Pyridine-Containing Electron-Transport Materials for Highly Efficient Blue Phosphorescent OLEDs with Ultralow Operating Voltage and Reduced Efficiency Roll-Off

A series of pyridine-containing electron-transport materials are developed as an electron-transport layer for the FIrpic-based blue phosphorescent organic light-emitting diodes. Their energy levels can be tuned by the introduction of pyridine rings in the framework and on the periphery of the molecules. Significantly reduced operating voltage is achieved without compromising external quantum efficiency by solely tuning the nitrogen atom orientations of those pyidine rings. Unprecedented low operating voltages of 2.61 and 3.03 V are realized at 1 and 100 cd m-2, giving ever highest power efficiency values of 65.8 and 59.7 lm W-1, respectively. In addition, the operating voltages at 100 cd m-2 can be further reduced to 2.70 V by using a host material with a small singlet-triplet exchange energy, and the threshold voltage for electroluminescence can even be 0.2-0.3 V lower than the theoretical minimum value of the photon energy divided by electron charge. Aside from the reduced operating voltage, a further reduced roll-off in efficiency is also achieved by the combination of an appropriate host material.