Abstract
We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A = CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br3- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.
Highlights
Introduction a Weizmann Institute ofScience, Department of Materials and Interfaces, 7610001, Rehovot, Israel c Bar Ilan University, Department of Chemistry, 5290002, Ramat Gan, Israel d Bar Ilan University, Bar-Ilan Institute for Adv
(4) Surface degradation is more pronounced than bulk degradation, due to kinetic effects, which can be hindered with tailored encapsulation and judicious electron transporting layer (ETL) and hole transporting layer (HTL).[17] (5) We identify and explain the mechanisms involved in the self-healing, an obvious step towards optimizing self-healing
We report the observed values of lifetime for the more photo-luminescent, blue-shifted material obtained by bleaching the Cs sample at the surface with high laser power (LP)
Summary
Halide Perovskites (HaPs) continue to have significant impact on the field of solar cells, thanks to their very low cost, ease of production, and competitive efficiencies that are comparable, if not superior, to those of established photovoltaic (PV) technologies.[1,2,3] In the wake of these PV results, other applications, such as light-emission, radiation detection, and electronics are explored.[4,5,6,7] Despite all these efforts, a critical issue hangs as a sword of Damocles over the entire field: stability. Various approaches for delaying or avoiding device degradation, due to external influences such as heat, light, or chemicals, have been tested.[8] Encapsulation of the solar cell and perovskite passivation through long alkylamine molecules,[9] as well as use of 2D variants of HaPs,[10,11] show promise for avoiding degradation; 1570 | Mater.
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