Vinyl polymer thin films having carbazole units were prepared by a new method combining physical vapor deposition and self-assembled monolayer (SAM) techniques. 3-(N-carbazolyl)propyl acrylate monomer was evaporated onto a gold substrate that had a VAZO 56 (DuPont) initiator attached as a SAM. The VAZO initiator was activated by irradiating ultraviolet light after depositing the monomer. Although the polymerization reaction can proceed even without the surface initiator, the SAM was effective in improving the surface smoothness, thermal stability, and film-substrate adhesion as a consequence of the formation of covalent chemical bonds between the film and the substrate. Thermal activation of the initiator was examined for the deposition polymerization of 9-H-carbazole-9-ethylmethacryrate. Substrate heating during the evaporation was not effective for accumulating thin films. On the other hand, performing postdeposition annealing on the film after deposition at room temperature resulted in the formation of a polymer thin film chemically bound to the substrate.
Organic light-emitting diodes were prepared by coevaporating 4,4 0 -N,N 0 -dicarbazolylbiphenyl (CBP) host material, tris(2phenylpyridinate) iridium(III) [Ir(ppy) 3 ] dopant material, and their vinyl derivatives for the emissive layer. It was found that the emission spectrum changes with time during device operation when both the host and the dopant molecules have vinyl groups. This spectral change corresponded to the transition of emission from vinyl-modified Ir(ppy) 3 to nonmodified Ir(ppy) 3 . The spectral transition was more prominent when an acrylate derivative was used in place of a vinyl derivative of CBP. A similar change in luminescence spectrum was observed when the film was polymerized by thermal annealing or electronassisted deposition. It is speculated that the spectral transition is caused by the copolymerization of host and dopant molecules during device operation. Such a current-induced polymerization can be a novel and convenient method of preparing polymeric thin films by the physical vapor deposition of functional monomers.
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