Investigation Of Preparation And Properties Of Cu₂ZnSn(S_x,Se_1-x)₄ And Cu₂ZnSnSe₄ Semiconductor Thin Films By Solvothermol Method

In recent years,I_2-II-IV-VI₄ compounds Cu₂ZnSnS₄?CZTS?,Cu₂ZnSn(S_x,Se_1-x)₄?CZTSSe?and Cu₂ZnSn Se₄?CZTSe?thin films are considered to be new generation solar cell materials instead of CIGS thin film due to their ideal tunable bandgap value between1.0 eV-1.5 eV by regulating element proportion of thin films,high absorption efficiency,earth abundant,low-cost and environment-friendly.So these thin films are considered to be the most potential new photovoltaic materials,which are suitable for the development of high-efficiency,green and low-cost solar cells.In this paper,the single-step solvothermal method is proposed to prepare CZTSSe and CZTSe thin films directly on FTO glass,by taking CuCl₂,ZnCl₂ and SnCl₂ as metal element sources,thiourea as sulfur source,Na₂SeSO₃ or selenourea as selenium source,CTAB as cationic surfactant.And then we investigate morphology,structures and optical properties of CZTSSe and CZTSe thin films by SEM,EDS,XRD,Raman spectrum and UV-Vis spectrum.And the results are summaried as followed:1.We prepare CZTSSe thin films directly by solvothermal method on FTO glass substrate.By varying the thiourea/selenourea ratio in the precursor solution,the nanostructure and the chalcogen composition of CZTSSe thin films can be effectively controlled,leading to a tunable bandgap from 1.51 eV-1.38 e V.Because of their strong ability of absorption in the visible and near-infrared range,CZTSSe thin films are suitable for the absorption layer of thin film solar cell.2.CZTSSe thin films are composed of uniform spherical particals.Their structures are kesterite and the XRD diffraction peaks move toward lower angles relative to those of CZTS,indicating that more selenium atoms are added into kesterite structure to replace the position of S atom,because the radius of Se^2-?0.198 nm?is larger than S^2-?0.184nm?,and the shift of diffraction peaks to lower angles indicates that more selenium atoms are indicated in the lattice to replace the position of sulfur atom to form CZTSSe.Meanwhile,with the decrease of the molar ratio of thiourea/selenourea in the precursor solution,the Raman characteristic peak near 334 cm^-1 associated with CZTS is gradually weakened,and the intensity of the Raman characteristic peaks near 190 cm^-1and 230 cm^-1 which are related to CZTSe are gradually enhanced.3.The ratio of Cu:Zn:Sn:?S+Se?in CZTSSe thin film deviates from the theoretical value of 2:1:1:4 of stoichometry Cu₂ZnSn(S_x,Se_1-x)₄ thin film,and the actual ratio of S/?S+Se?in the film is much lower than the molar ratio of thiourea to selenourea in the precursor solution.In addition,as shown in SEM-EDS element distribution diagrams,the distribution of Cu,Sn and S is uniform in the thin films,however the distribution of Zn and Se is uneven.It's difficult to control the ratio of each element in the film effectively for quintuple compounds,and this difficulty needs to be further studied.4.We prepare CZTSe thin films directly by solvothermal method on FTO glass substrate and find that the concentration of selenourea in the precursor solution obviously affects the morphology of CZTSe thin films.The prepared CZTSe thin films are composed of a large number of uniform spherical particles when the concentration of selenourea is under 0.2 M,and the diameter of spherical particles increases with the increase of selenourea concentration.When the concentration of selenourea increases to0.25 M,the morphology of the film is composed of nanosheets,and these nanosheets are distributed vertically or quasi vertically on FTO substrates uniformly.The CZTSe semiconductor thin films have a kesterite structure with bandgap of 1.17 eV-1.38 eV.