A DFT study on the stability and optoelectronic properties of Pb/Sn/Ge-based MA2B(SCN)2I2 perovskites

Abstract

Inorganic–organic perovskites have been considered as one type of promising photoelectric material. However, there are two aspects hindering their application in practice. One is the toxicity of metal lead and the other is the stability of the compound. Herein, the structure, stability, electronic properties, and optical properties of six novel perovskites, MA₂B(SCN)₂I₂ (B = Pb, Sn, Ge) and lead-less MA₂Pb_1−xSn_x(SCN)₂I₂ (x = 0.25, 0.50, 0.75), have been investigated employing the density functional theory method. The effects of the substitution of Sn/Ge for Pb and Sn-doping on the stability, electronic, and optical properties were also considered. The calculation of formation enthalpy and ab initio molecular dynamics simulations show that the six studied compounds possess good thermodynamic stability. The band structure, partial density of states, Bader charge analysis, electron localization function, and carrier's effective mass were simulated for evaluating the electronic properties of the six perovskites. The metal tin completely or partially replaces metal Pb of MA₂Pb(SCN)₂I₂ perovskites, reduces the band gap of the system and improves carrier's transport ability. The dielectric constants ε(ω), optical absorption coefficient α(ω), energy loss L(ω), refractivity index n(ω) and reflectivity R(ω) were determined for estimating the optical properties of the studied compounds. The introduction of metal tin also enhances the optical absorption range and optical absorption intensity in the UV-vis region. Ge-based perovskite has very similar electronic and optical properties to the MA₂Pb(SCN)₂I₂ compound, but is lead-free. Therefore, among the six studied perovskites, Sn-based systems could be ideal materials for use in the absorption layer of single junction perovskite solar cells, while the MA₂Ge(SCN)₂I₂ compound may also be a good substitute for the MA₂Pb(SCN)₂I₂ compound because it is lead-free with good stability. We hope this work can provide a theoretical foundation for the development of high-performance and lead-free/partially lead free perovskites.