Phosphorescent light-emitting diodes using triscarbazole/bis(oxadiazole) hosts: comparison of homopolymer blends and random and block copolymers

Abstract

Examples of blends of carbazole- and bis(oxadiazole)benzene-based side-chain polymers have recently been reported to be efficient host materials for phosphorescent emitters in organic light-emitting diodes. Here, the properties and performance of a physical blend of polynorbornene homopolymers with triscarbazole and bis(oxadiazole)benzene side chains are compared to those of random and block copolymers of the corresponding triscarbazole- and bis(oxadiazole)benzene-functionalized monomers. Green-emitting devices in which the blend is used a host for Ir(ppy)₃ are significantly more efficient than those based on copolymers. Differential scanning calorimetry and solid-state NMR data show that there is no macroscale separation between the two polymers in the blend. The NMR data suggest that there are significant differences in the dimensionality and characteristic length of nanoscale domain structures in the block copolymer and the blend. Use of Ir(pppy)₃ in place of Ir(ppy)₃ leads to even more efficient light-emitting diodes, with external quantum efficiencies of up to ca. 21% (at 100 cd m⁻²).