Thermodynamics and kinetics of light-induced phase segregation in mixed halide hybrid perovskites
Zehua Chen a, Geert Brocks a b, Peter Bobbert a, Shuxia Tao a
a Center for Computational Energy Research, Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
b Computational Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands
Poster, Zehua Chen, 080
Publication date: 25th November 2019

Mixed halide hybrid perovskites CH3NH3Pb(I1-x Brx)3 are excellent light absorbers for organic-inorganic solar cells with tunable band gap. However, light-induced halide phase segregation degrades their photovoltaic performance. Here, using density functional theory and kinetic Monte Carlo methods, we show that the band gap difference between the parent mixed halide and iodine-rich regions is the driving force for halide segregation. We also show that there is a competition between this driving force and entropic effects, leading to a dependence of the critical point where the driving force disappears on temperature and concentration of photo-generated carriers. Under continuous illumination, photo-generated carriers funnel into stochastically appearing iodine-rich regions and induce local halide anion rearrangements. Kinetic Monte Carlo simulations reveal that a high vacancy concentration accelerates halide segregation. The exchange of iodine and bromine facilitated by vacancies leads to the nucleation of regions with an increase of iodine and a decrease of bromine concentration. When illumination stops, the driving force vanishes, leading to a return to the entropically favorable mixed alloy. Our study deepens the understanding of both the thermodynamics and kinetics of light-induced halide phase segregation in mixed halide hybrid perovskites.

This research is supported by TU/e-DIFFER Storing Solar Energy program.

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