Research On Microstructure And Surface Bandgap Adjustment Of Electrodepositied Cu(In,Ga)Se₂ Thin Films

Cu(In,Ga)Se₂(Cu(In,Ga)Se₂,or CIGSe for short)thin film solar cell has attracted increasing research attention for energy conversion application because of its low cost,nontoxic materials,high absorption coefficient,and high conversion efficiency.Recently,the record efficiency of CIGSe solar cell prepared by vacuum process is23.35%,which is one of the most promising solar cells.Electrodeposition is famous for the preparation of metal precursors in vacuum-free conditions and high material utilization and deposition rate,leading to the reduction of manufacturing costs.However,the highest efficiency of CIGS solar cells by electrodeposition is only 17.3%,indicating that there are many problems to be solved,such as large roughness,nonuniform elements distribution,Ga accumulation at the back contact during post-selenization,high defect density,low surface bandgap,and high recombination at the CIGSe/Cd S interfaces.According to the issues mentioned above,the research contents of this dissertation are as follows:In the respect of precursors,the surface roughness of Cu film is limited by the parameters of the pulse electrodeposition,which is prone to“dendritic”clusters,resulting in nonuniform element distribution and affecting subsequent electrodeposition of In and Ga films.In this paper,bidirectional pulse electrodeposition method is applied to prepare smooth and dense Cu film.Reverse pulse has a smooth effect by dissolving the burrs of Cu film during the electrodeposition process.Meanwhile,by introducing a novel metal precursor structure(Mo/Sb/Cu/In/Ga),smooth and uniform Cu film can be obtained under a wide pulse condition(pulse frequency:1000?10000 Hz,current density:31.25?62.5 m A cm^-2),thus broadening the deposition conditions of Cu film.The thinnest thickness of the CIGSe absorber with the Sb film is successfully reduced to 0.36?m,while the thinnest one without the Sb film is approximately 0.7?m.In addition,CIGSe absorber with the Sb film shows good crystal and excellent interfacial properties.Finally,CIGSe thin-film solar cells with efficiencies of 5.25 and 11.27%are obtained with CIGSe absorber thicknesses of 0.36and 1.2?m,respectively.This work proposes a method of rapid high temperature pulse selenization for post selenization,which the surface temperature of the thin film can rapidly increase to 800?by pulse irradiation of halogen lamp,without damaging the glass substrate.The reaction mechanism of pulse seleniazation is studied.Pulse selenization accelerates the formation of CIGSe phase on the surface of CIGSe film.It can increase the Ga content on the surface of CIGSe film,alleviate the accumulation of Ga on the back of CIGSe film,and prepare the back field which is benefit for carrier collection.In addition,pulse selenization can reduce the back barrier of CIGSe solar cells,passivate the deep level defects and optimize the solar cell performance.The efficiency of CIGSe solar cell is improved to 12.35%by this method.To improve the CIGSe surface bandgap and solve CIGSe/Cd S interface problems,the surface of CIGSe absorber is coated by spin-coating Ga Cl₃,KSCN and Ga Cl₃+KSCN solutions,respectively,which is used to widen the surface band gap and process K⁺doping.With the increase of Ga content on the surface of CIGSe thin film treated with Ga Cl₃ solution,the minimum conduction band on the surface of CIGSe films are increased,and the conduction band offset(CBO)of CIGSe/Cd S interface decreases.CIGSe thin film treated with KSCN solution can effectively realize the doping of K⁺,so that the carrier concentration increases obviously,and the valence band maximum on the surface of CIGSe thin film moves downward.It helps to form the transmission barrier for holes and suppress the interface recombination.In addition,Ga content on the surface of CIGSe film and K⁺content in the film is increased obviously by the co-treatment of Ga Cl₃ and KSCN solutions.Compared with the KSCN treatment,K⁺content in CIGSe thin film with Ga Cl₃ and KSCN co-treatment is also slightly increased,and the carrier concentration is increased by one order of magnitude.The co-treatment of Ga Cl₃ and KSCN solution can simultaneously realize the minimum conduction band upward and the maximum valence band downward,which can effectively reduce the interface recombination.Meanwhile,the deep level defects can be passivated to form shallow defects K_Cu or K_In.The highest efficiency of CIGSe solar cell with the co-treatment of Ga Cl₃ and KSCN solution is 13.5%.The increase of efficiency is mainly due to the obvious increase of open circuit voltage from532 m V to 610 m V.To further widen the CIGSe surface bandgap,In₂S₃ thin film is prepared on the surface of CIGSe thin film for the first time by solution method.In the drying process,elements are diffused:Cu in the CIGSe absorption layer diffuses outward into In₂S₃film to form Cu In₅S₈,while S in In₂S₃ film diffuses outward into CIGSe film.Therefore,Cu-poor layer forms on CIGSe surface.The thickness of Cu-poor layer on CIGSe surface changes with the thickness of In₂S₃ film.It is found that the surface of CIGSe thin film changes from p type to n type after In₂S₃ treatment,which was beneficial to reduce the interface recombination and improve the CIGSe/Cd S interface properties and the solar cell performance.This method is not only applied to CIGSe cells prepared by post selenization after electrodeposition,but also to improve the surface performance of CIGSe solar cell by other post selenization methods.Finally,KSCN and In₂S₃ co-treatment are used to change the performance of CIGSe thin film.It is found that the alkali metal K⁺could be effectively doping,and the carrier concentration is obviously increased.The highest CIGSe efficiency is 14.35%.