Role of Tailoring Exciplex‐Forming Cohosts in the Longevity of Blue Phosphorescent Organic Light‐Emitting Diodes

Abstract Although exciplex hosts have been used to enhance the operational lifetime of blue phosphorescent organic light‐emitting diodes, their detailed design rule has not been established because of their unresolved degradation. In this work, the underlying mechanisms involved in device degradation are investigated, focusing on the tailor‐tuning effect of the exciplex host to maximize the device's lifetime with a given phosphorescent dopant. Analysis of trap dynamics shows that the stability improvement caused by adding an electron‐donating unit to the N‐type host does not originate from the stable positive polaron state. It is found that the exciplex host modified with minor substituents opens extra pathways for polaron transport and exciton generation rather than chemical stability, resulting in the spatial dispersion of polarons and excitons. The dispersion of excitons and polarons, in turn, enhances the recombination coefficient and suppresses exciton–exciton and exciton–polaron bimolecular interactions, enabling further improved device stability. With comprehensive analysis, a perspective on the role of the host in the degradation mechanism is presented.