Wide bandgap copolymers with vertical benzodithiophene dicarboxylate for high-performance polymer solar cells with an efficiency up to 7.49%

Article abstract of DOI:10.1039/C6TA07364F

In aiming to build novel wide band-gap high-performance photovoltaic donor materials, a vertical benzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxylate (V-BDTC) with a weak electron-withdrawing character was primarily developed. And its wide band-gap (WBG) copolymers PV-BDTC1 and PV-BDTC2 were designed and synthesized, which contain a traditional electron-donating unit of 4,8-disubstituted benzo[1,2-b:4,5-b′] dithiophene (BDT) derivative with diethylhexyloxy for the former and diethylhexylthiophenyl for the latter. It is found that the weak electron-withdrawing V-BDTC unit endows its copolymers with a WBG up to 2.09 eV and a deep HOMO energy level of ～5.67 eV. Furthermore, PV-BDTC2 exhibits much better photovoltaic properties than PV-BDTC1 in the solution-processing polymer solar cells (PSCs) with a higher open-circuit voltage (V_oc) of 1.03 V and an increased power conversion efficiency (PCE) of 7.49%. To the best of our knowledge, this PCE value is the highest level recorded for copolymers with a WBG over 2.0 eV in the PSCs to date, along with a remarkable V_oc over 1.0 V. This work provides a feasible strategy to develop a novel promising electron-withdrawing building block and its high-performance WBG copolymers based on the BDT unit.

Full text of DOI:10.1039/C6TA07364F

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DOI: 10.1039/C6TA07364F
Wide bandgap copolymers with vertical benzodithiophene
dicarboxylate for high-performance polymer solar cells with efficiency
up to 7.49%
Qiang Tao,‡a,b Tao Liu,‡d Linrui Duan,a,c Yufeng Cai,a Wenjing Xiong,a,b Pu Wang,a
HuaTan,a Gangtie Lei,a Yong Pei,a Weiguo Zhu*a,b, Renqiang Yang*c,Yanming Sun*d
a College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in
the Ministry of Education, Xiangtan University, Xiangtan 411105, China.
bSchool of Materials Science and Engineering, Jiangsu Collaborative Innovation Center
of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164,
China.
cQingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao 266101, China.
dHeeger Beijing Research and Development Center, School of Chemistry and
Environment, Beihang University, Beijing 100191, China.
*To whom correspondence should be addressed. Email:
(W. Z.) zhuwg18@126.com
(R. Y.)yangrq@qibebt.ac.cn
(Y. S.) sunym@buaa.edu.cn
‡The two authors contributed equally to this work.
1

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Abstract: In aiming to build novel wide band-gap high-performance photovoltDaOicI: 1d0.o10n3o9/rC6TA07364F
materials, a vertical benzo[1,2-b:4,5-b']dithiophene-2,6-dicarboxylate (V-BDTC) with
weak electron-withdrawing character was primarily developed. And its wide band-gap
(WBG) copolymers of PV-BDTC1 and PV-BDTC2 were designed and synthesized,
which contain a traditional electron-donating unit of 4,8-disubstituted benzo[1,2-b:4,5-b']
dithiophene (BDT) derivative with diethylhexyloxy for the former and diethylhexyl-
thiophenyl for the latter. It is found that the weak electron-withdrawing V-BDTC unit
endows its copolymers with a WBG up to 2.09 eV and a deep HOMO energy level of
∼5.67 eV. Furthermore, PV-BDTC2 exhibits much better photovoltaic properties than
PV-BDTC1 in the solution-processing polymer solar cells (PSCs) with a higher open-
circuit voltage (Voc) of 1.03 V and a rising power conversion efficiency (PCE) of 7.49%.
To the best of our knowledge, this PCE value here is the highest level recorded for
copolymers with a WBG over 2.0 eV in the PSCs to date, along with a remarkable Voc
over 1.0 V. This work provides a feasible strategy to develop a novel promising electron-
withdrawing building block and its high-performance WBG copolymers based on BDT
unit.
Keywords: Vertical benzodithiophene carboxylate, Benzodithiophene, Copolymer,
Organic solar cells.
2

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DOI: 10.1039/C6TA07364F
Introduction
Benzo[1,2-b:4,5-b']dithiophene (BDT) has emerged as an effective building block for
constructing organic photovoltaic materials since the first report in organic solar cells
(OSCs).1 As its inherent structural symmetry and rigid fused aromatic system are avail-
able to enhance electron delocalization and improve charge mobility,2 numbers of effi-
cient photovoltaic copolymers3-7 and small molecules (SMs) 8-11 have been developed by
incorporating the electron-rich BDT units with other electron-deficient building blocks.
High-efficiency photovoltaic materials have been obtained by design the BDT deriva-
tives used as electron-donating building blocks in OSCs.12-15 For instance, 5-alkylthio-
phene-2-yl-substituted benzo[1,2-b:4,5-b']dithiophene (BDTT) moiety has been used
extensively to construct two-dimensional (2D)-conjugated polymers in recent years.
12,16,17
In general, the derived BDT moieties are mostly modified with alkyl,18, 19 alkoxy4,
20 or aryl groups12, 13 at its 4, 8-positions, and incorporated into molecular backbone from
its 2, 6-positions, which can be defined as paralleled BDT (P-BDT) unit as shown in Fig.
1. There are also a few reports by Yang et al. on photovoltaic materials based on the
vertical BDT (V-BDT) moieties modified with electron-rich units from 2,6-positions. It
further provided another potential way to construct the D-A alternating copolymers via
incorporation from 4,8-positions of V-BDT.21, 22 The typical V-BDT based polymers
displayed relatively low HOMO energy level (lower than −5.2 eV) and gave a promising
power conversion efficiency (PCE) of 7.1% reported by Nallan et al., in which the PCE
value is compared well with that of the reported P-BDT-directed copolymers.23 While
most of effort has been devoted to develop donor units, it is incredible that there is only
3

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one record that the BDT unit was modified as an electron-accepting building block to
construct conjugated polymers, in which two carboxylate groups were attached into 4,
8-positions of the BDT unit to form a paralleled BDT dicarboxylate (BDTC).24 However,
this BDTC-based polymers suffered from an increasing HOMO levels over ‒ 4.8 eV and
exhibited a limited PCEs with a lower Voc value (~ 0.5 V) in their solar cells. Therefore,
further effort is needed to modify the BDT unit to become electron-withdrawing unit and
further obtain its copolymers with lower HOMO energy level. Inspired by the case of the
vertical BDT as donor unit, which has made its polymers exhibit a sufficiently decreasing
HOMO, in this work, we are full of interest to investigate whether the novel vertical
BDTC unit is suitable to use as electron-withdrawing unit and construct its high-
performance copolymers in PSCs.
Fig.1 Structural modifications of the BDT unit.
Towards this end, a novel accepter unit of vertical BDTC (V-BDTC) was developed by
appending dicarboxylate onto 2, 6-positions of the BDT unit. By copolymerization path-
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way of 4, 8-positions of the V-BDTC unit and diethylhexyloxy (or dietDhOyIl:h10e.1x0y39l/-C6TA07364F
thiophenyl)-substituted BDT unit, two donor−acceptor alternating copolymers, PV-BDT-
C1 and PV-BDTC2, have been designed and synthesized. As expected, both V-BDTC-
based copolymers display extremely deep HOMO energy level of −5.56 ~ −5.67 eV and
a very wide optical band gap of 2.07 ~ 2.09 eV owing to the weak electron-withdrawing
property of V-BDTC unit and relatively twisty polymer main chains. However,
PV-BDTC2 exhibits more improving photovoltaic properties than PV-BDTC1 in the
solution-processing polymer solar cells (PSCs) with a higher open-circuit voltage (Voc) of
1.03 V and an increasing PCE of 7.49%. In principle, an efficient wide band-gap (WBG)
polymer (Eg ≥1.9 eV) is essential for tandem solar cells to convert high-energy photons
and provide high PCE in subcells.25-27 As WBG materials often suffer from an inherent
defect of a narrow absorption, only a few of them have achieved high PCE over 7%,28-30
in particular when the band-gap exceeds 2.0 eV. We believe this WBG copolymer of
PV-BDTC2 with high PCE of 7.49% has great potential to be used for tandem cell
applications.
Experimental section
2.1 Measurements.
1H NMR and13C NMR spectra were collected on a Bruker DPX 400 spectrometer
using chloroform-d as solvent and tetramethylsilane (TMS) as internal standard at a
1
resonance frequency of 400 MHz for H and 100 MHz for13C at room temperature. The
molecular weight and polydispersity index (PDI) of polymers were determined by Waters
1515 gel permeation chromatography (GPC) using polystyrene as a standard and chloro-
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form (CHCl3) as the eluent at room temperature and 1,2,4-trichlorobenzene as the eluent
at 160 °C. Thermogravimetric analysis (TGA) was carried out using a Netzsch TG 209
analyzer at a heating rate of 10 °C min-1 up to 600 °C. The differential scanning
o
calorimetry (DSC) was measured on a TA DSCQ-10 instrument at a heating rate of 10 C
min-1 under a nitrogen atmosphere. UV–vis absorption spectra were recorded on a
Perkin-Elmer Lambda 25 spectrometer. Cyclic voltammograms (CV) were recorded on a
CHI 660 electrochemical workstation with a three-electrode setup contain -ing a working
electrode of platinum rod, a counter electrode of platinum wire and a reference electrode
of Ag/AgCl at scanning rate of 100 mV s–1. The CV measurements were carried out in an
anhydrous acetonitrile solution of 0.1 M nitrogen-saturated tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) calibrated with a ferrocene/ferrocenyl couple (Fc/Fc+) at
room temperature. The HOMO and LUMO energy levels (EHOMO and ELUMO) were
calculated by the following equations:
EHOMO = −e(Eox + 4.80) (1)
opt
ELUMO = EHOMO + Eg
(2)
The theoretical study was performed on the 6-31G** basis set in Gaussian 09 using the
density functional theory (DFT), as approximated by the B3LYP. Surface morphologies
were recorded by transmission electron microscopy (TEM), which was carried out on a
FEI Tecnai T20 with LaB6 operated at 200 kV. The polymer/PC71BM blend films used as
TEM measurement were placed onto a copper grid. Atomic force microscopy (AFM)
images were obtained by using a NanoMan VS microscope in tapping-mode. The
hole/electron mobilities were measured by using space-charge-limited current (SCLC)
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method and determined by fitting the dark current to the model of a single carrDieOrI: S10C.10L39C/C6TA07364F
using the Mott-Gurney equation.
Scheme 1 Synthetic routs of copolymers PV-BDTC1 and PV-BDTC2.
2.2 Materials.
All reagents and chemicals were purchased from commercial sources (Aldrich, Acros,
TCI) and used without further purification unless stated otherwise. Toluene and tetra-
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hydrofuran (THF) were distilled according to common methods. Column chromate-
graphy was carried out with Merck silica gel (200–300 mesh). The vertical BDTC
derivative of M1 used as one of two monomers was synthesized via four steps according
to the literature methods: (1) Oxidization of Compound 1 by chromium trioxide in the
presence of acetic anhydrideto give compound 2;31 (2) Condensation of compound 2 with
ethyl 2-mercaptoacetate to afford compound 3;32 (3) Interesterification of compound 3
with 2-butyloctanol to provide compound 4 in 74% yield; (4) Bromination of compound
4 with Br2 in the presence of FeCl3 to give the compound 5. Finally, the monomer M1
was obtained by the well-known Stille coupling reaction and then bromination by NBS
according to the procedures reported previously.33
2,5-Dibromobenzene-1,4-dicarbaldehyde (2)
To a suspension of compound 1 (3.9 g, 15mmol), acetic acid (20 mL) and acetic anhy-
o
dride (40 mL) was added dropwise sulfuric acid (14 mL) at 0 C under stirring. Then
CrO3 (6.0 g) was added in it in portions. The resulting mixture was stirred vigorously at
this temperature for a further 3 h. The formed greenish slurry was poured into ice-water
and filtered. The collected white solid was washed with water and cold methanol. The
diacetate was then hydrolyzed by refluxing with a mixture of water (20 mL), ethanol (20
mL) and sulfuric acid (2 mL) overnight. After the mixture was cooled, the pale yellow
solid was separated by filtration. It was purified by recrystallization from chloroform to
afford compound 2 as a light yellow solid (2.0 g, yield 48.0%).1H NMR (400 MHz,
CDCl3) δ 10.39 (s, 2H), 8.20 (s, 2H).13C NMR (100 MHz, CDCl3) δ 10.39, 8.20, 7.31,
1.66, 0.04.
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Diethyl benzo[1,2-b:4,5-b']dithiophene-2,6-dicarboxylate (3)
To a dry one-neck 250 mL flask filled with 100 mL DMF were added compound 2
(2.7g, 9.3 mmol), ethyl 2-mercaptoacetate (3.0 g, 28 mmol), K2CO3 (4.1 g, 30 mmol) and
CuO nanopowder (180 mg, 0.3 mmol). Protected by N2 flow, the reaction mixture was
heated to 80 °C and stirred overnight. After cooled to room temperature, the mixture was
poured into water. The aqueous phase was extracted with chloroform (3 × 25 mL). The
organic layer was combined and dried over Na2SO4. The solvent was removed off under
vacuum and the residue was purified by column chromatgraphy to provide compound 3
1
as a light yellow solid (1.2 g, yield 38.7%). H NMR (400 MHz, CDCl3) δ 8.37 (s, 2H),
8.10 (s, 2H), 4.43 (d, J = 6.7 Hz, 4H), 1.69 – 1.31 (m, 6H).13C NMR (100 MHz, CDCl3)
δ 162.55, 139.23, 138.32, 135.87, 129.42, 119.34, 110.02, 61.88, 14.35.
Benzo[1,2-b:4,5-b']dithiophene-2,6-bis(2'-butyloctyl) dicarboxylate (4)
Titanium (IV) oxide bis(2,4-pentaedionate)(20mg,0.08 mmol) was added to a suspen-
sion of compound 3 (1.0 g, 3 mmol) and 2-butyloctanol(2.0 g, 11 mmol) in anhydrous
toluene (40 mL). The mixture was heated to reflux for 10 h under Dean-Stark conditions.
Then 5mL water was added to the Dean-Stark trap to help removal of the generated
ethanol. Further titanium (IV) oxide bis(2,4-pentaedionate) (50 mg) and 2-butyloctanol
(1.0 g) were added and the mixture was heated to reflux for further 10 h. After cooled to
room temperature, the mixture was poured into water. The aqueous phase was extracted
with dichloromethane (CH2Cl2, 3 × 25 mL) and the combined organic layer was dried
over Na2SO4. The solvent was removed off under vacuum and residue was purified by
column chromatography to afford compound 4 as a light yellow solid (1.36 g, yield
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1
74.1%). H NMR (400 MHz, CDCl3) δ 8.38 (s, 2H), 8.09 (s, 2H), 4.28 (d, J = 5.6 Hz,
4H), 1.80 (d, J = 5.6 Hz, 2H), 1.47 – 1.12 (m, 32H), 0.89 (m, 12H). 13C NMR (100 MHz,
CDCl3) δ 162.64, 141.35, 139.27, 138.34, 136.72, 135.95, 129.89, 129.30, 122.31,
119.32, 69.88, 68.62, 66.72, 37.45, 34.15, 31.82, 31.44, 31.11, 29.63, 29.00, 27.78, 26.76,
23.74, 23.00, 22.66, 15.21, 14.10, 14.05. Elementary analysis calcd for C36H54O4S2: C
70.31, H 8.85, S 10.43. Found: C 71.74, H 8.83, S 10.17.
4,8-Dibromobenzo[1,2-b:4,5-b']dithiophene-2,6-bis(2'-butyloctyl)dicarboxylate (5)
To a mixture of compound 4 (1.36 g, 2.2 mmol) and anhydrous FeCl3 (150 mg) in
chloroform (80mL), bromine (0.24 mL, 4.6 mmol) was added dropwise at room
temperature protected from light. The mixture was refluxed overnight. Monitored by
TLC, bromine was further added if it was necessary. The mixture was cooled down to
room temperature and poured into aqueous of NaHSO3 (100 mL). The aqueous solution
was extracted with CH2Cl2 and the organic layer was collected. The solvent was removed
off under vacuum and residue was purified by column chromatography using a mixture
solvent of hexane and CH2Cl2 (4:1, v:v) as eluent to give 5 as a light yellow powder
(1.35g, yield 80%). 1H NMR (400 MHz, CDCl3) δ 8.27 (s, 2H), 4.30 (d, J = 5.6 Hz, 4H),
1.82 (d, J = 5.6 Hz, 2H), 1.48 – 1.16 (m, 32H), 0.91 (dt, J = 15.6, 6.4 Hz, 12H).13C NMR
(100 MHz, CDCl3) δ 161.99, 141.20, 137.68, 136.93, 129.74, 111.46, 69.08, 37.39, 31.84,
31.42, 31.09, 29.62, 29.01, 26.76, 23.00, 22.68, 14.14, 14.09. Elementary analysis calcd
for C36H52Br2O4S2: C 55.96, H 6.68, S 8.30. Found: C 56.20, H 5.59, S 8.61.
4,8-Di(thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-bis(2'-butyloctyl)dicarbo
-xylate (6)
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A solution of compound 5 (0.77 g, 1.0 mmol), tributyl(thiophen-2-yl)stannanDeO(I:11.01.12039g/,C6TA07364F
3.0 mmol) and Pd(PPh3)4 (60 mg) in 60 mL toluene was degassed with nitrogen and
heated to reflux for overnight. Cooled down to room temperature, the reaction mixture
was poured into water (100 mL) and extracted with CH2Cl2 (50 mL×3). The organic layer
was washed with brine, dried over anhydrous MgSO4, then distillated to remove off the
solvent. The residue was purified by column chromategraphy using a mixture solvent of
hexane and CH2Cl2 (2:1, v:v) as eluent to give compound 6 as a yellow solid (0.72 g,
yield 92.5%). 1H NMR (400 MHz, CDCl3) δ 8.30 (s, 2H), 7.59 (d, J = 4.9 Hz, 2H), 7.51
(d, J = 3.0 Hz, 2H), 7.34 – 7.27 (m, 2H), 4.23 (d, J = 5.8 Hz, 4H), 1.77 (d, J = 4.9 Hz,
2H), 1.63 (dd, J = 15.6, 8.2 Hz, 2H), 1.47 – 1.09 (m, 32H), 0.90 (dt, J = 15.2, 7.0 Hz,
12H).13C NMR (100 MHz, CDCl3) δ 162.58, 141.27, 137.95, 137.51, 136.07, 129.72,
128.70, 127.77, 127.29, 126.23, 68.82, 37.37, 31.77, 31.45, 31.10, 29.59, 28.96, 28.29,
26.77, 26.72, 22.96, 22.64, 17.31, 14.09, 14.03, 13.58. MALDI-TOF MS m/z: calcd for
C44H58 O4S4, 778.32; found, 778.25. Elementary analysis calcd for C44H58O4S4: C 67.82,
H 7.50, S 16.46. Found: C 69.25, H 7.54, S 16.26.
4,8-Bis(5-bromothiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-bis(2'-butyloctyl)
dicarboxylate (M1)
To a solution of compound 6 (0.72 g, 0.93 mmol) dissolved in 30 mL THF was added
N-bromosuccinimide (0.344 g, 1.95 mmol) in portions over a course of 30 min protected
from light and stirred for another 24 h at room temperature. Then 100 mL water was
added to quench the reaction. The mixture was extracted with CH2Cl2. The resulting
organic phase was washed with brine and dried over anhydrous MgSO4. The solvent was
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removed off under reduced pressure and the residue was purified by column chromate -
graphy on silica gel using a mixture solvent of hexane and CH2Cl2 (2:1, v:v) as eluent to
give M1 as an orange solid (0.70 g, yield 82.0%).1H NMR (400 MHz, CDCl3) δ 8.28 (d,
J = 0.8 Hz, 2H), 7.29 (m, 4H), 4.27 (d, J = 5.8 Hz, 4H), 1.82 (s, 2H), 1.51 – 1.18 (m,
32H), 0.91 (dd, J = 12.3, 6.7 Hz, 12H).13C NMR (100 MHz, CDCl3) δ 162.36, 141.13,
139.25, 137.37, 136.42, 130.68, 129.25, 129.17, 125.46, 114.53, 68.99, 37.31, 31.80,
31.41, 31.08, 29.61, 28.96, 26.72, 23.00, 22.68, 14.15, 14.10. Elementary analysis calcd
for C44H56Br2O4S4: C 56.40, H 6.02, S 13.69. Found: C 58.97, H 5.94, S 15.07.
Synthesis of PV-BDTC1
In a dry 25 mL flask, monomer M1 (187 mg, 0.2 mmol), BDT(154 mg, 0.2 mmol),
Pd2(dba)3 (3.2 mg) and P(o-Tol)3 (6.1 mg) were dissolved in a degassed toluene (6 mL).
The mixture was stirred and heated for 24 h at 100 °C. Cooled down to room temperature,
it was dropped into acetone to form polymer precipitation. The resulting polymer was
collected by filtration through 0.45 μm Teflon filter and was resolved in toluene, then
quickly passed through a short column. Its further purification was carried out by
Soxhlet-extraction with methanol, diethyl ether, and then chloroform. The collected
chloroform fraction was concentrated to a small volume and precipitated by adding
dropwise this solution into acetone. Finally, the polymer was collected by filtration using
a 0.45 μm Teflon filter and dried under vacuum at 40 °C overnight (200 mg, yield 90%).
Molecular weight: Mn = 31.7 kDa; Mw/Mn= 2.9.
Synthesis of PV-BDTC2
Using a similar polymerization procedure of PV-BDTC1 described above to prepare
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PV-BDTC2. A mixture of M1 (140 mg, 0.15 mmol) and BDTT (136 mg, 0.15 mmol)
was polymerized for 24 h to give PV-BDTC2 (150 mg, yield 73%). Molecular weight:
Mn = 35.2 kDa; Mw/Mn= 1.7.
2.3 Solar Cells Fabrication and Characterization
A traditional bulk heterojunction sandwich structure was used in the solar cell
fabrication. The ITO/glass substrates were ultrasonic-cleaned by detergent, deionizer
water, acetone and isopropanol for 15 min each in sequence. The resulting substrates
were dried under a stream of nitrogen and subjected to the treatment of Ar/O2 plasma for
5 min. A layer of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-
PSS) film (40 nm) was spin-coated on ITO and baked at 150 °C for 10 min in a glove
box. A blend solution of polymeric donor materials (PV-BDTC1 or PV-BDTC2) and
[6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) in o-dichlorobenzene (DCB) was
then spun cast onto the PEDOT:PSS layer. A mixing concentration of PV-BDTC1 or
PV-BDTC2 is 10 mg mL-1 in DCB. The photoactive layer with an optimal thickness of 90
nm was prepared with different polymers/PC71BM weight ratios and DIO concen-
trations at 1800 rpm for 40 s. Finally, 20 nm calcium and 100 nm aluminum films were
successively deposited on the active layer through a shadow mask at a vacuum pressure
of~5×10-5 Pa to form the top electrode. The active area was 0.04 cm2 for each cell. EQE
spectra were performed in a QEX10 solar cell measurement system. The hole mobilities
of PV-BDTC2 in both pure and blend films were investigated by employing the
hole-only devices with a structure of ITO/MoO3/polymer (or polymer/ PC71BM blend)/
MoO3/Al. The electron mobilities of blend films were performed in the electron-only
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devices with a structure of ITO/Al/polymer/PC71BM blend/Al.
RESULTS AND DISCUSSION
3.1 Synthesis and Thermal properties
The synthetic route of both V-BDTC based copolymers of PV-BDTC1 and PV-
BDTC2 is summarized in Scheme 1. They were obtained in good yields over 70% by
Stille coupling reaction of trimethylstannyl-substituted benzo[1,2-b:4,5-b']dithiophene
derivatives of BDTSnMe3 or BDTTSnMe3 with monomer M1. Both polymers are soluble
in common organic solvents such as chloroform, toluene and DCB at room temperature.
Number-average molecular weight (Mn) and polydispersity (PDI=Mw/Mn) of PV-BDTC1
and PV-BDTC2 are 31.7 kDa (2.93) and 35.2 kDa (1.70) at room temperature, respective
-ly. However, the resulting Mn values are decreased to 25.0 kDa for PV-BDTC1 with a
PDI value of 1.96 and 32.0 kDa for PV-BDTC2 with a PDI value of 2.04 at 160 oC due to
the different molecular aggregation effect. The larger Mn (PDI) difference value between
room temperature and 160 oC is exhibited for PV-BDTC1. It implies that PV-BDTC1 has
stronger molecular aggregation than PV-BDTC2 in room temperature.
Table 1 Optical, electrochemical and thermal properties of PV-BDTC1 andPV-BDTC2.
Solution
Film
EHOMO ELUMO
(eV)
Td
a
opt
Polymers
εmax
λabs
λabs
(nm)
fwhm
(nm) b (eV) c
Eg
(eV) d (oC)
(Lg-1cm-1)
(nm)
483
35
520
186
2.07
–5.56
–5.67
–3.49 326
–3.58 357
PV-BDTC1
PV-BDTC2
479
34
514
177
2.09
b
aAbsorption coefficient from the peak at long wavelength. Full width at half-maximum.
c Band gap estimated from optical absorption band edge of the film.
opt
dELUMO (eV) = EHOMO +Eg
.
The thermal properties of PV-BDTC1 and PV-BDTC2 are evaluated with TGA and
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DSC. The recorded TGA and DSC curves are shown in Fig. 2a and Fig. 2b, resDpOeIc: 1t0iv.10e3l9y/.C6TA07364F
The corresponding data are summarized in Table 1. High decomposition temperatures (Td)
of 326 °C for PV-BDTC1 and 357 °C for PV-BDTC2 are observed during the heating
process at a 5% weight loss, respectively. It indicates that PV-BDTC2 exhibits higher
thermal stability than PV-BDTC1, that because alkoxy groups are eliminated earlier than
aryl groups from their parent BDT unit, which is consistent with the observations for
other similar systems.34,35 On the other hand, no distinct endothermic and exothermic
peaks are exhibited for both polymers during the heating and cooling processes between
50 and 300 °C, as shown in Fig. 2b. It implies that both PV-BDTC1 and PV-BDTC2 tend
to present an amorphous state.
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Fig. 2 TGA (a) and DSC (b) curves of PV-BDTC1 and PV-BDTC2.
3.2 Optical Properties
Fig.3 displays the UV−vis absorption spectra of PV-BDTC1 and PV-BDTC2 in dilute
solutions and in their thin films. The absorption maximums are listed in Table 1. Similar
absorption spectra are observed for both polymers. Furthermore, these absorption profiles
exhibit a red-shifted peak in low-lying energy region by about 35 nm from in solution to
in solid state. In solutions, PV-BDTC1 and PV-BDTC2 display an absorption peak at 483
nm with a molar extinction coefficient (εmax) of 35 Lg-1 cm-1and 479 nm with a εmax value
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of 34 Lg-1 cm-1, respectively. Obviously, PV-BDTC2 shows a slightly blue-shifted
absorption profile compared to PV-BDTC1 in this solution state. It implies that PV-
BDTC1 with alkoxyl side chain adopts a slightly more coplanar orientation than PV-
BDTC2 with aryl side chain on the BDT unit, which is further supported by theoretical
calculation in Table S1 (see electron supporting information, ESI).
Fig. 3 UV-vis spectra of PV-BDTC1 and PV-BDTC2 in CHCl3 solutions and thin films.
The optical band gaps are estimated to be 2.07 eV for PV-BDTC1 and 2.09 eV for
PV-BDTC2 from their absorption band edges (Egopt) in thin films. Moreover, a very broad
absorption window in a range of 300-600 nm with a full width at half-maximum (fwhm)
of 186 nm for PV-BDTC1 and 177 nm for PV-BDTC2 is observed at the same time.
Some investigations have shown that wide band-gap (WBG) and low band-gap (LBG)
polymers can be simultaneously used in polymer tandem solar cells to obtain high
performance solar cells, in which the high-energy and the low-energy photons are
efficiently absorbed by WBG and LBG polymers, respectively.26,36,37 In this work, such
opt
high Eg over 2.0 eV and absorption feature should endow their great potential to be
used as novel WBG materials.
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In order to explore the influence of the acceptor of PC71BM on the absorption of their
polymeric blend films, the absorption spectra of the polymer/PC71BM blend films were
measured and shown in Fig. S1 (see ESI). In comparison with absorption spectra of the
pure films and the blend films in Figure 3 and Figure S1, it is found that the acceptor of
PC71BM plays an important role in tuning photo-response region from 600 nm to 700 nm
for the blend films, which should result in extension of EQE response. The similar
phenomenon was observed in other reported blend films. The large extension of photo
response here should be related to the polymer structure and the D-A interaction between
the polymer and PC71BM.38
3.3 Electrochemical Properties
Fig. 4 depicts the oxidative curves of PV-BDTC1 and PV-BDTC2 measured by the CV
method. A reversible oxidative behavior is observed with an onset of oxidation potential
at 0.76 V for PV-BDTC1 and 0.87 V for PV-BDTC2 vs Fc/Fc+, respectively. According
to these onsets of oxidation and the energy level of ferrocene (4.8 eV below vacuum
level),39 the HOMO energy level is estimated at −5.56 eV for PV-BDTC1 and −5.67 eV
for PV-BDTC2.
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Fig. 4 The oxidation curves of PV-BDTC1 and PV-BDTC2.
The electrochemical results reveal that attaching two carboxylate substitutions on the
BDT unit has made their polymers exhibit lower HOMO energy levels around − 5.6 eV,
which is available to increase open-circuit voltages (Voc) for their photovoltaic devices.
Similar phenomena were also recorded in the literatures.40,41 On the other hand, the
LUMO energy levels were calculated to be −3.49 and −3.58 eV for PV-BDTC1 and
PV-BDTC2, respectively, based on the equation ELUMO = EHOMO + Egopt. The offset of
LUMO energy level between one of polymers and PC71BM is ∼0.4 eV, which should be
able to provide sufficient driving force for exciton dissociation.42
3.4 Theoretical Calculations
To further understand the relationship among structures and energy levels of polymers,
we performed theoretical simulations for their molecular geometries and frontier orbitals
with the hybrid density functional theory (DFT) at the B3LYP/6-31G(d) level using
Gaussian 09W. It is suggested this method would over delocalize the wave-functions in
conjugated systems and favor more planar structures, but the calculated results are still
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comparable to some extent as both polymers share the same molecular backbone.43 The
optimal geometries and wave functions (HOMO and LUMO) of PV-BDTC1 and PV-
BDTC2 are shown in Fig. 5. And dihedral angles of the optimal geometries of PV-
BDTC1 and PV-BDTC2 are summarized in Table S1 (see ESI). It shows that the electron
densities of LUMO and HOMO are most localized on electron- deficient V-BDTC unit
and electron-rich BDT moiety, respectively. In comparison with the LUMO and LUMO
+1 orbitals of both polymers, there is little change in the electron density distribution,
which reveals the electron-accepting feature of V-BDTC moiety resulting from the subs-
titution of dicarboxylate functional group on vertical BDT unit. A similar phenomenon is
also found from HOMO to HOMO‒1 owing to the electron-rich property of BDT unit.
Compared to their optimal geometries and dihedral angles of two polymers, it is further
found that the dihedral angles between donor BDT and thiophene space is 20.39o in PV-
BDTC1, and 27.36o in PV-BDTC2. It further confirms that PV-BDTC2 has a slightly
twisty polymeric main chain, which is consistent with those results obtained from UV−
vis spectra and CV properties.
PV-BDTC1
PV-BDTC2
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LUMO+1
LUMO+1
LUMO
LUMO
HOMO
HOMO
HOMO-1
HOMO-1
Fig. 5 Optimized molecular geometries, molecular frontier orbitals of LUMO+1, LUMO,
HOMO, and HOMO−1 for PV-BDTC1 and PV-BDTC2.
3.5 Photovoltaic Properties
The processing conditions of the bulk-heterojunction polymer solar cells based on
polymers of PV-BDTC1 and PV-BDTC2 were preliminarily optimized by varying
polymer/PC71BM weight ratios from 1:1 to 1:2.5 and 1,8-diiodoctane (DIO) additive con
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-centrations from 1 wt% to 4 wt% under an illumination of AM1.5G simulated solar light
at 100 mW/cm2. The recorded photovolatic parameters of these solar cells are
summarized in Table S1 and S2 (see ESI). It is shown that an optimized PV-BDTC1/
PC71BM (or PV-BDTC2/PC71BM) weight ratio was 1:2. However, the DIO additive
made different influnce on photovoltaic performance in the PV-BDTC1 and PV-BDTC2
based cells. An optimized DIO concentration of 3 wt% was obtained in the PV-BDTC2-
based cells only. In contrast, adding DIO made the PV-BDTC1-based cells exhibit lower
photovoltaic properties. Fig. 6 depicts the typical current density-voltage (J–V) characte-
ristics of two polymers in these devices at optimized processing conditions and the
resulting device parameters are summarized in Table 2. A PCE of 3.25% with a Voc of
0.93 V in the PV-BDTC1-based cells and an increasing PCE of 5.62% with a Voc of 1.04
V in the PV-BDTC2-based cells are observed at an optimized polymer/PC71BM weight
ratio of 1:2 without any post treatment. When 3% DIO additive is added, a sufficently
enhanced PCE of 7.49% with greater Jsc of 10.37 mA cm-2 and fill factor (FF) of 70.0%
is achieved for the PV-BDTC2-based devices except for a sligthly decreased Voc of 1.03
V. However, this Voc value over 1.0 V in the PV-BDTC2-based devices is still higher
among the reported PSCs, which would be ascribed to its deeper HOMO energy levels of
-5.67 eV. To the best of our knowledge, this PCE value in the PV-BDTC2- based devices
represents the highest value so far for PSCs based on polymers with bandgaps over 2.0
eV.
Table 2. Photovoltaic parameters of the optimized polymer/PCBM-based solar cells
based on PV-BDTC1 and PV-BDTC2.
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a
b
c
D/A
ratio
Voc
(V)
Jsc
FF
(%)
PCE
(%)
Jsc DOI: 10.1P03C9/EC6(TaAv0e73)64F
Polymer/accepter
(mA cm-2)
(mA cm-2)
(%)
3.15
2.13
PV-BDTC1/PC71BM
PV-BDTC1/PC61BM
PV-BDTC2/PC71BM
PV-BDTC2/PC71BM d
PV-BDTC2/PC61BM d
1:2
1:2
1:2
1:2
1:2
0.93
0.91
1.04
1.03
1.02
5.90
5.99
8.93
10.37
9.87
59.39
39.5
60.8
70.0
61.2
3.25
2.25
5.61
7.49
6.32
5.75
5.88
8.82
10.10
9.55
5.53
7.33
6.18
b
c
aJsc measured from solar cells. Jsc estimated from EQE spectra. Obtained from 20 devices. d 3%DIO
additive.
Additionally, an average PCE of 7.33% was further obtained in twenty PV-BDTC2
based devices, indicating this class of devices has good reproducibility. In order to
explore the influence of PCBM acceptor, the photovoltaic properties of the polymer/
PC61BM devices were also examined, and depicted in Table 2 and Figure S2 (see ESI). A
lower PCE value was observed in both polymer/ PC61BM devices owing to its weaker
absorption in visible region compared to the polymer/PC71BM devices. In comparison
with all these devices, the PV-BDTC2/PC71BM device exhibited the best photovoltaic
properties whether the acceptor material is PC71BM or PC61BM. The recorded EQE
curves of the optimized polymer/PC71BM and polymer/PC61BM blend films under illu-
mination of monochromatic light, as respectively shown in Fig. 6b and Fig. S2, further
support why PV-BDTC2 exhibit better photovoltaic performance than PV-BDTC1 in the
devices. The photo-response wave-length ranges of the EQE curves were well-consistent
with the UV−vis absorption ranges for the polymer/PC71BM blend films (see Fig. S1).
The maximum EQE values of 78% and 50% are respectively observed for the PV-
BDTC2/PC71BM and PV-BDTC1/PC71BM blend films, which is higher than those corres
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-ponding values for the PV-BDTC2/PC61BM and PV-BDTC1/PC61BM blend fDilOmI: s10..1T03h9e/C6TA07364F
measured Jsc values of 10.10 and 5.75 mA/cm2 respectively in the PV-BDTC2 and
PV-BDTC1 based devices here are agreed well with the Jsc values estimated from the
EQE charcteristics. Therefore, the higher photo response efficiency for the PV-BDTC2/
PC71BM blend film can promote its solar cells exhibit higher Jsc values, then better
photovoltaic performance.
Fig. 6 (a) J – V curves of the optimized polymer/PC71BM-based solar cells under a
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simulated AM 1.5 G irradiation (100 mW cm−2); (b) EQE curves of the optimized
polymer/PC71BM-based solar cells under illumination of monochromatic light.
3.6 Film Morphology and Charge mobility
To analyze the origin of photovoltaic performance differences between the PV
-BDTC1- and PV-BDTC2-based solar cells, we carried out the investigation on their mor
-phologies of the polymer/PC71BM blend films by TEM. Fig. 7 shows the TEM images
of these polymer/PC71BM blend films with/without 3 wt% DIO solvent additive at a
polymer/PC71BM weight ratio of 1:2. It is seen that nanoscale phase separation of
polymer and PC71BM is observed in both polymer/PC71BM blend films without DIO
additive. The fibrillar-like light regions are attributed to the polymer aggregation.44
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Fig. 7 TEM images of the polymers/PC71BM blend films at a D/A weight ratio of 1:2
with or without DIO additive.
It is clear that the PV-BDTC2/PC71BM blend film exhibits smaller domain size than
the PV-BDTC1/PC71BM blend film, which can provide larger interface area for efficient
exciton diffusion and charge separation. This can be explained why the PV-BDTC2-
based devices show higher Jsc and FF than the PV-BDTC1-based devices without the
DIO additive. When DIO is employed, for the case of PV-BDTC1, the size of polymer/
PC71BM domains instead increases, which should limit the donor/acceptor interface and
result in a decreased performance.45,46 In contrast, for the case of PV-BDTC2, a more
uniform image with better bicontinuous morphology is observed under the presence of 3
wt% DIO, this is also confirmed by the atomic force microscopy (AFM) images featured
with interpenetrating network (see ESI figure S2), thus giving rise to a remarkably
increased FF and PCE values.
To further understand the device performance differences ofthePV-BDTC2-based solar
cells with/without DIO additive, charge transport behavior for hole and electron
mobilities were deduced from the space-charge-limited current (SCLC) measurements.47
Fig. 8 shows the current–voltage (J–V) characteristics and SCLC fittings of these hole-
and electron-only devices. The corresponding hole and electron mobilities are summa-
rized in Table 3. A hole mobility (µh) of 8.2×10-4 cm2V-1s-1and an electron mobility (µe)
of 2.28×10-3 cm2V-1s-1, as well as a µe/µh value of 2.77 are observed in the PV-BDTC2/
PC71BM based hole- and electron-only devices, respectively. While 3 wt% DIO was
added, the µh and µe values are increased to 9.7 ×10-4 and 2.32 × 10-3cm2V-1s-1 with a
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µe/µh value of 2.38, respectively. It is clear that more balanced charge carrier mobility is
achieved in the devices under the presence of 3 wt% DIO. The increasing carrier mobility
and less charge recombination loss should be mainly responsible for the improvement in
Jsc and overall performance.48,49
Fig. 8 J–V curve of the hole/electron-only PV-BDTC2/PC71BM-based devices: (a)
hole-only characteristics of pure films and blend films (with or without 3% DIO). (b)
electron-only characteristics of the blend films (with or without 3% DIO).
Table 3 Hole and electron mobilities of PV-BDTC2/PC71BM in the hole- and
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electron-only devices.
Samples
Hole mobility Electron mobility
µe/µh
µh /cm2V-1S-1
2.26  10-3
8.22  10-4
µe /cm2V-1S-1
PV-BDTC2
--
2.28  10-3
2.32  10-3
PV-BDTC2:PC71BM
PV-BDTC2:PC71BM (3% DIO)
2.77
2.38
9.72  10-4
Conclusion
In summary, a novel vertical dicarboxylate substituted benzodithiophene of V-BDTC
was primarily used as an electron-withdrawing building block to construct D-A
copolymers. Its copolymers of PV-BDTC1 and PV-BDTC2 with alkoxy- and thiophenyl-
substituted benzodithiophene (BDT) units have been obtained, respectively. Wide band
gaps of 2.07~2.09 eV with remarkable deep HOMO levels of –5.56 to –5.67 eV were
observed in both copolymers. However, the better photovoltaic properties were exhibited
in the PV-BDTC2-based solar cells with an impressive PCE up to 7.49% and high Voc of
1.03 V at a PV-BDTC2/PC71BM weight ratio of 1:2 and a DIO concentration of 3 wt%.
Its relatively wide optical band gap over 2.0 eV with high absorption coefficient, high
hole mobility, much balanced hole/electron mobility, and excellent film morphology of
the PV-BDTC2/PC71BM blend film are responsible for the remarkably improved photo-
voltaic performance of the PV-BDTC2-based solar cells. The PCE maximum with a high
Voc over 1.0 V obtained in this work is the highest level recorded for the conventional
BHJ PSCs based on WBG (>2.0 eV) polymers. The work provides a feasible strategy to
develop a novel family of the BDT-based D-A copolymers. It further indicates that
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V-BDTC is a potential electron-deficient building block in D-A-type copolyDmOeI:r1s0.10a3n9d/C6TA07364F
PV-BDTC2 is a promising WBG donor material for high-performance PSCs. Interesting-
ly, there remains considerable scope to fully explore the potential of the V-BDTC-based
copolymers in photovoltaic applications, such as choosing more suitable donor units to
construct novel copolymers, optimization of the device processing technique and use in
tandem solar cells.
Acknowledgements
Thanks to the financial support from the Major Cultivation and General Programs of the
National Natural Science Foundation of China (91233112, 21172187, 51403178), the
International Science & Technology Cooperation Program of China (No. 2014DFA-
52820). The Program for Innovative Research Cultivation Team in University of Ministry
of Education of China (1337304), the Natural Science Foundation of Hunan (2015JJ
3113), the Natural Science Foundation of Hunan (14JJ4019), Open Project for the
National Key Laboratory of Luminescent Materials and Devices (2014-skllmd-10),
Research Foundation of Hunan Education Bureau (13A102, YB2015B025, 14C1099),
the Hunan Postgraduate Science Foundation for Innovation (CX2014B257, CX2015-
B201) and the Natural Science Foundation of Xiangtan University (13QDZ23).
References
1
J. Hou, M. Park, S. Zhang, Y. Yao, L. Chen, J. Li, and Y. Yang, Macromolecules,
2008, 41, 6012-6018.
2
H. Pan, Y. Li, Y. Wu, P. Liu, B. Ong, S. Zhu, and G. Xu, J. Am. Chem. Soc., 2007,
29