Solution-processable naphthalene and phenyl substituted carbazole core based hole transporting materials for efficient organic light-emitting diodes

Article abstract of DOI:10.1039/c7tc03049e

Solution-processable molecular hole transporting materials (HTMs) are extremely crucial in order to realize low cost, high throughput, and roll-to-roll fabrication of large area organic light emitting diodes for display and lighting applications. In this report, a series of naphthalene and phenyl substituted carbazole core based HTMs, 3-(1-naphthyl)-9-(2-phenylvinyl)carbazole (NPVCz), 3,6-di-(1-naphthyl)-9-phenylvinylcarbazole (DNPVCz), and 3,6-diphenyl-9-(2-phenylvinyl)carbazole (DPPVCz) are successfully synthesized and characterized. The synthesized HTMs possess excellent solubility in common organic solvents. By using a fluorescent tris(8-hydroxyquinolinato)aluminium emitter, we demonstrate an enhancement of 135%, from 1.7 to 4.5 cd A^-1, in the current efficiency of an organic light emitting diode (OLED) by replacing the conventional HTM, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), with the NPVCz counterpart. Moreover, the current efficiency of a conventional tris[2-phenylpyridinato-C2,N]iridium(iii) based phosphorescent green OLED device increases from 46.4 to 66.2 cd A^-1 by substituting the NPB with NPVCz. These findings suggest that this type of solution-processable molecular HTM will be a promising contender for high efficiency OLED devices.

Full text of DOI:10.1039/c7tc03049e

View Article Online
View Journal
Journal of
Materials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. KUMAR, C. An,
S. Sahoo, R. Griniene, D. Volyniuk, J. V. Grazulevicius, S. Grigalevicius and J. Jou, J. Mater. Chem. C, 2017,
DOI: 10.1039/C7TC03049E.
This is an Accepted Manuscript, which has been through the
Volume
4 Number 1 7 January 2016 Pages 1–224
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Accepted Manuscripts are published online shortly after
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Page 1 of 27
Journal of Materials Chemistry C
Solution-processable naphthalene and phenyl substituted carbazole coVrieew Article Online
DOI: 10.1039/C7TC03049E
based hole transporting materials for efficient organic light-emitting diodes
Sudhir Kumara,b, Chih-Chia Ana, Snehasis Sahooa, R. Grinienec, D. Volyniukc, Juozas V.
Grazuleviciusc, Saulius Grigaleviciusc,*, Jwo-Huei Joua,*
aDepartment of Materials Science and Engineering, National Tsing-Hua University, No.101,
Kung-Fu Rd. Hsin-Chu 30013 Taiwan, R.O.C.
bInstitute for Chemical and Bioengineering, ETH Zürich, Vladimir Prelog Web 1, 8093
Zürich, Switzerland.
cDepartment of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu plentas 19, LT50254, Kaunas, Lithuania.
Abstract
Solution-processable molecular hole transporting material (HTM) is extremely crucial in
order to realize the low cost, high throughput, and roll-to-roll fabrication of large area organic
light emitting diodes for display and lighting applications. In this report, a series of
naphthalene and phenyl substituted carbazole core based HTMs, 3-(1-naphthyl)-9-(2-
phenylvinyl)carbazole (NPVCz), 3,6-di-(1-naphthyl)-9-phenylvinylcarbazole (DNPVCz),
and 3,6-diphenyl-9-(2-phenylvinyl)carbazole (DPPVCz) are successfully synthesized and
characterized. The synthesized HTMs possess excellent solubility in common organic
solvents. By using a fluorescent tris(8-hydroxyquinolinato)aluminium emitter, we
demonstrate an enhancement of 135%, from 1.7 to 4.5 cd A-1, in current efficiency of organic
light emitting diode (OLED) by replacing the conventional HTM, N,N’-di(1-naphthyl)-N,N’-
diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB), with the NPVCz counterpart. Moreover, the
current efficiency of a conventional tris[2-phenylpyridinato-C2,N]iridium(III) based
phosphorescent green OLED device increases from 46.4 to 66.2 cd m-2 by substituting the
NPB from the NPVCz. These findings suggest that such type of solution-processable
molecular HTMs will be promising contender for high efficiency OLED devices.
1. Introduction
Organic light emitting diodes (OLEDs) have attracted the great attention because of their
extensive applications in high quality displays and lightings.1,2 At the present time numerous
type of portable display devices and a few large size OLED televisions have already been in

Journal of Materials Chemistry C
Page 2 of 27
the market.2,3 At present, almost every single commercial OLED product is fabricatedViewbAyrticle Online
DOI: 10.1039/C7TC03049E
vacuum deposition because of its superlative features, such as precise layer thickness control,
high efficiency, and easy multilayer deposition in the intricate devices, which are highly
expensive because of low throughput and huge materials wastage.4 To make the resultant
products commercially profitable and large-area roll-to-roll fabrication, solution-processable
OLEDs with higher efficiencies are extremely demanded.5-9
Nowadays, it is well established that carrier transporting materials are crucial to enable
a balance carrier, electron and hole, transport from the counter electrodes (cathode and
anode). Moreover, these layers should facilitate a confinement of injected carriers within the
emissive layer to realize a high efficiency OLED devices.10-11 Numerous type of high triplet
energy electron transporting materials have been reported, while only a few hole transporting
materials (HTMs) have been realized for OLED devices.12 A high triplet energy HTM with
high mobility and can effectively increase the carrier injection balance and exciton
confinement, respectively. Typically, arylamines,13,14 carbazoles,15-18 carbazole-arylamine
hybrids,19 and tolylamino cyclohexane20 derivatives have been extensively studied as HTM in
OLEDs because of their high hole mobility, electron confining ability, and amorphous film
forming property. Amongst, arylamine derivatives, such as N,N’-diphenyl-N,N’-bis(1-
naphthyl)-(1,1’-biphenyl)-4,4’-diamine (NPB), N,N’-bis(3-methylphenyl)-N,N’-bis(phenyl)
benzidine (TPD), and 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC), have been
widely used as HTMs because of their high hole mobility and ease of thermal evaporation.13-
15,21,22
Though, their poor film forming ability, low morphological stability, and ease of
crystallization at room temperature caused the poor device efficiency or device failure in
solution-process.14,23
On the other hand, carbazole derivatives, such as tris(4-carbazoyl-9-ylphenyl)amine
(TCTA),24 3,6-bis(4-vinylphenyl)-9-ethylcarbazole (VPEC),25 and tris(4-carbazole)triphenyl

Page 3 of 27
Journal of Materials Chemistry C
amine,26 have been also studied as the HTM due to their rational hole mobility,DhOiI:g1h0.10tr3i9p/Cl7eTtC03049E
energy, amorphous film forming ability, and high thermal stability. Numerous solution-
processable long chain polymeric, and oligomeric type HTMs have been reported, while they
exhibited a poor device efficiency.27-32 However, only a few dendritic HTMs have been
investigated in OLED devices, and they have exhibited relatively higher efficiency than that
of polymer counterparts. For example, Moonsin et al.33 reported a maximum current
efficiency (ηCE) of 4.45 cd A-1 as a dendritic carbazole derivative, 3’,6’-bis[3,6-bis(carbazole-
9-yl)carbazole-9-yl]-bis[9-(2-ethylhexyl) carbazole-3,6-diyl, was spin-coated between the
hole injection layer of poly(3,4-ethylenedioxythiopene):poly(styrenesulfonate) (PEDOT:PSS)
and an emissive/electron transporting layer of tris(8-hydroxyquinolinato)aluminium (Alq3).
Usluer et al.34 reported a maximum ηCE of 7.7 cd A-1 by employing a newly synthesized
fluorene-carbazole dendrimer based HTM between the PEDOT:PSS and Alq3 layers.
Prachumrak et al.35 designed and synthesized a series of carbazole dendrimers based HTMs
for OLEDs. The resultant device exhibited a maximum ηCE of 5.11 cd A-1 by using a five
carbazole units based HTM, 3,6-bis[3’,6’-bis(3”,6”-di-tert-butylcarbazol-N”-yl)carbazol-N’-
yl]-N-dodecylcarbazole (G3C). Recently, solution-process feasible molecular HTMs attracted
a considerable attention in OLEDs over long-chain polymer or complex dendrimer
counterparts because of their easy synthesis, high degree of purity, defect free structure, and
higher hole mobility.36,37 Most importantly, molecular HTMs based OLEDs have shown
excellent reproducibility because of smaller chain entanglement than that of the long chain
polymer counterparts.37 The design of wet-process feasible molecular HTM is highly
challenging because they must have to offer a high solubility in common organic solvents
with excellent film forming property, film integrity, and amorphous film morphology. In the
past years, a few solution-processable molecular HTMs have been reported, for example, Wu
et al.38 synthesized a tri-carbazole functionalized fluorenyl core based compound 2,4,7-tri[2-
View Article Online

Journal of Materials Chemistry C
Page 4 of 27
View Article Online
(9-hexylcarbazole)-ethynyl]-9,9-dihexyl fluorene. The resultant multilayer OLED device
DOI: 10.1039/C7TC03049E
exhibited a maximum ηCE of 3.2 cd A-1. Thaengthong et al.39 synthesized a carbazole-type
amorphous HTM of 3′,6′-bis(carbazol-9-yl)-bis[9-(2-ethylhexyl)carbazol-3,6-diyl], and
employed in a double layer OLED device. Hence, it is worthy to devise a solution-
processable small molecular type HTM for OLED devices.
In this study, we demonstrate a series of naphthyl and phenyl substituted 9-(2-
phenylvinyl) carbazole type HTMs, 3,6-diphenyl-9-(2-phenylvinyl)carbazole (DPPVCz), 3-
(1-naphthyl)-9-(2-phenylvinyl)carbazole (NPVCz), and 3,6-di-(1-naphthyl)-9-phenylvinyl
carbazole (DNPVCz). These HTMs exhibit excellent solubility in common organic solvents
and robust thermal stability. Taking double layer fluorescent green device for example, at
1000 cd m-2, the ηCE is enhanced from 1.7 to 4.5 cd A-1 and a power efficiency (ηPE) from 1.7
to 2.6 lm W-1, as NPB is replaced by NPVCz. Moreover, conventional tris[2-
phenylpyridinato-C2,N]iridium(III), Ir(ppy)3, emitter based green phosphorescent device
shows a maximum ηCE of 66.2 cd A-1 (ηPE =70.0 lm W-1 and external quantum efficiency, ηext
= 18.3%) with solution-processable NPVCz, and improves by almost 40% compared to the
OLED based on the NPB. The high device efficiency is attributed to rational hole mobility
and effective electron confinement. Moreover, the phosphorescent device architecture
enabling therein an electron trap to facilitate the injection of the minor carriers against that of
a hole, leading to a more balance carrier injection.
2. Result and discussion
2.1 Synthesis
The synthesis of naphthyl and phenyl substituted 9-(2-phenylvinyl) carbazole derivatives,
NPVCz, DNPVCz, and DPPVCz, was carried out by multi-step synthetic routes shown in
Scheme 1. The key starting materials 3-iodo-9H-carbazole (2) and 3,6-di-iodo-9H-carbazole
(3) were synthesized from commercially available 9H-carbazole (1) by Tucker iodination

Page 5 of 27
Journal of Materials Chemistry C
with KI/KIO3 in acetic acid.40 The 3-iodo-9-(2-phenylvinyl) carbazole (4) and 3,6-di-iodo-9-
View Article Online
DOI: 10.1039/C7TC03049E
(2-phenylvinyl)carbazole (5) were synthesized, respectively, by condensation reaction of the
iododerivatives 2 and 3 with an excess of 2-phenylacetaldehyde under acidic conditions in
the presence of (±)-camphor-10-sulfonic acid. The naphthyl substituted carbazole derivatives
NPVCz and DNPVCz were obtained by Suzuki coupling reaction of the compounds 4 and 5,
correspondingly, with an excess of 1-naphtalene boronic acid. On the other hand, the
compound DPPVCz was synthesized by Suzuki coupling reaction of the compound 5 with an
excess of phenyl boronic acid. Molecular structures of all compounds were characterized by
1H NMR, IR, and HRMS techniques (see spectra in Fig. S1 to S5).
Scheme 1. Schematic illustration of the synthesis of the naphthyl and phenyl substituted 9-(2-
phenylvinyl) carbazole based hole transporting materials (HTMs), NPVCz, DNPVCz, and
DPPVCz.

Journal of Materials Chemistry C
Page 6 of 27
View Article Online
DOI: 10.1039/C7TC03049E
2.2 Photophysical properties
As shown in Fig. 1, the ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra of
the compounds NPVCz, DNPVCz, and DPPVCz were measured in tetrahydrofuran (THF) at
room temperature. The optical band gaps (Eg) were estimated from intersections of the
absorption spectra. The compounds NPVCz, DNPVCz, and DPPVCz show the Eg of 3.28 eV,
3.32 eV, and 3.29 eV, respectively. The triplet-energies (ET) were calculated from the first
triplet emission peak of the low temperature PL spectrum measured in cryogenic medium
(liquid N2) at 77K (Fig. S6), which exhibit the ET of 3.09 eV, 2.84 eV and 2.69 eV,
respectively, for NPVCz, DNPVCz and DPPVCz.
Figure 1. The ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra of the
naphthalene and phenyl substituted carbazole core based HTMs, (a) NPVCz, (b) DNPVCz,
(c) DPPVCz, and (d) NPB. All the data were measured by dissolving the HTMs in THF.

Page 7 of 27
Journal of Materials Chemistry C
View Article Online
Table 1. Photophysical, electrochemical, and thermal characteristics of the DnOoI: v10e.1l039h/oC7leTC03049E
transporting materials.
b)
c)
d)
e)
HTM
λabs
λem
(nm) (eV)
HOMOa) LUMOa) Eg
ET
Td
Tm
Tg
μh
(nm)
(eV)
2.12
2.21
1.82
2.40*
(eV)
3.15
3.10
3.11
3.10*
(eV)
3.09
2.82
2.69
(oC) (oC)
(oC)
(cm2.V-1.s-1)
2.4×10-4
1.5×10-4
3.5×10-5
NPVCz
326, 298, 247 381
5.27
5.31
4.93
5.50*
379
461
360
120
265
184
57
DNPVCz 308, 245
DPPVCz 299, 255
384
384
450
107
55
NPB
345, 215
2.30* 350
280[15a] 95[15a] 4.1×10-4 [15b]
a)HOMO and LUMO values were measured by cyclic voltammetry method. Semi-oxidation potential (E1/2
)
ox
could be calculated from (Ep1+Ep2)/(2-0.48). Where 0.48 is correction value obtained by the oxidation system
was added Fc+/Fc as the internal standard, and EHOMO = -(E1/2ox + 4.8). ELUMO = (EHOMO -Eg). b)decomposition
temperature. c)melting temperature. d)glass-transition temperature. e)hole mobility. Ref. 15.
2.3 Electrochemical properties
The electrochemical properties of all three HTMs were characterized by cyclic voltammetry.
The highest occupied molecular orbital (HOMO) energy levels were calculated by using the
oxidation potentials, which were estimated from the cyclic voltammograms (Fig. 2). The
resultant HOMO levels are -5.27, -5.31 and -4.93 eV for the NPVCz, DNPVCz, and
DPPVCz, respectively. The lowest unoccupied molecular orbital (LUMO) energy levels of
the HTMs were calculated from HOMO energy level and optical energy band gap (Eg), which
are -1.99, -1.99 and -1.64 eV, respectively. The determined oxidation potentials (E1/2ox),
energy bandgaps, HOMO and LUMO values are listed in Table 1.

Journal of Materials Chemistry C
Page 8 of 27
View Article Online
DOI: 10.1039/C7TC03049E
Figure 2. Cyclic voltammograms of the compounds (a) NPVCz, (b) DNPVCz, and (c)
DPPVCz.
2.4 Thermal and morphological properties
The thermal behavior of the compounds DPPVCz, NPVCz, and DNPVCz was characterized
by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)
techniques. As shown in Fig. S7, the melting temperatures (Tm) of the compounds are ranged
o
o
from 120 oC to 265 C, and the glass transition temperatures (Tg) from 55 oC to 107 C,
respectively. The thermal behavior of the HTMs is strongly depended on the type of
substituents (phenyl and naphthalene) and the number of substituent units.

Page 9 of 27
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03049E
Figure 3. Surface morphologies of the spin-coated films and molecular structures of the
novel HTMs (a) NPVCz, (b) DNPVCz, and (c) DPPVCz, and a typical HTM (d) NPB. All
the films were spin-coated from THF solutions onto the PEDOT:PSS layer as employed in
devices.

Journal of Materials Chemistry C
Page 10 of 27
View Article Online
As shown in Table 1, two phenyl groups substituted carbazole core based compound
DOI: 10.1039/C7TC03049E
o
DPPVCz compound shows the lowest Tg value of 55 C, while the Tg increases to 57 oC for
the asymmetric NPVCz compound with one naphthalene unit. The Tg of compound
extensively increases from 57 oC to 107 oC for the DNPVCz as an additional naphthalene unit
was introduced on to the carbazole core of the NPVCz compound. As shown in Fig. S8, the
compounds DPPVCz and NPVCz, DNPVCz exhibit a thermal decomposition temperatures
o
o
o
(Td) of 360 C, 379 C, and 461 C, respectively, corresponding to a 5% weight loss.
Amongst, the highest Td of the DNPVCz may arise because of the fused molecular structure
and two naphthalene substituents. The robust thermal characteristic of compounds may
facilitate the excellent film integrity under electrical excitation.41 Because an organic material
is less vulnerable to heat and forms stable amorphous film with increasing Tg. In contrast, the
low Tg organic compounds easily crystallize that affects film homogeneity, and crystal
boundaries raise the electrical resistance to the carriers. Therefore, high Tg materials result
into better film integrity and leads to better device performance.41
As shown in Fig. 3, the surface morphologies of the spin-coated films of the HTMs
were characterized by the atomic-force microscopy (AFM). The resultant root-mean-square
(rms) surface roughness values are 0.41, 0.38, 0.62, and 0.76 nm for the NPVCz, DNPVCz,
DPPVCz, and NPB, respectively. All the spin-coated films exhibit smooth surface topologies
without any crystallization. The bi-naphthyl units substituted carbazole core based
compound, DNPVCz, shows relatively low film roughness. It has been found that the newly
synthesized carbazole core based HTMs, NPVCz, DNPVCz, DPPVCz, exhibit relatively
much smoother film morphologies than that of typical NPB counterpart.
2.5 Hole mobility
The hole mobility (μh) of the compounds DPPVCz NPVCz, and DNPVCz was measured by
the time-of-flight (TOF) technique.42,43 The μh was calculated by the formula of μh =

Page 11 of 27
Journal of Materials Chemistry C
d2/(V.tT), where d is the layer thickness of the compounds, V applied bias, and tT carrier
View Article Online
DOI: 10.1039/C7TC03049E
transit time.44,45 The μh determined for the carbazole core based HTMs is shown as a function
of the square root of electric field (E1/2) in Fig. 4 (see transient photocurrent curves in Fig.
S9). The μh of the compounds DPPVCz, NPVCz and DNPVCz are ranged from 3.5×10-5 to
2.4×10-4 cm2/V·s, as shown in Table 1.
Figure 4. The hole mobility of the HTMs, NPVCz, DNPVCz and DPPVCz, as a function of
the square root of electric field.

Journal of Materials Chemistry C
Page 12 of 27
View Article Online
DOI: 10.1039/C7TC03049E
Figure 5. Schematic diagram of the energy levels of the (a) fluorescent and (b)
phosphorescent OLED devices containing the spin-coated films of four different HTMs,
NPVCz, DNPVCz, DPPVCz, and NPB.
2.6 Electroluminescent characteristics
Fig. 5a shows the schematic energy-level diagrams of the OLED devices studied herein.
Taking the fluorescent green OLED device for example, the fabrication process included
firstly spin-coating an aqueous solution of PEDOT:PSS at 4000 rpm for 20 s to form a hole-
injection layer (HIL) on a pre-cleaned ITO anode. Before spin-coating the following hole
transporting layer (HTL), the solution was prepared by dissolving three different
concentrations 2, 4 and 6 mg ml-1 of the HTMs (NPVCz, DNPVCz, and DPPVCz) in
tetrahydrofuran at room-temperature with stirring for 0.5 h. The resulting solution was then
spin-coated at 2500 rpm for 20 s under nitrogen. Followed were the depositions of the
emission layer/electron-transporting layer Alq3, the electron injection layer LiF, and the
cathode Al, by thermal evaporation in a vacuum chamber at a vacuum level of less than 5 ×
10−6 Torr. A standard device was also fabricated for comparison by using NPB as HTL. As
shown in Fig. 5b, the phosphorescent green device contain a 32 nm HIL of PEDOT:PSS,
which was spin-coated on a pre-cleaned ITO coated glass substrate. Subsequently, a 25 nm

Page 13 of 27
Journal of Materials Chemistry C
HTM and 20 nm EML were spin-coated on the PEDOT:PSS coated substrate. Finally, Vaiew3A5rticle Online
DOI: 10.1039/C7TC03049E
nm ETL of TPBi, a 1 nm EIL of LiF, and a 100 nm Al cathode, were deposited by the
thermal evaporation.
Table 2. Electroluminescence (EL) characteristics of double layer fluorescent OLED devices
by using newly synthesized carbazole derivatives, NPVCz, DPPVCz, and DNPVCz, and a
typical NPB as hole transporting layer.
Vdriv
(V)
ηPE
(lm/W)
ηCE
(cd/A)
1931 CIEx,y
Coordinates
Maximum
luminance
(cd/m2)
HTL
@ 100 /1,000 cd/m2
NPB
3.6/5.4
3.9/ 5.4
2.4/1.7
3.3/ 2.6
2.7/1.7
(0.32, 0.52)/(0.32, 052)
4218
NPVCz
4.1/ 4.5 (0.33, 0.54)/ (0.33, 0.54)
3.7/ 4.0 (0.34, 0.54)/(0.34, 0.54)
10360
DNPVCz 3.8/ 5.3
DPPVCz 4.1/5.9
3.0/ 2.4
2.0/1.7
4748
5829
2.6/3.1
(0.34, 0.55)/ (0.34, 0.55)
2.6.1. Fluorescent OLED devices
The current density (J), luminance (L), ηPE, and ηCE of two layer green fluorescent OLEDs
are presented in Fig. 6a. All the three HTMs based devices also show higher efficiencies and
luminance than that of typical NPB based device. At 1000 cd m-2, for example, fluorescent
green OLED with NPVCz or DPPVCz layer demonstrate a ηCE of 4.5 or 4.0 cd A-1, which are
almost 165 or 135% higher than that of typical HTM, NPB, based device. In addition,
naphthalene substituted carbazole core type HTMs, NPVCz and DNPVCz, based devices also
show a maximum luminance of 10360 cd m-2 and 4748 cd m-2, respectively, which are 146%
and 13% higher than NPB based device. The reason why NPVCz and DNPVCz based
devices show higher performance may be attributed to effective electrons and excitons
confinement within the Alq3 layer (EML/ETL), especially at high luminance, due to their
low-lying LUMO energy levels.9 Moreover, the high efficiency may attributed to smooth film
morphology of NPVCz that can effectively prevent any undesirable current leakage
throughout the device measurement. The resultant naphthyl substituted HTM, NPVCz, also
exhibited relatively higher device performance than that of the phenyl substituted DPPVCz

Journal of Materials Chemistry C
Page 14 of 27
View Article Online
counterpart (Table 2). In naphthalene substituted HTMs, we attribute the high device
DOI: 10.1039/C7TC03049E
performance to the following factors, (i) relatively higher hole mobility than that of the
phenyl substituted HTM, and (ii) matching HOMO energy levels that fascilitate injection of
holes into the Alq3 layer. All the four devices show an EL emission peak at 516 nm, which is
directly correspond to the Alq3 emission (Fig. S10).
Figure 6. Effects of HTMs on the (a) current density, (b) luminance, (c) power efficiency vs
luminance, and (d) current efficiency vs current density results of the fluorescent OLED
devices.
2.6.2. Phosphorescent OLED devices
Table 3 shows the electroluminescent characteristics of conventional Ir(ppy)3 based green
phosphorescent devices with various HTMs, NPVCz, DNPVCz and DPPVCz, and NPB. The

Page 15 of 27
Journal of Materials Chemistry C
View A-r2ticle Online
NPB based control device shows a ηPE of 30.6 lm W-1 and a ηCE of 41.4 cd A-1, aDt O1I0: 100.1c0d39/mC7TC03049E
(Fig. S11), which are almost 82% and 45% lower compared to the OLED devices based on
the NPVCz. Moreover, the maximum ηCE is markedly improved from 46.4 to 66.2 cd A-1
(max. ηPE from 48.5 to 70.0 lm W-1, and a max. ηext from 14.2% to 18.3%). As shown in Fig.
7, the NPVCz containing device demonstrated a ηCE of 60.1 cd A-1 (ηPE = 55.6 lm W-1 and ηext
= 17.1%), at 100 cd m-2. On the contrary, DNPVCz containing device showed a ηCE of 58.4
cd A-1 (ηPE = 54.8 lm W-1 and ηext = 16.1%), while a 45.1 cd A-1 (ηPE = 40.8 lm W-1 and ηext
=
12.5%) for the DPPVCz containing device. The reason why the NPVCz based device showed
the highest efficiencies among all the three studied HTMs may be attributed to three reasons,
namely high hole mobility, appropriate HOMO and LUMO energy levels, and sufficiently
high triplet energy.
Table 3. Effect of the carbazole based novel HTMs on the EL characteristics of
phosphorescent OLED devices.
HTL
Vdriv. (V)
ηPE (lm/W) ηCE (cd/A)
@ 100 /1,000 cd/m2
41.4/32.7
CIEx,y coordinates
Maximum luminance
(cd/m2)
NPB
4.2/5.9
3.5/4.6
3.4/4.7
3.5/4.7
30.6/17.8
55.6/34.3
54.8/26.6
40.8/24.5
(0.31, 0.63)/(0.31, 63)
(0.31, 0.63)/(0.31, 63)
(0.30, 0.63)/(0.30, 63)
(0.31, 0.63)/(0.31, 63)
11560
15140
9020
NPVCz
DNPVCz
DPPVCz
60.1/50.2
58.4/39.8
45.1/36.1
12280

Journal of Materials Chemistry C
Page 16 of 27
View Article Online
DOI: 10.1039/C7TC03049E
Figure 7. Effects of HTMs on the (a) current density and luminance, (b) power efficiency vs
luminance, (c) current efficiency vs luminance, and (d) EL spectra of the phosphorescent
OLED devices.
The NPVCz shows a highest hole mobility of 2.4 × 10-4 cm2.V0.5.s0.5, which is much
higher than that of the DPPVCz and DNPVCz counterparts. The NPVCz favors an efficient
hole injection in the device, and result a highest brightness among all four devices. Moreover,
the resultant device architecture shows an energy barrier of 0.07 eV for hole to enter from
HIL (PEDOT:PSS) and 0.33 or 0.73 eV to the Ir(ppy)3 guest or CBP host. Whilst, the barrier
is 0.11 eV for hole to enter from the HIL to the HTL DNPVCz and 0.29 or 0.69 eV further to
the guest or host. The higher hole mobility and low energy barrier between the HIL and HTL
may explain why device with NPVCz shows higher efficiencies.

Page 17 of 27
Journal of Materials Chemistry C
View Article Online
The reason why the naphthyl substituted carbazole derivatives, NPVCz anDdODI: 1N0.1P0V39/CC7zT,C03049E
based devices exhibit relatively higher efficiency than that of phenyl substituted carbazole
compound DPPVCz because of their higher ET values (Table 1). Moreover, all the three
HTMs demonstrated a higher ET than that of the CBP host (2.6 eV). The resultant HTMs can
effectively confine the electrons and excitons within the EML and result a bright emission
with high efficiencies.46
As to DPPVCz, its resultant device structure shows an extremely high barrier
for hole to enter into the guest or host, which in 0.67 or 1.07 eV. Although there is barely any
barrier for the hole to enter from the HIL, the tremendously high barrier to further inject into
the emissive layer is in a totally unfavorable position. This explains why the DPPVCz
containing device shows lowest performance among all four devices studied. The NPVCz
based device also show 54% and 92% higher in ηCE and ηPE, respectively, compared to the
conventional NPB based OLED (Table 2 and Fig. S11a and 11b). Fig. 7d and Fig. 11c show
the EL spectra of devices that were collected at 100 and 1000 cd m-2. The EL spectra of
devices remained identical when naphthyl and phenyl substituted carbazole derivatives were
utilized as the HTMs (Fig. 7d). Over the past years, numerous vinyl type compounds, such as
PVK and VPEC, exhibit better hole transporting characteristics in the solution-processed
devices than conventional aryl compounds, namely NPB and TPD.15,21,25,40 The excellent
performance of typical HTMs, PVK and VPEC, in solution-processed OLEDs motivated us
to design molecular HTMs based on vinyl linkage.47
The operational lifetime (t1/2), which is defined as a time when the luminance drops to
50% of initial luminance (L0), of phosphorescent OLEDs was carried-out by operating the
devices under a constant current with a varying initial brightness values at room temperature.
Fig. S12 shows the effect of HTMs, namely NPB, NPVCz, DNPVCz, and DPPVCz, on the
device lifetime. The DNPVCz based device shows a lifetime of 1.63 h at the L0 of 1914 cd m-

Journal of Materials Chemistry C
Page 18 of 27
2, while conventional NPB based device exhibits a lifetime of 1.0 h at the L0 of 2329 cdVimew-A2r.ticle Online
DOI: 10.1039/C7TC03049E
The t1/2 of DNPVCz device, at an initial brightness of 100 cd m-2, can be extrapolated to 246
h. The devices with the NPVCz and DPPVCz HTMs also demonstrate the extrapolated t1/2 of
160 h and 174 h, respectively, which are slightly lower than that of conventional NPB (208 h)
counterpart. The DNPVCz shows a highest thermal degradation temperature among all the
studied HTMs, NPVCz, DPPVCz, and NPB, that may also lead higher t1/2 under the electrical
excitation. Hence, it is worthy to investigate the degradation mechanism of vinyl linkage
based molecular HTMs. The t1/2 of solution-process OLEDs can be enhanced further by
rationalizing the molecular design of naphthalene-substituted carbazole core based HTMs.
These materials will open an avenue towards realization of high efficiency, long-lasting
OLEDs for displays and lighting applications.
3. Conclusion
In conclusion we have designed and synthesized a series of naphthyl and phenyl substituted
carbazole core based HTMs, NPVCz, DNPVCz, and DPPVCz, for high efficiency
fluorescent and phosphorescent OLED devices. All the three molecular HTMs showed
excellent solution-processability with much smooth film morphologies due to appropriate
thermal stabilities and high solubility in common organic solvents. These HTMs, especially
naphthyl derivatives NPVCz and DNPVCz, showed higher device efficiencies than that of
typical NPB counterpart. Fluorescent device shows an improvement of 165% and 52%, in ηCE
and ηPE, respectively, as typical NPB is replaced with the NPVCz. The NPVCz based
phosphorescent device showed a maximum ηPE and ηCE of 70.0 and 66.2 cd A-1, respectively,
and improves by over 40% compared to the OLED based on the NPB. The higher device
performances are resulted because of rational hole transporting ability, smooth surface
morphology, effective electron and exciton confinement function, and adequately high triplet
energies. The naphthyl substituted carbazole derivatives can serve as potential molecular

Page 19 of 27
Journal of Materials Chemistry C
View Article Online
HTMs in solution-processed OLED devices. These materials will open an avenue towards
DOI: 10.1039/C7TC03049E
realization of high efficiency and long-lasting OLEDs for displays and lighting applications.
4. Experimental
4.1 Synthetic detail and spectral data
All the precursor compounds required for the synthesis were purchased from commercial
sources and used as such without further purification. The 9H-carbazole (1),
phenylacetaldehyde, (±)-camphor-10-sulfonic acid, 1-naphtalene boronic acid, phenyl
boronic acid, bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2), and potassium
hydroxide were purchased from Aldrich and used as received without further purification. 3-
Iodo-9H-carbazole (2) and 3,6-diiodo-9H-carbazole (3) were obtained by a procedure of
Tucker.40
3-Iodo-9-(2-phenylvinyl)carbazole (4): Firstly, 3-iodo-9H-carbazole (Compound 2) (1.5 g, 5
o
mmol) was dissolved in toluene (15 ml) at 80 C. Subsequently, phenylacetaldehyde (1.32
ml, 11.3 mmol) was added dropwise into the stirred solution of Compound 2. Then a (±)-
camphor-10-sulfonic acid was added as a catalyst. The resultant mixture was stirred for 24 h
at 110 oC. Finally, the solvent was evaporated under vacuum and the resultant crude product
was purified by silica gel column chromatography using hexane/diethyl ether (vol. ratio 30:1)
as an eluent. The yield of Compound 4 was 0.56 g (50%).
H1 NMR spectrum (300 MHz, CDCl3, δ, ppm): 8.31 (d, 1H, J = 1.2 Hz, Ar); 7.95 (d, 1H, J =
7.2 Hz, Ar); 7.67 (dd, 1H, J1 = 1.5 Hz, J2 = 9.0 Hz, Ar); 7.63 (d, 1H, J = 8.7 Hz, Ar); 7.53 (d,
1H, J = 14.4 Hz, NCH=CHPh); 7.50 – 7.38 (m, 8H, Ar); 6.96 (d, 1H, J = 14.4 Hz,
NCH=CHPh). IR (KBr, cm−1): 3057, 3022 (C-H in Ar); 1652 (C=C); 1616, 1599, 1574,
1468, 1442 (C=C in Ar); 1374, 1359, 1267, 1235, 1218 (C-N in Ar); 946 (=C-H trans); 871,
793, 743, 688 (C-H in Ar); 543, 506 (C-J). MS (APCI+, 20 V): 397.3 ([M+H], 100 %).

Journal of Materials Chemistry C
Page 20 of 27
View Article Online
3,6-Diiodo-9-(2-phenylvinyl)carbazole (5): Firstly, a 3,6-di-iodo-9H-carbazole (Compound
DOI: 10.1039/C7TC03049E
o
3) (8.7 g, 20.86 mmol) was dissolved in toluene (70 ml) at 80 C. Afterward, a
phenylacetaldehyde (4.88 ml, 41.7 mmol) was added dropwise into the stirred solution of
Compound 3. Later, a (±)-camphor-10-sulfonic acid was added as catalyst. The solution was
stirred for 24 h at 110 oC. Finally, the solvent was evaporated under vacuum and the resultant
crude product was purified by silica gel column chromatography using hexane/diethyl ether
(vol. ratio 30:1) as an eluent. The yield of compound 7 was 8.1 g (75%).
H1 NMR spectrum (400 MHz, CDCl3, δ, ppm): 8.26 (d, 2H, J = 1.6 Hz, Ar); 7.71 (dd, 2H, J1
= 1.4 Hz, J2 = 8.7 Hz, Ar); 7.51 – 7.38 (m, 7H, Ar and NCH=CHPh); 7.32 (t, 1H, J = 6.9 Hz,
Ar); 6.94 (d, 1H, J = 14.1 Hz, NCH=CHPh). IR (KBr, cm−1): 3053, 3022 (C-H in Ar); 1650
(C=C trans); 1599, 1468, 1427 (C=C in Ar); 1367, 1357, 1284, 1234, 1216 (C-N in Ar); 923
(=C-H trans); 803, 793, 743,689 (C-H in Ar); 564, 558, 507 (C-J). MS (APCI+, 20 V): 522.2
([M+H], 100 %).
3-(1-Naphthyl)-9-(2-phenylvinyl)carbazole (NPVCz): In order to synthesize the target
compound NPVCz, a 0.3 g (0.75 mmol) of 3-iodo-9-(2-phenylvinyl)carbazole (Compound
4), a 0.26 g (1.51 mmol) of 1-naphtalene boronic acid, a 0.023 g (0.033 mmol) of
PdCl2(PPh3)2, and a 0.21 g (3.7 mmol) of powdered potassium hydroxide were dissolved in 5
o
ml of THF solvent. Then the resultant mixture was stirred for 24 h at 80 C under nitrogen
atmosphere. Subsequently, the reaction mixture was cooled and then quenched in an ice bath.
The resultant compound was extracted by using a chloroform solvent. The extract was dried
by using an anhydrous Na2SO4. Finally, the resultant extract was purified by silica gel
column chromatography using a mixture of ethyl acetate and hexane (vol. ratio 1:50) as an
eluent. Yield: 0.25 g (83%) of white crystals.

Page 21 of 27
Journal of Materials Chemistry C
H1 NMR spectrum (400 MHz, CDCl3, δ, ppm): 8.19 (d, 1H, J = 1.2 Hz, Ar); 8.07 (d, 1H, J =
View Article Online
DOI: 10.1039/C7TC03049E
7.2 Hz, Ar); 8.00 (d, 1H, J = 8.4 Hz, Ar); 7.92 (d, 1H, J = 8.0 Hz, Ar); 7.88 (dd, 1H, J1 = 1.8
Hz, J2 = 7.6 Hz, Ar); 7.82 (d, 1H, J = 8.0 Hz, Ar); 7.75 (d, 1H, J = 14.4 Hz, NCH=CHPh);
7.70 (d, 1H, J = 8.0 Hz, Ar); 7.62 (dd, 1H, J1 = 2.0 Hz, J2 = 8.4 Hz, Ar); 7.58 -7.39 (m, 9H,
Ar); 7.34 – 7.28 (m, 2H, Ar); 7.12 (d, 1H, J = 14.4 Hz, NCH=CHPh). IR (KBr, cm−1): 3053
(C-H in Ar); 1648 (C=C); 1622, 1600, 1565, 1486, 1455 (C=C in Ar); 1395 (naphthyl group);
1375, 1331, 1236 (C-N in Ar); 939 (=C-H trans); 889, 819, 776, 746 (C-H in Ar). MS
(APCI+, 20 V): 396.4 ([M+H], 100 %).
3,6-Di-(1-Naphthyl)-9-phenylvinylcarbazole (DNPVCz): In order to synthesize the target
compound DNPVCz, a 1.0 g (1.92 mmol) of 3,6-diiodo-9-(2-phenylvinyl)carbazole (5), a
0.68 g (5.76 mmol) of 1-naphtalene boronic acid, a 0.081 g (0.12 mmol) of PdCl2(PPh3)2 and
a 1.08 g (19.25 mmol) of powdered potassium hydroxide were dissolved in 15 ml of a THF
o
solvent. The resultant mixture was stirred for 24 h at 80 C under nitrogen atmosphere.
Subsequently, the reaction mixture was cooled and quenched in an ice bath. Then the product
was extracted by a chloroform solvent. The resultant extract was dried by using an anhydrous
Na2SO4. Finally, the extract was purified by silica gel column chromatography using a
mixture of ethyl acetate and hexane (vol. ratio 1:20) as an eluent. Yield: 0.5 g (50%) of white
crystals.
H1 NMR spectrum (400 MHz, CDCl3, δ, ppm): 8.13 (dd, 2H, J1 = 1.6 Hz, J2 = 7.2 Hz, Ar);
8.04 (d, 2H, J = 0.8 Hz, Ar); 7.73 (dd, 2H, J1 = 2.4 Hz, J2 = 7.2 Hz, Ar); 7.69 (d, 2H, J = 8.4
Hz, Ar); 7.54 (dd, 2H, J1 = 1.6 Hz, J2 = 8.4 Hz, Ar); 7.51 – 7.45 (m, 5H, Ar and
NCH=CHPh)); 7.38 (tr, 2H, J = 7.6 Hz, Ar); 7.32 -7.26 (m, 6H, Ar); 7.23 (tr, 2H, J = 7.6 Hz,
Ar); 7.13 (tr, 1H, J = 7.2 Hz, Ar); 7.12 (d, 1H, J = 14.4 Hz, NCH=CHPh). IR (KBr, cm−1):
3036 (C-H in Ar); 1650 (C=C); 1626, 1488, 1470, 1453 (C=C in Ar); 1392 (naphthyl group);

Journal of Materials Chemistry C
Page 22 of 27
View Article Online
1357 (C-N in Ar); 939 (=C-H trans); 882, 811, 800, 778, 690 (C-H in Ar). MS D(OAI:P10C.1I0+39,/C270TC03049E
V): 522.7 ([M+H], 100 %).
3,6-Diphenyl-9-(2-phenylvinyl)carbazole (DPPVCz): In order to synthesize the target
compound DPPVCz, a 1.5 g (2.9 mmol) of 3,6-diiodo-9-(2-phenylvinyl)carbazole (5), a 1.05
g (8.6 mmol) of phenyl boronic acid, a 0.12 g (0.17 mmol) of PdCl2(PPh3)2, and a 1.6 g (28.5
mmol) of powdered potassium hydroxide were dissolved in 20 ml of a THF solvent. The
o
resultant mixture was stirred for 24 h at 80 C under nitrogen atmosphere. Then the reaction
mixture was cooled and quenched by the addition of ice water. Finally, the quenched reaction
mixture was extracted by chloroform after that the resultant extract was dried by using the
anhydrous Na2SO4. The dry extract was purified by silica gel column chromatography using
the mixture of ethyl acetate and hexane (vol. ratio 1:40) as an eluent. Yield: 0.22 g (18%) g of
white crystals.
H1 NMR spectrum (300 MHz, CDCl3, δ, ppm): 8.18 (d, 2H, J = 1.8 Hz, Ha); 7.74 – 7.26 (m,
20H, Ar); 7.10 (d, 1H, J = 14.8 Hz, NCH=CHPh). IR (KBr, cm−1): 3057, 3028 (C-H in Ar);
1649 (C=C); 1625, 1598, 1475, 1456 (C=C in Ar); 1373, 1363, 1227 (C-N in Ar); 934, 925,
875, 868, 761, 696 (C-H in Ar). MS (APCI+, 20 V): 422.6 ([M+H], 100 %).
4.2 Materials and characterization
The indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 Ω
light transmittance greater than 84% were purchased from Luminescence Technology
Corporation. The hole injection material poly(3,4-ethylene-dioxythiophene)-
̸□ and a
poly(styrenesulfonate) (PEDOT:PSS) with a purity of 99.9% was purchased from Bayer
Taiwan. The electron transporting material TPBi was purchased from e-Ray Optoelectronic.
Typical fluorescent and phosphorescent green emitters, respectively, Alq3 and tris(2-phenyl-
pyridine) iridium (Ir(ppy)3 were purchased from Luminescence Technology Corporation.

Page 23 of 27
Journal of Materials Chemistry C
View Article Online
Lithium fluoride (LiF) (purity 99.95%) was purchased from Strem Chemicals Corporation.
DOI: 10.1039/C7TC03049E
Aluminum (Al) ingots (99.999%) were purchased from Showa Chemical Co. Ltd. All the
materials were used without further purification.
Column chromatography purifications were performed with silica gel (70-230 mesh) as
1
a stationary phase in a column with 50 cm long and 5 cm diameter. H NMR spectra were
recorded using a Varian Unity Inova (300 MHz) apparatus. Mass spectra of the compounds
were obtained on a Waters ZQ 2000 spectrometer in the positive ion mode. The PL spectra of
the compounds were recorded in THF at room temperature in quartz cuvettes using a
Fluorolog III photoluminescence spectrometer. The UV-vis spectra of the compounds were
recorded in THF at room temperature using a UV-Vis spectrophotometer.
Cyclic voltammetry (CV) experiments were performed in an electrochemical
workstation using a three electrode assembly comprising glassy carbon working electrode, a
non-aqueous Ag/AgCl reference electrode, and an auxiliary platinum electrode. The
experiments were performed at room temperature under nitrogen atmosphere in
dichloromethane using a 0.1 M tetrabutylammonium perchlorate (Bu4NClO4) as supporting
ox
electrolyte on a CH-instruments CH1604A potentiostat. The E1/2 values were determined
a
c
a
c
using (Ep + Ep )/2, where Ep and Ep are the anodic and cathodic peak potentials,
respectively.
Differential scanning calorimetry (DSC) measurements were carried out using a Bruker
Reflex II thermosystem. Thermogravimetric analysis (TGA) was performed using a Netzsch
STA 409 apparatus. The DSC and TGA curves were recorded in a nitrogen atmosphere at a
ramp rate of 10 oC min.-1.
4.3 Device fabrication and characterization

Journal of Materials Chemistry C
Page 24 of 27
View Article Online
Fig. 5 shows the schematic energy-level diagrams of the OLED devices studied herein.
DOI: 10.1039/C7TC03049E
Taking the fluorescent green OLED device for example, the fabrication process included
firstly spin-coating an aqueous solution of PEDOT:PSS at 4,000 rpm for 20 s to form a hole-
injection layer (HIL) on a pre-cleaned ITO anode. Before spin-coating the following hole
transporting layer (HTL), the solution was prepared by dissolving 4 mg ml-1 of the HTMs
(NPVCz, DNPVCz, DPPVCz, and NPB) in tetrahydrofuran at room-temperature with stirring
for 0.5 h. The resulting solution was then spin-coated at 2,500 rpm for 20 s under nitrogen.
Followed were the depositions of the emission layer/electron-transporting layer Alq3, the
electron injection layer LiF, and the cathode Al, by thermal evaporation in a vacuum chamber
at a vacuum level of less than 5 × 10−6 Torr.
For phosphorescent green device, a 32 nm HIL of PEDOT:PSS was spin-coated on a
pre-cleaned ITO coated glass substrates. Subsequently, molecular HTMs (NPVCz, DNPVCz,
DPPVCz and NPB) were spin-coated on the PEDOT:PSS coated substrates and a 20 nm
EML (10 wt% Ir(ppy)3 doped in the CBP host) was deposited on the annealed HTL via spin-
coating method (2000 rpm for 30 s). Later on, a 35 nm ETL of TPBi and a 1 nm EIL of LiF
were thermally-evaporated with 0.5 Å s-1 and 0.1 Å s-1. Finally, a 100 nm Al cathode layer
was deposited by the thermal-evaporation.
The luminance, CIE chromatic coordinates, and electroluminescent spectrum of the
resultant green OLEDs were measured by using a Photo Research PR-655 spectrascan and
CS100A luminance meter. A Keithley 2400 source meter was used to measure the current-
voltage (I–V) characteristics. The emission area of the devices was 25 mm2, which is defined
by the overlapping area between anode and cathode layers, and only the luminance in the
forward direction was measured.
Acknowledgement

Page 25 of 27
Journal of Materials Chemistry C
View Article Online
This work was financially supported by Ministry of Science and Technology DthOrI:o10u.1g0h39/tCh7eTC03049E
grant numbers of 100-2119-M-007-011-MY3 and 103-2923-E-007-003-MY3, Ministry of
Economic Affairs through the grant number MEA 102-EC-17-A-07-S1-181, and by Research
Council of Lithuania (grant No. TAPLLT1/14).
Declaration
There are no conflicts of interest to declare.
References
1 F. So, J. Kido, and P. Burrows, MRS Bull., 2008, 33, 663.
2 B. W. D’Andrade, S. R. Forrest, Adv. Mater., 2004, 16, 1585.
3 S. R. Forrest, Nature, 2004, 428, 911.
4 H. Kim, Y. Byun, R. R. Das, B. K. Choi, and P. S. Ahn, Appl. Phys. Lett., 2007, 91,
093512.
5 T.-W. Lee, T. Noh, H.-W. Shin, O. Kwon, J.-J. Park, B.-K. Choi, M.-S. Kim, D. W. Shin,
and Y.-R. Kim, Adv. Mater., 2009, 19, 1625.
6 J.-H. Jou, M.-F. Hsu, W.-B. Wang, C.-L. Chin, Y.-C. Chung, C.-T. Chen, J.-J. Shyue, S.-M.
Shen, M.-H. Wu, W.-C. Chang, C.-P. Liu, S.-Z. Chen and H.-Y. Chen, Chem. Mater., 2009,
21, 2565.
7 J.-H. Jou, Y.-M. Yang, S.-Z. Chen, J.-R. Tseng, S.-H. Peng, C.-Y. Hsieh, Y.-X. Lin, C.-L.
Chin, J.-J. Shyue, S.-S. Sun, C.-T. Chen, C.-W. Wang, C.-C. Chen, S.-H. Lai and F.-C.
Tung, Adv. Optical Mater., 2013, 1, 657.
8 J.-H. Jou, Y.-X. Lin, S.-H. Peng, C.-J. Li, Y.-M. Yang, C.-L. Chih, J.-J. Shyue, S.-S. Sun,
M. Lee, C.-T. Chen, M.-C. Liu, C.-C. Chen, G.-Y. Chen, J.-H. Wu, C.-H. Li, C.-F. Sung,
M.-J. Lee, and J.-P. Hu, Adv. Funct. Mater., 2014, 24, 555.
9 J.-H. Jou, S. Kumar, P.-H. Fang, A. Venkateswararao, K. R. J. Thomas, J.-J. Shyue, Y.-C.
Wang, T.-H. Li, and H.-H Yu, J. Mater. Chem. C, 2015, 3, 2182.
10 C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett., 1990, 57, 531.
11 D. O’Brien, A. Bleyer and D. G. Lidzey, J. Appl. Phys., 1997, 82, 2662.
12 J. H. Jou, S. Kumar, A. Agrawal, T.-H. Li, and S. Sahoo, J. Mater. Chem. C, 2015, 3, 2974.
13 C. Adachi, K. Nagai and N. Tamoto, Appl. Phys. Lett., 1995, 66, 2679.
14 B. E. Koene, D. E. Loy and M. E. Thompson, Chem. Mater., 1998, 10, 2235.

Journal of Materials Chemistry C
Page 26 of 27
15 Q. Huang, G. A. Evmenenko, P. Dutta, P. Lee, N. R. Armstrong and T. J. MDaOrkI:s10,.1J0.3V9iAe/wCm7ATr.tCic0le3O0n4li9nEe
Chem. Soc., 2005, 127, 10227.
16 S. H. Kim, J. Jang, and J. Y. Lee, Appl. Phys. Lett., 2007, 90, 223505.
17 S. O. Jeon, K. S. Yook, C. W. Joo, J. Y. Lee, K. Y. Ko, J. Y. Park, and Y. G. Baek, Appl.
Phys. Lett., 2008, 93, 063306.
18 R. J. Holmes, S.R. Forrest, R.C. Kwong, J.J. Brown, S. Garon, and M.E. Thompson, Appl.
Phys. Lett., 2003, 82, 2422.
19 M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, Appl. Phys. Lett., 2001, 79, 156.
20 J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. Xue, and F. So, J. Shi, Appl. Phys. Lett., 2008,
93, 123306.
21 L. H. Chan, R. H. Lee, C. F. Hsieh, H. C. Yeh and C. T. Chen, J. Am. Chem. Soc., 2002,
124, 6469.
22 P. M. Borsenberger, E. H. Magin, and J. J. Fitzgerald, J. Phys. Chem., 1993, 97, 8250.
23 S. Tokito, H. Tanaka, A. Okada, and Y. Taga, Appl. Phys. Lett. 1996, 69, 878.
24 Y. Kuwabara, H. Ogawa, H. Inada, N. Noma, and Y. Shirota, Adv. Mater., 1994, 6, 677.
25 J.-H. Jou, T.-H. Li, S. Kumar, C.-C. An, A. Agrawal, S.-Z. Chen, P.-H. Fang, G. Krucaite,
S. Grigalevicius, J. Grazulevicius, and C.-F. Sung, Org. Electron., 2015, 24, 254.
26 Y. H. Niu, M. S. Liu, J. W. Ka, J. Bardeker, M. T. Zin, R. Schofield, Y. Chi, and A. K.Y.
Jen, Adv. Mater., 2007, 19, 300.
27 Z. Jiang, T. Ye, C. Yang, D. Yang, M. Zhu, C. Zhong, J. Qin, and D. Ma, Chem. Mater.,
2011, 23, 771.
28 C. Zhong, C. Duan, F. Huang, H. Wu, and Y. Cao, Chem. Mater., 2011, 23, 326.
29 Y.-J. Cheng, M. S. Liu, Y. Zhang, Y.-H. Niu, F. Huang, J.-W. Ka, H.-L. Yip, and A. K.-Y.
Jen. Chem. Mater. 2008, 20, 413.
30 P. Zacharias, M. C. Gather, M. Rojahn, O. Nuyken, and K. Meerholz, Angew. Chem., Int.
Ed. 2007, 46, 4388.
31 B. Domercq, R. D. Hreha, Y.-D. Zhang, N. Larribeau, J. N. Haddock, C. Schultz, S. R.
Marder, and B. Kippelen, Chem. Mater. 2003, 15, 1491.
32 I. K. Yakushchenko, M. G. Kaplunov, O. N. Efimov, M. Y. Belov, and S. N. Shamaev,
Phys. Chem. Chem. Phys., 1999, 1, 1783.
33 P. Moonsin, N. Prachumrak, R. Rattanawan, T. Keawin, S. Jungsuttiwong, T. Sudyoadsuk,
and V. Promarak, Chem. Commun., 2012, 48, 3382.
34 O. Usluer, S. Demic, D. A. M. Egbe, E. Brickner, C. Tozlu, A. Privrikas, A. M. Ramil, and
N. S. Sariciftci, Adv. Funct. Mater. 2010, 20, 4152.

Page 27 of 27
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC03049E
35 N. Prachumrak, S. Pansay, S. Namuangruk, T. Kaewin, S. Jungsuttiwong, T. Sudyoadsuk,
and V. Promarak, Eur. J. Org. Chem. 2013, 2013, 6619.
36 T. Tsuboi, S.-W. Liu, M.-F. Wu, and C.-T. Chen, Org. Electron. 2009, 10, 1372.
37 L. Duan, L. Hou, T.-W. Lee, J. Qiao, D. Zhang, G. Dong, L. Wang, and Y. Qiu, J. Mater.
Chem. 2010, 20, 6392.
38 C. S. Wu, S. W. Fang, and Y. Chen, Phys. Chem. Chem. Phys., 2013, 15, 15121.
39 A. M. Thaengthong, S. Seangsuwan, S. Jungsuttiwong, T. Keawin, T. Sudyoadsuk, and V.
Promarak, Tetrahedron Lett., 2011, 52, 4749.
40 H. Tucker, J. Chem. Soc. 1926, 1, 548.
41 (a) J.-H. Jou, W.-B. Wang, S.-M. Shen, S. Kumar, I.-M. Lai, J.-J. Shyue, S. Lengvinaite, R.
Zostautiene, J. V. Grazulevicius, S. Grigalevicius, S.-Z. Chen and C.-C. Wu, J. Mater.
Chem., 2011, 21, 9546. (b) W. J. Shen, R. Dodda, C. C. Wu, F. I. Wu, T. H. Liu, H. H.
Chen, C. H. Chen and C. F. Shu, Chem. Mater., 2004, 16, 930. (c) H. Doi, M. Kinoshita, K.
Okumoto and Y. Shirota, Chem. Mater., 2003, 15, 1080. (d) J.-H. Jou, S. Sahoo, S. Kumar,
H.-H. Yu, P.-H. Fang, M. Singh, G. Krucaite, D. Volyniuk, J. V. Grazulevicius, S.
Grigalevicius, J. Mater. Chem.C, 2015, 3, 12297.
42 C. A. Amorim, M. R. Cavallari, G. Santos, F.J. Fonseca, A. M. Andrade, and S. Mergulhao,
J. Non-Cryst. Solids, 2012, 358, 484.
43 S. C. Tse, C. H. Cheung, S. K. So, in: F. So (Ed.), Organic Electronics Materials,
Processing, Devices and Applications, Taylor & Francis, London, 2010, pp. 71-74. Chapter
3.
44 W. Y. Hung, L. C. Chi, W. J. Chen, Y. M. Chen, S. H. Chou, K. T. Wong, J. Mater. Chem.,
2010, 20, 10113.
45 M. H. Tsai, T. H. Ke, H. W. Lin, C. C. Wu, S. F. Chiu, F. C. Fang, Y. L. Liao, K. T. Wong,
Y. H. Chen, C. I. Wu, ACS Appl. Mater. Interfaces, 2009, 1, 567.
46 J. Lee, N. Chopra, S.-H. Eom, Y. Zheng, J. Xue, F. So, and J. Shi, Appl. Phys. Lett. 93,
123306, 2008.
47 R.-Q. Png, P.-J. Chia, J.-C. Tang, B. Liu, S. Sivaramakrishnan, M. Zhou, S.-H. Khong, H.
S. O. Chan, J. H. Burroughes, L.-L. Chua, R. H. Friend, P. K. H. Ho, Nat. Mater. 2010, 9,
152.