Abstract
A series of phosphine oxide hosts, 4,6-bis(diphenylphosphoryl) dibenzothiophene (DBTDPO) and 4- diphenylphosphoryldibenzothiophene (DBTSPO), and electron transporting materials (ETM), 2-(diphenylphosphoryl)dibenzothiophene sulfone (2DBSOSPO), 3-(diphenylphosphoryl)dibenzothiophene sulfone (3DBSOSPO) and 4-(diphenylphosphoryl)dibenzothiophene sulfone (4DBSOSPO) were developed to support blue thermally activated delayed fluorescence (TADF) devices with high performance through optimizing intralayer and interlayer compatibility of emissive layers. On the basis of the triplet energy of ~3.0 eV for the hosts and ETMs, excitons can be effectively confined on DMAC-DPS. Compared to DBTSPO, DBTDPO can support the excellent distribution uniformity to blue TADF dye bis[4-(9,9-dimethyl–9,10-dihydroacridine) phenyl] sulfone (DMAC-DPS), owing to their configuration similarity; while 3DBSOSPO and 4DBSOSPO are superior in compatibility with the hosts due to the similar molecular polarity or configuration. Through adjusting the molecular configuration, the electrical performance of ETMs can be feasibly tuned, including the excellent electron mobility (μe) by the order of 10−3 cm2 V−1 s−1. As the result, DBTDPO and 4DBSOSPO endowed their four-layer blue TADF devices with the maximum current efficiency of 33.5 cd A−1 and the maximum external quantum efficiency more than 17%, which are impressive among the best blue TADF devices. It is showed that intralayer compatibility determines the maximum efficiencies, while interlayer compatibility influences efficiency stability.
Highlights
Benzene[35] and bis[2-[di(phenyl)phosphino]-phenyl]ether oxide (DPEPO)[36], were adopted to realize the efficient red, green, blue and white thermally activated delayed fluorescence (TADF) devices
Three prerequisites should be satisfied for high-performance blue TADF electron transporting materials (ETM): (i) T1 value approaching to 3.0 eV for exciton confinement; (ii) high electron affinity and mobility for charge flux balance and (iii) good compatibility with EML for suppressing interfacial quenching, viz. interlayer compatibility between EML and electron transporting layer (ETL)
Their T1 values are 2.98 and 2.90 eV to confine triplet excitons on DMADDPS. Their highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels are − 6.0 and − 2.5 eV. Even through their dipole moments are similar as ~4 Debye, symmetrically V-shaped DBTDPO is predominant to asymmetric DBTSPO in compatibility with DMAC-DPS, according to the “similarity-intermiscibility” theory
Summary
Benzene (mCP)[35] and bis[2-[di(phenyl)phosphino]-phenyl]ether oxide (DPEPO)[36], were adopted to realize the efficient red, green, blue and white TADF devices. Ishimatsu and Adachi et al demonstrated the reduction of photoluminescence quantum yield (PLQY) for TADF dyes in polar solvents, due to the increased nonradiative decay of S1 state, which reflected the significant influence of the interactions between hosts and TADF dopants on luminescence performance of their solid solutions[42]. It is showed that the requirements on the properties of host and ETM are similar but rather difficult to be qualified simultaneously, especially for intralayer and interlayer compatibility of EML In this case, it is imperative to figure out which one among these requirements is predominant and prioritized for molecular design. This work indicated the significance of intralayer and interlayer compatibility optimization and verified a feasible strategy of high-performance blue TADF diodes
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