| Kesterite copper zinc tin sulfide (Cu2ZnSnS4, CZTS) has been confirmed as an important photovoltaic material for scalable production of thin-film solar cells due to its earth-abundant and non-toxic elements, and an optimal direct band gap with the predicted theoretical maximum efficiency of 32.4%. As a result, it has attracted worldwide attention and the remarkable progress has been made by several groups over the past few years, using different fabrication approaches. To date, the reported highest power conversion efficiencies for the pure sulfide Cu2ZnSnS4, pure selenide Cu2ZnSnSe4 and mixed sulfo-selenide Cu2ZnSn(S, Se)4 have reached 8.4%,11.6%, and 12.6%, respectively, which however is lower than that of the theoretical limit. Key factors on CZTS are how to reduce its production cost, meanwhile, improve the power conversion conversion efficiency. Electrodeposition approach for preparing CZTS thin films has received a great deal of attention in recent years because of its simplicity, low equipment cost, cheap raw materials and room temperature growth. Furthermore, there is no need to use toxic solvents or ligands like hydrazine. This work focuses on investigating the electrodeposition mechanism for Cu-Zn-Sn-S precursors, electrolyte formula, electrodeposifion process parameters, and effects of the sulfidizing process on the properties of CZTS thin films and thus the performance of the as-resulted solar cells.To date, high-efficiency CZTS solar cells have been reported where a high-resistivity CdS thin film deposited by the traditional chemical bath deposition (CBD) is always employed as a buffer layer. However, the major drawback is that the CBD-CdS buffer layer typically produces a large amount of toxic Cd containing waste. In addition, ammonium hydroxide is usually utilized during the deposition process, which is highly volatile and toxic to human health. Besides, the volatility of ammonia changes the pH of the deposition bath solution and influences the performance of the CdS buffer layer. Moreover, a redundant treatment in a wastewater process greatly increases the cost. In this regard, we have demonstrated radio-frequency sputtering to process a CdS layer, which is suitable to reduce the environmental impact and can enhance the continuity of the deposition processes so that thin film solar cells can be manufactured by an in-line process for massive production. Here we developed a simple, facile, green and low-cost electrodeposition method for the fabrication of a high-quality CZTS absorber layer with control over the composition and pure phase. To the best of our knowledge, the efficiency of 7.23% is the highest value reported to date for CZTS solar cells prepared by the co-electrodeposition approach. The innovative research results obtained are as follows:1. CZTS absorber layers have been successfully deposited on tin-doped indium oxide coated glass (ITO/glass) substrates by sulfurization process of co-electrodepositied Cu-Zn-Sn-S precursor thin films at various annealing temperatures ranging from 500 to 580℃ for 30 min in an atmosphere of Ar/H2S (6.5%) or sulfur. The effects of sulfurization temperature on the structure, morphology, composition and optical property of CZTS thin films have been investigated.The results clearly show that the structure, morphology and optical properties of CZTS thin films depend mainly on the sulfurization temperature. XRD and Raman measurements reveal that the intensity of preferential orientation along the (112) direction becomes relatively more intense and sharp with increasing annealing temperature. It is found that the optimized sulfurization temperature is at 560℃, the precursor thin film after sulfurization forms a single phase CZTS with Cu-poor and Zn-rich composition, exhibits large densely packed grains with compact and faceted morphology, and its band gap is about 1.50 eV. Then an AZO/i-ZnO/CdS/CZTS/ITO/glass solar cell with about 2% efficiency has been obtained using the CZTS absorber sulfurized at 560℃.2. CZTS thin films have been successfully deposited on Mo-coated glass substrates by the rapid thermal processing sulfurization of co-electroplated Cu-Zn-Sn-S precursor thin films. Effects of single-deposition potential and double-deposition potential on the properties of CZTS films and CZTS solar cells were investigated. CZTS thin films have been prepared via a double-deposition potential process for the first time. The effects of double-deposition potential and single-deposition potential process on the structural and morphological properties of CZTS films have been studied by XRD, Raman, SEM and EDS measurements. It is found that double-deposition potential process benefits the formation of CZTS films with a relatively flat surface, full sulfurization and a thin MOS2 film compared with single-deposition potential process. Moreover, the J-V characteristics indicate that solar cell with a conversion efficiency of 3.68% can be obtained using CZTS absorber deposited by double-deposition potential process.3. We have systematically studied the influence of deposition time on the properties of CZTS thin films and the performance of the resulting solar cell devices. CZTS thin films with thicknesses ranging from 0.35 to 1.85 μm and micron-sized grains (0.5-1.5 μm) were synthesized using co-electrodeposited Cu-Zn-Sn-S precursors with different deposition times.It is found that the increase in deposition time from 5 to 30 min can quasi-linearly increase the thickness and micron-sized grain, significantly improves the crystallinity of the absorber, greatly reduces the series resistance, and consequently improves the device efficiency greatly by the boost of Jsc and Voc. However, the further increase of the deposition time to 40 min, the excessive increase in the thickness of the CZTS absorber leads to some degradation in different device parameters. It is necessary to control the electrodeposition time to ensure CZTS absorber with large grain size and appropriate film thickness.Here we have firstly introduced a sputtered CdS buffer layer for the development of CZTS solar cells, which enables breakthrough efficiencies up to 6.6%.4. CZTS thin films with fine control over composition and pure phase were fabricated by sulfurization of co-electroplated Cu-Zn-Sn-S precursors. Here we introduce a novel sputtered CdS buffer layer for the CZTS solar cells. For the first time, co-electrodeposited CZTS solar cells exceed the 7% efficiency threshold. We have systematically investigated the concentration of Cu2+ ion can influence the properties of CZTS absorber layers and the photovoltaic performance of resulting solar cell devices.Our results indicate that increasing Cu2+ concentration almost linearly increases the Cu content in the final CZTS thin films, greatly enhances the (112) preferred orientation, significantly improves the crystallinity of the absorber layer, remarkably reduces the ZnS secondary phase, and hence improves their photovoltaic performance. However, upon further increase the Cu2+ concentration degrades the crystal quality of the absorber layer, and forms the CuxS secondary phase, which is quite detrimental to device photovoltaic performance. Here we have introduced an innovative sputtered CdS buffer layer for CZTS solar cells that shows similar functionality level as a CBD-CdS buffer layer. This work not only opens up a very important and promising route to preparing high-efficiency CZTS solar cells but also provides a critical pathway to reduce the environmental impact and enhance the continuity of the solar modules.5. A simple and cost-effective co-electrodeposition process has been demonstrated to fabricate high-performance CZTS photovoltaic materials with composition tunability and phase controllability. Here we have investigated that controlling the Zn2+ concentration of the precursor solutions used to fabricate the CZTS thin films has a significant effect on some key properties of the absorber layers of our solar cell devices. These results suggest that increasing the concentration of Zn2+ linearly increases the Zn content in final composition of CZTS thin films, significantly improves the grain size and morphology of the absorber layers, and consequently improves their photovoltaic properties, especially the response to the medium wavelength. In contrast, upon further increase in the Zn2+ concentration degrades the crystal quality of the absorber layer, the more ZnS phase appears at the surface of CZTS thin film, and forms a rather rough morphology, which is harmful to the photovoltaic performance of the device. When the concentration of Zn2+ is optimized to 30 mM, a power conversion efficiency of 7.23% is achieved, which, to the best of our knowledge, is the highest efficiency for a co-electrodeposited CZTS solar cell with a sputtered CdS buffer layer to date. These findings offer a better understanding of the growth process of the CZTS absorber layers, which are critical for the precise control and adjustment of the composition, phase, structure and morphologies to facilitate high-efficiency CZTS solar cells. |
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