Improved Cu2OBased Solar Cells Using Atomic Layer Deposition to Control the Cu Oxidation State at the pn Junction

ambient. [ 7,15 ] Similar surface chemistry changes are common for many thin-fi lm solar cell materials including copper indium gallium selenide (CIGS). [ 16 ] In the past, chemical treatments have been employed to modify the surface chemistry of airexposed CIGS fi lms. [ 16,17 ] For Cu 2 O prepared by a thermal oxidation method, similar methods for Cu 2 O surface modifi cation, including wet etching, heat treatments, and a catalyst layer, have been introduced to remove the CuO surface layers, resulting in enhancements of solar cell performance. [ 5,7,18 ] However, even if the surface treatments remove the CuO layer completely, a nanometer-scale-thick CuO layer can re-grow when the surface is exposed to air thereafter. [ 15 ] Thus, a surface modifi cation process with high controllability should be performed in situ immediately prior to overlayer deposition during solar cell fabrication (e.g., buffer layer deposition) to achieve higher PCE. Recently, Wilson et al. proposed a method to control interface stoichiometry of Cu 2 O in situ by controlling the oxygen partial pressure during sputtering of a ZnO over-layer. [ 14 ]