Single layer light-emitting diodes from copolymers comprised of mesogen-jacketed polymer containing oxadiazole units and PVK

A series of copolymers PCt-co-Poly(N-vinylcarbazole) were synthesized through common radical polymerization, in which P-Ct as a kind of mesogen-jacketed liquid crystalline polymer was introduced, and the effects of copolymers composing variation on the optical properties of the polymers were studied. The structures and properties of the copolymers were characterized and evaluated by thermogravimetric (TGA), UV, photoluminescence (PL), cyclic voltammetry (CV), and electroluminescence (EL) analyses. All the polymers enjoy high thermal stability. PL peaks in the film show blue-shift compared with in solutions and fluorescent quantum efficiency decreased with the N-vinylcarbazole (nvk) content increasing, which supported the efficient energy transfer from nvk units to the oxadiazole units. CV revealed that, with the incorporation of nvk to the copolymer, these copolymers had high-lying HOMO energy levels ranging from -5.94 to -6.09 eV. Single-layer light-emitting diodes (LEDs) with the configuration of ITO/PEDOT/PCt-nvk/Mg:Ag/Ag were fabricated, which emit a blue light around 450 and 490 nm with a maximum luminance of 703 cd/m². The device performance varies with the content of nvk and device configuration, with device configuration (b) and PCt-nvk8 giving the best value of external quantum efficiency of 0.27%. We show here that by proper design copolymer structure and modification of device configuration can exhibit strong blue EL in higher external quantum efficiency.