Interactions between hole-transporting carbazole groups and electron-transporting 1,3,4-oxadiazole groups were studied by photoluminescence and electroluminescence ͑EL͒ spectroscopy, in blends of poly͑N-vinylcarbazole͒ with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole ͑PVK:PBD͒ and in random copolymers with carbazole and oxadiazole groups attached as side chains. Different excited-state complexes form in the blends, which exhibit exciplexes, and in the copolymers, which manifest electroplexes, due to topological constraints on the position of carbazole and oxadiazole units in the polymer. Both types of complex red-shift the EL spectra of the matrices compared with pure PVK homopolymer, although the shift is significantly greater for the electroplex. The presence of these complexes has a profound effect on the external quantum efficiency of dye-doped organic light-emitting diodes employing the blends or copolymers as matrices, as it strongly affects the efficiency of Förster energy transfer from the matrix to the dye. Single-layer devices doped with either coumarin 47 ͑C47͒, coumarin 6 ͑C6͒, or nile red ͑NR͒ were compared. Among the three dye-doped PVK:PBD devices, C6 doping yields the highest efficiency, while NR doping produced the most efficient copolymer devices, consistent with the degree of overlap between the EL spectrum of the matrix material and the absorption spectrum of the dye.
A method of masked dye diffusion to locally pattern the emissive color of organic light-emitting devices (OLEDs) over a large area is introduced. By using a large-area diffusion source, which may be a spin-coated doped polymer film, the entire process of masked diffusion of dye into a polymer film of an OLED to create an integrated three-color device has been demonstrated at atmospheric pressure. The materials used to demonstrate this method are the polymer poly(9-vinylcarbazole) combined with electron transport molecules, and the dyes bimane, coumarin 6, and Nile red.
New statistical copolymers with bipolar carrier transport abilities were synthesized through free radical copolymerization of N-vinylcarbazole (NVK, hole-transport monomer) with either of two substituted styrenes containing oxadiazole groups, which serve as electron transport monomers: 2-phenyl-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxadiazole, PVO, and 2-(4-tert-butylphenyl)-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxadiazole, BVO. In all cases, the charge transport moieties exist in side groups, and carrier transport proceeds by hopping. Copolymerization yields homogeneous statistical copolymers of widely variable composition and thus tunable carrier transport properties; the copolymers are transparent in the visible region and form good films. Compared with systems where the oxadiazole units are incorporated by simply blending a small-molecule oxadiazole into poly(N-vinylcarbazole), the glass transition temperatures of these copolymers are high, and there is no possibility for the oxadiazole units to phase-separate through recrystallization. The glass transition temperatures for the copolymers show positive deviations from a harmonic mixing rule, suggesting some interaction between the NVK and BVO residues; however, blends of the homopolymers show limited miscibility at best, indicating that copolymerization is essential to produce a homogeneous material. Incorporating the oxadiazole units reduces the hole transport ability of these copolymers somewhat relative to NVK homopolymer, but single-layer dye-doped devices emitting blue, green, and orange light fabricated from these copolymers all showed good efficiency.
The method of solvent-enhanced dye diffusion for patterning full-color (red, green, and blue) polymer light-emitting diode displays was investigated in detail. After local dry transfer of dye onto a device polymer film, the dye remains on the surface of the polymer layer and must be diffused into the polymer for efficient emission. Exposure of the polymer to solvent vapor at room temperature increases the dye-diffusion coefficient by many orders of magnitude, allowing rapid diffusion of the dye into the film without a long, high-temperature anneal that can degrade the polymer. The increase in diffusion is due to absorption of the solvent vapor into the polymer film, which increases the polymer thickness and decreases its effective glass transition temperature T g,eff. Measurements of the polymer in solvent vapor indicate that its thickness varies roughly linearly with pressure and inversely with temperature, with thickness increases as large as 15% often observed. A model based on Flory-Huggins theory is used to describe these results. The diffusion of the dye into the polymer was evaluated by photoluminescence and secondary-ion mass spectroscopy. This dye-diffusion increase is largest for high solvent-vapor partial pressures and, most surprisingly, is larger at lower temperatures than at higher temperatures. This anomalous temperature dependence is due to the increased solvent-vapor absorption and consequent reduction in the effective glass-transition temperature at lower temperatures.
In an organic light-emitting device with a polymeric matrix concurrently doped with two different dyes, the photoluminescence (PL) and electroluminescence (EL) spectra are observed to be very different, with both dyes emitting in PL and only one in EL at room temperature. A simple model based on charge trapping and thermal excitation is introduced to explain this observation. The EL spectral change of a device at 77 K is consistent with this model. In addition, using the same model, the strong dependence of the EL efficiency (but not PL) on the concentration of a single dye in an organic film can be understood. The materials used in this experiment are the polymer poly(9-vinylcarbazole) combined with electron transport molecules, and the dyes coumarin 47, coumarin 6, and Nile red.
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