Excitons in 2D Organic–Inorganic Halide Perovskites

Abstract

Two-dimensional layered perovskites (2DLPs) are solution-processed semiconductors that form natural quantum wells with high exciton binding energies. Excitonic properties can be tuned in multiple ways, including: electronic bandgap, exciton binding energy, and exciton lifetime. Exciton dynamics in 2DLPs are complex due to the hybrid organic–inorganic nature of the polarizable, relatively soft lattice. Spectroscopic signatures indicate strong exciton–lattice interactions through the formation of polarons, which are electronic excitations dressed in the surrounding deformation of the polar lattice. Exciton coupling to lattice motion in the form of local reorganization or optical and acoustic phonons is not yet fully understood. Layered perovskites are hybrid 2D materials, formed through the self-assembly of inorganic lead halide networks separated by organic ammonium cation layers. In these natural quantum-well structures, quantum and dielectric confinement lead to strongly bound excitonic states that depend sensitively on the material composition. In this article, we review current understanding of exciton photophysics in layered perovskites and highlight the many ways in which their excitonic properties can be tuned. In particular, we focus on the coupling of exciton dynamics to lattice motion and local distortions of the soft and deformable hybrid lattice. These effects lead to complex excited-state dynamics, presenting new opportunities for design of optoelectronic materials and exploration of fundamental photophysics in quantum confined systems. Layered perovskites are hybrid 2D materials, formed through the self-assembly of inorganic lead halide networks separated by organic ammonium cation layers. In these natural quantum-well structures, quantum and dielectric confinement lead to strongly bound excitonic states that depend sensitively on the material composition. In this article, we review current understanding of exciton photophysics in layered perovskites and highlight the many ways in which their excitonic properties can be tuned. In particular, we focus on the coupling of exciton dynamics to lattice motion and local distortions of the soft and deformable hybrid lattice. These effects lead to complex excited-state dynamics, presenting new opportunities for design of optoelectronic materials and exploration of fundamental photophysics in quantum confined systems. in a semiconductor, the energy difference between an electron in the highest energy level of the valence band and the lowest energy level of the conduction band; also referred to as the quasiparticle gap, Eg. a change in the electronic bandgap energy of a semiconductor following photoexcitation. In 2D semiconductors, a high density of photogenerated carriers will screen repulsive Coulomb interactions between charge carriers of the same sign, leading to a net decrease in the electronic bandgap energy. minimum energy required to ionize a bound electron–hole pair from its lowest energy eigenstate into uncorrelated free charge carrier states. The exciton binding energy, Eb, is usually given a positive sign for net attractive interaction. a measure of the maximum probability density of the Coulomb interaction between an electron and hole in an exciton. This value can serve as a proxy for the size of the exciton in a given material. the splitting of excitonic states into multiple sublevels, characterized by their energetic spacing, degeneracy, oscillator strength, and spin characteristics. the extent to which an exciton is delocalized will determine whether it is of Frenkel or Wannier character, based on how the excitonic electron–hole Coulomb interaction is screened by its surrounding dielectric environment. In a high-dielectric material, the attractive Coulomb interaction is well-screened and the exciton is Wannier-like. In this case the exciton has a large Bohr radius and delocalizes over many molecules or unit cells. However, in a low-dielectric material, the electron and hole are tightly bound because the Coulomb interaction is poorly screened and the exciton is Frenkel-like. The exciton Bohr radius is small and the exciton is highly localized to a single molecule or unit cell. a metric of the fluorescence efficiency of a material, equivalent to the number of photons emitted divided by the number of photons absorbed. a charge carrier that has created its own potential well via structural deformation of the surrounding lattice. A large polaron extends over multiple unit cells of the structural lattice and behaves like a free charge carrier, but with a heavier effective mass and reduced scattering. A small polaron is localized to a single structural site and moves by thermally activated site-to-site hopping. in a material whose dimensions are smaller than its exciton Bohr radius, an excitation will be confined in space, causing its energy levels to be quantized (i.e., discrete). a material constant that determines the magnitude by which spin-polarized bands are offset from the zone center (k = 0) in noncentrosymmetric compounds exhibiting strong spin-orbit coupling. similar to a small polaron, a self-trapped exciton is an electron–hole pair that has become localized to a single lattice site through displacement of nearby ions from their equilibrium positions. interaction between an electron’s spin and its orbital angular momentum that breaks state degeneracy. For heavy atoms such as lead, this relativistic effect is significant.