Copolyphenylenes with pendant benzimidazolyl and diethanolaminohexyloxy groups: Synthesis and electron-transporting application in PLEDs

Article abstract of DOI:10.1002/pola.28642

Two new electron-transporting copolyphenylenes P1NH and P2NH possessing balanced charges crucial to emission efficiency of polymer light-emitting diodes (PLEDs) have been synthesized and applied as an electron-transporting layer (ETL). The main chain structure is all para-linkage for P1NH and both para- and meta-linkage for P2NH, with the same pendant electron-withdrawing benzimidazolyl and polar diethanolaminohexyloxy groups. Both copolymers possess excellent thermal stability (T_d > 300 °C, T_g > 100 °C) due to their rigid backbones. In addition, the pendant groups effectively lower LUMO (～ ?2.70 eV) and HOMO (～ ?5.70 eV) levels, resulting in improved electron-transporting and hole-blocking capabilities. Multilayer yellow-emitting PLEDs with a configuration of ITO/PEDOT:PSS/SY/ETL/LiF/Al were successfully fabricated by the spin-coating process. The maximum luminance and maximum current efficiency of the P1NH-based device were 12,881 cd/m² and 10.94 cd/A, respectively, superior to the performance of P2NH-based device (4938 cd/m², 3.70 cd/A) and the device without ETL (8690 cd/m², 2.78 cd/A). Current results indicate that P1NH is highly effective in enhancing electron transport and device performance.

Full text of DOI:10.1002/pola.28642

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Copolyphenylenes with Pendant Benzimidazolyl and Diethanolaminohexyloxy
Groups: Synthesis and Electron-Transporting Application in PLEDs
Chih-Yang Tseng, Wen-Fen Su, Yun Chen
National Cheng Kung University, Department of Chemical Engineering, No. 1, University Road, Tainan 701, Taiwan
Correspondence to: Y. Chen (E-mail: yunchen@mail.ncku.edu.tw)
Received 26 March 2017; accepted 18 April 2017; published online 00 Month 2017
DOI: 10.1002/pola.28642
ABSTRACT: Two new electron-transporting copolyphenylenes
P1NH and P2NH possessing balanced charges crucial to emis-
sion efficiency of polymer light-emitting diodes (PLEDs) have
been synthesized and applied as an electron-transporting layer
ITO/PEDOT:PSS/SY/ETL/LiF/Al were successfully fabricated by
the spin-coating process. The maximum luminance and maxi-
mum current efficiency of the P1NH-based device were
2
12,881 cd/m and 10.94 cd/A, respectively, superior to the per-
2
(
ETL). The main chain structure is all para-linkage for P1NH
formance of P2NH-based device (4938 cd/m , 3.70 cd/A) and
2
and both para- and meta-linkage for P2NH, with the same pen-
dant electron-withdrawing benzimidazolyl and polar diethano-
laminohexyloxy groups. Both copolymers possess excellent
the device without ETL (8690 cd/m , 2.78 cd/A). Current results
indicate that P1NH is highly effective in enhancing electron
transport and device performance. V
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Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 00, 000–000
d g
thermal stability (T > 300 8C, T > 100 8C) due to their rigid
backbones. In addition, the pendant groups effectively lower
LUMO (ꢀ 22.70 eV) and HOMO (ꢀ 25.70 eV) levels, resulting
in improved electron-transporting and hole-blocking capabili-
ties. Multilayer yellow-emitting PLEDs with a configuration of
KEYWORDS: benzimidazolyl; copolyphenylene; charge transport;
diethanolaminohexyloxy; electron-transporting; light-emitting
diodes (LED); luminescence; PLEDs; solution process
INTRODUCTION Polymer light-emitting diodes (PLEDs) have
drawn considerable attention due to their bright prospect in
applications to large-area flexible displays and solid-state
metals, such as Al and Ag, as cathode. However, large barrier
height greatly hinders electron-injection from such cathodes.
Constantly, insertion of an electron-transporting layer (ETL)
between the emission layer and the cathode effectively pro-
1
–3
lighting.
Moreover, PLEDs are readily fabricated by solu-
9
4
,5
motes electron injection and transport. Promotion in electron
tion processes such as ink-jet printing and spin-coating,
injection and transport effectively balances carriers in the EML
to enhance device performance, because holes transport faster
than electrons in most emitting polymers. Accordingly, the
effective electron-transporting material is crucial for the fabri-
cation of highly efficient multilayer PLEDs.
which are usually difficult to be applied in the fabrication of
organic light-emitting diodes (OLEDs). Thermal evaporation
under high vacuum has been the common fabrication pro-
cess for the organic layers in OLEDs. However, there are
some disadvantages intrinsic to polymeric materials used in
PLEDs, which prevent their direct and widespread applica-
tions. For example, purification of polymer and control of
batch-to-batch variation are much more difficult than low-
molecular-weight counterparts are. Moreover, the hole-
transporting ability of electroluminescent polymers applied
in PLEDs is generally higher than electron-transporting abil-
ity, leading to an imbalance in electrons and holes in the
An ideal electron-transporting material should possess
appropriate lowest unoccupied molecular orbital (LUMO)
10
and highest occupied molecular orbital (HOMO) levels,
which determine its reduction and oxidation capability,
respectively, under an external bias. Lower LUMO level facili-
tates electron injection from the cathode, whereas deeper
HOMO level blocks holes from entering the ETL (confined in
the EML near interface). Electron-withdrawing moieties such
6
emission layer (EML). Consequently, low work function met-
1
1,12
13,14
als such as barium and calcium are widely employed as a
cathode to promote electron injection. However, the low
work-function metals are usually reactive, leading to unstable
as oxadiazolyl,
tives
triazolyl,
and benzimidazolyl deriva-
1
5,16
were constantly used to design electron-transporting
materials. Electron-withdrawing substituents in a compound
usually depress its HOMO and LUMO levels. Low LUMO level
is effective in promoting electron injection/transport because
7,8
under ambient conditions containing water and oxygen.
Therefore, it is highly desirable to use high work-function
Additional Supporting Information may be found in the online version of this article.
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of diminished energy barrier between adjacent bilayers. How-
electrode. The electrodes were immersed in an acetonitrile
17,18
ever, deep HOMO invariably blocks holes,
which effectively
solution containing 0.1 M (n-Bu) NClO4 as the electrolyte.
4
increases recombination ratio of the carriers. On the other
hand, electron-transporting materials are usually high polar
substituents, such as hydroxy, diethanolamino, and phosphonate
groups. This is because the dipole–dipole interaction between
The energy levels were calculated by using the ferrocene
(FOC) value of 24.8 eV with respect to the vacuum level,
which is defined as zero. The onset potentials of oxidation
and reduction were estimated by the intersection of two tan-
gent lines, which were drawn at the background and rising
current. The thickness of deposited films was determined
19–21
cathode and ETL not only facilitates electron injection
but
20,22
also confines the holes in the interface.
2,9-Dimethyl-4,7-
V
R
diphenyl-1,10-phenanthroline (BCP) and 2,2,2-(1,3,5-phenylene)-
tris(1-phenyl-1H-benzimidazole) (TPBI) were widely used
electron-transporting low molecular weight materials, whose
films were prepared by vacuum thermal deposition.
using a surface profiler (Alpha-Step 500). Surface morphol-
ogy of spin-coated polymer films was measured with an
atomic force microscope (AFM), equipped with a Veeco/
Digital Instrument Scanning Probe Microscope (tapping
mode) and a Nanoscope III controller.
17
In this study, we present the synthesis and optoelectronic
properties of two new electron-transporting random copoly-
phenylenes (P1NH and P2NH) attached with both electron-
withdrawing benzimidazolyl and polar diethanolaminohexy-
loxy pendant groups. The pendant groups not only facilitate
transport of electrons but also block holes in the interface.
Moreover, the copolyphenylenes are readily soluble in a mix-
Synthesis
All the reagents and materials were from commercial sources
and used without further purification unless otherwise noted.
Anhydrous tetrahydrofuran was obtained by distilling with
sodium under the nitrogen blanket overnight. Scheme 1 depicts
the synthetic routes for dibromo monomer M1, M2, and M3,
from which the random precursor copolymers P1 and P2 were
prepared (Scheme 2). Finally, the target electron-transporting
random copolymers P1NH and P2NH were synthesized from
P1 and P2, respectively, as shown in Scheme 3.
ture of C H Cl and isopropanol (v/v 5 2/5), which is an
2
2
4
orthogonal solvent to the emission layer, a poly(para-phenyl-
ene vinylene) copolymer “Super Yellow” (SY). Therefore,
multilayer PLEDs can be readily fabricated by sequential
spin-coating of PEDOT:PSS, SY, and ETL. Results of cyclic vol-
tammetric measurements substantiate that the LUMO and
HOMO levels are appropriate for the PLEDs, facilitating elec-
tron injection/transport and hole blocking, respectively, to
improve charge recombination. Especially, P1NH enhances
device performance significantly when used as an electron-
transporting layer.
2-(2,5-Dibromophenyl)-1-Phenyl-1H-Benzo[d]Imidazole
(M1)
The hydrolysis of methyl 2,5-dibromobenzoate (1) was con-
ducted in an aqueous solution of sodium hydroxide for 6 h,
followed by neutralization with HCl(aq) (1N) to obtain the
white powder of 3,5-dibromobenzoic acid. The 3,5-dibromo-
benzoic acid (2.799 g, 10 mmol) was added to thionyl chlo-
ride (10 mL), and the mixture was refluxed under the
nitrogen atmosphere for 4 h. After removal of residual thi-
onyl chloride by distillation, it was added with N-phenylben-
zene-1,2-diamine (1.843 g, 10 mmol), and triethylamine
EXPERIMENTAL
General Procedures
1
H NMR spectra were measured on a Bruker AV-500 MHz
spectrometer, with the chemical shifts reported in ppm using
tetramethylsilane (TMS) as an internal standard. Elemental
analysis (EA) of carbon, nitrogen, and hydrogen were per-
formed on an elemental analyzer (elementar vario EL III)
using acetanilide, nicotin amide and sulfanilic acid as the
standards. Thermogravimetric analysis (TGA) was conducted
under a nitrogen atmosphere using a PerkinElmer TGA-7
thermal analyzer, with a linear heating rate of 10 K/min
from room temperature to 800 8C. Thermal decomposition
(
2 mL), and dried tetrahydrofuran (THF, 40 mL). The mix-
ture was stirred at room temperature for 24 h, and then the
solvent was removed using a rotary evaporator. The residue
was dissolved in dichloromethane and extracted with water.
The organic phase was dried over anhydrous MgSO , and the
solvent was removed in vacuo. Adding acetic acid to the resi-
due and refluxed for 12 h. After cooling to room tempera-
ture, the mixture was precipitated from H O. The appearing
precipitates were collected by filtration and further purified
by column chromatography on SiO , using ethyl acetate and
4
2
temperature (T ) is the temperature at 5% weight loss of a
d
2
sample. Differential scanning calorimetric (DSC) measure-
ments were performed on a Mettler Toledo DSC 1 Star Sys-
tem, under a nitrogen atmosphere at a heating rate of 15 K/
n-hexane (v/v 5 1:4) as eluent to afford white powder of M1
(
2.183 g, 50.9%).
min. Glass transition temperature (T ) was determined from
1
g
H NMR (500 MHz, DMSO-d ): d (ppm) 7.97–7.91 (m, 1H),
6
the second heating scan. Uv/vis absorption spectra and pho-
toluminescence (PL) spectra were recorded on a Jasco V-550
spectrophotometer and a Hitachi F-4500 fluorescence spec-
trophotometer, respectively. Cyclic voltammograms (CVs)
were measured with a voltammetric apparatus (model CV-
7
7
.84–7.78 (d, 1H), 7.60–7.55 (m, 2H), 7.52–7.46 (m, 2H),
.45–7.30 (m, 6H). Anal. Calcd. (%) for C H Br N : C,
1
9
12
2 2
53.30; H, 2.83; N, 6.54. Found: C, 53.28; H, 2.82; N, 6.55.
2-(3,5-Dibromophenyl)-1-Phenyl-1H-Benzo[d]Imidazole
(M2)
50W from BAS) equipped with a three-electrode cell, at a
scan rate of 100 mV/s. The cell was composed of carbon as
the working electrode, an Ag/AgNO3 electrode as the refer-
ence electrode, and a platinum wire as the auxiliary
To a two-necked glass reactor were added with 3,5-dibromo-
benzoic acid (2: 2.801 g, 10 mmol) and thionyl chloride
(15 mL); the mixture was refluxed under nitrogen for 4 h.
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SCHEME 1 Synthesis of dibromo monomers M1, M2, and M3.
After the thionyl chloride was removed by distillation, N-phe-
hexane (v/v 5 1:4) as eluent, to afford white powder of M2
nylbenzene-1,2-diamine (1.842 g, 10 mmol), triethylamine
(2.562 g, 59.8%).
(
2 mL) and dried THF (40 mL) were added, and then stirred
1
6
H NMR (500 MHz, DMSO-d ): d (ppm) 7.91–7.88 (m, 1H),
.83–7.79 (d, 1H), 7.65–7.57 (m, 5H), 7.51–7.47 (m, 2H),
.37–7.29 (m, 2H), 7.22–7.19 (d, 1H). Anal. Calcd. (%) for
at room temperature for 24 h. After removing THF by a
rotary evaporator, the residue was dissolved in dichlorome-
thane and then extracted with water. The organic phase was
dried over anhydrous MgSO , and then the solvent was
removed in vacuo. The residue was added with acetic acid
7
7
C H Br N : C, 53.30; H, 2.83; N, 6.54. Found: C, 53.29; H,
2
19 12
2 2
4
.85; N, 6.57.
and refluxed for 12 h. The reaction mixture was poured into
a large amount of H
The appearing precipitates were collected and further puri-
fied by column chromatography, using ethyl acetate and n-
2
O after cooling to room temperature.
1,4-Bis(6-Bromohexyloxy)-2,5-Dibromobenzene (M3)
To a three-necked glass reactor were added 2,5-dibromohy-
droquinone (3: 1.072 mL, 4 mmol), 1,6-dibromohexane
SCHEME 2 Synthesis of random precursor copolymers P1 and P2.
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SCHEME 3 Synthesis of random copolymers P1NH and P2NH.
1
(
(
2.5 mL, 16 mmol), K CO (1.678 g, 12 mmol), and acetone
45 mL). The mixture was stirred at 60 8C under the nitro-
H NMR (500 MHz, DMSO-d ): d (ppm) 8.26–6.43 (m, 30H),
2
3
6
4.10–3.82 (m, 4H), 3.48–3.30 (m, 4H), 1.78–1.60 (m, 8H),
1.42–1.28 (m, 8H).
gen atmosphere for 15 h. It was poured into a large amount
of water and extracted with chloroform. The organic layer
was collected and dehydrated over anhydrous MgSO , fol-
4
Precursor Copolymer P2
lowed with the removal of the solvent by a rotary evapora-
tor. The resulted residue was further purified by column
To a two-necked glass reactor were added with M2 (0.856 g,
2
mmol), M3 (1.188 g, 2 mmol), 1,4-benzenediboronic acid
chromatography on Al2O using ethyl acetate and n-hexane
3
bis(pinacol) ester (4: 1.320 g, 4 mmol), K CO (aq) (2 M, 6
2
3
(v/v 5 1:6) as eluent, to afford white powder of M3 (1.283 g,
mL), Pd(PPh ) (72 mg), and p-dioxane (60 mL). The follow-
3
4
5
4.1%).
ing reaction and purification procedures were analogous to
those employed in the synthesis of P1. The product was pale
yellow powder of P2 (1.171 g, 68.2%).
1
H NMR (500 MHz, Acetone-d ): d (ppm) 7.27–7.25 (s, 2H),
6
4.07–4.02 (t, 4H), 3.52–3.47 (t, 4), 1.91–1.85 (m, 4H), 1.83–
1
1.76 (m, 4H), 1.58–1.48 (m, 8H). Anal. Calcd. (%) for
3
H NMR (500 MHz, CDCl ): d (ppm) 8.05–6.92 (m, 28H),
C H Br O : C, 36.40; H, 4.41. Found: C, 36.41; H, 4.46.
1
8
26
4
2
4.06–3.85 (m, 4H), 3.56–3.30 (m, 4H), 1.88–1.70 (m, 8H),
.48–1.35 (m, 8H).
1
Precursor Copolymer P1
To two-necked glass reactor were added with M1
0.476 g), M3 (0.661 g), 1,4-benzenediboronic acid bis(pina-
col) ester (4: 0.734 g), K CO (2M, 4 mL), Pd(PPh3)4
a
Copolymer P1NH
To two-necked glass reactor were added with P1
(
a
(200 mg), diethanolamine (1000 mg), dried THF (20 mL)
and N,N-dimethylformamide (DMF, 5 mL). The mixture was
stirred at 60 8C under the nitrogen atmosphere for 48 h.
After cooling to room temperature, the mixture was poured
into a large amount of water. The appearing precipitates
were collected by filtration, dissolved in chloroform and
extracted with water. The organic phase was collected and
filtered through a Celite pad. The chloroform was removed
in vacuo to obtain a residue, which was re-dissolved in DMF
and precipitated from n-hexane to afford white powder of
P1NH (158 mg, 74.5%).
2
3(aq)
(
40 mg) and p-dioxane (40 mL). The mixture was stirred at
90 8C under the nitrogen atmosphere for 23 h. Then bromo-
benzene and phenylboronic acid pinacol ester were succes-
sively injected into the mixture to end-cap the resulting
polymers. The reaction time was 2 h after each injection.
After cooling to room temperature, the mixture was precipi-
tated from a mixture of methanol and water (v/v 5 10:1).
The appearing precipitates were sequentially washed with
methanol, water, and acetone to get pale yellow powder of
P1 (0.582 g, 67.8%).
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1
2 2
FIGURE 1 H NMR spectrum of P2NH in CD Cl . [Color figure can be viewed at wileyonlinelibrary.com]
1
H NMR (500 MHz, CD Cl ): d (ppm) 8.28–6.36 (m, 34H),
.05–3.90 (m, 4H), 3.60–3.40 (m, 8H), 2.68–2.42 (m, 12H),
.86–1.68 (m, 4H), 1.54–1.22 (m, 12H).
deposited onto the ETL as the cathode by vacuum thermal
2
2
2
6
4
1
deposition (10
torr). The luminance (L)-voltage, (V)-cur-
rent density, (J) characteristics, and electroluminescence
spectra (EL) of the devices were recorded using a combina-
tion of a Keithley 2400 source meter and an Ocean Optics
usb2000 fluorescence spectrophotometer.
Copolymer P2NH
To a two-necked glass reactor were added with P2 (400 mg),
diethanolamine (2000 mg), dried THF (30 mL) and DMF
(
7.5 mL). The following reaction and purification procedures
RESULTS AND DISCUSSION
were analogous to those employed in the synthesis of P1NH.
The product was white powder of P2NH (382 mg, 90.5%).
Synthesis and Characterization
The electron-transporting random copolyphenylenes P1NH
and P2NH with pendant benzimidazolyl and diethanolamino-
hexyloxy groups were prepared from precursor polymers P1
and P2, respectively (Scheme 3). The P1 was synthesized
from M1, M3 and 1,4-benzenediboronic acid bis(pinacol) ester
1
H NMR (500 MHz, CD Cl ): d (ppm) 8.10–6.90 (m, 34H),
.06–3.88 (m, 4H), 3.70–3.40 (m, 8H), 2.68–2.46 (m, 12H),
.87–1.66 (m, 4H), 1.58–1.26 (m, 12H).
2
2
4
1
Device Fabrication and Characterization
(4) by the Suzuki coupling reaction, whereas the P2 was pre-
Patterned indium tin oxide (ITO) glass substrate was cleaned
sequentially in an ultrasonic bath (50 8C) with neutral deter-
gent, deionized water, acetone, and isopropyl alcohol. The
cleaning time was 20 min for each step, followed by rinsing
with deionized water. Then the substrate was baked at 120 8C
for 20 min to remove the deionized water, followed by treat-
ing in a UV-ozone chamber for 20 min. The hole-injection
layer (HIL) was spin-coated (3000 rpm) from a solution
of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
pared from the coupling of M2 and M3 with 4 (Scheme 2).
The weight-average molecular weight (polydispersity index)
of P1 and P2 were 4405 g/mol (1.41) and 7908 g/mol
(
1.49), respectively. In the fabrication of multilayer PLEDs, the
electron-transporting polymers should be soluble in high
polar solvents such as methanol and isopropanol. The high
polar alcoholic solvents are unable to dissolve the SY emission
layer (orthogonal solvents) that they are suitable for the prep-
aration of ETL solution. Otherwise, the solvents used in the
coating of the ETL will re-dissolve or damage the emission
layer (SY). Accordingly, P1 and P2 were further modified with
surfactant-like diethanolaminohexyloxy pendant groups to
(
PEDOT:PSS, from Bayer) onto the cleaned ITO surface and
then annealed at 150 8C for 20 min. The emission layers
were spin-coated (4000 rpm) from a solution of poly(para-
phenylene vinylene) derivative, Super Yellow (SY) from
Merck, on top of the PEDOT:PSS layer, followed by annealing
at 70 8C for 20 min. The electron-transporting layer (ETL)
was spin-coated onto the emission layer, from P1NH or
15
enhance their solubility in high polar solvents. Broad vibra-
21
tion absorption of hydroxy groups around 3300 cm in FTIR
spectra (Supporting Information Figs. S1 and S2) indicates
that the diethanolaminohexyloxy groups are successfully
attached to P1NH and P2NH.
2 2 4
P2NH solution in C H Cl and 2-isopropanol (v/v 5 2/5),
and annealed at 70 8C for 20 min. The concentrations of
P1NH and P2NH solutions were 1.5 and 1.0 mg/mL, respec-
tively. Finally, LiF (0.5 nm) and Al (90 nm) were sequentially
1
The H NMR spectra of dibromo monomers (M1, M2, M3)
and precursor polymers (P1, P2) are shown in Figures S3–
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FIGURE 2 (a) Thermogravimetric traces of P1NH and P2NH at heating rate of 10 K/min under the nitrogen atmosphere; (b) differ-
ential scanning calorimetric traces of P1NH and P2NH at a heating rate of 15 K/min. [Color figure can be viewed at wileyonline-
library.com]
S5 and S6–S7, respectively. Peak areas attributed to protons
in (a) aromatic protons and (b) diethanolaminohexyloxy pen-
dant groups were used to estimate the ratios of two repeat-
ing units in P1NH and P2NH (Scheme 3). The ratios are 5:9
and P2NH revealed very similar, two-stage thermal decom-
position characteristics. Moreover, they demonstrated high
thermal decomposition temperatures (T , at 5% weight loss),
d
with T being 306 and 304 8C, respectively. The first stage
d
(
Supporting Information Fig. S8) and 5:8 (Fig. 1), respec-
weight loss (ca. 30%) is attributable to the thermal decom-
position of soft diethanolaminohexyloxy side chains. The sec-
ond stage weight loss is caused by the decomposition of
polyphenylene main chain and benzimidazolyl pendant
groups. Furthermore, the residual weight at 800 8C of both
polymers were still higher than 30%, suggesting that carbon-
ization of phenylene backbone and benzimidazolyl side
groups may occur during thermal treatment under the nitro-
gen atmosphere. Thermal transitional behaviors were stud-
ied using a differential scanning calorimeter at a heating rate
of 15 K/min. As shown in Figure 2(b), the glass transition
tively, indicating that both M1 and M2 are more reactive
than M3 in coupling with 1,4-benzenediboronic acid bis(pi-
nacol) ester (4) (Scheme 2). It seems that electron-
withdrawing nature of the benzimidazolyl groups in M1 and
M2 promotes coupling reactivity. The reduced contents of
polar diethanolaminohexyloxy groups lead to limited solubil-
ity of P1NH and P2NH in alcohol. However, both P1NH and
P2NH are soluble in a mixture of dichloroethane and isopro-
panol (v/v 5 2/5). More importantly, the emitting SY layer is
resistant to this mixture solvent, that is, not seriously cor-
roded during spin coating of P1NH or P2NH solution.
Accordingly, fabrication of multilayer PLEDs with P1NH or
P2NH as the ETL are possible by using wet processes, such
as spin coating.
g
temperatures (T ) of P1NH and P2NH were 113 and 108 8C,
respectively, high enough for practical application in PLEDs.
High thermal stability prevents degradation of the polymer
during thermal deposition of cathode and device operation.
Thermal and Photo-Physical Properties
Figure 3 shows the absorption and photoluminescence (PL)
spectra of P1NH and P2NH in THF solution and film state,
with the characteristic data summarized in Table 1. P2NH
reveals main absorption band around 301 nm accompanied
Thermal decomposition behaviors of P1NH and P2NH were
investigated by TGA under the nitrogen atmosphere, at a
scanning rate of 10 K/min. As shown in Figure 2(a), P1NH
FIGURE 3 Absorption spectra and photoluminescence spectra of (a) P1NH and (b) P2NH in THF solution and film state. [Color
figure can be viewed at wileyonlinelibrary.com]
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2
4
TABLE 1 Optical Properties of P1NH and P2NH
Absorption kmax
eV). This indicates that pendant benzimidazolyl groups
effectively lower the HOMO and LUMO levels of P1NH and
P2NH. The LUMO levels are close to that of commonly used
electron-transporting material TPBI (22.70 eV), ensuring
capability in electron transport/injection. Moreover, the
LUMO levels of both polymers (ETL) are close to 22.8 eV of
emission SY layer, facilitating electron transport from the
ETL to SY layer. On the other hand, their HOMO levels
(
nm)
PL kmax (nm)
a
b
a
b
pt
c
Polymer
THF
Film
THF
Film
E
g
8
(eV)
P1NH
P2NH
296, 337
301, 347
300, 346
308, 357
417
414
424
421
3.10
3.13
(
25.70 eV, 25.61 eV) are much lower than 25.0 eV of SY,
a
22
In 10 mg/mL THF solution.
b
which is deep enough to block holes in the interface
between SY layer and ETL. These characteristics are usually
effective in raising charge balance and emission efficiency.
The films were obtained by spin-coating (1000 rpm) their solutions
).
The values were estimated by
absorption in absorption spectra (film state).
(5 mg/mL in CHCl
3
c
pt
g onset
E 8 5 1240/konset eV, k : onset
Electroluminescent Properties
Single Carrier-Transporting Property
with a shoulder at ca. 347 nm. For P1NH the peaks shift
To investigate injection/transport capabilities of the electron-
transporting P1NH and P2NH, electron-only and hole-only
multilayer PLEDs were fabricated and the variations of cur-
rent density with operating voltage measured. The hole-only
devices used high work function Au (25.1 eV) as a cathode
instead of Al, with a configuration of ITO/PEDOT:PSS/SY/
hypsochromically to about 296 and 337 nm; the one at
337 nm becomes the main peak. The absorption band
around 300 nm is attributed to benzimidazole-centered p-p*
2
3
transitions,
whereas the longer wavelength absorption
band is arisen from the p-p* transitions of extended conjuga-
tion including backbone and benzimidazolyl groups. The
2
1
pt
ETL/Au (100 nm). High work function of Au makes elec-
tron injection very difficult due to high barrier height, that
is, the holes contribute to the current exclusively. Similarly,
the electron-only devices [Glass/PEDOT:PSS/Ag(100 nm)/
SY/ETL/Al(90 nm)] replaced anode ITO by Ag for the pur-
optical energy bandgaps (E 8 s) of P1NH and P2NH are
g
3.10 and 3.13 eV, respectively, as estimated from the absorp-
tion edges in the film state. In THF solution, the photolumi-
nescence maxima of P1NH and P2NH situate at 417 and
414 nm, respectively, which shift slightly to 424 and 421 nm
2
6,27
pose of preventing hole-injection.
The function of
in the film state. The redshift in film state is attributable to
aggregate formation caused by intermolecular interactions.
PEDOT:PSS layer is making smooth glass surface instead of
promoting hole-injection.
Electrochemical Properties
For hole-only devices (HODs), both P1NH and P2NH reduce
current density significantly [Fig. 5(a,c), hollow symbol] and
the reduction increases slightly with increasing thickness.
This indicates that both polymers reveal good hole-blocking
properties; therefore, the hole-blocking ability increases
gradually with increasing thickness. Moreover, P1NH demon-
strates stronger hole-blocking properties than P2NH, sub-
stantiated from a larger decrease in current density under
the same voltage. For example, the current density of hole-
only device based on P1NH (25 nm) is smaller than that
based on P2NH (27 nm). This is because P1NH possesses
deeper HOMO level (25.70 eV) than P2NH (25.61 eV) as
shown in Figure 6.
The HOMO and LUMO levels of P1NH and P2NH were esti-
mated from the onset oxidation and reduction potentials
obtained in cyclic voltammetric measurements (Fig. 4, Table
2). The HOMO and LUMO levels of P1NH are 25.70 and
22.70 eV, respectively, whereas those of P2NH are 25.61
and 22.58 eV. Both HOMO and LUMO levels are much lower
than the values of poly(p-phenylene) (25.20 eV, 22.04
On the contrary, for electron-only devices (EODs) the current
density increases slightly after the insertion of the ETL
between SY layer and Al cathode, indicating that both P1NH
and P2NH possess electron injection/transporting character-
istics. However, the current densities are nearly equal under
the same operating voltages [Fig. 5(b,d)], suggesting that
P1NH and P2NH reveal parallel electron injection/transport-
ing ability due probably to the fact that they possess the
same electron-withdrawing benzimidazolyl pendant groups.
The results of single-carrier PLEDs strongly indicate that
both P1NH and P2NH promote electron injection/transport
and block holes simultaneously. This characteristic is benefi-
cial to the carrier balance in PLEDs, since for most
FIGURE 4 Cyclic voltammograms of P1NH and P2NH in 0.1 M
(
4 4
n-Bu) NClO ; scan rate: 100 mV/s. [Color figure can be viewed
at wileyonlinelibrary.com]
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TABLE 2 Electrochemical Properties of P1NH and P2NH
a
a
b
c
el
d
pt
e
Polymer
P1NH
E
onset(ox) vs. FOC (V)
E
onset(red) vs. FOC (V)
E
HOMO (eV)
E
LUMO (eV)
E
g
(eV)
E
g
8
(eV)
0.90
0.81
–
22.10
25.70
25.61
25.20
26.70
22.70
22.58
22.04
22.70
3.00
3.03
–
3.10
3.13
–
P2NH
22.22
f 24
PPP
–
–
g 25
TPBI
–
–
–
a
e
pt
E
E
FOC 5 0.08 V vs. Ag/AgNO
3
.
E
g
8
5 1240/konset(abs. spectrum) (nm).
24
b
c
f
HOMO 5 2(Eonset(ox), FOC 1 4.8) eV.
PPP: poly(p-phenylene).
TPBI: 1,3,5-Tris(N-phenyl-benzimidizol-2-yl)benzene.
g
25
E
LUMO 5 2(Eonset(red), FOC 1 4.8) eV.
d
el
g
E
5|LUMO-HOMO|.
conjugated emitting polymers such as SY hole transport sur-
passes electron transport.
to inject and transport, due to much larger barrier height than
holes under an external field. Moreover, in the emission layer
holes would transport faster than electrons, attributed to fully
conjugated characteristics of SY. Accordingly, the carriers’ injec-
tion/transport are imbalanced in the emission layer. However,
carriers balance is highly required in obtaining enhanced
device efficiency and reducing turn-on voltage.
As an Electron-Injection and Transporting Layer
We first fabricated multilayer PLEDs (ITO/PEDOT:PSS/SY/
ETL/Al) to investigate the applicability of P1NH and P2NH
as an electron injection/transporting layer (EIL/ETL). The
EIL/ETL was inserted between SY emission layer and Al
cathode; and PEDOT:PSS was used as a hole-injection layer.
The polymeric layers of PEDOT:PSS, SY, and ETL were suc-
cessively deposited by spin-coating processes. Figure 6
shows the energy level diagrams of each component layer.
Without the ETL, electrons should be much more difficult
Figure 7 depicts luminance versus voltage, current density
versus voltage, current efficiency versus current density, and
electroluminescence (EL) spectra of the PLEDs. The corre-
sponding optoelectronic data are summarized in Table 3.
2
The turn-on voltage (at 10 cd/m ) decreased from 5.7 V to
FIGURE 5 Current density versus voltage characteristics for hole-only (HOD) and electron-only (EOD) devices. The electron-
transporting layer (ETL) was P1NH (a,b) or P2NH (c,d). The devices without ETL were also fabricated for comparison. [Color figure
can be viewed at wileyonlinelibrary.com]
8
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of P1NH-based devices increased with increasing thickness.
This is the consequence of improved electron injection/
transport and hole-block by the inserted P1NH layer. It is
evident that P1NH is superior to P2NH in terms of perfor-
mance improvement. In summary, the following characteris-
tics of P1NH give rise to the performance improvement.
First, suitable LUMO level of P1NH (Fig. 6) is beneficiary to
electron injection. Second, deeper HOMO level (25.70 eV)
raises the hole-blocking capability, which effectively blocks
holes in the EML interface near ETL. Finally, fully extended
para-conjugated backbone increases electron-transporting
capability.
There is an obvious redshift in the emission color, as
observed from the red-shift of EL spectra [Fig. 7(d)] and var-
iation in CIE coordinate from (0.39, 0.59) to (0.47, 0.53)
FIGURE 6 Energy-level diagrams of PEDOT:PSS, SY, P1NH,
and P2NH.
(
Table 3). This redshift is probably caused by the additional
emission from aggregates formed at the interface between
SY and P1NH. To reduce this redshift and increase chromatic
purity, the recombination region of electrons and holes
should be in the central region of the EML, i.e., away from
the interface. Therefore, we also fabricated multilayer PLEDs
having conventional LiF as the electron injection layer and
P1NH or P2NH as the electron-transporting layer. In this
way, the LiF promote electron injection greatly to shift the
recombination zone away from the interface. The details will
be discussed in next section.
ca. 4.0 V after inserting P1NH or P2NH, attributable to
improved electron injection/transport. However, although
the current efficiency was raised slightly from 0.22 cd/A to
around 0.30 cd/A for devices based on P2NH, the maximum
2
2
luminance deceased from 1101 cd/m to 501–799 cd/m .
On the contrary, the P1NH-based devices revealed significant
enhancement in both maximum luminance (4302–6216 cd/
2
m ) and current efficiency (4.37–7.18 cd/A) depending on
ETL thickness. Furthermore, the performance enhancement
FIGURE 7 (a) Luminance and (b) current density versus voltage characteristics of PLEDs. (c) Current efficiency and (d) EL spectrum
of PLEDs. Device structure: ITO/PEDOT:PSS/SY/ETL/Al (90 nm). [Color figure can be viewed at wileyonlinelibrary.com]
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a
TABLE 3 Optoelectronic Properties of PLEDs having P1NH or P2NH as the ETL
b
2 c
d
e
f
ETL Thickness (nm)
None
V
on (V)
B
max (cd/m )
CEmax (cd/A)
LPEmax (lm/W)
CIE (x, y)
5.7
4.1
3.8
4.4
4.3
3.6
4.0
1,101
4,302
5,154
6,216
501
0.22
0.07
1.57
2.45
2.74
0.15
0.24
0.17
(0.39, 0.59)
(0.43, 0.56)
(0.47, 0.53)
(0.46, 0.53)
(0.43, 0.55)
(0.43, 0.55)
(0.44, 0.55)
g
P1NH (5 nm)
4.38
g
P1NH (25 nm)
5.81
g
P1NH (35 nm)
7.18
g
P2NH (4 nm)
0.27
g
P2NH (20 nm)
799
0.38
g
P2NH (27 nm)
757
0.35
a
e
f
Device structure: ITO/PEDOT:PSS/SY/ETL/Al (90 nm).
Turn-on voltage at 10 cd/m .
Maximum luminous power efficiency.
The 1931 CIE coordinate at maximum luminance.
Thickness was measured by a surface profiler (Alpha-StepVR 500).
b
c
2
g
Maximum luminance.
d
Maximum current efficiency.
2
Lithium fluoride (LiF) is a well-known efficient electron-
injection material. Multilayer PLEDS with a configuration of
ITO/PEDOT:PSS/SY/(ETL)/LiF(0.5 nm)/Al(90 nm) were fab-
ricated, using LiF as the electron-injection layer (EIL) and
P1NH or P2NH as the electron-transporting layer (ETL). Fig-
layer (Table 3: 5.7 V, 1101 cd/m , 0.22 cd/A, and 0.07 lm/
W). Evidently, there is a drastic reduction in turn-on voltage
and a significant enhancement in device performance, attrib-
utable to promoted electron injection and balanced carriers.
However, when P2NH was inserted as ETL, maximum lumi-
2
ure
8
and Table
4
demonstrates the optoelectronic
nance was drastically decreased to about 4940 cd/m
characteristics of the PLEDS. First, the performance of the
PLEDs without the ETL, ITO/PEDOT:PSS/SY/LiF(0.5 nm)/
Al(90 nm), were investigated. The turn-on voltage, maximum
luminance, current efficiency, and power efficiency were 3.3
V, 8690 cd/m , 2.78 cd/A, and 1.00 lm/W, respectively. The
performances are significantly superior to those without LiF
although turn-on voltage remained almost unchanged (3.3–
3.4 V). The current efficiencies (3.19–3.70 cd/A) and power
efficiencies (2.14–2.27 lm/W) were slightly improved, due to
hole-blocking property of P2NH. In contrast, when P1NH
was inserted as the ETL, the turn-on voltage was further
decreased to 2.9–3.0 V, accompanied by significant
2
FIGURE 8 (a) Luminance and (b) current density versus voltage characteristics of PLEDs. (c) Current efficiency and (d) EL spectrum
of PLEDs. Device structure: ITO/PEDOT:PSS/SY/ETL/LiF (0.5 nm)/Al (90 nm). [Color figure can be viewed at wileyonlinelibrary.com]
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a
TABLE 4 Optoelectronic Properties of PLEDs with ETL/HBL
b
c
2
d
e
f
ETL
V
on (V)
L
max (cd/m )
CEmax (cd/A)
LPEmax (lm/W)
CIE (x, y)
None
3.3
3.0
2.9
3.4
3.3
8,690
11,963
12,881
4,938
4,940
2.78
9.13
10.94
3.70
3.19
1.00
6.25
7.90
2.14
2.27
(0.41, 0.58)
(0.46, 0.53)
(0.44, 0.55)
(0.44, 0.55)
(0.45, 0.54)
P1NH (25 nm)
P1NH (35 nm)
P2NH (20 nm)
P2NH (27 nm)
a
d
e
f
Device structure: ITO/PEDOT:PSS/SY/ETL/LiF (0.5 nm)/Al (90 nm).
Maximum current efficiency.
b
c
2
Turn-on voltage at 10 cd/m .
Maximum luminance.
Maximum luminous power efficiency.
The 1931 CIE coordinates at maximum luminance.
2
enhancement in maximum luminance (11,963–12,881 cd/
m ), current efficiency (9.13–10.94 cd/A) and power effi-
ciency (6.25–7.90 cd/W). Moreover, the increase in thickness
3.70 cd/A) and the device without the ETL (8690 cd/m and
2.78 cd/A). Current results indicate that P1NH is potential
EIL/ETL materials for optoelectronic devices.
2
of P1NH leads to slight improvement in device performance.
Clearly, the additional LiF layer effectively promotes electron
injection. However, high electron injection rate does not
guarantee high charge recombination ratio, because some
holes may reach the cathode (quenching) without meeting
any electron. Therefore, inserted P1NH layer not only pro-
motes electron transport but also effectively blocks the holes
from quenching by the cathode. The hole-blocking P1NH
layer confines most of the holes in the emission layer due to
its low HOMO levels (25.70 eV). These two factors act
together to increase the carriers’ recombination ratio in the
EML, which is an essential requirement in obtaining efficient
PLEDs. The EL spectra and CIE coordinates remain almost
the same after inserting the ETL [Fig. 8(d), Table 4], substan-
tiating that the emission is mainly contributed from the SY
layer.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support from
the Ministry of Science and Technology of Taiwan through grant
MOST 104-2221-E-006-162.
REFERENCES AND NOTES
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