoparticles were prepared by the method reported by Gra¨tzel and co-workers.14,15 Nanocrystalline TiO2 electrodes (apparent area, 0.5 × 0.5 cm2; thickness, ca. 14 µm) were prepared on a glass substrate coated with a transparent conducting oxide (TCO, F-doped SnO2) by a screen-printing technique. The detailed procedure for preparing the TiO2 films has been reported elsewhere.20 The coumarin dyes (0.3 mM) were dissolved in a 50:50 (vol %) solution of tert-butyl alcohol (Kanto Chemical) and acetonitrile (AN, Kanto, dehydrated for organic synthesis). The solvents were used as obtained from the suppliers without further purification. Deoxycholic acid (DCA, Tokyo Kasei Kogyo Co. Ltd.) was added to the dye solution at a concentration ranging from 5 to 70 mM. The TiO2 films were immersed in the dye solutions and then kept at 25 °C for at least 12 h to allow the dye to adsorb onto the TiO2 surface. The UV-vis absorption spectra of coumarin dyes adsorbed on a transparent TiO2 film were measured in transparency mode with a Shimadzu UV-3101PC spectrophotometer. FT-IR absorption spectra were measured with a Perkin-Elmer Spectrum One spectrometer with an attenuated total reflection (ATR) system equipped with a ZnSe prism. The oxidation potentials of the dyes adsorbed on the TiO2 films were measured by differential pulse voltammetry using an ALS Electrochemical Analyzer, model 610B. The measurements were carried out in a 0.1 M tetrabutylammonium perchlorate (TBAP)/acetonitrile electrolyte with a dye-coated TiO2 electrode as the working electrode, a Pt counter electrode, and a Ag/Ag+ (0.01 M AgNO3-0.1 M TBAP/ acetonitrile) reference electrode. The potential was calibrated against the ferrocene redox couple at 0.38 V vs SCE. 3. Photovoltaic Measurements of Solar Cells. The electrochemical cell (two-electrode type) used for photovoltaic measurements consisted of a dye-coated TiO2 electrode, a counter electrode, a polyethylene film spacer (25 µm thick), and an organic liquid electrolyte. The counter electrode was a Pt film sputtered on a TCO-coated glass plate. The electrolyte consisted of 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI), 0.1 M LiI, (19) 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, 597-606. (20) Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783-785.

Figure 1. (a) UV-vis absorption spectra of NKX-2586 adsorbed on a transparent TiO2 film at various DCA concentrations: (‚ ‚ ‚) DCA 0 mM, (- - -) DCA 5 mM, (- -) DCA 20 mM, and (s) DCA 40 mM; (b) normalized spectra. and 0.05 M I2 in methoxyacetonitrile (MAN) or AN with or without TBP (Aldrich). Reagent-grade LiI (Wako) and I2 (Wako) were used for the electrolyte. MAN (Aldrich and Tokyo Kasei) and TBP were distilled before use. DMPImI was purchased from Tomiyama Pure Chemical Industries Ltd. The photovoltaic performance of the DSSCs based on coumarin dyes was measured with a source meter (Advantest, R6246). We employed an AM 1.5 solar simulator (Yamashita Denso Co., YSS150A with a 1000-W Xe lamp and an AM filter) as the light source. The incident light intensity was calibrated with a standard Si solar cell produced by Japan Quality Assurance Organization. Action spectra of the monochromatic incident photon-to-current conversion efficiency (IPCE) for the solar cell were measured with a CEP-99W system (Bunkoh-keiki Co., Ltd.).

Results and Discussion 1. Effect of DCA Coadsorption on UV-Vis Absorption Spectra of Dyes. The amount of NKX-2586 adsorbed on the TiO2 surface decreased with increasing DCA concentration: a decrease of about 70% was observed when the DCA concentration was increased to 40 mM (Figure 1a). Like the dye, DCA is able to adsorb onto the TiO2 surface by way of its carboxyl group. Bauer et al. reported that after a TiO2 film is dipped into a solution containing the black dye and a CA derivative (taurochenodeoxycholic acid), no IR absorption bands for the CA derivative are observed on the TiO2 film. They concluded that the CA derivative is not a coadsorbate in the case of the black dye.12 In the case of NKX-2586, however, IR absorption bands for DCA were observed on a TiO2 film (not shown) that was dipped into the dye/DCA solution, which suggests that DCA was coadsorbed with NKX-2586 on the TiO2 surface. The coadsorption of DCA would decrease the amount of adsorbed dye on the TiO2 surface.

Nanocrystalline TiO2 Solar Cells

Langmuir, Vol. 20, No. 10, 2004 4207 Table 1. Electrochemical Properties of the Dyes Adsorbed on TiO2 Electrode with and without DCAa dye

DCA/mM

Eoxb/ V vs SCE

E0-0b/ eV

Eox-E0-0/ V vs SCE

NKX-2586

0 40 0 40

0.71 0.70 0.70 0.67

1.65 1.79 1.74 1.74

-0.94 -1.09 -1.04 -1.07

NKX-2677

a Conditions: working, a dye-coated TiO electrode; counter, a 2 Pt electrode; reference, an Ag/Ag+ in 0.01 M AgNO3-0.1 M TBAP/ b AN; electrolyte, 0.1 M TBAP/AN. Eox was estimated by differential pulse voltammetry. c E0-0 was estimated from the onset of absorption spectra of the dyes adsorbed on the TiO2 electrode.

Figure 2. (a) UV-vis absorption spectra of NKX-2677 adsorbed on a transparent TiO2 film at various DCA concentrations: (‚ ‚ ‚) DCA 0 mM, (- - -) DCA 20 mM, (- -) DCA 50 mM, and (s) DCA 70 mM; (b) normalized spectra.

The absorption spectrum of NKX-2586 narrowed when the DCA concentration was increased, as shown in the normalized absorption spectra (Figure 1b). The absorption spectrum of NKX-2586 adsorbed on TiO2 without DCA was broader than the spectrum of the dye in solution (not shown). Strong interactions between the adsorbed dye molecules and the oxide surface are known to lead to aggregate formation, and consequently, broadening of the absorption spectra as well as an increase in dimer absorption have been observed when the dyes are adsorbed on the oxide surfaces.21,22 The oxide surfaces play an important role in organizing and orienting the adsorbed dye molecules. The broadening of the absorption spectrum of NKX-2586 on TiO2 might be due to dye-TiO2 interactions, dye-dye interactions, or both, and coadsorption of DCA diminished these interactions on the TiO2 surface. As was the case for NKX-2586, the amount of NKX2677 adsorbed on the TiO2 film also decreased with increasing DCA concentration (Figure 2a). When 70 mM DCA was added to the solution, the amount of adsorbed dye was slightly more than one-half that adsorbed in the absence of DCA. The shape of the spectra did not change when the DCA concentration was increased from 20 to 70 mM, whereas the spectrum in the absence of DCA was slightly broadened (Figure 2b). Thus, the influence of DCA on the spectrum of NKX-2677 was obviously smaller than that for NKX-2586. We attribute this difference to a strong intermolecular π-π stacking interaction due to the thiophene moieties, as observed in substituted oligothiophenes.23 The amount of NKX-2677 adsorbed on a TiO2 film (10 µm) without DCA was 2.0 × 10-7 mol cm-2, (21) Gopidas, K. R.; Kamat, P. V. J. Phys. Chem. 1989, 93, 64286433. (22) Kamat, P. V. Chem. Rev. 1993, 93, 267-300.

which was larger than the amounts of other dyes, e.g., 1.7 × 10-7 mol cm-2 for NKX-2586 and 1.3 × 10-7 mol cm-2 for the N3 dye.1 These results suggest that strong interaction between the thiophene moieties of NKX-2677 results in a highly ordered adsorption of the dyes on the TiO2 surface. 2. Electrochemistry of Dyes Adsorbed on Nanocrystalline TiO2 Electrodes. Oxidation potentials of aromatic amines24 and metal complexes14,25 adsorbed on nanocrystalline TiO2 electrodes were estimated by cyclic voltammetry using a dye-coated TiO2 electrode as a working electrode. The redox reaction is considered to occur first on transparent conducting oxide, such as F-doped SnO2, substrates as the trigger step.24,25 We estimated the oxidation potentials of NKX-2586 and NKX2677 adsorbed on nanocrystalline TiO2 electrodes by differential pulse voltammetry. The electrochemical properties of the dyes adsorbed on a nanocrystalline TiO2 electrode are summarized in Table 1. The oxidation potential (Eox) of NKX-2586 adsorbed on a TiO2 film is not affected by the presence of DCA, as shown in Table 1. Eox-E0-0 (which corresponds to the LUMO level of the dye) was changed in the presence of DCA, and we attribute this change to the different values of E0-0, which was estimated from the onset of absorption: -0.94 V without DCA and -1.09 V with DCA. No large changes in the Eox and Eox-E0-0 values of NKX-2677 adsorbed on a TiO2 film were observed in the presence of DCA (Table 1). 3. Effect of DCA Coadsorption on Photovoltaic Performance of Solar Cells. The action spectra of incident photon-to-current conversion efficiency (IPCE) for DSSCs based on NKX-2586 and NKX-2677 at various DCA concentrations are shown in Figure 3. The IPCEs are given by the following equation

IPCE (%) )

1240 [eV‚nm] × Jph[mA cm-2] λ [nm] × Φ [mW cm-2]

× 100 (1)

where Jph is the short-circuit photocurrent density for (23) (a) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 87168721. (b) Stecher, R.; Gompf, B.; Mu¨nter, J. S. R.; Effenberger, F. Adv. Mater. 1999, 11, 927-931. (c) Rep, D. B. A.; Roelfsema, R. Adv. Mater. 2000, 12, 563-566. (d) Gesquie´re, A.; Abdel-Mottaleb, M. M. S.; De Feyter, S.; De Schryver, F. C.; Schoonbeek, F.; van Esch, J.; Kellogg, R. M.; Feringa, B. L.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L. Langmuir 2000, 16, 10385-10391. (24) Bonhoˆte, P.; Gogniat, E.; Tingry, S.; Barbe´, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Gra¨tzel, M. J. Phys. Chem. B 1998, 102, 1498-1507. (25) (a) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319-5324. (b) Trammell, S. A.; Meyer, T. J. J. Phys. Chem. B 1999, 103, 104-107. (c) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkara, J. M.; Meyer, G. J. Langmuir 1999, 15, 7047-7054. (d) Yanagida, M.; Yamaguchi, T.; Kurashige, M.; Hara, K.; Katoh, R.; Sugihara, H.; Arakawa, H. Inorg. Chem. 2003, 42, 7921-7931.

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Figure 3. Action spectra of IPCE for DSSCs based on coumarin dyes at various DCA concentrations: (a) NKX-2586 (‚ ‚ ‚) DCA 0 mM, (- - -) DCA 5 mM, (- -) DCA 20 mM, and (s) DCA 40 mM; (b) NKX-2677 (‚ ‚ ‚) DCA 0 mM, (- - -) DCA 20 mM, (- -) DCA 50 mM, and (s) DCA 70 mM.

monochromatic irradiation and λ and Φ are the wavelength and the intensity, respectively, of the monochromatic light. Interestingly, the IPCE of the DSSC based on NKX-2586 improved with increasing DCA concentration, whereas the amount of dye on the TiO2 surface decreased (Figure 3a). A similarly improved IPCE performance was also observed for a DSSC based on NKX-2677 (Figure 3b). Ghosh and Bard reported that a strong interaction between neighboring molecules on the substrates leads to deactivation of the excited state via self-quenching processes in the case of Ru(bpy)32+ (bpy ) 2,2′-bipyridine) adsorbed on clay materials.26 When Zn(bpy)32+ is coadsorbed with Ru(bpy)32+ on the surface, the self-quenching rate is dramatically reduced due to dilution of Ru(bpy)32+ molecules.26 He et al. reported that 3R,7R-dihydroxy-5βcholic acid (cheno) and TBP as coadsorbates improved the IPCE performance of DSSCs based on zinc phthalocyanine photosensitizers.9 They suggested that reduced surface aggregation due to the coadsorbates, which suppresses quenching processes due to energy transfer or chargetransfer reactions between the aggregated molecules and/ or between molecules in the aggregates and monomers, leads to the improved IPCE performance. In addition, Khazraji et al. reported that the IPCE performance of a merocyanine dye-sensitized TiO2 solar cell is improved by the addition of a coadsorbate, which prevents formation of a dimer whose electron-injection performance is lower than that of the monomer.27 The improved IPCE performance of DSSCs based on the coumarin dyes may arise from the prevention of the deactivation of the excited state via quenching processes between dyes and the resulting improved electron-injection yield from the dye into TiO2. In addition, the Eox-E0-0 level of NKX-2586 with DCA is (26) Ghosh, P. K.; Bard, A. J. J. Phys. Chem. 1984, 88, 5519-5526. (27) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 4693-4700.

Hara et al.

Figure 4. Current-voltage curves obtained for DSSCs based on coumarin dyes at various DCA concentrations with 0.6 M DMPImI, 0.1 M LiI, and 0.05 M I2 in MAN: (a) NKX-2586 (‚ ‚ ‚) DCA 0 mM, (- - -) DCA 5 mM, (- -) DCA 20 mM, and (s) DCA 40 mM; (b) NKX-2677 (‚ ‚ ‚) DCA 0 mM, (- - -) DCA 20 mM, (- -) DCA 50 mM, and (s) DCA 70 mM. Table 2. Photovoltaic Performance of DSSCs Based on NKX-2586 and NKX-2677 at Various DCA Concentrationsa dye

DCA/mM

Jsc/mA cm-2

Voc/V

ff

η/%

NKX-2586

0 5 20 40 0 20 50 70

13.7 14.5 14.6 14.7 15.3 16.2 16.9 16.6

0.45 0.49 0.51 0.54 0.42 0.46 0.49 0.49

0.50 0.46 0.51 0.49 0.44 0.44 0.47 0.49

3.1 3.3 3.8 3.9 2.8 3.3 3.9 4.0

NKX-2677

a Conditions: irradiated light, AM 1.5 (100 mW cm-2); photoelectrode, TiO2 (14 µm thickness and 0.25 cm2); electrolyte, 0.6 M DMPImI-0.1 M LiI-0.05 M I2 in MAN.

0.15 V more negative than that for the dye without DCA (Table 1). It is also possible that the more negative EoxE0-0 level observed for NKX-2586 with DCA led to a higher electron-injection yield from the dye into TiO2 (conduction band edge level ) -0.7 V vs SCE2) due to a large energetic driving force. Figure 4 and Table 2 show the photovoltaic performance of DSSCs based on NKX-2586 and NKX-2677 under AM 1.5 irradiation (100 mW cm-2) at various DCA concentrations. The solar energy-to-electricity conversion efficiency, η, under white-light irradiation (e.g., AM 1.5) can be obtained from the following equation

η (%) )

Jsc[mA cm-2] × Voc[V] × ff I0[mW cm-2]

× 100

(2)

where I0 is the photon flux (e.g., ca. 100 mW cm-2 for AM 1.5), Jsc is the short-circuit photocurrent density under irradiation, Voc is the open-circuit voltage, and ff is the fill factor. The dark current characteristics corresponding to

Nanocrystalline TiO2 Solar Cells

Langmuir, Vol. 20, No. 10, 2004 4209 Table 3. Photovoltaic Performance of a DSSC Based on NKX-2677 at Various TBP Concentrations TBP/M

Jsc/mA cm-2

Voc/V

ff

η/%

0.0 0.5 1.0 2.0

17.1 15.6 15.3 14.7

0.47 0.66 0.69 0.73

0.45 0.70 0.71 0.70

3.6 7.2 7.5 7.5

a Conditions: dye, NKX-2677; coadsorbate, DCA 40 mM; irradiated light, AM 1.5 (100 mW cm-2); photoelectrode, TiO2 (14 µm thickness and 0.25 cm2); electrolyte, 0.6 M DMPImI-0.1 M LiI0.05 M I2-TBP in AN.

Figure 5. FT-IR-ATR absorption spectra of (a) a NKX-2677coated TiO2 film and (b) a bare TiO2 film after being dipped into (‚ ‚ ‚) AN, (- - -) 0.5 M TBP in AN, and (s) 0.1 M LiI and 0.5 M TBP in AN. The inset is the FT-IR absorption spectrum of neat TBP. -

the reaction between the electrons in TiO2 and I3 ions are also shown in Figure 4. The Jsc values for the DSSCs based on NKX-2586 and NKX-2677 increased with increasing DCA concentration: these improved Jsc values are reflected in the improved IPCE performance, as shown in Figure 3. Interestingly, the Voc also improved with increasing DCA concentration in both the DSSCs (Table 2). The improved Voc is probably caused by suppression of recombination between the injected electrons and I3ions (dark current) due to the DCA coadsorption, as reported for DSSCs based on chlorophyll-derivative photosensitizers.6 Thus, DCA improved both the Jsc and the Voc of the solar cell, which resulted in improved total efficiency: the efficiency rose from 3.1% to 3.9% for NKX2586 as the DCA concentration was increased from 0 to 40 mM, and the efficiency rose from 2.8% to 4.0% for NKX2677 as the DCA was increased from 0 to 70 mM. 4. TBP Adsorption on the TiO2 Surface. Figure 5 shows the FT-IR absorption spectra of TiO2 films (bare and NKX-2677-coated) measured after the films were immersed in acetonitrile or 0.5 M TBP in acetonitrile or 0.1 M LiI with 0.5 M TBP in acetonitrile. The absorption peaks at 1596 and 1408 cm-1 were assigned to the CdC stretching band of the pyridine moiety of TBP. When the NKX-2677-coated TiO2 film was immersed in a 0.5 M TBP acetonitrile solution, no absorption peaks attributed to TBP were observed in the spectrum (Figure 5a). However, the absorption peaks assigned to TBP (e.g., 1607 and 1415 cm-1) were observed in the film immersed in the solution of 0.1 M LiI with 0.5 M TBP in acetonitrile. This result clearly indicates that TBP molecules were adsorbed on the surface of the NKX-2677-coated TiO2 film in the presence of LiI. Slight shifts in the absorption peaks were probably caused by the interaction of TBP with the dye, the TiO2 surface, or both. Li cations could adsorb and/or

Figure 6. Current-voltage curves obtained for a DSSC based on NKX-2677 (DCA 40 mM) at various TBP concentrations with 0.6 M DMPImI, 0.1 M LiI, and 0.05 M I2 in AN: (‚ ‚ ‚) 0 M TBP, (- - -) 0.5 M TBP, (- -) 1 M TBP, and (s) 2 M TBP.

intercalate on the TiO2 surface.28 The TBP molecules must interact with Li cations present near the TiO2 surface due to a Coulombic interaction. In the case of the bare TiO2 film (Figure 5b), TBP was slightly adsorbed on the TiO2 surface even without LiI (dashed line). The amount of TBP adsorbed on the surface increased remarkably in the presence of LiI (solid line), as was the case for the dye-coated film. This result clearly indicates that TBP molecules were adsorbed on the TiO2 surface in the presence of LiI and suggests that TBP molecules interacted with Li cations on the TiO2 surface in the absence of the dyes. Comparison of the intensity of the absorption at 1600 cm-1 (the CdC stretching band) in Figure 5a and b suggests that the amount of TBP adsorbed on the surface decreased by about 50% upon adsorption of NKX-2677. TBP molecules were probably adsorbed on the sites not occupied by the dyes. 5. Effect of TBP on the Photovoltaic Performance of a DSSC Based on NKX-2677. The photovoltaic performance and I-V characteristics of a DSSC based on NKX-2677 (DCA 40 mM) under AM 1.5 irradiation and dark conditions at various TBP concentrations (0 M to 2 M) in the electrolyte are given in Table 3 and Figure 6. The Voc and ff values were remarkably improved by the addition of TBP and as the TBP concentration was increased, whereas the Jsc value decreased slightly with increasing TBP concentration. Consequently, the total conversion efficiency increased remarkably from 3.6% for 0 M TBP to 7.5% for 1 and 2 M TBP (Table 3). The onset of dark current is remarkably shifted toward larger voltage at higher TBP concentrations, which indicates that (28) (a) Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1993, 97, 14261430. (b) Enright, B.; Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1994, 98, 6195-6200. (c) Hagfeldt, A.; Vlachopoulos, N.; Gra¨tzel, M. J. Electrochem. Soc. 1994, 141, L82-L84. (d) Liu, Y.; Hagfeldt, A.; Xiao, X.-R.; Lindquist, S.-E. Sol. Energy Mater. Sol. Cells 1998, 55, 267-281.

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recombination between the electrons and I3- ions is suppressed (Figure 6). This suppression of dark current would lead to the improved Voc and ff. TBP is known to prevent the dark current and thus to improve the Voc of DSSCs.1,2,9,17 From the spectral data shown in Figure 5, it is possible that TBP blocks the dark current on the TiO2 surface. In addition, TBP adsorption on the TiO2 surface might shift the conduction band edge level of TiO2 negatively, owing to TBP’s basicity. In the case of a DSSC based on a coumarin dye, NKX-2311, whose LUMO level is more positive than that of NKX-2677, the added TBP remarkably diminishes the Jsc value.18 This remarkable decrease was probably caused by a decrease in a driving force for electron transfer from the dye to TiO2 (i.e., ∆G) due to the negative shift of the conduction band of TiO2 in the presence of TBP.18 The Jsc value of a DSSC based on a phthalocyanine sensitizer also decreased with added TBP in the electrolyte.9 In the case of the DSSC based on NKX2677, the Jsc value decreased slightly with increasing TBP concentration (Table 3). Taking into consideration these results, it is possible that this negative shift of the conduction band level of TiO2 due to the TBP adsorption on the TiO2 surface also resulted in improved Voc. The smaller decrease in the Jsc value with increasing TBP concentration for NKX-2677 relative to NKX-2311 is probably due to the more negative Eox-E0-0 value of NKX2677 compared to that of NKX-2311.18,20 In the absence of TBP, the shapes of the I-V curves under irradiation and in the dark were different: the photocurrent decreased from 0.15 V, while the onset of the dark current was 0.35 V (this behavior was also observed in Figure 4). This result suggests that recombination between the injected electrons in TiO2 and the dye cations or I3- ions in the electrolyte dominantly occurred under irradiation. Actually, the differences in the I-V curve shapes observed under irradiation and in the dark for bulk semiconductor-electrolyte junction solar cells have been explained in terms of dominant occurrences of several types of recombination: e.g., recombination between conduction band electrons and acceptor ions in the electrolyte, the electron-hole recombination in the bulk and in the space charge region.29 Charge recombination between the injected electrons in TiO2 and the cations of a Ru complex (N3 dye) is reported to be a relatively slow process under conditions in which negative bias is low.30 Taking into consideration this point, recombination between the injected electrons and I3- ions under irradiation would lead to the decrease in photocurrent at (29) Reichman, J. Appl. Phys. Lett. 1980, 36, 574-577. (30) Haque, S. A.; Yachibana, Y.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. B 1998, 102, 1745-1749.

Hara et al.

relatively low voltage, although the detailed mechanism is not clear at present. As shown in Figure 6, the addition of TBP prevented the decrease of photocurrent at relatively low voltage, which led to a remarkable improvement in the Voc and ff values. The effect of TBP on the surface would be similar to the effect of metal oxide blocking layers deposited on the TiO2 surface, which inhibit recombination between injected electrons and I3- ions and consequently lead to improved Voc of the solar cells, as reported by several groups.31 Conclusions Coadsorption of DCA improved both the Jsc and the Voc values for DSSCs based on NKX-2586 and NKX-2677, but coadsorption decreased the amount of adsorbed dye on the TiO2 surface. For NKX-2586, DCA changed the absorption spectra and the LUMO level of the dye. The influence of DCA observed for NKX-2677 adsorbed on the TiO2 surface was smaller than that observed for NKX2586, which we attributed to a strong intermolecular π-π stacking interaction due to the thiophene moieties. The improved Jsc value observed for a DSSC based on NKX2586 may arise from prevention of the deactivation of the excited state via quenching processes between dyes and the resulting improved electron-injection yield from the dye into TiO2 or a more negative LUMO level of the dye in the presence of DCA, resulting in a large energetic driving force for the electron injection. The improved Voc of the DSSCs is probably caused by suppression of recombination between the injected electrons and I3- ions (dark current) due to the DCA coadsorption. FT-IR absorption measurements suggested that in the presence of LiI, TBP molecules were adsorbed on the TiO2 surface at sites not occupied by the dyes. The Voc and ff values for a DSSC based on NKX-2677 were remarkably improved by the addition of TBP and as the TBP concentration increases, whereas the Jsc values decreased slightly with increasing TBP. Consequently, the total conversion efficiency increased remarkably from 3.6% to 7.5%. The improved Voc was probably due to suppression of the recombination between the electrons and I3- ions (dark current). Acknowledgment. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under Japan’s Ministry of Economy Trade and Industry. LA0357615 (31) (a) Zaban, A.; Chen, S. G.; Chappel, S.; Gregg, B. A. Chem. Commun. 2000, 2231-2232. (b) Kay, A.; Gra¨tzel, M. Chem. Mater. 2002, 14, 2930-2935. (c) Diamant, Y.; Chen, S. G.; Melamed, O.; Zaban, A. J. Phys. Chem. B 2003, 107, 1977-1981. (d) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475-482.