Spin-coated highly efficient phosphorescent organic light-emitting diodes based on bipolar triphenylamine-benzimidazole derivatives

A new series of star-shaped bipolar host molecules, tris(4'-(1-phenyl-1H-benzimidazol-2-yl)biphen-yl-4-yl) amine (TIBN), tris(2'-methyl-4'-(1-phenyl-1H-benzimidazol-2-yl)biphenyl-4-yl)amine (Me-TIBN), and tris(2,2-dimethyl-4'-(1-phenyl-1H-benzimidazol-2-yl)biphenyl-4-yl)amine (DM-TIBN), that contain hole-transporting triphenylamine and electron-tran sporting benzimidazole moieties are designed based on calculations using density functional theory and successfully prepared. The theoretical calculation of energy levels of TIBN derivatives affords helpful ideas to design molecules with a favorable localization of highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) levels and a predefined enhancement of the triplet energy gap. The THIN derivatives are employed as hosts to fabricate phosphorescent organic light-emitting diodes (OLEDs) by the two methods of spin-coating and vacuum deposition. Notably, the spin-coated Me-TIBN and DM-TIBN devices exhibit a much better performance than the vacuum-deposited ones, in which the spin-coated DM-TIBN device (47 500 cd m(-2), 27.3 cd A(-1), 7.3 lm W-1) is outstanding with respect to other seminal work for solution-processed OLEDs. More importantly, the new concept of localizing HOMO and LUMO levels for bipolar molecules is illustrated, and a facile strategy to tailor the energy levels by breaking the conjugation of hole- and electron-transporting moieties is demonstrated.