Design Strategy of Anthracene-Based Fluorophores toward High-Efficiency Deep Blue Organic Light-Emitting Diodes Utilizing Triplet-Triplet Fusion.

Abstract

In contrast to the red and green regions, conventional fluorescent emitters continue to serve as blue emitters in commercialized organic light-emitting diodes. Many researchers have studied anthracene moieties as blue emitters, given their appropriate energy levels and good emission properties. We herein report two new deep blue-emitting anthracene derivatives that include p-xylene as moieties connecting the anthracene cores to side groups. We enhanced the efficiency by maximizing triplet-triplet fusion (TTF) without sacrificing emission color. The large steric hindrance imposed by the methyl groups of p-xylene creates a perpendicular geometry between p-xylene and the neighboring aromatic rings. Any extension of π-conjugation is thus disrupted, and the isolated core anthracene moiety emits a deep blue color with a high photoluminescence quantum yield. Moreover, the extensive steric hindrance suppresses vibration and rotation because the molecules are rigid. The high horizontal dipole ratio attributable to the large aspect ratio increases the outcoupling efficiency of the emitted light. Furthermore, the charge mobility and triplet harvesting ability are enhanced by decreasing the bulkiness of the side groups. Molecular dynamics simulation revealed that the bulkiness of the side group significantly impacted molecular density, which in turn affected the charge transport and TTF. We used two molecules, 2PPIAn (containing a phenyl side group) and 4PPIAn (containing a terphenyl side group), to form nondoped emission layers that exhibited maximum external quantum efficiencies of 8.9 and 7.1% with Commission Internationale de L'Eclairage coordinates of (0.150, 0.060) and (0.152, 0.085), respectively.