For devices with a-TiO2 and c-TiO2 protection layers, the bias voltage was -0.15 V vs Ag│AgCl. For devices with the SiOx protection layer, the bias voltage was -0.55 V vs Ag│AgCl. After 10 mC cm -2 of charge had passed, cyclic voltammetry in 1 M H2SO4 under 0.5 sun was performed (25-50 cycles) to condition the catalyst and improve the fill factor.