A high-voltage type solar cell consisting of a TiO2 photocatalyst on the anode, an Ag-TiO2 photocatalyst on the cathode, and the water/O-2 redox system is enabled and optimized by controlling the Ag size (similar to 4 nm) and core-shell structure at the proximity of the indium tin oxide cathode, the photocatalyst film thickness (3.1 +/- 0.3 mu m), and the configuration of photoelectrodes in the solar cell (SC). The active sites for O-2 photoreduction were Ag-0 in situ formed on cathode irradiated by UV-visible light as evidenced by Ag K-edge extended X-ray absorption fine structure. Ag-Ag coordination number increased from 1.6 under aqueous HCl electrolyte to 9.6-11 under the conditions of SC tests, suggesting homogeneous Ag size distribution and maximized Ag-0 active sites. The effective Ag-0 shell was present only under the irradiation of UV-visible light; Ag-0 sites were transformed into Ag2O sites when the light was turned off. By the optimization of Ag content and Ag-TiO2 film thickness on photocathode and light irradiation configuration from photocatalyst/electrode side on anode/cathode, an opencircuit voltage of 1.55 V exceeding most of the SC reported so far. Furthermore, cell power of 18.7 mu W cm(-2) was also achieved.