Optimizing CdS thin films as optical windows in solar cells: A comparative study of cryogenic and classical techniques

In this study, CdS thin films were produced in a quasi-closed volume using two different techniques (classical and cryogenic thermal evaporation techniques) between the 100–573 K substrate temperature, and their characteristic properties (structural, electrical, and optical properties) were investigated. While CdS thin films were produced at 373 K, 473 K, and 573 K substrate temperatures in the classical technique (hot), they were produced at 100–300 K substrate temperature range with 50 K steps in the cryogenic technique (cold). The X-Ray Diffraction (XRD) analysis showed that the CdS thin films grew in a hexagonal structure in the (002) plane at all substrate temperatures. According to the field emission scanning electron microscope (FESEM) images, the thin films produced at 200 K substrate temperature consisted of equally sized spherical grains. This situation shows that the soliton growth mechanism occurs at a substrate temperature of 200 K during the film production process with the cryogenic technique. Due to the characteristic properties of the soliton waves occurring on the substrate surface in the soliton growth mechanism (mass transport), the films grow in a tight-packed form. Therefore, the produced films consist of clusters of equal size, providing a homogeneous surface and a uniform thickness. In addition, Atomic Force Microscope (AFM) and optical analyses showed that the CdS thin film produced at 200 K substrate temperature had the smallest average surface roughness value (Ra) and the highest optical transmittance value. It was found that the energy band gap (2.37–2.47 eV) and resistivity (1.25 × 10³–5.39 × 10³ Ω-cm) values ​​of CdS thin films increased with decreasing substrate temperature. The carrier density increased with decreasing substrate temperature (3.91 × 10¹⁷–1.73 × 10¹⁶ cm⁻³). Energy Dispersive Spectroscopy (EDS) analysis showed that the films grew stoichiometrically at substrate temperatures of 473 K and 200 K. The results brought out that in case of using of the produced CdS thin films at a substrate temperature of 200 K by the new cryogenic technique as an optical window layer could provide significant increases in efficiency in solar cells. In addition, ideal substrate temperature values ​​for CdS thin films that can be used in photodevice production were determined for both techniques.