Photoluminescence (PL) is a powerful tool for investigating the optoelectronic properties of perovskite semiconductors. These materials exhibit exceptional luminescence characteristics, including high quantum yields, tunable emission wavelengths, and narrow emission linewidths. The PL behavior of perovskites is closely linked to their structural and compositional properties, making it a critical probe for understanding phase stability, ion migration, and halide segregation phenomena. Both hybrid organic-inorganic perovskites (HOIPs) and all-inorganic perovskites exhibit distinct PL signatures, which provide insights into their underlying physical mechanisms.

Perovskite semiconductors adopt the general crystal structure ABX3, where A is a monovalent cation (e.g., methylammonium (MA), formamidinium (FA), cesium (Cs)), B is a divalent metal (e.g., lead (Pb), tin (Sn)), and X is a halide anion (e.g., iodine (I), bromine (Br), chlorine (Cl)). The PL properties are highly sensitive to the local crystal environment, including defects, strain, and phase purity. For example, the PL emission peak position directly correlates with the bandgap, which can be tuned by varying the halide composition. Mixed-halide perovskites, such as MAPb(I1-xBrx)3, exhibit composition-dependent emission wavelengths spanning the visible spectrum, from approximately 1.5 eV (820 nm) for pure iodide to 2.3 eV (540 nm) for pure bromide compositions.

Phase stability is a critical factor influencing PL behavior. Many perovskites undergo phase transitions under varying temperatures or environmental conditions. For instance, MAPbI3 transitions from a tetragonal to orthorhombic phase below 160 K, accompanied by a blue shift in PL emission due to bandgap widening. Similarly, FAPbI3 exhibits a phase transition from a photoactive black phase (α-phase) to an inactive yellow phase (δ-phase) at room temperature unless stabilized by additives or strain engineering. The metastability of these phases can lead to hysteresis in PL intensity and spectral shifts under illumination or thermal cycling. All-inorganic perovskites like CsPbI3 also face similar challenges, with the black perovskite phase being stable only above 300 °C unless nanocrystalline or strain-stabilized.

Ion migration is another key phenomenon affecting PL in perovskites. Under illumination or electric fields, halide ions and vacancies can migrate, leading to changes in local composition and defect distribution. This results in PL quenching, spectral shifts, or even phase segregation. For example, in mixed-halide perovskites like MAPb(I1-xBrx)3, illumination can induce halide segregation, forming iodide-rich and bromide-rich domains. This manifests as a splitting of the PL spectrum into multiple peaks corresponding to different local bandgaps. The extent of segregation depends on factors such as excitation intensity, halide composition, and material morphology. Ion migration is also linked to defect-assisted non-radiative recombination, which reduces PL quantum efficiency. Studies have shown that passivating surface and grain boundary defects with ligands or additives can significantly enhance PL intensity and stability.

Halide segregation is particularly pronounced under high excitation densities, such as those encountered in laser excitation or under solar concentration. The process is reversible in some cases, with the mixed phase recovering upon removal of illumination, but repeated cycling can lead to permanent degradation. The activation energy for halide migration has been estimated to be in the range of 0.1 to 0.5 eV, depending on the composition and defect density. All-inorganic perovskites like CsPb(Br1-xIx)3 exhibit similar segregation behavior but often with higher activation energies due to the absence of organic cation mobility. The dynamics of segregation and recovery can be probed using time-resolved PL, which reveals the timescales of domain formation and relaxation.

Temperature-dependent PL studies provide further insights into the excitonic and defect-related processes in perovskites. At low temperatures, PL spectra often show sharp excitonic peaks with minimal inhomogeneous broadening, while at room temperature, electron-phonon interactions and thermal disorder lead to broader emission bands. The exciton binding energy, which ranges from a few meV to tens of meV depending on the material, can be extracted from the thermal quenching behavior of PL intensity. For example, CsPbBr3 exhibits a binding energy of around 40 meV, while MAPbI3 has a lower value of approximately 15 meV due to its higher dielectric screening. The presence of shallow traps can also be inferred from the non-exponential decay of time-resolved PL, with trap densities often estimated to be in the range of 10^15 to 10^16 cm^-3 in untreated films.

The role of dimensionality in PL properties has also been explored, particularly in layered (2D) perovskites. These materials exhibit quantum confinement effects, leading to higher exciton binding energies and narrower PL linewidths compared to their 3D counterparts. For example, (PEA)2PbI4 (PEA = phenylethylammonium) shows a sharp PL peak with a binding energy exceeding 200 meV due to the dielectric confinement within the organic layers. The interlayer energy transfer in mixed 2D-3D perovskites can be studied using spatially resolved PL mapping, revealing efficient funneling of excitons to lower-energy domains.

Surface and interface effects are crucial in determining PL efficiency. Non-radiative recombination at surfaces and grain boundaries is a major loss mechanism, particularly in polycrystalline films. Chemical passivation strategies, such as treatment with Lewis bases or halide salts, have been shown to reduce trap densities and enhance PL quantum yields. For instance, treatment of MAPbI3 films with pyridine derivatives can increase the PL intensity by an order of magnitude, with corresponding improvements in carrier lifetimes. Similarly, encapsulation with hydrophobic layers can mitigate environmental degradation, preserving PL stability under ambient conditions.

In conclusion, the photoluminescence properties of perovskite semiconductors are governed by a complex interplay of phase stability, ion migration, and halide segregation. Both hybrid and all-inorganic perovskites exhibit rich PL behavior that reflects their underlying structural and electronic dynamics. Advances in material processing, defect passivation, and environmental protection have led to significant improvements in PL efficiency and stability, paving the way for further optimization of these materials for optoelectronic applications.