Monolithic Photoelectrochemical Tandem Devices consisting of Tunnel Oxide Passivated Contact Silicon and BiVO₄ enabling Unassisted Water Splitting

A tandem photoelectrochemical (PEC) water-splitting device for solar hydrogen production consists of two light absorbers with different bandgaps. Silicon photoelectrodes have been widely investigated in PEC applications because their bandgap (1.12 eV) is suitable for the bottom cell of a tandem device. We apply a tunnel oxide passivated contact (TOPCon) on the front and back sides of a Si wafer to prepare a TOPCon Si PEC device. Photocathodes and photoanodes based on TOPCon Si exhibit remarkable versatility across a wide pH spectrum (0–14), generating photovoltages ranging from 640–650 mV under 1 sun illumination, marking the highest values achieved among crystalline Si photoelectrodes. TOPCon Si demonstrates excellent thermal stability, enduring a high processing temperature of up to 600 °C for 1 h in ambient air. These advantages of TOPCon Si provide high efficiency and great design flexibility for monolithic tandem cells. TOPCon Si is integrated with BiVO₄, a large bandgap top cell consisting of earth-abundant and non-toxic elements, in monolithic construction—a wireless PEC tandem cell. A dual-junction monolithic tandem cell consisting of NiOOH/FeOOH/BiVO₄/SnO₂/Ta:SnO₂(TTO)/ TOPCon Si yields a maximum photocurrent density of 1.4 mA cm^-2 (equal to the solar-to-hydrogen conversion efficiency of 1.72%) when illuminated with air mass 1.5 G simulated solar irradiation, which is the highest value among dual-junction monolithic PEC cells except for III-V materials. Transparent and conductive TTO grown by pulsed laser deposition serves as a recombination layer to achieve effective integration. In addition, the TTO provides chemical and physical protection, allowing the surface of the TOPCon Si to exhibit 24 h of tandem cell stability under weak base electrolyte conditions. The SnO₂ hole-blocking layer inserted between TTO and BiVO₄ enhances the charge separation of BiVO₄, allowing device to achieve high efficiency. Furthermore, we present artificial leaf-type monolithic tandem cells consisting of NiFe/BiVO₄/SnO₂/TTO/TOPCon Si/Ag/Ti/Pt. Additionally, if time permits, I will also discuss our recent research involving the utilization of halide perovskite/Cu(In,Ga)Se₂ tandem devices for unassisted water splitting.