Copolyfluorenes containing pendant bipolar carbazole and 1,2,4-triazole groups: Synthesis, characterization, and optoelectronic applications

Article abstract of DOI:10.1002/pola.24831

Three novel copolyfluorenes (P1-P3) containing pendant bipolar groups (2.5-7.7 mol %), directly linked hole-transporting carbazole and electron-transporting aromatic 1,2,4-triazole, were synthesized by the Suzuki coupling reaction and applied to enhance emission efficiency of polymer light-emitting diodes based on conventional MEH-PPV. The bipolar groups not only suppress undesirable green emission of polyfluorene under thermal annealing, but also promote electron- and hole-affinity of the resulting copolyfluorenes. Blending the bipolar copolyfluorenes with MEH-PPV results in significant enhancement of device performance [ITO/PEDOT:PSS/MEH-PPV+P1, P2 or P3/Ca(50 nm)/Al(100 nm)]. The maximum luminance and luminance efficiency were enhanced from 3230 cd/m² and 0.29 cd/A of MEH-PPV-only device to 15,690 cd/m² and 0.81 cd/A (blend device with MEH-PPV/P3 = 94/6 containing about 0.46 wt % of pendant bipolar residues), respectively. Our results demonstrate the efficacy of the bipolar copolyfluorenes in enhancing emission efficiency of MEH-PPV.

Full text of DOI:10.1002/pola.24831

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Copolyfluorenes Containing Pendant Bipolar Carbazole and 1,2,4-Triazole
Groups: Synthesis, Characterization, and Optoelectronic Applications
Chia-Shing Wu,1,2 Yun Chen1,2
1Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
2Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, Taiwan
Correspondence to: Y. Chen (E-mail: yunchen@mail.ncku.edu.tw)
Received 22 April 2011; accepted 11 June 2011; published online 7 July 2011
DOI: 10.1002/pola.24831
ABSTRACT: Three novel copolyfluorenes (P1-P3) containing
pendant bipolar groups (2.5–7.7 mol %), directly linked hole-
transporting carbazole and electron-transporting aromatic
1,2,4-triazole, were synthesized by the Suzuki coupling reaction
and applied to enhance emission efficiency of polymer light-
emitting diodes based on conventional MEH-PPV. The bipolar
groups not only suppress undesirable green emission of poly-
fluorene under thermal annealing, but also promote electron-
and hole-affinity of the resulting copolyfluorenes. Blending the
bipolar copolyfluorenes with MEH-PPV results in significant
enhancement of device performance [ITO/PEDOT:PSS/MEH-
PPVþP1, P2 or P3/Ca(50 nm)/Al(100 nm)]. The maximum lumi-
nance and luminance efficiency were enhanced from 3230 cd/m2
and 0.29 cd/A of MEH-PPV-only device to 15,690 cd/m2 and 0.81
cd/A (blend device with MEH-PPV/P3 ¼ 94/6 containing about
0.46 wt % of pendant bipolar residues), respectively. Our results
demonstrate the efficacy of the bipolar copolyfluorenes in
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enhancing emission efficiency of MEH-PPV. 2011 Wiley Period-
icals, Inc. J Polym Sci Part A: Polym Chem 49: 3928–3938, 2011
KEYWORDS: bipolar; carbazole; conjugated polymers; light-emit-
ting diodes (LED); luminescence; polyfluorene; triazole
INTRODUCTION Since the discovery of green-yellow electro-
luminescence of poly(p-phenylene vinylene) (PPV) by
Holmes and coworkers in 1990,1 polymeric light-emitting
diodes (PLEDs) continue to attract intensive interests due to
their potential applications in large-area display and solid-
state lighting.2 In general, polymeric materials possess many
advantages over inorganic ones, such as facile fabrication
via spin-coating, roll-to-roll processing and other printing
methods,3,4 and susceptible to structural modification. How-
ever, for conventional electroluminescent MEH-PPV {poly[2-
methoxy-5-(20-ethylhexyloxy)-1,4-phenylenevinylene]}, holes
are more readily injected and transported than electrons,
leading to unbalanced carriers currents in PLEDs.5–9 Maintain-
ing a balance between electron and hole currents in PLEDs is
an important factor for achieving high device efficiency.
Various methods have been attempted to improve the device
efficiency. One is to insert an additional electron injection/
transport layer between the emitter and the cathode.10–12
Nevertheless, fabrication of the multilayer polymer LEDs is
usually a difficult task, since the emitting layer might be re-
dissolved during subsequent spin-coating of the electron injec-
tion/transport layer. Single-layer devices are preferred from
the viewpoints of process simplicity and cost effectiveness.
zole, 1,2,4-triazole (TAZ), triphenylamine or carbazole (CAZ)
derivatives. However, crystallization-induced degradation and
thermal breakdown during device operation might readily
happen on account of their poor thermal stability.13–15
Therefore, many research groups prepared thermally stable
polymers carrying electron- and/or hole-transporting groups
in an attempt to improve device performance.16–22 For
instance, Jung et al. synthesized thermally stable fluorene-
based poly(iminoarylene)s with a triarylamine unit as hole-
injection layers to enhance efficiency (with max. brightness:
12,370 cd/m2).18(c) Alam et al. synthesized electron-trans-
porting conjugated poly(2,2’-(3,3’-dioctyl-2,2’-bithienylene)-
6,6’-bis(4-phenylenequinoline)) (POBTPQ) as acceptor to
enhance performance of devices based on poly(2-methoxy-
5-(2’-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) or
poly(3-octylthiophene) (POT).20(f) Similarly, Tonzola et al.
improved performance of (MEH-PPV)-based device using a
soluble n-type conjugated copolymers incorporated with
bis(phenylquinoline) and region-regular dialkylbithiophene
in the backbone.20(h) Lim et al. synthesized an organosilicate
polymer based on N,N’-diphenyl- N,N’-bis(4-((E)-2-(triethoxy-
silyl)vinyl)phenyl)biphenyl-4,4’-diamine (TEVS-TPD) and
used as a hole injection/transport layer to enhance OLED
device performance, as compared to the device with
PEDOT:PSS.22(c) To our knowledge, few researches have been
focused on directly blending functional polymers containing
Another method is to fabricate single-layer LEDs which
blend charge injection/transport molecules such as oxadia-
Additional Supporting Information may be found in the online version of this article.
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bipolar groups to enhance emission efficiency of another
light-emitting polymer. In addition, in our previous study, the
device performances were significantly enhanced (11,090
cd/m2 and 0.56 cd/A), using blends of the pendant bipolar
copolyfluorenes (with triphenylamine and 1,2,4-triazole moi-
eties) and MEH-PPV as emitting layer.21(d)
In this work, we demonstrate the synthesis of a new bipolar
monomer (M1) and the following preparation of three co-
polyfluorenes (P1-P3) containing 2.5–7.7 mol % pendant
bipolar groups by the Suzuki coupling reaction. The bipolar
unit consists of electron-transporting aromatic 1,2,4-triazole
directly linked with hole-transporting carbazole. The copoly-
fluorenes not only enhance the injection of holes and elec-
trons but also lessen excimer formation under thermal
annealing due to non-planar structure of the bipolar groups.
Finally, the influence of bipolar contents on device perform-
ance is investigated, using blends of the copolyfluorenes and
MEH-PPV as emitting layer. Both maximum luminance and
maximum luminance efficiency of the blends-based PLEDs
are further enhanced to 15,690 cd/m2 and 0.81 cd/A,
respectively, relative to those of our previous study.21(d) Con-
sequently, the copolyfluorenes containing pendant bipolar
groups are applicable to effectively enhance emission effi-
ciency of PPV-based polymers.
EXPERIMENTAL
Measurements
Newly synthesized compounds were identified by NMR spec-
tra, FT-IR spectra and elemental analysis (EA). 1H NMR and
13C NMR spectra were obtained on a Bruker AMX-400 MHz
or an AV500 MHz FT-NMR spectrometer, with the chemical
shifts reported in ppm using tetramethylsilane (TMS) as an
internal standard. The FT-IR spectra were measured as KBr
disk on a Fourier transform infrared spectrometer, model
7850 from Jasco. Elemental analysis was carried out on a
Heraus CHN-Rapid elemental analyzer. The weight-average
molecular weight (Mw) and polydispersity index (PDI) of
polymers were measured with a gel permeation chromato-
graph (GPC), using chloroform (CHCl3) as eluent and mono-
disperse polystyrenes as calibration standards. Thermogravi-
metric analysis (TGA) of polymers was performed under
SCHEME 1 Synthesis of monomer M1 and structures of M0,
TAZ, and CAZ.
carbon to form as thin films. The electrodes were immersed
in acetonitrile containing 0.1 M (n-Bu)4NClO4 as electrolyte.
The energy levels were calculated using ferrocene (FOC) as
standard (ꢁ4.8 eV with respect to vacuum level).23
Materials
9,9-Dihexyl-2,7-dibromofluorene (Aldrich), 9,9-dihexylfluor-
ene-2,7-bis(trimethyleneborate) (Aldrich), aliquat 336 (Alfa
Aesar), tetrakis(triphenylphosphine) palladium [Pd(PPh3)4]
(Acros), methyl 2,5-dibromobenzoate (Acros), benzoyl
chloride (Acros), phosphorous pentachloride (PCl5, Riedel-
Dehaen), 9-phenyl-9H-carbazole (CAZ, Aldrich) N,N-dime-
thylaniline (Acros), dimethyl sulfoxide (DMSO, Tedia), tolu-
ene (Tedia), chloroform (CHCl3, Tedia) and other solvents
were of commercial sources and used without further puri-
fication. 1-((2,5-Dibromophenyl)chloromethylene)-2-(chlor-
o(phenyl)methylene) hydrazine (4) was prepared from
2,5-dibromobenzohydrazide (2) by reacting consecutively
with benzoyl chloride and phosphorous pentachloride
(PCl5) (Scheme 1). 4-(9H-carbazol-9-yl)benzenamine (8)
was synthesized via direct reduction of 9-(4-nitrophenyl)-
9H-carbazole (7) by hydrazine monohydrate, in which 7
was obtained from 1-fluoro-4-nitrobenzene (5) and 9H-
carbazole (6). For a comparative study, triazole model
ꢀ
nitrogen atmosphere at a heating rate of 20 C /min using a
Perkin–Elmer TGA-7 thermal analyzer. Thermal transition
properties of polymers were investigated using a differential
scanning calorimeter (DSC), PerkinElmer DSC 7, under nitro-
gen atmosphere at a heating rate of 10 ꢀC/min. Absorption
and photoluminescence (PL) spectra were measured with a
Jasco V-550 spectrophotometer and a Hitachi F-4500 fluores-
cence spectrophotometer, respectively. Cyclic voltammograms
were recorded with a voltammetric analyzer (model CV-50W
from Bioanalytical Systems) at room temperature under
nitrogen atmosphere. The measuring cell was made up of a
glassy carbon as the working electrode, an Ag/AgCl electrode
as the reference electrode, and a platinum wire as the auxil-
iary electrode. The concentration of low MW compounds
(M0, TAZ and CAZ) in the cell was 3 mg/1 mL acetonitrile,
while polymers (PF, P1-P3) were directly coated on glassy
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(Scheme 2).24 For instance, stoichiometric amounts of 9,9-
dihexylfluorene-2,7-bis(trimethyleneborate) (9: 0.251 g, 0.50
mmol), 9,9-dihexyl-2,7-dibromofluorene (10: 0.222 g, 0.45
mmol), 9-(4-(3-(2,5-dibromophenyl)-5-phenyl-4H-1,2,4-tria-
zol-4-yl)phenyl)-9H-carbazole (M1: 0.031 g, 0.05 mmol),
10 mg of (PPh3)4Pd and several drops of Aliquat 336 were
added to a mixture containing toluene (10 mL) and 2 M
aqueous solution of K2CO3 (8 mL). The mixture was refluxed
for 48 h under nitrogen atmosphere. Extra monomer 9
(16 mg, 0.03 mmol) was added to the reaction mixture and
stirred for additional 12 h. Finally, mono-functional bromo-
benzene (10 mg, 0.07 mmol) was added and stirred for 12 h
to end-cap polymer chain with phenyl group. After cooling to
room temperature, the reaction mixture was precipitated
from methanol and distilled water (v/v ¼ 10/1). The precip-
itate was collected by filtration, dissolved in chloroform and
reprecipitated from methanol twice. Then it was Soxhlet
extracted with acetone for 48 h to remove trace oligomers
SCHEME 2 Synthesis of bipolar copolyfluorenes (P1-P3) and
polyfluorene (PF).
1
and catalyst residues and then dried in vacuo to give P2. H
NMR (CDCl3, ppm): d 7.86–7.84 (m, 2H, Ar-H), 7.71–7.67 (m,
4H, Ar-H), 7.65 (m, 4H, Ar-H), 7.59–7.56 (m, 7H, Ar-H),
7.52–7.34 (m, 6H, Ar-H), 2.12 (s, 4H, ACH2A), 1.26–1.25 (s,
4H, ACH2A), 1.14–1.11 (s, 12H, ACH2A), 0.84–0.78 (s, 6H,
ACH3). FTIR (KBr pellet, cmꢁ1): m 2930 (Ar-H), 1509 (C¼¼N),
1455 (C¼¼C), 1274 (CAN). Anal. Calcd. (%) for P2: C, 89.36;
H, 9.80; N, 0.84. Found: C, 87.34; H, 9.12; N, 0.53.
3,4,5-triphenyl-4H-1,2,4-triazole (TAZ) containing electron-
transporting core was also prepared from aniline and
1,2-bis(chloro(phenyl)methylene)hydrazine. The synthetic
procedures of bipolar model compound M0 were similar to
those of bipolar M1.
Synthesis of Bipolar Monomer (M1) and Model
Compound (M0) (Scheme 1)
The synthetic procedures of PF, P1 and P3 were analogous
to those used in the preparation of P2, except with varied
molar feed ratios in 9, 10 and M1. PF: 9 (0.251 g, 0.50
mmol), 10 (0.246g, 0.50 mmol). 1H NMR (CDCl3, ppm): d
7.85–7.83 (m, 2H, Ar-H), 7.71–7.67 (m, 4H, Ar-H), 2.12 (s,
4H, ACH2A), 1.25 (s, 4H, ACH2A), 1.13 (s, 12H, ACH2A),
0.84–0.78 (m, 6H, ACH3). FTIR (KBr pellet, cmꢁ1): m 2928
(Ar-H), 1455 (C¼¼C). Anal. Calcd. (%) for PF: C, 90.26; H,
9.74. Found: C, 88.37; H, 9.43. P1: 9 (0.251 g, 0.50 mmol),
10 (0.231 g, 0.47 mmol) and M1 (0.018 g, 0.03 mmol). 1H
NMR (CDCl3, ppm): d 7.86–7.84 (m, 2H, Ar-H), 7.71–7.67 (m,
4H, Ar-H), 7.65 (m, 4H, Ar-H), 7.59–7.56 (m, 7H, Ar-H),
7.52–7.34 (m, 6H, Ar-H), 2.12 (s, 4H, ACH2A), 1.25–1.23 (s,
4H, ACH2A), 1.14–1.11 (s, 12H, ACH2A), 0.84–0.78 (s, 6H,
ACH3). FTIR (KBr pellet, cmꢁ1): m 2928 (Ar-H), 1509 (C¼¼N),
1455 (C¼¼C), 1274 (CAN). Anal. Calcd. (%) for P1: C, 89.54;
H, 9.95; N, 0.51. Found: C, 88.27; H, 9.37; N, 0.38. P3: 9
(0.251 g, 0.50 mmol), 10 (0.197 g, 0.40 mmol) and M1
A
mixture of 1-((2,5-dibromophenyl)chloromethylene)-
2-(chloro(phenyl)methylene) hydrazine (4: 0.87 g, 2 mmol)
and 4-(9H-carbazol-9-yl)benzenamine (8: 0.52 g, 2 mmol) in
10 mL of N,N-dimethylaniline was stirred at 135 C for 12 h
ꢀ
under nitrogen atmosphere. After adding aqueous solution of
HCl (30 mL, 2 N), the mixture was stirred for additional 30
minutes. The precipitated solid was collected by filtration,
dried in vacuo, and recrystallized from acetone to afford
9-(4-(3-(2,5-dibromophenyl)-5-phenyl-4H-1,2,4-triazol-4-yl)-
phenyl)-9H-carbazole (M1) (54%); mp >250 ꢀC. 1H NMR
(CD2Cl2, ppm): d 8.15 (s, 1H, Ar-H), 8.13 (s, 1H, Ar-H), 7.68
(d, J ¼ 2.0 Hz, 1H, Ar-H), 7.60–7.56 (m, 4H, Ar-H), 7.54–7.41
(m, 7H, Ar-H), 7.36–7.29 (m, 6H, Ar-H). FTIR (KBr pellet,
cmꢁ1): m 1507 (C¼¼N), 1262 (CAN), 1070 (CABr). Anal.
Calcd. (%) for C32H20Br2N4: C, 61.96; H, 3.25; N, 9.03. Found:
C, 61.65; H, 3.18; N, 8.96.
9-(4-(3,5-Diphenyl-4H-1,2,4-triazol-4-yl)phenyl)-9H-carbazole
(M0), model compound of bipolar unit, was synthesized by
analogous procedures used in the preparation of monomer
M1 with a yield of 61%; mp >250 ꢀC. 1H NMR (CDCl3,
ppm): d 8.18–8.15 (d, J ¼ 7.7 Hz, 2H, Ar-H), 7.67–7.65 (d,
J ¼ 8.6 Hz, 2H, Ar-H), 7.57–7.55 (m, 4H, Ar-H), 7.47–7.32
(m, 14H, Ar-H). FTIR (KBr pellet, cmꢁ1): m 1509 (C¼¼N),
1270 (CAN). Anal. Calcd. (%) for C32H22N4: C, 83.09; H,
4.79; N, 12.11. Found: C, 83.00; H, 4.81; N, 12.14.
1
(0.062 g, 0.10 mmol). H NMR (CDCl3, ppm): d 7.86–7.84 (m,
2H, Ar-H), 7.72–7.68 (m, 4H, Ar-H), 7.65 (m, 4H, Ar-H),
7.59–7.56 (m, 7H, Ar-H), 7.52–7.34 (m, 6H, Ar-H), 2.13
(s, 4H, ACH2A), 1.25–1.23 (s, 4H, ACH2A), 1.50 (s, 12H,
ACH2A), 0.84–0.78 (s, 6H, ACH3). FTIR (KBr pellet, cmꢁ1): m
2930 (ArAH), 1509 (C¼¼N), 1455 (C¼¼C), 1274 (CAN). Anal.
Calcd. (%) for P3: C, 88.91; H, 9.45; N, 1.64. Found: C, 87.66;
H, 8.82; N, 1.28.
Synthesis of Bipolar Copolyfluorenes (P1-P3) and
Poly(9,9-dihexylfluorene) (PF)
Copolyfluorenes (P1-P3) containing pendant bipolar groups
and poly(9,9-dihexylfluorene) (PF) were prepared by the
Suzuki coupling reaction using (PPh3)4Pd as the catalyst
Fabrication of Polymer Light-Emitting Diodes
Double-layer polymer light-emitting diodes (ITO/PEDOT:
PSS/polymer/Ca/Al) were fabricated to investigate their
optoelectronic characteristics. Transparent indium tin oxide
(ITO) glass was successively cleaned with neutraler
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TABLE 1 Polymerization Results and Characterization of
Polymers
reiniger/de-ionized water (1/3 ¼ v/v) mixture, de-ionized
water, acetone and 2-propanol, and then dried in vacuo over-
night. Aqueous dispersion of PEDOT:PSS was spin-coated on
top of the cleaned ITO glass as hole-injection layer and dried
at 150 ꢀC for 15 min. Emitting layer was then deposited by
spin-coating (1000 rpm) onto the PEDOT:PSS layer from a
polymer solution in 1,2-dichlorobenzene (10 mg/mL).
Finally, calcium and aluminum were consecutively vacuum-
deposited as cathode using a vacuum coater at a pressure of
2 ꢂ 10ꢁ6 Torr. Luminance versus voltage and current den-
sity versus voltage characteristics of the devices were meas-
ured using a combination of a Keithley power source (model
2400) and an Ocean Optics usb2000 fluorescence spectro-
photometer. The optoelectronic measurements were con-
ducted in a glove-box filled with nitrogen.
Yield Mn
Polymer (%)
Mw
Tg
Td
y
(ꢂ104)a (ꢂ104)a PDIa (ꢀC) (ꢀC)b (%)c
PF
P1
P2
P3
a
66
64
59
60
2.06
1.61
1.21
0.87
4.79
3.40
2.63
1.70
2.33
91 440
0
2.11 101 469
2.17 104 475
1.95 120 482
2.5
3.2
7.7
Mn, Mw, and PDI of the polymers were determined by gel permeation
chromatography using polystyrene standards.
b
The temperature at 5 wt % loss under nitrogen atmosphere.
The molar fractions of M1 residues were estimated from elemental
c
analysis.
Thermal decomposition temperatures (Td) (at 5 wt % loss)
and glass transition temperatures (Tg) of the copolyfluorenes
were evaluated with thermal gravimetric analysis (TGA) and
differential scanning calorimetry (DSC), respectively. The re-
sidual weights of P1-P3 and PF (at 800 ꢀC) are above 50%
under nitrogen atmosphere (Supporting Information Fig. S3).
As shown in Table 1, the thermal decomposition tempera-
tures (Td: 469–482 ꢀC) of the copolyfluorenes (P1-P3) are
higher than that of PF (Td: 440 ꢀC). Moreover, the copoly-
fluorenes reveal higher glass-transition temperatures (101–
120 ꢀC) than PF (91 ꢀC) and the Tg increases gradually with
increasing M1 residues in main chain. This is attributable to
rigid and non-planar structure of M1 residues which not only
raises chain rigidity but also restrict chain mobility of the
copolyfluorenes.21(c),26 Accordingly, the copolyfluorenes exhibit
better thermal stability than PF; i.e., their thermal decomposi-
tion (469–482 ꢀC) and glass-transition temperꢀatures (101–
RESULTS AND DISCUSSION
Characterization of Bipolar Monomer (M1) and
Copolyfluorenes (P1-P3)
Scheme 1 illustrates the synthetic routes employed in the
preparation of novel bipolar monomer M1. 9-(4-nitro-
phenyl)-9H-carbazole (7)25(d) and 4-(9H-carbazol-9-yl)ben-
zenamine (8)25(d) were prepared according to the proce-
dures reported previously. The copolyfluorenes (P1-P3)
were synthesized by the Suzuki coupling reaction of 9,9-
dihexylfluorene-2,7-bis(trimethyleneborate) (9) with func-
tionalized dibromo aromatic monomers (10 and M1)
(Scheme 2), using Pd(PPh3)4 as the reaction catalyst and Ali-
quat 336 as the phase-transfer catalyst. The feed ratios of
monomer M1 in the preparation of P1, P2, and P3 were 3,
5, and 10 mol %, respectively. Extra monomer 9 and mono-
functional bromobenzene were added to end-cap polymer
chain after the polymerization. Supporting Information (Fig.
ꢀ
ꢀ
120 C) are higher than those of PF (440 C, 91 C).
Photophysical Properties of Model Compound M0 and
Bipolar Copolyfluorenes (P1-P3)
1
S1) shows the H NMR, 13C NMR, DEPT135, HAH COSY, and
CAH HMQC spectra of M1. Assignments of each carbon and
proton were assisted by the two-dimensional NMR spectra
[Supporting Information Fig. S1(d,e)], which agree well with
the proposed molecular structure of M1. The existence of
M1 residue in P3 is confirmed by 13C NMR and DEPT135
spectra of M1, PF and P3 as shown in Supporting Informa-
tion Figures S1(b,c) and S2. In 13C NMR spectra of P3 chemi-
cal shifts of M1 residue are obviously observed at 132.2
ppm (12), 128.8 ppm (2), 128.6 ppm (4), 128.4 ppm (3),
and 127.2 ppm (14). The actual molar percents of M1 resi-
dues in P1, P2 and P3 are 2.5, 3.2 and 7.7 mol %, respec-
tively, as estimated from the elemental analysis data. This
indicates that the bipolar monomer M1 is less reactive than
9,9-dihexyl-2,7-dibromofluorene (10), which is probably due
to steric hindrance and electron-deficient characteristic of its
bipolar moiety. The copolyfluorenes are readily soluble in
common organic solvents such as toluene, chloroform, chloro-
benzene and 1,2-dichlorobenzene. Their weight-average
molecular weights (Mw) and polydispersity indexes (PDI) are in
the range of (1.70–4.79)ꢂ104 and 1.95–2.33 (Table 1), respec-
tively, as determined by gel permeation chromatography using
mono-disperse polystyrenes as calibration standards.
Figure 1 illustrates the absorption and photoluminescence
(PL) spectra of bipolar model compound (M0), carbazole
model (CAZ) and triazole model (TAZ) in CHCl3 with the
characteristic optical data summarized in Table 2. The
absorption maximum and main emission peak of the bipolar
model M0 in CHCl3 appear at about 283, 292 nm (with
shoulders at 310, 323 and 337 nm) and 347 nm (with a
shoulder at 359 nm), respectively. These absorption and
emission bands are mainly attributed to carbazole part as
judged from their spectral features (similar to those of CAZ).
However, the absorption and emission of M0 are slightly
blue-shifted (3–5 nm) relative to those of CAZ due to the
presence of electron-accepting triazole moiety. Moreover, the
PL spectra of M0 were almost the same whether they were
excited with 261 nm (absorption maximum of TAZ) or 286
nm (absorption maximum of CAZ), suggesting efficient intra-
molecular energy transfer from TAZ to CAZ moiety under
the photo-excitation.
Figure 2 illustrates the absorption and photoluminescence
(PL) spectra of poly(9,9-dihexylfluorene) (PF) and bipolar
copolyfluorenes (P1-P3) in CHCl3 and as films spin-coated
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FIGURE 1 Absorption and PL spectra of M0, CAZ, and TAZ in
chloroform (1 ꢂ 10ꢁ5 M) (kex ¼ 283 nm for M0, 286 nm for CAZ
and 261 nm for TAZ). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
from CHCl3 solutions, with the characteristic optical data
summarized in Table 2. The absorption maxima of P1-P3 in
CHCl3 locate at 386, 385 and 380 nm with only small
shorter-wavelength absorptions at about 292 nm. The major
absorption of P1-P3 (380–386 nm) can be attributed to the
p-p* transitions of their conjugated backbone, whereas the
shorter-wavelength absorption (ca. 292 nm) is definitely ori-
ginated from bipolar M1 residues. The major absorption
shifts slightly from 389 nm (PF) to 380 nm (P3) because
the conjugation of polyfluorene main chain is interrupted by
the bipolar moieties. This can be substantiated by the exis-
tence of twist angles (ca. 90ꢀ) between heterocyclic ring and
adjacent benzene rings of bipolar M1 units (Fig. 3), simu-
lated by minimizing energy via semi-empirical MNDO calcu-
lations in the gas states.27 Solid state absorption maxima of
PF and P1-P3 are slightly red-shifted (5–7 nm) relative to
solution state ones due to aggregate formation via intra- or
inter-chain interactions. Copolyfluorenes P1-P3 exhibit PL
peaks at 418–419 nm which are very close to 419 nm of PF.
Moreover, the PL spectra of P1-P3 were almost the same
whether they were excited with 292 nm (absorption maxi-
mum of M0) or 380 nm, confirming efficient Fo¨rster energy
transfer from bipolar M1 residues to fluorene segments
FIGURE 2 Absorption and PL spectra of PF and P1-P3: (a) in
chloroform (1 ꢂ 10ꢁ5 M) (kex ¼ 389 nm for PF, 386 nm for P1,
385 nm for P2 and 380 nm for P3), (b) in film state (kex ¼ 391
nm for PF, 391 nm for P1, 390 nm for P2 and 387 nm for P3).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
under the photo-excitation. Partial overlap between emission
spectrum of M1 residues and absorption spectrum of PF
contributes greatly to the energy transfer.21(c),26,28 It is note-
worthy that the emission spectrum of PF partially overlaps
with the absorption spectrum of MEH-PPV, indicating that
energy transfer from PF to MEH-PPV can be expected.21(c)
Based on this characteristic we fabricated efficient PLEDs
using blends of MEH-PPV and P1-P3 as emitting layer and
will be discussed later.
TABLE 2 Optical Properties of Model M0 and Polymers
UV-vis kmax
Solution (nm)a
UV-vis kmax
PL kmax
Solution (nm)b
PL kmax
Film (nm)b
c
Molecule/Polymer
Film (nm)
UPL
M0
CAZ
TAZ
PF
283,292,310s,323s,337s
—
347,359s
352,364s
346
—
—
—
—
261,286,293,327s,341s
—
—
261
—
—
389
391
419,441s
419,441s
419,441s
418,441s
425,449s
425,449s
424,449s
424,449s
0.82
0.79
0.77
0.72
P1
292,386
292,385
292,380
293,391
293,390
293,387
P2
P3
a
c
In chloroform (1 ꢂ 10ꢁ5 M).
U
PL: Determined in CHCl3, relative to quinine sulfate in 1N H2SO4(aq) at
b
s: wavelength of shoulder.
a concentration of 10ꢁ5 M (UPL ¼ 0.55).
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FIGURE 3 Optimized geometry of a model bipolar compound
obtained from semiempirical MNDO calculation. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Relative PL quantum yields (UPL) of PF, P1, P2, and P3 in
chloroform (at a concentration of 10ꢁ5 M) are 0.82, 0.79,
0.77, and 0.72, respectively. The UPLs of the copolyfluorenes
are slightly lower than that of PF, probably caused by tor-
sion-induced non-radiative deactivation occurred at the non-
planar bipolar groups.29 Polyfluorenes usually show undesir-
able green emission after thermal annealing or under device
operation, which has been attributed to enhanced intermolecu-
lar interactions.30 To examine stability in emission color, PF
and P1-P3 films were thermally annealed at 150 ꢀC for 24 h
to observe their PL spectral variations. As shown in Figure 4,
the green emission appears at about 500–600 nm in PF film
after the thermal treatment; however, it is significantly sup-
pressed in P1-P3 films. This result suggests that the non-planar
bipolar M1 residues are effective in suppressing the intermo-
lecular interactions in the copolyfluorenes.
FIGURE 5 Cyclic voltammograms of (a) M0, TAZ, and CAZ are
3 mg/1 mL acetonitrile (b) PF and P1-P3 coated on glassy car-
bon working electrode (scan rate: 100 mV/s). [Color figure can
be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Electrochemical Investigations
mated by the equations: EHOMO (eV) ¼ ꢁ (Eox,FOC þ 4.8) and
ELUMO (eV) ¼ ꢁ (Ered,FOC þ 4.8), where Eox,FOC and Ered,FOC
are the onset oxidation and onset reduction potentials,
respectively, relative to the ferrocene/ferrocenium couple
whose energy level is already known (ꢁ4.8 eV). The cyclic
voltammograms are shown in Figure 5, with the representa-
tive electrochemical data summarized in Table 3. Bipolar
model M0 would emit blue-green light corresponding to its
band-gap (2.80 eV), but its main emission peak (347 nm)
with a shoulder at 359 nm obviously strays from blue-green
emission. To elucidate above interesting phenomena, we also
estimated the LUMO and HOMO energy levels of bipolar
model M0, triazole model TAZ and carbazole model CAZ
from their cyclic voltammograms. The LUMO levels of M0,
TAZ and CAZ are ꢁ2.79, ꢁ2.71, and ꢁ2.45 eV, and the esti-
mated HOMO levels are ꢁ5.59, ꢁ6.12, and ꢁ5.53 eV, respec-
tively (Table 3). Obviously, the LUMO and HOMO energy lev-
els of M0 are close to LUMO level of TAZ and HOMO level of
CAZ, respectively. This indicates that the LUMO and HOMO
levels of M0 are attributed to triazole and carbazole moi-
eties, respectively. Accordingly, when M0 is subjected to
external potentials, the reduction starts from electron-defi-
cient aromatic 1,2,4-triazole and the oxidation begins from
electron-rich 9-phenyl-9H-carbazole groups, respectively.
Cyclic voltammetry (CV) was employed to investigate the
electrochemical properties of PF, bipolar copolyfluorenes P1-
P3, bipolar model M0, triazole model TAZ and carbazole
model CAZ. Their HOMO and LUMO energy levels were esti-
FIGURE 4 PL spectra of PF and P1-P3 after annealing in vac-
uum at 150 ꢀC for 24 h. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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TABLE 3 Electrochemical Properties of M0, TAZ, CAZ and Polymers (PF, P1-P3)
Molecule/Polymer
Eox vs. FOC (V)a
Ered vs. FOC (V)a
EHOMO (eV)b
ELUMO (eV)b
Eegl (eV)c
Eogpt (eV)d
M0
TAZ
CAZ
PF
0.79
1.32
0.73
0.85
0.84
0.83
0.81
ꢁ2.01
ꢁ2.09
ꢁ2.35
ꢁ2.31
ꢁ2.30
ꢁ2.28
ꢁ2.25
ꢁ5.59
ꢁ6.12
ꢁ5.53
ꢁ5.65
ꢁ5.64
ꢁ5.63
ꢁ5.61
ꢁ2.79
ꢁ2.71
ꢁ2.45
ꢁ2.49
ꢁ2.50
ꢁ2.52
ꢁ2.55
2.80
3.41
3.08
3.16
3.14
3.11
3.06
—
—
—
2.91
2.91
2.90
2.90
P1
P2
P3
a
c
EFOC¼ 0.48 V vs. Ag/AgCl.
Electrochemical band gap: Eegl ¼ LUMO – HOMO.
b
d
EHOMO¼ - (Eox, FOC þ 4.8) eV; ELUMO ¼ - (Ered, FOC þ 4.8) eV.
Optical band gap: Eogpt ¼ hc/konset
.
In this study, copolyfluorenes P1-P3 contain 2.5–7.7 mol %
of bipolar M1 residues derived from electron-transporting
triazole and hole-transporting carbazole. The estimated
HOMO energy levels of PF, P1-P3 are ꢁ5.65, ꢁ5.64, ꢁ5.63,
and ꢁ5.61 eV, whereas the estimated LUMO levels are
ꢁ2.49, ꢁ2.50, ꢁ2.52, and ꢁ2.55 eV, respectively. The HOMO
levels of P1-P3 are raised gradually from ꢁ5.65 eV to ꢁ5.61
eV with an increase in M1 residues (2.5–7.7 mol %; Fig. 6),
suggesting that their hole affinity is in the order of P3 > P2 >
P1. On the contrary, the LUMO levels of P1-P3 are lowered
slightly (ꢁ2.49 to ꢁ2.55 eV) with increasing M1 residues,
meaning that their electron injection ability is improved in the
order of P3 > P2 > P1. Accordingly, both hole and electron
affinity of P1-P3 is slightly enhanced with increasing M1 resi-
dues, resulting in improved carriers injection and transport.
Both LUMO and HOMO levels of bipolar model M0 (ꢁ2.79 and
ꢁ5.59 eV) are lower than those of conventional MEH-PPV
(ꢁ2.70 and ꢁ5.02 eV) because of the electron-withdrawing
aromatic 1,2,4-triazole moiety.21(b,c),31 Therefore, blending MEH-
PPV with P1-P3 containing bipolar residues would increase
electron affinity; the hole affinity, however, would be reduced
simultaneously. The blending might be an effective way to
improve charges injection and transport balance in MEH-PPV,
whose hole injection and transport are more facilitated.
Electroluminescent Enhancement of MEH-PPV by
Bipolar Copolyfluorenes
Polymer light-emitting devices using P1-P3 as emitting layer
[ITO/PEDOT:PSS/P1-P3/Ca(50 nm)/Al(100 nm)] were fabri-
cated to study their current density and luminance (vs. bias)
characteristics. The turn-on voltages of P1-P3 devices
increased from 5.3 to 7.3 V with increasing M1 moieties
(from 2.5 to 7.7 mol %). In addition, the maximum lumi-
nance and maximum luminance efficiency of P1-P3 devices
decreased from 410 to 230 cd/m2 and from 0.15 to 0.10 cd/
A, respectively, with increasing M1 moieties (2.5–7.7 mol %).
Moreover, the LUMO and HOMO levels of bipolar model M0
(ꢁ2.79 and ꢁ5.59 eV) are encompassed within those of PF
(ꢁ2.49 and ꢁ5.65 eV) (Fig. 6). Accordingly, the performance
degradation at high M1 contents is probably caused by
charges trapping in the bipolar moieties that reduce charge
recombination in polyfluorene segments.32,33 As mentioned
earlier, both LUMO and HOMO levels of bipolar model M0
are lower than those of MEH-PPV (Fig. 6). This suggests that
the M1 residues will promote electron injection/transport
and block hole transport in MEH-PPV device. To elucidate
this reasoning, MEH-PPV was blended with bipolar P1-P3
and used as emitting layers to solve its common defect of
unbalanced charges injection and transport. Double-layer
polymer light-emitting diodes [ITO/PEDOT:PSS/(MEH-
PPVþP1, P2 or P3)/Ca(50 nm)/Al (100 nm)] were fabri-
cated to investigate their device performances. The weight
ratios of MEH-PPV to the bipolar copolyfluorenes (MEH-
PPV/P1, MEH-PPV/P2 and MEH-PPV/P3) were controlled at
91/9, 94/6 and 97/3, respectively, to adjust the ratios of
bipolar residues to be about 0.2 wt %. Figure 7 shows the
current density and luminance versus bias characteristics of
the blend devices, with the representative data summarized
in Table 4. The maximum luminance efficiency of the neat
MEH-PPV device (0.29 cd/A) is moderate relative to those
reported in literatures.20(a–c,h,i),22(d,e) The discrepancy of
luminance efficiency among these EL devices is probably due
to their device structural difference. The blend devices ex-
hibit slightly higher turn-on voltages (3.3–4.1 V) than MEH-
PPV device (3.3 V), indicating that electron injection
enhancement by the bipolar residues seems inferior to their
hole-blocking effect that higher bias is needed to turn on the
devices.21(c),34 However, the maximum luminance and
FIGURE 6 Energy band diagrams of M0, TAZ, CAZ, PF, P1-P3,
and MEH-PPV. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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tents of fluorene segments (from copolyfluorenes). For
instance, P3-based blend device reveals the best EL perform-
ance, due probably to the least content of fluorene segments
(ca. 2.8 wt %) relative to P1- and P2-based devices (ca. 8.8
wt % and 5.8 wt %). This means that higher percents of flu-
orene segments in emitting layer (as in P1- and P2-based
devices) leads to inferior device performance. As shown in
Figure 6, the HOMO (ꢁ5.02 eV) and LUMO energy levels
(ꢁ2.70 eV) of MEH-PPV lies between those of poly(9,9-
dihexylfluorene) (PF: ꢁ5.65 eV, ꢁ2.49 eV). Therefore,
charges injection and transport ability of the blend is
reduced with increasing contents of fluorene segments.
The influence of bipolar residues’ contents on electrolumi-
nescent performance was also investigated using P3-based
blend devices as an example. The current density and lumi-
nance versus bias characteristics are shown in Figure 8, with
the corresponding performance data summarized in Table 4.
The turn-on voltages increase slightly from 3.3 to 4.8 V with
increasing P3 contents (3–15 wt %) because hole-blocking
effect of the bipolar residue surpasses its enhancement in
electron injection.21(c),34 To clarify the hole-blocking effect of
P3 in the blends, hole-only devices [ITO/PEDOT:PSS/(MEH-
PPVþ P3)/Au(20 nm)/Al (100 nm)] were fabricated to
investigate their current density versus bias characteristics.
As shown in Figure 9, the curve shifts horizontally to higher
bias as P3’s ratio is increased from 0 to 6 wt %, indicating
diminished current density under the same bias. For
instance, at an electric field of 9 ꢂ 105 V/cm the current
density decreases from 1000 mA/cm2 to about 700 mA/cm2,
which is obviously due to increased hole-blocking at high P3
contents (6 wt %). The maximum luminance and the maxi-
mum luminance efficiency of the device are enhanced from
3230 cd/m2 and 0.29 cd/A to 15,690 cd/m2 and 0.81 cd/A,
respectively, as the weight ratio of P3 is increased from 0 to
6%. This performance is superior to that obtained for our
previous copolyfluorene with 8.2 mol % pendant bipolar
groups derived from triphenylamine and aromatic 1,2,4-tria-
zole (11,090 cd/m2 and 0.56 cd/A).21(d) The EL performance
difference is probably due to varied LUMO and HOMO
energy levels, resulting from different chemical structures
(carbazole vs. triphenylamine) and contents of the bipolar
FIGURE 7 (a) Current density versus bias and (b) luminance
versus bias characteristics of PLEDs using blends of MEHPPV
and P1-P3 (3–9 wt %) as emitting layer. Device configuration:
ITO/PEDOT:PSS/MEH-PPV þ P1, P2 or P3 (90–110 nm)/Ca(50
nm)/Al(100 nm). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
maximum luminance efficiency of the blend devices are sig-
nificantly enhanced to 8780–13,350 cd/m2 and 0.56–0.79
cd/A, respectively, when compared with 3230 cd/m2 and
0.29 cd/A of the MEH-PPV device. Apparently the bipolar
M1 residues in emitting layer effectively balance charges
injection and transport under device operation. In addition,
the device performance seems mainly dependent on the con-
TABLE 4 Optoelectronic Performance of Polymer Light-Emitting Diodesa
Emitting–Layer
Compositionb
Turn on
Voltage (V)c
Maximum Luminance
Efficiency (cd/A)
Maximum
Luminance (cd/m2)
CIE Coordinates (x, y)d
MEH-PPV
3.3
4.1
3.5
3.3
3.6
3.6
4.3
4.8
0.29
0.56
0.67
0.79
0.81
0.71
0.56
0.44
c
3,230
8,780
(0.54, 0.46)
(0.49, 0.50)
(0.49, 0.50)
(0.50, 0.49)
(0.48, 0.51)
(0.52, 0.48)
(0.48, 0.51)
(0.53, 0.47)
MEH-PPV/P1 (91/9)
MEH-PPV/P2 (94/6)
MEH-PPV/P3 (97/3)
MEH-PPV/P3 (94/6)
MEH-PPV/P3 (91/9)
MEH-PPV/P3 (88/12)
MEH-PPV/P3 (85/15)
a
11,950
13,350
15,690
11,470
9,620
5,060
Device structure: ITO/PEDOT:PSS/emitting layer/Ca/Al.
The weight ratios are given in parentheses.
The voltage required for the luminance of 10 cd/m2.
The CIE coordinates at maximum luminance.
b
d
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FIGURE 9 Current density versus bias characteristics of the
hole-only devices using blends of MEH-PPV and P3 (0–6 wt %)
as emitting layer. Device structure: ITO/PEDOT:PSS/MEH-PPV þ
P3 (90–110 nm)/Au(20 nm)/Al(100 nm). [Color figure can
be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
tion in maximum luminance and maximum current effi-
ciency; i.e., diminish to 5060 cd/m2 and 0.44 cd/A at MEH-
PPV/P3 ¼ 85/15. The results of morphology investigation
can reasonably explain this performance reversion at higher
P3 contents. As shown in Figure 10, the root-mean-square
surface roughness of the blend films increases from 0.97 to
1.33 nm with increasing P3 weight ratio from 6 to 15%. The
increased surface roughness at high P3 weight ratio is due
to its limited compatibility with MEH-PPV. Therefore, the
quick performance degradation of the blend device is prob-
ably attributed to higher surface roughness formed during
film preparation.
FIGURE 8 (a) Current density versus bias and (b) luminance
versus bias characteristics of PLEDs using blends of MEHPPV
and P3 (3–15 wt %) as emitting layer. Device structure: ITO/
PEDOT:PSS/MEH-PPV þ P3 (90–110 nm)/Ca(50 nm)/Al(100 nm).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
groups (7.7 vs. 8.2 mol %). Accordingly, the significantly
enhanced device performance in P3-based device is mainly
resulted from more balanced charges recombination. How-
ever, further increase in P3 contents leads to quick degrada-
However, at low P3 content (MEH-PPV/P3 ¼ 94/6) the de-
vice performance is effectively enhanced. As shown in Figure
11, the electroluminescent emission of all devices is exclu-
sively originated from MEH-PPV, with full width at half-
FIGURE 10 AFM images of blend films (Scan size: 5 ꢂ 5 lm2): (a) MEH-PPV/P3 ¼ 94/6, RMS roughness ¼ 0.97 nm, (b) MEH-PPV/
P3 ¼ 85/15, RMS roughness ¼ 1.33 nm.
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ing additives in effectively improving device performance of
MEH-PPV and other conjugated polymers.
The authors thank the National Science Council of Taiwan for fi-
nancial aid through project NSC 98-2221-E-006-002-MY3.
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common organic solvents and thermally stable (Td at 5 wt %
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