Stefan Wiese

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.

Fiber optic sensors for an in-situ monitoring of moisture and pH value in reinforced concrete

Concrete structures such as social buildings and bridges are important economic goods. Thus, maintenance and preservation of these structures are of major interest. Buildings of reinforced concrete are exposed to a variety of damaging influences. In particular, moisture has an important influence on the lifetime of concrete structures. This is caused by the involvement of free water in corrosion of the steel, and the fact that water acts as transport medium for damaging ions such as chloride, sulfate, carbonate and ammonium. Thus, we designed and developed an integrated fiberoptical sensor system, which allows in-situ non- destructive long-term monitoring of concrete structures. As moisture indicator we use a pyridinium-N-phenolat betainital dye, which shows a strong solvatochromic behavior in the ultraviolet-visible spectral range (UV-VIS). The dye is embedded in a polymer matrix, whose moderate polarity is enhanced by free water diffusing into the sensor. This leads to a continuous hypsochromic shift of the absorption spectrum according to the water concetration. Another appropriate dye is 4-amino-N-methylphthalimid, which shows a similar behavior in its fluorescent spectra, and presently we are developing its derivatives and suitable polymer matrices. The determination of the pH-value of concrete is of major importance for the assessment of acidic attacks which may lead to serious damage in reinforced concrete, as the embedded steel structures exhibit long-term stability (i.e. resistance to corrosion) only at pH-values of 9 or higher. Therefore we have developed a fiberoptical sensor system for the measurement of pH-values in concrete consisting of pH- indicator dyes immobilized in a highly immobilized in a highly hydrophilic polymer matrix. Any change in pH-value of the wet concrete material is indicated by a color change of the dye/polymer system. The sensor system displays long term stability even in aggressive media of pH12 - 13.