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 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.