Monolithically Integrated Thin-Film/Silicon Tandem Photoelectrodes for High Efficiency and Stable Photoelectrochemical Water Splitting

Recipient University of Michigan (PI: Zetian Mi)

Water Splitting Technology PEC

Status Awarded

Abstract In this project, we propose to develop double-junction thin film photoelectrodes monolithically integrated on a Si platform, with the objective to achieve both high efficiency (solar-to-H2 efficiency of 15%) and long-term stability (>1,000 hours). Photoelectrochemical (PEC) water splitting with a solar-to-H2 efficiency ~30% can be potentially achieved by using a double-junction device consisting of two semiconductors with energy bandgaps of 1.75 eV for the top cell and 1.13 eV for the bottom cell. In this project, we propose to use Si as the bottom light absorber, taking advantage of its narrow energy bandgap and prevalence in industry. We have recently demonstrated that Ta3N5, BCTSSe, and InGaN are well suited to serve as the top light absorber of double-junction photoelectrodes. They can be controllably doped n or p-type and exhibit large light absorption coefficients and superior charge carrier transport properties. Recent studies have further shown that the conduction and valence band edge positions of Ta3N5 and In0.5Ga0.5N straddle the water splitting potentials, promising photovoltage larger than 1.23 V. We have further demonstrated the unique scheme of nanowire tunnel junction on a Si platform, which can exhibit remarkably low resistivity and will be used to connect the top light absorber (Ta3N5, BCTSSe, or InGaN films) with the Si bottom junction. The use of nanowire tunnel junction can further reduce the formation of defects and dislocations in the top light absorber, due to the effective lateral surface stress relaxation. Moreover, the photoelectrode surface will be protected by an ultrathin N-rich GaN coating against photocorrosion and oxidation, a recent innovation by the applicants that has already led to highly stable overall water splitting reaction. This project will also leverage the extensive computational materials diagnostics, modelling, and optimization, materials characterization, and device measurement expertise at LBNL, LLNL, and NREL. Through these interdisciplinary and collaborative studies, our team will address the photoelectrode materials growth/synthesis, integration, surface protection, catalyst and PEC characterization challenges and demonstrate high efficiency and stable PEC water splitting on low cost, large area Si substrate.