Highly efficient blue thermally activated delayed fluorescence organic light emitting diodes based on tercarbazole donor and boron acceptor dyads

Article abstract of DOI:10.1039/c9tc05950d

Two new blue TADF emitters, namely, 9′-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-9,9′′-diphenyl-9H,9′H,9′′H-3,3′:6′,3′′-tercarbazole (3CzTB) and 9′-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-6,6′′-dimethyl-9,9′′-diphenyl-9H,9′H,9′′H-3,3′:6′,3′′-tercarbazole (M3CzB), for application in highly efficient organic light-emitting diodes are reported. These emitters consist of tercarbazole as donor and rigid oxygen bridged boron as acceptor units. Despite having the same donor and acceptor segments, an alkyl modification on the periphery of the donor and acceptor units altered their physicochemical properties and the electrochemical stability of the emitters. Blue TADF light-emitting devices fabricated with these emitters exhibited a maximum external quantum efficiency (EQE) and luminance of 30.7%, 18 160 cd m^-2 and 29.1%, 11 690 cd m^-2 for M3CzB and 3CzTB, respectively. The operational lifetime (LT₅₀) of the stable blue TADF device fabricated with these emitters was 81 h and 60.5 h at 400 nits with the maximum EQE and CIE color coordinates of 14.4%, (0.13, 0.19) and 7.6%, (0.14, 0.10) for M3CzB and 3CzTB, respectively.

Full text of DOI:10.1039/c9tc05950d

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DOI: 10.1039/C9TC05950D
ARTICLE
Highly Efficient Blue Thermally Activated Delayed Fluorescence
Organic Light Emitting Diodes Based on Tercarbazole Donor and
Boron Acceptor Dyads
Received 00th January 20xx,
Accepted 00th January 20xx
Durai Karthik, Dae Hyun Ahn, Jae Hong Ryu, Hyuna Lee, Jee Hyun Maeng, Ju Young Lee* and Jang
Hyuk Kwon*
DOI: 10.1039/x0xx00000x
Two new blue TADF emitters, namely, 9'-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-9,9''-
diphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole (3CzTB) and 9'-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-6,6''-
dimethyl-9,9''-diphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole (M3CzB) for application in highly efficient organic light-emitting
diode are reported. These emitters consist of tercarbazole as donor and rigid oxygen bridged boron as acceptor units.
Despite having the same donor and acceptor segments, an alkyl modification on the periphery of donor and acceptor units
altered their physicochemical properties and electrochemical stability of the emitters. Blue TADF light-emitting devices
fabricated with these emitters exhibited maximum external quantum efficiency (EQE) and luminance of 30.7%, 18,160 cd/m2
and 29.1%, 11,690 cd/m2 for M3CzB and 3CzTB, respectively. An operational lifetime (LT50) of stable blue TADF device
fabricated with these emitters exhibited 81 hrs and 60.5 hrs at 400 nits with maximum EQE and CIE color coordinates of
14.4%, (0.13, 0.19) and 7.6%, (0.14, 0.10) for M3CzB and 3CzTB, respectively.
However, highly efficient TADF material cannot only be
developed with low ∆EST, it also requires high
photoluminescence quantum yield (PLQY) and high horizontal
Introduction
Organic light-emitting diodes (OLEDs) have emerged as the
potential light source in lighting and display technologies over
conventional light sources due to their low power consumption,
high brightness, less weight, large viewing angle and low cost.1,2
Although OLEDs have commercialized in solid-state and flexible
displays, their efficiency and operational lifetime are still a
bottleneck challenge in display technology. In practical, the
usage of blue emitters are still behind green and red emitters in
terms of efficiency.3 Though high-efficiency blue
phosphorescence (PH) emitters are available, they suffer from
high cost, color purity and device operating lifetime.4-6
Alternatively, blue TADF materials have developed with
superior efficiency and color purity.7-9 However, TADF emitters
also suffer from operating lifetime.10,11 Hence, the development
of potential TADF emitters satisfying both high efficiency and
device lifetime is highly desirable. In general, efficient TADF
materials should have small singlet (S1) and triplet (T1) energy
gap (∆EST) for facile reverse intersystem crossing (RISC)
process.12 An effective strategy to reduce the ∆EST is reducing
the overlap between highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO).
dipole orientation.13,14 A rigid/planar conjugated molecular
skeleton can bring both high PLQY and horizontal dipole
orientation.15 Importantly, donor and acceptor interaction
maintaining high S1 and T1 energy required for blue emission is
also an essential consideration. Along with the above
prerequisites, the TADF material should be thermally and
electrochemically stable in order to achieve durability of the
device. Thus, the design of efficient blue TADF emitter is
challenging and requires a deep understanding of structure-
property relations. Therefore, the development of highly
efficient blue TADF material requires careful selection of donor
and acceptor segments.
Among the available several donors, carbazole and its
derivatives are promising candidates for blue OLED materials
due to their high triplet energy, high PLQY and rigid delocalized
electronic structure.16 Among the carbazole-based donors,
tercarbazole donor is interesting as it offers high horizontal
orientation and high PLQY due to the long rod-like extended
conjugation. It was demonstrated that the tercarbazole donor-
based material exhibited higher efficiency than mono and
bicarbazole donor-based materials due to higher horizontal
dipole orientation factor (0.95) of tercarbazole than mono
(0.66) and bicarbazole (0.89) donor-based materials.17 Later, it
was also demonstrated that the tercarbazole-based TADF
emitter showed higher efficiency and longer device lifetime
than indolocarbazole and bicarbazole donor-based TADF
emitters due to the low ∆EST and short delayed exciton lifetime
of the former.18 Though tercarbazole-based materials showed
Organic Optoelectronic Device Laboratory, Department of Information Display,
Kyung Hee University, Seoul 02447, Republic of Korea. E-mail: jhkwon@khu.ac.kr,
juyoung105@khu.ac.kr.
Electronic Supplementary Information (ESI) available:
A detailed synthetic
experimental procedure, solvatochromism, time resolved photoluminescence
spectra. TGA and DSC graphs, device diagram, 1H and 13C NMR spectra of new
materials. See DOI: 10.1039/x0xx00000x
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superior performances than other carbazole donor-based believed that the alkyl modification of acceptor would tune the
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materials, their device efficiency and durability together are not acceptor strength resulting in change iDnOtI:h1e0.1d0o39n/oCr9-TaCc0c5e9p5t0oDr
satisfactory. Generally, the prevention of active 3,6-positions of interaction. Thus, we envisaged that an alkyl modification on
carbazole-based donor materials showed enhanced lifetime the donor/acceptor moieties could influence the material
due to the electrochemical stability of the materials by stability, physicochemical and electroluminescence properties.
inhibiting electrochemical polymerization reaction.19-21 So, the The synthetic scheme of the new emitters is shown in Scheme
prevention of such active positions of tercarbazole donor could 1. The starting materials 3Cz and 4 were synthesized according
enhance the durability of the device. On the other hand, boron- to the reported procedures15, 23 and the new acceptor 3 was
based acceptors are promising for high efficiency TADF synthesized as the similar procedure used for 4 using phenol as
devices.22 Recently, our group reported highly efficient deep starting material instead of 4-(tert-butyl)phenol. The new donor
blue TADF emitters based on symmetrical oxygen bridged boron M3Cz was synthesized in two steps starting from 3-methyl-9-
acceptor.23 The high efficiency originated from the planar phenyl-9H-carbazole (1). Finally, the target materials were
extended conjugation of boron acceptor which led to horizontal synthesized by palladium catalyzed Buchwald-Hartwig cross
dipole orientation factor of 0.89. Another advantage of boron coupling amination reaction between the corresponding donor
acceptor is that it has relatively weak acceptor nature than and acceptor. The structure of the newly synthesized materials
triazine and sulfone-based acceptors which helps to control the were characterized by 1H and 13C NMR and high resolution mass
donor-acceptor interaction so as to maintain the high singlet spectrometry (HRMS).
energy (S1) suitable for blue emission.
In this work, we report two new blue TADF emitters 9'-(2,12-di-
O
O
H
N
tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-
B
B
O
O
Br
yl)-9,9''-diphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole
(3CzTB)
N
H
N
N
and 9'-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-
6,6''-dimethyl-9,9''-diphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole
(M3CzB) based on tercarbazole donor linked with boron
acceptor. The structure of the materials is shown in Fig. 1a. An
incorporation of methyl group at active positions of
tercarbazole increased the electrochemical stability of M3CzB.
On the other hand, incorporation of tert-butyl group on the
boron acceptor decreased the donor-acceptor interaction
leading to high ∆EST of 3CzTB. The blue TADF device fabricated
with these emitters showed maximum external quantum
efficiency (EQE) of 30.7% and 29.1% for M3CzB and 3CzTB,
respectively, Also, a stable blue TADF device with M3CzB
exhibited operating lifetime (LT50) of 81 hrs with maximum EQE
of 14.4% and CIE coordinates of (0.13, 0.19) at 400 nits. To the
best of our knowledge, this is the best result of tercarbazole-
based blue TADF device satisfying both efficiency and lifetime
together reported so far.
(a)
(b)
N
N
1
2
M3Cz
O
(a) = NBS, DMF, 0 °C,
(b) = Pd(PPh3 4, K2CO3, THF/H2O, reflux,
(c)
B
Br
)
O
(c) = Pd(dba)2, t-Bu3PHBF4, t-BuONa, toluene/110 °C
3
M3CzB
3CzTB
H
N
O
O
+
B
Br
(c)
N
N
3Cz
4
Scheme 1. Synthetic route of the emitters M3CzT and 3CzTB.
Theoretical investigation
Density functional theory (DFT) calculation was performed to
optimize the ground state geometry and time dependent DFT
(TDDFT) was performed to analyze the electronic properties of
3CzTB and M3CzB using B3LYP method with 6-311g basis set on
Gaussian 16. The dihedral angle between the donor and
acceptor is 52.09° and 51.48° for 3CzTB and M3CzB,
respectively. The HOMO is located on the tercarbazole donor
with small contribution on the acceptor segment. The LUMO is
localized on the entire acceptor moiety and on the nitrogen
atom of central carbazole group as shown in Fig. 1b. The well
separated HOMO and LUMO orbitals would give small ∆EST. The
calculated ∆EST is 0.10 eV for both materials. The small ∆EST of
these materials would expect to give good TADF performances.
The oscillator strength of these materials was also calculated to
be 0.2161 and 0.1888 for 3CzTB and M3CzB, respectively. The
difference in their oscillator strength may originate from their
different electronic nature which is tuned by the alkyl groups.
Results and discussion
Molecular design and synthesis
As described in the introduction section tercarbaozle donor and
symmetrical oxygen bridged boron acceptor has been selected
to design highly efficient blue TADF emitters. In order to
increase the electrochemical stability of the emitter, the methyl
group incorporation at the active 6,6'-positions of tercarbazole
donor would be ideal to prevent the electrochemical
polymerization reaction under applied voltage. So, we
introduced
a new 6,6'-dimethyl substituted tercarbazole
(M3Cz) donor as shown in Scheme 1. Also, to determine the
relative electrochemical stability of M3Cz-donor, methyl group
modified tercarbazole (3Cz) donor was also compared. On the
other hand, the tert-butyl modified boron acceptors (4 – with
tert-butyl and 3 – without tert-butyl) have been selected to
attach with the tercarbazole-based donor segment. It is
2 | J. Name., 2012, 00, 1-3
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0.23 eV and 0.14 eV, respectively. These values are adequate in
View Article Online
DOI: 10.1039/C9TC05950D
order to have efficient reverse intersystem crossing (RISC)
process. Despite having similar triplet energies, the smaller ∆EST
of M3CzB is due to strong ICT character which decreased the S1
energy. Further, these results indicate that the incorporation of
methyl group on the donor segment have negligible influence
on the triplet states, whereas incorporation of tert-butyl groups
on the acceptor part have significant influence on the ICT
character which greatly alters the ∆EST of the materials.
Fig. 1. (a) Molecular structures of new materials. (b) Molecular orbitals and their
energy levels.
Photophysical properties
The absorption and photoluminescence properties of these
materials were evaluated in dilute toluene solutions. The
related spectra of these materials are shown in Fig. 2 and the
data are listed in Table 1. The lower wavelength absorption
originates from the tercarbazole delocalized π-π* transition and
the higher wavelength absorption attributed to the
intramolecular charge transfer (ICT) from the tercarbazole
donor to boron acceptor. The material M3CzB exhibited blue-
shifted and lower ICT absorption peak than 3CzTB suggests
lower ICT interaction of the former in the ground state. Both
materials showed deep blue emission in solution. In contrary to
the absorption spectra, M3CzB exhibited red-shifted
photoluminescence (PL) than 3CzTB suggests more donor
acceptor interaction in the excited state of the former material.
This behaviour is due to the absence of tert-butyl group in
acceptor segment of M3CzB increased the acceptor strength
resulting in higher ICT interaction. To confirm this ICT
interaction in the excited state PL solvatochromism of these
materials were measured and shown in Fig. S1. The degree of
red-shift extension from non-polar hexanes (HX) to polar
methylenechloride (MC) is more in case of M3CzB that is 113
nm shift, whereas 3CzTB showed 104 nm shift. This result
indicates that the better donor-acceptor interaction of M3CzB
would show better TADF behaviour than 3CzTB. The low
temperature PL (LTPL) spectra were measured to evaluate the
T1 energy of these materials. Both materials showed similar
LTPL spectra with vibronicaly fine profile suggests the triplet
state originated from the triplet local excited state (3LE).
Further, to confirm the origination of the triplet state, LTPL
spectra of both corresponding donors and acceptors were
recorded in toluene solution and shown in Fig. S2. This result
shows the triplet origination of dopants are dominated by their
donor segments rather than their acceptors. The singlet and
triplet energies of these materials were obtained from the
onset of room temperature PL (RTPL) and LTPL spectra,
respectively. The calculated ∆EST values of 3CzTB and M3CzB are
Fig. 2. Absorption and photoluminescence spectra of materials recorded in toluene.
The temperature dependant time resolved photoluminescence
decay of the emitters were recorded for the 20 wt% doped in
DBFPO films and shown in Fig. 3. The delayed portion increases
on increasing the temperature for both materials indicates the
existence of TADF properties. Both materials showed bi-
exponential decay corresponding to prompt and delayed
emission. The prompt and delayed lifetimes of the emitters
3CzTB and M3CzB are 23.40 ns, 9.32 µs and 88.20 ns, 7.84 µs,
respectively. The M3CzB showed little higher delayed portion
and lower delayed lifetime than 3CzTB due to low ∆EST of the
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former. This indicates the M3CzB possess facile RISC process for literature.25, 26 On the other hand, the M3CzB sVhieowwAertdiclesOanmlinee
the efficient conversion of triplet to singlet excitons. The oxidation peak on all five cycles indicatesDthOeI: p10r.e10v3e9n/tCio9TnCo0f59su50cDh
photoluminescence quantum yield (PLQY) of both materials oligomerisation reaction due to the presence of methyl group
were also evaluated in 20 wt% doped DBFPO films. The material at 6,6'-positions of tercarbazole donor. This result suggests the
M3CzB showed little higher PLQY than 3CzTB attributed to methyl group on the terminal tercarbazole donor increased the
higher delayed portion of the former. To know the ease of RISC electrochemical stability of M3CzB. Thermal properties of the
process different rate constants of the excited state processes materials were evaluated by using thermo gravimetric analysis
were calculated using the reported method24 and listed in Table (TGA) and differential scanning calorimetry (DSC) and shown in
푘
S1. The M3CzB showed higher RISC rate ( 푟푖푠푐) and lower triplet Fig. S3. The thermal decomposition temperature (Td) (5% wt
푘푡
non-radiative rate
(
)
푛푟
than 3CzTB. This confirms the loss) of the materials were found to be 513 °C and 527 °C for
involvement of higher delayed portion in M3CzB than 3CzTB is 3CzTB and M3CzB, respectively. In addition, these materials
due to the low ∆EST. Hence, it is expected that the M3CzB would showed very high glass transition temperature (Tg) of 220 °C and
show higher TADF performance in its OLED device.
219 °C for 3CzTB and M3CzB, respectively. These results suggest
that these materials possess high thermal stability which is
Fig. 3. Temperature dependent time resolved photoluminescence spectra of
emitters in 20 wt% doped DBFPO films of (a) 3CzTB and (b) M3CzB.
Electrochemical and thermal properties
To evaluate the redox potentials of the materials cyclic
voltammogram was performed using thin film of the materials.
The HOMO values were calculated from the oxidation potential
of the materials. The calculated HOMO values are −5.63 eV and
−5.62 eV for 3CzTB and M3CzB, respectively. The LUMO values
were estimated from the HOMO and band gap values. The band
gap values were obtained from their absorption onset. The
slightly lower band gap of M3CzB (2.95 eV) than 3CzTB (2.99 eV)
suggests the enhanced donor-acceptor interaction of the
former. The calculated LUMO values are −2.64 eV and −2.67 eV
for 3CzTB and M3CzB, respectively. The methyl group in the
M3CzB increased the HOMO level by 0.01 eV and tert-butyl
group in 3CzTB increased the LUMO level by 0.03 eV. Though
both materials possess similar donor and acceptor skeleton, a
small change in their band gap and LUMO is due to the tert-
butyl
group on the acceptor. This result indicates that the methyl
groups have negligible influence on the donor strength,
whereas the tert-butyl groups have small influence on the
acceptor strength. The electrochemical stability of these
materials were evaluated with five repeated cycles of cyclic
voltammogram as shown in Fig. 4. The 3CzTB showed one
oxidation on first cycle whereas on second cycle onwards it
showed a new lower oxidation peak which is originated from
the dimer of the oxidized species formed through
oligomerisation reaction via the terminal 6,6'-positions of
tercarbazole donor. This kind of electrochemical
oligomerisation reaction of carbazole is known in the
Fig. 4. Cyclic voltammogram of materials measured in thin film up to five repeated
cycles.
beneficial for use in stable OLED devices.
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DOI: 10.1039/C9TC05950D
Table 1. Photophysical and electrochemical properties of materials.
a
a
a
a
b
b
5
b
푘푟푖푠푐 (10 )
ʎabs
ʎem
S1
T1
∆EST
(eV)a
0.23
0.14
ΦFb
(%)
87.8
92.8
τp
τd
HOMO
(eV) c
-5.63
-5.62
LUMO
(eV) c
-2.64
-2.67
Band gap
(eV) c
2.99
(nm)
380
376
(nm)
433
445
(eV)
3.16
3.06
(eV)
2.93
2.92
(ns)
23.40
88.20
(μs)
9.32
7.84
(s−1)
1.02
1.40
3CzTB
M3CzB
2.95
a Recorded in toluene solution (10−5M), b measured for 20 wt% in DBFPO host, c measured for neat film.
methylphenyl)benzenamine]) used as hole transporting layer
HTL), DCDPA (3,5-di(9H-carbazol-9-yl)-N,Ndiphenylaniline) used
as exciton blocking layer (EBL), DBFPO (2,8-bis
(diphenylphosphine oxide) dibenzofuran) used as host and EBL
Electroluminescence properties
Electroluminescence performance of these materials were
evaluated by fabricating blue TADF OLED device with a
configuration of ITO (50 nm)/HATCN (7 nm)/TAPC (50
nm)/DCDPA (10 nm)/DBFPO:20 wt% dopant (25 nm)/DBFPO (5
nm)/TPBi (15 nm)/LiF/Al (1.5/100 nm), where ITO (indium tin
and
TPBi
(1,3,5-tris(1-phenyl-1H-benzo[d]imidazole-2-
yl)benzene) used as electron transporting layer (ETL), LiF/Al
used as cathode. The energy level diagram of the structure of
the materials used for the fabrication of devices are shown in
Fig. S4. The electroluminescence performances of these
materials are shown in Fig. 5 and the data are presented in
Table 2.
oxide)
hexaazatriphenylenehexacarbonitrile) used as hole injection
layer, TAPC (4,4′-cyclohexylidenebis[N,N-bis(4-
used
as
anode,
HATCN
(1,4,5,8,9,11-
Fig. 5. Electroluminescence properties. (a) J-V-L curve, (b) device efficiency vs luminance, (c) current efficiency vs luminance and (d)
electroluminance spectra.
emissive layer in the presence of polar host medium. In addition
The current density of M3CzB-based device is higher than 3CzTB to that while rationalizing the EL performances using HOMO
indicates the balanced charge collection at the emissive layer in and LUMO levels charge dipoles formed at the interface of
the former device. Although the difference of HOMO and LUMO organic layers should also be considered. The existence of
level is very small between the two emitters, the more bipolar charge transfer induced dipoles of the emitter and the charge
nature of M3CzB than 3CzTB could help charge balance in the dipoles formed at the interface of adjacent layers leads to the
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shift in the molecular level of organic materials.27, 28 So, the dipole values and the PLQYs of the two emitters, VtiheweAertxicpleeOcntleinde
DOI: 10.1039/C9TC05950D
difference in current densities and operating voltages observed maximum EQE values are almost close with the experimental
due to the formation of band offsets. The observed maximum EQE values. Therefore, this result indicates that the high EQE of
luminance of M3CzB and 3CzTB devices are 18,160 cd/m2 and these emitters is enhanced by horizontal dipole orientation. The
11,690 cd/m2, respectively. The external quantum efficiency EQE at 1000 cd/m2 was 21.6% and 13.5% for M3CzB and 3CzTB,
(EQE) of these materials were evaluated to be 30.7% and 29.1% respectively. The efficiency roll-off is severe in case of 3CzTB
for M3CzB and 3CzTB, respectively. The high efficiency of these (53.6%) when compared to M3CzB (29.6%) is due to large ∆EST
emitters may attributed to horizontal dipole orientation which and long delayed lifetime. These devices showed high current
is induced by their donor and acceptor molecular skeletons. To efficiency of 46.7 cd/A and 36.4 cd/A for M3CzB and 3CzTB,
confirm this, we have evaluated the horizontal orientation by respectively. The EL of these devices showed red-shifted peak
using TRPL method.29 The experiment was performed for the 20 than the solution PL spectra due to the polar host medium.32
wt% doped DBFPO films of 3CzTB and M3CzB emitters and The CIE color coordinates of these devices are in the blue region
compared with the simulation as shown in Fig. 6. As expected, of (0.14, 0.19) and (0.14, 0.26) for M3CzB and 3CzTB,
these emitters showed horizontal orientation factor of 0.82 and respectively. This is the highest efficiency of tercarbazole-based
0.81 for 3CzTB and M3CzB, respectively. The successful blue TADF OLEDs reported so far with CIEy color coordinates
prediction of maximum EQE with the horizontal dipole vs PLQY <0.40.
was demonstrated.30,31 Thus, on considering the horizontal
Table 2. Electroluminescence properties of materials.
Max.
CE (cd/A)
EQE (%)
Von (v)
luminance
(cd/m2)
11,690
EL (nm)
CIE
Max/@1000 cd/m2
Max/@1000 cd/m2
3CzTB
M3CzB
3.1
3.0
36.4/17.1
46.7/31.4
29.1/13.5
30.7/21.6
470
478
(0.14, 0.19)
(0.14, 0.26)
18,160
on considering the CIE coordinates, M3CzB and 3CzTB showed
deeper blue compared to TDBA-DI. Thus, the M3CzB based
device is the highly efficient blue TADF device at this color
coordinates based on tercarbazole donor-based materials
reported so far.18
Fig. 6. Measured and simulated radiative decay rate and dipole orientation of
20wt% doped in DBFPO films of (a) 3CzTB and (b) M3CzB.
The device lifetime of the emitters 3CzTB and M3CzB were
evaluated by fabricating stable OLED devices. In order to make
a stable device some materials were changed as shown in Fig.
34
S5.33,
The device lifetime (LT50) of these devices were
evaluated at 400 nits. The lifetime of devices are 60.5 hrs and
81 hrs for 3CzTB and M3CzB, respectively as shown in Fig. 7. The
EQE and CIE of the devices are 7.6% and (0.14, 0.10), 14.4% and
(0.13, 0.19) for 3CzTB and M3CzB, respectively. The longer
device lifetime of M3CzB is attributed to the presence of methyl
group on the terminals of the tercarbazole donor and relatively
short delayed exciton lifetime. Further, nearly two fold increase
of EQE is attributed to low ∆EST. However, the EQE is lower than
the DBFPO hosted devices due to low triplet energy level of
mCBP-CN host resulting in some triplet energy loss. It is
interesting to compare the lifetime of these devices with the
lifetime of TDBA-DI device.23 The TDBA-DI doped in mCBP-CN
host exhibited lifetime (LT50) of 55.2 hrs with EQE and CIE of
21.36% and (0.14, 0.27), respectively, at 1,000 nits. However,
Fig. 7. Electroluminescent device operating lifetime (LT50) at 400 nits.
Conclusion
Two highly efficient blue TADF emitters M3CzB and 3CzTB
based on tercarbazole donor and boron acceptor were designed
and synthesized to have high PLQY and horizontal dipole
orientation. Alkyl modification on the donor and acceptor units
tuned their donor-acceptor interaction and electronic nature
which altered their photophysical and electrochemical
6 | J. Name., 2012, 00, 1-3
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Page 7 of 9
Journal of Materials Chemistry C
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Journal Name
properties. The absence of tert-butyl group on the acceptor of 0.1
ARTICLE
M
tetrabutyl ammonium perchlorate (Bu4NClO4),
View Article Online
M3CzB emitter enhanced the ICT character which was acetonitrile solution were used as cDoOuIn: t1e0r.1,03w9/oCr9kTiCn0g59a5n0Dd
confirmed by lower ∆EST, higher red-shift in PL solvatochromism reference electrodes, respectively. For supporting electrolyte,
and lower bandgap than 3CzTB emitter. Additionally, the 0.1 M tetrabutyl ammonium perchlorate in acetonitrile solution
presence of methyl group at 6,6'-positions of tercarbazole was used. Using an internal ferrocene/ferrocenium (Fc/Fc+)
donor increased the electrochemical stability of M3CzB emitter. standard, the potential values were converted to the saturated
The blue TADF OLEDs fabricated with these emitters showed calomel electrode (SCE) scale. The optical band-gap was
maximum EQE/current efficiency/luminance of 30.7%/46.7 determined from absorption onset. The LUMO level of each
cdA−1/18,160 cdm−2 and 29.1%/36.4 cdA−1/11,690 cdm−2 for material was calculated from both the optical band gap and
M3CzB and 3CzTB, respectively. The high efficiency of these HOMO level.
devices may attributed to horizontal dipole orientation of these
emitters originated from the rigid/planar donor and acceptor Optical simulation
skeletons. In addition to that, the stable blue TADF device
The optical simulations of the OLED devices were performed by
fabricated with these emitters exhibited device operating
using the semiconducting emissive thin film optics simulator
lifetime (LT50) of 81 hrs and 60.5% hrs with maximum EQE/CIE
(SETFOS 4.3) simulation program, which enabled to input the
coordinates of 14.4%/(0.13, 0.19) and 7.6%/(0.14, 0.10) for
refractive index (n), extinction coefficient (k), dipole orientation
factor of the emitters, thickness of layers and the emission
M3CzB and 3CzTB, respectively. This result is the best efficient
blue TADF device based on tercarbazole TADF emitters reported
spectra of emissive layer.
so far. Thus, we strongly believe that this design tactic would
help to design new efficient blue TADF emitters.
Device fabrication and characterization
To fabricate OLEDs, Indium-Tin-Oxide (ITO) coated glass
substrates (50 nm, sheet resistance of 10 Ω/square,) were
Experimental Section
sequentially cleaned in ultrasonic bath with acetone, and
Materials and characterization
isopropyl alcohol for 10 minutes each, and then rinsed with
All reagents for synthesizing the materials were purchased from
Sigma–Aldrich, TCI (SEJINCI) and used as such. 1,4,5,8,9,11-
hexaazatriphenylene-hexacarbonitrile (HATCN) was purchased
from EM Index. 1,1-bis[(di-4-tolylamino) phenyl] cyclo-hexane
(TAPC), and 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-
benzimidazole) (TPBi) were purchased from Jilin OLED Material
Tech Co., Ltd. The other materials, 3,5-di(9H-carbazol-9-yl)-N,N-
diphenylaniline (DCDPA), 2,8-bis(diphenylphosphine oxide)
deionized water. Finally, substrates were dried using nitrogen
followed by UV-ozone treatment for 10 minutes. All organic
layers and metal cathode were deposited on the pre-cleaned
ITO glass by vacuum evaporation technique under a vacuum
pressure of ~1×10-7 torr. The deposition rate of all organic layers
in was about 0.5 Å/s. Similarly, the deposition rate of LiF and Al
were maintained at 0.1 Å/s, 4.0 Å/s, respectively. Finally, all
devices were encapsulated using glass cover and UV curable
resin inside the nitrogen filled glove box. The OLED area was 4
mm2 for all the samples studied in this work. J-V and L-V
characteristics of fabricated OLED devices were measured by
using Keithley 2635A SMU and Konica Minolta CS-100A,
respectively. EL spectra and CIE 1931 color coordinates were
obtained using Konica Minolta CS-2000 spectroradiometer. All
measurements were performed in ambient condition.
dibenzofuran (DBFPO) were synthesized by using the previously
36
reported methods.35,
For the characterization of photo-
physical properties in solution, all materials were prepared in
toluene solution at the concentration of 1 × 10−5 M. The UV-vis
absorption
spectrum
was
measured
by
V-750
Spectrophotometer (Jasco). The solution PL spectrum and low
temperature (77 K) PL spectrum were measured by FP-8500
Spectrofluorimeter (Jasco). The low temperature PL spectra was
measured after 30ms time delay. The absolute PLQY values in
doped films were measured by connecting an integrating
sphere to the same spectrofluorimeter. These doped films were
also used for transient PL measurement. Transient PL was
measured when photon counts were reached until 10,000 in a
nitrogen environment using Quantaurus-Tau fluorescence
lifetime measurement system (C11367-03, Hamamatsu
Photonics Co.). Thermal properties of newly synthesized
materials were investigated by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). The glass
transition temperature (Tg) was measured by DSC graph and the
decomposition temperatures (Td) of emitters measured by TGA
at 5% weight loss. Electrochemical analyses were performed
using EC epsilon electrochemical analysis equipment. To
measure the cyclic voltammetry (CV) characteristics of TADF
emitters, platinum, carbon wire and Ag wire in 0.01 M AgNO3,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Research Foundation
of Korea Grant (No. NRF-2019R1I1A1A01058272) and the
Technology Innovation Program (20006464, Development of
RGB delayed fluorescent material and ink-jet printing
technology with quantum efficiency more than 20% for mass
production of large-area OLED) funded By the Ministry of Trade,
Industry & Energy (MOTIE, Korea). And this work was also
supported by Samsung Display Co., Ltd.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal of Materials Chemistry C
Page 8 of 9
ARTICLE
Journal Name
30
S.-Y. Kim, W.-I. Jeong, C. Mayr, Y.-S. Park, K.-H. Kim, J.-H.
Lee, C.-K. Moon, W. Brütting anDdOJI.:-1J0. .K10im39,/CA9dTvC.0F5u9n50cDt.
Mater., 2013, 23, 3896-3900.
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TOC
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DOI: 10.1039/C9TC05950D
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