Toward Ultra-Efficient Oleds: Approaches Based On Low Refractive Index Materials

Organic light-emitting diodes (OLEDs) have successfully established themselves as light sources vital in high-end mobile displays and television due to various advances made for a wide range of technical aspects from materials, devices, and processes to air-tight packaging and systems. However, a significant portion of the generated excitons in OLEDs are still lost to various channels. Among them, excitation of surface plasmon polaritons (SPPs) is a critical source of exciton loss, the recovery of which often involves structuring in nanoscale. Herein, a nano-pattern-free approach is reviewed to reducing SPP loss that makes use of materials with low refractive index. Because of the limited availability of such low refractive index materials, a method to use a low extraordinary refractive index of birefringent organic semiconductors is explored. With the proposed method combined with common lens-based outcoupling schemes and high refractive index substrates, it is shown that an ultrahigh external quantum efficiency of approximate to 72% in a green, single-junction phosphorescent OLED is within the reach. Transient photoluminescence measurement is used to monitor the change in Purcell factor incurred by the use of the birefringent low refractive index layer and to further verify the validity of the proposed concept.