Solution-processed solid solution of a novel carbazole derivative for high-performance blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/adma.201001392

Blue light: Incorporation of two fluorenyl rings into a phenyl group at the C9 position of fluorene builds a bulky and rigidly tetrahedral framework, which is functionalized by two carbazole groups. This molecule possesses excellent thermal and morphologi

Full text of DOI:10.1002/adma.201001392

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Solution-Processed Solid Solution of a Novel Carbazole
Derivative for High-Performance Blue Phosphorescent
Organic Light-Emitting Diodes
By Shanghui Ye, Yunqi Liu,* Jianming Chen, Kun Lu, Weiping Wu, Chunyan Du,
Ying Liu, Ti Wu, Zhigang Shuai, and Gui Yu
Since Forrest’s group advanced the phosphorescent dye to
improve light-emission efficiencies,[1] organic light-emitting
diodes (OLEDs) have made dramatic progress. Because of the
spin-orbit coupling, both the singlet and triplet states can be
transferred to the photons, resulting in near 100% internal
quantum efficiency, and extremely high efficiencies have
been achieved.[2–4] Recently, Kido and coworkers reported a
very efficient blue phosphorescent OLED based on an iridium
(III) bis[(4,6-difluorophenyl)pyridinato-N,C2picolinate (FIrpic)
emitter with recorded efficiencies of 55 and 46 lm W−1 , which
corresponded to external quantum efficiencies of 26% and 25%,
respectively, at a practical luminance of 100 and 1000 cd m−2 .[5]
In such methodology, the phosphorescent dye is dispersed
into a wide energy gap host material to eliminate triplet–triplet
annihilation and concentration quenching, and thus to develop
suitable host materials is equally important for highly efficient
phosphorescent OLEDs. Up to date, the most efficient OLEDs
are still being fabricated by vacuum vapor deposition of a mul-
tilayer structure based on small molecules, which involves a
complex fabrication process. But for low cost general lighting
applications or display on large area, a simple solution-based
approach is desirable, provided the efficiency of the devices is
not compromised.[6] Even though polymer light-emitting diodes
can be easily fabricated by solution casting, they are generally
difficult to synthesize and purify for electronic devices. There-
fore, to develop low-molecular-weight host materials suitable
for solution processing will possess both advantages. The
largest challenge for small molecules lies in the poor solubility
accompanied by the tendency to crystallize and failure to form
high quality films. Therefore, it is of significant importance to
design and synthesize amorphous small molecules with high
purity and that are highly solubility to provide high quality
films.[7,8]
Although a large number of new phosphorescent dyes have
been demonstrated in the literature, the most efficient one
has still proven to be FIrpic. However, FIrpic is a greenish-
blue emitter with a typical emission peak around 474 nm and
a shoulder at 500 nm, and the corresponding Commission
Internationale de L’Eclairage (CIE) coordinates x, y are around
(0.15, 0.35), depending on the emission band profile.[9–13] For an
efficient host material, it must possess a higher triplet energy
level than that of the dopant to ensure effectively exothermic
energy transfer. In addition, it must dissolve the dopant well to
prevent aggregation or ground-state association to depress spec-
troscopic broadening.[14] Thirdly, it must have good film-forming
and morphological stability as well as thermal and chemical sta-
bility. Meeting all the requirements is very demanding.
In this contribution, we report the use of a phenyl-substituted
fluorene dimer as a bulky and rigid core functionalized by two
carbazole groups to build a high performance amorphous mol-
ecule. The newly synthesized compounds is 1,4-bis(9-(4-(3,6-
di-tert-butyl-9H-carbazol-9-yl)phenyl)-9H-fluoren-9-yl)benzene
(DTCPFB), whose structure and synthetic route is shown in
Scheme 1. We choose carbazole and fluorene as building blocks
because of their excellent photophysical properties. Due to the
non-conjugation linkage mode, the resulting compound pos-
sesses high triplet energy (2.85 eV), excellent miscibility to the
dopant, and good film-forming and morphological stability as
well as thermal and chemical stability, and solution-processed
high-performance blue phosphorescent OLEDs with a satisfac-
tory efficiency of 24.0 cd A−1 or 8.8 lm W−1 were achieved. We
also proposed that the host must have low polarity to avoid polar-
ization-induced spectroscopic red-shifts for realizing authentic
emission of the dopant in a guest-host emissive system.
∗
[ ] S. H. Ye, Prof. Y. Q. Liu, J. M. Chen, Dr. K. Lu, Dr. W. P. Wu,
C. Y. Du, Y. Liu, T. Wu, Prof. Z. G. Shuai, Prof. G. Yu
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (P. R. China)
The structure and properties relationship was investigated
by density functional theory (DFT) calculation (B3LYP; 6-31G∗),
and the results showed that DTCPFB was a highly twisted non-
coplanar tetrahedral molecule, with two active groups of car-
bazole and the central phenyl-substituted fluorene arranged
in a tetrahedral configuration. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels were located in the carbazole and fluorenyl
ring, respectively (see Supporting Information, Figure S1).
The separation between the HOMO and LUMO is preferable
E-mail: liuyq@iccas.ac.cn
S. H. Ye, J. M. Chen, C. Y. Du, Y. Liu, T. Wu
Graduate School
Chinese Academy of Sciences
Beijing 100039 (P. R. China)
Prof. Z. G. Shuai
Department of Chemistry
Tsinghua University
Beijing 100084 (P. R. China)
DOI: 10.1002/adma.201001392
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Scheme 1. Synthetic route of the resulting compound DTCPFB and intermediates.
for efficient hole- and electron-transfer properties, as demon-
strated in the literature.[15] These geometrical characteristics
should effectively prevent intermolecular interactions between
the π-systems and thus suppress the crystallization of molecules
and improve the morphological stability of the thin film. The
calculation results also indicate that the fluorenyl skeleton has
little effect on the electronic structures of the carbazole moiety
combined through the non-conjugated method.
DTCPFB exhibited excellent thermal stability with a 5%
weight-loss decomposition temperature (Td) as high as 442 °C
and a high glass transition temperature (Tg) of 242 °C as
examined by thermogravimetric analysis and differential scan-
ning calorimetry (DSC) (Figure S2). The Tg was significantly
higher than the commonly used materials of bis(carbazol-9-yl)
biphenyl (CBP) and N,N-dicarbazolyl-3,5-benzene (mCP) (62
and 65 °C, respectively).[16,17] Such high Td and Tg endow our
compound with excellent thermal stability, making vacuum
deposition a possible method to fabricate homogeneous and
amorphous films. To further investigate the thermal and mor-
phological stability as well as miscibility to the dopant as a
potential host material, we employed atomic force microscopy
(AFM) to explore the surface image of DTCPFB samples doped
with variant concentrations of FIrpic dopant. The topographical
images revealed that our samples provided smooth and uni-
form surfaces, free of pinholes, particle aggregation, or phase
separation. Importantly, they were thermally stable against
heating as witnessed by annealing the films at 110 °C for 12 h
(Figure 1). This suggested that our compound possesses high
thermal and morphological stabilities as well as good misci-
bility to the dopant, which can be attributed to the bulging tet-
rahedral configuration of the fluorene framework as well as the
high molecular weight.
The singlet and triplet energies were investigated by exam-
ining the UV and photoluminescence (PL) spectra, which
showed that our compound possesses high singlet and triplet
energies (3.46 and 2.85 eV, respectively, corresponding emis-
sion of 358 and 435 nm). The higher triplet energy relative to
the commonly used host material of CBP (2.56 eV)[17] and the
blue phosphorescent emitter of FIrpic (2.65 eV),[18] or that of
mCP (2.90 eV),[18] was favorable for efficient energy transfer. In
Figure 1. AFM imagine of the DTCPFB films doped with 10%, 15%, 20%, and 30% of FIrpic, top, before heating, bottom, after heating at 110 °C for
12 h under nitrogen atmosphere.
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Figure 2. a) Current density –voltage–luminance curves and b) luminance efficiency as a function of current density of devices with variant doping
concentrations of FIrpic.
30% doped device with a luminance efficiency of 24.0 cd A−1
or 8.8 lm W−1 at 8.7 V, with a corresponding brightness of
165 cd m−2 , while the other devices showed moderate efficien-
cies with device a having the lowest of 18.0 cd A−1 or 7.2 lm W−1 .
Despite this, its efficiency was still higher than the literature
value reported by Qiu’s group (12.7 cd A−1 )[12] and even compa-
rable to the vacuum deposited multilayer devices. For example,
Wong et al. reported a triphenylsilylcarbazole device with an
efficiency of 24 cd A−1 or 16 lm W−1 at a practical brightness
of 100 cd m−2 ;[19] Yang et al. reported a triphenylamine device
with a maximum efficiency of 25 cd A−1 or 17 lm W−1 ;[21] For-
rest et al. reported a mCP device with a maximum efficiency of
8.9 lm W−1 and a CBP device of 6.3 lm W−1 .[18,22] Another thing
worth mentioning is that the maximum efficiency was achieved
at practical luminance of 100–1000 cd m−2 , which indicates a
greater use for commercial applications.[23] For instance, device
d has a luminance efficiency of 22.5 cd A−1 at 100 cd m−2 , and
19.3 cd A−1 at 1000 cd m−2 .
The obvious advantages of our devices over the large majority
of other devices lie in the electroluminesence (EL) spectra, as
shown in Figure 3. Our devices displayed bluer emission with
an emission peak at 467.9–470.4 nm and a vibrational peak
at 489.3–491.3 nm with a narrow full-width at half maximum
(FWHM) of 47–53 nm for devices a–d. For example, device a
showed the bluest emission with a FWHM of 47 nm with CIE
coordinate x, y values of (0.141, 0.229), which is the bluest emis-
sion based on a FIrpic emitter.
We probed this phenomenon and found that the dopant
FIrpic was a polar molecule whose emission was affected by
the polarity of the matrix, with a bluer emission in a low or
non-polar solvent (Figure 3 and Table S1). Secondly, our host
material is a low polarity material which was proved by com-
paring the PL spectra of the doped samples with the PL spectra
of FIrpic in dilute solutions of variant polarities. Thirdly, our
host material and FIrpic can form a solid solution, not just a
simple blend as proved by thermal measurement and phase
contrast optical microscopy (Figure S5 and 6). The obvious
advantage of a solid solution lies in the pure emission of the
dopant molecule, similar to the PL spectrum in dilute liquid
solution, as demonstrated in the literature.[24] In analogy to the
solvatochromism of a normal liquid solution, the emission of
the solid solution was also affected by the host polarity arising
from the difference between the chromophore–chromophore
the UV spectra two distinct groups of peaks at 286, 297, and
310 nm, and 334 and 348 nm, attributed to the fluorene and
carbazole, respectively, were observed (Figure S3). Moreover,
the negligible bathochromic shift (within 1 nm) in the peak
positions of the UV and PL spectra indicated negligible inter-
molecular interactions between DTCPFB molecules in the solid
packing state, which was consistent with the molecular struc-
ture, a high twisted tetrahedral configuration, which effectively
prevented intermolecular interactions.
Moreover, our compounds possessed excellent electrochem-
ical reversibility as proved by cyclic voltammetry (CV) analysis
(Figure S4), which is sharply contrasting to mCP which showed
irreversible redox behavior.[19] This can be attributed to the
introduction of two tert-butyl groups at the C3 and C6 positions
of carbazole. Excellent reversible redox properties suggested
that our compound underwent reversible electrochemical
doping and dedoping processes during potential sweeps, which
is desirable for charge transporting materials. The HOMO and
LUMO energy levels were calculated to be −5.63 and −2.16 eV,
respectively. Compared with mCP (HOMO = −5.9 eV, LUMO =
−2.4 eV)[20] our compound showed improved HOMO and
LUMO energy levels, which means a greater match with the
indium tin oxide (ITO) anode. On the other hand, a higher
LUMO energy level was better for obstructing electrons to over-
flow the emissive layer and encounter the anode, which leads
to more efficient recombination. High thermal and chemical
stability, excellent miscibility to the dopant, and a high triplet
energy level (2.85 eV), plus good film forming properties and
morphological stability were achieved simultaneously in our
compound, as mentioned above, and thus an ideal host mate-
rial was expected.
The electroluminescent properties were investigated using
the same samples by capping an electron-transporting and
hole-blocking layer, 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-
2-yl)benzene (TPBI), on which the cathode was deposited.
The current density–voltage–luminance (J–V–L) character-
istics are depicted in Figure 2. The current density increases
as the doping concentration increases, with the 30% doped
device showing the maximum current density among the four
devices at a given voltage. Accordingly, the turn-on voltage (Von,
recorded at 1 cd m−2 ) decreased as the doping concentration
increased from 6.8 V for the 10% doped device to 6.0 V for
the 30% device. The maximum efficiency was achieved in the
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Figure 3. a) PL spectra of FIrpic in dilute solutions (1.1 × 10−5 mol L−1 ). b) PL and c) EL spectra of DTCPFB films doped with varying concentrations
of FIrpic at 9 V.
interactions and chromophore–matrix interactions.[25,26] Thus
the more blue emission was attributed to the lower polarity,
excellent miscibility, and morphological stability of our host
material, as discussed above, which effectively separated and
clamped the dopant, and depressed the ground-state associa-
tion, leaving a similar single molecule emission of the dopant
in the solid solution.
In summary, a novel solution processable carbazole and flu-
orene-based amorphous molecule has been designed and syn-
thesized. Utilizing this compound as a host material and FIrpic
as a dopant of a solid solution, high-performance blue phospho-
rescent OLEDs have been achieved arising from the excellent
morphological stability, miscibility, low polarity, and high triplet
energy of the host material. We believe this work will provide
a guide to blue phosphorescent emission in molecular design
and device fabrication.
filtered, and evaporated under reduced pressure. The crude residue was
dissolved in acetic acid (100 mL) and a catalytic amount of aqueous HCl
(12 N) was added, and then the whole mixture was heated to reflux for 10 h.
After cooling to room temperature, the mixture was condensed under
reduced pressure. The crude residue was purified by re-crystallization from
1
toluene to afford 6.87 g of pure DBPFB, yield 96%. H NMR (400 MHz,
CDCl3, δ): 7.73 (d, J = 7.6 Hz, 4H), 7.36–7.22 (m, 16H), 6.99 (d, J =
7.6 Hz, 8H). 13C NMR (400 MHz, CDCl3, δ): 150.6, 145.0, 143.7, 140.1,
131.3, 129.9, 128.0, 127.8, 127.7, 126.1, 120.7, 120.2, 64.7. EIMS (m/z
(%)): calcd for C44H28Br2, 716.5; found, 716.3 (100).
1,4-Bis(9-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-9H-fluoren-9-yl)
benzene (DTCPFB): A mixture of DBPFB (1 mmol, 0.716 g), 3,6-di-tert-
butyl-9H-carbazole (0.573 g, 2.05 mmol), CuI (0.019 g, 0.010 mmol),
and K2CO3 (0.580 g, 4.2 mmol) in 2 mL of 1,2-dichlorobenzene, 0.05 mL
of 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), and
0.010 g of 18-crown-6 was stirred at 170 °C for 8 h under a nitrogen
atmosphere. After cooling to room temperature the mixture was added
to 20 mL of water and 60 mL of CH2Cl2, and the organic phase was
separated and concentrated under reduced pressure to give a gray solid,
which was subjected to column chromatography (CH2Cl2 and petroleum
ether as eluant, 1:2, v/v, R = 0.3) to yield 0.835 g of the pure target
product of DTCPFB; yield 75%. 1H NMR (400 MHz, CDCl3, δ): 8.09
(d, J = 1.1 Hz, 4H), 7.80 (d, J = 7.5 Hz, 4H), 7.48 (d, J = 7.6 Hz, 4H),
7.41–7.35 (m, 16H), 7.32 (t, J = 7.9 Hz, 8H), 7.17 (s, 4H), 1.44 (s, 36H).
13C NMR (400 MHz, CDCl3, δ): 151.1, 144.4, 144.0, 142.7, 140.2, 139.1,
136.7, 129.5, 128.1, 127.8, 127.7, 126.4, 126.2, 123.5, 123.3, 120.3, 116.1,
109.3, 65.0, 34.7, 32.0. MALDI-TOF MS (m/z (%)): calcd for C84H76N2,
1113.5 ; found, 1113.1 (100). Anal. calcd for C84H76N2: C 90.60, H 6.88,
N 2.52; found: C 90.58, H 6.89, N 2.51.
Experimental Section
General information: All starting materials were obtained from
commercial suppliers and used without further purification; solvents
were purified according to the standard procedure. 1,3,5-Tris(1-phenyl-
1H-benzo[d]imidazol-2-yl)benzene was purified by gradient sublimation
three times.
1,4-Bis(9-(4-bromophenyl)-9H-fluoren-9-yl)benzene (DBPFB): 2-Bromo-
biphenyl (3.495 g, 15 mmol) and tetrahydrofuran (THF, 50 mL) were
mixed and cooled to −78 °C and then n-BuLi (2.5 M in hexane, 6 mL,
15 mmol) was added dropwise. The whole solution was stirred at this
temperature for 45 min followed by the dropwise addition of a solution
of 1,4-bis(4-bromobenzoyl)benzene (4.441 g, 10 mmol) in THF (50 mL).
The resulting mixture was gradually warmed to ambient temperature and
kept stirring for 12 h, after which 50 mL of saturated aqueous NaHCO3
was added to quench the reaction. The mixture was exacted with CH2Cl2
(3 × 60 mL) and the combined organic layers were dried over MgSO4,
1,4-Bis(4-bromobenzoyl)benzene[27] and 3,6-di-tert-butyl-9H-carbazole
were prepared according to the literature procedure.[28]
Device Fabrication and Measurements: In
a general procedure,
ITO-coated glass substrates were etched, patterned, and washed
with detergent, deionized water, acetone, and ethanol in turn.
Polyethylenedioxythiophene: polystryrene sulfonate was spin-coated
to smooth the ITO surface and promote hole injection, and then the
emissive layer was spin-coated from chlorobenzene solution, on which
an electron-transporting/hole-blocking layer was deposited by vacuum
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deposition at a pressure of 8 × 10−5 Pa. In all devices, a thin (8 Å) Cs2CO3
layer served as the electron-injecting layer at the Ag electrode interface
and a 100-nm-thick Ag capping layer were deposited through a shadow
mask. The active area of the device was 6 mm2. EL spectra were
collected using a Hitachi F-4500 spectrophotometer and chromaticity
coordinates were measured with a SpectraScan PR650 photometer.
J–V–L measurements were recorded simultaneously using a Keithley
4200 semiconductor parameter analyzer and a Newport multifunction
2835-C optical meter, with luminance measured in the forward direction.
All device characterizations were carried out under ambient laboratory
air at room temperature.
[7] Y. Zhou, Q. G. He, Y. Yang, H. Z. Zhong, C. He, G. Y. Sang, W. Liu,
C. H. Yang, F. L. Bai, Y. F. Li, Adv. Funct. Mater. 2008, 18, 3299.
[8] Y. Shirota, J. Mater. Chem. 2005, 15, 75.
[9] S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, F. Sato, Appl. Phys.
Lett. 2003, 83, 569.
[10] P. I. Shih, C. H. Chien, C. Y. Chuang, C. F. Shu, C. H. Yang, J. H.
Chen, Y. Chi, J. Mater. Chem. 2007, 17, 1692.
[11] J. Q. Ding, B. H. Zhang, J. H. Lu, Z. Y. Xie, L. X. Wang, X. B. Jing,
F. S. Wang, Adv. Mater. 2009, 21, 4983.
[12] L. D. Hou, L. Duan, J. Qiao, W. Li, D. Q. Zhang, Y. Qiu, Appl. Phys.
Lett. 2008, 92, 263301.
[13] J. Lee, J. I. Lee, J. Y. Lee, H. Y. Chu, Organic Electronics 2009, 10, 1529.
[14] G. Q. Zhang, S. E. Kooi, J. N. Demas, C. L. Fraser, Adv. Mater. 2008,
20, 2099.
[15] Z. Y. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T.
Kajita, M. A. Kakimoto, Adv. Funct. Mater. 2008, 18, 584.
[16] V. Adamovich, J. Brooks, A. Tamayo, A. M. Alexander, P. I. Djurovich,
B. W. D’Andrade, C. Adachi, S. R. Forrest, M. E. Thompson, New J.
Chem. 2002, 26, 1171.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
[17] M. H. Tsai, Y. H. Hong, C. H. Chang, H. C. Su, C. C. Wu, A.
Matoliukstyte, J. Simokaitiene, S. Grigalevicius, J. V. Grazulevicius,
C. P. Hsu, Adv. Mater. 2007, 19, 862.
[18] R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong, J. J. Brown,
S. Garon, M. E. Thompson, Appl. Phys. Lett. 2003, 82, 2422.
[19] M. H. Tsai, H. W. Lin, H. C. Su, T. H. Ke, C. C. Wu, F. C. Fang, Y. L.
Liao, K. T. Wong, C. I. Wu, Adv. Mater. 2006, 18, 1216.
[20] R. J. Holmes, B. W. D’Andrade, S. R. Forrest, X. Ren, J. Li, M. E.
Thompson, Appl. Phys. Lett. 2003, 83, 3818.
[21] Z. Q. Jiang, Y. H. Chen, C. Fan, C. L. Yang, Q. Wang, Y. T. Tao, Z. Q.
Zhang, J. G. Qin, D. G. Ma, Chem. Commun. 2009, 3398.
[22] C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo,
M. E. Thompson, S. R. Forrest, Appl. Phys. Lett. 2001, 79, 2082.
[23] P. I. Shih, C. Y. Chuang, C. H. Chien, E. W. G. Diau, C. F. Shu, Adv.
Funct. Mater. 2007, 17, 3141.
[24] J. Liu, Y. J. Shi, Y. Yang, Appl. Phys. Lett. 2001, 79, 578.
[25] V. Bulovic, A. Shoustikov, M. A. Baldo, E. Bose, V. G. Kozlov, M. E.
Thompson, S. R. Forrest, Chem. Phys. Lett. 1998, 287, 455.
[26] C. F. Madigan, V. Bulovic, Phys Rev Lett 2003, 91, 247403.
[27] V. Percec, R. S. Clough, P. L. Rinaldi, V. E. Litman, Macromolecules
1994, 27, 1535.
Acknowledgements
The authors acknowledge financial support from the National Natural
Science Foundation of China (20825208, 60736004, 20721061),
the National Major State Basic Research Development Program
(2006CB806203, 2006CB932103, 2009CB623603), and the Chinese
Academy of Sciences.
Received: April 18, 2010
Revised: May 9, 2010
Published online: June 10, 2010
[1] M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E.
Thompson, S. R. Forrest, Nature 1998, 395, 151.
[2] C. Adachi, M. A. Baldo, M. E. Thompson, S. R. Forrest, J. Appl. Phys.
2001, 90, 5048.
[3] L. X. Xiao, S. J. Su, Y. Agata, H. L. Lan, J. Kido, Adv. Mater. 2009, 21,
1271.
[4] S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, K. Y. Ko, J. Y. Park, Y. G.
Baek, Appl. Phys. Lett. 2008, 93, 063306.
[5] S. J. Su, E. Gonmori, H. Sasabe, J. Kido, Adv. Mater. 2008, 20, 4189.
[6] M. K. Mathai, V. E. Choong, S. A. Choulis, B. Krummacher, F. So,
Appl. Phys. Lett. 2006, 88.
[28] N. D. McClenaghan, R. Passalacqua, F. Loiseau, S. Campagna, B.
Verheyde, A. Hameurlaine, W. Dehaen, J. Am. Chem. Soc. 2003, 125,
5356.
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Adv. Mater. 2010, 22, 4167–4171
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