Solvatochromic Dye Sensitized Nanocrystalline Solar Cells 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 TBA 4 [Ru(CN) 4 (dcb)], where TBA is tetrabutylammonium and dcb is 4,4′-(CO 2 - ) 2 -2,2′-bipyridine, was synthesized and attached to optically transparent nanocrystalline (anatase) TiO 2 films, abbreviated [Ru(CN) 4 (dcb)]/TiO 2 . 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 TBA 4 [Ru(CN) 4 (dcb)] and at 470 and 500 for [Ru(CN) 4 (dcb)]/ TiO 2 , respectively. Pulsed light excitation of TBA 4 [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)]/TiO 2 yields an absorption difference spectrum attributed to an interfacial charge separated state, [Ru III (CN) 4 (dcb)]/TiO 2 (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)]/TiO 2 were prepared whose spectral sensitivity and efficiency were a function of the solvent used with 0.5 M LiI and 0.05 M I 2 . 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) TiO 2 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 TiO 2 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 we demonstrate solvatochromic solar cells based upon [Ru(CN) 4 (dcb)] 4- , where dcb is 4,4′-(CO 2 - ) 2 -2,2′-bipyridine, anchored to nanocrystalline TiO 2 films, abbreviated [Ru- (CN) 4 (dcb)]/TiO 2 and shown below. The results demonstrate that solvent can be used to tune the spectral response and efficiencies of regenerative solar cells and provides funda- mental information on the solvation environment of semi- conductor bound molecules. Experimental Section. The synthesis and characterization of TBA 4 [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 ac- etonitrile were obtained from Burdick & Jackson and were used as received. NANO LETTERS 2002 Vol. 2, No. 6 625-628 10.1021/nl0255395 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/30/2002