Research On CIGSe Thin Film Fabricated By Pulse Electrodeposited Cu/In/Ga Metallic Precursor And Selenization Sulfuration Processes

Cu?In,Ga?Se₂ thin film solar cell,with characteristics of stable,high radiation tolerance,and high conversion efficiency,etc.,has been one of the most promising solar cells,the highest conversion efficiency of CIGSe thin film solar cell is 22.6%.CIGSe thin films with high quality can be fabricated by evaporation and sputtering metal precursors followed by selenization.Electrodeposition,as one of the most important research direction,has the advantages of high deposition speed,efficient utilization of raw materials,low instrumental cost,and feasibility in making large area films.However,the highest conversion efficiency of CIGSe thin film solar cells,prepared by electrodeposited metal precursor followed by selenization process,is about 16%,which is mainly due to the following issues:large roughness,nonuniform elements distribution,and impurity incorporation of the electrodeposited Cu/In/Ga metal precursors.Additionally,the commen issues of post-selenization include poor crystallization of CIGSe thin film,Ga accumulation at the back contact during selenization,and difficult control of film band gap.The research was investigated to solve the issues mentioned above,details are as follows:In the respect of precursors,the pulse electrodeposition method was imployed to deposit Cu/In/Ga metal precursors with little roughness,less impurity incorporation,and uniform element distribution.The film roughness and composition unifomity was influenced by the“dendritic”clusters formed on the surface of electrodeposited Cu films on Mo substrates.The flat uniform and compact Cu film without“dendritic”clusters on Mo substrate was fabricated by increasing the pulse peak current density,which induced the increasing of nucleation rate and decreasing of nucleus size.The morphology of In film electrodeposited on Cu film was usually“island-shape”.High cathodic polarization and discrete growth,and the energy of the nucleus transferred to water during T_off to prevent the nucleus merging together,made the flat,uniform,and compact In film without“island-shape”morphology.The parasitic reaction of hydrogen evolution happened during Ga electrodeposition process,which decreases the depositing efficiency of Ga and film quality.The Ga current efficiency and film quality were improved by increase the current density,which helped the electrode potential shifted to the region that favored the metal deposition.In the respect of post-selenization,the three step process separatly annealing in the atmosphere of Se/N₂/Se was employed to solve the problem of poor crystalization quality of CIGSe thin film and Ga accumulation.The Ga diffusion in the film was controlled by the selenization degree of the first step,which would influent the vacancies in the film.CIGSe thin films with single phase and high quality were fabricated when the Se/?Cu+In+Ga?ratio was in the range of 0.74 and 0.96.The diffusion distance of Ga in the film was controlled by the annealing time of the second step,which helped to form grading band gap.Large-grained CIGSe films were prepared with Cu/?In+Ga?ratio in the range of 0.7 and 0.9.Additionally,when the Cu/?In+Ga?ratio was less than 0.9,the thickness of MoSe₂ layer increased with the diffusion of Ga,and the dicrease of Cu content.Finally,the CIGSe thin film solar cell with conversion efficiency of 11.12%,fill factor of 66.61%was obtained by optimizing the three step selenizaiton process,which kept the precursor ratios of Cu/?In+Ga?and Ga/?In+Ga?in the range of 0.78⁰.82 and 0.29⁰.30,respectively.Pulse rapid annealing process method was proposed and performed at the first time to control of S content and diffusion depth in the CIGSe film,which intended to solve the problems of low surface band gap of CIGSe thin film and poor band gap control during post-selenization process.The S content in the film surface increased with the number of pulse increase The diffusion depth of S in the film was decreased with the decrease of the substrate temperature and T_on.The CIGSSe thin film solar cell with the conversion efficiency of 12.01%,the open circuit voltage of 532mV,the current density of 35.08 mA cm^-2,and the fill factor of 62.90%was fabricated by optimizing the sulfuration process of pulse rapid annealing.