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Cesium power: low Cs(+) levels impart stability to perovskite solar cells.
Title Cesium power: low Cs(+) levels impart stability to perovskite solar cells.
Authors Deepa, M.; Salado, M.; Calio, L.; Kazim, S.; Shivaprasad, S.M.; Ahmad, S.
Journal Phys Chem Chem Phys
DOI 10.1039/c6cp08022g
Abstract

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 (MA0.15FA0.85)Pb(I0.85Br0.15)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 : Pb(2+), I : N and Br : Pb(2+) 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.