Synthesis of new conjugated polymers as hole injection layer and performance of OLED devices

Article abstract of DOI:10.1080/15421406.2011.600145

The fluorene-based poly(dioctylfluorene-alt-biphenylamine)s with styrylpyridyl group were synthesized by using Pd-catalyzed polycondensation reaction. These hole injection/ transport polymers showed very good solvent resistance after photo-crosslinking which could facilitate the subsequent spin coating of the emitting layer polymer solution. Moreover these polymers could be patterned by using distyrylpyridyl alkyl monomer (DSM) as crosslinking agents. The OLED devices with configuration of ITO/HIL/Alq3:NPD/LiF/Al in which synthesized polymer was used as hole injection layer (HIL) were fabricated and their performance was compared with the commercially available PEDOT:PSS as HIL layer.

Full text of DOI:10.1080/15421406.2011.600145

This article was downloaded by: [University of Connecticut]
On: 16 June 2014, At: 04:29
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis of New Conjugated Polymers as
Hole Injection Layer and Performance of
OLED Devices
Tae Wook Yoo a , Chul Park b , Nguyen Thi Mai b , Dong Uk Kim c &
Lee Soon Park a c d
a Department of Sensor & Display Engineering , KyungPook National
University , Daegu , 702-701 , Korea
b Department of Polymer Science , Kyungpook National University ,
Daegu , 701-702 , Korea
c Department of Science Education , Daegu National University of
Education , Daegu , 705-715 , Korea
d Mobile Display Research Center , Kyungpook National University ,
Daegu , 701-702 , Korea
Published online: 18 Oct 2011.
To cite this article: Tae Wook Yoo , Chul Park , Nguyen Thi Mai , Dong Uk Kim & Lee Soon Park (2011)
Synthesis of New Conjugated Polymers as Hole Injection Layer and Performance of OLED Devices,
Molecular Crystals and Liquid Crystals, 551:1, 69-77, DOI: 10.1080/15421406.2011.600145
To link to this article: http://dx.doi.org/10.1080/15421406.2011.600145
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Mol. Cryst. Liq. Cryst., Vol. 551: pp. 69–77, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.600145
Synthesis of New Conjugated Polymers as Hole
Injection Layer and Performance of OLED Devices
TAE WOOK YOO,1 CHUL PARK,2 NGUYEN THI MAI,2
DONG UK KIM,3 AND LEE SOON PARK1,3,4,∗
1Department of Sensor & Display Engineering, KyungPook National University,
Daegu 702-701, Korea
2Department of Polymer Science, Kyungpook National University,
Daegu 701-702, Korea
3Department of Science Education, Daegu National University of Education,
Daegu 705-715, Korea
4Mobile Display Research Center, Kyungpook National University,
Daegu 701-702, Korea
The fluorene-based poly(dioctylfluorene-alt-biphenylamine)s with styrylpyridyl group
were synthesized by using Pd-catalyzed polycondensation reaction. These hole injec-
tion/transport polymers showed very good solvent resistance after photo-crosslinking
which could facilitate the subsequent spin coating of the emitting layer polymer so-
lution. Moreover these polymers could be patterned by using distyrylpyridyl alkyl
monomer (DSM) as crosslinking agents. The OLED devices with configuration of
ITO/HIL/Alq3:NPD/LiF/Al in which synthesized polymer was used as hole injection
layer (HIL) were fabricated and their performance was compared with the commer-
cially available PEDOT:PSS as HIL layer.
Keywords: Organic Light Emtting Diode; OLED; Synthesis; Conjugated polymers
Introduction
Conjugated polymers have drawn much attention due to their potential applications to
polymer light-emitting devices (PLED) [1,2]. To date many types of conjugated polymers
e.g poly(acetylene)s, poly(thiophene)s, poly(fluorene)s, and poly(phenylene)s have been
synthesized [3–6]. Among these polymers, poly(fluorene)s are well-known in the polymer
OLEDs due to their easy processability, long term stability and high fluorescence quantum
efficiency [7]. The common method to get thin film using polymer is solution casting such
as spin coating and inkjet printing. However this method is difficult to apply in OLED
because the first layer formed by solution coating can be partly dissolved by the solvent
used in the following spin coating process. Thus a challenge of the solution process OLED
is to control the solubility of OLED materials. One of the approaches is to develop novel
polymers containing crosslinkable moieties which not only become insoluble in common
organic solvents but also can simultaneously be patterned like a negative photoresist using
standard photolithographic techniques. In this work, we synthesized a series of crosslinkable
∗Corresponding author. E-mail: lspark@knu.ac.kr
[389]/69

70/[390]
T. W. Yoo et al
conjugated copolymers based on alternating fluorene and phenylamine units by using
Buchwald-Hartwig reaction in order to investigate their potential as hole injection (HIL)
or transporting (HTL) materials in the OLED devices. The synthesized polymers could be
patterned by using distyrylpyridyl alkyl monomer (DSM) as crosslinking agents.
Experimental
(1) Synthesis of Conjugated Polymers
The synthetic scheme of intermediate (2-BSP) and monomer (2-APSP) for the conjugated
copolymers are shown in Scheme 1 [8–10].
N
H3C
Br
CHO
+
2-picoline
4-bromobenzaldehyde
Acetic anhydride
Reflux/120oC/24hr
N
H
H
Br
C
C
2-BSP
CH3
CH3
O
O
3 mol% Pd(PPh3)4
Toluene/aq.K2CO3/100oC
H2N
B
CH3
CH3
N
H
H2N
C
C
H
2-APSP
Scheme 1. Synthesis of monomer.
The synthesis of crosslinkable HIL copolymer was carried out by Buchwald-Hartwig
reaction as shown in Scheme 2.
First, 9,9-dioctyl-2,7-dibromofluorene (DODBF) (1.82 mmol), 4-aminobiphenyl (1.64
mmol), 2-(4-aminophenyl-styryl)pyridine (2-APSP) (0.18 mmol), sodium-tert-butoxide
(NaO-Bu) (5.47 mmol), and tris(dibenzylideneacetone) dipalladium (Pd2(dba)3) (0.05
mmol) were dissolved in anhydrous toluene (50 ml) at room temperature. The catalyst
tris-tert-butylphosphine (P(t-Bu)3) (0.27 mmol) was added to the reaction mixture by using
syringe. The polymerization was carried out at 100∞C for 72h to get yellow solution.
After cooling to room temperature, the product solution was extracted with ammonium
water and dichloromethane, washed with water and dried with MgSO4. The residue was
concentrated by rotary evaporator and the organic fraction was precipitated in methanol to
give the yellow product in 92% yield [11].

Synthesis of New Conjugated Polymers as Hole Injection Layer
[391]/71
C8H17 C8H17
C8H17 C8H17
NH2
NaO-Bu
Pd2(dba)3 *
P(t-Bu)3
NH2
N
N
*
+
+
Br
n
Br
Toluene
100oC
(DODBF)
(n mol)
(x mol)
CH
HC
HC
HC
N
(B7-3F-x)
N
(Where: x + y = n)
(2-APSP) (y mol)
Scheme 2. Synthesis of HIL polymer.
(2) Synthesis of Crosslinking Agents
Crosslinking agents were synthesized as shown in Scheme 3 with the example of typical
crosslinking agent (DSM-6).
O
O
N
N
0.2 mole Acetic acid
Reflux, 150oC, 24h
HO
CHO +
CH3
+
H3C
O
CH3
H3COO C
C
H
C
H
Acetic anhydride
2.0 mole
4-Hydroxybenzaldehye 2-Picoline
2-ASP
1.0 mole
1.2 mole
KOH/EtOH
90oC/12hr
N
N
H3C
C
O
O
C
H
C
H
HO
C
H
C
H
2-HSP
2-ASP
K2CO3
KI/DMF
N
+
Br-(CH2)n-Br
C
H
C
H
O
(CH2)n
DSM
O
C
H
C
2-HSP
2mole
H
90oC/48hr
N
Dibromoalkane
1mole
n = 4,6,7, 8, 9
Scheme 3. Synthesis of crosslinking agent.
Potassium carbonate (2.312 g, 15.20 mmol) and potassium iodide (0.253 g, 1.5 mmol)
were added to a solution of 2-HSP (3.00 g, 15 mmol) in DMF (40 ml) with magnetic stir
bar. 1,6-Dibromohexane was then added to the mixture. The reaction mixture was heated
at 90◦C for 48 hours. After cooling to room temperature, the reaction mixture was washed
with 20% Na2CO3 and with distilled water (50 mL × 2). The separated organic layer was
dried over anhydrous MgSO4, filtered, and concentrated by a rotary evaporator. The solid
was dissolved in CH2Cl2 (15 mL) and precipitated in n-hexane to get solid product (70%
yield) [12–16].

72/[392]
T. W. Yoo et al
N
C
H
C
H
O
R
O
C
H
C
H
N
∗
N
C
H
C
H
N
UV
∗
∗
∗
∗
C
N
C
N
∗
N
R
O
O
C
H
CH
H
C
HC
N
∗
N
N
C
C
R: (CH2)6 ; (CH2)8; (CH2)9
∗
∗
∗
Scheme 4. Crosslinking reation of fluorene-base conjugated polymer.
(3) HIL Patterning with Crosslinking Agent DSM
A photoresist system (DSMn-B7-3F) containing the synthesized copolymers B7-3F-3 and
DSM crosslinking agent was used to fabricate crosslinked HIL layer. The crosslinking
reaction between copolymer B7-3F-3 and DSM are shown in Scheme 4.
Table 1. Characterization of fluorine-based conjugated polymers
Monomer
(mole%)
Conjugated
Polymer
Yield
(%)
x
y
Mw
PDI
B7-3F-1
B7-3F-3
B7-3F-7
B7-3F-9
90
70
30
10
10
30
70
90
29,000
27,000
20,000
19,000
2.38
1.71
1.92
2.07
92
94
87
84
x : biphenyl amine.
y : 2-APSP comonomer.

Synthesis of New Conjugated Polymers as Hole Injection Layer
Result and discussion
Characterization of Copolymers
[393]/73
The yields of copolymers by the Buchwald-Hartwig reaction were relatively high in
the range of 84 to 94% and the molecular weights of copolymers were in the range of
29,000–19,000 g/mol as shown in Table 1.
392
In solution
B7-3F-3
DSM6-3F-3
DSM8-3F-3
DSM9-3F-3
300
400
500
600
700
Wave length (nm)
402
In film
B7-3F-3
DSM6-3F-3
DSM8-3F-3
DSM9-3F-3
400
500
600
700
Wave length (nm)
Figure 1. UV-Vis absorbance spectra of DSMn-B7-3F-3.

74/[394]
T. W. Yoo et al
Table 2. Structure of OLED devices
OLEDs
HIL
EML
EIL
Cathode
No. 1
No. 2
No. 3
No. 4
No. 5
PEDOT:PSS
B7-3F-3
DSM6-B7-3F-3
DSM8-B7-3F-3
DSM9-B7-3F-3
Alq3
Alq3
Alq3:NPB (3:1)
Alq3
LiF
LiF
LiF
LiF
LiF
Al
Al
Al
Al
Al
Alq3:NPB (3:1)
Photoreactivities of Copolymers
The photoreactivities of crosslinkable conjugated polymers as well as photoresist system
DSMn-B7-3F were analyzed by UV−visible and PL spectrophotometer. In the presence of
DSM as crosslinking agent, the photoresist system DSMn-B7-3F can form a crosslinked
net work as shown in Scheme 4.
The UV absorbance spectra of conjugated polymer (B7-3F-3) and photoresist system
(DSMn-3F-3 series) showed maximum UV absorbance at 392 and 402 nm in solution and
solid film, respectively, as shown in Figure 1. The shape decrease of the UV maximum
peaks indicated that the (2π+2π) photo-cycloaddition had occurred. The PL spectra of
the photoresist system exhibited maxima at 434 and 542 nm in solution and solid film,
respectively as shown in Figure 2. It was noted that the PL intensity increased with the
spacer number (n in DSMn) in the photoresist system.
OLED Performance
A series of OLED devices with different structures were made to investigate HIL properties
of the synthesized fluorine-based conjugated polymers as shown in Table 2. In these de-
vices, the HIL polymer (B7-3F-3) andphotoresist system (DSMn-B7-3F) were used as hole
injection layers with ∼60 nm thickness. Tris(8-hydroxyquinoline)aluminum and bis[N-(1-
naphthyl)-N-phenyl]benzidine (Alq3:NPB) (wt/wt 3:1) were used as emitting layer.
As shown in Table 3 the OLED devices indicated that the B7-3F-3 uncrosslinked
copolymer (Device No. 2) exhibited higher luminance than commercially available HIL
Table 3. Performance of OLED devices with new HIL layer
Sample
Property
Step
No. 1
No. 2
No. 3
No. 4
No. 5
Spin coating speed (rpm)
1st (30s)
2nd (10s)
3000
4500
66
1581
5
0.17
0.29
0.52
3000
4000
34
1852
5
0.18
0.29
0.52
4500
7000
45
2241
4
0.008
0.29
0.52
4500
7000
72
484
5
0.002
0.29
0.52
4500
7000
68
1280
4
0.005
0.29
0.52
Thickness (nm)
Luminance (cd/m2)
Drive voltage (V)
Current efficiency (cd/A)
Color coordinate
x
y

Synthesis of New Conjugated Polymers as Hole Injection Layer
[395]/75
In solution
B7-3F-3
434
DSM6-3F-3
DSM8-3F-3
DSM9-3F-3
429
400
500
600
Wave length (nm)
430
In film
519
542
B7-3F-3
DSM6-3F-3
DSM8-3F-3
DSM9-3F-3
500
600
Wave length (nm)
Figure 2. Photoluminescence spectra of DSMn-B7-3F-3.
polymer PEDOT:PSS(Device No.1) with the same configuration. The OLED No.4 with
crosslinked copolymer DSM8-B7-3F-3 as HIL layer exhibited lower luminance than com-
mercially available
PEDOT:PSS (Device No.1) with the same device configuration. It was due to the high
surface roughness of DSM8-3F-3 HIL layer as examined by SEM images. When small
amount of NPB was added as dopant in the Alq3 EML layer, the OLED (No.3) with
DSM6-3F-3 as HIL exhibited higher luminance than the B7-3F-3 copolymer as well as
PEDOT:PSS as the HIL layers.

76/[396]
T. W. Yoo et al
35000
B7-3F-3
DSM6-3F-3
DSM8-3F-3
DSM9-3F-3
30000
25000
20000
15000
10000
5000
0
4
6
8
10
12
14
Voltage (V)
Figure 3. Voltage versus current density profile of OLED devices.
Figure 3 shows the current density versus voltage of the OLED devices. It was noted
that the OLED devices with crosslinked DSMn-B7-3F-3 series as HIL layer could withstand
higher applied voltage than the OLED device with uncrosslinked B7-3F-3 as HIL layer.
Acknowledgment
This work was supported by the Regional Innovation Center Program (ADMRC) of the
Ministry of Knowledge Economy.
Conclusion
In conclusion we successfully synthesized DSM crosslinking agents and new fluorene-
based polymers (B7-3F-X) with photo-crosslinkable styrylpyridyl group. The performance
of OLED devices fabricated with crosslinked hole-transport layer (DSM6-B7-3F-3) was
better than the one made by commercially available HIL polymer PEDOT:PSS. It was
also that the OLED devices with crosslinked DSMn-B7-3F-3 series as HIL layer could
withstand higher applied voltage than the OLED device with uncrosslinked B7-3F-3 as
HIL layer.
References
[1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L.
Burns, A.B. Holmes, Nature, 347, 539 (1990).
[2] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, A.
Stocking, Science, 273, 884 (1996).
[3] M. Kertesz, C.H. Choi, S. Yang, Chem. Rev., 105, 3448 (2005).
[4] J.–L. Bredas, D. Beljonne, V. Coropceanu, J. Cornil, Chem. Rev., 104, 4971 (2004).

Synthesis of New Conjugated Polymers as Hole Injection Layer
[397]/77
[5] D. T. McQuade, A.E. Pullen, T.M. Swager, Chem. Rev., 100, 2537 (2000).
[6] U. H. F. Bunz, Chem. Rev., 100, 1605 (2000).
[7] J. W. Shin, D. S. Choi, D. M. Shin. Mol. Cryst. And Liq. Cryst. 370, 17 (2001).
[8] T.-Q. Nguyen, I. B. Martini, J. Liu, and B. J. Schwartz. J. Phys. Chem. B, 104 (2), 237 (2000).
[9] C. B. Aakeroy, N. Scgultheiss, J. Desper and C. Moore. Crys Eng Comm, 9, 421 (2007).
[10] H.O. Nam, M. S. Jeong, O. J. Sohn, J. I. Rhee, J. H. Oh, Y. J. Kim and S.W. Lee. Inorganic
Chemistry Communications, 10, 195 (2007).
[11] B. J. Jung, J. I. Lee, H. Y. Chu, L.M. Do and H. K. Shim. Macromolecules 35, 2282 (2002).
[12] Lester Horwitz. J. Org. Chem., 21 (9), 1039 (1956).
[13] D. M. Huck, H. Loc. Nguyen, S. J. Coles, M. B. Hursthouse, B. Donnio and D. W. Bruce.
J. Mater. Chem., 12, 2879 (2002).
[14] (a) M. H. Hubacher, and S. Doernberg, J. Pharm. Sci., 53, 1067 (1964) (b) G. Chen, W. Shan,
Y. Wu, L. Ren, J. Dong and Z. Ji, Chem. Pharm. Bull., 53, 1587 (2005).
[15] Y. J. He, J. H. Hou, Z. A. Tan and Y. F. Li, J. App. Polym. Sci, 115, 532 (2010).
[16] C. L. Toh, J. W. Xu, X. H. Lu, C. B. He, J. Polym. Sci: Part A: Polymer Chemistry, 43, 4731
(2005).