High‐Efficiency and Low Efficiency Roll‐Off Blue‐Emission Crystalline Organic Light‐Emitting Diodes by embedding Triplet‐Triplet Annihilation Nanoaggregates

Abstract Crystalline host matrix (CHM)‐nanoaggregates (NA) structure combines the advantages of a crystalline host matrix and high‐performance luminescent materials. This structure emerges as a highly effective strategy for achieving efficient blue organic light‐emitting diodes (OLEDs). This approach can tackle the current challenge of achieving low voltage and high brightness in amorphous OLEDs. In this work, triplet‐triplet annihilation (TTA) material NA are embedded into CHM and sensitize the fluorescent dopant (D) BD1. As a result, a CHM‐TTANA‐D structure device is created, demonstrating an outstanding external quantum efficiency (EQE) of 8.46%. Notably, the efficiency roll‐off is mitigated. The device maintains an EQE of ≈7.50% at 1000 cd m − 2 , which is the highest values among blue crystalline OLEDs. The device leverages the benefits of CHM and exhibits swift turn‐on and a rapid increase in brightness and current density. This leads to a significant improvement in blue photon output and a reduced series‐resistance Joule heat loss ratio. The findings underscore the promising nature of the CHM‐TTANA‐D structure as a favorable design paradigm for high‐performance crystalline thin‐film OLEDs (C‐OLEDs).