Manufacturing of organic components

Design study of a lamp which utilizes quadratic OLEDs as emitting material

Organic electronic components such as OLEDs and organic solar cells are thin and of large area. They are no more than a few ten thousandths of a millimeter thick and they are not mechanically resilient. Therefore they are deposited onto a suitable carrier material, the substrate. Common substrates are glass or plastic foil; alternatively metals or other smooth surfaces may also be used.
The different materials, of which the component is made, are deposited onto the substrate in thin films, one after the other. Two different manufacturing processes can be used here: Vacuum deposition or solution-based processing. For the production of OLEDs – technologically the most advanced application of organic semiconductors – as well as OLED lighting vacuum processing has been adopted. For solar cells however both processes are currently undergoing further development.

Solution-based processing

Solution-based processing is similar to printing with a conventional ink jet printer or a printing machine. The organic materials are dissolved in a liquid solvent, just like the color pigments in normal printer ink are dissolved in a liquid. Following this analogy, the solution is also referred to as ‘electronic ink’. This ink – carefully dosed – is applied to the substrate. The solvent is volatile, so once it has evaporated only the organic semiconductor remains on the substrate as a thin film.
Solution-based processing is the simplest and technologically the most field-tested method for the fabrication of thin material films. It is suitable for processes in which films in the region of several micrometers or thicker are needed and where only one or very few layers of organic semiconductor are to be deposited. For components which comprise several clearly separated layers, specific combinations of organic semiconductors and solvents have to be used so that one film can be deposited onto an already existing one without etching this first film and thus causing an unwanted mixing of the two materials. Polymers (very large molecules) are often used here since they are less susceptible to solvents once they have been deposited. Processing on flexible substrates in a ‘roll to roll’ procedure is more easily possible, similar to newspaper printing machines.

Vacuum processing

For vacuum processing the – high purity – material is available in the form of a powder. It is heated and then sublimated1 or evaporated, similar to water at high temperatures. The substrate is placed above the heated material source so that the evaporated material can condense on its surface as a thin film – similar to a mirror or window glass on which water vapor condenses as a thin film of water drops. Both material source and substrate must be placed in a vacuum, i.e. in an evacuated chamber.2
Vacuum processing allows for the fabrication of any number of layers on top of each other without limitation. Substrates here can be large, up to several square meters (3m x 6m are possible today). Layer thickness, evaporation speed and the mixing of several materials can be fine-tuned and controlled with great precision. The materials consist of smaller molecules as these can easily be evaporated without any chemical damage to the molecule itself (e.g. ‘scorching’ inside the crucible). Vapor coating of individual substrate plates (e.g. displays) in a vacuum chamber is today’s state of the art and is already being used in the mass production of OLED displays. The ‘roll to roll’ procedure in vacuum is currently under development, for example at the company Heliatek and Fraunhofer COMEDD in Dresden.

 

 


1: The word ‘sublimate’ refers to the direct transition from the solid state to the gaseous state without a melting step in between. This phenomenon can, for example, be observed for snow during a cold, dry week in winter, when piles of snow reduce in size without the occurrence of melt water. Instead, the snow sublimates, thus increasing air humidity.

2: The vacuum is necessary for two reasons: For one, it prevents turbulences and the formation of clouds during evaporation so that the material can move directly and uniformly from source to substrate and there form a smooth film. Secondly it protects sensitive materials from disturbing influences such as humidity or atmospheric oxygen.


2012 by OES, Johannes Widmer