Synthesis of multifunctional monomers for patterning pixel define layer of organic light emitting diode

Article abstract of DOI:10.1080/15421406.2018.1470710

Organic light emitting diode (OLED) is widely used as displays for small devices like cellular phone, tablet and wearable devices as well as large devices like TV. Photolithography utilizing black negative-tone photoresist is the vital process for pattern

Full text of DOI:10.1080/15421406.2018.1470710

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of multifunctional monomers for
patterning pixel define layer of organic light
emitting diode
Sung Hoon Park, Genggongwo Shi, Duck Min Seo, Minji Kim, Jeseob Kim &
Lee Soon Park
To cite this article: Sung Hoon Park, Genggongwo Shi, Duck Min Seo, Minji Kim, Jeseob Kim
& Lee Soon Park (2018) Synthesis of multifunctional monomers for patterning pixel define layer
of organic light emitting diode, Molecular Crystals and Liquid Crystals, 663:1, 168-173, DOI:
10.1080/15421406.2018.1470710
To link to this article: https://doi.org/10.1080/15421406.2018.1470710
Published online: 04 Jun 2018.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Synthesis of multifunctional monomers for patterning pixel
define layer of organic light emitting diode
Sung Hoon Parka, Genggongwo Shia, Duck Min Seoa, Minji Kima, Jeseob Kimb,
and Lee Soon Parka
aSchool of Material Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan,
Korea; bCCTech Inc., Hwaseong, Korea
ABSTRACT
KEYWORDS
Multifunctional monomer;
negative tone photoresist;
organic light emitting diode;
photolithography; pixel
define layer
Organic light emitting diode (OLED) is widely used as displays for small
devices like cellular phone, tablet and wearable devices as well as large
devices like TV. Photolithography utilizing black negative-tone photore-
sist is the vital process for patterning the black pixel define layer (PDL).
In this work we report the syntheses of thermally-stable multifunctional
monomers and evaluation of their contribution to the patterning of
black PDL in OLED panel.
1. Introduction
It is well known that the visibility of organic light emitting diode (OLED) display is better
than that of thin film transistor liquid crystal display (TFT-LCD), since OLED is self-emitting
device while TFT-LCD has non-emitting mechanism [1, 2]. However, it is still necessary to
increase the visibility of OLEDs, especially when used outdoors. The main reason for reduced
outdoor visibility of the OLED is the reflection of ambient light from the OLED panel. So that
the ¼ λ polarizing film is inserted and black matrix (BM) patterns are fabricated on top of the
thin film encapsulation (TFE) layer of OLED panel [3∼5], as shown in Fig. 1.
The emission of separate red, green and blue light from the pixels of OLED becomes pos-
sible by fabrication of rectangular parallelepiped microstructures on the pixel define layer
(PDL) of OLED panel. This process is currently carried out by the photolithographic method
including coating of positive-tone photosensitive polyimide, UV exposure through the fine
photomask followed by development with aqueous basic solution. The use of positive-tone
photosensitive polyimide to make PDL pattern is not very effective in reducing the reflection
of ambient light, since the color of polyimide PDL patteren is yellow to brown [6, 7]. In order
to solve this problem a negative-tone photoresist containing black pigment was investigated
in this work.
The black photoresist is composed of photoinitiator, photosensitizer, binder polymer, black
pigment, dispersant and multifunctional monomer. The role of multifunctional monomer
in the black photoresist is to make crosslinked structure in the UV light irradiated area of
the spin coated black PDL. The multifunctional acrylate monomers such as pentaerythritol
CONTACT Lee Soon Park
Gyeonggi-do , Korea.
parkls@unist.ac.kr
CCTech Inc., , Sicheong-ro  beon-gil, Paltan-myeon, Hwaseong,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
169
Figure . Illustration of PDL in OLED panel.
tetraacrylate (PETA) has been used for this purpose. However the thermal stability of the
patterned black PDL is not high enough with the multifunctional monomer such as PETA.
The black photoresist needs high thermal stability since the OLED panel fabrication processes
after the patterning of black PDL require to withstand a temperature up to 300°C [8, 9]. We
synthesized new multifunctional monomers and tested them in the photolithographic process
for both good patterning and high thermal stability of the black PDL patterns in the OLED
panel.
2. Experimental methods
In the syntheses of multifunctional monomers, common reagents and solvents were pur-
chased and used without further purification. Synthesized compounds were characterized by
1H, 13C NMR (400 MHz FT-NMR Model: 400-MR DD2 Mag. Res. System at 298 K).
Synthesis of MFM-2. 9,9-Bis[4-(2-Glycidyloxyethyl)phenyl]fluorene (1, 1.3876 g,
3 mmol), acrylic acid (0.8647 g, 12 mmol), 2,6-di-tert-buthyl-4-methylphenol (BHT,
0.2644 g, 1.2 mmol) and tetrabutylphosphonium bromide (TBPB, 0.2036 g, 0.6 mmol) was
added to 10 mL of toluene and stirred at 90°C for 7 hours. After cooling down, the reaction
mixture was diluted with ethyl acetate and washed with saturated NaHCO3(aq). The organic
layer was dried with anhydrous Na2SO4 and finally evaporated to dryness to afford MFM-2.
1
Yield: 0.945 g, 52%. H NMR (CDCl3, 400 MHz): δ 7.82–7.84 (d, 2H), 7.32–7.44 (m, 8H),
7.20–7.16 (m, 4H), 6.83–6.91 (d, 4H), 6.48–6.52 (d, 2H), 6.18–6.25 (m, 2H), 5.89–5.94
(d, 2H), 4.21–4.56 (m, 8H).
O
O
OH
O
O
O
OH
O
O
O
acrylic acid, TBPB
PhMe, BHT
O
O
1
MFM-2
Synthesis of MFM-3. Acryloyl chloride (3, 2.4 mL, 30 mmol) was added to suspension of
gallic acid monohydrate (2, 0.940 g, 5 mmol) in 5 mL EtOAc. The mixture was then stirred

170
S. H. PARK ET AL.
at 60°C for 4 hours before pouring into excess water. The organic layer was washed with sat-
urated NaHCO3(aq), separated and dried with anhydrous Na2SO4, and finally evaporated to
dryness to afford MFM-3 as oil. Yield (not pure): 2.28 g, 137%. 1H NMR (CDCl3, 400 MHz):
δ 12.52 (s, 1H), 7.55 (s, 1H), 5.89–6.32 (m, 6H).
O
O
OH
HO
O
O
4
HO
O
O
EtOAc
O
OH
+
Cl
O
H2O OH
2
O
MFM-3
Synthesis of MFM-4-1. Acryloyl chloride (3, 8.0 mL, 100 mmol) was added into 20 mL
dimethyl acetamide (DMAc) solution of trisaminomethane (5, 1.21 g, 10 mmol) at RT. The
mixture was then stirred at 60°C for 4 hours before pouring into excess water. The oil was
extracted with dichloromethane, dried with anhydrous Na2SO4, and finally evaporated to dry-
ness to afford MFM-4-1 as oil. Yield (not pure): 5.514 g, 82%. 1H NMR (CDCl3, 400 MHz): δ
6.41–6.46 (d, 6 H), 6.12–6.23 (d, 2H), 6.22–6.27 (d, 6H), 5.88–5.90 (d, 2H), 5.65–5.68 (s, 6H),
4.62 (d, 12H).
O
O
O
OH
O
3
DMAc
N
H
+
O
OH
OH
Cl
O
H2N
O
O
5
MFM-4-1
Synthesis of MFM-4-2. Methacryloyl chloride (4, 6.0 mL, 60 mmol) was added into 10 mL
DMAc solution of trisaminomethane (5, 1.21 g, 10 mmol) within 10 min and the mixture
was stirred at RT for 18 hours. After pouring into excess water, the oil was extracted with
dichloromethane, dried with anhydrous Na2SO4, and finally evaporated to dryness to afford
MFM-4-2 as oil. Yield: 2.7 g, 69%. 1H NMR (CDCl3, 400 MHz): δ 6.52–6.62 (d, 1H), 6.12–6.23
(d, 3H), 5.62–5.70 (d, 3H), 5.30–5.39 (d, 1H), 4.61 (s, 6H), 1.93–1.94 (d, 12H).
O
O
4
OH
O
O
DMAc
+
OH
OH
Cl
H2N
N
H
O
O
O
O
5
MFM-4-2

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
171
Synthesis of MFM-4-3. Allyl bromide (6.914 mL, 80 mmol) in 15 mL of acetone was added
dropwise to suspension of gallic acid monohydrate (2, 1.881 g, 10 mmol) and K2CO3 (13.8 g,
100 mmol) in acetone within 30 min. After stirring overnight at reflux, the mixture was filtered
and the solid was washed with acetone. The mother liquid was evaporated to dryness and the
residue was purified by silica-gel column chromatography (eluent: EtOAc/hexane = 1/20) to
afford MFM-4-3. Yield: 721 mg, 22%. 1H NMR (CDCl3, 400 MHz): δ 7.31 (s, 2H), 6.03–6.12
(m, 4H), 5.17–5.46 (m, 8H), 4.79–4.82 (d, 2H), 4.61–4.66 (d, 6H).
O
OH
OH
O
HO
O
K2CO3
acetone
+
O
Br
O
O
H2O OH
2
6
MFM-4-3
3. Results and discussions
A typical formulation of a negative-tone black photoresist is shown in Table 1. In this pho-
toresist propylene glycol monomethyl ether acetate (PGMEA) was used as common solvent to
dissolve or mix all the components of the black photoresist. This was because the black mill-
base (LT-1, from SKC htm Korea, 23% solid in PGMEA) containing lactam black pigment
was prepared to a well dispersed state (average partical size 50 nm) in PGMEA. If any one
component in black photoresist is not completely soluble in PGMEA, it could make micro-
coagulums, which will lead to saw-toothed pattern of black PDL.
The photolithographic process was as following. The black photoresist was spin coated
on the wafer (Si/SiO2 200 nm) with a stepwise speed of 300 rpm/5 sec, 500 rpm/5 sec, and
5,000 rpm/50 sec. After soft bake at 120°C for 100 sec, the dry black photoresist thin film was
exposed to UV irradiation (120 mJ/cm2) through the photomask. After hard bake at 120°C
for 100 sec, the black PDL thin film was developed with AZ-300 MIF developer (tetramethy-
lammonium hydroxide 2 wt% in water, Bayer Co., Germany) for 120 sec followed by water
rinsing and drying with N2 gun.
The black PDL pattern in Fig. 2(a) shows that the black photoresist sample PT-1 containing
acrylate type multifunctional monomer (PETA) gave sharp pattern of black PDL. The black
Table . Formulation of black photoresist for PDL patterning.
Compnents/Photoresist
PT-
PT-
PT-
PT-
PT-
PT-
Photoinitiator
Irgacure 




















Irgacure TPO
Darocur ITX
SR- (SMS Co., Korea)
PETA
Photosensitizer
Binder Polymer ( wt% in PGMEA)
Multifunctional Monomers





MFM-
MFM-


MFM--
MFM--


MFM--
BYK-


Dispersant





Black Millbase ( wt% in PGMEA)
Solvent
LT- (SKC htm. Co., Korea)
PGMEA












Total
Thermal Stability
wt%



—

—



—

—
 wt% loss temp. (TGA)

172
S. H. PARK ET AL.
Figure . The optical microscope images of black PDL patterns obtained with (a) PT-, (b) PT-, (c) PT-
photoresists.
photoresist sample PT-3 also showed good black PDL pattern as shown in Fig. 2(b). How-
ever other photoresist samples failed to give acceptable PDL patterns. The analyses were as
following; PT-1 photoresist had MFM-2 monomer with two acrylates (f = 2) so that it could
not give enough photo-crosslinking under 120 mJ/cm2 UV exposure density. Similarly, PT-2
also had f = 3 multifunctional monomer (MFM-3). Although PT-3, 4, 5 samples had all four
acrylate groups (f = 4) in the multifunctional monomers, only PT-3 resulted in good black
PDL pattern. This could be explained by the structures of the multifunctional monomers.
The multifunctional monomers MFM-4-2 had methacrylates compared to acrylates in MFM-
4-1 thus causing steric hindrance in photo-crosslinking reaction due to the methyl group
(Fig. 2(c)). The MFM-4-3 monomer had allylic group which could cause autoinhibition reac-
tion, a well-known phenomenon in the free radical polymerization, in the photo-crosslinking
process.
4. Conclusion
In this work 5 multifunctional monomers (with f = 2, 3, 4) were used to make black photore-
sists and tested in photolithographic patterning of black PDL for OLED panel. Of the mul-
tifunctional monomers the one with four acrylate groups (MFM-4-1) gave sharp black PDL
pattern, which could be explained by effective photo-crosslinking reaction with the highest
double bond number density based on unit mass of multifunctional monomers.
It was also noted that the thermal stability of black photoresist PT-3 was higher than that
of PT-0 with PETA as multifunctional monomer. This seemed to be due to more effective
crosslinking reaction caused by the higher double bond number density of MFM-4-1 than
that of PETA monomer which has higher molecular weight than MFM-4-1.
Funding
This work was supported by the Technology Innovation Program (or Industrial Strategic Technology
Development Program(10063289, Development of High Temperature Negative tone Photosensitive
Black Resin and Fabrication Process for Pol-less AMLED Devices) funded by the Ministry of Trade,
Industry & Energy (MOTIE, Korea).
Competing interests
The authors declare that they have no competing interests.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
173
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