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A detailed study of charge transport in electrophosphorescent organic semiconductors is reported. The study shows that by increasing the generation of the dendrimer, the mobility is lowered and the level of disorder in the film is not increased, as reflected in identical levels of tail broadening for the first- and second-generation material.

Neutron reflection (NR) has been utilized to study the physical structure of the light-emitting polymer poly[2-(2'-ethylhexyloxy)-5-methoxy-l,4-phenylenevinylene) (MEHPPV). Films of soluble MEHPPV (S-MEHPPV) and insoluble MEHPPV (I-MEHPPV) prepared via a chloro precursor polymer were investigated. For S-MEHPPV spin-coated films were found to contain two layers, one of low-density and thickness of ≈ 1360 Å with the second layer being thinner, ≈240 Å, but denser. The thicker layer only covered about 30% of the substrate surface. In contrast, I-MEHPPV formed uniform films across the substrate. We found that during the thermal conversion of the chloro precursor polymer the film thickness was maintained and it was only on cooling that the films contracted. Importantly, the thermal expansion and contraction of I-MEHPPV was reversible. NR showed that thermal annealing of I-MEHPPV had little affect on the films physical structure while for S-MEHPPV only the thinner layer was changed and became more dense. Photoluminescence spectra of S-MEHPPV before and after annealing showed that the densification observed in the neutron reflection measurements was mirrored in the emission properties of the polymer film. [ABSTRACT FROM AUTHOR]

We demonstrate efficient tunable blue electroluminescence from blends of two solution-processible light-emitting dendrimers. These materials can be blended to form optical quality thin films with no phase-separation effects, irrespective of the blend ratio. External quantum efficiencies of 1% have been measured for the blend systems and the emission color can be tuned from deep blue (emission peak 401 nm) to blue green (477 nm) by blend composition. A power efficiency of 1.5 lm/W (at 200 Cd/m2 and 5.4 V) is measured for a single layer, first-generation blue-green fluorene-thiophene dendrimer. These results show that by choice of a dendrimer structure with common branching units and surface groups, dissimilar cores can be blended with excellent miscibility. This provides a simple way of tuning the color of organic light-emitting diodes. [ABSTRACT FROM AUTHOR]

High-efficiency single-layer-solution-processed green light-emitting diodes based on a phosphorescent dendrimer are demonstrated. A peak external quantum efficiency of 10.4% (35 cd/A) was measured for a first generation fac-tris(2-phenylpyridine) iridium cored dendrimer when blended with 4,4′-bis(N-carbazolyl)biphenyl and electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene at 8.1 V. A maximum power efficiency of 12.8 lm/W was measured also at 8.1 V and 550 cd/m[SUP2]. These results indicate that, by simple blending of bipolar and electron-transporting molecules, highly efficient light-emitting diodes can be made employing a very simple device structure. [ABSTRACT FROM AUTHOR]

Measurements of the mobility of a first-generation (G1) bis-fluorene cored dendrimer have been performed on spin-coated samples of 500 nm thickness using the charge-generation-layer time-of-flight (TOF) technique. A 10 nm perylene charge generation layer was excited by the 532 nm line of a Q-switched Nd:YAG laser and the generated carriers swept through the dendrimer film under an applied field. We observe nondispersive hole transport in the dendrimer layer with a room-temperature mobility μ = 2.0 × 10[sup -4] cm²/V s at a field of 0.55 MV/cm. There is a weak field dependence of the mobility and it increases from μ= 1.6 × 10[sup -4] cm²/V s at 0.2 MV/cm to μ = 3.0 × 10[sup -4] cm²/Vs at 1.4 MV/cm. These results suggest that the measurement of mobility by TOF in spin-coated samples on thickness scales relevant to organic light-emitting diodes can yield valuable information, and that dendrimers are promising materials for device applications. [ABSTRACT FROM AUTHOR]