Managing charge balance and triplet excitons to achieve high-power-efficiency phosphorescent organic light-emitting diodes

High-efficiency phosphorescent organic light-emitting diodes (PhOLEDs) have been achieved by using an ortho-linked triphenylamine/oxadiazole hybrid, namely p-TPA-o-OXD, as host material. The high HOMO level of the compound (5.25 eV) can facilitate efficient hole injection from 1, 4-bis[(1-naphthylphenyl)amino] biphenyl (NPB) layer to the emissive layer. As a result, low driving voltages and high power efficiencies have been attained as compared to the carbazole-based hosts with lower HOMO levels and higher injection barriers under similar device structures. By introducing 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) to replace 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/tris(8-hydroxyquinoline)aluminum (Alq3) as hole blocking/electron transporting layer, followed by tuning the thicknesses of hole-transport NPB layer to manipulate the charge balance, high-efficiency PhOLEDs have been achieved, with maximum current efficiency/power efficiency/external quantum efficiency of 85.7 cd/A/99.7 lm/W/22.2% for green (ppy)₂Ir(acac)- based devices, 55.2 cd/A/64.2 lm/W/19.0% for yellow (fbi)₂Ir(acac)- based devices, and 11.4 cd/A/11.8 lm/W/14.8% for deep red (piq) ₂Ir(acac)-based devices. By inserting 10 nm of p-TPA-o-OXD as self-triplet exciton blocking layer between hole transporting and emissive layer to confine triplet excitons, device performances have been further improved in green PhOLED, with peak current efficiency/power efficiency/external quantum efficiency of 90.0 cd/A/97.7 lm/W/23.5%.