Design of Efficient Exciplex Emitters by Decreasing the Energy Gap Between the Local Excited Triplet ((3)LE) State of the Acceptor and the Charge Transfer (CT) States of the Exciplex

A series of thermally activated delayed fluorescence (TADF) exciplex based on the TX-TerPy were constructed. The electronic coupling between the triplet local excited states ((3)LE) of the donors and acceptor and the charge transfer states had a great influence on the triplet exciton harvesting and Φ(PL). Herein, based on this strategy, three donor molecules TAPC, TCTA, and m-MTDATA were selected. The local triplet excited state ((3)LE) of the three donors are 2.93, 2.72 and 2.52 eV in pure films. And the (3)LE of TX-TerPy is 2.69 eV in polystyrene film. The energy gap between the singlet charge transfer ((1)CT) states of TAPC:TX-TerPy (7:1), TCTA:TX-TerPy (7:1) and the (3)LE of TX-TerPy are 0.30 eV and 0.20 eV. Finally, the Φ(PL) of TAPC:TX-TerPy (7:1) and TCTA:TX-TerPy (7:1) are 65.2 and 69.6%. When we changed the doping concentration of the exciplex from 15% to 50%, the ratio of the triplet decreased, and Φ(PL) decreased by half, perhaps due to the increased energy gap between (1)CT and (3)LE. Therefore, optimizing the (1)CT, (3)CT, and (3)LE facilitated the efficient exciplex TADF molecules.