Electroluminescence of pure poly(N-vinylcarbazole) and its blends with a multiblock copolymer

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Abstract
Electroluminescence (EL) in pure poly(N-vinylcarbazole) (PVK) has been observed in a simple single active layer EL device with the structure of ITO/PVK/Al (or Ca). The emitted light is violet with a maximum intensity at 426 nm; a part of the spectrum lies within the UV region. Blends of PVK with a conjugated-nonconjugated multiblock copolymer (CNMBC) previously described are also electroluminescent. In these blends a new emission peak in the EL spectra appeared and can be attributed to the emission from an exciplex formed by the two polymers. As the composition of the aforementioned blend changes from a PVK-poor to a PVK-rich ratio, the emitted light changes from green to blue. Further, blends containing 97 wt. % PVK and 3 wt. % CNMBC yielded an EL spectrum with a single emission peak different in location from either that of pure PVK or the pure CNMBC; the latter is blue with a maximum intensity at 476 nm. The EL output of the blends can be clearly seen in a brightly lit room at ambient temperature. An electron transport layer consisting of butyl-2-(4-biphenyl)-5-(4-tert-butylphenyl-1,3,4-oxadiazole) (PBD) in PMMA on the cathodic side of the EL device greatly increased the brightness of the emitted light to 200 cd/m2 in the case of a PVK-rich blend, but it had less effect on a PVK-poor blend or in pure PVK and had little effect on the pure form of the CNMBC.