Studies On The Growth Regulation And Mechanism Of CTS Mesophase During The Preparation Of High-quality CZTSSe Absorber

Developing new high-efficiency thin-film solar cells is one of the effective ways to alleviate energy and environment crisis.At present,the photoelectric conversion efficiency of copper indium gallium selenium（CIGS）and cadmium telluride（Cd Te）thin film solar cells has reached 21.4%and 23.35%,respectively,showing a good market application prospect.However,the scarcity of the two components In and Ga and the toxicity of Cd will restrict their future large-scale application and long-term development.In recent years,Cu₂Zn Sn（S,Se）₄ thin film solar cells have attracted great attention from researchers because of their abundant reserves of component elements,adjustable band gap and high optical absorption coefficient.According to theoretical calculation,its Schockley-Queisser efficiency limit is as high as 32%,which is regarded as the most potential complementary and alternative material for CIGS thin film solar cells.However,the current conversion efficiency of 14.9%is far behind the maximum conversion efficiency of CIGS（23.35%）and its own theoretical efficiency limit（32%）,and there is still a lot of room for improvement.The research shows that one of the key factors restricting the efficiency improvement of CZTSSe thin film solar cells is the CZTSSe absorption layer itself.CZTSSe is a multi-compound semiconductor material with sensitive element ratio,more structural freedom and small stable region in the phase diagram,so it is difficult to synthesize a single phase and well-crystallized CZTSSe thin film material.The deviation of each component and small chemical potential change during the growth of materials will lead to binary heterophase（Cu₂S,Cu₂Se,Zn S,Zn Se,Sn S,Sn Se,Sn S₂,Sn Se₂,etc.）,ternary heterophase and vacancy defect(V_S,V_Se,V_Cu,V_Zn,V_Se,V_Se,V_Zn,V_Sn),substitution defects(Cu_Zn,Cu_Sn,Sn_Cu,Zn_Sn,Sn_Zn),and more complex defect clusters(Cu_Zn+Zn_Cu,Sn_Zn+Zn_Sn,Cu_Sn+Sn_Cu,V_Cu+Zn_Cu,V_Zn+Sn_Zn),The existence of these defects will cause the change of the band gap of the non-radiative composite and absorption layer materials,resulting in a large loss of the device open-circuit voltage(V_OC).Numerous basic studies have shown that there are multiple reaction pathways in the synthesis of CZTSSe films,Cu₂（S,Se）+Sn（S,Se）+Sn_liquid+Zn SSe→Cu₂Zn Sn（S,Se）₄ and Cu₂Sn（S,Se）₃+Zn SSe→Cu₂Zn Sn（S,Se）₄ showed a competitive mechanism.Compared with the reaction of CZTSSe generated by many binary compounds,the reaction path of Cu-Sn-S（Se）ternary structure and Zn SSe reaction is shorter,and the formation energy is lower,which can effectively avoid the formation of related binary phases.However,the specific reaction path of CTS intermediate phase assisted formation of high quality CZTSSe absorption layer and its regulatory effect and mechanism on CZTSSe grain growth remain unclear.In view of the above problems,this paper starts from optimization and control of the reaction path,uses Zn/CTS/Mo prefabricated layer structure to prepare high-performance CZTSSe absorption layer,studies its growth process,and illustrates the reaction path of CZTSSe absorption layer generation based on CTS mesophase.The mechanism and effect of CTS mesophase assisted formation of high quality CZTSSe grains were revealed.The specific research content and results of this paper are as follows:1.CTS absorption layer was prepared by magnetron sputtering.Firstly,the CTS absorption layer was sequentially prepared by DC sputtering by magnetron sputtering method.The results show that the Sn/Cu/Mo structure prepared CTS absorption layer film has good crystal quality and no secondary phase exists in the film.There are large holes on the surface of the Cu/Sn/Mo films,which contain the secondary phase of Sn S.On this basis,the composition of CTS absorption layer prepared by Sn/Cu/Mo structure was optimized.Finally,the photoelectric conversion efficiency of 2.96%was obtained.2.Performance optimization of CTS solar cell devices.In order to further improve the device performance of CTS thin film solar cells,Na doping is applied to the CTS absorption layer by spinning coated Na₂S solution to improve the device performance.When the concentration of Na₂S is 0.2M,the device performance is the best.The results show that Na doping increases the grain size,improves the crystallinity of the absorption layer,reduces the recombination at the interface,and improves the carrier collection ability of the device.Finally,the photoelectric conversion of CTS solar cell devices increased from 2.96%to 4.45%,V_OC increased from 220 m V to 290 m V.3.CZTSSe solar cells were prepared based on CTS mesophase.Firstly,metal Zn was sputtered on CTS layer by magnetron sputtering method,and CZTSSe absorption layer was prepared by Zn/CTS/Mo prefabricated layer structure.The effect of different temperature on the crystal quality and device performance of CZTSSe absorption layer was investigated.The results show that CZTSSe thin film prepared at 550℃has high crystal quality and the best device performance.CZTSSe solar cells with V_OCof 0.49 V and PCE of 9.39%were obtained.On this basis,the growth process of CZTSSe film was studied.At low temperature,the CTS phase did not change,and Zn S reacted with S to form Zn S.At the temperature rising,a small amount of CZTS was generated at the interface between Zn S and CTS.The reaction process can avoid the formation of binary quadratic phase,which is conducive to the preparation of high performance CZTSSe absorption layer thin films.