Magneto-Optical Studies of The Exciton Fine Structure in Two-Dimensional Perovskites

Two-dimensional (2D) lead halide perovskites are a very interesting group of novel semiconducting materials, considered an alternative for applications in photovoltaics and optoelectronics. Their structure can be seen as the “natural” quantum wells, where slabs of metal-halide octahedral units are surrounded from both sides by large organic cations, acting as potential barriers. As a consequence of the quantum and dielectric confinement, the exciton binding energy can reach several hundreds of millielectronvolts, which results in significant splitting of states of the exciton fine structure. This makes them attractive objects for the investigation of exciton physics, since all excitonic effects are greatly enhanced in this system. Significant state spacing and whether or not the lowest excitonic state interacts with photons are also crucial aspects affecting the performance of a device based on 2D perovskites. Gaining a deeper understanding of the exciton fine structure is therefore very important from the point of view of potential applications.<br/>We combine the optical spectroscopy techniques with the use of magnetic field to investigate the exciton fine structure of perovskite compounds with the general formula (PEA)2(MA)n-1PbnI3n+1, where n=1,2,3,4 denotes the number of octahedra layers within a slab. For the thinnest quantum wells (n=1) we reveal the full exciton fine structure, including the optically inactive dark state. We also observe the bright state oriented out-of-plane of the crystal, located above the in-plane states - contrary to the theoretical predictions. For the first time in 2D perovskites, we observe the lower lying, brightened dark exciton state in the Faraday configuration. Knowing the energies of all four states of the exciton fine structure, we were able to quantify the bright-dark state splitting and the dark exciton g-factor along the c-axis of the crystal, which allowed us to estimate the values of the electron and hole g-factors along this direction [1,2].<br/>Further magnetooptical spectroscopy studies on the perovskite compounds with n=2, 3 and 4 provide valuable information about the evolution of the properties of 2D perovskites with the change of confinement strength, as well as a solid base for further studies of the band structure and excitons in lead halide perovskites.<br/>[1] K. Posmyk et al., Journal of Physical Chemistry Letters 13, 4463-4469 (2022)<br/>[2] K. Posmyk et al., Advanced Optical Materials, 2300877 (2023)