Electron-phonon coupling in low-dimensional metal halide perovskites

Abstract

Metal halide perovskites (MHPs) are versatile, low-cost materials for optoelectronic applications like solar cells and LEDs, offering tunable architectures that enhance optical absorption and carrier mobility. Low-dimensional metal halide perovskites (LDMHPs) have specifically gained attention for their superior stability and molecular-level quantum confinement, where structural constraints create 2D layers, 1D chains, or 0D clusters of the inorganic framework. The "soft" lattice nature of LDMHPs facilitates rapid structural distortions upon excitation as the charge carriers of the quasiparticles called excitons, formed by bound electron-hole pairs through Coulomb forces, interact with lattice ions through phonons. This relationship is governed by electron-phonon coupling (EPC), which forms quasiparticles called polarons that significantly dictate optoelectronic properties such as photoluminescence (PL). Understanding the mechanisms and strength of EPC is therefore essential for developing high-efficiency materials, particularly for white light illumination sources capable of achieving substantial global energy savings. In this context, this thesis focuses on the study of EPC in LDMHPs, primarily concerning their optoelectronic properties, such as low-temperature PL. The first family of LDMHPs studied was the 0D halide perovskites A2SnBr6 (A = Cs, Rb). For Cs2SnBr6, the analysis revealed a strong EPC, as evidenced by multiphonon scattering, leading to intense overtones up to the fourth order. Such strength of the EPC was confirmed by the Huang–Rhys factor S = 24.4 from the PL phenomenon arising from self-trapped excitons (STE) with the contribution of phonon modes related to the Sn–Br asymmetric stretching predominating in the electronic transition. Furthermore, for Rb2SnBr6, temperature-dependent PL measurements, complemented by other characterization techniques and theoretical calculations, revealed broadband emission with a significant Stokes shift attributed STE state. The Fröhlich mechanism, mediated by interactions between excitonic charge carriers and longitudinal optical (LO) phonons, primarily accounted for the emission broadening through phonon-assisted radiative recombination. The EPC strength was evaluated through the Huang–Rhys factor S = 34, confirming strong correlations between electronic and vibrational properties. The possible mechanism of STE formation was evaluated by the Fröhlich parameter α of 2.78 for electrons and 4.41 for holes, which pointed out a major contribution of the hole-polaron quasiparticle on exciton trapping. Finally, the last family of LDMHPs investigated consisted of low-dimensional lead bromide perovskites with chiral spacers based on α-methylbenzylamine (MBA). It was found that the use of a racemic mixture of this organic molecule yielded a reduction of dimensionality from the 2D (R-/S-MBA)2PbBr4, obtained when using the enantiomers R- and S-MBA of the spacer, to the 1D (Rac-MBA)3PbBr5·H2O phase, which exhibited a different PL behavior compared to the 2D phases and two regimes of EPC with Huang–Rhys factors of S = 67 above 110 K and S = 189 below this threshold that were further correlated with the degree of octahedra distortions of the inorganic framework upon lowering temperature. The results obtained through this thesis pave the way for developing new and improved devices and technologies towards optoelectronic applications.