Research On Selenization And Sulfization Of Electrodeposited Cu-In-Ga Metallic Precursor

The highest efficiency of Copper Indium Gallium diselenide (CIGSe) thin film solar cells has reached20.4%, which was far more than other thin-film solar cells. And with the advantages of low cost, no resistance and stability, it became one of the most promising photovoltaic devices. Electrodeposition which had the advantages of the high material utilization rate and low cost input was one of the non-vaccum preparation processes, and became the important research direction of the preparation of CIGSe solar cells. However because of the problem of the poor crystalline quality, the difficulty of the incorporation of optimum Ga in the film and the optimization of film bandgap, at present, the efficiency of the CIGSe thin-film solar cells prepared by electrodeposition was still low. For these three main problems, this paper has researched on the electrodeposition of CIGSe thin-film solar cell in two parts which were the preparation process of the precursor layer and the selenization process.In the preparation process of the precursor layer, the difficulty of the Ga electrodeposition on the Cu/In substrate has been mainly resolved in this paper. In-depth research has been taken on the solution of the Ga electrodeposition by the simulation software and electrochemical experiments. Found that KNH2SO3as the electrolyte can significantly improve the solubility of Ga and the stability of the solution. The solution in low pH value had higher stablity, and the current efficiency of the Ga electrodeposition was not influenced by the pH value at more negative potential(<-3V). The hydrogen evolution reaction was the main factor which affected the quality of Ga thin film by electrodepositing. Because of the adsorption on the Cu/In substrate, Triethanolamine and glucose could effectively decrease the hydrogen evolution reaction, increase the current efficiency of Ga and improve the quality of the film. Besides that, the combined use of triethanolamine and glucose would show more levelling effect on the Ga film by the electrochemical impedance test. Therefore the Ga solution with Ga2(SO4)3as the main salt, KNH2SO3as the electrolyte, Triethanolamine and glucose as additives, pH value below2was confirmed, which could long-term electrodeposite high quality Ga films as stable.For the first time found the Ga would diffuse into the film and through the In layer, and then change with CuIn alloy to CuGa2alloy as phase transition in the Ga electrodeposition process on the Cu/In substrate, and put forward the physical mechanism. It was because the In film is loose and the size of the Ga atom is small, the diffusion of the Ga is easy. And the melting point and the alloy formation energy of Ga is low, Ga could easily alloy with other metal. In addition, the crystal package of CuGa2alloy is much smaller than CuIn alloy, and the structure is more like Cu and In, CuIn alloy easily change to CuGa2alloy as phase transition. Through the SIMS test, the actual structure of Ga film electrodepositing on the Cu/In substrate was Cu/CuGa2/In.In the selenization process, single-phase CIGSe thin film was obtained in this paper by using the plasma activation Se source in order to solve the difficulty of the incorporation of optimum Ga in the film. Combining with the characteristics of the metallic precursor by electrodepositing, the adhesion of the CIGSe film to the Mo substrate was enhanced in the alloying process before the selenization. Systematic research on the effect of gas and plasma power on the CIGSe film has been taken in the plasma selenization process. H2could help the large molecules Sen(2≤n≤8) crack to the smaller ones as a catalyst and improve the activity of Se. Using the plasma activation Se source could eliminate the phenomenon of CuInSe2-CuGaSe2(CISe-CGSe) two-phase separation, obtain single-phase CIGSe thin film at the specific plasma power. Comparative analysis between the reaction kinetics and the experimental results showed that the formation of Ga-Se phase is the control step of the formation of single-phase CIGSe. According to this conclusion, the optimization of plasma selenization process in stages was put forward, and the CIGSe thin film solar cell by optimized had higher efficiency. The efficiency is9.42%.This paper also has researched on sulpho-selenization of the CIGSe film to solve the problem of the low bandgap of the CIGSe thin film surface. The research found that there is appearing a phenomenon of Ga and S accumulating at the surface and backside of the film in the sulpho-selenization process of electrodepositing CIGSe film, different with the film by sputtering or other preparations. Through the research on selenization, sulfization and sulpho-selenization of the Cu-In-Ga metallic precursor, the the mechanism of this phenomenon has been revealed. It was because the CIGSe film is incompletely selenization and has more grain boundaries and holes, S has higher activity than Se, more easily diffuse into the backside of the film and react with Cu-Ga alloy and Ga, so that make Ga diffuse along the grain boundaries to the surface of the film and accumulate there. In addition, Cu-(Se,S) phase preferentially reacted with In-S phase, and inhibited the formation of CISe phase so that In-Se phase diffused into the film, the distribution of In became more uniform. According to this conclusion, the optimization was put forward that change the substrate temperature of selenization process in order to optimize the film bandgap, and reduce the substrate temperature of sulpho-selenization in order to prevent the Cu-(Se,S) phase accumulating at the surface of the film. Finally, the CIGSSe thin film solar cell with the efficiency up to10.4%was obtained.