High efficiency thermally activated delayed fluorescence based on 1,3,5-tris(4-(diphenylamino)phenyl)-2,4,6-tricyanobenzene

Article abstract of DOI:10.1039/c5cc00511f

A trigonal donor-acceptor molecule of 1,3,5-tris(4-(diphenylamino)phenyl)-2,4,6-tricyanobenzene (3DPA3CN) was synthesized to exhibit efficient thermally activated delayed fluorescence. By doping 3DPA3CN into a wide energy gap host, the film had a photoluminescence quantum efficiency of 100% with the reverse intersystem crossing efficiency of 100%. An OLED including the emitter exhibited a very high external quantum efficiency (η_EQE) of 21.4%.

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High efficiency thermally activated delayed
fluorescence based on 1,3,5-tris(4-(diphenylamino)
phenyl)-2,4,6-tricyanobenzene
Cite this: DOI: 10.1039/x0xx00000x
Masatsugu Tanedaa, Katsuyuki Shizua, Hiroyuki Tanakaa, and Chihaya Adachia,b,c,*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
trigonal donor-acceptor molecule of 1,3,5-tris(4- materials. This is known as thermally activated delayed fluorescence
(diphenylamino)phenyl)-2,4,6-tricyanobenzene (3DPA3CN) (TADF) and is evident in thirdꢀgeneration luminescent materials.
was synthesized to exhibit efficient thermally activated TADF can realize an ultimate electroluminescence (EL) efficiency
delayed fluorescence. By doping 3DPA3CN into a wide through efficient upconversion from the lowest triplet excited state
energy gap host, the film had a photoluminescence quantum (T1) to the lowest singlet excited state (S1) through reverse
efficiency of 100% with the reverse intersystem crossing intersystem crossing (RISC).3 Our recent study showed that TADF
efficiency of 100%. An OLED including the emitter exhibited strongly depends on the distribution and separation of the highest
very high external quantum efficiency (
η
EQE) of 21.4%.
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) in a single molecule. The preparation of a
Since Tang and VanSlyke reported the first, practical multilayered
organic electroluminescent (EL) device in 1987,1 organic lightꢀ
emitting diodes (OLEDs) have been studied extensively. These
studies have resulted in the commercialization of technology for flatꢀ
panel displays. One of the most critical issues in improving OLED
performance is the enhancement of the external quantum efficiency.
sufficiently small energy gap between S1 and T1 (ꢀEST) enables
efficient upconversion of triplet excitons from T1 to S1, leading to
nearly 100% singlet exciton formation (ηST = 1). We note that
because the separation of the HOMO and the LUMO reduces the
radiative transition probability, a small overlap of the HOMO and
LUMO is necessary to ensure both a small ꢀEST and a large ΦPL. It
is also important to prevent vibrational deactivation processes that
The external quantum efficiency,
ηEQE, is given by γηSTΦPLΦout,
where is charge carrier balance, ηST is efficiency of exciton
γ
compete with the radiative decay.
generation by carrier recombination, ΦPL is the photoluminescence
(PL) quantum yield of the emissive layer, and Φout is the light outꢀ
coupling efficiency. In particular, for the enhancement of emissive
excitons, active management of the triplet excitons is the most
important issue because singlets and triplets form in a ratio of 1:3.2
After the development of fluorescent materials, room
temperature phosphorescent materials attracted much attention
because both singlet and triplet excitons can be harvested for light
emission as phosphorescence. Heavy metal complexes such as
tris(phenylpyridine)iridium and their derivatives can convert the
singlet excitons into triplet excitons through intersystem crossing
(ISC) and the triplet excitons then decay radiatively. Although such
phosphorescent metal complexes can harvest all excitons for light
emission (ηST and ΦPL = 1), stable complexes with emission in the
deep blue region of the spectrum are few. Additionally, the precious
metals used in these complexes are expensive.
Fig. 1 The molecular structure and HOMO and LUMO of the 3DPA3CN calculated
at the B3LYP/6-31G level of theory.
In this study, we designed the trigonal molecule 1,3,5ꢀtris(4ꢀ
(diphenylamino)phenyl)ꢀ2,4,6ꢀtricyanobenzene (3DPA3CN). We
expect that all six substitutions of the benzene ring with two
different types of alternating substituents results in a decrease of
vibrational deactivation because of the steric hindrance. Density
functional theory (DFT) calculations4 suggested that the introduction
More recently, we proposed an alternative emissive mechanism
instead of the conventional fluorescent and phosphorescent
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of a phenylene linker bridging the diphenylamino groups and decay time of 550
ꢁs and had an efficiency of 10.3%. The
tricyanobenzene results in the separation of the HOMO and LUMO details of the transient decay characteristics are supplied in ESI.
of 3DPA3CN while keeping a moderate overlap between them (Fig. Because both spectra were nearly identical, the delayed
1).
component was attributed to TADF. From these PL
We expect that this planar structure would induce a horizontal characteristics, we estimated the numerical values of ISC rate
molecular orientation. In our previous study, we showed that the van (kISC)=1.66 x 107, RISC rate (kRISC) =2.03 x 103, prompt
der Waals’ interaction between a substrate surface and a molecule fluorescence rate (kp) =1.61 x 108, TADF rate (kd) =1.83 x 103,
with a planar geometry induces a horizontal molecular orientation. and nonradiative decay rate (knr) < 105
Additionally, molecules with a donorꢀacceptor structure induce
intermolecular interactions that can also influence the molecular
orientation.5 When the orientation of the transition dipole moment of
the emitter molecules is parallel to a substrate, we can enhance the
Φout of an OLED significantly.6 Although the ηEQE of OLEDs has
been limited by the rather low outcoupling efficiency, a horizontal
.
orientation of the emitters should enhance the
Φ
out by up to 45%.7
In this work, we synthesized 3DPA3CN and investigated the
fundamental photophysical and OLED characteristics. 1,3,5ꢀtris(4ꢀ
(diphenylamino)phenyl)ꢀ2,4,6ꢀtrifluorobenzene was synthesized as a
precursor of 3DPA3CN by Suzuki coupling of 1,3,5ꢀtribromoꢀ2,4,6ꢀ
trifluorobenzene10
and
2ꢀ(phenylyl)ꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀ
dioxaborolane. The precursor was treated with KCN in DMSO to
obtain 3DPA3CN. The product was purified by sublimation (see
further detail of the synthesis of 3DPA3CN in the Supporting
Information).
Fig. 3 The transient PL decay profile of a 6wt% 3DPA3CN:DPEPO film measured at
300 K (solid line) and 100 K (broken line). The inset shows the PL spectra of the co-
deposited film.
The ultravioletꢀvisible absorption and PL spectra of 3DPA3CN
in toluene are shown in Fig. 2. The broad absorption peak observed
at around 420 nm can be assigned to the charge transfer (CT)
transition from the diphenylamino groups to the tricyanobenzene
moiety. Upon irradiation with ultraviolet light, 3DPA3CN in
degassed toluene emitted green light with a peak wavelength of 506
nm and ΦPL of 82%. When 3DPA3CN (6 wt%) was coꢀdeposited
with bis(2ꢀ(diphenylphosphino)phenyl)ether oxide (DPEPO)11 on a
quartz substrate, the doped thin film had a green emission with a
ΦPL of 100%, indicating complete suppression of nonꢀradiative
decay. The peak wavelength of the spectrum was 533 nm. The
To evaluate ꢀEST quantitatively, the temperature dependence of
RISC was measured. An Arrhenius plot was used to determine the
activation energy of the RISC. We note that it was rather difficult to
estimate the gap from the difference of fluorescence and
phosphorescence spectra, since 3DPA3CN showed no clear
phosphorescence even at low temperature. We note that the
activation energy would be slightly higher than the
∆EST.The rate
constant of the RISC (kRISC) can be estimated from experimentally
determined rate constants and the ΦPL values of the prompt and
delayed components at each temperature using13
ꢀꢆꢀd ꢇd
ꢀꢁꢂꢃꢄ
ꢅ
Eq. (1)
ꢀisc ꢇp
where kp and kd are the rate constants of the prompt and delayed
fluorescence components, respectively, kISC is the rate constant of
the transition of excitons from the T1 to the S1 state, and Φp and Φd
are the PL quantum yields of the prompt and delayed components,
respectively. Figure 4 shows kRISC calculated using Equation (1)
plotted against 1/T for T = 200–300 K. It was assumed that kISC was
independent of temperature. The value of
plot was 103 meV. Although the value of
ꢀ
EST from the Arrhenius
Fig. 2 The absorption (solid line) and emission (broken line) spectra of 3DPA3CN in
ꢀ
EST is not small, ΦPL of
degassed toluene.
the 3DPA3CN doped into DPEPO film was 100%, indicating that
the trigonal structure of 3DPA3CN suppresses nonradiative
vibrational losses. We note that an mCP host also provided 100% PL
quantum efficiency.
bathochromic shift is likely to be caused by the polarity of
DPEPO.12 Transient PL characteristics of the doped thin film
(Fig. 3) showed both prompt and delayed emissions at 300 K.
The prompt component had a transient decay time of 6.2 ns
with an efficiency of 89.7% and the delayed component had a
2 | J. Name., 2012, 00, 1-3
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with a host material with a low concentration cannot be determined
because there is not sufficient detection sensitivity, and therefore
ellipsometry is usually unsuitable. However, in the case of the
6wt%ꢀ3DPA3CN:DPEPOꢀdoped film, the strong absorption band of
3DPA3CN observed at around 420 nm was completely isolated from
the absorption spectrum of DPEPO. This absorption band was used
to detect the orientation of 3DPA3CN using ellipsometry. As shown
in Fig. 5b, 3DPA3CN had a tendency to orient towards the
horizontal in the DPEPO host layer. The average angle between the
molecules and the substrate was confirmed to be 30.8° with the
numerical values of ke=0.0106 and k0=0.0350 at the wavelength
of 432 nm
To confirm that high
orientation of 3DPA3CN would lead to a high
OLED with the structure of indium tin oxide (ITO)ꢀcoated glass/
N,N’ꢀdiꢀ[(1ꢀnaphthalenyl)ꢀN,N’ꢀdiphenyl]ꢀ(1,1’ꢀbiphenyl)ꢀ4,4’ꢀ
.
Φ
PL, a small
ꢀ
EST and a slight horizontal
EQE, we prepared an
η
Fig. 4 An Arrhenius plot of kRISC where kISC was set to 4 × 10−7 s−1. The straight line
fit (least-squares regression) was used to determine the activation energy. The
evaluated ꢀEST was 103 meV.
diamine
(α−NPD)/ 3,3’ꢀbis(Nꢀcarbazole)ꢀ1,1’ꢀbiphenyl (mCBP)/
To investigate the molecular orientation in thin films, the optical
anisotropy was measured using variable angle spectroscopic
ellipsometry (VASE).8,9 If the molecular orientation is anisotropic,
the optical properties parallel and perpendicular to the substrate will
be different, leading to large differences in the refractive indices and
extinction coefficients (k) in the parallel (ordinary) and
3DPA3CN (6 wt%) in DPEPO/1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzimidazol
ꢀ2ꢀyl) benzene (TPBi)/ LiF/ Al. In this architecture, αꢀNPD is the
hole transport layer, mCBP is the electron blocking layer,
3DPA3CN in DPEPO is the emissive layer, TPBi is the electron
transport layer, LiF is the electron injection layer, and Al is the
cathode. As shown in Fig. 6, a maximum ηEQE of 21.4 ± 3% at the
current density J = 0.001 mA/cm2 was obtained. We believe that the
horizontal orientation of 3DPA3CN in an emissive layer would
perpendicular (extraordinary) directions. Figure
5 shows the
anisotropy of the extinction coefficients obtained using an analysis
of the ellipsometric parameters from 245 to 1000 nm at seven
different incident angles. To determine the molecular orientation, an
orientation order parameter S9,14 was calculated using
contribute to enhance the Φout
.
ꢓ
ꢗꢘꢙꢗꢚ
ꢌ ꢍ
ꢈ ꢅ ꢉꢊ ꢋ ꢊ ꢎꢏꢐ ꢒꢑ ꢋ ꢔꢕ ꢅ ꢗ
ꢖ
ꢑ
Eq. (2)
ꢘꢛꢑꢗꢚ
where <…> is the ensemble average,
θ
is the angle between the
molecular πꢀplane and the direction perpendicular to the substrate
surface, and ko and ke are the ordinary and extraordinary extinction
coefficients at the peak wavelength. S = −0.5 if the molecules are
completely parallel to the surface and S = 0 if they are randomly
oriented. The ellipsometric analysis showed that 3DPA3CN was
oriented horizontally in the amorphous films and the average angle
between the molecules and the substrate was 29.5° with the
numerical values of ke=0.1304 and k0=0.4710 at the wavelength
of 432 nm (Fig. 5a). The orientation of the guest materials doped
Fig. 6 The dependence of the external electroluminescence quantum
efficiency on the current density. The inset shows the EL spectrum.
In conclusion, we showed that 3DPA3CN has excellent
photophysical characteristics of a small ꢀEST of 103 meV and a high
ΦPL of 100% in a 6wt%ꢀ3DPA3CNꢀdoped DPEPO film. The
preferential molecular orientation in both neat and doped films
resulted in a fairly high ηEQE of 21.4%. We believe that further
optimization of the device parameters will improve the ηEQE
.
This research was supported by the Japan Society for the
Promotion of Science (JSPS) through the “Funding Program for
WorldꢀLeading Innovative R&D on Science and Technology
(FIRST Program)”, initiated by the Council for Science and
Technology Policy (CSTP).
Fig. 5 Extinction coefficients (k) of (a) a neat 3DPA3CN film and (b) a 6wt%
3DPA3CN-doped DPEPO film in parallel (ordinary, solid line) and perpendicular
(extraordinary, broken line) directions. k was obtained from the analysis of the
ellipsometric parameters measured from 245 to 1000 nm or from 315 to 1000 nm
at seven different incident angles.
Notes and references
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DOI: 10.1039/C5CC00511F
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Center for Organic Photonics and Electronics Research (OPERA), 14 a) J. Y. Kim, D. Yokoyama, C. Adachi, J. Phys. Chem. C, 2012, 116
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Kyushu University, 744 Motooka, Nishi, Fukuoka 819ꢀ0395, Japan. 8699ꢀ8706; b) I. M. Ward, Structure and Properties of Oriented Polymers,
Eꢀmail: adachi@cstf.kyushuꢀu.ac.jp
Applied Science Publishers, London, 1975.
b
JST, ERATO, Adachi Molecular Exciton Engineering Project, c/o
OPERA, Kyushu University, 744 Motooka, Nishi, Fukuoka 819ꢀ0395,
Japan
c
International Institute for Carbon Neutral Energy Research (WPIꢀ
I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819ꢀ0395,
Japan
†
Electronic Supplementary Information (ESI) available: Experimental
and synthesis details, additional photophysical data, and supplementary
figures and tables. See DOI: 10.1039/c000000x/
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