<p>Quaternary compounds’ remarkable optical and electrical qualities have sparked a lot of attention, especially considering their possible use in solar energy conversion. This paper concentrated on photon interaction and the electronic transition nature in KA₂CuS₄ (A = Y, Nd) semiconductors. A greater cohesive energy in KNd₂CuS₄ results from Nd’s greater lattice stability as a lanthanide which has a larger atomic radius and stronger f-electron interaction than Y. The results from the band profile analysis confirm that these compounds have a direct band gap nature. Based on the density of states, the d orbitals of copper add up to the valence because of its relatively lower energy levels and contribution to bond formation and anti-bonding states. For possible use in optoelectronic applications, linear optical characteristics including complex dielectric function components, absorbance coefficients, energy loss functions, real light-dependent conductive properties, reflective properties, index (RI), and extinction coefficient were calculated and discussed. These compounds were suitable for thermoelectric applications, as shown by remarkable thermoelectric features. It is anticipated that the results will significantly influence the development of effective integrated electronic devices that are light-dependent and their useful applications.</p>