title: Cesium power: low Cs+levels impart stability to perovskite solar cells.
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)3lattice, 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.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285, Sangareddy, Telangana, India.
authors.forename: Melepurath
authors.identifier: http://orcid.org/0000-0001-7070-5100
authors.initials: M
authors.lastname: Deepa
authors.affiliation: Abengoa Research, Abengoa, C/Energía Solar no 1, Campus Palmas, Altas, Sevilla, 41014, Spain. shahzada.ahmad@abengoa.com.
authors.forename: Manuel
authors.initials: M
authors.lastname: Salado
authors.affiliation: Abengoa Research, Abengoa, C/Energía Solar no 1, Campus Palmas, Altas, Sevilla, 41014, Spain. shahzada.ahmad@abengoa.com.
authors.forename: Laura
authors.initials: L
authors.lastname: Calio
authors.affiliation: Abengoa Research, Abengoa, C/Energía Solar no 1, Campus Palmas, Altas, Sevilla, 41014, Spain. shahzada.ahmad@abengoa.com.
authors.forename: Samrana
authors.initials: S
authors.lastname: Kazim
authors.affiliation: International Centre for Materials Science, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India.
authors.forename: S M
authors.initials: SM
authors.lastname: Shivaprasad
authors.affiliation: Abengoa Research, Abengoa, C/Energía Solar no 1, Campus Palmas, Altas, Sevilla, 41014, Spain. shahzada.ahmad@abengoa.com.
authors.forename: Shahzada
authors.identifier: http://orcid.org/0000-0002-1218-2556
authors.initials: S
authors.lastname: Ahmad