Design considerations for a hybrid amorphous silicon/photoelectrochemical multijunction cell for hydrogen production

Triple-junction amorphous silicon (a-Si) solar cells demonstrating photovoltaic (PV) efficiencies up to 12.7% and open-circuit voltages up to 2.3 V have recently been deposited onto stainless-steel foil substrates by the University of Toledo for photoelectrochemical (PEC) tests conducted by the University of Hawaii. The fundamental design strategy for producing such high efficiency in multijunction amorphous silicon devices involves careful current matching in each of the junctions by adjustment of the absorption spectra through bandgap tailoring. Integrated electrical/optical models are frequently used to aid in the optimization procedure, as well documented in the PV literature. Typically, the top nip junction in an a-Si triple-junction cell is designed to absorb most strongly in the 350–500 nm range. In principle, this top cell could be replaced by a PEC junction with strong absorption in a similar range to form a water-splitting photoelectrode for hydrogen production. This photoelectrode could be fabricated on SS with the back surface catalyzed for the hydrogen evolution reaction, and the front surface deposited with an a-Si:nipnip/ITO/SC structure. The top layer semiconductor (SC), which forms the PEC junction with an electrolyte, must have appropriate conduction band alignment for the oxygen evolution reaction, and the junction must be strongly absorbing in the 350–500 nm region for current matching. Possible candidate SC materials include dye-sensitized titanium dioxide (TiO2), tungsten trioxide (WO3), and iron oxide (Fe2O3). This paper discusses the specific design considerations for high solar-to-hydrogen conversion efficiency in a hybrid solid-state/PEC photoelectrode, and describes the use of integrated electrical/electrochemical/optical models developed at the University of Hawaii for the analysis of such hybrid structures. Important issues include the bias-voltage and current-matching requirements in the solid-state and electrochemical junctions, as well as fundamental quantum efficiency considerations.