S. Karg

Polymeric anodes for improved polymer light-emitting diode performance

We have studied polyaniline and polyethylenedioxythiophene transparent electrodes for use as hole-injecting anodes in polymer light emitting diodes. The anodes were doped with a variety of polymer and monomer-based acids and cast from either water or organic solvents to determine the effect of the dopant and solvent on the hole-injection properties. We find that the anodes with polymeric dopants have improved device quantum efficiency and brightness relative to those with small molecule dopants, independent of conductivity, solvent, or type of conducting polymer. For the most conducting polymer anodes [σ>2(Ωcm)−1], diodes could be made without an indium tin oxide underlayer. These diodes show substantially slower degradation.

Ambipolar light-emitting organic field-effect transistor

We demonstrate a light-emitting organic field-effect transistor (OFET) with pronounced ambipolar current characteristics. The ambipolar transport layer is a coevaporated thin film of α-quinquethiophene (α-5T) as hole-transport material and N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) as electron-transport material. The light intensity is controlled by both the drain–source voltage VDS and the gate voltage VG. Moreover, the latter can be used to adjust the charge-carrier balance. The device structure serves as a model system for ambipolar light-emitting OFETs and demonstrates the general concept of adjusting electron and hole mobilities by coevaporation of two different organic semiconductors.

Increased brightness and lifetime of polymer light-emitting diodes with polyaniline anodes

Abstract The properties of light-emitting diodes based on poly(2-methoxy,5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) with indium-tin oxide (ITO) anodes are compared with those in which a layer of polyaniline (PAni) is coated between the ITO and the emissive polymer. It is found that the PAni injection layer yields higher current, brighter emission and a lower rate of degradation than ITO. The electrical behavior is discussed in terms of bulk and contact current limitation. The improvement in degradation is attributed to reduced oxidation as the PAni layer provides a barrier for the passage of oxygen out of the oxide. In addition, improvement in microscopic short formation indicates planarization of the anode interface by the PAni film.

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.

Electrical and optical characterization of poly(phenylene-vinylene) light emitting diodes

Abstract We investigated the electrical and optical properties of poly( p -phenylene-vinylene) and light emitting devices based on this polymer. At room temperature these devices can be well described by a Schottky diode configuration. Dark I–V characteristics reveal a good diode behaviour with a maximum rectification ratio of 10 6 . White light illumination yields an open-circuit voltage of more than 1 V and a power conversion efficiency of about 0.1%, suggesting the possible application as solar cells. Complex impedance gives information about acceptor dopant concentration, diffusion voltage and depletion width. At low temperatures the I–V characteristics become symmetrical and electroluminescence appears in both current directions, indicating that the Schottky junction is not necessary for the observation of electroluminescence in conjugated polymers.

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 ...

Polymeric anodes for organic light-emitting diodes

Abstract Polymer light-emitting diodes based on PPV, for example MEH-PPV, are known to be susceptible to photo-oxidative degradation. The formation of the carbonyl species in the polymer results in quenching of the luminescence. In addition the oxidation process reduces the conjugation of the polymer, leading to lower charge carrier mobilities and consequently higher operating voltages. Previous in situ FTIR studies revealed that even in a dry inert atmosphere polymer oxidation occurs, and that ITO can act as the source of oxygen. In order to explore further the nature of the oxidation mechanism and to provide guidance for its elimination, we have studied the behavior of MEH-PPV LEDs prepared with conducting polymer anodes. When a layer of polyaniline is present between the ITO and the MEH-PPV the device characteristics improve dramatically: the injection voltage drops, the luminous efficiency increases and, most significantly, the rate of decay of the luminance decreases by up to two orders of magnitude. These data not only confirm that ITO is a source of oxygen, but also imply that the oxidation mechanism is due to direct interfacial reaction. We compare several different forms of polyaniline, with different dopants, as well as a derivative of a polythiophene.

Light-emitting diodes based on poly-p-phenylene-vinylene: II. Impedance spectroscopy

Electrical impedance measurements on poly-p-phenylene-vinylene (PPV) light-emitting diodes in the frequency range between 100 Hz and 10 MHz are reported. A significant difference can be revealed between the device characteristics of light-emitting diodes eliminated on indium-tin-oxide (ITO) and those of other high-work-function metals (e.g., Au). Thermal conversion of the precursor polymer on ITO substrates results in a p-type doping of the conjugated polymer PPV. Hence, devices in the configuration ITO/PPV/Al display Schottky behavior, which can be modeled by a simple equivalent circuit of two RC elements in series, representing a bulk and a junction region. The low-frequency device capacitance displays a pronounced voltage dependence and, from a detailed analysis, the ionized acceptor concentration NA, the diffusion potential VD, and the width of the space charge region w can be obtained. Typical values for NA are 1016–1017 cm−3, and for VD within the range 1–1.5 V, resulting in a width w of the space c...

Charge carrier mobility in poly(p-phenylenevinylene) studied by the time-of-flight technique

The charge carrier transport in poly(p-phenylenevinylene) (PPV) is investigated by the time-of-flight technique. Mobilities of positive carriers in PPV are determined and the dispersive character of transport is established. The dispersion parameters are analyzed in the frame of a multiple trapping model. The drift mobility of the positive carriers is in the range of 10−5 cm2/V s at room temperature for an electric field of 105 V/cm and increases with increasing field and temperature. The mobility shows thermally activated behavior with an activation energy of about 0.75 eV at zero field. It is shown that the experimental results can be interpreted by polaron transport.