Hyperbranched fluorene-alt-carbazole copolymers with spiro[3.3]heptane-2,6-dispirofluorene as the core and their application in white polymer light-emitting devices

Article abstract of DOI:10.1039/c5ra05713b

A series of hyperbranched copolymers with fluorene-alt-carbazole as the branches and three-dimensional-structured spiro[3.3]heptane-2,6-dispirofluorene (SDF) as the core were synthesized by one-pot Suzuki polycondensation. 4,7-Dithienyl-2,1,3-benzothiadiazole (DBT) as the orange-light emitting unit was introduced into the backbones to obtain white-light emission. The thermal, photoluminescent (PL), electrochemical and electroluminescent (EL) properties of the copolymers were investigated. The copolymers show great thermal stabilities by the introduction of a carbazole moiety. Besides, the HOMO energy levels of the copolymers were enhanced and the hole injection was improved because of the hole-transporting ability of the carbazole unit. The hyperbranched structures suppress the interchain interactions efficiently, and help to form amorphous films. The copolymers exhibit efficient EL performance as a result of the hyperbranched structure with the incorporation of the carbazole moiety. A quite low turn-on voltage of 5.3 V, a maximal luminance of 7409.5 cd m^-2 and a luminous efficiency of 4.27 cd A^-1 were achieved with a CIE coordinate of (0.32, 0.26) for the PFCzSDF10DBT10 (10 mol% of SDF and 0.1 mol% of DBT) device. The hyperbranched framework based on fluorene-alt-carbazole branches and SDF core are attractive candidates for solution-processable white polymer light-emitting device (WPLED) applications.

Full text of DOI:10.1039/c5ra05713b

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PAPER
Hyperbranched fluorene-alt-carbazole copolymers
with spiro[3.3]heptane-2,6-dispirofluorene as the
core and their application in white polymer light-
emitting devices†
Cite this: RSC Adv., 2015, 5, 49662
Yuling Wu,ab Jie Li,ab Wenqing Liang,ab Junli Yang,ab Jing Sun,ab Hua Wang,ab
c
ab
and Wei Huangd
*
Xuguang Liu, Bingshe Xu
A series of hyperbranched copolymers with fluorene-alt-carbazole as the branches and three-dimensional-
structured spiro[3.3]heptane-2,6-dispirofluorene (SDF) as the core were synthesized by one-pot Suzuki
polycondensation. 4,7-Dithienyl-2,1,3-benzothiadiazole (DBT) as the orange-light emitting unit was
introduced into the backbones to obtain white-light emission. The thermal, photoluminescent (PL),
electrochemical and electroluminescent (EL) properties of the copolymers were investigated. The
copolymers show great thermal stabilities by the introduction of a carbazole moiety. Besides, the HOMO
energy levels of the copolymers were enhanced and the hole injection was improved because of the
hole-transporting ability of the carbazole unit. The hyperbranched structures suppress the interchain
interactions efficiently, and help to form amorphous films. The copolymers exhibit efficient EL
performance as a result of the hyperbranched structure with the incorporation of the carbazole moiety.
A quite low turn-on voltage of 5.3 V, a maximal luminance of 7409.5 cd mꢀ2 and a luminous efficiency
of 4.27 cd Aꢀ1 were achieved with a CIE coordinate of (0.32, 0.26) for the PFCzSDF10DBT10 (10 mol% of
SDF and 0.1 mol% of DBT) device. The hyperbranched framework based on fluorene-alt-carbazole
branches and SDF core are attractive candidates for solution-processable white polymer light-emitting
device (WPLED) applications.
Received 31st March 2015
Accepted 26th May 2015
DOI: 10.1039/c5ra05713b
www.rsc.org/advances
the glass transition temperature (Tg) of the polymers.4,8,9
Recently, a number of hyperbranched electroluminescent
polymers with the three principle colors (red,10 green11,12 and
Introduction
Hyperbranched polymers have recently received considerable
interest as light-emitting materials for displays, lighting and
other photonic devices.1–5 Firstly, they are easy to be synthesized
by one-pot polymerization with comparable properties to
dendritic polymers or other well dened polymers.6,7 Secondly,
hyperbranched polymers with a three-dimensional structure
can prevent the aggregation of polymer chains, making the
material form amorphous lms with good quality, and increase
blue13) have been synthesized, and both emission efficiency and
thermal stability were effectively improved with respect to their
linear analogies.14–16 However, white-light-emitting hyper-
branched polymers have rarely been reported. Meanwhile,
white polymer light-emitting devices (WPLEDs) have been
widely recognized owing to their potential applications in large-
area full-color and exible displays combined with a color lter,
backlights and solid lighting sources, and their great advan-
tages such as high stability, low price, as well as easy fabrication
process.17–19 A general approach to obtain white-light emission
from a single molecule or polymer is to use blue-light emitting
and complementary orange-light emitting units.19–21 In our
previous research,22 white-light emitting from copolymers
aKey Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan
University of Technology, Taiyuan, 030024, China. E-mail: xubs@tyut.edu.cn
bResearch Center of Advanced Materials Science and Technology, Taiyuan University of
Technology, Taiyuan, 030024, China
cCollege of Chemistry and Chemical Engineering, Taiyuan University of Technology,
Taiyuan, 030024, China
based on
9,9-dioctyluorene and
4,7-dithienyl-2,1,3-
dKey Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of
Advanced Materials, Nanjing University of Posts and Telecommunications (NUPT),
Nanjing, 210046, China
benzothiadiazole (DBT) was realized through the incomplete
¨
Forster resonance energy transfer (FRET) from the uorene
segment to DBT unit.23 Polyuorenes (PFs) are shown to be the
most promising blue light-emitting materials because of high
photoluminescence quantum efficiency, and relatively good
chemical and thermal stabilities.24–26 However, it is hard to
† Electronic supplementary information (ESI) available: 1H NMR and 13C NMR
characterization of DBrCz and 1H NMR characterization of the copolymers and
PFCzSDF10DBT7. UV-vis absorption of DBT and PL spectra of PFCz in CHCl3
solution (10ꢀ5 mol Lꢀ1), and PL spectra of DBT in CHCl3 solution (10ꢀ5 mol
L
ꢀ1). See DOI: 10.1039/c5ra05713b
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balance charge injection owing to the large band gap between at a scanning rate of 50 mV sꢀ1 at room temperature under
the HOMO and LUMO energy levels.27 Carbazole is a well-known nitrogen atmosphere. A Pt plate was used as the counter elec-
hole-transporting unit as a result of the electron-donating trode, and a saturated calomel electrode was used as the refer-
capabilities associated with its nitrogen atom.13,28 For this ence electrode.
reason, to reduce the band gap between the work function of
PEDOT:PSS and the HOMO energy levels of the copolymers, and
further improve the electroluminescent (EL) performance of the
copolymers, carbazole incorporated into the copolymer back-
bone is a promising strategy.
Device fabrication and characterization
Patterned glass substrates coated with indium tin oxide (ITO)
(20 U squareꢀ1) were cleaned by a surfactant scrub, washed
successively with deionized water, acetone and isopropanol in
In this paper, hyperbranched copolymers with 3,6-carbazole-
alt-2,7-uorene (PFCz) and DBT branches and spiro[3.3]
heptane-2,6-dispirouorene (SDF) core (10 mol%) were con-
structed. The three-dimensional-structured SDF exhibits great
morphological stability and intense uorescence,29 and
furthermore, its steric hindrance can prevent rotation of the
adjacent aryl groups, which reduces close packing and inter-
molecular interactions between the chromophores in the solid-
state.30 In order to obtain white-light emission, the orange light-
emitting unit DBT was introduced with different contents from
0.05 mol% to 0.10 mol%. Such a highly branched framework
provides a highly efficient white-light electroluminescence.
an ultrasonic bath, and then dried at 120 ꢁC in
a
heating chamber for
ethylenedioxythiophene):
8
h.
A
40 nm-thick poly(3,4-
acid)
poly(styrenesulfonic
(PEDOT:PSS) hole injection layer was spin-coated on top of ITO
and baked at 120 ꢁC for 20 min. Thin lms (50 nm thick) of the
copolymers as the emitting layer were deposited on top of the
PEDOT:PSS layer by spin-coating the chlorobenzene solution of
the copolymers, followed by thermal annealing at 110 ꢁC for 20
min. Then an electron-transporting layer of 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi, 35 nm) and LiF (1 nm)
and Al (150 nm) as the cathode were deposited by vacuum
evaporation under a base pressure of 5 ꢂ 10ꢀ4 Pa.31 The EL
spectra and CIE coordinates were measured with a PR-655
spectra colorimeter. The current–voltage–forward luminance
curves were measured using a Keithley 2400 source meter and a
calibrated silicon photodiode.
Experimental section
Materials and characterization
9,9-Dioctyluorene-2,7-bis(trimethyleneboronate) (M2, 99.5%)
was purchased from Synwitech. Tetrahydrofuran (THF) and
toluene were distilled using standard procedures. Other
Syntheses
solvents were used without further purication unless other- Spiro[3.3]heptane-2,6-di-(20,200,70,700-tetrabromospirouorene)
wise specied. All reactions were carried out using Schlenk (TBrSDF),32,33
and
4,7-bis(2-bromo-5-thienyl)-2,1,3-
techniques under dry nitrogen atmosphere. 1H NMR and 13C benzothiadiazole (DBrDBT)34–36 were synthesized according to
NMR spectra were measured on a Bruker DRX 600 spectrom- the published literature.
eter, and chemical shis were reported in ppm using tetrame-
3,6-Dibromo-N-(2-ethylhexyl)-carbazole (DBrCz).37,38 3,6-
thylsilane as an internal standard. Elemental analysis (EA) was Dibromo carbazole (6.50 g, 20 mmol) and adequat tetrabutyl
performed with a Vario EL elemental analyzer. Molecular ammonium bromide (TBAB) were added to toluene (100 mL)
weights and polydispersities of the copolymers were deter- and potassium hydroxide solution (16 mol Lꢀ1, 15 mL). Aer the
mined using gel permeation chromatography (GPC) on an solution was stirred for 1 h, 2-ethylhexylbromide (4.26 mL,
HP1100 high performance liquid chromatograph (HPLC) 25 mmol) was added. The mixture was stirred for 12 h under
system equipped with a 410 differential refractometer, and a reuxing, and another 12 h at room temperature. Aer cooling
refractive index (RI) detector, with polystyrenes as the standard to room temperature, the mixture was extracted with water and
and THF as the eluent at a ow rate of 1.0 mL minꢀ1 at 30 C. methylene chloride (CH2Cl2), dried over anhydrous magnesium
ꢁ
The UV-visible absorption spectra were determined on a Hitachi sulfate and concentrated. The crude product was puried by
U-3900 spectrophotometer and the PL emission spectra were column chromatography on silica gel with petroleum
obtained using a Horiba FluoroMax-4 spectrophotometer at ether : CH2Cl2 ¼ 5 : 1 as eluent to give DBrCz as colorless
room temperature. Thermogravimetric analysis (TGA) of the viscous uid (6.73 g, 15 mmol) in a 77% yield. 1H NMR
copolymers was conducted on a Setaram ꢀt1hermogravimetric (600 MHz, CDCl3) d (ppm): 7.81 (d, J ¼ 1.8 Hz, 2H, Ph), 7.34 (dd,
analyzer at a heating rate of 10 ꢁC min
under nitrogen J1 ¼ 1.8 Hz, J2 ¼ 9 Hz, 2H, Ph), 6.93 (d, J ¼ 8.4 Hz, 2H, Ph), 3.65
atmosphere. Differential scanning calorimetry (DSC) measure- (t, J ¼ 6.6 Hz, 2H, CH2), 1.78–1.73 (m, 1H, CH), 1.22–1.04 (m, 8H,
ments were performed at both heating and cooling rates of 5 ꢁC CH2), 0.77 (t, J ¼ 6.6 Hz, 3H, CH3), 0.73 (t, J ¼ 7.8 Hz, 3H, CH3);
minꢀ1 under nitrogen atmosphere, using DSC Q100 V9.4 Build 13C NMR (600 MHz, CDCl3) d (ppm): 142.58, 131.83, 126.24,
287 apparatus. Atomic force microscopy (AFM) measurements 126.03, 114.81, 113.52, 50.46, 42.22, 33.86, 31.67, 27.27, 25.94,
were performed on an SPA-300HV from Digital Instruments Inc. 16.95, 13.80. Elemental anal. calcd for C20H23Br2N: C 54.94, H
(Santa Barbara, CA) at a tapping mode. Cyclic voltammetry (CV) 5.30, N 3.20; found: C 54.90, H 5.33, N 3.21.
measurements were performed on an Autolab/PG STAT302
General procedure for the synthesis of copolymers
electrochemical workstation with the thin lm of the copolymer PPFCzSDF10DBT5–PFCzSDF10DBT10. To a solution of pre-
on the working electrode in a solution of tetrabutylammonium determined amount of the monomers (DBrCz, M2, TBrSDF and
hexauorophosphate (Bu4NPF6, 0.1 M) in acetonitrile (CH3CN) DBrDBT) in toluene (20 mL) was added an aqueous solution (5
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mL) of potassium carbonate (2 M) and a catalytic amount of (ppm): 7.98–7.48 (–ArH–), 6.93–6.78 (–ArH–), 3.39–3.02 (–CH2–),
Pd(PPh3)4 (2.0 mol%). Aliquat 336 (1 mL) in toluene (5 mL) was 2.21–1.75 (–C–CH2–), 1.22–0.88 (–CH2–), 0.86–0.45 (–CH3).
added as the phase transfer catalyst. The mixture was vigorously
stirred at 90 ꢁC for 3 days. Phenylboronic acid was then added to
the reaction mixture, followed by stirring at 90 C for an addi-
ꢁ
Results and discussion
tional 12 h. Finally, bromobenzene was added in the same way
by heating for 12 h again. When cooled to room temperature,
the reaction mixture was washed with 2 M HCl and water. The
organic layer was separated, and the solution was added drop-
wise to excess methanol. The precipitates were collected by
ltration, and dried under vacuum. The solid was Soxhlet
extracted with acetone for 72 h and then passed through a short
chromatographic column using toluene as the eluent to afford
the copolymers.
Synthesis and characterization
As shown in Scheme 1, a series of hyperbranched copolymers
based on N-alkyl and 9-C-alkyl substituted 3,6-carbazole-co-2,7-
uorene branches with SDF (10 mol%) as the branch point were
synthesized by one-pot Suzuki polycondensation in relatively
high yields. To obtain white-light emission, the organe-light-
emitting unit DBT was incorporated into the framework with
the feed ratios of 0.05 mol%, 0.07 mol%, 0.08 mol% and
0.10 mol%, and the corresponding copolymers are named as
PFCzSDF10DBT5, PFCzSDF10DBT7, PFCzSDF10DBT8 and
PFCzSDF10DBT10, respectively. The synthetic and structural
results of PPFCzSDF10DBT5–PFCzSDF10DBT10 are summa-
rized in Table 1.
The functional groups for Suzuki polycondensation were
bromine and trimethyleneboronate. The monomers with
bromine group include N-(2-ethylhexyl)-carbazole, the branch
point spiro[3.3]heptane-2,6-dispirouorene (SDF), and 4,7-
dithienyl-2,1,3-benzothiadiazole (DBT, #0.1 mol%), and the
monomers with the trimethyleneboronate group were 9,9-dio-
ctyluorene. Thus, the uorene and carbazole monomers
distributed alternately in the synthesized polymer branches.
Because of the same feed ratios of monomers DBrCz, M2 and
TBrSDF for copolymers PPFCzSDF10DBT5–PFCzSDF10DBT10,
their 1H NMR spectra were quite similar (Fig. S3, ESI,† the
proton signals of DBT have little effect because of its low
content), revealing the similar backbone structures of the
copolymers. Taking PFCzSDF10DBT7 as an example (Fig. S4,
ESI†), the actual ratio of monomers DBrCz, M2 and SDF was
PFCzSDF10DBT5. DBrCz (0.153 g, 0.35 mmol), M2 (0.354 g,
0.55 mmol), TBrSF (0.071 g, 0.1 mmol) and DBrDBT (0.2 ml, 2 ꢂ
1
10ꢀ3 mol Lꢀ1). Green powder, yield: 62.9%. H NMR (CDCl3) d
(ppm): 7.88–7.57 (–ArH–), 6.93–6.89 (–ArH–), 3.41–2.93 (–CH2–),
2.21–1.89 (–C–CH2–), 1.18–0.96 (–CH2–), 0.93–0.55 (–CH3).
PFCzSDF10DBT7. DBrCz (0.153 g, 0.35 mmol), M2 (0.354 g,
0.55 mmol), TBrSF (0.071 g, 0.1 mmol) and DBrDBT (0.28 ml,
1
2 ꢂ 10ꢀ3 mol Lꢀ1). Light yellow powder, yield: 65.8%. H NMR
(CDCl3) d (ppm): 7.89–7.56 (–ArH–), 6.93–6.81 (–ArH–), 3.42–2.93
(–CH2–), 2.21–1.88 (–C–CH2–), 1.19–0.98 (–CH2–), 0.94–0.60
(–CH3).
PFCzSDF10DBT8. DBrCz (0.153 g, 0.35 mmol), M2 (0.354 g,
0.55 mmol), TBrSF (0.071 g, 0.1 mmol) and DBrDBT (0.32 ml, 2 ꢂ
1
10ꢀ3 mol Lꢀ1). Green powder, yield: 59.3%. H NMR (CDCl3) d
(ppm): 8.06–7.42 (–ArH–), 6.94–6.77 (–ArH–), 3.45–3.02 (–CH2–),
2.24–1.87 (–C–CH2–), 1.19–0.95 (–CH2–), 0.94–0.64 (–CH3).
PFCzSDF10DBT10. DBrCz (0.153 g, 0.35 mmol), M2 (0.354 g,
0.55 mmol), TBrSF (0.071 g, 0.1 mmol) and DBrDBT (0.4 ml, 2 ꢂ
1
10ꢀ3 mol Lꢀ1). Gray powder, yield: 61.4%. H NMR (CDCl3) d
Scheme 1 Synthesis of the hyperbranched copolymers.
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Table 1 Polymerization results and characterizations of the copolymers
GPC
Copolymers
nDBrCz
nM2
nTBrSDF
nDBrDBT
Yield (%)
Mn
PDI
PFCzSDF10DBT5
PFCzSDF10DBT7
PFCzSDF10DBT8
PFCzSDF10DBT10
0.35
0.35
0.35
0.35
0.55
0.55
0.55
0.55
0.10
0.10
0.10
0.10
5 ꢂ 10ꢀ4
7 ꢂ 10ꢀ4
8 ꢂ 10ꢀ4
10 ꢂ 10ꢀ4
62.9
65.8
59.3
61.4
9547
2.29
2.63
2.43
1.60
10 535
10 854
10 366
calculated by comparing the peak integral intensities of the suggested to benet from the presence of a large amount of
proton signals of a-CH2 (d 4.0–4.5), b-CH (d 1.8–2.2) of N-(2- carbazole groups, which are known to exhibit excellent thermal
ethylhexyl)-carbazole, b0-CH2 of 9,9-dioctyluorene (d 1.8–2.2), and chemical stabilities.39 The relatively high glass transition
and the spiro[3.3]heptane of SDF (d 3.0–3.5), and the results temperatures (Tg, inset of Fig. 1) of the copolymers at around
ꢁ
(nDBrCz : nM2 : nSDF 0.35 : 0.54 : 0.1) was quite close to the feed 180 C are found in the DSC curves, which indicates the good
ratio of the monomers (0.35 : 0.55 : 0.1). The number-average morphological stabilities of the copolymers. The high Tgs of the
molecular weights (Mn) of the copolymers determined by GPC resulting copolymers could be attributed to their rigid hyper-
ranged from 9547 to 10 854 with a polydispersity index (PDI) branched structures and the introduction of carbazole unit to
from 1.60 to 2.63. The resulting copolymers are readily soluble the copolymer backbones.
in common organic solvents such as CHCl3, THF and toluene.
Photophysical properties
Thermal properties
The UV-vis absorption of DBT and the PL spectra of uorene-alt-
The TGA and DSC data of the copolymers are shown in Fig. 1
and Table 2. The onset decomposition temperatures (Td,
measured at a 5% weight loss) range from 400 to 447 ꢁC under
nitrogen. The high thermal stability of the copolymers is
carbazole copolymer (PFCz) are shown in Fig. S5a.† The
absorption of DBT and the emission of PFCz show good spectral
overlap, indicating the efficient FRET from the PFCz segment to
DBT unit can be expected. The normalized UV-vis absorption
and PL spectra of the copolymers in dilute solution are shown in
Fig. 2a. The absorption bands at around 365 nm are due to the
p–p* transitions of poly(uorene-alt-carbazole) backbones,27
which are blue-shied about 20 nm compared with the
uorene-based hyperbranched copolymer.40 This notable
hypochromatic shi is attributable to the interruption of the
conjugation of the copolymer backbone by the introduction of
the 3,6-carbazole linkage. In the PL spectra, the copolymers
exhibit emission bands at 416 nm and a slight vibronic shoulder
at 436 nm, which can be attributed to the formation of a charge-
transfer (CT) state in the poly(uorene-alt-carbazole) branches.
The PL spectra showed about 6 nm blue-shi respect to that of
the uorene-based copolymer because of the decreased conju-
gated length in these uorene-alt-carbazole based copolymers.40
No distinct absorption or emission peaks of the DBT unit can be
observed in the spectra because of its comparatively low
contents in the copolymers, and the FRET is exclusively intra-
chain in dilute solution.41
Fig. 1 TGA and DSC (insert) curves of the copꢀo1lymers in nitrogꢀe1n
atmosphere with a heating rate of 10 ꢁC min and 5 ꢁC min
respectively.
,
Table 2 Thermal and photophysical properties of the copolymers
Dilute solution
Solid lm
labs (nm)
Copolymer
Td (ꢁC)
Tg (ꢁC)
labs (nm)
lPL (nm)
lPL (nm)
PFCzSDF10DBT5
PFCzSDF10DBT7
PFCzSDF10DBT8
PFCzSDF10DBT10
405
447
400
424
186
186
179
178
364
362
365
371
416, 435
416, 436
416, 436
415, 436
370
367
368
372
418, 439
417, 437
419, 440
419, 439, 603
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copolymers are at about ꢀ5.10 eV, which are relatively close to
the work function of PEDOT (ꢀ5.2 eV) and thus facile hole
injection into the emission layer (EML) can be expected.43 On
the other hand, the LUMO levels of copolymers (from ꢀ2.08 eV
to ꢀ2.13 eV) are comparatively distant to the work function of
LiF/Al (ꢀ2.9 eV), indicating that there is a barrier for electron
injection. The results demonstrate that the incorporation of the
carbazole moiety into the uorene backbone could effectively
increases the hole-transporting ability of the materials, and at
the same time, the potential barrier of electron injection is
aggrandized. Thus, an electron transport layer (ETL) will be
needed in the following PLED structure.
Film morphology
The morphology of the spin-coated lms of the copolymers,
which is a key factor for the PLED fabrication, was investigated
by atomic force microscopy (AFM) at a tapping mode. Fig. 4
shows the AFM images of PPFCzSDF10DBT5–PFCzSDF10DBT10
lms prepared by spin-coating chlorobenzene solutions of the
copolymers (10ꢀ5 M) on the quartz substrates. Generally, all the
lms show at and smooth surface without any pinhole defects.
The results imply that the three-dimensional structured SDF
branch point can help to form homogeneous lms with good
quality. The uniform amorphous morphology is favorable for
PLED fabrication.
Electroluminescent properties
Fig. 2 UV-vis absorption and PL spectra of the copolymers: (a) in
CHCl3 solution (10ꢀ5 M) and (b) in solid film.
The devices were fabricated with the conguration ITO/
PEDOT:PSS (40 nm)/copolymer (50 nm)/TPBi (35 nm)/LiF
(1 nm)/Al (150 nm). The relative-energy-level diagram of the
devices is shown in Fig. 5. As mentioned above, the incorpora-
tion of carbazole unit greatly increases the hole-transporting
ability of the materials but leads to a barrier for electron
injection into the emission layer. Here 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi) is used as the electron-
injection layer to facilitate electron transport.10
The electroluminescence spectra of the copolymers are
shown in Fig. 6, and the device data are summarized in Table 4.
The main peaks at 420 nm with a shoulder at 440 nm are
analogous to their PL counterparts. The peaks in the long-
In lms, the copolymers exhibit UV-vis absorption bands at
around 370 nm owning to the p–p* transitions of the poly-
(uorene-alt-carbazole) backbones (Fig. 2b and Table 2). In the
PL spectra, the maximum emission bands of copolymers are at
about 418 nm, along with a shoulder at around 439 nm,
showing no obvious bathochromic shi with respect to those in
dilute solution. This result indicates that the hyperbranched
molecular structure can prevent the aggregation of the copol-
ymer chains efficiently. For PFCzSDF10DBT10, the emission
band of DBT centered at 603 nm can be observed as a result of
both intra- and interchain FRET from uorene-alt-carbazole
unit to DBT, which showed a bathochromic shi of 43 nm
compared to that of pure DBT as a result of the conjugation
effect of the copolymer system.23,41,42
Electrochemical characteristics
The electrochemical behaviors of the resulting copolymers were
investigated by cyclic voltammetry (CV), as shown in Fig. 3 and
Table 3. The oxidation potentials (Eox) vary slightly from 0.58 to
0.60 V. The HOMO levels of copolymers are calculated accord-
ing to the empirical formulas EHOMO ¼ ꢀ(Eox + 4.5) (eV).41 The
LUMO levels are deduced from the HOMO levels and the optical
band gaps (Eg) determined from the onset value of the
absorption spectrum in lm in the long-wavelength direction
(Eg ¼ 1240/ledge). The HOMO levels of the hyperbranched Fig. 3 Cyclic voltammograms of the copolymers.
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Table 3 Electrochemical properties of the copolymers
Copolymers
labs (onset) (nm)
Eg (eV)
Eonset/OX (V)
HOMO (eV)
LUMO (eV)
PFCzSDF10DBT5
PFCzSDF10DBT7
PFCzSDF10DBT8
PFCzSDF10DBT10
416
412
418
419
2.98
3.01
2.97
2.96
0.60
0.59
0.60
0.58
ꢀ5.11
ꢀ5.09
ꢀ5.10
ꢀ5.08
ꢀ2.13
ꢀ2.08
ꢀ2.13
ꢀ2.12
Fig.
4
AFM images (10 ꢂ 10 mm) of the copolymer films: (a) PFCzSDF10DBT5, (b) PFCzSDF10DBT7, (c) PFCzSDF10DBT8 and (d)
PFCzSDF10DBT10.
rate of the intensity of the peak at 610 nm compared to that at
420 nm accelerates as the voltage increases (Fig. 8b). The results
reveal that the intra-, interchain FRET from PFCz unit to DBT
and the charge trapping of DBT with narrow-band gap happen
simultaneously in the electroluminescence process, and the
charge trapping is more efficient under higher voltage.23 So the
white-light emission of copolymer PFCzSDF10DBT10 was
Fig. 5 The energy-level diagram of the devices.
wavelength around 610 nm are gradually enhanced with
increasing the contents of the orange-light emission unit DBT
from PFCzSDF10DBT5 to PFCzSDF10DBT10. White-light emis-
sion was obtained with CIE coordinates located near (0.33,
0.33), and the CRI values are around 90 when the contents of
DBT were 0.08 mol% and 0.10 mol%. PFCzSDF10DBT8 exhibits
cold white light, and PFCzSDF10DBT10 shows warm white light
(Fig. 7).
Taking PFCzSDF10DBT10 as an example, the mechanism of
white light emission was investigated from the EL spectra under
voltages varying from 8 V to 13 V (Fig. 8a). It is clearly seen that
the spectral stabilities at 420 and 440 nm are extremely well and
the broad peaks in the long-wavelength around 610 nm are
gradually enhanced with increasing the voltages. The growth
Fig. 6 Electroluminescence spectra of the copolymer PLEDs at a
voltage of 13 V.
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Table 4 EL performances of the PLEDs
Lmaxb (cd mꢀ2) (at the voltage (V))
CEmax (cd Aꢀ1
)
LEmax (lm Wꢀ1
)
CIE (x,y)
a
Copolymer
Von (V)
PFCzSDF10DBT5
PFCzSDF10DBT7
PFCzSDF10DBT8
PFCzSDF10DBT10
5.11
5.24
5.49
5.34
7280 (12.6)
4.38
4.02
4.21
4.27
1.64
1.31
1.19
1.45
(0.22,0.20)
(0.25,0.24)
(0.28,0.31)
(0.32,0.26)
7188.5 (13.2)
7266.4 (13.5)
7409.5 (13.5)
a
Turn-on voltage (at 1 cd mꢀ2). b Maximum luminance at applied voltage.
Fig. 7 CIE coordinates of the copolymer PLEDs: (a) PFCzSDF10DBT5,
(b) PFCzSDF10DBT7, (c) PFCzSDF10DBT8 and (d) PFCzSDF10DBT10.
obtained at /V from the blue-light emitting of PFCz segments
and the complementary orange-light emitting from DBT
through both incomplete intra-, interchain FRET and the
charge trapping processes mentioned above.
Fig. 9a shows the current density–voltage–brightness (J–V–L)
characteristics of the devices. The devices exhibit moderate low
turn-on voltage of 5.1–5.5 V (Table 4), which can be attributed to
the small energy barrier between PEDOT:PSS and EML. The
maximum luminance (ꢃ7400 cd mꢀ2) and maximum current
efficiency (ꢃ4.2 cd Aꢀ1) of the devices are quite analogous as a
result of the homologous molecular structure of the copoly-
mers. As can be seen from Fig. 9b, the efficiencies decrease
quite slowly with increasing current density, suggesting that
these hyperbranched copolymers and their devices have good
stabilities. Further investigations on the optimization of the
device performance are ongoing in our laboratory.
Fig.
8
Electroluminescence
spectra
of
the
copolymer
PFCzSDF10DBT10 (a), and the intensity ratio of the 610 nm of DBT
emission to the 420 nm of PFCz emission of the copolymer
PFCzSDF10DBT10, i.e. RDBT/PFCz (b).
efficiency from uorene-alt-carbazole segments to DBT unit is
remained in the hyperbranched systems. With the introduction
of carbazole moiety into the backbone, the hyperbranched
copolymers show great thermal stability with Tds ranging from
400 to 447 ꢁC and Tgs ranging from 178 to 186 ꢁC, respectively.
The HOMO energy levels of the copolymers were close to the
work function of PEDOT:PSS because of the carbazole segment,
which facilitates hole injection from PEDOT:PSS to EML in the
PLEDs. Thus, the hyperbranched copolymers exhibit good EL
properties with a low turn-on voltage ꢀo2f about 5 V, the
Conclusions
In conclusion, a series of hyperbranched 3,6-carbazole-alt-2,7-
uorene copolymers with SDF as the core were prepared by
Suzuki polycondensation. The hyperbranched structures
suppress the interchain interactions efficiently, and help to
form amorphous spin-coating lms. The emission bands of the
copolymers showed no obvious bathochromic shis in solid
lms with respect to those in dilute solution. The FRET
maximum luminance of 7409.5 cd m
(at 13.5 V) and
maximum current efficiency of 4.38 cd Aꢀ1. PFCzSDF10DBT8
and PFCzSDF10DBT10 devices realized white light-emitting
with CIE coordinates of (0.28, 0.31) and (0.32, 0.26),
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Acknowledgements
The authors are grateful to the “Program for New Century
Excellent Talents (NCET) in University” (NCET-13-0927), the
International Science & Technology Cooperation Program of
China (2012DFR50460), the National Natural Science Founda-
tion of China (21071108, 21101111, 61274056, 61205179,
61307030, 61307029), the Qualied Personnel Foundation of
Taiyuan University of Technology (QPFT) (no: tyutrc-201255a),
and the Shanxi Provincial Key Innovative Research Team in
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