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

Abstract

Dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO2 yield energy conversion efficiencies ∼10%. 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 (∼10 nm) with a thin ZnO outer shell and larger ZnO particles (∼100 nm). The short-circuit photocurrent and efficiency of these cells are ∼17 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., I3−) 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.