Positron Annihilation Lifetime Spectroscopy of Single Crystalline CH 3 NH 3 PbBr 3 : Experiments and Ab Initio Calculations

Abstract

One of the most promising materials for tandem solar cells, light-emitting diodes and γ-rays detectors is methylammonium lead bromide (CH3NH3PbBr3). Its actual use is crucially determined by our ability to control its electronic defects, which affect its optoelectronic properties. To date, identifying the chemical nature of these defects and quantifying their concentration remain open questions that require a clear answer to improve device efficiency. In this regard, positron annihilation lifetime spectroscopy (PALS) has much to contribute, thanks to its ability to identify and quantify vacancy concentration. However, previous studies performed with PALS have reported contradictory results on the positron lifetime in a defect-free crystal, the nature of the dominant defect, and the defect concentration. There is also disagreement between previous calculations using density functional theory (DFT), which report differing values for the bulk positron lifetime and for the lifetimes related to vacancies. Furthermore, there is disagreement as to the structure that best describes the room temperature disorder of the perovskite structure. In the present work, we clarify these points through new measurements and new ab initio calculations. Single crystals of CH3NH3PbBr3 with cubic structure, grown by the antisolvent method to lateral dimensions of several millimeters, are investigated. PALS measurements are performed, at room temperature in both air and vacuum, as well as in vacuum over the temperature range of 295–350 K. We observe two lifetime components, associated with a dominant defect and the reduced bulk positron lifetime. The temperature dependence of the lifetimes indicates the presence of negatively charged defects with an estimated defect density of (1.2 ± 0.3) × 1016 cm–3. Our ab initio calculations using two-component DFT confirm that the lifetime component associated with defects is due to lead vacancies, as suggested in a recent work. Our contribution is to clarify that the calculated value strongly depends on the local environment of the vacancy and not necessarily on the disorder of the structure used to describe the perovskite crystal. In particular, our calculations indicate that the lifetime of a positron trapped at a lead vacancy can vary between 353 and 388 ps depending on the local environment, although it tends to the highest value for the energetically most favorable lead vacancy. This agrees well with the value measured in our CH3NH3PbBr3 crystals, 382 ps. Comparing with the experimental and calculated results from other authors, we conclude that the growth conditions strongly influence the chemical nature and the concentration of the defects present in the bulk of this material.