Controlling the Average Number of Spin-Circulation in Thermally Activated Delayed Fluorescence by Purcell Effect

Abstract

Thermally activated delayed fluorescence (TADF) can exhibit a specific exciton behavior, which is called the spin-flip process between singlet and triplet excited states. Through continuous spin-circulation, TADF can harvest triplet excitons, realizing highly efficient organic light-emitting diodes (OLEDs). However, the spin-flip processes conversely induce exciton accumulation at a high current density region during TADF-OLEDs operation. To overcome this problem, the fast RISC rate constant has been highlighted, but it caused the concomitant increment of the ISC rate constant. This only increases the spin-circulation rate but could not suppress the exciton accumulation thoroughly. In this study, we control the average number of spin-circulation (𝑛̅) in TADF-OLEDs by optical engineering approach, i.e., the Purcell effect. In conclusion, the enhanced Purcell factor (F) realized the decreased average number of spin-circulation in TADF-OLEDs, and it could reduce exciton accumulation and relax the device efficiency roll-off behavior and OLEDs operational lifetime.