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

Broader context Cuprous oxide (Cu2O) is a candidate material for photovoltaic and photoelectrochemical device applications due to its suitable band gap and low processing cost. Furthermore, due to the natural abundance of its component elements in the atmosphere and crust, it is a candidate for terawatt scale solar energy production. Given the electronic band gap of Cu2O is 2.1 eV, the detailed balance energy conversion efficiency limit is 20%. However, the efficiency record for Cu2O stands at 5.38%. Currently, device efficiencies are limited in part because there is no method for creating a reproducible, stoichiometric interface between Cu2O and heterojunction partners. Cu2O is a uniquely reactive semiconductor due to its low enthalpy of formation ( 168.7 kJ mol ) and the existence of multiple stable Cu oxidation states. The present work presents a method for the controlled modication of the chemical state of Cu at the interface of Cu2O/ZnO by modifying the O2 partial pressure during emitter deposition. It was determined that stoichiometric interfaces had open circuit voltages approaching the thermodynamic limit set by the band offset of the hetThe interface stoichiometry of cuprous oxide (Cu2O)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. Open-circuit voltage measurements for ZnO/Cu2O heterojunctions under AM1.5 illumination were measured and it was found that a stoichiometric interface can achieve the voltage entitlement dictated by the band alignment, whereas the non-stoichiometric interface showed large open-circuit voltage deficits. These results highlight not only the need for stoichiometric interfaces in Cu2O devices, but also a reproducible experimental method for achieving stoichiometric interfaces that could be applied to any potential heterojunction partner. Additionally, valence-band offset measurements indicated changing the interface stoichiometry shifted the band alignment between Cu2O and ZnO, which accounts for the variation in previously reported band offset values. erojunction, and thus are necessary for optimal device performance.