| The thin film solar cell Cu2ZnSnS4(CZTS) is a promising material to replace traditional Cu(In,Ga)Se2. CZTS is a p-type semiconductor with a highly theoretical photoelectric conversion efficiency of 32.2 % and large optical absorption coefficient of 104 cm-1. It is the new orientation for the new thin films solar cell.Currently, the main problems of CZTS thin-film cells are the existence of mixed phase and the poor microstructure. For the hetero phase, the formation is related to the distribution of metal element if the composition of absoption layer is controlled fittly. Hetero phase in absorption layer seriously impacts on the electrical properties of the absorption layer, and further deteriorates the photoelectric conversion efficiency. Generally, larger grain size, having a preferred orientation and densed absorbent layer microstructure, are beneficial to the thin film solar cells of higher efficiency, however, in the CZTS, it has high reation activity the between copper and the sulphur. Additionally, the zinc and zinc sulphiudes have high vapor pressure, which results in seriously diffusion of element and decreases the desity, grain size and orientation degree. Most investigation showed, CZTS film usually has a poor microstructure. Based on the above analysis, the purpose of this paper is to improve electrical properties by controlling element distribution in the films and suppressing element diffusion during the annealing treatment.In this paper, in order to improve composition uniformity of films, suppress the element segregation and decrease the content of hetero phase, magnetron sputtering was used for the preparation of thin films. Metal sulphides was used as targets to increase the sulphur content in the precursor, which make the formation of metal sulphides before the annealing treatment. It is benefical to suppress the element diffusion and enhance the microstructure. Additionally, we prepared the precursor of CZTSSe by adding the Se element in targets. And, the influence of the doping content of Se on the phase structure, microstructure and optical properties of the CZTS films was investigated. Base on the results and discussion, we obtain the following conclusions:(1) Zn has a higher vapor pressure, at the same time, ZnS has the high formation temperature. During the annealing treatment, Zn diffuses to the surface of the film. Those factors lead to ZnS hetero phase synthesis in the surface of the film within the neighboring region;(2) For the CZTS film deposition by the Cu2 S, ZnS and SnS2 targets. It is beneficial to suppressing the formation of the hetero phase ZnS by decreasing the heat treatment time and increasing sulfide temperature, In addition, the formation of the ZnS hetero will decrease Zn content of CZTS, which affects the band gap of the film.(3) It is beneficial to improve the degree of crystallization of precursor film by increase of the S content in the precursor. It also improves the density of thin film after the annealing treatment and distribution uniformity of elements. Those factors decrease the impurity phase content in the film.(4) With the increase amount of Se in CZTS thin films, Raman spectra of peak CZTS type and CZTSe type peak moves to lower frequency gradually. The position of peak has a linear correlation with Se content. The band gap decreases from CZTS to CZTSe gradually. |
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