Dirk Ammermann

Luminescence enhancement in microcavity organic multilayer structures

Abstract We have studied the photoluminescence properties of aluminum-tris(8-hydroxychinoline) (Alq 3 ) and tris (4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono)-1,10-phenanthroline europium(III) (Eu(TTFA) 3 Phen) sandwiched between a planar Fabry-Perot microcavity structure. A strong influence of the position of the emission thin film on the luminescence spectra has been observed. The emission intensity normal to the substrate surface is drastically enhanced by an order of a magnitude if the active layer is placed at the antinode of the standing wave in the cavity. In comparison to non-resonant Alq 3 structures, the full width at half-maximum (FWHM) of the spectrum is reduced from 100 to about 20 nm. We also show that the Eu(TTFA) 3 Phen luminescence is spatially directed due to the inherent linewidth of only 5 nm.

Organic heterostructures for electronic and photonic devices

Abstract The successful fabrication of organic semiconductor devices for both electronic and photonic applications is discussed. Complex layer sequences of various organic semiconductor thin films, different metallizations, and indium tin oxide layers can be grown by means of the organic molecular beam deposition (OMBD) technique. 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. The optimization of the device structure for reduced forward voltages and high cutoff frequencies in the GHz regime is discussed, and a single balanced mixer with improved frequency conversion at low power levels is shown. Secondly, organic light emitting diodes (OLED) with bright emission in the blue, green, and red spectral region and with low operation voltages are presented. Embedding emissive organic thin films into planar Fabry-Perot microcavities light intensity enhancement, spectral narrowing, and spatial redistribution of the emission is achieved. Finally a 5×7 pixel organic matrix display is introduced.

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

Abstract The organic molecular beam deposition technology allows the reproducible growth of complex layer sequences of various organic semiconductors in combination with dielectric films, different metallizations and indium-tin-oxide layers. The successful fabrication of devices for both electronic and photonic applications is discussed. 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 and secondly, organic light emitting diodes with bright emission in the blue, green and red spectral region and with low operation voltages are presented.

Influence of the process vacuum on the device performance of organic light-emitting diodes

Abstract We demonstrate that by growing organic light-emitting diodes (OLEDs) under ultra high vacuum (UHV) conditions at a base pressure of 10 −9 mbar with the organic molecular beam deposition (OMBD) technique, the device performance can be significantly improved. Our devices consist of a CuPc (copper phthalocyanine) hole injection layer, a 4,4′-bis(3-methylphenylphenylamino)-biphenyl (TAD) hole transport layer and an aluminum-tris-(8-hydroxychinoline) (Alq 3 ) emitter layer. The operating voltage is reduced from 7.6 to 5.4 V compared to a device realized under high vacuum (HV) conditions at 10 −6 mbar. Beside this we also found a decrease of quantum efficiency and an increased formation of black spots in our OLEDs at higher base pressures of the fabrication process.

Photonic Devices Based on Crystalline Organic Semiconductors for Optoelectronic Integrated Circuits.

Crystalline organic thin films are deposited under ultrahigh vacuum conditions at substrate temperatures from 77 K to 300 K. Due to the weak van der Waals bonding forces between the organic molecules, lattice matching is not required for layer growth. Structures can be prepared both on dielectric or amorphous substrates and on top of completely processed III-V or Si wafers. In this paper, we discuss the fabrication and performance of organic semiconductor electroluminescent devices, waveguides, and photodetectors.

Characterization and Optimization of High Brightness Organic Light Emitting Diodes (OLEDs)

Introduction Light emitting diodes based on organic thin films (OLEDs) are of great interest because of their future application in active large-area flat color displays [1,2]. A multilayer structure consisting of hole transport layer, emission layer, and electron transport layer allows to achieve bright electroluminescent emission in the visible spectral region at low driving voltages [3,4]. We report on growth, characterization, and optimization of high brightness OLEDs for the green spectral region. Organic Materials and Device Preparation The molecular structures of the organic semiconductor materials used for the electroluminescent devices described here are shown in Fig 1. The metal-organic complex CuPc (copper phthalocyanine) exhibits preferentially hole-transporting properties and the 1,3,4-oxadiazole-derivative PBD (2-(4-biphenylyl)-5-(4tert-butylphenyl)-l,3,4-oxadiazole) serves as electron transport layer. The metal chelate complex Alq (tris(8-hydroxychinoline) aluminum) is known for its high fluorescence yield in the green spectral region.

More efficient algorithms for symbolic network analysis: supernodes and reduced loop analysis

In this paper, two efficient approaches will be discussed that support linear network analysis: supernode analysis (SNA) and reduced loop analysis (RLA). By means of some selected example networks, these methods will be demonstrated and, thus, it will be shown that calculations can be dramatically simplified. In this way, all network situations can be handled. There are obvious advantages to SNA as it combines the MNA and the straightforward manual processing of the network. A very efficient solution strategy is obtained without source shifting and other common, less directed methods being used. SNA/RLA and symbolic algebra fit extremely well together. Thus an algorithm that supports the symbolic calculation of networks by means of supernodes which has been conceptualized and implemented in the analog design expert system EASY will be presented in detail. Above the educational aspect, it should be noted that the computer can now take a systematic approach to MNA and network analysis in general.

Optimization of Current-Voltage Characteristics of Organic-on-Inorganic Heterostructure Diodes

Low barrier quasi-Schottky diodes for future microwave mixer applications are presented. The diodes are based on a heterojunction between an inorganic III-V semiconductor (GaAs, InP) and the aromatic compound PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) as a crystalline organic semiconductor. Measurements of the static I-V-characteristic with emphasis on the quadratic I-V behavior in forward direction and the use in microwave systems are discussed.

Device structures and materials for organic light-emitting diodes

Organic light emitting devices (OLEDs) are promising candidates for light-weight color flat panel displays. Different device structures with emission in the blue, green, and red spectral region are discussed with respect to their optical and electrical characteristics. Blue OLEDs based on OXD-8 as emitter molecule show quantum efficiencies of 0.9% (2.2 cd/A, 0.6 lm/W), green emitting devices based on Alq3 achieve values of 1.4% (4.9 cd/A, 1.3 lm/W). Electroluminescence with colors tunable from yellow-green to red is obtained with DCM doped Alq3 layers. To investigate the device physics, a thin DCM:Alq3 sensor film is inserted into an Alq3 emitter layer. Position and current dependent spectral characteristics allow to explain the device behavior. Carrier injection, transport, recombination, exciton diffusion and decay are identified as the crucial processes responsible for the operation of OLEDs.