Ansgar Werner

Low-voltage organic electroluminescent devices using pin structures

We have realized a small-molecule organic light-emitting diode where the intrinsic emitter layer is sandwiched by n- and p-doped transport layers with appropriate blocking layers. The diodes based on this pin concept have exponential forward characteristics up to comparatively high current densities. The diodes reach high brightness at very low operating voltage: for instance, 1000 cd/m2 at a voltage of 2.9 V. Despite the highly doped transport layers, the devices reach very high efficiency for the given emitter system up to high brightness.

Very-low-operating-voltage organic light-emitting diodes using a p-doped amorphous hole injection layer

We demonstrate the use of a p-doped amorphous starburst amine, 4, 4′, 4″-tris(N, N-diphenyl- amino)triphenylamine (TDATA), doped with a very strong acceptor, tetrafluoro- tetracyano-quinodimethane by controlled coevaporation as an excellent hole injection material for organic light-emitting diodes (OLEDs). Multilayered OLEDs consisting of double hole transport layers of p-doped TDATA and triphenyl-diamine, and an emitting layer of pure 8-tris-hydroxyquinoline aluminum exhibit a very low operating voltage (3.4 V) for obtaining 100 cd/m2 even for a comparatively large (110 nm) total hole transport layer thickness.

Low-voltage inverted transparent vacuum deposited organic light-emitting diodes using electrical doping

We demonstrate low-voltage inverted transparent vacuum deposited organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The devices comprise an intrinsic 8-tris-hydroxyquinoline aluminum (Alq3) emitting layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers. They exhibit low driving voltages (∼4 V for a luminance of ∼100 cd/m2). The devices are about 50% transparent in the Alq3 emission region and emit green light from both sides with a total external current efficiency of about 2.5 cd/A.

High-efficiency electrophosphorescent organic light-emitting diodes with double light-emitting layers

We demonstrate high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs) with double light-emitting layers (D–EMLs) by doping both hole and electron transport hosts with fac tris(2-phenylpyridine)iridium [Ir(ppy)3] simultaneously. The D–EMLs PHOLEDs show significantly improved efficiency (peak external quantum efficiency of about 12.6%, corresponding to a current efficiency of 44.3 cd/A) compared to the conventional PHOLEDs with a single EML and either hole or electron transport host doped with Ir(ppy)3. We attribute this improvement mainly to reduced losses of triplet excitons into regions that are not doped by phosphorescent emitter molecules.

Pyronin B as a donor for n-type doping of organic thin films

We present an approach to stable n-type doping of organic matrices using organic dopands. To circumvent stability limitations inherent in strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, pyronin B chloride is studied as a dopant in a 1,4,5,8-naphthalene tetracarboxylic dianhydride matrix. Conductivities up to 2×10−4 S/cm are obtained, which is two orders of magnitude higher than obtained previously using bis(ethylenedithio)-tetrathiafulvalene as a dopant [A. Nollau, M. Pfeiffer, T. Fritz, and K. Leo, J. Appl. Phys. 87, 4340 (2000)]. Field-effect measurements are used to prove n-type conduction. Other matrices which can be doped are N,N′-dimethyl-perylene-3,4,9,10-tetracarboxylic diimide and fullerene C60, frequently used in organic solar cells. Visible light and Fourier-transform infrared spectroscopy confirm the donor properties of pyronin B.

Exceptionally efficient organic light emitting devices using high refractive index substrates.

Organic light emitting devices (OLEDs) are now used in commercial cell phones and flat screen displays, but may become even more successful in lighting applications, in which large area, high efficiency, long lifetime and low cost are essential. Due to the relatively high refractive index of the organic layers, conventional planar bottom emitting OLEDs have a low outcoupling efficiency. Various approaches for enhancing the optical outcoupling efficiency of bottom emitting OLEDs have been introduced in the literature. In this paper we demonstrate a green bottom emitting OLED with a record external quantum efficiency (42%) and luminous efficacy (183 lm/W). This OLED is based on a high index substrate and a thick electron transport layer (ETL) which uses electrical doping. The efficient light outcoupling is modeled by optical simulations.

High‐efficiency p‐i‐n organic light‐emitting diodes with long lifetime

Abstract— High-performance organic light-emitting diodes (OLEDs) are promoting future applications of solid-state lighting and flat-panel displays. We demonstrate here that the performance demands for OLEDs are met by the PIN (p-doped hole-transport layer/intrinsically conductive emission layer/n-doped electron-transport layer) approach. This approach enables high current efficiency, low driving voltage, as well as long OLED lifetimes. Data on very-high-efficiency diodes (power efficiencies exceeding 70 lm/W) incorporating a double-emission layer, comprised of two bipolar layers doped with tris(phenylpyridine)iridium [Ir(ppy)3], into the PIN architecture are shown. Lifetimes of more than 220,000 hours at a brightness of 150 cd/m2 are reported for a red PIN diode. The PIN approach further allows the integration of highly efficient top-emitting diodes on a wide range of substrates. This is an important factor, especially for display applications where the compatibility of PIN OLEDs with various kinds of substrates is a key advantage. The PIN concept is very compatible with different backplanes, including passive-matrix substrates as well as active-matrix substrates on low-temperature polysilicon (LTPS) or, in particular, amorphous silicon (a-Si).

Acridine orange base as a dopant for n doping of C60 thin films

We present a study on n doping of C60 thin films by acridine orange base [3,6-bis(dimethylamino)acridine(AOB)] combining conductivity, field effect, and Seebeck measurements. An increase of more than six orders of magnitude in conductivity is observed for a doping ratio of 6mol%, accompanied by a decrease in the activation energy from 0.64to0.15eV compared to the undoped C60. We observe a clear doping effect immediately after sample preparation, but also a further activation by annealing or illumination. The field effect and Seebeck measurements confirm n-type conduction of C60 thin films and show that deep donor states are formed in AOB-doped C60 thin films. A field effect mobility of 0.2cm2∕Vs is achieved for a doping level of 1.8mol%. Near Infrared (NIR) and Fourier transform infrared (FTIR) spectra demonstrate electron transfer from the dopant to the matrix: For C60 doped with AOB, C60− is present in NIR absorption and FTIR spectra. On the other hand, a peak corresponding to acridine orange [3,6-bis(d...

Field dependence of thermally stimulated currents in Alq3

The trap properties of the commonly used organic light-emitting diode emitter material tris-8-(hydroxyquinoline) aluminum (Alq3) have been investigated using thermally stimulated currents. Based on a model of the field dependence of the thermally stimulated currents, a trap density of 1.3×1017 cm−3 for depths ranging from 0.05 to 0.7 eV is obtained, indicating considerable influence on charge carrier statistics. A field-induced lowering of trap depth was observed and explained in the framework of the Poole–Frenkel effect.

Realization of organic pn-homojunction using a novel n-type doping technique

We present a novel n-type doping technique for organic semiconductors using the metal complex bis(terpyridine)ruthenium as a strong donor. Owing to its low oxidation potential, the reduced neutral form of the donor complex allows an electron transfer to the matrix. This enables n-type conduction that has been seldom reported in metallophthalocyanine systems doped with organic compounds. The n-type zinc-phthalocyanine layers are characterized by the conductivity and the field-effect measurements. By sequential coevaporation of p- and n-doped layers, we have prepared the first stable and reproducible organic homojunction of zinc-phthalocyanine. The diode exhibits surprisingly high built-in voltage attractive e.g. for organic solar cell applications. The temperature dependence of the current-voltage characteristics does not follow the standard Shockley theory of pn-junctions. We explain the behavior of the ideality factor and the saturation current by deviations from the classical Einstein relation at low temperatures.