A design strategy of bipolar host materials for more than 30 times extended lifetime in phosphorescent organic light-emitting diodes using benzocarbazole and quinazoline

Article abstract of DOI:10.1039/c7tc02973j

Stable host materials based on benzocarbazole and quinazoline were synthesized to establish a design strategy of bipolar hosts for elongated lifetime in orange phosphorescent organic light-emitting diodes (PHOLEDs). Three host materials with benzocarbazole and quinazoline connected directly or through a phenyl linker were compared in terms of the device lifetime of the orange PHOLEDs. The connection between benzocarbazole and quinazoline through a phenyl linker was an effective way of improving the lifetime of the red PHOLEDs by more than twice and the molecular orbital distribution of the hosts was well correlated with the lifetime of the PHOLEDs. In particular, the lifetime of the PHOLEDs was extended by more than 30 times compared to a conventional PHOLED with a common CBP host.

Full text of DOI:10.1039/c7tc02973j

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC02973J
Design strategy of bipolar host materials for more than 30 times extended lifetime
in phosphorescent organic light-emitting diodes using benzocarbazole and
quinazoline
Sung Moo Kim, Ju Hui Yoon, Si Hyun Han, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2066, Seobuꢀro, Janganꢀgu, Suwonꢀsi, Gyeonggiꢀdo, 440ꢀ746, Korea
Fax: (+) 82ꢀ31ꢀ299ꢀ4716
Eꢀmail: leej17@skku.edu
* To whom correspondence should be addressed
Abstract
Stable host materials based on benzocarbazole and quinazoline were synthesized to establish a design
strategy of bipolar hosts for elongated lifetime in orange phosphorescent organic lightꢀemitting diodes
(PHOLEDs). Three host materials with benzocarbazole and quinazoline connected directly or through a
phenyl linker were compared in terms of device lifetime of the orange PHOLEDs. The connection
between benzocarbazole and quinazoline through a phenyl linker was an effective way of improving
the lifetime of the red PHOLEDs by more than twice and the molecular orbital distribution of the hosts
was well correlated with the lifetime of the PHOLEDs. In particular, the lifetime of the PHOLEDs was
extended by more than 30 times compared to a conventional PHOLEDs with a common CBP host.
Key words : benzocarbazole, quinazoline, orange device, long lifetime, phosphorescent device
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Introduction
The operational lifetime along with quantum efficiency and driving voltage is one of the most
important characteristics in the organic lightꢀemitting diodes (OLEDs). The lifetime of OLEDs has
been improved significantly over the last twenty years by developing stable materials and device
structures. However, the lifetime of the OLEDs still needs to be further improved in order to be used
for various purposes.[1,2] Particularly, the lifetime of high efficiency OLEDs like phosphorescent
OLEDs (PHOLEDs) is required to be extended.[3ꢀ9]
There are various methods of improving the lifetime of the PHOLED and those methods are mostly
related with device structure or materials. Although device structure engineering avoiding the damage
of the devices by degradation pathways was critical to the lifetime of PHOLEDs[10], a significant
progress of the device lifetime was accompanied by new material development. In particular, material
engineering of hosts was one of the most useful methods for improved lifetime in the PHOLEDs.[11ꢀ15]
A lot of hosts were engineered to be stable under electrical operation of the PHOLEDs through
bipolar charge carrying characteristics and aromatic molecular structures to withstand electrical stress
during device driving. For instance, triazine and carbazole type hosts which are bipolar hosts with an
aromatic chemical moieties offered long lifetime in the PHOLEDs.[16] Another example is
cyanobenzene and carbazole type hosts which resist device degradation. Other than these, there are
many hosts which can be stably operated with little degradation of luminance and most of them are
bipolar host materials. Therefore, the development of bipolar hosts is to be continued for exploration of
stable hosts for longꢀliving PHOLEDs.[17,18]
In this work, three stable host materials with chemical building blocks of benzocarbazole and
quinazoline, 7ꢀ(2ꢀphenylquinazolinꢀ4ꢀyl)ꢀ7Hꢀbenzo[c]carbazole (BZCꢀQ), 7ꢀ(3ꢀ(2ꢀphenylquinazolinꢀ4ꢀ
yl)phenyl)ꢀ7Hꢀbenzo[c]carbazole (BZCꢀmPQ) and 7ꢀ(4ꢀ(2ꢀphenylquinazolinꢀ4ꢀyl)phenyl)ꢀ7Hꢀ
benzo[c]carbazole (BZCꢀpPQ), were developed to increase the lifetime of orange PHOLEDs. The
presence of a phenyl linker between benzocarbazole and quinazoline was correlated with the lifetime of
orange PHOLEDs doped with
a
(2,4ꢀpentanedionato)bis[2ꢀ(2ꢀquinolinyl)phenyl]iridium(III)
2

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(Ir(pq)2acac) emitter. BZCꢀmPQ and BZCꢀpPQ using a phenyl linker were found to be superior to
BZCꢀQ without a phenyl linker in terms of the lifetime of orange PHOLEDs and their device lifetime
was more than 30 times longer than that of a common CBP device at initial luminance of 1000 cd/m2.
Experimental
General information
Sodium
tertꢀbutoxide
(NaO(tꢀbu)),
(Pd2(dba)3),
triꢀtertꢀbutylphosphine
(P(tꢀbu)3),
tris(dibenzylideneacetone)dipalladium(0)
tetrakis(triphenylphosphine)palladium(0)
(Pd(PPh3)4) (P&H Tech Co.), naphthalenꢀ1ꢀyl boronic acid, 1ꢀbromoꢀ2ꢀnitrobenzene, 4ꢀchloroꢀ2ꢀ
phenylquinazoline, and sodium hydride (Aldrich) were used without further purification. Sodium
hydroxide (NaOH), toluene, and tetrahydrofuran (THF) (Duksan Sci. Co.) were also used as received.
Synthesis
1-(2-Nitrophenyl)naphthalene (1)
A mixture of naphthalenꢀ1ꢀyl boronic acid (2.5 g, 14.5 mmol), 1ꢀbromoꢀ2ꢀnitrobenzene (2.94 g, 14.5
mmol) and Pd(PPh3)4 (0.84 g, 0.73 mmol) in THF (60 ml) and NaOH (1.74 g, 43.5 mmol) in water (20
ml) were added into a flask. The reaction mixture was stirred and refluxed for 1 h under a nitrogen
atmosphere. The reaction mixture was diluted with methylene chloride and washed with water. The
organic layer was dried with anhydrous MgSO4 and the solvent was removed in vacuum. The crude
product was purified by column chromatography on silica gel using a methylene chloride/nꢀhexane
eluent. The product was obtained as a yellow powder (3.1 g, Yield 86%).
1H NMR (500 MHz, DMSO): δ 8.16 (d, 1H, J=8.0 Hz), 8.01 (t, 2H, J=8.0 Hz), 7.87 (t, 1H, J=8.0 Hz),
7.77 (t, 1H, J=8.0 Hz), 7.57 (m, 3H), 7.47 (t, 1H, J=7.5 Hz), 7.40 (d, 1H, J=7.0 Hz), 7.36 (d, 1H, J=8.0
Hz).
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7H-benzo[c]carbazole (2)
A mixture of 1ꢀ(2ꢀnitrophenyl)naphthalene (3.1 g, 12.5 mmol) and triphenylphosphine (9.8 g, 37.5
mmol) in 1,2ꢀdichlorobenzene (60 ml) was added into a flask. The reaction mixture was stirred and
refluxed for 4 h under a nitrogen atmosphere. The reaction mixture was diluted with ethyl acetate and
washed with water. The organic layer was dried with anhydrous MgSO4 and the solvent was removed
in vacuum. The crude product was purified by column chromatography on silica gel using ethyl
acetate/nꢀhexane. The product was obtained as a brown powder (1.9 g, Yield 70%).
1H NMR (500 MHz, DMSO): δ 11.40 (s, 1H), 8.77 (d, 1H, J=8.5 Hz), 8.57 (d, 1H, J=8.5 Hz), 8.05 (d,
1H, J=8.0 Hz), 7.91 (d, 1H, J=9.0 Hz), 7.75 (d, 1H, J=8.5 Hz), 7.70 (t, 1H, J=8.0 Hz), 7.64 (d, 1H,
J=8.0 Hz), 7.45 (m, 2H), 7.31 (t, 1H, J=7.5 Hz). MS (APCIꢀ) m/z: 216.5 [(MꢀH)ꢀ]
7-(2-phenylquinazolin-4-yl)-7H-benzo[c]carbazole (BZC-Q)
Sodium hydride (0.03 g, 1.3 mmol) was added to a stirred solution of 7Hꢀbenzo[c]carbazole (0.22 g,
1.00 mmol) in dry DMF (30 mL). After stirring for 10 min, 4ꢀchloroꢀ2ꢀphenylquinazoline (0.29 g, 1.20
mmol) was added to the flask. The reaction mixture was stirred at room temperature for 1 h under a
nitrogen atmosphere at room temperature. The reaction mixture was diluted with methylene chloride
and washed with water. The organic layer was dried with anhydrous MgSO4 and the solvent was
removed in vacuum. The crude product was purified by column chromatography on silica gel using
ethyl acetate /nꢀhexane. The product was obtained as a yellow powder (0.33 g, Yield 77%).
1H NMR (500 MHz, DMSO): δ 8.98 (d, 1H, J=8.5 Hz), 8.82 (d, 1H, J=7.5 Hz), 8.57 (m, 2H), 8.31 (d,
1H, J=8.5 Hz), 8.14 (m, 2H), 7.97 (d, 1H, J=9.0 Hz), 7.83 (t, 1H, J=8.0 Hz), 7.68 (m, 3H), 7.60 (m, 7H).
13C NMR (125 MHz, DMSO): δ 159.89, 157.56, 153.39, 139.30, 137.90, 136.95, 135.55, 131.19,
129.59, 129.25, 128.96, 128.88, 128.76, 128.36, 128.17, 127.82, 127.62, 125.28, 125.25, 124.11,
124.05, 123.36, 122.20, 119.23, 116.10,112.96, 111.88. MS (APCI+) m/z: 422.1 [(M+H)+]
4-(3-Bromophenyl)-2-phenylquinazoline (3)
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A mixture of 4ꢀchloroꢀ2ꢀphenylquinazoline (1.0 g, 4.15 mmol), (3ꢀbromophenyl)boronic acid (1.0 g,
4.98 mmol), Pd(PPh3)4 (0.24 g, 0.21 mmol) in THF (50 ml), and NaOH (0.5 g, 12.45 mmol) in water
(20 ml) were added into the flask. The reaction mixture was stirred and refluxed for 1 h under a
nitrogen atmosphere. The reaction mixture was diluted with ethyl acetate and washed with water. The
organic layer was dried with anhydrous MgSO4 and the solvent was removed in vacuum. The crude
product was purified by column chromatography on silica gel using ethyl acetate/nꢀhexane. The
product was obtained as a white powder (0.9 g, Yield 58%).
1H NMR (500 MHz, DMSO): δ 8.61 (m, 2H), 8.14 (m, 1H), 8.05 (m, 2H), 7.87 (m, 2H), 7.72 (t, 1H,
J=9.0 Hz), 7.60 (m, 5H). MS (APCI+) m/z: 361.1 [(M+H)+]
7-(3-(2-Phenylquinazolin-4-yl)phenyl)-7H-benzo[c]carbazole (BZC-mPQ)
A mixture of 7Hꢀbenzo[c]carbazole (0.4 g, 1.84 mmol), 4ꢀ(3ꢀbromophenyl)ꢀ2ꢀphenylquinazoline (0.80
g, 2.21 mmol), NaO(tꢀbu) (0.5 g, 5.52 mmol), P(tꢀbu)3 (50%) (0.07 g, 0.18 mmol) and Pd2(dba)3 (0.08
g, 0.09 mmol) in toluene (30 ml) was added into the flask. The reaction mixture was stirred and
refluxed for 12 h under a nitrogen atmosphere. The reaction mixture was diluted with methylene
chloride and washed with water. The organic layer was dried with anhydrous MgSO4 and the solvent
was removed in vacuum. The crude product was purified by column chromatography on silica gel
using methylene chloride /nꢀhexane. The product was obtained as a white powder (0.75 g, Yield 82%).
1H NMR (500 MHz, DMSO): δ 8.91 (d, 1H, J=8.5 Hz), 8.76 (d, 1H, J=8.0 Hz), 8.63 (m, 2H), 8.31 (d,
1H, J=8.5 Hz), 8.18 (m, 2H), 8.15 (m, 2H), 8.08 (m, 1H), 8.03 (m, 3H), 7.79 (m, 3H), 7.70 (d, 1H,
J=8.5 Hz), 7.55 (m, 5H), 7.47 (t, 1H, J=7.5 Hz). 13C NMR (125 MHz, DMSO): δ 166.93, 159.02,
151.30, 139.34, 139.00, 137.83, 137.35, 136.81, 134.57, 130.85, 130.63, 129.69, 129.24, 129.14,
129.07, 128.92, 128.73, 128.61, 128.16, 127.74, 127.44, 126.78, 124.97, 123.44, 123.24, 123.13,
122.08, 121.16, 121.08, 114.84, 111.65, 110.36. MS (APCI+) m/z: 498.2 [(M+H)+]
4-(4-Bromophenyl)-2-phenylquinazoline (4)
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Except that (4ꢀbromophenyl)boronic acid (1.0 g, 4.98 mmol) is replaced by (3ꢀbromophenyl)boronic
acid, the synthesis process of 4ꢀ(4ꢀbromophenyl)ꢀ2ꢀphenylquinazoline(4) and 4ꢀ(3ꢀbBromophenyl)ꢀ2ꢀ
phenylquinazoline(3) was the same. The product was obtained as a white powder (1.1 g, Yield 75%).
1H NMR (500 MHz, DMSO): δ 8.60 (m, 2H), 8.15 (d, 1H, J=8.5 Hz), 8.10 (d, 1H, J=8.0 Hz), 8.05 (m,
1H), 7.98 (m, 2H), 7.72 (t, 1H, J=7.5 Hz), 7.59 (m, 3H), 7.56 (m, 2H). MS (APCI+) m/z: 361.1
[(M+H)+]
7-(4-(2-Phenylquinazolin-4-yl)phenyl)-7H-benzo[c]carbazole (BZC-pPQ)
Except that 4ꢀ(4ꢀbromophenyl)ꢀ2ꢀphenylquinazoline (0.80 g, 2.21 mmol) is replaced by 4ꢀ(3ꢀ
bromophenyl)ꢀ2ꢀphenylquinazoline, the synthesis process of BZCꢀpPQ and BZCꢀmPQ was the same.
The product was obtained as a white powder (0.7 g, Yield 77%).
1H NMR (500 MHz, DMSO): δ 8.92 (d, 1H, J=8.0 Hz), 8.77 (d, 1H, J=8.0 Hz), 8.69 (m, 2H), 8.35 (d,
1H, J=8.5 Hz), 8.26 (m, 2H), 8.21 (d, 1H, J=8.5 Hz), 8.12 (m, 2H), 8.03 (d, 1H, J=9.0 Hz), 7.98 (d, 2H,
J=8.5 Hz), 7.80 (m, 3H), 7.71 (d, 1H, J=8.0 Hz), 7.59 (m, 5H), 7.51 (t, 1H, J=9.0 Hz). 13C NMR (125
MHz, DMSO): δ 167.17, 159.08, 151.34, 139.25, 138.11, 137.75, 137.46, 136.37, 134.61, 131.98,
130.91, 129.25, 129.19, 129.06, 128.77, 128.21, 128.13, 127.76, 127.44, 126.97, 125.00, 123.49,
123.29, 123.16, 122.10, 121.25, 121.12, 114.92, 111.73, 110.46. MS (APCI+) m/z: 498.2 [(M+H)+]
Device fabrication and measurements
Orange PHOLEDs were fabricated using the sublimed BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ hosts. The
hosts were evaluated in the device stack structure of indium tin oxide (ITO, 120 nm)/ N,N′ꢀbisꢀ[4ꢀ
(phenylꢀmꢀtolylꢀamino)ꢀphenyl]ꢀbiphenylꢀ4,4′ꢀdiamine (DNTPD) (60 nm)/ N,N,N'N'ꢀtetra[(1,10ꢀ
biphenyl)ꢀ4ꢀyl]ꢀ(1,10ꢀbiphenyl)ꢀ4,4'ꢀdiamine (BPBPA) (30 nm)/ host:Ir(pq)2acac (30 nm, 3%)/ 1,3,5ꢀ
tris(Nꢀphenylbenzimidazoleꢀ2ꢀyl)benzene (TPBi) (35 nm)/ LiF (1.5 nm)/ Al (200 nm). A common
vacuum thermal evaporation process was used to deposit the organic materials, LiF, and Al. Single
carrier devices of ITO (120 nm)/ DNTPD (60 nm)/ BPBPA (30 nm)/ host (30 nm)/ DNTPD (10 nm)/
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Al (200 nm) (hole only device) and ITO (120 nm)/ TSPO1 (10 nm)/ Host (30 nm)/ TPBi (35 nm)/ LiF
(1.5 nm)/ Al (200 nm) (electron only device) were also fabricated by thermal evaporation process.
Voltage scan of the fabricated devices was carried out using Kiethley 2400 source measurement unit
and luminance was collected using CS1000 spectroradiometer. Constant current driving method was
used to test the lifetime of the PHOLEDs at an initial luminance of 1,000 cd/m2.
Results and discussion
BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ were designed as the compounds which can function as stable host
materials in orange PHOLEDs. For material stability, aromatic based rigid moieties were selected in the
buildꢀup of the hosts. Common carbazole moiety was replaced with a rigid aromaticꢀbased
benzocarbazole and common pyridine or pyrimidine unit was switched with a rigid quinazoline unit.
The benzocarbazole and quinazoline units may behave as a hole transport and an electron transport unit
due to electron rich and electron poor characters, respectively. Therefore, bipolar charge transport
properties are expected from host materials in addition to molecular stability.[19,20]
The benzocarbazole was synthesized as a hole transport unit of the bipolar host materials by ring
closing reaction of nitrated phenylnathalene using triphenylphosphine. Direct coupling of the
benzocarbazole with 4ꢀcholorꢀ2ꢀphenylquinazoline produced the BZCꢀQ host. Coupling of the
benzocarbazole with 4ꢀ(3ꢀbromophenyl)ꢀ2ꢀphenylquinazoline prepared by Suzuki coupling of 4ꢀchloroꢀ
2ꢀphenylquinazoline and 3ꢀbromophenylboronic acid yielded BZCꢀmPQ. The same synthetic procedure
was used for the preparation of BZCꢀpPQ by using 4ꢀ(4ꢀbromophenyl)ꢀ2ꢀphenylquinazoline instead of
4ꢀ(3ꢀbromophenyl)ꢀ2ꢀphenylquinazoline. Production yield of the three compounds was over 70% after
wet purification by column chromatography and recrystallization followed by dry purification by
temperature gradient sublimation.
Electronic molecular orbital of the host materials was calculated using a density functional theory
method implemented in Gaussian 09 program with a B3LYP 6ꢀ31G basis set and the highest occupied
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molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) pictures of BZCꢀQ, BZCꢀ
mPQ and BZCꢀpPQ are shown in Figure 1. The three host materials shared the similar HOMO and
LUMO distribution in that the HOMO is mostly localized in the benzofurocarbazole unit and the
LUMO is mostly dispersed in the quinazoline unit. However, they were differentiated in the orbital
distribution in that the HOMO of BZCꢀQ is slightly extended from the benzocarbazole unit to the
quinazoline unit, whereas the HOMO of BZCꢀmPQ and BZCꢀpPQ is isolated in the benzocarbazole
unit without any HOMO extension to the quinazoline unit. The HOMO of BZCꢀmPQ and BZCꢀpPQ
was positioned only in the phenyl linker and benzocarbazole. The same result was observed in the
LUMO calculation results. LUMO extension from the quinazoline unit to the benzocarbazole was
detected only in the BZCꢀQ host. In the case of BZCꢀmPQ and BZCꢀpPQ, the LUMO was detected in
the phenyl linker and quinazoline unit. The presence of the phenyl linker played a role of preventing
HOMO extension from the hole transport benzocarbazole unit to the electron transport type quinazoline
unit and LUMO extension from the electron transport type quinazoline to the hole transport type
benzocarbazole. In the geometrical structure of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ, the dihedral angle
quinazoline and adjacent aromatic units was relatively small compared to other dihedral angles due to
weak hydrogen bonding between N of quinazoline and H of adjacent aromatic units. For instance, the
dihedral angle between benzocarbazole and phenyl linker was 57.1 o, whereas that between quinazoline
and phenyl linker was 44.1 o in the BZCꢀmPQ host. The same trend was observed in the BZCꢀpPQ host.
Photophysical properties of the host materials were examined by ultravioletꢀvisible (UVꢀvis),
photoluminescence (PL) and low temperature PL analysis. Normalized UVꢀvis absorption, solid PL (in
polystyrene) and low temperature PL spectra are presented in Figure 2. Overall UVꢀvis absorption
spectra of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ were similar, but the absorption edge of the absorption
spectra appeared at short wavelength in the BZCꢀmPQ host by shortened conjugation. Paraꢀ connection
of benzocarbazole and quinazoline lead to long wavelength shift of the absorption edge by extensive
conjugation, while strong charge transfer character of BZCꢀQ resulted in the long wavelength
absorption edge. Solid PL emission obtained from polystyrene film doped with BZCꢀQ, BZCꢀmPQ and
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BZCꢀpPQ were detected at 435 nm, 433 nm and 437 nm, respectively. The first phosphorescence
emission peaks at 77 k were observed at 493 nm, 497 nm and 502 nm. Therefore, the triplet energies of
BZCꢀQ, BZCꢀmPQ, and BZCꢀpPQ were 2.52 eV, 2.49 eV, and 2.47 eV, respectively. Slight decrease of
triplet energy of BZCꢀmPQ from 2.52 eV of BZCꢀQ to 2.49 eV is caused by extension of conjugation
by the phenyl unit, and the reduction from 2.49 eV of BZCꢀmPQ to 2.47 eV is due to further extended
conjugation by paraꢀ orientation of the aromatic units.
Oxidation and reduction characteristics of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ were estimated using
cyclic voltammetry scans. Separate oxidation and reduction scan data of BZCꢀQ, BZCꢀmPQ and BZCꢀ
pPQ are shown in Figure 3. The ionization potential/electron affinity values of BZCꢀQ, BZCꢀmPQ and
BZCꢀpPQ determined from the electrochemical analysis were ꢀ6.15 eV/ꢀ3.37 eV, ꢀ6.06 eV/ꢀ3.38 eV and
ꢀ6.07 eV/ꢀ3,40 eV, respectively. The ionization potential of BZCꢀQ was deeper than that of BZCꢀmPQ
and BCZꢀpPQ because of the HOMO extension to the electron deficient quinazoline unit. The LUMO
was not very different although slightly shallow LUMO was validated in the BZCꢀQ by the LUMO
extension to the benzocarbazole unit. It seems that LUMO stabilizing effect of benzocarbazole by weak
electron accepting character resulted in the small change of the LUMO. Therefore, the HOMOꢀLUMO
gap was widened in the BZCꢀQ without the phenyl linker, whereas it was narrowed in the BZCꢀmPQ
and BZCꢀpPQ by separated HOMO and LUMO.
Using differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA), we estimated
the glass transition temperature (Tg) and thermal decomposition temperature (Td) as the thermal
properties of the host materials. DSC and TGA curves of host materials are shown in Figure 4. The Tgs
of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ were observed at 103 °C, 108 °C, and 118 °C from second
heating scan data of DSC measurement. Compared with the Tg of 62 °C of CBP, three hosts with lower
or similar molecular weight had much higher Tg by rigid molecular structure. Benzocarbazole is more
rigid than carbazole by the embedded naphthalene unit instead of phenyl of carbazole, and the
quinazoline is also a rigid unit by extended aromatic structure. Moreover, the hydrogen bonding
between N of quinazoline and H of neighboring unit also rigidify the molecular structure. Therefore,
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high Tg over 100 °C was observed in the three host materials. The phenyl linker increased the Tg of
BZCꢀmPQ and BZCꢀpPQ, and the paraꢀ orientation of aromatic units lead to increased Tg in the BZCꢀ
pPQ. Td at 5% weight loss was detected at 378 °C, 430 °C and 438 °C in the TGA thermogram. The Tg
and Td were correlated each other and all host materials exhibited high Td above 370 °C. Photophysical
and thermal analysis results of the host materials are summarized in Table 1.
From the material characterization results, BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ can be considered as the
potential host materials for improved lifetime in orange PHOLEDs due to the material properties such
as triplet energy over 2.4 eV for exothermic energy transfer, thermal stability represented by high Tg
o
over 100 C, and small HOMOꢀLUMO gap for bipolar carrier injection and transport. Therefore,
orange PHOLEDs with the BZCꢀQ, BZCꢀmPQ and BCZꢀpPQ host materials doped with Ir(pq)2acac
were developed. Device characteristics of the CBP, BZCꢀQ, BZCꢀmPQ and BCZꢀpPQ based orange
PHOLEDs are presented in Figure 5. Current density, voltage, and luminance relationship of the BZCꢀ
Q, BZCꢀmPQ and BCZꢀpPQ devices suggested that the BZCꢀmPQ and BZCꢀpPQ devices showed
higher current density and luminance than the BZCꢀQ device. In order to understand the current density
and luminance dependence on the molecular structure of the hosts, single carrier device data of the
three hosts were compared. Figure 6 shows current densityꢀvoltage characteristics of the single carrier
devices. Both hole and electron current densities were higher in the BZCꢀmPQ and BZCꢀpPQ devices
than in the BZCꢀQ device and this can be explained by the HOMO and LUMO distribution of the three
hosts because the HOMO is related with hole transport and the LUMO is related with electron transport.
In the HOMO and LUMO simulation results, the HOMO of BZCꢀmPQ and BZCꢀpPQ was dispersed
over the benzocarbazole and phenyl linker, but that of BZCꢀQ was distributed over the benzocarbazole
and quinazoline. As the quinazoline is an electron transport unit rather than hole transport unit, the hole
transport is better in the BZCꢀmPQ and BZCꢀpPQ host than in the BZCꢀQ host by large HOMO
overlap between molecules for hole hopping. The same explanation can be applied to the electron
current density of the three hosts. Therefore, the current density of the BZCꢀmPQ and BZCꢀpPQ orange
PHOLEDs was higher than that of the BZCꢀQ PHOLED.
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Although the current density and luminance were high in the BZCꢀmPQ and BZCꢀpPQ devices, QE
data in Figure 7 suggested that BZCꢀQ is better than other host materials to improve the QE of the
orange PHOLEDs. Maximum QEs of the BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ devices were 12.7%, 10.5%
and 11.5%, respectively. The different QEs of the three devices can be correlated with the hole and
electron current densities of the single carrier devices. Although the hole and electron current densities
were low in the BZCꢀQ device, they were balanced in the BZCꢀQ emitting layer, increasing the QE of
the orange PHOLEDs. The rather low QE of the BZCꢀmPQ device is due to too high hole current
density compared to the electron current density. The BZCꢀpPQ device was in between the BZCꢀQ
device and BZCꢀpPQ device. Other device parameters like energy transfer may not be the major factors
because the triplet energy of the three hosts was higher than that of the Ir(pq)2acac emitter (2.2 eV). All
host materials performed better than a common CBP in terms of maximum QE (9.6%) due to bipolar
charge transport character. Electroluminescence spectra in Figure 8 also substantiated that the three
hosts harvested phosphorescence of Ir(pq)2acac.
Lifetime of the BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ devices was also evaluated to examine the effect of
molecular structure on the operational lifetime of the orange PHOLEDs. Constant current driven
luminance change of the orange PHOLEDs from initial luminance of 1,000 cd/m2 was traced and it is
shown in Figure 9. The operational lifetimes of the BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ devices were
176 h, 401 h and 379 h up to 80% of initial luminance, respectively. The lifetime of the BZCꢀmPQ and
BZCꢀpPQ device was more than doubled relative to that of the BZCꢀQ device, and was more than 30
times longer than that of the common CBP device. In the case of the CBP device, the lifetime was only
11 h at the same test condition. It is difficult to simply explain the origin of the improved lifetime, but
three key factors seem to be responsible for the extended lifetime in the BZCꢀmPQ and BZCꢀpPQ
devices. Firstly, intrinsically extended aromatic character of the benzocarbazole and quinazoline
moieties can be proposed as one key factor for the improved lifetime. Extension of the aromatic
character in the molecular structure can stabilize the molecule by evenly sharing positive or negative
charges during electrical operation, which relieves degradation of the hosts and elongates the lifespan
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of the PHOLEDs. The three host materials are similar considering the aromatic character of the
o
chemical structures, but they are superior to the common CBP. Secondly, high Tg over 100 C of the
three hosts is a lifetime extending factor because morphological change during device driving is
suppressed. Although high Tg cannot explain the lifetime difference between the three hosts, the high Tg
is an advantage of the three hosts compared to CBP. Thirdly, the HOMO and LUMO isolation only in
the hole transport type benzocarbazole and electron transport type quinazoline can be regarded as one
of key factors for the increased lifetime. Isolation of the HOMO and LUMO indicates that holes and
electrons would dominantly remain in the benzocarbazole and quinazoline unit. It has been known that
electron rich unit is destabilized by negative polaron and electron poor unit is destabilized by positive
polaron[21]. Therefore, the isolation of HOMO in the electron rich benzocarbazole and LUMO in the
electron poor quinazoline is ideal for suppressing host degradation. This was accomplished in the BZCꢀ
mPQ and BZCꢀpPQ hosts, improving the lifetime of the orange PHOLEDs. However, the HOMO and
LUMO were not completely isolated in the BCZꢀQ device, which lead to the relatively short lifetime of
the BCZꢀQ device compared to the BZCꢀmPQ and BZCꢀpPQ devices. This result demonstrates that the
phenyl linker plays a key role of improving the lifetime of the devices by blocking molecular orbital
crossover to the neighboring unit. Therefore, it is essential to insert the phenyl linker in the molecular
design of bipolar host materials having hole transport and electron transport units.
Conclusions
Three stable host materials, BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ, have been designed for application
to orange PHOLED for long lifetime. In particular, BZCꢀmPQ and BZCꢀpPQ host showed a much
longer lifetime by more than 30 times than common CBP due to their stable molecular structure.
Therefore, the molecular design consisted of benzocarbazole and quinazoline can be effective to
elongate lifetime of the orange PHOLEDs.
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Acknowledgements
This work was supported by Basic Science Research Program (2016R1A2B3008845) through the
National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future
Planning.
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List of tables
Table 1. Photophysical and thermal properties of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
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Table 1. Photophysical and thermal properties of 1Cz BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
Photophysical analysis Thermal analysis
Emission HOMO LUMO Bandgap Tg Td
Triplet
Energy
(eV)
Material
(nm)
(eV)
(eV)
(eV)
(℃)
(℃)
BZCꢀQ
435
ꢀ6.15
ꢀ3.37
2.78
2.52
103
378
BZCꢀ
mPQ
433
437
ꢀ6.06
ꢀ6.07
ꢀ3.38
ꢀ3.40
2.68
2.67
2.49
108
118
430
438
BZCꢀ
pPQ
2.47
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List of figures
Scheme 1. Synthetic scheme of 1Cz BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
Figure 1. HOMO / LUMO distribution and geometrical structure of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
Figure 2. UVꢀvis, solid PL (in polystyrene) and low temperature PL spectra of toluene solution of
BZCꢀQ (a), BZCꢀmPQ (b) and BZCꢀpPQ (c).
Figure 3. CV curves for the oxidation and reduction of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
Figure 4. DSC (a) and TGA (b) thermograms of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ at a heating rate of
10 ℃/min under a nitrogen atmosphere.
Figure 5. Current density–voltage–luminance plots of PHOLEDs with of CBP, BZCꢀQ, BZCꢀmPQ and
BZCꢀpPQ.
Figure 6. Current densityꢀvoltage plots of hole (a) and electron (b) single carrier devices of BZCꢀQ,
BZCꢀmPQ and BZCꢀpPQ.
Figure 7. Quantum efficiency plots of CBP, BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ PHOLEDs against
luminance.
Figure 8. EL spectra of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ PHOLEDs at a luminance of 1,000 cd/m2.
Figure 9. Lifetime curves of BZCꢀQ:Ir(pq)2acac, BZCꢀmPQ: Ir(pq)2acac and BZCꢀpPQ: Ir(pq)2acac
devices. Lifetime was measured at an initial luminance of 1,000 cd/m2.
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Scheme 1. Synthetic scheme of 1Cz BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
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Figure 1. HOMO / LUMO distribution and geometrical structure of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
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(b)
UV-vis
Solid PL
LTPL
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure 2. UVꢀvis, solid PL (in polystyrene) and low temperature PL spectra of toluene solution of
BZCꢀQ (a), BZCꢀmPQ (b) and BZCꢀpPQ (c).
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Figure 3. CV curves for the oxidation and reduction of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ.
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Figure 4. DSC (a) and TGA (b) thermograms of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ at a heating rate of
10 ℃/min under a nitrogen atmosphere.
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40
35
30
25
20
15
10
5
100000
10000
1000
100
CBP
BCZ-Q
BCZ-mPQ
BZC-pPQ
10
1
0
0.1
0
2
4
6
8
10
Voltage(V)
Figure 5. Current density–voltage–luminance plots of PHOLEDs with of CBP, BZCꢀQ, BZCꢀmPQ and
BZCꢀpPQ.
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Figure 6. Current densityꢀvoltage plots of hole (a) and electron (b) single carrier devices of BZCꢀQ,
BZCꢀmPQ and BZCꢀpPQ.
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16
14
12
10
8
CBP
6
BCZ-Q
4
BCZ-mPQ
BZC-pPQ
2
0
1
10
100
1000
10000
Luminance(cd/m2)
Figure 7. Quantum efficiency plots of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ PHOLEDs against luminance.
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Figure 8. EL spectra of BZCꢀQ, BZCꢀmPQ and BZCꢀpPQ PHOLEDs at a luminance of 1,000 cd/m2.
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Figure 9. Lifetime curves of BZCꢀQ:Ir(pq)2acac, BZCꢀmPQ: Ir(pq)2acac and BZCꢀpPQ: Ir(pq)2acac
devices. Lifetime was measured at an initial luminance of 1,000 cd/m2.
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