Breaking the Efficiency Limit of Deep-Blue Fluorescent OLEDs Based on Anthracene Derivatives

Triplet harvesting is important for the realization of high-efficiency fluorescent organic light-emitting diodes (OLEDs). Triplet-triplet annihilation (TTA) is one triplet-harvesting strategy. However, for blue-emitting anthracene derivatives, the theoretical maximum radiative singlet-exciton ratio generated from the TTA process is known to be 15% in addition to the initially generated singlets of 25%, which is insufficient for high-efficiency fluorescent devices. In this study, nearly 25% of the radiative singlet-exciton ratio is realized by TTA using an anthracene derivative, breaking the theoretical limit. As a result, efficient deep-blue TTA fluorescent devices are developed, exhibiting external quantum efficiencies of 10.2% and 8.6% with Commission Internationale de l'Eclairage color coordinates of (0.134, 0.131) and (0.137, 0.076), respectively. The theoretical model provided herein explains the experimental results considering both the TTA and reverse intersystem crossing to a singlet state from higher triplet states formed by the TTA, clearly demonstrating that the radiative singlet ratio generated from TTA can reach 37.5% (total radiative singlet-exciton ratio: 62.5%), well above 15% (total 40%), despite the molecule having S-1, T-2 < 2T(1) < Q(1) energy levels, which will lead to the development of high-efficiency fluorescent OLEDs with external quantum efficiencies exceeding 28% if the outcoupling efficiency is 45%.