Magnetron Sputtering Sulfide Target To Prepare Copper-zinc-tin-sulfur (Cu ₂ ZnSnS ₄ ) Thin Film Solar Cells

The thin film solar cells with optical absorber layer composed of the copper-zinc-tin-sulphide-based（Cu₂ZnSnS₄（CZTS）） compound has attracted much attention as a promising alternative for light absorber materials, because it meets two necessary conditions for efficient solar cells, one is the direct nature of band gap and the other is its width within a certain optimal range（1.45¹.5 eV） for photovoltaic cells. Moreover, the consists of CZTS elements are earth abundant with low cost and environmentally friendly. In this paper, the CZTS thin films were fabricated by sulfurization of sputtered precursors using ZnS, Sn and CuS targets, and the CZTS thin film solar cells were preparated. The csystal structual, surface morphology,chemical composition, purity of phase, electrical properties and optical properties were characterized by field emission scanning electron microscopy（FE-SEM）, X-ray diffraction（XRD）, X-ray energy dispersive spectrometer（EDS）, Raman spectroscopy,Hall system and Ultraviolet-visible spectrophotometer（UV-vis）. Finally, the performance of fabricated CZTS thin film solar cells was measured by I-V.The main work and the analysed results are as follows:（1） The precursors with different stacking sequences of ZnS-SnS-Cu were deposited by magnetron sputtering. Then the precursors were thermally treated at low temperature and followed by sulfurization at high temperature in sulfur vapor. The quality of thin films which was fabricated by different stacking sequences was investigated. It has been found the precursor films with the stacking sequence of Glass/Mo/ ZnS/ SnS/ Cu and Glass/Mo/ ZnS/ Cu/ SnS showed good adhesion after the thermal treatment at low temperature, but a part of the CZTS thin film peeled off after sulfurization at high temperature. Whereas a part of the precursor films with the stacking sequences of Glass/Mo/ Cu/ ZnS/ SnS and Glass/Mo/ Cu/ ZnS/ SnS peeled off after the thermal trement at low temperature, and most of them peeled off after sulfurization. Besides, the CZTS thin film which was fabricated by the stacking sequence of Substrate/ SnS/ ZnS/ Cu and Substrate/ SnS/ Cu/ ZnS peeled off completely after the thermal treatment at low temperature. This was probably becausethe SnS and Cu have high thermal expansion coefficient, which was not matching with the thermal expansion coefficient of Mo layer well. So the peeled off after the thermal treatment at low temperature or sulfurization at high temperature when SnS layer or Cu layer was on the bottom.（2） Using ZnS,Sn and CuS targets, the precursors with different stacking sequences were deposited by magnetron sputtering. In the same way, the precursors were thermally treated at low temperature and followed by slufurization at high temperature in sulfur vapor. The quality of thin films which was fabricated by different stacking sequences and different sulfurization conditions were investigated.It has been found that all different stacking sequences have good adhesion, none of them peeled off after the thermal treatment at low temperature and later sulfurization at high temperature. Besides, the quality of CZTS thin film which was prepared by the stacking sequences of ZnS/Sn/CuS is bettert than others. Furthermore, the surface of CZTS thin films which was sulfurized at 580℃ is smooth and dense, and has big grain size without other second phases.（3） The CZTS thin film solar cells were fabricated according to the structure of SLG/ Mo/p-CZTS/n-CdS/i-ZnO/n-ITO/Al. and the performances of solar cells was investigated through comparing the CZTS thin films which were fabricated by different conditions. The studying found that the best CZTS solar cells with power conversion efficiency of 3.59 % has been achieved using the stacking sequence of Glass/ Mo/ ZnS/ Sn/ CuS. Nevertheless, the CZTS thin film solar cell with the worst performance power conversion efficiency of 2.57 % was obtained using the stacking sequence of Glass/Mo/CuS/ZnS/Sn. Maybe this is because the different stacking sequences had a great influence on element distribution during the precursors sulfurization at high temperature, thus affecting the quality of the CZTS thin film and the performance of the CZTS thin-film solar cells.（4） The CdS buffer layer was prepared also by vacuum method, and applied it to CZTS thin film solar cells. Finally, the other process steps of the CZTS thin film solar cells were fabricated at the same conditions, and the properties of CZTS thin film solar cells were investigated. Though the same thickness of CdS buffer layer deposited by CBD and by magnetron sputtering, It has been found that the CZTS thinfilm solar cells with 3.59 % power convention efficiency using CdS buffer layer prepared by CBD is obviously better than that of CZTS solar cells with 3.19 %efficiency using CdS buffer layer prepared by magnetron sputtering. This maybe because some oxides on the CZTS film surface were etched by the reaction solution containing ammonia during chemical bath deposition CdS layer. On the contrary, the surface of CZTS absorber layer could be damged by the high energy particles duraing the process of sputtering CdS layer.As a result, the CdS prepared by CBD is better than that by sputtering as a buffer layer for the CZTS solar cells.