Synthesis of new iridium complexes and their electrophosphorescent properties in polymer light-emitting diodes

Several new iridium complexes with p-substituted 2-phenylpyridine (R-PPy) ligands have been synthesized and characterized. The complexes were incorporated into phosphorescent polymer light-emitting devices using soluble poly[1,4-bis(6'-cyano-6'-methylheptyloxy)phenylene] (CNPPP) as the host and the resultant materials compared with Ir(PPy)(3)-doped devices. Green electrophosphorescence was observed, with peak emission at about 495 and 515 nm. Among the devices fabricated, highly efficient polymer light-emitting diodes were obtained with CNPPP doped with fac-tris[2-(4-tert-butylphenyl)pyridinato]iridium. An external quantum efficiency of 4.4% (photoluminescence/electroluminescence) and a luminous efficiency of 10 cd A(-1) were obtained at 120 cd m(-2). These values remain at 4.2% and 10 cd A(-1), respectively, at 2500 cd m(-2). The improvement is attributed to improved interaction between the guest and host, and to better and more complete energy transfer from the host singlet to the guest triplet state. These results demonstrate that efficient electrophosphorescence is not limited to small molecule organic light-emitting diodes, it can also be achieved in devices made with polymer hosts.