Side chain engineering and conjugation enhancement of benzodithiophene and phenanthrenequnioxaline based conjugated polymers for photovoltaic devices

Article abstract of DOI:10.1002/pola.27643

A series of donor-acceptor conjugated polymers incorporating benzodithiophene (BDT) as donor unit and phenanthrenequnioxaline as acceptor unit with different side chains have been designed and synthesized. For polymer P1 featuring the BDT unit and alkoxy

Full text of DOI:10.1002/pola.27643

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Side Chain Engineering and Conjugation Enhancement of
Benzodithiophene and Phenanthrenequnioxaline Based Conjugated
Polymers for Photovoltaic Devices
Ying Sun, Chao Zhang, Bin Dai, Baoping Lin, Hong Yang, Xueqin Zhang, Lingxiang Guo,
Yurong Liu
School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing 211189, Jiangsu Province,
People’s Republic of China
Correspondence to: B. Lin (E-mail: lbp@seu.edu.cn)
Received 6 February 2015; accepted 16 March 2015; published online 10 April 2015
DOI: 10.1002/pola.27643
ABSTRACT: A series of donor-acceptor conjugated polymers
incorporating benzodithiophene (BDT) as donor unit and phe-
nanthrenequnioxaline as acceptor unit with different side
chains have been designed and synthesized. For polymer P1
featuring the BDT unit and alkoxy chains substituted phenan-
threnequnioxaline unit in the backbone, serious steric hin-
drance resulted in quite low molecular weight. The
implementation of thiophene ring spacer in polymer P2 greatly
suppressed the interannular twisting to extend the effective
conjugation length and consequently gave rise to improved
absorption property and device performance. In addition, utiliz-
ing the alkylthienyl side chains to replace the alkyl side chains
at BDT unit in polymer P3 further enhanced the photovoltaic
performance due to the increased conjugation length. For
polymer P4, translating the alkoxy side chains at the phenan-
threnequnioxaline ring into the alkyl side chains at thiophene
linker group enhanced molecular planarity and strengthened
p2p stacking. Consequently improved absorption property and
increased hole mobility were achieved for polymer P4. Our
results indicated that side chain engineering not only can influ-
ence the solubility of polymer but also can determine the poly-
mer backbone planarity and hence the photovoltaic properties.
V
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2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2015, 53, 1915–1926
KEYWORDS: conjugated polymers; planarity; polymer solar cell;
side chains; steric hindrance; structure-property relations; two
dimensional; thin films
INTRODUCTION Polymer solar cells (PSCs) have attracted
considerable research attention due to their potential for
being light weight, low cost, flexible, green and sustainable
energy sources. So far, most of the efficient PSCs are based
on bulk-heterojunction (BHJ) structure with an electron
donor material and an electron acceptor material blended
together to form a nanosized bicontinuous interpenetrating
network, which can provide large heterointerface for exciton
dissociation as well as continuous pathways for efficient
charge transporting. In the past two decades, PSC perform-
ance has been significantly improved through new materials
development, morphology tuning, device engineering and
transfer (ICT), which can tune the energy level, enhance the
5
,6
light absorption ability and improve the charge mobility. To
further explore strategies to improve solar cell performance,
two dimensional(2D) D-A conjugated polymers with conju-
7
–9
gated side chains have been synthesized and studied.
Due
to the enhanced conjugation, the 2D conjugated polymer pos-
sesses more interchain p2p overlaps and consequently shows
1
0,11
broad absorption bands.
Furthermore, the 2D polymer is
supposed to have higher charge mobility as the degree of elec-
tronic communication between the polymer chains can be
7
improved by the conjugated side chains. However, if the con-
jugated side chains are too bulky and oversized, the planarity
of the polymer backbone may be disrupted and thereby the
1
–4
interface optimization.
1
2,13
packing ability can be decreased.
So it is critical to develop
Polymer structure optimization is one of the effective
approaches toward improving the PSC efficiency. Incorporat-
ing an electron donor(D) unit and an electron acceptor(A) unit
in the linear backbone to construct alternating D-A copolymer
has been the widely employed strategy in designing new poly-
appropriate building block for 2D conjugated polymer system
to achieve both of the desirable optoelectronic properties and
good backbone planarity.
2,3-Diphenylquinoxaline has been widely employed as the
acceptor unit to construct low-band gap polymers because of
5
mer. D-A copolymer can provide strong intramolecular charge
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the strong electronegativity of the two nitrogen atoms and
chemical reagents were purchased from Aldrich or other
commercial sources and used without further purification.
1
4
stable quinoid structure.
Compared to 2,3-diphenylqui-
noxaline, phenanthrenequnioxaline can provide higher pla-
narity and better intermolecular packing due to its more
Measurement and Characterization
H NMR spectra were obtained on Bruker HW500 MHz spec-
trometer (AVANCE AV-500). Molecular weight and polydis-
persity were measured from gel permeation chromatography
(GPC) (a Waters 1515 pump and a differential refractometer)
1
4
1
fused structure and extended conjugation length. So phe-
nanthrenequnioxaline can be a highly promising unit for the
construction of two dimensional donor-acceptor copolymer.
However, the fused rigid structure of phenanthrenequnioxa-
line leads to very poor solubility, which limits its application
in constructing the conjugated polymer. Introducing alkyl
side chain in the fused ring is an effective way to improve
the solubility.
2
1
with THF as eluent at a flow rate of 1 mL min
in room
temperature. UV-vis spectra were tested using the ultraviolet
spectrophotometer (UV-3600, Japan). Differential scanning
calorimetry (DSC) spectra were obtained on a TA Instru-
ꢀ
21
ments (SDT-Q600) with a heating rate of 10 C min and a
Furthermore, the position of side chain linked to the polymer
backbone can significantly influence the photovoltaic proper-
ties of the polymers, which has been indicated by many pub-
21
nitrogen flow of 50 mL min . The thermogravimetric analy-
sis (TGA) were performed using a thermo gravimetric ana-
ꢀ
21
lyzer (STA 409) under a heating rate of 10 C min and a
1
5–17
lished results.
Consequently, side chain engineering is
21
nitrogen flow of 100 mL min . Cyclic voltammetries were
necessary for the further optimization of the properties of
the phenanthrenequnioxaline based D-A conjugated
polymers.
carried out on an electrochemical workstation (CHI 760C)
with a three-electrode cell in acetonitrile with 0.1 M of tetra-
butylammonium hexafluorophosphate using a scan rate of
2
1
100 mV.s with ITO, Ag/AgCl and Pt mesh as the working
Motivated by these considerations, we designed and synthe-
sized a series of 2D D-A conjugated polymers, named as P1,
P2, P3, and P4, comprising the benzodithiophene(BDT)
donor unit and phenanthrenequnioxaline acceptor unit with
different alkyl, alkoxyl, and alkylthienyl side chains (Fig. 1).
In spite of the similar backbone structure, the polymers
exhibited significantly different optical and photovoltaic
properties in term of different side chains. For polymer P1,
severe steric hindrance caused by the peripheral H-atoms in
benzodithiophene and the incorporated solubilizing alkyl
chains in phenanthrenequnioxaline ring led to very low
molecular weight of polymer and limited absorption prop-
erty. When the thiophene ring was incorporated into poly-
mer backbone, steric hindrance was reduced and backbone
planarity was enhanced, giving rise to the higher photovol-
taic properties of P2. Compared to P2, replacing the alkyl
side chain with alkylthienyl in P3 increased the effective con-
jugation length of the polymer, which were translated into
broader and stronger absorption and better photovoltaic per-
formance. In addition, to further reduce the steric hindrance,
we moved the alkyl chains at phenanthrenequnioxaline ring
to thiophene ring. As expected, the synthesized polymer P4
showed smaller band gap and higher PCE. Overall, our
results indicated that the photovoltaic properties of the phe-
nanthrenequnioxaline based D-A polymer can be effectively
improved by coplanar geometries tuning and side chain
engineering.
electrode, reference electrode and counter electrode, respec-
tively. AFM images were obtained on a Veeco multimode
AFM with a Nanoscope III controller under tapping mode.
Synthesis of Monomers and Copolymers
Synthesis of Compound M1
Compound 1 (0.5586 g, 2.1 mmol) and phenanthraquinone
(
0.382 g, 2.1 mmol) were dissolved in 50 mL acetic acid and
ꢀ
the mixture was stirred at 80 C overnight. The solution was
cooled to room temperature and extracted with
CH Cl (33100 mL) after neutralization with saturated
2
2
NaHCO . Organic layer was washed with water (2 3
3
1
00 mL), dried over Na SO . After evaporation, the residue
2 4
was purified by column chromatography on silica gel with
chloroform as eluent to give the products as pale yellow
1
solid with the yield of 56%. H-NMR (300 MHz, CDCl , ppm)
3
d 5 9.55 (dd, 2H), 8.64 (d, 2H), 8.07 (s, 2H), 7.88–7.84 (m,
1
3
4
1
H). C-NMR (CDCl , 125 MHz, d): 152.26; 140.17; 136.28;
3
30.38; 128.89; 127.81; 126.36; 125.09; 121.14; 115.62;
73.37; 30.32; 28.79; 27.93; 27.77; 24.52; 21.15; 12.59. Anal.
calcd for: C44 : C, 65.51%; H, 7.25%; N, 3.47%.
H
2 2 2
58Br N O
Found: C, 63.95%; H, 6.10%; N, 4.07%.
Synthesis of Compound M2
A mixture of compound 2(1.33 g, 2.1 mmol) and phenan-
thraquinone (0.382 g, 2.1 mmol) in 50 mL acetic acid was
ꢀ
heated to 80 C and stirred overnight. After that, the mixture
was cooled to room temperature and neutralized with satu-
rated NaHCO . Acetic acid was removed by extraction with
3
EXPERIMENTAL
CHCl (33100 mL) and wash with water (2 3 100 mL). The
2
Materials
organic layer was dried over Na SO4 and the residue was
2
Compound 1 and compound 2 were prepared as reported
purified by column chromatography on silica gel with petro-
leum ether/dichloromethane (4/1) as eluent to afford the
product as yellow white solid in 88% yield. H-NMR (300
1
8,19
and the synthesis of compound D1 and D2 followed the
2
0,21
1
published literature procedures.
Toluene and tetrahydro-
furan (THF) were purified by distillation from sodium under
MHz, CDCl , ppm)d 5 9.48 (d, 2H), 8.60 (d, 2H), 7.75 (m,
3
nitrogen atmosphere and N, N-dimethylformamide (DMF)
4H), 4.23(t, 4H), 2.01(m, 4H), 1.20–1.52(m, 36H), 0.87(t,
6H). C-NMR (CDCl , 125 MHz, d): 153.20; 140.50; 138.16;
3
1
3
was distilled under vacuum after dried over CaH . All other
2
1
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1
1
2
33.69; 132.07; 130.92; 130.63; 129.88; 127.94; 127.84;
27.21; 126.10; 124.31; 122.81; 74.23; 31.97; 30.45; 29.76;
9.71; 29.69; 29.55; 29.42; 26.09; 22.74; 14.16. Anal. calcd
brine, then dried over Na SO . After evaporation, the resi-
2 4
due was purified by column chromatography on silica gel
with petroleum ether/dichloromethane(4/1) to give the
products as an orange solid with 92% yield. 1H-NMR (300
for: C H N O S : C, 75.39%; H, 6.90%; N, 4.00%. Found: C,
4
4 48 2 2 2
74.21%; H, 6.76%; N, 3.84%.
MHz, CDCl , ppm) d 5 9.55(d, 2H), 8.55(d, 2H), 8.19(s, 2H),
3
8.18(m, 6H), 7.24(s, 2H), 2.76(t, 4H), 1.78(m, 4H), 1.20–
Synthesis of Compound 3
1.52(m, 20H), 0.95(t, 6H). Anal. calcd for: C H N O S : C,
5
2 64 2 2 2
Compound M2 (0.455 g, 0.564 mmol) and tributyl(2-thie-
nyl)tin (0.442 g, 1.184 mmol) were dissolved in 50 mL anhy-
drous toluene under nitrogen. Afterward, tetrakis
76.80%; H, 7.93%; N, 3.44%. Found: C, 79.02%; H, 7.28%;
N, 4.17%.
(
triphenylphosphine) palladium(49.08 mg, 0.070 mmol) was
Synthesis of Compound M4
The compound 4 (0.3412g, 0.51mmol) was dissolved in
ꢀ
added. The reaction mixture was heated to 100 C with vig-
orous stirring until reaction completion by TLC monitor
5
1
0 mL THF, to which N-bromosuccinimide (NBS) (0.19 g,
.05 mmol) was added. After stirring the mixture at room
(
24 h). The mixture was poured into water (100 mL) and
extracted with dichloromethane. The organic layer was
washed with water and brine, then dried over Na SO . After
temperature for 2h in the dark, the solvent was removed
under reduced pressure, and then the residue was purified
by column chromatography on silica gel with petroleum
ether/dichloromethane(4/1) as eluent to afford the product
as a brown solid with the yield of 88%. 1H-NMR (300 MHz,
2
4
evaporation, the residue was purified by column chromatog-
raphy on silica gel with petroleum ether/dichloromethane
(
of 89%. 1H-NMR (300 MHz, CD Cl , ppm) d 5 9.32 (m,
2
4/1) to give the products as an orange solid with the yield
3
3
3
CDCl , ppm) d 5 9.45(d, 2H), 8.59(d, 2H), 8.18(s, 2H), 7.89(d,
H),8.50 (m, 2H), 8.12 (m, 2H), 7.70(m, 6H), 7.45(m, 2H),
4
2
1
1
2
C
6
H), 7.60 (s, 2H), 2.72 (t, 4H), 1.75 (m, 4H), 1.25–1.41 (m,
0H), 0.92 (t, 6H). C-NMR(CDCl , 125 MHz, d): 142.82,
42.50, 138.73, 138.60, 132.41, 131.63, 130.34, 128.05,
27.77, 126.95, 123.90, 122.90, 114.17, 31.71, 30.63, 29.66,
9.52, 29.30, 29.15, 22.66, 14.17. Anal. calcd for:
1
3
4
.22(t, 4H), 2.03(m, 4H), 1.21–1.51(m, 36H), 0.89(t, 6H). C-
13
3
NMR (CDCl , 125 MHz, d): 153.20; 140.50; 138.16; 133.69;
3
1
1
2
32.07; 130.92; 130.63; 129.88; 127.94; 127.84; 127.21;
26.10; 124.31; 122.81; 74.23; 31.97; 30.45; 29.76; 29.71;
9.69; 29.55; 29.42; 26.09; 22.74; 14.16. Anal. calcd for:
44
H
46Br
2
N
2
S
2
: C, 63.92%; H, 5.61%; N, 3.39%. Found: C,
C H N O S : C, 75.39%; H, 6.90%; N, 4.00%. Found: C,
7
4
4 48 2 2 2
3.92%; H, 5.59%; N, 3.41%.
4.21%; H, 6.76%; N, 3.84%.
Polymer P1
Synthesis of Compound M3
Compound D1 (148.4 mg, 0.2 mmol) and compound M2
The compound 3 (0.414 g, 0.51 mmol) was dissolved in
(
160.8 mg, 0.2 mmol) were dissolved in a mixture of tolu-
5
1
0 mL THF, to which N-bromosuccinimide (NBS) (0.19 g,
.05 mmol) was added in the dark. After stirring the mixture
ene (5 mL) in a 25 mL two-neck flask. The resulting solu-
tion was carefully degassed and the catalyst of Pd (dba)3
2
at room temperature for 2h, the solvent was removed under
reduced pressure, and then the residue was purified by col-
umn chromatography on silica gel with petroleum ether/
dichloromethane (2/1) as eluent to afford the product as an
orange red solid with the yield of 91%. 1H-NMR (300 MHz,
CD Cl , ppm)d 5 9.21(d, 2H), 8.45 (m, 2H), 7.91(d, 2H),
(
6 mg) and P (o-tol)3 (15 mg) were added. The reaction
mixture was refluxed for 48 h under a nitrogen atmosphere
and then cooled down to room temperature. The solution
was added dropwise into methanol (200 mL) and the pre-
cipitated polymer was collected by filtration and subse-
quently purified by soxhlet extraction with acetone, hexane
and chloroform. The concentrated polymer solution in chlo-
roform was reprecipitated into methanol and the resulting
polymer was obtained by filtration and dried under vacuum
3
3
7
.71(m, 4H), 7.24(m, 2H), 4.22(t, 4H), 2.03(m, 4H), 1.21–
1
3
1
1
1
.51(m, 36H), 0.89(t, 6H). C-NMR (CDCl , 125 MHz, d):
52.83; 140.51; 137.31; 135.23; 132.00; 131.30; 130.15;
29.99; 128.99; 127.95; 127.23; 123.37; 122.74; 115.75;
3
n w
(140 mg, 66%). M 5 2.21 kDa, M 5 4.64 kDa, PDI 5 2.10.
74.28; 31.98; 30.52; 29.79; 29.73; 29.59; 26.14; 22.75;
1
5
4.17. Anal. calcd for: C H Br N O S : C, 61.54%; H,
.41%; N, 3.26%. Found: C, 61.25%; H, 5.16%; N, 3.01%.
4
4
46
2 2 2 2
Polymer P2
Compound D1 (148.4 mg, 0.2 mmol) and compound M3
(193.6 mg, 0.2 mmol) were dissolved in a mixture of toluene
(10 mL) in a 25 mL two-neck flask. The resulting solution
was carefully degassed and the catalyst of Pd(PPh ) (8 mg)
were added. The reaction mixture was heated to 80 C for
1 h under a nitrogen atmosphere and then cooled down to
room temperature. The solution was added dropwise into
methanol (200 mL) and the precipitated polymer was col-
lected by filtration and subsequently purified by soxhlet
extraction with acetone, hexane and chloroform. The concen-
trated polymer solution in chloroform was reprecipitated
into methanol and the resulting polymer was obtained by
Synthesis of Compound 4
compound M1 (0.2471 g, 0.564 mmol) and 2-(4-octyl-2-
thienyl)- 4,4,5,5-tetramethyl- 1,3,2 -dioxaborolane (0.3816 g,
3
4
ꢀ
1.184 mmol) were dissolved in 50 mL anhydrous toluene
under nitrogen. Then 2M aqueous potassium carbonate
solution (5 mL) was added. Afterward, tetrakis (triphenyl-
phosphine) palladium(49.08 mg, 0.070 mmol) was added.
ꢀ
The reaction mixture was heated to 100 C with vigorous
stirring until reaction completion by TLC. The mixture was
poured into water (100 mL) and extracted with dichloro-
methane. The organic layer was washed with water and
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SCHEME 1 Molecular engineering routes and chemical structures of polymers. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
filtration and dried under vacuum (173 mg, 71%).
toluene (10 mL) in a 25 mL two-neck flask. The resulting
solution was carefully degassed and the catalyst of
Pd(PPh3)4 (8 mg) were added. The reaction mixture was
M 5 32.21 kDa, M 5 57.66 kDa, PDI 5 1.79.
n
w
ꢀ
Polymer P3
heated to 80 C for 1 h under a nitrogen atmosphere and
Compound D2 (181.2 mg, 0.2 mmol) and compound M3
then cooled down to room temperature. The solution was
added dropwise into methanol (200 mL) and the precipi-
tated polymer was collected by filtration and subsequently
purified by soxhlet extraction with acetone, hexane and
chloroform. The concentrated polymer solution in chloro-
form was reprecipitated into methanol and the resulting
polymer was obtained by filtration and dried under vac-
(
(
193.6 mg, 0.2 mmol) were dissolved in a mixture of toluene
10 mL) in a 25 mL two-neck flask. The resulting solution
was carefully degassed and the catalyst of Pd(PPh ) (8 mg)
3
4
ꢀ
were added. The reaction mixture was heated to 80 C for
h under a nitrogen atmosphere and then cooled down to
1
room temperature. The solution was added dropwise into
methanol (200 mL) and the precipitated polymer was col-
lected by filtration and subsequently purified by soxhlet
extraction with acetone, hexane and chloroform. The concen-
trated polymer solution in chloroform was reprecipitated
into methanol and the resulting polymer was obtained by fil-
uum (168 mg, 66%). M
PDI 5 2.12.
n w
5 19.88 kDa, M 5 42.15 kDa,
Photovoltaic Device Fabrication
First, the ITO-coated glass substrates were sequentially
cleaned with detergent, deionized water, acetone, and isopro-
pyl alcohol. Then PEDOT:PSS layer (ꢁ45 nm) was spin
coated from the its water solution (BaytronVR PVP AI 4083,
tration and dried under vacuum (211 mg, 76%). M 5 25.84
n
kDa, M 5 59.69 kDa, PDI 5 2.31.
w
Polymer P4
filtered at 0.45 mm) onto the ITO glass after UV-ozone treat-
Compound D2 (181.2 mg, 0.2 mmol) and compound M3
ment for 20 min. Then the substrate was dried on a hotplate
at 120 C for 30 min. On top of the PEDOT:PSS layer, the
ꢀ
(
164.8 mg, 0.2 mmol) were dissolved in a mixture of
1
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SCHEME 2 Synthetic routes for phenanthrenequnioxaline derivatives.
BHJ film was spin-coated in a nitrogen filled glove box from
a blending dichlorobenzene (DCB) solution of polymer with
was employed as the acceptor unit to construct the two
dimensional donor acceptor polymers with the benzodithio-
phene as the donor unit. Alkoxy and alkyl chains were intro-
duced to the fused ring to improve the solubility.
Furthermore, the coplanar geometry of the polymer was sys-
tematically tuned and side chains was modulated, which gen-
erated a series of polymer P1, P2, P3, and P4.
ꢀ
PC BM which was stirred at 80 C overnight and filtered
7
1
with a 0.2 mm PTFE filter. Before thermal evaporation, the
ꢀ
BHJ was annealed at the temperature of 110 C for 10 min.
Finally, calcium (200 nm) and aluminum (100 nm) were
thermally evaporated onto the active layer under high vac-
27
uum (< 2 3 10 Torr). The current density – voltage (J –
V) characteristics were measured with a Keithley 2400 SMU
source measurement unit and an Oriel Xenon lamp (450 W)
using an AM1.5 filter as the solar simulator. A standard sili-
con solar cell with a KG5 filter was employed to calibrate
the device results. To fabricate the hole only devices, the
same procedure was followed except that the calcium was
replaced by MoO3.
The synthetic routes of the monomers was described in
Scheme 2. The dibromo phenanthrenequnioxaline (com-
pound M1) was synthesized according to the published ref-
erence with the benzodithiazole as the starting material.
Due to the poor solubility of the compound M1, alkoxy sub-
stituents were introduced to the phenanthrenequnioxaline
ring to improve the solubility. The synthetic routes of com-
pound M2 started from the synthesis of alkoxy substituted
1
4
benzodithiazole, which was prepared following the reported
RESULTS AND DISCUSSION
18,19
procedure.
The conversion of the compound 2 to the tar-
get monomer M2 was conducted through the condensation
reaction. Stille coupling reaction of the M2 with the tribu-
tyl(thiophen-2-yl)stannane resulted in the compound 3 and
subsequent NBS bromination afforded the compound M3.
Synthesis of Monomers and Polymers
Scheme 1 illustrated our design concept and chemical struc-
tures for the polymers. Considering the high planarity and
extended conjugation length, phenanthrenequnioxaline unit
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SCHEME 3 Synthetic routes for the polymers.
The synthesis of M4 involved the stille coupling reaction of
M1 with 4-n-Octyl-2-thiopheneboronic acid pinacol ester and
subsequent NBS bromination.
charge carrier mobilities.
A
high molecular weight is
required for the polymer to ensure the long conjugation
length and promote the intermolecular charge transport and
consequently enhance the device efficiency. So the polymer
P1 is expected to induce limit optical property and poor
photovoltaic performance due to the low molecular weight.
All the polymers were precipitated from methanol at first
and then were purified by subsequent soxhlet extraction
with acetone, hexane and chloroform, subsequently. All the
polymers exhibited good solubility in chlorobenzene and
dichlorobenzene which are the common solvents for PSC.
As illustrated in the Scheme 3, stille cross-coupling reactions
of the dibromo monomers (M2, M3 and M4) with the bis-
stannyl compounds (D1 and D2) resulted in the four poly-
mers P1-P4. For the polymer P1, the polymerization is quite
difficult to carry out because the serious steric hindrance
between the peripheral H-atoms in benzodithiophene and
the incorporated solubilizing alkyl chains in phenanthrene-
qunioxaline ring made the transmetalation difficult to pro-
2
2
ceed. Although we tried to extend the reaction time and
employed the more effective tris(dibenzylidene-acetone)di-
palladium(0)(Pd dba ) as catalyst and tri(o-tolyl) phosphine
Thermal Properties
Excellent thermal stability is critical and necessary for the
2
3
2
3
practical application of the photovoltaic materials. Ther-
mogravimetric analysis (TGA) was employed to investigate
the thermal stability of polymers. As shown in Figure 1, the
decomposition temperatures (Td) with 5% weight loss for
P1, P2, and P3 were determined to be the similar value of
(
P(o-tol) ) as the corresponding ligand, the molecular weight
3
of P1 was still very low. The gel permeation chromatogra-
phy(GPC) tests indicated that the number average molecular
weight of the resulted polymer P1 is only 2.21 kg/mol. For
the other three polymers, the molecular weights are rela-
tively reasonable and the detailed data are listed in the Table
ꢀ
ꢀ
ꢀ
around 340 C, 332 C, and 342 C. However, polymer P4
showed
a much higher decomposition temperature of
ꢀ
1. The molecular weight of conjugated polymers plays a key
452 C. This behaviour can be explained by that the dialkoxy
groups at the phenanthrenequnioxaline unit as well as alkyl
role in determining their optical, electrical properties and
1
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should originate from the intramolecular charge transfer
6
,25
(
ICT).
Considering the quite low molecular weight of polymer P1,
the blue shifted p2p* transition absorption band can be
explained by the limit conjugation length. Furthermore, due to
the serious steric hindrance, polymer P1 also showed a weak-
est ICT wavelength band, which was enhanced when going
from solution to film. With the thiophene as the linker group,
the polymer P2 possessed more distinctive ICT wavelength
band, indicating reduced steric hindrance and increased con-
2
6
jugation length. In addition, a red shifted absorption spectra
of P3 can be observed when the alkyl side chain in the BDT
unit was changed into the alkylthienyl side chain, suggesting
that the p2p* stacking and ICT were progressively enhanced
due to the enlarged conjugation length in the 2-D copolymers.
Furthermore, when the alkyl side chains were introduced into
the thiophene ring and the alkoxy side chains were removed
from phenanthrenequnioxaline unit, the absorption spectra of
P4 was more red-shifted and even developed a pronounced
shoulder in the longer wavelength. This phenomena can be
ascribed to be the better planarity geometry of polymer P4
FIGURE 1 TGA thermograms of the polymers (heating rate:
20 K/min). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
chains at the BDT ring are the relatively weak sites to
2
4
1
6
undergo thermal decomposition.
Therefore, polymer P4
compared to polymer P3.
exhibited superiors thermal stability due to the absence of
alkoxy chain at phenanthrenequnioxaline and translating the
alkyl chain to the alkylthienyl chain at BDT unit. It should be
noted that there were two decomposition stages for polymer
P3. The first stage should be caused by the decomposition of
dialkoxy groups at the phenanthrenequnioxaline unit while
the second one can be due to the decomposition of alkylth-
ienyl chain at the BDT unit. Differential scanning calorimetry
Polymer P2 and P3 in solid state showed similar absorption
spectra in comparison with them in solution. However, for
polymer P4, the absorption spectra in film was significantly
broadened, probably due to the better planarization of the
2
7,28
backbone and increased packing ability in the solid state.
The optical band gaps determined from the film absorption
onset are 1.87 eV, 1.73 eV, 1.65 eV, and 1.54 eV for P1, P2,
P3, and P4, respectively. The broad absorption and low
bandgap of polymer P4 suggested its promising potential
application in the polymer solar cell.
(
DSC) tests showed that all the four polymers showed nei-
ther endothermic nor exothermic peaks in the temperature
ꢀ
range of 20–300 C, which indicated that the polymers tend
to be amorphous.
It is well acknowledged that the maximum achievable Voc of
bulk heterojunction PSC is directly related to the difference
between highest occupied molecular orbital (HOMO) level of
Optical and Electrochemical Properties
the donor material and lowest unoccupied molecular orbital
Figure 2 depicted the normalized UV-vis absorption spectra
of the four polymers in chloroform as well as in film. The
relevant optical properties data were listed in Table 1. In
solution, two distinct absorption bands can be observed for
all the four polymers. The absorption band at shorter wave-
length can be ascribed to the delocalized excitonic p2p*
transition while the absorption band at longer wavelength
29
(
LUMO) level of the acceptor. Besides, adequate LUMO
level offset between the donor and acceptor is required for
the efficient exciton dissociation and decreased charge
recombination.
To evaluate the molecular energy levels of the polymers, cyclic
voltammetry (CV) tests of the polymer films were performed
TABLE 1 Molecular weights, optical, and electrochemical properties of the polymers
UV-vis absorption
In CHCl
3
In film
M
n
M
w
k
max
k
onset
k
max
k
onset
Bandgap
(eV)
HOMO
[eV]
LUMO
[eV]
Polymer
[kg/mol]
[kg/mol]
PDI
[nm]
[nm]
[nm]
[nm]
P1
P2
P3
P4
2.21
4.64
2.10
1.79
2.31
2.12
388
428
646
733
599
710
739
806
392
428
674
701
663
717
752
805
1.87
1.73
1.65
1.54
25.27
25.25
25.21
25.12
23.40
23.52
23.65
23.58
32.21
25.84
19.88
57.66
59.69
42.15
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FIGURE 2 UV-vis absorption spectra of polymers in chloroform solution (a) and in film (b). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
1
with the ITO as the working electrode and Ag/Ag as the ref-
erence electrode in 0.1 mol/L tetrabutylammonium tetrafluor-
23.40 eVꢁ23.65 eV and the large difference between the
LUMO levels of polymers and fullerene derivative (24.0 eV)
can allow for the BHJ photovoltaic devices to achieve efficient
4 6
oborate (Bu NPF ) acetonitrile solution. Figure 3 illustrated
the cyclic voltammograms and the HOMO energy levels of
polymers were calculated from the equations of HOMO-
3
0
exciton dissociation.
5
2[Eox 1 4.80] eV, in which Eox is the onset of oxidation
Theoretical Analysis
potential. The measured results were calibrated using a ferro-
cene/ferrocenium redox couple. The HOMO energy levels
were determined to be 25.27 eV, 25.25 eV, 25.21 eV, and
To understand the geometries and orbital energies, we per-
formed the density functional theory (DFT) calculations
with the Gaussian 09 program at the level of B3LYP/6-
31G(d). Considering the molecular weights of polymers,
oligomer approach for P1 and periodic boundary conditions
25.12 eV for P1, P2, P3, and P4, respectively. The similar
HOMO level of polymer P2 and P3 suggested that the introduc-
tion of 2-D structure probably had no distinct effect on the Voc
of the photovoltaic devices. The n-doping process was not
shown here as it was very difficult to get an obvious n-doping
signal for the polymers. The LUMO energy levels of polymer
was determined from the difference between the optical
bandgap and the HOMO energy level of the polymer. The
detailed energy level values are shown in Table 1. The
LUMO levels of the polymers were determined to be around
3
1
(PBC) calculations
for P2–P4 were adopted to gain
insight into the influence of the side chain variation on the
optical and electrochemical properties of the polymers. All
alkyl groups in the polymers were replaced by methyl
group to simplify the calculation. The optimized geome-
tries, dihedral angles, simulated electron density distribu-
tions and the calculated HOMO and LUMO levels were
shown in Figure 4. Polymer P1 exhibited a very twisted
conformation and the dihedral angle between the benzodi-
thiophene ring and phenanthrenequnioxaline ring was cal-
ꢀ
culated to be 31.5 . This large dihedral angle can lead to
enhanced torsional relaxation and reduced conjugation
length, which can account for the limited absorption spec-
trum of P1. With the introduction of thiophene ring as the
linker group, polymer P2 showed a more planar geometry
with the dihedral angle between thiophene and phenan-
ꢀ
threnequnioxaline ring calculated to be 19.4 and the dihe-
dral angle between thiophene and benzodithiophene ring
ꢀ
calculated to be 21.4 . Furthermore, the molecular geome-
try is much more planar for polymer P4 compared to P2
and P3 and the dihedral angle between thiophene and phe-
ꢀ
nanthrenequnioxaline unit was determined to be 15.9 . The
more planar conformation can lead to enhanced energy
migration rate along the backbone and improved p-orbital
overlap by restricting the intrachain rotation, thus allowing
3
2
FIGURE 3 Cyclic voltammograms of polymers with a scan rate
of 100 mV/s. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
for enhanced p–p stacking and lower bandgaps. Those
results are well consistent with the previous experimental
optical property measurements.
1
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FIGURE 4 Density functional theory calculated molecular geometries and HOMO, LUMO wavefunctions for polymers. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
As shown in Figure 4, the HOMO wavefunctions are delocal-
ized over the conjugated polymer backbone while the LUMO
wavefunctions are mainly localized on phenanthrenequnioxa-
line ring. The calculated results were calibrated using the fol-
Photovoltaic Properties
Conventional bulk heterojunction polymer solar cells were
fabricated to investigate the photovoltaic properties of the
polymers. The device configuration was ITO/PEDOT:PSS/
polymer:PC BM/Ca/Al and the device performance was
3
3
lowing equations :
7
1
22
characterized under simulated 100 mW cm
AM 1.5 G illu-
HOMO50:683HOMODFT=B3LYP21:92eV
mination. PC BM was used as the acceptor material because
7
1
LUMO50:683LUMODFT=B3LYP21:59eV
of its better light absorption ability in comparison with
PC61BM. The BHJ active layer was formed by spin-coating
the blends from o-DCB solutions. The blending ratio of the
polymer and PC BM was tuned and highest device efficien-
cies were achieved at the ratio of 1:2 for the polymers. Due
to the low molecular weight and serious steric hindrance,
device based on polymer P1 showed a negligible device
Despite the small discrepancy existed between the calculated
and experimental energy level results, the variation trends
were similar. And the difference was probably due to the
interchain interactions which existed on the polymer films
but was not considered during theoretical calculation.
7
1
FIGURE 5 Respective J–V curves (a) and EQE curves (b) of optimized photovoltaic devices. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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TABLE 2 Photovoltaic parameters of optimized devices for polymer: PC71BM
Blend
Standard
2
Polymer
Ratio
Voc (V)
J
sc (mA/cm )
FF
PCE (%)
Deviation (%)
P2
P3
P4
1:2
1:2
1:2
0.79
0.80
0.76
8.59
9.21
11.6
0.42
0.50
0.49
2.85
3.68
4.32
0.26
0.39
0.32
performance. So the photovoltaic property of P1 will not be
discussed here. The film thickness for optimized active layer
is 120 nm, 98 nm and 90 nm for P2, P3, and P4 respectively.
Figure 5(a) showed the current density2voltage(J-V) charac-
teristics for polymer P2, P3, and P4 based on the optimized
conditions and the device characterization data were sum-
marized in Table 2.
associated with the conjugation enhancement in two dimen-
3
4
sion and planarity geometry tuning. Furthermore, the corre-
sponding external quantum efficiency (EQE) wavelength
dependencies of the optimized photovoltaic devices were
2
measured under illumination with 100 mW/cm (AM1.5G). As
shown in Figure 5(b), the P3:PC BM device exhibited similar
7
1
photo response with P2:PC BM at the range of 350–650 nm.
7
1
However, the photo response of P3:PC BM was extended to
7
1
Compared to polymer P1, the device using the blend of P2:
longer wavelengths of 700–750 nm because of its smaller
bandgap. Compared to P2:PC BM, the EQE curve of the
2
PC BM yielded a significantly increased J of 8.59 mA/cm , a
7
1
sc
7
1
FF of 0.42 and a PCE of 2.85%, which can be ascribed to the
improved absorption property and packing ability of P2. Simi-
larly, for the polymer P3 with the enlarged 2-D conjugation
device based on P4: PC71BM showed widest and strongest
photo response at the range of 350–800 nm, which was in
agreement with the red-shifted absorption spectrum and
higher current density of the device.
length, the device achieved a higher PCE of 3.68% with a J of
sc
2
9
.21 mA/cm , a V of 0.80 V and a FF of 0.50. In addition the
oc
device based on P4 gave rise to an even higher J of 11.6 mA/
cm and a PCE of 4.32%. This improvement of Jsc is highly
Towards the optimization of the photovoltaic properties of
polymers, the BHJ film morphology should be carefully tuned
sc
2
FIGURE 6 Tapping mode AFM height images(a) and phase images (b) of the optimized BHJ film. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
1
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CONCLUSIONS
In summary, a series of two dimensional donor acceptor con-
jugated polymers P1, P2, P3, and P4 were synthesized based
on the benzodithiophene donor unit and phenanthrenequ-
nioxaline acceptor unit with different alkyl, alkoxyl, and
alkylthienyl side chains, following the design concept of con-
jugation enlargement and coplanar geometry tuning. A corre-
lation between the molecular structure, thermal stability,
optical and electrochemical properties and photovoltaic
properties of the conjugated polymers was investigated and
discussed in details. Polymer P1 incorporating the BDT unit
and alkoxy chains substituted phenanthrenequnioxaline unit
showed limited absorption ability and negligible photovoltaic
properties due to the low molecular weight and serious
steric hindrance. For polymer P2, the introduction of the thi-
ophene ring as the linker reduced the steric hindrance and
improved the backbone planarity. Consequently, enhanced
absorption ability and increased device efficiency were
achieved for polymer P2 based PSC. In comparison with
polymer P2 with the alkyl side chains at BDT unit, polymer
P3 with the alkylthienyl side chains showed a reduced
bandgap and enhanced device performance, corresponding
to the increased conjugation length. Furthermore, when the
alkoxy chains on phenanthrenequnioxaline were translated
to the alkyl chains on the thiophene ring, the polymer P4
gave rise to even better photovoltaic performance. The
results showed clearly that the more effective solar energy
harvesting capability and the higher charge transport mobil-
ity were the main reasons for the improved PSC
performance.
FIGURE 7 Current density and voltage (J–V) curves of the hole-
only devices (V 5Vapplied -Vbi, Vbi5 0.1 V). [Color figure can be
viewed in the online issue, which is available at wileyonline
library.com.]
since it can have a significant effect on the device perform-
3
5,36
ance.
Ordered, interpenetrating network of donors and
acceptors should be formed for optimal charge generation
and transport. The surface morphology property of the poly-
mer:PC BM blending film based on the optimized conditions
7
1
was investigated by employing atomic force microscopy
AFM). Figure 6 depicted the height and phase images of the
(
polymer: PC71BM blend film. All the BHJ films exhibited
quite smooth surface and there were no obvious large-scale
phase separation can be observed, suggesting the good mis-
cibility of the polymer with PC BM. The surface roughness
7
1
was 1.33 nm, 0.66 nm, 0.81 nm for polymer P2, P3, and P4
based BHJ film, respectively. The higher Jsc and FF of
P4:PC71BM and P3:PC71BM based devices may also be
ACKNOWLEDGMENTS
The authors thank the National Natural Science Foundation of
China (no. 21304018, 21374016), Jiangsu Provincial Natural
Science Foundation of China (no. BK20130619, BK20130617)
and Postdoctoral Research Funding Plan of Jiangsu Province
related to the more uniform film compared to P2:PC BM.
7
1
To further explore the decisive factors affecting device per-
formance, the hole mobilities of the polymers were meas-
ured by employing the hole-only device with the structure of
ITO/PEDOT:PSS/Polymer: PC BM/MoO /Al. Figure 7 shows
(
1302095C).
7
1
3
the J-V characteristics of the hole only devices. The hole
mobility of the polymer was extracted by modeling the dark
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