Effect of Annealing Temperature on TiO₂ZnO CoreShell Aggregate Photoelectrodes of Dye-Sensitized Solar Cells

Photoelectrodes made of submicrometer-sized aggregates of ZnO nanocrystallites coated with TiO2 layer by atomic layer deposition with different annealing temperatures were investigated for dye-sensitized solar cells. Although the annealing at 350 C is commonly used for the ZnO aggregates in order to preserve the desired large surface area and mesoporous structure, the presence of ALD-TiO2 layer on the ZnO aggregates would enable to increase the annealing temperature up to 450 C without a loss of surface area by suppressing surface diffusion of the ZnO atoms. The morphology and crystal size of the aggregates and BET results supported the fact that ALDTiO2 layer coated on the ZnO aggregates is effective in preventing the surface diffusion of the ZnO atom at elevated annealing temperature. Electrochemical property of the photoelectrodes including the charge transfer resistance was significantly affected by the annealing temperature. Electrochemical impedance spectroscopy revealed that the charge transfer resistance at the ZnO aggregate/electrolyte interface, where recombination occurs, increased with the annealing temperature. The larger charge transfer resistance is thought to reduce the surface recombination and thus contributes to the increase in the open circuit voltage (Voc) of the DSCs as much as ∼100 mV. The increase in fill factor as well as Voc of the cell was observed corresponding to the increasing annealing temperature, and these improvements finally lead to more than 30% enhancement in the efficiency of DSCs.