Shijing Sun² Federico Brivio¹ Fengxia Wei⁴ Yue Wu⁵ Zeyu Deng¹ Gregor Kieslich³ Paul Bristowe¹ Tonio Buonassisi² Anthony Cheetham¹

2, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
1, University of Cambridge, Cambridge, , United Kingdom
4, A*Star, Singapore, Singapore, , Singapore
5, National University of Singapore, Singapore, , Singapore
3, Technische Universität München, Munich, , Germany

The commercialization of perovskite solar cells (PSCs) have been hindered by their chemical, structural and thermal instability. There is an increasing interest in compositional tuning of perovskites to improve their intrinsic stability. In this study, extrinsic and intrinsic strain in cubic perovskites, methylammonium (MA) and formamidinium (FA) lead bromide were examined in thin-film, powder and single crystal forms to investigate the role of A-site cations on the structural-property relationships. Based on laboratory and synchrotron X-ray diffraction and density functional theory (DFT), our quantitative analysis reveal the presence of anisotropic strain in the cubic perovskites, where the lattice strain is the highest on {111} and lowest on {100}. Comparative studies on FAPbBr₃ and MAPbBr₃ showed that the cation dynamics and hydrogen bonding of FA contributes to the higher strain as well as more compliant bulk properties. Our results shed light on the rational perovskite compositional design to minimize strain induced decomposition.