Study Of Cu?In,Ga??S,Se?₂ Thin Films And Devices Fabricated Using Single Target Sputtered Quaternary Chalcopyrite Targets

Cu?In,Ga??S,Se?₂?CIGSSe?is considered to be one of the most promising solar cell materials owing to its direct bandgap with adjustable bandgap,high power conversion efficiency,long stability,and radiation resistance.Recently,researchers developed an approach using a sputter a single quaternary alloy target to prepare CIGSSe absorber layers.It is an effective method for large-scale production of CIGSSe solar cells because it shorten the process time and improve material utilization and production efficiency.However,there are some disadvantages including the difficulty of adjusting the energy band of thin film and the growth of CIGSSe grains.These constrain the performance improvement of CIGSSe solar cells fabricated by single target sputtering.In this thesis,two methods are proposed to solve these problems:a)exploring the optimum annealing conditions to improve the crystallization properties of CIGSSe films.In addition,sulfur atmosphere was used during annealing to adjust the surface energy band in order to control band profile;b)raising the substrate temperature during sputtering to enlarge the grains size and optimize the crystallinity of the film.The single quaternary targets of CuInGaSe₂?CIGSe?and CuInGaS₂?CIGS?are studied and the main contents of this thesis are as follows:?1?Preparation of precursor films by sputtering quaternary CIGSe and CIGS targets?Cu:In:Ga:Se/S=21.9:19.3:7.4:51.4 at%?followed by annealing.The results show that the optimum annealing conditions for CIGSe precursor films is 575oC with 64 mg sulfur are used for annealing.The conversion efficiency of CIGSe solar cell prepared under this condition is 0.64%.For CIGS precursor films,the optimum annealing conditions is-400oC-575oC with 64 mg of sulfur and 40 mg of selenium are used.The obtained conversion efficiency of CIGS solar cell is 1.31%.One of the main reasons for the low device efficiency is contributed to be the small grains size and poor crystallinity of the thin films.?2?Based on the problems mentioned in section?1?,the precursor films are deposited by sputtering with the heating of substrates to increase grains size and crystallinity in the section.The results show that the crystallinity of CIGSe films is improved obviously.For CIGSe films,1?m grain size of CIGSe films is observed,which is larger than that of obtained films mentioned in section?1?.For CIGS films the grains size is between 100 and 250 nm,which is similar to that of section?1?.No obvious change is observed.Although large grain size is realized for CIGSe films,it is found that the composition is remarkable changed compared with the target,resulting in low resistivity and low efficiency of obtained solar cells.The efficiencies of the CIGSe and CIGS solar cells are 1.26%and 1.21%,respectively.?3?HBr aqueous solution was used to treat the thin film in this section,based on the serious composition deviation of sputtered film mentioned in section?2?.The results indicate that Cu and Ga elements of CIGSe films are selective etched by HBr solution,resulting in the decrease of Cu/?In+Ga?ratio and the improvement of resistivity the CIGSe films.The conversion efficiency of the CIGSe solar cells is increased to 1.73%owing to the improvement of the open circuit voltage.The higher conversion efficiency is considered to be realized by further optimizing etching parameters.Based on the results mentioned above,it is considered that for CIGSe targets,sputtering a single target with the heating of substrates is a suitable preparation method.By adjusting the substrate temperature,the sputtering power,and the working pressure,a CIGSe film having a high crystallinity and a reasonable elemental composition can be obtained,and if necessary,the film can be etched and adjusted by an acidic bromine solution.For CIGS targets,the method of sulfide selenization after sputtering without heating is relatively suitable for the treatment of the absorption layer.