Dual Encapsulation of Electron Transporting Materials To Simplify High-Efficiency Blue Thermally Activated Delayed Fluorescence Devices

The charge flux balance and interfacial optimization are two core concerns when simplifying blue thermally activated delayed fluorescence (TADF) diodes, which reflects the more stringent demand on carrier transporting materials (CTM) as the embodiment of the contradiction between charge transportation and quenching suppression with the opposite requirement on intermolecular interactions. Herein, phenylbenzimidazole (PBI) was used as the core substituted with two diphenylphosphine oxide (DPPO) groups to form six dual-encapsulated charge-exciton separation (CES)-type electron transporting materials (ETM) with the collective name of xyPBIDPO. Through tuning the substitution positions of DPPO group, its two functions of resonance and steric effects were integrated and optimized to enhance charged moiety encapsulation without cost of reducing electroactivity. As the result, among xyPBIDPO, mmPBIDPO successfully realizes the balance of favorable electrical performance and interfacial interaction suppressions in virtue of its doubled mesa-substitution structure and roughly symmetrical configuration, rendering the good electron affinity of 2.8 eV, the high electron mobility by the level of 10(-6) cm(2) V-1 s(-1) and effective PBI-core encapsulation. Consequently, mmPBIDPO was used to extremely simplify the blue TADF devices with the state-of-the-art performance from trilayer and quadruple-layer configurations, such as the maximum external quantum efficiency (EQE) beyond 20% and improved efficiency stability. This work not only established a solid example of CES-type ETM for high-performance simple structured blue TADF devices but also provided the direction of developing this kind of materials in the future.