Organic light-emitting diodes (OLEDs) usually exhibit a low light-outcoupling efficiency of only 20%. Typically, more than 30% of the available power is lost to surface plasmons (SPs). Consequently, the overall efficiency could be strongly enhanced by recovering SP losses. Therefore, three suitable techniques for extracting SPs-index coupling, prism coupling, and grating coupling-are discussed from a theoretical point of view and investigated experimentally in simplified OLED-like structures. The basic physical processes are clarified by systematic variations of the involved layer thicknesses and by excited state lifetime measurements. In addition, the analysis of the results is supported by optical simulations based on a dipole model. Finally, the advantages and disadvantages of each method, their potential efficiency for recovering SP losses, as well as the applicability in OLEDs are compared.
We focus on the determination of the internal luminescence quantum efficiency of a green-emitting organic light-emitting diode (OLED). By considering different geometrical configurations of OLED thin-film stacks, we elucidate the role of the internal luminescence quantum efficiency of the emitter in the thin-film microcavity. Combining optical simulations with experimental results, a comprehensive efficiency analysis is performed. Here the electroluminescence of a set of OLEDs is characterized. Additionally, the devices are characterized using time-resolved photoluminescence measurements. The experimental data are analyzed using optical simulations. This analysis leads to a quantification of internal luminescence quantum efficiency and allows conclusions about competing mechanisms resulting in nonradiative recombination of charge carriers.
The internal quantum efficiency of organic light-emitting diodes (OLEDs) 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. In this work we use numerical simulations to optimize light outcoupling
from different OLED stacks. First, we change the distance of the emission zone to the cathode, which minimizes
the excitation of surface plasmons. Then the influence of different dipole orientation of the emitter material on
the light outcoupling is studied. Finally, a metal-free, transparent OLED stack reported by Meyer et al.,<sup>1</sup> where
no plasmons can be excited, is investigated for improved outcoupling efficiency.
We present a novel surface plasmon resonance (SPR) sensor based on an integrated planar and polychromatic light
source. The sensor comprises an organic light emitting diode (OLED) and a metallic sensing layer located on opposite
sides of a glass prism. We successfully fabricated and tested prototype sensors based on this approach by the use of
different prism geometries and OLEDs with blue, green and red emission color. We investigated the angular and
wavelength dependent SPR dispersion relation for sensing layers consisting of silver and gold of different thicknesses in
contact with air. Further on we demonstrated the sensor function by real time monitoring of temperature changes inside
an adjacent water reservoir as well as by recording the dissolving process of sodium chloride in water. This shows that
the configuration can in principle be used for bio-sensing applications.
The presented technique offers the advantage that there is no necessity to couple light from external bulky sources such
as lasers or halogen lamps into the sensing device which makes it particularly interesting for miniaturization. The
presented SPR configuration can be monolithically integrated on one common substrate. Furthermore it is compatible
with the planar glass light pipe platform for SPR sensing and the two-color approach for the determination of the
thickness and the dielectric constant of thin films in a single experiment.
The internal quantum efficiency of organic light-emitting diodes (OLEDs) 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. In this work we use numerical
simulations to identify and quantify different loss mechanisms. Changing various simulation parameters, for
example layer thicknesses, enables us to study their influence on the fraction of light leaving the OLED. With
these simulations we therefore can enhance the light output of the OLED stack.
We present simulations of bottom-emitting OLEDs based on the green emitter tris-(8-hydroxyquinoline) aluminum
(Alq<sub>3</sub>) with transparent indium tin oxide anode and a metallic cathode, as well as microcavity devices
with two metallic electrodes. The results of the simulations are compared with experimental data on the angular
dependent emission spectra and published effi;ciency data.