Technology And Fundamentals For Electrodeposition Of Metal Selenide Thin Films

Metal selenide thin films are one of the most important semiconductor materials, which have extensive applications in many fields such as solar cells and sensors, and attracted special attention from academic to industrial community. Deposition of metal selenide thin films is the premise of these applications. Among various deposition technologies, electrodeposition is a non-vacuum, low cost and high deposition rate process, that is suitable for large area, continuous preparation of thin films with multicomponent and controlled properties.In this dissertation, technology and fundamental for electrodeposition of metal selenide thin films have been studied. First, the basic thermodynamic features of metal selenides electrodeposition have been revealed by potential-pH diagram mapping and analysis. Second, the electrochemical behaviour of selenium electrodepositon which often occurs firstly during metal selenides electrodeposition was studied by cyclic voltammetry (CV) and chronoamperometry (CA). Based on above studies, the electrodeposition mechanism, process optimization and characterization of CoSe, Sb2Se3 and Cu(In,Ga)Se2 were investigated. At last, a dynamic model of metal selenides electrodeposition was developed, which can realize the composition prediction and applied in Sb2Se3 and CuInSe2 electrodeposition. Innovative research results obtained are as follows:(1) The thermodynamics laws of metal selenides electrodeposition were interpreted. Potential-pH principle diagrams of metal-selenium-H2O and potential-pH diagram of typical metal-selenium-H2O were drawn. It is demonstrated that the electrodeposition of metal selenide thin films is performed in weak acid electrolyte (pH=1?3) and proceeds by Kroger mechanisms, whose deposition potential is usually more positive than that of Se electrodeposition. Different metal selenides have different deposition potentials and pH regions. The more positive standard redox potential of metal has the more positive deposition potential of corresponding selenide and the larger pH region.(2) Nucleation and growth mechanisms of selenium electrodeposition were revealed. The electrodeposition of Se proceeds by four-electron direct reduction of H2SeO3 or a comproportionation reaction between H2SeO3 and product of its six-electron reduction. Four-electron direct reduction involves two processes: pre-deposition which occurs at low overpotential and bulk deposition which occurs at high overpotential. High overpotential would lead to the occurrence of six-electron reduction. The nucleation and growth process exist in both four and six-electron reduction potential regions. The deposition of Se shows a 2D growth under instantaneous nucleation followed by a multilayer spiral growth mode at four-electron reduction potential region and a 3D growth under progressive nucleation at six-electron reduction potential region. The deposition temperature, pH and H2SeO3 concentration have significant effects on reaction characteristics, but do not change the mode of nucleation and growth in six-electron reduction potential region.(3) The electrodeposition mechanisms of CoSe thin films have been realized and the preparation process has been established. The deposition of Co into solid phase can proceed through two different routes:surface induced reduction by Se and reaction with H2Se from CV analysis. Se-rich CoSe thin films with compact and homogeneous morphology, hexagonal crystal structure and minimum stress were obtained at deposition potential of-0.5V vs. SCE, deposition temperature of 50?and pH value of 2.0. The electrodeposited CoSe film exhibits an optical absorption coefficient of higher than 105 cm-1 and an optical band gap of 1.53±0.01 eV, which is suitable for absorb layer materials of solar cells. The electrodeposited CoSe film shows a Se-rich bulk composition and p-type bulk conductivity, and a Se-poor surface composition and n-type surface conductivity. This electrical characteristic is benefit for formation of a buried homojunction which can reduce recombination of photo-generated carriers.(4) The electrodeposition mechanisms of Sb2Se3 thin films have been realized and the preparation process has been established. During Sb2Se3 thin film electrodeposition, Se deposition occurs first and then Sb. The deposited Se will induce the reduction of Sb, on the other hand, the deposited Sb also will promote the deposition of Se. Thin film with Se-poor composition has better morphology than that with Se-rich composition. The optimized deposition condition is as follows: deposition potential of-0.55V vs. SCE, deposition temperature of 25?and pH value of 2.0. After annealing, Sb2Se3 thin film have inconspicuous change in composition and band gap, but shows more compact morphology, improved crystallinity and orthorhombic Sb2Se3 phase with (212) preferred growth orientation. However, annealing worsens electrical properties, exhibiting reduce of photo-current and photo-voltage and negative shift of flat band potential, due to the formation of Sb second phase with high electrical activity which acts as recombination center of photo-generated carriers.(5) The incorporation mechanisms of In and Ga, and the effects of complexing agent sodium sulfamate during Cu(In,Ga)Se2 thin films electrodeposition were studied. The incorporation of indium into the solid phase occurs via surface-induced reduction by initially deposited copper selenides to form CuInSe2, reaction with H2Se to form In2Se3, or direct reduction of In3+to In. The insert of gallium into the growing films proceeds by reaction with H2Se to form Ga2Se3, or Ga2O3/Ga(OH)3 from the hydrolysis of Ga3+. Sodium sulfamate can inhibit the reduction of Cu2+, Cu+and H2SeO3, and accordingly hinder the formation of copper selenides. The influence regularities of deposition process on film composition and morphology were also understood, and it is found that annealing in Ar+H2S atmosphere can remove the oxygen and widen the band gap, and not lead to the deterioration of morphology.(6) A dynamic model of metal selenides electrodeposition was constructed and applied in Sb2Se3 and CuInSe2 electrodeposiiton. It is shown that this model realized the prediction of atom ratio for different main salts concentration ratio successfully. Known from the analysis of model, for Sb2Se3, negative shift of temperature from-0.5V(vs.SCE) to -0.6V(vs.SCE) or increasing deposition temperature from 25?to 50?can promote the increase of Sb content, while the adding of complexing agent tartaric acid or citric acid will inhibit the deposition of Sb. For CuInSe2, the codeposition of Cu and Se can be carried out for a large range of main salts concentration ratio; when co-depositing Cu and In, the mole fraction of Cu in thin film will higher than that of Cu2+in electrolyte; and for codeposition of In and Se, In3+ concentration should much higher than H2SeO3 concentration. It is found that the error of the prediction results can be reduced after correcting the differences of site selectivity constant caused by the change of deposition potential.