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

摘要

Dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO_2 yield energy conversion efficiencies ～10%. The efficiencies of similar cells with films of other oxide materials (SnO_2, ZnO) are well below the above value. However, the cells made from SnO_2―ZnO composite films give efficiencies comparable to TiO_2 cells. Two types of composite systems with SnO_2 and ZnO are possible. In the first type, SnO_2 crystallites are covered with an ultra-thin (< 1 nm) outer shell of ZnO_2 and in the second type, the film comprises SnO_2 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 SnO_2 could effectively counteract recombinations of electrons with acceptors in the electrolyte (e.g., I_3~-) and increase the efficiency although SnO_2 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.