The development of stable materials, processable on alarge area,is a prerequisite for perovskite industrialization. Beyond the perovskiteabsorber itself, this should also guide the development of all otherlayers in the solar cell. In this regard, the use of NiO x as a hole transport material (HTM) offers severaladvantages, as it can be deposited with high throughput on large areasand on flat or textured surfaces via sputtering, a well-establishedindustrial method. However, NiO x may triggerthe degradation of perovskite solar cells (PSCs) when exposed to environmentalstressors. Already after 100 h of damp heat stressing, a strong fillfactor (FF) loss appears in conjunction with a characteristic S-shaped J-V curve. By performing a wide range of analysison cells and materials, completed by device simulation, the causeof the degradation is pinpointed and mitigation strategies are proposed.When NiO x is heated in an air-tight environment,its free charge carrier density drops, resulting in a band misalignmentat the NiO x /perovskite interface and inthe formation of a barrier impeding hole extraction. Adding an organiclayer between the NiO x and the perovskiteenables higher performances but not long-term thermal stability, forwhich reducing the NiO x thickness is necessary.