Chalcogenide perovskites have emerged as lead-free, stable photovoltaic materials, having promising optoelectronic properties. However, a detailed theoretical study on excitonic properties is rather demanding task due to the huge computational cost and, therefore, is hitherto unknown. Here, we report the excitonic properties of chalcogenide perovskites AZrS 3 (A = Ca, Sr, Ba) using state-of-the-art hybrid density functional theory and many-body perturbation theory (within the framework of GW and BSE). We find the exciton binding energy (E B ) is larger than that of conventional halide perovskites. We also observe, by computing the electron−phonon coupling parameters, a more stable chargeseparated polaronic state as compared to that of the bound exciton. The ionic contribution to dielectric screening is found to be negligible in this class of materials. On the basis of the direct band gap and the absorption coefficient, the estimated spectroscopic limited maximum efficiency is quite good when these materials are considered as promising environmentally friendly perovskites suitable for photovoltaics.

The lead-free double perovskite material (viz. Cs2AgBiCl6) has emerged as an efficient and environmentally friendly alternative to lead halide perovskites. To make Cs2AgBiCl6 optically active in the visible region of solar spectrum, band gap engineering approach has been undertaken. Using Cs2AgBiCl6 as a host, band gap and optical properties of Cs2AgBiCl6 have been modulated by alloying with M(I), M(II), and M(III) cations at Ag-/Bi-sites. Here, we have employed density functional theory (DFT) with suitable exchange-correlation functionals in light of spin–orbit coupling (SOC) to determine the stability, band gap and optical properties of different compositions, that are obtained on Ag–Cl and Bi–Cl sublattices mixing. On analyzing 64 combinations within Cs2AgBiCl6, we have identified 19 promising configurations having band gap sensitive to solar cell applications. The most suitable configurations with Ge(II) and Sn(II) substitutions have spectroscopic limited maximum efficiency (SLME) of 32.08% and 30.91%, respectively, which are apt for solar cell absorber.

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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 (EB) accurately. Therefore, we have employed Wannier–Mott and DFPT approaches to compute EB, 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.

Ruddlesden−Popper (RP) phases of Ba n+1 Zr n S 3n+1 are an evolving class of chalcogenide perovskites in the field of optoelectronics, especially in solar cells. However, detailed studies regarding its optical, excitonic, polaronic, and transport properties are hitherto unknown. Here, we have explored the excitonic and polaronic effect using several firstprinciples based methodologies under the framework of Many Body Perturbation Theory. Unlike its bulk counterpart, the optical and excitonic anisotropy are observed in Ba n+1 Zr n S 3n+1 (n = 1−3) RP phases. As per the Wannier−Mott approach, the ionic contribution to the dielectric constant is important, but it gets decreased on increasing n in Ba n+1 Zr n S 3n+1 . The exciton binding energy is found to be dependent on the presence of large electron−phonon coupling. We further observed maximum charge carrier mobility in the Ba 2 ZrS 4 phase. As per our analysis, the optical phonon modes are observed to dominate the acoustic phonon modes, leading to a decrease in polaron mobility on increasing n in Ba n+1 Zr n S 3n+1 (n = 1−3).