July 8, 2016

One of the main drawbacks of OLEDs is the limited amount of functional colours available owing to the small number of organic molecules which are able to undergo electroluminescence. Several design concepts originally involved controlling and varying the OLEDs’ emission colours over a wide spectral range, which is technically far from a trivial task to achieve with an individual device.

Therefore, the need for (semi)transparent OLEDs was obvious, since a stack of two such devices would allow a rather easy control of the effective colour output by driving them independently from each other. Researchers at Holst Centre / TNO have developed a combination of materials, including transparent materials for both the top and bottom electrode which allowed the production of OLEDs transmitting up to 80 % of the incident light. The possibility of laminating such a transparent OLED on top of a non-transparent one with a different emission colour has been demonstrated to allow tuning the light emission by separately controlling the driving voltages for the top and bottom device. For future applications in product design, this colour tuning is expected to be a decisively advantageous feature, adding a lot of opportunities to the designer’s toolbox.

Another approach to change OLED colour was the development of down-conversion layers, investigated within the LTM project by a dedicated research team from the Wolfson Centre in Brunel University.

Since the emission of OLEDs is composed of a single wavelength (i.e. monochromatic light) the use of filters to modify the output colour is not possible and ultimately leads to a reduction in the intensity. There is, however, a solution represented by light down-conversion based on the principle of fluorescence. Fluorescent materials are able to absorb specific wavelengths of the light and convert them in different colours via an internal excitation-relaxation process of the electrons.

The research team at Brunel University / WMP has developed different layers for the colour conversion of blue and green OLEDs. These are silicone based layers which have been functionalised with hybrid organic-inorganic fluorescent materials and have been produced and tested demonstrating outstanding colour conversion efficiency, optical stability and allow for the dispersion of wide range of organic compounds in an otherwise, non-miscible environment. More recently, thin, transparent and flexible coatings based on fluorescent molecules embedded in polymer matrices have been developed. Their chemical-physical properties allow for easy application directly on the top layers of OLEDs by screen printing technology.


Corresponding Author Holst Centre / TNO: 

Pim Groen [], Robert Abbel []


Corresponding Author Brunel / WMP: 

Chris Frampton [], Andrea Mazzocut []