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/mm2) and violet laser (3.06 eV, 8×1017 ph/s/mm2) irradiation using synchrotron X-ray diffraction. The perovskites undergo a complex degradation landscape under irradiation charcterized 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/mm2) rather than radiation flux (W/mm2) 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 ablate 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.

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