Transition of the NiO_x Buffer Layer from a p-Type Semiconductor to an Insulator for Operation of Perovskite Solar Cells

To overcome the fundamental limitation of the low power conversion efficiency (PCE) of perovskite solar cells (PSCs) incorporated with a magnetron sputtered p-type NiO x as a hole transport layer (HTL), the effects of various sputtering conditions of the NiO x layer on MAPbI 3 PSC performance are comprehensively investigated. Based on the stoichiometry control of the bulk and surface of the NiO x layer by controlling oxygen partial pressure during the sputtering process, it is found that PSCs with a sputtered NiO x HTL prepared in an Ar atmosphere exhibits higher PCEs than those of devices with an NiO x HTL conventionally grown under Ar/O 2 mixed conditions. To elucidate the working principle, influences of defects in the NiO x layer grown under different O 2 partial pressure conditions are thoroughly investigated. The sputtered NiO x films under pure Ar conditions with a low Ni 3+ /Ni 2+ ratio show high hole extraction ability due to moderate hole conductivity, high optical transparency, and the improved band alignment with a MAPbI 3 perovskite layer. In particular, the NiO x film sputtered in pure Ar ambience results in higher photovoltaic performance with a negligible hysteresis behavior due to excellent crystallinity, the large grain size, and the improved interface morphology of the MAPbI 3 layer. In contrast, PSCs incorporated with an NiO x film sputtered under Ar/O 2 mixed conditions possess a noncontinuous MAPbI 3 perovskite layer having voids and cracks at the interface between MAPbI 3 /NiO x layer. These results indicate that a sputtered p-type NiO x HTL in Ar ambience can be an alternative route to the conventional solution-processed NiO x HTL for mass production of large-area PSCs.