Selenization Optimization Of CZTSSe Solar Thin Film Prepared By Magnetron Sputtering

Among many thin-film solar cells,the commercialized Ga As and Cu（In,Ga）Se₂（CIGS）have efficiencies as high as 29.1% and 23.35%,respectively.However,the scarcity of Ga,In and the toxicity of As compounds limit their large-scale production and application.Therefore,it is urgent to find clean and pollution-free semiconductor materials with high photoelectric efficiency.The crystal structure of Cu₂ Zn Sn（S,Se）₄（CZTSSe）is similar to CIGS,and it is a direct band gap semiconductor material.Its constituent elements are non-toxic and rich in reserves,the optical absorption coefficient is up to 10⁴ cm^-1,the band gap is adjustable in the range of 1.0～1.5 e V,and the theoretical conversion efficiency is high,these characteristics make it become one of the most promising absorbing layer materials with great development potential for solar cell.At present,the highest photoelectric conversion efficiency of CZTSSe thin-film solar cells prepared in the laboratory is only 12.62%,which is a large gap from its theoretical conversion efficiency（32.2%）.The main reason restricting the improvement of device efficiency is the loss of open circuit voltage.The absorption layer of CZTSSe is the core component of the battery device.The quality,composition,and internal defects of the thin film all affect the open circuit voltage of the final device,thereby affecting the photoelectric conversion efficiency of the cell.CZTSSe material has a large variety of elements and a narrow phase stability region,so it is easy to generate secondary phases and defects during the film preparation.In addition,it is difficult to control the stability and activity of selenium saturated vapor pressure during the high temperature selenization process by the traditional rapid annealing selenization method.,which will lead to an insufficient selenization.The small particle layer between the absorption layer and the Mo back electrode layer will cause serious interfacial recombination and leakage current,which directly affects the composition uniformity and crystal quality of the CZTSSe absorption layer.Aimed to resolve these problems,this paper prepares the CZTSSe absorption layer film based on magnetron sputtering CZTS precursor layer after selenization.In order to improve the crystalline quality of the absorption layer film and increase the open circuit voltage of the cell,we carry out a series of exploration and optimization on the selenization method of the absorption layer.First,we explored the basic process of preparing CZTSSe absorbing layer films by quaternary single-target magnetron sputtering followed by selenization,and studied in detail the effects of sputtering power and selenium powder amount on the film quality;Then,Se was deposited on the top of the precursor layer by thermal evaporation to prepare the CZTS/Se precursor layer.The top layer of Se was used to simplify the growth of the CZTSSe absorber layer.The open-circuit voltage of the solar cell is effectively increased by adjusting the thickness of Se and the selenization temperature.In order to improve the reactivity of selenium and achieve the purpose of continuous supply selenium,a new self-made selenization device is used.The effects of cracking temperature and heating rate of the selenium source on the crystallinity of the absorption layer and its performance is explored.The main research contents and results of this thesis are as follows:1.CZTSSe thin film solar cell based on quaternary single-target magnetron sputtering.First,a flat and dense CZTS precursor layer was prepared by magnetron sputtering.Subsequently,the precursor layers were annealed in a selenium-free and selenium-containing atmosphere,respectively.The surface of the absorbing layer prepared under selenium-free conditions was loose and there were more porous holes at the bottom of the film.It shows that the selenium atmosphere is beneficial to further improve the crystallinity of the absorption layer film,thereby improving the performance of the device.Then,during the selenium atmosphere,the influence t of sputtering power on the CZTSSe thin film was investigated in detail.As a result,it was found that an absorber film having relatively good crystallinity was obtained when the sputtering power was 90 W.The photoelectric conversion efficiency was 5.09%.In order to further improve the cells performance,the influence of the amount of selenium on the absorption film was also studied.The results showed that when the sputtering power is 90 W and the amount of selenium is 200 mg,a thin-film solar cell with an efficiency of 6.77% is obtained,which is mainly due to the improvement of the quality of selenization.2.Effect of selenization on CZTSSe film after in-situ selenium supply.A layer of Se is deposited on the CZTS precursor layer surface by thermal evaporation to realize the in-situ supply of selenium to the CZTSSe film.The effect of Se layer thickness on the quality of CZTSSe thin films was investigated.It was found that the XRD peak position of the film shifted to a small angle as the Se thickness increased.When the Se thickness was 1.5 μm,the grain size of the absorber film was larger and the device performance better.Subsequently,the effect of temperature on the performance of the CZTSSe thin film was explored.The results showed that the crystallinity and compactness of the films increased when the annealing temperature increased.When the annealing temperature was 540℃,the absorber film is denser and the crystal grains are larger.The open circuit voltage of the device is increased to 466 m V.3.The effect of pyrolysis of selenium on the performance of CZTSSe thin film solar cells.A dual temperature zone rapid annealing furnace and a self-made selenium cracking activation system were used to pyrolyze the selenium source at high temperature to explore the effect of the selenium source cracking temperature on the quality of the absorber film.Through characterization,it was found that increasing the temperature of the selenium source is beneficial to enhance the selenium activity and improve the film crystallinity.When the temperature is 600 ℃,the photoelectric conversion efficiency of the device is 5.99%.In order to further improve the device performance,the effect of heating rate on the quality of CZTSSe thin film was investigated in detail.And its optical and electrical characteristics were characterized and analyzed.The results showed that the grain the size of the absorbing layer film prepared under slow heating conditions is relatively large,the film crystalline quality is better,and the photoelectric efficiency is 7.9%.This is mainly due to the slow heating-up and the activity of selenization,which is conducive to the slow and continuous supply of selenium and facilitates the sufficient selenization of the CZTS precursor layer film.