Low-Temperature Cross-Linkable Small Molecules for Fully Solution-Processed OLEDs

Article abstract of DOI:10.1002/chem.201803308

Cross-linkable hole-transporting, host, and electron-transporting materials with a new cross-linking group, uracil, are designed and synthesized. These compounds exhibited good solubility in common organic solvents and excellent solvent resistance after cross-linking at a low temperature of 120 °C. The OLED was fabricated by all-solution processing using cross-linkable synthetic compounds, except for the electrodes. This device exhibited a current efficiency of 39.2 cd A^?1 and a power efficiency of 15.3 lm W^?1.

Full text of DOI:10.1002/chem.201803308

A Journal of
Accepted Article
Title: Low-Temperature Cross-linkable Small Molecules for Fully
Solution-Processed OLEDs
Authors: Sungkoo Lee and Min Hye Seo
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Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201803308
Link to VoR: http://dx.doi.org/10.1002/chem.201803308
Supported by

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COMMUNICATION
Low-Temperature Cross-linkable Small Molecules for Fully
Solution-Processed OLEDs
Sungkoo Lee* and Min Hye Seo
Abstract: Cross-linkable hole-transporting, host, and electron-
transporting materials with a new cross-linking group, uracil, are
designed and synthesized. These compounds exhibited good
solubility in common organic solvents and excellent solvent resistance
after cross-linking at a low temperature of 120 °C. The OLED was
fabricated by all-solution processing using cross-linkable synthetic
compounds, except for the electrodes. This device exhibited a current
efficiency of 39.2 cd/A and a power efficiency of 15.3 lm/W.
that can be cross-linked by heating or light irradiation and that are
not soluble in organic solvents after cross-linking.[10] In the upper
layer coating, the cross-linked underlayer is not affected by the
solvent of the upper layer, and it makes the fabrication of
multilayer OLEDs possible. The cross-linkable groups chosen for
previous studies include thermally and/or photochemically
initiated moieties such as acrylates, benzocyclobutenes (BCBs),
trifluorovinyl ethers, oxetanes, norborneol, or styrenes.[12-17] The
cross-linking materials can be divided into two process types:
photochemical and thermal. The photochemical process needs
photochemical initiators and ultraviolet (UV) radiation for cross-
linking, which add the problems of quenching of excitons from
radical initiators and causing photodamage to the OLED materials
by UV light, respectively.
Organic light-emitting diodes (OLEDs) have attracted attention
due to their excellent performance characteristics, such as low
weight, low power consumption, wide viewing angle, and short
response time.[1-3] Generally, OLEDs for display and lighting
applications are fabricated by vacuum deposition or solution
processing. Solution processes are more desirable for large-size
or flexible displays because of the possibility of employing cost-
effective large-area manufacturing processes, such as inkjet, roll–
to–roll, and screen printing.[4-7] It is also applicable to various
substrates including plastic substrates.[8,9] To date, the best way
to improve the performance of OLEDs is to form the OLEDs into
a multilayer structure. The multilayer structure of OLEDs is
typically fabricated by continuously depositing materials using the
thermal evaporation process. In solution processing of OLEDs,
when the new layer is coated, the underlayer can be dissolved by
the solvent of the upper layer, which makes it difficult to fabricate
a multilayer structure. As a result, few reports have been made
on the fabrication of multilayer OLEDs using the solution-process,
and all currently commercialized OLEDs have been fabricated by
the thermal deposition process.[10,11]
In order to solve the problem of producing a successive layer in
solution processing of OLEDs, two methods have been used:
orthogonal solvents or cross-linkable materials. The first method
uses materials that can be coated with orthogonal solvents. That
is because the orthogonal solvent dissolves only the upper layer
and not the underlayer. The multilayer OLEDs can be fabricated
by using materials soluble in orthogonal solvents such as water,
alcohols because organic materials used in OLEDs are usually
soluble in organic solvents. Nevertheless, this method has the
disadvantage that only a limited number of materials are
compatible with this solubility criterion. The second approach to
fabricate multilayer OLEDs is the use of cross-linkable materials
On the other hand, thermally cross-linking materials can be a
better choice because they do not cause similar problems in
OLED operation. Thermally cross-linkable materials generally
have functional groups such as styrene, benzocyclobutene, and
trifluorovinyl ether.[18,19] Ma et al. reported an OLED that had a
maximum external quantum efficiency of more than 10% using a
BCB-based cross-linked copolymer as the hole-transport layer.
This cross-linking process was accomplished by two hours of
heating at 200 °C.[20] Tang et al. utilized styryl groups to modify
carbazole and 1,2,4-triazole moieties to create a cross-linkable
[19]
bipolar host material. The host material with styryl groups can
be thermally cross-linked between 172–190 °C. Most of the
thermally cross-linkable materials mentioned above need curing
at temperatures around 200 °C. Although cross-linking was
achieved, the thermal cross-linking process requires high
temperatures and/or several hours. However, since most plastic
materials that can be used as substrates for flexible OLEDs are
unstable at temperatures above approximately 150 °C, fabrication
processes at low temperatures are crucial. As a result, lowering
of the thermal cross-linking temperature is an important
requirement in the development of flexible, solution-processed
OLEDs, and a large amount of research is currently being
dedicated to the development of cross-linkable materials capable
of cross-linking at low temperatures; however, research on OLED
materials for low-temperature cross-linking has rarely been
reported.
Lee et al. recently reported that hole-transporting materials with a
bicarbazole core and alkoxystyrene-based cross-linking
[
21]
functionalities were thermally cross-linked at 120 °C. Solution-
processed multilayer OLEDs using physically cross-linking
[
a]
Dr. S. Lee, M. H. Seo
[22,23]
materials were first reported by C. C. Cheng and co-workers.
IT Convergence Materials Group
Korea Institute of Industrial Technology (KITECH)
Choongnam-do 31056, Korea
E-mail: sungkoo@kitech.re.kr
They demonstrated that uracil-functionalized poly(3-thiophene)
was able to form a physically cross-linked layer, resulting in high
thermal stability and excellent hole injection/transporting
capabilities in the solid state. Cheng et al. also synthesized an
adenine-based polymer and used it as a hole-transporting
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
material, thereby providing a stable hole-transporting layer to and BmPyPB are generally known as materials used for the hole-
OLED devices.[24]
Solution processing also has the advantage of being compatible transporting layer (ETL), respectively, in thermally evaporated
with a much wider range of materials, including small molecules OLEDs. As a result, u-TCTA was synthesized as the hole-
and polymers, as well as dendrimers. Polymers can show transporting material, u-CzTP as the host material, and u-
transporting layer (HTL), emitting layer (EML), and the electron-
relatively good morphological stability and resistance to molecular
diffusion compared to small molecules. The drawback of solution
processing for the polymer is that it is generally more difficult to
purify these materials, thereby causing critical problems in
achieving long-lived devices.[11,25] On the other hand, small
molecules tend to undergo morphological changes, such as
recrystallization after solvent removal, but they have the
advantage of achieving high efficiency and long-term stability due
to a well-defined structure with high purity. However, there have
been many reports on solution-processed OLEDs using polymers,
but not many reports on using small molecules.[6,26-28]
BmPyPB as the electron-transporting material.
The purpose of this paper is the development of OLED small
molecules for solution processing that are cross-linked at low
temperatures and are well soluble in organic solvents. For low-
temperature cross-linking, it is necessary to introduce a new
cross-linking group which is not a known high-temperature cross-
linking group. In this study, we have introduced uracil group as a
cross-linking group. There are many functional groups being
capable of hydrogen-bonding such as the pyrimidine, acrylate,
urethane, et al.[
29,30]
Uracil has the advantage of being capable of
hydrogen bonding between uracils and forming strong hydrogen
bonds. Uracil can also be used as a self-healing material using
hydrogen-bonding property.[31,32] Therefore, we selected the uracil
group as a hydrogen bonding functional group. The synthesized
compounds based on the uracil group are capable of being
solution processed and thermally cross-linked at temperatures as
low as 120 °C. As a result, we synthesized small molecules with
the uracil functional group, which has been used as a hole-
transporting, host, and electron-transporting material in OLED
devices. We chose triphenylamine as the core of the hole-
transporting material and carbazole and pyridine as that of the
host material and electron-transporting material, respectively. The
optical properties and photophysical properties of the synthesized
compounds were fully investigated. We also fabricated an OLED
device containing four solution-processed layers (hole-injection
layer, hole-transporting layer, emitting layer, and electron-
u-TCTA
transporting
layer)
and
then
characterized
the
electroluminescence properties of this device. The relationship
between molecular structure and the electroluminescence
properties of the OLED device was systematically investigated
herein.
u-CzTP
The synthesized chemical structures and hydrogen-bonded
structures are shown in Figure 1. A series of uracil-functionalized
side-chain compounds were synthesized based on small
molecules previously reported in the literature, which were used
as the hole-transporting layer, the emitting layer, and the electron-
transporting layer. All compounds were designed to have different
u-BmPyPB
Figure 1. Cross-linked u-TCTA, u-CzTP, and u-BmPyPB.
cores; the hole-transporting material, u-TCTA, utilized
a
triphenylamine core, whereas the host material, u-CzTP, had a
carbazole core, and the electron-transporting material, u-
BmPyPB, had a pyridine core. u-TCTA had the structure in which
the uracil group was bonded to the side chain of the TCTA, which
was a triphenylamine-based OLED material. u-CzTP and u-
BmPyPB had the structure in which the uracil group was bonded
to the side chain of the CzTP and BmPyPB, which were
carbazole- and pyridine-based OLED materials. TCTA, CzTP,
u-TCTA was synthesized at 32% yield by an amino-
dehalogenation reaction between 1-(3-((9H-carbazol-2-
yl)oxy)propyl)pyrimidine-2,4(1H,3H)dione
bromophenyl)amine. u-CzTP and u-BmPyPB were prepared at
2% and 47% by a Suzuki reaction and an amide reaction,
and
tris(4-
3
respectively. The chemical structures of the synthesized
1
compounds were confirmed by H NMR. The synthetic routes are
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COMMUNICATION
shown in Scheme S1 (Supporting Information). The synthesized
compounds showed better solubility than the thermal-evaporated
compounds with similar structures. For each of the synthesized
compounds, the uracil group was connected to the OLED core
structure, which endows good solubility in common solvents,
facilitating film fabrication using solution-processing techniques.
The solubility of synthesized compounds is summarized in Table
S1.
The thermal properties of the synthesized compounds were
examined by differential scanning calorimetry (DSC) at a heating
rate of 10 °C/min and the thermogravimetric analysis (TGA) in a
nitrogen atmosphere at a scanning rate of 10 K/min. DSC
thermograms of compounds are shown in Figure S4. In the
g
synthesized compounds, the glass transition temperature (T )
was not observed. The thermograms of u-TCTA and u-BmPyPB
had only the endothermic peak of 287 °C and 284 °C, respectively.
That is assigned to the melting temperature. u-CzTP had the
crystallization temperature of 153 °C and the melting temperature
of 190 °C, respectively. u-HTL, u-EML, and u-ETL exhibited high
thermal stability with decomposition temperatures (T )
d
(corresponding to 5 % weight loss) of 416 °C, 419 °C, and 547 °C,
respectively (Figure S5).
The photophysical properties of the three synthetic compounds
were investigated by room-temperature absorption and
photoluminescence measurements of solid films. The
synthesized compounds had short wavelengths of maximum light
absorption (λmax) due to limited conjugation in the molecular
structures. The absorption peaks of u-TCTA, u-CzTP, and u-
BmPyPB were observed at 354 nm, 300 nm, and 304 nm,
respectively. The synthesized compounds exhibited strong
absorption bands in the range of 240–350 nm that can be
attributed to the η–π* and π–π* transitions of the whole molecules.
The photoluminescence emission of u-CzTP was observed at 390
nm, which was overlapped by the absorption of Ir(mppy)
range of 350–430 nm; therefore, efficient energy transfer from u-
CzTP to Ir(mppy) can be expected. T1 energy levels of the
3
in the
100
3
synthetic compounds were determined from phosphorescence
spectra. T1 levels of u-TCTA, u-CzTP, and u-BmPyPB were
estimated to be 2.38 eV, 2.74 eV, and 2.67 eV, respectively. The
u-CzTP as a host material had a higher triplet energy than the
80
60
40
3
2.54 eV of Ir(mppy) . Therefore, the triplet exciton can be confined
u-TCTA
u-CzTP
u-BmPyPB
in the emitting layer, and the triplet exciton quenching of Ir(mppy)
at the HTL/EML and/or EML/ETL interfaces can be minimized.
3
80
100
120
o
Crosslinking Temperature ( C)
Table 1. Photophysical and electrochemical properties of u-TCTA, u-CzTP,
and u-BmPyPB.
Figure 2. Solvent resistance results of cross-linked u-TCTA, u-CzTP, and u-
BmPyPB.
Compound
λ
max (abs)
T
[eV]
1
HOMO
[eV]
E
[eV]
g
LUMO
[eV]
[
nm]
u-TCTA
u-CzTP
354
300
304
2.38
2.74
2.67
5.47
5.62
6.74
3.40
3.54
3.80
2.07
2.08
2.94
The solvent resistance of the thermally cross-linked films was
investigated using UV/Vis spectroscopy. Solvent resistance test
samples were prepared by dissolving 30 wt% of the synthesized
compounds in chlorobenzene and then spin coating the solutions
onto a glass substrate. The films were cross-linked at 120 °C for
u-BmPyPB
3
0 min under a nitrogen atmosphere. After measuring the UV/Vis
Electrochemical properties of the synthetic compounds were
examined using cyclic voltammetry, with ferrocene as an internal
standard and the cyclic voltammograms of them are shown in
Figure S3. The synthesized compounds showed a reversible
oxidation behavior, indicating that they can be used in OLEDs.
The HOMO energy levels of u-TCTA, u-CzTP, and u-BmPyPB
were estimated from an oxidation onset potential to be -5.47 eV,
absorption of the cross-linked films, the films were dipped in
chlorobenzene for 5 min. After removing the solvent, the UV/Vis
spectra of the washed films were measured again. The
comparison of the UV/Vis absorbance before and after dipping
provided us with the solvent resistance created by cross-linking
the films. The solvent resistance results for the cross-linked films
are shown in Figure 2. Figure 2 shows that all of the compounds
cross-linked at 120 °C had almost 100% solvent resistance. It is
expected that the difference in the solvent resistance of cross-
linked materials at low temperatures is due to the difference in
solubility of each material. The absorption spectra of the cross-
linked compounds are almost the same before and after the
dipping process, indicating that the cross-linked compounds with
uracil have complete solvent resistance. This also indicated that
the hydrogen bonding between the uracil groups gave effective
solvent resistance, such that the OLED materials were not soluble
in the organic solvents. The detailed absorption spectra of the
synthetic compounds with different cross-linking temperatures
-
5.62 eV, and -6.74 eV, respectively. The LUMO energy levels of
the u-HTL, u-EML, and u-ETL were calculated to be -2.07 eV, -
.08 eV, and -2.94 eV, respectively, by adding the optical band
2
gap to the HOMO energy level. The properties of the synthesized
compounds are summarized in Table 1. The HOMO/LUMO
energy of TCTA, CzTP, and BmPyPB were reported to be 5.7
eV/2.4 eV, 5.9 eV/2.5 eV, and 6.6 eV/2.6 eV, respectively. The
HOMO/LUMO energies of the synthesized compounds with uracil
groups are similar to those of the compounds with the same core
structure. This indicated that the uracil group did not affect the
photophysical and electrochemical properties of the core
structure.
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before and after dipping in the solvent are shown in Figure S6. As
shown in Figure 1, the uracil groups of the synthesized
compounds are hydrogen-bonded to each other by heating, so
that the coated synthetic compounds form thin films having chain
structures or network structures. As a result, the thin films are not
dis-solved in organic solvents due to the solvent resistance
provided by these structures. The cross-linking of these com-
pounds can be analyzed using Fourier transform infrared (FT–IR)
spectroscopy as shown in Figure S7. In the FT–IR spectra, non-
cross-linked films showed free NH stretching vibration peaks
molecules was fabricated with the following structure:
ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) (30 nm)/u-TCTA (20 nm)/u-CzTP + Ir(mppy)
3
(8%) (40 nm)/u-BmPyPB (30 nm)/LiF (1 nm)/Al. In device, all
of the organic layers were solution processed, and the LiF and
Al were vacuum deposited to form
a cathode. The
PEDOT:PSS layer, acting as a hole-injection layer, was spin
coated onto the ITO glass. The emitting layer was composed
of 92 wt% u-CzTP as a host material and 8 wt% of the green
3
phosphorescent dopant, Ir(mppy) . u-TCTA was used as the
-1
around 3,400~3,470 cm ; however, after cross-linking the
intensities of the free NH vibration peaks at about 3,400~3,470
hole-transporting material. u-BmPyPB was used as the
exciton-blocking and electron-transporting material. LiF and Al
were thermally deposited on the top of the u-BmPyPB layer,
successively. For the solution-process of all the organic layers
in OLEDs, the uppermost layer, u-BmPyPB in this case, does
not need to be cross-linked, only solution-process is sufficient.
However, we used cross-linked u-BmPyPB in order to show
that the uracil group could be applied to all layers of OLEDs.
The current density–voltage, luminance–voltage, current
efficiency–luminance, and power efficiency–luminance
characteristics are shown in Figure 3. As shown in the inset of
Figure 3(b), the EL spectrum of the device shows the emission
-1
cm were decreased and the intensities of the NH stretching
peaks at about 3180 cm-1 were increased, indicating that the
interaction occurred through hydrogen bonding between the uracil
groups.
r
40
30
20
10
0
04
1
1
1
1
peak of Ir(mppy)
emission at around 390 nm. This indicates that efficient energy
transfer occurred from u-CzTP to the dopant Ir(mppy) , and
3
at 510 nm, without a trace of the host
03
3
02
that the two transporting materials, u-TCTA and u-BmPyPB,
well-confined the triplet exciton within the emitting layer. The
relatively low turn-on voltages of 4.6 V was observed for
01
2
device. At a given constant luminance of 5,000 cd/m , the
current and power efficiencies were 35.5 cd/A and 10.1 lm/W,
respectively. The maximum current and power efficiencies
were 39.2 cd/A and 15.3 lm/W, respectively. The solution-
processed device exhibited a lower driving voltage, indicating
that the charge injection and transport were relatively efficient.
This means that the HOMO energy level of u-TCTA matched
the energy level of PEDOT:PSS. The relatively low driving
voltage of the solution-processed device might be attributed to
the relatively low energy barrier between PEDOT/PSS and u-
TCTA as shown in Figure S8. The device fabricated using the
cross-linkable small molecules exhibited comparable device
performance to the device fabricated by the thermal
evaporator. This indicates that the new cross-linkable
materials can serve as good alternatives to vacuum-deposition
materials.
Since the device configuration has not been optimized and
manufacturing environment is not good in the present work,
further performance improvement may be expected by fine-
tuning of device structure and improvement of the
manufacturing environment. Future research will more
examine the lifetime characteristics of devices with the fine-
tuning of the device.
100
0
4
8
12
16
Voltage (V)
50
40
30
20
10
0
20
1
1
5
0
5
0
0.03
0.02
0.01
0.00
400
500
600
A
700
800
0
2000
4000
6000
8000
10000
Luminance (cd/m2)
Figure 3. (a) Current density and luminance as a function of voltage and (b)
current efficiency and power efficiency as a function of luminance for the
solution processed PhOLED.
In summary, low molecular mass OLED materials with uracil
that are capable of cross-linking have been successfully
synthesized. The uracil group improved the solubility of the
small-molecule compounds and made them usable for the
solution process. The thermal cross-linking of OLED materials
with the uracil group occurred at approximately 120 °C without
the addition of an initiator and the production of byproducts. In
the film state, the hydrogen bonding between uracil groups
results in the enhancement of solvent resistance, and as a
result, it has become possible to manufacture multilayer thin
To further investigate the actual device aspects of the
synthesized compounds, we fabricated OLEDs by solution
processing of all the layers, except for the electrode, using the
synthesized compounds. A fully solution-processed multi-layer
phosphorescent OLED (PhOLED) using cross-linkable small
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Chemistry - A European Journal
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COMMUNICATION
films using a solution process. The OLED was fabricated by
all-solution processing using cross-linkable synthetic
compounds, except for the electrodes. This device exhibited a
current efficiency of 39.2 cd/A and a power efficiency of 15.3
lm/W. These results demonstrate that OLED materials with the
uracil group have the potential to be used for solution
processing of OLEDs that require multilayer thin films.
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This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201803308
COMMUNICATION
COMMUNICATION
S. Lee*, M. H. Seo
Low Temperature Cross-linkable
small molecules for Fully Solution-
Processed Multilayer OLEDs
Thermal-crosslinking
Text for Table of Contents
OLED materials with uracil that are capable of cross-linking have been successfully synthesized. The hydrogen bonding between the
uracil groups gave effective solvent resistance, such that the OLED materials were not soluble in the organic solvents. The OLED was
fabricated by all-solution processing using cross-linkable synthetic compounds, except for the electrodes. The device fabricated using
the cross-linkable small molecules exhibited comparable device performance to the device fabricated by the thermal evaporator. This
indicates that the new cross-linkable materials can serve as good alternatives to vacuum-deposition materials.
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201803308
COMMUNICATION
Text for Table of Contents
Figure 1. Cross-linked u-TCTA, u-CzTP, and u-BmPyPB.
Figure 1 showed hydrogen-bonded structures of the synthesized materials. This figure shows the hydrogen-binding between the uracil
groups that occurs between the oxygen of the carboxyl group and amine hydrogen.
Figure 2. Solvent resistance results of cross-linked u-TCTA, u-CzTP, and u-BmPyPB.
Cross-linking studies of the synthesized materials were performed by varying the curing temperatures. Figure 2 provided the solvent
resistance created by the cross-linked films with the comparison of the UV/Vis absorbance before and after washing at varying the
curing temperature.
Figure 3. (a) Current density and luminance as a function of voltage and (b) current efficiency and power efficiency as a function of
luminance for the solution processed PhOLED
Figure 3 shows (a) Current density and luminance curves and (b) current efficiency and power-efficiency curves of solu-tion-processed
OLED and vacuum deposited OLED.
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