Electroluminescence performances of 1,1-bis(4-(N,N-dimethylamino)phenyl)-2, 3,4,5-tetraphenylsilole based polymers in three cathode architectures

Article abstract of DOI:10.1007/s11426-013-4875-z

A new silole monomer with two 4-(N,N-dimethylamino)phenyl substitutions on silicon atom as designed and synthesized. Three copolymers PF-N-HPS1, PF-N-HPS10 and PF-N-HPS20 were then obtained by copolymerizations of 2,7-fluorene derivatives with the silole monomer at feed ratios of 1%, 10%, and 20%. Their UV-vis absorption, electrochemical, photoluminescent, and electroluminescent (EL) properties were investigated. PF-N-HPS possessed HOMO levels of -5.25 - 5.58 eV, and showed green emissions. Using PF-N-HPS as the emissive layer, three different polymer light-emitting diodes were fabricated as device A with ITO/PEDOT/PF-N-HPS/Al, device B with ITO/PEDOT/PF-N-HPS/Ba/Al, and device C with ITO/PEDOT/PF-N-HPS/TPBI/Ba/Al. For the device A, PF-N-HPS only showed very low EL efficiency of 0.06-0.33 cd/A, indicating that the Al cathode could not inject electron efficiently to the emissive polymers containing the 4-(N,N-dimethylamino)phenyl groups. For the device B, low work function Ba supplied better electron injections, and the EL efficiency could be improved to 0.85-1.44 cd/A. TPBI with a deep HOMO level of -6.2 eV could enhance electron transport and hole blocking. Thus modified recombinations and largely elevated EL efficiency of 4.56-7.96 cd/A were achieved for the device C. The separation of the emissive layer and metal cathode with the TPBI layer may also suppress exciton quenching at the cathode interface.

Full text of DOI:10.1007/s11426-013-4875-z

SCIENCE CHINA
Chemistry
August 2013 Vol.56 No.8: 1129–1136
doi: 10.1007/s11426-013-4875-z
• ARTICLES •
Electroluminescence performances of
1,1-bis(4-(N,N-dimethylamino)phenyl)-2,3,4,5-tetraphenylsilole
based polymers in three cathode architectures†
LIU ZhiTian1, 2, HU SuJun1, ZHANG LinHua2, CHEN JunWu1*, PENG JunBiao1
& CAO Yong1
1State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, China
2School of Materials Science & Engineering, Wuhan Institute of Technology, Wuhan 430073, China
Received December 27, 2012; accepted January 30, 2013; published online April 17, 2013
A new silole monomer with two 4-(N,N-dimethylamino)phenyl substitutions on silicon atom as designed and synthesized.
Three copolymers PF-N-HPS1, PF-N-HPS10 and PF-N-HPS20 were then obtained by copolymerizations of 2,7-fluorene deriva-
tives with the silole monomer at feed ratios of 1%, 10%, and 20%. Their UV-vis absorption, electrochemical, photolumines-
cent, and electroluminescent (EL) properties were investigated. PF-N-HPS possessed HOMO levels of 5.25– 5.58 eV, and
showed green emissions. Using PF-N-HPS as the emissive layer, three different polymer light-emitting diodes were fabricated
as device A with ITO/PEDOT/PF-N-HPS/Al, device B with ITO/PEDOT/PF-N-HPS/Ba/Al, and device C with ITO/PEDOT/
PF-N-HPS/TPBI/Ba/Al. For the device A, PF-N-HPS only showed very low EL efficiency of 0.06–0.33 cd/A, indicating that
the Al cathode could not inject electron efficiently to the emissive polymers containing the 4-(N,N-dimethylamino)phenyl
groups. For the device B, low work function Ba supplied better electron injections, and the EL efficiency could be improved to
0.85–1.44 cd/A. TPBI with a deep HOMO level of 6.2 eV could enhance electron transport and hole blocking. Thus modi-
fied recombinations and largely elevated EL efficiency of 4.56–7.96 cd/A were achieved for the device C. The separation of
the emissive layer and metal cathode with the TPBI layer may also suppress exciton quenching at the cathode interface.
silole-containing polymers, N,N-dimethylaminobenzene, polyfluorene, electroluminescence
the ring carbons of the butadiene, can be realized, mean-
1 Introduction
while large fused-rings containing a silole core, e.g., diben-
zosilole and dithienosilole, can be formed. Some siloles
possess outstanding optoelectronic properties, as high elec-
tron affinity and fast electron mobility were observed for
those fused with electron-deficient aromatic rings [3]. A
series of 2,3,4,5-tetraphenylsiloles with different 1,1-
disubstitutions displayed novel aggregation-induced emis-
sion (AIE) [46], giving high PL quantum yields (PL) up
to 100% in an amorphous form and 80% in a crystalline
form [7, 8]. In addition, the organic light-emitting diodes
(OLEDs) [9, 10] could generate very high electrolumi-
nescent (EL) efficiency, for example, an external quan-
tum efficiency (EL) of 8% was realized when 1-methyl-
Siloles or silacyclopentadienes [1] are a group of five-
membered heterocycles, with structural characterization of
σ*-* conjugation arising from the σ* orbital of two σ*
bonds on the silicon atom and the * orbital of the butadi-
ene moiety. Such σ*-* interaction usually has lower
LUMO levels than other five-membered systems such as
pyrroles, furans an thiophenes [2]. Fully substituted siloles,
i.e, with two groups on the silicon atom and four groups on
*Corresponding author (email: psjwchen@scut.edu.cn)
†Recommended by QIU Yong (Tsinghua University)
© Science China Press and Springer-Verlag Berlin Heidelberg 2013
chem.scichina.com www.springerlink.com

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Liu ZT, et al. Sci China Chem August (2013) Vol.56 No.8
1,2,3,4,5-pentaphenylsilole was used as emissive layer,
and an EL of 7% was reported for 1,1,2,3,4,5- hexa-
phenylsilole (HPS) [11]. Considerable interest has been
attracted to silole-containing polymers due to their
unique optoelectronic properties [12]. High PL of >80%
and EL of ~3% have been achieved for the polymer
light-emitting diodes (PLEDs) with silole-containing
polymers as the emissive layer [13, 14]. Another example
is the application of dithienosilole-based polymers, which
gave high hole mobility of an order of 102 cm2/Vs in
field effect transistors and energy conversion efficiency
over 7% in polymer solar cells (PSC) [1517]. Moreover,
dibenzosilole-based polymers showed good luminescent
and photovoltaic properties [18].
2 Experimental section
2.1 Material
All manipulations involving air-sensitive reagents were
performed under an atmosphere of dry argon. All reagents,
unless otherwise specified, were obtained from Aldrich,
Acros, and TCI Chemical Co., and were used as received.
All solvents were carefully dried and distilled under nitro-
gen flow.
2.2 Synthesis of monomers
Bis(4-(N,N-dimethylamino)phenyl)-diphenylethynyl-silane
(1)
The polymers with 2,5-difunctionalized siloles produced
interesting emissions, and the band gap could be tuned
through different 2,5-difunctionalizations. For example,
green emissions were observed for the polymers with
2,5-diphenylsilole [13] and 2,5-difluorenylsilole [19] as the
building blocks, while red emissions were reported for the
2,5-dithienylsilole-based polymers [20]. Other silole mon-
omers with 3,4- [21] or 1,1-difunctional groups were also
synthesized. However, the 1,1-difunctionalized silole
monomers are very limited, only involving 1-methyl-1-
phenyl or 1,1-dimethyl substitutions in a previous report
[22].
In recent years, alcohol or water soluble conjugated
polymers have exhibited excellent performances in interfa-
cial modifications of cathodes for PLEDs and PSCs [23].
For instance, the alcohol solubility of a polyfluorene PFN
was greatly improved by introducing two N,N-dimethyl-3-
aminopropyl groups to its 9-position [24]. The application
of PFN as the interfacial layer of the cathode of PLEDs re-
sulted in powerful electron injections from air-stable high
work-function metals (Au, Ag and Al), with device effi-
ciency comparable to those of low work-function metals
(Ca, Ba) [25]. For the cathode modifications of normal and
inverted PSCs, the open-circuit voltage, short-circuit current,
and fill factor, the three photovoltaic parameters for solar
cells, could be greatly elevated to give high PSC efficiency
of >9% [2628].
In this work, a new hexaphenylsilole monomer with
2,5-di(4-bromophenyl)-substitutions was successfully syn-
thesized by a lithium naphthalenide-mediated cyclization
reaction, in which two 4-(N,N-dimethylamino)phenyl
groups were attached to the silicon atom. The resultant mon-
omer 1,1-bis(4-(N,N-dimethylamino)phenyl)-3,4-diphenyl-
2,5-bis(4-bromophenyl)silole, was then copolymerized with
fluorene monomers [2931] to afford three different 1,1-
bis(4-(N,N-dimethylamino)phenyl)-2,3,4,5-tetraphenylsilole-
containing polyfluorenes (PF-N-HPS). Their UV absorption,
electrochemistry, and PL properties were characterized.
Furthermore, three EL devices with Al, Ba/Al, and
TPBI/Ba/Al as the cathodes were applied to evaluate
their electron injection and transport properties.
Magnesium powder (480 mg, 20 mmol) was added to a so-
lution of 4-bromo-N,N-dimethylaniline (2.00 g, 10 mmol) in
THF (100 mL) under nitrogen atmosphere. The reaction
was initiated by iodine (10 mg) at room temperature, and
further refluxed under heating for 2 h to provide the crude
(N,N-dimethylamino)phenylmagnesium bromide. This Gir-
gnard reagent was added dropwise to solution of tetrachlo-
rosilane (0.63 mL, 5 mmol) in THF (10 mL), and the re-
sulting mixture was stirred for 1 h at room temperature to
produce the dichlorosilane. Meantime, phenylethynyl lith-
ium was prepared in another reaction flask as followed. To
a solution of phenylacetylene (1.1 mL, 10 mmol) in THF
(20 mL) was added n-butyl lithium (4 mL, 2.5 M solution in
hexane) dropwise at 0 °C, and the reaction mixture was
stirred at this temperature for 1 h. With both the di-
chlorosilane and phenylethynyl lithium in hand, the desired
diaryl-bis(phenylethynyl)silane 1 could be assembled. The
phenylethynyl lithium mixture was added dropwise to the
dichlorosilane solution, and the reaction was performed for
1 h at room temperature. An aqueous solution of HCl (1 M)
was added and the mixture was extracted with diethyl ether.
The organic layer was washed with brine and dried over
MgSO4. The solvents were concentrated by vacuum evapo-
ration and the crude product was purified by silica gel
chromatography using ethyl acetate/petroleum ether (1:8,
v/v) as eluent to afford a white solid (1.43 g, 61% yield),
which was further purified by recrystallization in toluene/
1
n-heptane to give the title compound 1 (1.21 g). H NMR,
(300 MHz, CDCl3), δ (ppm): 7.74 (d, J = 8.8 Hz, 4H),
7.60 (t, J = 6.3 Hz, 6H), 7.34 (m, 8H), 6.82 (d, J = 9.0 Hz,
4H), 3.00 (s, 12H). 13C NMR (75 MHz, CDCl3), δ (ppm):
136.18, 132.42, 132.31, 128.76, 128.20, 128.15, 123.06,
111.97, 107.51, 89.55, 40.21.
1,1-Bis(4-(N,N-dimethylamino)phenyl)-3,4-diphenyl-2,5-bis
(4-bromophenyl)silole (2)
Granular lithium (84 mg, 12 mmol) was added into a solu-
tion of naphthalene (1.58 g, 12.1 mmol) in THF (12 mL),
and the resulting mixture was stirred at room temperature
under a nitrogen atmosphere for 5 h, readily forming lithium

Liu ZT, et al. Sci China Chem August (2013) Vol.56 No.8
1131
naphthalenide (LiNaph). Then a solution of bis(4-(N,N-
dimethylamino)phenyl)-bis(phenylethynyl)-silane 1 (1.41 g,
3 mmol) in THF (5 mL) was added dropwise at 0 °C. The
mixture was stirred for 10 min after the ice bath was re-
moved. Then the system was cooled back to 0 °C,
ZnCl2-TMEDA (3 g, 12 mmol) and 20 mL of THF were
added. The resulting mixture was stirred for 1 h at this tem-
perature to give the 2,5-dizincated silole intermediate for
the following Pd-catalyzed transformation. 4-Bromo-
iodobenzene (1.7 g, 6 mmol) and PdCl2(PPh3)2 (100 mg,
0.15 mmol) were added to the above reaction mixture, and
the system was heated to 50 °C and stirred for 48 h. An
aqueous solution of HCl (1 M) was added and the mixture
was extracted with diethyl ether. The organic phase was
washed with brine and dried over anhydrous MgSO4. After
the solvents were evaporated under reduced pressure, the
crude product was purified by silica gel chromatography
using ethyl acetate/petroleum ether (1:8, v/v) as eluent to
afford a yellow solid (1.31 g, 56% yield), which was further
purified by recrystallization in toluene/n-heptane to give the
greenish powder (508 mg, 70%). (GPC): Mw 28800; Mw/Mn
1.25 (Table 1). Anal. Calcd. C 88.25, H 9.18, N 1.28; Found:
C 86.27, H 9.08, N 1.09.
2.4 PLED fabrication and characterization
Following previously reported procedures, the PLEDs were
fabricated with PF-N-HPS1~20 as emissive layers [14].
Patterned indium-tin oxide (ITO; ~15 Ω/square) coated glass
substrates were cleaned with acetone, detergent, distilled
water, and 2-propanol, subsequently in an ultrasonic bath.
After treatment with oxygen plasma, poly(3,4-ethylenedi-
oxythiophene) (PEDOT) was spincoated onto the ITO sub-
strate, followed by drying in a vacuum oven. A thin film of
electroluminescent copolymer was coated onto the anode as
emissive layer. The TPBI hole blocking layer and metal
cathode (Ba/Al or Al) were evaporated on the top of an EL
polymer layer in a vacuum coating machine. Current-
voltage (I-V) characteristics of the devices were recorded
with a Keithley 236 source meter. The luminance was
measured with a calibrated photodiode. The external quan-
tum efficiency was verified by measurement in the inte-
grating sphere (IS-080, Labsphere), and luminance was cal-
ibrated by using a PR-705 SpectraScan spectrophotometer
(Photo Research).
1
title compound 2 (1.2 g). H NMR, (300 MHz, CDCl3), δ
(ppm): 7.51 (d, J = 8.8 Hz, 4H), 7.11 (d, J = 8.9 Hz, 4H),
7.05 (m, 6H), 6.86(m, 4H), 6.77(m, 8H), 3.01 (s, 12H).
13C NMR (75 MHz, CDCl3), δ (ppm): 155.83, 151.57,
139.83, 139.03, 138.73, 137.21, 136.88, 130.82, 129.91,
127.54, 126.38, 119.31, 115.48, 112.11, 40.00.
3 Results and discussion
2.3 Synthesis of copolymers
3.1 Synthesis and characterization
All polymerizations were carried out by palladium(0)-
catalyzed Suzuki coupling reactions of the diboronic ester
monomer 3 with one equivalent of combined dibromo
monomers 2 and 4 under nitrogen protection (Scheme 1).
All monomers were purified by recrystallization. Three dif-
ferent ratios of fluorene monomers to the silole monomer
(99:1, 90:10, and 80:20) were investigated. A tpytical proce-
dure for the polymerization of PF-N-HPS20 is given below.
Carefully purified 2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9-dioctylfluorene 3 (642 mg, 1 mmol),
2,7-dibromo-9,9-dioctylfluorene 4 (328 mg, 0.6 mmol),
1,1-bis(4-(N,N-dimethylamino)phenyl)-3,4-diphenyl-2,5-bis
(4-bromophenyl)silole 2 (169 mg, 0.4 mmol), (PPh3)4Pd (6
mg, 0.005 mmol) and several drops of Aliquat 336 were
dissolved in a mixture of toluene (10 mL) and aqueous 2 M
Na2CO3 (2 mL). The mixture was refluxed under nitrogen
for 24 h. At the end of the polymerization, a small amount
of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dioctylfluorene was added, and the reaction continued for
12 h to remove the bromine end groups. In a similar way,
bromobenzene was added as a monofunctional end-capping
reagent to remove the boronic ester end group. The mixture
was then poured into vigorously stirred methanol. The pre-
cipitate was collected by filtration and washed with acetone
to remove oligomers and catalyst residues. The product was
dried in vacuum for 24 h to yield PF-N-HPS20 as a yellow-
The synthesis of the monomer 2 and polymers PF-N-HPS
are shown in Scheme 1. The Grignard reagent prepared
from 4-N,N-dimethylamino-benzene reacted with silicon
tetrachloride to give the dichlorosilane intermediate, which
was treated with phenylethynyl lithium to afford bis(4-(N,N-
dimethylamino)phenyl)-bis(phenylethynyl)-silane (1) in 61%
yield. The target monomer 1,1-bis(4-(N,N-dimethylamino)
phenyl)-3,4-diphenyl-2,5-bis(4-bromophenyl) silole 2 was
prepared in a one-pot fashion. The cyclization of 1 was ac-
complished using lithium naphthalenide (LiNaph) at 0 °C to
afford 2,5-dilithiosilole, which was readily converted to
2,5-dizincated silole under ZnCl2-TMEDA conditions. The
subsequent selective Negishi coupling reaction with
4-bromo-iodobenzene generated the target monomer 2 in
56% overall yield.
Three different silole-containing polymers PF-N-HPS1,
PF-N-HPS10, and PF-N-HPS20 were synthesized from
monomer 2 and two 2,7-fluorene monomers (3 and 4) by
Suzuki coupling reaction in 6581% yield. The feed ratios
of monomer 2 and 2,7-fluorene monomers (the combination
of 3 and 4) were 1%, 10%, and 20% respectively. Light
emitting energy transfer conjugated polymers with a ran-
dom chemical structure demonstrated that the quenching of
the light emission of chromophores could be largely sup-
pressed. The average weight molecular weights (Mw) of

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Liu ZT, et al. Sci China Chem August (2013) Vol.56 No.8
Scheme 1 Synthetic routes of the silole monomer and the polymers.
Table 1 Molecular weights and elemental analyses of the copolymers
Elemental analysis a)
H
Copolymer
Mw
Mw/Mn
C
N
PF-N-HPS1
PF-N-HPS10
PF-N-HPS20
50000
30900
28800
1.78
1.43
1.25
87.29(89.54)
86.74(88.89)
86.27(88.25)
9.82(10.33)
9.44(9.76)
9.08(9.18)

0.64(0.68)
1.09(1.28)
a) Data given in the parentheses are contents in the feed compositions (leaving groups excluded).
PF-N-HPS1, PF-N-HPS10, and PF-N-HPS20, estimated by
gel permeation chromatography (GPC), were 50000, 30900
and 28800 with narrow polydispersity index as 1.78, 1.43
and 1.25, respectively. The elemental analyses of the co-
polymers are listed in Table 1. The C, H, and N contents of
the copolymers are very close to those of the feed composi-
tions (leaving groups excluded).
3.2 UV absorption properties
The UV-vis absorption spectra of PF-N-HPS1, PF-N-
HPS10 and PF-N-HPS20 in THF solution are shown in
Figure 1. The three different copolymers show similar ab-
sorption with absorption maxima around 390 nm, which are
mainly attributed to the -* absorption of fluorene seg-
ment [13]. However, the intensities of absorption edges are

Liu ZT, et al. Sci China Chem August (2013) Vol.56 No.8
1133
Figure 2 Cyclic voltammetry of the copolymer films.
Figure 1 UV absorption spectra of the copolymers in THF solution (1 
104 M).
obviously different, indicating the influence of silole con-
tent. With higher silole content, the absorption at 450 nm
increases gradually.
The absorption spectra of the films are very close to
those of their THF solutions (data not shown). The optical
band gaps of the copolymers are estimated as 2.59, 2.67 and
2.81 eV, based on the onset wavelengths in the absorption
spectra of the films (Table 2). With the increase of silole
content, the optical band gaps decrease gradually.
3.3 Electrochemical properties
The electrochemical behaviors of the copolymers were in-
vestigated by cyclic voltammetry (CV), with the corre-
sponding curves were shown in Figure 2. The CV curves
were referenced to a saturated calomel electrode (SCE). The
onsets of oxidation potentials (Eox) measured by p-doping
scan process are between 5.58 and .25 eV. The LUMO
levels are calculated based on their HOMO levels and opti-
cal band gaps. As listed in Table 2, the LUMO levels are in
the range between 2.60 and 2.77 eV.
Figure 3 Photoluminescence spectra of the copolymers. (a) THF solution
(4  105 M); (b) thin solid film. Excitation wavelength: 390 nm.
3.4 Photoluminescence properties
Photoluminescence (PL) spectra of the copolymers are
shown in Figure 3. Photoluminescence spectra of the co-
polymers in a dilute THF solution (4  105 M) are shown in
Figure 3(a), with the emission maxima listed in Table 3.
The PL spectrum of PF-N-HPS1 exhibits only two blue
emissive peaks of a polyfluorene at 418 and 440 nm. Be-
cause no obvious signature of green emission is observed
from HPS segment, the two blue emissive peaks can be as-
cribed to the low content of silole (1%). The results also
indicate that the intrachain excitation energy transfer from
fluorene segments to silole units is quite limited in diluted
solution. With the increase of HPS contents in PF-N-HPS10
and PF-N-HPS20, a new emissive peak appears at ~506 nm,
Table 2 Optical band gaps and electrochemical properties of the copolymer films
Copolymer
PF-N-HPS1
PF-N-HPS10
PF-N-HPS20
Optical band gap (eV)
Eox (V)
1.18
HOMO a) (eV)
5.58
LUMO b) (eV)
2.77
2.60
2.66
2.81
2.67
2.59
0.87
5.27
5.25
0.85
a) Calculated according to HOMO = e(Eox + 4.4); b) calculated from HOMO level and the optical band gap.

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Liu ZT, et al. Sci China Chem August (2013) Vol.56 No.8
with the silole emissions displaying relative intensity of
30% and 60% to the emission of the fluorene segments,
respectively. This observation clearly demonstrates the con-
tribution of intrachain excitation energy transfer.
the three devices are listed in Table 4.
It was reported that the application of polyfluorenes with
trialkylamine side chain as the emissive layer in PLEDs
with Al cathode could show higher EL efficiency, in com-
parison to Ba/Al cathode [24, 25]. This was attributed to the
formation of interfacial dipole between the trialkylamine
groups of the emissive polymer and the Al cathode, which
decreased the barrier height for electron injection. Moreover,
efficient electron injection from high work function Au
cathode could be achieved [25]. Interestingly, in our work,
the turn-on voltages (Von) of PF-N-HPS1~20 in device A all
exceed 10 V, and the maximum brightness (Lmax) values are
only 38~116 cd/m2, showing very low EL efficiencies (Ta-
ble 4). The results indicate that 4-(N,N-dimethylamino)-
phenyl groups on PF-N-HPS failed to achieve good electron
injection from the Al cathode. Therefore, the electron injec-
tion ability from a high work function metal cathode may be
quite different for PF-N-HPS with aromatic amine from
those with the trialkylamine groups.
Photoluminescence spectra of the thin solid copolymer
films are shown in Figure 3(b), with the PL peaks listed in
Table 3. The PF-N-HPS1, with a very low content of silole,
still gives a large ratio of emission from fluorene segments.
The blue emissive peak is slightly red-shifted, while the
green-emissive band from the silole units reaches a relative
intensity of 75% in comparison to the emission of fluorene
segments, which can be ascribed to the significant contribu-
tion of interchain excitation energy transfer in the solid state
[13, 14]. For the PF-N-HPS10 with higher silole content on
the main chain, the silole emission becomes a major PL
peak and the emission from the fluorene segment only dis-
plays a 10% relative intensity. In the PL spectrum of
PF-N-HPS20 with a silole content of 20%, the emissive
band from the fluorene segment almost disappears.
The maximum luminous efficiencies (LEmax) in device B
are improved in comparison to device A. The LEmax values
of PF-N-HPS1–20 are 1.44, 0.92, and 0.85 cd/A respective-
ly, which are 4–8 times higher than those of device A. The
Von values of device B are all below 5 V, and the Lmax val-
ues are between 1460 and 2260 cd/m2. The EL spectra of
device B are shown in Figure 4(a). A main EL emission
peak (ELmax) of PF-N-HPS1 appears at 522 nm (Table 4)
due to the emission from HPS units, while a weak emission
from fluorene segments still exists. The ELmax of
PF-N-HPS10 is at 538 nm, with a weak shoulder peak at
600 nm. For the PF-N-HPS20, the ELmax moves to 605 nm,
with the emissive band at 538 nm becoming a shoulder
peak. But in device C, there is no emission peak at ~600 nm
(Figure 4(b)). Therefore, the emission peaks at ~600 nm
may be resulted from the possible interactions between the
polar N,N-dimethylamino groups on the silole and the Ba
cathode. The electron deficient silole rings could act as ef-
fective electron traps, causing exciton recombination zone
much close to the cathode [14].
3.5 Electroluminescence properties
Three different PLEDs were prepared with the following
configurations: ITO/PEDOT (50 nm)/PF-N-HPS (80
nm)/Al for device A, ITO/PEDOT (50 nm)/PF-N-HPS (80
nm)/Ba/Al for device B, and ITO/PEDOT (50 nm)/PF-
N-HPS (80 nm)/TPBI (30 nm)/Ba/Al for device C. In the
device C, the TPBI layer was inserted as electron transport
and hole blocking material [14]. The EL performances of
Table 3 Photoluminescence properties of the copolymers
Solution a)
Film
Copolymer

PLmax (nm)
418, 440

PLmax (nm)
PF-N-HPS1
PF-N-HPS10
PF-N-HPS20
434, 461, 493
430, 500
506
418, 440, 506
418, 440, 508
a) THF solution: 4105 mol/L; excitation wavelength: 390 nm
Table 4 EL properties of the copolymers in three device configurations
b)
c)
d)
Device
Emissive
layer
Von
LEmax
Lmax
CIE coordinates
ELmax
ELmax
configuration a)
(V)
(cd/A)
0.33
0.12
0.06
1.44
0.92
0.85
4.78
7.96
4.56
(cd/m2)
116
(x, y)
(nm)
(%)
A
A
A
B
B
B
C
C
C
PF-N-HPS1
PF-N-HPS10
PF-N-HPS20
PF-N-HPS1
PF-N-HPS10
PF-N-HPS20
PF-N-HPS1
PF-N-HPS10
PF-N-HPS20
10.8
10.4
10.0
4.7
522
534
0.13
0.04
0.02
0.58
0.37
0.34
1.91
3.18
1.82
–
50
–
540
38
–
522
2260
1460
1690
2070
4880
1340
0.35, 0.47
0.40, 0.53
0.44, 0.49
0.24, 0.36
0.31, 0.48
0.35, 0.53
4.9
538
4.5
538, 605
440, 507
520
8.0
6.4
8.1
527
a) Device A: ITO/PEDOT (50 nm)/PF-N-HPS (80 nm)/Al, device B: ITO/PEDOT(50 nm)/PF-N-HPS (80 nm)/Ba/Al, device C: ITO/PEDOT (50
nm)/PF-N-HPS (80 nm)/TPBI(30 nm)/Ba/Al. The active area for all devices is 0.17 cm2; b) turn-on voltage; c) maximum luminous efficiency; d) maximum
brightness.

Liu ZT, et al. Sci China Chem August (2013) Vol.56 No.8
1135
phenyl)-3,4-diphenyl-2,5-bis(4-bromophenyl)silole, was pre-
pared from tetrachlorosilane. The copolymerization of this
silole with flurorene derivatives was achieved using Suzuki
coupling to afford PF-N-HPS1, PF-N-HPS10 and
PF-N-HPS20. The HOMO levels of the copolymers are
between 5.25 and 5.58 eV. The copolymers show green
emissions due to excitation energy transfer from fluorene
segments to hexaphenylsilole units. The PLED perfor-
mances of the polymers were investigated with three dif-
ferent cathode configurations (Al, Ba/Al, and TPB/Ba/Al).
The efficiencies with Al as cathode are only 0.06–0.33 cd/A,
indicating that the 4-(N,N-dimethylamino)phenyl groups in
PF-N-HPS cannot achieve effective electron injection from
the Al cathode. With Ba/Al as cathode, the electron injec-
tion is improved and the efficiencies are elevated to
0.85–1.44 cd/A. By utilizing the electron transport and hole
blocking functions of the TPBI layer, the TPBI/Ba/Al cath-
ode can boost the efficiencies to 4.56–7.96 cd/A. The sepa-
ration of the emissive layer and metal cathode with the
TPBI layer may also suppress exciton quenching at the
cathode interface.
This work was supported by the National Natural Science Foundation of
China (21225418, 51003080), the National Basic Research Program of
China (2013CB834705), the Youth Science Plan for Light of the Morning
Sun of Wuhan City (201271031385), State Key Laboratory of Luminescent
Materials and Devices (2012-09), and Natural Science Foundation of
Hubei Province (2012FFB04705).
Figure 4 EL spectra of PF-N-HPS. (a) Device B; (b) device C.
In device C, a TPBI layer was inserted as electron
transport and hole blocking material, separating the emis-
sive layer from Ba/Al cathode. As shown in Figure 4(b), the
EL spectra are somewhat similar to the corresponding PL
spectra, with the ELmax red-shifted to the PLmax. In the EL
spectrum of PF-N-HPS1, a blue emission from fluorene
segments is observed, which is remarkably weakened for
PF-N-HPS10 with higher silole content. In the EL spec-
trum of PF-N-HPS20, this blue emission peak totally dis-
appears, with ELmax of 527 nm as the exclusive green-
emission from silole units. In comparison with device B,
the EL efficiencies of device C are significantly improved.
As listed in Table 4, the LEmax values of PF-N-HPS1–20
are 4.78, 7.96 and 4.56 cd/A respectively, which are 3–8
times higher than those of device B. Among the three emis-
sive copolymers, PF-N-HPS10 displays the highest ELmax
of >3%, with a Von of 6.4 V and an elevated LEmax of 4880
cd/m2. The improvement of EL performance in device C
can be explained as follows: the insertion of the TPBI layer
not only enhanced the electron transport and hole blocking,
but also separated the PF-N-HPS emissive layer from the
metal cathode, which could greatly suppress the cathode
quenching.
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