Statistical copolymers with side-chain hole and electron transport groups for single-layer electroluminescent device applications

New statistical copolymers with bipolar carrier transport abilities were sy nthesized through free radical copolymerization of N-vinylcarbazole (NVK, h ole-transport monomer) with either of two substituted styrenes containing o xadiazole groups, which serve as electron transport monomers: 2-phenyl-5-{4 -[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxadiazole, PVO, and 2-(4-tert-butyl phenyl)-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxzdiazole, BVO. In all cases, In all cases, the charge transport moieties exist in side groups, an d carrier transport proceeds by hopping. Copolymerization yields homogeneou s statistical copolymers of widely variable composition and thus tunable ca rrier transport properties; the copolymers are transparent in the visible r egion 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 ar e high, and there is no possibility for the oxadiazole units to phase-separ ate through recrystallization. The glass transition temperatures for the co polymers shaw positive deviations from a harmonic mixing rule, suggesting s ome interaction between the NVK and BVO residues; however, blends of the ho mopolymers show limited miscibility at best, indicating that copolymerizati on is essential to produce a homogeneous material. Incorporating the oxadia zole units reduces the hole transport ability of these copolymers somewhat relative to NVK homopolymer, but singlelayer dye-doped devices emitting blu e, green, and orange light fabricated from these copolymers all showed good efficiency.