Importance of Purcell factor for optimizing structure of organic light-emitting diodes

Top-Emitting Quantum Dot Light-Emitting Diodes: Theory, Optimization, and Application

The superior optical properties of colloidal quantum dots (QDs) have garnered significant broad interest from academia and industry owing to their successful application in self-emitting QD-based light-emitting diodes (QLEDs). In particular, active research is being conducted on QLEDs with top-emission device architectures (TQLEDs) owing to their advantages such as easy integration with conventional backplanes, high color purity, and excellent light extraction. However, due to the complicated optical phenomena and their highly sensitive optoelectrical properties to experimental variations, TQLEDs cannot be optimized easily for practical use. This review summarizes previous studies that have investigated top-emitting device structures and discusses ways to advance the performance of TQLEDs. First, theories relevant to the optoelectrical properties of TQLEDs are introduced. Second, advancements in device optimization are presented, where the underlying theories for each are considered. Finally, multilateral strategies for TQLEDs to enable their wider application to advanced industries are discussed. This work believes that this review can provide valuable insights for realizing commercial TQLEDs applicable to a broad range of applications.

Influence of Light-Matter Interaction on Efficiency of Quantum-Dot Light-Emitting Diodes

Light-matter interaction can affect the radiative decay rate of excitons and thus the emission quantum yield (QY) of quantum dots (QDs). In this work, light-matter interaction and outcoupling efficiency of QD light-emitting diodes with different structures are investigated experimentally and theoretically. We show that the external quantum efficiency (EQE) of top-emitting devices is higher than that of the bottom-emitting devices, which is mainly due to the stronger light-matter interaction and higher light outcoupling efficiency of the top-emitting structures. In addition, we show that the QY enhancement induced by light-matter interaction is more significant for low-QY QDs. Our results suggest that top-emitting structures are more powerful for improving the EQE of devices built with low-QY QDs.

Accurate optical optimization of light-emitting diodes with consideration of coupling between Purcell factor and transmittance coefficient

The calculation method for light emission efficiency splits external quantum efficiency (EQE) into internal quantum efficiency (IQE) and light extraction efficiency (LEE) independently. Consequently, the IQE connected to Purcell factor and the LEE are calculated separately. This traditional method ignores the interplays between the Purcell factor and transmittance coefficient in spectral domain, which all strongly depend on emitting directions. In this work, we propose a new figure of merit to describe the light emission process accurately by using the direction-dependent Purcell factor and transmittance coefficient simultaneously. We use a specific LED structure as a numerical example to illustrate the calculation method and optimization procedure.

All-Polymer Microcavities for the Fluorescence Radiative Rate Modification of a Diketopyrrolopyrrole Derivative

Controlling the radiative rate of emitters with macromolecular photonic structures promises flexible devices with enhanced performances that are easy to scale up. For instance, radiative rate enhancement empowers low-threshold lasers, while rate suppression affects recombination in photovoltaic and photochemical processes. However, claims of the Purcell effect with polymer structures are controversial, as the low dielectric contrast typical of suitable polymers is commonly not enough to provide the necessary confinement. Here we show all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative, which allows a change in the fluorescence lifetime. Radiative and nonradiative rates were disentangled systematically by measuring the external quantum efficiencies and comparing the planar microcavities with a series of references designed to exclude any extrinsic effects. For the first time, this analysis shows unambiguously the dye radiative emission rate variations obtained with macromolecular dielectric mirrors. When different waveguides, chemical environments, and effective refractive index effects in the structure were accounted for, the change in the radiative lifetime was assigned to the Purcell effect. This was possible through the exploitation of photonic structures made of polyvinylcarbazole as a high-index material and the perfluorinated Aquivion as a low-index one, which produced the largest dielectric contrast ever obtained in planar polymer cavities. This characteristic induces the high confinement of the radiation electric field within the cavity layer, causing a record intensity enhancement and steering the radiative rate. Current limits and requirements to achieve the full control of radiative rates with polymer planar microcavities are also addressed.

Ultrahigh Resolution Pixelated Top-Emitting Quantum-Dot Light-Emitting Diodes Enabled by Color-Converting Cavities

Realizing pixelated quantum-dot light-emitting diodes for high-resolution displays remains a challenging task because of the difficulty of fine patterning the quantum-dots. In this study, instead of patterning the quantum-dots, the color-converting cavities for realizing high-resolution pixelated emission are developed. By defining the thicknesses of the transparent electrodes (phase tuning layers) through a photolithographic process, the resultant cavities can selectively convert the unpatterned quantum-dot white emission as saturated red, green, and blue emission with a brightness of 22170, 51930, and 3064 cd m^-2 at 5.5 V, respectively. The developed method enables the realization of ultrahigh density red, green, and blue emission for a display with a resolution of ≈1700 pixel-per-inch and a color gamut of 111% National Television System Committee; together with the advantages of quantum-dot patterning-free, color-filter-free and high brightness, the demonstrated architecture could find potential applications in various displays ranging from cell phone to emerging virtual reality and augmented reality displays.

Bright and Stable Quantum Dot Light-Emitting Diodes

Quantum dot light-emitting diodes (QLEDs) are one of the most promising candidates for next-generation displays and lighting sources, but they are barely used because vulnerability to electrical and thermal stresses precludes high brightness, efficiency, and stability at high current density (J) regimes. Here, bright and stable QLEDs on a Si substrate are demonstrated, expanding their potential application boundary over the present art. First, a tailored interface is granted to the quantum dots, maximizing the quantum yield and mitigating nonradiative Auger decay of the multiexcitons generated at high-J regimes. Second, a heat-endurable, top-emission device architecture is employed and optimized based on optical simulation to enhance the light outcoupling efficiency. The multilateral approaches realize that the red top-emitting QLEDs exhibit a maximum luminance of 3 300 000 cd m^-2 , a current efficiency of 75.6 cd A^-1 , and an operational lifetime of 125 000 000 h at an initial brightness of 100 cd m^-2 , which are the highest of the values reported so far.

Design and optical characterization of an efficient polarized organic light emitting diode based on refractive index modulation in the emitting layer

Luminescent liquid Crystal (LC) material is regarded as the most promising material for polarized organic light emission due to their intrinsic characteristics including orderly alignment and luminescence. Nevertheless, the optical extraction efficiency of LC based organic light emitting diodes (OLEDs) devices still requires significant effort and innovation towards real-world applications. In this paper, we propose the design of a highly linearly polarized light-emission from OLEDs with integrated refractive index nanograting in the emissive layer (EML) based on photo aligned luminescent liquid crystal material. The simulation results indicate that the geometrically optimized polarized device yields an external quantum efficiency (EQE) up to 47% with a polarized ratio up to 28 dB at a 550 nm emission wavelength. This conceptual design offers a new opportunity to achieve efficient polarized organic luminescence, and it is (to the best of our knowledge) the first approach that enhances the light extraction of OLEDs based on luminescent liquid crystal via index grating in the EML.

Metasurface-driven OLED displays beyond 10,000 pixels per inch

Optical metasurfaces are starting to find their way into integrated devices, where they can enhance and control the emission, modulation, dynamic shaping, and detection of light waves. In this study, we show that the architecture of organic light-emitting diode (OLED) displays can be completely reenvisioned through the introduction of nanopatterned metasurface mirrors. In the resulting meta-OLED displays, different metasurface patterns define red, green, and blue pixels and ensure optimized extraction of these colors from organic, white light emitters. This new architecture facilitates the creation of devices at the ultrahigh pixel densities (>10,000 pixels per inch) required in emerging display applications (for instance, augmented reality) that use scalable nanoimprint lithography. The fabricated pixels also offer twice the luminescence efficiency and superior color purity relative to standard color-filtered white OLEDs.

Multi-objective collaborative optimization strategy for efficiency and chromaticity of stratified OLEDs based on an optical simulation method and sensitivity analysis

The low efficiency and dissatisfactory chromaticity remain as important challenges on the road to the OLED commercialization. In this paper, we propose a multi-objective collaborative optimization strategy to simultaneously improve the efficiency and ameliorate the chromaticity of the stratified OLED devices. Based on the formulations derived for the current efficiency and the chromaticity Commission International de L'Eclairage (CIE) of OLEDs, an optical sensitivity model is presented to quantitatively analyze the influence of the layer thickness on the current efficiency and the CIE. Subsequently, an evaluation function is defined to effectively balance the current efficiency as well as the CIE, and a collaborative optimization strategy is further proposed to simultaneously improve both of them. Simulations are comprehensively performed on a typical top-emitting blue OLED to demonstrate the necessity and the effectivity of the proposed strategy. The influences of the layer thickness incorporated in the blue OLED are ranked based on the sensitivity analysis method, and by optimizing the relative sensitive layer thicknesses in the optical views, a 16% improvement can be achieved for the current efficiency of the OLED with desired CIE meantime. Hence, the proposed multi-objective collaborative optimization strategy can be well applied to design high-performance OLED devices by improving the efficiency without chromaticity quality degradation.

Simulation method for study on outcoupling characteristics of stratified anisotropic OLEDs

We derive explicit power dissipation functions for stratified anisotropic OLEDs based on a radiation model of dipole antennas inside anisotropic microcavity. The dipole field expressed by vector potential is expanded into plane waves whose coefficients are determined by scattering matrix method, and then an explicit expression is derived to calculate the energy flux through arbitrary interfaces. Taking advantage of the formulation, we can easily perform quantitative analysis on outcoupling characteristics of stratified anisotropic OLEDs, including outcoupling efficiency, normalized decay rate and angular emission profile. Simulations are carried out on a prototypic stratified OLED structure to verify the validity and capability of the proposed model. The dependencies of the outcoupling characteristics on various emission feature parameters, including dipole position, dipole orientation, and the intrinsic radiative quantum efficiency, are comprehensively evaluated and discussed. Results demonstrate that the optical anisotropy in different organic layers has nonnegligible influences on the far-field angular emission profile as well as outcoupling efficiency, and thereby highlight the necessity of our method. The proposed model can be expected to guide the optimal design of stratified anisotropic OLED devices, and help to solve the inverse outcoupling problem for determining the emission feature parameters.