Study On The Preparation And Properties Of Co-doped Cu₂ZnSnS₄ Thin Films

Cu₂ZnSnS₄(CZTS)thin film has the advantages of abundant element reserves,environmental protection,low production cost,and high absorption coefficient.However,the highest photoelectric conversion efficiency of CZTS thin film solar cells is only 12.62%.Doping is an effective way to improve the optical and electrical properties of CZTS thin films.Co-doping CZTS with two elements can obtain the advantages of improving optical and electrical properties more effectively than single doping.At present,the mechanism of co-doping on CZTS is still unclear.Therefore,this dissertation will carry out research on the preparation,characteristics and doping mechanism of co-doped CZTS films.In this thesis,the sol-gel method and magnetron sputtering method were used to prepare Ag and Mn co-doped CZTS films and Ag and Ge co-doped CZTS films.Study the influence of each dopant on the co-doped CZTS film by characterizing the structure,microscopic strain,dislocation density,cross-sectional morphology,composition,element chemical valence and optical properties.The first-principles method is further used to calculate the energy band structure,density of states,and optical properties of Ag and Mn co-doped,Ag and Ge co-doped CZTS.Combining with experimental results,the mechanism of co-doping on CZTS was studied.The work is summarized as follows:(1)Ag and Mn co-doped CZTS thin film was successfully prepared by the sol-gel method.Changing the Ag/(Ag+Cu)and Mn/(Mn+Zn)ratio of the sol can adjust the Ag and Mn doping amount of the CZTS film.Adding Ag doping on the basis of Mn single-doped CZTS film to form Ag and Mn co-doping can improve the crystallinity of CZTS film,increase the particle size and direct band gap of CZTS film.The Ag-4d electrons introduced by Ag doping affect the energy level near the Fermi level,resulting in the increase of the CZTS band gap after Ag doping.Experimental characterization and theoretical calculations show that the absorption coefficient of Ag and Mn co-doped CZTS film reaches 10⁴ cm^-1.Compared with single doping of Ag,Ag and Mn co-doped can increase the crystallinity of CZTS,reduce micro-strain and dislocation density in CZTS crystal,increase film particles,reduces the Urbach energy and the tail state of the CZTS film,and reduce the thickness of MoS₂ interface layer between CZTS and Mo CZTS by limiting the diffusion of S in CZTS.The chemical valences of Cu,Zn,Sn,S,Ag,and Mn in the co-doped CZTS thin film are+1,+2,+4,-2,+1,+2,respectively.Mn doping affects the valence band top and conduction band bottom energy levels near the Fermi level so that the band gap of CZTS is reduced.The change of the lattice constant is affected by the atomic radius and the interaction force between atoms.(2)In the process of preparing co-doped CZTS films,the sulfurization temperature is an important factor.As the vulcanization temperature increases,the grain size becomes larger,the particles on the surface of the film become larger,and the crystallinity of the film is improved.When the temperature increased from 450°C to 580°C,the microscopic strain and dislocation density of the co-doped CZTS crystal decreased.The temperature rising cause the lattice expansion and the S atoms gain enough energy to move between CZTS and Mo,which results the MoS₂ becoming thinner.(3)Ag and Ge co-doped CZTS thin film was successfully prepared by magnetron sputtering.Changing the sputtering time of the Ge target and replacing the target with different Ag/(Ag+Cu)ratio can adjust the Ag and Ge doping amount of the CZTS film.With the increase of Ge doping content,the film becomes flat and the pores on the surface decrease,the particles on the surface of the film become larger,and the crystallinity of the film is improved.Ge-4s introduced by Ge doping affects the energy level at the bottom of the conduction band,thereby increasing the direct band gap of the CZTS film.The absorption coefficient of the CZTS film co-doped with Ag and Ge,which is characterized by experiment and theoretical calculation,reaches the10⁴ cm^-1.Compared with Ge single doping,Ag and Ge co-doping can increase the crystallinity of CZTS and increase the film particles when the Ag doping ratio is low.when the Ag doping ratio is high,the particles on the surface of the film become smaller and the absorption coefficient becomes smaller.Ag doping can increase the band gap.To sum up,co-doping can exert the control effect of each dopant on CZTS individually.Ag doping can reduce defects in the crystal and increase the open circuit voltage.Mn doping can thin the MoS₂ interface layer between CZTS and Mo,improve the electrical properties and reduce the microscopic strain and dislocation density in the film.Ge can promote the stability of the CZTS structure,inhibit the formation of Sn secondary phase and reduce the volatilization of Sn compounds.Co-doping synergizes the positive effects of various dopants on CZTS,which is an effective way to improve the optical and electrical properties of the film.Iit is expected to improve the photoelectric conversion efficiency of CZTS thin-film solar cells.