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

Eric T Hoke; Daniel J Slotcavage; Emma R Dohner; Andrea R Bowring; Hemamala I Karunadasa; Michael D McGehee

doi:10.1039/c4sc03141e

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

Chem Sci. 2015 Jan 1;6(1):613-617. doi: 10.1039/c4sc03141e. Epub 2014 Nov 4.

Authors

Eric T Hoke¹, Daniel J Slotcavage¹, Emma R Dohner², Andrea R Bowring¹, Hemamala I Karunadasa², Michael D McGehee¹

Affiliations

¹ Department of Materials Science and Engineering , Stanford University , 476 Lomita Mall , Stanford , California 94305 , USA . Email: mmcgehee@stanford.edu.
² Department of Chemistry , Stanford University , 337 Campus Drive , Stanford , California 94305 , USA . Email: hemamala@stanford.edu.

Abstract

We report on reversible, light-induced transformations in (CH₃NH₃)Pb(Br _x I_1-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.