Preparation And Research Of Copper Zinc Tin Germanium Selenium (CZTGSe) Material And Thin Film Solar Cell

A quaternary compound semiconductor material,copper,zinc,tin and selenium?Cu₂ZnSnSe₄,CZTSe?film has always been attracted a great attention as the photovoltaic absorber material due to a p-type direct band gap semiconductor?Eg?1.0eV?,non-toxic,earth abundant composition and high absorption coefficient(>10⁴cm^-1).However,the power conversion efficiency?PCE?of CZTSe thin film solar cell is far lower than the theoretical efficiency?>30%?.The serious interface recombination,the relatively small band gap,the poor crystal quality and so on,which results in lower open circuit voltage(V_oc)and fill factor?FF?,are the mainly limiting factors to improve the PCE.In this thesis,for breaking this bottleneck which limits the improvement of the PCE of CZTSe thin films solar cell,it is proposed to use the substitution of cation?Ge?and anion?S?with Sn and Se by the same chemical group,respectively.Firstly,the CZTS precursors were prepared by RF magnetron sputtering CuS,ZnS and Sn targets.Subsequently,the CZTSe absorber layer was obtained by alloying at low temperature and annealing in selenium atmosphere at high temperature.The effect of Ge incorporation at different locations of the Mo layer on the quality of surface film was investigated,and the optimum Ge incorporation position was demonstrated.In order to clarify the optimal diffusion depth of Ge element in CZTSe film,the effect of the different Ge contents on the diffusion depth of Ge in CZTSe film was studied,and the Cu₂ZnSn_1-xGe_xSe₄?CZTGSe?film,as an electron blocking layer at the back side of CZTSe absorber layer,was achieved.Meanwhile,a high-quality crystallinity,suitable chemical composition and uniform grain size of CZTGSe was obtained.In addition,in order to achieve the structure of CZTSe/CZTSSe,the S concentration gradient of CZTSe absorber layer was roughly studied by using a plasma system.Finally,CZTSe-based thin film solar cells with the element concentration gradient were fabricated,and the corresponding power conversion efficiency was achieved,respectively.The main research contents were summarized as following:?1?The CZTS precursors were prepared by radio frequency?RF?magnetron sputtering Sn,CuS and ZnS targets,following annealed at a low temperature and selenised at high temperature.The effects of different selenium annealing conditions on the crystal quality,surface,cross-section morphology and photoelectric properties of CZTSe films were investigated.?2?The CZT?G?Se precursors were formed by magnetron sputtering with different Ge contents in the Mo layer and then preheated under the same annealing conditions to obtain the CZT?G?Se films.The effects of the different Ge contents on the quality of the CZT?G?Se film and the diffusion depth of Ge element were investigated,respectively.?3?In order to constitute a band gap grading?1.0eV-CZTSe/1.13eV-CZTSSe?film,the CZTSSe film with a wider band gap compared to the CZTSe film was prepared under the surface of CZTSe film by plasma system.And the effect of sulfurization process under the plasma system on the propriety of CZTSSe film was studied.?4?The films formed by above optimized methods were applied to CZTSe-based thin film solar cells with the structures of SLG/Mo/CZTSe/CdS/i-ZnO/ITO/Ni-Al,SLG/Mo?Ge?/CZTGSe/CZTSe/CdS/i-ZnO/IT O/Ni-AlandSLG/Mo/CZTSe/CZTSSe/CdS/i-ZnO/ITO/Ni-Al,respectively.Meanwhile,the effects of two kinds of optimized CZTSe-based thin film solar cells and pure CZTSe thin film solar cells on device parameters were detected.The research results showed that:?1?For pure CZTSe film without Ge doping,the CZTSe precursors were subjected to selenization process at 500°C,540°C and580°C for 13 min and 20min,respectively.It was shown that the CZTSe films obtained under the selenization temperature of 550°C for 13 min had better crystallinity.Meanwhile,the uniform and compact surface,the larger grain size,the proper composition and the better purity phase were attained by the sample annealed at 550°C for 13 min.?2?The CZT?G?S precursors were prepared by doping Ge content at different positions of Mo layer.The results showed that when the Ge layer was deposited on the bottom and surface of Mo layer,there were different degrees of shedding for the CZTGS films after the low temperature alloying and selenium annealing at high temperature.Only the samples in which the Ge layer was deposited into the Mo layer demonstrated the good crystallinity,the relatively uniform and dense surface morphology as well as larger grains size 1²?m.It could be observed by XPS that the valence state of Ge was+4 among Ge-incorporated CZTSe film,and it could be confirmed that the Ge incorporation of CZTSe films occupied the Sn position and also could be deduced that Sn⁺⁴ of CZTGSe films was replaced by Ge⁺⁴.The CZTGSe films were fabricated by doping various Ge content into Mo layer under the same annealing conditions?540?/13min?.The results revealed that the Ge distribution in CZTGSe films exhibited a relatively better concentration gradient?the diffusion depth of 350nm?by sputtering Ge target at 25W for 20min.At the same time,the crystal quality of the film was improved due to the diffusion of Ge to the upper surface.However,too little Ge contents could result in a short diffusion depth of Ge element or even no diffusion in the absorber layer.On the contrary,if the amount of Ge was too higher,it caused a long diffusion depth of Ge element.?3?The CZTSSe film with S concentration gradient was obtained by plasma system.The results implied that the S content of CZTSSe film via sulfurization under Ar atmosphere using plasma system showed a decrease trend from surface to bottom.In addition,according to the GIXRD characterization,the results showed that the position of the main peak?112?of CZTSe film was gradually shifted to the main peak?112?of CZTS film as the incident angle decreased.Compared to the pure CZTSe film,the better roughness,the more uniform grain and the larger band gap of CZTSSe could be prepared under the plasma system,which also could be concluded by the result of PL and AFM measurement.?4?CZTSe-based thin film solar cells based on different optimized absorption were developed and the results showed that the PCE of CZTGSe thin film solar cells with Ge concentration gradient reached 3.69%;the PCE of CZTSSe thin film solar cell with S concentration gradient was approximately 3.28%,Each of them was higher than the efficiency of pure CZTSe thin film solar cell.