Excitons and the Rashba effect in 2D perovskites*

Halide perovskites exhibit remarkable excitonic properties. Radiative recombination in cube-shaped CsPbBr₃ NCs is extremely fast at low temperature, which was previously proposed to arise from a bright exciton ground state. Reducing the dimensionality has the potential to increase recombination rates even further, due to the large exciton binding energy and narrow photoluminescence (PL). Two approaches to dimensionality reduction are collodial nanoplatelets and 2D perovskites.

In the first part of the talk, the exciton fine structure in 2-8 layer Cs_n-1Pb_nBr_3n+1 NPLs is revealed by merging temperature- and time-resolved PL with effective mass modeling taking quantum confinement and dielectric confinement anisotropy into account. This approach exposes a thickness-dependent bright-dark exciton splitting up to 32.3 meV with a dark ground state. The model also reveals a 5-16 meV splitting of the bright exciton states with transition dipoles polarized parallel and perpendicular to the NPL surfaces, the order of which is reversed for the thinnest NPLs, as confirmed by time-resolved PL measurements. The derived model can be generalized for any isotropically or anisotropically confined nanostructure.

In part 2, we turn to two-dimensional hybrid organic–inorganic perovskites, in which a strong Rashba interaction creates a “Rashba exciton”, i.e., an exciton whose bulk dispersion reflects large spin–orbit Rashba terms in the conduction and valence bands and thus has minima at non-zero quasi-momenta. Placing Rashba excitons in quasi-2D cylindrical quantum dots, we calculate size-dependent levels of confined excitons and their oscillator transition strengths. We consider the implications of this model for light-emitting devices, discuss generalizations of this model to 3D NCs, and establish criteria under which a bright ground exciton state could be realized.