|Interface Stoichiometry Control in ZnO/Cu2O Photovoltaic Devices|
|Samantha S. Wilson, Yulia Tolstova, Harry A. Atwater
California Institute of Technology, Pasadena, CA, United States
Cu2O is a promising and earth abundant alternative to traditional photovoltaic materials (CIGS, CdTe, Si etc.) because of its low cost, high availability, and straightforward processing. Cu2O has potential as an absorber in photovoltaics with a direct optical gap of 1.9 eV and a detailed balance energy conversion efficiency for a homojunction of η ~25%. It is a native p-type semiconductor with relatively high absorbance in the visible region above its energy gap. Long minority carrier diffusion lengths and high hole mobilities have also been reported. One of the major challenges to Cu2O photovoltaics has been the difficulty of controlling the interface stoichiometry due to the reactive nature of Cu2O. One of the major challenges to Cu2O photovoltaics has been the difficulty of creating devices with high quality interfaces. Cu2O is a uniquely reactive oxide due to its low enthalpy of formation, which is -176 kJ/mol. Furthermore, Cu2O may be both oxidized and reduced easily, as it is in the copper (I) oxidation state. We report a method to measure the stoichiometry of Cu2O at an interface and also the effect of interface composition on heterojunction device performance. The interface stoichiometry was controlled by adjusting the deposition conditions of the ZnO and measured by XPS of thin heterosturctures. We found that deposition conditions that resulted in zinc rich ZnO yielded stoichiometric interfaces while conditions that resulted in oxygen rich ZnO yielded oxygen rich interfaces. Current-voltage characteristics of Cu2O/ZnO heterojunctions indicate open circuit voltages of Voc ~ 530 mV for devices where the Cu2O layer is stoichiometric at the interface and Voc ~ 100 mV for devices where Cu2O is oxygen rich at the interface.