High-performance bipolar host materials for blue TADF devices with excellent external quantum efficiencies

Article abstract of DOI:10.1039/c6tc00385k

New bipolar host molecules composed of carbazole, pyridoindole, and dibenzothiophene (DBT) were synthesized for blue thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs). 2,8-Di(9H-carbazol-9-yl)dibenzo[b,d]thiophene, 9-(8-(9H-carbazol-9-yl)dibenzo[b,d]thiophen-2-yl)-9H-pyrido[2,3-b]indole, and 2,8-bis(9H-pyrido[2,3-b]indol-9-yl)dibenzo[b,d]thiophene were prepared based on the selective reactivity at the 2,8-positions of DBT. The new symmetric and asymmetric host materials exhibited high triplet energies (2.89-2.95 eV). 4,5-Di(9H-carbazol-9-yl)phthalonitrile (2CzPN) was selected as an emitting dopant for achieving sky-blue emissions in TADF-OLEDs. 2CzPN-doped TADF-OLEDs, whose configuration is ITO (50 nm)/HATCN (7 nm)/TAPC (75 nm)/host:6% 2CzPN (20 nm)/TmPyPB (50 nm)/LiF (15 nm)/Al (100 nm), showed low driving voltages and high external quantum efficiencies (EQEs). These results are attributed to the well-controlled bipolar character of the host giving a better charge balance in the emitting layer. In particular, the device containing ZDN:6% 2CzPN showed an unprecedentedly high EQE of 25.7% (at 0.074 mA cm^-2).

Full text of DOI:10.1039/c6tc00385k

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Cho, J. H. Kwon and D. H. Choi, J. Mater. Chem. C, 2016, DOI: 10.1039/C6TC00385K.
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DOI: 10.1039/C6TC00385K
Journal Name
ARTICLE
High-performing Bipolar Host Materials for Blue Thermally
Activated Delayed Fluorescent Devices with Excellent External
Quantum Efficiencies
Received 00th January 20xx,
Accepted 00th January 20xx
Ju Sik Kang,a Tae Ryang Hong,a Hyung Jong Kim,a Young Hoon Son,b Raju Lampande, b Byoung Yeop
Kang,b Chiho Lee, a Jong-Kwan Bin,c Bang Sook Lee,c Joong Hwan Yang,c JinWuk Kim,c Sungnam
Park, a Min Ju Cho,*a Jang Hyuk Kwon,*b and Dong Hoon Choi *a
DOI: 10.1039/x0xx00000x
www.rsc.org/
New bipolar host molecules composed of a carbazole, pyridoindole, and dibenzothiophene (DBT) were synthesized for
blue thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs). 2,8-Di(9H-carbazol-9-
yl)dibenzo[b,d]thiophene, 9-(8-(9H-carbazol-9-yl)dibenzo[b,d]thiophen-2-yl)-9H-pyrido[2,3-b]indole, and 2,8-bis(9H-
pyrido[2,3-b]indol-9-yl)dibenzo[b,d]thiophene were prepared based-on the selective reactivity at the 2,8-positions of DBT.
The new symmetric and asymmetric host materials exhibited high triplet energies (2.89–2.95 eV). 4,5-Di(9H-carbazol-9-
yl)phthalonitrile (2CzPN) was selected as an emitting dopant for achieving sky-blue emissions in TADF-OLEDs. 2CzPN-
doped TADF-OLEDs whose configuration is ITO (50 nm)/HATCN (7 nm)/TAPC (75 nm)/Host:6% 2CzPN (20 nm)/TmPyPB (50
nm)/LiF (15 nm)/Al (100 nm) had low driving voltages and high external quantum efficiencies (EQEs). These results are
attributed to the well-controlled bipolar character of host giving better charge balance in the emitting layer. In particular,
the device containing ZDN : 6% 2CzPN had an unprecedentedly high EQE of 25.7% (at 0.074 mA/cm2).
having higher S1 and T1 energy levels than those of the dopant,
efficient energy transfer to the dopant, low carrier injection
barriers from both the carrier’s transporting layers, and
Introduction
High quantum efficiency is an important factor in producing
competitive organic light-emitting diode (OLED) devices. Until
now, the focus has been on phosphorescent OLEDs (PhOLEDs),
which have high quantum efficiencies based on the spin
selection rule.[1,2] In 2012, Adachi et al. described a new
concept, namely thermally activated delayed fluorescence
(TADF)-OLED systems, with dopant/host materials, for
obtaining very high quantum efficiencies.[3] Reverse
intersystem crossing (RISC) distinguishes TADF-OLEDs from
PhOLEDs or conventional fluorescent OLEDs. Effective RISC
occurs when the emitting material has a small energy gap (ΔEST)
between the singlet (S1) and triplet (T1) energy states. Facile
up-conversion from the T1 to S1 energy state in emitting
materials can enhance the external quantum efficiency (EQE)
by increasing the number of excitons in the S1 state.[3–6] One of
the keys to achieving a high EQE is that the host material for
the emitting dopant must satisfy crucial requirements such as
efficient exciton confinement. The host materials in TADF-
OLEDs are similar to those in PhOLEDs, although some minor
prerequisites need to be fulfilled. The host material can be a
bipolar material or a mixture of two dissimilar unipolar
materials.[7,8] For a mixture of two different unipolar hosts,
charge carriers can be easily injected into the emitting layer,
because of the uncomplicated tuning of the energy level
alignment with adjacent transport layers. However, the use of
a bipolar host has the advantage of fewer interfacial contacts
between adjacent layers, resulting in a lower density of
trapping sites at interfaces.[9,10] Irrespective of strong
recognition of host materials, there are few host materials
which were demonstrated compared to emitting dopants in
TADF-OLEDs. High-performance host materials therefore need
to be developed to enhance the light-emitting efficiencies of
TADF-OLEDs. Among the many host materials for TADF-OLEDs,
blue host molecules have been significantly less studied,
because it is difficult to obtain high T1 energies and
appropriate highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels for facile
charge injections. In particular, achieving charge balance of the
bipolar host is a barrier to obtain successful blue TADF-OLED
host materials.[11,12]
In previous studies, 2,8-bis(diphenylphosphoryl)dibenzo
[b,d]thiophene (PPT) and 1,3-di(9H-carbazol-9-yl)benzene
(mCP) were used as host materials in 4,5-di(9H-carbazol-9-
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purification.
2,8-Dibromodithiophene
(2),[17]
2,8-
yl)phthalonitrile (2CzPN)-based sky-blue TADF-OLEDs; the
corresponding devices had EQEs of around 8% and 13.6%, with
the aid of auxiliary layers made of mCP and PPT to increase
charge injection and transport.[3,13] However, the
performances of 2CzPN-based devices have not yet been
optimized. Despite the high T1 energies of mCP (2.90 eV) and
PPT (3.02 eV), their biased carrier-transport capabilities cannot
achieve the desired charge balance in the emitting layer,
resulting in low EQEs in the 2CzPN devices.
diiododibenzothiophene (4),[28] 9H-pyrido[2,3-b]indole,[26] and 2,8-
dibromodibenzothiophene 5,5-dioxide (7)[29] were prepared using
previously reported methods. Dipyrazino[2,3-f:2',3'-h]quinoxaline-
2,3,6,7,10,11-hexacarbonitrile (HATCN), 1,1- bis[4-[N,N′-di(p-
tolyl)amino]phenyl]cyclohexane
(TAPC),
and
1,3,5-tri[(3-
pyridyl)phen-3-yl]benzene (TmPyPB) for thermally activated
delayed fluorescence-organic light-emitting diode fabrication were
purchased from the LUMTEC Co.
Carbazole which has a rigid molecular frame, sufficiently
high T1 value, and good hole-transport property has often
been used to construct host materials with desired structures
for high-T1 blue host materials.[14] In addition,
dibenzothiophene (DBT), which is thermally stable and has two
reaction sites, has been used as the core unit, because it can
be easily substituted with various electron-donating or -
withdrawing moieties such as carbazoles,[15-19] triarylamines,[20]
cyano groups,[21] and diphenylphosphine oxide;[15,22] these can
alter the intrinsic electronic characteristics of the bipolar host.
In addition to these moieties with electron affinities, pyri-
S
N
N
N
N
N
N
ZDZ
S
do[2,3-b]indole (i.e.,
α-carboline), which has a pyridine-
N
containing heteroaromatic ring, has been identified as a useful
electron-accepting moiety. Recently, new carboline-based host
materials consisting of carbazole and simple core materials
have been shown to have highly efficient bipolar
characteristics; the corresponding blue PhOLED exhibited
significantly improved device efficiency.[23–27] However, there
has been no report of the use of structures containing
carbazole and carboline moieties in blue TADF-OLEDs and their
corresponding device efficiencies. It is therefore worth
investigating how varying the number of carbazole and
carboline moieties in DBT-based host materials through direct
linkages affects the TADF-OLED performance.
ZDN
S
N
N
NDN
In this study, we synthesized three different host molecules
with carbazole, carboline, and DBT moieties in one molecular
structure, i.e., 2,8-di(9H-carbazol-9-yl)dibenzo[b,d]thiophene
Figure 1. Molecular structures of ZDZ, ZDN, and NDN
Synthesis of 2,8-di(9H-carbazol-9-yl)dibenzo[b,d]thiophene (3,
ZDZ).
(
ZDZ), 9-(8-(9H-carbazol-9-yl)dibenzo[b,d]thiophen-2-yl)-9H-
pyrido[2,3-b]indole (ZDN), and 2,8-bis(9H-pyrido[2,3-b]indol-9-
yl)dibenzo[b,d]thiophene (NDN). In these host molecules, we
tethered carbazole and carboline units directly to the 2,8-
positions of the DBT unit. ZDZ bears two carbazole units, ZDN
has one carbazole and one carboline unit, and NDN has two
carboline units, enabling them to display different bipolar
characters (Figure 1). The three host molecules have similar T1
levels, i.e., greater than 2.89 eV. The 2CzPN-doped TADF-
OLEDs had lower driving voltages (4.7–5.0 V) and significantly
higher EQEs than did devices containing an mCP host. In
particular, the TADF-OLED device with a ZDN host had an
unprecedentedly high EQE of around 25.7 %.
2,8-Dibromo[b,d]dibenzothiophene (4 g, 11.7 mmol), 9H-
carbazole (4.4 g, 26.3 mmol), tribasic potassium phosphate
(12.4 g, 58.5 mmol), copper(I) iodide (0.8 g, 4.2 mmol), and
trans-1,2-diaminocyclohexane (1.3 g, 11.4 mmol) were mixed
in toluene (110 mL). After purging for 30 min under Ar gas, the
solution was stirred and refluxed for 12 h. The solution was
cooled to room temperature and extracted with MC and water.
The extracted organic layer was washed twice with water,
dried with anhydrous magnesium sulfate, and evaporated
under vacuum. The resulting crude product was purified by
silica-gel column chromatography with MC/n-hexane (1:3) as
the eluent. The obtained product was recrystallized from
1
methanol, yielding 2.0 g (33 %) of pure product. H NMR (300
Experimental
MHz, CDCl3):
(d, 2H), 7.44-7.28 (m, 14H). 13C NMR (100 MHz, CDCl3) :
δ
(ppm) 8.32 (s, 2H), 8.18-8.14 (d, 4H), 7.75-7.72
(ppm)
δ
141.38, 139.47, 136.82, 135.15, 126.83, 126.32, 124.62, 123.65,
120.83, 120.66, 120.34, 109.80. Mass m/z [(M+H)+] Calcd. for
C36H22N2S: 514.15, found 515.73. Elemental analysis Calcd. for
Materials
All chemicals for synthesizing the host materials were pur-chased
from Aldrich, TCI, and Alfa Aesar, and were used without further
2 | J. Name., 2012, 00, 1-3
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C36H22N2S: N (5.44%), C (84.02%), H (4.31%), S (6.23%), found
N (5.44%), C (83.53%), H (4.26%), S (6.25%).
Scheme 1. Synthetic procedures for host materials: (i) bromine, methylene chloride (MC), 72 h, room temperature (rt); (ii)
carbazole, K3PO4, CuI, 1,2-diaminocyclohexane, toluene, 12 h, reflux; (iii) carboline, K3PO4, CuI, 1,2-diaminocyclohexane,
toluene, 12 h, reflux; (iv) LiAlH4, diethyl ether, 5 h. rt.
Synthesis
of
9-(8-iododibenzo[b,d]thiophen-2-yl)-9H- with MC and water. The extracted organic layer was washed
pyrido[2,3-b]indole (5)
twice with water, dried with anhydrous magnesium sulfate,
2,8-Diiodo[b,d]dibenzothiophene (10 g, 22.9 mmol), 9H- and evaporated under vacuum. The resulting crude product
pyrido[2,3-b]indole (3.8 g, 22.6 mmol), tribasic potassium was purified by silica-gel column chromatography with MC/n-
phosphate (24 g, 113 mmol), copper(I) iodide (2.2 g, 11.6 hexane (1:3) as the eluent. The crude product was
mmol), and trans-1,2-diaminocyclohexane (2.5 g, 21.9 mmol) recrystallized from methanol, yielding 1.5 g (70 %) of pure
1
were mixed in toluene (500 mL). After purging for 30 min product. H NMR (300 MHz, CDCl3):
δ (ppm) 8.46 (s, 1H), 8.40
under Ar gas, the solution was stirred and refluxed for 12 h. (d, 2H), 8.31 (s, 1H), 8.16-8.11 (m, 5H), 7.79 (d, 1H), 8.69 (d,
The solution was cooled to room temperature and extracted 1H), 7.51-7.22 (m, 10H). 13C NMR (100 MHz, CDCl3):
(ppm)
δ
with MC and water. The extracted organic layer was washed 152.40, 146.79, 141.46, 140.51, 139.78, 139.49, 137.00, 136.62,
twice with water, dried with anhydrous magnesium sulfate, 134.94, 133.66, 128.67, 127.34, 126.99, 126.64, 126.28, 124.52,
and evaporated under vacuum. The resulting crude product 124.38, 123.58, 121.30, 121.24, 121.09, 121.01, 120.60, 120.24,
was purified by silica-gel column chromatography with MC/n- 116.62, 116.48, 110.42, 109.87, 104.99. Mass m/z [(M+H)+]
hexane (1:3) as the eluent. The crude product was Calcd. for C36H21N3S: 515.15, found 516.15. Elemental analysis
recrystallized from methanol, yielding 2.2 g (20 %) of pure Calcd. for C36H21N3S : N (8.15%), C (81.53%), H (4.11%), S
1
product. H NMR (300 MHz, CDCl3):
δ (ppm) 8.51 (d, 1H), 8.48 (6.22%), found N (8.09%), C (81.05%), H (4.05%), S (6.36%).
(s, 1H), 8.45 (d, 1H), 8.37 (s, 1H), 8.18 (d, 1H), 8.08 (d, 1H), 7.78
(d, 2H), 7.66 (d, 1H), 7.51 (d, 1H), 7.43-7.27 (m, 3H).
Synthesis
of
2,8-bis(9H-pyrido[2,3-b]indol-9-yl)dibenzo
[b,d]thiophene 5,5-dioxide (8)
2,8-Dibromodibenzothiophene 5,5-dioxide (8 g, 21.4 mmol), 9H-
Synthesis of 9-(8-(9H-carbazol-9-yl)dibenzo[b,d]thiophen-2-
yl)-9H-pyrido[2,3-b]indole (6, ZDN)
pyrido[2,3-b]indole (7.6 g, 45.2 mmol), tribasic potassium
9-(8-Iododibenzo[b,d]thiophen-2-yl)-9H-pyrido[2,3-b]indole (2 phosphate (22 g, 103.6 mmol), copper(I) iodide (2 g, 10.5 mmol),
g, 4.2 mmol), 9H-carbazole (0.7 g, 4.2 mmol), tribasic po-
tassium phosphate (4.5 g, 21.2 mmol), copper(I) iodide (0.4 g,
2.1 mmol), and trans-1,2-diaminocyclohexane (0.5 g, 4.4 mmol)
were mixed in toluene (150 mL). After purging for 30 min
under Ar gas, the solution was stirred and refluxed for 12 h.
The solution was cooled to room temperature and extracted
and trans-1,2-diaminocyclohexane (2.4 g, 21.0 mmol) were mixed in
toluene (800 mL). After purging for 30 min under Ar gas, the
solution was stirred and refluxed for 12 h. The solution was cooled
to room temperature and extracted with MC and water. The
extracted organic layer was washed twice with water, dried with
anhydrous magnesium sulfate, and evaporated under vacuum. The
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