Electroluminescent properties of heterocyclic conjugated random copolymers containing phenothiazine and carbazole moieties

Article abstract of DOI:10.1080/15421400600922232

Electroluminescent properties of polymer light-emitting diode (PLED)s fabricated from heterocyclic conjugated random copolymers, poly(PZV-ran-CZV), containing (N-2-ethylhexyl)-3,6-phenothiazinylene vinylene (PZV), (N-2-ethylhexyl)-3,6-carbazolylene vinylene (CZV) and phenylene vinylene (PV) unit as a light emitting layer were investigated. PLEDs made with corresponding alternating copolymers, poly(PZV-PV) and poly(CZV-PV), as a light emitting layer were also studied and exhibited EL maximum at 570 nm and 500 nm, respectively. Single-layered PLEDs made with poly(PZV-ran-CZV) copolymers, however, showed EL maxima shifted to longer wavelength. Double-layered PLED made with poly(PZV-ran-CZV) copolymer as a light emitting layer and Alq₃ as an electron transporting layer exhibited enhanced EL emission compared to that of single-layered PLED.

Full text of DOI:10.1080/15421400600922232

Mol. Cryst. Liq. Cryst., Vol. 459, pp. 129=[409]–140=[420], 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600922232
Electroluminescent Properties of Heterocyclic Conjugated
Random Copolymers Containing Phenothiazine
and Carbazole Moieties
Yoon Soo Han
Hoyoung Kim
Daegu Gyeongbuk Institute of Science & Technology, Daegu, Korea
Hyeon Jin Seo
LG Philips LCD, Gumi-city, Gyeongbuk, Korea
Younghwan Kwon
Research Institute of Nano Technologies, Department of Chemical
Engineering, Daegu University, Gyeongsan, Gyeongbuk, Korea
Youngjune Hur
Advanced Display Manufacturing Research Center,
Kyungpook National University, Daegu, Korea
Dong-Uk Kim
Department of Science Education, Daegu National University
of Education, Daegu, Korea
Jung Moo Kim
Hyun Suk Kim
Lee Soon Park
Department of Polymer Science, Kyungpook National University,
Daegu, Korea
Electroluminescent properties of polymer light-emitting diode (PLED)s fabricated
from heterocyclic conjugated random copolymers, poly(PZV-ran-CZV), containing
(N-2-ethylhexyl)-3,6-phenothiazinylene vinylene (PZV), (N-2-ethylhexyl)-3,6-carb-
azolylene vinylene (CZV) and phenylene vinylene (PV) unit as a light emitting
layer were investigated. PLEDs made with corresponding alternating copolymers,
poly(PZV-PV) and poly(CZV-PV), as a light emitting layer were also studied and
This work was supported by the Regional Innovation Center Program of the Ministry
of Commerce, Industry and Energy of Korea.
Address correspondence to Lee Soon Park, Department of Polymer Science,
Kyungpook National University, Daegu 702-701, Korea. E-mail: lspark@knu.ac.kr
129=[409]

130=[410]
Y. S. Han et al.
exhibited EL maximum at 570 nm and 500 nm, respectively. Single-layered PLEDs
made with poly(PZV-ran-CZV) copolymers, however, showed EL maxima shifted
to longer wavelength. Double-layered PLED made with poly(PZV-ran-CZV) copo-
lymer as a light emitting layer and Alq3 as an electron transporting layer exhibited
enhanced EL emission compared to that of single-layered PLED.
Keywords: carbazolylene vinylene; heterocyclic conjugated copolymer; Horner-
Emmons reaction; phenothiazinylene vinylene; polymer light-emitting diode
INTRODUCTION
Research on conjugated polymers has been motivated by their promis-
ing application in polymer light emitting diodes (PLEDs), since R. H.
Friend et al. reported PLED made from poly(p-phenylenevinylene)
(PPV). To date, it has been generally accepted that the electronic
properties and chemical nanostructures of conjugated polymers have
played a key role on the emission color, quantum efficiency, and lumi-
nance=voltage profile of PLEDs [1,2]. In a PLED, the conjugated poly-
mer sandwiched between two electrodes must act as a hole transport,
electron transport, and emission layer at a same time for efficient
operation of PLEDs. However, a few types of conjugated polymers
have been satisfied with the requirement mentioned above. Difficult-
ies associated with fabricating high efficient PLEDs arise principally
due to unbalanced injection rate of holes and electrons from both
opposite electrodes, resulting in low recombination efficiency of
injected holes and electrons in polymer emissive layer. Thus, one
approach to further improve electroluminescent properties such as
electroluminescent efficiencies, fine-tuning of balanced rate of carrier
(electron and hole) injection=transporting and recombination effi-
ciency in PLEDs can be to construct multi-layered PLEDs consisting
of hole injection layer (HIL), hole transporting layer (HTL), emitting
layer (EML), etc [3–6]. Aromatic amines such as N,N0-diphenyl-N,N0-bis
(3-methylphenyl)-(1,10-biphenyl)-4,40-diamine and carbazole have been
reported to have hole transporting capability [7,8]. On the other hand,
organic compounds such as 2-(4-biphenylyl)-5-(40-tert-butylphenyl)-
1,3,4-oxadiazole and anthracene derivatives were found to have elec-
tron transporting capability [9–11].
In this work, we describe electroluminescent properties of conju-
gated random copolymers with hole transporting phenothiazine and
carbazole moieties in the main chain. Single- and multi-layered
PLEDs were fabricated by spin-coating the conjugated random copoly-
mer as a light-emitting layer on ITO glass and their electrolumines-
cent properties were studied.

Electroluminescent Properties of Heterocyclic Copolymers
131=[411]
EXPERIMENTAL
Monomer Synthesis
N-(2-ethylhexyl)-3,6-diformylphenothiazine (DFPZ) [12]
Phosphoryl chloride (122.7 g, 0.8 mol) was added dropwise over a
period of 30 min to a mixture of N-(2-ethylhexyl)phenothiazine (EHPZ)
(12.36 g, 0.04 mol) and N,N-dimethylformamide (DMF) (58.5 g, 0.8 mol)
in 1,2-dichloroethane (15 ml) in an ice bath. The solution was refluxed
1
for 72 h. DFPZ was obtained as liquid in 69% yield. H-NMR (CDCl3),
d: 9.73 (m, 2H, aldehydic hydrogen), 6.88, 7.56 (m, 6H, Ar H), 3.73
(t, 2H, >NꢀCH2ꢀ), 1.17–1.85 (m, 9H, ꢀCH2ꢀ, >CHꢀ), 0.76–0.82 (m,
6H, ꢀCH3).
N-(2-ethylhexyl)-3,6-diformylcarbazole (DFCZ) [13]
Phosphoryl chloride (122.7 g, 0.8 mol) was added dropwise over a per-
iod of 30 min to a mixture of N-(2-ethylhexyl)carbazole (EHCZ) (11.18 g,
0.04 mol) and DMF (58.5 g, 0.8 mol) in 1,2-dichloroethane (15 ml) in an
ice bath. The solution was refluxed for 72 h. DFCZ was obtained as vis-
1
cous liquid in 65 % yield. H-NMR (CDCl3), d : 10.11 (2H, aldehydic
hydrogen), 7.49–8.62 (6H, Ar H), 4.23 (2H, >NꢀCH2ꢀ), 2.05 (1H,
ꢀCH2ꢀ, >CHꢀ), 1.26–1.38 (8H, ꢀCH2ꢀ), 0.84–0.97 (6H, ꢀCH3).
1,4-Bis(diethoxyphosphinylmethyl)benzene (PHBZ) [12]
PHBZ was synthesized by reacting a,a0-dichloro-p-xylene (33 g,
0.2 mol) with triethylphosphite (33 g, 0.2 mol) at 120ꢁC for 12 hr. The
unreacted triethylphosphite was removed under reduced pressure. The
1
product was obtained as colorless liquid and yield was 93%. H-NMR
(CDCl3), d: 7.33(s, 4H, Ar H), 4.00(s, 8H, PꢀOCH2ꢀ), 3.13(d, 4H,
ArꢀCH2ꢀP), 1.25(t, 12H, ꢀCH3), IR (KBr), cmꢀ1: 3021(Ar CH; weak
=
signal), 2952(aliphatic CH), 1515(Ar C C), 1243(P¼O).
Polymerization
Poly(N-2-ethylhexyl-3,6-phenothiazinylene vinylene-alt-1,4-
phenylene vinylene) [poly(PZV-PV)] [12]
To a solution of DFPZ (1.85 g, 0.005 mol) and PHBZ (1.90 g,
0.005 mol) in anhydrous tetrahydrofuran (THF) (40 ml) was added
dropwise a solution of potassium tert-butoxide (tBuOK) (4.73 g of
purity 95%, 0.04 mol) in dry THF (20 ml) at room temperature under
t
nitrogen atmosphere. After the addition of BuOK the solution was
stirred for 7 hr to give a polymer product as precipitate. The resulting
mixture was concentrated under reduced pressure, and the residue
was washed with methanol and water. After filtration and vacuum
drying the poly(PZV-PV) was obtained as orange color powder. IR

132=[412]
Y. S. Han et al.
(KBr), cmꢀ1: 3019(Ar CH), 2923(aliphatic CH), 2853(>NꢀCH2ꢀ), 1579,
=
1508(Ar C C), 956(trans vinylene).
Poly(N-2-ethylhexyl-3,6-carbazolylene vinylene-alt-1,4-
phenylene vinylene) [poly(CZV-PV)]
Poly(CZV-PV) were synthesized by a synthetic method similar to
that of poly(PZV-PV) using DFCZ and PHBZ. IR (KBr), cmꢀ1:
3007(Ar CH), 2915(aliphatic CH), 2853(>NꢀCH2ꢀ), 1561, 1470(Ar
=
C C), 960(trans vinylene).
Poly[(N-2-ethylhexyl-3,6-phenothiazinylene vinylene-alt-1,4-
phenylene vinylene)-ran-(N-2-ethylhexyl-3,6-carbazolylene
vinylene-alt-1,4-phenylene vinylene)] [poly(PZV-ran-CZV)]
Heterocyclic copolymers, poly(PZV-ran-CZV)s containing N-2-ethyl-
hexyl-3,6-phenothiazinylene vinylene(PZV), N-2-ethylhexyl-3,6-carba-
zolylene vinylene(CZV), and phenylene vinylene(PV) unit in different
ratio were synthesized from DFPZ, DFCZ, and PHBZ with different
feed ratio of comonomers. A brief synthetic scheme of the heterocyclic
conjugated copolymers is shown in Scheme 1.
Fabrication of PLEDs
Single-layered PLEDs
Single-layered PLEDs were fabricated by using these conjugated
copolymers as emitting materials. Indium-tin oxide (ITO) coated glass
SCHEME 1 Synthetic route to heterocyclic conjugated copolymers.

Electroluminescent Properties of Heterocyclic Copolymers
133=[413]
with a sheet resistance of 30 X=& and a thickness of 1.08 mm was cut
into 2.0 ꢂ 2.0 cm2, and electrode area was prepared by photoetching
technique. It was sequentially cleaned in an ultrasonic bath of acet-
one, methanol, and mixture of isopropyl alcohol and water (1:1 by
vol.) solution. Filtered solutions of conjugated polymer in 1,1,2,2-tetra-
chloroethane were cast on patterned ITO glass by spin coating and the
solvent was evaporated to give emitting layers. Thin Al film as cathode
was vacuum deposited using Thermal Evaporator System (KVT-420,
Korea Vacuum Co.) on top of the emitting layer by thermal method
(below 2.0 ꢂ 10ꢀ5 torr) at a rate of 20 A=sec to a thickness of about
˚
˚
1800 A to give single-layered PLEDs (ITO=conjugated polymer=Al).
Multi-layered PLEDs
Double-layered PLEDs were fabricated with Alq3 as an additional
electron transporting layer between the emissive layer and the metal
cathode. For ITO=conjugated polymer=Alq3=Al double-layered PLED,
after spin-coating and drying of polymeric emissive layer, vacuum
deposition of Alq3 was performed on top of the emissive layer at a rate
˚
˚
of 3–5 A=sec until 250 A thickness, followed by vacuum deposition of Al
˚
metal with a thickness of 1800 A to give the double-layered PLED. The
area of Al electrode deposited was approximately 0.4 cm2.
With insertion of PEDOT:PSS[poly(3,4-ethylenedioxythiophene):
poly(styrene sulfonic acid)], which is well-known as both hole injection
and transporting layer, double-layered PLED(ITO=PEDOT:PSS=
conjugated polymer=Al)s were made. Triple-layered PLED(ITO=
PEDOT:PSS=conjugated polymer=Alq3=Al)s were also fabricated.
After spin-coating and drying of PEDOT:PSS followed by emissive
polymer layer, Alq3 layer was vacuum deposited on top of copolymer
thin film layer.
Measurements
The 1H-NMR and FT-IR spectra of synthesized compounds were taken
on a Varian Unity Plus 300 and Jasco FT=IR-620 spectrometer, respect-
ively. Molecular weight was measured by Waters gel permeation chro-
matograph (GPC) equipped with a 410 differential refractometer, 510
HPLC pump, and Styragel HR 5E column using THF as an eluent
against polystyrene standards at room temperature. Thermal analysis
was performed with differential scanning calorimeter (DSC) (TA Instru-
ments, TA4000=Auto DSC 2910 system) and thermogravimetric analy-
zer (TGA) (Seiko instruments Inc., TG=DTA 320) with heating rate of
20ꢁC=min under nitrogen atmosphere. UV-visible absorption spectra
of the copolymers were obtained by Shimadzu UV-2100. Electrolumi-
nescence (EL) spectra and color coordinates were measured by using

134=[414]
Y. S. Han et al.
Spectroscan PR 704 (Photoresearch Inc.). Luminance with voltage was
obtained by using a dc power supply connected with Model 8092 A Digi-
tal Multimeter (Hyun Chang Product Co. Ltd) and luminance meter
(Minolta LS-100) equipped with close-up lens (No. 110, U 40.5 mm) at
room temperature, respectively.
RESULTS AND DISCUSSION
Synthesis and Characterizations
Heterocyclic random copolymers, poly(PZV-ran-CZV)20, poly(PZV-ran-
CZV)30 and poly(PZV-ran-CZV)40, containing N-2-ethylhexyl-3,6-
phenothiazinylene vinylene (PZV), N-2-ethylhexyl-3,6-carbazolylene
vinylene (CZV), and phenylene vinylene (PV) unit in different ratio
were synthesized by Honer-Emmons reaction of DFPZ, DFCZ and
PHBZ with different feed ratio of comonomers, i.e., DFPZ=DFCZ=
PHBZ¼15=35=50, 25=25=50, 35=15=50, respectively. From the elemental
analysis, we found that the content of PZV, CZV, and PV repeating unit
within poly(PZV-ran-CZV)20 had 18, 32, and 50 mole%, respectively.
With similarity, poly(PZV-ran-CZV)30 had 29 mol% of PZV, 21 mol%
of CZV and 50 mol% of PV in the random copolymer, and poly
(PZV-ran-CZV)40 had 38, 12, and 50 mol%, respectively. Alternating
copolymers, poly(PZV-PV) and poly(CZV-PV), were also synthesized
for comparison.
1
Synthesized monomers and polymers were characterized with H-
NMR and FT-IR, and their resulting spectral data were listed in
experimental section. Other characterizations such as molecular
weight and thermal decomposition temperature were also conducted.
The molecular weights of synthesized conjugated polymers ranged
from 14,000 to 18,000 g=mol. TGA analysis indicated that synthesized
polymers were stable up to about 360ꢁC under nitrogen. The good solu-
bility in common organic solvents at room temperature is one of the
important properties of the synthesized copolymers for the fabrication
of PLEDs by the simple spin casting method. The observed good solu-
bility of these poly(CZV-PV), poly(PZV-PV), and poly(PZV-ran-CZV)
copolymers in common organic solvents seemed to be originated from
the presence of heterocyclic aromatic ring in the main chain and
also due to the relatively long ethylhexyl groups attached both on
the phenothiazine and carbazole moieties.
Electro-Optic Properties of Single-Layered PLEDs
UV absorption spectra of synthesized copolymers, poly(CZV-PV),
poly(PZV-ran-CZV) and poly(PZV-PV), are shown in Figure 1. The

Electroluminescent Properties of Heterocyclic Copolymers
135=[415]
FIGURE 1 UV-visible absorption spectra of conjugated copolymers.
absorption maxima (kmax,UV) of the copolymer solution were varied
from 401 to 425 nm, and the band gap energy estimated from the
absorption edge (kedg,UV) decreased with increasing the amount of
the PZV unit.
PLEDs fabricated with poly(CZV-PV) and poly(PZV-PV) as light
emitting layers exhibited EL maxima at 500 and 570 nm, respectively,
as shown in Figure 2. Figure 3 shows emission spectra from PLEDs
made with random copolymers, poly(PZV-ran-CZV)20, 30, and 40.
PLEDs made with poly(PZV-ran-CZV) copolymers showed EL maxima
shifted to longer wavelength than 570 nm of ITO=poly(PZV-PV)=Al
PLED even though poly(PZV-ran-CZV) had lower PZV content than
that of poly(PZV-PV). On the contrary, PLEDs fabricated with
blends(1:1 by weight) of poly(PZV-CV) and poly(CZV-CV) alternating
copolymers as light emitting layers exhibited EL maxima at 564 nm.
The electro-optical data such as kmax,UV, kedg,UV, kmax,EL, Stoke’s
shift, and color coordinates measured from UV-visible absorption
and EL spectrum are summarized in Table 1. Random copolymers,
poly(PZV-ran-CZV)20, 30, and 40, showed larger Stoke’s shift than
that of alternating copolymers, poly(CZV-PV) and poly(PZV-PV). This
large Stoke’s shift in random copolymers which have various chain
segments may be due to the larger possibility of excimer formation
than that of alternating copolymers. Of the poly(PZV-ran-CZV) copoly-
mers used, the PLED fabricated with poly(PZV-ran-CZV)30 copolymer
exhibited the highest Stoke’s shift of 190 nm. It has been well known

136=[416]
Y. S. Han et al.
FIGURE 2 Emission spectra of PLEDs made with alternating copolymers.
that the emission at excimer formation is observed in longer wave-
length than normal emission peak [14,15]. Therefore, large shift of
EL emission by poly(PZV-ran-CZV)30 may be due to the enhanced
excimer formation at this copolymer composition compared to
alternating copolymers and other random polymers with different
copolymer composition.
FIGURE 3 Emission spectra of PLEDs made with random copolymers.

137=[417]

138=[418]
Y. S. Han et al.
FIGURE 4 Color coordinates of (a) poly(CZV-PV), (b) poly(PZV-ran-CZV)20,
(c) poly(PZV-ran-CZV)30, (d) poly(PZV-ran-CZV)40, and (e) poly(PZV-PV) in
CIE 1931 chromaticity diagram.
Color coordinates of singlelayer PLED (ITO=conjugatedpolymer=Al)s
were compared with those of standard red, green, blue, and white color
established by NTSC (national television system committee) [16] in
Figure 4. The emission color could be determined from these color coordi-
nates as measured by Spectroscan PR-704 (Photoresearch Inc.) and
Kelly’s map [17] in CIE (Commission Internationale de l’Eclairage)
1931 chromaticity diagram [18]. The EL light from poly(CZV-PV),
poly(PZV-ran-CZV)20, 30, 40, and poly(PZV-PV) showed green, yellow,
yellowish orange, yellowish orange, and yellow, respectively. Three
random copolymers, poly(PZV-ran-CZV)20, 30, and 40, showed similar
value of CIE coordinates and their EL emission in yellowish orange
region. Emission maxima(kmax,EL) of these copolymers showed small
amountofbathochromic shiftwithincreasingPZV unitinthemainchain.
Emission Properties of Double-Layered PLEDs
We also fabricated multi-layered PLEDs using poly(PZV-ran-CZV)30
as an emitting layer with the insertion of PEDOT:PSS as a hole

Electroluminescent Properties of Heterocyclic Copolymers
139=[419]
injection=transporting layer, Alq3 as an electron transporting layer, or
both of PEDOT:PSS and Alq3 layers. The double-layered PLED, ITO=
poly(PZV-ran-CZV)30=Alq3=Al, exhibited enhanced EL emission[220
nit at 20 V] compared to those of ITO=poly(PZV-ran-CZV)30=Al[50
nit at 20 V], double-layered ITO=PEDOT:PSS=poly(PZV-ran-CZV)30=
Al[40 nit at 20 V], and triple-layered ITO=PEDOT:PSS=poly(PZV-ran-
CZV)30=Alq3=Al[70 nit at 20V]. From the comparison of double-layered
ITO=poly(PZV-ran-CZV)30=Alq3=Al and single-layered ITO=poly(PZV-
ran-CZV)30=Al, the enhanced luminance by insertion of Alq3 as an
electron injection=transporting layer in the double-layered PLED was
attributed to the balanced rate of electron=hole injection and effective
charge confinement in the interface of poly(PZV-ran-CZV)30=Alq3.
Results on decreased luminance in double-layered ITO=PEDOT:
PSS=poly(PZV-ran-CZV)30=Al PLED indicated that inserting PED-
OT:PSS as a hole injection=transporting layer adversely disrupted
the balance of electron and hole injection in the PLED. Therefore, it
can be concluded that heterocyclic conjugated copolymers synthesized
have hole transporting property under the induced electric field. This
may be due to the presence of phenothiazine and carbazole moieties in
these polymers since the aromatic amines were well known to be a
hole transport agent in the EL devices.
CONCLUSIONS
Horner-Emmons reaction was found to be a useful synthetic method
for the preparation of EL copolymers with designed microstructure.
Of the synthesized copolymers, PLED fabricated with poly(PZV-ran-
CZV)30 copolymer exhibited larger Stoke’s shift compared to those of
other PLEDs. This might be due to the enhanced excimer formation
of poly(PZV-ran-CZV) copolymer at 29 mol % of PZV repeating units
in the polymer chain. Double-layered ITO=poly(PZV-ran-CZV)30=
Alq3=Al PLED exhibited better improved EL emission than those of
other multi-layered PLEDs. These results were attributed to the
balanced rate of electron=hole injection and effective charge confine-
ment in the interface of poly(PZV-ran-CZV)30=Alq3. It could be
concluded, therefore, that synthesized heterocyclic copolymers have
good hole transporting and electron blocking capability.
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