Development of CZTS thin film for fibrous solar cells via magnetron sputtering

Abstract

With the development of miniaturization and portability of electronic devices, especially the popularity of wearable devices, it challenges the wearable power technology. Flexible solar energy power system is one of the most reasonable solutions. Correspondingly, concept of fiber-shaped solar cells has been put forward and attracted numerous attentions in recently years. Cu2ZnSnS4 (CZTS) as a semiconductor with earth-abundant elements, shows promising potentials in application of thin-film solar cells, and has attracted numerous attentions due to its excellent properties including an ideal bandgap of 1.5 eV, high absorption coefficient, low cost, and non-toxicity. Still, there is no scientific research on preparing CZTS-based fibrous solar cells. Therefore, the current project aims to develop and investigate CZTS-based fibrous thin-film solar cell via magnetron sputtering technology. Firstly, CZTS thin films, as absorb layer of solar cells, were deposited onto soda lime glass, and explored by varying the deposition argon flow rate. By characterization of chemical composition, it is found the argon flow rate can affect the stoichiometry of the obtained CZTS precursors, and the final CZTS thin film show a stoichiometry of 2.1: 1: 1.25: 4.1 towards Cu:Zn:Sn:S which is near to the theoretical one (2:1:1:4). Meanwhile, it is confirmed that the film deposited at an optimized parameter of argon flow rate with 300 sccm showed greatest crystallinity, morphologies, few secondary phases, as well as a bandgap of 1.48 eV. Based on the effect exploration of argon flow rates, CZTS thin films were further deposited via magnetron sputtering with varied deposition powers. Similar to the argon flow rate, deposition power also contributed to the formation of CZTS precursors with different elemental ratios. The prepared CZTS thin films were characterized with respect to the chemical composition evaluation, morphology analysis, crystallinity investigation, secondary phases detection, and optical performance confirmation. As a result, the final CZTS thin film deposited at following optimized parameters - base pressure of 5×10-4 Pa, argon flow rate of 300 sccm, and deposition power of 200 W, achieves a stoichiometry of 1.9: 1: 0.96: 3.99, and possesses good crystallinity, few secondary phases, and an Eg value of 1.52 eV. Subsequently, the effect of the sulfurization annealing process was investigated by varying dwelling temperature. The effects of element ratio on the quality of CZTS thin films with the varied sulfurization temperature were focused on, and optimized. In detail, the effects of elemental ratios on morphologies, crystallinity, phase changes, variation of bandgap, as well as electrical performance were deeply analyzed. And the final obtained CZTS thin film sulfurized at 500°C achieves good performance with an Eg of 1.48 eV, carrier concentration of 6.72×1018 cm3, mobility of 4.105 cm2/v·s, and resistivity of 2.26×10-3 Ω·m. Besides, it also confirms that the Eg could be adjusted by controlling the amount of S, loss of Sn, and annealing temperature. Then, the effects of heating rate on the properties of CZTS thin films were explored. Here, the secondary phases and disorder degree of CZTS thin films were investigated as key points, which will further affect the performance of assembled devices. And new characterization and analysis methods were introduced to characterize the prepared films via combination of Raman mapping, peak fitting, and calculation. The CZTS thin film sulfurized at final confirmed conditions (500°C with a heating rate of 20°C/min) shows the lowest disorder degree and excellent crystallinity. Before preparing CZTS based thin-film solar cells on stainless-steel filament, the feasibility was firstly explored by using stainless-steel foil as a substrate to assemble solar cells. In the current stage, sodium incorporation strategies of CZTS films were carried out to improve the performance of final devices. It is well noted that the bifacial incorporation method is more helpful to prepare CZTS thin films with good uniformity, enlarged crystal size, as well as good device performance. This work is helpful for the next stage of fabricating CZTS-based fibrous thin-film solar cells. Finally, CZTS-based fibrous thin-film solar cells were successfully prepared on stainless-steel filament through magnetron sputtering technology. A pre-oxidation process was put forward to prevent the sulfurization of stainless-steel substrate, meanwhile, the bifacial sodium incorporation method was also applied in the fabrication process. The results indicate that the pre-oxidation process can effectively prevent the reaction between stainless-steel filament and sulfur during the sulfurization process, and the final obtained fibrous devices show the best performance with a conversion efficiency of 1.59%, an FF of 40.1%, Voc of 348 mV, and Jsc of 11.5 mA/cm2. The fabrication and investigation of CZTS-based fibrous thin-film solar cells provide a potential candidate for further use in smart garments and power for portable and wearable electronic devices in the future.