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Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff.

ABSTRACT: Aromatic organic deep-blue emitters that exhibit thermally activated delayed fluorescence (TADF) can harvest all excitons in electrically generated singlets and triplets as light emission. However, blue TADF emitters generally have long exciton lifetimes, leading to severe efficiency decrease, i.e., rolloff, at high current density and luminance by exciton annihilations in organic light-emitting diodes (OLEDs). Here, we report a deep-blue TADF emitter employing simple molecular design, in which an activation energy as well as spin-orbit coupling between excited states with different spin multiplicities, were simultaneously controlled. An extremely fast exciton lifetime of 750?ns was realized in a donor-acceptor-type molecular structure without heavy metal elements. An OLED utilizing this TADF emitter displayed deep-blue electroluminescence (EL) with CIE chromaticity coordinates of (0.14, 0.18) and a high maximum EL quantum efficiency of 20.7%. Further, the high maximum efficiency were retained to be 20.2% and 17.4% even at high luminance.

SUBMITTER: Kim JU

PROVIDER: S-EPMC7156453 | biostudies-literature | 2020 Apr

REPOSITORIES: biostudies-literature

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Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff.

Kim Jong Uk JU Park In Seob IS Chan Chin-Yiu CY Tanaka Masaki M Tsuchiya Youichi Y Nakanotani Hajime H Adachi Chihaya C

Nature communications 20200414 1

Aromatic organic deep-blue emitters that exhibit thermally activated delayed fluorescence (TADF) can harvest all excitons in electrically generated singlets and triplets as light emission. However, blue TADF emitters generally have long exciton lifetimes, leading to severe efficiency decrease, i.e., rolloff, at high current density and luminance by exciton annihilations in organic light-emitting diodes (OLEDs). Here, we report a deep-blue TADF emitter employing simple molecular design, in which ...[more]

PMID: 32286281

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Project description:The similarity of thermally activated delayed fluorescence (TADF) dyes and their hosts as pure organic molecules makes hosts predominant in intermolecular interactions and crucial to exciton harvesting and utilization in TADF diodes. DPEPO is the most popular high-energy-gap blue TADF host with steric ortho-substituted diphenylphosphine oxide (DPPO) groups for intermolecular interaction suppression, but suffers from serious efficiency roll-off due to its weak electroactivity. On the contrary, para-substituted DPPO with small steric hindrance is superior in intramolecular electronic coupling. In this work, four constitutional isomers of DPEPO are constructed as diphenylether (DPE) with two diphenylphosphine oxide (DPPO) groups substituted at either the 2 or 4 position, namely 22'DPEPO (viz.DPEPO), 24DPEPO, 24'DPEPO and 44'DPEPO, respectively. On the basis of separation configuration, the steric effect and electroactivity of ortho- and para-substituted DPPOs are successfully integrated in 24'DPEPO, accompanied by remarkably reduced intermolecular interactions due to its unsymmetrical configuration. Compared to its congeners, 24'DPEPO has a rigid structure and locally excited states similar to 22'DPEPO for interaction suppression and improved charge mobility comparable to 44'DPEPO for charge flux balance. Significantly, by virtue of the predominant orientation effect of ortho-DPPO on the T1 location, its T1 state is extremely condensed onto a single phenyl, protected from intermolecular interactions by its remaining five phenyls at its maximum extent. Consequently, 24'DPEPO endowed its DMAC-DPS-based deep-blue devices with state-of-the-art performance, including high color purity with chromaticity coordinates of (0.16, 0.17), external quantum efficiency (EQE) beyond 20% and EQE roll-off as low as 32% at 1000 cd m-2. It is shown that the device performance of 24'DPEPO was far beyond simple integration of those of 22'DPEPO and 44'DPEPO, verifying the significance of host optimization.

Dataset Information

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff.

Publications

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff.

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