Highly efficient exciplex organic light-emitting diodes incorporating a heptazine derivative as an electron acceptor

Article abstract of DOI:10.1039/c4cc01590h

Highly efficient exciplex systems incorporating a heptazine derivative (HAP-3MF) as an electron acceptor and 1,3-di(9H-carbazol-9-yl)benzene (mCP) as an electron donor are developed. An organic light-emitting diode containing 8 wt% HAP-3MF:mCP as an emitting layer exhibits a maximum external quantum efficiency of 11.3%. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01590h

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Highly efficient exciplex organic light-emitting
diodes incorporating a heptazine derivative as an
electron acceptor†
Cite this: Chem. Commun., 2014,
50, 6174
Received 3rd March 2014,
Accepted 15th April 2014
Jie Li,a Hiroko Nomura,a Hiroshi Miyazakiab and Chihaya Adachi*ac
DOI: 10.1039/c4cc01590h
www.rsc.org/chemcomm
Highly efficient exciplex systems incorporating a heptazine derivative and acceptors.6 Over the past two decades, interest in exciplexes
(HAP-3MF) as an electron acceptor and 1,3-di(9H-carbazol-9-yl)- formed at solid interfaces and in physically blended structures has
benzene (mCP) as an electron donor are developed. An organic expanded.7 With judicious molecule selection, exciplex-based organic
light-emitting diode containing 8 wt% HAP-3MF:mCP as an emitting light-emitting diodes (ExOLEDs) with EQEs as high as 10.0% have
layer exhibits a maximum external quantum efficiency of 11.3%.
been obtained.8 However, it is still challenging to explore highly
efficient exciplex systems because exciplex formation is usually
Organic light-emitting diodes (OLEDs) have attracted tremendous accompanied with a large red-shift of emission spectra, which tends
interest because of their promise in optoelectronic devices, especially to decrease photoluminescence (PL) quantum efficiency (PLQE) as
flat-panel displays and general lighting.1 Phosphorescent OLEDs well as electroluminescence (EL) performance.
containing transition metal complexes can exhibit very high external
In this communication, we report an exciplex system with a
quantum efficiencies (EQEs) as a result of effective use of both singlet blended structure containing 1,3-di(9H-carbazol-9-yl)benzene (mCP)
and triplet excitons.2 Although OLEDs based on conventional fluor- as an electron donor, and 2,5,8-tris(4-fluoro-3-methylphenyl)-
escent materials typically show a limited EQE of 5% because only 1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3MF) as an electron accep-
singlet excitons can be harvested under electrical excitation,3 two new tor. The molecular structures of mCP and HAP-3MF are depicted in
tactics to harvest triplet excitons through triplet–triplet annihilation Fig. 1. mCP is a widely used host molecule possessing two electron-
(TTA)4 and thermally activated delayed fluorescence (TADF)5 have donating carbazole moieties.9 HAP-3MF, which is composed of a
been exploited to markedly enhance the EQE of fluorescent OLEDs. heptazine core and three 2-fluorotoluene groups, was designed and
In particular, TADF molecules have attracted much more interest synthesized as an electron acceptor. 2-Fluorotoluene groups were
because almost 100% internal quantum efficiencies can be achieved. introduced to increase the solubility and maintain the electron-
The crucial design strategy of TADF molecules is to possess a small withdrawing ability of HAP-3MF. The synthesis of HAP-3MF is
energy gap (DEst) between the lowest singlet (S1) and triplet (T1) described in the ESI.† We chose a heptazine derivative as an
excited states to allow efficient intersystem crossing (ISC). An elabo- electron acceptor in the exciplex system because it exhibits a
rate molecular design is necessary to obtain a small DEst because it is number of intriguing thermal,10 optical,5g,11 and electronic12 proper-
proportional to the exchange energy between the highest occupied ties. Although heptazine derivatives are promising candidates for
molecular orbital (HOMO) and the lowest unoccupied molecular OLEDs, no exciplex system containing a heptazine derivative has
orbital (LUMO) of electron donating and accepting moieties, respec- been reported. Herein, we examine the photophysical characteristics
tively. Alternatively, small DEst can be readily realized by exciplex and EL performance of an HAP-3MF:mCP exciplex system.
formation via intermolecular charge transfer between electron donors
a Department of Chemistry and Biochemistry, and Center for Organic Photonics and
Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi,
Fukuoka 819-0395, Japan. E-mail: adachi@cstf.kyushu-u.ac.jp
b Functional Materials Laboratory, Nippon Steel and Sumikin Chemical Co., Ltd,
46–80 Nakabaru, Sakinohama, Tobata, Kitakyushu, Fukuoka 804-8503, Japan
c International Institute for Carbon Neutral Energy Research (WPI-I2CNER),
Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
† Electronic supplementary information (ESI) available: Synthesis scheme,
experimental details and additional photophysical data. See DOI: 10.1039/
Fig. 1 Molecular structures of mCP and HAP-3MF.
c4cc01590h
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The HOMO and LUMO levels of HAP-3MF are ꢀ6.0 and
ꢀ3.4 eV, which were estimated by ultraviolet photoelectron spectro-
scopy and subtracting the energy gap from the HOMO level,
respectively (Fig. S1, ESI†). The thermal stability of HAP-3MF was
measured by thermogravimetric-differential thermal analysis; it
exhibited fairly high thermal stability with an initial decomposition
temperature of 413.5 1C (Fig. S2, ESI†). The ultraviolet-visible
(UV-vis) absorption and PL spectra of mCP, HAP-3MF, and
25 wt% HAP-3MF:mCP films are presented in Fig. 2a. Absorption
peaks at 329 and 342 nm originate from mCP, and the dominant
absorption peak of HAP-3MF is centered at 334 nm. The UV-vis
Fig. 3 Transient PL decay properties of an 8 wt% HAP-3MF:mCP exciplex
system. (a) Prompt and delayed PL spectra under vacuum conditions at
300 K. (b) Prompt and delayed PL spectra under vacuum conditions at
50 K. (c) Transient PL decay in air and under vacuum conditions at 300 K.
absorption spectrum of a 25 wt% HAP-3MF:mCP blend film is Considering the exciplex mechanism, the transient PL decay proper-
similar to that of an mCP one. No new absorption band appeared ties of an 8 wt% HAP-3MF:mCP exciplex system were characterized
at longer wavelengths, indicating the absence of charge transfer (Fig. 3). The well-overlapped prompt and delayed emission spectra
between mCP and HAP-3MF. Both mCP and HAP-3MF solid films at 300 K confirm that all photons are generated from the same
show well-resolved PL spectra resulting from p–p* or n–p* emis- excited state (Fig. 3a). A considerable overlap was also observed
sions.13 In contrast, the PL of the blend film exhibits typical between the prompt and delayed spectra at 50 K (Fig. 3b), indicating
exciplex characteristics with a peak at around 550 nm, which is that the exciplex system possesses a fairly small DEst. Fig. 3c shows
red-shifted compared with those of mCP and HAP-3MF films. the transient PL decay of an 8 wt% HAP-3MF:mCP film in air and
Interestingly, the exciplex system shows a rather high PLQE of under vacuum conditions at 300 K. The transient decay can be
55.7%, which is much higher than that of a HAP-3MF film with a divided into prompt and delayed components. The prompt compo-
PLQE of 12.7%. This is unusual behavior compared with conven- nent with a transient lifetime of 184 ns can be ascribed to conven-
tional exciplex systems, which usually possess PLQEs lower than tional fluorescence-based exciplex emission, while the delayed ones
that of each single layer. Furthermore, the red-shift between the PL with transient lifetimes of 1.7 and 5.7 ms are generated from the
peaks of HAP-3MF (526 nm) and a 25 wt% HAP-3MF:mCP blend delayed exciplex emission after up-conversion of triplet exciplex
film (550 nm) is just 24 nm. The photon energy of the exciplex is excitons into the singlet excited state as discussed in a previous
therefore only slightly lower than that of HAP-3MF. The similar report.6 While we cannot fully characterize the origin of the multiple
HOMO levels of mCP and HAP-3MF suppress the red-shift of decay components, it should be caused by the inhomogeneous
exciplex emission. Moreover, the half width at half maximum distribution of HAP-3MF molecules in the mCP host matrix. Here,
(HWHM) of the exciplex emission is narrow compared with those we note that the transient PL decay curves in air and under vacuum
observed for other conventional exciplex systems.6–8 The HWHM of conditions overlap well, suggesting that oxygen has almost no
83 nm of the blend film is only slightly larger than that of HAP-3MF influence on the PL characteristics, probably because of the rigid
alone (66 nm). These unique PL characteristics could be ascribed to and tightly packed environment of an mCP host layer. As a
the rigid and relatively planar geometries of HAP-3MF and mCP consequence, the PLQE of an 8 wt% HAP-3MF:mCP exciplex system
molecules, which tend to result in tight molecular packing of in air is 65.8%, which is nearly equal to that in nitrogen (66.1%). We
p-conjugated moieties and very strong intermolecular interactions.7 can approximately estimate the PLQE of the prompt component,
To obtain an optimized exciplex system, HAP-3MF:mCP blend Fprompt = 18.6%, while that of the delayed component, Fdelayed
=
films with various weight ratios were fabricated and characterized. 47.5% in nitrogen. The transient PL properties of 25 wt% and
The UV-vis absorption and PL spectra of these films are depicted in 50 wt% HAP-3MF:mCP exciplex systems are analogous to that of
Fig. 2b, and the PLQEs of HAP-3MF:mCP exciplex systems with 8 wt% HAP-3MF:mCP (Fig. S3 and S4, ESI†).
various weight ratios in air and nitrogen are summarized in Table
Using the exciplex-based TADF characteristics, ExOLEDs
S1, ESI.† We found that the 8 wt% HAP-3MF:mCP exciplex system containing HAP-3MF:mCP with various weight ratios were
exhibits a remarkably high PLQE of 66.1% and a rather small PL fabricated. The device structure was ITO/a-NPD (30 nm)/TCTA
spectral red-shift of 12 nm compared with that of an HAP-3MF film. (10 nm)/HAP-3MF:mCP (20 nm)/DPEPO (10 nm)/TPBI (40 nm)/
LiF (0.8 nm)/Al (100 nm), where ITO is indium tin oxide, a-NPD
is N,N0-di(naphthalen-1-yl)-N,N0-diphenylbenzidine used as a
hole transport layer, TPBI is 1,3,5-tris(N-phenylbenzimidazole-
2-yl)benzene used as an electron transport layer, and LiF and Al
act as the cathode. Thin tris(4-(9H-carbazol-9-yl)phenyl)amine
(TCTA) and bis(2-(diphenylphosphino)phenyl) ether oxide
(DPEPO) layers were inserted to block electrons coming from
the cathode and holes from the anode, respectively, and
simultaneously confine the excitons in the emitting layer. The
device structure and the energy diagram are presented in
Fig. 2 (a) UV-vis absorption and PL spectra of mCP, HAP-3MF, and 25 wt%
Fig. 4a. In accordance with PLQE trends, the ExOLED contain-
ing 8 wt% HAP-3MF:mCP exhibited the best EL performance.
HAP-3MF:mCP in solid films at 300 K. (b) UV-vis absorption and PL spectra
of HAP-3MF:mCP blend films with various weight ratios at 300 K.
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efficient exciton up-conversion and a high PLQE of 66.1%. Using
this exciplex system as an emitting layer in an ExOLED, a rather
high EQE of 11.3% was obtained. These findings are of funda-
mental interest for the development of highly efficient OLEDs
based on exciplex systems. Through elaborate molecular design
and careful selection of electron donors and acceptors, we believe
that ExOLEDs with enhanced efficiencies can be expected.
This work was supported in part by the Funding Program for
World-Leading Innovative R&D on Science and Technology
(FIRST) and the International Institute for Carbon Neutral
Energy Research (WPI-I2CNER) sponsored by MEXT.
Notes and references
Fig. 4 EL characteristics of an ExOLED containing an 8 wt% HAP-
3MF:mCP blend film as an emitting layer. (a) Device structure and energy
level diagrams (values are in eV). (b) EL spectra recorded at various current
densities. (c) Current density–voltage–luminance (J–V–L) characteristics.
(d) EQE as a function of current density.
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In summary, we designed a highly efficient 8 wt% HAP-
3MF:mCP exciplex system with a very small DEst, resulting in
6176 | Chem. Commun., 2014, 50, 6174--6176
This journal is ©The Royal Society of Chemistry 2014