First-principles optimization of thermoelectric and optoelectronic performance of KSn1−xZnxI3 (x = 0, 0.25, 0.5, 0.75, 1) perovskites for sustainable energy harvesting

Abstract

In the pursuit of non-toxic and high-efficiency perovskite solar cell materials, this study investigates the enhancement of thermoelectric and optoelectronic properties of Zn-doped KSn_1−xZn_xI₃ (x = 0, 0.25, 0.5, 0.75, 1) perovskites. The study uses first-principles density functional theory (DFT) with the Vienna Ab initio Simulation Package (VASP). Structural analysis confirms a transition from orthorhombic (Pnma) to monoclinic (Pm) phases. All the compositions exhibit thermodynamic, mechanical, and dynamic stability. Electronic properties reveal a robust bandgap range of 1.47–1.96 eV (GGA-PBEsol) and 2.34–3.02 eV (HSE06), positioning these materials as promising candidates for the top cell in a tandem solar cell and UV-optoelectronics. An indirect-to-direct band structure transition occurs at 50% Zn doping, which primarily enhances the stiffness, Pugh's ratio (2.39–2.70), and Poisson's (0.316–0.335) ratio of the lattice for KSn_1−xZn_xI₃. The elastic modulus (E), shear modulus (G), and bulk modulus (B) in KSn_1−xZn_xI₃ also significantly increased upon addition of Zn in the compound. These behaviors indicate that although there is better lattice stiffness in the material, there is still very good ductility for making flexible devices. Near-perfect mechanical isotropy has been achieved in KZnI₃ with a universal elastic anisotropy factor (A^U) of only 0.15. This low level of anisotropic elastic behavior indicates that KZnI₃ is unlikely to experience micro- fracture during or after manufacturing. Thermoelectric analysis shows that KSnI₃ maintained a high Seebeck coefficient of 230 µV K⁻¹ at low temperature, while KZnI₃ showed a 225 µV K⁻¹ Seebeck coefficient at elevated temperature. A high figure of merit (ZT) is achieved by both pristine compounds at high temperature, with values of 1.01 for KSnI₃ and 1.27 for KZnI₃. Furthermore, for optical properties, a high absorption coefficient of 7.32 × 10⁵ cm⁻¹ is observed by 25% Zn doping at UV-visible range. These findings make Zn-doped KSnI₃ perovskite material suitable for efficient, non-toxic, low-cost optoelectronic and thermoelectric devices.