Realtime structural characterisation of thin film OLED stacks during thermally induced diffusion - the importance of glass transitions

Abstract

Organic Light Emitting Diode (OLED) devices are ubiquitous in the displays of many modern electronic devices, including televisions and mobile phones. High efficiency OLEDs are typically made as a sequentially deposited multilayer, with different organic semiconductor layers being required for hole/electron injection/transport, as well as light emission. The operational lifetime, efficiency and fidelity of these OLEDs depends on the structural stability of this multilayer stack, and understanding the factors that affect this stability are crucial in developing new devices. We have systematically characterised the kinetics of structural evolution in these systems as they experience thermally induced diffusion. These leading edge experiments are performed on the Platypus neutron reflectometer, which has been pioneering event mode acquisition techniques to capture quickly changing reflection signals as the multilayers inter-diffuse. Selective deuteration of these semiconductor materials is also critical in providing neutron contrast between each of the layers, without which the experiment could not be carried out. Our investigations reveal the importance of glass transition temperatures on the stability of these OLED systems and provide clear guidelines for material choices when designing new devices. Indeed, with knowledge of each of the T \'s one can predict the way in which diffusion occurs. For example, use of a high Tg emissive layer does not necessarily prevent diffusion from taking place.