The Effect of Purcell Cavity on Thermally Activated Delayed Fluorescence Lifetime

Organic light emitting diodes (OLEDs) have emerged as a promising display technology for smartphones, monitors, televisions and wearable electronics. One open question is to obtain both high efficiency emission and long operational lifetime in the deep blue spectrum. The lifetime bottleneck arises from bimolecular annihilation involving the long-lived triplet state.¹ The high energies delivered in these processes result in molecular fragmentation and the creation of defect sites that act as luminance quenchers. Recent studies have shown evidence that the Purcell effect can reduce the triplet density in Ir-based blue phosphorescent OLEDs, and hence the probability of destructive annihilation events.² In this work, we extend the study of the Purcell effect to blue thermally activated delayed fluorescence (TADF) OLED transient decay and operational lifetime. We observe an exciton radiative decay lifetime 0.7 times in the structure with optical cavity effect compared to a conventional structure. Temperature-dependent delayed emission lifetime characterization reveals the cavity affects both singlet and triplet excitons. We demonstrate more than 1.5 times increase in device operational lifetime of the plasmon-exciton-polariton enhanced Purcell device compared to the control. This work establishes the generality of Purcell effect in decreasing the triplet density and extending operational stability of OLEDs across a variety of emission processes.<br/><br/>1. Giebink, N. C., et al. (2008). Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions. J. Appl. Physics, 103, 044509. https://doi.org/10.1063/1.2884530<br/>2. Zhao, H., et al. (2024). Stable blue phosphorescent organic LEDs that use polariton-enhanced Purcell effects. Nature, 626, 300-305. https://doi.org/10.1038/s41586-023-06976-8