Limit of incorporating cesium cations into formamidinium-methylammonium based mixed halide perovskite solar cells

Incorporating formamidinium (FA) into methylammonium (MA) based perovskite has brought significant thermal and environmental stability including best device performance. It has been shown that addition of Cesium (Cs) makes perovskite robust in terms of thermodynamic stability as well. We explore the means of incorporating Cs into a base perovskite of mixed cation (FA/MA) and mixed halide (I/Br) that has a proven track record of high performance through an inter-diffusion approach. With this approach, it has been shown that perovskites form a smooth film without any residual PbI2 and exhibit higher absorbance. Though the residual PbI2 disappeared with the increase in added Cs, the film morphology became rough for Cs concentration higher than 15%. Addition of small amounts of PbCl2 allowed inclusion of more Cs content, which resulted in smooth film surface and further improved device performance.