Fabrication And Optimization Of Zn(S,O) Buffer Layer For Solution-Processed Cadmium-free CIGS Thin Film Solar Cells

The energy crisis and environmental problems caused by fossil energy are becoming more and more serious,and the development of clean energy is more and more important.In 2021,the strategic decision of"carbon peaking and carbon neutrality"has made clean energy a hot topic again.Among different clean energies,solar cells have been widely studied due to many advantages such as no pollution,no noise,low maintenance cost,and long service lifetime.The current efficiency of copper indium gallium selenide（CIGS）thin-film solar cells has approaching that of the most widely commercialized silicon-based solar cells.Compared with crystalline silicon solar cells,CIGS thin-film solar cells also have advantages including tunable band gap,high light absorption coefficient,and light weight.At present,high-efficiency CIGS thin film solar cells are prepared by vacuum method.Solution method does not require complex and expensive equipment,which is not only conducive to large-scale production but also suitable for flexible roll-to-roll production.However,the CIGS thin-film solar cells prepared by the solution method all use Cd S as the buffer layer,which has the risk of polluting the biological environment.In this thesis,the CIGS absorber is prepared by solution method,and the Zn（S,O）buffer layer is prepared by chemical bath deposition.The effects of water bath solution concentration,annealing method,and film thickness on the quality of the Zn（S,O）films deposited on CIGS absorber layers have been systematically studied,and a uniform and dense Zn（S,O）buffer layer has been obtained.The performance differences of the CIGS solar cells with Cd S and Zn（S,O）buffer layers have also been investigated.Further,（Zn,Mg）O has been fabricated to replace Zn O window layer and CIGS solar cells with four buffer/window combinations of Cd S/Zn O,Cd S/（Zn,Mg）O,Zn（S,O）/Zn O and Zn（S,O）/（Zn,Mg）O have been fabricated and the differences in their photovoltaic performance have been investigated.The main contents and conclusions of this thesis are as follows:（1）The effect of the chemical bath deposition conditions on the uniformity and density of Zn（S,O）buffer layer and device performance.The chemical reactions of Zn（S,O）and Cd S in water bath solution were analyzed,and the difference between Zn（S,O）and Cd S deposited by chemical bath was studied.The results show that the solubility product of Cd S is much smaller than that of Zn（S,O）,so the deposition efficiency of Zn（S,O）is lower than that of Cd S.A higher concentration of the water bath solution is required to obtain a uniform and dense Zn（S,O）buffer layer film.Because Zn（S,O）is inevitably mixed with hydroxide during the deposition process,which seriously affects the device performance,it is necessary to anneal the Zn（S,O）buffer layer to remove hydoroxide.By using the two-step annealing method with reduced deposition time,thermal stress during annealing can be prevented and film damage can be effectively suppressed,resulting in uniform and dense Zn（S,O）buffer layer and significantly improved device performance.A CIGS solar cell with an efficiency of 8.0%was obtained.（2）The effect of post light-treatment on the performance of CIGS solar cells with Zn（S,O）buffer layer.The effects of illumination times on the performance of Zn（S,O）devices were explored,the stability of the device performance gain by post-illumination treatment was studied,and the performance differences of Cd S and Zn（S,O）buffer layer CIGS solar cells were compared.The results show that a 60-min light post-treatment of device with Zn（S,O）buffer layer solar cells can reduce heterojunction interface defects and facilitates better energy band alignment,which significantly increases the device V_OC and FF and improves device efficiency to 11.6%.After further optimization of the absorber selenization condition,CIGS solar cell with an high efficiency of 12.0%has been obtained with a Zn（S,O）buffer layer.The wider band gap of Zn（S,O）achieves higher device than Cd S buffer layer.（3）The effect of wide bandgap（Zn,Mg）O window layer on the performance of CIGS solar cells.（Zn,Mg）O window layer was fabricated and optimized to relace Zn O layer and the performance of CIGS solar cells with four combinations of Cd S/Zn O,Cd S/（Zn,Mg）O,Zn（S,O）/Zn O and Zn（S,O）/（Zn,Mg）O buffer/window layers were compared.The optimal deposition rate of（Zn,Mg）O thin films was found to be 1.6nm/min with sputtering power of 80 W.Using 30 nm（Zn,Mg）O instead of Zn O as the window layer improves device Jsc for both Cd S and Zn（S,O）devices in varied levels.However,due to the interface problem between（Zn,Mg）O and Cd S,the efficiency of Cd S devices is not improved,while the energy bands of（Zn,Mg）O and Zn（S,O）are more suitable,and the efficiency of Zn（S,O）devices is improved.In this thesis,fabricatin of Zn（S,O）buffer layer via chemical bath deposition has been studied for high-efficiency solution-processed Cd-free CIGS thin film solar cells.CIGS solar cells with efficiency comparable to that using tranditional Cd S buffer layers are achievd.The results provide a realistic basis for the commercialization of solution-processed Cd-free CIGS thin-film solar cells.