Tilman A. Beierlein

Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling

A dielectric capping layer has been used to increase the light output and to tune the spectral characteristics of top-emitting, phosphorescent organic light-emitting devices (OLEDs). By controlling the thickness of the dielectric layer deposited on top of a thin metal cathode, the transmittance of the top electrode can be adjusted. Maximum light output is not achieved at highest cathode transmittance, indicating that the interplay between different interference effects can be controlled by means of the capping-layer thickness. Furthermore, we demonstrate that the electrical device characteristic is not influenced by the capping layer. The strength of our concept in particular lies in the fact that the optical and the electrical device performance can be optimized separately. Using the capping-layer concept, we have achieved an OLED efficiency of 64 cd/A with pure green emission.

Tuning the emission characteristics of top-emitting organic light-emitting devices by means of a dielectric capping layer: An experimental and theoretical study

The emission characteristics of top-emitting organic light-emitting devices (OLEDs) have been studied experimentally and theoretically to derive a quantitative understanding of the effect of a dielectric capping layer. We demonstrated that the angular intensity distribution and the spectral characteristics can be tuned and the light outcoupling enhanced simply by varying the optical thickness of a dielectric layer deposited on top of a semitransparent metal electrode. With the capping-layer concept, the outcoupled light intensity in forward direction was increased by a factor of 1.7, and concomitantly a high color purity achieved. An optical model based on a classical approach was used to calculate the emission characteristics. The excellent agreement between measured and simulated data shows that the capping layer controls the interplay between different interference effects such as wide-angle and multiple-beam interference occurring in top-emitting OLEDs. The strength of the capping layer concept is in ...

Simulating electronic and optical processes in multilayer organic light-emitting devices

A detailed investigation of the device operation of a blue-emitting multilayer organic light-emitting device (OLED) using an electronic device model is presented. In particular, a transient electroluminescence overshoot at turn-on is found to originate from charge and recombination confinement effects at internal interfaces. The location of the emission zone is obtained from the electronic model and its experimental determination exemplified by a sensing layer method. Moreover, the optimization of emission intensity and color is discussed for a red-emitting OLED. The thin-film interference effects are analyzed with help of an optical device model.

4.1: A 20‐inch OLED Display Driven by Super‐Amorphous‐Silicon Technology

A 20-inch, largest OLED display in the world is demonstrated which is driven by “Super Amorphous Silicon” technology. It has been widely believed that the characteristics of amorphous silicon TFT is not sufficient to drive OLED display. This paper turns over the hypothesis to prove that amorphous silicon can be applied to the large active-matrix driven displays. Novel approaches to enable amorphous silicon to drive bright and long life OLED display are shown to open a bright future to realize larger OLED televisions.

Influence of trapped and interfacial charges in organic multilayer light-emitting devices

Trapped and interfacial charges have significant impact on the performance of organic light-emitting devices (OLEDs). We have studied devices consisting of 20 nm copper phthalocyanine (CuPc) as the buffer and hole-injection layer, 50 nm N, N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) as the hole transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq3) as the electron transport and emitting layer sandwiched between a high-work-function metal and a semitransparent Ca electrode. Current-voltage measurements show that the device characteristics in the negative bias direction and at low positive bias below the built-in voltage are influenced by trapped charges within the organic layers. This is manifested by a strong dependence of the current in this range on the direction and speed of the voltage sweep. Low-frequency capacitance-voltage and static charge measurements reveal a voltage-independent capacitance in the negative bias direction and a significant increase between 0 and 2 V in the given device configuration, indicating the presence of negative interfacial charges at the NPB/Alq3 interface. Transient experiments show that the delay time of electroluminescence at low voltages in these multilayer devices is controlled by the buildup of internal space charges, which facilitates electron injection, rather than by charge-carrier transport through the organic layers. To summarize, our results clearly demonstrate that the tailoring of internal barriers in multilayer devices leads to a significant improvement in device performance.

Kelvin probe investigations of metal work functions and correlation to device performance of organic light-emitting devices

Abstract Using the vibrating capacitor Kelvin probe technique, we have determined the contact potential difference (CPD) between a reference electrode and various metals acting as charge carrier injecting contacts in organic light-emitting devices (OLEDs). These investigations show that the work function of anode materials for OLEDs such as Pt, Au, and indium tin oxide depends strongly on the surface treatment and can be increased by more than 1 eV via oxygen plasma or UV-ozone cleaning. The device performance of multilayer OLEDs consisting of these anodes, copper-phthalocyanine (CuPc), N , N ′-di(naphthalene-1-yl)- N , N ′-diphenyl-benzidine (NPB), tris-(8-hydroxyquinolinato)aluminum (Alq 3 ), and a low-work-function metal cathode is correlated with the results of the CPD measurements. However, our investigations indicate that, apart from the measured work function, other factors such as the surface roughness and the binding energy of oxygen to the metal surface can significantly influence the injection properties and the long-term stability of the devices.

Influence of space charges on the current–voltage characteristic of organic light-emitting devices

Abstract The role of space charges on the device characteristics of multilayer organic light-emitting devices (OLEDs) is investigated. We studied OLEDs consisting of copper phthalocyanine (CuPc) as buffer and hole injection layer, N , N ′-di(naphthalene-1-yl)- N , N ′-diphenyl-benzidine (NPB) as hole transport layer, and tris(8-hydroxyquinolinato)aluminum (Alq 3 ) as electron transport and emitting layer sandwiched between high and low work function metal electrodes. Detailed current–voltage measurements show that the device characteristics at low bias depend strongly on sweep direction as well as on sweep speed, indicating that space charges accumulate within the organic layers. On the one hand these space charges increase the electric field for electron injection at the cathode, on the other hand they screen the applied electrical field and thus determine the steepness of the current–voltage characteristics. Reducing these space charges by fabricating optimized structures where the limiting interfaces between the different organic layers are graded results in a significantly enhanced current flow and higher brightness at a given voltage.

Investigation of internal processes in organic light-emitting devices using thin sensing layers

Abstract Systematic studies are a prerequisite for a detailed understanding of the internal processes in organic semiconductors and devices, which is of great importance for optimizing organic light-emitting diode performance. Devices based on small molecules are especially well-suited for introducing thin layers (

59.1: Invited Paper: Optoelectronic OLED Modeling for Device Optimization and Analysis

Organic light-emitting devices (OLEDs) consist of a stack of multiple thin film layers whose thicknesses influence both the optical and electronic performance. Upon injection and transport, the charge carriers may recombine to form excitons that diffuse and decay radiatively, thus leading to distinct recombination and emission zone profiles that determine device performance. Suitable simulation tools that allow a better understanding and efficient optimization of organic optoelectronics devices and materials are desirable.

22.4: Invited Paper: Tuning Solution Processed OLEDs Towards R2R Printing

Solution processed OLEDs offer the potential for highest throughput production processes such as roll-to-roll (R2R) printing. We use polymer mixtures with additives to evaluate and optimize the performance. Such multi-component systems open up a huge parameter space by varying layer thicknesses, material combinations and concentrations. Manual device fabrication would be time-consuming and expensive. A new custom built and automated fabrication and characterization setup accelerates material screening, testing and optimization. Furthermore, a systematic set of devices is the basis for reliable comparisons with simulations.