First iridium complex end-capped polyfluorene: Improving device performance for phosphorescent polymer light-emitting diodes

Two types of fluorene-based copolymers, with 1-phenylisoquinoline-iridium complexes incorporated into the polyfluorene main chain by either embedding (P1 and P2) or end-capping (P3) manners via ancillary ligand beta-diketonate were synthesized by the Suzuki polycondensation reaction and characterized by H-1 NMR, C-13 NMR, elemental analysis, and GPC. The electrochemical investigation reveals that the HOMO and LUMO energy levels of the monomeric iridium complexes fall within those of the parent polyfluorene, implying that the iridium complexes in the polymers could function as traps for both electrons and holes under electrical excitation. The different connection manners between the iridium complex and polyfluorene backbone have a significant effect on their photophysical and electroluminescent properties. The absorption spectra of P1-P3 are mostly characteristic of the polyfluorene backbone. The PL spectra of P1 and P2 are dominated by emission from the iridium complex at 625 nm, whereas for P3 the emission at 425 nm from the polyfluorene backbone is more intense than the emission at 622 nm from the iridium complex. The PL decay measurements show that P3 has a longer triplet lifetime at 1.05 us with monoexponential mode than those of P1 and P2 with biexponential mode. Polymer light-emitting diodes with the configuration of ITO/PEDOT/PVK/ P1, P2, or P3/Ba/Al were fabricated. The EL spectra of all of the devices show exclusively phosphorescent emission at 626-633 nm dominated by the charge-trapping mechanism. The device using P3 as the emitting layer displays significantly higher efficiency than those based on P1 and P2, which is attributed mainly to the fact that P3 suffers much less from triplet exciton back-transfer from the iridium complex to the polyfluorene backbone than P1 and P2. A red-emitting polymer light-emitting diode with an emission peak at 633 nm, a maximum external quantum efficiency of 1.70% at a current density (J) of 3.58 mA/cm(2), and a maximum luminance of 706 cd/m(2) at 18 V was achieved.