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Issue 24, 2020
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A hybridized local and charge transfer excited state for solution-processed non-doped green electroluminescence based on oligo(p-phenyleneethynylene)

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Abstract

We herein report a new highly efficient green emissive hot-exciton molecule, 1,4-bis((4′-diphenylamino-3-cyano-[1,1′-biphenyl]-4-yl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-TPA-CNPE) that consists of an extended D′–π′–A–π–D–π–A–π′–D′ molecular π-system with diphenylamino end units (D′) and ethynylene/phenylene spacers (π/π′). The new molecule exhibits high photoluminescence (PL) quantum efficiencies (ΦPL = 0.95 (solution) and 0.45 (spin-coated neat thin-film)), and a strong PL solvatochromic behavior revealing significant changes in excited state energies/characteristics (locally excited (LE) → hybridized local and charge transfer (HLCT) → charge-transfer (CT)) depending on solvent polarity. Highly efficient (radiative exciton yield (ηr) = 50–59% ≫ 25%) green-emitting OLEDs were fabricated in a conventional device architecture by employing (non-)doped thin-films reaching a maximum current efficiency (CEmax) of 12.0 cd A−1 and a maximum external quantum efficiency (EQEmax) of 5.5%. The emission profile of the non-doped OLED has CIE 1976 (u′, v′) chromaticity coordinates of (0.10, 0.55) corresponding to a night vision imaging system (NVIS) compatible Green A region. 2EHO-TPA-CNPE-based OLED devices of industrial relevance were also fabricated by ink-jet printing the emissive layer and by fabricating an inverted architecture, which possessed respectable device performances of 2.4–6.1 cd A−1. The solid-state solvation effect in OLED devices yields HLCT electronic behavior resulting in high ηr's, which is confirmed by TDDFT to originate from energetically/spatially favorable reverse intersystem crossings (RISCs) (T2/3 → S1). As a unique observation, delayed fluorescence due to this RISC was evident in the PL decay lifetime measurement with a ns-scale lifetime of ∼10 ns. These results clearly allow a better understanding of the structure–photophysical property–electroluminescence relationships in this new class of oligo(p-phenyleneethynylene)-based hot-exciton molecules, and it could open up new opportunities for high-performance solution-processed optoelectronic/sensing applications.

Graphical abstract: A hybridized local and charge transfer excited state for solution-processed non-doped green electroluminescence based on oligo(p-phenyleneethynylene)

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Supplementary files

Article information


Submitted
11 Mar 2020
Accepted
21 Apr 2020
First published
22 Apr 2020

J. Mater. Chem. C, 2020,8, 8047-8060
Article type
Paper

A hybridized local and charge transfer excited state for solution-processed non-doped green electroluminescence based on oligo(p-phenyleneethynylene)

H. Usta, D. Alimli, R. Ozdemir, E. Tekin, F. Alkan, R. Kacar, A. G. Altas, S. Dabak, A. G. Gürek, E. Mutlugun, A. F. Yazici and A. Can, J. Mater. Chem. C, 2020, 8, 8047
DOI: 10.1039/D0TC01266A

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