Organic Light-Emitting Diodes

4-pixel large area OLEDs on 96 mm x 96 mm glass substrate
4-pixel large area OLEDs on 96 mm x 96 mm glass substrate

An organic light-emitting diode (OLED) is a class of light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound. Compared with traditional LEDs, OLEDs have a lot more flexibility in pixel design as the pixel size can be easily tuned from several squared microns to tens of squared centimetres and the pixel shape can be in any patterns. As the next generation display technology, OLEDs show huge advantages over currently the most popular displays – liquid crystal displays (LCDs) – as they present a better contrast ratio, higher refresh rate and more vivid colours. 

This group has over 20 years of experience on OLED research and is equipped with the most advanced instruments for OLED production and characterization. From ultra-fast excitation light sources to various time-scale spectra detection equipment, we can perform world leading photophysics leading towards the development of new efficient OLED materials.

Thermally activated delayed fluorescent OLEDs

OLEDs have been classified into three different generations: Generation 1- OLEDs based on fluorescent materials; Generation 2 – OLEDs based on phosphorescent materials and Generation 3 –  OLEDs based on thermally activated delayed fluorescent (TADF) materials, which is the most active area of OLEDs research today. The great advantage of the TADF OLEDs over the other two generations is given by the fact that these emitters can transform excitation energy into light with 100% internal efficiency using purely organic molecules.

blue TADF OLED – pixel size 2 mm x 1 mm
Blue TADF OLED – pixel size 2 mm x 1 mm

The TADF mechanism relies on thermal energy to harvest triplet states (dark states) into emissive singlet states by reverse intersystem crossing (RISC). Using such a design strategy, these heavy-metal free OLEDs can achieve more than 25% external quantum efficiency (EQE).  Recent improvements in the EQEs and roll-offs of TADF OLEDs show that they are becoming competitive, meaning it is just a matter of time until these emitters became commercially available.

We are actively involved on the study of TADF emitters, with the goal of addressing the current challenges of their molecular design, underlying mechanisms and device performance. The TADF team works mainly on materials giving light emission in the red and blue regions of the visible spectrum, using both thermal evaporation and solution-process routes for device fabrication.

Our group strongly collaborates with the Zysman-Colman group in the chemistry department at the University of St Andrews. Together, there are over 10 researchers developing more efficient and stable TADF emitters. Moreover, we have widely international collaboration with many groups around the world especially Wroclaw University of Science and Technology in Poland, and Kyoto and Kyushu universities in Japan.

Large area and flexible OLEDs

Flexible OLED
Flexible OLEDs

The two biggest advantages of OLEDs are its capability to scale up to large areas and the ability to fabricate on flexible substrates. Large area OLEDs are of great interest in the field of advanced display and lighting because they are area light sources. The highly uniform light output, without using diffusers, makes it a simple structure for fabrication. Furthermore, by simply thermally evaporating or blending the materials with different emission wavelengths together, desirable emission spectra can be easily obtained. Other than that, large area OLEDs can be fabricated through solution-processes such as blade coating or ink-jet printing, which largely reduces the cost for manufacturing. These technologies also apply to flexible OLEDs fabrication, which will broaden the applications of OLED in flexible electronics and wearable electronics.

We are interested in making efficient large area and flexible OLED with both thermal evaporation and solution-process. The main challenges are to achieve high brightness and uniform light output with an adequate device operation lifetime. In particular, we are dedicated to push the boundary of OLEDs in biomedical applications such as photodynamic therapy and muscle contraction sensing.