Effect of inserting a thin buffer layer on the efficiency in n-ZnO/p-Cu2O heterojunction solar cells

Transparent conducting Al-doped ZnO (AZO)/nondoped ZnO/Cu2O heterojunction solar cells were fabricated by inserting a thin nondoped ZnO film as a buffer layer between an n +-AZO thin film and a p-Cu2O sheet prepared by thermally oxidizing a Cu sheet. The effect of inserting the buffer layer on the obtainable photovoltaic properties in n +-AZO/n-ZnO/p-Cu2O heterojunction solar cells was investigated to improve the conversion efficiency. An improvement of photovoltaic properties was obtained by optimizing the thickness as well as the deposition conditions in the nondoped ZnO thin-film buffer layer. The obtained improvement of photovoltaic properties may be attributable mainly to an increase of the barrier height formed in the np junction, resulting from the inserted buffer layer functioning as an n-type ZnO layer as well as an enhancement of carrier lifetimes near the interface between the nondoped ZnO thin-film buffer layer and the Cu2O. A high efficiency of 4.08% was obtained in an AZO/nondoped ZnO/Cu2O heterojunction solar cell fabricated using a buffer layer prepared at room temperature and an O2 gas pressure of 1.0 Pa with a thickness of 50 nm on a Cu2O sheet and measured under simulated AM1.5G solar.