Triisopropylsilylethynyl substituted benzodithiophene copolymers: Synthesis, properties and photovoltaic characterization

Article abstract of DOI:10.1039/c4tc02107j

We demonstrated herein the facile synthesis of triisopropylsilylethynyl (TIPS) functionalized benzo[1,2-b:4,5-b′]dithiophene (BDT). Three new TIPSBDT-based donor-acceptor alternating copolymers were further developed by Pd-catalyzed Stille coupling. The effect of accepting unit structure on the optical, electrochemical and energy levels of the polymer was studied. The positive impact of PFN layer, high-boiling solvents processing, polar solvent treatment and solvent annealing on the performance of polymer:PC₇₁BM bulk heterojunction solar cells was revealed. The best devices delivered a power conversion efficiency of 5.46% when blend films processed using o-dichlorobenzene with 3 vol% DIO and treated with the optimization of THF annealing and insertion of PFN layer. The device performance was correlated with the morphology evolution of blend films processed with solvent choice, methanol treatment and THF annealing.

Full text of DOI:10.1039/c4tc02107j

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ARTICLE TYPE
Triisopropylsilylethynyl substituted benzodithiophene copolymers:
synthesis, properties and photovoltaic characterization
Enwei Zhu,a,‡ Guoping Luo,b,‡ Yun Liu,a Jiangsheng Yu,a Fujun Zhang,c Guangbo Che,d Hongbin Wu,*,b
Weihua Tang*,a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
We demonstrated herein the facile synthesis of triisopropylsilylethynyl (TIPS) functionalized benzo[1,2ꢀ
b:4,5ꢀb']dithiophene (BDT). Three new TIPSBDTꢀbased donorꢀacceptor alternating copolymers were
further developed by Pdꢀcatalyzed Stille coupling. The effect of accepting unit structure on the optical,
10 electrochemical and energy levels of the polymer was studied. The positive impact of PFN layer, highꢀ
boiling solvents processing, polar solvent treatment and solvent annealing on the performance of
polymer:PC71BM bulk heterojuction solar cells was revealed. The best devices delivered a power
conversion efficiency of 5.46% when blend films processed using oꢀdichlorobenzene with 3 vol% DIO
and treated with the optimization of THF annealing and insertion of PFN layer. The device performance
15 was correlated with the morphology evolution of blend films processed with solvent choice, methanol
treatment and THF annealing.
successful D units for highꢀefficient PSC materials.21ꢀ27 Featuring
good planarity with two fused thiophene units, BDT allows
feasible substitutions on benzene core to fineꢀtune the energy
50 levels of the resulted derivatives.28 Triisopropylsilylethynyl
functionalized BDT (TIPSBDT) was firstly developed by Li et al,
whose low bandgap copolymer delivered a PCE of 4.33% by
blending with PC71BM.29 TIPSBDTꢀquinoxaline alternating
polymer contributed a PCE of 2ꢀ6%.30 All these reports inspired
55 us to employ TIPSBDT for constructing new efficient
photovoltaic polymers with deep HOMO energy levels. The
incorporation of bulky TIPS groups on BDT cores can not only
improve the solubility and oxidative stability of the polymers but
also promote the πꢀorbital overlap for higher charge transport.
60 The ethynylene group as an important πꢀelectron linker extended
the conjugated system between backbone and silicon atom to
facilitate high charge transfer along the side chains. In addition,
the DꢀA copolymers incorporating electronꢀdeficient moieties
1 Introduction
In the past decades, polymer solar cells (PSCs) have attracted
great interest as a promising nextꢀgeneration green technology in
20 comparison of inorganic siliconꢀbased cells due to their potential
advantages of costꢀeffective, lightweight, and largeꢀscale rollꢀtoꢀ
roll production.1ꢀ5 Great progress has been made for PSCs with
bulk heterojunction (BHJ) structures, where a polymer/fullerene
blend with bicontinuous interpenetrating networks acts as active
25 layer. Tremendous efforts in developing new materials,
optimizing film morphology and designing new device
architectures have pushed the power conversion efficiency (PCE)
of the PSCs.6ꢀ8 Excitingly, the stateꢀofꢀtheꢀart PCEs have been
over 8%, with a champion record of 10.6%.9ꢀ12 Although both the
30 innovation of the material and the optimization of the device
structure have led to fairly desirable results, further PSCs
commercial application is challenging before the solution of some
key issues, such as dependable encapsulation technology to
secure longꢀlife working devices and robust largeꢀarea processing
35 for device making.2,9 It is still necessary to find ideal conjugated
polymers combining high PCEs with convenient processability in
order to meet the requirements of commercialization of PSCs.
Currently, most research is focused on the development of low
bandgap polymers with strong and broad absorption, lowꢀlying
40 highest occupied molecular orbital (HOMO) energy levels and
high charge mobility. Among various successful polymers
utilized for highꢀefficient solar cells, donorꢀacceptor (DꢀA)
alternating structure taking advantage of the intramolecular
charge transfer (ICT) is one of the most effective approaches to
45 achieve low bandgap polymers.13ꢀ20 To date, benzo[1,2ꢀb:4,5ꢀ
b']dithiophene (BDT) has been proven to be one of the most
such
65 dithienylbenzo[c][1,2,5]oxadiazole
demonstrated good potential for highꢀperformance PSCs.
as
thiazolo[5,4ꢀd]thiazole
(TTz)20,31
units
,
or
also
(DTBO)22
From the view of designing new polymers to achieve highꢀ
performance PSCs, we are intrigued to know how the alternating
of electronꢀdeficient units including TTz, DTBO and
70 thiadiazolo[3,4ꢀg]quinoxaline (DTQx)32 with TIPSBDT unit can
tune the bandgap, absorption and photovoltaic properties of the
resulted polymers. On the other hand, the correlation study
between device processing strategies and morphology of
polymer/fullerene blend in BHJ cells is the key consideration in
75 the pursuit of highꢀperformance PSCs. The device performance
can be significantly improved by optimizing the morphology in
active layer with insertion of alcohol/waterꢀsoluble conjugated
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polymer named poly [(9,9ꢀbis(3’ꢀ(N,Nꢀdimethylamino) propyl)ꢀ
2,7ꢀfluorene)ꢀaltꢀ2,7ꢀ(9,9ꢀdioctylfluorene)] (PFN) layer,24,25 use
toluene. The molecular weight and polydispersity index (PDI)
were measured by GPC analysis. Polymers exhibited a numberꢀ
of processing additives,16,26,27 solvent annealing,33,34 and polar 45 average molecular weight (Mn) of 10ꢀ15 kDa, with
a
solvent treatment.14,22 For example, a PCE of 8.4% was achieved
for PBDTꢀDTNT:PC71BM based devices by inserting a PFN
interlayer to optimize the interface between photoactive layer
with electrode.24 Ge et al. reported the hitherto highest PCE of
corresponding PDI ranging from 1.38 to 1.48. The thermal
5
stability of three polymers was investigated with TGA analysis,
with the TGA traces plotted in Figure S11. A degradation
temperature (Td, corresponding to 5% weight loss) of polymer
7% for furanꢀcontaining PSCs through simply solvent treatment 50 was found to be 310oC for PTIPSBDTꢀTTz, 380oC for
before deposition of the metal electrodes.22
PTIPSBDTꢀBO, and 360oC for PTIPSBDTꢀDTQx. These high Td
values indicate the good thermal stability of the polymers, which
are beneficial for their application in optoelectronic devices.6
10
In this study, we demonstrated the facile synthesis of TIPSBDT
via Sonogashira coupling between trimisopropylsilyl acetylene
and BDT triflate. Three BDT based new copolymers (i.e.,
PTIPSBDTꢀTTz, PTIPSBDTꢀBO and PTIPSBDTꢀDTQx) were
further developed. We investigated the synthesis, thermal
2.2 Optical Properties
55 The UVꢀvis absorption spectra of three polymers in dilute CF
solutions (~1×10ꢀ5 M) and in thin films are shown in Figure 1,
with the corresponding absorption data summarized in Table 1.
As shown, all polymers exhibit two distinct absorption bands.
The absorption at ultraviolet region can be ascribed to the πꢀπ*
60 transition of their main chain units and the absorption at the
visible region can be attributed to the CT absorption of polymer
main chains.6 PTIPSBDTꢀTTz solution showed long absorption
band in 600ꢀ680 nm region with an absorption maximum (λmax) at
616 nm. Compared with its solution, PTIPSBDTꢀTTz film
65 exhibited 12 nm redꢀshifted λmax and 70 nm redꢀshifted
absorption edge (λonset), indicating the strong intermolecular
interaction and aggregation occurring in the solidꢀstate of
polymer. PTIPSBDTꢀDTBO film exhibited broad absorption
spectra with a λmax at 619 nm and absorption shoulder in the
70 range of 500ꢀ700 nm, which was ~20 nm redꢀshifted in
comparison to its dilute solution. This may be explained with the
effective intermolecular πꢀπ packing interactions in its solid state.
PTIPSBDTꢀDTQx films, however, exhibited a remarkably redꢀ
shifted absorption (~ 80 nm) with a λonset up to 950 nm in
75 comparison to its dilute solution, suggesting the strongest ICT
absorption along polymer backbones caused by the strong
electronꢀwithdrawing properties of DTQx units as well as the
large conjugated fusedꢀring system. Meanwhile, the longꢀ
wavelength band of PTIPSBDTꢀDTQx is almost completely
80 located in the nearꢀinfrared area and determines the ability of this
material to absorb photons of the solar spectrum with the lowest
energy.32,35
15 stability, optical and electrochemical properties, and pristine
photovoltaic characteristics of the resulting polymers. Further
device optimization was conducted by insertion of PFN layer,
screening of solvent and additive, solvent annealing and polar
solvent treatment. Encouragingly, PTIPSBDTꢀTTz:PC71BM
20 devices exhibited a great enhancement in PCE, with the highest
efficiency of 5.46% obtained. The enhanced device performance
was further correlated with morphology study.
2 Results and discussion
2.1 Synthesis and characterization
25 The synthetic routes for donating monomer TIPSBDT as well as
polymers are outlined in Scheme 1. The TIPSBDT monomer was
synthesized in 90% yield using Sonogashira coupling between
trimisopropylsilyl acetylene and BDT triflate (1). BDT triflate
was developed as a versatile coupling reagent for various Pdꢀ
30 catalyzed crossꢀcoupling for direct access of 4,8ꢀbifunctionalized
BDTs.28 With tin reagent of TIPSBDT at hand, its
polycondensation
with
dibrominated
electronꢀaccepting
monomers was conducted via Pdꢀcatalyzed Stille coupling to
afford three new TIPSBDTꢀbased copolymers. After Soxhlet
35 extraction with methanol, hexane and chloroform in sequence, the
pure polymers were obtained with a yield of 82% for PTIPSBDTꢀ
TTz, 75% for PTIPSBDTꢀDTBO, 80% for PTIPSBDTꢀTPD and
88% for PTIPSBDTꢀDTQx.
Fig.1 UV-vis absorption spectra of (a) polymer solutions in diluted CHCl3
85
and (b) polymer films drop-cast on quartz.
The optical bandgap (Egopt) of polymers can be estimated with
the λonset values of polymer films according to the empirical
opt
opt
equation Eg = 1240/λonset. The Eg of polymers were thus
calculated to be 1.69, 1.76 and 1.31 eV for PTIPSBDTꢀTTz,
40 Scheme 1 Synthetic route to TIPSBDT-based donor-acceptor copolymers.
90 PTIPSBDTꢀDTBO and PTIPSBDTꢀDTQx, respectively. The
small bandgaps (<2.0 eV) of three polymers were successfully
achieved by the strong ICT effect between the donating and the
All polymers exhibit good solubility in common organic
solvents such as THF, chloroform (CF), chlorobenzene (CB) and
2
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2.4 Photovoltaic properties
accepting units. Obviously, the bandgaps of TIPSBDT polymers
can be easily tuned by adjusting the electron withdrawing units.
To evaluate the potential of the polymers in PSCs application,
BHJ cells were fabricated with the device configuration of
ITO/PEDOT:PSS(40nm)/Active layer (~90 nm)/with or w/o PFN
50 (5 nm)/Al (100 nm). Firstly, the PSC devices were fabricated by
spinꢀcoating the active layers from CF solutions. Different weight
ratios of copolymer donor to PC71BM (1:1 and 1:2) were studied.
We found that the optimal D:A ratio was 1:2 for PTIPSBDTꢀTTz
while 1:1 for other three polymers. The JꢀV curves of the PSC
55 devices under solar illumination (AM1.5G, 100 mW cmꢀ2) and
under dark are shown in Figure S12, with the deduced parameters
summarized in Table S1. PTIPSBDTꢀTTz, PTIPSBDTꢀDTBO
and PTIPSBDTꢀDTQx based PSCs showed a relatively high Voc
of 0.79, 0.80, and 0.73 V, respectively. The discrepancies may be
60 explained with the deeper lowꢀlying HOMO energy levels of the
first three polymers (Figure 2b). Among of them, PTIPSBDTꢀ
DTQx based cells exhibited quite low Jsc and FF, resulting in a
PCE lower than 0.15%. This may be explained with its deepꢀ
lying LUMO level, close to that of PC71BM, which cannot ensure
65 sufficient driving force to promote the charge transfer from the
polymer donor to fullerene acceptor.5
Table 1 Optical and electrochemical properties of polymers
a
a
b
b
opt
λmax
λonset
λmax λonset
Eg
HOMO LUMO
Polymer
(nm) (nm) (nm) (nm) (eV)c
(eV)d
(eV)e
PTIPSBDTꢀTTz 616
PTIPSBDTꢀDTBO 615
PTIPSBDTꢀDTQx 762
665
680
907
628
619
843
734 1.69
702 1.76
ꢀ5.53
ꢀ5.50
ꢀ5.40
ꢀ3.84
ꢀ3.74
ꢀ4.09
950 1.31
b
aUVꢀvis absorption in dilute CHCl3 solution, UVꢀvis absorption in thin
opt
ox
5
films, cEg = 1240/λonset dHOMO = ꢀe(Eonset + 4.4) (eV), eLUMO =
,
opt
HOMO + Eg
2.3 Electrochemical properties
The highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels of the
10 conjugated polymers can be determined with CV measurements.
The CV traces of polymers films on Pt electrode are shown in
Figure 2a, with the potential of ferrocene 0.40 V vs SCE used as
internal standard. On the basis of 4.8 eV below vacuum for the
energy level of Fc/Fc+, the HOMO level of the polymers is
15 calculated from the onset oxidation potentials (Eoxonset) with
onset
equation: HOMO
=
ꢀe(Eoxonset+4.4) (eV). The Eox
of
As known, PFN could lead to better interfacial contacts
through decreasing the series resistance, benefiting for the
enhanced electron collection of the cathode as well as decreased
70 possibility of holeꢀelectron recombination in the active layer.9,24
The effect of PFN interlayer on the device performance was
studied for our PSCs. As expected, inserting the cathode
interfacial layer PFN had much improvement on the performance
of devices (Table S1). For instance, for PTIPSBDTꢀTTz:PC71BM
75 (1:2) device, the device with Al cathode displayed an initial PCE
of 0.89%, while a high PCE of 1% was obtained for device when
PFN was introduced as the cathode interlayer. The improved PCE
was attributed mainly to the obvious enhancement in Voc and FF.
This enhancement was also observed for PTIPSBDTꢀDTBO and
80 PTIPSBDTꢀDTQx based devices. Particularly, PTIPSBDTꢀ
DTBO:PC71BM (1:1) device exhibited a remarkable enhancement
of Voc from 0.8 V to 0.96 V. The observed enhancement of Voc
could be revealed by the JꢀV characteristics of the devices in the
dark (Figure S12).
PTIPSBDTꢀTTz, PTIPSBDTꢀDTBO, and PTIPSBDTꢀDTQx was
observed to be 1.09, 1.06 and 1.00 eV, respectively. Accordingly,
the HOMO energy level of PTIPSBDTꢀTTz, PTIPSBDTꢀDTBO,
20 PTIPSBDTꢀTPD and PTIPSBDTꢀDTQx is determined as ꢀ5.53, ꢀ
5.50 and ꢀ5.40 eV, respectively.
Since only oxidation peaks of polymer films were obtained, the
LUMO levels of polymers were calculated using HOMO and
Egopt according to the equation: LUMO = HOMO+Egopt (eV). The
25 LUMO level of PTIPSBDTꢀTTz, PTIPSBDTꢀDTBO and
PTIPSBDTꢀDTQx was thus calculated to be ꢀ3.84, ꢀ3.74 and ꢀ
4.09 eV, respectively. The HOMOꢀLUMO energy diagrams of
the polymers and PC71BM are shown in Figure 2b. Apparently,
all these polymers exhibited deep HOMO levels close to the ideal
30 value (ca. ꢀ5.4 eV)36 for highꢀefficiency PSCs, indicating that
these polymers when used as donors for BHJ cells could generate
desirable Voc and maintain good chemical stability at ambient
conditions. Three polymers exhibited almost identical HOMO
energy levels, due to the presence of the same BDT donating
35 moiety. Importantly, PTIPSBDTꢀTTz and PTIPSBDTꢀDTBO
also show good LUMO energy levels, which are beneficial for
the electrons transfer from polymer donors to PC71BM, because
the efficient exciton separation is provided that the energy offset
between the polymer and fullerene (LUMO) is at least 0.3 eV.
40 These results further demonstrated the ability of tuning the
energy levels by changing accepting units for DꢀA polymers.
85
The low device performance for devices processed from CF
solutions may indicate the poor solubility of polymer:P71CBM in
CF. At present, the widely processing solvents for spinꢀcoating
are high boilingꢀpoint solvents of CB8,17 and oꢀDCB11,16 due to
their good solvation properties and low evaporation rates, which
90 allows the blend of polymer/fullerene to organize into optimal
microstructure. On the other hand, the DIO as solvent additive is
often used to be the cosolvent due to its prolonged evaporation
time and preferential solubility for PC71BM, leading to the
formation of aggregation of the conjugated polymer and
95 increment of the degree of phase separation. Based on this
strategy, we tried to use the mixed solvent of CB or oꢀDCB with
3 vol% DIO as an additive for processing the active layer to
optimize the Photovoltaic Performance.16,26 Figure 3 show the Jꢀ
V characteristics of polymer/PC71BM devices processed using a
100 mixed solvent of CB or oꢀDCB with 3 vol% DIO. The effect of
insertion of PFN interlayer and methanol treatment of active layer
Fig. 2 (a) Oxidation scans of cyclic voltammograms of polymer thin films
in 0.1 M Bu4NPF6 acetonitrile solutions at 50 mV/s scan rate, (b) the
45
HOMO-LUMO energy levels of the copolymers.
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those of the analogues without PFN layer. This result agrees well
with the results reported by Wu et al. that the PFN interlayer led
50 to simultaneous enhancement of all parameters in the based
PTB7:PC71BM device.9
on device performance was studied for PTIPSBDTꢀTTz:PC71BM
(1:2) and PTIPSBDTꢀDTBO:PC71BM (1:1) based devices. The
photovoltaic parameters of the devices are summarized in Table 2.
Table 2 Comparison of photovoltaic properties of polymer/PC71BM
based devices parepared from CB:DIO or oꢀDCB:DIO (97:3, v/v) with
PFN interlayer and methanol treatment
5
PCEmax(%)f
D:A
(wt%)
Voc
Jsc
FF
Active layera
Treatment
(V) (mA/cm2) (%)
As castb
PFNb,d
0.64
0.76
9.12
9.47
9.63
9.84
10.5
10.4
3.74
5.76
6.51
5.24
6.72
6.48
33.8
37.5
41.5
43.4
53.5
54.8
30.8
35.7
39.0
34.6
42.4
41.0
1.97 (1.86)
2.70 (2.64)
3.08 (2.99)
3.29 (2.96)
4.99 (4.38)
5.02 (4.62)
0.82 (0.76)
1.83 (1.64)
2.41 (2.30)
1.14 (1.09)
2.77(2.50)
2.52 (2.18)
PTIPSBDTꢀ
TTz:
PC71BM
Methanolb,e 0.77
1:2
1:1
As castc
PFNc,d
0.77
0.89
Methanolc,e 0.88
As castb
PFNb,d
0.71
0.89
Methanolb,e 0.95
PTIPSBDTꢀ
DTBO:
PC71BM
As castc
PFNc,d
0.63
0.97
Methanolc,e 0.95
aCell structure was ITO/PEDOT:PSS(40 nm)/Active layer(~90 nm)
/Al(100 nm); bThe active layers spinꢀcoated from the mixed solvent of CB
+3 vol% DIO; cThe active layers spinꢀcoated from the mixed solvent of oꢀ
Fig. 3 (a) J-V characteristics of photovoltaic devices processed using
CB:DIO (97:3 v/v) solvent based on (a) PTIPSBDT/PC71BM (1:2, w/w), and
10 DCB +3 vol% DIO; dThe PFN layer was inserted; eactive layers were
treated by methanol; fThe average PCE was given in parentheses.
55
60
(b) PTIPSBDT-DTBO/PC71BM (1:1, w/w) with different fabricated
conditions under illumination of AM1.5G (100 mW/cm2); J-V
characteristics of photovoltaic devices processed using o-DCB:DIO (97:3
v/v) solvent based on (c) PTIPSBDT/PC71BM (1:2, w/w), and (d) PTIPSBDT-
DTBO/PC71BM (1:1, w/w) with different fabricated conditions under
illumination of AM1.5G (100 mW/cm2).
The
asꢀcast
device
structure
was
ITO/PEDOT:PSS/polymer:PC71BM/Al. As shown in Table 2,
15 devices fabricated using CB:DIO or oꢀDCB:DIO (97:3, v/v)
mixture solvent exhibited significantly increased performance in
comparison to those using CF. For example, when processed
using CB+3% DIO, PTIPSBDTꢀTTz device obtained an
increased PCE from 0.89% to 1.97%, while PTIPSBDTꢀDTBO
20 devices showed an increased PCE from 0.62% to 0.82%. When
processed using oꢀDCB:DIO (97:3, v/v), the device incorporating
PTIPSBDTꢀTTz, PTIPSBDTꢀDTBO and PC71BM achieved
further improved from 1.97% to 3.29% and from 0.82% to 1.14%,
respectively. The enhancement in PCE for devices processed
25 from different solvents may be explained with the solubility of
PC71BM and the morphology of active layer, where smaller
domain sizes for PC71BM were observed. Similar enhancement of
PCE for device processed using CB instead of CF was also found
for the TIPSBDT polymer based devices.8,29
The active layer through simple polar solvent treatment was
reported to form fiberlike interpenetrating morphologies
benefiting from the increased hole mobilities as well as moreꢀ
balanced charge transport, which resulted in the simultaneous
65 enhancement of device parameters of PSCs.14,22 However, much
less attention has been paid to their beneficial influence on the
performance of PSCs. We tried to employ methanol as polar
solvent to optimize the performance of PSCs. Inspiringly, after
methanol exposure, the PSCs achieved dramatically improved
70 performance. From Table 3, one can see that the influence of
methanol treatment depends on the processing solvent. The
PTIPSBDTꢀTTz devices spinꢀcoated with CBꢀDIO (3%) solvent
exhibited a much increased PCE of 3.08%, with Jsc increased
from 5.76 to 9.63 mA/cm2, Voc from 0.64 to 0.76 V and FF from
75 33.8% to 37.5%. On the basis of the same structure of
ITO/PEDOT:PSS(40 nm)/Active layer(~90 nm) /Al(100 nm), the
PCE of devices based on PTIPSBDTꢀTTz with oꢀDCB:DIO (3%)
as spinꢀcoating solvent was further significantly improved to 5.02%
with a Jsc of 10.4 mA/cm2, a Voc of 0.88 V, and an FF of 54.8%.
80 The trend of the device based on PTIPSBDTꢀDTBO with
methanol treatment was similar to that of the aboveꢀmentioned
PTIPSBDTꢀTTz. The PTIPSBDTꢀDTBO devices with CB:DIO
(3%) mixed solvent also showed a significantly increased PCE
from 0.82% to 2.41% after the exposure of active layers to
85 methanol. The further increased PCE of 2.52% was achieved in
the PSCs based on PTIPSBDTꢀDTBO with oꢀDCB:DIO (3%)
solvent.
30
Compared with the asꢀcast devices, devices with PFN
interfacial layer showed simultaneous enhancement in all device
parameters, resulting in dramatically increased PCE values. For
instance, PTIPSBDTꢀTTz: PC71BM (1:2) device fabricated from
CBꢀDIO (3%) solvent gave a much improved PCE from 1.97% to
35 2.70%, with Voc increased from 0.64 to 0.77 V, Jsc from 9.12 to
9.63 mA/cm2 and FF from 33.8% to 41.5%. The PTIPSBDTꢀ
TTzꢀbased device fabricated from oꢀDCBꢀDIO (3%) obtained an
increased PCE from 2.70% to 4.99% with a largely increased Voc
from 0.76 V to 0.89 V and FF from 37.5% to 53.5% (Figure 3c,
40 Table 2). For the PTIPSBDTꢀDTBOꢀbased devices, an improved
PCE of 1.83% was achieved with the spinꢀcoating solvent of 97%
CB/3% DIO compared to that of CF. With oꢀDCB:DIO (97:3,
v/v) as spinꢀcoating solvent, the corresponding devices showed
further increased PCE from 1.83% to 2.77% accompanied with
45 simultaneously improvement of Voc from 0.89 V to 0.97 V and FF
from 37.5% to 42.4%. A noteworthy fact is that the devices with
PFN layer show a dramatically improvement in the Voc relative to
Solvent annealing of active layer is well recognized to enhance
its light absorption and the hole mobility by inducing the selfꢀ
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organization of polymers, resulting in improved efficiency for
broad response ranges covering 380ꢀ660 nm were obtained and a
PSCs.33,34 THF was herein chosen as the low boilingꢀpoint 50 relatively high IPCE value above 35% in the region of 400ꢀ650
solvent to study the effect of solvent annealing on device
performance. The JꢀV characteristics of
ITO/PEDOT:PSS/polymer:PC71BM/PFN/Al devices with or w/o
THF annealing are shown in Figure 3aꢀd and device data were
nm. The IPCE values of PTIPSBDTꢀDTBOꢀbased devices were
observed to be lower than those of PTIPSBDTꢀTTz devices,
which agreed with its lower Jsc values than PTIPSBDTꢀTTz based
devices.11 The Jsc of PTIPSBDTꢀTTz calculated from the integral
5
summarized in Table 4. With “solvent annealing”, PTIPSBDTꢀ 55 of IPCE curves is consistent with the Jsc value obtained from the
TTz:PC71BM (1:2) device with CB:DIO (97:3,v/v) as the spinꢀ
coating solvent achieved an increased PCE of 2.96% (by 9.6%),
JꢀV curves in the Figure 3c.
10 with dramatic increase in Voc from 0.76 to 0.81 V and slight
increase of Jsc from 9.47 to 9.75 mA/cm2 but unchanged FF.
When replacing CB with oꢀDCB as parent solvent, the
PTIPSBDTꢀTTzꢀbased device gave the highest PCE of 5.46%
with a Jsc of 9.57 mA/cm2, a Voc of 0.91 V, and an FF of 62.7%. It
15 was identically observed that The PTIPSBDTꢀDTBO:PC71BM
(1:1,w/w) device prepared from CB:DIO (97:3, v/v) solvent also
presented an increment (by 25%) in PCE from 1.83 to 2.29%,
with FF increased from 35.7 to 42.8% and Voc increased from
0.89 to 0.95 V after solvent annealing. Incorporating 3% DIO
20 into the spinꢀcoating solvent of oꢀDCB, however, the PTIPSBDTꢀ
DTBO device showed a declined PCE of 2.10% attributed to the
reduced Jsc. These results indicate that the solvent annealing is
effective to improve device performance, which are attributed to
the healing of the disruption in the ordering of polymer donors
25 induced by incorporating PC71BM molecules in the blend
system.34
Fig. 4 IPCE spectrum of best performing polymer:PC71BM solar cells
under illumination of AM 1.5G (100 mW/cm2).
60 2.6 Morphology
In order to gain more insight for the observed difference in
device performance, tappingꢀmode atomic force microscopy
(AFM) was employed to characterize the surface morphological
properties of PTIPSBDTꢀTTz/PC71BM and PTIPSBDTꢀ
65 DTBO/PC71BM blend films prepared with different D/A weight
ratios, processing solvents, methanol solvent exposure and THF
annealing. When increasing the weight ratio of polymer/PC71BM
from 1:1 to 1:2, the domains size of PC71BM increased for all
active layers processed using CF (Figure 5aꢀb and 5cꢀd). The
70 relatively large (100ꢀ300 nm) globular clusters assigned to
PC71BM domains were observed in PTIPSBDTꢀTTz, implying
poor uniform distribution of PTIPSBDTꢀTTz and PC71BM.
However, the phase images of PTIPSBDTꢀDTBO:PC71BM films
showed obscure interfaces between two phases with uniform
75 domain sizes for the dispersed PC71BM phase. The
polymer:PC71BM blend films show a root mean square roughness
(RMS) of 12.142 and 3.469 nm in the topography for
PTIPSBDTꢀTTz and PTIPSBDTꢀDTBO, respectively. When
spinꢀcoating the active layers from CB solution with 3% DIO
80 additive, the active layers presented much finer and more uniform
morphology, as shown as the right column in Figure 5c and 5f.
The big difference in film morphology indicates that the
polymer:PC71BM blend had better solubility in CB solution with
3% DIO than CF. Nanoscale phase separation was observed in all
85 active layers with rather small RMS (3.4~4.1 nm). Especially,
PTIPSBDTꢀTTz/PC71BM (1:2) films exhibited good phase
separation and dramatically decreased RMS (4.123 nm). Under
the same condition, PTIPSBDTꢀDTBO:PC71BM(1:1) blend films
exhibited much improved miscibility between PTIPSBDTꢀDTBO
90 and PC71BM, leading to the formation of closely interpenetrating
networking of fibrous phases. The film showed increased Rms
from 2.551 to 3.420 nm when shifting solvent from CF to CB+
Table 4 Comparison of the photovoltaic properties of polymer/PC71BMꢀ
based devices with or w/o THF annealing
Active layersa
D:A Treating Voc
Jsc
FF
PCEmax (%)
(wt%) time (s) (V) (mA/cm2) (%)
0b
0.76
9.47
9.75
10.5
37.5 2.70 (2.80)
37.5 2.96 (2.61)
53.5 4.99 (4.38)
30b,d 0.81
PTIPSBDTꢀ
TTz:PC71BM
1:2
1:1
0c
30c,d
0b
0.89
0.919.57 (10.1)e 62.7 5.46 (5.34)
0.89
5.76
5.63
6.72
4.42
35.7 1.83 (1.64)
42.8 2.29 (2.20)
42.4 2.77 (2.50)
50.0 2.15 (2.10)
30b,d 0.95
PTIPSBDTꢀ
DTBO:PC71BM
0c
0.97
0.97
30c,d
aCell structure was ITO/PEDOT:PSS(40 nm)/Active layer(~90 nm)/THF
30 solvent treatment 30 s /PFN (5 nm)/Al(80 nm);. bThe active layers spinꢀ
c
coated from the mixed solvent of CB +3 vol% DIO; The active layers
d
spinꢀcoated from the mixed solvent of oꢀDCB +3 vol% DIO; The active
layer was THF annealing for 30s; eThe value in the parentheses is
integrated current calculated from IPCE spectrum.
35 2.5 IPCE
The incident photonꢀtoꢀcurrent efficiency (IPCE) of the solar
cells was measured under the illumination of monochromatic
light. Figure 4 shows the IPCE curves of the best performance
PTIPSBDTꢀTTz and PTIPSBDTꢀDTBO devices, which are also
40 used to verify the measured Jsc values. PTIPSBDTꢀ
TTz:PC71BM(1:2)/PFN/Al devices processed using CB+3 vol%
DIO exhibited a high photoꢀtoꢀcurrent response in the region of
400~700 nm. With a maximum IPCE of 55.5% peaking at 560
nm, the PTIPSBDTꢀTTz devices exhibit a broad photoꢀresponse
45 of over 50% in the range of 450ꢀ650 nm, suggesting a highly
efficient photo conversion process and balanced charge transport
in the devices. Although PTIPSBDTꢀDTBO:PC71BM devices
presented lower IPCE values than PTIPSBDTꢀTTz devices,
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3% DIO. The improved morphology PTIPSBDTꢀDTBO:PC71BM
incorporating PTIPSBDTꢀDTBO with PC71BM, the dark
(1:1) film are in good agreement with the enhanced Jsc observed 35 morphologies of Figure 5j were disappeared and bright regions
for the devices.16 As shown in Figure 5g and 5i, different phase
images occurred when the blend films of the active layers were
processed with the oꢀDCB solution with 3% DIO in comparison
whit that of CB solution with 3% DIO. For example, the blend
occurred in Figure 5k. For the PTIPSBDTꢀTTz/PC71BM (1:2)
blend film, a more uniform distribution of PTIPSBDTꢀTTz and
PC71BM and the formation of fiberlike interpenetrating
morphologies at the length scale of ∼20 nm with RMS of 24.5
5
film incorporating PTIPSBDTꢀTTz with PC71BM showed 40 nm after THF annealing were observed, which was evidenced
continuous fibril domains phase with similar RMS of 4.290 nm,
which is favorable for the charge transportation, affording
from Figure 5i. Obviously, the selfꢀorganization of the active
layer was induced and continuous interpenetrating network films
were produced after THF annealing, which is essential for the
exciton separation and charge transport. This is consistent with its
10 significantly improved FF.9,33 The PTIPSBDTꢀDTBOꢀbased
blend film, however, showed fibril domains phase accompanied
with appearance of the dark regions owning to the aggregated 45 highest efficiency and FF. Similarly, dramatic changes in the film
PC71BM.
morphology of the blend comprising PTIPSBDTꢀDTBO and
PC71BM (Figure 5l) were observed compared to that of devices
without THF annealing. The selfꢀorganization of the active layer
formed with RMS of 10.2 nm in the phase images and the
50 appropriate phase separation was also responsible for the
enhancement of FF.
3. Conclusions
We have reported the facile synthesis of TIPSBDT unit via the
Sonogashira coupling between trimisopropylsilyl acetylene and
55 BDT triflate. By alternating with different accepting units a series
of TIPSBDTꢀbased DꢀA polymers were further developed. The
impact of accepting unit structures on the optical, thermal,
electrochemical and photovoltaic properties of the resulted
polymers was investigated in details. PTIPSBDTꢀDTQx
60 exhibited an absorption extending to 1000 nm due to the strong
electronꢀwithdrawing
property
of
DTQX
unit.
The
ITO/PEDOT:PSS/polymer:PC71BM/PFN/Al devices processed
using CB solutions with 3 vol% DIO delivered a PCE of 2.70%
and 1.83% for PTIPSBDTꢀTTz and PTIPSBDTꢀDTBO
65 respectively. Incorporating 3% DIO into the spinꢀcoating oꢀDCB
solutions, the asꢀprepared device obtained from blends of
PTIPSBDTꢀTTz and PTIPSBDTꢀDTBO with PC71BM achieved a
PCE of 4.99% and 2.77%, respectively. Cheerfully, the device
incorporating PTIPSBDTꢀTTz with PC71BM that was processed
70 involving THF annealing and oꢀDCBꢀDIO (3%) exhibited the
highest PCE of 5.46% with Jsc of 9.57 mA/cm2, a Voc of 0.91 V,
and an FF of 62.7%.In addition, the performance of the device
with the structure of ITO/PEDOT:PSS/ PTIPSBDTꢀ
TTz:PC71BM/Al was found to be further improved from 3.08% to
75 5.02% with exposure of the active layers to methanol and oꢀ
DCBꢀDIO (3%) instead of CBꢀDIO (3%) as spinꢀcoating solvent.
The morphology studies revealed that polymer:PC71BM had
better solubility in CB solution with 3vol% DIO than in CF.
Active layers processed from CB+3% DIO exhibited nanoscale
80 phase separation and low surface roughness, which contributed
for the efficient charge transfer to achieve high Jsc and FF for the
devices. For the PTIPSBDTꢀTTz:PC71BM blend, phase
separation in oꢀDCBꢀDIO (3%) spinꢀcoated film was much better
than that of CBꢀDIO (3%). The further methanol exposure to the
85 blend film gave rise to the optimized morphology for
PTIPSBDTꢀTTz and PTIPSBDTꢀDTBO respectively, which was
responsible for simultaneous enhancement of Voc, Jsc, and FF in
PSCs. It is worth noting the films prepared from oꢀDCBꢀDIO
(3%) exhibited morphologies of obvious selfꢀorganization after
90 THF annealing. Especially, the interpenetrating network films
15 Fig. 5 AFM topographic images (5×5 μm) for thin films of (a) PTIPSBDT-
TTz:PC71BM (1:1, w/w), (b) PTIPSBDT-TTz:PC71BM (1:2, w/w), (c)
PTIPSBDT-TTz/PC71BM (1:2) using CB+3% DIO, (d) PTIPSBDT-
DTBO:PC71BM (1:1, w/w), (e) PTIPSBDT-DTBO:PC71BM (1:2, w/w), (f)
PTIPSBDT-DTBO/PC71BM (1:1) using CB+3% DIO, (g) PTIPSBDT-
20 TTz/PC71BM (1:2) using o-DCB+3% DIO, (h) PTIPSBDT-TTz/PC71BM (1:2)
using o-DCB+3% DIO with methanol treatment, (i) PTIPSBDT-TTz/PC71BM
(1:2) using o-DCB+3% DIO with THF annealing, (j) PTIPSBDT-
DTBO/PC71BM (1:1) using o-DCB+3% DIO, (k) PTIPSBDT-DTBO/PC71BM
(1:1) using o-DCB+3% DIO with methanol treatment, (i) PTIPSBDT-
25
DTBO/PC71BM (1:1) using o-DCB+3% DIO with THF annealing.
Subsequently, In the case of PTIPSBDTꢀTTz:PC71BM blends,
it could be observed that the blend film with solvent exposure
exhibited relatively much smaller fibril domains phase between
the polymer and PC71BM with a reduced RMS of 4.06 nm
30 (Figure 5h). This observation indicates that the surface
composition distribution of the blend film may be changed by
methanol treatment, which was responsible for the dramatically
enhancement of the PCE. For the methanolꢀtreated films
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with the length scale of ∼20 nm were produced for PTIPSBDTꢀ
concentrated in vaccuo. The crude compound was recrystallized
TTz. This work demonstrates the correlation of the morphology
evolution of blend films and photovoltaic properties of the
with isopropyl alcohol to give the title compound as a pale yellow
solid (1.9 g, 88%). H NMR (500 MHz, CDCl3, ppm) δ: 7.66 (s,
1
resulted polymers and offers simple and effective methods to 60 2H), 1.23 (m, 42H), 0.47 (s, 18H).
5
improve the efficiency of PSCs by interfacial engineering, solvent
choice, polar solvent treatment and solvent annealing.
Synthesis of polymers. The polymers PBDTꢀTTz, PTIPSBDTꢀ
DTBO and PBDTꢀDTBTQx were synthesized by Stille coupling
reaction. The general synthetic procedure was described as
follows. TIPSBDTꢀTin (0.2 mmol) and TTz (or DTBO and
65 DTBTQx) based dibromo monomer (0.2 mmol) were dissolved in
a mixed solvent of anhydrous toluene (5 mL) and DMF (1 mL).
The mixture solution was purged with nitrogen for 15 min before
Pd(PPh3)4 (12 mg, 0.01 mmol) was added. After degassed for
another 15 min, the mixture solution was stirred at 105°C for 48
4. Experimental
4.1. Materials
All starting materials and reagents were purchased from Sigmaꢀ
10 Aldrich Chemical Co. and used without further purification. THF
was dried over sodium/benzophenone and freshly distilled before
use.
4,8ꢀDihydroxybenzo[1,2ꢀb:4,5ꢀb']dithiophene,28
TTz,20 70 h. The reaction mixture was added dropwise to methanol (200
DTBO22 and DTQx32 were synthesized according to previously
reported procedures. Detailed synthesis and purification
15 procedures for the intermediate compounds and characterization
details are given in the Electronic Supplementary Information
(ESI).
4,8ꢀBis(trifluoromethanesulfonyloxy)ꢀbenzo[1,2ꢀb:4,5ꢀ
b']dithiophene (1).28 Under N2 atmosphere, to a suspension of
20 4,8ꢀdihydroxybenzo[1,2ꢀb:4,5ꢀb']dithiophene (6 g, 27 mmol) and
dry pyridine (6.6 mL) in dichloromethane (150 mL) was slowly
mL). The collected solid was then subjected to Soxhlet extraction
with methanol and acetone. Subsequently, the residue was
washed with chloroform and concentrated using rotary
evaporator. By reꢀprecipitating from methanol and drying in
75 vaccuo, the title polymer was obtained for characterization.
1
PTIPSBDT-TTz: a purple solid (240 mg, 82%); H NMR (500
MHz, CDCl3, ppm) δ: 7.61ꢀ7.60 (br, 2H), 7.55ꢀ7.53 (br, 2H), 4.4ꢀ
4.1 (br, 4H), 1.8ꢀ1.0 (br, 82H), 0.8ꢀ0.6 (br, 42H); GPC: Mn=12.5
kDa, Mw/Mn (PDI)=1.38; Elemental anal. cald. for
added with trifluoromethanesulfonic anhydride (13.6 mL, 80 80 [(C48H128N2O2S6Si2)n]: C 56.85, H 12.72, N 2.76, S 18.97; Found:
mmol) at 0oC. After the mixture was stirred at 0oC for 12 h, water
(100 mL) and hydrochloric acid (100 mL, 1 M) were added. The
25 resulting mixture was extracted with dichloromethane (3×80 mL).
The combined organic layers were dried over MgSO4 and
C 56.79, H 12.70, N 2.71, S 18.96. PTIPSBDT-DTBO: a blue
solid (200 mg, 70%); H NMR (500 MHz, CDCl3, ppm) δ: 8.5ꢀ
8.3 (br, 4H), 7.9ꢀ7.8 (br, 2H), 4.1ꢀ4.0 (br, 4H), 1.7ꢀ0.88 (br, 72H);
GPC: Mn=13.2 kDa, PDI=1.40; Elemental anal. cald. for
1
concentrated in vaccuo. The residue was purified through column 85 [(C62H82N2O3S4Si2)n]: C 68.33, H 7.77, N 2.57, S 11.77; Found:
chromatography on silica gel eluted with petroleum ether/ethyl
acetate (19:1, v/v) to give the pure title compound as a white
30 solid (9 g, 70%). 1H NMR (500 MHz, DMSOꢀd6, ppm) δ: 8.22 (d,
J = 5.5 Hz, 2H), 7.57 (d, J = 5.5 Hz, 2H).
C 68.29, H 7.76, N 2.55, S 11.75. PTIPSBDT-DTQ: a black
solid (240 mg, 88%); H NMR (500 MHz, CDCl3, ppm) δ: 8.96ꢀ
8.95 (2H), 7.85ꢀ7.82 (2H), 7.61ꢀ7.59 (2H), 7.33ꢀ7.32 (2H), 6.92ꢀ
6.91 (2H), 3.9ꢀ3.8 (8H), 1.5ꢀ0.88 (138H); GPC: Mn=15.2 kDa,
1
4,8ꢀDi(triisopropylsilylethynyl)ꢀbenzo[1,2ꢀb:4,5ꢀb']dithiophene 90 PDI=1.48; Elemental anal. cald. for [(C108H154N4O4S5Si2)n]: C
(TIPSBDT). The bistriflate 1 (3 g, 6.1 mmol), triisopropylsilyl
acetylene (3.44 mmol, 15.2 mmol), Pd(PPh3)2Cl2 (85.5 mg, 0.06
35 mmol, 10 mol%) and CuI (45 mg, 0.12 mmol, 20 mol%) were
placed in a roundꢀbottomed flask (50 mL) under nitrogen. DMF
(7 mL) and diisopropylamine (7 mL) was added via a septum.
The mixture was stirred for 12 h at 100oC then poured onto
aqueous hydrochloric acid (1 mL, 1 M). The resulting mixture
40 was extracted with dichloromethane (3×10 mL), and the
combined organic layer was dried over MgSO4 and concentrated
in vaccuo. The residue was purified by column chromatography
on silica gel eluted with hexane to give the title compound as a
72.43, H 8.78, N 3.13, S 8.95; Found: C 72.40, H 8.86, N 3.10, S
8.96.
4.2. Characterization
1H NMR and 13C NMR spectra were characterized on Bruker
95 AVANCE 500ꢀMHz (BioꢀSpin Corporation, Europe) spectrometer
with CDCl3 as solvent and tetramethylsilane (TMS) as internal
standard. Gel permeation chromatography (GPC) analysis was
measured with on a Waters 717ꢀ2410 instrument with polystyrene
as the standard and THF as eluent (flow rate 1.0 mL/min).
100 Ultravioletꢀvisible (UVꢀvis) absorption spectra were recorded on
a UVꢀvis instrument Evolution 220. Thermogravimetric analysis
1
yellow solid (3 g, 90%). H NMR (500 MHz, DMSOꢀd6, ppm) δ:
(TGA) analyses were conducted with
a TA instrument
45 7.60 (d, J = 5.5 Hz, 2H), 7.55 (d, J = 5.5 Hz, 2H), 1.20 (m, 42H).
2,6ꢀBis(trimethyltin)ꢀ4,8ꢀbis(triisopropylsilylethynyl)ꢀ
TGA/SDTA851e with heating rate of 20oC minꢀ1 under nitrogen
gas flow. The electrochemical cyclic voltammetry was conducted
105 on an electrochemical workstation (CHI660D, Chenhua
Shanghai) with Pt plate as working electrode, Pt slice as counter
electrode, and saturated calomel electrode (SCE) as reference
electrode in tetrabutylammonium hexafluorophosphate (Bu4NPF6,
0.1 M) acetonitrile solutions at a scan rate of 50 mV/s. Atomic
110 force microscopy (AFM) was recorded on a Dimension Icon
AFM in tapping mode.
benzo[1,2ꢀb:4,5ꢀb’]diꢀthiphene (TIPSBDTꢀTin). Under the
protection of N2, to a suspension of compound 2 (2 g，3.6 mmol)
and tetramethylethylenediamine (1.2 mL, 7.56 mmol) in THF (80
50 mL) was slowly added nꢀBuLi (3.15 mL, 7.56 mmol, 2.4 M) at ꢀ
78oC. After stirring at low temperature for
1
h,
trimethyltinchloride (10.8 mmol) was added in one portion. The
reaction mixture was allowed to warm to room temperature
overnight. The reaction mixture was quenched with water. The
55 residue was extracted with chloroform and washed with brine and
water. The combined organic layer was dried over MgSO4 and
4.3. Fabrication and characterization of polymer solar cells
The BHJ solar cells were prepared with a device structure of
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‡ These authors contributed equally.
ITO/PEDOT:PSS (40 nm)/active layer (~90 nm)/with or w/o PFN
(5 nm)/Al (100 nm). The ITO glass substrates were
unltrasonicated sequentially in deionized water, acetone, toluene
and isopropanol. Immediately prior to device fabrication, the
substrates were treated by oxygen plasma for 4 min. Firstly,
60
65
70
75
1
2
3
4
5
6
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poly(3,4ꢀethylenedioxythiophene)/poly(styrene
sulfonate)
(PEDOT/PSS, H. C. Starck) thin film (40 nm) was spinꢀcoated
and then baked at 50oC for 15 min. Secondly, the
polymer:PC71BM (1:1, 1:2, weight ratio) blend active layer, with
10 a nominal thickness of ~90 nm, was spinꢀcoated on top of the
PEDOT/PSS from a solvent of chloroform (CF) or mixed solvent
of chlorobenzene/1,8ꢀdiiodoctane (DIO) (95:5 vol%) (10 mg/mL)
at 1000 rpm for 2 min. Thirdly, for the insertion of PFN, the PFN
material was dissolved in methanol under the presence of small
15 amount of acetic acid and its solution was spinꢀcoated on the top
of the obtained active layer to form a thin interlayer of 5 nm. For
methanol treatment, after drying of the active layer under
vacuum, methanol solvent exposure was carried out by spinꢀ
coating methanol on the top of active layers at 2000 rpm for 30 s.
20 For treatment of THF annealing, the devices before PFN/Al
deposition were transferred to a glass Petri dish containing THF
solvent and covered with a glass cap. After solvent annealing, the
samples were dried overnight (>12 h) at room temperature in a
N2ꢀfilled glovebox. Al electrode (100 nm) was evaporated
25 through a shadow mask to define the active area of the devices
(2×8 mm2). The currentꢀvoltage (JꢀV) characteristics and PCE
were measured with a Keithley 2400 sourcemeter under 1 sun,
AM 1.5G spectrum from a solar simulator (Oriel model 91192) at
room temperature in a nitrogen filled glovebox. Solar simulator
30 illumination intensity was determined using a monocrystal silicon
reference cell (HamamatsuS1133, with KGꢀ5 visible color filter)
calibrated by the National Renewable Energy Laboratory
(NREL).
7
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Acknowledgements
35 This work is supported by the National Natural Science
Foundation of China (Grant No. 21074055), Program for New
Century Excellent Talents in University (NCETꢀ12ꢀ0633), The
Jiangsu Province Natural Science Fund for Distinguished Young
Scholars (BK20130032), Doctoral Fund of Ministry of Education
40 of China (No. 20103219120008), and the Fundamental Research
Funds for the Central Universities (30920130111006).
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† Electronic Supplementary Information (ESI) available: Synthesis of
accepting monomers, NMR spectra and devices data for cells processed
using CF. See DOI: 10.1039/b000000x/
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