First principle studies of the lead-free all-inorganic halide double perovskites Cs₃InX₆ (X = Cl, Br and I)

The optoelectronic character, photovoltaic performance, structural and thermodynamic stability of halide double perovskites (HDPs) Cs 3 InX 6 (X [Formula: see text] Cl/Br/I) are investigated using first-principles calculations based on density functional theory (DFT) for possible optoelectronic applications. In the structure composition of Cs 3 InX 6 HDPs, eight Cs clusters occupy cubic octahedral cavities within the CsInI 6 framework, where Cs[Formula: see text] and In[Formula: see text] formed corner-sharing CsX 6 and InX 6 octahedra. The thermodynamic stability of these materials is confirmed by their positive entropy change ([Formula: see text]S), as a positive [Formula: see text]S results in a negative change in Gibbs free energy. The specific heat capacity curves comply with the Dulong–Petit law and satisfy the equipartition theorem of classical mechanics. These HDPs exhibit a direct bandgap nature, with the conduction band (CB) and valence band (VB) edges located at the [Formula: see text] symmetry point. The spin–orbit coupling (SOC) effect typically shifts the CB and VB edges toward the Fermi level, leading to a reduction in the band gaps. The top of the VB is primarily composed of strongly hybridized p orbitals of In and X, which are crucial for carrier transport properties due to their role in facilitating valence electron excitation into the CB with minimal photon energy. The optical band gaps of Cs 3 InI 6 , Cs 3 InBr 6 and Cs 3 InCl 6 are 1.80, 3.05 and 4.10[Formula: see text]eV, respectively, which lie within the ultraviolet (UV) and visible regions of the optical spectrum. The calculated power conversion efficiency (PCE) of Cs 3 InI 6 is 13.92%, with a quantum efficiency (QE) of [Formula: see text]90% in the UV energy range (50–250[Formula: see text]nm). The calculated optoelectronic properties and stability metrics of the studied HDPs offer valuable insights into their potential applications in photovoltaic and light-emitting devices due to their suitable band gap and higher quantum efficiency.