Large-area Organic Light-emitting Diodes Research Articles

The Improvement of Luminous Uniformity of Large-Area Organic Light-Emitting Diodes by Using Auxiliary Electrodes

The study developed a large emission area of flexible blue organic light-emitting diodes (BOLED) on a polyethylene terephthalate/ Indium tin oxide (PET/ITO) substrate using a polycyclic skeleton ν-DABNA Thermally Activated Delayed Fluorescence (TADF) material. Initially, a 1 × 1 cm2 blue OLED was fabricated to optimize the layer thickness. The blue OLED structure consisted of PET/ITO/HATCN/TAPC/UBH-21:ν-DABNA/TPBi/LiF/Al. However, as the emission area increased to 3.5 × 3.5 cm2, the current density decreased due to the resistance of PET/ITO, leading to luminance non-uniformity. To address this issue, auxiliary Au lines were added to the ITO anode to enhance current injection. Despite this, when the Au lines reached a thickness of 30 nm, average light emission was disrupted. To improve the luminescence characteristics of large-area PET/ITO OLEDs, a capping and planarization layer of PEDOT:PSS was applied. Grid uniformity revealed a significant increase in overall luminance uniformity from 74.1% to 87.4% with the addition of auxiliary Au lines. Further increases in grid line density slightly reduced uniformity but enhanced brightness, resulting in brighter, flexible, large-area blue OLED lighting panels.

Efficient electron injection layer for thermal stability of top emission phosphorescent organic light emitting diodes

Thermal stability holds significant importance in both high-resolution and large-area organic light-emitting diodes (OLEDs) due to its potential impacts on pixel shrinkage, thereby adversely affecting visual quality and long-term device functionality. The thermal instability could be from the thermal diffusion of metal ions or small molecules occurring at the interface between the electron injection layer (EIL) and the cathode material, influenced by differing surface properties and binding strength. In this study, we meticulously engineered magnesium fluoride (MgF2) as EIL to mitigate the aforementioned challenges. Important physical properties associated with the EIL/cathode interaction were systematically analyzed. Employing Ag:Yb (2.5:1) as the cathode, we achieved notable enhancements in current efficiency and a reduced turn-on voltage for a green device operating under optical microcavity conditions. Thermal degradation test conducted over 240 h on fabricated devices revealed that employing MgF2 as the EIL markedly enhanced thermal stability compared to devices utilizing Yb as the EIL reference. The robust EIL/cathode system observed herein is attributed to the high binding energy between the EIL and cathode materials utilized in this study.

Improving the Uniformity of Top Emitting Organic Light Emitting Diodes Using a Hybrid Electrode Structure

AbstractSome applications of organic light‐emitting diodes (OLEDs) require large area, high light output, and high uniformity. It is difficult to achieve these attributes simultaneously because of voltage drops in the contacts, which cannot easily satisfy high optical transparency and electrical conductivity simultaneously. In large area OLEDs, thin electrodes with high sheet resistance induce voltage drops across the devices, leading to non‐uniform distribution of light. However, thick electrodes with low sheet resistance decrease the light output due to low transmittance. To overcome this trade‐off, a multilayer hybrid electrode based on Ag (20 nm)/WO3/Ag (20 nm)/WO3 is designed to obtain high electrical conductance with low optical loss. Compared to conventional devices using a single Ag (40 nm) top electrode, there is a considerable increase in the external quantum efficiency (EQE) of the device using this electrode (from 11.5% to 25.5% at 1000 cd m−2), while maintaining similar sheet resistance. In addition, a large area (≈57 cm2) OLED with the hybrid electrode demonstrates a luminance uniformity of 77% as compared to a device using single silver electrode with uniformity of 66%. Therefore, the proposed Ag/WO3/Ag/WO3 hybrid electrode is a promising choice for the fabrication of efficient and uniform large‐area OLEDs.

Systematic strategy for high-performance small molecular hybrid white OLED via blade coating at ambient condition

Large-area organic light-emitting diodes (OLEDs) fabricated by solution process with low-cost are essential for practical applications. However, maintaining desirable uniformity and smoothness of prepared films are challenging with the enhancement of film size and layers. Reported herein is a systematic strategy of solvent and device structure engineering for designing small molecular hybrid white OLED (WOLED) via blade-coating. Benefiting from the uniform blade-coated films with low roughness about 1 nm as well as the balanced charge carrier transport, the blade-coated WOLED with hybrid fluorescence and phosphorescence (small area 0.1 cm 2 ) shows the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of 29.0 cd A −1 , 30.4 lm W −1 , and 11.6%, respectively. And the WOLEDs achieve low turn-on voltage ( V on ) of about 2.8 V. Meanwhile, the all-vacuum-based WOLED reaches maximum CE, PE, and EQE of 40 cd A −1 , 44.3 lm W −1 , and 15.9%, respectively. Remarkably, a larger area of the device (2 cm × 2 cm) with uniform emitting reaches peak EQE of 6.8% by blade-coating, notably, only a small amount of solution about 0.9 μL/cm 2 was consumed for blade-coated films. The high-performance and low-cost WOLED via blade-coating is ascribed to the facile strategy to improve the film morphology and the energy-level matching, which provides huge potential in display and flat panel lighting. Benefiting from the solvent and device structure engineering, the blade-coated hybrid white organic light-emitting diodes (WOLEDs) reach external quantum efficiencies (EQEs) close to 12% (small-area, 0.1 cm 2 ) and 6.8% (2 cm × 2 cm) with a small amount of solution about 0.9 μL/cm 2 . • Fabricating a hybrid white organic light-emitting diodes (WOLEDs) by blade-coating at ambient conditions. • Selecting the ideal solvents (solvent engineering) and electron-transporting layer for the highly efficient blade-coating WOLED which reaches the maximum EQE, power efficiency (PE), and current efficiency (CE) of 11.6%, 30.4 lm W −1 , and 29.0 cd A −1 , respectively. • Fabricating a blade-coated WOLED (2 cm × 2 cm) owning peak EQE of 6.8% with a small amount of solution of 0.9 μL/cm 2 .

Analysis of a bulk-surface thermistor model for large-area organic LEDs

The existence of a weak solution for an effective system of partial differential equations describing the electrothermal behavior of large-area organic light-emitting diodes (OLEDs) is proved. The effective system consists of the heat equation in the threedimensional bulk glass substrate and two semilinear equations for the current flow through the electrodes coupled to algebraic equations for the continuity of the electrical fluxes through the organic layers. The electrical problem is formulated on the (curvilinear) surface of the glass substrate where the OLED is mounted. The source terms in the heat equation are due to Joule heating and are hence concentrated on the part of the boundary where the current-flow equation is posed. The existence of weak solutions to the effective system is proved via Schauder’s fixed-point theorem. Moreover, since the heat sources are a priori only in L^1 , the concept of entropy solutions is used.

Enhancing hole injection by processing ITO through MoO3 and self-assembled monolayer hybrid modification for solution-processed hole transport layer-free OLEDs

Compared with current vacuum-deposited multilayer organic light-emitting diodes (OLEDs), solution-processed simplified-structured OLEDs are more appealing by merits of low costs and promising potentials in large-area OLEDs. Nevertheless, it remains challenging due to intermixing between adjacent organic layers. Herein, indium tin oxide (ITO) surface was deposited with molybdenum trioxide (MoO3) and then modified with self-assembled monolayer (SAM) of pentafluorobenzyl phosphonic acid. The SAM modification process made the MoOx film rich in oxygen vacancies, which ensured effective hole injection from the hybrid modified ITO directly to the emitting layer. This strategy facilitates HTL-free OLEDs and then completely avoids interlayer intermixing. Using the hybrid modified ITO as anode, HTL-free solution-processed green phosphorescent OLED was prepared and achieved a maximum current efficiency of 63.24 cd A−1 and a maximum external quantum efficiency of 18.18%. Meanwhile, preferable operational stability than standard multilayer OLEDs was obtained. For red phosphorescent OLED, a maximum external quantum efficiency of 17.49% was achieved. This work proposed a new way of adjusting cation valence to improve hole injection, contributing to solution-processed OLED fabrication.

Highly efficient tandem PHOLEDs with lithium-doped BPhen/NDP-9-doped TAPC as a charge generation layer

The development of large-area organic light-emitting diode (OLED) displays requires a highly efficient tandem device architecture and an easily processable charge generation layer (CGL) with a low voltage drop and high optical transparency. In this study, we investigated and applied a doped organic n-CGL/p-CGL using thermal vacuum deposition in tandem OLED devices. A doping concentration of 1.0 wt.% for Li in 4, 7-Diphenyl-1, 10-phenanthroline (BPhen) was optimal for the n-CGL with 8 wt.% for 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene)-malononitrile (NDP-9)-doped N,N-bis(4-methylphenyl)benzenamine (TAPC) as a p-CGL. Maximum luminous efficiencies of 42.5 and 63.4 cd/A and a 4,000 cd/m2 current density for the target luminance values of 11.2 and 6.5 mA/cm2 were demonstrated for double-stack and triple-stack tandem blue phosphorescent OLED devices, respectively. Implementing these highly efficient tandem device structures will improve the overall lifetime of OLED displays by lowering their operating current density at the target luminance.

A triple layer printed blue OLED with blurred interface

Multilayer printing is a method for producing large area organic light-emitting diode (OLED) devices with many advantages including not using shadow mask, high material utilization and low production cost. However, the issue of the dissolution of the underneath layer at the interface when depositing an upper layer, makes it difficult to fabricate a multilayer printed device. A concept of a ‘blurred interface’ had been proposal previously and demonstrated in multilayer-printed orange and yellow-green OLED devices. In this paper, the blurred interface concept is employed for fabricating multilayer blue OLEDs and successfully demonstrated in both double layers or triple layers printed devices. With optimized ink formulation and printing parameters settings, uniform and stable blue multilayer printed OLEDs have been fabricated.

Dimension reduction of thermistor models for large-area organic light-emitting diodes

<p style='text-indent:20px;'>An effective system of partial differential equations describing the heat and current flow through a thin organic light-emitting diode (OLED) mounted on a glass substrate is rigorously derived from a recently introduced fully three-dimensional <inline-formula><tex-math id="M1">\begin{document}$ p(x) $\end{document}</tex-math></inline-formula>-Laplace thermistor model. The OLED consists of several thin layers that scale differently with respect to the multiscale parameter <inline-formula><tex-math id="M2">\begin{document}$ \varepsilon>0 $\end{document}</tex-math></inline-formula>, which is the ratio between the total thickness and the lateral extent of the OLED. Starting point of the derivation is a rescaled formulation of the current-flow equation in the OLED for the driving potential and the heat equation in OLED and glass substrate with Joule heat term concentrated in the OLED. Assuming physically motivated scalings in the electrical flux functions, uniform a priori bounds are derived for the solutions of the three-dimensional system which facilitates the extraction of converging subsequences with limits that are identified as solutions of a dimension reduced system. In the latter, the effective current-flow equation is given by two semilinear equations in the two-dimensional cross-sections of the electrodes and algebraic equations for the continuity of the electrical fluxes through the organic layers. The effective heat equation is formulated only in the glass substrate with Joule heat term on the part of the boundary where the OLED is mounted.</p>

Two-Color Pixel Patterning for High-Resolution Organic Light-Emitting Displays Using Photolithography.

Nowadays, the display industry is endeavoring to develop technology to provide large-area organic light-emitting diode (OLED) display panels with 8K or higher resolution. Although the selective deposition of organic molecules through shadow masks has proven to be the method of choice for mobile panels, it may not be so when independently defined high-resolution pixels are to be manufactured on a large substrate. This technical challenge motivated us to adopt the well-established photolithographic protocol to the OLED pixel patterning. In this study, we demonstrate the two-color OLED pixels integrated on a single substrate using a negative-tone highly fluorinated photoresist (PR) and fluorous solvents. Preliminary experiments were performed to examine the probable damaging effects of the developing and stripping processes upon a hole-transporting layer (HTL). No significant deterioration in the efficiency of the develop-processed device was observed. Efficiency of the device after lift-off was up to 72% relative to that of the reference device with no significant change in operating voltage. The procedure was repeated to successfully obtain two-color pixel arrays. Furthermore, the patterning of 15 μm green pixels was accomplished. It is expected that photolithography can provide a useful tool for the production of high-resolution large OLED displays in the near future.

Full Field Radiant Flux Distribution of Multiple Tilted Flat Lambertian Light Sources

The emergence of visible light communication (VLC) as a subset of optical wireless communication (OWC) in the early 2000s has turned any light-emitting diode (LED) source into a potential data transmitter. The design process for any VLC or OWC system typically involves a link budget analysis performed by studying the signal-to-noise ratio (SNR) at the receiver. Since this SNR strongly depends on the radiant flux collected by the receiver, an accurate model for this parameter is required. The point-source model has been widely used since 1979 and generally provides a good approximation of the received radiant flux. However, it might be less accurate for typical extended lighting sources like LED panels or large-area organic LEDs. In this paper, the radiant flux distribution of flat Lambertian rectangular or circular sources is derived from a vector analysis of their irradiance. It is then validated through actual measurements in the case of a circular source. The resulting extended-source models thus better capture the light beam pattern of such transmitters to enable a more accurate link budget. It provides at the same time almost identical results to the point-source model for small sources and can, therefore, be seen as a natural extension of this already widely used model. INDEX TERMS LED pattern, irradiance pattern, light power pattern, non-imaging optics, link budget, optical wireless communication, visible light communication.

Impact of thermal transport parameters on the operating temperature of organic light emitting diodes

Excess heat in organic light emitting diodes (OLEDs) that is produced during their operation may accelerate their degradation and may cause an inhomogeneous brightness distribution, in particular in large area OLEDs. Assessing the quantitative impact of heat excess is difficult, because all decisive processes related to charge transport and emission via charge recombination are thermally activated. For example, electric currents that are elevated due to larger temperatures cause additional Joule heating and, hence, increase the device temperature even further. Here, we establish how parameters responsible for heat transport, i.e., the thermal conductivity of the organic layers and the heat transfer coefficient between the device surface and the environment, govern the temperature inside the OLED. Relying on three-dimensional drift-diffusion simulations that self-consistently couple thermally-activated charge transport and heat transport, we establish that the thermal conductivity of organic layers is not a bottleneck for heat transport, because the encountered layer thicknesses in realistic device geometries prevent heat accumulation. The heat transfer to the ambient environment is the key parameter to dissipate excess heat from the device. Intentionally elevated operating temperatures, which may improve the OLEDs’ electric performance, are not necessarily beneficial, as any increase in operating temperature reduces the device stability. The thermal effects, being decisive for the OLED temperature, occur in device layers beyond the electrically active region. We propose analytical expressions that relate the temperature in the device for a given point of operation to the heat transfer to the environment and the substrate.

Photometric and Electrical Characterizations of Large-Area OLEDs Aged Under Thermal and Electrical Stresses

The aim of this paper is to identify the electrical signatures of degradations of large-area organic light-emitting diodes (OLEDs) (41 cm² active area), subjected to various stress conditions. Three Philips GL55 OLEDs were stressed under three distinct temperature values: 23 °C, 40 °C, and 60 °C at a stress current density of J = 15 mA/cm² (rated current density: Jn = 9.49 mA/cm²). Under thermal and electrical stresses, an increase of the operating voltage was observed with stress time but thermal stress alone did neither affect the operating voltage nor the luminance values up to 60 °C.

Radiated Emissions of an Array of OLED Luminaries

This paper presents the analysis of the radiated emissions of large-area organic light-emitting diode (OLED) luminaries with respect to the electromagnetic (EM) compatibility defined by the CISPR 15 standard. Large-area luminaries consist of an array of OLED modules. The radiated emissions of the individual modules overlap in the frequency domain. The radiated emissions of the modules are analyzed by an EM model, which estimates large loop antenna measurements. The probability of the radiated emissions overlap in the frequency domain when using the standard measurement procedure is addressed by a statistical model. The measurements of an example OLED wall light agree with the estimates of the EM model of the radiated emissions, while the numerical calculations confirm the probability of radiated emissions overlap in the frequency domain and its distribution. The probability of the radiated emissions overlap in the large-area OLED luminary is significant and the design of the individual OLED module has to take into account the size of the large-area luminary and the number of modules it consists of.

Cloaking of metal grid electrodes on Lambertian emitters by free-form refractive surfaces.

We discuss invisibility cloaking of metal grid electrodes on Lambertian light emitters by using dielectric free-form surfaces. We show that cloaking can be ideal in geometrical optics for all viewing directions if reflections at the dielectric-air interface are negligible. We also present corresponding white-light proof-of-principle experiments that demonstrate close-to-ideal cloaking for a wide range of viewing angles. Remaining imperfections are analyzed by ray-tracing calculations. The concept can potentially be used to enhance the luminance homogeneity of large-area organic light-emitting diodes.

Backside Contacting for Uniform Luminance in Large-Area OLED

ABSTRACTWe have investigated organic light emitting diode (OLED) backside contacting for the enhancement of luminance uniformity as a superior alternative to gridlines. In this approach, the low-conductivity OLED anode is supported by a high-conductivity auxiliary electrode and vertically contacted through via holes. Electrical simulations of large-area OLEDs have predicted that this method allows comparable luminance uniformity while sacrificing significantly less active area compared to the common gridline approach.The method for fabricating backside contacts is comprised of five steps: (1) Thin-film encapsulation of the OLED, (2) Patterning of the OLED surface with lithography (resist mask defining via hole positions), (3) Via hole formation to the bottom anode by a plasma etching process, (4) Organic residues removal and sidewall insulation. (5) Contacting of the anode with a high-conductivity auxiliary electrode.Backside-contacted OLEDs processed by organic vapor phase deposition show high luminance uniformity. Scanning electron microscopy pictures and electrical breakthrough measurements confirm efficient sidewall insulation.

Roll-to-roll preparation of silver-nanowire transparent electrode and its application to large-area organic light-emitting diodes

Silver-nanowires (AgNWs) transparent electrodes were successfully fabricated on a polyethylene terephthalate (PET) film substrate by roll-to-roll processes. The AgNWs were embedded in a polymer resin so that the surface was flat enough to fabricate organic light-emitting diodes (OLEDs) without an additional surface planarization. For the fabrication of the embedded AgNWs electrodes, a polyimide film was utilized as an AgNWs-carrying film to realize a roll-to-roll process for the first time. Large-area OLEDs were then deposited directly on the embedded AgNWs electrodes by a roll-to-roll vacuum deposition process. The OLEDs with the embedded AgNWs electrodes on a PET film showed 30–40% better performance than those with commercial indium-tin-oxide electrodes on glass substrates. The surface roughness, sheet resistance, and optical transmittance of the embedded AgNWs electrodes on a PET film were around 3 nm, 5 Ω/square, and 85%, respectively. The sheet-resistance uniformity of the embedded AgNWs electrode was higher than 90% in 15 cm × 45 cm area.

Grid Optimization of Large-Area OLED Lighting Panel Electrodes

Luminance loss resulting from potential drop on the transparent indium tin oxide (ITO) electrode due to its relatively high resistivity is one of the most essential issues in the design of large-area organic light-emitting diode (OLED) lighting panels. One solution is to pattern metal grid with low sheet resistance on the ITO electrode. However, the shape, height, and width of the metal grid element have a great influence on the final luminance uniformity of the device. In this paper, a method is proposed to optimize these parameters in order to get the best luminance uniformity for large-area OLED lighting panels. The method takes two grid geometry parameters—height and width—into account and predicts the highest relative luminance by finite element method simulations under different operating voltages.

P-Laplace thermistor modeling of electrothermal feedback in organic semiconductor devices

In large-area organic light-emitting diodes (OLEDs), spatially inhomogeneous luminance at high power due to inhomogeneous current flow and electrothermal feedback can be observed. To describe these self-heating effects in organic semiconductors, we present a stationary thermistor model based on the heat equation for the temperature coupled to a p-Laplace-type equation for the electrostatic potential with mixed boundary conditions. The p-Laplacian describes the non-Ohmic electrical behavior of the organic material. Moreover, an Arrhenius-like temperature dependency of the electrical conductivity is considered. We introduce a finite-volume scheme for the system and discuss its relation to recent network models for OLEDs. In two spatial dimensions, we derive a priori estimates for the temperature and the electrostatic potential and prove the existence of a weak solution by Schauder’s fixed-point theorem.

Multilayer slot-die coating of large-area organic light-emitting diodes

We investigate the slot-die coating process for the fabrication of large-area OLED lighting panels. Of many OLED layers, aqueous polymer-based hole injection layer (HIL) and small molecule-based hole transport layer (HTL) are formed using large-area slot-die coating. We are faced with three technical issues related with slot-die coating such as the flow down of an aqueous polymer solution near the inner perimeter of an insulator bank, pinhole-like surface in solution-processed small-molecule films, and the dissolution between two stacked layers with different solvents. We have suppressed those phenomena to a great extent and demonstrated that OLEDs with slot-die coated multiple layers show almost the same device performance as a reference OLED device with spin-coated HIL and vacuum-evaporated HTL. The peak-to-peak roughness of the slot-die coated bilayer (HIL/HTL) films is observed to be less than 12.5nm. The OLED device with the slot-die coated bilayer film exhibits the power efficiency of 27.2lm/W at 1000cd/m2, which is even higher than that (25.5lm/W) of the reference device.