Efficient dye-sensitize photoelectrochemical cells made from nanocrystalline tin(IV) oxide-zinc oxide composite films

Dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO2 yield energy conversion efficiencies similar to10%. The efficiencies of similar cells with films of other oxide materials (SnO2, ZnO) are well below the above value. However, the cells made from SnO2-ZnO composite films give efficiencies comparable to TiO2 cells. Two types of composite systems with SnO2 and ZnO are possible. In the first type, SnO2 crystallites are covered with an ultra-thin (< 1 nm) outer shell of ZnO2 and in the second type, the film comprises SnO2 crystallites (similar to10 nm) with a thin ZnO outer shell and larger ZnO particles (similar to100 nm). The short-circuit photocurrent and efficiency of these cells are similar to17 mA cm(-2), 19 mA cm(-2) and 7%, 8% respectively. This paper explains in detail how a thin shell of ZnO on SnO2 could effectively counteract recombinations of electrons with acceptors in the electrolyte (e.g., I-3(-)) and increase the efficiency although SnO2 and ZnO are individually not good materials for dye-sensitized photoelectrochemical cells. In the second type, larger ZnO crystallites reduce the rate of geminate recombinations, in addition to the effect of the outer shell.