Preparation And Controlling Composition And Structure Of Copper Indium Selenide/Copper Indium Selenide Sulfur Thin Films By Pulse Electrodeposition

Solar cells converting solar energy into electrical energy are an important means to completely solvethe energy crisis and environmental pollution. Among these, CIS-based solar cell has many advantages,such as excellent optical and electrical properties, high conversion efficiency etc, is recognized as one ofthe solar cell is the most large-scale industrial applications. Electrochemical synthesis CIS-based film hasaroused great interest of photovoltaic researchers due to simple equipment, low cost, large area deposition.In previous studies, direct current （DC） potentiostatic mode as main preparation techniques ofelectrodeposition CIS absorber layer has been extensively studied. However, this mode has limitedregulation on CIS film growth process due to only one controlled parameter （deposition potential）, whichresult in the chemical composition of the CIS film is difficult to effectively controlled. And Cu-Se phaseeasily precipitates in CIS film during the following annealing process. These largely reduce thephotoelectric conversion efficiency of cells. By designing appropriate electrodeposition system, combinedwith the advantage of pulse mode and constant current deposition mode not only can reduce solutionconcentration polarization during electrodeposition process using the advantages of pulse mode, improvethe microstructure morphology of the thin films and enhance the binding between film and substrate. Butalso can make use of the advantages of DC mode to effectively control the stoichiometric ratio of the film,preparing a large area uniform and dense CIS film. Furthermore, we prepared a doping gradientCuIn（Se,S）₂absorbing layer with elemental S by selenium/vulcanization process for one stepelectrodeposition CIS film to improve the photoelectric conversion efficiency of CIS cells and reduce thecosts. The highest conversion efficiency of the battery has now reached21.3%.Based on the above considerations, this thesis introduces the pulse power with DC mode to carry outresearch of CIS thin films. To develop new methods and ways to prepare CIS material, we explore theoptimum conditions and reveale the nucleation and growth mechanism to regulate the composition,morphology, crystal structure and the binding force between film and substrate. The work mainly includesthe following three parts:（1） The study on the composition regulation and Structure of CuInSe₂thin films by a pulse electrode-position technique with the galvanostatic modeCuInSe₂（CIS） thin films were prepared by pulse-plating electrodeposition technique over FTO glasssubstrate in the galvanostatic mode. Compact large area CIS thin films with uniform structure and standardstoichiometric composition were obtained through optimizing the pulse parameters. The chemicalcomposition and structure of the film were studied in detail. The results showed that the chemicalcomposition of electrodeposition CIS film and the Cu2-xSe phases in the film could be effectively controlledby the pulse parameters. Specifically, the composition of the electrodeposition CIS film could be linearlyregulated by the pulse current, and a suitable pulse frequency can inhibit the production of Cu2-xSe phase inthe CIS to improve the photoelectric properties of thin films. （2） The study on nucleation，growth process and electrodeposition mechanism of CuInSe₂The study of CIS thin film growth and electrodeposition progress indicated Se⁴⁺ions are firstlyadsorbed and reduced to Se⁰which occupied most of the surface nucleation sites of FTO. Then the Cu²⁺started to deposit and reacted with Se⁰to form Cu2-xSe nuclei. Finally, In3+began to deposit and theformations of Cu2-xSe in turn reacted with In3+ions to form CuInSe₂nuclei. Furthermore, the initialnucleation process of CIS thin films indicated that the Se⁴⁺ions were first restored and occupied most of thesurface nucleation sites due to the complexation of Na-citrate for copper ions, which was very significantlydifferent from the previous reports that Cu²⁺first absorption and reduction at the initial nucleation stage ofCIS film. These results may provide an important insight for the electrodeposition preparation CISmaterials.（3） Electrochemical synthesis and characterization of CuIn（Se,S）₂films with graded compositionCuIn（Se,S）₂thin films with with graded composition were prepared on FTO substrate by controllingthe sulfurization temperature and Cu/In of CuInSe₂thin films. The process included the single step pulseelectrodeposition of CuInSe₂precursor followed by a selenide and sulfurization procedure. The resultindicated the sulfurization conditions and the ratio of Cu/In in precursor film significantly affected thegraded composition and structure of CuIn（S,Se）₂the film. With the temperature of sulfurization increasesfrom250to550°C, the Eg linearly increases from0.98to1.67eV. The X-ray diffraction indicated thestrong dependence of S incorporation into CIS films on the Cu/In ratio of the precursor. The CuIn（S,Se）₂films with strong crystalline and graded structure of CuInSe₂-CuIn（S, Se）₂-CuInS2were prepared at thenear-stoichiometric Cu/In ratio at lower sulfurization of300℃. This work provides a simple and low-costtechnique for preparing large area CISS absorber layers with graded composition.