Wide-Energy-Gap Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes.

Two novel wide-energy-gap molecules, 1,3-bis(9-phenyl-9H-fluoren-9-yl)benzene (mDPFB) and 1,4-bis(9-phenyl-9H-fluoren-9-yl)benzene (pDPFB), suitable for blue phosphorescent host materials, were designed and prepared. The thermal, photophysical, and electrochemical properties as well as electroluminescence characteristics were fully investigated. Both mDPFB and pDPFB possess high singlet (4.0 eV) and triplet (2.8 eV) energies, well energy-matched with the deep-blue phosphorescent dopant, iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). The single crystals of the newly synthesized compounds mDPFB and pDPFB were grown, and their crystal structures were determined by X-ray diffraction. Both molecules adopt a rigid twisted tetrahedral crystallographic conformation, and no facial to facial π−π stacking exists in the solid state. The substituents at the 1,4-positions of benzene enhance the twisted degree of the fluorenyl rings and have predominant influence on the thermal and morphological properties. Both molecules possess excellent thermal stability with decomposition temperatures (Td) of 354 and 386 °C in nitrogen, respectively. Blue phosphorescent organic light-emitting diodes (OLEDs) were fabricated using pDPFB or mDPFB as host materials and FIr6 as a blue phosphorescent dopant with the configuration of ITO/NPB/mCP/host:FIr6/BCP/Ca:Ag. Devices based on the pDPFB host showed a maximum luminous power efficiency of 18.9 lm/W, with CIE coordinates (x, y) of (0.18, 0.33), improved by a factor of over 10 compared with the reference devices based on N,N′-dicarbazolyl-3,5-benzene (mCP) host. We discussed the mechanism and demonstrated that highly efficient blue phosphorescent OLEDs can be achieved in designing host materials which possess matched energy levels with the phosphorescent dopant, with high miscibility to the dopant, and with excellent film-forming ability and thermal and morphological stability. [ABSTRACT FROM AUTHOR]