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Published: 2019-02-12

New possibilities for light-emitting plastics with patterned light

NEWS The precise thickness of ultra-thin films of light-emitting plastics is important for their brightness. By layering two plastic films on top of each other, and writing in one of the films with a sharp stylus, light-emitting patterns like for example a signature can be created. Mattias Lindh, who has studied the operation of light-emitting electrochemical cells, writes this in his PhD thesis.

Text: Ingrid Söderbergh

There are potential uses of light that require flexible and versatile light-sources with emission from large and patterned areas.

– The unique light-sources we work with, which have the complicated name light-emitting electrochemical cells, are very well suited to fulfil these criteria, and at the same time the fabrication can be cost effective and environmentally friendly, says Mattias Lindh, PhD student at the Department of Physics at Umeå University.

Light-emitting plastics can already today be found in, for example, television and smartphone displays and other kinds of portable electronic devices, light-emitting indicators and as illumination for cars and aircrafts with special demands. The knowledge about how light is created and affected in the light-sources is, however, comparatively limited.

In his thesis, Mattias Lindh shows that the precise thickness of a film of light-emitting plastics is decisive for how bright and efficient the light-sources will be. Reflections within the light sources give rise to interference—that is, light waves which interact and can enhance or decrease the intensity—which is affected by the thickness of the film. To get efficient and bright devices the film should be made with just the right thickness; so that it matches a maximum, where the light waves interfere constructively.

But there are also other loss channels in the light-sources, and through computer simulations it can be determined which these loss channels are, and how big they are. From the simulation results Mattias Lindh in his thesis suggests a number of ways for how to design future optimized light-sources that deliver maximum brightness with high efficiency.

– For example, it is possible to change the order of the films or change the flatness of the interfaces between them, that would affect the reflections of the light, he says.

Mattias Lindh also studied how light-emitting electrochemical cells operate on a more detailed microscopic level, and realized that light-emitting patterns can be created by putting two thin plastic layers on top of each other. The first layer can be printed with an inkjet printer and form a kind of display with high resolution and adaptable patterns. Using this technique the need for expensive and complicated methods to create the pattern is lifted, and the patterns instead form directly during fabrication.

Another possibility is to scratch one of the films with a sharp stylus like a needle or a mechanical pencil, which results in manually formed and personal light-emitting patterns. This way it is possible to produce light-emitting signatures or marks that would be very difficult to forge, which can be important for authentication of valuable or security classed documents and products.

– In addition to the scientific publications, me and my colleagues are also pleased to have been able to submit a patent application related to our findings on patterned light-emission. It is an exciting process and a good merit, but it is a lot of work writing a patent application, says Mattias Lindh.

Future applications for the plastic light-source could for example be light-emitting band aids that can improve wound healing or activate drugs locally, or light-emitting security features in passports and similar important and valuable documents.

Mattias Lindh grew up in Lomma outside of Malmö in Sweden. He has a degree in Engineering Physics from Umeå University. He also has a background as a fjell- and wilderness guide, and a genuine interest in nature and the environment.

Read the whole thesis digitally

Text: Ingrid Söderbergh

About the dissertation:

On Friday March 1, Mattias Lindh, Department of Physics at Umeå University, defends his thesis titled: On the operation of light-emitting electrochemical cells. Swedish title: Om hur ljusemitterande elektrokemiska celler fungerar.
The dissertation takes place at 9:15 in room KBE301, Umeå University
Faculty opponent is Professor Sebastian Reineke, Technische Universität Dresden

Press photo. Photo: Ingrid Söderbergh