Efficient delayed fluorescence via triplet-triplet annihilation for deep-blue electroluminescence

Article abstract of DOI:10.1039/c4cc01851f

Four 2-(styryl)triphenylene derivatives (TSs) were synthesized for deep-blue dopant materials. By using a pyrene-containing compound, DMPPP, as the host, the TS-doped devices exhibited significant delayed fluorescence via triplet-triplet annihilation, providing the highest quantum efficiency of 10.2% and a current efficiency of 12.3 cd A^-1. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01851f

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Efficient delayed fluorescence via triplet–triplet
annihilation for deep-blue electroluminescence†
Cite this: Chem. Commun., 2014,
50, 6869
P.-Y. Chou,*a H.-H. Chou,a Y.-H. Chen,a T.-H. Su,a C.-Y. Liao,a H.-W. Lin,b
W.-C. Lin,b H.-Y. Yen,a I.-C. Chena and C.-H. Cheng*a
Received 12th March 2014,
Accepted 9th May 2014
DOI: 10.1039/c4cc01851f
www.rsc.org/chemcomm
Four 2-(styryl)triphenylene derivatives (TSs) were synthesized for deep- electroluminescence and exhibit very high EL efficiencies with
blue dopant materials. By using a pyrene-containing compound, CIEy less than 0.14. Enhanced by delayed fluorescence (DF), the
DMPPP, as the host, the TS-doped devices exhibited significant delayed device doped with TSTA shows the highest external quantum
fluorescence via triplet–triplet annihilation, providing the highest efficiency (EQE) of 10.2%, a current efficiency of 12.3 cd Aꢀ1
,
quantum efficiency of 10.2% and a current efficiency of 12.3 cd Aꢀ1
.
and a luminance of 68 670 cd mꢀ2. The EQEs and current
efficiencies of these doped OLEDs increased with an increase in
Recently, it has been reported that the performance limitation of luminance, without compromising on the efficiency, owing to
fluorescent OLEDs can be overcome by the up-conversion of triplet the increased TTA contribution at larger applied voltages.3,4
to singlet states via two possible mechanisms, namely, thermally
DMPPP, a pyrene-containing compound (Fig. 1), has been used
activated delayed fluorescence (TADF)1 and triplet–triplet annihila- as a host material for fluorescent blue devices.5 With a solid
tion (TTA).2,3 In the case of TADF, the small singlet–triplet energy quantum efficiency (FPL) of 0.85 and a singlet energy gap of 3.2 eV,
gap (DEST) of the material enables triplet excitons to undergo DMPPP is considered to be a suitable host for TS-doped devices. The
reverse intersystem crossing to form singlet excitons. Although FPLs of TSs in cyclohexane are near 0.90. However, the FPLs of
TADF materials can provide very high device efficiency, most DMPPP films doped with 2% TSTA, TSNA, or TSMA were as high as
TADF-based OLEDs show large efficiency roll-off and low device 0.99 except for that of the film doped with TSCz. In addition, a
luminance.1 Therefore, TTA materials with high EL efficiencies complete energy transfer of DMPPP excitons to TSTA leading to only
are considered to be potential candidates for practical use.
TSTA emission was observed for the 2% TSTA–DMPPP film (Fig. S3,
In this communication, we report four new deep-blue dopants ESI†). The high PL efficiencies of TSs in cyclohexane, the efficient
containing 2-(styryl)triphenylene (TS) as the core moiety, namely energy transfer from DMPPP to TSs and the restricted rotation of
TSCz, TSTA, TSNA, and TSMA (Fig. 1). The deep-blue OLEDs the aromatic rings in the dopant and host molecules in the film that
fabricated using 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) reduces the fluorescence quenching caused by rotation likely
doped with one of the TSs as an emitting layer show delayed account for the very high FPLs of the TS-doped thin films.6 The
UV absorption spectra of TSs in toluene and the photoluminescence
(PL) spectrum of DMPPP are shown in Fig. S1 (ESI†). The maximum
absorptions of TSs appear in the range of 350–400 nm, characteristic
of p–p* transitions. Compared with TSCz, TSMA, TSTA, and TSNA
show better overlap of the absorption spectra with the PL spectrum
of DMPPP. These results plausibly account for the observed lower
FPL of the TSCz-doped DMPPP film. Further photophysical and
thermal properties of TSs (see Fig. S1–S4, ESI†) are summarized in
Fig. 1 Molecule structures of host and dopants.
Table 1. Due to the high photoluminescence quantum efficiencies
of DMPPP and TSs which lead to little or no intersystem crossing
from singlet to triplet excited states, we were unable to measure
their phosphorescent PL spectra at 77 K. Similarly, we measured the
transient PL of the DMPPP film and of TSs in degassed toluene and
no DF was observed for these compounds. These results exclude the
possibility that DMPPP and TSs are TADF materials, in view of the
fact that TADF materials in general show delayed fluorescence in
a Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
E-mail: chcheng@mx.nthu.edu.tw
b Department of Materials Science and Engineering, National Tsing Hua University,
Hsinchu 30013, Taiwan
† Electronic supplementary information (ESI) available: General information,
synthesis, 1H and 13C NMR spectra and photophysical, thermal and electro-
luminescence properties. See DOI: 10.1039/c4cc01851f
This journal is ©The Royal Society of Chemistry 2014
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Table 1 Photophysical and thermal properties of TSs
HOMOc, LUMOd (eV)
Eg (eV)
Tg , Td (1C)
FPL
a
a
b
e
f
g
h
Compd
Abssol (nm)
PLsol (nm)
PLfilm (nm)
TSCz
TSTA
TSNA
TSMA
358
308, 386
386
420, 442
441, 462
441, 462
451
469
470
469
476
5.37, 2.57
5.38, 2.65
5.38, 2.65
5.23, 2.53
2.80
2.73
2.73
2.70
112, 431
108, 431
119, 448
100, 421
0.85 (0.92)
0.89 (40.99)
0.90 (40.99)
0.90 (0.99)
308, 392
a
b
c
Absorption and PL spectra measured in toluene with concentration = 1 ꢁ 10ꢀ5 M. PL spectra measured in the thin film. Determined by using
d
e
a photoelectron spectrometer (AC-II). LUMO = HOMO ꢀ Eg. Band gaps were estimated from the optical absorption threshold in the films.
f
g
Glass-transition temperature determined by DSC with a heating rate of 20 1C minꢀ1 under N2. Onset decomposition temperature, as measured
with 5% mass loss by TGA with a heating rate of 20 1C minꢀ1 under N2. Fluorescence quantum efficiency, relative to 9,10-diphenylanthracene in
cyclohexane (FPL = 0.90); the data presented in the parentheses are the solid state photoluminescence quantum efficiency of the dopants in the
DMPPP thin film measured by an integral sphere with the excitation wavelength of 310 nm.
h
the transient PL because the small DEST leads to the possibility of For device A, the EQE and current efficiency vs. luminance was
T1 - S1 to produce DF.1 The calculated HOMO and LUMO density nearly flat. On the other hand, the EQEs and current efficiencies
maps of the TS series (Fig. S5, ESI†) also do not support that they are of devices B, C, and D gradually increased with an increase in
TADF materials.1
luminance from 10 to 30000 cd mꢀ2. These results are rather
Subsequently, we fabricated OLEDs using the emission layer intriguing, as most OLED devices show obvious efficiency roll-off.
containing DMPPP doped with one of the TS compounds, Furthermore, devices B, C, and D exhibited very high EQEs of
designated as A, B, C, and D, respectively. The device structure 10.2, 8.5 and 8.9%, respectively, and excellent current efficiencies
consists of ITO/NPNPB: 10% MoO3 (5 nm)/NPNPB (80 nm)/NPB of 12.3, 8.9 and 10.3 cd Aꢀ1, respectively. It should be noted that
(10 nm)/DMPPP: 5% dopant (25 nm)/BAlq2 (20 nm)/LiF (1 nm)/ the EQEs of all four OLEDs fabricated in this study are much
Al (100 nm). A thin layer of NPNPB (N,N0-di-phenyl-N,N0-di-[4- higher than the conventional upper limit of 5%. Furthermore, the
(N,N-di-phenyl-amino)phenyl]benzidine) doped with 10% MoO3 operating lifetime of all the four OLEDs was tested under an
was used as the hole injection layer, in order to reduce the hole initial luminance of 2000 cd mꢀ2 (Fig. S8, ESI†). Device C showed
injection barrier.7 All the devices fabricated in this study showed the longest operation lifetime of about 210 h, which is projected
deep-blue EL emissions, with maximum peaks at around 450 nm to be about 680 h at 1000 cd mꢀ2 according to the empirical
1.7
and CIEy r 0.14 (Fig. S6, ESI†). Fig. 2 shows the dependence of lifetime acceleration function, L0 ꢁ t50 = const.8
the EQE and the current efficiency of the devices on luminance.
To gain further insights into the observed unusually high
efficiency of the doped blue devices, we also fabricated non-doped
devices E and F with the same configuration as that of devices A–D,
except that only DMPPP and TSTA were used, respectively, in the
emitting layer. Both devices E and F exhibited relatively low device
efficiency compared with that of the doped devices (Table 2). It is
likely that the high concentration of these molecules in the emitting
layer leads to concentration-quenching and lowers the efficiency of
these non-doped devices.9 Subsequently, we measured the transient
EL of devices A–F at 450 nm (Fig. 3a). As the decay lifetime of singlet
excitons is generally within several ns, the results clearly show
microsecond-scale DF for all devices including the non-doped
devices E and F. These surprising results indicate that both DMPPP
and dopants (TSs) are able to produce DF. We also measured the
transient EL of the DMPPP-based devices doped with different
concentrations of TSTA in the emitting layer, namely, 2, 5, 10,
15 and 100% (Fig. 3b). The results reveal that the dopant used and
Fig. 2 EQEs and current efficiencies vs. luminance for devices A–E.
Table 2 Electroluminescence performance of devices A–F
EMLa
Von
EQEc
Zc
Zp
Lmax
lmax
CIE (x, y)h
t50
b
d
e
f
g
i
Device
A
B
C
D
E
F
5% TSCz
5% TSTA
5% TSNA
5% TSMA
DMPPP
4.4
3.8
3.6
3.4
4.8
3.6
6.7 (10.6 V)
10.2 (15.0 V)
8.5 (14.2 V)
8.9 (15.0 V)
3.5 (8.2 V)
7.1 (10.6 V)
12.3 (15.0 V)
8.9 (14.2 V)
10.3 (15.0 V)
3.2 (9.6 V)
3.8 (5.2 V)
5.7 (3.8 V)
4.8 (4.6 V)
8.7 (3.4 V)
1.3 (7.2 V)
3.8 (5.0 V)
33 510 (18.6 V)
68 670 (18.2 V)
49 060 (17.2 V)
58 640 (18.0 V)
10 840 (18.0 V)
30 584 (20.0 V)
448
454
450
460
444
470
0.15, 0.10
0.14, 0.14
0.14, 0.11
0.14, 0.14
0.16, 0.10
0.14, 0.25
75
100
210
35
—
—
TSTA
4.2 (7.0 V)
7.6 (7.0 V)
a
b
c
d
Emission layer of devices A–F: DMPPP/5% dopant. The voltage at 10 cd mꢀ2 (V). Maximum external quantum efficiency (%). Maximum
current efficiency (cd Aꢀ1). Maximum power efficiency (lm Wꢀ1). Maximum luminance (cd mꢀ2). The electroluminescence wavelength (nm).
e
f
g
h
i
Recorded at 8 V. Operation lifetime at an initial luminance of 2000 cd mꢀ2 under constant current.
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a second-order process proportional to the square of the triplet-
exciton concentration; we expected that the triplet excitons
generated by the device are nearly proportional to the current
density of the device. Similarly, the dependence of luminescence
on current density as shown in Fig. S15 (ESI†) is a non-linear
curve. The luminescence increased more than linearly with an
increase in current density, a characteristic of TTA-type devices.12
The other evidence to support TTA is that the decay rate of the
transient EL intensity is second-order with respect to the transient
EL intensity (Fig. S12 and S13, ESI†).
In summary, we have demonstrated that DMPPP/TS-based
devices are highly efficient affording the highest EQE of 10.2%
and a maximum current efficiency of 12.3 cd Aꢀ1. Both DMPPP
and TS appear to exhibit TTA-type delayed EL. This is the first
time that pyrene- and triphenylene-containing materials with
TTA properties were used in OLEDs. In contrast to the TADF-
based OLEDs, which generally have low maximum luminance
and high roll-off efficiencies, the present devices A–D provide
very high luminance in the range of 33 510–68 670 cd mꢀ2
without compromising on the efficiency.
We thank the Ministry of Science and Technology of the
Republic of China (NSC-102-2633-M-007-002) for financial support
of this research and the National Center for High-Performance
Computing (Account number: u32chc04) of Taiwan for providing
the computing time.
Notes and references
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structure is similar to that of device B except for the concentration of
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