Radiation induced structural phase transition and damage in methyl ammonium lead perovskites studied by focused-beam synchrotron X-ray diffraction

Abstract

We investigate the structural degradation of methylammonium lead iodide (MAPbI₃) thin films under focused hard X-ray (10 keV, 5 × 10¹³ ph s⁻¹ mm⁻²) and violet laser (3.06 eV, 8 × 10¹⁷ ph s⁻¹ mm⁻²) irradiation using synchrotron X-ray diffraction. The perovskites undergo a complex degradation landscape under irradiation characterized by marked structural transformations, including phase decomposition into PbI₂, increased disorder, crystallite size reduction across all phases, loss of preferential orientation, and the unexpected formation of an orthorhombic MAPbI₃ phase. This new phase coexists with the original tetragonal structure, highlighting a novel irradiation-driven phase transition pathway. Photon flux (ph s⁻¹ mm⁻²) rather than radiation flux (W mm⁻²) is the crucial parameter that triggers the partial phase transition to the orthorhombic structure which presents anomalously large lattice parameters. The transition is proposed to be activated by radiation-induced ion migration to lattice interstitial sites which restrict MA motion reducing entropy and locally increasing the orthorhombic transition temperature. This orthorhombic phase is surprisingly robust against radiation compared to the tetragonal one. Bi³⁺ doped MAPbI₃ presents higher disorder and smaller crystal size due to the heterovalent doping but, while radiation partially ablates the perovskite, PbI₂ formation is inhibited due to the preferred formation of BiI₃. A CVD-deposited overlayer of BiI₃ on MAPbI₃ offers effective protection against combined X-ray and laser damage. These results advance our understanding of radiation-induced degradation in hybrid perovskites, revealing a previously unrecognized structural transformation pathway and highlighting that photon energy and fluence govern distinct and competing degradation mechanisms with potential implications for the stability and performance of perovskite-based optoelectronic devices.