M, Deepa and Salado, M and Calio, L and Kazim, S and Shivaprasad, S M and Ahmad, S
(2017)
Cesium Power: Low Cs+ Levels Impart Stability to Perovskite Solar Cells.
Physical Chemistry Chemical Physics, 19 (5).
pp. 4069-4077.
ISSN 1463-9076
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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 efficiencies. Heretofore unexplored, we probed the Cs/MA/FA (Cesium/methyl ammonium/formamidinium) cation based perovskites by X-ray photoelectron spectroscopy (XPS) and connected their compositional features with their solar cell performances. Cs+ content was found to be optimum at 5%, when incorporated in the (MA0.15FA0.85)Pb(I0.85Br0.15)3 lattice, for 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 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 anticpated to be of paramount significance to further engineering the composition and push device performances.
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