Molecular Design of Highly Efficient Thermally Activated Delayed Fluorescence Hosts for Blue Phosphorescent and Fluorescent Organic Light-Emitting Diodes

Abstract

Recently, thermally activated delayed fluorescence (TADF) materials have become the most promising hosts for realizing high-performance phosphorescent and fluorescent organic light-emitting diodes (OLEDs) because of their ability to upconvert triplet excitons to singlet excitons. However, despite a few TADF hosts having been introduced for low energy phosphorescent and fluorescent dopants, developing host materials with TADF properties for blue phosphorescent and TADF OLEDs is still a great challenge to date. In this study, bipolar hosts exhibiting TADF behavior and high triplet energy, consisting of the carbazole group as the donor, diphenylsulphone moiety as the acceptor, and m-bitolyl as the π-conjugated bridge, are synthesized and applied for the first time to blue devices. The ΔE ST value of the TADF host is tuned via the introduction of a cyano group in the carbazole moiety due to the increase of the LE contribution in the CT excited state. Detailed photophysical studies confirm the efficient TADF properties of bipolar hosts. The blue phosphorescent and TADF devices using BT-01 as the host give external quantum efficiencies of 31.8% and 25.5%, respectively. The blue devices based on the BT-01 host exhibit superior electroluminescence performance and more reduced efficiency roll-off compared with those hosted by BT-02, ascribed to the faster reverse intersystem crossing process on the BT-01 host. These excellent results manifest that the use of the bipolar host with TADF behavior is a promising approach for the realization of highly efficient blue phosphorescent and TADF devices in the future.

High Efficiency Deep‐Blue Phosphorescent Organic Light‐Emitting Diodes with CIE x, y (≤ 0.15) and Low Efficiency Roll‐Off by Employing a High Triplet Energy Bipolar Host Material

“…Whereas the two weak absorption peaks at 328 and 341 nm were assigned as characteristic peak of Cz‐centered n–π* transition. Due to steric hindrance of 2,2′‐dimethylbiphenyl system, there was no CT absorption peak observed at longer wavelength region in the UV–vis absorption spectra . The estimated optical E g of m‐ CBPPO was 3.5 eV (Table ).…”

“…The EL maximum of 462 nm without any vibrational shoulder peak and a narrow FWHM of 52 nm was observed from the EL spectrum showing high color purity. From the previous reports, it is observed that most blue‐emitting Ir(III) complexes possess vibrational shoulder peak due to larger oscillator strength at longer wavelengths, resulting in a poor color purity in deep‐blue PHOLEDs. From the observed EL spectra, there was no identifiable emission from the host m‐ CBPPO, suggesting the efficient energy transfer from m‐ CBPPO to Ir(cb) 3 .…”

“…Besides, the electron withdrawing nature of Ph 2 P=O assists to maintain the charge balance and enhance the recombination efficiency in EML . The incorporation of 2,2′‐dimethylbiphenyl as π‐conjugated bridge in the host material m‐ CBPPO leads to steric hindrance as well as increase in the twist angle, which are the important factors for high E T of 3.02 eV . To this end, we presume that high E T for m‐ CBPPO is because of their electronic interaction between D and A units are depressed by the steric hindrance of two methyl groups in the core structure.…”

“…Nevertheless, they result in insufficient charge balance and exciton confinement in the emitting layer imparting poor device performance due to higher lowest unoccupied molecular orbital energy (LUMO) level of −2.4 eV for mCP and deeper highest occupied molecular orbital energy (HOMO) level of −7.0 eV for silane‐based host materials . Recently, thermally activated delayed fluorescence (TADF)‐based organic molecules as hosts have attracted great attention owing to their potential of achieving 100% internal quantum efficiency . In particular, Lin et al reported an impressive external quantum efficiency (EQE) of 31.8% with Commission International de l'Eclairage (CIE) 1931 coordinates of (0.16, 0.36) using TADF host with blue Ir(III) complexes …”

“…Recently, thermally activated delayed fluorescence (TADF)‐based organic molecules as hosts have attracted great attention owing to their potential of achieving 100% internal quantum efficiency . In particular, Lin et al reported an impressive external quantum efficiency (EQE) of 31.8% with Commission International de l'Eclairage (CIE) 1931 coordinates of (0.16, 0.36) using TADF host with blue Ir(III) complexes …”