Improving the Stability of Green Thermally Activated Delayed Fluorescence OLEDs by Reducing the Excited-State Dipole Moment

Highly efficient organic light-emitting diodes (OLEDs) employing metal-free thermally activated delayed fluorescence (TADF) emitters have attracted much attention in recent years. Efficient TADF mainly occurs in charge-transfer (CT) molecules. Here, we demonstrate that a small dipole moment for TADF emitters in the S1 state is key for device stability by comparing two green TADF emitters with the same moieties but different types of linker. The compound 1AC-TRZ with a 9,9-dimethyl-9,10-dihydroacridine donor and a triphenyltriazine acceptor attached at the 1- and N-positions of carbazole has a smaller excited-state dipole moment (11 D) than the 3- and N-substituted analog 3AC-TRZ (30 D). Although the emission spectra and efficiencies of their devices are similar, the operational lifetime of the 1AC-TRZ-based device is much longer than that of the 3AC-TRZ-based device. Time-resolved emission spectra indicate that the S1 energy of 3AC-TRZ in doped films is higher than that of 1AC-TRZ at the start of nanosecond solvation relaxation. A photodegradation experiment suggests that the short-term existence (2.9 eV) excitons can hasten the upconversion-induced degradation in films. Our study reveals a relationship between solid-state solvation and OLED stability and provides a new guideline for designing stable green and blue TADF emitters.