NANO LETTERS
Solvatochromic Dye Sensitized Nanocrystalline Solar Cells
2002 Vol. 2, No. 6 625-628
Roberto Argazzi and Carlo Alberto Bignozzi* Departimento di Chimica, UniVersita di Ferrara, Via L. Borsari, No. 46, 44100 Ferrara, Italy
Mei Yang, Georg M. Hasselmann, and Gerald J. Meyer* Department of Chemistry, Johns Hopkins UniVersity, Baltimore, Maryland 21218 Received March 1, 2002; Revised Manuscript Received March 29, 2002
ABSTRACT The coordination compound TBA4[Ru(CN)4(dcb)], where TBA is tetrabutylammonium and dcb is 4,4′-(CO2-)2-2,2′-bipyridine, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO2 films, abbreviated [Ru(CN)4(dcb)]/TiO2. The metal-to-ligand-charge-transfer (MLCT) absorption and emission bands were found to shift in wavelength with solvent. The absorption maximum of the low energy MLCT band was observed at 517 nm in acetonitrile and 535 nm in dimethylformamide for TBA4[Ru(CN)4(dcb)] and at 470 and 500 for [Ru(CN)4(dcb)]/ TiO2, respectively. Pulsed light excitation of TBA4[Ru(CN)4(dcb)] in acetonitrile produces a long-lived emissive MLCT excited state, τ ) 30 ns. Pulsed light excitation of [Ru(CN)4(dcb)]/TiO2 yields an absorption difference spectrum attributed to an interfacial charge separated state, [RuIII(CN)4(dcb)]/TiO2(e-). This state forms within 10 ns and returns cleanly to ground-state product within milliseconds. Regenerative solar cells based on [Ru(CN)4(dcb)]/TiO2 were prepared whose spectral sensitivity and efficiency were a function of the solvent used with 0.5 M LiI and 0.05 M I2. The maximum incident photon-to-current efficiency (IPCE) was measured at 480 nm (25%) in acetonitrile and 510 nm (5%) in dimethylformamide. This work reports a new approach for controlling the spectral sensitivity of solar cells and for probing the solvation of molecules anchored to nanocrystalline semiconductor surfaces.
Introduction. The dye-sensitized nanocrystalline (anatase) TiO2 solar cell reported by O’Regan and Gratzel about 10 years ago has renewed interest in molecular approaches to energy conversion.1,2 The wide band gap TiO2 is transparent in the visible region and thus the color of the dye sensitized solar cells is mainly determined by the dye molecules and the redox-active electrolyte. This is unlike the situation encountered with solid-state photovoltaics based on silicon, which are always dark and may be exploited for niche applications as solar or “smart” windows.3 One method to achieve colorful solar cells is to utilize different dye molecules. In this regard, extensive research efforts have been initiated to extend the spectral sensitivity to the near-IR region thereby increasing the solar cell efficiency under sunlight conditions.4-7 An alternative approach is to utilize a solvatochromic dye and simply control the solvent or solvent mixture in the electrolyte. This is the subject of this letter. Coordination compounds of the general type [M(CN)4(bpy)]2-, where M is Fe, Ru, or Os and bpy ) 2,2′bipyridyine, are known to be highly solvatochromic.8-12 Previous researchers have proposed that the remarkable solvatochromism in this class of compounds results from donor-acceptor interactions with the cyanides. In this paper 10.1021/nl0255395 CCC: $22.00 Published on Web 04/30/2002
© 2002 American Chemical Society
we demonstrate solvatochromic solar cells based upon [Ru(CN)4(dcb)]4-, where dcb is 4,4′-(CO2-)2-2,2′-bipyridine, anchored to nanocrystalline TiO2 films, abbreviated [Ru(CN)4(dcb)]/TiO2 and shown below. The results demonstrate that solvent can be used to tune the spectral response and efficiencies of regenerative solar cells and provides fundamental information on the solvation environment of semiconductor bound molecules.
Experimental Section. The synthesis and characterization of TBA4[Ru(CN)4(dcb)] will be published in a future manuscript. We note that the preparation of the unsubstituted bpy compound, [Ru(CN)4(bpy)]2-, has been previously reported.9,11 Tetrahydrofuran, dimethylformamide, and acetonitrile were obtained from Burdick & Jackson and were used as received.
Table 1. Spectroscopic Properties of TBA4[Ru(CN)4(dcb)] and [Ru(CN)4(dcb)]/TiO2 at 298 ( 2 K compound
solvent
TBA4[Ru(CN)4(dcb)] TBA4[Ru(CN)4(dcb)] TBA4[Ru(CN)4(dcb)] [Ru(CN)4(dcb)]/TiO2 [Ru(CN)4(dcb)]/TiO2 [Ru(CN)4(dcb)]/TiO2
acetonitrile tetrahydrofuran dimethylformamide acetonitrile tetrahydrofuran dimethylformamide
absorbance,a emission,b nm nm 515 ( 4 530 ( 4 535 ( 4 470 ( 10 450 ( 10 500 ( 20
750 ( 10 765 ( 10 770 ( 10 645 ( 10 650 ( 10 650 ( 10
a Absorbance maxima of low energy MLCT bands. b Uncorrected emission maxima.
Figure 1. Absorbance spectra of (a) [Ru(CN)4(dcb)]4- in neat acetonitrile (solid line), tetrahydrofuran (dash-dotted line) and dimethylformamide (dashed line) and (b) [Ru(CN)4(dcb)]/TiO2 in acetonitrile (solid line), tetrahydrofuran (dash-dotted line) and dimethylformamide (dashed line) and of unsensitized TiO2 in acetonitrile (dotted line).
Colloidal TiO2 films were prepared by a previously described sol-gel technique that produces mesoporous films of ca. 10 µm thickness.13 For absorption studies the films were coated onto glass slides rather than conductive glass. The binding of [Ru(CN)4(dcb)]4- to TiO2 films was realized by soaking TiO2 films in millimolar solutions of TBA4[Ru(CN)4(dcb)] in acetonitrile overnight. Absorption spectra were acquired at ambient temperature in air using a Hewlett-Packard 8453 diode array spectrometer. Transient absorbance spectra were acquired under an Ar atmosphere as previously described.14 The photocurrent action spectra were obtained in a twoelectrode sandwich cell arrangement that has been previously described.13 Briefly, ∼10 µL of electrolyte was sandwiched between a TiO2 electrode and a Pt coated tin oxide electrode. The electrolytes consisted of 0.5 M LiI and 0.05 M I2 in actetonitrile, dimethylformamide or tetrahydrofuran. The light source was a 450 W Xe lamp coupled to an f/0.22 monochromator. Photocurrents were measured with a Keithly model 617 digital electrometer and the incident irradiances were measured with a calibrated silicon photodiode from UDT Technologies. Results and Discussion. The absorbance spectra of [Ru(CN)4(dcb)]4- in neat acetonitrile (ACN), tetrahydrofuran (THF), and dimethylformamide (DMF) are shown in Figure 1a. Two absorption bands were observed whose maxima depend on the solvent are well described as metal-to-ligand charge-transfer, MLCT, Ru(II) f dcb. Figure 1b shows [Ru(CN)4(dcb)]/TiO2 in the same solvents and solvatochromism is also clearly observed. The [Ru(CN)4(dcb)]/TiO2 appears yellow in acetonitrile while pinkish in dimethylformamide. Measurements at wavelengths less than 400 nm were difficult to accurately obtain due to the strong fundamental absorption of TiO2. The compounds were found to be emissive in both fluid solution and when anchored to TiO2. The absorption and emission maxima are given in Table 1. No sensitizer 626
Figure 2. Nanosecond transient absorbance difference spectra recorded at the indicated times after pulsed 532.5 nm light excitation in acetonitrile at room temperature: (a) [Ru(CN)4(dcb)]4- (9) 0, (b) 20, and (2) 50 ns and (b) [Ru(CN)4(dcb)]/TiO2 (9) 0, (b) 100, and (2) 500 ns.
desorption was observed over a period of 2 days for [Ru(CN)4(dcb)]/TiO2 films immersed in these solvents. The solvation environment of the semiconductor bound molecules can be qualitatively assessed by the data in Figure 1. The MLCT absorption bands of [Ru(CN)4(dcb)]/TiO2 are blue-shifted relative to those in fluid solution but do shift significantly with solvent. This is consistent with the sensitizers being partially solvated by the polar TiO2 surface and the external solvent. The absorption bands appear much broader on the TiO2 surface, resulting in larger uncertainty in the maxima, Table 1. The broadened MLCT bands likely reflect surface heterogeneity. The data indicate that [Ru(CN)4(dcb)]4- is a useful molecular probe of solvation environment and surface homogeneity of these mesoporous nanocrystalline TiO2 films. Figure 2a shows transient absorbance difference spectrum of [Ru(CN)4(dcb)]4- in acetonitrile immediately following pulsed 532.5 nm laser excitation. A positive band centered at ∼370 nm was observed, which is consistent with the MLCT excited state of [Ru(CN)4(dcb)]4- and previous Nano Lett., Vol. 2, No. 6, 2002
The IPCE is the product of three terms: the quantum yield for charge injection (φ), the efficiency of collecting the injected electrons in the external circuit (η), and light harvesting efficiency (LHE) which is the fraction of radiant power absorbed by the sensitizers2 IPCE ) (φ)(η)(LHE)
Figure 3. Normalized photoaction spectra of [Ru(CN)4(dcb)]/TiO2 in 0.5 M LiI/0.05 M I2 acetonitrile (9) or dimethylformamide (b) electrolytes. The inset is the unnormalized spectra in 0.5 M LiI/ 0.05 M I2 acetonitrile (9), dimethylformamide (b) electrolytes or tetrahydrofuran (().
literature reports.9 The kinetics were first-order and independent of the monitoring wavelength with τ ) 30 ( 4 ns. Figure 2b shows transient absorbance difference spectra of [Ru(CN)4(dcb)]/TiO2 in acetonitrile after pulsed light excitation. A bleach between 380 and 630 nm and a weak positive band after 630 nm were observed. On the basis of previous reports, the transient difference spectra are assigned to a charge-separated state with an electron in TiO2 and an oxidized Ru center, abbreviated [RuIII(CN)4(dcb)]/TiO2(e-). The charge separated state is formed within our instrument response function, indicating that kinj > 108 s-1. Recombination of the injected electron with the oxidized dye is complete on a millisecond time scale. Rapid electron injection and slow back electron transfer are consistent with previous studies on ruthenium dye molecules bound to TiO2 through carboxylate linkages.2,14 Figure 3 shows the photocurrent action spectra of [Ru(CN)4(dcb)]/TiO2 in 0.5 M LiI and 0.05 M I2 in acetonitrile, dimethylformamide, and tetrahydrofuran electrolytes. The IPCE is the incident photon-to-current conversion efficiency that is defined by IPCE )
(1240, eV‚nm)(photocurrent density, µA/cm2) (λ, nm)(irradiance, µW/cm2)
(1)
The sensitizer surface coverage was approximately the same for the samples shown in Figure 3, yet the spectral sensitivity and efficiency of [Ru(CN)4(dcb)]/TiO2 films were highly dependent on the solvent. The [Ru(CN)4(dcb)]2-/TiO2 showed an IPCE maximum value of 40% at 510 ( 10 nm in tetrahydrofuran, 25% at 460 ( 10 nm in acetonitrile, and 7% at 510 ( 10 nm in dimethylformamide. Repetitive trials showed that the sensitized photocurrents were reproducible and stable. Nano Lett., Vol. 2, No. 6, 2002
(2)
The latter term is easily quantified by visible absorption spectroscopy and accounts for the solvent dependent changes in spectral sensitivity of the solar cell. The LHE is related to the molecular extinction coefficient that decrease for the parent bpy compound as ACN (5100 M-1 cm-1) > DMF (4600 M-1 cm-1) > THF (4400 M-1 cm-1).12 For the photocurrent measurements, the surface coverage was systematically varied and it was found that the LHE term cannot explain the large changes in the magnitude of the IPCE. The emission from the surface bound sensitizers indicates that φ is not unity. However, transient absorption measurements in both acetonitrile and in dimethylformamide, reveal no evidence for MLCT excited states following pulsed 532.5 nm light excitation of [Ru(CN)4(dcb)]2-/TiO2, indicating that φ is near unity and that the weak emission is probably from a small population of sensitizers ( E°[Ru(III/II) DMF], Figures 1 and 3. Consistent with this trend, the measured Ru(III/II) potentials for the parent bpy compound are E°(ACN) ) -0.14 V and E°(DMF) ) -0.28 V vs Fe(C5H5)2+/Fe(C5H5)2.12 This solvent induced shift would alter the driving force for iodide oxidation and charge recombination, both of which could affect η. However, the situation is much more complicated as the external solvent may also influence the position of the conduction band edge and the energetics for iodide oxidation. Therefore, more detailed studies are required to determine the factors that cause η to be solvent dependent. In conclusion, dye-sensitized solvatochromic solar cells with [Ru(CN)4(dcb)]/TiO2 were prepared and characterized for the first time. Solvents tune the color, efficiency, and spectral response of the solar cell. The results demonstrate that [Ru(CN)4(dcb)]4- sensitizers are molecular probes of the solvation environment and surface heterogeneity of nanocrystalline TiO2 interfaces. The results appear to be general and may be extended to other solvatochromic probes 627
and semiconductor surfaces. Fundamental studies designed to quantify the energetics and dynamics of these fascinating sensitized nanostructures are currently underway in our laboratories. Acknowledgment. The Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy, is gratefully acknowledged for the research support of G.J.M. The work of C.A.B. was supported in part by the European Union under the project Human Potential RTN, Micro-Nano, Contract number HPRNCT-2000-00028. References (1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (2) For recent reviews see: (a) Kamat, P. V. Chem. ReV. 1993, 93, 267. (b) Qu, P.; Meyer, G. J. In Electron Transfer in Chemistry; Balzani, V., Ed.; 2001; Chapter 2, Part 2, Vol. IV, p 355. (c) Gratzel, M. Nature 2001, 414, 338. (3) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague. J.; Gregg, B. A. Nature 1996, 383, 608. (4) 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.
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(5) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1994, 33, 5741. (6) (a) Argazzi, R.; Bignozzi, C. A.; Hasselmann, G. M.; Meyer, G. J. Inorg. Chem. 1998, 37, 4533. (b) Islam, A.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida, M.; Murata, S.; Takaashi, Y.; Sugihara, H.; Arakawa, H. Chem. Lett. 2000, 490. (7) 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.; Kay, A.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (8) Toma, H. E.; Takasugi, M. S. J. Solution Chem. 1983, 12, 547. (9) Bignozzi, C. A.; Chiroboli, C.; Indelli, M. T.; Scandola, M. A. R.; Varani, G.; Scandola, F. J. Am. Chem. Soc. 1986, 108, 7872. (10) Winkler, J. R.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1987, 109, 3470. (11) Garcia Posse, M. E.; Katz, N. E.; Baraldo, L. M.; Polonuer, D. D.; Colombano, C. G.; Olabe, J. A. Inorg. Chem. 1995, 34, 1830. (12) Timpson, C. T.; Bignozzi, C. A.; Sullivan, B. P.; Kober, E. M.; Meyer, T. J. J. Phys. Chem. 1996, 100, 2915. (13) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319. (14) Kelly, C. A.; Farzard, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999, 15, 731.
NL0255395
Nano Lett., Vol. 2, No. 6, 2002
