H. Heil

Mechanisms of injection enhancement in organic light-emitting diodes through an Al/LiF electrode

The mechanisms of enhanced electron injection into the electron transport layer of Alq3 [tris(8-hydroxyquinoline)-aluminum] via LiF interlayers are studied by means of I–V characteristics, secondary ion mass spectroscopy (SIMS), and Kelvin probe measurements. Devices for single carrier injection were prepared using aluminum electrodes, Alq3 electron transport layers, and thin intermediate layers of LiF. It was found that devices deposited in the order Alq3-LiF-aluminum have a much higher electron injection capability than structures deposited in the order aluminum-LiF-Alq3. SIMS depth profile analysis reveals that the evaporation of Al on LiF leads to a spatial separation of Li and F probably induced by a chemical reaction of Al with LiF. Simple thermodynamic calculations support the energetic feasibility of such a reaction. Titanium cathodes in the same layer sequence also exhibit electron injection enhancement, probably due to their similar chemical reactivity. However, electron injection from Ag electr...

The influence of mechanical rubbing on the field-effect mobility in polyhexylthiophene

This paper reports on improvements of the field-effect mobility in regioregular head-to-tail coupled poly(3-hexylthiophene) based transistors by mechanically induced alignment of polymer chains in the active layer. It is demonstrated that mechanical rubbing perpendicular to the source drain contacts can increase the field-effect mobility up to 800% whereas rubbing parallel to the source drain contacts results in a reduced mobility. The polymer alignment is thereby deduced from optically polarized transmission spectroscopy on polymer-coated quartz glass substrates and is shown to directly correlate with the electrical behavior of a bottom-gate field-effect transistor. The influence of layer thickness on rubbing is investigated and it is shown that annealing after mechanical rubbing at high temperature can further increase the alignment. Differences between thick drop-cast and thin spin-coated films are explained in terms of different solvent evaporation rates, allowing the material to order to a different degree. This interpretation is deduced from characteristic optical and electrical features of the differently prepared poly(3-hexylthiophene) films.

Space-charge limited current in regioregular poly-3-hexyl-thiophene

Temperature dependent current–voltage characteristics of an organic diode based on a thin film of regioregular poly-3-hexyl-thiophene (P3HT) are compared with results of a theoretical model assuming space-charge limited currents with a density of states (DOS), nonmonotonous in energy. This DOS was deduced from experiments utilizing the method of thermally stimulated currents. Both, experiments and theory result in an almost power-law dependence of j∝Vm, where the exponent m increases with decreasing temperature assuming a value of m=2 at room temperature. This effect can be accounted for by filling of deep traps at lower temperatures. Transport of charge carriers in P3HT seems to be limited by hopping in disordered regions rather than by the transport via extended states within crystalline grains.

Tris(dibenzoylmethane)(monophenanthroline)europium(III) based red emitting organic light emitting diodes

The concept of a blue light emitting diode containing a hole blocking layer (HBL) is generalized to efficiently excite dye molecules introduced in the form of an emission layer on the hole transporting side. The concept allows the excitation of an emitting molecule even if the molecule does not act primarily as a recombination center. On the basis of this concept pure red emitting organic light emitting diodes were produced utilizing tris(dibenzoylmethane)(monophenanthroline)europium(III) [Eu(dbm)3phen] as the emitting layer in an Al/LiF/Aluminum tris(8-hydroxyquinoline) (Alq3)/HBL/Eu(dbm)3phen/N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD)/copper-phthalo- cyanine (CuPc)/indium tin oxide structure. Additionally, the luminance output of such a device was improved by partially adding a hole transport material to the emitting layer in order to increase the penetration of holes into the emission region. A maximum luminance output of 200 cd/m2 at 15 V was achieved without optimizing the layer thickness. Fu...

Electronic traps and percolation paths in electroluminescent polymers

Trap related properties of the electroluminescent polymers poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV, OC1C10–PPV) and poly[(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]-[(4′-(3″,7″-dimethyloctyloxy)phenyl-5)-1,4-phenylenevinylene] are investigated in organic light emitting diode (OLED) devices. By means of the thermally stimulated currents (TSC) technique the densities of occupied states, trap polarities and charge transport mechanisms are studied. The fractional TSC technique in combination with optical trap filling reveals broad density-of-occupied-states distributions for both polymers, ranging from 200 to 600 meV located between the highest occupied molecular orbital and lowest unoccupied molecular orbital levels. Conventional TSC experiments in combination with electrical trap filling allow to distinguish between electron and hole traps. In OC1C10–PPV both trap polarities are detected whereas in the copolymer only hole traps and no electron traps were fo...

Sunlight stability of organic light-emitting diodes

This paper reports on the photodegradation of organic light-emitting diodes (OLEDs) due to exposure to visible and near-ultraviolet light. Such exposure affects strongly the device performance, e.g., the electroluminescence intensity and the device current decrease considerably, however, the photoluminescence remains unaffected. This photodegradation was investigated on various production-relevant classes of high-quality polymer semiconductors with different energy gaps, i.e., a yellowish-green polyphenylenevinylene derivative, a red polyfluorene derivative, and a blue polyspiro derivative. It will be demonstrated that the action spectrum of the photodegradation is strongly correlated with the fundamental absorption of the polymer itself. The indium-doped tin oxide (ITO) polymer interface was identified as the interface where photodegradation takes place, while the bulk of each layer (ITO or polymer) turned out to be insensitive to irradiation with visible or near-ultraviolet light. Furthermore, it is demonstrated that the photoinduced damage can be suppressed by inserting appropriate interlayer systems.

Light-emitting field-effect transistor: simple model and underlying functional mechanisms

We report on light emission on organic thin film transistors of tetracene and polyfluorene (Poly(9,9-di(ethylhexyl)fluorene) (PF2/6)). The utilized transistor structure is a bottom gate configuration with interdigitated source and drain electrodes on a Si/SiO2 substrate with a channel length of 5 μm. Light emission occurs above a source drain voltage of 30V even if the gate voltage is higher than the drain voltage. The light output can be controlled by the gate voltage. The light emission occurs close to the drain electrode as observed by light microscope images of operating transistors. In order to understand the functional principle of a light emitting transistor a resistor capacitor equivalent circuit model has been utilized to describe charge carrier transport, carrier distribution and the electrical potential distribution in such a device. The model extends the common thin film transistor theory for unipolar charge transport to ambipolar charge transport. Analytical expressions for output and transfer characteristics as well as for the potential and charge carrier distributions are obtained. Further, the effect of contact resistors on the output and transfer characteristics are simulated. The model is used to explain the underlying mechanisms of the present devices. Imperfections on the contact electrodes, most probably due to under-etching of the electrodes are seen as the main reason for the electron injection.

New concepts for light-emitting transistors

In this study we report on new concepts to generate light emission in organic thin film transistors. The initial physical understanding of light emission from tetracene based field-effect transistors was proposed to be originated from a strong underetching of the drain and source electrodes. This underetched electrodes in combination with the evaporated tetracene is thereby believed to generate a virtual OLED at the drain electrode. Accumulated holes have to leave the gate oxide interface to reach the drain electrode by crossing the bulk of the organic semiconductor. Light then occurs by injection of electrons in a large electric field in the bulk. Todays transistors do not show the underetching anymore but are still emitting light only at the drain electrode, again supporting the initial interpretation of a defect state at the edge of the drain electrode. In this context the question how electrons can overcome a potential barrier of 2.7 eV is still open. Therefore an investigation of the gold tetracene interface by UPS and XPS techniques has been started and preliminary data indicate the unexpected result that the barrier for electrons is comparable to that for holes. In a further step the generation of an ambipolar transistor by interface doping with calcium was tried and an n-type pentacene transistor could be fabricated but the strategy failed for tetracene. Finally an electrochemical interface doping was performed by the application of Lithium triflate in PEO to a thin interface layer between gate oxide and tetracene. This leads to light emission but unfortunately also to the loss of the gate voltage influence. Based on these results a possible strategy will be presented.