Water and oxygen induce undesired phase transitions in cesium-formamidinium lead halide perovskites
Juanita Hidalgo a, Juan-Pablo Correa-Baena a, Yu An a, Susan Schorr b, Joachim Breternitz b
a School of Materials Science and Engineering, Georgia Institute of Technology, North Avenue, Atlanta, United States
b Dept. Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin, 12489 Berlin, Germany
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Juanita Hidalgo, presentation 100
DOI: https://doi.org/10.29363/nanoge.hopv.2022.100
Publication date: 20th April 2022

Lead halide perovskites have shown to be promising materials for solar cells, having achieved a maximum power conversion efficiency greater than 25%. However, the lead halide perovskite structure is highly unstable under different environmental conditions such as humidity. Single-cation cesium (Cs) and formamidinium (FA) lead iodide perovskites suffer from phase instability at room temperature. The mixed Cs-FA has led to a more stable perovskite structure [1], but not when exposed to humidity. The mechanisms that lead to crystallographic phase transitions upon exposure to humidity, in Cs-FA based lead halide perovskites, are still not well understood. Given that compositional mixing has been a strategy to stabilize the structure, it is essential to unravel the humidity-driven structural degradation pathway of these materials to design robust and efficient materials.

Herein, we studied the phase transformations of Cs-FA lead halide perovskites upon exposure to humidity through in-situ grazing incidence wide angle X-ray scattering (GIWAXS). We investigated the effect of the humidity carrier gas (i.e. nitrogen, air) and the effect of the halide composition in the crystal phases upon exposure to humidity. The degradation kinetics were observed and quantified, demonstrating that humidity in air accelerated the formation of the hexagonal non-perovskite delta-FAPbI3 phase. Moreover, a different degradation route was observed in the mixed iodine-bromine composition compared to the pure-iodine one. Bromine addition hindered the transformation of the cubic perovskite into delta-FAPbI3 but degraded into the orthorhombic non-perovskite delta-CsPbI3. We found that the origins of the phase transitions of the perovskite go beyond humidity: the carrier gas and the elements present in the solid solution play an important role in determining the degradation process. In addition, solar cells were fabricated, and the non-perovskite crystal phases were correlated with the opto-electronic properties of the material.

This work was performed in part of the Georgia Tech Institute for Electronics and Nanotechnology, a memeber of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation USA. J.H acknowledges the Georgia Tech Graduate Assistance in Areas of National Need GAANN, and GEM fellowship for carreer funding. J.H also acknowledges the DAAD scholarship for the German Academic Exchange program. Y.A acknowledges financial support from the National Science Foundation of China. We all acknowledge beamline scientists Dr. Ruipeng Li from the National Synchrotron Light Source II, Brookhaven National Laboratory, and Dr. Barry Lai from the Advanced Photon Source, Argonne National Laboratory and the US Department of Energy. 

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