Role of Ag1+ substitutional defects on the electronic and optical properties of n-type CdS thin films semiconductor for sustainable and stable window layer in solar cells technology

Cadmium sulfide (CdS) has been investigated extensively for II generation solar cell as an efficient window layer with n type conductivity. The significance of CdS is because of its large energy band gap tune-ability and native n-type conductivity. In this report, energy band gap engineering of CdS films was carried out by means of controlled silver (Ag) doping to modify their optical properties along with electrical properties. Closed space sublimation (CSS) technique has been employed to synthesize as-deposited CdS thin films. One of the advantages of CSS grown films is roughness which offers more trapping/absorbing of light. Ion exchange solution process was carried out for Ag-doping, which resulted in widening of the band gap from 2.15 eV to a value of 2.45 eV, showing the increased capability of solar energy harvesting. Un-doped CdS thin films showed a high resistivity of the order of ∼106 Ω-cm, which reduced to only ∼102 Ω-cm with the Ag-doping, as probed by four-point probe, offering great advantage for the system to be used in II-VI solar cells. Un-doped and Ag-doped films showed n-type conductivity. We studied the Ag-doping effects on the surface morphological and correlated the morphological modification with changing optical and electrical behaviour of CdS thin films. It is believed that enhanced properties offer great potential for these films to be used for the solar cell applications.