Effect Of Indium Doping On Cu₂ZnSnS₄ Thin Film Solar Cell

In recent years,with the aggravation of environmental pollution caused by fossil fuels’combustion and the gradual depletion of fossil fuels,solar cells are a critical breakthrough in future energy demand as an essential green energy tool for people to use solar energy.Among many kinds of solar cells,the development of thin-film solar cells is the most effective way to reduce the cost.Cu₂Zn Sn S₄（CZTS）thin-film material has a direct band gap structure,high light absorption coefficient(>10⁴ cm^-1),and the band gap（Eg～1.5 e V）matching the solar spectrum,The thickness of about1μm can absorb most of the solar radiation.These advantages make the CZTS thin-film solar cell one of the solar cells with excellent scale application potential.In this paper,CZTS and In-doped CZTS thin films and solar cells were successfully prepared by the sol-gel method.Firstly,using ethylene glycol methyl ether as the solvent,copper acetate,zinc acetate,stannous chloride as the metal source,and thiourea as sulfur source respectively,the CZTS sol was successfully prepared by baking solution in a 50℃constant temperature water bath for 2 hours.The thermogravimetric analysis determined 300℃is the best temperature for sol pre-sintering.The CZTS precursor film is prepared by repeating the spin-coating and pre-sintering process in the air.After high-temperature sulfurization,the CZTS film is successfully obtained.CZTS absorption layer thin films with copper-poor and zinc-rich,good crystal quality,smooth and compact surface were successfully prepared by optimizing the sulfization process parameters.XRD and Raman analysis showed that the CZTS absorption layer thin films was kesterite phase structure and grew preferentially along（112）crystal plane,with band gap about 1.50 e V.In the same time,CZTS thin-film solar cells with photoelectronic conversion efficiency more than 3%were also obtained.Secondly,In₂S₃ was deposited on Mo coated soda lime glass substrate by magnetron sputtering In₂S₃ target.The effect of In₂S₃ interface layer between the CZTS absorption layer and back Mo electrode was studied.Through the analysis of the structure,morphology,and photoelectric properties of the CZTS films,the results show that the In₂S₃ interface layer can inhibit the decomposition of the back surface of the CZTS absorption layer film to a certain extent,promote the grain growth,reduce the influence of the secondary phase caused by the decomposition of the back CZTS on the photoelectric properties of the CZTS device,improve the short-circuit current density and fill factor.As a result,CZTS thin film solar cells with 4.47%efficiency were developed by introducing In₂S₃ interface layer.However,the open-circuit voltage loss is still large.CZTS sol containing In was spin-coated at the bottom,and CZTS sol without In was spin-coated on the top of the precursor film using the spin coating and pre-baking process.Through XPS analysis of CZTS film after sulfurization,it is found that In element presents gradient distribution along the bottom of the absorption layer.The CZTS thin film solar cells with bottom In graded distribution are also fabricated and the I-V test shows that the s open-circuit voltage is significantly improved due to the increase of the carrier concentration at the bottom and the reduction of Sn _Zndeep level defects in the absorption layer.In the preparation of CZTS sol,indium chloride,copper acetate,zinc acetate,stannous chloride,and thiourea were dissolved in ethylene glycol methyl ether to explore the effects of different In concentration on the phase structures,surface morphologies,and photoelectric properties of CZTS films.The composition test shows that In is evenly distributed in the absorption layer and S/mental>1,indicating that the sulfurization has been carried out entirely.There are no secondary phases in CZTIS films with different In doping concentrations.With the increase of In/（Sn+In）ratio from 0 to 3%,the crystalline quality of the CZTS absorption layer is improved,the surface holes disappear,and the grain size becomes larger.As the In concentration continues to increase,the grain growth of the absorption layer deteriorates,and the CZTS on the back surface decomposes.This is because the In concentration is too high,which leads to the saturation of In in the absorption layer,and deteriorates the grain growth of the absorption layer.When In doped content into the absorption layer is 3%,the performance of CZTS thin-film solar cells is the best,and the corresponding photoelectric conversion efficiency is 5.11%.