High radiation hardness is the primary requirement for application of lead halide perovskite semiconductors in X-ray detectors for medical diagnostics and solar panels for space missions. Multiple reports show that perovskite absorber films and solar cells indeed could successfully tolerate high electron, proton and neutron fluences as well as gamma rays and x-rays [1-2]. Among different types of ionizing radiation, gamma rays have very high penetration ability and hence could hardly be mitigated using simple shielding used to suppress the damage from proton and also electron fluences. Thus, the investigation of the radiation hardness of lead halide perovskites with respect to gamma rays is essentially important from fundamental point of view and also in the context of the emerging applications.

Herein, we present the results of our systematic study of model lead halide perovskite materials: MAPbI₃, FAPbI₃, (CsMA)PbI₃ and (CsMAFA)PbI₃, where MA and FA are methylammonium and formamidinium cations, respectively [3]. We show that among the studied materials FAPbI₃ is the only one which does not degrade after receiving the ultrahigh radiation doses up to 20 MGy and thus represents highly promising absorber material for radiation-tolerant solar cells. Other complex lead halides produce different aging products upon exposure to gamma rays including metallic lead and PbI₂.

 Infrared near-field optical microscopy revealed the radiation-induced depletion of organic cations from the grains of MAPbI₃ and their accumulation at the grain boundaries. Using a set of complementary techniques, we evidenced that multication (CsFA)PbI₃ and (CsMAFA)PbI₃ perovskites undergo a facile phase segregation to domains enriched with Cs, MA and FA cations. This new degradation pathway is quite similar to the gamma-ray-induced halide phase segregation we observed previously for Cs_0.15MA_0.10FA_0.75Pb(Br_0.17I_0.83)₃ material [4-5]. The revealed aging pathways could be successfully mitigated through the rational compositional engineering of complex lead halides using (1) partial lead substitution and (2) the formation of the mixed dimensional 2D/3D absorber materials. The perovskite solar cells maintained 80-90% of their initial performance after exposure to extreme doses of gamma rays approaching 1 MGy, which is unprecedented result for all types of photovoltaic cells.

To summarize, our findings suggest that the radiation hardness of the rationally designed perovskite semiconductors could go far beyond the impressive threshold of 20 MGy we set herein for FAPbI₃ films and 1 MGy we demonstrate for completed perovskite solar cells. Thus, the unique radiation hardness of complex lead halides opens many exciting opportunities for practical implementation of these materials in detectors for medical diagnostics and solar cells operating in harsh radiation environments.