Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal–halide perovskites

Mostafa Othman; Quentin Jeangros; Daniel A. Jacobs; Moritz H. Futscher; Stefan Zeiske; Ardalan Armin; Anaël Jaffrès; Austin G. Kuba; Dmitry Chernyshov; Sandra Jenatsch; Simon Züfle; Beat Ruhstaller; Saba Tabean; Tom Wirtz; Santhana Eswara; Jiashang Zhao; Tom J. Savenije; Christophe Ballif; Christian M. Wolff; Aïcha Hessler-Wyser

doi:10.1039/D4EE00901K

Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal–halide perovskites†

Mostafa Othman,*^a Quentin Jeangros,^b Daniel A. Jacobs,^a Moritz H. Futscher,

^c Stefan Zeiske,^d Ardalan Armin,

^d Anaël Jaffrès,^a Austin G. Kuba,

^a Dmitry Chernyshov,

^f Sandra Jenatsch,

^g Simon Züfle,

^g Beat Ruhstaller,^g Saba Tabean,^ei Tom Wirtz,^ei Santhana Eswara,

^ei Jiashang Zhao,^h Tom J. Savenije,

^h Christophe Ballif,^ab Christian M. Wolff

*^a and Aïcha Hessler-Wyser*^a

Author affiliations

* Corresponding authors

^a Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Electrical and Micro Engineering (IEM) Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), Neuchâtel, Switzerland
E-mail: mostafa.othman@epfl.ch, christian.wolff@epfl.ch, aicha.hessler@epfl.ch

^b Centre d’Electronique et de Microtechnique (CSEM), Rue Jaquet-Droz 1, Neuchâtel, Switzerland

^c Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland

^d Sustainable Advanced Materials (Ser-SAM), Department of Physics, Swansea University, Swansea SA2 8PP, UK

^e Advanced Instrumentation for Nano-Analytics (AINA), Luxembourg Institute of Science and Technology (LIST), Materials Research and Technology Department, 41 Rue du Brill, Belvaux L-4422, Luxembourg

^f Swiss-Norwegian Beamlines at the European Synchrotron Radiation Facility, 71 Avenue des Martyrs, Grenoble, France

^g Fluxim AG, Katharina-Sulzer-Platz 2, Winterthur, Switzerland

^h Department of Chemical Engineering, Delft University of Technology, Delft, The Netherlands

ⁱ University of Luxembourg, 2 Avenue de l’Université, Esch-sur-Alzette L-4365, Luxembourg

Abstract

The technique of alloying FA⁺ with Cs⁺ is often used to promote structural stabilization of the desirable α-FAPbI₃ phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in Cs_xFA_1−xPbI₃ perovskite thin-films and solar-cell devices. Selected-area electron diffraction patterns combined with microwave conductivity measurements reveal that fine Cs⁺ tuning (Cs_0.15FA_0.85PbI₃) leads to a minimization of stacking faults and an increase in the photoconductivity of the perovskite films. Ultra-sensitive external quantum efficiency, kelvin-probe force microscopy and photoluminescence quantum yield measurements demonstrate similar Urbach energy values, comparable surface potential fluctuations and marginal impact on radiative emission yields, respectively, irrespective of Cs content. Despite this, these nanoscopic defects appear to have a detrimental impact on inter-grains’/domains’ carrier transport, as evidenced by conductive-atomic force microscopy and corroborated by drastically reduced solar cell performance. Importantly, encapsulated Cs_0.15FA_0.85PbI₃ devices show robust operational stability retaining 85% of the initial steady-state power conversion efficiency for 1400 hours under continuous 1 sun illumination at 35 °C, in open-circuit conditions. Our findings provide nuance to the famous defect tolerance of halide perovskites while providing solid evidence about the detrimental impact of these subtle structural imperfections on the long-term operational stability.

Energy & Environmental Science

Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal–halide perovskites†

Abstract

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Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal–halide perovskites

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