Structure-Property Relationship of Pyridine-Containing Triphenyl Benzene Electron-Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

Three triphenyl benzene derivatives of 1,3,5-tri(m-pyrid-2-yl-phenyl)benzene (Tm2PyPB), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm3PyPB) and 1,3,5-tri(m-pyrid-4-yl-phenyl) benzene (Tm4PyPB), containing pyridine rings at the periphery, are developed as electron-transport and hole/exciton-blocking materials for iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N, C-2')picolinate (Flrpic)-based blue phosphorescent organic light-emitting devices. Their highest occupied molecular orbital and lowest unoccupied molecular orbital (LUMO) energy levels decrease as the nitrogen atom of the pyridine ring moves from position 2 to 3 and 4; this is supported by both experimental results and density functional theory calculations, and gives improved electron-injection and hole-blocking properties. They exhibit a high electron mobility of 10(-4)-10(-3) cm(2) V-1 s(-1) and a high triplet energy level of 2.75 eV. Confinement of Flrpic triplet excitons is strongly dependent on the nitrogen atom position of the pyridine ring. The second exponential decay component in the transient photoluminescence decays of Firpic-doped films also decreases when the position of the nitrogen atom in the pyridine ring changes. Reduced driving voltages are obtained when the nitrogen atom position changes because of improved electron injection as a result of the reduced LUMO level, but a better carrier balance is achieved for the Tm3PyPB-based device. An external quantum efficiency (EQE) over 93% of maximum EQE was achieved for the Tm4PyPB-based device at an illumination-relevant luminance of 1000 cd m(-2), indicating reduced efficiency roll-off due to better confinement of Firpic triplet excitons by Tm4PyPB in contrast to Tm2PyPB and Tm3PyPB.