The EC color-changing layer is the core of the whole system, and the performance of the device mainly depends on it. This layer can be either an anode coloring material or a cathode coloring material. The shape, composition and structure of EC films have a significant influence on the vertical preparation methods, and the films obtained by different methods are generally different. But it also provides a choice for improving methods, preparing films with better performance and developing more practical devices. Among the inorganic electrochromic materials, WO and MoO are the two most promising oxides, and there are many studies on them. Because they belong to the same family and have many similar properties, they are introduced here.
Experience shows that in order to obtain high electrochromic activity, it is necessary to have some oxygen defects in the film [55.65], that is, wO-, or MoO–, and the electrochromic effect of the film is better. When preparing MoO thin films by sputtering], if the partial pressure of oxygen in the vacuum chamber is small, MoO deviating from the stoichiometric ratio will be obtained. Film; When the oxygen partial pressure is high, the film of MoO, which is close to the stoichiometric ratio, is obtained, and the discoloration efficiency of the former is obviously higher than that of the latter. Moreover, the discoloration efficiency of amorphous films or microcrystalline films is higher than that of crystals [S2,89]. Chemical vapor deposition (CVD) method was used, and W(CO) was used. The color change efficiency of the amorphous film obtained as the starting material is 230cm2/C, while that of the crystalline film is only 41cm2/c52]. The reasonable explanation is that the amorphous thin film has a high degree of imperfection, an open network structure, a large number of oxygen defects and grain boundaries, which are very beneficial to ion transport, so the color change effect is good. In addition, the thickness of the film also has a certain influence on the electrochromic properties. Thin films (less than 200nm) have fast coloring/decoloring speed, while thick films (> 500nm) have deep discoloration, but the decoloring time is long [83].
The existence of a certain amount of water in the film also has an important influence on the electrochromic behavior of the film. It is beneficial to contain a small amount of water, but if it is too much, it will corrode the film after many cycles and shorten the electrochromic life of the film. Al-
Nordessun[61] studied the dissolution behavior of WO-film in aqueous phase in detail. At 25℃, wO. The solubility in water is 1.3× 10 ~ 3mol (pH = 7). With the decrease of pH, WO. Polytungstate ions will be formed in the solution, and adding
Dissolution of fast film. Amorphous WO films deposited by vacuum evaporation have a fast bath decomposition speed and a large bath decomposition weight. Theoretically, 1ml of water can dissolve the WO film with a thickness of 4nm and a size of 1cm’ at most, that is, it is less than the thickness of a molecular layer. However, the actual situation is far more than that. The dissolved WO far exceeds the limit of precipitation, so there must be the formation of tungstic acid ions in the solution. The test results show that there are meta-tungstate and psudo-meta-
Tungstate ion, as shown in figure 17.9. Its basic unit is a triangle formed by three WO octahedrons with the same side. M-t is a tetrahedron composed of four basic units, and m-t-like is composed of two basic units. This trimer unit exists in a large number of compounds related to WO, forming a low symmetric structure of 3,5,6 yuan ring.