Interface stoichiometry control to improve device voltage and modify band alignment in ZnO/Cu2O heterojunction solar cells

Abstract

The interface stoichiometry of cuprous oxide (Cu2O) prepared by thermal oxidation was controlled by adjusting the O2 and Zn partial pressures during ZnO sputter deposition and measured by high-resolution X-ray photoelectron spectroscopy of ultrathin (<3 nm) ZnO films on Cu2O. Current-voltage characteristics of ZnO/Cu2O heterojunctions under AM1.5 1-sun illumination were collected for three different interfacial compositions: (1) mixed Cu2O and Cu, (2) stoichiometric Cu2O, and (3) and mixed Cu2O and CuO. ZnO/Cu2O heterojunctions with stoichiometric interfaces achieved open circuit voltages of VOC = 530 ± 4 mV. VOC = 347 ± 30 mV and VOC = 109 ± 11 mV were achieved for devices where the interface contained Cu and CuO, respectively. Thus a stoichiometric interface is essential for achieving a high voltage photovoltaic device. Additionally, valence-band offset measurements indicated the presence of CuO changed the band alignment between Cu2O and ZnO, which could account for some of the variation in previously reported values.