Solar fuels are a promising way to store solar energy seasonally. This paper proposes an earth-abundant heterostructure to split water using a photovoltaic-electrochemical device (PV-EC). The heterostructure is based on a hybrid architecture of a thin-film (TF) silicon tandem on top of a c-Si wafer (W) heterojunction solar cell (a-Si:H (TF)/nc-Si:H (TF)/c-Si(W)) The multijunction approach allows to reach enough photovoltage for water splitting, while maximizing the spectrum utilization. However, this unique approach also poses challenges, including the design of effective tunneling recombination junctions (TRJ) and the light management of the cell. Regarding the TRJs, the solar cell performance is improved by increasing the n-layer doping of the middle cell. The light management can be improved by using hydrogenated indium oxide (IOH) as transparent conductive oxide (TCO). Finally, other light management techniques such as substrate texturing or absorber bandgap engineering were applied to enhance the current density. A correlation was observed between improvements in light management by conventional surface texturing and a reduced nc-Si:H absorber material quality. The final cell developed in this work is a flat structure, using a top absorber layer consisting of a high bandgap a-Si:H. This triple junction cell achieved a PV efficiency of 10.57%, with a fill factor of 0.60, an open-circuit voltage of 2.03 V and a short-circuit current density of 8.65 mA/cm ² . When this cell was connected to an IrO _x /Pt electrolyser, a stable solar-to-hydrogen (STH) efficiency of 8.3% was achieved and maintained for 10 hours.

Wireless photoelectrochemical (PEC) devices promise easy device fabrication as well as reduced losses. Here, the design and fabrication of a stand-alone ion exchange material-embedded, Si membrane-based, photoelectrochemical cell architecture with micron-sized pores is shown, to overcome the i) pH gradient formation due to long-distance ion transport, ii) product crossover, and iii) parasitic light absorption by application of a patterned catalyst. The membrane-embedded PEC cell with micropores utilizes a triple Si junction cell as the light absorber, and Pt and IrO _x as electrocatalysts for the hydrogen evolution reactions and oxygen evolution reactions, respectively. The solar-to-hydrogen efficiency of 7% at steady-state operation, as compared to an unpatterned η _PV of 10.8%, is mainly attributed to absorption losses by the incorporation of the micropores and catalyst microdots. The introduction of the Nafion ion exchange material ensures an intrinsically safe PEC cell, by reducing the total gas crossover to <0.1%, while without a cation exchange membrane, a crossover of >6% is observed. Only in a pure electrolyte of 1 m H ₂ SO ₄ , a pH gradient-free system is observed thus completely avoiding the build-up of a counteracting potential.