Preparation Of Copper-zinc-tin-sulfur Thin Films By DC Magnetron Sputtering And Study Of Element Substitutio

The Copper-Zinc-Tin-Sulfur(CZTS)thin film is considered as one of the most suitable absorbing layer materials for thin film solar cells due to its proper band gap,high optical absorption coefficient and non-toxic and earth-abundant raw materials.The single sputtering deposition method,sputtering from a Cu-Zn-Sn-S quaternary ceramic target,has been extensively investigated for preparing CZTS thin films,because it has obvious advantages in simplifying the process and achieving uniform and stable deposition in large areas.Radio-frequency(RF)sputtering has been usually used in this deposition and relevant research.However,RF sputtering devices are expensive and radiative,which would weaken the advantages of the single target sputtering and is not conducive to industrial production.Therefore,direct current(DC)sputtering,which is more inexpensive,fast and non-radiative,is used for the CZTS film deposition with a Cu-Zn-Sn-S target instead of RF sputtering in this paper.Specific research contents and results are as follows:(1)The relationship between the working pressure and the stable sputtering power range was studied.The results show that the highest stable sputtering power increases with the decreasing of working pressure,and the stable sputtering power range can be widened.In order to obtain a suitable stable sputtering power range,the working pressure should be controlled at 0.5 Pa or lower.(2)The effects of sputtering power on the composition,crystal structure,morphology and optical properties of the prepared films were studied.The results show that the Cu-poor and Zn-rich kesterite CZTS thin films can be prepared at the power of 35 W and 50 W.The film prepared at the power of 50 W has lower average grain size than that of 35 W,but the deviation of Cu and Sn content in this fillm is smaller.So the film prepared at 50 W has smaller Eg(approximately 1.65 eV)and bigger optical absorption coefficient.(3)By fixing sputtering power at 50 W and changing the substrate temperature from room temperature to 300℃ the effects of substrate temperature on the composition,crystal structure,morphology and optical properties of the films were investigated.Researches show that S content in the films decreases with the increase of substrate temperature;Zn content firstly decreases with the increase of substrate temperature within 200℃,and then increases with the increase of temperature when the substrate temperature exceeds 200℃.At the substrate temperature of 120℃,the deviation of the Cu and Sn of the film is further reduced,so the band gap of the film has dropped to 1.58 eV.(4)The prepared films were sulfurized in the range of 450-700℃,and the effects of sulfurization temperature on the composition,crystal structure,morphology and optical properties of the films were studied.The results show that S content of the films could be improved when the films were sulfurized within 600℃;S loss of the films would happen when the sulfurization temperature exceeds 600℃;but no loss of Sn is observed even under a higher sulfurization temperature like 600 and 700℃.Pure kesterite and relatively homogeneous and dense CZTS thin films could be obtained after sulfurized at 550℃.This CZTS film has a high optical absorption coefficient(>104 cm-1)and a suitable band gap about 1.55 eV.(5)Fe-substituted and Ge-substituted CZTS thin films were prepared by the direct current sequential sputtering CZTS target and metal target followed by a sulfuration process.The effects of the substitution amount of Fe and Ge on the phase compositions and optical properties of the films were studied.Researches show that Cu2FeSn3S8 and Cu2Zn1-xFexSnS4 would be formed in the Fe-substituted films;with the increased of substituted Fe,Fe content in Cu2Zn1-xFexSnS4 would increase and the FeS2 phase would begin to form and gradually dominate.The absorption coefficient of Fe-substituted films in long wavelength bands increases with the increase of Fe content.As for Ge-substituted CZTS films,Ge is easy to freeze Sn out of the lattice and occupy the lattice position of Sn to form CZTGS;the extruded Sn would react with S to form SnS2 during the sulfuration process.After the substitution of Ge,the optical absorption coefficient of the films decreases.