A proper doping amount will improve tungsten oxide thin film property. TiO2-doped tungsten oxide thin film lowers its defect. Ni and Co doped thin film can reduce polarization voltage, which can improve stability of tungsten oxide thin film. About doping vario-property, people usually use quantum-mechanical theory to calculate impurity level, set up color center model, valence shift model and small polaron model to analyze coloring mechanism, use electrochromic reaction to analyze bleaching state and coloring state. Proper doping amount provides more electron which improved conductivity of material. If it is over doped, there would be too much impurities accumulated and destroy crystal structure, thus would lower the conductivity.
Thin film material are composed of very small crystalline which can be single or multi crystalline. For a certain electric material, we assume that it is composed of NA atom and NB doped atom. In the crystalline, when the doping amount is high enough, atom would be replaced by impurity and forms a BA, when another BA forms, they meet together which causes accumulation of impurities. It destroy the crystalline structure which exceeds previous doping amount. For the sol-gel method and evaporation method, chemical stoichiometric ratio is very accurate, temperature of thin film is low and the spread of impurities are uniform. So it lowers the possibility of accumulation. For the magnetron sputtering method, preparing temperature is lower, spread of impurities are uneven, increase the possibility of accumulation. For vacuum vapor evaporation method, it requires higher preparing temperature, spread of impurities are uneven, increase the possibility of accumulation. For chemical vapor deposition method, producing temperature changes a lot, increase the possibility of accumulation.
For a certain kind of thin film crystalline structure and preparation method, we can identify crystalline ligancy and powder changes of atom during formation of thin film, calculating the optimum doping amount x in a wide value range. For sol-gel method, the optimum doping amount is x＝14.2857％；for magnetron sputtering and electric beam evaporation method, the optimum doping amount is x＝8.6647％；for vapor evaporation method, the optimum doping amount is x=5.2554%；for chemical vapor deposition method, x=3.1876%. These results do not referring to the doping elements which need to be proved in experiment.