Strategy for achieving efficient electroluminescence with reduced efficiency roll-off: Enhancement of hot excitons spin mixing and restriction of internal conversion by twisted structure regulation using an anthracene derivative

Phosphorescent and thermally activated delayed fluorescence (TADF) emitters have met a bottleneck due to the efficiency roll-off problem caused by the accumulation of the lowest triplet excited states with a relatively long lifetime. To achieve a high performance electroluminescence (EL) device with a simultaneously high radiative exciton ratio and a low efficiency roll-off in lighting applications at high luminance, we have demonstrated a novel strategy to construct an efficient hot exciton channel by effective spin mixing of the high-lying singlet and the triplet charge transfer (CT) excited states and restriction of the internal conversion (IC) from the high-lying triplet CT state to the lower triplet state. We selected luminous anthracene and electron-withdrawing triphenylphosphine oxide groups with a high triplet energy level to build four moderated D-A structure molecules which possess a high-lying CT hot exciton. Furthermore, the electronic coupling between the D and A unit can be precisely reduced by means of twisted structure regulation. In our series of novel synthesized materials, 9-[4-(diphenyl-phosphinoyl)-2-methyl-phenyl]-anthracene (An9-MePo) achieved optimization of the hot exciton radiation process and restriction of the IC. Both the maximum radiative exciton ratio of 74% and 72%, and the low efficiency roll-off of 10.6% and 15.9% at a luminance of 10000 cd m^-2 were achieved in a practical lighting application by the An9-MePo based nondoped and doped EL devices, respectively. Our results provide a novel approach to the fabrication of an efficient hot exciton channel, proving the great potential of this EL molecule design strategy for practical lighting applications.