Exploring the potential of lead-free double perovskites K₂BBiBr₆ (B= Cu, Ag) for advanced optoelectronic and thermoelectric applications: A DFT study

In this study, we investigate the properties of double perovskite compounds K₂CuBiBr₆ and K₂CuBiBr₆ using density functional theory and semi-classical Boltzmann transport theory. Our goal is to identify materials optimized for optoelectronic and thermoelectric applications. The electronic property analysis shows that K₂CuBiBr₆ and K₂CuBiBr₆ exhibit semiconductor behavior, with indirect bandgap values of 1.73eV and 2.83eV, respectively. The compounds' energetic and structural stability is demonstrated by their tolerance factor values and negative formation and cohesive energies. Moreover, full compliance with the Born elastic stability criteria confirms their resilience to deformation. The optical properties, including dielectric function, absorption coefficient, reflectivity, and refractive index, reveal high absorption coefficients exceeding 1.7×10⁵ cm^-1 in the visible range and over 7×10⁵ cm^-1 in the near-UV region. Their low reflectivity, around 20% in the visible spectrum, results in a notable improvement in light absorption, thus significantly increasing the efficiency of light-to-electricity conversion, which is highly beneficial for photovoltaic cells. High Seebeck coefficients, electrical conductivity, and merit factor make these materials promising candidates for thermoelectric power generation and cooling applications.