Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics† †Electronic supplementary information (ESI) available: Experimental details, PL, PDS spectra and XRD patterns. See DOI: 10.1039/c4sc03141e Click here for additional data file

• Published in: Chemical science (Cambridge) Vol. 6; no. 1; pp. 613 - 617
• Main Authors: Hoke, Eric T., Slotcavage, Daniel J., Dohner, Emma R., Bowring, Andrea R., Karunadasa, Hemamala I., McGehee, Michael D.
• Format: Journal Article
• Language: English
• Published: Royal Society of Chemistry 04.11.2014
• Subjects: Chemistry
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c4sc03141e

A reversible photo-induced instability has been found in mixed-halide photovoltaic perovskites that limits the open circuit voltage in solar cells. We report on reversible, light-induced transformations in (CH 3 NH 3 )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.