Doping
Doping
Doping is the most critical step for manufacturing transparent conductive film, because it is usually difficult to find material with both good transparency and conductivity. However, for manufacturing transparent conductive film, doping can integrate these two properties on one material.
Generally, doping is the process of intentionally adding impurities into the material to change materials’ electric properties. The impurities are called dopants. It is usually used in manufacturing semiconductors, but it is also widely applied in making raw material of transparent conductive film. The amount of impurities doped into the material is called contamination level.[27]
The working principle of doping is heat diffusion. Under high temperature, the dopants’ atoms move actively between the lattice and interstitials of the crystal structure of the doped material to take the place of the original material atoms.
The doping techniques can be classified into solid state proto-diffusion, liquid state proto-diffusion, and gas-state proto diffusion. In transparent conductive film, solid state diffusion is usually applied.
When doping, first, put the dopant material in a crucible and insert the substrate slice in a groove. Put the groove and crucible side by side in a diffusion furnace tube. Blow in inert gases such as argon and then heat it up to 800-900 ℃. The dopant atom will be taken to the surface of substrate slices by the flow of hot inert gas. After heating for long enough time, the dopants will diffuse into the substrate material.[26]
The process is as shown below.
Figure5.1 process of doping
[26]https://www.sciencedirect.com/topics/physics-and-astronomy/doping-materials
[27]https://www.sciencedirect.com/topics/chemistry/doping-material
Coating
Coating
The transparent conductive film is usually made by spin coating different kind of “ink” onto etched PVC or glass substrate to make the final products. The ink is the solution of the nanoparticles of the doped material.
In the first step, the nanoparticle of ITO, AZO, AgNWs, or other common raw material for TPCF are usually modified by organic small molecules such as fatty acid methyl ester to make sure it can disperse evenly in the organic solvent and form homogeneous phase ink. The solvent varies but are usually organic, too.[28]
Secondly, the glass (here we just take glass as an example) substrate will be thoroughly cleaned with ultrasonic cleaning machine in acetone and methanol. For the case of glass substrate, it usually go through the radiation of UV light for several time to introduce more hydroxyl groups onto the surface in order to enhance the binding force with coating layer.
Then, spin coating the substrate at certain temperature in protective air atmosphere to get the preliminary products.
The process is shown in the figure below.
figure 5.2 process of coating
The coating process seems simple but is actually a property-determining process since coating layer should have uniform thickness and smooth surface. Also, the products cannot function well if there is little local contamination. And the coating layer should have enough adhesion strength to avoid stripping off.[29]
Heat treatment
After coating, the material is still not uniformly distributed and there is still residual solvent on the layer. So, the film usually goes through a heating treatment to remove the residual ink and refine the grains of coating material to change it microstructure and acquire homogeneous layer.[30]
Heat treatment is relatively simple. Put the film into oven, under protective atmosphere, and heat it up to certain temperature for a while. The heating temperature depends on the type of substrates. For glass substrate, it is usually 450 ℃. Also, the treating time can affect the grain size of the ITO or AZO and the conductivity of the film is highly related to the grain size of ITO and AZO.[31]
figure5.3 process of heat treatment
The surface of ITO film after heat treatment (Sn%=10%)
[30]https://pubs.acs.org/doi/full/10.1021/la4033282
[31]https://www.sciencedirect.com/science/article/pii/S1005030211600790