Light Extraction Mechanism Research Articles

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

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. 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-hydroxyquinoline) aluminum with transparent indium tin oxide anode and a calcium/aluminum cathode.

13.4: A 20.8‐inch WXGA Full Color AMOLED Display by Integrating Scattering Reflector with Micro‐Bumps

AbstractIn order to improve light extraction efficiency from organic light emitting diodes (OLED), we report a novel structure of top‐emission OLED by integrating a scattering reflector with micro‐bumps into the substrate. The mechanism of light extraction has been investigated by evaluating the dependency of flux on the shape of the micro‐bumps. It was confirmed that the scattering effect of the micro‐bumps improves the light extraction of OLED. Moreover, the scattering reflector has been applied on a 20.8‐inch WXGA full color AMOLED display. This is the world's first report that actually proves a light extraction technology, integrating scattering reflector, applied on AMOLED display.

Light extraction from OLEDs for lighting applications through light scattering

Abstract OLEDs are gaining increasing interest for lighting applications as large-area light sources. Their lifetime and overall efficiency can be increased by the optimization of light outcoupling. In this article, scattering films on the viewer’s side of the substrate to increase the outcoupling efficiency of OLEDs for lighting applications are examined. Experimental results show that the increase of outcoupling efficiency is dependent from the absolute number of scattering particles in the matrix. Theoretical considerations expect an increase of outcoupling efficiency of up to 70%. Experimental results yield an increase of outcoupling efficiency of about 22%. A software model based on raytracing to simulate light outcoupling from OLEDs through a scattering layer is introduced to gain a deeper understanding of the light extraction mechanism. We found that the scattering anisotropy factor g determined using the Henyey–Greenstein phase function and the effective absorbance of the OLED device have strong impact on outcoupling efficiency.

Light-extraction mechanisms in high-efficiency surface-textured light-emitting diodes

We present a detailed quantitative analysis of the light extraction and loss mechanisms in high-efficiency GaAs-AlGaAs surface-textured thin-film light-emitting diodes (LEDs). The analysis is based on a Monte Carlo simulation. Most input parameters, including scattering of photons at the textured surface, sub-bandgap absorption, and absorption at the metal mirror are obtained from experiments or from literature. The simulation also takes into account the effect of photon recycling and the realistic geometry of the diodes. The only remaining fitting parameter is the internal quantum efficiency, which is deduced to be about 80% at room temperature for the experimentally realized 850-nm LEDs with an external quantum efficiency of 44%. The analysis shows further that the most important loss mechanism is reabsorption in the active layer, and in particular in those parts of the active layer that are not electrically pumped. This conclusion is also valid for other types of high-efficiency LEDs. We could furthermore verify the validity of the Monte-Carlo simulation results by conducting experiments at low temperatures, where nonradiative recombination processes are reduced, resulting in the internal quantum efficiency approaching unity. The measured external quantum efficiency at 90 K is 68%, which is close to the theoretically predicted efficiency for a perfect active layer. The results demonstrate that the light extraction from surface-textured LEDs is fully understood and can be quantitatively modeled by a simple raytracing algorithm.

Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes

The transmission properties of semiconductor surfaces can be changed by surface texturing. We investigate these changes experimentally and find that an enhancement of the angle-averaged transmission by a factor of 2 can be achieved with optimum texturing parameters. This enhanced transmission provides an additional light extraction mechanism for high-efficiency surface-textured light-emitting diodes. External quantum efficiencies of 46% and 54% are demonstrated before and after encapsulation, respectively.