Synthesis and characterization of fluorene-derived PU as a thermo cross-linked hole-transporting layer for PLED

Article abstract of DOI:10.1016/j.polymer.2012.03.023

Novel hole-transporting polyurethane, denoted as P1, resulting from the condensation of 9, 9-bis(4-hydroxyphenyl)fluorene and isophorone diisocyanate (denoted as IPDI) has been developed. When P1 is thermally consolidated in the presence of 2-(phosphonooxy)ethyl methacrylate (P2M), it forms a distinguished hole-transport layer that leads to an extremely good performance of the phosphorescent PLED. In the study, the device of ITO/PEDOT: PSS/P1-P2M/Ir(ppy) ₃-t-PBD-PVK/Mg/Ag shows a high current efficiency of 27.6 cd/A and a low turn-on voltage of 6 V. In particular, the stable output efficiency of 17-22 cd/A within the range of 420-4400 cd/m² at 12-20 V makes P1 a promising hole-transport material for phosphorescent PLED applications.

Full text of DOI:10.1016/j.polymer.2012.03.023

Polymer 53 (2012) 2001e2007
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of fluorene-derived PU as a thermo cross-linked
hole-transporting layer for PLED
,c,
Ching-Nan Chuang a, Chao-Hui Kuo a, Yu-Shan Cheng a, Chih-Kai Huang a, Man-kit Leung a
*
,
,b,
Kuo-Huang Hsieh a
*
a Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan
b Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
c Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel hole-transporting polyurethane, denoted as P1, resulting from the condensation of 9, 9-bis(4-
hydroxyphenyl)fluorene and isophorone diisocyanate (denoted as IPDI) has been developed. When P1
is thermally consolidated in the presence of 2-(phosphonooxy)ethyl methacrylate (P2M), it forms
a distinguished hole-transport layer that leads to an extremely good performance of the phosphorescent
PLED. In the study, the device of ITO/PEDOT: PSS/P1-P2M/Ir(ppy)3-t-PBD-PVK/Mg/Ag shows a high
current efficiency of 27.6 cd/A and a low turn-on voltage of 6 V. In particular, the stable output efficiency
of 17e22 cd/A within the range of 420e4400 cd/m2 at 12e20 V makes P1 a promising hole-transport
material for phosphorescent PLED applications.
Received 19 December 2011
Received in revised form
5 March 2012
Accepted 8 March 2012
Available online 15 March 2012
Keywords:
PLED
Polyurethane
Hole-transporting material
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
optimized of the device structure. For the first strategy, the incorpo-
ration of hole-transporting moieties on a main or side chain, such as
Organic electronic polymers have attracted considerable interest
because of theirapplications inpolymer light-emitting diodes (PLEDs)
[1e5], organic thin-film transistors (OTFTs) [6e8], flexible flat-panel
display [9] and photovoltaic cells [10,11]. Among the classes of elec-
troluminescent(EL)conjugated polymers,poly(p-phenylenevinylene)
s (PPVs) [12] and poly(9,9-dialkylfluorene)s (PFs) [13] are two families
that have attracted a lot of attention during the past decade. PFs and
their derivatives are promising blue-light-emitting materials that are
widely used in PLEDs due to their excellent thermal and chemical
stability, as well as their high photoluminescence (PL) quantum yield
triphenylamine, is usually adopted to improve hole-injection from
anode [24,25]. For the second strategy, multilayer devices are required
and fabricated by adding an extra hole-transporting layer (HTL),
such as poly(styrenesulphonate):poly(3,4-ethylenedioxythiophene)
(PEDOT: PSS), to reduce the hole-injection barrier from anode [26,27].
An ideal device should have a smooth charge carrier injection, and
balanced charge transport properties in the polymer layer. The pres-
enceof a high injection barrierusuallyresults ina highdriving voltage,
which leads to an increased thermal loading for the polymer layer;
therefore, a good hole-transporting layer (HTL) plays a very important
role in fabricating a high efficiency multilayer PLED. It bridges the
hole-injection from an indium tin oxide (ITO) anode into a light-
emitting layer (EML), which results in balanced charge-injection/
transport and better device performance. Material to be used as an
efficient HTL in multilayer PLEDs has to possess very good solvent
resistance for multilayer processing. To achieve this purpose, either
photo- or thermally cross-linked hole-transporting materials or
a suitable solvent combination are usually employed to consolidate
the bottom layer [28e37]. Recently, Wong reported that fluorene
derivatives show good charge transport behavior [38,39]. Certain
challenges are posed, however, for carrier-transport properties that
arebothfundamentalandpracticalinterest, becauselinearconjugated
polymers in films generally have a strong tendency to form crystalline
domains. Although sporadic reports exist oncarrier-transport
[14e17] and p-electron excessive (electron-rich) in nature and hence
have better hole-injection and hole-transporting ability [18e21].
However, the mismatched HOMO energy level of 5.8 eV [22,23] builds
up a high hole-injection barrier, and an unbalance charge injection
could lead to poor performance of the organic electronic devices. To
solve this problem, there are two strategies have been adopted, one is
through appropriate design of chemical structure and the other is
* Corresponding authors. Institute of Polymer Science and Engineering, National
Taiwan University, Taipei 106, Taiwan. Tel.: þ886
2
33663044; fax: þ886
2
33661673.
E-mail addresses: mkleung@ntu.edu.tw (M.-k. Leung), khhsieh@ntu.edu.tw
(K.-H. Hsieh).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.03.023

2002
C.-N. Chuang et al. / Polymer 53 (2012) 2001e2007
freshly distilled before use. Tetrabutylammonium perchlorate
OH
HO
(TBAP) was recrystallized twice from ethyl acetate and vacuum-
dried for two additional days. The other chemicals were used
without further purification. The chemical structures for all of the
products were confirmed by 1H NMR spectroscopy, mass spectra
(FAB) and elemental analyses.
N
N
HO
OH
M2
M1
2.2. Characterization methods
Scheme 1. Structure of M1 and M2.
1H NMR spectra were measured on a Bruker 400 MHz spec-
trometer. Elemental analyses were measured using an EA Heraeus
Vario EL-3 analyzer. FT-IR spectra were recorded on a Jasco-480
spectrometer. UVeVis analyses were obtained from a Jasco V-570
UVeVis spectrophotometer. The number-average and weight-
average molecular weight of the polymers were determined by
properties as a function of polymer lengths in crystalline states of
some conjugated polymers, such as polythiophenes and polyacenes
[40e44], however, they give no representation of the generally
amorphous (disordered) situation in polymer films. To obtain a truly
amorphous film to study, diaryl substituents have been introduced at
the C9 of fluorene, leading to enhanced morphological stability of the
amorphous phaseandexhibiting intriguing non-dispersive ambipolar
carrier-transport properties. The other method was to isolate the
function group, such as ether linkage, silane, alkyl chain, urethane, etc.
The usage of this kind of linkage realizes an unlimited mixing of active
side groups and causes a stable morphology, since both migration and
aggregation are significantlysuppressed by fixing theactive molecules
as side groups [45,46].
Polyurethanes (PUs) are common polymers that are widely used
in industrial applications due to their properties: elasticity, flexi-
bility, thermal stability, and excellent chemical resistance [47e52].
Several studies on the applications of PUs on PLEDs have recently
been reported [53e57]. In our previous reports [36,47,54,55], we
demonstrated that polymers with urethane linkages enhance hole-
injection performance between PEDOT: PSS and EML.
In this work, two series of polyurethanes were synthesized and
characterized: the first series comprised the homopolymers of
fluorene type polyurethane (P1) and triphenylamine type poly-
urethane (P5). The second series comprised the copolymers of
triphenylamine-co-9, 9-bis(4-hydroxyphenyl)fluorene derivatives
(P2eP4) [58e62]. PUs were ideal candidates in our study due to
their metal-ion-free synthetic pathway. A low level of metal-ion
contaminants is an essential requirement for high performance
electronic polymers. PUs were prepared from condensation of diols
and diisocyanates, in which no metal-ion-containing reagents were
involved so that metal-ion contaminations could be avoided.
a Waters GPC-480 system with a column of AM GPC Gel (10 mm)
from the American Polymer Standard Company. Dimethylforma-
mide (DMF) was used as eluent and polystyrene as the standard in
the GPC experiments. TGA and DSC were performed on a TGA
PerkineElmer TGA-7 and a DSC Du Pont 2010 analyzer under
nitrogen atmospheric conditions at a heating rate of 10 ꢀC minꢁ1
.
The thickness of the polymer films was measured using an Alpha
step Dektak 3030 profilometer. PL spectra of the polymers were
recorded using a Hitachi-4500 spectrofluorometer. Cyclic voltam-
metric measurements of the material were made in DMF with 0.1 M
tetrabutylammonium perchlorate (TBAP) as the supporting elec-
trolyte at a scan rate of 100 mV/s. Platinum wires were used as both
the counter and working electrodes, and silver/silver ions (Ag in
0.1 M AgCl solution) was used as the reference electrode. Ferrocene
was used as an internal standard, and the potential values were
obtained and converted to vs. SCE (saturated calomel electrode)
[63]. Electroluminscence was recorded on a Minolta CS-100A
instrument. The IeV and LeV characteristics of the devices were
measured by integrating a Keithley 2400 source-meter as the
voltage and current source and a Minolta CS-100A instrument as
the Luminance detector. All of the measurements and device
fabrications were performed at room temperature in a dust-
controlled environment.
2.3. Device fabrication
Two kinds of device structures were adopted in this study. The
first type of device was fabricated with ITO/PEDOT: PSS/PUs/
Ir(ppy)3 þ PVKþt-PBD/Mg/Ag where there was no thermal cross-
linked agent contained in the PUs layer. The second device con-
tained a cross-linked agent [2-(phosphonooxy)ethyl methacrylate
(P2M)] in the PU layer which was thermally cross-linked prior to
metal vapor deposition. The ITO surface was cleaned by sonication,
and rinsed sequentially in de-ionized water, Triton-100 water
solution, de-ionized water, acetone, and methanol. For the previous
structure, the hole-injection material: PEDOT: PSS was spin-coated
2. Experimental
2.1. Materials
N, N0-bis(4-hydoxyphenyl)-N, N0-diphenylbenzidine was
prepared in our previous work [55]. Reagent grade chemicals and
solvents were purchased from Aldrich, ACROS, Fluka, and Lancaster
Chemical Co. THF, dichloromethane and DMF were dried over
sodium/benzophenone, P2O5 and calcium hydride respectively and
OH
NCO
HO
O
1.DMF , 50 oC,24h
NH
O
+
OCN
NH
2.
O
O
OH
X
P1
Scheme 2. PUs synthetic route of the fluorene type (P1).

C.-N. Chuang et al. / Polymer 53 (2012) 2001e2007
2003
N
O
O
NH
O
HN
O
HN
O
N
O
H
N
O
O
X
Y
Feeding ratio
P2 (X:Y = 75 : 25 )
P3 (X:Y = 50 : 50 )
P4 (X:Y = 25 : 75 )
P5 (X:Y = 0 : 100 )
Scheme 3. PUs structure of copolymers: P2eP5.
at 5000 rpm, 60 s, on top of the ITO glass and dried on a hot plate at
130 ꢀC for 30 min within a vacuum. The PU solution was spin-
coated at 4000 rpm, 60 s, onto the prepared ITO/PEDOT-PSS
anode and dried under reduced pressure for 30 min at 120 ꢀC; it
contained 2.5% P2M of PU which was spin-coated at 4000 rpm, 60 s
and dried under reduced pressure for 30 min at 150 ꢀC in order to
form cross-linkable film. The emitting layer solution of concentra-
isophorone diisocyanate (IPDI, 5.25 mmol) and dried DMF (25 mL)
were charged in a two-necked flask, and stirred for 24 h at 50 ꢀC
under nitrogen. 4-tert-Butylphenol (0.5 mL) was added and stirred
for 24 h to terminate the reaction. When the reaction was complete,
the reaction mixture was poured into methanol and the desired PU
would precipitate. The PU was then collected and purified by dis-
solving in DMF and reprecipitated from methanol several times.
The synthesis process is illustrated in Scheme 2. The polymers were
characterized by 1H NMR and FT-IR.
tion of Ir(ppy)3 (4 mg)
þ
PVK(50 mg)
þ
t-PBD(20 mg)/in
chloroform(4 mL) was spin-coated at 3000 rpm, 90 s, on the surface
of ITO/PEDOT: PSS/PU. The Mg and Ag contacts were deposited on
ITO/PEDOT-PSS/PU/EML at pressure below 10ꢁ6 torr. The deposi-
tion rates of Mg and Ag cathodes were 1.0 Å/s and 4.0 Å/s, giving an
active area of 0.125 cm2.
P1: Anal. Calcd: C, 76.86; H, 6.64; N, 5.63%. Found: C, 75.94; H,
7.24; N: 5.03%. 1H NMR (400 MHz in DMSO-d6):
d7.90e7.88 (m, 2H),
7.40e6.98 (m, 8H), 6.64e6.62 (m, 2H), 1.52e1.26 (m, 6H), 1e0.87
(m, 9H), IR(KBr): 3318, 1725, 1600, 1450, 820.
P5: Anal. Calcd: C, 74.1; H, 6.60; N, 9.0%. Found: C, 73.9; H, 6.84;
N: 8.8%. 1H NMR (400 MHz, DMSO-d6):
d7.6e7.4 (m, 4H), 7.3e7.2
(m, 4H), 7.1e6.9 (m, 18H), 3.7e3.6 (m, 4H), 1.2e0.7 (m, 15H), IR
(KBr): 3321, 1725, 1600, 1450, 820.
P2, P3 and P4: 9, 9-Bis(4-hydroxyphenyl)fluorene and N,N’-
bis(4-hydoxyphenyl)-N,N0-diphenylbenzidine with the feeding
ratio of 3:1, 1:1 and 1:3, IPDI and dried DMF were charged in a two-
necked flask, and stirred for 24 h at 50 ꢀC under nitrogen. 4-tert-
Butylphenol was added and stirred for 24 h to terminate the
reaction. When the reaction was complete, the reaction mixture
2.4. Monomer synthesis
9,9-Bis(4-hydroxyphenyl)fluorene (M1) was prepared from the
condensation of 9-fluorenone and phenol using cationic ion-
exchange resins as catalysts in 83% yields [64e67]. M2 was
synthesized as described in our previous literature [55]. Scheme 1
is the structure of M1 and M2.
2.5. Synthesis of fluorene type polyurethane as hole-transporting
layer P1 to P5
1.0
P1
P2
P3
P4
P5
The general synthetic procedures for the polymers are described
as follows [54,55]:
P1 and P5: 9, 9-Bis(4-hydroxyphenyl)fluorene (5 mmol) or N,-
N’-bis(4-hydoxyphenyl)-N, N0-diphenylbenzidine (5 mmol),
Table 1
Average molecular weights and thermal properties of PUs: P1eP5.
0.5
Polymer
Mw
Mn
PDI
Tg (ꢀC)c
Td (ꢀC)b
a
P1
P2
P3
P4
P5
7900
5000
6300
7600
7500
5900
3500
4500
7200
4000
1.33
1.43
1.40
1.05
1.87
165
173
175
176
174
274
296
290
282
274
a
Determined by GPC by eluting with DMF, by comparison with polystyrene
0.0
standards.
250
300
350
400
450
500
b
Temperature at which a 5% weight loss occurred was determined at a heating
rate of 10 ꢀC/min under a nitrogen atmosphere.
Wavelength (nm)
c
The value of Tg was determined at a heating rate of 10 ꢀC/min under a nitrogen
atmosphere.
Fig. 1. UVeVis absorption spectra of P1eP5 in DMF solution.

2004
C.-N. Chuang et al. / Polymer 53 (2012) 2001e2007
10000
P1
P2
P3
P4
P5
1000
100
10
1
0
5
10
15
20
25
30
380
400
420
440
460
480
Voltage (V)
Wavelength (nm)
Fig. 4. The BeV of the P1eP5 in DP1 to DP5 including standard device STD: --- DP1,
-C- DP2, -:- DP3, - - DP4, -A- DP5, - - STD.
Fig. 2. PL spectra of P1eP5 in a solid film.
=
;
Table 2
Optical properties of PUs: P1eP5.
P4: Anal. Calcd: C, 74.39; H, 6.65; N, 7.25%. Found: C, 75.13; H,
6.8; N: 7.1%. 1H NMR (400 MHz in DMSO-d6):
7.96e7.88 (m, 16H),
7.37e7.18 (m, 17H), 6.96e6.6 (m, 15H), 1.7e1.4 (m, 12H), 1.27e0.7
d
Polymer
UVeVis lmax
PL lmax solutionb
Optical
Egap (eV)
HOMO
(eV)
LUMO
(eV)
solutiona (nm)
(nm)
(m, 18H), IR (KBr): 3324, 1725, 1600, 1450, 820.
P1
P2
P3
P4
P5
281(272)c
318(308)
318(308)
320(309)
315(305)
410(408)d
410(419)
413(418)
409(413)
414(420)
3.19
3.10
3.17
3.13
3.10
5.33
5.25
5.12
5.26
5.30
2.14
2.15
1.95
2.13
2.20
3. Results and discussion
3.1. Synthesis and characterization
a
Concentration: 1 ꢂ 10ꢁ5 M in DMF.
b
c
The condensation polymerization of the polyurethane in dried
DMF were synthesized with commercially available IPDI as the non-
conjugated linkages incorporation of 9,9-bis(4-hydroxyphenyl)flu-
orene (M1) and N,N0-bis(4-hydoxyphenyl)-N,N0-diphenylbenzidine
(M2), with feeding ratios of 1:0, 3:1, 1:1, 1:3 and 0:1 being intro-
duced. The molecular weight data of the prepared PUs including Mn,
Mw, PDI value were determined by gel-permeation chromatography
(GPC) analysis against polystyrene standard in DMF. In addition, the
GPC results and their thermal properties, including Tg and Td were
listed inTable 1. It can be seen that the Mn and Mw values of these PUs
are in the range of 3500e7200 and from 5000 to 7900, respectively,
with PDI less than 1.87.
Concentration: 1 ꢂ 10ꢁ6 M in DMF.
The value in parentheses represents the value of labsmax(nm) of the polymer as
a solid film.
d
The value in parentheses represents the value of lPLmax(nm) of the polymer as
a solid film.
was poured into methanol and the desired PUs would precipitate.
The PUs were collected and purified by dissolving in DMF and
reprecipitated from methanol and toluene for several times in
order to remove any oligomeric residue. Scheme 3 is the PU
structure of the copolymer P2eP5. The polymers were character-
ized by 1H NMR and FT-IR.
P2: Anal. Calcd: C, 74.59; H, 6.44; N, 7.74%. Found: C, 75.79; H,
6.76; N: 7.79%. 1H NMR (400 MHz in DMSO-d6):
d7.96e7.88 (m,
3.2. Thermal properties
16H), 7.37e7.18 (m, 17H), 6.97e6.6 (m, 15H), 1.52e1.22 (m, 12H),
1.3e0.7 (m, 18H), IR (KBr): 3318, 1725, 1600, 1450, 820.
The thermal behavior of the P1eP5 was measured by ther-
mogravimetric analysis (TGA) and differential scanning calorimetry
(DSC). The thermal decomposition temperatures (Td, 5% weight
loss) and the glass transition temperatures (Tg) of five PUs are also
listed in Table 1. All polymers showed high Tg’s above 160 ꢀC and
P3: Anal. Calcd: C, 74.39; H, 6.55; N, 7.5%. Found: C, 74.59; H,
6.78; N: 7.37%. 1H NMR (400 MHz in DMSO-d6):
d7.96e7.88 (m,
16H), 7.37e7.18 (m, 17H), 6.97e6.6 (m, 15H), 1.52e1.22 (m, 12H),
1.3e0.7 (m, 18H), IR (KBr): 3324, 1725, 1600, 1450, 820.
LUMO(eV)
1.95 eV
2.14 eV 2.15 eV
2.13 eV
2.20 eV
2.4 eV 2.4 eV
2.6 eV
PBD
Mg-Ag
3.7 eV
Ir(ppy)3
PVK
P4
P5
P1
P2
P3
ITO
PEDOT
5.0 eV
4.8 eV
5.12 eV
5.26 eV
5.33 eV
5.30 eV
5.25 eV
5.46 eV
5.90 eV
HOMO(eV)
6.2 eV
Fig. 3. Energy level diagram of the ITO/PEDOT: PSS/P1eP5/EML/Mg/Ag device.

C.-N. Chuang et al. / Polymer 53 (2012) 2001e2007
2005
35
30
25
20
15
10
5
300
400
500
600
700
800
0
Wavelength (nm)
0
5
10
15
20
25
Voltage (V)
Fig. 6. The electroluminescence spectra of P1eP5 in DP1 to DP5 including standard
device STD: --- DP1, -C- DP2, -:- DP3, - - DP4, -A- DP5, - - STD.
=
;
Fig. 5. The current efficiency of P1eP5 in DP1 to DP5 including standard device STD:
--- DP1, -C- DP2, -:- DP3, - - DP4, -A- DP5, - - STD.
=
;
3.4. Electrochemical properties
Td’s above 274 ꢀC. The high thermal resistance of the polymers
would be beneficial for the device fabrication, against the high
temperature conditions during the thermal vapor deposition of the
metallic cathode. Thermal stability of the polymer is an important
factor for PLED application. The thermal properties were highly
related to the performance as well as the lifetime of the devices.
Materials having too low of Tg and Td may decompose or cause
morphological change, deformation and degradation under oper-
ating conditions and thus create obstacles in transporting holes
and electrons in the device, hampering the device performance
[49,50].
Cyclic voltammetry measurements have previously been
demonstrated to investigate the energy levels of the three poly-
mers. When the applied potential reached the oxidation (reduc-
tion) potential, the redox reaction took place on the surface of the
electrode and redox current occurred. This critical potential is
termed “onset potential”. The onset potential was determined by
the obvious turning of the curve observed in the CV diagram. The
corresponding highest occupied molecular orbital (HOMO) was
estimated with standard ferrocene potential at 4.8 eV. The esti-
mated values of P1eP5 are shown in Table 2. Eg was determined by
the equation 1240/lonset(nm) [68,69], where the lonset was referred
to as the value of onset absorption wavelength. The lowest-
unoccupied molecular orbital (LUMO) energy levels were then
estimated according to the following equation: ELUMO ¼ EHOMOꢁEg.
According to the above analysis, the HOMO values of all these
polymers were found to be between 5.12 and 5.33. This result
indicates that these are potentially good hole-transport materials
for PLED devices. On the other hand, the LUMO values of these
polymers were found to be between 1.95 and 2.20. The optical
range gap of all these polyurethanes are within the range of
3.10e3.19 eV and the energy level of the device was therefore
constructed, as shown in Fig. 3.
3.3. Optical properties
The optical properties of P1eP5 were measured both in solution
and in solid thin-film. Transparent uniform films of P1eP5 were
prepared on a quartz substrate by spin-coating DMF solutions.
Their spectra are shown in Figs. 1 and 2. Table 2 summarizes their
absorption maxima. Homopolymer P1 (only fluorene type) shows
that absorption peaked at 272 nm. P2eP5 exhibited an additional
absorption band peaking in the region of 350 nm, which were
assigned to p-p* electronic transition of the triphenylamine group.
No spectral wavelength shift in their solution spectra was observed,
indicating that the intra-chain between triphenylamine and fluo-
rene was minimal. The triphenylamine absorption reduced along
with a decrease in the M2 content, which is consistent with their
feed ratios. The photoluminescence (PL) spectra of P1-P5 in solid
film are illustrated in Fig. 2. Their fluorescence peaked around
420 nm and exhibited similar band widths.
3.5. Electroluminescence properties
The first series of [ITO/PEDOT: PSS (30 nm)/PU (30 nm)/
Ir(ppy)3 þ PVKþt-PBD (65 nm)/Mg (2 nm)/Ag (100 nm)] PLEDs
were fabricated, using the successive layer-by- layer spin-coating
approach to establish
a multilayer structure. Although the
apparent electroluminescence performance of this series looked
Table 3
Devices performances of the P1eP5 in system (1).
Device
Turn-on voltage
at 1 cd/m2 (V)
Max. brightness
Efficiency (cd/A)/
voltage (v)
(cd/m2)/voltage (V)
DP1
DP2
DP3
DP4
DP5
STD
6.5
9
8
8.5
7.5
8
5390/21.5
6800/23
5540/23
5950/25.5
9910/21
5190/23
21.4/23.5
18.8/22
19.6/18.5
12.2/25.5
26.8/18
17.2/19
Scheme 4. Structure of cross-linking agent P2M.

2006
C.-N. Chuang et al. / Polymer 53 (2012) 2001e2007
Table 4
Device performance of P1eP5 in system (2).
30
Device
Turn-on voltage (V)
at 1 cd/m2
Max. brightness
Max. efficiency
(cd/A)/voltage (V)
(cd/m2)/voltage (V)
25
DDP1
DDP2
DDP3
DDP4
DDP5
STD
6
9
8.5
9
7.5
8
9560/21.5
8560/24.5
7290/23.5
7920/25
9530/19
5190/23
27.6/21.5
18/24.5
16.5/23.5
16.2/25
27/19
17.2/19
20
15
*Operational lifetime of DDP1 was compared with DP1 and STD (see Supplementary
information).
10
5
attractive, the device performance was relatively irregular. The
maximum luminescence obtained for DP1-DP5 respectively was
5390, 6800, 5540, 5950 and 9910 cd/m2, with maximum current
efficiency of 21.4, 18.8, 19.6, 12.2 and 26.8 cd/A, as illustrated in
Figs. 4 and 5. The brightness-voltage and current efficiency-voltage
results of the devices are summarized in Table 3. Since the PU layer
had not been thermally cross-linked during the layer-by-layer spin-
coating process, we suspected that a swelling or partial re-
dissolution of the PU film would occur during spin-coating of the
light-emitting layer, causing an irregularity in the performances of
the devices. The electroluminescence spectra of DP1-DP5 and STD
are shown in Fig. 6. The EL spectra illustrates that the maximum
emission peak locates at the 510 nm wavelength. This indicates the
hole-transport PU layer inserted between PEDOT: PSS and EML
wouldn’t affect the EML emission.
However, it is noteworthy to point out that the turn-on voltage
for the device of DP1 was particularly low. In our study, the turn-on
voltage was reduced down to 6.5 V, which is about 1.5 V lower than
that of the STD device (8V). The efficiency was up to 21.4 cd/A which
was higher than that of 17.2 cd/A for the STD device. More inter-
esting is the stable output efficiency between voltages of 10 V and
20 V, corresponded to the EL output range of 100e4000 cd/m2. The
unexpectedly good outcomes from the DP1 device encouraged us to
further explore any possibilities to improve the construction of the
device.
0
0
5
10
15
20
25
Voltage (V)
Fig. 8. The current efficiency of P1eP5 in DDP1 to DDP5 including standard device
STD: --- DDP1, -C- DDP2, -:- DDP3, - - DDP4, -A- DDP5, - - STD.
=
;
presence of the phosphate group on P2M helped the cross-linking
agent to be doped into the PEDOT: PSS layer, thereby improving the
surface contact between the PEDOT: PSS and the PU layers.
Table 4 summarizes the brightness and current efficiency of the
DDP1-DDP3 devices. The results are also illustrated in Figs. 7 and 8
respectively. Luminescence levels of up to 9560, 8560, 7290, 7920
and 9530 cd/m2 were recorded for devices DDP1-DDP5. The
maximum current efficiency of the devices are 27.6, 18, 16.5, 16.2
and 27 cd/A, under operating voltages of: 21.5, 24.5, 23.5, 25 and
19 V, respectively. It is noteworthy to point out while the perfor-
mance of DDP2-DDP5 were similar, as shown in Figs. 7 and 8, the
performance of DDP1 stood out from the others. The DDP1 device
exhibited significant improvement in current efficiency (27.6 cd/A),
higher than STD by 17.2 cd/A, with a turn-on voltage drop to 6 V.
Particularly, the low turn-on voltage of 6 V and the stable output
efficiency of 17e22 cd/A within the range of 420e4400 cd/m2 at
12e20 V suggested that P1 was an extremely effective hole-
transport material. On the other hand, the performance of the M2
containing polymers was relatively poor, in comparison to that of
P1. It is probably due to the electron-rich structure of M2 which can
form very stable radical cations after oxidation and therefore
possibly hamper the hole-mobility of the polymer matrix. In
To solve the problem of the partial re-dissolution of PU film
during the layer-by- layer spin-coating process, the PU layer was
blended with 2-(phosphonooxy)ethyl methacrylate (P2M, 2.5 wt%,
as shown in Scheme 4) as a cross-linking agent. The layer was
thermally treated at 150 ꢀC for 30 min to induce cross-linking
before the light-emitting layer was spin-coated on top. The
10000
1000
100
10
1
0
5
10
15
20
25
300
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Fig. 7. The BeV of the P1eP5 in DDP1 to DDP5 including standard device STD: ---
DDP1, -C- DDP2, -:- DDP3, - - DDP4, -A- DDP5, - - STD.
Fig. 9. The electroluminescence spectra of P1eP5 in DDP1 to DDP5 including standard
device STD: --- DDP1, -C- DDP2, -:- DDP3, - - DDP4, -A- DDP5, - - STD.
=
=
;
;

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This work was supported by the Ministry of Economic Affairs
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