Light extraction and optical loss mechanisms in organic light-emitting diodes: Influence of the emitter quantum efficiency

Abstract

The internal quantum efficiency of organic light-emitting diodes ͑OLEDs͒ can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used; however, the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are, for example, waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. Additionally, absorption in the organic layers and the electrodes can play a role. In this work we use numerical simulations to identify and quantify different loss mechanisms. Changing simulation parameters, for example, the distance of the emitter material to the cathode or thicknesses of the various layers, enables us to study their influence on the fraction of light leaving the OLED. An important parameter in these simulations and for the actual device is the radiative quantum efficiency q, which is defined as the efficiency of radiative exciton decay in an unbounded space filled by the emitting dye and its matrix. The simulations show that due to microcavity effects the radiative decay channel can be considerably changed in an OLED as compared to free space emission of a dipole. Thus the knowledge of the radiative quantum efficiency is crucial for the optimization of OLEDs. As an example, we present simulations of bottom-emitting OLEDs based on the well-known green emitter tris-͑8-hydroxyquinoline͒ aluminum with transparent indium tin oxide anode and a calcium/aluminum cathode.

Simulations of emission from microcavity tandem organic light-emitting diodes

“…The simulation for each orientation and spatial position involves a sum over (a) Bloch vectors and a (b) finite set of reciprocal lattice vectors G. This rigorous solution of Maxwell's equations inside the OLED explicitly accounts for the reflected electric fields at the position of the dipole, that modify the radiation rate of dipoles in the OLED over their free-space value-an essential feature of OLED emission. 18 However, it does not account for the reabsorption and re-emission of photons within the OLED layer since our simulation is done for a single source at each spatial position.…”

More light from organic light-emitting diodes

“…Quantitative calculations, treating the emitting molecules as classical electrical dipoles (cf. the sketch on the title page of this article), reveal that in planar OLED stacks typically around 50% of the light is trapped in waveguided and plasmon modes [7]. As an example, Fig.…”

Development of Extremely High Efficacy White OLED with over 100 lm/W

“…For example, fabrication of nanostructures between organic layer and metal cathode [7][8][9], back-cavity structure with thin metal cathode [10], and horizontally oriented emissive layer [11]. In this study, we paid attention to the distance from emissive zone to the metal cathode [12]. In order to quantitatively specify each mode in OLED, a bottom emitting monochrome OLED on the high refractive index substrate was investigated by the optical simulation.…”