First‐Principles Investigation of Structural, Electronic, Optical, Mechanical, and Thermoelectric Properties of Lead‐Free ASnI ₃ (A = K, Rb, Cs) Halide Perovskites

The structural, electronic, optical, mechanical, and thermoelectric properties of lead‐free halide perovskites ASnI 3 (A = K, Rb and Cs) were investigated using density functional theory (DFT) simulations within the Wien2k framework. Structural optimizations were performed using the Wu‐Cohen GGA (WC‐GGA) functional, while electronic properties were studied using the modified Becke–Johnson (mBJ) potential. The calculated Goldschmidt tolerance factors are equal to 0.803, 0.820, and 0.854 for KSnI 3 , RbSnI 3 , and CsSnI 3 , respectively, indicating structural compatibility with the perovskite framework. Calculations of band structures and density of states reveal that these materials exhibit semiconducting behavior with direct bandgaps of 0.20 eV for KSnI 3 , 0.23 eV for RbSnI 3 , and 0.15 eV for CsSnI 3 . The narrow direct R→R transition indicates a strong potential for photovoltaic applications. Optical, elastic, and thermoelectric properties were analyzed using the PBE‐GGA scheme. Optical investigation shows that the static dielectric constant ( ε 1 (0)) ranges from 10.38 to 10.95, the refractive index varies between 2.25 and 3.31, and strong absorption occurs at energies above 6.48 eV, signifying potential for UV‐visible optoelectronic applications. The calculated elastic constants satisfy the Born stability criteria, confirming that all investigated perovskites are mechanically stable. The compounds also exhibit ductile behavior, as indicated by B/G ratios of 2.09–2.36, Poisson's ratios of 0.29 and 0.31, and positive Cauchy pressures, highlighting their potential for practical applications. The Seebeck coefficient at room temperature reveals that KSnI 3 and RbSnI 3 are n‐type materials, whereas CsSnI 3 is p‐type. Overall, the theoretical findings align well with experimental reports, confirming the reliability and predictive value of the simulations for designing functional perovskites‐based devices. This study presents, for the first time, a detailed theoretical investigation of cubic ASnI 3 (A = K, Rb and Cs) halide perovskites, providing valuable insights into their role in optoelectronic and photovoltaic applications.