Efficiency enhancement using a Zn1− x Ge x -O thin film as an n-type window layer in Cu2O-based heterojunction solar cells

Efficiency enhancement was achieved in Cu2O-based heterojunction solar cells fabricated with a zinc–germanium-oxide (Zn1− x Ge x -O) thin film as the n-type window layer and a p-type Na-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing Cu sheets. The Ge content (x) dependence of the obtained photovoltaic properties of the heterojunction solar cells is mainly explained by the conduction band discontinuity that results from the electron affinity difference between Zn1− x Ge x -O and Cu2O:Na. The optimal value of x in Zn1− x Ge x -O thin films prepared by pulsed laser deposition was observed to be 0.62. An efficiency of 8.1% was obtained in a MgF2/Al-doped ZnO/Zn0.38Ge0.62-O/Cu2O:Na heterojunction solar cell.