Single-phase indium-doped cupric oxide films prepared through DC reactive magnetron sputtering: Influence of substrate temperature on microstructure, optical and electrical behavior

Abstract

Cupric oxide (CuO) films with high electrical conductivity are crucial to their potential application in heterojunction solar cells. However, intrinsic CuO films usually have low electrical conductivity. The doping of proper ions into CuO is effective for the improved electrical conductivity. Herein, a series of monoclinic, indium-doped cupric oxide (CuO:In) films was prepared at different substrate temperature (Tsub) values via low-cost DC reactive magnetron sputtering. The influence of Tsub was intensively studied on the film’s microstructure, optical and electrical behaviors. A conducting transition from p type to n type occurs at 400 °C Tsub for CuO:In films rather than 500 °C Tsub for intrinsic CuO films. The decrease in critical transition temperature is mainly caused by the substitution of copper by indium. With increase of Tsub, the film crystallization is enhanced and the point defect content is reduced in the films, thereby increasing the film’s mobility and free carrier concentration. Increased Tsub helps the effective substitution of copper by indium and thus forms the n-type conduction. Compared with undoped CuO film, the CuO:In films have quite low mobility, improved electrical conductivity and large optical absorption edge. This finding indicates that CuO:In films can be used as absorption layer material in heterojunction solar cells rather than as hole-transporting layer material in perovskite solar cells.