Controllably tuning excited-state energy in ternary hosts for ultralow-voltage-driven blue electrophosphorescence.

Phosphorescent organic light-emitting diodes (PHOLEDs), with 100% theoretical internal efficiency, are being rapidly developed as a most promising approach to meet the urgent and extensive demand of energy-efficient and portable digital terminals and lighting sources. Thanks to the recent breakthrough of highly efficient blue PHOLEDs and outcoupling technologies, PHOLEDs in full color can already realize extremely high efficiencies that approach those of fluorescent tubes (about 70 LmW ). Nevertheless, as the hosts in the emitting layers (EMLs) should have higher triplet excited energy levels (T1) to confine the excitons on phosphorescent guests, the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gaps in PHOLEDs are often much larger than their fluorescent counterparts, which consequently result in poor energy-level alignment and thus higher driving voltages. This drawback not only complicates the design of driving circuit, but also directly reduces power efficiency (PE). Thus, the low-voltage-driving high-efficiency PHOLED remains the biggest challenge. Su, Kido, et al. have reported green PHOLEDs with extremely low operating voltages of 2.18 V for onset and 2.41 V at 100 cdm 2 through good management of the interfacial contact between electron transporting layers and anodes. However, there are only a few blue PHOLEDs that achieve low driving voltages; for example, applicable luminance at a driving voltage of less than 3 V. The formidable challenge is the high barriers for carrier injection and transportation deriving from the prerequisite of extremely high T1 of the hosts, for example, 2.85 eV (0.2 eV higher than that of blue phosphor iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic; Scheme 1). This issue actually reflects the intrinsic