Study On The Performance Modification Of CZTSSe Thin Films Prepared By Sol-Gel Method And Solar Cells

Cu₂Zn Sn（S,Se）₄（CZTSSe）thin films are composed of abundant materials,safe and non-toxic,so they have received more and more attention.It is a general photovoltaic technology route to obtain CZTSSe thin film by high temperature selenization process after preparing precursor film by sol-gel method.Researchers all over the world have conducted years of research on CZTSSe thin-film solar cells prepared by the sol-gel method and have obtained rich theoretical and experimental experience,but there is still a big gap between their theoretical conversion efficiency.The purpose of this paper is to prepare high efficiency,environmental protection,and low cost CZTSSe thin film solar cells,mainly for CZTSSe absorber layer materials.Firstly,the preparation process of the CZTSSe film and other layers of the solar cell are introduced.The non-vacuum preparation process of the CZTSSe is optimized and the defect control of the CZTSSe film device by atomic layer deposition（ALD）growth Al₂O₃ is discussed.Then,the effect of introducing cationic Ag into the absorber layer in different ways on the performance of photovoltaic devices is explored.The N-type surface layer and pre-gradient distribution of Ag are constructed.Finally,a conversion efficiency of 12.8%of the effective area is achieved.The efficiency by the doping of alkali metal Na in CZTSSe thin film solar cells are also studied.The detail research contents of this dissertation are as follows:The process of preparing precursors by sol-gel method and then preparing CZTSSe thin films by high-temperature selenization is studied.The raw material content during the preparation of the precursor solution and the selenium content during the selenization process are important parameters for obtaining high-quality absorber film,so they are regulated and optimized.It is found that when the raw material content of Cu/（Zn+Sn）is 0.80 and Zn/Sn is 1.10 in precursor solution,a single-phase and well-crystallized CZTSSe absorber film is obtained.The best thin-film solar device is obtained under the preparation conditions of 0.5g selenium,and its highest conversion efficiency is 9.7%,which lays a good foundation for the development of subsequent work.A large number of defects and defect clusters distributed in CZTSSe thin-film solar cells are one of the reasons leading to the increase of device recombination and the decrease of V_OC.The regulation of ALD-Al₂O₃ on the performance and defects of CZTSSe devices is studied.The effects of Al₂O₃ thickness and growth temperature on CZTSSe films and devices are studied in this paper respectively,and the physical mechanism of the improvement of the device performance is analyzed.It is found that Cu _Zn defects and Sn-related deep defects are largely suppressed because of the decrease of Sn²⁺and the increase of Sn⁴⁺in the film by ALD-Al₂O₃ on precursor.At the same time,the recombination inside the absorber bulk and the front interface are both reduced,and the carrier lifetimes are increased.Finally,CZTSSe thin film device with Al₂O₃ has the highest conversion efficiency of 11.0%and an open circuit voltage(V_OC)of 483.5 m V.Cation doping and energy band engineering are important methods to realize high-efficiency thin-film photovoltaic devices.A novel Ag doping method is proposed,that is,the ultra-thin Al₂O₃ is prepared by atomic layer deposition on the Cu₂Zn Sn S₄（CZTS）precursor film,and the N-type Ag₂Zn Sn S₄（AZTS）is prepared by the spin coating method,and then subjected to conventional high-temperature selenization process.The distribution of Ag element is successfully controlled,and finally the construction of the N-type layer on the surface of the absorber layer is realized in the kesterite structure film.The performance of CZTSSe thin-film solar cells has been significantly improved with a conversion efficiency of 12.8%（effective area）and a low open circuit voltage(V_OC,def)of 0.546 V.In addition,physical characterization and analysis of the reasons for the increase in efficiency have been carried out,and it is found that there is a surface inversion layer and the Ag gradient distribution.The presence of the inversion layer increases the surface band gap,thereby increasing the open circuit voltage.The surface valence band gradient distribution on surface prevents the diffusion of photo-generated holes to the CZTSSe/Cd S interface and reduces the interface recombination.At the same time,the increased width of the depletion region facilitates the collection of carriers.This provides a new idea for the preparation of high-efficiency copper-based kesterite solar cells.Alkali metal treatment is an optimized strategy for realizing high-efficiency solar cells.Investigate the physical reasons for improving the conversion efficiency of alkali metal Na-doped CZTSSe thin-film solar cells.It is found that an appropriate amount of Na doping can significantly improve the crystalline quality of the absorber layer,while improving the properties of the heterojunction interface and reducing the optical loss in the Cd S buffer layer.Based on the three variables caused by Na-doped CZTSSe thin-film solar cells in the experiment,the wider band gap of the absorber layer,increased carrier concentration,and thinner buffer layer,the simulation experiments are carried out:The increase of V_OC is affected by both increased carrier concentration and the widened band gap,both of which will increase the quasi-Fermi level difference(E_Fn-E_Fp).The change of J_SC is the combined effect of the thinner Cd S thickness and the increase of carrier concentration.The former enhances the short-wave response of the solar cell,while the latter weakens the long-wave response.The thickness of Cd S plays a more important role,so the final J_SC of the device can be increased from 32.7m A/cm² to 33.9 m A/cm².