Mixology of MA1-xEAxPbI3 Hybrid Perovskites: Phase Transitions, Cation Dynamics and Photoluminescence

Mixing of molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance stability and performance of the optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed methylammonium (MA)-ethylammonium (EA) MA1-xEAxPbI3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase transition temperatures indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with different symmetry. We use the broadband dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe signatures of the dipolar glass phase formation. Our findings are supported by the density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing indicating suitability of these compounds for optoelectronic applications.