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 S 1 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 S 1 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 (<1 ns) of high energy (>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.