Theoretical Insights of Excitonic Effect in Lead Bromide Perovskites*

Exciton binding energy is an important factor in photovoltaics as the formation of excitons influences the charge separation in solar cells. However, a detailed theoretical study of excitonic properties is rather demanding due to huge computational cost. We have systematically applied several state-of-the-art advanced first-principles based methodologies, viz., hybrid density functional theory combined with Spin–Orbit Coupling (SOC), Many Body Perturabtion Theory (MBPT), model-BSE, Wannier–Mott, and Density Functional Perturbation Theory (DFPT) approaches, to understand the excitonic properties by taking a prototypical model system of lead bromide perovskites, viz., APbBr₃ [A=CH₃NH₃⁺ (MA), HC(NH₂)₂⁺ (FA), Cs⁺]. We show that via conventional procedure using GW/BSE approach along with SOC effect, it is very challenging to converge the BSE calculation to obtain the correct position of the excitonic peak to compute the exciton binding energy (E_B) accurately. Therefore, we have employed Wannier–Mott and DFPT approaches to compute E_B, where we find that the contribution of ionic dielectric screening is essential. In addition, we have calculated the exciton lifetime, which is in agreement with the trend observed (FAPbBr₃ > MAPbBr₃ > CsPbBr₃) for electron–phonon coupling. The role of cation “A” for achieving the long-lived exciton lifetime is also explained and well understood.