Pure Deep Blue Light-Emitting Diodes from Alternating Fluorene/Carbazole Copolymers by Using Suitable Hole-Blocking Materials

Article abstract of DOI:10.1021/ma030535n

The influences of the carbazole content on the photophysical, electrochemical, and electroluminescent properties of alternating fluorene/carbazole copolymers PFnCz (n = 1, 2, 3) with well-defined chemical structures have been systematically investigated. The incorporation of carbazole units into the polyfluorene (PF) backbone resulted in a blue shift of both the absorption and photoluminescence (PL) emission peaks, improved PL thermal stability, raised HOMO energy levels, and thus facilitated hole injection into the copolymers. Pure deep blue electroluminescence (EL) with narrow with (full width at the half-maximum) (39-52 nm) and negligible low-energy emission bands was successfully achieved from the PFnCz copolymers by using l,3,5-tris(4′-fluorobiphenyl-4-yl)benzene (F-TBB) as a hole-blocking layer and Alq₃ as an electron injection/transporting layer. This device configuration stabilized the blue emission from the PF derivatives. An efficiency of 0.72 cd/A at a luminance of 100 cd/m² was obtained even with aluminum metal as the cathode.

Full text of DOI:10.1021/ma030535n

2442
Macromolecules 2004, 37, 2442-2449
Pure Deep Blue Light-Emitting Diodes from Alternating Fluorene/
Carbazole Copolymers by Using Suitable Hole-Blocking Materials
J ia n p in g Lu ,† Ye Ta o,*,† Ma r ie D’ior io,† Yu n in g Li,‡ J ia n fu Din g,‡ a n d Mich a el Da y‡
Institute for Microstructural Sciences and Institute for Chemical Process and Environmental
Technology, National Research Council of Canada, 1200 Montreal Road,
Ottawa, ON K1A 0R6, Canada
Received October 22, 2003; Revised Manuscript Received J anuary 14, 2004
ABSTRACT: The influences of the carbazole content on the photophysical, electrochemical, and
electroluminescent properties of alternating fluorene/carbazole copolymers PFnCz (n ) 1, 2, 3) with well-
defined chemical structures have been systematically investigated. The incorporation of carbazole units
into the polyfluorene (PF) backbone resulted in a blue shift of both the absorption and photoluminescence
(PL) emission peaks, improved PL thermal stability, raised HOMO energy levels, and thus facilitated
hole injection into the copolymers. Pure deep blue electroluminescence (EL) with narrow fwhms (full
width at the half-maximum) (39-52 nm) and negligible low-energy emission bands was successfully
achieved from the PFnCz copolymers by using 1,3,5-tris(4′-fluorobiphenyl-4-yl)benzene (F-TBB) as a hole-
blocking layer and Alq3 as an electron injection/transporting layer. This device configuration stabilized
the blue emission from the PF derivatives. An efficiency of 0.72 cd/A at a luminance of 100 cd/m2 was
obtained even with aluminum metal as the cathode.
In tr od u ction
mainly responsible for this strong low-energy emission.6
Scherf and co-workers have shown that the end-capping
of polyfluorenes with bis(4-methylphenyl)phenylamine
can almost completely suppress the keto defect emission
band.7 Another serious problem associated with poly-
fluorene homopolymer is the significant energy barrier
for hole injection from the ITO anode since its HOMO
energy level is as low as 5.8 eV.8 As a result, an addi-
tional hole transport layer (HTL) is necessary in poly-
fluorene-based PLED structures to achieve acceptable
device performance. Poly(3,4-ethylenedioxythiophene)-
poly(styrenesulfonate) (PEDOT-PSS)9 and camphor-
sulfonic acid-doped polyaniline10 are widely used as the
hole injection/transport layer in blue PLED devices.
Although these conducting polymers have been shown
to increase hole injection, none of them is perfect
because of their significant absorption in the visible
region and the unstable interface they form with ITO.11
Another approach to address the deficiency in hole
injection is the modification of chemical structures of
PFs through the incorporation of electron-donating
moieties, such as thiophene,12 triphenylamine,13 and
carbazole,4b into the polymer backbone.
The first report in 1990 on the electroluminescence
(EL) from poly(p-phenylenevinylene)1 triggered a large
amount of interest in polymeric light-emitting diodes
(PLEDs). Since their initial appearance, many improve-
ments have been made in polymer-based devices, in-
cluding emission across the entire visible spectrum, low
drive voltage, good efficiency, and high brightness.2
However, the development of blue-light-emitting poly-
mers remains the subject of intense research in both
academia and industry due to their potential application
in full-color flat panel displays. They can be used either
as the active layer in PLEDs or as the host material for
internal color conversion techniques.3
In the past decade, fluorene-based conjugated poly-
mers (PFs) have emerged as a very promising class of
blue-light emitting materials for use in PLEDs because
of their high photoluminescence (PL) and electrolumi-
nescence quantum efficiencies, thermal stability, good
solubility, and facile functionalization at the C-9 position
of fluorene.4 However, the application of polyfluorenes
in PLEDs has been hampered because of the trouble-
some formation of a tailed emission band at long
wavelengths (> 500 nm) during device fabrication and
operation, leading to both a color instability and reduced
efficiency. Some researchers attributed this instability
to the undesired interchain aggregation and/or excimer
formation and demonstrated that the introduction of
bulky groups at the C-9 position of the fluorene or
introduction of cross-linkable moieties such as styryl
groups tended to suppress the excimer emission and to
improve the thermal stability of PL spectra.5 However,
recent results indicated that keto defects in the polymer
backbone, originating probably from photo- and/or elec-
trooxidative degradation of 9,9,-dialkylated PFs, were
In this study, we prepared a series of alternating
fluorene/carbazole copolymers PFnCz (n ) 1, 2, 3) with
well-defined chemical structures and systematically
investigated the influences of the carbazole content on
photophysical, electrochemical, and electroluminescent
properties of the resulting polymers. Cyclic voltammet-
ric studies have shown that the HOMO energy levels
of copolymers PFnCz can be raised by increasing the
carbazole content. Consequently, the hole injection was
greatly improved, as proven by the current density-
electric field strength characteristics of the hole-only
devices consisting of the PFnCz film sandwiched be-
tween ITO and Au electrodes. As the main-chain
conjugation is interrupted by the presence of the 3,6-
carbazole units, the absorption and PL emission peaks
of the alternating copolymers were blue-shifted with an
increase in the carbazole content. Compared to PF
† Institute for Microstructural Sciences.
‡ Institute for Chemical Process and Environmental Technology.
* To whom all correspondences should be addressed: Tel (613)
998-2485; Fax (613) 990-0202; e-mail Ye.Tao@nrc-cnrc.gc.ca.
10.1021/ma030535n CCC: $27.50 Published 2004 by the American Chemical Society
Published on Web 03/05/2004

Macromolecules, Vol. 37, No. 7, 2004
Alternating Fluorene/Carbazole Copolymers 2443
Ch a r t 1
sealed in a soft glass rod were used as the quasi-reference
electrode, counter electrode, and working electrode, respec-
tively. The Ag quasi-reference electrode was calibrated using
a ferrocene/ferrocenium redox couple as an external standard
prior to measurements.
EL Device F a br ica tion a n d Testin g. ITO-glass sub-
strates (15 Ω/0) were patterned by the conventional photoli-
thography using an acid mixture of HCl and HNO3 as the
etchant. After patterning, the samples were rinsed in deionized
water and then ultrasonicated sequentially in acetone and
2-propanol. The active area of each EL device was 5 × 6 mm2.
A thin layer of PFnCz copolymer was spin-coated onto the ITO
substrate from its chloroform solution (ca. 12 mg/mL) at 2000
rpm for 60 s. The thickness of the resulting film was measured
using a Dektak3 surface profilometer and found to be around
100 nm. A hole-blocking layer (20 nm) was then vacuum-
deposited on top of the polymer film at 2 × 10-7 Torr, followed
by the deposition of Alq3 thin film (10 nm). The device
fabrication was completed by the evaporation of LiF and
aluminum cathode. The devices were tested in air under
ambient conditions with no protective encapsulation. EL
spectra, device luminances, and current-voltage characteris-
tics were recorded using a combination of a Photo Research
PR-650 SpectraScan and a Keithley 238 source meter.
4-Acetyl-4′-br om obip h en yl (1). 4-Bromobiphenyl (10.0 g,
42.9 mmol) was dissolved in 60 mL of CH2Cl2 at 0 °C under a
blanket of nitrogen. In a separate 150 mL three-necked flask
equipped with an addition funnel, a condenser, and a nitrogen
inlet, AlCl3 (10.00 g, 74.9 mmol) and CH2Cl2 (80 mL) were
added. Acetyl chloride (3.80 mL, 53.4 mmol) was added to the
AlCl3 suspension. The pale greenish mixture of the AlCl3/CH3-
COCl complex with uncomplexed AlCl3 was then added drop-
wise to the 4-bromobiphenyl solution over a period of 1 h. Upon
the addition of the AlCl3/CH3COCl complex, the reaction
solution turned dark yellow immediately. After the completion
of the addition, the reaction mixture was further stirred at 0
°C for another 5 h. The reaction mixture was then poured into
ice-HCl (300 mL of broken ice/30 mL of concentrated HCl).
The organic layer was separated, washed with water, and dried
with MgSO4. The crude product was isolated as a pale yellow
powder by distilling the solvent out under reduced pressure
and purified by silica gel column chromatography using CH2-
Cl2 as the eluent. After drying at 45 °C in vacuo overnight,
8.21 g (69.6% yield) of a white crystalline powder was obtained.
Mp: 129.7 °C. TLC (silica, CH2Cl2): one spot, Rf ) 0.32. 1H
NMR (CDCl3): δ (ppm) 8.02 (d, 2H, J ) 8.4 Hz), 7.64 (d, 2H,
J ) 8.4 Hz), 7.59 (d, 2H, J ) 8.4 Hz), 7.48 (d, 2H, J ) 8.4 Hz),
2.63 (s, 3H).
homopolymer, all of the PFnCz copolymers examined
in this study exhibited much more stable emission
spectra with very little change after annealing at 150
°C under vacuum for 24 h. This is probably due to the
interruption of the linearity of the polymer backbones
by the 3,6-carbazole units. A pure deep blue PLED with
negligible emission in the 500-600 nm region has been
achieved from the PFnCz copolymers by using 1,3,5-tris-
(4′-fluorobiphenyl-4-yl)benzene (F-TBB) as a hole-block-
ing layer and Alq3 as an electron injection/transporting
layer. An efficiency of 0.72 cd/A at a luminance of 100
cd/m2 was obtained even with aluminum metal as the
cathode. However, when the F-TBB was replaced by
1,3,5-tris(4′-cyanobiphenyl-4-yl)benzene (CN-TBB), de-
vice performance degraded, leading to the broadening
of the emission peak and a much lower efficiency. This
means that the choice of hole-blocking material has a
large impact on the device performance.
Exp er im en ta l Section
Ma ter ia ls. Reagent grade solvents and chemicals were used
as received unless otherwise noted. Aluminum chloride, 4-bro-
mobiphenyl, acetyl chloride, copper(I) cyanide, and trifluo-
romethanesulfonic acid were purchased from Aldrich. 4-Flu-
orobiphenyl was obtained from Fluorochem USA. Acetonitrile
used in the electrochemical measurements was refluxed in the
presence of calcium hydride under argon and distilled prior
to use. Alternating 9,9-dioctylfluorene/9-octylcarbazole copoly-
mers PFnCz (n ) 1, 2, 3) were synthesized using the Suzuki
coupling reaction of 9-octylcarbazole-3,6-bis(ethyleneboronate)
and the corresponding fluorene oligomer dibromo compounds
in refluxing toluene in the presence of Pd(PPh3)4 (0.5% equiv),
2 M NaCO3 (3.3 equiv), and a phase-transfer catalyst, trica-
prylylmethylammonium chloride (Aliquat 336). The chemical
structures of the resulting copolymers PFnCz are shown in
Chart 1, and the detailed synthetic procedures were published
elsewhere.14
1,3,5-Tr is(4′-br om obip h en yl-4-yl)ben zen e (Br -TBB), 3.
To a 100 mL three-necked round-bottom flask fitted with a
magnetic stirrer, a condenser, and a nitrogen inlet was added
4-acetyl-4′-bromobiphenyl, 1 (4.40 g, 0.016 mol), trifluo-
romethanesulfonic acid (0.8 mL), and toluene (35 mL). The
reaction solution was stirred at reflux under nitrogen for about
14 h. After cooling, the crude product was collected by
filtration, washed with methanol, and purified by recrystal-
lizing twice from chloroform. After drying at 90 °C in vacuo
overnight, 1.60 g (38.9%) of a white crystalline powder was
obtained.
Ch a r a cter iza tion . 1H NMR spectra were obtained in
CDCl3 on a 400 MHz Varian Unity Inova spectrometer. Thin-
layer chromatography (TLC) was carried out on Silica gel 60
F (254) precoated sheets (Aldrich). The molecular weights of
fluorene/carbazole copolymers were determined using gel
permeation chromatography (GPC) on a Waters model 515
HPLC equipped with µ-Styragel columns (103, 104, and 105)
using THF as an eluent with polystyrene standards. UV-vis
absorption spectra were recorded on a Varian Cary 50 spec-
trophotometer. Fluorescence measurements were carried out
on a Spex Fluorolog 3 spectrometer. The absorption λmax of
samples were used as the excitation wavelengths (λexc) for the
measurement. The differential scanning calorimetry (DSC)
analysis of small molecules and fluorene/carbazole copolymers
was performed under a nitrogen atmosphere on a TA Instru-
ments DSC 2920 at heating rates of 10 °C/min. High-resolution
mass spectrometry was carried out by the mass spectroscopy
facility in the Chemistry Department, University of Ottawa.
Cyclic voltammetry (CV) measurements were conducted in a
0.1 M Bu4NPF6 acetonitrile solution using a Solartron SI 1287
potentiostat at a scan rate of 50 mV s-1 at room temperature
under nitrogen. A silver wire (2 mm diameter), a platinum
wire (0.5 mm diameter), and a platinum disk (1 mm diameter)
Mp: 280.7 °C. TLC (silica, CH2Cl2): one spot, Rf ) 0.92. 1H
NMR (CDCl3): δ (ppm) 7.86 (s, 3H), 7.79 (d, 6H, J ) 8.4 Hz),
7.69 (d, 6H, J ) 8.4 Hz), 7.59 (d, 6H, J ) 8.4 Hz), 7.53 (d, 6H,
J ) 8.4 Hz). MS: found M+ 768; calcd M+ 768.
1,3,5-Tr is(4′-cya n obip h en yl-4-yl)ben zen e (CN-TBB), 5.
A 100 mL three-necked round-bottom flask fitted with a
magnetic stirrer, a condenser, and a nitrogen inlet was charged
with Br-TBB 3 (1.60 g, 2.08 mmol), CuCN (1.12 g, 12.5 mmol),
and N-methyl-2-pyrrolidinone (15 mL). The reaction mixture
was heated to reflux under nitrogen. Upon heating, both
compound 3 and CuCN dissolved into NMP to form a greenish
yellow solution. The reaction solution was stirred at reflux for
about 4 h. During stirring, a brown solid precipitated out of
the solution. The hot reaction mixture was poured into a
solution of FeCl3 (8.0 g), concentrated HCl (4 mL), and H2O
(24 mL), and stirred at 50 °C for 20 min. The crude product

2444 Lu et al.
Macromolecules, Vol. 37, No. 7, 2004
Sch em e 1. Syn th etic Rou te to F -TBB a n d CN-TBB
was collected by filtration and washed sequentially with
aqueous HCl solution, H2O, and methanol. Purification by
column chromatography using CH2Cl2 as an eluent afforded
1.05 g (83.1% yield) of a white powder.
Mp1: 265.5 °C; Mp2: 333. 6 °C (major peak). Tg: 116.0 °C.
TLC (silica, CH2Cl2): one spot, Rf ) 0.33. 1H NMR (CDCl3): δ
(ppm) 7.90 (s, 3H), 7.85 (d, 6H, J ) 8.8 Hz), 7.77 (s, 12H), 7.75
(d, 6H, J ) 8.8 Hz). High-resolution MS: found M+ 609.2216;
calcd M+ 609.2205.
4-Acetyl-4′-flu or obip h en yl (2). With the same procedure
as described for the synthesis of the compound 1, 4-fluorobi-
phenyl (10.00 g, 58.1 mmol) was reacted with acetyl chloride
(5.20 mL, 73.1 mmol) in the presence of AlCl3 as catalyst.
Purification by column chromatography afforded 10.36 g of a
white powder in 83.3% yield.
Mp: 104.3 °C. TLC (silica, CH2Cl2): one spot, Rf ) 0.43. 1H
NMR (CDCl3): δ (ppm) 8.03 (d, 2H, J ) 8.4 Hz), 7.64 (d, 2H,
J ) 8.4 Hz), 7.59 (dd, 2H, J 1 ) 8.8 Hz, J 2 ) 5.2 Hz), 7.16 (t,
2H, J ) 8.8 Hz), 2.63 (s, 3H).
TBB) was first synthesized by Shirota via the Suzuki
coupling reaction of 1,3,5-tris(4-iodophenyl)benzene with
4-fluorophenylboronic acid.15 The energy gap of F-TBB
was reported to be about 3.85 eV. In this study, we
synthesized this compound by another approach, namely
a condensation reaction. The synthetic route is outlined
in Scheme 1. Besides F-TBB, we also synthesized CN-
TBB with an intension to improve the electron injection/
transport property by replacing the fluorine in F-TBB
with the stronger electron-withdrawing cyano group.
The powder sample of CN-TBB obtained from column
chromatography showed two crystallization tempera-
tures at 172.3 and 211.8 °C, respectively, and one
melting point at 333.6 °C in the first DSC analytical
scan. Meanwhile, the second DSC scan following slow
cooling in nitrogen exhibited a glass transition temper-
ature at 116.0 °C, a broad crystallization peak around
250 °C, and a double melting exotherm at 265.5 and
333.6 °C. The third DSC scan was identical to the second
scan. This thermal behavior associated with polymor-
phism has been observed for a number of amorphous
molecular materials.15 Both F-TBB and CN-TBB can
form stable amorphous thin films by vacuum deposition
and exhibit glass transition temperatures at 89.4 and
116.0 °C, respectively. The UV-vis absorption and
fluorescence spectra of the vacuum-deposited 20 nm
thick films of F-TBB and CN-TBB are shown in Figure
1. As can be seen from Figure 1, the replacement of
fluorine with the cyano group causes a red shift in both
absorption and PL emission peaks, indicating that CN-
TBB has a longer effective conjugation length and lower
energy gap. The physical properties of the F-TBB and
CN-TBB are summarized in Table 1.
1,3,5-Tr is(4′-flu or obip h en yl-4-yl)ben zen e (F -TBB), 4.
With the same procedure as described for the synthesis of the
compound 3, 4-acetyl-4′-fluorobiphenyl, 2 (4.40 g, 0.021 mol),
was stirred in refluxing toluene in the presence of trifluo-
romethanesulfonic acid. The crude product was purified by
silica gel column chromatography using CH2Cl2 as the eluent,
followed by recrystallization from toluene/hexanes. Yield: 1.55
g (38.5%). This was further purified by train sublimation at
10-5 Torr.
Mp: 237.0 °C (lit.15 236 °C). Tg: 89.4 °C. TLC (silica, CH2-
1
Cl2): one spot, Rf ) 0.92. H NMR (CDCl3): δ (ppm) 7.88 (s,
3H), 7.80 (d, 6H, J ) 8.4 Hz), 7.69 (d, 6H, J ) 8.4 Hz), 7.63
(dd, 6H, J 1 ) 9.2 Hz, J 2 ) 5.2 Hz), 7.17 (dd, 6H, J 1 ) 9.2 Hz,
J 2 ) 8.0 Hz).
Resu lts a n d Discu ssion
Syn t h esis a n d P h ysica l P r op er t ies of H ole-
Block in g Ma ter ia ls F -TBB a n d CN-TBB. As a hole-
blocking material, the compound should have a high
oxidation potential and large HOMO-LUMO energy
band gap. 1,3,5-Tris(4′-fluorobiphenyl-4-yl)benzene (F-
Th er m a l a n d Op tica l P r op er ties of Alter n a tin g
Cop olym er s P F n Cz. DSC curves of the PFnCz copoly-
mers show no crystallization and melting peaks but only
glass transition temperatures (Tgs) ranging from 98 to

Macromolecules, Vol. 37, No. 7, 2004
Alternating Fluorene/Carbazole Copolymers 2445
F igu r e 1. UV-vis absorption and fluorescence spectra of
vacuum-deposited 20 nm thick films of F-TBB (solid line) and
CN-TBB (dashed line).
F igu r e 2. UV-vis absorption spectra of PFnCz films (nor-
malized to 100 nm) spin-coated on quartz slides.
Ta ble 1. P h ysica l P r op er ties of Hole-Block in g Ma ter ia ls
F -TBB a n d CN-TBB
abs
max
compound
Tg (°C)a
λ
(nm)b
λem (nm)b
F-TBB
CN-TBB
89.4
116.0
294
314
379
396
a
b
Determined by DSC. Measured on vacuum-deposited 20 nm
thick films.
113 °C. The Tgs of PFnCz increase with the amount of
carbazole in the copolymer. On the contrary, poly(9,9-
dioctylfluorene) (POF) had a crystallization temperature
of 113 °C and a melting point at 159 °C. This clearly
indicates that the presence of the carbazole units in
these copolymers effectively suppresses the crystalliz-
ability (or chain aggregation) of the polymer chains.
Table 2 summarizes the physical properties of copoly-
mers PFnCz.
UV-vis absorption and PL spectra of PFnCz films
spin-coated on quartz slides are shown in Figures 2 and
3, respectively. These two spectra indicate that the
absorption and emission peaks are gradually blue-
shifted to shorter wavelengths with an increase in the
carbazole content in the copolymer. This is due to the
interruption of the main-chain conjugation by the pres-
ence of the 3,6-carbazole units. The PL efficiencies (Φfl)
of these copolymers were measured in dilute THF
solution by comparing emission with that of a standard
solution of 9,10-diphenylanthracene in cyclohexane (Φfl
) 0.95) at room temperature.16 In general, these co-
polymers showed slightly decreased PL efficiencies (Φfl
) 0.68-0.76) compared with POF (Φfl ) 0.84) because
of their shorter fluorene sequences. One striking prop-
F igu r e 3. PL spectra of PFnCz films spin-coated on quartz
slides.
erty of these copolymers was that their PL spectra
showed remarkable stability having very little change
on annealing in a vacuum oven at 150 °C (a temperature
well above their glass transition temperatures) for 24
h. In contrast, the emission spectrum of the annealed
POF film red-shifted ∼10 nm, and a featureless broad
Ta ble 2. P h ysica l P r op er ties of Alter n a tin g P F n Cz Cop olym er s a n d P OF Hom op olym er
c
polymer
Mn (×103)
PDI
Tg (°C)
λambasx (nm)a
λfelm (nm) As-spa
λfelm (nm) Annb
Φfl
PF1Cz
PF2Cz
PF3Cz
POF
26.0
35.9
31.0
47.2
2.29
2.51
2.65
3.45
113
108
98
348
364
370
385
412
415, 439
419, 443,471
422, 447,481
413
416, 439
421, 445, 474
430, 455, 485, 512
0.68
0.76
0.76
0.84
ndd
a
b
Obtained from the pristine films spin-coated on quartz slides. Films annealed at 150 °C under vacuum for 24 h. c Measured in THF
d
solution relative to 9,10-diphenylanthracene (Φfl ) 0.95 in cyclohexane) as a reference. No noticeable Tg was observed; instead, a
crystallization peak at 113 °C and a melting peak at 159 °C were noted.

2446 Lu et al.
Macromolecules, Vol. 37, No. 7, 2004
Ta ble 3. Electr och em ica l P r op er ties a n d En er gy Levels of F -TBB, CN-TBB, a n d P F n Cz
ox
1/2
ox
p/2
red
1/2
red
p/2
sample
E
or E , Va
E
or E , Va
E
HOMO,b eV
ELUMO,b eV
Eg, eV (Echem)c
Eg, eV (UV)d
F-TBB
CN-TBB
PF1Cz
PF2Cz
PF3Cz
POF
1.91
1.78
0.95
0.96
1.01
1.25
-2.01
-1.83
-2.36
-2.34
-2.21
-1.95
6.34
6.21
5.38
5.39
5.43
5.68
2.42
2.60
2.07
2.09
2.22
2.48
3.92
3.61
3.31
3.30
3.22
3.20
3.63
3.33
3.06
2.98
2.95
2.91
ox
1/2
red
1/2
a
E
and E
stand for half-wave oxidation (p-doping) and reduction (n-doping) potentials vs an Ag quasi-reference, respectively.
ox
red
Estimated from E1/2 by using empirical equations: EHOMO ) E1/2 + 4.43 eV and ELUMO ) E1/2 + 4.43 eV. c Electrochemical band gaps
determined using Eg ) EHOMO - ELUMO
b
d
.
Optical energy gap calculated from the edge of the electronic absorption band.
peak centered at 512 nm appeared. These spectral
changes were observed previously for fluorene ho-
mopolymers and were believed to be due to the forma-
tion of interchain aggregates at high temperatures.17
We think the interruption of the linearity of the polymer
backbone by the 3,6-carbazole units can hamper close
chain packing.18
Electr och em ica l Stu d y. To investigate the electro-
chemical properties of CN-TBB, F-TBB, and copolymers
PFnCz and estimate their HOMO and LUMO energy
levels, cyclic voltammetry (CV) was carried out using a
platinum disk (1 mm diameter), a platinum wire (0.5
mm diameter), and a silver wire (2 mm diameter) as
the working electrode, counter electrode, and quasi-
reference electrode, respectively. CV measurements
were conducted in a 0.1 M Bu4NPF6 acetonitrile solution
at room temperature under nitrogen with a scan rate
of 50 mV s-1. PFnCz copolymers were coated on the
working electrode, while small molecules F-TBB and
CN-TBB were directly dissolved in acetonitrile. The Ag
quasi-reference electrode was calibrated using ferrocene/
ferrocenium (Fc/Fc+) redox couple (4.8 eV below the
vacuum level)17 as an external standard, and the E1/2
of the Fc/Fc+ redox couple was found to be 0.37 V vs
the Ag quasi-reference electrode. Therefore, the LUMO
F igu r e 4. Electric field strength-hole current density char-
acteristics of the hole-only devices ITO/PFnCz/Au.
This implies that the electron-charged state (reductive
state) of PF1Cz is rather unstable.
and HOMO energy levels of the materials can be
ox
1/2
estimated using the empirical equations ELUMO ) E
The hole injection/transporting properties of these
copolymers were further investigated using “hole-only”
devices with a configuration of ITO/PFnCz/Au. The
thickness of the spin-coated polymer films ranged from
90 to 110 nm, depending on the molecular weights of
the copolymers. The concentration of POF solution was
reduced to 10 mg/mL in order to give similar thickness.
As shown in Figure 4, the hole current densities of the
devices at the same electric field strength increase with
increased carbazole contents. This result is analogous
to their HOMO levels as determined by cyclic voltam-
metry and confirms the improved hole injection and/or
transporting by the incorporation of the carbazole units.
Th e Role of Hole-Block in g La yer s. Although the
incorporation of the carbazole units can raise the HOMO
energy levels, the LUMO levels of the copolymers are
also raised, leading to larger energy barriers for electron
injection. As anticipated, the single layer device of ITO/
PF2Cz/LiF(1 nm)/Al gave a very weak emission (1.9 cd/
m2) even at a current density of 63 mA/cm2. A very
possible reason for this extremely low EL efficiency is
that a major fraction of holes just passed through the
device and did not combine with electrons to form
excitons; i.e., holes are the majority carrier in these
devices. To improve the device performance, a hole-
blocking layer has to be used to balance the charge
carriers in the emissive layer. The hole-blocking mater-
ial should have a high oxidation potential and large
HOMO-LUMO energy band gap. Shirota first demon-
strated that F-TBB can be used as a hole-blocking
red
+ 4.43 eV and EHOMO ) E
+ 4.43 eV, respectively,
are the half-wave potentials for
1/2
ox
red
1/2
where E
and E
1/2
oxidation and reduction relative to the Ag quasi-refer-
ence electrode, respectively. Table 3 summarizes the
electrochemical properties of the materials used in this
study. Both CN-TBB and F-TBB underwent reversible
cathodic reduction. However, the anodic oxidation pro-
cess of F-TBB was only partially reversible. This partial
reversibility does not prevent it from acting as a hole
blocker because it serves to pass on electrons from the
electron-transport layer to the emitting layer.15 As
compared with F-TBB, CN-TBB has a lower LUMO
level and relatively smaller energy gap due to its longer
effective conjugation length and the electron-withdraw-
ing nature of the cyano groups.
The POF thin films showed typical CV curves similar
to those reported previously for this polymer.19 Both the
cathode and anode sweeps were reversible. As can be
seen from Table 3, the incorporation of the carbazole
units raises the HOMO energy levels from 5.68 to 5.38
eV and also slightly increases the energy band gaps of
the copolymers. HOMO levels rise up with increasing
the carbazole content.
The electrochemical band gaps are higher than the
optically measured ones probably due to the interfacial
energy barrier for charge injection.20 It should be noted
that the alternating copolymer PF1Cz had an irrevers-
ible CV curve in the n-doping process, although PF2Cz
and PF3Cz underwent reversible cathodic reduction.

Macromolecules, Vol. 37, No. 7, 2004
Alternating Fluorene/Carbazole Copolymers 2447
F igu r e 6. EL spectra of PF2Cz for devices A, B, and C.
F igu r e 5. Voltage-luminance (a) and voltage-efficiency (b)
curves of PF2Cz for devices A, B and C. A: ITO/PF2Cz/F-TBB
(20 nm)/Alq3 (10 nm)/LiF/Al; B: ITO/PF2Cz/CN-TBB (20 nm)/
Alq3 (10 nm)/LiF/Al; and C: ITO/PF2Cz/CN-TBB (30 nm)/LiF/
Al.
material in small molecular blue-violet emitting diodes.
In our study, we investigated the effects of F-TBB and
CN-TBB on the device performance with PF2Cz as the
emitting layer. Three types of devices were designed and
fabricated: ITO/PF2Cz/F-TBB(20 nm)/Alq3 (10 nm)/LiF/
Al (device A), ITO/PF2Cz/CN-TBB(20 nm)/Alq3 (10 nm)/
LiF/Al (device B), and ITO/PF2Cz/CN-TBB(30 nm)/LiF/
Al (device C). As can be seen from Figure 5, the choice
of the blocking layer has a strong impact on device
performance. To our surprise, the F-TBB was much
superior to CN-TBB, giving higher luminance and EL
efficiency. The turn-on voltages, corresponding to a
luminance of 1 cd/m2, increased from 10 to 14 V when
F-TBB was replaced by CN-TBB. Figure 6 displays the
EL spectra of devices A, B, and C. No emission from
Alq3 was observed from devices A and B, indicating both
CN-TBB and F-TBB can function as hole-blocking
materials. The fwhms (full width at the half-maximum)
of the EL spectra, however, increased from 52 nm
(device A) to 69 nm (device B). The CIE 1931 chromatic-
ity coordinates of the emission of devices A, B, and C
are (0.172, 0.087), (0.194, 0.162), and (0.188, 0.144),
respectively. It is clear that from device A to device B
the emission colors changed from deep blue to blue. In
F igu r e 7. EL spectra of PF2Cz (solid line), PF3Cz (dashed
line), and POF (dotted line) from devices ITO/PFnCz/F-TBB/
Alq3/LiF/Al.
device C, no Alq3 was used; however, its EL spectrum,
I-V curve, and luminance efficiency are quite similar
to those of device B. Therefore, we think that although
CN-TBB can function as a hole-blocking and electron-
transporting layer, it may form exciplexes with PF2Cz
at the interface, leading to the broadening of the EL
emission and poor luminance efficiency.
EL P r op er ties of P F n Cz. In this paper, we also
studied the effect of carbazole contents on the EL
performance by fabricating double-layer devices with a
configuration of ITO/PFnCz/F-TBB/Alq3/LiF/Al. The
copolymer PF1Cz was not studied here due to its too
short a conjugation length and low PL efficiency. As can
be seen from Figure 7, a pure deep blue EL with
negligible emission in the 500-600 nm region can be
achieved from the PFnCz copolymers by using F-TBB
as a hole-blocking layer and Alq3 as an electron injec-

2448 Lu et al.
Macromolecules, Vol. 37, No. 7, 2004
driving voltage was probably due to the aluminum
cathode used in the devices and nonoptimized layer
thicknesses, while the efficiencies at a practical lumi-
nance of 100 cd/m2 (0.54 cd/A for PF2Cz and 0.72 cd/A
for PF3Cz) are comparable to the literature data,4d,21,22
considering the use of the air stable aluminum cathode,
which provided less ideal electron injection into the
devices as compared to the commonly used calcium
electrode. In addition, our device configuration provides
stable deep blue EL with negligible emission in the
500-600 nm region.
Con clu sion s
In a summary, we have systematically studied the
photophysical, electrochemical, and electroluminescent
properties of a series of alternating fluorene/carbazole
copolymers. Both absorption and PL emission peaks of
the alternating copolymers were blue-shifted with an
increase in the carbazole content due to the interruption
of the main-chain conjugation by the presence of the 3,6-
carbazole units. Meanwhile, the HOMO energy levels
were also raised, and thus the hole affinity was im-
proved. Pure deep blue emission with narrow fwhms
(39-52 nm) and negligible low-energy emission bands
was successfully achieved from the PFnCz copolymers
by using F-TBB as a hole-blocking layer and Alq3 as an
electron injection/transporting layer. It seems that this
kind of device structure can stabilize the blue emission
from PF derivatives. In future work, we intend to focus
on the device optimization to further improve the EL
efficiency and luminance, such as treating ITO sub-
strates with Ar/O2 plasma, optimizing the emissive layer
thickness, and using a better cathode material to replace
the aluminum.
Su p p or tin g In for m a tion Ava ila ble: General procedure
for the preparation of alternating 9,9-dioctylfluorene/9-octyl-
carbazole copolymers PFnCz (n ) 1, 2, 3) and their 1H NMR
spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
Refer en ces a n d Notes
F igu r e 8. Electric field strength-luminance (a) and electric
field strength-efficiency (b) characteristics of PF2Cz, PF3Cz,
and POF.
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