Corrosion and Passivation of p-GaInP2 Photocathodes at Light-Dark Boundaries

The tandem GaAs:GaInP₂ PV:PEC (photovoltaic:photoelectrochemical) cell has achieved a record 12.4% solar-to-hydrogen water splitting efficiency, but the instability of the p-GaInP₂ PEC junction greatly limits device lifetime. We have developed surface modifications to stabilize p-GaInP₂ for 300+ hours, but failure can often be associated with corrosion that initiates at photoelectrode edges, beneath stationary bubbles, and at defects.  Common to these conditions that initiate corrosion, is the presence of sharp light-dark boundaries, which may accelerate corrosion. In this work, we investigate photo-corrosion behavior of p-GaInP₂ at light-dark boundaries. A light-dark boundary was simulated by illuminating a ~2 mm diameter region at the center of a 6 mm diameter photoelectode and a set of durability tests were performed in 3M sulfuric acid at constant current.  After each test, samples were analyzed with optical microscopy, profilometry, and SEM with EDS and the corrosion products in solution were quantified with ICP-MS.  We observe a corrosion trench at the outer perimeter of the illuminated region and spatially-dependent corrosion features that correspond with the placement of the counter electrode as well as a region where corrosion is inhibited that corresponds with the direction of evolving H₂ bubbles.  To explain these observations, we will present a set of experiments and measurements to show how spatial variation in surface band bending and H₂ saturation can explain corrosion characteristics at light-dark boundaries.