Performance Optimization Of CZTSe Thin Film Solar Cells Fabricated By Magnetron Sputtering

The thin film solar cells have attracted wide attention due to their advantages of high efficiency,flexibility and low cost.Among them,CZTSSe thin film solar cells are considered to be one of the most promising photovoltaic materials because of its abundant raw materials storage,safe and non-toxic,high theoretical photoelectric efficiency and light absorption coefficient.The magnetron sputtering method is more suitable for large-area industrial production due to its has uniform flatness and can be programmed of the operation process.In laboratory research,the photoelectric conversion efficiency of CZTSSe thin film solar cells prepared by magnetron sputtering has reached 12.3%.Due to the serious open circuit voltage loss of CZTSSe thin film solar cells,the photoelectric conversion efficiency is low and cannot meet the requirements of practical applications.The crystal quality of the CZTSSe absorbers,the level matching degree of the interface and the defect type and concentration of the material all have crucial influences on the performances of the devices.The CZTSe absorber layer is the core component of the solar cells,and its crystal quality and optical bandgap directly determine the photoelectric properties of the device.On the one hand,the Cu_Znn anti-position deep defect can be suppressed effectively in the absorption layer,when the film are copper-poor and zinc-rich（Cu/（Zn+Sn）≈0.8,Zn/Sn≈1.2）.If the grain size is too small,the number of grain boundaries will increase.This leads to an increase in the recombination probability for the electron-hole pair at the grain boundary,an increase in the recombination loss,and then a serious loss in the open circuit voltage.In addition,the crystalization quality of the film also affects the contact with the molybdenum back electrode.For example,the presence of a thick layer of small grains at the bottom of the light absorbing layer also causes severe recombination loss.On the other hand,the bandgap of CZTSe material is about 1.0 eV,while 1.5 eV is the ideal bandgap for photovoltaic materials.Therefore,for the development of efficient CZTSe devices,except optimizing the high quality CZTSe absorption layer,it is also very important to find a new method to effectively regulate the bandgap of CZTSe devices,which can improve the open-circuit voltage of CZTSe devices.Aimed to resolve the above problems,this paper takes magnetron sputtering as the basic method,and a series of researches was carried out for the purpose of preparing high quality CZTSe films and optimizing their performance.Firstly,the basic process of preparing metal precursor and the subsequent post-selenization was studied.The photoelectric conversion efficiency of the device reached 7.8%.Based this,the antimony element was doped into the CZTSe absorption layer by Se and Sb₂Se₃ co-selenization,which improved the crystal quality of CZTSe film and then increased the open circuit voltage of the device.Finally a photoelectric conversion efficiency of 9.6%was achieved.The S/（S+Se）ratio was adjusted by the low-temperature vulcanization method to optimize the bandgap of the absorption layer,thereby further improving the open circuit voltage and photoelectric conversion efficiency of the device.The main research contents are as follows:1.Optimization the preparing process of CZTSe thin film solar cells.The CZTSe absorption layer was prepared by magnetron sputtering the copper,tin,zinc pure metal target followed by post-selenization.It was found that the film grain was small and there was a thick small particle layer,and the device efficiency was only 3.7%.We use a two-step selenization process to improve its performance.The results showed that the selenium powder had entered the metal pre-formed layer during the alloying process,which not only promoted the fusion between the single-layer metals,but also inhibited the generation of internal defects and compensated for the deficiency of selenium during the post-selenization process.The grain was enlarged after the annealing treatment in the high temperature stage.At the same time,it reduced the thickness of the small grain layer.In the end,we obtained an efficiency of 7.8%based on the optimized CZTSe absorber.2.Influences of Se and Sb co-selenization on the performance of CZTSe thin film solar cells.We discussed the properties of CZTSe films when the Cu/Sn/Zn presursor under the co-annealing conditions of Se and Sb₂Se₃.It was found that when the high temperature at 560°C,the grain size of the film was larger and the device performance was better.Then,the effects of different amounts of Sb₂Se₃ on the phase structure,crystal quality,antimony content distribution and device properties of CZTSe absorber layer films under the same annealing conditions were investigated.The results showed that the structure and bandgap of the material were not significantly changed and it can effectively reduce the thickness of the small grain layer and promote grain growth when the Sb₂Se₃ was 30 mg.Therefore,the film quality of CZTSe was much better and the cell device efficiency was up to 9.6%.3.Effect of low temperature vulcanization on the performance of CZTSSe thin film solar cells.By introducing sulfur directly into the low temperature annealing process,the S/（S+Se）ratio can be accurately adjusted,the bandgap of the absorption layer can be optimized,and the open circuit voltage of the device can be increased.The structure,film crystal quality and device properties of CZTSSe materials were analyzed by different S/（S+Se）ratio.It was found that the XRD diffraction angle shifted to a large angle and the crystal quality was slightly poor when the sulfur content increases.However,the bandgap increased and the open circuit voltage could increase to about 480 mV.