Predictive analysis of Cu and Ni substitution effects on the structural, optoelectronic and thermoelectric behavior of CdS: A first-principles approach

Abstract

Employing the Full-potential linearized augmented plane wave plus Hubbard U method (FLAPW + U), this study investigates the electronic and optical properties of pristine CdS and its Cu- and Ni-substitution variants. The results show that pure CdS exhibits a direct band gap ($E_g^{\Gamma -\Gamma }$) of 2.32 eV, confirming its nature as a direct band gap semiconductor. The analysis of the total and partial density of states (TDOS and PDOS) reveals the contributions of various electronic bands. In particular, the d/d orbitals of Cu and Ni were found to hybridize significantly with the p/d orbitals of S and Cd, especially near the Fermi level. This hybridization leads to enhanced p/d and d/d charge transfers between Cu/Ni and S/Cd atoms, causing a transition from a direct band gap semiconductor to a semi-metallic state with increasing Cu/Ni substitution levels, thus narrowing the band gap and significantly enhancing the conductivity of CdS. However, as noted, substituting with Cu/Ni causes a transition from a semiconductor to a semi-metallic state, leading to the closure of the band gap. The optical transitions from the valence band to the unoccupied d/d states of Cu/Ni are distinct from the typical conduction band to valence band transitions, but still enhance the optical absorption properties. This clarification resolves the apparent contradiction regarding the band gap. The presence of substituents further significantly improves optical transitions from the valence band to the un-occupied d/d states of Cu/Ni, offering an advantage over the native optical transitions in CdS. The presence of substituents further significantly improves optical transitions between the valence band and the un-occupied d/d states of Cu/Ni, offering an advantage over the native optical transitions in CdS. However, as the substituting levels increase, the material transitions from a semiconductor to a semi-metallic state, leading to a closure of the band gap, and the typical conduction band to valence band transitions are no longer present. Despite the closing of the band gap, the optical transitions involving the d/d states of Cu/Ni still enhance the optical absorption properties. The reflectivity reaches approximately 27 % in the high-energy region. Additionally, as Cu/Ni substituents levels increase, the absorption spectra shift towards the blue region, highlighting an enhancement in optical absorption within the visible range for CdS:Cu and CdS: Ni. The study employed BoltzTrap code analysis to examine the temperature-dependent properties of these materials. The study evaluated thermal and electrical conductivities, the Seebeck coefficient, and other relevant metrics. The first-principles calculation of the optical and thermoelectric characteristics paves the way for future experimentation with their application in renewable energy devices.