High-efficiency electrophosphorescent fluorene-alt-carbazole copolymers N-grafted with cyclometalated Ir complexes

A series of electrophosphorescent conjugated fluorene-alt-carbazole copolymers are synthesized by Suzuki polycondensation. A diketone-ended alkyl chain which is grafted in the N-position of carbazole serves as a ligand to form a pendant cyclometalated Ir complex with 1-phenylisoquinoline (Piq), 2-naphthylpyridine (Napy), and 2-phenylquinoline (Phq). The PL efficiencies of PFCzIrPiq and PFCzIrPhq copolymers are around 60% and 70%, respectively. EL emission from the backbone of PFCzIrPiq and PFCzIrPhq copolymers is completely quenched even though the Ir complex contents in the polymers are as low as 0.5 mol%. The device of PFCzIrPiq05 copolymer shows the highest external quantum efficiency of 4.9% ph/el and the luminous efficiency of 4.0 cd/A with 240 cd/m(2) at a bias voltage of 7.7 V and a peak emission of 610 nm. The device efficiency of PFCzIrPiq05 copolymer still remains high (QE(ext) = 3.4% ph/el and LE = 2.9 cd/A) with a luminance of 2978 cd/m(2) even at a current density of 100 mA/cm(2). The enhancement of the device performance could be due to the higher triplet energy and meanwhile to suitable HOMO and LUMO levels for efficient charge injection by introducing a carbazole unit into the polyfluorene backbone at the 3,6-linkage and blending PBD into the copolymers.