Deciphering the Role of Strain Effect in the Evolution of the Electronic Structure in Organic Lead Halide Perovskites

Abstract

Organic lead halide perovskite (OLHP) materials have made notable improvements in their optical and electronic properties, expanding their spectrum of possible applications. However, their long-term stability and performance are often compromised by ion migration within the perovskite structure. One effective solution to address this issue is the stabilization of the perovskite structure through compositional engineering. Nevertheless, such modifications also alter certain perovskite functionalities. The present study delves into the intricate chemical and electronic structure properties of A-site and X-site substituted OLHPs, APbX 3 (where A = methylammonium, MA+; and formamidinium, FA+; X = Cl and Br) single crystals using X-ray photoemission spectroscopy in conjunction with UV-vis spectroscopy. The study reveals that the cationic size difference of MA+ and FA+ ions and the electronegativity of Br<overline> and Cl<overline> influence the binding energy peak shifts due to the insertion of strain effects in the PbX 6 octahedral network. The Elliott model fitting of the absorbance spectra yields an exciton binding energy and a continuum energy state at the conduction band edge that marks the onset of the band gap energy and extends to the higher energies within the conduction band. Higher exciton binding energy is recorded in MA-based systems as compared to FA. A closer look at the Pb 4f core level spectra shows the emergence of metallic Pb0 doublets with increased X-ray exposure, which is significantly suppressed in FA-based systems, pointing to their increased stability toward X-ray irradiation. These vital insights into the electronic structures of APbX 3 pave the way for the optimization of perovskite materials, ultimately guiding the design of optimal structures from an applications perspective.