Design Strategy for 25% External Quantum Efficiency in Green and Blue Thermally Activated Delayed Fluorescent Devices

Article abstract of DOI:10.1002/adma.201502053

Carbazole- and triazine-derived thermally activated delayed fluorescent (TADF) emitters, with three donor units and an even distribution of the highest occupied molecular orbital, achieve high external quantum efficiencies of above 25% in blue and green T

Full text of DOI:10.1002/adma.201502053

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Design Strategy for 25% External Quantum Efficiency
in Green and Blue Thermally Activated Delayed
Fluorescent Devices
Dong Ryun Lee, Mounggon Kim, Sang Kyu Jeon, Seok-Ho Hwang, Chil Won Lee,
and Jun Yeob Lee*
Development of high efficiency organic light-emitting diodes
(OLEDs) has been one of key issues of OLEDs because con-
ventional fluorescent OLEDs exhibit low external quantum
efficiency (EQE) by nonradiative loss of triplet excitons.[1]
This hurdle can be overcome by inducing radiative transition
of triplet excitons by singlet-triplet mixing[2] or up-conversion
of triplet excitons into singlet excitons.[3] The former process
is used for phosphorescent emission and the latter process is
utilized for thermally activated delayed fluorescent (TADF)
emission. The two methods were proven to be successful to
improve the EQE of OLEDs. Especially, the EQE of the phos-
phorescent OLEDs could reach above 25% in all red, green, and
blue devices.[4–7] However, the EQE of the TADF device is still
lower than that of phosphorescent OLEDs. In particular, the
EQE of blue TADF OLEDs is not in the same level as that of
phosphorescent OLEDs although EQE close to 20% has been
demonstrated.[8–10]
In general, molecular design of the TADF emitters is the
most important factor to develop high efficiency TADF devices.
It has been known that the TADF emitters need to be designed
to have separated highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) for small
singlet-triplet energy gap (ΔEST) and weakly overlapped HOMO
and LUMO for efficient light emission.[3] The former condition
is desired for up-conversion of triplet excitons into singlet exci-
tons and the latter condition is required for high photolumines-
cence (PL) quantum yield. Therefore, both conditions should
be met to develop highly efficient TADF devices.
via a linker[8–15] and the other approach was to link the donor
and acceptor moieties without a linker.[16] Typically, the molec-
ular design with the aromatic linker between the donor and
acceptor moieties provided high EQE. Additionally, wide disper-
sion of the HOMO and LUMO in a donor unit was proposed
as an effective way of enhancing the quantum efficiency of the
blue TADF devices.[8] However, the design strategy in previous
work could provide EQE only close to 20%,[8,9] which is inferior
to that of phosphorescent OLEDs. Therefore, a new molecular
design approach to improve the EQE of the TADF OLEDs is
desired.
Herein, we provide a design approach to realize high EQE
above 25% in green and blue TADF OLEDs. A molecular design
strategy to evenly distribute the HOMO of the TADF emitters
over the donor units by introducing the same donor units in
the molecular structure as many as possible was successful to
reach above 25% EQE in green and blue TADF devices. The
design concept was verified by comparing targeted green and
blue TADF emitters with control material with uneven HOMO
distribution or less donor units.
One green emitter and one blue emitter were prepared
as target materials to obtain high EQE in the TADF OLEDs
and control materials were compared with the target mate-
rials. Diphenyltriazine was an acceptor moiety and carba-
zole or 3,6-dimethylcarbazole was a donor moiety of the
TADF emitters. Two compounds with three donor units in
the backbone structure, 9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin-
2-yl)benzene-1,2,3-triyl)tris(9H-carbazole)
(TCzTrz)
and
Based on this design criteria, various red, green, and blue
TADF emitters were synthesized.[8–16] A variety of donor
moieties were combined with acceptor moieties via an aro-
matic linker based on different design strategies. One design
approach was to link a donor moiety with an acceptor moiety
9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)
tris(3,6-dimethyl-9H-carbazole) (TmCzTrz), were developed
as the green and blue TADF emitters for high EQE. To prove
the effectiveness of the design strategy to include many
donor units, a diphenyltriazine derivative with two carbazole
units,
9,9′-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)
bis(9H-carbazole) (DCzTrz), reported in our previous work
was assessed as the blue control material with two donor
units.[10] Additionally, one diphenyltriazine derivative with
one 3,6-dimethylcarbazole and two carbazole units in place of
three carbazole or three 3,6-dimethylcarbazole units of TCzTrz
and TmCzTrz, 9,9′-(2-(3,6-dimethyl-9H-carbazol-9-yl)-5-(4,6-
diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole)
(DCzmCzTrz), was also prepared to elucidate the necessity of
uniform distribution of the HOMO over the entire donor units.
The molecular structure of the four TADF emitters used to
prove the design approach is shown in Figure 1a. The synthetic
scheme is depicted in Scheme 1 and synthetic procedures
D. R. Lee, S. K. Jeon, Prof. J. Y. Lee
School of Chemical Engineering
Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon
Gyeonggi 440-746, South Korea
E-mail: leej17@skku.edu
M. Kim, Prof. S.-H. Hwang, Prof. C. W. Lee
Department of Polymer Science and Engineering
Dankook University
126, Jukjeon-dong, Suji-gu, Yongin
Gyeonggi 448-701, South Korea
DOI: 10.1002/adma.201502053
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Figure 1. a) Molecular structure of DCzTrz, TCzTrz, TmCzTrz, and DCzmCzTrz. b) HOMO and LUMO distribution of TCzTrz, TmCzTrz, and DCzmCzTrz.
are explained in the Supporting Information in detail. All
compounds were prepared by coupling reaction, but two-
step coupling reaction was used to synthesize DCzmCzTrz.
3,6-Dimethylcarbazole was coupled with a central F unit of
2,4-diphenyl-6-(3,4,5-trifluorophenyl)-1,3,5-triazine because the
reactivity of the central F atom is higher than the other F units
by the strong electron withdrawing character of two F units sur-
rounding the central F unit.
2,4-diphenyl-6-(3,4,5-trifluorophenyl)-1,3,5-triazine
diate by controlling molar ratio of 3,6-dimethylcarbazole to
interme-
In order to reach high EQE in the TADF devices, high PL
quantum yield of the TADF emitters is essential and it is closely
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Scheme 1. Synthetic scheme of TCzTrz, TmCzTrz, and DCzmCzTrz.
related with HOMO–LUMO overlap of the TADF emitters and
reverse intersystem crossing by up-conversion. Therefore, elec-
tronic calculation of the HOMO and LUMO was conducted to
study the HOMO–LUMO overlap and photophysical charac-
terization was carried out to extract PL related photophysical
parameters. Figure 1b shows electronic calculation results of the
TADF emitters which describes the HOMO and LUMO distri-
bution. The HOMO and LUMO were isolated, and small overlap
of the HOMO and LUMO was detected in all TADF emitters.
Relating the chemical structure of the DCzTrz[10] and TCzTrz
TADF emitters with the HOMO distribution, the HOMO was
evenly dispersed over all carbazole units incorporated in the
molecular structure. Although three carbazole units were
attached to the central phenyl ring of TCzTrz, the HOMO dis-
persion was uniform over all three carbazole units. In addition,
the three carbazole units expanded the HOMO to the central
phenyl linker extensively, and overlap of the HOMO and LUMO
was observed. Detailed calculated parameters are summarized
in Table S1, Supporting Information. All three TADF emitters
are expected to possess small ΔEST for TADF behavior.
The chemical structure of TCzTrz, TmCzTrz, and DCzm-
CzTrz was also correlated with the HOMO and calculated
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Table 1. Photophysical properties of DCzTrz, TCzTrz, TmCzTrz, and DCzmCzTrz.
UV–vis absorption edge
[nm]
Singlet energya)
[eV]
Triplet energyb)
[eV]
PLQY solutionc) PLQY filmd)
Delayed fluorescence lifetime
[µs]
ΔEST
[eV]
[%]
[%]
DCzTrz[10]
TCzTrz
411
414
447
427
3.09
2.96
2.86
2.93
2.86
2.80
2.79
2.73
0.23
0.16
0.07
0.20
43 (17)
100 (47)
99 (39)
84 (52)
–
31.0
13.5
13.3
9.7
100 (81)
100 (86)
98 (89)
TmCzTrz
DCzmCzTrz
a) Singlet energy was calculated from the onset wavelength of fluorescent emission; b)Triplet energy was calculated from the onset wavelength of phosphorescent emission;
c)PLQY measurement was performed in toluene solution after 10 min of nitrogen bubbling; d)PLQY measurement was performed using 30% TADF emitting material doped
DPEPO film. Values in parentheses: before nitrogen bubbling.
photophysical parameters. In the case of TCzTrz and TmC-
zTrz, the HOMO was uniformly distributed over all donor
units because the same donor unit was substituted. How-
ever, the HOMO of DCzmCzTrz was mostly localized on the
3,6-dimethylcarbazole by relatively strong electron richness of
3,6-dimethylcarbazole compared to carbazole. Nonuniform
HOMO distribution was observed in the DCzmCzTrz, while
uniform HOMO distribution was observed in the TCzTrz and
TmCzTrz. The LUMO of TCzTrz, TmCzTrz, and DCzmCzTrz
was similarly dispersed over the diphenyltriazine moiety. The
HOMO and LUMO were overlapped in the phenyl linker,
which connects the donor and acceptor moieties.
TCzTrz, TmCzTrz, and DCzmCzTrz. PL quantum yields of the
three emitting materials are listed in Table 1 in addition to PL
quantum yield of DCzTrz.
Basic photophysical parameters were collected from UV–vis
and PL measurements, and are summarized in Table 1. UV–
vis absorption, room temperature PL and low temperature PL
data are presented in Figure S1 in the Supporting Informa-
tion. Comparing DCzTrz and TCzTrz, additional donor unit
lowered the singlet and triplet energy of the TADF emitter by
strengthened donating power and extended conjugation struc-
ture. Comparing TCzTrz, TmCzTrz, and DCzmCzTrz, rela-
tively strong electron donating character of 3,6-dimethylcarba-
zole to carbazole lowered the singlet energy of TmCzTrz and
DCzmCzTrz. The relatively low singlet energy of TmCzTrz to
that of DCzmCzTrz is due to strong electron donating char-
acter by three 3,6-dimethylcarbazole units of TmCzTrz com-
pared to one 3,6-dimethylcarbazole unit of DCzmCzTrz. ΔEST
of DCzTrz, TCzTrz, TmCzTrz, and DCzmCzTrz were 0.23,
0.16, 0.07, and 0.20 eV, respectively. From the ΔEST of DCzTrz
and TCzTrz, it can be suggested that the additional donor
unit reduces the ΔEST and increases the PL quantum yield by
improved reverse intersystem crossing of the TADF emitters.
In the case of TCzTrz and TmCzTrz, the ΔEST was small in
the TmCzTrz because the donor strength of the donors was
different.
Transient PL measurement of TCzTrz, TmCzTrz, and DCzm-
CzTrz was carried out from 100 K to room temperature to mon-
itor delayed fluorescence behavior. Transient PL decay of the
three TADF emitters is described in Figure 2. The delayed fluo-
rescence component of the three TADF emitters was intensified
at high temperature, which proposed that the delayed emission
of the three emitters is activated by thermal energy. Excited
state lifetimes for delayed emission of TCzTrz, TmCzTrz, and
DCzmCzTrz were 13.5, 13.3, and 9.7 µs, respectively. Com-
pared with the lifetime for TADF emission of other triazine and
carbazole derived TADF emitters, the lifetime of TCzTrz, TmC-
zTrz, and DCzmCzTrz was relatively short because of facile up-
conversion by small ΔEST. The small ΔEST of the three TADF
emitters induced efficient up-conversion, short lifetime for
TADF and high PL quantum yields. Lifetime for the delayed
emission of DCzTrz with two carbazole units was 31.0 µs,[10]
which was much longer than that of TCzTrz with three carba-
zole units. Prompt and delayed emission spectra are shown in
Figure S2 in the Supporting Information. Delayed emission
spectra of TCzTrz, TmCzTrz, and DCzmCzTrz were almost
identical to prompt emission spectra of each TADF emitter,
Photoluminescence (PL) quantum yields of DCzTrz,
TCzTrz, TmCzTrz, and DCzmCzTrz in toluene measured
using an integrating sphere after N2 bubbling were 43%,[10]
100%, 99%, and 84%, respectively. TCzTrz and TmCzTrz
showed almost 100% PL quantum yield under N2 compared
to 47% and 39% in ambient conditions, supporting up-con-
version of triplet excitons for radiative transition by reverse
intersystem crossing process. From the PL quantum yield
data, ΦTADF/ΦT representing relative up-conversion efficiency
of triplet excitons was calculated, where ΦTADF is PL quantum
yield of TADF and ΦT is quantum yield of intersystem crossing
from singlet excited state to triplet excited state.[8] The ΦTADF
values of TCzTrz, TmCzTrz, and DCzmCzTrz were 53, 60, and
32%, respectively. The ΦTADF/ΦT values were 100%, 98%, and
67% for TCzTrz, TmCzTrz, and DCzmCzTrz, which indicates
that triplet excitons of TCzTrz and TmCzTrz are completely
converted into singlet excitons, while only 67% of triplet exci-
tons of DCzmCzTrz are transformed into singlet excitons. This
result is due to small ΔEST of TCzTrz and TmCzTrz, which is
presented in Table 1. Therefore, TCzTrz and TmCzTrz may be
more efficient than DCzmCzTrz to harvest triplet excitons for
singlet emission. PL quantum yield of TADF emitters doped
in bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) host
was also measured and the triplet to singlet conversion effi-
ciency was high in the TCzTrz and TmCzTrz. ΦTADF values of
TCzTrz, TmCzTrz, and DCzmCzTrz in the DPEPO host were
19%, 14%, and 9%, respectively, and the ΦTADF/ΦT values
100%, 100%, and 82%. From the PL quantum yields of DCzTrz
and TCzTrz, it can be assumed that introduction of three
donor units is better than introduction of two donor units for
up-conversion of triplet excitons. Similarly, it can be presumed
that even HOMO distribution is preferred to uneven HOMO
distribution to reach high PL quantum yield and triplet to
singlet conversion efficiency from the PL quantum yields of
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Figure 3. Quantum efficiency-luminance curves of TCzTrz, TmCzTrz, and
DCzmCzTrz devices. Doping concentrations of TCzTrz, TmCzTrz, and
DCzmCzTrz were 40%, 30%, and 20%, respectively.
for each TADF emitter. EQE of the TADF devices was plotted
according to the luminance of the devices in Figure 3. Cur-
rent density and luminance data are presented in the Sup-
porting information (Figure S3). EQE of the TADF devices
was calculated based on the assumption that light emission
profile follows Lambertian distribution after confirming that
angle dependent radiation intensity of the devices agrees with
Lambertian emission pattern (Figure S4, Supporting Informa-
tion). Correlating the number of donor units with the EQE,
the increase of the number of donor units enhanced the EQE.
Maximum EQE of the TCzTrz and DCzTrz devices were 25.0%
and 17.8%,[10] respectively. A dramatic improvement of the
EQE was realized by increasing the number of carbazole units
attached to the central phenyl linker. The EQE of 25.0% of the
TCzTrz blue device is higher than 20.6% of the state of the
art EQE of the blue TADF device reported by Adachi and co-
workers.[8] More than 20% improvement in the EQE was made
by adopting a molecular design with three carbazole units.
Comparing TCzTrz, TmCzTrz, and DCzmCzTrz devices, the
EQE was in the order of TCzTrz (25.0%) ≈ TmCzTrz (25.5%)
> DCzmCzTrz (21.3%). Although the DCzmCzTrz device with
one 3,6-dimethylcarbazole and two carbazole units exhibited
high EQE, the TCzTrz and TmCzTrz device worked better
than the DCzmCzTrz device. Both green TmCzTrz and blue
TCzTrz devices showed high quantum efficiency above 25%.
The better EQE of the TCzTrz and TmCzTrz devices than that
of DCzmCzTrz device can be correlated with the PL quantum
yield of the three emitters. In particular, the triplet to singlet
up-conversion efficiency can explain the high EQE of the
TCzTrz and TmCzTrz devices. In the current TADF devices,
dominant light-emission mechanism is direct charge trapping
by TADF emitters rather than energy transfer because of poor
hole injection character of the DPEPO host. The charge trap-
ping generates singlet and triplet excitons at a ratio of 1–3 in
the TADF emitters, so the triplet exciton harvesting for sin-
glet exciton conversion is the dominant factor for the EQE of
the devices. As explained in the PL quantum yield data, the
triplet to singlet conversion efficiency was relatively high in
Figure 2. Transient decay curves of TCzTrz, TmCzTrz, and DCzmCzTrz
from 100 to 297 K.
which indicates that the delayed emission is caused by TADF
process.
Device performances of the TADF emitters were charac-
terized by doping the TADF emitters in a high triplet energy
DPEPO host. Hole transport layer structure was optimized
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Table 2. Quantum efficiency and color coordinate of DCzTrz, TCzTrz,
TmCzTrz, and DCzmCzTrz devices.
Quantum efficiency
[%]
Power efficiency
[lm W−1
Color coordinate
at 500 cd m−2
]
500 cd m−2
14.9
500 cd m−2
12.7
Maximum
17.8
Maximum
22.4
DCzTrz[10]
TCzTrz
(0.15, 0.16)
(0.18, 0.33)
(0.25, 0.50)
(0.23, 0.46)
25.0
16.7
42.7
18.0
TmCzTrz
DCzmCzTrz
25.5
17.3
52.1
18.6
21.3
14.6
42.4
16.8
molecular design strategy to include more donor units and
distribute the HOMO uniformly would allow the development
of high EQE TADF devices.
Figure 4. EL spectra of TCzTrz, TmCzTrz, and DCzmCzTrz devices.
Doping concentrations of TCzTrz, TmCzTrz, and DCzmCzTrz were 40%,
30%, and 20%, respectively.
Experimental Section
Synthesis: Detailed synthesis of TCzTrz, TmCzTrz, and DCzmCzTrz is
explained in the Supporting Information.
the TCzTrz and TmCzTrz, which increased the EQE in the
TADF devices. The EQE calculated from the PL quantum
yield of the TADF emitters is summarized in Table S2,
Supporting Information, which coincided with the measured
EQE values of the TADF devices. Therefore, the device perfor-
mances were well correlated with the PL emission characteris-
tics of the TADF emitters. Other parameters such as fluores-
cent rate constant had little effect on the device performances.
From the EQE of the TADF devices, it can be concluded that
the uniform dispersion of the HOMO over many donor units
of the TADF emitters is critical to the device performances of
the TADF emitters. High optimum doping concentration of
the TADF emitters is due to the balanced hole and electron
density at high doping concentration due to better hole injec-
tion by TADF emitters.
Device Fabrication and Measurement: This measurement method
was the same as reported in previous work.[10] The device structure on
indium tin oxide substrate was PEDOT:PSS (60 nm)/TAPC (20 nm)/
mCP (10 nm)/DPEPO:TADF emitter (25 nm)/TSPO1 (5 nm)/TPBI
(20 nm)/LiF (1 nm)/Al (200 nm). In the device structure, PEDOT:PSS,
TAPC, mCP, TSPO1, and TPBI represent poly(3,4-ethylenedioxythio-
phene):poly(styrenesulfonate),
4,4′-cyclohexylidenebis[N,N-bis(4-
methylphenyl)aniline], 1,3-bis(N-carbazolyl)benzene, diphenylphosphine
oxide-4-(triphenylsilyl)phenyl, and 1,3,5-tris(N-phenylbenzimidazole-2-yl)
benzene. Doping concentrations of TCzTrz, TmCzTrz, and DCzmCzTrz
in the DPEPO host were 40%, 30%, and 20%, respectively, which were
optimized doping concentrations in terms of EQE.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Electroluminescence (EL) spectra of the TADF devices char-
acterized using spectroradiometer are presented in Figure 4.
Comparison of the EL spectra of TCzTrz and DCzTrz devices
revealed that the addition of more carbazole units induced
bathochromic shift of the EL emission. EL peak positions Acknowledgements
of TCzTrz and DCzTrz devices were 480 and 459 nm,[10]
This research was supported by Basic Science Research Program through
respectively. The EL spectra of the TCzTrz, TmCzTrz, and
DCzmCzTrz devices were also different and the presence of
3,6-dimethylcarbazole altered the EL peak position to long
wavelength. EL peak positions of TCzTrz, TmCzTrz, and
DCzmCzTrz devices were 480, 500, and 496 nm, respec-
tively, without any other emission. Color coordinates of all
TADF devices are listed in Table 2 in addition to other device
performances.
the National Research Foundation of Korea (NRF) funded by the Ministry
of Education, Science, and Technology (2013R1A1A2007991) and
Ministry of Science, ICT, and future Planning (2013R1A2A2A01067447).
Received: April 29, 2015
Revised: June 25, 2015
Published online: August 26, 2015
In conclusion, a molecular design approach to realize high
EQE in the TADF devices by increasing the number of donor
units and dispersing the HOMO evenly over the donor units
was effective to obtain high PL quantum yield close to 100%
and high EQE above 25% in the green and blue TADF OLEDs.
The increase in the number of donor units contributed to
reduce the ΔEST of the TADF emitter and uniform dispersion
of the HOMO enhanced PL quantum yield and triplet to sin-
glet conversion efficiency of the TADF emitter. Therefore, the
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