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 (MA_0.15FA_0.85)Pb(I_0.85Br_0.15)₃ 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 : Pb²⁺, I : N and Br : Pb²⁺ 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.