Collective motion of methylammonium cations affects phase transitions and self-trapped exciton emission in A-site engineered MAPbI ₃ films

Hybrid organic-inorganic halide perovskites are celebrated for their exceptional optoelectronic properties and facile fabrication processes, making them prime candidates for next-generation photovoltaic and optoelectronic devices. By incorporating larger organic cations at the A-site, a novel class of '3D hollow perovskites' has been developed, exhibiting enhanced stability and tunable optoelectronic properties. This study systematically explores the structural, phase transition, and photophysical characteristics of {en}MAPbI₃ thin films with varying ethylenediammonium (en²⁺) content. The incorporation of less polar en²⁺ expands the perovskite unit cell, prolongs carrier lifetimes, and disrupts MA⁺ dipole-dipole interactions, thereby lowering the tetragonal-to-orthorhombic phase transition temperature. Temperature-dependent photoluminescence studies reveal that en²⁺ incorporation reduces the intensity and Stokes shift of self-trapped exciton emission at low temperatures, which are attributed to the diminished collective rotational dynamics of MA⁺ cations. These findings underscore the critical role of A-site cation dynamics in modulating phase stability and excitonic behaviour within hybrid halide perovskites, deepening our understanding of the interplay between organic cations and the inorganic framework and highlighting the potential of 3D hollow perovskites for stable and tunable optoelectronic applications.