Investigation of the role of oxygen in close-spaced sublimated CdS layer for enhancing the efficiency of CdS/CdTe thin film solar cells

Abstract

As an alternative to silicon solar cells, polycrystalline thin-film heterojunction solar cells stand out due to their promising efficiencies and low manufacturing cost. In CdS/CdTe thin film solar cells, cadmium telluride (CdTe) is employed as the absorber material and cadmium sulfide (CdS) is used as the window material. In this work, the close-spaced sublimation (CSS) method was used to deposit a CdS window layer. We have conducted a comprehensive study of oxygen incorporation into the CSS CdS window layer to enhance the efficiency of CSS CdS/CdTe thin film solar cells. UV- visible spectroscopy was used for the optical characterization of the CdS layer, and a solar simulator was employed to study the current-voltage characteristics of the CSS CdS/CdTe solar cell. The optimum bandgap of 2.44 eV was obtained for the CdS films incorporating 20% O2 with Ar ambient and the films thicknesses were maintained around 220 nm. Bandgap values tend to increase with the increase in O2 concentration. Also, according to the SEM and EDX analysis, 20% O2 concentration revealed the highest oxygen incorporation with a denser microstructure. Oxygen diffusion into the CdS crystal structure is expected to substantially affect the films' crystallite size, lattice strain, and dislocation densities. The highest average efficiency of 13.8% with an open-circuit voltage (VOC) of 717 mV, a short circuit current (JSC) of 34 mA/cm2, and a fill factor (FF) of 52% was obtained for the dot cells with an area of 0.12 cm2 , comprising CSS-deposited CdS layer incorporating 20% of oxygen under the AM 1.5 illumination.