Due to their outstanding properties, e.g., good contrast, wide viewing angle, low power consumption, and self-emission organic light-emitting (OLE) displays on the basis of conjugated polymers are on the verge of commercialization. Two major disadvantages of the current processing technique for the polymers—spin coating—are the material waste and the difficulties involved in patterning multichrome or even full-color displays. Therefore, we investigated the screen-printing technique for the production of OLE displays. In this letter, we present performance data and images of screen-printed OLE diodes. They are already comparable to spin-coated ones. We observed luminance of 10 000 cd/m2 at 8 V and peak efficiencies exceeding 10 cd/A for green diodes. These data indicate that printed organic displays have the potential to replace “classical” spin-coated devices.
We present a series of low-molecular-weight materials
based on
cyclic phosphazenes for the use as host materials in blue phosphorescent
organic light-emitting diodes. Substituted phenyl rings are attached
to the central phosphazene ring either via phosphorus–oxygen
bonds to yield phenoxy-substituted derivatives or via direct phosphorus–carbon
bonds to yield phenyl-substituted derivatives. The phenoxy substituted
cyclic phosphazenes were prepared by nucleophilic substitution of
the six chlorine atoms in hexachlorocyclotriphosphazene with phenoxy
groups, whereas the phenyl substituted cyclic phosphazenes were formed
in a cyclocondensation reaction of three equivalents of substituted
phosphinic amides. The phenyl substitution leads to materials with
superior thermal properties compared to the phenoxy substitution.
Because of the nonconjugated linkage to the phosphazene core, the
host materials have very high triplet energies of more than 3 eV.
In an OLED device using one compound as host for the saturated blue
phosphorescent emitter Ir(dbfmi), a peak power efficiency of 7.6 lm
W–1 and a peak luminance of 5000 cd m–2 were achieved.
We fabricate simple, solution processed layers containing two kinds of phosphorescent, organic dyes in a poly(methyl methacrylate) (PMMA) matrix. After optical excitation, the close proximity of the emitter molecules with different triplet energies T 1 causes a transfer of excitons from the more highly excited dye to the lower one. Optical characterization by photoluminescence (PL) and time resolved PL (TRPL) measurements leads to qualitative and quantitative investigations of the underlying exciton transfer principle. It can be shown that Dexter transfer is not able to describe the experimentally obtained data while an established model for exciton migration between fluorescent molecules, based on Förster theory, presents a consistent picture. Using this model, experimentally obtained values for the characteristic interaction distance R 0 agree well with theoretical predictions by basic Förster theory.
This work investigates an eddy current-based non-destructive testing (NDT) method to characterize corrosion of pipes under thermal insulation, one of the leading failure mechanisms for insulated pipe infrastructure. Artificial defects were machined into the pipe surface to simulate the effect of corrosion wall loss. We show that by using a giant magnetoresistance (GMR) sensor array and a high current (300 A), single sinusoidal low frequency (5–200 Hz) pipe-encircling excitation scheme it is possible to quantify wall loss defects without removing the insulation or weather shield. An analysis of the magnetic field distribution and induced currents was undertaken using the finite element method (FEM) and analytical calculations. Simple algorithms to remove spurious measured field variations not associated with defects were developed and applied. The influence of an aluminium weather shield with discontinuities and dents was ascertained and found to be small for excitation frequency values below 40 Hz. The signal dependence on the defect dimensions was analysed in detail. The excitation frequency at which the maximum field amplitude change occurred increased linearly with the depth of the defect by about 3 Hz/mm defect depth. The change in magnetic field amplitude due to defects for sensors aligned in the azimuthal and radial directions were measured and found to be linearly dependent on the defect volume between 4400–30,800 mm3 with 1.2 × 10−3−1.6 × 10−3 µT/mm3. The results show that our approach is well suited for measuring wall loss defects similar to the defects from corrosion under insulation.
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