Carbazole dendronised triphenylamines as solution processed high T g amorphous hole-transporting materials for organic electroluminescent devices

Article abstract of DOI:10.1039/c2cc16878b

Carbazole dendrimers up to 4th generation were synthesized. They showed significantly high T_g, amorphous and stable electrochemical properties, and great potential as solution processed hole-transporting materials for OLEDs. Alq3-based green de

Full text of DOI:10.1039/c2cc16878b

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Chemical Communications
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Volume 48 | Number 28 | 7 April 2012 | Pages 3365–3464
ISSN 1359-7345
COMMUNICATION
Vinich Promarak et al.
Carbazole dendronised triphenylamines as solution processed high Tg amorphous hole-transporting materials
for organic electroluminescent devices

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Cite this: Chem. Commun., 2012, 48, 3382–3384
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COMMUNICATION
Carbazole dendronised triphenylamines as solution processed high Tg
amorphous hole-transporting materials for organic electroluminescent
devicesw
Preecha Moonsin, Narid Prachumrak, Ratanawalee Rattanawan, Tinnagon Keawin,
Siriporn Jungsuttiwong, Taweesak Sudyoadsuk and Vinich Promarak*
Received 7th November 2011, Accepted 18th January 2012
DOI: 10.1039/c2cc16878b
Carbazole dendrimers up to 4th generation were synthesized.
They showed significantly high Tg, amorphous and stable electro-
chemical properties, and great potential as solution processed
hole-transporting materials for OLEDs. Alq3-based green devices
exhibited high luminance efficiency and CIE coordinates of
4.45 cd Aꢀ1 and (0.29, 0.53), respectively.
forming a bulky structure, in particular dendrimer structure.9,14,15
Recent progress in organic synthesis provides the build-up of
dendritic molecules bearing well designed building blocks in
the core, branching point and on the surface.15,16 Owing to its
excellent hole-transporting ability, high charge carrier mobility,
high thermal, morphological and photochemical stability, and
easy functionalization at the 3,6 or N-positions, carbazole has
been used as a building block to form many HTMs.9–12,17
Therefore, we herein implemented all required aspects in the
chosen dendrimers (GnCT, Fig. 1). Carbazole as the branching
unit and triphenylamine as the core offer a perfect hole-
transporting ability with high charge carrier mobility. The
strongly twisted carbazole unit in the dendron and the presence
of tert-butyl groups as the surface offer bulky dendrimers
with high solubility, and thereby yield electrochemically and
thermally stable amorphous thin film with high glass transition
temperatures.9
Organic light-emitting diodes (OLEDs) have attracted a great
deal of attention due to their applications such as full-color
flat panel displays and general illumination.1 The past decade
has seen great progress in both material development and
device fabrication techniques.2–5 One of the key developments
is the use of hole-transporting layers (HTLs) for hole injection
from the anode into the emitting layer providing significant
improvement in the performance of the device.6 In general, high
glass transition temperature (Tg) amorphous hole-transporting
materials (AHTMs) are needed for highly efficient and long
lifetime OLED devices. The most commonly used HTMs,
N,N0-diphenyl-N,N0-bis(1-naphthyl)-(1,10-biphenyl)-4,40-diamine
(NPB) and N,N0-bis(3-methylphenyl)-N,N0-bis(phenyl)benzidine
(TPD), exhibit many attractive properties such as high charge
carrier mobility and ease of sublimation. However, their low
Tg (65 1C for TPD and 100 1C for NPB), ease of crystallization
and low morphological stability usually lead to degradation and
short lifetime of the devices.7 Although many arylamine- and
carbazole-based AHTMs with high Tg such as carbazole end-
capped molecules,8,9 9,9-bis(4-[bis-(4-carbazol-9-yl-biphenyl-4-yl)-
amino]phenyl)fluorenes,10 and triarylamine-based starburst
molecules11–13 have been reported, solution-processed analogues
remain rare and largely unexplored in OLEDs. It is known that
a suppressing crystallization formation and improvement in
morphological stability of the molecule can be achieved by
The convergent synthetic routes toward carbazole dendrons
(Gn-H, n = 1–4) and their triphenylamine dendrimers (GnCT,
n = 1–4) are described in the ESI.w The dendrons up to 4th
generation were synthesized in good yields using an iterative
Center for Organic Electronic and Alternative Energy, Department of
Chemistry and Center for Innovation in Chemistry, Faculty of
Science, Ubon Ratchathani University, Ubon Ratchathani, 34190,
Thailand. E-mail: pvinich@ubu.ac.th; Fax: +66 4528 8379;
Tel: +66 4535 3400 ext. 4510
w Electronic supplementary information (ESI) available: Synthetic
details and characterization, NMR spectra, optimized geometrical
structures, optical spectra, TGA/DSC plots, XRD patterns, AFM
images, CV plots, OLED device data. See DOI: 10.1039/c2cc16878b
Fig. 1 Molecular structures of GnCT.
c
3382 Chem. Commun., 2012, 48, 3382–3384
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Table 1 Physical data and device performance of the dendrimers
Solution
em/nm
Film
em/nm
HOMO/LUMOa (eV)
Lmax/cd mꢀ2
Z/cd Aꢀ1
CIE(x, y)
b
Von /V
Compd
Device
l
FF
l
Tg/1C
G1CT
G2CT
G3CT
G4CT
NPB
I
389
402
403
403
450
0.37
0.30
0.29
0.12
0.45
395
402
404
404
440
215
321
368
401
100
ꢀ5.21/ꢀ1.81
ꢀ5.36/ꢀ1.96
ꢀ5.38/ꢀ2.00
ꢀ5.38/ꢀ2.01
ꢀ5.40/ꢀ2.40
3.13
2.96
2.92
2.88
2.90
1835
24 165
25 390
25 397
20 373
3.75
4.20
4.47
4.32
3.22
0.29, 0.53
0.29, 0.53
0.29, 0.53
0.29, 0.53
0.29, 0.53
II
III
IV
V
Calculated from HOMO = ꢀ(4.44 + Eonset) and LUMO = HOMO + Eg. Turn-on voltage at luminance of 1 cd mꢀ2
.
a
b
cycle of two orthogonal reactions namely Ullmann coupling
and detosylation reactions. Ullmann coupling of each Gn-H
with tri(p-iodophenyl)amine 3 afforded GnCT as white solids
in reasonable good yields of 75–78%. The structures of all
dendrons and dendrimers were characterized unambiguously with
reported so far. The amorphous characteristics of GnCT were
further studied by powder X-ray diffraction (XRD) using
silicon wafer as a substrate (ESIw). For G1CT, a series of
sharp peaks were recorded at 2y of 171, 181 and 211 (the
corresponding d values are 5.2, 4.9 and 4.3 A) which were
attributed to the p–p stacking of carbazole segments. In higher
generation dendrimers, these peaks became broad amorphous
peaks at the same positions. The morphologies of the dendrimers
were also characterized by atomic force microscopy (AFM). The
films of all dendrimers spin-coated with chloroform : toluene
(1 : 1) solution showed quite smooth surface, indicating their
good film formation abilities (ESIw). For G1CT, a few pinholes
were observed. The presence of pinholes in the film might cause
large leakage of current.18 We believe that small molecular size
and crystallization ability of G1CT cause the pinhole formation
in the film.
1
FTIR, H-NMR, 13C-NMR spectroscopy and MALDI-TOF
mass spectrometry (ESIw). These dendrimers show good
solubility in most organic solvents. As a result, uniform and
stable amorphous thin films of these dendrimers can be obtained
from a solution spin coating process. The geometrical structures
of the dendrimers optimized using the HF/6-31G (d,p) method
revealed increasingly sterically hindered structures which are
pushed towards a sphere with the core at the center and the
outer face covered by the surface groups (ESIw). Such structural
characteristics can influence some of their electronic and
physical properties.
The optical properties of GnCT were investigated in both
solution and solid state (Table 1 and ESIw). The solution
and solid state absorption spectra of these dendrimers show
characteristic absorption features of both carbazole and
triphenylamine. The absorption edges slightly red-shifted from
G1CT to G4CT, suggesting that the generation number of
the dendrons has little effect on electronic properties of the
dendrimers. The energy band gaps (Eg) of the dendrimers
deduced from the absorption edge were nearly identical
(3.40–3.37 eV), despite the somewhat different molecular sizes.
The solution photoluminescence (PL) spectra of GnCT
showed an emission band in the blue purple region with a
featureless pattern. PL spectra of G2CT–G4CT were nearly
the same and red shifted (15 nm) as compared to the PL of G1CT.
Fluorescence quantum yields (FF) of GnCT determined in
CH2Cl2 gradually decreased from 0.37 to 0.12 as the generation
number of the dendrimer increased. In thin film, a slight red shift
(6 nm) in PL spectra of G1CT from its solution PL was observed,
while those of G2CT–G4CT were identical to their corresponding
solution PL, indicating weak intermolecular electronic inter-
actions in the less bulky G1CT dendrimer.
Cyclic and differential pulse voltammetry (CV and DPV)
studies on these materials revealed quasireversible oxidation
processes with no distinct reduction process being detected in
all samples (ESIw). Multiple CV scans displayed identical CV
curves with no additional peak at lower potential on the cathodic
scan (Epc) being observed, indicating electrochemically stable
molecules. The HOMO level of G1CT (5.21 eV) calculated from
the onset of the oxidation was slightly higher than those of
higher generation dendrimers (5.36–5.38 eV). Besides, the
LUMO energy levels of these materials were calculated to be
between 1.81 and 2.01 eV by subtracting the energy band gaps
from HOMO levels (Table 1).
To investigate HTM abilities of GnCT, green OLEDs were
fabricated using these dendrimers as hole-transporting layers
(HTL); indium tin oxide (ITO)/PEDOT:PSS/HTL(50 nm)/
Alq3(40 nm)/LiF(0.5 nm):Al(150 nm), with tris(8-hydroxy-
quinoline)aluminium (Alq3) as the green light-emitting (EML)
and electron-transporting layers (ETL) (Fig. 2a). The HTL was
spin-coated using chloroform : toluene (1 : 1) solution with
controlled thickness. Under applied voltage, all devices exhibited
a bright green emission with peaks centered at 518 nm and CIE
coordinates of (0.29, 0.53). The electroluminescence (EL) spectra
of these diodes were identical, and matched with the PL spectrum
of Alq3, the EL of the reference device (device V) and also other
reported EL spectra of Alq3-based devices (Fig. 2b).8–13,17 No
emission at the longer wavelength owing to exciplex species
occurred at the interface of HTL and ETL materials, which is
often observed in the devices fabricated from HTL with planar
molecular structure, was detected.19 In our case, the formation of
exciplex species could be prevented by the bulky nature of the
carbazole dendron. From these results and in view of the fact
that the barrier for electron migration at the Alq3/HTL
To gain insight into the thermal and morphological properties
of these dendrimers differential scanning calorimetry (DSC) was
carried out (ESIw). The results suggested that they were thermally
stable amorphous materials with glass transition temperature
(Tg) increasing from 215 1C (G1CT) to 401 1C (G4CT) as the
generation number of the dendron increased because of the rigid
structure of the carbazole dendron (Table 1 and ESIw). These
Tg values were substantially higher than those of the commonly
used HTMs (NPB, TPD), and many reported carbazole and
triphenylamine derivatives.8–12,17 To our knowledge, G4CT
is among one of the highest Tg AHTMs that have been
c
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Chem. Commun., 2012, 48, 3382–3384 3383

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In conclusion, we have presented a facile and efficient
synthesis of rigid carbazole dendrons up to 4th generation using
simple Ullmann coupling and detosylation reactions. Their
triphenylamine dendrimers showed chemically stable redox
and thermally stable amorphous properties with substantially
high glass transition temperatures (Tg) up to 401 1C. The
abilities of these dendrimers as solution-processed HTLs for
green OLEDs in terms of device performance and thermal
property were greater than those of a common hole-transporter
NPB. These dendrimers may also be promising materials for
long lifetime device applications, especially for high temperature
applications in OLEDs or other organic optoelectronic devices.
The use of these carbazole dendrons by forming dendritic
structures with other fluorescent or nonfluorescent core units
might be an effective way to prepare high Tg amorphous
materials for device applications.
This work was financially supported by the Thailand Research
Fund (MRU5080052). We acknowledge the scholarship support
from Center of Excellence for Innovation in Chemistry
(PERCHCIC), Ubon Ratchathani University, and the Strategic
Scholarships for Frontier Research Network for Research
Groups (CHE-RES-RG50) from the OHEC. Mr Preecha
Moonsin was supported by CHE Ph.D. Scholarship.
Fig. 2 (a) Structure, (b) EL spectra and (c) voltage–current–luminance
(J–V–L) characteristics of the OLEDs; device I (’), device II (K),
device III (m), device IV (.) and device V (%).
interface (B1.00 eV) is nearly five times higher than those for
hole migration at the HTL/Alq3 interface (B0.21 eV), under
the present device configuration GnCT would act only as
HTM, and Alq3 would act preferably as an electron blocker
more than as a hole blocker and charge recombination thus
confined to the Alq3 layer. More importantly, a stable emission
was obtained from all diodes with the EL spectra and CIE
coordinates did not change over the entire applied voltages
(ESIw). The light turn-on voltage at 1 cd mꢀ2 for all devices
was in the range of 2.88–3.13 V and the operating voltage at
100 cd mꢀ2 was in the range of 3.80–4.40 V, indicating that good
performance is achieved for all the devices (Fig. 2c and Table 1).
The device characteristics in terms of maximum brightness
(Lmax), turn on voltage and maximum luminous efficiency (Z)
clearly demonstrated that the hole-transporting abilities of
G2CT–G4CT were greater than NPB-based device (device
V). Device III having compound G3CT as HTL exhibited
the best performance with a high maximum brightness of
25 390 cd mꢀ2 for green OLED at 10.80 V, a low turn on
voltage of 2.92 V, a maximum luminous efficiency of 4.47 cd Aꢀ1
and a maximum external quantum efficiency of 0.21% (Fig. 2c
and ESIw). In order to explain the different efficiencies of the
OLED devices, analysis of band energy diagrams of all devices
was performed and it revealed that the injection barriers for
the gathered holes to transfer from the HTL to Alq3 are
0.59 eV (device I) and B0.42 eV (devices II–IV) (ESIw).
Accordingly, migration of a hole from the HTL to Alq3 layers
is more effective in devices II–IV compared to device I,
resulting in efficient charge recombination in the Alq3 emitting
layer and better device performance. It has been demonstrated
that the efficiency of an OLED depends both on the balance of
electrons and holes and the FF of the emitter.20 Although many
HTMs have been reported, in terms of the amorphous morphology,
significantly high Tg, solution processability and device efficiency,
these dendrimers are among good HTMs reported.
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3384 Chem. Commun., 2012, 48, 3382–3384
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