This Small Business Innovative Research Phase I Project will explore, demonstrate, and develop cost-effective designs and production technologies for semiconductor monolithic photovoltaic/photoelectrochemical (PV-PEC) devices for hydrogen production. This work is motivated by the recent report of a 12.5% efficient laboratory device made by metalorganic chemical vapor deposition on a single-crystal GaAs substrate. It is estimated that an order of magnitude cost reduction is necessary to make this technology economic. This approach is based on selective epitaxial growth of III-V compound semiconductors by simple closed-spaced vapor transport and solution growth processes on a large-grain polycrystalline silicon sheet material. Cells based on heteroepitaxy of GaAs, InP, InGaP and GaN on polycrystalline silicon will be evaluated. The simple structure of PV-PEC cells relative to other semiconductor devices such as transistors, photodiodes, lasers, and LEDs, makes the proposed selective epitaxy process especially advantageous in achieving high material quality at low cost with high-throughput processing. This approach will yield the needed cost reductions without significantly sacrificing performance.<br/><br/>This is a moderate- to high-risk payoff project that seeks an economically viable, clean, renewable process for hydrogen generation.