Influence of potassium doping on the structural, optical, and optoelectrical properties of ZnS thin films for photovoltaic applications

Abstract

Zinc sulfide (ZnS) is an important n-type semiconductor exhibiting remarkable electrical and optical properties. The present study used the nebulizer spray pyrolysis technique to produce undoped and potassium-doped ZnS thin films using an economical spray pyrolysis method at different potassium concentrations (2.5, 5, and 7.5 wt%). The XRD results indicate a hexagonal structure for ZnS and potassium-doped ZnS thin films. Examining the structural characteristics reveals that the crystallite size (D) of the ZnS and potassium-doped ZnS films was expanded as the potassium content was elevated from 2.5 to 7.5 wt%. The strain and dislocation density of the examined potassium-doped ZnS layers were diminished by augmenting the potassium concentration in the ZnS films. The linear optical parameters of the examined potassium-doped ZnS films were estimated by recording the reflectance and transmittance spectra in the wavelength 200–2500 nm. The refractive index values of the potassium-doped ZnS layers were enhanced by raising the potassium concentration in the studied samples. Moreover, the energy gap (E_g) calculations refer to the ZnS and potassium-doped ZnS films having direct optical transition, and the E_g values were reduced from 3.64 to 2.97 eV by the increase in the potassium concentration. The Wemple and DiDomenico model study shows that the dispersion energy and oscillator strength of the examined potassium-doped ZnS layers were boosted by raising the potassium concentration while the oscillator energy was dropped. The optoelectrical indices analysis displays the enhancement of the plasma frequency, optical mobility, optical carrier concentration, electrical conductivity, and optical dielectric constants while enlarging the potassium concentration. Meanwhile, the boost in potassium contents enhances the nonlinear absorption coefficient and nonlinear refractive index of the ZnS and potassium-doped ZnS films. The hot probe procedure refers to these samples as n-type semiconductors. The results indicated that these samples could be used as a new window layer for solar cells.