Highly efficient fluorene- and benzothiadiazole-based conjugated copolymers for polymer light-emitting diodes

A series of highly efficient conjugated polymers have been synthesized for application in polymer light-emitting diodes (PLEDs). These polymers were based on the copolymerization between 9,9-dihexylfluorene and benzothiadiazole (BT). In several cases, a third comonomer was added to adjust the charge injecting and transporting properties of the polymers. All of these copolymers exhibited strong green emission at around 540 nm which can be attributed to either the charge transfer between an electron-rich segment and an electron-deficient BT-containing segment of the polymers or the Förster energy transfer between different polymer chains. These copolymers also exhibited very high photoluminescence quantum efficiencies up to 55%. A double-layer device using one of the copolymers as the emissive layer exhibited a low turn-on voltage (3.4 V), a very high external quantum efficiency (6.0%), and a high brightness of 59 400 cd/cm².