Collective Motion of Methylammonium Cations Affects Phase Transition and Self-Trapped Exciton Emission in MAPbI3 Films

Hybrid organic-inorganic halide perovskites are celebrated for their exceptional optoelectronic properties and facile fabrication methods, making them prime candidates for next-generation photovoltaic and optoelectronic devices. However, their inherent instability limits practical applications. To address this, a new class of 3D hollow perovskites, denoted as {en}MAPbI3, has been developed by partially replacing methylammonium (MA+) with ethylenediammonium (en) cations. This study systematically investigates the structural, phase transition, and photophysical properties of {en}MAPbI₃ thin films with varying en content. Incorporating en cations expands the unit cell, suppresses the tetragonal-to-orthorhombic phase transition, and disrupts dipole-dipole interactions among MA⁺ cations. Temperature-dependent photoluminescence (PL) and X-ray diffraction (XRD) reveal that en incorporation stabilizes the tetragonal phase while diminishing the collective rotational dynamics of MA⁺ cations, crucial for phase stability and self-trapped exciton (STE) formation. These insights advance the understanding of 3D hollow perovskites and highlight their potential for stable and tunable optoelectronic applications.