Doped Organic Light-Emitting Diodes Based on Random Copolymers Containing Both Hole and Electron Transport Groups
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phase separation. This can be overcome by covalently attaching the oxadiazole groups to
H-
N
1
the polymer chain. In this work we describe the
incorporation of both hole-transport carbazole and electron-transport oxadiazole groups into a single copolymer and its application in dye doped LEDs. Fig. 1 shows the structures of the monomers and copolymers (COPs); the oxadiazole monomer (BVO) is very similar in structure to PBD, and so should show similar electronic properties, but BVO contains aNN
-C t
0 70N
0
0
N
polymerizable styryl group at one end so as to allow easy incorporation into a polymer by free-radical copolymerization.
/:zzN
N A
EXPERIMENT BVO
Materials Synthesis and Characterization
NVK
COP
Fig. 1. Molecular structures of the monomers and copolymers
NVK was purchased from Aldrich. The detailed synthesis of BVO will be published elsewhere [5]. Polymerization was carried out in tetrahydrofuran (THF) and azobisisobutyronitrile (AIBN) was used as initiator. The copolymers were purified by repeated reprecipitation from THF into methanol. Compositions of the copolymers were determined by UV-vis spectral analysis using an HP 8452A diode array spectrometer. Molecular weights of the copolymers were determined by gel permeation chromatography (GPC), using a Waters 590 pump, 60 cm PLgel 5Rj MIXED-C column, a Waters 490E programmable multiwavelength UV detector, and THF as the solvent. Reported molecular weights are relative to narrow-distribution polystyrene standards. Glass transition temperatures (Tg) of the copolymers were measured using a Perkin-Elmer DSC7 differential scanning calorimeter at a scan rate of 10 0C/min. Device Fabrication and Testini ITO substrates were precleaned ultrasonically and were treated with oxygen plasma prior to spin coating of the polymer layer [4]. The dye-doped polymer layer (either PVK, PVK:PBD, or copolymer) was spin coated onto the ITO substrate from a 28 mg/ml solution (total solids) in 1,2-dichloroethane at 3500 to 4000 rpm in a nitrogen glove box. This produced a polymer film thickness of 700-1000 A as measured by stylus profilometry. Then a layer of Mg:Ag (10:1) with thickness of 1000 A was evaporated at a pressure of 2x 10- 5 Torr. The active area of the device was about 8 mm2.2 A layer of Ag with thickness of 1000 A was then deposited as a cap. For holeonly devices, a layer of gold was evaporated instead of the Mg:Ag and Ag layers. The current voltage and photocurrent - voltage characteristics were measured on a HP 4145B Semiconductor Parameters Analyzer and calibrated Si photodiode system under nitrogen. RESULTS To have good control over the copolymer composition, the radical reactivity ratios of the two monomers were determined by running several small-scale polymerizations with varying ratios of NVK:BVO in the monomer feed. Table I gives the compositions of the monomer mixture and
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the corresponding copolyTable I. Monomer feed and copolymer compositions. mers formed in these smallMonomer Feed (fNvK) 0.9489 0.8952 0.7975 scale polymerizations. The
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