The role of dark triplets in the degradation of blue phosphorescent OLEDs

The relative instability of high efficiency blue phosphorescent Organic Light Emitting Devices (OLEDs) is an important and longstanding challenge. Conventional models of degradation emphasize the dynamics of emissive triplet excitons on phosphorescent molecules, neglecting the potential role of non-emissive, ‘dark’, triplet excitons on host and charge blocking molecules. In this talk, bright and dark triplet dynamics are probed in-situ using surface plasmon polariton-coupled modulation and magnetic exciton annihilation modulation, respectively. It is observed that dark triplet excitons dominate both the instability and efficiency roll-off of a high-performance blue phosphorescent OLED. Dissociating dark triplets both improves device stability and reduces efficiency losses at high brightness by a factor of 2. The combined benefit to stability of Purcell engineering and dark triplet management is a factor of 3.5. The results demonstrate that dark triplet excitons reduce the potential benefit of Purcell engineering, emphasizing the importance of in-situ characterization and control of dark triplets to realize the goal of stable blue phosphorescent OLEDs.