Effect of bandgap tunability on the physical attributes of potassium-based K2CuBiX6 (X = I, Br, Cl) double perovskites for green technologies

[Display omitted] •The structural, elastic, electronic, transport, and optical properties of lead-free double perovskites K2CuBiX6 (where X can be I, Br, or Cl) have been thoroughly examined through first-principles calculations.•The electronic band structure shows an increment in the energy gap after the replacement of Iodine with Bromine and Chlorine, which is also endorced by the density of states (DOS). An optical band gap with large absorption in the visible and UV region reveal the potential of K2CuBiX6 (X = I, Br, Cl) for the applications in optoelectronics.•The perovskites exhibit substantial potential for renewable energy applications, as they demonstrate significant ZT values, which suggest their availability for thermoelectric devices. Potassium-based perovskites hold significant potential to revolutionize renewable technology by enabling more cost-effective and sustainable energy devices. The DFT incorporated in the Wein2K interface is used to provide an extensive investigation of the structural, mechanical, electrical, optical, and thermoelectric behavior of double perovskites K2CuBiX6 (X = I, Br, Cl). Structural stability is confirmed through optimization curves, tolerance factor and octahedral tilting. Mechanical properties support the ductile character of K2CuBiX6 (X = I, Br, Cl). Electronic band structures reveal that perovskite K2CuBiI6 initially possesses a bandgap of 0.6 eV. This bandgap is increased to 0.97 eV when (I) atom is replaced with (Br) atom. Furthermore, a prominent shift in the bandgap value occurs when the bromine (Br) atom is replaced with a chlorine (Cl) atom, resulting in a bandgap of 1.3 eV. The Kramer-Kronig equations are used to evaluate the optical characteristic, which shows significant absorption in the visible range for all structures, allowing them to be utilized in optoelectronics. Numerous thermoelectric characteristics are calculated using the Boltzmann semi-classical theory. The perovskites exhibit substantial potential for renewable energy applications, as they demonstrate significant ZT values, along with elevated Seebeck coefficients and electrical conductivities, which enhance their suitability for utilization in clean energy technologies.