Perovskite semiconductors have emerged as a fascinating class of materials due to their unique excitonic properties, which are governed by their structural and electronic characteristics. The excitonic behavior in perovskites is influenced by factors such as dimensionality, crystal structure, and temperature, making them a rich platform for studying fundamental light-matter interactions. This article explores the exciton binding energies, Mott transitions, and biexciton formation in perovskites, with a focus on comparing three-dimensional (3D) and two-dimensional (2D) variants. Additionally, temperature-dependent exciton dynamics are discussed to provide a comprehensive understanding of their behavior.

Excitons in perovskites are Coulomb-bound electron-hole pairs, and their properties are largely determined by the dielectric environment and carrier effective masses. In 3D perovskites, such as methylammonium lead iodide (MAPbI3), the exciton binding energy typically ranges between 10 to 50 meV due to the high dielectric constant and relatively small effective masses of charge carriers. This low binding energy results in weakly bound excitons that can easily dissociate into free carriers at room temperature. In contrast, 2D perovskites, which consist of layered structures separated by organic spacers, exhibit significantly higher exciton binding energies, often in the range of 100 to 400 meV. The increased binding energy arises from quantum and dielectric confinement effects, which enhance the Coulomb interaction between electrons and holes.

The dimensionality of perovskites also plays a critical role in exciton stability and Mott transitions. In 3D perovskites, the screening of Coulomb interactions by the high dielectric constant reduces the exciton binding energy, facilitating the formation of free carriers even at low densities. This makes 3D perovskites more susceptible to Mott transitions, where excitons dissociate into an electron-hole plasma at high carrier densities or elevated temperatures. The critical density for Mott transitions in 3D perovskites is typically around 10^17 to 10^18 cm^-3. On the other hand, 2D perovskites require much higher carrier densities, often exceeding 10^19 cm^-3, to achieve a Mott transition due to their larger exciton binding energies. The stronger confinement in 2D systems also leads to more stable excitonic states, making them less prone to dissociation under similar conditions.

Biexcitons, which are bound states of two excitons, exhibit distinct behavior in 3D and 2D perovskites. In 3D systems, biexciton binding energies are relatively small, usually less than 10 meV, due to the weak Coulomb interaction between excitons. This results in biexcitons that are stable only at low temperatures or high excitation densities. In 2D perovskites, however, biexciton binding energies can reach 20 to 50 meV, owing to the enhanced Coulomb interaction in confined geometries. The formation of biexcitons in 2D perovskites is more favorable, and they can persist at higher temperatures compared to their 3D counterparts. The stability of biexcitons in 2D systems has implications for nonlinear optical applications, where strong exciton-exciton interactions are desirable.

Temperature-dependent exciton dynamics further highlight the differences between 3D and 2D perovskites. In 3D perovskites, exciton dissociation dominates at elevated temperatures due to thermal energy overcoming the weak binding energy. This leads to a rapid decrease in exciton population as temperature increases, with excitons becoming virtually absent above 200 K in many cases. In contrast, 2D perovskites maintain a robust exciton population even at room temperature due to their higher binding energies. However, temperature-dependent broadening of exciton lines occurs in both systems, primarily due to electron-phonon interactions. The linewidth broadening follows a linear or superlinear trend with temperature, reflecting the coupling between excitons and optical phonons.

The exciton-phonon coupling strength also varies between 3D and 2D perovskites. In 3D systems, the coupling is relatively weak, leading to moderate linewidth broadening with temperature. In 2D perovskites, the coupling is stronger due to the confined geometry and reduced screening, resulting in more pronounced linewidth broadening. Additionally, the exciton dynamics in 2D perovskites are influenced by the organic spacers, which can introduce disorder and localized states. These effects lead to inhomogeneous broadening and complex exciton relaxation pathways, including trapping and detrapping processes.

The excitonic properties of perovskites are also affected by structural phase transitions. For example, 3D perovskites undergo tetragonal-to-cubic phase transitions at elevated temperatures, which can modify the exciton binding energy and dynamics. In 2D perovskites, the layered structure introduces additional complexity, with phase transitions involving changes in the orientation or conformation of organic spacers. These transitions can alter the dielectric environment and confinement potential, thereby influencing exciton behavior.

In summary, the excitonic properties of perovskites are highly dependent on their dimensionality, with 3D and 2D systems exhibiting distinct behaviors in terms of binding energies, Mott transitions, and biexciton formation. The weak exciton binding in 3D perovskites facilitates free carrier generation but limits exciton stability at high temperatures. In contrast, 2D perovskites offer stronger exciton confinement and higher binding energies, making them suitable for applications requiring stable excitonic states. Temperature-dependent studies reveal the role of electron-phonon interactions and structural phase transitions in shaping exciton dynamics. These fundamental insights into perovskite excitonics provide a foundation for exploring their potential in optoelectronic and quantum devices beyond photovoltaics.