Till 2016, the best perovskite solar cells use mixed organic cations methylammonium (MA) and formamidinium (FA) and mixed halides I and Br. Unfortunately, MA/FA perovskites are sensitive to processing conditions because of their intrinsic structural and thermal instability. The films frequently contain detrimental impurities and tend to be less crystalline explaining the large variability observed by many labs.

Adding small amounts of inorganic cesium (Cs) in a ‘‘triple cation’’ (Cs/MA/FA) configuration results in highly monolithic grains of more pure perovskites. The films are more robust to subtle variations during the fabrication process enabling a breakthrough in terms of reproducibility where efficiencies larger than 20% are reached on a regular basis. Despite the magic tricks the Cs does to the triple cation perovskites, the role of the Cs in improving the stability of of the triple cation perovskites is far from understood.

The objective of this project is to apply a set of chemical bonding analysis techniques studying the stability of (1) the mixing of MA/FA and Br/I in the perovskite compounds as well as (2) the role of inorganic cations, such as Cs, possibly also Rb in triple-cation perovskites. This study will allow us to identify several types of chemical bonding contribution (ionic, covalent and hydrogen bond) to the overall bond strength of the relevant perovskite alloys. The results of the project will be a significant step in fundamental understanding of the composition dependent stability performance of the state-of-the-art perovskite solar cells materials and provide design rules for further improvement in terms of long term stabilities of the perovskite solar cells.