In this study we investigate and develop the integration of thin film silicon solar cells in photoelectrochemical (PEC) water-splitting devices. Thin film silicon solar cells utilize layer stacks of amorphous hydrogenated silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) and readily provide photovoltages well above the thermodynamically required 1.23 V to drive the oxygen and hydrogen evolution reactions at photocurrent densities that approach 10 mA/cm2. However, the use of such solar cells in integrated PEC devices imposes a considerable challenge concerning chemical stability and the overall device design. This is the topic of the present study with a focus on a variation of metal layers at the solar cell/electrolyte interface.

Tandem junction solar cells with open-circuit voltages Voc above 1.8 V and photovoltaic conversion efficiencies above 11%, were applied as photocathodes for water-splitting. A metallic layer at the solid/electrolyte interface was implemented for multiple purposes: (i) it functions as a back reflector for transmitted photons and thereby increases the probability of photon absorption and charge carrier generation in the silicon and thus enhances the photocurrent of the device, (ii) it forms a good electronic contact to the silicon semiconductor and provides an efficient charge transfer at the solid/electrolyte interface by providing catalytic activity, hence lowering the overpotential for hydrogen evolution, and (iii) it protects the silicon solar cell against corrosion. Considering this broad range of demands, the metal interlayer has to be chosen carefully. Here, we systematically evaluated thin layers (approx. 300 nm) of Ti, Ni, Ag, Cu, Au, Al, and Pt with regards to their stability and catalytic activity. First, the optical and electrochemical properties of the individual metal layers were evaluated on glass substrates. In a second step the metals were deposited on photocathodes and photoelectrochemical measurements were conducted both in acidic and alkaline solutions, because both the stability against corrosion and the catalytic activity depend on the pH value of the used electrolyte. Ag was found to be the most efficient reflector, while Pt and Ni showed the best electrochemical stability and catalytic activity.

Modeling the integrated PEC system in terms of a series connection of a solar cell and an electrolysis cell shows good agreement with experimental results and allows for an analysis of the losses and a prediction of efficiency limits for thin film silicon based water-splitting devices from the solar cell performance.