Cesium power: low Cs levels impart stability to perovskite solar cells.

Towards increasing the stability of perovskite solar cells, the addition of Cs is found to be a rational approach. Recently triple cation based perovskite solar cells were found to be more effective in terms of stability and efficiency. Heretofore they were unexplored, so we probed the Cs/MA/FA (cesium/methyl ammonium/formamidinium) cation based perovskites by X-ray photoelectron spectroscopy (XPS) and correlated their compositional features with their solar cell performances. The Cs content was found to be optimum at 5%, when incorporated in the (MAFA)Pb(IBr) lattice, because the corresponding device yielded the highest fill factor compared to the perovskite without Cs and with 10% Cs. XPS studies distinctly reveal how Cs aids in maintaining the expected stoichiometric ratios of I : Pb, I : N and Br : Pb in the perovskites, and how the valence band (VB) edge is dependent on the Cs proportion, which in turn governs the open circuit voltage. Even at a low content of 5%, Cs resides deep within the absorber layer, and ensures minimum distortion of the VB level (compared to 0% and 10% Cs perovskites) upon Ar sputtering, thus allowing the formation of a stable robust material that delivers excellent solar cell response. This study which brings out the role of Cs is anticipated to be of paramount significance to further engineer the composition and improve device performances.