Study of Kesterite Cu2ZnSnS4 Thin Films prepared from Cu2ZnSnS4 Nanoparticle Inks

Due to the recent rapid industrial and population growth, there has been an increase in the demand for energy which has led to a strong reliance on non-renewable energy sources which has negative environmental effects. Due to its unlimited sources and absence of environmentally harmful green house gas emissions, solar energy has emerged as an effective candidate for renewable energy. Cu2ZnSnS4 (CZTS) has been widely investigated as an absorber for thin film solar cells. It can replace Cu(In, Ga)Se2 (CIGS), and CdTe since it consists of non-toxic, abundant, and cheap elements. Its high absorption coefficient, optimal band gap, and naturally abundant non-toxic elemental constituents give it several advantages over most thin film absorber materials. However, the present performance of CZTS solar cells is still below their theoretical limit. Recently, there has been a rise in interest in improving the performance and lowering the production cost of solar cells based on CZTS. In this study, CZTS nanocrystals were synthesized by one of the non-vacuum techniques: the CZTS absorber is synthesized by using the hot injection technique, and it is responsible for synthesizing the crystalline layer that performs as the solar cell device absorber. CZTS nanoparticles were synthesized using a chemical synthesis process, with sulphur being injected into a solution of hot metallic precursor ions. It was revealed that the reaction temperature and time greatly influenced their composition, structure, and optical properties. In this research, a method was proposed to fabricate nonstoichiometric CZTS thin films. Using soda lime glass and molybdenum substrates, the CZTS thin films were deposited on the different substrates by drop casting and spin coating methods, followed by preheating in air and post-deposition annealing treatments, in two different atmospheres, H2S and N2. The CZTS thin films were annealed at different annealing temperatures. The morphology, composition, structure, and optical properties of the CZTS thin films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, and UV-vis spectroscopy. The influence of the annealing on the morphology, composition, and structure of the films has been studied. From the XRD patterns of the CZTS thin films, the peaks of all CZTS thin films are indexed to tetragonal CZTS (PDF 026-0575) and can be attributed to the (112), (220), and (312) planes of kesterite CZTS, respectively. The Raman spectra of the samples give evidence that in all spectra the dominant peaks are located at 330–338 cm−1 , indicating that these samples contain peaks associated with the Cu2ZnSnS4 phase. A significant influence on the energy bandgap of CZTS thin films was observed during the annealing of thin films. In addition, the fabrication of CZTS/CdS heterojunctions was explored to gain a better understanding of the properties of the interface reactions that take place between the Mo/CZTS and CZTS/CdS layers. the effect of high annealing temperature on a Mo/CZTS/CdS heterojunction was carried out at temperatures ranging from 450°C to 550°C at an annealing time of one hour in H2S and N2 atmosphere . Annealing promotes cadmium diffusion from the n-type semiconductor to the absorber. According to various studies, Cd diffusion improved the performance of thin-film solar cells. However, at annealing temperatures of 500 and 550 °C, the presence of a Cu-related secondary phase was observed, which is detrimental because it introduces shunting routes that lower the devices’ FF due to its metallic nature and low resistivity profile. In addition, Mo combines with S at the CZTS/Mo interface to form MoS2, which leads to losses of Voc, Jsc, and FF in CZTS devices. The findings presented here indicate that there is significant interdiffusion of the elements from the different layers in a Mo/CZTS/CdS heterojunction, which occurs at the temperatures commonly used to fabricate CZTS devices. Further improvements in CZTS devices will require strategies to control this interdiffusion.