Hans-Hermann Johannes

Large Area Electronics Using Printing Methods

After the demonstration of the first organic FET in 1986, a new era in the field of electronic began: the era of organic electronics. Although the reported performance of organic transistors is still considerably lower compared to that of silicon transistors, a new market is open for organic devices, where the excellent performance of silicon technology is not required. Several commercial applications for organic electronics have been suggested: organic RFID tags, electronic papers, imagers, sensors, organic LED drivers, etc. The main advantage of organic technologies over silicon technologies is the possibility of making low-cost, large area electronics. The main processes which allow patterning with suitable resolution on a large areas are printing methods. Here we will provide an overview of methods that can be useful in the low-cost production of large area electronics.

Efficient and Long-Time Stable Red Iridium(III) Complexes for Organic Light-Emitting Diodes Based on Quinoxaline Ligands

We report the design and characterization of three heteroleptic orange-red phosphorescent iridium(III) complexes bearing two 2-(4-fluorophenyl)-3-methyl-quinoxaline (fpmqx) cyclometalated ligands combined with three different ancillary ligands, triazolylpyridine (trz), picolinate (pic), and acetylacetonate (acac). All of these complexes emit an orange to red color in the spectral range of 605-628 nm in dichloromethane. Strong spin-orbit coupling of the iridium atom allows the formally forbidden mixing of singlet and triplet states. Because of the structureless phosphorescent line shapes and low Stokes shifts between triplet metal-to-ligand charge-transfer ((3)MLCT) absorption and phosphorescent emission, we propose that emission originates predominantly from the (3)MLCT state with a lesser admixture of totally ligand-based (3)(pi-pi*) states. The influence of 5d-electron densities of the iridium center on highest occupied molecular orbitals leads to high emission quantum yields in toluene (Phi(p) = 0.39-0.42) and to short triplet lifetimes. Cyclovoltammetry measurements show reversible oxidation peaks from 0.74 to 0.92 V and reversible reduction waves with potentials ranging from -1.58 to -2.05 V versus Cp(2)Fe/Cp(2)Fe(+). All complexes have been applied in simple test devices and also in stable, long-living devices to evaluate their electroluminescent device performances, for which we especially report the influence of the chosen ancillary ligands on emission colors, efficiencies, and device lifetimes. We obtained narrowband emission ranging from 613 to 630 nm with a full width at half-maximum of 64-71 nm, and a maximum in power efficiency of eta(p) = 14.6 lm/W at a current density of J = 0.01 mA/cm(2) for [(fpmqx)(2)Ir(pic)]. The operating lifetimes of [(fpmqx)(2)Ir(trz)] in both neat and mixed matrixes were longer than that of the established stable tris(1-phenylisoquinolinato)iridium(III) [Ir(piq)(3)]. From the lifetime measurements, it becomes clear that the stability is strongly correlated to the type of ancillary ligand. An extrapolated lifetime of 58 000 h with an initial brightness of 1000 cd/m(2), together with a very low voltage increase of 0.2 V over a time period of 1000 h (starting voltage of 4.1 V), was achieved. Such a high device lifetime is attributed to the chemical stability of all materials toward both charge carriers and excitons.

A strategy towards p-type doping of organic materials with HOMO levels beyond 6 eV using tungsten oxide

The authors present a concept to p-dope organic hole transport materials with highest occupied molecular orbital (HOMO) levels on the order of 6 eV (e.g.4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) or 4,4′-bis(carbazol-9-yl)biphenyl (CBP)) by using WO3 as acceptor. Owing to its large work function, WO3 allows for efficient electron transfer from organic semiconductors with HOMO levels in this region. The effect of p-type doping is evidenced by optical absorption spectroscopy, electrical transport characteristics and Kelvin probe analysis. Moreover, OLED devices have been realized which exhibit 30% enhanced power efficiency due to the p-type doping. The results are expected to have a significant impact on the development of organic (opto-)electronic devices.

Low-voltage organic electroluminescence device with an ultrathin, hybrid structure

We have prepared organic light-emitting diodes with a narrow recombination zone confined by an organic double-heterojunction structure using both polymer and small molecules (a hybrid structure). In these light-emitting diodes, we used very thin small molecule layers, down to a total thickness of 40 nm, to achieve an exponential forward characteristic. These layers were evaporated on a highly conductive layer of PEDT:PSS for a high-yield process and for good charge injection at the anode. Although no doping processes were applied during device fabrication, either at the injecting electrodes or in the Alq3 layer, the diodes attained high brightness at very low voltage, for instance, 10.000 cd/m2 at voltage of 4.7 V.

Organic molecular beam deposition: technology and applications in electronics and photonics Invited Lecture

Organic semiconductors have been intensively studied over the past decades. The potential of this new class of materials for photonic and electronic device applications is demonstrated by successful fabrication of organic and organic-on-inorganic heterostructures for electroluminescent devices, photodetectors, and microwave diodes. The fabrication technology of organic semiconductor devices for both electronic and photonic applications is discussed. In contrast to spin-on or dipping techniques for fabrication of polymeric films, organic compounds with low molecular weight are sublimated under ultra high vacuum (UHV) conditions. The organic molecular beam deposition (OMBD) technology employed allows the reproducible growth of complex layer sequences with a defined thickness of various organic semiconductors in combination with dielectric films, different metallizations, and indium–tin oxide layers. Growth rates from 1–5 nm min-1 and substrate temperatures from 77 to 350 K are used. Organic-on-inorganic heterostructure diodes based on crystalline thin PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) films on III–V semiconductors are investigated with regard to microwave applications with reduced forward voltage and high cut-off frequencies in the GHz regime. Secondly, efficient organic light emitting diodes with bright emission in the blue [1-AZM-Hex (N,N′-disalicylidene-1,6-hexanediaminate)zinc(II)], green, [Alq3 (tris(8-hydroxyquinoline)-aluminum)], and red (Eu complexes) spectral region and with low operation voltages are presented. In general an onset voltage of 2.7 V, efficiencies up to 7 lm W-1 and a luminance up to 2×105 cd m-2 (CW, RT) are attained for N,N′-diphenyl-quinacridone doped Alq3 devices. An undoped device can be operated up to 5000 h without any loss in brightness and just a small increase of the driving voltage of about 2 V. Embedding emissive organic thin films with a narrow spectral characteristic into planar Fabry–Perot microcavities, a light intensity enhancement and a spatial redistribution of the emission is achieved.

Pyrrolo[1,2-a]quinoxalines: Novel Synthesis via Annulation of 2-Alkylquinoxalines

In an attempt to synthesize a novel homoleptic complex 3 from 2-methyl-3-phenylquinoxaline 1 and Ir(acac)(3) for application as a triplet emitter in OLEDs (organic light-emitting diodes) no cyclometalation was observed. Instead, an annulation to 1-methyl-4-phenylpyrrolo[1,2-a]quinoxaline 2 was observed. Since pyrroloquinoxalines are potentially bioactive and few paths for their synthesis are known, selected reactions and conditions were investigated, suggesting Ir(acac)(3) as catalyst and proving glycerol to be a reactant.

Conformal and highly luminescent monolayers of Alq3 prepared by gas-phase molecular layer deposition.

The gas-phase molecular layer deposition (MLD) of conformal and highly luminescent monolayers of tris(8-hydroxyquinolinato)aluminum (Alq3) is reported. The controlled formation of Alq3 monolayers is achieved for the first time by functionalization of the substrate with amino groups, which serve as initial docking sites for trimethyl aluminum (TMA) molecules binding datively to the amine. Thereby, upon exposure to 8-hydroxyquinoline (8-HQ), the self-limiting formation of highly luminescent Alq3 monolayers is afforded. The growth process and monolayer formation were studied and verified by in situ quartz crystal monitoring, optical emission and absorption spectroscopy, and X-ray photoelectron spectroscopy. The nature of the MLD process provides an avenue to coat arbitrarily shaped 3D surfaces and porous structures with high surface areas, as demonstrated in this work for silica aerogels. The concept presented here paves the way to highly sensitive luminescent sensors and dye-sensitized metal oxides for future applications (e.g., in photocatalysis and solar cells).

Conduction mechanisms in thin atomic layer deposited Al2O3 layers

Thin Al2O3 layers of 2–135 nm thickness deposited by thermal atomic layer deposition at 80 °C were characterized regarding the current limiting mechanisms by increasing voltage ramp stress. By analyzing the j(U)-characteristics regarding ohmic injection, space charge limited current (SCLC), Schottky-emission, Fowler-Nordheim-tunneling, and Poole-Frenkel-emission, the limiting mechanisms were identified. This was performed by rearranging and plotting the data in a linear scale, such as Schottky-plot, Poole-Frenkel-plot, and Fowler-Nordheim-plot. Linear regression then was applied to the data to extract the values of relative permittivity from Schottky-plot slope and Poole-Frenkel-plot slope. From Fowler-Nordheim-plot slope, the Fowler-Nordheim-energy-barrier was extracted. Example measurements in addition to a statistical overview of the results of all investigated samples are provided. Linear regression was applied to the region of the data that matches the realistic values most. It is concluded that ohmi...

Regimes of leakage current in ALD-processed Al2O3 thin-film layers

A recently known phenomenon of thin oxide layers with thicknesses below approximately 40 nm is the increase in their breakdown electric field, called disruptive strength, towards lower thicknesses. This offers the possibility of examining the current–electric field characteristics at higher electric field strengths without an early electric breakdown. In this paper, we report on the identification of a current regime of trap-free square law and the buildup of an S-shaped current–electric field characteristic curve. This observation for atomic layer deposition (ALD)-processed Al2O3 layers has not been mentioned in the literature so far. Additionally, a modern model of space charge limited current is used to fit the S-shaped characteristic and extract the associated parameters, such as mobility, density of states, and the energy band gap between the conduction band and the trap state. In this context, the Poole–Frenkel effect is neglected in the model to fit our measurements towards the current increase after the trap filled limit.

Ethynylated, vinylated, and hetarylated indodicarbocyanines by palladium-catalyzed cross-coupling reactions

Abstract Palladium-catalyzed cross-coupling reactions of chain halogenated indodicarbocyanines 2b,c with arylethynes, styrenes and hetarenes afford the ethynyl, vinyl, and hetaryl derivatives 4 . The structure of 4b is confirmed by X-ray structure determination.