Efficient narrowband organic light-emitting devices based on multi-resonance TADF emitters with secondary donor

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been considered to be one of the most desirable materials for high-quality organic light emitting diodes (OLEDs) owing to their small fullwidth at half-maximum (FWHM) and outstanding device performance. However, there are still some inherent issues that restrict their further practical applications, such as slow reverse intersystem crossing (RISC) rate (kRISC), and severe intermolecular aggregation. Herein, three MR-TADF emitters, SPAC-tCzBN, SPBAC-tCzBN and o-SPAC-tCzBN, were designed and synthesized by introducing the secondary donor into tCzBN, which successfully realized the modification of the excited states and therefore achieved the enhanced RISC process. Furthermore, the rigid and steric molecular geometry greatly block the intermolecular forces between adjacent molecule and then reduce the concentration-induced quenching and spectral broadening. Consequently, OLEDs based on these emitters showed state-of-the-art external quantum efficiencies (EQEs) of 34.1%-36.5% with narrow FWHM of 24 nm - 28 nm. Meanwhile, the emission peaks and FWHM in EL spectra show much-promised less-sensitive to the doping concentration owing to suppressed intermolecular interactions. This work demonstrates a novel and effective MR-TADF molecular design strategy to realize high-quality OLEDs with extreme color purity and remarkable device efficiency.