Electrodeposition Of CdTe Semiconductor Thin Film From Alkaline Aqueous Electrolytes And Performance Study

Cadmium telluride（CdTe）,an important II-VI compound semiconductor with a direct band gap of 1.44 e V,a high optical absorption coefficient in the visible spectrum and high theoretical conversion efficiency（30%）,is considered as an ideal adsorber material for photovoltaics.Rapid and tremendous progress has been made in CdTe solar cell in the past few years,and the newly reported module efficiency（18.6%）even overpasses the full area efficiency of the market-dominated multicrystalline Si module（17.7%）.Conventional production methods of CdTe solar are sputtering,close space sublimation,vapor transport deposition,electrodepostion,et al.Compared to others techniques,Electrochemical method has shown importance in the synthesis of films,since it is a low-temperature,low-cost,and facile technique with good control of the deposited composition.However,in most cases,CdTe films are electrodeposited in the acid solution.Problems with the electrochemical technique lies mainly in the low solubility of tellurium dioxide,Corrosion of Cd S thin films,Te excess,porosity,poor interface quality and the complex structure.Although a more acidic media has the larger solubility to Te O₂ and corresponds to an improved deposition rate of CdTe film,the rapid deposition leads to a great many stacking defects in films,which deteriorate the device efficiency.There are only limited reports in ammoniacal alkaline solution.The present thesis adopts a novel alkaline solution system for electrochemical deposition of CdTe thin films.The electrochemical deposition mechanisms were explored,and the effect of process parameters on the film structure and properties were studied.The main contents and innovation points of this thesis are as follows:1. The electrodeposition mechanism of CdTe films from Na OH alkaline solution system.The voltammetric behaviors of Te O₃^2-,Cd²⁺with different p H were studied.At p H 9.5,for the solution without Cd Cl₂,the reduction potential of Te O₃^2-was at-0.3 V which was more positive.For the solution without Te O₃^2-,reduction potential of Cd²⁺was not affected by p H.The electrochemical properties of Na OH alkaline solution system indicates that electrochemical potential range of electrodiposited CdTe thin films is-0.8V to-1.2V.CdTe thin film were deposition at four potentials.The phase composition of the as-prepared CdTe films varied with the deposition potential.More WZ phase is formed in the films at the more positive deposition potential.The increased Te content in film should contribute to the formation of WZ phase.Annealing leads to the phase transformation from the WZ phase to the ZB phase.After annealing,films of the proper stoichiometry have good crystallinity and the narrower bandgap.The WZ phase and its transformation as well as its relationship to the film composition provide solid proof for the low efficiency of the low-temperature CdTe film,and will help to resolve the problems during the electrodeposition.2.The study on electrodeposition of CdTe thin films in TMAH alkaline solution systemHowever,to reach the required basicity carboxylic complex solutions were adjusted with a great deal of inorganic alkali,such as Na OH,which inevitably introduced alkali metal ions into the deposited CdTe films.It is said that the alkali metal ions have recently been demonstrated to be electrically active impurities in CdTe solar cells and seriously compromise the device performance.we develop a novel alkaline electrochemical process to eliminate the possible detriment of alkali metal ions on CdTe films.The Tetramethylammonium hydroxide（TMAH）has stronger basicity than the inorganic alkali and good solubility to Te O₂.Meanwhile,the effect of the deposition temperature,photon irradiation,the deposition potential and the complexation ratio of NTA/Cd²⁺on the composition,structure properties of CdTe polycrystalline semiconductor thin films were studied.The deposition rate of the films increased with the increasing of the deposition temperature.It was found that the electrolyte deposition temperature of 80℃,deposition potential at-1.0 V,the deposited films were with good crystallinity and to the cubic（111）preferred orientation growth.The current density was enhanced by irradiation the cathodic surface and,the films obtained under illumination consisted of near stoichiometric CdTe at-0.8 V,simultaneously the phase films composition was changed.By increasing the ratio of NTA and Cd²⁺,the orientation of deposited thin films changed from（111）to（220）.