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Lead iodide perovskites have attracted considerable interest in the upcomingphotovoltaic technologies and optoelectronic devices. Therefore, an accuratetheoretical description of the electronic and optical properties especially tounderstand the excitonic effects in this class of materials is of scientificand practical interest. However, despite several theoretical researchendeavours in past, the most accurate analysis of the key electronic parametersfor solar cell performance, such as optical properties, effective mass, excitonbinding energy (E$_B$) and the radiative exciton lifetime are still largelyunknown. Here, we employ state-of-the-art first-principles based methodologiesviz. hybrid functional(HSE06) combined with spin-orbit coupling (SOC),many-body perturbation theory (GW, BSE), model-BSE (mBSE), Wannier-Mott (WM)and Density Functional Perturbation Theory (DFPT). By taking a prototypicalmodel system viz. APbI$_3$ (A = Formamidinium (FA), methylammonium (MA), andCs), an exhaustive analysis is presented on the theoretical understanding ofthe optical, electronic and excitonic properties. We show that tuning of exactexchange parameter ($\alpha$) in HSE06 calculations incorporating SOC, followedby single shot GW, and BSE play a pivotal role in obtaining a reliablepredictions for the experimental bandgap. We demonstrate that mBSE approachimproves the feature of optical spectra w.r.t experiments. Furthermore, WMapproach and ionic contribution to dielectric screening (below 16 meV)ameliorate the E$_B$. Our results reveal that the direct-indirect band gaptransition (Rashba splitting) may be a factor responsible for the reducedcharge carrier recombination rate in MAPbI$_3$ and FAPbI$_3$. The role ofcation ''A'' for procuring the long-lived exciton lifetime is well understood.This proposed methodology allows to design new materials with tailoredexcitonic properties.

Capturing Excitonic Effects in Lead Iodide Perovskites from Many-Body Perturbation Theory