Illumination-Induced Photoluminescence Enhancement in Metal Halide Perovskites: Revisiting Mechanisms and Emerging Perspectives

Metal halide perovskites (MHPs) have emerged as leading materials for next-generation optoelectronic technologies, owing to their remarkable photophysical properties such as tunable bandgaps, long carrier diffusion lengths, and high radiative efficiencies. While light-induced degradation pathways, ion migration, defect formation, and phase segregation, have been extensively studied, an equally critical yet underrecognized phenomenon is the reversible enhancement of photoluminescence (PL) under continuous illumination. This anomalous brightening behaviour challenges prevailing degradation-centric narratives and suggests a constructive light–matter interaction. Such effects can be leveraged to enhance photovoltaic parameters, such as open-circuit voltage (Voc) and fill factor (FF), offering new routes for optimizing perovskite device performance under illumination. However, the physical origins of this effect remain fragmented across the literature, lacking a cohesive theoretical framework. This review critically evaluates the proposed mechanisms behind PL enhancement, including defect passivation, ionic redistribution, and lattice dynamics, and introduces a unifying conceptual model based on lattice energy reservoirs (LERs): metastable, phonon-rich states that transiently store and recycle vibrational energy to re-activate trapped carriers. This framework explains key unresolved phenomena such as reversible PL gain, subgap emission, and excitation-fluence-dependent behaviour. By situating these insights within the mixed ionic-electronic nature of MHPs, we show how illumination can act not only as a stressor, but as a functional tuning parameter. This perspective offers new pathways for engineering light-responsive, high-performance, and more stable perovskite-based optoelectronics.