Large-area Organic Light-emitting Diodes Research Articles

Effects of a 2 nm thick Al2O3 buffer layer in metal auxiliary electrode on lifetime and stable operation of large-area organic light emitting diodes

We have investigated the buffer layer effect in the auxiliary electrode on the lifetime and stable operation of large-area organic light-emitting diodes (OLEDs). A 2-nm-thick Al2O3 layer was introduced between the Mo and Al in a conventional Mo–Al–Mo metal auxiliary electrode structure. The structure of the buffer layer inserted auxiliary electrode is Mo–Al–Al2O3–Al–Al2O3–Al–Mo. By introducing the 2-nm-thick Al2O3 buffer layers, the etch rate of the Al layer was effectively controlled, which can prevent the formation of undercut structures in the metal auxiliary electrode caused by the different etch rates of Mo and Al. A trapezoidal etch profile was obtained from the Al2O3 buffer layer inserted metal auxiliary electrode structure; the profile did not contain undercuts or voids. The buffer layer inserted auxiliary electrode was used when fabricating a 30×120mm2 green emission OLED lighting panel. The lifetime of the OLEDs devices with buffer layer introduced metal auxiliary electrode reached 40,000h for the LT 70 condition, which is around four times longer than that obtained from OLEDs without the buffer layers.

Improved light outcoupling of organic light-emitting diodes by randomly embossed nanostructure

In this paper, we proposed a very economical and simple method to fabricate the randomly structured organic light emitting diode (OLED) to enhance light outcoupling efficiency. The random pattern is obtained by directly spin-coating the mixture solution of polystyrene (PS) nanospheres and PEDOT:PSS on an ITO/glass substrate and then removing PS nanospheres. The results demonstrate that using such a randomly embossed PEDOT:PSS as the hole injection layer in Alq3-based OLED produces the maximum current efficiency of 3.34cd/A with the enhancement factor of 18.0% and the maximum power efficiency of 2.16lm/W with the enhancement factor of 67.9% compared with the typical planar Alq3-based OLED (with no PEDOT:PSS layer). On one hand, the enhanced efficiency is associated with the enhanced injection/transport of holes. On the other hand, the enhanced efficiency is also attributed to the extraction of some trapped optical modes over a broad range of wavelengths. This work offers a promising method for the fabrication of high performance, low-cost and large area OLEDs.

Optimization of large-area OLED current distribution grids with self-aligned passivation

The luminance homogeneity of large-area organic light emitting diodes (OLEDs) is limited by the sheet resistance of the transparent electrode. A large lateral voltage drop inside the electrode and thus brightness inhomogeneity result from the high sheet resistance of transparent conductors. To improve sheet resistance, a low-resistance metal grid is often included. To prevent shorting, the grid needs to be passivated. However, since passivation further decreases the device active area, accurate alignment of the passivation layer is crucial. We report simulations of a Joule-heating-based self-alignment method for the passivation layer. The Joule heating model was divided into two sub-models: a current model to study current distribution on grid scale during Joule heating, and thermoelectric model to study heat transfer in the system. Grid design rules – minimum line pitch and the geometry of the grid – necessary for a successful Joule heating process were studied. The line group design was found to be the best option for a current distribution grid. The minimum line pitch limited by heat transfer was 0.8mm on an indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrate. With the line group design, maximum luminance output was achieved with a pitch of 4mm, when the sheet resistance of the metal lines was 0.01Ω/□. This fulfils the demands placed by the Joule heating process.

Fabrication and implementation of large-area organic light-emitting-diode devices on direct patterned backplanes

In this paper, we report on a new cost-effective fabrication technology for large-area organic light-emitting diode (OLED) devices that we have developed. We fabricated OLED lighting devices on a directly patterned backplane using a dry process without employing conventional photolithography patterning technology. The indium–tin-oxide (ITO) anode, metal cathode, insulator, and emissive organic layer patterns were all formed directly on the substrates during the sputtering and evaporation steps by using a shadow mask without using etching or lift-off methods. We fabricated and characterized green phosphorescent emission OLED devices with an emission area of 30 × 120 mm2, based on backplanes formed by direct patterning technology. Application of direct patterning technology reduced the total number of processing steps to 4 from the 26 steps required by conventional photo-patterning technology, making it possible to reduce large-scale production cost. Although the process steps were reduced considerably, the typical characteristics were comparable to those of photo-patterned OLEDs. Furthermore, the lifetime of the OLEDs based on direct patterned backplane was observed to be 27,200 h, which is approximately 97% of the lifetime of conventionally patterned OLEDs.

Optical determination of the junction temperature of OLEDs

This paper describes a novel optical measurement technique for the in situ determination of the spatial temperature distribution at the organic layer level in large-area organic light-emitting diodes (OLEDs). The local junction temperature of OLEDs is a very important factor with respect to the luminance uniformity. Moreover the variation of local temperatures leads to a non-uniform depreciation of its light output, which in turn increases the luminance non-uniformity over time and affects the lifetime expectancy of the OLED.Junction temperature maps can be derived from easily measurable luminance maps after a dedicated calibration procedure. The underlying model based on the relationship between the local diode electrical behavior and the light emission at different temperatures. The method is illustrated for the self-heating of a commercially available OLED for lighting, and validated by comparison with infrared thermography.

Passive cooling of large-area organic light-emitting diodes

The authors investigated passive cooling of large-area organic light-emitting diodes (OLEDs) with special focus on convective cooling. Electro-optical and thermal behaviour of large-area OLED devices are therefore modelled using finite element method (FEM) and computational fluid dynamics (CFD) simulations. Resulting temperature and luminance distributions are compared with measurement data at different driving conditions and test setups. The investigation yields that including laterally resolved convection coefficients from CFD simulations greatly improves model accuracy compared to simpler convection estimations. These findings are important for OLED and their heat spreader design especially for features like flexible, transparent, high-power and or large-area due to their specific limitations for heat spreading and or their high heat spreading requirements.

Analytical model for current distribution in large-area organic light emitting diodes with parallel metal grid lines

In this study, an analytical solution for the current distribution of a large-area organic light emitting diodes (OLEDs) with parallel equidistant gridlines is derived. In contrast to numerical methods, this analytical solution allows for a very quick scan of the OLED design space, even for very large OLEDs, providing insight how different model parameters affect each other. The assumptions within the analytical derivation are verified with finite element simulations of the same OLED. Furthermore, the analytically calculated light distribution was experimentally verified by measuring the light distribution on a large-area OLED.

Continuous blade coating for multi-layer large-area organic light-emitting diode and solar cell

A continuous roll-to-roll compatible blade-coating method for multi-layers of general organic semiconductors is developed. Dissolution of the underlying film during coating is prevented by simultaneously applying heating from the bottom and gentle hot wind from the top. The solvent is immediately expelled and reflow inhibited. This method succeeds for polymers and small molecules. Uniformity is within 10% for 5 cm by 5 cm area with a mean value of tens of nanometers for both organic light-emitting diode (OLED) and solar cell structure with little material waste. For phosphorescent OLED 25 cd/A is achieved for green, 15 cd/A for orange, and 8 cd/A for blue. For fluorescent OLED 4.3 cd/A is achieved for blue, 9 cd/A for orange, and 6.9 cd/A for white. For OLED with 2 cm by 3 cm active area, the luminance variation is within 10%. Power conversion efficiency of 4.1% is achieved for polymer solar cell, similar to spin coating using the same materials. Very-low-cost and high-throughput fabrication of efficient organic devices is realized by the continuous blade-only method.

ITO-free large-area organic light-emitting diodes with an integrated metal grid

We report on ITO-free large-area organic light-emitting diodes (OLEDs) fabricated on glass substrates comprising α-NPD as a hole transport layer (HTL) and coevaporated CBP:Ir(ppy)(3) as the emission layer. Indium-tin-oxide (ITO) was replaced with a conductive polymer electrode and an electroplated thick metal grid was used to improve the homogeneity of the potential distribution over the transparent polymer electrode. An electrical model of a metal grid integrated OLED shows the benefits of the use of metal grids in terms of improving the uniformity of the light emitted as the area of the OLED increases as well as the conductivity of the transparent electrode decreases. By integrating metal grids with polymer electrodes, the luminance increases more than 24% at 6 V and 45% at 7 V compared to the polymer electrode devices without a metal grid. This implies that a lower voltage can be applied to achieve the same luminance, hence lowering the power consumption. Furthermore, metal grid integrated OLEDs exhibited less variation in light emission compared to devices without a metal grid.

Electrothermal characterization of large-area organic light-emitting diodes employing finite-element simulation

We investigated and modeled the electrothermal behavior of a 5 × 5 cm 2 organic light-emitting diode (OLED) with electrothermal measurements and finite-element simulation. A hybrid electrothermal model consisting of finite and lumped elements was proposed. Heat distribution of large-area OLED was measured by infrared spectroscopy. We have achieved an excellent agreement of measured and simulated results. The simulation confirms a strong influence of temperature on current distribution for large-area OLED. It turns out that the design of homogeneous devices requires knowledge about electrical and thermal aspects. Another result anticipates that the switching behavior of OLED strongly correlates with thermal relaxation. The model is a valuable tool to simulate luminance distribution and local aging allowing strong improvement of device development.

Ink Jet Technology for Large Area Organic Light-Emitting Diode and Organic Photovoltaic Applications

Due to its flexibility and ease of patterning, ink jet printing has become a popular technique for the noncontact deposition of liquids, solutions, and melts on a variety of substrates at lateral resolutions down to 10 μm. This article presents a study of ink jet printing of homogeneous layers of Orgacon™ (Agfa-Gevaert, Belgium), a water-based dispersion of poly(3,4-ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS). The printed PEDOT:PSS layer can be used as a transparent electrode in organic light-emitting diodes (OLEDs). Fundamental aspects of the interaction between the ink jet ink and the substrate and the resulting homogeneity of the active layer in relation to OLED device performance are investigated. The optimized PEDOT:PSS ink formulation is shown to improve layer homogeneity, resulting in a uniform light output and device efficiency. Ink jet printing is shown to be capable of fabricating 25×25 mm OLED devices that have equivalent efficiency and light uniformity to the ones produced by spin coating.

Speedup of Dynamic Response of Organic Light-Emitting Diodes

We demonstrate that large-area organic light-emitting diodes (OLEDs) exhibit slow dynamic response due to their capacitor-like behaviors. In particular, the discharge dynamics of large-area OLEDs is observed to be relatively slower, compared with the charge dynamics. To find ways to increase the response speed of large-area OLEDs, we make an in-depth study of transient electroluminescence (EL) of OLEDs in response to voltage pulses (i.e., pulse separation), device configurations (i.e., device length), and various material parameters (e.g., carrier mobility, exciton lifetime, and energy level offset). The EL response speed can be increased by applying high bias voltage and reducing the device length. Meanwhile, the pulse separation affects the response speed of the second pulse. As the pulse separation is decreased, the ratio of the delay time of the second pulse to the delay time of the first pulse is found to be reduced, whereas the ratio of the peak luminance of the second pulse to the peak one of the first pulse is getting increased. We have also shown that the dependence of the delay time on the carrier mobility, exciton lifetime, and energy level offset is less pronounced. This study may provide design guidelines of OLEDs for practical applications including ac-driven displays and next-generation visible-light communications (VLCs).

Luminance Uniformity of Large-Area OLEDs With an Auxiliary Metal Electrode

One of the key issues of a large-area organic light-emitting diode (OLED) for flat panel lighting applications is to enhance the uniformity of light emission. In this work, we have investigated the effect of an auxiliary metal (chrome) electrode in association with a device configuration on the luminance uniformity of a large-area (15 times 15 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) white OLED. We demonstrate that the ratio between the effective horizontal resistance of anode (indium-tin-oxide (ITO) with the grid patterned metal electrode) and the vertical resistance of the OLED device is the critical factor to determine the luminance uniformity. Moreover, the luminance uniformity is shown to be a function of the current density and degraded with increasing current density. Namely, the OLED panel with the 200-mu m-wide metal lines exhibits the luminance uniformity as high as 90% at 200 mA and 85% at 500 mA.

Large-area organic light-emitting diode technology

Large-area organic light-emitting diode technology