Siegfried Dirr

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 Light Emitting Diodes with Reduced Spectral and Spacial Halfwidths

Organic light emitting diodes (OLEDs) with reduced spectral and spacial halfwidths for full-color imaging applications are investigated. The electroluminescence behaviour of the lanthanide complexes Eu(TTFA)3Phen and Tb(ACAC)3Phen having sharp emission lines in the red and green spectrum, respectively, is examined for different OLED structures. Device optimization criteria with respect to multilayer design and material choice for this type of device are given. A Fabry-Perot microcavity OLED is investigated to narrow the spectral width of commonly used organic materials (e.g. Alq3) from 100 nm to only 13 nm, demonstrating blue and green emission. In addition, the forward emission at the resonance wavelength is 8 times higher in intensity accompanied by a reduced spacial halfwidth. The directivity is further improved using materials with inherent narrow spectral widths. A spacial full-width at half maximum (FWHM) of 12° is demonstrated if the europium complex Eu(TTFA)3Phen is embedded into a microcavity structure. Compared to a non-cavity device as a reference, the emission intensity in this direction is significantly enhanced by a factor of about 8.

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.

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.

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.

Improved lifetime and efficiency of organic light-emitting diodes for applications in displays

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 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 conditions. The organic molecular beam deposition 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.

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.

Metal Semiconductor Metal Photodetectors and Modulators for Long Wavelength Applications: Optimization of Solid Source Molecular Beam Epitaxy of AlInAs/(Al)GaInAs-Heterostructures

Solid source molecular beam epitaxial (MBE) growth of AlInAs/(Al)GaInAs heterostructures for applications in metal-semiconductor-metal (MSM) photodetectors and electrooptic MSM modulators has been optimized with regard to high frequency performance. The influence of the thickness of an AlInAs barrier enhancement layer on the dynamical characteristics is also investigated. In contrast to the commonly chosen thickness of about 10 nm we show that an increase to 100 nm AlInAs results in bandwidth enhancement and noise reduction. Using these improved structures with breakdown voltages exceeding V B = 100 V for electrooptic switching we attain an on-off-ratio of 19:1