Novel Cross-Linkable Blue Light Emitting Material and Its High Stability OLEDs by Solution Process
The solution process holds great promise for organic light-emitting diode (OLED) fabrication owing to its minimal material loss, simple processing and low equipment investment. However, solution-processed blue OLEDs still face the challenges of low electroluminescence efficiency and poor working stability. In this study, two new cross-linkable blue light-emitting molecules, v-4CzBn and v-5CzBn, were synthesized. They featured a multicarbazole-substituted benzonitrile donor–acceptor structure as the emitting core, with two vinyl phenyl units on the carbazole rings serving as cross-linking groups. A singlet–triplet energy gap of ΔEST ≤ 0.10 eV and a high reverse intersystem crossing rate (kRISC > 10^6 s−1) were achieved because the three-dimensionally confined covalent network structure formed through a thermal cross-linking reaction limited intramolecular motions and vibrational relaxations of luminescent units. Moreover, this structure suppressed irreversible morphological changes and structural deterioration of light-emitting units due to aggregation or crystallization, improving the light-emitting performance of the device. Nondoped solution-processed OLEDs with the structure of ITO/PEDOT:PSS/TFB/S-4CzBn/TPBi/LiF/Al exhibited blue emission with a peak at 488 nm, achieving a maximum external quantum efficiency of 12.01%, a maximum luminance of 11,141.15 cd m−2, and a T50 lifetime of 1375.66 h@100 cd m−2. This result represents the longest operational lifetime reported to date for solution-process devices with cross-linked emitting layers.