Web Release Date: February 16,
First Iridium Complex End-Capped Polyfluorene: Improving Device Performance for Phosphorescent Polymer Light-Emitting Diodes
and
Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, P. R. China, and Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China
Received: September 15, 2007
In Final Form: November 12, 2007
Abstract:
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
-diketonate were synthesized by the Suzuki polycondensation reaction and characterized by 1H NMR, 13C
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 
s 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/cm2, and a maximum
luminance of 706 cd/m2 at 18 V was achieved.
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