Enhancing the Out-Coupling Efficiency of Organic Light-Emitting Diodes Using Two-Dimensional Periodic Nanostructures

Abstract

The out-coupling efficiency of planar organic light emitting diodes (OLEDs) is only about 20% due to factors, such as, the total internal reflection, surface plasmon coupling, and metal absorption. Two-dimensional periodic nanostructures, such as, photonic crystals (PhCs) and microlenses arrays offer a potential method to improve the out-coupling efficiency of OLEDs. In this work, we employed the finite-difference time-domain (FDTD) method to explore different mechanisms that embedded PhCs and surface PhCs to improve the out-coupling efficiency. The effects of several parameters, including the filling factor, the depth, and the lattice constant were investigated. The result showed that embedded PhCs play a key role in improving the out-coupling efficiency, and an enhancement factor of 240% was obtained in OLEDs with embedded PhCs, while the enhancement factor of OLEDs with surface PhCs was only 120%. Furthermore, the phenomena was analyzed using the mode theory and it demonstrated that the overlap between the mode and PhCs was related to the distribution of vertical mode profiles. The enhancement of the extraction efficiency in excess of 290% was observed for the optimized OLEDs structure with double PhCs. This proposed structure could be a very promising candidate for high extraction efficiency OLEDs.

Parameter Optimization of Light Outcoupling Structures for High-Efficiency Organic Light-Emitting Diodes

“…The EQE and the period Λ have a modest linear correlation between each other with r = 0.403 (Subfigures a, b and c). However, from Figure 4 we already expected a non-linear correlation with an optimal period of about 1000 -1500 nm, which is in agreement with simulations from literature [43][44][45].…”

“…For example, R. Pfeifer et al [43] suggest that gratings with a period of Λ = 800 nm and height of H = 60 nm are the most effective for achieving the maximal EQE of green OLEDs. Another study [44] shows simulations for photonic crystals embedded in OLEDs and states that the optimal period is 800 -1000 nm and height is 300 nm. These two parameters are also calculated for wrinkled substrates by J.…”

To Enhance Light Extraction for Organic Light-Emitting Diodes by Body Modification of Substrate

“…For simplicity, the metal layer is regarded as ideal reflecting layer, and the absorption parameter of organic material is set to zero. Refractive indexes of organic material, ITO anode, and glass substrate are 1.8, 1.8, and 1.5, respectively, referencing from former reports [23][24][25].…”

An efficient non-Lambertian organic light-emitting diode using imprinted submicron-size zinc oxide pillar arrays

“…From the experiment results, PC grating resulted in significant drop in efficiency enhancement when the filling factor is too large or too small, which agrees with previous research. 17 In conclusion, we have fabricated high efficiency OLEDs with ZnO pillar array at the backside of the glass substrate using PRINT technique. The device with the ZnO pillar array pitch of 650 nm (FF of 0.3) achieved a maximum EQE of 38.5 6 0.1%, which corresponds to 75.0% enhancement in total light output.…”

“…Among the above mentioned techniques, PCs attracted much attention because of their capability to control photons in various ways by designing different photonic nanostructures. 9,10,16,17 However, most of these techniques involve several steps necessary for fabricating PCs such as chemical vapour deposition (CVD), electron beam (EB) lithography, and reactive ion etching (RIE), etc., 9 which are very time consuming and cost inefficient for industrial production. The pattern replication in non-wetting templates (PRINTs) technique, 16,18,19 in contrast, is a simple, cost-effective method to fabricate the PC gratings with high throughput.…”