Oligofluorene-based push-pull type functional materials for blue light-emitting diodes

Article abstract of DOI:10.1016/j.jphotochem.2011.12.008

A series of new oligofluorene-based push-pull type blue light-emitting functional materials, namely, 2-(9H-carbazole-9-yl)-7-(4-cyanophenyl)-9,9- dihexylfluorene (F1), 7-(9H-carbazol-9-yl)-7′-(4-cyanophenyl)-2,2′- bi(9,9-dihexylfluorene) (F2), 7-(9H-carba

Full text of DOI:10.1016/j.jphotochem.2011.12.008

Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Oligofluorene-based push–pull type functional materials for blue
light-emitting diodes
Ying Lina,b, Yu Chena,∗, Teng-Ling Yec, Zhi-Kuan Chenb,∗, Yan-Feng Daic, Dong-Ge Mac,∗
a Shanghai Key Laboratory of Functional Materials Chemistry, Institute of Applied Chemistry, East China University of Science and Technology, 130 Meilong Road,
Shanghai 200237, China
b Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore
c State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences,
Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of new oligofluorene-based push–pull type blue light-emitting functional materi-
Received 25 September 2011
Received in revised form 4 December 2011
Accepted 14 December 2011
Available online 24 December 2011
als, namely, 2-(9H-carbazole-9-yl)-7-(4-cyanophenyl)-9,9-dihexylfluorene (F1), 7-(9H-carbazol-9-
yl)-7ꢀ-(4-cyanophenyl)-2,2ꢀ-bi(9,9-dihexylfluorene) (F2), 7-(9H-carbazole-9-yl)-7ꢀꢀ-(4-cyanophenyl)-
2,2ꢀ:7ꢀ,2ꢀꢀ-ter(9,9-dihexylfluorene) (F3), and 7-(9H-carbazole-9-yl)-7ꢀꢀꢀ-(4-cyanophenyl)-2,2ꢀ:7ꢀ,2ꢀꢀ:7ꢀꢀ,2ꢀꢀꢀ
-
quarter(9,9-dihexylfluorene) (F4) were synthesized and characterized. Their onset decomposition
temperatures for the thermal bond cleavage and the glass-transition temperatures were in general
increased with increasing number of fluorene units. In dilute toluene solution, the oligofluorenes exhib-
ited main absorption peaks in the range of 343–370 nm, photoluminescence maxima from 403 to 410 nm,
and absolute quantum yields (˚PLs) of higher than 87%. In contrast, the absorption spectra of these
compounds in the thin films had no large differences from those in the solutions except for the slight
peak red-shifts (2–8 nm). The main emission maxima of F1, F2, and F3 in the thin films were located at
418–420 nm, while the main emission of F4 was found to be shifted to 446 nm, followed by a shoulder
peak at 421 nm. The ˚PLs of these thin films were estimated in the range of 59.2–68.7%. The existence of
the electron-pull and -push end groups could effectively tune the energy levels of the oligofluorenes. By
using the organic light emitting device (OLED) configuration of ITO/PEDOT:PSS/oligofluorenes/TPBi/LiF/Al
by solution-process, F4 displayed the best performance: the lowest turn-on voltage (4.1 V) and highest
maximum luminance (2180 cd/m2) with maximal current efficiency of 1.17 cd/A. When F4 was fabricated
into the optimized device of ITO/MoO3/NPB/CBP:F4(1:4)/TPBi/LiF/Al by vapor deposition, highest bright-
ness of 5135 cd/m2 and current efficiency of 1.76 cd/A were achieved with the Commission Internationale
de l’Eclairage (CIE) coordinates of (0.16, 0.09).
Keywords:
Oligofluorene
OLED
Cyanophenyl
Carbazole
Push–pull type functional materials
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
the well-established green- and red-emitting materials, stable and
high-performance blue light-emitting materials and corresponding
As very promising active functional materials, ␲-conjugated
organic oligomers with high chemical purity and well-defined
molecular structures have attracted considerable interest owing
to their great potential for applications in organic thin film elec-
tronic devices such as organic light-emitting diodes (OLEDs) [1–3],
organic photovoltaic (OPV) cells [4–6], and organic field effect tran-
sistors (OFETs) [7–9]. During the past 20 years, much progress has
been made in the field of OLEDs that represent a promising tech-
nology for large, flexible, lightweight, flat-panel displays that could
replace liquid-crystal displays or cathode-ray tubes. Compared to
optimized device structures are still under development. Among
the many reported organic oligomers used as the blue emitters,
fluorene-based oligomers with two or three fluorenyl repeat units
having different functional groups at the C-9 position of the fluo-
rene unit have been used as a good blue light source and/or, as the
host material that can be used to realize other color emissions, as a
result of their excellent thermal stability, high fluorescent quantum
yield, and outstanding electroluminescent behavior [10–14].
It is necessary to increase the injection and transport of the elec-
trons and holes to produce more excitons in the active polymers.
Many hole-transporting groups such as triarylamine and carbazole
have been covalently incorporated into the oligofluorenes by a
variety of reactions as part of the backbone, the end of side chain,
branch points of star, and end-capping groups. For example, Ma
et al. [15] reported the synthesis of spirobifluorene trimers with
∗
Corresponding authors.
E-mail addresses: chentangyu@yahoo.com (Y. Chen),
zk-chen@imre.a-star.edu.sg (Z.-K. Chen), mdg1014@ciac.jl.cn (D.-G. Ma).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.12.008

56
Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
OH
NC
B
OH
N
H
Br
Br
N
CN
Br
N
Pd(PPh3)4, K2CO3,
CuI, K2CO3,
1,10-phenanthroline,
DMF, reflux
toluene, H2O, 80 o
C
1
2
F1
CN
B
Pd(PPh3)4, K2CO3,
toluene, H2O, 80 o
C
HO
OH
O
O
O
O
O
B
B
B
2
CN
Br
N
O
2
NC
Pd(PPh3)4, K2CO3,
PdCl2(dppf), KOAc,
NC
toluene, H2O, 80 o
C
dioxane, 80 o
C
4
3
F2
OH
B
NC
N
H
Br
OH
Br
N
Br
CN
N
3
3
3
CuI, K2CO3,
Pd(PPh3)4, K2CO3,
toluene, H2O, 80 o
1,10-phenanthroline,
DMF, reflux
C
6
F3
5
Pd(PPh3)4, K2CO3,
toluene, H2O, 80 o
4
C
CN
N
4
F4
Scheme 1. Synthetic routes of F1–F4.
peripheral carbazole groups as side chains. These compounds have
higher HOMO energy levels and better hole-injection abilities in
comparison with the trimers without carbazoles. A series of multi-
H shaped oligofluorenes surrounded by triphenylamine units
through the C-9’s of the central fluorene with pure blue electro-
luminescence and high HOMO energy levels were studied by Tao
et al. [16]. Four starburst oligomers bearing a 4,4ꢀ,4ꢀꢀ-tris(carbazol-
9-yl)-triphenylamine (TCTA) core and six oligofluorene arms were
also reported to demonstrate that the presence of the TCTA was
crucial for the hole-transporting [17]. Using Ullmann coupling
reaction, Promarak et al. [18] prepared a series of N-carbazole
end-capped oligofluorenes with 1, 2, and 3 fluorene units and
found that these molecules exhibited red shifts in absorption
and photoluminescence spectra with respect to the number of
fluorene units and excellent electrochemical reversibility. In
such a system energy or exciton can efficiently transfer from the
peripheral carbazole via the lone electron pair of the nitrogen atom
to the oligofluorene backbone. Using either palladium catalyzed
cross-coupling reaction or nickel-catalyzed reductive dimeriza-
tion, the same authors designed and synthesized a series of new
N-carbazole end-capped oligofluorene–thiophenes with one, two,
three, and four thiophene rings [19]. The terminal carbazole and
fluorene moieties of the resulting materials are beneficial for
their morphology, conjugation length, and solubility. Although
these carbazole end-capped oligomers were potential blue light-
emitting materials with improved hole affinity, the corresponding
OLED device performance was deficient. To address this problem,
covalent grafting of electron-withdrawing and donating groups
onto the two sides of the oligofluorene backbone, respectively,
would be an effective way to improve the charge injection of
blue light-emitting oligomers. In the family of electron deficient
units such as cyano group [20–23], oxadiazole [24,25], quinoline
[26,27], pyridine [28,29], and others, the cyano group has been
demonstrated to possess prominent capacities of electron affinity
and luminance [30,31]. For these reasons, we, for the first time,
prepared a series of oligofluorenes end-capped by cyanophenyl and
carbazole units: 2-(9H-carbazole-9-yl)-7-(4-cyanophenyl)-9,9-
dihexyl- fluorene (F1), 7-(9H-carbazol-9-yl)-7ꢀ-(4-cyanophenyl)-
2,2ꢀ-bi(9,9-dihexylfluorene) (F2), 7-(9H-carbazole-9-yl)-7ꢀꢀ-(4-
cyanophenyl)-2,2ꢀ:7ꢀ,2ꢀꢀ-ter(9,9-dihexylfluorene) (F3), and 7-(9H-
carbazole-9-yl)-7ꢀꢀꢀ-(4-cyanophenyl)-2,2ꢀ:7ꢀ,2ꢀꢀ:7ꢀꢀ,2ꢀꢀꢀ-quarter(9,9-
dihexylfluorene) (F4), as shown in Scheme 1. As
a result,
when F4 was fabricated into an optimized device of
ITO/MoO3/NPB/CBP:F4(1:4)/TPBi/LiF/Al by vapor deposition,
highest brightness of 5135 cd/m2 and current efficiency of
1.76 cd/A were achieved with the CIE coordinates of (0.16, 0.09).
2. Experimental
2.1. General
Organic solvents were purified, dried, and distilled under
dry nitrogen. 2,7-dibromo-9,9-dihexylfluorene (1) was pur-
chased from Sigma–Aldrich. 4-Cyanophenyl-boronic acid and

Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
57
bis(pinacolato)diboron were bought from Boron Molecular. Cop-
2.3. Synthesis of
per(I) iodine and 1,10-phenanthroline were from Merck and
Lancaster, respectively. Tetrakistriphenylphosphine palladium(0)
[Pd(PPh3)4] and dichloro 1,1ꢀ-bis(diphenylphos-phino)ferrocene
palladium (II) [PdCl2(dppf)] were purchased from Strem Chemicals.
Potassium acetate, carbazole, and potassium carbonate were from
Alfa Aesar, Fluka, and Fisher Scientific, respectively. 7,7ꢀꢀ-Dibromo-
2,2ꢀ:7ꢀ,2ꢀꢀ-ter(9,9-dihexylfluorene) (5) was synthesized according
to the literature reported [32].
Nuclear magnetic resonance spectra were recorded on a Bruker
400 spectrometer at a resonance frequency of 400 MHz for 1H
and 100 MHz for 13C in deuterated solution with a tetramethyl-
silane (TMS) as a reference for the chemical shifts. Elemental
analyses were conducted on a Flash 1112 Series elemental ana-
lyzer. The matrix assisted laser desorption ionization time-of-flight
mass spectra (MALDI-TOF–MS) of F1–F4 were measured on a
Bruker Autoflex II TOF/TOF mass spectrometer (Bruker Dalton-
ics). Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out using TA instruments TGA-
Q500 and DSC-Q100 modules, respectively. The UV–vis absorption
and photoluminescence (PL) spectra were recorded on a Shimadzu
UV-3101 scanning spectrophotometer and on a Perkin-Elmer LS
55 fluorescence spectrometer, respectively. The absolute PL quan-
tum yields were obtained with a HORIBA-JOBIN-YVON fluoromax-4
spectrofluorometer.
2-bromo-7-(4-cyanophenyl)-9,9-dihexylfluorene (3)
Pd(PPh3)4 (37 mg, 3.5% mmol) was added into a mixture
of 2,7-dibromo-9,9-dihexylfluorene (1) (3.446 g, 7.00 mmol), 4-
cyanophenylboronic acid (514 mg, 3.50 mmol) and potassium
carbonate (1.693 g, 12.25 mmol) in argon atmosphere. Degassed
toluene (35 mL) and water (7 mL) was added into the mixture by
a syringe and heated to reflux with continuous stirring in the dark
for 24 h under the protection of nitrogen. After cooling to room
temperature, the organic layer was separated and the aqueous
layer was extracted with ethyl acetate. The combined organic layer
was washed with water, brine and dried over sodium sulphate.
The solvent was removed under reduced pressure and the crude
product was purified by silica gel column chromatography with
5:1 n-hexane/ethyl acetate to give 3 as a white powder (1.291 g,
71.6%). 1H NMR (CDCl3, 400 MHz): ı/ppm = 7.76 (s, 1H), 7.75 (s, 4H),
7.60–7.56 (m, 2H), 7.51–7.47 (m, 3H), 2.00–1.96 (m, 4H), 1.14–1.04
(m, 12H), 0.78–0.74 (t, 6H, J = 6.8 Hz), 0.65–0.62 (t, 4H, J = 6.8 Hz);
13C NMR (CDCl3, 100 MHz): ı/ppm = 153.32, 151.46, 145.98, 140.80,
139.32, 138.39, 132.61, 130.22, 127.77, 126.43, 126.34, 121.68,
121.55, 121.38, 120.37, 118.96, 110.83, 55.67, 40.26, 31.45, 29.60,
23.76, 22.54, 13.94. Anal. calcd for C32H36BrN: C, 74.70; H, 7.05; N,
2.72. Found: C, 74.32; H, 7.12; N, 3.02.
Cyclic voltammetry was performed on a three-electrode AUTO-
LAB (model PGSTAT30) workstation in deaerated acetonitrile
containing nBu4NPF6 (0.10 M) as supporting electrolyte at 298 K. A
conventional three-electrode cell was used with a platinum work-
ing electrode (surface area of 0.3 mm2) and a gold wire as the
counter electrode. The Pt working electrode was routinely pol-
ished with polishing alumina suspension and rinsed with acetone
before use. The measured potentials were recorded with respect to
the Ag/AgCl reference electrode. All electrochemical measurements
were carried out under an atmospheric pressure of argon.
The thickness of the film for the OLED device was mon-
itored and calibrated by Dektak 6M Profiler (Veeco). The EL
2.4. Synthesis of 2-(4-cyanophenyl)-7-(4,4,5,5-tetramethyl-
1,3,2-dioxaborole-2-yl)-9,9-dihexylfluorene
(4)
6 mL degassed dioxane was added into
a mixture of 3
(1.028 g, 2.00 mmol), bis(pinacolato)diboron (558 mg, 2.20 mmol),
potassium acetate (294 mg, 3.00 mmol), and PdCl2(dppf) (60 mg,
0.06 mmol) with continuously stirring. The mixture was heated
at 80 ◦C for 2 h under nitrogen protection. After cooling to room
temperature, the dark suspension was extracted with ethyl acetate
and washed with water. The organic layers were combined and
dried over sodium sulphate. After the solvent was removed, the
crude product was purified by silica gel column chromatography
with 5:1 n-hexane/ethyl acetate to give 4 as an off-white solid
(680 mg, 60.5%). 1H NMR (CDCl3, 400 MHz) ı/ppm = 7.84–7.80 (m,
2H), 7.77–7.72 (m, 6H), 7.58–7.54 (m, 2H), 2.05–1.99 (m, 4H), 1.40
(s, 12H), 1.10–1.03 (m, 12H), 0.76–0.72 (t, 6H, J = 6.8 Hz), 0.63–0.61
(br, 4H). 13C NMR (CDCl3, 100 MHz) ı/ppm = 152.42, 150.27, 146.19,
143.19, 141.75, 138.33, 133.92, 132.58, 128.99, 127.78, 127.74,
126.19, 121.60, 120.68, 119.32, 119.02, 110.71, 83.81, 55.39, 40.21,
31.44, 29.63, 24.07, 23.74, 22.54, 13.95. Elemental Analysis (EA):
calcd for C38H48BNO2: C81.27, H8.61, N2.49; found: C81.50, H 8.77,
N2.19.
spectra were performed by
a JY SPEX CCD3000 spectrome-
ter. Current–voltage–brightness characteristics were measured by
using a Keithley 2400 source measurement unit with a calibrated
silicon photodiode. All the device testing was carried out in ambient
atmosphere.
2.2. Synthesis of
2-bromo-7-(9H-carbazole-9-yl)-9,9-dihexylfluorene (2)
A mixture of 2,7-dibromo-9,9-dihexylfluorene (1) (4.923 g,
10.00 mmol), carbazole (1.087 g, 6.50 mmol), copper(I) iodide
(124 mg, 0.65 mmol), 1,10-phenanthroline (234 mg, 1.30 mmol)
and potassium carbonate (1.973 g, 14.30 mmol) was purged with
nitrogen, and then 10 mL of anhydrous DMF was added. The
mixture was heated to reflux for 24 h. After cooling to room tem-
perature, the dark suspension was poured into ice-water. The
precipitate was collected and purified by silica gel column chro-
matography eluting with 20:1 hexane/dichloromethane to give
2 as a white solid (2.866 g, 76.2%). 1H NMR (CDCl3, 400 MHz):
ı/ppm = 8.18–8.16 (d, 2H, J = 7.6 Hz), 7.89–7.87 (d, 1H, J = 8.0 Hz),
7.64–7.62 (d, 1H, J = 8.8 Hz), 7.55–7.51 (m, 4H), 7.42–7.41 (d, 4H,
J = 3.6 Hz), 7.33–7.29 (m, 2H), 2.20–1.96 (m, 4H), 1.16–1.10 (m,
12H), 0.81–0.76 (t, 6H, J = 6.8 Hz), 0.76–0.72 (m, 4H); 13C NMR
(CDCl3, 100 MHz): ı/ppm = 153.29, 152.29, 141.07, 139.38, 136.88,
130.27, 126.35, 125.99, 125.95, 123.47, 121.88, 121.48, 121.24,
120.95, 120.40, 119.96, 119.84, 109.75, 55.76, 40.22, 31.51, 29.59,
23.88, 22.54, 13.97; Elemental Analysis (EA): calcd for C37H40BrN:
C76.80, H6.97, N2.42; found: C76.48, H7.03, N2.67.
2.5. Synthesis of 7-bromo-7ꢀꢀ-(9H-carbazole-9-yl)-2,2ꢀ:7ꢀ,2ꢀꢀ-
ter(9,9-dihexylfluorene)
(6)
A mixture of 7,7ꢀꢀ-dibromo-2,2ꢀ:7ꢀ,2ꢀꢀ-ter(9,9-dihexylfluorene)
(5) (2.624 g, 2.27 mmol), carbazole (253 mg, 1.51 mmol), cop-
per(I) iodide (29 mg, 0.15 mmol), 1,10-phenanthroline (54 mg,
0.30 mmol) and potassium carbonate (458 mg, 3.32 mmol) was
purged with nitrogen, and then 20 mL of anhydrous DMF was
added. The mixture was heated to reflux for 24 h. The cooled
dark suspension was poured into ice-water to precipitate solid.
The solid was collected by filtration and purified by silica gel col-
umn chromatography eluting with 50:1 hexane/dichloromethane
to give 6 as an off-white solid (952 mg, 50.7%). 1H NMR (CD2Cl2,
400 MHz): ı/ppm = 8.22–8.20 (d, J = 7.6 Hz, 2H), 8.02–8.00 (d,
J = 8.0 Hz, 1H), 7.93–7.44 (m, 21H), 7.35–7.31 (m, 2H), 2.19–2.05
(m, 12H), 1.17–1.11 (m, 36H), 0.89–0.70 (m, 30H); 13C NMR (CDCl3,

58
Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
100 MHz): ı/ppm = 153.50, 153.32, 152.92, 151.88, 151.86, 151.85,
151.16, 150.96, 141.18, 141.08, 140.99, 140.45, 140.43, 140.20,
140.14, 140.13, 139.88, 139.55, 139.32, 139.27, 136.41, 130.04,
126.38, 126.31, 126.27, 126.25, 126.22, 125.93, 125.89, 123.45,
121.95, 121.57, 121.51, 121.45, 121.07, 121.01, 120.84, 120.38,
120.16, 120.04, 119.89, 109.85, 55.59, 55.50, 55.40, 40.34, 40.29,
40.24, 31.52, 31.46, 29.66, 29.65, 29.63, 24.00, 23.89, 23.78, 22.56,
22.54, 14.00, 13.99; Elemental Analysis (EA): calcd for C87H104BrN:
C84.02, H8.43, N1.13; found: C84.01, H8.59, N1.26.
2.8. Synthesis of 7-(9H-carbazole-9-yl)-7ꢀꢀ-(4-cyanophenyl)-
2,2ꢀ:7ꢀ,2ꢀꢀ–ter(9,9-dihexyl-fluorene)
(F3)
Preparation, see the synthesis of F1. Batch:6 (380 mg,
0.31 mmol), 4-cyanophenylboronic acid (147 mg, 1.00 mmol),
potassium carbonate (150 mg, 1.09 mmol), Pd(PPh3)4 (3.5 mg,
0.3% mmol). F3 was obtained as a light yellow powder (258 mg,
63.6%). 1H NMR (CD2Cl2, 400 MHz): ı/ppm = 8.21–8.19 (d, J = 8.0 Hz,
2H), 8.02–8.00 (d, J = 8.0 Hz, 1H), 7.93–7.72 (m, 17H), 7.68–7.66
(m, 2H), 7.62–7.59 (m, 2H), 7.50–7.46 (m, 4H), 7.35–7.33 (m, 2H),
2.16–2.12 (m, 12H), 1.15–1.14 (br, 36H), 0.81–0.78 (m, 30H); 13C
NMR (CDCl3, 100 MHz): ı/ppm = 152.92, 152.13, 151.90, 151.88,
151.85, 146.20, 141.63, 141.21, 141.18, 140.98, 140.47, 140.44,
140.20, 140.18, 140.13, 139.56, 139.47, 137.87, 136.42, 132.60,
127.74, 126.39, 126.35, 126.33, 126.27, 125.93, 125.90, 123.45,
121.96, 121.60, 120.84, 120.38, 120.32, 120.16, 120.05, 119.89,
119.04, 110.68, 109.85, 55.59, 55.48, 55.41, 40.37, 31.53, 31.47,
29.67, 29.65, 24.00, 23.90, 23.88, 22.54, 14.00, 13.97; MALDI-
TOF–MS: calcd. for C94H108N2: m/z = 1265.85; found: m/z = 1265.09
(M+). Elemental Analysis (EA): calcd for C94H108N2: C89.19, H8.60,
N2.21; found: C88.76, H8.77, N2.47.
General procedures for Suzuki coupling reaction taking F1 as an
example:
2.6. Synthesis of
2-(9H-carbazole-9-yl)-7-(4-cyanophenyl)-9,9-dihexylfluorene
(F1)
Pd(PPh3)4 (6 mg, 0.6% mmol) was added into
a mixture
of 2 (289 mg, 0.50 mmol), 4-cyanophenylboronic acid (147 mg,
1.00 mmol) and potassium carbonate (242 mg, 1.75 mmol) in a
glove-box. Degassed toluene (5 mL) and water (1 mL) was added
into the mixture by a syringe. The mixture was allowed to heat
to reflux with continuous stirring in the dark for 24 h under
the protection of nitrogen. After cooling to room temperature,
the organic layer was separated and the aqueous layer was
extracted with ethyl acetate. Organic layers were combined and
washed with water, brine and then dried over sodium sulphate.
The solvent of the organic solution was removed under reduced
pressure and the crude product was purified by silica gel col-
umn chromatography with 5:1 n-hexane/ethyl acetate to give F1
as a white powder (205 mg, 68%). 1H NMR (CD2Cl2, 400 MHz):
ı/ppm = 8.20–8.18 (d, J = 7.6 Hz, 2H), 8.01–7.99 (d, J = 7.6 Hz, 1H),
7.93–7.91 (d, J = 7.6 Hz, 1H), 7.85–7.79 (m, 4H), 7.71–7.68 (m,
J = 8.0 Hz, 1H), 7.61–7.59 (m, 3H), 7.49–7.43 (m, 4H), 7.34–7.30 (m,
2H), 2.12–2.08 (t, 4H), 1.17–1.12 (br, 12H), 0.80–0.77 (m, 10H);
13C NMR (CDCl3, 100 MHz): ı/ppm = 153.00, 152.14, 146.01, 141.09,
141.07, 139.57, 138.31, 137.00, 132.63, 127.80, 126.51, 126.01,
125.96, 123.50, 121.95, 121.63, 121.21, 120.49, 120.42, 119.99,
118.98, 110.85, 109.77, 55.69, 40.31, 31.51, 29.62, 23.97, 22.53,
13.96; MALDI-TOF–MS: calcd. for C44H44N2: m/z = 600.35; found:
m/z = 600.32(M+); Elemental Analysis (EA): calcd for C44H44N2:
C87.96, H7.38, N4.66; found: C 87.52, H7.56, N4.92.
2.9. Synthesis of 7-(9H-carbazole-9-yl)-7ꢀꢀꢀ-(4-cyanophenyl)-
2,2ꢀ:7ꢀ,2ꢀꢀ:7ꢀꢀ,2ꢀꢀꢀ-quarter(9,9-dihexylfluorene)
(F4)
Preparation, see the synthesis of F1. Batch:
6 (312 mg,
0.25 mmol), 4 (140 mg, 0.25 mmol), potassium carbonate (121 mg,
0.88 mmol), Pd(PPh3)4 (2 mg, 0.2% mmol). F4 was obtained as a
light yellow powder (260 mg, 65.1%). 1H NMR (CDCl3, 400 MHz):
ı/ppm = 8.20–8.18 (d, J = 8.0 Hz, 2H), 7.97–7.95 (d, J = 8.4 Hz, 1H),
7.89–7.58 (m, 27H), 7.49–7.42 (m, 4H), 7.34–7.30 (t, J = 7.6 Hz,
2H), 2.12–2.09 (br, 16H), 1.14–1.11 (br, 48H), 0.86–0.76 (br,
40H); 13C–NMR (CDCl3, 100 MHz): ı/ppm = 152.94, 152.15, 151.90,
151.86, 146.22, 141.66, 141.25, 141.20, 141.02, 140.68, 140.64,
140.42, 140.41, 140.39, 140.24, 140.22, 140.21, 140.05, 140.01,
139.55, 139.46, 137.87, 136.42, 132.62, 127.76, 126.39, 126.35,
126.25, 126.23, 125.94, 125.91, 123.46, 121.97, 121.60, 120.85,
120.39, 120.32, 120.16, 120.03, 119.89, 119.05, 110.69, 109.86,
55.61, 55.49, 55.41, 55.40, 40.37, 31.54, 31.47, 29.69, 29.67,
24.01, 23.91, 23.89, 22.55, 14.00, 13.98; MALDI-TOF–MS: calcd for
C119H140N2: m/z = 1598.11; found: m/z = 1598.49 (M+). Elemental
Analysis (EA): calcd for C119H140N2: C89.42, H8.83, N1.75; found:
C88.88, H9.29, N1.83.
2.7. Synthesis of 7-(9H-carbazol-9-yl)-7ꢀ-(4-cyanophenyl)-2,2ꢀ-
2.10. Device fabrication
bi(9,9-dihexylfluorene)
(F2)
The solution-processed OLEDs were assembled in a sand-
wich structure: ITO/PEDOT:PSS/oligomer/TPBi/LiF/Al. The glass
substrates covered by indium tin oxide (ITO) were pre-cleaned
mechanically with deionized water, methanol, and acetone.
A 50 nm-thick layer of poly(ethylenedioxy)thiophene–polystyren-
esulphonic acid (PEDOT:PSS) was spin-coated on top of the ITO
electrode treated by oxygen plasma. Afterwards, the system was
baked at 120 ◦C for 30 min in a vacuum oven and subsequently
moved into a glovebox. A 60 nm-thick emitting layer (F1–F4) was
spin-coated from toluene solution on the PEDOT:PSS layer, fol-
lowed by being annealed at 120 ◦C for 30 min to remove residual
solvent. TPBi (40 nm), LiF (1 nm), and aluminum (100 nm) were vac-
uum deposited successively under the pressure <10−4 Pa. All the
solutions were filtered through 0.22 ␮m membranes before pro-
cess. The optimized device of F4 was fabricated in the configuration
of ITO/MoO3 (8 nm)/NPB (50 nm)/CBP:F4 (1:4 wt%, 40 nm)/TPBi
(25 nm)/LiF (1 nm)/Al (150 nm), in which each layer was ther-
mally deposited under the pressure <10−4 Pa. The thicknesses of
Preparation, see the synthesis of F1. Batch:
2 (145 mg,
0.25 mmol), 4 (140 mg, 0.25 mmol), potassium carbonate (121 mg,
0.88 mmol), Pd(PPh3)4 (2 mg, 0.2% mmol). F2 was obtained as a
off-white powder (152 mg, 65.4%). 1H NMR (CD2Cl2, 400 MHz):
ı/ppm = 8.20–8.18 (d, J = 8.0 Hz, 2H), 8.01–7.99 (d, J =7.6 Hz, 1H),
7.93–7.58 (m, 15H), 7.49–7.44 (m, 4H), 7.34–7.31 (m, 2H), 2.14–2.12
(br, 8H), 1.15–1.11 (br, 24H), 0.85–0.76 (m, 20H); 13C NMR (CDCl3,
100 MHz): ı/ppm = 152.91, 152.13, 151.94, 151.88, 146.19, 141.59,
141.17, 141.02, 140.79, 140.13, 139.69, 139.59, 137.93, 136.48,
132.61, 127.75, 126.41, 126.39, 126.34, 125.93, 125.91, 123.45,
121.96, 121.59, 120.87, 120.39, 120.35, 120.18, 119.90, 119.03,
110.70, 109.84, 55.60, 55.51, 40.37, 40.33, 31.52, 31.46, 29.65, 23.99,
23.87, 22.54, 13.99, 13.97; MALDI-TOF–MS: calcd. for C69H76N2:
m/z = 932.60; found: m/z = 932.59 (M+); Elemental Analysis (EA):
calcd for C69H76N2: C88.79, H 8.21, N3.00; found: C88.44, H8.32,
N3.24.

Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
59
100
80
60
40
20
0
F1
F2
-1
-2
-3
-4
F1
F2
F3
F4
F3
F4
50
100
150
200
250
100
200
300
400
500
600
700
800
Temperature (ºC)
Temperature (ºC)
Fig. 1. TGA thermograms of the samples at a heating rate of 10 ◦C/min measured in
dry nitrogen conditions.
Fig. 2. DSC traces of F1, F2, F3, and F4 in the second heating process (heating rate:
10 ◦C/min) measured in dry nitrogen conditions.
the vapor-deposited films were monitored by frequency counter
and calibrated by Dektak 6 M Profiler (Veeco), while those of the
spin-coated films were controlled by Dektak 6 M Profiler (Veeco)
directly. The current density–voltage–luminance characteristics
and EL spectra were measured by using a Keithley 2400 source
measurement unit with a calibrated silicon photodiode and by a
JY SPEX CCD3000 spectrometer, respectively. All the device tests
were carried out in ambient atmosphere.
that, at the end of the heating–up process, the residues of F1, F2,
F3, and F4 were 2.3, 35.5, 54.1, and 53.6 wt%, respectively. This sim-
ilar trend as that of Td indicated that the more fluorene units, the
better thermal stability could be obtained. The DSC curves of F1, F2,
and F3 in their second heating cycles showed endothermic baseline
shifts at 45, 64, and 70 ◦C, respectively, at which the glassy states
transformed into the supercooled liquid phases (Tg). The DSC trace
of F4 exhibited unconspicuous baseline shift, thereby its Tg was dif-
ficult to be identified. In relation to the data reported [12], it could
be concluded that in comparison with the cyanophenyl group, the
carbazole had better capacity of raising the Td and Tg values.
The UV/vis absorption and PL spectra of F1–F4 in dilute toluene
solutions (∼10−6 M) and in the thin films are shown in Fig. 3.
When measured in solution, the maximum absorptions related
to the ␲–␲* transition of the oligomer backbones were in the
range of 343–370 nm and increased as the number of the fluo-
rene units. This is due to that more fluorene units resulted in
longer conjugated length. It was in great consistent with the car-
bazole contents that the relative intensity of the shoulder peak at
ꢀ = 294 nm, which were originated from the end-capped carbazole
units [18,19], decreased orderly from F1 to F4. By using the wave-
length of maximum absorption as the excitation wavelength, the PL
spectrum of F1 in dilute toluene solution exhibited a single emis-
sion peak at ꢀ = 403 nm, and the other three oligomers showed an
emission maximum at 403–410 nm and a shoulder at 422–434 nm.
In contrast to the dilute solution absorption spectra, the thin film
absorption spectra of F1–F4 displayed a small red-shifts of 2–8 nm
due to the weak aggregation effects. The main emission peaks of
F1, F2, and F3 in thin films were located at 418–420 nm, while the
maximum emission of F4 was shifted to 446 nm with a shoulder
peak at 421 nm. This phenomenon was much different from that
of the quarterfluorene [11], of which the emission main peak was
unchanged in the solid state when compared to that in solution.
3. Results and discussion
Starting with 2,7-dibromo-9,9-dihexylfluorene (1), 2-bromo-
7-(9H-carbazole-9-yl)-9,9-dihexylfluorene (2) and 2-bromo-7-(4-
cyanophenyl)-9,9-dihexylfluorene (3) were obtained through the
Ullmann condensation and Suzuki coupling reaction, respec-
tively. 3 was converted to its corresponding bronic ester 4 when
the reaction was catalyzed by PdCl2(dppf). 2 was reacted with
4-cyanophenylboronic acid or 4 to give F1 or F2. 7-Bromo-
7ꢀꢀ-(9H-carbazole-9-yl)-2,2ꢀ:7ꢀ,2ꢀꢀ-ter(9,9-dihexylfluorene) (6) was
synthesized by the condensation of the dibromo-substituted ter-
fluorene 5 and carbazole. Suzuki coupling reaction of 6 with
4-cyanophenylboronic acid or 4 was employed for the synthe-
sis of F3 or F4, respectively. The molecular structures of the four
light-emitting compounds were validated by 1H NMR, 13C NMR,
MALDI-TOF–MS (Figs. S1–S3, see supporting information), and ele-
mental analysis (EA). These oligofluorenes are highly soluble in
common organic solvents such as toluene, dichloromethane, chlo-
roform, and tetrahedronfuran. Table 1 summarizes the results of
MALDI-TOF–MS, TGA, DSC, and photophysical properties of the
compounds. TGA and DSC curves are presented in Figs. 1 and 2,
respectively. The onset decomposition temperatures (Td) for the
thermal bond cleavage of the oligofluorenes were increased from
352 to 394 ◦C with increasing number of fluorene units. Besides
Table 1
Physical properties of the series of blue light-emitting oligofluorenes.
a
a
b
b
c
d
Compound
MS
600.32
932.59
1265.09
1598.49
Td (◦C)
Tg (◦C)
ꢀabs (nm)
ꢀem (nm)
ꢀabs (nm)
ꢀem (nm)
˚
(%)
˚
(%)
PL
PL
F1
F2
F3
F4
352
383
389
394
45
64
70
–
343 (294)
355 (294)
365 (294)
370 (294)
403
403 (422)
408 (430)
410 (434)
346 (296)
363 (296)
368 (296)
372 (296)
418
418
97.0
97.8
87.5
90.5
59.2
66.7
66.8
68.7
420 (437)
446 (421)
a
Maximum absorption/emission wavelengths in dilute toluene solutions (10−6 M).
Maximum absorption/emission wavelengths in the thin films. All the shoulders are shown in the brackets.
Absolute fluorescence quantum yields in dilute toluene solutions.
b
c
d
Absolute fluorescence quantum yields in the thin films.

60
Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
(a)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
F1
F2
F3
0.4
F4
0.2
0.0
300
350
400
450
500
550
Wavelength (nm)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
F1
F2
F3
F4
300
350
400
450
500
550
Fig. 4. Cyclic voltammograms of F1–F4 in 0.1 M nBu4NPF6 dichloromethane solution
at room temperature. Sweep rate 100 mV s−1
.
Wavelength (nm)
Fig. 3. UV–vis absorption and PL emission spectra (ꢀex = ␭abs max) of F1–F4 (a) in
in Fig. 4(a). The highest occupied molecular orbital (HOMO)
energy levels of the oligomers were calculated from their corre-
dilute toluene solution (10−6 mol/L) and (b) in the thin films.
sponding onset oxidation potentials according to the equation
ox
Thus, it could be inferred that the end groups of F4 led to larger
geometry change of the molecule in the solid state, and the elec-
trons in its high singlet excited state were easier to relax to lower
singlet excited state. For these reasons, the main emission of F4 was
due to the electron transition from the lower excited state to ground
state. By utilizing the HORIBA–JOBIN–YVON instrument, the abso-
lute quantum yields (˚PLs) of F1, F2, F3, and F4 in solutions were
evaluated as 97.0, 97.8, 87.5, and 90.5%, respectively. The ˚PLs of
their thin films drop-casted onto quartz plates were in the range of
59.2–68.7%. These findings indicated that these compounds were
excellent fluorescent materials. In the solid state, F4 possessed the
highest quantum yield among these compounds owing to its largest
fluorescent fluorene number.
HOMO = −(E
+ 4.40)eV with the ferrocene oxidation potential
onset
as the standard, where the lowest unoccupied molecular orbital
(LUMO) energy levels could be estimated from the corresponding
HOMO energy levels and optical band gaps (Eg) [33]. All the
compounds showed similar reversible oxidation process in the
toluene solution. The onset oxidation potentials of F1, F2, F3,
and F4 were 1.23, 1.18, 1.16, and 1.15 V, respectively. From these
data, their HOMO values were estimated to be −5.63, −5.58,
−5.56, and −5.55 eV, respectively. By utilizing the equation of
LUMO = Eg + HOMO, the LUMO energy levels of F1, F2, F3, and F4
could be calculated as −2.47, −2.52, −2.53, and −2.55 eV, respec-
tively. When the number of the fluorene units increased, the HOMO
energy level rose whereas the LUMO level fell, implying that when
the distance between the electron-withdrawing cyanophenyl
group and the electron-donating carbazole group extended, the
The electrochemical properties of these fluorene-based
oligomers were investigated by cyclic voltammetry (CV), as shown
Table 2
Band gaps and energy levels of the oligofluorenes.
a
ox
onset
b
e
Compound
Eg (eV)
E
(V)
HOMOc (eV)
LUMOd (eV)
HOMOe (eV)
LUMOe (eV)
Eg (eV)
F1
F2
F3
F4
3.16
3.06
3.03
3.00
1.23
1.18
1.16
1.15
−5.63
−5.58
−5.56
−5.55
−2.47
−2.52
−2.53
−2.55
−5.38
−5.25
−5.20
−5.17
−1.87
−1.85
−1.85
−1.82
3.51
3.40
3.35
3.35
a
Determined from UV–vis absorption spectra in toluene solutions.
b
c
ox
E
: onset oxidation potential.
onset
ox
HOMO = −(E
+ 4.40) eV.
onset
d
e
Estimated from HOMO levels and the optical band gaps: LUMO = HOMO + Eg.
Calculated by density-functional theory (DFT) molecular simulation (B3LYP/6-31G(d) level).

Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
61
Fig. 5. (a) Current density–voltage (I–V), (b) luminance–voltage (L–V), (c) current efficiency–current density, and (d) power efficiency–current density characteristics of the
ITO/PEDOT:PSS/ETL/TPBi/LiF/Al devices.
molecule was easier to be oxidized or reduced. In other words,
the intramolecule interaction caused by the cyanophenyl and
carbazole units became less intensive as the amount of the flu-
orene spacers increased. Moreover, the HOMO and LUMO levels
of these cyanophenyl and carbazole end-capped oligofluorenes
were lower than those of the carbazole end-capped oligofluorenes
[18], suggesting that the hole and electron transporting properties
could be effectively modulated through the end groups. To gain
insights into the electronic structures, computational studies
were carried out on the oligofluorenes using density-functional
methods at the B3LYP/6-31G(d) level, as shown in Fig. 4(b). For
each oligomer, the majority of the electron distribution of the
HOMO was found to be located over the carbazole and neigh-
boring fluorene entities; whereas the electron distribution of the
(a)
(b)
1.0
1.0
0.8
0.6
0.4
0.2
0.0
7 V
6 V
7 V
8 V
9 V
0.8
0.6
0.4
0.2
0.0
8 V
9 V
10 V
300 350 400 450 500 550 600 650 700
Wavelength (nm)
300
400
500
600
700
Wavelength(nm)
(d) 1.0
(c)
1.0
7 V
0.8
0.6
0.4
0.2
0.0
6 V
7 V
8 V
9 V
0.8
0.6
0.4
0.2
0.0
8 V
9 V
10 V
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength(nm)
Fig. 6. EL spectra of (a) F1, (b) F2, (c) F3, (d) F4 recorded at different applied voltages with the device configuration of ITO/PEDOT:PSS/ETL/TPBi/LiF/Al.

62
Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
Table 3
EL performance of the devices based on the series of blue light-emitting oligofluorenes.
c
d
e
h
i
j
Emissive
layer
Vturn-on
(V)
Lmax
VL,max
(V)
ꢁc@100 cd/m2 f
(cd/A)
ꢁc@1000 cd/m g
(cd/A)
ꢁc max
ꢁp max
␭
EL
(nm)
CIE1931 k (x, y)
(cd/m2)
(cd/A)
(lm/w)
F1a
4.7
5.1
4.7
4.1
6.9
714
1410
1507
2180
5135
10.3
9.7
9.7
10.9
15.3
0.84
1.28
1.19
1.17
1.70
–
0.86
1.38
1.25
1.17
1.76
0.50
0.76
0.74
0.68
0.85
437
443
446 (423, 475)
450 (426, 481)
416 (442)
(0.17, 0.12)
(0.18, 0.19)
(0.18, 0.19)
(0.18, 0.20)
(0.16, 0.09)
F2a
0.90
0.89
0.97
1.42
F3a
F4a
CBP/F4b
a
These results were obtained with the device configuration of ITO/PEDOT:PSS/ETL/TPBi/LiF/Al.
The device structure was ITO/MoO3/NPB/CBP:F4(1:4)/TPBi/LiF/Al.
Turn-on voltage corresponding to 1 cd/m2 of luminance.
Maximum luminance.
Driving voltage corresponding to maximum luminance.
Current efficiency at the luminance of 100 cd/m2.
Current efficiency at the luminance of 1000 cd/m2.
Maximum current efficiency.
b
c
d
e
f
g
h
i
Maximum power efficiency.
Maximum EL wavelength.
j
k
CIE coordinates measured at the VL,max
.
LUMO was located mainly over the cyanophenylfluorene entities.
As summarized in Table 2, the calculated results, including the
HOMO, LUMO, and HOMO–LUMO gaps, were all slightly higher
than the corresponding electrochemically measured values. The
calculated HOMO levels of F1, F2, F3, and F4 were −5.38, −5.25,
−5.20, and −5.17 eV, respectively. The increase of the conjugation
length elevates the HOMO energy levels of the parent oligomer, but
has no large effect to the LUMO levels. The molecular simulation
results show that the carbazole end-groups serve as electron
donors and the cyanophenyl terminals as electron acceptors.
To investigate the electroluminescence (EL) properties of the
oligomers, the OLED devices were fabricated using the configura-
tion of ITO/PEDOT:PSS(50 nm)/compound(60 nm)/TPBi(40 nm)/LiF
(1 nm)/Al(100 nm). Poly(ethylenedioxy)thiophene mixed with
poly(styrene sulphonic acid) (PEDOT:PSS) was used as the
the device based on F4 displayed the lowest turn-on voltage
(4.1 V) and highest maximum luminance (2180 cd/m2) among
the devices. The highest luminances of F1, F2, and F3 were 714,
1410, and 1507 cd/m2, respectively. These results indicated that
the amount of the fluorescent fluorene units had direct relation
with the brightness. When F2 and F3-based devices displayed
maximum brightness, their operating voltage were both 9.7 V,
whereas, for the F1 and F4-based devices, the voltages were 10.3
and 10.9 V, respectively. Thereby, the EL spectra of these devices
would not be recorded in the same range of voltages (Fig. 6). The
current efficiencies (ꢁcs) of the devices were calculated by the
equation of ꢁc = L/(10I), in which L was the luminance, and I was
the corresponding current density at the same voltage. Although
the maximum ꢁc of F4 (1.17 cd/A) was not as high as that of F2
(1.38 cd/A), the curve of F4 decayed slowest, even at the bright-
ness of 1000 cd/m2. The current efficiency value of the F4-based
device was 0.97 cd/A, while the values of F2 and F3 were 0.90 and
0.89 cd/A, respectively. The maximum power efficiencies (ꢁps) of
hole
injection
layer.
1,3,5-Tris(1-phenyl-1H-benzimidazol
-2-yl)benzene (TPBi) and LiF were introduced as electron trans-
porting and injection layers, respectively. As shown in Fig. 5,
Fig. 7. (a) Current density–voltage–luminance (I–V–L) characteristics, (b) current efficiency–current density relationship, (c) plots of power efficiency–current density, and
(d) normalized EL spectra recorded from 7 to 10 V of the ITO/MoO3/NPB/CBP:F4(1:4)/TPBi/LiF/Al device.

Y. Lin et al. / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 55–64
63
these compounds were found to be in the range of 0.50–0.76 lm/w,
and still, the decline of F4 was the gentlest. All the devices emitted
deep blue light when they were operating. It was worthy to note
that the EL curves of F1 and F2 were almost voltage-independent
but those of F3 and F4 had larger change when driven at higher
voltages. This could be explained as follows: for F1 and F2, the
molecular chains were short and the end groups hindered the
␲–␲ stacking effect under electric field. When the intensity of the
electric field increased, F3 and F4, in which the end groups might
have weak aggregation-suppressing function to the longer molec-
ular chains, were easy to rearrange and aggregate into cluster,
therefore the long-wavelength EL emission was intensified. When
these devices were operated at their corresponding VL,max (see
Table 3), the CIE coordinates (x, y) of F1, F2, F3, and F4 were (0.17,
0.12), (0.18, 0.19), (0.18, 0.19), and (0.18, 0.20), respectively. All the
CIE coordinates were in the blue region of the CIE 1931 diagram.
We have known that F4 was the brightest oligofluo-
rene among the four blue light-emitting compounds. To
further improve its device performance, we fabricated the
doped device of ITO/MoO3(8 nm)/NPB(50 nm)/CBP:F4(1:4 wt%,
was lower. The absolute ˚PLs of the oligofluorenes measured in
thin films were estimated in the range of 59.2–68.7%. By using
CV measurements together with the optical band gaps in solu-
tions, the HOMO(LUMO) energy values of F1, F2, F3, and F4
were estimated to be −5.63(−2.47), −5.58(−2.52), −5.56(−2.53),
and −5.55(−2.55) eV, respectively. The device based on solution-
processed F4 displayed the lowest turn-on voltage (4.1 V) and high-
est maximum luminance (2180 cd/m2) with maximal current effi-
ciency of 1.17 cd/A among those based on the oligofluorenes in the
configuration of ITO/PEDOT:PSS/oligofluorenes/TPBi/LiF/Al. When
the intensity of the electric field increased, F3 and F4 with longer
molecular chain with end-groups were easy to rearrange and aggre-
gate, therefore their EL spectra were voltage-dependent. With the
vapor-deposited device of ITO/MoO3/NPB/CBP:F4(1:4)/TPBi/LiF/Al,
improved device performance and voltage-independent EL spectra
with the CIE coordinates of (0.16, 0.09) were obtained.
Acknowledgements
The authors are grateful for the financial support of the National
Natural Science Foundation of China (21074034), Ministry of Edu-
cation of China (309013), the Fundamental Research Funds for the
Central Universities, Shanghai Municipal Educational Commission
for the Shuguang fellowship (08GG10, to Y.C.) and the Shanghai
Eastern Scholarship (to Y.C.).
40 nm)/TPBi(25 nm)/LiF
(1 nm)/Al(150 nm)
using
vapor
deposition method. MoO3 and 4,4ꢀ-bis[N-(1-naphthyl)-N-
phenylamino]biphenyl (NPB) were used as hole injection and
transporting layers, respectively. In the emitting layer, 25 wt%
of 4,4ꢀ-N,Nꢀ-dicarbazole-biphenyl (CBP) was doped into F4. The
device performance results were shown in Fig. 7. Compared with
the undoped device based on F4, the turn-on voltage of the doped
device was raised to 6.9 V, which might be caused by the hole-
blocking effect of CBP [34]. The doped device exhibited maximum
luminance of 5135 cd/m2, operating voltage tolerance from 6.9 to
15.3 V, highest current efficiency (ꢁc max) of 1.76 cd/A, and power
efficiency (ꢁp max) of 0.85 lm/w, respectively. The improved device
performance might be resulted from the increased purity and
morphology via the deposition method and the increase of the
exciton formation in the emitting layer due to the hole-confining
capacity of CBP. In addition, the EL spectra of the CBP:F4 device
were voltage-independent, indicating that the existence of CBP
could effectively inhibit the rearrangement and aggregation of the
F4 molecules under electric field. The CIE coordinates (x, y) of this
device was (0.16, 0.09), which located at the deep blue area of CIE
1931 diagram.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2011.12.008.
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