Pressure-Induced Detrapping from Self-Trapped Excitons to Free Excitons toward Enhanced Emission and Piezochromism in Ruddlesden–Popper (110)-Oriented Perovskites

Two-dimensional (2D) lead halide perovskites are emerging as excellent materials for optoelectronic applications including light-emitting diodes, photovoltaics, and photodetectors, owing to their efficient excitonic emission originating from both self-trapped excitons (STEs) and free excitons (FEs). Recently, many efforts have been focused on enhancing the emission intensity, modulating the dominant emission mechanism, and establishing direct correlations between optical properties and underlying structural motifs. Across the hybrid organic-inorganic layers in 2D perovskites, the nanorange modulation of the density, stiffness, strain, and ionicity can be efficiently tuned by external stimuli, including the substrate-film strain. Here, we report a pressure-induced narrowing of the band gap and an enhancement of STE and FE emission in the topology of the (110)-oriented Ruddlesden-Popper (RP) perovskite ACE₂PbBr₄ (ACE = acetamidinium), accompanied by the rare phenomenon of the reversible detrapping process from STEs to FEs under compression. Specifically, the STE-related emission exhibits a 6.4-fold increase of intensity up to 2.56 GPa, while the FE emission dominates under higher pressures, up to 11.11 GPa, causing a pronounced change in the emission color, from orange-yellow to greenish-blue, across the compression range. In situ single-crystal X-ray diffraction and Raman spectroscopy reveal that these emission changes arise from the rarely observed pressure-induced reduction of lead bromide octahedral distortion and confinement in the direction of the corrugated structure and changes in amine-framework interactions, as well as pressure-induced phase transitions (PTs) occurring near 2, 5, 5.9, and 6.7 GPa. These findings elucidate the structure-property relationship in (110)-oriented RP perovskites and underscore the utility of strain engineering for realizing light-emitting materials with tailored and enhanced functionalities.