Organic and inorganic metal halide perovskites have emerged in recent years as revolutionary semiconductor materials for lighting and energy harvesting applications. The unprecedented success of organic-inorganic lead halide perovskites results from their unique combination of optoelectronic properties. Namely, they exhibit strong broadband absorption, together with long carrier lifetime and diffusion length, which makes them almost ideal materials for energy harvesting.

These materials, characterized by ABX3 structure, seem to defy conventional wisdom concerning semiconductors. Despite their preparation with room temperature wet chemistry (essentially bucket chemistry), which is prone to result in a high defect concentration, these materials are almost immune to the influence of defects. In contrast to conventional semiconductors, their ionic crystal lattice has comparatively low energy phonon modes (which cannot be described within the widely used harmonic approximation) and low elastic constant, which sometimes allows one to consider perovskites as crystalline liquids with a dynamic lattice disorder rather than hard semiconductor material. In general this set of properties is not associated with high optoelectronic quality. Remarkably, in perovskites the mixed crystalline/liquid/glassy behavior is inherent and even crucial in explaining their outstanding performance. Therefore, metal-halide perovskite form a most peculiar system with an unexpected and unprecedented synergy of mechanical and optoelectronic properties.

Giant fine structure splitting in a bulk MAPbBr3 Single Crystal

In an ideally pure semiconductor, the lowest energy electronic excitation is a bound electron–hole pair (exciton). The exchange interaction between electron and the hole spins lifts the degeneracy between dark singlet and bright multiplet excitonic states producing a fine structure.

The pattern of excitonic states produced by the exchange interaction strongly depends on the symmetry of the system. In structures with sufficiently low symmetry, the degeneracy of the bright states should be completely lifted. However, in general, it is expected that the bright exciton FSS is much smaller than the bright–dark state splitting, which is typically a few tens of microelectronvolts in common inorganic semiconductors. Because the symmetry breaking is generally also small, the bright exciton FSS has never been observed in any bulk semiconductor.

We have observed the bright exciton FSS in a high quality bulk MAPbBr3 single crystal, in which all the extrinsic sources of symmetry breaking and quantum confinement enhancement can be specifically excluded. Using magneto-optical spectroscopy, we observe the FSS of the exciton 1s transition with a splitting as large as 200 μeV (see Fig. 1).

FIG. 1  (a) σ1 and σ2 components of the reflectance spectrum of a bulk MAPbBr3 single crystal measured at zero and 7 T. (b) Reflectance spectrum at 1 T divided by the zero-field. (c) Shift of the σ1 and σ2 components as a function of magnetic field. (d) Zeeman splitting and diamagnetic shift (e) Comparison of the reflectance shift with the PL spectra feature assigned to free exciton emission.

For more details please see Baranowski et al. Nano Letters 19, 7054 (2019)