Fluorene-thiophene copolymer wire on TiO2: Mechanism achieving long charge separated state lifetimes

Insertion of interfacial molecules in bulk heterojunction and dye sensitized solar cells is effective to retard charge recombination reactions, and thus to improve solar cell performance. So far, to extend charge separated state lifetime, the molecule was designed to increase distance between an n-type and a p-type semiconductors to reduce their electronic coupling. Here we investigated a series of thiophene-fluorene molecular wires on the TiO2 nanoporous surface, and propose a model to explain a long lived charge separated state. The polymer wire acts as a sensitizer aligned in parallel to the TiO2 surface, and injects an electron into the TiO2 with electron injection efficiency of >80 %. Time-resolved microwave conductivity measurements suggest that a generated hole can be mobile, and we found with DFT calculation that a hole appears to be localized at the thiophene units which are not directly attached to the TiO2 surface. Charge recombination between the mobile electron in the TiO2 and the hole at the thiophene units is retarded to >100 ms, compared to the reaction at the monomer/TiO2 interface with ~5 ms. Monte Carlo simulation supports that this slow charge recombination occurs with the localization of the hole at the thiophene units.