Modeling solar-to-hydrogen efficiencies in tandem photoelectrochemical cells with band-edge pinned electrodes

Tandem photoelectrochemical cells are attracting major attention as devices for green hydrogen generation. However, reaching the thermodynamic limit for solar-to-hydrogen (STH) efficiency seems unrealistic, being thus relevant reporting efficiencies for tandem devices in different achievable scenarios. Here, a straightforward model applicable to both photoelectrodes and based on the Gärtner approach is applied for a tandem device, which enables predicting front and back illumination photocurrent-potential curves for compact semiconductor electrodes. For a tandem cell with the photoanode as the top electrode, calculated STH efficiencies are reported for four representative scenarios based on key photoelectrode properties such as absorption coefficients, doping levels or minority carrier diffusion lengths. As a result, calculated STH efficiencies under realistic conditions reach values in the range of 20-25%. In contrast with published models, the photoanode thickness is also explicitly considered, revealing that relatively efficient tandem cells can be developed, even for electrode materials having similar band gaps.