A theoretical perspective on the electronic and optical properties of layered halide perovskites: Symmetry analysis of quantum-well electronic structure and charge/energy transfer processes beyond the quantum-well picture

Layered halide perovskites are a class of semiconductors produced via chemical approaches and showing unique opto-electronic properties. Pioneered by Prof. David Mitzi in the 1990s, these materials have a natural quantum-well electronic structure (the band-gap of the inorganic frame is embedded into that of the organic spacers) and feature positive transport properties, effective visible-light absorption, narrow excitonic emission, etc. The full exploitation of these perovskites cannot go without deep understanding of their native electronic and optical properties, as accessible, via complex experimental set-ups and computationally expensive ab-initio calculations. However, interpretation of experimental and theoretical results can greatly benefit from basic “pencil-and-paper” symmetry analysis. Based on group-theory, we discuss how the change in the dimensionality influences the corresponding electronic and optical properties. In the direct band gap quantumwell scenario, we highlight how the breaking of the chemical connectivity influences the electronic structure, in terms of hybridization of the composing atoms (see Figure a). We also discuss the corresponding evolution of the optical properties, with focus on the lowest energy exciton fine structure. The present analysis will be then extended to the case of defective-halide perovskite frames, a class of systems recently reported in the literature. As an alternative scenario, substitution of commonly used saturated organic spacers with molecules featuring extended π-conjugation breaks the quantum-well picture. Indeed, the band-gap closing due to extended π- system can result in the (de)stabilization of the frontier orbitals of the spacer, compared to those of the inorganic frame, leading to the formation of a type II heterojunction at the organic/inorganic interface (see Figure b). We discuss the related charge/energy transfer processes on the basis of recently reported PbX (X=Cl,Br) layered perovskite frame incorporating tetrazine derivative as spacer. Featuring resonances both for the single particle states and excitonic states, these systems allow to discuss several diversion channels for the relaxation of carriers generated in the perovskite, in the detail.