Transition of the NiO_xBuffer 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 NiOx as a hole transport layer (HTL), the effects of various sputtering conditions of the NiOx layer on MAPbI3 PSC performance are comprehensively investigated. Based on the stoichiometry control of the bulk and surface of the NiOx layer by controlling oxygen partial pressure during the sputtering process, it is found that PSCs with a sputtered NiOx HTL prepared in an Ar atmosphere exhibits higher PCEs than those of devices with an NiOx HTL conventionally grown under Ar/O2 mixed conditions. To elucidate the working principle, influences of defects in the NiOx layer grown under different O2 partial pressure conditions are thoroughly investigated. The sputtered NiOx films under pure Ar conditions with a low Ni3+/Ni2+ ratio show high hole extraction ability due to moderate hole conductivity, high optical transparency, and the improved band alignment with a MAPbI3 perovskite layer. In particular, the NiOx 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 MAPbI3 layer. In contrast, PSCs incorporated with an NiOx film sputtered under Ar/O2 mixed conditions possess a noncontinuous MAPbI3 perovskite layer having voids and cracks at the interface between MAPbI3/NiOx layer. These results indicate that a sputtered p-type NiOx HTL in Ar ambience can be an alternative route to the conventional solution-processed NiOx HTL for mass production of large-area PSCs.