The Effect of Purcell Cavities on the Lifetime of Thermally Activated Delayed Fluorescent Emitters

ABSTRACT One of the most pressing challenges to improving organic light emitting diode (OLED) displays and lighting is to balance high efficiency and long operational lifetime in the deep blue spectrum. Recent studies have shown that the Purcell effect reduces the triplet density and hence the probability for destructive energy‐driven triplet annihilation events that limit the lifetime in phosphorescent OLEDs. In this work, we extend the study of the Purcell effect to two different classes (metal‐containing and metal‐free) of thermally activated delayed fluorescent (TADF) emitters. Using a representative of each class, we observe a doubling of the emission rate of carbene‐metal‐amide (cMa) TADF molecules with large intersystem crossing (ISC) rates due to coupling to surface plasmon polariton (SPP) modes in a Ag cathode. Temperature‐dependent photophysical characterization reveals that the cavity equally increases both singlet and triplet exciton radiative rates. The larger emission rate results in a 1.3 times enhancement in the device lifetime of a cMa TADF OLED with 1931 Commission Internationale d'Eclairage color space chromaticity coordinates of CIE xy = (0.37, 0.55) in a weak Purcell cavity, exhibiting an LT80 (i.e., the time for the luminance to decay to 80% of its initial value) of 184 ± 5 h at an initial luminance of 1500 cd/m ² . In comparison, the Purcell cavity has a much smaller impact on the emission rate of a metal‐free emitter due to the smaller endergonic ISC rate than the singlet transition rate. As a result, such OLEDs with CIE xy = (0.19, 0.41) do not show increased device stability. This study highlights the key characteristics needed in TADF emitters to leverage the Purcell effect for improving device lifetime.