Photovoltaic properties of thin film heterojunctions with cupric oxide absorber

In this work, we report on the fabrication, characterization, and photovoltaic properties of sputter-deposited, thin film heterojunctions combining p-type cupric oxide (CuO) absorber with n-type ZnO. The structural investigation reveals highly crystalline, columnar growth of the layers and confirms that the absorber's phase is purely CuO, with only negligible traces of Cu 2O. The optical characterization yields for CuO an indirect bandgap of 1.2 eV and a direct optical transition at approximately 3 eV. The short circuit current, open circuit voltage, fill factor, and power conversion efficiency of the heterojunction solar cells were extracted as a function of the CuO thickness under AM1.5 G (1 kW/m2) illumination. From the observed dependencies, we conclude that the photovoltaic performance is compromised by a restricted carrier collection efficiency, caused by the small carrier lifetime in CuO. Indeed, the carrier population is found to decay with time constants of 40 and 460 ps. A maximum power conversion efficiency of 0.08% was obtained for the solar cell with CuO thickness of 500 nm.