Markus Fröbel

Optimizing the internal electric field distribution of alternating current driven organic light-emitting devices for a reduced operating voltage

The influence of the thickness of the insulating layer and the intrinsic organic layer on the driving voltage of p-i-n based alternating current driven organic light-emitting devices (AC-OLEDs) is investigated. A three-capacitor model is employed to predict the basic behavior of the devices, and good agreement with the experimental values is found. The proposed charge regeneration mechanism based on Zener tunneling is studied in terms of field strength across the intrinsic organic layers. A remarkable consistency between the measured field strength at the onset point of light emission (3–3.1 MV/cm) and the theoretically predicted breakdown field strength of around 3 MV/cm is obtained. The latter value represents the field required for Zener tunneling in wide band gap organic materials according to Fowler-Nordheim theory. AC-OLEDs with optimized thickness of the insulating and intrinsic layers show a reduction in the driving voltage required to reach a luminance of 1000 cd/m2 of up to 23% (8.9 V) and a cor...

Ultrathin Silver Electrodes for Transparent Organic Light-Emitting Diodes

The performance of ultrathin silver electrodes is compared to conventional indium tin oxide when used in transparent white organic light-emitting diodes. These electrodes provide a valid alternative.

Three-terminal RGB full-color OLED pixels for ultrahigh density displays

In recent years, the organic light-emitting diode (OLED) technology has been a rapidly evolving field of research, successfully making the transition to commercial applications such as mobile phones and other small portable devices. OLEDs provide efficient generation of light, excellent color quality, and allow for innovative display designs, e.g., curved shapes, mechanically flexible and/or transparent devices. Especially their self emissive nature is a highly desirable feature for display applications. In this work, we demonstrate an approach for full-color OLED pixels that are fabricated by vertical stacking of a red-, green-, and blue-emitting unit. Each unit can be addressed separately which allows for efficient generation of every color that is accessible by superpositioning the spectra of the individual emission units. Here, we use a combination of time division multiplexing and pulse width modulation to achieve efficient color mixing. The presented device design requires only three independently addressable electrodes, simplifying both fabrication and electrical driving. The device is built in a top-emission geometry, which is highly desirable for display fabrication as the pixel can be directly deposited onto back-plane electronics. Despite the top-emission design and the application of three silver layers within the device, there is only a minor color shift even for large viewing angles. The color space spanned by the three emission sub-units exceeds the sRGB space, providing more saturated green/yellow/red colors. Furthermore, the electrical performance of each individual unit is on par with standard single emission unit OLEDs, showing very low leakage currents and achieving brightness levels above 1000 cd/m2 at moderate voltages of around 3–4 V.

84-3: Invited Paper: Organic Light-Emitting Diode Beam Shaping: Pixel Design for Variable Angular Emission Profile Control

Organic light-emitting diodes (OLEDs) are the leading self-emitting pixel technology in current and future small and large area displays. Once integrated with a certain layer architecture into the backplane layout, their emission colour and angular distribution is set by the optical properties of the layered system. In this paper, we demonstrate a pixel design that allows for actively controlled variation of the angular emission profile of the individual vertical pixel. For this, a tandem device is developed that comprises two units optimized for different angular emission pattern. We constrained the system to operate in a narrow emission band to maintain monochromaticity of the individual pixel. We discuss this concept for a red phosphorescence-based OLED stack and give an outlook based on simulations for the other primary display colours green and blue. The tandem unit can be operated with only two electrodes making use of the AC/DC driving concept, where the outer electrodes are in direct connection. In this paper, we will discuss the potential, status, and technology challenges for this concept.

Real-time beam shaping without additional optical elements

Providing artificial light and enhancing the quality of the respective light sources is of continued interest in the fields of solid state, condensed matter, and semiconductor physics. Much research has been carried out to increase the luminous efficiency, lifetime and colour stability of such devices. However, the emission characteristics of a given light source do not necessarily comply with today’s often sophisticated applications. Here, beam shaping addresses the transformation of a given light distribution into a customized form. This is typically achieved by secondary optical elements often sporting elaborate designs, where the actual light source takes up only a small fraction of the system’s volume. Such designs limit the final light source to a single permanent operation mode, which can only be overcome by employing mechanically adjustable optical elements. Here we show that organic light-emitting diodes (OLEDs) can enable real-time regulation of a beam shape without relying on secondary optical elements and without using any mechanical adjustment. For a red light-emitting two-unit OLED architecture, we demonstrate the ability to continuously tune between strongly forward and strongly sideward emission, where the device efficiency is maintained at an application-relevant level ranging between 6 and 8% of external quantum efficiency for any chosen setting. In combination with additional optical elements, customizable and tuneable systems are possible, whereby the tuning stems from the light source itself rather than from the use of secondary optics.Organic Light-Emitting Diodes: Promising beam-shaping properties uncoveredA simple, efficient design incorporating organic light-emitting diodes could enable the development of customized, adjustable light sources for many applications. The ability to manipulate the shape of a beam of light emitted by a device is useful in many scenarios, for example switching between normal and private screen display modes on a mobile phone. However, current beam-shaping designs require extra components, making devices bulkier and more complex. Now, Felix Fries, Sebastian Reineke and co-workers at the Technische Universität Dresden, Germany, have used organic light-emitting diodes (OLEDs) to generate real-time beam shaping simply by manipulating the input power signal. The team’s design comprises two red-emitting OLEDs; by sending a carefully-modulated power signal through both OLEDs simultaneously they could customize the resulting beam of light. They hope to expand their research into full red-green-blue colour displays.