Role of defects in MAPbI3 to modulate optical absorption, and solar efficiency
We address the role of Sn-substitution and Pb-vacancy (Pb-⊠) in reducing the Pb concentration and enhancing the stability of mixed perovskite CH3NH3Pb1-X-YSnX⊠YI3 using state-of-the-art hybrid density functional theory. The role of spin orbit coupling (SOC) and electron self-interaction error are examined carefully. Despite SOC has a significant role in artificially shifting the electronic bands, it doesn't affect the relative hierarchy of formation energies of different defected configurations. However, electron self-interaction plays an important role in determining the same. We find a semi-local functional (e.g. PBE) gives completely opposite trend to determine thermodynamic stability w.r.t HSE06+SOC. We obtain, to reduce the Pb-content from pristine MAPbI3, Sn-substitution has a favorable thermodynamic stability than Pb-⊠. This has been inferred from the lower formation energy of (charged) Sn substitution in the Pb8I24 framework as compared to a (charged) Pb-⊠ vacancy creation. For n-type and p-type host, the Sn substitution is more preferable than Pb-⊠ formation.
Further, the optical properties and spectroscopic limited maximum efficiency (SLME) for thermodynamically stable conﬁgurations (Sn-substitutions) are also calculated. We have achieved highest efficiency of 33% for MA8Pb7Sn1I24, and for other cases upto 50% Sn, the efficiency is reasonably good as compared to pristine. The stability of the doped perovskites are also confirmed at higher temperatures using ab initio Molecular Dynamics (AIMD) simulation. The lower formation energy, structural stability, direct p-p/s-p transition, high optical absorption coefficient (red shifted), stability at operational temperature and maximum solar cell efficiency of the CH3NH3Pb1−XSnXI3 (0 ≤ X ≤ 0.5) makes it a promising candidate for highly efficient solar cell absorber.
Journal Reference: Phys. Rev. B 101, 054108 (2020).