Siyu Yan, Nakita K. Noel, University of Oxford, Oxford, United Kingdom
While metal halide perovskites have shown remarkable power conversion efficiencies in photovoltaic applications, their limited long-term stability remains a significant barrier to commercialisation. Previous studies have demonstrated that trace impurities present in perovskite precursor materials can influence the crystallisation dynamics of perovskite thin films and hence, affect crystal structure, morphology and optoelectronic properties. However, the nature of the impurities in formamidinium iodide (FAI) and their effect on film quality and device performance are still underexplored. In this work, we carry out an analysis of the impurities present in commonly used commercial FAI sources, and probe their impact on the composition, structure, and optoelectronic quality of the resulting perovskite thin films and devices. We find that while some impurities in these precursors can improve the optoelectronic properties of solution-processed perovskite thin films, in vapour-processed films, their presence alters the sublimation behaviour of FAI, favouring irreversible degradation pathways which lead to the formation of sym-triazine. This results in films which deviate significantly from the target stoichiometry, and do not fully convert into the desired photoactive phase, eventually causing poor material stability. Our results highlight the importance of understanding and controlling impurity concentrations in perovskite precursor materials as a route to enhancing both process reproducibility and the performance of perovskite solar cells.