Simple CBP isomers with high triplet energies for highly efficient blue electrophosphorescence

Two simple CBP isomers, namely m-CBP and o-CBP, were designed and synthesized by finely tuning the linking topology between the carbazole and the central biphenyl units of CBP, and their thermal, photophysical and electrochemical properties were investigated. Such simple modification of the linking topology endows the CBP isomers with high triplet energy and relative high thermal and morphological stability. The high triplet energies of m-CBP and o-CBP ensure efficient energy transfer from the host to the phosphor and triplet exciton confinement on the phosphor, as indicated by the transient photoluminescence decay of 3 wt% FIrpic doped into m-CBP and o-CBP. Blue phosphorescent devices employing FIrpic as guest and the two CBP isomers as hosts exhibit high efficiencies. The best EL performance is achieved for the o-CBP-based device, with a maximum current efficiency of 29.9 cd A(-1), and a maximum external quantum efficiency of 14.2%, which are over 2 times higher than those of CBP.