Thermally cross-linkable styrene-based host materials for solution-processed organic light-emitting diodes

Article abstract of DOI:10.1166/jnn.2019.16688

Thermally cross-linkable host materials, DV-TPACZ, DV-TPADBCZ, and TV-TPBI, were designed and synthesized for solution-processed organic light-emitting diodes (OLEDs). The synthesized styrene-functionalized host materials were thermally cross-linked by cu

Full text of DOI:10.1166/jnn.2019.16688

Article
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Nanoscience and Nanotechnology
Vol. 19, 4705–4709, 2019
Copyright © 2019 American Scientific Publishers
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Thermally Cross-Linkable Styrene-Based Host Materials
for Solution-Processed Organic Light-Emitting Diodes
Jae-Ho Jang1, Hea Jung Park1, Ji-Ho Kang2, Jeong Yong Park1, Min Chul Suh2, and Do-Hoon Hwang1ꢀ∗
1Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 46241, Korea
2Department of Information Display, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, Korea
Thermally cross-linkable host materials, DV-TPACZ, DV-TPADBCZ, and TV-TPBI, were designed
and synthesized for solution-processed organic light-emitting diodes (OLEDs). The synthesized
ꢀ
styrene-functionalized host materials were thermally cross-linked by curing at 150–200 C without
using a polymerization initiator. Excellent solvent resistance was observed for all cured host films.
They exhibited low highest occupied molecular orbital energy levels of 5.4–5.7 eV, which indicated
a low hole injection barrier from the hole transport layer to the emissive layer. A solution-processed
red phosphorescent OLED with 5 wt% (MPHMQ)2Ir (tmd) dopant in the thermally cross-linkable
DV-TPACZ host exhibited a current efficiency of 5.3 cd/A, power efficiency of 3.2 lm/W, and external
quantum efficiency of 3.6%.
Keywords: Thermally Cross-Linkable Host, Organic Light-Emitting Diode, Phosphorescence,
Styrene, Solvent Resistance.
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1. INTRODUCTION
to quenching of excitons by the photochemical initiator.10
In contrast, thermally cross-linkable materials are attrac-
tive because they do not degrade the performance of the
light-emitting device. In particular, styrene-based materials
have attracted a lot of attention as thermally cross-linkable
host materials for OLEDs, because free radicals are gen-
erated from a Diels-Alder styrene dimer by curing at
150–200 ꢀC.11–14
In this study, we designed and synthesized styrene-
functionalized thermally cross-linkable host materials,
DV-TPACZ, DV-TPADBCZ, and TV-TPBI, based on
4-(9H-carbazolyl)triphenylamine (TPACZ), 4-(3,6-di-
Organic light-emitting diodes (OLEDs) have been
considered as the most competitive candidates for solid-
state lighting and new-generation full-color flat-panel dis-
plays owing to advantages such as fast response, wide
visual angle, low operating voltage for low power con-
sumption, high brightness, high contrast, and the possi-
bility to produce thin, large-area, and flexible devices.1ꢀ2
Solution-processed OLEDs that are prepared by the incor-
poration of phosphorescent dopants in macromolecules
or polymer hosts have generated a lot of interest in
recent years because they can harvest both singlet and
triplet excitons and also overcome the demerits of vac-
uum deposition processes.3–7 However, multilayer OLED
fabrication from solutions requires that each underlying
layer be resistant to the solvent used for the deposition of
the next layer. A simple approach to solve this problem
involves the use of an orthogonal solvent for spin-casting
to deposit the subsequent layers. Yet another approach
involves using a cross-linkable material, which can be
solution-processed by spin-casting and then transformed
into an insoluble film by light or thermal treatment.8ꢀ9
However, the light-induced cross-linking process may lead
tert-butyl-9H-carbazolyl)triphenylamine
(TPADBCZ),
and 2,2’,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benz-
imidazole) (TPBI). The synthesized host materials with
styrene groups were thermally cross-linked by curing at
ꢀ
150–200 C without any polymerization initiator. Excel-
lent solvent resistance was observed for all cured host
material films. In particular, a solution-processed red
phosphorescence device using DV-TPACZ as a thermally
cross-linkable host was fabricated and its performance
was evaluated. The detailed synthetic routes and chemical
structures of the thermally cross-linkable host materials
are outlined in Scheme 1.
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 8
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doi:10.1166/jnn.2019.16688
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Thermally Cross-Linkable Styrene-Based Host Materials for Solution-Processed Organic Light-Emitting Diodes
Jang et al.
H
N
Br
Br
Br
R
R
R
R
R
(HO)2B
(R=Hor(CH3)3)
Br
N
N
N
N
N
K2CO3,THF
Pd2(dba)3, P-tBu3
t-BuONa,THF
R
Br
2:R=H
1
DV-TPACZ:R=H
DV-TPADBCZ:R=(CH3)3
3:R=(CH3)3
Br
N
N
N
N
Br
(HO)2B
N
N
N
N
N
N
Pd(PPh3)4
K2CO3,THF
N
N
Br
TV-TPBI
4
Scheme 1. Synthesis of DV-TPACZ, DV-TPADBCZ, and TV-TPBI.
2. EXPERIMENTAL DETAILS
2.1. Materials
were dissolved in tetrahydrofuran. A solution of 2 M
K2CO3 and Aliquat 336 (0.10 g, 0.26 mmol) was added
to the reaction flask and the mixture was refluxed for
24 h with stirring under a nitrogen atmosphere. Subse-
quently, the reaction mixture was cooled to room tem-
perature and extracted with ethyl acetate. The combined
All chemicals were purchased from Sigma-Aldrich and
Alfa Aesar and used without further purification. Com-
pounds 2 and 4 were synthesized according to the meth-
ods described in previous reports.15ꢀ16 All spectra matched
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with previously reported data.
organic extracts were dried over MgSO . The solvent
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4
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was removed under reduced pressure to obtain a crude
residue. The crude product was then purified by silica
gel column chromatography (ethyl acetate/hexane, 1/2,
v/v) to obtain DV-TPACZ as a pale green solid (0.8 g,
51%). DV-TPADBCZ and TV-TPBI were prepared using
compounds 3 and 4, respectively, following the same
procedure.
2.2. Synthesis of Thermally Cross-Linkable
Host Materials
2.2.1. Synthesis of 4-Bromo-N-(4-bromophenyl)-N-
(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)
phenyl)Aniline(3)
Compound
1
(5.0 g, 10.4 mmol), 3,6-di-tert-
DV-TPACZ (Yield = 51%): 1H NMR (300 MHz,
CDCl3ꢁꢂ (ppm): 8.15 (d, 2H, J = 9.0 Hz), 7.57 (m, 8H),
7.46 (m, 10H), 7.29 (m, 8H), 6.77 (dd, 2H, J = 11,
18 Hz), 5.80 (d, 2H, J = 18 Hz), 5.27 (d, 2H, J = 11
Hz); MALDI-TOF (M+, C46H34N2ꢁ: Calcd. 613.88, found
614.27.
butylcarbazoleoride (1.53 g, 5.46 mmol), sodium tert-
butoxide (2.10 g, 21.8 mmol), tri-tert-butylphosphine
(0.66 mL of 1 M solution, 0.66 mmol), and Pd2(dba)3
(0.3 g, 0.33 mmol) were dissolved in toluene and refluxed
with stirring for 24 h under a nitrogen atmosphere.
Subsequently, the reaction mixture was cooled to room
temperature and extracted with ethyl acetate. The com-
bined organic extracts were dried over MgSO4. The
solvent was removed under reduced pressure to give a
crude residue. The crude product was purified by silica
gel column chromatography (ethyl acetate/hexane, 1/4,
v/v) and compound 3 was obtained as a pale green solid
DV-TPADBCZ (Yield = 35%): 1H NMR (300 MHz,
CDCl3ꢁꢂ (ppm): 8.19 (s, 2H), 7.60 (m, 8H), 7.46 (m,
10H), 7.27 (m, 6H), 6.78 (dd, 2H, J = 16.9, 11.4 Hz), 5.79
(d, 2H, J = 16.9 Hz), 5.22 (d, 2H, J = 11.4 Hz), 1.46
(s, 18H); MALDI-TOF (M+, C54H50N2ꢁ: Calcd. 725.51,
found 726.40.
1
(1.71 g, 43%). H NMR (300 MHz, CDCl3ꢁꢂ (ppm): 8.18
1
TV-TPBI (43%): H NMR (300 MHz, CDCl3ꢁꢂ (ppm):
(s, 2H), 7.41 (m, 12H), 7.20 (d, 2H, J = 7.5 Hz), 7.02
(d, 2H, J = 7.5 Hz), 1.47 (s, 18H); MALDI-TOF (M+,
C38H36Br2N2ꢁ: Calcd. 677.23, found 678.12.
7.90 (s, 3H), 7.80 (d, 3H, J = 7.5 Hz), 7.54 (d, 6H,
J = 8.1 Hz), 7.43 (m, 12H), 7.26 (m, 9H), 7.13 (d,
6H, J = 8.1 Hz), 6.75 (dd, 3H, J = 11.4, 17.7 Hz),
5.80 (d, 3H, J = 17.7 Hz), 5.30 (d, 3H, J = 11.4 Hz);
MALDI-TOF (M+, C69H48N6ꢁ: Calcd. 959.89, found
960.39.
2.2.2. Synthesis of Host Materials
Compound 2 (1.5 g, 2.64 mmol), 4-vinylphenylboronic
(0.47 g, 3.17 mmol), and Pd(PPh3ꢁ4 (0.09 g, 0.08 mmol)
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Thermally Cross-Linkable Styrene-Based Host Materials for Solution-Processed Organic Light-Emitting Diodes
2.3. Measurements and Device Fabrication
and 4, were prepared following literature procedures.15ꢀ16
Three host materials, DV-TPACZ, DV-TPADBCZ, and
TV-TPBI were synthesized by a palladium-catalyzed
Suzuki cross coupling reaction between 2, 3, or 4
and (4-ethenylphenyl)boronic acid and characterized by
1H-NMR, 13C-NMR, and MALDI-TOF mass spectrometry.
Measurements and device fabrication were performed fol-
lowing methods described in previous reports.11ꢀ13–16
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization
Thermally cross-linkable host materials, DV-TPACZ, DV-
TPADBCZ, and TV-TPBI were synthesized as shown
in Scheme 1. The halogenated compounds, 2, 3,
3.2. Photophysical Properties
Absorption and emission spectra of the synthesized host
materials were measured in 10−5 M CH2Cl2 solutions.
DV-TPACZ and DV-TPADBCZ exhibited similar absorp-
tions at ∼290 and 350 nm, which were attributed to
the n–ꢃ∗ transitions of the carbazole moieties and the
ꢃ–ꢃ∗ charge transitions from carbazole to triphenylamine,
respectively.17–19 TV-TPBI showed a strong absorption at
∼290 nm that was attributed to the benzimidazole moi-
ety. DV-TPACZ, DV-TPADBCZ, and TV-TPBI exhibited
photoluminescence (PL) emissions at 447 nm, 453 nm,
and 388 nm, respectively. The optical band gap energies
of the host materials were 3.20, 3.10, and 3.70 eV for
DV-TPACZ, DV-TPADBCZ, and TV-TPBI, respectively,
as determined by the absorption edges. TV-TPBI showed
the largest energy band gap among the three synthesized
host materials. The triplet energy levels of DV-TPACZ,
DV-TPADBCZ, and TV-TPBI were measured to be 2.70,
2.50, and 2.58 eV, respectively.
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3.3. Electrochemical Properties
Copyright: American Scientific Publishers
The electrochemical properties of the synthesized host
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materials were investigated by cyclic voltammetry
(CV). The measurements were conducted using their
dichloromethane solutions with tetrabutylammonium per-
chlorate as the electrolyte. The highest occupied molecular
Figure 1. DSC curves of DV-TPACZ, DV-TPADBCZ, and TV-TPBI.
Figure 2. UV-vis absorption spectra of cured (a) DV-TPACZ, (b) DV-TPADBCZ, and (c) TV-TPBI films before and after rinsing with organic solvents
(methylene chloride (MC), tetrahydrofuran (THF), chlorobenzene (CB), and toluene (Tol)).
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Thermally Cross-Linkable Styrene-Based Host Materials for Solution-Processed Organic Light-Emitting Diodes
Jang et al.
orbital (HOMO) energy levels of DV-TPACZ, DV-
TPADBCZ, and TV-TPBI were estimated to be −5.60,
−5.40, and −5.70 eV versus Fc/Fc+, respectively. The
lowest unoccupied molecular orbital (LUMO) energy lev-
els of DV-TPACZ, DV-TPADBCZ, and TV-TPBI were
calculated from the differences between the optical band
monitored using UV-Vis absorption spectroscopy. The thin
films were obtained by spin-coating from dichloromethane
solutions followed by thermal curing at 150 ꢀC for 30 min
under N2 atmosphere. The results in Figure 2 show that the
DV-TPACZ, DV-TPADBCZ, and TV-TPBI films exhibit
unchanged UV-Vis absorbances after thermal curing fol-
lowed by washing with common solvents such as methy-
lene chloride, tetrahydrofuran, chlorobenzene, and toluene.
These results confirmed that these host materials possessed
suitable solvent resistance for multilayer integration by the
solution process in OLEDs.
gaps and the HOMO energy levels (ꢄEband gap = ELUMO
−
E
HOMOꢁ to be −2.40, −2.30, and −2.0 eV, respectively.
The HOMO and LUMO energy levels of the synthesized
host materials were well matched with those of the pre-
viously reported (MPHMQ)2Ir(tmd) (HOMO: −5.10 eV,
LUMO: −3.10 eV) red dopant.20
3.5. Electrophosphorescent OLEDs
Among the three synthesized host materials, DV-TPACZ
was selected as the thermally cross-linkable host in
red phosphorescent OLED with a device configura-
tion of ITO (50 nm)/PEDOT:PSS (40 nm)/SHTD-
510 (17 nm)/DV-TPACZ:(MPHMQ)2Ir(tmd) (95:5%)
(30 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). The
optimized device performance was obtained at 5 wt%
concentration of the (MPHMQ)2Ir(tmd) dopant. The
device containing (MPHMQ)2Ir(tmd) dopant with ther-
mally cross-linked DV-TPACZ host showed the EL
3.4. Thermal Properties and Solvent Resistances
The thermal properties of the host materials were
investigated by thermogravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC). The 5 wt% loss
temperatures (Tdꢁ of DV-TPACZ, ꢀ DV-TPADBCZ, and
TV-TPBI were 469, 428, and 486 C, respectively. This
result indicated that the synthesized compounds were suf-
ficiently thermally stable to be applied in OLEDs as
host materials. Figure 1 shows the first- and second-
heating DSC curves of DV-TPACZ, DV-TPADBCZ, and
TV-TPBI. These curves exhibited exothermic peaks in
ꢀ
the range of 143–210 C, which correspond to the ther-
mal polymerization of the styrene group.11–14 The curing
temperatures (TCLꢁ of DV-TPACZ, DV-TPADBCZ, and
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ꢀ
TV-TPBI were 154, 143, and 210 C, respectively. DV-
Copyright: American Scientific Publishers
TPACZ and DV-TPADBCZ showed endothermic peaks at
131 and 123 C, respectively, which were followed by the
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ꢀ
exothermic peaks, indicating that styrene polymerization
occurred in the partial melting state. In the second heat-
ing scan, no peaks were observed in the temperature range
ꢀ
of 140–200 C, which likely corresponded to the cross-
linking of the styrene-functionalized host materials after
thermal treatment. There has been no consensus on the
correct thermal cross-linking mechanism of the styryl unit
even though several mechanisms were proposed in previ-
ous reports. Two well-known mechanisms on the cross-
linking reaction have been suggested by Mayo and Flory.13
The solvent resistances of the cross-linked films were
Figure 4. (a) Current density–voltage–luminance (J–V–L) and (b) exter-
nal quantum efficiency-current density-current efficiency (EQE-J-CE)
curves of the DV-TPACZ-based device.
Figure 3. The energy level diagram of the fabricated device.
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Jang et al.
Thermally Cross-Linkable Styrene-Based Host Materials for Solution-Processed Organic Light-Emitting Diodes
maximum peak at 606 nm with CIE coordinates of (0.62,
0.36) unaffected by the applied voltage with a full-width
at half-maximum (FWHM) values of 65 nm. The turn-
on voltage (Vturn−on, defined as the bias at a luminance
of 1 cd/m2ꢁ of the device was 5.5 V, current efficiency
of 5.3 cd/A, power efficiency of 3.2 lm/W, and exter-
nal quantum efficiency of 3.6%. The device performances
suggest that the energy levels of the employed materi-
als are well matched (Fig. 3), leading to facile injection
of holes and electrons, their transport across layers, and
recombination. DV-TPACZ also showed hole-transporting
property after film curing despite its low device
performance. The I-V-L characteristics and the cur-
rent efficiency–Luminance–EQE characteristics of the red
phosphorescent OLEDs are shown in Figure 4.
Research Foundation (NRF) grants funded by the Korean
government (Grant Nos. NRF2017R1A2A2A05001345
and 2011-0030013 through GCRC SOP).
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4. CONCLUSION
Herein, we have described the synthesis of novel ther-
mally cross-linkable styrene-functionalized host materi-
als, DV-TPACZ, DV-TPADBCZ, and TV-TPBI, which
were applied in solution-processed red phosphorescent
OLEDs. These host materials showed thermal sta-
bility and excellent solvent resistance. The solution-
processed OLED device based on the red phosphorescent
(MPHMQ)2Ir(tmd) dopant in the thermally cross-linked
DV-TPACZ host exhibited a current efficiency of 5.8 cd/A,
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Acknowledgment: This research was supported by
the Ministry of Trade, Industry and Energy (MOTIE,
Korea) under the Industrial Technology Innovation Pro-
gram. No. 10067715, and with the assistance of National
Received: 29 May 2018. Accepted: 10 September 2018.
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