Green and red electrophosphorescent devices consisting of cabazole/triarylamine-based polymers doped with iridium complexes

Article abstract of DOI:10.1080/15421400701548506

Two types of polymers were synthesized such as poly[N-(2-ethylhexyl) carbazole-alt-N-(4-aminophenyl)carbazole] (PECAC) and poly[N-(2-ethylhexyloxy- phenyl) carbazole-alt-N-(4-aminophenyl)carbazole] (PEPCAC). These polymers are designed to have carbazole g

Full text of DOI:10.1080/15421400701548506

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Green and Red
Electrophosphorescent Devices
Consisting of Cabazole/
Triarylamine-Based Polymers
Doped with Iridium Complexes
Hui Wang a , Jeong-Tak Ryu a , Dong Uk Kim b , Yoon
Soo Han c , Lee Soon Park d , Ho-Young Cho d , Sam-
Jong Lee d & Younghwan Kwon e
a College of Information and Communication
Engineering, Daegu University , Gyeongsan,
Gyeongbuk, Korea
b Department of Science Education, Daegu National
University of Education , Daegu, Korea
c Daegu Gyeongbuk Institute of Technology , Daegu,
Korea
d Department of Polymer Science , Kyungpook
National University , Daegu, Korea
e Department of Chemical Engineering , Daegu
University , Gyeongsan, Gyeongbuk, Korea
Published online: 22 Sep 2010.
To cite this article: Hui Wang , Jeong-Tak Ryu , Dong Uk Kim , Yoon Soo Han , Lee
Soon Park , Ho-Young Cho , Sam-Jong Lee & Younghwan Kwon (2007) Green and
Red Electrophosphorescent Devices Consisting of Cabazole/Triarylamine-Based
Polymers Doped with Iridium Complexes, Molecular Crystals and Liquid Crystals,
471:1, 279-291, DOI: 10.1080/15421400701548506
To link to this article: http://dx.doi.org/10.1080/15421400701548506

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Mol. Cryst. Liq. Cryst., Vol. 471, pp. 279–291, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701548506
Green and Red Electrophosphorescent Devices
Consisting of Cabazole/Triarylamine-Based
Polymers Doped with Iridium Complexes
Hui Wang
Jeong-Tak Ryu
College of Information and Communication Engineering,
Daegu University, Gyeongsan, Gyeongbuk, Korea
Dong Uk Kim
Department of Science Education, Daegu National University
of Education, Daegu, Korea
Yoon Soo Han
Daegu Gyeongbuk Institute of Technology, Daegu, Korea
Lee Soon Park
Ho-Young Cho
Sam-Jong Lee
Department of Polymer Science, Kyungpook National University,
Daegu, Korea
Younghwan Kwon
Department of Chemical Engineering, Daegu University,
Gyeongsan, Gyeongbuk, Korea
Two types of polymers were synthesized such as poly[N-(2-ethylhexyl)carbazole-alt-
N-(4-aminophenyl)carbazole] (PECAC) and poly[N-(2-ethylhexyloxy-phenyl) car-
bazole-alt-N-(4-aminophenyl)carbazole] (PEPCAC). These polymers are designed
to have carbazole groups with wide band gap and a triarylamine moiety with a
hole transporting property. Green and red phosphorescent polymer light emitting
diodes (PhPLEDs) were fabricated with an emitting layer consisting of either
PECAC or PEPCAC as a host matrix with suitable iridium complexed, Ir(ppy)3
as a green or IR-PIQCH as a red dopant, respectively. Red PhPLEDs doped with
IR-PIQCH exhibited maximum EL emission peaks at 620 nm with higher
luminescence and lower driving voltage, due to effective energy transfer from host
This work was supported by Daegu University Research Grant 2006.
Address correspondence to Prof. Y. Kwon, Department of Chemical Engineering,
Daegu University, Gyeongsan, Gyeongbuk 712-714, Korea. E-mail: y_kwon@daegu.ac.kr
279

280
H. Wang et al.
polymers to IR-PIQCH red dopant. Based on HOMO and LUMO levels of the
polymers and Ir(ppy)3, however, the synthesized polymers could not be served as
suitable host polymers for Ir(ppy)3 green dopant. Ir(ppy)3 doped PhPLEDs
exhibited red shift and wide range of EL emission.
Keywords: carbazole; dopant; host; phosphorescence; PLED; triarylamine
INTRODUCTION
Increasing interest has been paid to carbazole-based polymers because of
their well-known optical properties and hole transporting ability in
organic light emitting diodes (OLEDs). Such good properties for series
of carbazole materials are due to the electron-donating capabilities with
its nitrogen atom [1–4]. Among them, polyvinylcarbazole (PVK) was
widely used and investigated as non-conjugated vinyl polymer with
carbazole attached in the side chain [5]. Recently, conjugated polymers
with carbazole as main chain built-unit have been studied extensively.
Polymers incorporated of 3,6-carbazole into varieties of aromatic and het-
erocyclic comonomers have been synthesized. For instance, the synthesis
of well-defined copolymers of 3,6-carbazole with phenylenes, thiophenes
or fluorenes has led to a significant improvement in the performance of
these polymeric materials and to a better understanding of their struc-
ture-property relationships [6–8]. Considering good hole-transporting
property and wide band gap energy for the conjugated polycarbazole, it
can be used as a host segment in copolymerization with various comono-
mers and used as host-matrix materials in OLEDs.
Triarylamine derivatives with low molecular weight are the most
common class of hole transporting materials, such as N,N0-diphenyl-
N,N0-bis(3-methylphenyl)-1,10-biphenyl-4,40-diaimine (TPD). Whereas,
TPD shows low glass transition temperature at 60ꢀC that causes crys-
tallization and thermal breakdown during the operation of OLEDs. So,
N,N0-diphenyl-N,N0-bis(a-naphthyl)benzidine (NPB) has been widely
used instead of TPD [9–10]. Further, high Tg polymers containing
TPD unit were synthesized. Normally, triarylamine-based hole-
transporting materials were usually synthesized by Ullmann coupling
[10], Pd-catalyzed amination reaction [11], and Suzuki coupling reac-
tion and showed high hole-mobility with high Tg [12–13].
Phosphorescent organic light emitting diodes (PhOLEDs) have been
attracting much attention because of their high quantum efficiency.
The phosphorescent molecules allow harvesting light not only from
the singlet but also from the triplet excitons so that 100% internal
quantum efficiency can be achieved theoretically [14]. High efficient

Green and Red Electrophosphorescent Devices
281
PhOLEDs have been reported by doping dopants from blue to red
emissive phosphorescence in low molecular weight organic host. Phos-
phorescent polymer LEDs (PhPLEDs) are also investigated greatly
with the advantage that they can be fabricated by simple solution pro-
cesses, such as spin-casting, screen-printing, or inject-printing at room
temperature [15]. However, PhPLEDs show lower quantum efficiency
than PhOLEDs with low molecular weight organic host, because of the
phase separation or aggregation of dopant. It was reported from
devices of PVK doped with FIrpic that maximum external quantum
efficiency of 1.3% and power efficiency of 0.8 lm=W were obtained
[16]. So it is important to select the proper polymeric hosts and
efficient phosphorescent dopants.
In this article, we prepared poly[N-(2-ethylhexyl) carbazole-alt-N-
(4-aminophenyl) carbazole] (PECAC) and poly[N-(2-ethylhexyloxy-
phenyl) carbazole-alt-N-(4-aminophenyl) carbazole] (PEPCAC) by
Pd-catalyzed polycondensation. From the view point of design concept,
carbazole groups in these polymers were introduced both to main
and side chains, as well as the polymer chains were linked by hole-
transporting triarylamine groups. Photophysical properties of these
polymers and the performance of PhPLEDs fabricated by doping a
green, Ir(ppy)3, or a red, IR-PIQCH, phosphorescent dopant in PECAC
and PEPCAC hosts were investigated.
EXPERIMENTAL
Materials
2-Ethylhexylbromide (97%), 3,6-dibromocarbazole (97%), 4-iodophenol
(99%), copper (I) iodide (98%), trans-1,2-cyclohexane-diamine (ligand)
(99%), tin (II) chloride (SnCl2, 98%), sodium tert-butoxide (NaO-t-Bu,
97%), tris(dibenzylidene-acetone)dipalladium(0) (Pd2(dba)3), and
tri-tert-butylphosphine (P(t-Bu)3, 90%) were purchased from Aldrich
Chemical Co. Reagent grade carbazole, 4-iodonitrobenzene, potassium car-
bonate anhydrous (K2CO3) and potassium phosphate tribasic (K3PO4)
were obtained from Tokyo Kasei Co. and used as received. Solvents were
reagent grades and purified prior to use. Bis(2-methyl-8-quinolinato)-
4-phenlphenolate aluminum (BAlq), tri(8-hydroxyquinolate)aluminum
(Alq3) and lithium fluoride (LiF) were obtained from Tokyo Kasei Co.
and used as received. Ir(ppy)3 was purchased from Asung Co. IR-PIQCH
was synthesized according to the procedure reported previously [17].
PEDOT was received from LG electronics Co. Aluminum (99.99%) was
purchased from CERAC, USA. Indium-tin oxide (ITO) coated glass with
a sheet resistance of 50 X=& was obtained from Sinan SNP Co.

282
H. Wang et al.
Synthesis of Polymers
PECAC was prepared according to the procedure reported previously
[18]. Two monomers for PEPCAC were prepared as shown in
Scheme 1. The polymerization was carried out according to reference
[19], as shown in Scheme 2.
N-(2-Ethylhexyloxylphenyl)-3, 6-dibromocarbazole (Monomer
1). Monomer 1 was synthesized in two steps. First, 4-iodophenol
(10 mmol) was dissolved in 100 ml of acetonitrile, then K2CO3
(20 mmol) was added. The mixture was stirred in N2 atmosphere at
60ꢀC. After 30 minutes, 2-ethylhexylbromide (12 mmol) was dissolved
in 25 ml of acetonitrile and added to the flask dropwise. The reaction
was kept for 48 hours. K2CO3 was filtered out, and the solvent was
evaporated, then the crude product was washed with water and dried
with MgSO4. The pure white liquid product was obtained by silica
column chromatography (eluent: hexane). (Yield: 77%). 1H-NMR
(300 MHz in CDCl3): d 7.33 (d, 2H, ArꢁH), 6.79 (d, 2H, ArꢁH), 3.79
(d, 2H, ꢁOꢁCH2ꢁ), 1.71 (m, 1H, ꢁCHꢁ), 1.29 ꢂ 1.46 (m, 8H, ꢁCH2ꢁ),
0.91 (m, 6H, ꢁCH3); 13CꢁNMR (300 MHz in CDCl3): 158.4, 132.0,
116.2, 112.4, 70.6, 39.2, 30.4, 29.0, 23.8, 23.0, 14.1, 11.1.
SCHEME 1 Synthetic route to monomers.

Green and Red Electrophosphorescent Devices
283
SCHEME 2 Synthesis of the polymers.
In the second step, 4-iodo-(2-ethylhexyloxyl)benzene (7.5 mmole),
3,6-dibromocarbazole (9.0 mmol), K3PO4 (15.8 mmol), CuI (0.16 mmol)
and trans-1, 2-cyclohexane-diamine (0.90 mmol) were added to
1,4-dioxane (100 ml) at room temperature. The reaction mixture was
refluxed for 24 hours. After cooling the mixture to room temperature,
the dissolved materials were filtered. 1,4-Dioxane was evaporated, and
ethyl acetate was added. The mixture was washed with distilled

284
H. Wang et al.
water, and dried with MgSO4. The final product was obtained through
column chromatography (hexane: ethylacetate ¼ 9:1v=v). White solid
product was obtained (Yield: 90%). 1H-NMR (300 MHz in CDCl3):d
8.18 (d, 2H, ArꢁH), 7.47 (d, 2H, ArꢁH), 7.36 (d, 2H, ArꢁH), 7.16 (d,
2H, ArꢁH), 7.08 (d, 2H, ArꢁH), 3.94 (d, 2H, ꢁOꢁCH2ꢁ), 1.77 (m,
1H, ꢁCHꢁ), 1.26 ꢂ 1.57 (m, 8H, ꢁCH2ꢁ), 0.91 (m, 6H, ꢁCH3);
13CꢁNMR (300 MHz in CDCl3): 159.1, 140.3, 129.2, 128.9, 128.3,
123.6, 123.1, 115.7, 112.7, 111.5, 70.8, 39.3, 30.5, 29.1, 23.8, 23.1,
14.1, 11.1.
N-(4-aminophenyl)carbazole (Monomer 2). First, a mixture of
4-iodonitrobenzene (20 mmole), carbazole (15.24 mmmol) and K3PO4
(42 mmol) were added to 1,4-dioxane (100 ml) at room temperature.
After stirring for 30 minutes, CuI (0.4 mmol) and trans-1,2-cyclohexa-
nediamine (2.4 mmol) were added to the mixture. The reaction mix-
ture was refluxed for 24 hours. After cooling the mixture to room
temperature, the dissolved materials were filtered. 1,4-Dioxane was
evaporated, and ethyl acetate was added. The mixture was washed
with distilled water, and dried with MgSO4. The final product was
obtained through column chromatography (hexane: ethylacetate ¼
1
9:1 v=v) with the yield of 98%. H-NMR (300MHz in CDCl3): d 8.58 (d,
1H, ArꢁH), 8.55 (d, 1H, ArꢁH), 8.26 (m, 1H, ARꢁH), 8.23 (m, 1H,
ArꢁH), 8.01 (d, 1H, ArꢁH), 7.98 (d, 1H, ArꢁH), 7.60 (t, 1H, ArꢁH),
7.58 (t, 1H, ArꢁH), 7.51 ꢂ 7.45 (m, 2H, ArꢁH), 7.39 ꢂ 7.33 (m, 2H,
ArꢁH); 13C-NMR (300 MHz in CDCl3): d 144.35, 140.70, 131.85,
127.94, 127.34, 126.36, 124.82, 121.91, 121.33, 110.65; GC-MS m=z ¼ 288
288 (mþ).
In the second step, a mixture of N-(4-nitrophenyl)carbazole
(5 mmol), SnCl2 (30.4 mmol) and EtOH (15 ml) was heated at 70ꢀC
for 4 h. After the mixture was cooled to room temperature, 1 M NaOH
solution was added until the mixture became alkaline. After extraction
with ethyl acetate, the combined organic layers was washed with
brine, dried with MgSO4 and evaporated. The final compound was
1
obtained with the yield of 98%. H-NMR (300 MHz in CDCl3):d 8.15
(d, 1H, ArꢁH), 8.12 (d, 1H, ArꢁH), 7.42 ꢂ 7.22 (m, 8H, ArꢁH), 6.87
(d, 1H, ArꢁH), 6.84 (d, 1H, ArꢁH), 3.81 (s, 2H, ArꢁNH2); 13CꢁNMR
(300 MHz in CDCl3):d 145.89, 141.45, 128.48, 125.70, 122.92, 120.16,
119.39, 115.89, 109.75; GC-MS m=z ¼ 258 (mþ).
Device Fabrication
ITO coated glass was cut into 5.0 cm ꢃ 5.0 cm, and electrode area was
prepared by photo-etching technique. It was sequentially cleaned in an

Green and Red Electrophosphorescent Devices
285
ultrasonic bath of acetone, methanol, and mixture of isopropyl
alcohol and water (1:1 v:v) solution. PhPLEDs were fabricated
with a configuration of ITO=PEDOT=Polymers þ iridium complex
(10 wt%)=BAlq (30 nm)=Alq3 (20 nm)=LiF (1 nm)=Al (120 nm). The hole
injecting PEDOT layer was spin-coated with a solution of 1 wt% in iso-
propyl alcohol onto ITO substrate in two steps at 1000 rpm for 10 s and
1000 rpm for 20 s, and then baked on hot plate at 80ꢀC for 10 minutes.
Subsequently, an emissive layer was spin-coated with a solution
(1 wt%) of polymer and iridium complex in 1,1,2,2-tetrachloroethane,
after filtration using MFS filter (0.45 mm pore size), at 1200 rpm for
10 s and dried at 100ꢀC for 10 minutes. The emissive thin films pre-
pared had uniform surface with thickness of about 60–70 nm. Finally,
hole-blocking BAlq, electron transporting Alq3, and LiF=Al were suc-
cessively deposited under pressure <10ꢁ6 torr. The active area of
the PhPLEDs was 2 mm ꢃ 2 mm.
Characterization
NMR spectra were recorded on a Varian Unity Plus 300 with solvent of
CDCl3. GC Mass (GC-MS) spectra were recorded on a QP 5050 mass
spectrometer. Weight and number average molecular weight were
measured by gel permeation chromatograph (GPC) equipped with
Styragel HR 5E column using THF as an eluent against polystyrene
standards at room temperature. Thermogravimetric analysis (TGA)
was performed by using a Seiko EXSTAR 6000 TG=DTA 6300 thermal
analyzer at a heating of 20ꢀC=min under N2 atmosphere. UV-visible
absorption spectra were taken with Shimadzu UV-2100. The photo-
luminescence (PL) spectra excited by He-Cd laser at 325 nm were
monitored by Optical Multichannel Analyzer (Laser Photonics, OMA
system). The ionization potential (IP) was measured by a photoelec-
tron spectroscopy (Riken Keiki AC-2). Electroluminescence (EL) spec-
tra were measured by using Spectroscan PR 704 (Photoresearch Inc).
Current-luminescence-voltage (I-V-L) profiles of devices were obtained
by using dc power supply connected Model 8092 A Digital Multimeter
and luminance meter (Minolta LS-100).
RESULTS AND DISCUSSION
As outlined in Scheme 1, alkylation of 4-iodophenol with 2-ethyl-
hexylbromide for the synthesis of monomer 1 was easily obtained,
which could impart higher solubility of synthesized polymers in com-
mon organic solvents. For the synthesis of monomer 2, N-(4-nitro-
phenyl)carbazole was first synthesized via Ullmann coupling of

286
H. Wang et al.
TABLE 1 Physical Property of Polymers
Polymer
Yield (%)
PDI
Mw
Mn
Td (ꢀC)a
Weight loss at 800ꢀC
PECAC
PEPCAC
80
81
1.54
1.70
5700
7800
3700
4600
465
464
36.8%
27.8%
aTemperature resulting in 5% weight loss based on initial weight.
4-iodonitrobenzene and carbazole with a high yield. Then, the
reduction of N-(4-nitrophenyl)-carbazole with SnCl2 catalyst in
methanol gave monomer 2. Synthesized copolymers are designed to
have blue emitting carbazole moieties both in main chains and side
chains, as well as the molecular chains are liked by triarylamine units,
as depicted in Scheme 2. Results on physical properties of the synthe-
sized polymers such as molecular weights, molecular weight distribu-
tions and decomposition temperature (Td) were summarized in Table 1.
Maximum UV-Vis absorption and PL emission peaks of PECAC and
PEPCAC were measured both in solution and solid state, as can be
seen in Figure 1. In their dilute CHCl3 solution, maximum absorption
peaks of PECAC and PEPCAC were observed at 312 nm and 320 nm;
in their film state, they were observed at 336 nm and 346 nm, respect-
ively. With excitation at their kmax,UV in either solution or solid state,
FIGURE 1 UV-Vis absorption and PL emission spectra of polymers.

Green and Red Electrophosphorescent Devices
287
PL emission of two polymers was in the range of blue emission. In
dilute CHCl3 solution, maximum PL emission peak (kmax,PL) of PECAC
and PEPCAC was observed at 453 nm and 455 nm; in their film state,
they were observed at 466 nm and 467 nm. Compared with PL spectra
in solution, red shift of PL emission spectra in the solid state of PECAC
and PEPCAC was observed, indicating an aggregation of polymer
chains in the solid state.
The optical and electrochemical properties of the polymers are
listed in Table 2. The band gap energies of the synthesized polymers
were calculated from UV-Vis absorption and PL emission spectra.
The band gap energies of PECAC and PEPCAC were measured to be
2.97 eV and 3.25 eV, respectively. The wide band gap energy of poly-
mers is required to be qualified as good host materials in an emitting
layer. Structural characteristics of materials play a crucial role in
adjusting the band gap, and wider band gap for PEPCAC means less
conjugation in the molecular chains, which might be originated from
the fact that substitution of 2-ethylhexyloxylphenyl groups on
nitrogen atom increased the dihedral angle between the aryl unit
and carbazole to decrease the degree of planarity and conjugation of
the system [20]. The polymers showed HOMO levels of ꢁ5.01 eV
and ꢁ5.17 eV, higher than those of low molecular weight analogs such
as TPD (ꢁ5.5 eV) or NPB (ꢁ5.7 eV) [19]. Such energy levels of the poly-
mers may provide a closer match to the work function of ITO anode
(4.7 eV) or PEDOT layer (5.0 eV) when they are used as hole transport-
ing materials in PLEDs.
In order to investigate the electroluminescent performance with the
synthesized polymers as host matrix, PhPLEDs were fabricated with
theconfigurationofITO=PEDOT=Polymersþ iridium complex(10 wt%)=
BAlq (30 nm)=Alq3 (20 nm)=LiF (1 nm)=Al (120 nm). The energy
TABLE 2 Optical and Electrochemical Properties of the Polymers
Solution
Films
kmax,UV
(nm)
kmax,PL
(nm)
kmax,UV
(nm)
kmax,PL
(nm)
Band
gap (eV)a
HOMO
(eV)b
LUMO
(eV)c
Polymer
PECAC
PEPCAC
312
320
453
455
336
346
466
467
2.97
3.25
ꢁ5.01
ꢁ5.17
ꢁ2.04
ꢁ1.92
aCalculated from the crosspoint of UV-Vis and PL spectrum.
bMeasured by a RIKENKeiki AC-2.
cEstimated from the HOMO and band gap.

288
H. Wang et al.
FIGURE 2 Schematic view of energy levels of devices.
diagram of the devices is shown in Figure 2. Figures 3 (a) and (b)
showed the current–voltage (I–V) and luminescence–voltage (L–V)
curves of PhPLEDs, respectively. For devices doped with Ir(ppy)3,
the maximum luminescence and driving voltage were 453 cd=cm2
and 8 V for PECAC, and 77 cd=cm2 and 14 V for PEPCAC. In the case
of IR-PIQCH doped PhPLEDs, the maximum luminescence and driv-
ing voltage were 576 cd=cm2 and 6 V for PECAC, and 59 cd=cm2 and
12 V for PEPCAC, respectively. It appeared that, due to matching of
HOMO energy levels, the devices with PECAC as a host matrix in
PhPLEDs showed lower turn-on voltage.
Figure 4 presents electroluminescencent spectra of PhPLEDs. The
maximum emission wavelengths of Ir(ppy)3 doped devices were mea-
sured at 546 nm. However, emission by the triplet state of Ir(ppy)3
has been reported to be 515 nm [21]. This difference in the maximum
emission wavelength of Ir(ppy)3 and Ir(ppy)3-doped devices accounts
for the inefficient band gap energy of the host materials to the guest
Ir(ppy)3 dopant, especially lower HOMO level of host polymers (ꢁ5.0
for PECAC and ꢁ5.2 eV for PEPCAC) than ꢁ5.5 eV of Ir(ppy)3. In
IR-PIQCH doped devices, on the other hand, the effective energy
transfer from host polymers to IR-PIQCH red dopant was observed,
showing that the maximum EL emission at 620 nm was matched to
IR-PIQCH triplet emission.

Green and Red Electrophosphorescent Devices
289
FIGURE 3 Current–voltage curves (a) and luminescence–voltage curves
(b) of PhPLEDs.
CONCLUSIONS
Novel polymers such as PECAC and PEPCAC were successfully
synthesized and utilized as host polymers in PhPLEDs. PhPLEDs
were fabricated with the configuration of ITO=PEDOT=
Polymers þ iridium complex (10 wt%)=BAlq (30 nm)=Alq3 (20 nm)=LiF
(1 nm)=Al (120 nm). Due to insufficient band gap energy of the synthe-
sized polymers to Ir(ppy)3 green dopant, Ir(ppy)3 doped PhPLEDs
exhibited red shift and wide range of EL emission. On the other
hand, red electrophosphorescence was observed with emission peaks

290
H. Wang et al.
FIGURE 4 Electroluminescent spectra of PhPLEDs.
at approximately 620 nm in IR-PIQCH doped devices. Efficient energy
transfer from host polymers, PECAC or PEPCAC, to IR-PIQCH red
dopant confirmed that the synthesized polymers could be suitable
host materials for red dopants. PhPLED doped with IR-PIQCH in
PECAC as an emitting layer exhibited luminescence of 576 cd=cm2
and driving voltage of 6 V. Further optimization of device efficiencies
by utilizing PECAC host and IR-PIQCH red dopant could be a
continuing study.
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