Chain dynamics and quasi-melting transitions in mixed-halide layered perovskites by NMR spectroscopy

Abstract

Dynamics of the organic cations in hybrid materials can modify the structure and optoelectronic properties, determining the performance in, e.g., photovoltaic devices. In pure-halide BA₂PbI₄ and BA₂PbBr₄, the n-butylammonium (BA⁺) chains undergo quasi-melting phase transitions at ca. −10 °C and 115 °C, respectively, but the chain dynamics in mixed-halide compositions were unknown. Here, we measure the amplitude of motion for each BA⁺ carbon as a function of temperature for pure and mixed-halide compositions using ¹H→¹³C cross-polarisation build-up experiments. We find that motional amplitude increases with temperature with clear steps across the phase transitions in the end-members. Mixed-halide samples exhibit intermediate dynamics, with halide mixing partly or fully suppressing the concerted phase transitions due to the substitutional disorder. This disorder is probed by ¹³C lineshape analysis, becoming dynamically averaged at high temperature. Using the measured amplitude of motion, the correlation times for motion are calculated from ¹³C T₁ relaxation as a function of temperature and used to derive the activation energy. This detailed picture of the chain dynamics can inform the design of spacer cations for (mixed-halide) 2D perovskite photovoltaics and passivating layers.