ABSTRACT Understanding the exciton evolution relating to high‐lying excited triplet state ( T n , n≥2) is crucial for fully utilizing the triplet excitons in organic light‐emitting diodes (OLEDs). However, due to the optical‐inaccessible features of T n , the role of T n in the emission processes and how they affect device performance are still debated. Here, we studied the exciton dynamics of T n by using four anthracene derivatives through magneto‐electroluminescence (MEL) and transient electroluminescence (TrEL) measurements complemented by theoretical calculation. The results show that in materials with fast radiative rate and hence inhibited high‐lying intersystem crossing (hISC) process, triplet–triplet annihilation (TTA) could enhance the efficiency of OLEDs. In contrast, for materials with slow radiative rate, the hISC process complement IC and TTA up‐conversion processes degrade device performance. Finally, the optimized OLEDs with negligible efficiency roll‐off are fabricated by manipulating the exciton evolution. The device delivers a maximum external quantum efficiency of 10% at a high luminance of over 20 000 cd m −2 , and no efficiency roll‐off is observed. These findings give a new understanding of the excitons on T n and TTA up‐conversion process that could be beneficial for the fabrication of high‐performance OLEDs in the future.
Chen et al. (Wed,) studied this question.