A Rational Molecular Design Strategy of TADF Emitter for Achieving Device Efficiency Exceeding 36%

An excellent thermally activated delayed fluorescence (TADF) emitter requires a sophisticated molecular design strategy to incorporate structural features to simultaneously achieve high photoluminescence quantum yield (PLQY) and high horizontal emission dipole ratio (Theta(//)). This work reports the uses of heteroarenes and dicarbonitrile benzenes to design four new acceptors PymCN, PyoCN, PmmCN, and PmoCN, which are linked to a common donor dimethylacridine (DMAC) for making new TADF emitters. The emission wavelength, Delta E-ST, k(risc), k(r), and the resulting PLQY of the target TADF emitters are governed by the combined natures of the heteroaryl bridges (Py vs Pm) and the CN-substituted patterns (o-CN vs m-CN). The photophysical and device characteristics reveal the best acceptor to be PyoCN, which is further coupled with spiroacridine to afford a new emitter SpiroAC-PyoCN with an enhanced PLQY of 100% compared to that (91%) of the DMAC-based counterpart DMAC-PyoCN. Furthermore, linking PyoCN with spiro-bisacridine (SBAC) gives an A-D-A-configured TADF emitter SBAC-PyoCN with both enhanced PLQY (100%) and Theta(//) (90%). The device employing SBAC-PyoCN as emitter renders a maximum external quantum efficiency up to 36.1% owing to its unity PLQY and superior light out-coupling efficiency. This rational molecular design strategy provides a feasible means to achieve an excellent TADF emitter design.