Spiro-Configured Bipolar Host Materials for Highly Efficient Electrophosphorescent Devices

In this study, we synthesized and characterized a series of spirobifluorene-based bipolar compounds (D2?ACN, DNPACN, DNTACN, and DCzACN) in which a dicyano-substituted biphenyl branch, linked orthogonally to a donor biphenyl branch bearing various diarylamines, acted as an acceptor unit allowing fine-tuning of the morphological stability, triplet energy, bipolar transport behavior, and the HOMO and LUMO energy levels. The promising physical properties of these new compounds, together with their ability to transport electrons and holes with balanced mobilities, made them suitable for use as host materials in highly efficient phosphorescent organic light-emitting diodes (PhOLEDs) with green iridium-based- or red osmium-based phosphors as the emitting layer (EML). We adopted a multilayer structure to efficiently confine holes and electrons within the EML, thus preventing exciton diffusion and improving device efficiency. The device incorporating D2?ACN doped with the red emitter [Os(bpftz)2(PPhMe2)2] (bpftz=3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate) gave a saturated red electrophosphorescence with CIE coordinates of (0.65, 0.35) and remarkably high efficiencies of 20.3?% (21 cd?A-1) and 13.5 Lm?W-1 at a practical brightness of 1000 cd?m-2.