Cu2Zn(Sn:Ge)S and Cu2Zn(Sn:Si)S4 absorbent thin films production and characterization

Photovoltaics are devices that convert solar energy directly into electrical energy. Thanks to this transformation, solar cell studies have accelerated, but the high cost of electricity generation produced by solar cells has increased the use of thin film semiconductors. Thin films are films of a few μm thickness by arranging the atom or molecule separately on the surface to be coated. Recently, compared to chalcogen thin films, CZTS thin films are preferred because they are abundant in nature, do not contain toxic materials, have a direct band gap of 1.4-1.5 eV and a high absorption coefficient of 104 cm-1. The aim of this study is to examine the effect of new elements experimentally and theoretically in detail while producing CZTS thin films (especially instead of Sn). In this study, the purpose of replacing Sn atoms with Si and Ge atoms is to reduce the production cost and increase the absorption of light in Cu2Zn(Sn:Si)S4 and Cu2Zn(Sn:Ge)S4. In the study, Cu2Zn(Sn:Si)S4 and Cu2Zn(Sn:Ge)S4 films were formed on glasses by using Spin coating method in order to use Cu2Zn(Sn:Si)S4 and Cu2Zn(Sn:Ge)S4 thin films more effectively in PV systems. The changes in the structural, morphological and optical properties of Cu2Zn(Sn:Si)S4 and Cu2Zn(Sn:Ge)S4 formed with different substitution ratios (x=0, 0.25, 0.50, 0.75 and 1) and different H2S (30 and 40 ccm) ratios were investigated depending on the production conditions. The optical properties of the formed thin films were investigated by ultraviolet and visible light absorption spectroscopy (UV-VIS), their structural properties were investigated by X-ray diffraction (XRD) and Raman spectroscopy, and their morphological properties were investigated by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). In the study, it were determined that the crystal size of the films decreased from 32.15 and 33.53 nm to 15.67 and 22.82 nm with the substitution of Ge into the CZTS structure, after more Ge substitutions, it increased up to 36.39 and 28.34 nm for Cu2ZnGeS4 thin films, and also the dislocation density and micro-strain values of Cu2ZnGeS4 thin films were inverse. The Raman spectrum of the samples proved the formation of blue-shifted questerite structures by adding Ge content to the structures. SEM images showed that the film surfaces annealed under 30 ccm Ar:H2S flux rate were more homogeneous than the film surface annealed under 40 ccm Ar:H2S flux rate. Findings from SEM images were confirmed with AFM images. It was observed that the optical band gap of CZTS thin films increased from 1.53 eV to 2.05 eV with Ge content. Substitution of Si into the CZTS structure resulted in a shift of the peak from 28.52° to 28.61° at the main peak in the XRD analysis and showed that the crystal size of the center sharply decreased and increased both the dislocation density and strain values. In addition, the absence of secondary phases was observed with Raman spectrum. In the SEM images, it was observed that the Cu2ZnSnS4 and Cu2ZnSnS4 crystals were uniformly distributed along the surface of the thin film samples. In addition, it was determined that there was much more and small agglomeration on the surface of the Cu2ZnSiS4 sample compared to the Cu2ZnSnS4 surface. UV-vis measurements revealed an increase in optical band gap from 1.51 to 3.22 eV with the increase of Si content in the CZTS structure. The results showed that the optical bandgap of CZTS thin films can be changed by Si substitution into the structure without acquiring any secondary phase. As a result of the study, it has been shown that Si and Ge substitution can be used in various applications that require higher band gap semiconductors compared to conventional CZTS structures.