An exciton, a quasi-particle consisting of an electron and a hole bound by Coulomb interaction, represents the lowest electronic excitation in a perfect semiconductor. The exchange interaction, which couples the spins of the electron and hole, affects excitonic states, leading to fine structure splitting. This interaction lifts the degeneracy of states with different angular momenta, separating the bright and dark exciton states. The energy separation and ordering of exciton states can significantly impact the optoelectronic properties of materials. The emerging field of two-dimensional organic-inorganic halide perovskites (2DP) offers a novel platform to explore exciton fine structure splitting (FSS) and its potential applications. In these materials, quantum and dielectric confinement enhance the Coulomb interaction, resulting in a much larger FSS compared to conventional low-dimensional systems. The splitting of excitonic states in 2DP can reach tens of meV, which is orders of magnitude greater than in epitaxial structures or nanocrystals. This large splitting, combined with the excellent optical properties of 2DP and the ease of engineering their band structure and quantum confinement, makes them an ideal system for studying exciton FSS physics.

Here, I present our recent findings, revealing a complete spectrum of excitonic states within its fine structure and available knobs to tune exchange interaction. Furthermore, I demonstrate that the excitonic properties of 2D perovskites are significantly influenced by carrier-lattice interactions, resulting in a complex interplay between exciton fine structure and phonons that profoundly affects their optical response. I will focus on the phonon bottleneck effect between bright and dark exciton states. Despite the substantial splitting between these states, which can reach tens of meV, 2D perovskites exhibit surprisingly intense photoluminescence emission even at cryogenic temperatures, indicating a non-Boltzmann distribution of excitons. However, the reason for this high bright-state occupation has remained unclear. Using magneto-optical spectroscopy, I will show that the exciton population is characterized by a higher temperature than the crystal lattice. To explain this observation, we employed detailed microscopic and material-specific many-particle theory to investigate the formation, relaxation, and decay dynamics of excitons. Our modelling reveals that the energy mismatch between the exciton fine structure and phonons leads to a pronounced phonon bottleneck effect, highlighting the importance of exciton fine structure and carrier-phonon interaction in the optical response of metal halide perovskites.