Tunnel Oxide Passivated Contact on Silicon Photoelectrodes for Tandem Photoelectrochemical Water-splitting Devices

Crystalline silicon has been considered one of the most promising materials for the bottom cell of a tandem photoelectrochemical (PEC) water-splitting device because of its appropriate band gap (1.12 eV) as well as the industrial maturity. The tandem PEC water-splitting device, especially when made of earth-abundant materials, would be a sustainable way of producing hydrogen fuels; thus, considerable efforts have been made to acquire the best performance out of the large band gap top cell and Si bottom cell. Despite these efforts, there have been only a few studies that enable unassisted solar water splitting by using a tandem PEC device with a silicon bottom cell because 1) the photovoltage from the tandem device is not sufficient to drive the water splitting reaction by itself, and 2) poor thermal stability of a Si bottom cell severely limits the choice of the top cell material and processing conditions. Therefore, improving the photovoltage and thermal stability of the Si bottom cell would provide a great opportunity to realize tandem PEC devices for solar fuel applications.<br/><br/>In this work, we report silicon photoelectrodes based on a tunnel oxide passivated contact (TOPCon), which exhibits superior properties as a potential bottom cell for a monolithic tandem device. Our TOPCon Si photoelectrodes yielded photovoltages in the range of 640 - 650 mV in both photocathode and photoanode configurations under a broad range of pH (0 – 14) conditions of working electrolytes, which are the highest values reported for photoelectrodes based on crystalline silicon. Furthermore, the TOPCon device retained its superior property after thermal annealing at 600 °C for 1 hour in air. The capability of operating in the both configurations (photocathode and photoanode) over a broad range of pH and high thermal stability provides great flexibility in the design of a tandem PEC device: TOPCon Si is capable of stable operation in any pH and is compatible with a top cell semiconductor of any type (n-type or p-type) that may require a high processing temperature. As a preliminary study of a monolithic tandem device based on TOPCon Si, we tested wired tandem devices using a BiVO₄ photoanode or halide perovskite photocathode as a large band gap counterpart of TOPCon Si photoelectrodes to show the feasibility of unassisted solar water splitting. The BiVO₄-TOPCon Si and perovskite-TOPCon Si tandem devices yielded solar-to-hydrogen conversion efficiencies of 0.24 and 3.6 %, respectively. Notably, STH efficiency from perovskite-TOPCon Si (3.6 %) is one of the highest values from tandem PEC devices based on earth-abundant elements.