Study On The Preparation And Properties Of Absorber Layer And Cadmium-Free Buffer Layer For CZTS(Se) Solar Cells

With the development of society,fossil energy depletion and environmental pollution are becoming more and more serious,so it is imperative to develop renewable energy,especially solar energy.Copper-zinc-tin-sulfur-based（Cu₂Zn Sn S₄,CZTS）solar cells have attracted much attention because of the high raw material abundance,high absorption coefficient,adjustable band gap,environmental friendliness,and high theoretical efficiency.However,the power conversion efficiency is only 13%at present,which is far from the theoretical efficiency.The main reasons are the extremely high open-circuit voltage loss,and the recombination of photogenerated carriers caused by the interior defects in the absorber layer and at the junction interface.Therefore,the preparation of high-quality absorber layer and solving the interfacial contact problem are crucial for improving performance of devices.In this work,the following studies are carried out and focused on the CZTS（Se）absorber layer and novel buffer layer:1.It was systematically studied that the effects of annealing parameters on the morphology,phase composition,and photoelectric properties of CZTS and CZTSSe absorber layers to obtain high-quality films.The CZTS precursor film was prepared by the sol-gel method firstly,and then annealed in Ar gas to get the CZTS layer.The influence of annealing temperature and holding time on the grain growth of the absorber layer were studied,and thereafter a CZTS absorber layer with large grain size and few holes was obtained.In order to obtain an absorber layer with a more suitable band gap,Se source was introduced to adjust the band gap during annealing,the effects of annealing temperature and amount of selenium powder on the quality of the absorber layer were studied.The CZTSSe absorber layer composed of a single phase reduces the recombination of photogenerated carriers.The selenization process did not change the chemical valence state of the elements,but improved the stability of the film and adjusted the energy band structure.2.The introduction of antimony selenide（Sb₂Se₃）into the CZTS precursor film can promote the grain growth of the CZTS layer,and the mechanism of Sb₂Se₃promoting the grain growth is proposed.Sb₂Se₃ was designed to be deposited on the bottom,middle,top,middle/top of the absorber layer,respectively,and then annealed in Ar gas to systematically investigated the improvement of Sb₂Se₃ deposition position on the morphology,phase composition and photoelectric properties of the absorption layer.It was found that Sb₂Se₃ can promote the grain growth of the absorber layer,and the promotion effect of the double Sb₂Se₃ layer is more obvious.The thermal decomposition characteristics of Sb₂Se₃ at different temperatures were studied,and it was found that Sb₂Se₃ volatilized at high temperature decomposed into Sb and Se vapors,but the volatilization ability of Sb was greater than that of Se.The mechanism that Sb₂Se₃ promotes grain growth is proposed:During the annealing process,Sb flows between grain boundaries to provide energy for grain growth,and Se replaces S with S during the flow to release heat to promote grain growth.The dual effect of Sb and Se is the main reason for Sb₂Se₃ to promote grain growth in the absorber layer.3.A novel cadmium-free MgZnS buffer layer was prepared by magnetron sputtering.When a cadmium sulfide（Cd S）film is deposited on the surface of the absorber layer by the traditional chemical bath deposition,it is difficult to fully adhere to the absorber layer due to the surface tension of the liquid.When the device is made,it is easy to form a shunt channel,which seriously affects the efficiency of the device.A new type of cadmium-free Mg Zn S buffer layer was prepared by co-sputtering Zn S target and Mg target.The effects of substrate temperature,sputtering power and Mg doping amount on the morphology,crystallinity,phase composition,transmittance and band gap of the buffer film were systematically studied.A new cadmium-free buffer layer with tight fit and high light transmittance was obtained,and overcome the problems of narrow forbidden band width,poor crystallinity,non-dense film,and environmental pollution of the buffer layer film.It’s the fundaments to prepare for environmentally friendly CZTS-based solar cells.