A review for nickel oxide hole transport layer and its application in halide perovskite solar cells

As an indispensable part of traditional perovskite solar cells (PSCs), the hole transport layer (HTL) plays an important role in enhancing the performance of PSCs by increasing hole collection, blocking electrons, and protecting the perovskite from external influences such as moisture, heat and oxygen. As a commonly used hole transport material (HTM), nickel oxide (NiOx) shows significant promise in various photovoltaic devices owing to its intrinsic superior chemical stability, high hole mobility, and low fabricating cost. Tremendous progress has been achieved in the power conversion efficiency (PCE) of NiOx-based halide PSCs in recent years. However, as a typical HTL, NiOx is easily affected by synthesis methods and conditions subsequent to treatment in application. This comprehensive review summarizes the preparation methods and strategies of NiOx subsequent to treatment. Meanwhile, more discussions focus on doping and surface modification strategies of NiOx film and the corresponding performance improvement for devices. It also includes diverse compositions and structure variation of the perovskite active layer. Finally, we discuss the current challenges of using NiOx HTL in PSCs and make recommendations for further development of efficient and stable NiOx-based PSCs.