Improving intrinsic stability of perovskite solar cells by additives

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Aiming to find out how to improve the stability of lead halide perovskite solar cells, our interdisciplinary team investigated several stabilising additives developed at EPFL during the first funding period of SPP2196. We studied their effect on solar cells with Cs0.05MA0.05FA0.90PbI3 absorbers with power conversion efficiency higher than 21% under thermal stress, while controlling the film morphology, crystallographic properties, optical properties, charge carrier mobility and open circuit voltage. The results obtained so far clearly indicate an improved stability of the films and devices prepared with functionalised additives, especially without compromising the performance efficiency. In addition, we have made some further interesting observations with regard to ion migration and photoluminescence quenching efficiency and found a strong influence of the mesoporous electron transport layer on all these effects. We therefore propose a targeted research project to definitively clarify the interaction mechanisms and the origin of the effect of pre-selected ionic liquids as well as the microscopic mechanism behind them, but also to prove their applicability to other perovskite compositions. On the experimental side, we will apply two transient techniques to study, on the one hand, relatively fast processes of free carrier recombination using the microwave-based TRMC method and on the other hand relatively slow processes related to ion migration using transient open circuit voltage (OCVD). The outcome of the research project will be the understanding of the mechanisms of interaction between ionic liquids and perovskite bulk as well as interfaces, and finally the development of concepts to improve the intrinsic stability of perovskite solar cells in general. Particular attention is paid to ion migration during device operation, which contributes to destabilisation of the perovskite layer. At present, it is not clear which exact mechanisms lead to this ion migration and how it can be minimised, e.g. to avoid possible reactions with the metal oxide or other internal interfaces, but also to ensure a defined electric field distribution within the solar cell so that ist electrical behaviour can be reliably described and predicted. Last but not least, when using stabilising additives, which are also ions, it is also very important to distinguish between intrinsic and extrinsic ions in the perovskite absorber, which has not been possible experimentally so far.