Importance of disorder and lattice anharmonicity on the exciton fine structure and exciton-phonon coupling in halide perovskite nanocrystals for light emission

Halide perovskite nanocrystals are a promising route to efficient light emitting devices, and more recently as potential single photon emitters. Their radiative recombinations are governed by excitons. The bright or dark character of the ground state is debated. A Rashba effect correlated with a local polarization was proposed in 2018 as a general mechanism for inversion of bright and dark level ordering. But direct spectroscopic signatures of the dark exciton emission in the low-temperature photoluminescence of single FAPbBr3 and CsPbI3 nanocrystals under magnetic fields rather yield a dark singlet below the bright triplet. The dark-bright splitting values are in fair agreement with numerical estimations of the long-range electron–hole exchange interaction. The intense luminescence of perovskite nanocrystals is alternatively attributed to a reduced bright-to-dark phonon-assisted relaxation. The phonon side bands in nanocrystals match neutron scattering phonon spectroscopy results obtained at low temperature. More, a recent classification of 3D perovskites into ferroelectric or hyperferroelectric materials, points toward the role of the depolarization effect for the reduction of a spontaneous polarization in nanocrystals. The role of lattice disorder and the strong anharmonicity of the lattice is however far from being well-understood.[1] P. Tamarat, J.-B. Trebbia, M. I. Bodnarchuk, R. Erni, M. V. Kovalenko, J. Even & B. Lounis, The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state. Nature Materials 18, 717–724 (2019).[2] P. Tamarat, L. Hou, J.-B. Trebbia, A. Swarnkar, L. Biadala, Y. Louyer, M. I. Bodnarchuk, M. V. Kovalenko, J. Even, B. Lounis, The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals, Nature Comm. 11, 6001 (2020)[3] A. Ferreira, S. Paofai, A. Létoublon, J. Ollivier, S. Raymond, B. Hehlen, B. Rufflé, S. Cordier, C. Katan, J. Even & P. Bourges, Direct evidence of weakly dispersed and strongly anharmonic optical phonons in hybrid perovskites, Comm. Phys. 3, 48 (2020).[4] G. Roma, A. Marronnier & J. Even, From latent ferroelectricity to hyperferroelectricity in alkali lead halide perovskites, Phys Rev. Mat. 4, 092402(R) (2020).