| Molybdenum(Mo)thin films have been widely used in aerospace,electronic display,communication and photovoltaic new energy due to its excellent electrical,thermal and chemical stability.Especially for the new thin film solar cells,the low contact resistance and good thermal stability make the Mo film as an ideal material for the back electrode of thin film solar cells,which plays a key role in improving the efficiency and stability of thin film solar cells.At present,the main problem to be solved for Mo film as back electrode is the contradiction between adhesion and electrical properties.In order to develop Mo film back electrode with good electrical properties and good adhesion,the process optimization was carried out from the selection of original structure of Mo target,heating method of single layer Mo film,structural adjustment of bilayer Mo film and heat treatment,etc.And its influence on the structure and photoelectric properties of Mo film was explored to prepare good Mo films with good electrical and adhesive properties.The microstructure of the Mo films was characterized by XRD,SEM,STEM and AFM.The electrical and optical properties of the Mo films were analyzed by Hall effect tester and UV Vis spectrophotometer.The effects of target structure,heating method,film thickness matching,sputtering mode,sputtering pressure and annealing temperature on the structure,electrical and optical properties of Mo films were systematically studied.Using Mo film as back electrode,CIGS thin film solar cells were prepared.The main research contents are as follows:1.The relationship between the original microstructure of the target and the structure and properties of Mo films was established.The effects of rolling temperature and rolling direction on the structure and properties of Mo target and Mo films were investigated.The effects of rolling direction and rolling temperature on the structure,grain size and distribution of Mo target materials were studied,and the relationship between the structure and properties of Mo films prepared with them was also studied.The target materials with better performance were selected to provide guarantee for the preparation of films with good performance.The results show that the Mo target prepared by cross rolling at 1200℃ has fine grain size and uniform distribution,and the preferred orientation of(110)crystal plane is stronger.The Mo film prepared with the target has more uniform and compact structure,smoother surface and the best photoelectric performance.2.The structure and photoelectric properties of single-layer Mo films were optimized by different heating methods.Single layer Mo thin films were prepared by DC magnetron sputtering.Different heating methods,including substrate heating(in situ heating during sputtering),annealing treatment(heating after sputtering)and dual heating processes,were used to investigate the effects of different heating methods on the microstructure and properties of single-layer metal Mo films.The results show that:compared with the substrate heating,the annealed Mo film has better photoelectric properties,and the metal Mo film prepared by double heating method has the most uniform and dense grain distribution,smoother surface,the least defects and excellent photoelectric performance,but the adhesion slightly decreases.The efficiency of CIGS solar cells is 12.8%,which is 43.8%higher than that of the solar cells prepared at room temperature.3.The effects of sputtering mode and thickness matching on the structure,adhesion and photoelectric properties of the bilayer Mo films were studied.Bilayer Mo thin films were prepared by DC/DC and RF/DC sputtering modes,respectively.The effects of thickness matching of the upper and bottom layers on the structure and photoelectric properties of the bilayer Mo films with the different ratios of the bottom layer thickness to the whole film thickness were studied.In the same sputtering mode,with the increase of thickness of bottom layer,the adhesion of Mo films increases obviously,and the resistivity of Mo films also increases,but the optical reflectivity increases gradually.Under the same thickness matching condition,although the electrical properties of the bilayer Mo films prepared by RF/DC method are slightly lower than those by DC/DC method,their optical properties and bonding properties are better.The comprehensive evaluation results show that the RF/DC mode is the best method to prepare bilayer Mo films with thin bottom layer.4.The microstructure and element distribution of the interface between soda-lime glass substrate and Mo film were studied by HRTEM and EDS at the atomic scale,and the bonding mechanism between glass/Mo film interface was revealed.The results show that the bonding mode between glass and Mo film is diffusion bonding,Mo atoms diffuse to the glass substrate,Ca and Na atoms diffuse to the side of Mo film,forming a diffusion gradient interface layer.The sputtering pressure of bottom layer has a great influence on the formation and thickness of the gradient interface layer.When the sputtering pressure of bottom layer increases,the number of sputtered atoms per unit time increases,which promotes the diffusion between atoms on both sides of the interface,and the gradient interface layer between glass and Mo film thickens.When the sputtering pressure of bottom layer is 0.4 Pa,the thickness of the gradient interface layer is 7.5 nm,and 1.2 Pa sputtering pressure of bottom layer corresponds the gradient interface layer of 12 nm.The thicker the gradient interface layer is,the stronger the adhesion between the film and the substrate.5.The mechanism of the influence of the bottom pressure on the diffusion,microstructure and adhesion of the interface elements,and the effect of the subsequent annealing temperature on the microstructure and photoelectric properties of the bilayer Mo films are explained.The larger the bottom pressure is,the thicker the small crystal nucleus layer forms at the interface between glass and Mo film,the smaller the grain size is,the more the amorphous particles are,and the more the defects are.The thicker the diffusion layer(gradual interface)is,the better the adhesion is,but the conductivity is reduced.When the sputtering pressure of the bottom layer and the top layer is 0.4 Pa and 0.3 Pa,respectively,the resistivity of the bilayer Mo film annealed at 400℃ is as low as 9.2×10-6 Ω·cm,and the average reflectivity is up to 55%.The optimal performance is achieved,which realizes the unity of good adhesion and excellent photoelectric properties.The efficiency of CIGS solar cells used above Mo film as back electrode is as high as 13.5%without antireflection layer,which is 51.7%higher than that of solar cells with Mo film as electrode at room temperature under the same conditions,which lays a good foundation for improving the photoelectric conversion efficiency and stability of solar cells. |
PDF Full Text Request