Quantitative Analysis of Exciton Dynamics: Investigating the Role of the Efficiency-Enhancement Layer for Blue Fluorescent Organic Light-Emitting Diodes

One effective approach to overcoming the efficiency limits of triplet–triplet fusion (TTF)-based blue organic light-emitting diodes is to utilize the efficiency enhancement layer (EEL), which can increase efficiency through simple modifications in the device structure. However, the physical mechanism of the EEL, particularly exciton dynamics, has not been thoroughly investigated. In this study, the exciton utilization efficiency (EUE) was calculated by considering the host and EEL as individual emission components to analyze the exciton dynamics of the EEL. The materials that exhibit high-lying reverse intersystem crossing (hRISC) characteristics 2-(4-(10-(3-(9H-carbazol-9-yl)phenyl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (PAC) and 5-(4-(10-phenylanthracen-9-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (ATDBF) were used as an EEL. The maximum external quantum efficiency (EQEmax) of the device without utilizing the EEL was measured at 3.0%. However, when PAC and ATDBF were utilized as the EEL in the device, the EQEmax significantly increased to 4.4 and 4.5%, respectively, resulting in an efficiency improvement of approximately 47–50% compared to when the EEL was not used. To ascertain whether the efficiency enhancement is induced through the hRISC process of the EEL, transient electroluminescence and magnetoelectroluminescence measurements were carried out. The results indicated that the triplets harvested into radiative singlets in the EEL device occurred through the TTF and hRISC processes. Finally, numerical simulations were carried out to evaluate the efficiency contributions from various mechanisms in the nondoped device, including TTF induced from the host (ηTTF.H), TTF induced from the EEL (ηTTF.E), and hRISC induced from the EEL (ηhRISC.E) in addition to 25% of EUE from singles via hole–electron recombination. The calculated ηTTF.H, ηTTF.E, and ηhRISC.E were 3.8, 0.6, and 4.7% for the PAC-based device and 5.5, 0.9, and 12.3% for the ATDBF-based device, respectively. Consequently, while the device using the host alone exhibited an EUE of 31.8%, the incorporation of PAC and ATDBF elevated the EUE to 34.1 and 43.7%, respectively, highlighting significantly enhanced EUE.