Top-emitting Organic Light-emitting Diodes Research Articles

Color-Tunable, Spectra-Stable Flexible White Top-Emitting Organic Light-Emitting Devices Based on Alternating Current Driven and Dual-Microcavity Technology

White top-emitting organic light-emitting diodes (TOLEDs) are attractive due to their freedom of choice in substrates. However, tunable color and stable spectra for white TOLED remain considerable ...

5‐4: High Efficiency Top‐Emission Organic Light Emitting Diodes Realized Using Newly Developed Low Absorption Pure Ag cathode Configuration

We report, highly efficient red top emission organic light emitting diodes (TEOLEDs) having thin pristine Ag cathode unit with low absorption, uniform surface morphology and good electron injection property. TEOLEDs with Ag cathode unit exhibit almost 1.4 times improvement in current efficiency as well as low driving voltage compared with widely used Mg:Ag cathode and almost identical lifetime with Mg:Ag cathode.

5‐1: Distinguished Paper: Eliminate Angular Color Shift in Top Emitting OLEDs through Cavity Design

OLEDs suffer from viewing angle dependent spectral shift due to microcavity effects. To address this issue, we introduce a novel top‐emitting OLED with a dielectric spacer which forms multiple cavity modes. The resulting device shows almost no color shift at different viewing angles.

Highly efficient top-emission organic light-emitting diode on an oxidized aluminum anode

In today's manufacturing of organic light-emitting diode on silicon for microdisplay technologies, a top-emitting OLED (TEOLED) is required to be fabricated on top of an active-matrix circuitry located on the silicon backplane. This requires a highly reflective anode to enhance the luminance output. However, during the production process of a TEOLED, the hole injection efficiency and electrical conductivity may be suppressed by environmental exposure, in particular, moisture and oxygen. Given this, aluminum is an unfavorable reflective anode due to the primary concern of its native insulating oxide layer. The native oxide tends to grow during the patterning of the metal anode. In this paper, we have discovered that, by utilizing an Al2O3/MoO3 heterojunction anode structure, a highly efficient device can be made to achieve a current efficiency of 94 cd/A at a luminance of 1000 cd/m2. X-ray/ultraviolet photoelectron spectroscopy measurements show the formation of molybdenum gap states and favorable energy level alignment for hole injection.

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

The ratio of spontaneous emission inside a diode structure to that in free space is called the Purcell factor (F(λ)). The structure of organic light-emitting diodes (OLEDs) has a significant influence on the spontaneous emission rate of dipole emitters. Therefore, to describe the optical properties of OLEDs, it is essential to incorporate F(λ) in the description. However, many optical studies on OLEDs continue to be conducted without considering F(λ) for simplicity's sake. Hence, in this study, using carefully designed bottom- and top-emitting OLEDs, we show that the external quantum efficiency obtained without considering F(λ) can be over- or under-estimated, and in some cases, the margin of error may be significant. We also reveal that the subtle distribution of the electroluminescence spectrum can be explained properly only by including F(λ). Both these results stipulate the importance of including F(λ) to maintain a quantitative agreement between theoretical and experimental data. Hence, the inclusion of F(λ) is important for designing OLEDs with enhanced efficiency or desired spectral characteristics.

Effect of Dipole Orientation on the Angular Emission Characteristic of a Three-Dimensional Top-Emitting Organic Light-Emitting Diode with Square Pixel Boundary

We numerically investigate the effect of the dipole orientation on the light emission characteristics of a three-dimensional top-emitting organic light-emitting diode (TOLED) with square pixel boundary, based on the finite element method. The spatial profiles of the optical power flows and the output angular emission characteristics are calculated in various horizontal positions of a single dipole with the x-, y-and z-directed dipole orientations. The total angular emission characteristics of the TOLED are obtained by summing the individual angular emission characteristics of 100 dipole emitters, which are assumed to be uniformly distributed within the square pixel boundary. The angular emission pattern of the y-directed dipole emitters exhibits an approximately 10% higher intensity with a narrower linewidth than that of the x-directed dipole emitters. This difference in the angular emission patterns of the x- and y-directed dipole orientations can be attributed to the dipole orientation dependent internal radiation pattern, which has different spectral power densities as a function of the internal emission angle and light polarization. In addition, the average angular emission characteristics of the randomly oriented dipole emitter are calculated by averaging over those of the x- and y- directed dipole emitters.

基于微结构阵列基板的高效顶发射OLED器件

基于微结构阵列基板的高效顶发射OLED器件

Theoretical Modeling of the Internal Power Flow and Absorption Loss of the Air Mode Based on the Proposed Poynting Vector Analysis in Top-emitting Organic Light-emitting Diodes

We propose the Poynting vector analysis of the air mode in a top-emitting organic light-emitting diode (OLED) by combining the transfer matrix method and dipole source term. The spatial profiles of the time-averaged optical power flow of the air mode are calculated inside and outside the multilayer structure of the OLED with respect to the thickness of the semi-transparent top cathode and capping layer (CPL). We elucidate how the micro-cavity effect controlled by the thickness variation of the semi-transparent top cathode or CPL affects the internal optical power and absorption loss inside the OLED multilayer and the external optical power coupled into the air. When the calculated absorption loss and external power obtained by the proposed Poynting vector and currently-used point dipole models are compared, two calculation results are identical, which demonstrates the validity of the two models.

Highly efficient and foldable top-emission organic light-emitting diodes based on Ag-nanoparticles modified graphite electrode

Abstract Top-emission flexible organic light-emitting devices (TE-FOLEDs) are highly suitable for next generation display due to their numerous assets including top-emitting configuration and mechanical flexibility. One major challenge in TE-FOLEDs is to prepare a deformable and reflective bottom electrode capable of effective carrier injection. In this paper, a new strategy for efficient and foldable TE-FOLEDs is demonstrated. It is based on a highly conductive Ag-nanoparticles (Ag-NPs) modified graphite that is used as a flexible bottom electrode. The good reflectance to full-color emission (>59% over the whole visible wavelength range), ultralow sheet resistance (

Optical modeling of the emission zone profile and optimal emitter position based on the internal field profile of the air mode in organic light-emitting diodes.

We propose a theoretical formulation to calculate the internal profile of the air mode in the organic light-emitting diode (OLED) on the combination of the transfer matrix method and source-term method. The spatial distributions of the air mode are calculated in a top-emitting OLED with respect to the light polarization, extraction angle, dipole orientation, and dipole position. Air modes are also calculated on the basis of the previously used external source model, where the input optical wave is injected from the air into the OLED multilayer. Comparison of the calculated air modes between two models checks the validity of the external source model. In addition, we propose an improved formula to determine the optimal emitter positions that maximize the two-beam interference of the micro-cavity effect. In the improved formula, a non-ideal reflection phase shift at a reflective metal anode is treated as the skin depth of the air mode. Finally, the effect of the dipole orientation on the air mode is investigated. Compared with the air mode emitted by the horizontally oriented dipole, the air mode generated by the vertically oriented dipole has relatively small intensity and shows the opposite dependence of the emitter position variation. The calculation results of the internal profile of the air mode within the emission layer are matched with the profile of the emission zone obtained by output radiant flux on the basis of the currently used point dipole model.

Non-linear relation between emissive dipole orientation and forward luminous efficiency of top-emitting organic light-emitting diodes

Abstract Studies related to emitting dipole orientation (EDO) have been widely carried out to obtain highly efficient organic light-emitting diodes (OLEDs) in a bottom-emitting structure. Herein, we report on the relation between EDO and the forward luminous efficiency of top-emitting OLEDs (TEOLEDs). The forward emission of OLED device is solely contributed by the horizontal dipole. However, the effect of horizontal and vertical dipole on the various optical modes in OLEDs structure is significantly different, resulting in changing the Purcell factor for the ratio of EDO. Therefore, as horizontally oriented dipole ratio (HODR) is increased, the efficiency is also increased but is not linearly increased. When Ir(ppy)2(acac) with predominantly horizontal EDO (74% HODR) was applied to top-emitting OLEDs, the device efficiency in the normal direction was improved by 26% compared with OLEDs based on Ir(ppy)3 with isotropic EDO (68% HODR), which is larger than the HODR difference between two dopants.

Optical Analysis of Power Distribution in Top-Emitting Organic Light Emitting Diodes Integrated with Nanolens Array Using Finite Difference Time Domain.

Recently, we have addressed that a formation mechanism of a nanolens array (NLA) fabricated by using a maskless vacuum deposition is explained as the increase in surface tension of organic molecules induced by their crystallization. Here, as another research using finite difference time domain simulations, not electric field intensities but transmitted energies of electromagnetic waves inside and outside top-emitting blue organic light-emitting diodes (TOLEDs), without and with NLAs, are obtained, to easily grasp the effect of NLA formation on the light extraction of TOLEDs. Interestingly, the calculations show that NLA acts as an efficient light extraction structure. With NLA, larger transmitted energies in the direction from emitting layer to air are observed, indicating that NLAs send more light to air otherwise trapped in the devices by reducing the losses by waveguide and absorption. This is more significant for higher refractive index of NLA. Simulation and measurement results are consistent. A successful increase in both light extraction efficiency and color stability of blue TOLEDs, rarely reported before, is accomplished by introducing the highly process-compatible NLA technology using the one-step dry process. Blue TOLEDs integrated with a N, N'-di(1-naphthyl)- N, N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine NLA with a refractive index of 1.8 show a 1.55-times-higher light extraction efficiency, compared to those without it. In addition, viewing angle characteristics are enhanced and image blurring is reduced, indicating that the manufacturer-adaptable technology satisfies the requirements of highly efficient and color-stable top-emission displays.

P‐176: Distinquished Poster Paper: The Suppression of Viewing Angle Dependence of Top Emission Organic Light Emitting Diodes Having Strong Microcavity Characteristics by Applying Concave Patterned Anode

In order to obtain the stable viewing angle characteristics of top emission organic light emitting diodes (TEOLEDs) having strong microcavity characteristics, we fabricated nano‐porous film on the glass substrate before depositing highly reflective anode. And then, we could obtain the concave patterned anode by depositing anode on the nano‐porous film and fabricated TEOLED composed of concave patterned layers. From this approach, we could successfully obtain not only the stable color shift and luminance distribution with viewing angle but also high efficiency caused by uneven morphology. In addition, we found that the driving voltage of TEOLED could be reduced due to increased surface area effect of the concave patterned anode, so that the power efficiency was enhanced by about 15% in comparison with reference device.

6‐3: Distinguished Paper: Highly Efficient Fluorescent Blue Materials and Their Applications for Top Emission OLEDs

We developed new fluorescent blue dopants (BDs) for achieving high efficient blue organic light emitting diode (OLED). The new BD showed both high photoluminescent quantum yield >0.9 and highly horizontal orientation (S'>0.9) in the doped film with keeping a chemical stability by introducing suitable substituents. We developed hole transporting materials and optimized the combination of hole transporting layers to decrease a carrier accumulation at the interface between electron blocking layer/emission layer (EML). We found that the external quantum efficiency dependency from low to high current density was turned flat by promoting hole injection into EML. The Top Emission OLED using the new BD and the optimized device architecture exhibited high efficiency of L/J/CIEy around 200 at CIEy=0.043.

Transparent electrode for top emission organiclight-emitting diode

Organic light-emitting diode (OLED) has been successfully used in mobile display (small area) and television display (large area). For small area display screens, the top emission structure is mostly adopted because of the limitation of display area. Top emission OLED (TEOLED) can get higher power efficiency at the same current density than traditional bottom emission devices because of its higher aperture-opening ratio. However, the most important part of top emission device is the preparation of transparent electrode. In summary, according to the method and material, TEOLED transparent electrode is divided into the following categories: (1) Transparent conductive oxide electrode (TCO); (2) ultrathin composite metal electrode; (3) dielectric/metal/dielectric (DMD) composite electrode; (4) nanomaterial electrodes, etc. The general oxide has great optical transmittance in the visible light range, but the thermal evaporation temperature is very high. It needs to be fabricated by sputtering, which damages the organic layer. The composite metal electrode can be made by thermal evaporation, which is simple and easy. It is adopted by most commercial products by now, however its transparency is relatively low. Nano materials have high transparency and can be fabricated for flexible electrodes, but because of the solution processing, it is easier to destroy the organic layer. DMD composite electrode not only has high transmittance, but also can use thermal evaporation method to form film, it is expected to become better choice of top electrode for TEOLED. In order to achieve the objective of producing top emitting OLED with excellent properties, it is most important to find enough transparent, high conductive, ohmic contact, film forming process without destroying organic layer and stable electrode material. Up to now, the researchers have proposed several methods for preparing barrier layer-ITO, pulsed laser deposition transparent conducting oxide, thermal evaporated ultrathin composite metal, DMD composite, nano-materials, etc. In this paper, the applications of these materials in the TEOLED transparent electrode are reviewed. By comparing the advantages and disadvantages of thin film electrode, the research progress of TEOLED transparent electrode is reviewed.

Suppression of the color shift of microcavity organic light-emitting diodes through the introduction of a circular polarizer with a nanoporous polymer film

ABSTRACTA circular polarizer (CP) film modified by a nanoporous polymer film (NPF) was applied to improve the viewing angle characteristics of a top-emission organic light-emitting diode (TEOLED) without any serious pixel blur phenomenon. From this approach, the angular color shift (Δu′v′) was significantly improved from 0.096 to 0.056, and the angular luminance distribution was also expanded to the Lambertian distribution direction. The black color had a slightly faded tint, however, like the turn-off condition of the plasma display panel (PDP), regardless of the introduction of NPF below or above the CP. Very interestingly, the surface reflectance of the device with NPF-coated CP was not significantly different from that of the device with non-NPF CP. The pixel blur level according to the distance between the scattering medium and the light source was also investigated. As a result, it was found that the pixel blur phenomenon can be greatly suppressed by applying a thinner encapsulation lid with an NPF film.

Enhanced device performances of a new inverted top-emitting OLEDs with relatively thick Ag electrode.

To improve the device performances of top-emitting organic light emitting diodes (TEOLEDs), we developed a new inverted TEOLEDs structure with silver (Ag) metal as a semi-transparent top electrode. Especially, we found that the use of relatively thick Ag electrode without using any carrier injection layer is beneficial to realize highly efficient device performances. Also, we could insert very thick overlying hole transport layer (HTL) on the emitting layer (EML) which could be very helpful to suppress the surface plasmon polariton (SPP) coupling if it is applied to the common bottom-emission OLEDs (BEOLEDs). As a result, we could realize noteworthy high current efficiency of approximately ~188.1 cd/A in our new inverted TEOLEDs with 25 nm thick Ag electrode.

The suppression of viewing angle dependence of top emission organic light emitting diodes having strong microcavity characteristics by applying concave patterned anode

AbstractIn order to obtain the stable viewing angle characteristics of top emission organic light emitting diodes (TEOLEDs) having strong microcavity characteristics, we fabricated nano‐porous film on the glass substrate before depositing highly reflective anode. And then, we could obtain the concave patterned anode by depositing anode on the nano‐porous film and fabricated TEOLED composed of concave patterned layers. From this approach, we could successfully obtain not only the stable color shift and luminance distribution with viewing angle but also high efficiency caused by uneven morphology. In addition, we found that the driving voltage of TEOLED could be reduced due to increased surface area effect of the concave patterned anode, so that the power efficiency was enhanced by about 15% in comparison with reference device.

Light modulation of top emission organic light emitting diodes showing strong microcavity effect by applying multilayered scattering film.

In order to suppress the viewing angle dependence of top emission organic light emitting diodes (TEOEDs) on a strong microcavity structure, we prepared multi-layered nano scattering film which was consisted with transparent planarizing layer and hazy crosslinked scattering layer. Through such an approach, we could obtain not only a stable color shift and luminance distribution with viewing angle but also a negligible pixel blur level. Meanwhile, we investigated a black tint level of TEOLEDs after attachment of circular polarizer (CP) on various nano scattering films because nano scattering film deteriorates a black level. We found that the black level could be improved from the black tint by reducing the refractive index difference between planarizing layer and scattering layer.

Origin of a sharp spectral peak near the critical angle in the spectral power density profile of top-emitting organic light-emitting diodes

We theoretically investigate the origin of the sharp spectral peak near the critical angle in the spectral power density profile of a top-emitting organic light-emitting diode (OLED). On the basis of the point dipole model, spectral power densities of the top-emitting OLED are calculated with respect to the thickness of the capping layer (CPL). Sharp peaks in the spectral power density profile are observed at certain CPL thicknesses near the spectral interface between the radiation- and waveguide-mode regions, which is determined by the critical angle. By calculating the waveguide-mode spatial profile, the sharp peak in the spectral power density profile is identified as the radiation-mode-like waveguide mode, which has a very large evanescent length near the cut-off condition of waveguide modes. By decomposing the total spectral power density into the out-coupled light and absorption loss components, we demonstrate that the sharp spectral peak near the critical angle can enhance the relative power contribution of the absorption loss.