Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics

TLDR

The study investigates whether photoexcitation induces halide segregation that creates trap states in mixed‑halide perovskites. The authors propose that photoexcitation drives halide segregation into iodide‑rich minority and bromide‑enriched majority domains, with the iodide‑rich regions acting as recombination‑center traps. They observe reversible, light‑induced transformations: a 1.68 eV photoluminescence peak and increased sub‑bandgap absorption appear within a minute of 1‑sun illumination, XRD peak splitting indicates phase segregation, and all changes revert within minutes in the dark, implying that this instability could limit voltages and affect photostability in perovskite solar cells.

Abstract

We report on reversible, light-induced transformations in (CH3NH3)Pb(BrxI1−x)3. Photoluminescence (PL) spectra of these perovskites develop a new, red-shifted peak at 1.68 eV that grows in intensity under constant, 1-sun illumination in less than a minute. This is accompanied by an increase in sub-bandgap absorption at ∼1.7 eV, indicating the formation of luminescent trap states. Light soaking causes a splitting of X-ray diffraction (XRD) peaks, suggesting segregation into two crystalline phases. Surprisingly, these photo-induced changes are fully reversible; the XRD patterns and the PL and absorption spectra revert to their initial states after the materials are left for a few minutes in the dark. We speculate that photoexcitation may cause halide segregation into iodide-rich minority and bromide-enriched majority domains, the former acting as a recombination center trap. This instability may limit achievable voltages from some mixed-halide perovskite solar cells and could have implications for the photostability of halide perovskites used in optoelectronics.