Solvatochromic Dye Sensitized Nanocrystalline Solar Cells

The coordination compound TBA(4)[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 TBA(4)[Ru(CN)(4)(dcb)] and at 470 and 500 for [Ru(CN)(4)(dcb)]/TiO2, respectively. Pulsed light excitation of TBA(4)[Ru(CN)(4)(dcb)] in acetonitrile produces a long-lived emissive MLCT excited state, tau = 30 ns. Pulsed light excitation of [Ru(CN)(4)(dcb)]/TiO2 yields an absorption difference spectrum attributed to an interfacial charge separated state, [Ru-III(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 Lil 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.