X-Ray Induced Modification of the Photophysical Properties of MAPbBr₃ Single Crystals

Single crystals of methylammonium lead tribromide (MAPbBr₃) are among the most promising materials for next generation X- and gamma-ray detectors. Indeed, they combine a low-cost fabrication process with high stopping power, large resistivity and low trap density. These properties allowed devices based on MAPbBr₃to reach outstanding sensitivities, overcoming those of commercially available detectors based on cadmium zinc telluride and amorphous selenide. Despite these promising results, the radiation hardness of this material has not been fully characterized yet and thorough studies on the effects of ionizing radiation on its photophysical properties are still lacking. In our study, we characterized the optical, structural, and compositional properties of MAPbBr₃single crystals before and after X-ray irradiation. We focused on the dose range that is most interesting for diagnostic applications, up to hundreds of grays (Gy), using a medical imaging X-ray tube operated in ambient conditions.<br/>We characterized the optical absorption properties of the crystals by means of Surface Photovoltage Spectroscopy (SPS), a technique that allows to gather the whole absorption spectrum of thick (&gt; 1mm) crystals without having to rely on two separate measurements such as transmittance and reflectance spectroscopy. We collected SPS spectra on a pristine sample and after subsequent 40 Gy X-ray irradiations. We found that the typical excitonic feature (T1) in the absorption spectrum of the material disappears after exposure to ionizing radiation, while a new excitonic resonance (T2) appears. By fitting the spectra we found that binding energies of T1 and T2 excitonic species are 39 and 76 meV, respectively. Surprisingly, after 7 days T2 disappears and the T1 species is recovered.<br/>We found the T2 species not to be emissive, as no additional feature is observed in photoluminescence (PL) spectra after irradiation. We also found the PL intensity to be negatively affected by X-ray irradiation, in agreement with the disappearance of the T1 species and possibly indicating the formation of non-radiative recombination centers in the material.<br/>To check if this effect could be related to X-ray induced structural deformation, we performed high resolution X-ray diffraction (HRXRD) measurements on pristine and irradiated samples. We did not find any relevant change in intensity or position of the diffraction peaks, indicating that radiation does not induce strain in the crystal. Therefore, we moved to a compositional analysis by means of X-ray photoelectron spectroscopy (XPS) to investigate the relative amount and the chemical state of different elements at the sample’s surface before and after irradiation. We found that irradiation in ambient conditions produces bromine and nitrogen vacancies, as well as water in situ. To explain this observation, we propose a X-ray triggered chemical reaction of MAPbBr₃with atmospheric oxygen. Based on these results, we propose that the appearance of the T2 excitonic specie is related to the creation of bromine vacancies by X-rays. Such vacancies can change the local lattice polarization, modifying the dielectric screening of the electron-hole pairs, and thus the exciton binding energy. The recovery effect observed by SPS is attributed to the filling of the vacancies by environmental oxygen and water.