Hybrid organic-inorganic metal halides (HOIMH) are a large family of hybrid materials that consist of 3D, 2D, 1D, or 0D anionic metal halide framework and organic cations. The greater structural flexibility of the lower-dimensional HOIMH allows the incorporation of various inorganic structures (such as corner-, edge-, face-shared octahedra, tetrahedra, etc.) and organic cations [with different sizes and different type of bonding (σ or π bonding)]. Electronic structure calculations of a series of low-dimensional HOIMH show that the large compositional and structural space can be explored to engineer the band alignment at the interface. Excitons can be localized at either the organic or the inorganic component for specific optical applications (such as lighting and radiation detection).
The 0D HOIMH, in which the metal-halide anions are spatially separated by organic or inorganic cations, exhibit strong exciton self-trapping, which may lead to efficient exciton emission. Photoluminescence quantum efficiency (PLQE) higher than 90% at room temperature has been reported for several 0D HOIMH, including (C4N2H14X)4SnX6 (X = Br, I). On the other hand, inorganic Cs4PbBr6 exhibits UV emission, which is quenched at T > 100 K, and strong green emission, whose origin is still under debate. Hybrid density functional calculations on (C4N2H14X)4SnX6 (X = Br, I) and Cs4PbBr6 show large exciton binding energies and exothermic exciton trapping at deep halogen vacancy levels inside the band gap. The calculated excitation and emission energies of excitons are in good agreement with experimental values. Our results suggest that suppressing exciton migration and the subsequent energy loss at defects are critical for efficient luminescence. The thermally-activated exciton migration should be limited by the large exciton binding energy and the weak coupling between inorganic metal-halide octahedra, which are the luminescent centers. However, fast exciton migration may occur through the resonant exchange of the excitation energy provided that the conditions of the wavefunction overlap between luminescent centers and the spectral overlap between excitation and emission are both satisfied. These conditions can be prevented by incorporating large molecular cations in 0D HOIMH, which suppress the electronic coupling between luminescent centers and allow more extended exciton relaxation and consequently large Stokes shift that prevents the spectral overlap. Our results explain the high PLQE observed in (C4N2H14X)4SnX6 (X = Br, I) and the thermal quenching of luminescence in Cs4PbBr6. The frequently observed green luminescence in Cs4PbBr6 is likely the result of exciton emission from CsPbBr3 inclusions within the bulk of Cs4PbBr6.