W. Rieß

Electron mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via transient electroluminescence from single- and multilayer organic light-emitting diodes

Transient electroluminescence (EL) from single- and multilayer organic light-emitting diodes (OLEDs) was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide (ITO)/tris(8-hydroxy-quinoline)aluminum (Alq3)/magnesium (Mg):silver (Ag), whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine (CuPc)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/Alq3/Mg:Ag. Apparent model-dependent values of the electron mobility (μe) in Alq3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the ...

Ambipolar organic field-effect transistor based on an organic heterostructure

Ambipolar charge injection and transport are a prerequisite for a light-emitting organic fieldeffect transistor (OFET). Organic materials, however, typically show unipolar charge-carrier transport characteristics. Consequently, organic thin-film field-effect transistors based on a single material as active layer can typically either be operated as p- or as n-channel device. In this article we show that by using a heterostructure with pentacene as hole-transport and N,N′-Ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27) as electron-transport material, ambipolar characteristics, i.e., simultaneous p- and n-channel formation, can be observed in a single device. An OFET structure is investigated in which electrons and holes are injected from Mg top and Au bottom contacts into the PTCDI-C13H27 and pentacene layers, respectively. Our device exhibits electron and hole mobilities of 3×10−3 and 1×10−4 cm2/V s, respectively. This device architecture serves as a model structure for ambipolar field-e...

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 ...

Highly efficient and stable organic light-emitting diodes

Abstract Doped and undoped multilayer organic light-emitting diodes containing copper phthalocyanine as the hole-injection layer, aromatic diamines as the hole-transport layers and (tris(8-hydroxy-quinoline)aluminum) as the electron-injection, electron-transport, and emitting layer were fabricated. The influence of the individual layer thicknesses and doping locations (electron-transport layer and/or hole-transport layer) on the current-voltage characteristics, luminous efficiency, and stability was investigated. Optimization of these parameters led to highly efficient (above 8 lm/W) and stable organic light-emitting diodes.

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.

Organic–inorganic multilayer structures: a novel route to highly efficient organic light-emitting diodes

Abstract A novel device structure for organic light-emitting diodes (OLEDs) is described, which consists in the most general case of an alternating sequence of thin inorganic and organic layers sandwiched between two electrodes. Compared to conventional OLEDs, these devices have a significantly enhanced current flow, increased brightness, and higher luminous efficiency at a given voltage. These improvements in performance can be attributed to increased and more balanced charge-carrier injection as well as charge-carrier confinement effects, which together lead to higher radiative recombination probability.

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 (

Influence of alkoxy substituents on the exciton binding energy of conjugated polymers

Abstract Scanning tunneling microscope (STM) spectroscopy and optical absorption measurements were used to determine the exciton binding energy (Eb) of poly(p-phenylenevinylene) (PPV). We find Eb=0.48±0.14 eV, which is significantly higher than the value of Eb reported previously for an alkoxy-PPV derivative. Furthermore, an analysis of photoluminescence (PL) and STM excited cathodoluminescence (CL) spectra, performed on PPV and on alkoxy-PPV derivatives, reveals that the energy separation between the dominant peaks in the vibronic progression is significantly smaller in the alkoxy-substituted compounds. The reduction of Eb appears to be related to the influence of the alkoxy side-chains, inducing a reduction of both the Coulomb interaction as well as the molecular structural relaxation energy of the exciton.