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As an emerging candidate material for photovoltaic applications, metal halide perovskites are well-known for their high power conversion efficiency and fabrication convenience. Mixed-halide perovskites, specifically those with bromide (Br) and iodide (I) anions, are a popular choice for the light-absorbing layer due to their wide bandgap, a critical consideration as perovskite-based tandem solar cells make a major effort towards commercialization. However, the photo-induced halide phase segregation in mixed halide perovskites under illumination can severely damage the device lifetime, so it is of great importance to realize appropriate strategies to slow down or inhibit this photo-segregation phenomenon. In this work, we applied both doping (using Cu and Li-Cu dopants) and surface modification (using two different phosphonic acid-based self-assembled monolayers) strategies to the nickel oxide (NiOx) hole transporting layer (HTL) under perovskite films in solar cell devices to adjust its hole collection capability and carefully examine the photo-segregation process via in situ UV-visible absorption spectrum monitoring. This enables us to elucidate the relationship between the properties of HTL and the photo-segregation process and work towards an optimal HTL. Hole trapping, leading to iodide oxidation and extraction, is a major pathway leading to halide ion mobility. We expect the existence of a HTL can prevent accumulation of holes in the bulk perovskite film and thus slow down the process of photo-segregation. Obtained results for both 2D and 3D mixed halide perovskite films are discussed in detail.
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