Efficiency analysis of organic light-emitting diodes based on optical simulation

In spite of huge progress in improving the internal quantum efficiency of organic light-emitting diodes (OLEDs), these devices still suffer from poor light out-coupling. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmons at metallic electrodes. Their relative strength and the mutual dependence on the OLED structure have been studied both experimentally and by numerical simulation. Here, we consider the impact of the radiative quantum efficiency of the emitter material on predictions of light extraction from OLEDs. Competing processes resulting in non-radiative recombination of charge carriers usually reduce the emitter quantum efficiency in a real device. We show that optical simulation leads to erroneous conclusions when neglecting these competing processes. Furthermore, we demonstrate a method, which allows determining both the radiative quantum efficiency and the charge recombination factor via simulation based analysis of experimental data. This analysis of device efficiency is applied on a set of red-emitting electrophosphorescent devices.     

Light Extraction From Solution-Based Processable Electrophosphorescent Organic Light-Emitting Diodes

Molecular dye dispersed solution processable blue emitting organic light-emitting devices have been fabricated and the resulting devices exhibit efficiency as high as 25 cd/A. With down-conversion phosphors, white emitting devices have been demonstrated with peak efficiency of 38 cd/A and luminous efficiency of 25 lm/W. The high efficiencies have been a product of proper tuning of carrier transport, optimization of the location of the carrier recombination zone and, hence, microcavity effect, efficient down-conversion from blue to white light, and scattering/isotropic remission due to phosphor particles. An optical model has been developed to investigate all these effects. In contrast to the common misunderstanding that light out-coupling efficiency is about 22% and independent of device architecture, our device data and optical modeling results clearly demonstrated that the light out-coupling efficiency is strongly dependent on the exact location of the recombination zone. Estimating the device internal quantum efficiencies based on external quantum efficiencies without considering the device architecture could lead to erroneous conclusions