High efficiency non-dopant blue organic light-emitting diodes based on anthracene-based fluorophores with molecular design of charge transport and red-shifted emission proof

Abstract

A new series of 9,10-diphenylanthracene (DPA)-based blue fluorophores have been synthesized and characterized for organic light-emitting diode (OLED) applications. These fluorophores have a bulky substituent, such as triphenylsilane in TPSDPA and mesitylene in TMPDPA, on the C-2 position. The C-2 substituent also includes electron transporting diphenylphosphine oxide in PPODPA and dimesitylene borane in BMTDPA, or hole transporting N-phenylnaphthalen-1-amine in NPADPA. For TMPDPA blue fluorophores, 9,10-diphenyl substituents of the anthracene core are further attached to hole-transporting 9H-carbazole in CBZDPA and electron-transporting 2-phenyl-1,3,4-oxadiazole in OXDDPA. Absorption and emission spectroscopic properties of all DPA-derived fluorophores, either in solution or in the condensed phase, were fully characterized and the HOMO/LUMO energy levels of these fluorophores were determined. The frontier molecular orbitals of the DPA derivatives were analysed by theoretical methods to determine the possible intramolecular charge transfer (ICT) characteristics. Whereas the blue emission is best preserved in TMPDPA, in which the non-conjugated bulky mesitylene group suppresses red-shifted emissions, the ICT is attributed to the deterioration in the emissions of NPADPA and BMTDPA. In the solid state, PPODPA suffered from red-shifted and weakened emissions because of adverse crystallization, which is promoted by the dipolar nature of the diphenylphosphine oxide substituent. Non-dopant OLEDs were fabricated with DPA, TPSDPA, TMPDPA, PPODPA, CBZDPA, and OXDDPA. Except for PPODPA, the electroluminescence efficiency of these DPA derivatives was significantly improved compared with that of the DPA OLEDs. In particular, CBZDPA and OXDDPA OLEDs exhibited the best external quantum efficiency of 4.5% and 4.0% with a true blue colour, with CIE_x,y coordinates of (0.17, 0.17) and (0.16, 0.18), respectively. The improved electroluminescence efficiency can be attributed to the molecular charge transport design of CBZDPA and OXDDPA.