Incorporation of a CN group into mCP: a new bipolar host material for highly efficient blue and white electrophosphorescent devices

A novel bipolar host material mCPCN has been designed and synthesized by incorporating the electron-accepting CN group into the well-known benchmark host material mCP. Compared to mCP, the incorporation of the simple and small CN group significantly improves thermal/morphological stabilities (Tg = 97 °C and Td = 313 °C) and increases the electron affinity, while keeping electronic transition energies unaltered and maintaining a high triplet energy (ET = 3.03 eV). Characteristics of single-carrier devices containing mCPCN indicate its rather balanced hole/electron injection and transport properties. Highly efficient blue phosphorescent organic light emitting devices (PhOLEDs) with maximum external quantum, current and power efficiencies of 26.4%, 58.6 cd A−1, and 57.6 lm W−1, respectively, were achieved using mCPCN as the bipolar host material and bis(4′,6′-difluorophenyl)pyridinato-N,C2′iridium(III) picolinate (FIrpic; ET = 2.65 eV) as the triplet emitter. Furthermore, blue PhOLEDs adopting mCPCN exhibit impressively low efficiency roll-offs, retaining high quantum efficiencies of ∼25% at 1000 cd m−2 and ∼20% even at 8000 cd m−2. mCPCN has also been successfully used in implementing highly efficient white PhOLEDs having external quantum efficiencies in excess of 23%.