Improving the light extraction efficiency of polymer LEDs using microcavities and photonic crystals

A significant fraction of light generated within an organic light emitting diode (OLED) is often trapped within the structure within waveguide modes and is unable to escape usefully from the device. Addressing this issue is of significant importance, as it potentially offers a route to improve the external efficiency of OLEDs. Here, we discuss a number of methods to improve light extraction efficiency from conjugated-polymer LEDs. Firstly we explore the use of low finesse optical microcavities to redistribute trapped-light into externally propagating modes. The improvements obtained by simply adopting a microcavity structure on its own are rather small, however we then show that they can be improved significantly by improving the reflectivity of the cathode. Finally, we show that by engineering a photonic crystal beneath the anode of a polymer LED, a significant improvement in external efficiency (by a factor of 2) can be achieved. Such an approach is anticipated to be readily scalable to a manufacturing environment.