Preparation And Characterization Of CdS Thin Films By Urea-assisted Homogeneous Precipitation And Its Application In CdTe Solar Cells

Cadmium telluride?CdTe?thin-film solar cells are promising hotovoltaic energy conversion devices with great development potential and application value.As a light absorbing layer,CdTe has an optical bandgap of 1.45-1.5 eV,which is in good agreement with the solar spectrum.The film with only 2?m thickness can absorb more than 90%of solar radiation energy.Cadmium sulfide?CdS?is a direct bandgap semiconductor material.The theoretical bandgap is 2.42 eV.It is an ideal window material.Therefore,CdTe and CdS are very suitable for the preparation of thin film solar cells.Although significant progress has been made in the manufacturing of CdTe solar cells in the past decade,there are still some key issues to be resolved.These key issues can be summarized as follows:?1?How to achieve effective doping of CdTe;?2?How to prepare a low contact resistance electrode;?3?How to prepare a window layer CdS film to meet the following requirements:The thin film is sufficiently thin,uniform and dense,with good adhesion and low material resistance.?4?How to effectively control the interdiffusion of CdS and CdTe contact surfaces.Homogeneous deposition method is used to prepare CdS thin film in this paper.In the experiment,FTO,cadmium chloride?CdCl₂?and thiourea?CS?NH₂?₂?was used as the substrate,cadmium source and sulfur source,respectively.Urea?CO?NH₂?₂?is used to provide hydroxide ions for the preparation of CdS thin films.The effects of water bath temperature,CO?NH₂?₂ concentration,CdCl₂ concentration,CS?NH₂?₂concentration and deposition time on the quality of CdS thin films were studied.The results of X-ray diffraction?XRD?analysis showed that the as-prepared CdS thin films had a wurtzite structure.The bath temperature was 90°C,the urea concentration was 1.5 M,and the cadmium-sulfur concentration ratio was 0.02 M:0.2 M.The deposition time is 1 h.The as-prepared film is even and dense,which is suitable as the window layer of CdTe thin-film battery.The as-prepared CdS thin film is applied to the preparation of CdTe thin film solar cells.CdTe thin films are prepared by magnetron sputtering on CdS thin films.The preparation conditions are as follows:pressure was set at 0.7 Pa,sputtering power was set at 90 W,deposition time was 0.5 h,target base distance was 7 cm,and the thickness of CdTe film was about 1?m.Gold is directly plated on the surface of the processed CdS/CdTe heterojunction film as an electrode using an ion sputtering apparatus.The effect of CdS film thickness on the efficiency of thin-film solar cells was studied.It was found that the thickness of CdS thin film less than 200 nm can easily peel off when sputtered CdTe thin film.When the thickness of CdS film is more than200nm,the light absorption performance of CdS film may become stronger,resulting in the decrease of the photoelectric performance of the solar cell.The effects of annealing treatment on CdS thin films in different environments and their effects on solar cell performance were studied.It was found that the annealing of CdS films under different annealing conditions?air,nitrogen,vacuum?will affect the resistivity of the films.The resistance of CdS films annealing in nitrogen is the smallest,and the performance of solar cells is the best.The effect of annealing temperature on the CdS/CdTe heterojunction film under nitrogen atmosphere was studied.The best annealing temperature was 400°C.When the temperature is higher than 400°C,the film will appear holes.When the temperature is lower than 400°C,it is not beneficial for the formation of interface between CdTe and CdS.The effect of annealing on the preparation of the electrode was studied.It was found that the annealing of the counter electrode can promote the combination of the gold electrode and the cadmium telluride film.The annealing temperature of 150°C is beneficial to improve the battery performance.