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

Abstract

Methylammonium lead tribromide (MAPbBr₃) perovskite single crystals demonstrate to be excellent direct X-ray and gamma-ray detectors with outstanding sensitivity and low limit of detection. Despite this, thorough studies on the photophysical effects of exposure to high doses of ionizing radiation on this material are still lacking. In this work, we present our findings regarding the effects of controlled X-ray irradiation on the optoelectronic properties of MAPbBr₃ single crystals. Irradiation is carried out in air with an imaging X-ray tube, simulating real-life application in a medical facility. By means of surface photovoltage spectroscopy, we find that X-ray exposure quenches free excitons in the material and introduces new bound excitonic species. Despite this drastic effect, the crystals recover after 1 week of storage in dark and low humidity conditions. By means of X-ray photoelectron spectroscopy, we find that the origin of the new bound excitonic species is the formation of bromine vacancies, leading to local changes in the dielectric response of the material. The recovery effect is attributed to vacancy filling by atmospheric oxygen and water.

Keywords: X-ray photoelectron spectroscopy; excitons; hybrid lead halide perovskites; ionizing radiation; methylammonium lead bromide; surface photovoltage spectroscopy.

Figure 1

(a,b) Surface photovoltage and (c,d) photoluminescence spectra of an MAPbBr₃ single crystal as a function of X-ray dose for the (a,c) irradiated sample and as a function of time for the (b,d) control sample. The gray curves in (a,c) show the SPS and PL spectra after 1 week of storage.

Figure 2

Fitting of SPS spectra by means of Elliott’s formula at (a) 0 Gy and (b) 200 Gy. The blue circles represent the experimental data; the red curve shows the fit result. The blue shaded areas represent the band-to-band (continuum) fit component. The orange and red areas represent the T1 and T2 excitonic fit components, respectively.

Figure 3

Results of the XPS analysis on the three MAPbBr₃ single crystals exposed to 0 Gy (control sample), 60 Gy, and 120 Gy X-ray radiation in air. (a–d) XPS spectra of Pb 4f, Br 3d, N 1s, and O 1s. Each graph shows the experimental data as dots and the fitting curve as black solid lines. The components of the fitting curves for the 0 Gy sample are displayed as shaded areas. (e) Elemental ratios with respect to the total amount of Pb calculated from the XPS spectra as a function of absorbed dose, both immediately after irradiation (opaque bars) and after 1 week of storage (semitransparent bars).