A highly efficient OLED based on terbium complexes

A neutral ligand 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphineoxide)-carbazole (DPPOC) and its complex Tb(PMIP)3DPPOC (A, where PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) were synthesized. DPPOC has a suitable lowest triplet energy level (24,691 cm−1) for the sensitization of Tb(III) (5D4: 20,400 cm−1) and a significantly higher thermal stability (glass transition temperature 137 °C) compared with the familiar ligand triphenylphosphine oxide (TPPO). Experiments revealed that the emission layer of the Tb(PMIP)3DPPOC film could be prepared by vacuum co-deposition of the complex Tb(PMIP)3(H2O)2 (B) and DPPOC (molar ratio = 1:1). The electroluminescent (EL) device ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB; 10 nm)/Tb(PMIP)3 (20 nm)/co-deposited Tb(PMIP)3DPPOC (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP; 10 nm)/tris(8-hydroxyquinoline) (AlQ; 20 nm)/Mg0.9Ag0.1 (200 nm)/Ag (80 nm) exhibited pure emission from terbium ions, even at the highest current density. The highest efficiency obtained was 16.1 lm W−1, 36.0 cd A−1 at 6 V. At a practical brightness of 119 cd m−2 (11 V) the efficiency remained above 4.5 lm W−1, 15.7 cd A−1. These values are a significant improvement over the previously reported Tb(PMIP)3(TPPO)2 (C).