Magnesium doped spinel NiCo2O4 for improved hole extraction in efficient inverted perovskite solar cells

Abstract

In recent years, p-type oxide semiconductors have been widely used as hole transport materials (HTMs) in halide perovskite solar cells (PSCs). Spinel NiCo2O4 has attracted much attention in this regard due to its environmental friendliness and rich elemental composition. In this work, a solution-processed Mg-doped NiCo2O4 film was prepared for the first time as HTM for inverted planar PSCs. The density functional theory calculations reveal that the Mg doping increases the electronic states at the Fermi energy level of NiCo2O4 to promote the carrier transport. Ultraviolet photoelectron spectroscopy (UPS) further shows that the valance band maximum (VBM) of the Mg-doped NiCo2O4 is increased by 0.08 eV compared to that of pristine NiCo2O4, thus reducing the energy level mismatch at the HTM/perovskite interface. As a result, the Mg-doped NiCo2O4 HTM enables a promising power conversion efficiency (PCE) of 16.71% for the inverted MAPbI3 PSCs, which is 18% higher than that of the pristine NiCo2O4 devices (14.07%). The improved PCE is attributed to the improved hole extraction and better matching of the interfacial energy levels of the Mg-doped NiCo2O4. Our result provides novel prospect for the electronic structure manipulation of spinel NiCo2O4 by element doping.