Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells

Abstract

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAPbI3)0.85(MAPbBr3)0.15) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

The Role of Grain Boundaries on Ionic Defect Migration in Metal Halide Perovskites

“…The signal for Pb, on the other hand, is higher in the exposed region compared to the surrounding regions (Figure 1b 4 ). At first this may seem counter‐intuitive, however consistent with the EDX images of I and C, the increase of the Pb signal just denotes an increase of the concentration of Pb with respect to the other elements, I and C. Whether this is due to the concomitant formation of I 2 and Pb 0[ 29 ] or collapse of the tetragonal structure into PbI 2 due to the removal of MA + and I − , [ 30 ] goes beyond the aim of this work. Indeed, the reduced concentration of I and C in the illuminated region can further enhance the Pb signal in EDX imaging because it reduces the reabsorption of Pb‐emitted X‐rays by MA + and I − .…”

Photoinduced Anion Segregation in Mixed Halide Perovskites

“…These and other like examples [64][65][66][67][68] show that, in addition to halide photosegregation into I-rich and Br-rich domains, other ongoing chemistries occur in the perovskite due to the presence of mobile anions. These chemistries and, most notably, I --related ones, are not contained within existing models for light-induced halide segregation.…”

Room‐Temperature Ion‐Exchange‐Mediated Self‐Assembly toward Formamidinium Perovskite Nanoplates with Finely Tunable, Ultrapure Green Emissions for Achieving Rec. 2020 Displays

“…While alloying with I − can intuitively be an effective approach to shift the emission to the target window, the incorporation of I − is found to lower the photoluminescence (PL) QY. And HPs with mixed halides suffer from easy phase separation during long‐term operation . What's even worse, the anion exchange reaction is nonquantitative, which usually causes a large degree of PL shift (>10 nm) rather than a delicate tuning.…”