Dual-functional materials for interface modifications in solid-state dye-sensitised TiO2 solar cells

The concept of solid-state dye-sensitised TiO2 solar cells with an organic semiconductor as hole-transport medium is studied in detail by examining the dye-hole conductor interface. The facile transfer of holes from Ru-dye core to the hole conductor requires suitable interface modifiers which have the function of dye and hole transport moiety, but with exactly positioned anchor groups and antenna functions. The synthesis and characterisation of such novel low molecular weight multifunctional molecules carrying dye units and triphenylamine moieties are presented and their influence as interface modifiers is studied. This interface modification results in doubling the external quantum efficiency of current conversion via improved charge transfer at the dye-hole conductor interface. Moreover, the recombination processes at this interface are drastically suppressed, which leads to higher open-circuit voltage and consequently higher power-conversion efficiency. The concept is also extended to polymers to obtain dye-centred polymeric hole conductors which carry a single Ru-dye unit in the middle of the poly(vinyltriphenylamine) chain that acts as hole-conductor polymer. The polymerisation was carried out by atom-transfer radical polymerisation of 4-bromostyrene followed by polymer amination and finally metallation with Ru-bis(bipyridyl) precursors.