Impact of Orientational Glass Formation and Local Strain on Photo-Induced Halide Segregation in Hybrid Metal-Halide Perovskites.

Band gap tuning of hybrid metal-halide perovskites by halide substitution holds promise for tailored light absorption in tandem solar cells and emission in light-emitting diodes. However, the impact of halide substitution on the crystal structure and the fundamental mechanism of photo-induced halide segregation remain open questions. Here, using a combination of temperature-dependent X-ray diffraction and calorimetry measurements, we report the emergence of a disorder- and frustration-driven orientational glass for a wide range of compositions in CH3NH3Pb(Cl x Br1-x )3. Using temperature-dependent photoluminescence measurements, we find a correlation between halide segregation under illumination and local strains from the orientational glass. We observe no glassy behavior in CsPb(Cl x Br1-x )3, highlighting the importance of the A-site cation for the structure and optoelectronic properties. Using first-principles calculations, we identify the local preferential alignment of the organic cations as the glass formation mechanism. Our findings rationalize the superior photostability of mixed-cation metal-halide perovskites and provide guidelines for further stabilization strategies.T.W.J. acknowledges financial support from the Schiff Foundation. T.W.J. and M.A.J. acknowledge support from the EPSRC Cambridge NanoDTC, EP/L015978/1. S.F. acknowledges funding from the EPSRC and the Studienstiftung des deutschen Volkes. S.A.B. acknowledges the support from the EPSRC Centre for Doctoral Training in Graphene Technology (EP/L016087/1). S.F., T.W. and F.D. acknowledge funding from an EPSRC NI grant (EP/R044481/1). S.E.D. and F.D. acknowledge the Winton Program for the Physics of Sustainability. F.D. acknowledges funding from the DFG Emmy Noether Program. Y.L. and R.H.F acknowledge support from the Simons Foundation (Grant 601946). S.P.E. was funded via an EPSRC iCASE (Award 1834544) and via the Royal Society (RP\R1\180147). This work was performed using resources provided by the Cambridge Service for Data Driven Discovery (CSD3) operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council (capital grant EP/P020259/1), and DiRAC funding from the Science and Technology Facilities Council (www.dirac.ac.uk). B.M. acknowledges support from the Gianna Angelopoulos Programme for Science Technology and Innovation. We are grateful for computational support from the UK national high performance computing service, ARCHER, for which access was obtained via the UKCP consortium and funded by EPSRC grant ref EP/P022561/1