Enhancement of photovoltaic and thermoelectric properties of A-site doped A3−xA′xIO (A/A′=K, Rb, Cs) anti-perovskites with room temperature phonon stabilization: a DFT and molecular dynamics study

Abstract

In this work, alkali-metal A-site-doped A_3−xA′_xIO (A/A′ = K, Rb, Cs) anti-perovskites are systematically investigated using Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations. The structural, mechanical, thermal, electronic, optical, and thermoelectric properties are analyzed using DFT, while MD & Ab initio MD simulations are employed to examine anharmonic lattice stabilization and thermal stability, respectively, at room temperature. The effect of larger alkali-metal substitution is analyzed for three pristine cubic anti-perovskites (K₃IO, Rb₃IO, and Cs₃IO), and for 33.33% and 66.67% A-site doping in six tetragonal anti-perovskites (K₂CsIO, KCs₂IO, K₂RbIO, KRb₂IO, Rb₂CsIO and RbCs₂IO). A-site doping results in bandgap narrowing and bowing, with reduced direct band gaps in the ranges 0.33–0.61 eV (GGA-PBE), 0.53–0.74 eV (GGA-RPBE), and 1.41–1.67 eV (HSE06). It leads to enhanced optical absorption (0.4–0.5 × 10⁵ cm⁻¹) in the visible spectrum region. Notably, in terms of thermoelectric efficiency, KRb₂IO and K₂CsIO exhibit the highest ZT values of 0.5–0.6, at their melting point (T_m ∼490 K). At higher temperatures, K₂CsIO achieves the highest PF of 7 × 10¹¹ W K⁻² m⁻¹ s⁻¹. All compounds are mechanically and thermodynamically stable. At 0 K, Cs₃IO, KRb₂IO and RbCs₂IO show negative phonon modes, indicating dynamical instability. However, at 300 K, MD simulations stabilize all phonon modes through anharmonic phonon renormalization, which is further validated by the Dulong–Petit limit. The doped phases also show improved Shockley–Queisser limit efficiency, ductility and machinability. These results establish A-site-doped non-toxic anti-perovskites as promising candidates for photovoltaic and thermoelectric applications, while also demonstrating MD to be an effective method for assessing room temperature dynamic stabilization.