Preparation And Properties Of CdTe Thin Films By Electrochemical Deposition

With the increase of energy crisis and environmental pollution problems, much attentionis paid to solar cells. Improving the photoelectric conversion efficiency and reducing theproduction cost of solar cells are still the major problems in photovoltaic industry. CdTe is adirect band gap semiconductor, which band gap energy matchs well with the solar spectrum.Due to its higher absorption coefficient, the theoretical conversion efficiency of CdTe solarcells is about28%to30%. CdTe thin films solar cell is recognized as efficient, stable andcheap photovoltaic device. Because of its advantages of simple process, less equipmentinvestment, low energy consumption, large-scale preparation, the electrochemical depositiontechnique has great potential in reducing cost. At present, the electrochemical deposition ofCdTe thin films is mostly conducted in stronge acidic solution system, having the lowdeposition rate and high requirements for equipment. The present thesis adopts a novelalkaline solution system for electrochemical deposition of CdTe thin films. Theelectrochemical deposition mechanisms were explored, and the effect of process parameterson the film structure and properties were studied.Firstly, CdS polycrystalline thin films were prepared on FTO and ITO conductivesubstrate by chemical bath method. The solution pH, temperature and annealing conditionswere optimized to obtain the CdS films suitable for electrochemically depositing CdTe thinfilm. NTA was the complexing agent in alkaline solution. The electrochemical properties ofsolution were tested by cyclic voltammetry. A three-electrode system and a stirring methodwere adopted for depositing CdTe thin films on CdS substrates. XRD, FESEM, UV-Vis andAC impedance were used to characterize the film composition, surface morphology,transmittance, electrical and optical properties of CdS and CdTe respectively. The effect oflight excitation, the deposition potential, annealing treatment on the composition, structureand properties of CdTe films were studied. The electrochemical mechanisms of CdTe thinfilm in alkaline solution were discussed.Study on CdS thin films prepared by CBD demonstrated that, the film growth rateincreased first and then decreases with increasing the solution pH. With the rising of thetemperature, the reaction speed and the nucleation density are increased while the grain size is decreased. Optimized growth conditions for CdS thin film are2.5mM Cd(COOH)2,25mMNH4COOH，12mM SC(NH2)2，pH=10.5，and85℃. After60min growth,90nm CdS thinfilms with cubic phase were prepare. CdCl2annealing promotes transfermation of CdS filmsfrom cubic phase to hexagonal phase transition.The electrochemical properties of alkaline electrolytes indicates that electrochemicalpotential range of electrodeposited CdTe semiconductor thin films is-0.6V to-1.2V vsHg2SO4. with the potential shiftting to the negative end, the deposition rate was increased, andthe Te/Cd ratio was gradually decreased. The light illumination not only increased depositionrate of CdTe films, and the deposition current density increased from0.07mA/cm2to0.28mA/cm2, but also improved the crystallinity of CdTe films. Mott-Schottky testsconfirmed that CdTe thin films prepared by electrodeposition in alkaline solution is a p-typesemiconductor, which is different from n-type CdTe semiconductor films prepared in acidicelectrolytes. After annealing, CdTe thin films showed random orientation, good crystallinity,and large grain size. After annealing in CdCl2atmosphere, the (220) diffraction peak intensityexceeds the (111) diffraction peak intensity, showing obviously random orientation. TheCdTe semiconductor/electrolyte interface energy level model was established based on theabove results. The interface energy level model illuminated that the light excitedphotogenerated electron-hole pairs in p-type CdTe semiconductor, and the formed interfacialspace charge depletion region under the cathodic deposition potential, promote electronstransferring to solution, and as a result the deposition rate is improved.