Perovskite–Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting

Photoelectrochemical water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to optoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic–inorganic halide perovskite (CH3NH3PbI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic electro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematite's photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe2O3 photoanodes. The total photopotential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias.