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Energy Conversion and Storage; Plasmonics and Optoelectronics
Breaking Efficiency Limit of Fluorescent OLEDs by Hybridized Local and Charge-Transfer Host Materials Lingfeng Chen, Shitong Zhang, Hui Li, Runfeng Chen, Lu Jin, Kai Yuan, Huanhuan Li, Ping Lu, Bing Yang, and Wei Huang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02138 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018
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The Journal of Physical Chemistry Letters
Breaking Efficiency Limit of Fluorescent OLEDs by Hybridized Local and Charge-Transfer Host Materials Lingfeng Chen†, Shitong Zhang‡, Hui Li†, Runfeng Chen†*, Lu Jin†, Kai Yuan†, Huanhuan Li†, Ping Lu‡, Bing Yang‡* and Wei Huang§* †
Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory
for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nan-jing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China. ‡
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun
130012, P.R. China. §
Key Laboratory of Flexible Electronics & Institute of Advanced Materials, National Synergistic
Innovation Center for Advanced Materials, Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, P.R. China.
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ABSTRACT. Hybridized local and charge-transfer (HLCT) states with “hot exciton” properties are effective in harvesting high-lying triplet excitons for electroluminescence in organic lightemitting diodes (OLEDs). Here, we propose a technique based on HLCT mechanism at the highlying excited states to develop HLCT-sensitized fluorescent (HLCT-SF) OLEDs using HLCT host molecules and metal-free fluorescent dopants for highly efficient OLEDs. A maximum external quantum efficiency (EQE) up to 6.3% and exciton utilizing efficiency (EUE) of 64% were achieved, apparently exceeding the upper limits of EQE (5%) and EUE (25%) in conventional fluorescent OLEDs. The HLCT-SF process via the long-range Förster resonance energy transfer from the singlet excited states of HLCT host to that of fluorescent guest is efficient in harvesting “hot triplet excitons” by efficient high-lying reverse intersystem crossing and the newly proposed HLCT-SF OLEDs represent an important advance in realizing high-performance OLEDs.
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KEYWORDS. Hybridized Local and Charge-Transfer, Sensitizing, fluorescent OLEDs, OLEDs.
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The Journal of Physical Chemistry Letters
Organic light-emitting diodes (OLEDs) have attracted tremendous attention due to their great potential in flat-panel displays and solid-state lighting technologies.1,
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Fluorescent OLED
(FOLED) materials, which are metal-free with tunable, ultra-fast and very stable emission, are of significant importance for commercial OLED applications.3 However, due to the fact that only 25% electronically excited excitons in singlet states are available for fluorescent emission, the device efficiencies of FOLEDs are usually much lower than that of phosphorescent OLEDs (PhOLEDs) based on the noble metal complex emitters.4 To harvest both singlet and triplet excited states for electroluminescence (EL) in metal-free organic optoelectronic molecules, various strategies have been proposed, including triplet-triplet annihilation,5 large singlet exciton formation,6 hybridized local and charge-transfer (HLCT) excited states,7 and thermally activated delayed fluorescence (TADF).8 Among them, it is no doubt that TADF should be the most famous and efficient one to achieve 100% internal quantum efficiency (IQE) without using phosphorescent metal complexes.9 Owing to the efficient reverse intersystem crossing (RISC) from the lowest triplet excited state (T1) to the lowest singlet excited one (S1), the 75% electronically generated triplet exciton on T1 can be transformed to singlet exciton on S1 for delayed fluorescence, leading to very high external quantum efficiency (EQE) of TADF OLEDs comparable to that of PhOLEDs.10 In addition to be used as emitters in OLEDs, TADF molecules were also found to be efficient as host materials of phosphorescent complexes owing to their inherent bipolar charge injection and transport features with donor-acceptor (D-A) molecular structures.11 More interestingly, when TADF molecules were used as host materials of conventional fluorescent dopant, singlet excitons of the TADF host, both directly generated ones and up-converted ones from the electronically generated triplets, can be transferred to the dopant singlets via Förster resonance energy transfer (FRET) and then decay radiatively to give the TADF-sensitized fluorescence.12 By preventing the
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unfavorable Dexter energy transfer (DET) from triplets of host to that of dopant using low doping concentration (
