N-CuInSe₂ based photoelectrochemical cells: Improved, stable performance in aqueous polyiodide through rational surface and solution modifications

Surface optimization of n-CuInSe₂ crystals by chemical etching in Br₂/MeOH and subsequent thermal oxidation results in >10% efficient photanodes in neutral aqueous polyiodide. The etchant removes a near-stoichiometric surface layer and thermal oxidation leads to In-O bond formation. We find from (photo) electrochemical studies in acetonitrile and in the iodide electrolyte that, upon semiconductor decomposition, Cu ions are released into the solution, consistent with solid state chemical studies that show the existence of the chalcopyrite phase also towards Cu-poor compositions. Addition of Cu ions, as also reported by S. Menezes, H. J. Lewerenz, and K. J. Bachmann [Nature (London) 305, 615 (1983)] or Cu and In ions to the iodide solution drastically decreases photoanode decomposition. Covering the electrode with an extra film of oxidized indium results in true stabilization, at up to 11.7% conversion efficiency under 120-mW/cm² tungsten-halogen illumination. Because, in the presence of Cu ions and under illumination, iodide solutions are not completely stable towards oxidation by O₂, especially if the solution is acidic, an air-free electrolyte needs to be used.