In these devices, structures with large conducting paths of several millimeters in size alternate with very small structures of several micro- or nanometers in size. So far, these conducting paths have been produced in various production steps. Now, the researchers at the INM Leibniz-Institute for New Materials have succeeded in producing macroscopic and microscopic conducting paths in a single production step.
"For this purpose, we add a photoactive layer, which consists of metal oxide nanoparticles, to a substrate," says Peter William de Oliveira, Head of the Program Division "Optical Materials". "Afterwards, we apply a colorless, UV-resistant silver complex," adds de Oliveira. By exposing this stack of layers to light, the silver complex is decomposed at the photoactive layer, and the silver ions are reduced to silver. This technique has several advantages:
It is fast, flexible and non-toxic. With only one step, followed by rinsing with water, the production is completed in only a few minutes. Further process steps for the post-treatment can be omitted. Even a heat treatment is not absolutely necessary. With this technique, the researchers at INM obtain layers with a thickness of up to 100 nanometers and a specific conductivity, which corresponds to a quarter of the specific conductivity of pure silver. A heat treatment of 120°C increases the conductivity up to half of silver.
With this basic principle, the researchers at INM are able to apply very individually conductive paths of various sizes onto substrates such as glass or plastic. "There are three different methods we can use depending on the requirement: The "writing" by UV laser is very suitable for the first, custom-made production and the testing of a new design of the conducting path. But this method is too time-consuming for mass production," says the physicist de Oliveira.Photo masks, which are only permeable to UV light at the requested positions, can be used for the structuring. "However, the production of these masks is expensive and environmentally intense. For a "semi-continuous process", they are particularly appropriate for solid substrates such as glass ", says the materials expert. However, they are not appropriate for roll-to-roll processing, as they are mainly made of quartz glass and are not flexible.
The researchers are working intensely on a third method, i.e. the use of so-called transparent stamps: "These stamps replace mechanically the silver complex, and where is not silver, there is no conducting path," says de Oliveira. "With these stamps, we can form structures with the size of a few micrometers. The stamps are made of a soft polymer, thus, we can apply them on a roll. As they are transparent, we are working on implementing the UV source directly into the roll. Hence, the first steps for a roll-to-roll-process would be done," the head of the program division summarizes. With this process, conducting paths of various sizes can be produced on substrates such as polyethylene foils or polycarbonate foils in large scale.
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