Iridium complexes embedded into and end-capped onto phosphorescent polymers: optimizing PLED performance and structure–property relationships

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Abstract
A series of novel fluorene-alt-carbazole or fluorene-based copolymers have been synthesized by Suzuki polycondensation. The iridium complexes with the cyclometalated ligand 2-p-tolyl-benzothiazole were incorporated into the copolymers by either embedding or end-capping into the backbone via an ancillary β-diketonate ligand. The copolymers were characterized by 1H NMR, 13C NMR, and GPC. The TGA/DSC measurements indicate that the copolymers with carbazole units in the main chain show better thermal stability than those with only fluorene units. Electrochemical investigations reveal that the HOMO and LUMO energy levels of the monomeric iridium complexes fall within those of the parent polymers. The absorption spectra of the polymers are mostly characteristic of the polymer backbone. The PL spectra of the polymers show phosphorescent emission at ca. 560 nm with a shoulder at ca. 594 nm, and their PL decay measurements indicate that the end-capped polymerP4 has a longer triplet lifetime with a mono-exponential mode than those of the embedded polymersP1–3 with biexponential modes. Light-emitting diodes using the copolymers as the emitting layer under different device configurations were fabricated. For P1–3, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) doping in copolymers shows remarkably enhanced efficiencies, whereas for P4, the introduction of a PVK layer significantly improves the device performance. Noticeably, the devices using P4 as the emitting layer display significantly higher efficiencies than those based on P1–3, which is mainly attributed to the fact that P4 suffers much less from triplet exciton back transfer from the iridium complex to the polymer backbone than P1–3 do. An orange–red-emitting PLED with an emission peak at 599 nm, a maximum external quantum efficiency of 2.19% at a current density (J) of 0.3 mA cm−2, and a maximum luminance of 2347 cd m−2 at 17 V was achieved from the device ITO/PEDOT/PVK/P4/Ba/Al.