Recombination inhibitive structure of organic dyes for cobalt complex redox electrolytes in dye-sensitised solar cells

Abstract

A very high open-circuit voltage (V_oc) has recently been achieved in dye-sensitised solar cells (DSSCs) using cobalt complex redox couple electrolytes because of the high positive redox potential of the electrolyte. However, the obtained V_oc is lower than the expected value owing to fast back electron transfer reactions from the TiO₂ surface to the redox species. Recombination may be retarded by introduction of steric barrier groups into the dye structure to block the approach of the cobalt complex towards the TiO₂ surface. Herein, carbazole dyes with various structural features in each dye component, i.e., number of n-hexylthiophene units in the linker, bulky appendage in the donor group, and alkyl chain length in the thiophene linker moiety, were studied in order to retard the fast back electron transfer reactions. There was a positive relationship between V_oc and the number of n-hexylthiophene units, bulkiness of the appendage group in the donor segment, and shorter alkyl chains in the thiophene linker moiety. The increased V_oc was attributed to the retardation of charge recombination, demonstrating that longer and larger molecules exert better blocking function.