Importance of Vacancies and Doping in the Hole-Transporting Nickel Oxide Interface with Halide Perovskites

Nickel oxide (NiO) is a commonly used contact material for a variety of thin-film optoelectronic technologies based on organic or hybrid materials. In such setups, interfaces play a crucial role as they can reduce, if not kill, the device performances by bringing additional traps or energy barriers, hindering the extraction of charge carriers from the active layer. Here, we computationally examine a prototype halide perovskite architecture, NiO/MAPbI3 (MA = CH3NH3+), that has shown excellent photovoltaic performance and, in particular, a large open-circuit voltage. We show that efficient hole collection is achieved only when considering the role of vacancies induced by standard material deposition techniques. Specifically, Ni vacancies lead to nearly perfect valence band energy level alignment between the active layer and the contact material. Finally, we show how Li doping greatly improves the performances of the device and further propose alternative dopants. Our results suggest the high tunability of NiO interfaces for the design of optimized optoelectronic devices far beyond that of halide perovskites.