Abstract
A fast reverse intersystem crossing rate (kRISC) is an ongoing pursuit for exciplex-forming host with thermally activated delayed fluorescence (TADF) for improved performances of organic light-emitting diodes. However, design rules regarding the development of exciplex with a high kRISC are still elusive. Here, the influence of heavy atom effect on kRISC of exciplex is investigated with bromine (Br)-substituted acceptors. It is proved that heavy atom effect induced by Br atom can enhance the spin-orbital coupling of exciplex systems and thus promote the kRISC process of exciplex. Compared with the reference one (kRISC of 5.4 × 105 per second), Br-containing exciplex exhibits an increased kRISC by almost an order of magnitude, being 4.56 × 106 per second. Based on those exciplex-forming hosts, devices based on a conventional fluorescent emitter and a TADF emitter are fabricated, and it is observed that the faster the RISC process of exciplex-forming host is, the higher the device efficiency can be obtained. Those results here should provide an effective strategy to accelerate the kRISC of exciplex-forming host for advanced device performances.
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