Synthesis and characterization of fluorene-based copolymers as electron-transporting materials for PLEDs

Owing to their low cost, easy processing, and the possibility of flexible fabrication, polymer light-emitting diodes (PLEDs) are emerging as an important class of materials. Despite promising characteristics, the relatively easy ionization of the well-known low-work-function cathodes such as Ca and Ba prevents the full usage of these materials. Herein, we report the syntheses of three alcohol-soluble conjugated polymers with different conjugation lengths and electron affinities as electron injection and transport materials for PLEDs: poly9,9-bis(2-dihexylaminoethoxy)fluorene-co-tetrafluorobenzene] (PFOH-1), poly9,9-bis(2-dihexylaminoethoxy)fluorene-co-thiophene] (PFOH-2), and poly9,9-bis(2-dihexylaminoethoxy)fluorene-co-benzo-thiadiazole] (PFOH-3). For comparison, devices using Al, Ca, and Al cathodes were also fabricated. The device based on the Al cathode showed lower performance with a luminescence efficiency of 0.93 cd/A and a luminance of 248 cd/m²; that based on the low-work-function metal Ca as the cathode showed a near-threefold increase in luminescence efficiency at 2.51 cd/A and brightness at 856 cd/m² owing to greatly enhanced electron injection from the cathode; and the device employing the PFOH-3/Al cathode exhibited a luminescence efficiency of 2.35 cd/A and a brightness of 667 cd/m² at a current density of 35 mA/cm², which is comparable with the performance of the device with the Ca cathode.