Blue light emitting diode internal and injection efficiency AIP Advances 2, 032117 (2012) Enhancement of hole injection and electroluminescence characteristics by a rubbing-induced lying orientation of alpha-sexithiophene J. Appl. Phys. 112, 024503 (2012) Study of field driven electroluminescence in colloidal quantum dot solids J. Appl. Phys. 111, 113701 (2012) Indium incorporation and emission properties of nonpolar and semipolar InGaN quantum wells

Light-emitting diodes have been fabricated from self-assembled multilayers of poly(p-phenylene vinylene) (PPV) and two different polyanions; polystyrene sulfonic acid (SPS) and polymethacrylic acid (PMA). The type of polyanion used to assemble the multilayer thin films was found to dramatically influence the behavior and performance of devices fabricated with indium tin oxide and aluminum electrodes. Light-emitting devices fabricated from PMA/PPV multilayers were found to exhibit luminance levels in the range of 20–60 cd/m2, a thickness dependent turn-on voltage and classical rectifying behavior with rectification ratios greater than 105. In sharp contrast, the devices based on SPS/PPV exhibited near symmetric current–voltage curves, thickness independent turn-on voltages and much lower luminance levels. The significant difference in device behavior observed between these two systems is primarily due to a doping effect induced either chemically or electrochemically by the sulfonic acid groups of SPS. It was also found that the performance of these devices depends on the type of layer that is in contact with the Al top electrode thereby making it possible to manipulate device efficiency at the molecular level.

Thin film light emitting diodes based on multibilayer combinations of the conjugated polymer poly(phenylene vinylene) (PPV) and different polyanions were fabricated via the use of a simple layer-by-layer molecular-level processing scheme. Molecular-level manipulation of the type and sequence of PPV bilayers used to construct the films was found to produce devices with dramatically improved luminance levels and efficiencies. It was also found that device efficiency could be improved by controlling the nature of the thin film/electrode interface.

PPV based light emitting thin film devices were fabricated using a layer-by-layer deposition technique involving the alternate spontaneous adsorption of a PPV precursor polymer and either poly(styrene-4-sulfonate) (SPS) or poly(methacrylic acid) (PMA). It was demonstrated that the polyanion used to self-assemble the PPV precursor strongly influences the characteristics and performance of the resulting LEDs. Devices fabricated with PPV created in the presence of SPS exhibited symmetric I–V curves, low luminance levels and very high current densities while PPV/PMA devices exhibited luminance levels in the range of 10–60 cd/m2 and classical rectifying behavior. These dramatic differences are primarily due to a low level of p-type doping activated during the thermal conversion of PPV and/or during device operation that confers excellent hole carrier transport capabilities to the PPV/SPS combination. Fabrication of a multi-slab type heterostructure device comprised of a PPV/SPS block (hole transporting block) and a PPV/PMA block (emitting block) resulted in improved performance with luminance levels significantly higher than previously obtained for a single slab PPV/PMA device (typically > 100 cd/m2). It was also demonstrated that the presence of very thin (about 20–30 Å thick) insulating layers at the Al/polymer interface improves device efficiency by a factor of 2–4.

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