Donor-modified asymmetric N/B/O multi-resonance TADF emitters for high-performance deep-blue OLEDs with the BT.2020 color gamut

Abstract

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials of polycyclic heteroaromatics are attractive narrowband emitters in wide-color-gamut organic light-emitting diodes (OLEDs). However, deep-blue MR-TADF emitters with CIE coordinates fulfilling the BT.2020 standard and high efficiency still remain a significant challenge. Herein, two novel emitters NBO-mSAF and NBO-pSAF were developed by incorporating an electron donor, 10H-spiro[acridine-9,9′-fluorene] (SAF), at the para-position of the oxygen atom and the para-position of the boron atom in the nitrogen/boron/oxygen (N/B/O) ternary doped asymmetric MR skeleton. With appropriate electron-donating capacity and rigid spiro-structure, SAF was selected as the donor so that the long-range charge transfer triplet state (³LRCT) is induced to accelerate the reverse intersystem crossing (RISC) process, while the ¹LRCT aligns higher than the short-range CT state (¹SRCT) of the N/B/O core to retain the MR characters. As a result, these optimized emitters exhibit deep-blue TADF with narrow spectra and a high RISC rate constant of 3.4 × 10⁵ s⁻¹. In hyperfluorescence OLEDs with a TADF emitter DMAC-DPS as the sensitizer and PPF as the host, NBO-mSAF and NBO-pSAF achieved maximum external quantum efficiencies (EQE_max) of 26.7% and 25.2%. Interestingly, improved performance was realized in a traditional device configuration with a single bipolar host 26DCzPPy but without any sensitizer, where NBO-mSAF realized a higher EQE_max of 29.5% and CIE (0.128, 0.114), and NBO-pSAF exhibited an EQE_max of 20.5% and CIE of (0.147, 0.048). Narrow full width at half maximum (FWHM) values of 26–28 nm were achieved in both devices. Among all the deep-blue N/B/O type MR-TADF emitters with CIEx ≤ 0.15 and CIEy ≤ 0.12 ever reported so far, NBO-mSAF exhibited a highest EQE_max of 29.5%, which is even higher than those obtained with sensitizers, while the CIEy = 0.048 of the NBO-pSAF device is close to the standard blue (0.046) according to BT.2020, with a decent EQE of 20%.