The influence of dye structure on charge recombination in dye-sensitized solar cells

Replacing the nonyl groups on the solar cell dye Ru(4,4′-carboxylic acid-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2 (Z-907) with amino groups results in a marked decrease in solar cell performance. This is despite the fact that the amino derivative (Z-960) has more favourable light absorption characteristics than Z-907 when used with thick nanocrystalline TiO2 layers. Electron transfer to the electrolyte from the exposed fluorine-doped tin oxide (FTO) substrate is particularly fast in cells employing the Z-960 dye if a compact TiO2 blocking layer is not used. The kinetics of electron transfer from the nanocrystalline TiO2 layer in DSCs employing Z-960 are comparable to those of bare TiO2 and ca. 2 to 5 times faster than for cells employing Z-907. The faster charge recombination in cells employing Z-960 lowers open-circuit photovoltage and results in very significant charge collection losses that lower short-circuit photocurrent. Voltammetric measurements show that surface modification of FTO electrodes with Z-960 results in slightly more facile charge transfer to acceptor species in triiodide/iodide electrolytes in the dark. A simpler molecule, p-aminobenzoic acid, more dramatically catalyses this charge transfer reaction. Conversely, chemical modification of FTO electrodes with Z-907 or p-toluic acid retards charge transfer kinetics. Similar results are obtained for nanocrystalline TiO2 electrodes modified with these benzoic acid derivatives. These results strongly imply that surface adsorbed molecules bearing amino groups, including dye molecules, can catalyse charge recombination in dye-sensitized solar cells.