Study Of The Preparation Of Chemical Bath Deposited Cds Thin Films

CIGS thin film solar cell is one of the most potential development solar cells,forit high efficiency and low costs. CdS thin film is used as buffer layer in CIGS thinfilm solar cells. The thin film could improve the interface state of CIGS/CdS/ZnO,protect CIGS film,and play a decisive role in improving the efficiency of solarcell.CBD is a low cost growth technique capable of producing good quality CdS thinfilm over large area.At present, the growth mechanism of CBD-CdS thin film is notclear. The film growth rate and the precursors (cadmium acetate, thiourea) utilizationhave been ignored, only a few scholars have researched that.In this paper,using the CBD method,CdS thin film were deposited on glass froma chemical bath containing cadmium acetate,thiourea,ammonia and ammoniumacetate.By analyzing the growth mechanism of CBD-CdS thin film,we found that themain deposition parameters are solution components, bath temperature and stirringspeed.Thus,I designed a five levels and six factors orthogonal experiment.Using rangeanalysis method,this paper have analysed the variation of the film growth rate and theprecursors utilization with deposition parameters,in order to find out the optimumprocess.Under the optimum process conditions,we studied the influence of depositiontime on the surface morphology,crystal properties and optical properties of CdS thinfilm.After analysis of the growth mechanism of CBD-CdS thin film and experimentalresults, we made conclusions as follows:Along with the concentration of free Cd2+and S2-increased,the film growth rateincreases, the dominant growth mechanism of CdS thin film changes from ion-by-ionmechanism to cluster by cluster mechanism. Mechanisms of influence of differentdeposition paramters on the film growth rate are different.Through the variouschemical reactions, each reactant changed the concentration of free Cd2+and S2-,therise of temperature not only change the concentration of free Cd2+and S2-,alsopromote Cd(NH3)4+,OH-,SC(NH2)2ions diffusion and adsorption onto the substrate;the stirring speed increases,speeding up mass transfer velocity of OH-,SC(NH2)2andthe transfer of deposited particles,thus affecting the growth rate of the thin film.The influence extent of different deposition paramters on the film growth rateand the precursor utilization is different. The influence extent of different depositionparamters on the film growth rate has been ranked from large to small: bath temperature,ammonium acetate,stirring speed,cadmium acetate,thiourea,ammonia.Onthe cadmium acetate utilization:cadmium acetate,ammonium acetate,bathtemperature,stirring speed, thiourea,ammonia. Thiourea concentration is the maininfluencing factor of thiourea utilization,and the rest deposition paramters aresubordinate effect factors.First consider the quality of the CdS film,followed the precursorutilization.Under this premise,the optimum process conditions for preparing CdS thinfilm have been determined by orthogonal experiment as follows: Cd(CH3COO)24mM,SC(NH2)214mM, CH3COONH430mM, NH3·H2O400mM,temperature of70℃,stirring speed of200r/min. Under the optimum process conditions,CdS films wereprepared,when deposition time was25min,the film was uniform,adhesive and fewdefects, and also high light transmittance, and a direct band gap(2.52eV).It is verysuitable for buffer layer of CIGS thin film solar cell.