Improvement of device efficiency in PIN-OLEDs by controlling the charge carrier balance and intrinsic outcoupling methods

Abstract

Organic light-emitting diodes (OLEDs) with high power efficiency are desirable for lighting applications. A prerequisite for high power efficiency is the achievement of low driving voltages, which can be done via charge carrier doping of the transport layers in PIN organic light-emitting diodes (PIN- OLEDs). We have looked at how to combine low voltage with high current efficiency and long lifetime and have therefore investigated different ways of changing the charge carrier balance in PIN-OLEDs. The carrier supply to the emitting layer was adjusted by electron and hole-blocking layers to allow for an undisturbed exciton formation and radiative decay while keeping the total voltage low. In further investigations, we developed highly efficient and stable white PIN bottom emission OLED devices using novel evaporation processable outcoupling enhancement materials inside the PIN OLED stack. In white bottom emission OLEDs the use of this outcoupling enhancement material in combination with a standard micro-lens arrays (MLA) outcoupling film can yield an efficiency enhancement of up to 1.8. By combining the well-balanced stack with the internal outcoupling approach we achieved a power efficiency of 51.9 lm/W. With additional flat external outcoupling, the power efficiency has been further increased to 60 lm/W. The color coordinates are 0.470/0.429 with color rendering index (CRI) of 87. The 50% lifetime of the OLED could be estimated to 90,000 h.