Low-bias quenching due to spontaneous orientation polarization and its impact on OLED efficiency

Much effort has been directed at understanding organic light-emitting device (OLED) efficiency and the role of bimolecular quenching in efficiency roll-off. Quenching is less widely discussed at low-bias, where populations are reduced. Here, we describe lock-in-based photoluminescence measurements on working phosphorescent OLEDs to demonstrate that this assumption is not generally valid, and that significant exciton-polaron quenching is present even prior to turn-on. Exciton-polaron quenching arises with holes accumulated due to spontaneous orientation polarization in the electron transport layer. This low-bias quenching is found to strongly determine maximum achievable efficiency, suggesting a need to refine materials selection and device design rules.