New Semiconducting Polymer Based on Benzo[1,2-b:4,5-b′]diselenophene Donor and Diketopyrrolopyrrole/Isoindigo Acceptor Unit: Synthesis, Characterization and Photovoltaics

Article abstract of DOI:10.1002/cjoc.201500270

Two new D-A type polymers PBDSe-DPP and PBDSe-ID were synthesized to explore new ideal semiconducting polymers, by conjugating acceptor unit diketopyrrolopyrrole/isoindigo to a donor unit benzo[1,2-b:4,5-b′]diselenophene, which is designed by substituting the sulfur atom with a selenium atom in the benzo[1,2-b:4,5-b′]dithiophene. The thermal, optical, electrochemical, photoelectric and photovoltaic properties of the two polymers were studied systematically. Relatively high open circuit voltage (0.7 and 0.75 V) and fill factor (>65%) were demonstrated for both polymers. Huge increase (by 64% and 120%) of the short circuit current density was achieved for both polymer based devices by using additive compared to the corresponding reference without additive, resulting in decent power conversion efficiency of 3.7% and 2.5% respectively with only simple optimizing consideration. We believe this class of BDSe polymer possesses a good potential to be alternatives of active material for photovoltaic applications. D-A type polymers PBDSe-DPP and PBDSe-ID were synthesized to explore new photovoltaic semiconductors by conjugating acceptor unit diketopyrrolopyrrole/isoindigo to donor unit benzo[1,2-b:4,5-b′]diselenophene. The thermal, optical, electrochemical, photoelectric and photovoltaic properties of the obtained polymers were studied systematically. Relatively high open circuit voltage (0.7 and 0.75 V) and fill factor (>65%) were demonstrated for both polymers. Huge increase (by 64% and 120%) of the short circuit current density was achieved for both polymer based devices by using additive compared to their references, resulting in decent power conversion efficiency.

Full text of DOI:10.1002/cjoc.201500270

FULL PAPER
DOI: 10.1002/cjoc.201500270
New Semiconducting Polymer Based on
Benzo[1,2-b:4,5-b′]diselenophene Donor and
Diketopyrrolopyrrole/Isoindigo Acceptor Unit:
Synthesis, Characterization and Photovoltaics
Zhongjie Xu,a Guozheng Shi,a Jianyu Yuan,a Emily Glenn,b Hai-Qiao Wang,*,a and Wanli Ma*,a
a Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional
Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, China
b University of Waterloo, 200 University Avenue West, Waterloo Ontario, N2L3G1, Canada
Two new D-A type polymers PBDSe-DPP and PBDSe-ID were synthesized to explore new ideal semiconduct-
ing polymers, by conjugating acceptor unit diketopyrrolopyrrole/isoindigo to a donor unit benzo[1,2-b:4,5-b']di-
selenophene, which is designed by substituting the sulfur atom with a selenium atom in the benzo[1,2-b:4,5-b']-
dithiophene. The thermal, optical, electrochemical, photoelectric and photovoltaic properties of the two polymers
were studied systematically. Relatively high open circuit voltage (0.7 and 0.75 V) and fill factor (＞65%) were
demonstrated for both polymers. Huge increase (by 64% and 120%) of the short circuit current density was
achieved for both polymer based devices by using additive compared to the corresponding reference without addi-
tive, resulting in decent power conversion efficiency of 3.7% and 2.5% respectively with only simple optimizing
consideration. We believe this class of BDSe polymer possesses a good potential to be alternatives of active material
for photovoltaic applications.
Keywords semiconducting polymer, benzo[1,2-b:4,5-b']diselenophene, synthesis, characterization, photovoltaics
Introduction
ered one of the most effective building-blocks for ideal
polymers for efficient PSCs.[15,17,18] On the other hand,
benzo[1,2-b:4,5-b']diselenophene (BDSe) donor unit
obtained by using electron-rich selenophene to substi-
tute the thiophene moiety, has also been developed as it
can lower the band gap of polymers.[19,20] Besides that,
the heavier Se atom can improve the carrier mobility
between molecules as the enhanced inter-chain transfer
integrals.[19,21,23] And moreover, selenophene-based co-
polymers show higher absorption coefficient to the solar
spectrum than thiophene-based conjugated polymers.[24]
Therefore superior properties like high mobility and
strong absorbance in a broad region could be expected
for BDSe based copolymers, which possess big poten-
tial to develop ideal active material for PSCs. Recently,
the research of a selenophene-based conjugated polymer
in organic field-effect transistor device was reported by
Kang et al.[19,25,26] A high mobility of ~0.1 cm2•V−1•s−1
improved to 4.97 cm2•V−1•s−1 was demonstrated for this
selenophene-based conjugated polymer.[19,27] At the
same time, Yu et al. reported the synthesis of a series of
copolymers containing selenopheno[3,4-b]selenophene
and benzodiselenophene, which delivered power con-
Polymer solar cells (PSCs) have gained considerable
attention during the past decades due to the fact that
they are inexpensive, lightweight, easily solution-proc-
essable and capable of flexible fabrication.[1-3] Bulk-
heterojunction PSCs consisting of interpenetrating
polymer donor and fullerene acceptor nanoscale net-
works offer a competitive approach to efficient and
cost-effective solar energy technology.[4-9] To achieve a
high efficiency for PSCs, the biggest challenge at mo-
lecular level is to develop an ideal p-type conjugated
polymer which possesses simultaneously the following
properties: sufficient solubility for processing and mis-
cibility with a n-type material, low band gap for broad
absorption and strong light absorption coefficient to
capture as many solar photons as possible, and reason-
able high hole-mobility for efficient carrier trans-
port.[10-13]
As an electron-rich donor unit, benzo[1,2-b:4,5-b']-
dithiophene (BDT) has been used to synthesize donor-
acceptor (D-A) type low band gap polymers due to its
ability to promote polymer’s planarity, molecular π-π
stack and charge carrier mobility,[14−16] which is consid-
*
E-mail: whqbme@gmail.com, wlma@suda.edu.cn; Tel.: 0086-0512-65880945; Fax: 0086-0512-65882846
Received April 2, 2015; accepted May 8, 2015; published online June 10, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500270 or from the author.
Chin. J. Chem. 2015, 33, 909—916
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Lambda 750. The electrochemical cyclic voltammetry
was carried out on a Zahner IM6 electrochemical work-
station. Tapping-mode AFM images were obtained by a
Veeco Multimode V instrument. Grazing incidence
X-ray diffraction (GIXD) experiments were conducted
at Shanghai Synchrotron Radiation Facility (SSRF) on
diffraction beam line (BL14B1).
version efficiency of 6.87% for polymer solar cell when
blended with PC71BM.[28] And very recently, Ashraf et
al. reported selenophene-based polymers, for which
good field effect mobility of 1.6 cm2•V−1•s−1 and power
conversion efficiency (PCE) of 7.6% for bulk-hetero-
junction solar cell was demonstrated,[29] suggesting
promising potential for BDSe based polymers in PSC
applications.
Synthesis of monomers and polymers
As acceptor units, diketopyrrolopyrrole (DPP) and
isoindigo (ID) have often been adopted in the design of
D-A type polymers[30-35] because of their excellent
properties. Both DPP and ID possess the ability to
achieve suitable lowest unoccupied molecular orbital
(LUMO) for polymers to match well with the fullerene
acceptors. Their strong electron withdrawing ability and
their intrinsic energy band structure properties benefit
the exciton dissociation.[36-39] On the other hand, both
units tend to provide broad absorption extending to the
near-infrared region.[35,40-42] Besides the influence on
polymer’s LUMO, DPP and ID can also help to lower
the polymer’s highest occupied molecular orbital
(HOMO) level, hence increase the open circuit voltage
(VOC).[43-45]
Herein in this work, we report new D-A type poly-
mers, PBDSe-DPP and PBDSe-ID. Different from the
polymers reported by Yu et al., we have utilized a sim-
ple and easy-to-synthesize side-chain free unit BDSe as
donor, and different unit DPP or ID as the acceptor. The
thermal, photophysical, electrochemical and photo-
voltaic properties of the two polymers were investigated
in detail. Both polymers show a broad absorption in the
region of 350－750 nm, relatively low optical band gap,
and suitable HOMO energy levels. And decent device
performance can be achieved for both polymers as well,
based on simple optimizations with additive.
The synthesis of monomer 1,4-dibromo-2,5-
di(octyn-1-yl)benzene (2) and benzo[1,2-b:4,5-b']bis-
(selenophene) (3): The compounds 2 and 3 were synthe-
sized according to previous reference,[41] experimental
details can be found in the supporting information.
2,6-Bis(trimethylstannyl)benzo[1,2-b:4,5-b']bis(sele-
nophene) (4): A solution of benzo[1,2-b:4,5-b′]dise-
lenophene (446 mg, 1.57 mmol) in anhydrous tetrahy-
drofuran (50 mL) was cooled to 0 ℃, and left for 0.5 h.
Then n-BuLi (2.4 mol/L, 1.3 mL) was added in drop-
wise in 10 min. The solution was stirred for 1 h at room
temperature. The solution of trimethyltin chloride (3.45
mmol) in tetrahydrofuran (5 mL) was added to the mix-
ture and then cooled to 0 ℃ and stirred for 2 h. Then
the mixture was warmed up to room temperature in 2 h.
Water (150 mL) was added to the mixture, which was
then extracted with diethyl ether (30 mL×3). The or-
ganic layer was dried with anhydrous Na2SO4. The sol-
vent was removed by rotary evaporation and residue
was purified by re-crystallization in ethanol to produce a
white solid (700 mg, 73%). EI-MS m/z: 613.9 (M＋); 1H
NMR δ: 8.73 (s, 2H), 8.06 (s, 2H), 0.79 (s, 18H).
Synthesis of PBDSe-DPP
In a 50 mL reaction tube, monomer 4 (220 mg, 0.22
mmol), monomer 5 (132 mg, 0.22 mmol), tri(o-tolyl)-
phosphine (20 mg, 0.08 mmol) and Pd2(dba)3 (10 mg,
0.001 mmol) were added under a nitrogen atmosphere.
Then anhydrous toluene (6 mL) was injected into the
reaction tube through a syringe. After being stirred for
12 h at 110 ℃, the reaction mixture was cooled down
to room temperature and then precipitated in methanol
(130 mL). The precipitate was filtered and repeatedly
washed with methanol (200 mL, 24 h), hexane (200 mL,
24 h) and chloroform (200 mL, 10 h) in a Soxhlet appa-
ratus. The chloroform fraction was concentrated and
precipitated in methanol. The precipitate was filtered
and dried in a vacuum for 24 h at 90 ℃. PBDSe-DPP
was obtained as dark green solid (130 mg, 52.7%). GPC:
Experimental
Materials and characterization
Tetrahydrofuran and toluene were dried over so-
dium/benzophenone ketyl and distilled before use.
N,N-Dimethylformamide was distilled over CaH2.
Chloroform, methanol, and dichloromethane were used
as received. All other chemicals were purchased from
Sigma-Aldrich and used as received without further pu-
rification.
1H NMR was performed on a Varian NMR system
300 MHz spectrometer with tetramethylsilane (TMS; δ
0) as the internal standard. Mass spectra were obtained
by using a ThermoFisher Scientific GC/MS Trace-ISQ
mass spectrometer. Gel permeation chromatography
(GPC) analysis was carried out on a Waters 717－2410
instrument with polystyrenes as the standard and tri-
chlorobenzene as eluant (flow rate 1.0 mL/min). The
thermo gravimetric analysis (TGA) was conducted with
a Perkin Elmer TGA4000 under nitrogen atmosphere at
a heating rate of 10℃/min. Ultraviolet-visible absorp-
tion spectra were recorded on a Perkin Elmer model
1
Mn＝8.0 kg/mol; poly dispersity index (PDI)＝1.79. H
NMR (300 MHz, CDCl3) δ: 7.98－6.98 (br, 10H, ArH),
4.24 (br, 4H), 2.60－2.37 (br, 2H), 1.80－1.27 (br, 64H),
1.27－0.81 (br, 12H).
Synthesis of PBDSe-ID
PBDSe-ID was prepared from monomer 4 (220 mg,
0.22 mmol) and monomer 6 (138 mg, 0.22 mmol) by
using the same method which was used for PBDSe-DPP.
A green solid was obtained with a yield of 50.0% (125
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New Semiconducting Polymer: Synthesis, Characterization and Photovoltaics
1
mg). GPC: Mn＝9.8 kg/mol; PDI＝2.40. H NMR (300
MHz, CDCl3) δ: 9.15－9.30 (br, 2H), 7.85－7.90 (br,
2H), 6.90－7.25 (br, 6H), 3.0－2.50 (br, 6H), 1.95－
1.50 (br, 64H), 1.50－0.81 (br, 12H).
mined by fitting the dark current to the model of a sin-
gle carrier SCLC, which is described by the following
equation:[33]
9
V 2
d3
J＝ ε0εr μh
Fabrication and characterization
8
Polymer solar cells were fabricated with a general
structure of ITO/PEDOT:PSS (40 nm)/polymer: PCBM/
LiF (0.6 nm)/Al (100 nm). Patterned ITO glass sub-
strates were cleaned by ultrasonic treatment with deter-
gent, acetone, deionized water and isopropyl alcohol,
respectively, and further treated with UV-ozone for 10
min to remove organic residue. A thin film of
PEDOT:PSS (40 nm) was spin-coated on ITO substrates
and annealed at 150 ℃ for 10 min. Blends of PBDSe-
DPP (or PBDSe-ID) and PC61BM of different ratios
were dissolved in chloroform containing 10% (V/V) di-
chlorobenzene (DCB) (the optimal parameters, 5 mg
polymer, 20 mg PC61BM, and 1 mL solvent), filtered
through a 0.45 mm poly(tetrafluoroethylene) filter,
spin-coated at different speeds for 40 s. 0.6 nm LiF (0.2
Å/s) and 100 nm Al (2 Å/s) layers were then thermally
evaporated on the active layer at a pressure of 1.0×
where J is the current, μh is the zero-field mobility, ε0 is
the permittivity of free space, εr is the relative permittiv-
ity of the material, d is the thickness of the active layer,
and V is the effective voltage. The effective voltage can
be obtained by subtracting the built-in voltage (Vbi) and
the voltage drop (Vs) from the substrate’s series resis-
tance from the applied voltage (Vappl), V＝Vappl−Vbi−Vs.
Results and Discussion
Synthesis and thermal properties
The synthetic routes for the monomers and polymers
are illustrated in Scheme 1. Compounds 2 and 3 were
prepared according to literature.[46] Compound 2 was
accomplished by substituting the iodine atom with TMS
in compound 1. Compound 3, benzo[1,2-b:4,5-b']bis-
(selenophene), was synthesized based on the ring clos-
ing reaction of compound 2. Monomer 4 was obtained
based on the reaction using compound 3, chlorotri-
methylstannane and n-BuLi in dried THF. Compounds 5
and 6 were synthesized according to literature.[40,41]
Since no side-chain engineering was carried out for the
donor unit, which simplifies the synthesis procedure for
the polymers PBDSe-DPP and PBDSe-ID. To ensure
sufficient solubility for both polymers, long side-chain
2-octyldodecyl (R) was adopted in both DPP and ID
acceptor units.
10－ mbar through a shadow mask (active area 7.25
6
mm2). The current density-voltage characteristics of the
photovoltaic cells were measured using a Kheithey 2400
(I-V) digital source meter under a simulated AM 1.5G
solar irradiation at 100 mW•cm− 2 (Newport, Class AAA
solar simulator, 94023A-U). The light intensity was
calibrated by a certified Oriel Reference Cell (91150V).
Hole-only devices were fabricated to measure the
hole-mobility of polymers by the space charge limited
current (SCLC) method, with structure ITO/PEDOT/
polymer:PCBM/MoO3/Al. The hole-mobility is deter-
Scheme 1 The synthetic routes of different monomers and copolymers
I
TMS
Br
Br
Se
Se
Se powder
n-BuLi, THF
TMSA
THF
Sn
Sn
Me3SnCl
Se
Se
NaBH4, NMP
Br
Br
I
TMS
4
1
2
3
R
N
R
N
Se
O
O
Br
S
S
Se
S
S
n
Br
O
N
O
Se
Sn
N
Sn
R
R
Se
PBDSe-DPP
5
Pd2(dba)3, P-(o-tol)3
R
R
N
Toluene, DMF
O
N
O
Se
C8H17
Br
Br
R :
n
Se
C10H21
N
R
O
N
R
O
6
PBDSe-ID
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The molecular weight and poly dispersity index (PDI)
of the resultant polymers PBDSe-DDP and PBDSe-ID,
synthesized by Stille poly condensations, are listed in
Table 1. Both polymers show relatively good solubility
since they can be dissolved in common solvents like
tetrahydrofuran, chloroform and chlorobenzene, at room
temperature. At the same time, the molecular weight is
very low which may affect the cell performance as
higher molecular weight polymer usually has a better
device performance.
The second absorption band with λs,max at 695 and 674
nm respectively, can be attributed to a localized transi-
tion between donor-acceptor charge transfer states.[47]
Compared to the solution sample, both polymer films
showed narrowed absorption at the second absorption
band. This phenomenon could be the result of the en-
hanced inter-chain interaction in the solid films, which
is most likely related to the increased π-π stacking of the
backbones in the film.[48,49]
Table 1 Molecular weights and thermal properties of the poly-
mers
PDIa Td /℃
b
a
a
Mp /(kg•mol−1) Mw /(kg•mol−1)
Polymer
PBDSe-DPP
PBDSe-ID
8.6
14.3
23.5
1.79
2.40
389
398
10.3
a Determined by GPC using polystyrene standards and THF as
eluent. b 5% weight loss temperatures measured by TGA under
nitrogen.
The thermal stability of the two polymers was ex-
amined by TGA and their results are listed in Table 1.
As shown in Figure 1, both polymers show good ther-
mal stability with onset decomposition temperature (Td)
corresponding to 5% weight loss at 389 ℃ and 398 ℃
respectively, which is adequate for normal applications
in PSCs.
Figure 2 Absorption spectra of polymers in chloroform solution
and thin films.
The cyclic voltammetry measurement was used to
study the electrochemical behavior of the polymers. The
corresponding cyclic voltammetry curves and data of
PBDSe-DPP and PBDSe-ID films are shown in Figure
S1 (see the Supporting Information) and summarized in
Table 2. The onset potential of oxidation (Eox) is ob-
served to be 1.02 and 1.20 eV for PBDSe-DPP and
PBDSe-ID, respectively. The HOMO energy levels of
PBDSe-DPP and PBDSe-ID were calculated to be −5.15
eV and −5.31 eV, respectively (EHOMO
＝
ox
−( E
−0.69)−4.8 eV), which suggests a relatively
onset
ideal VOC. These deep HOMO energy levels are due to
the introduction of the acceptor unit in the backbone.
PBDSe-ID shows
a
deeper HOMO level than
PBDSe-DPP, predicting a higher VOC of PSCs. The
LUMO energy levels of PBDSe-DPP and PBDSe-ID
Figure 1 TGA curves of copolymers at scan rate of 10 ℃/min
under N2 atmosphere.
were measured to be −3.59 eV and −3.61 eV (ELUMO
＝
−( Eorendset −0.69)−4.8 eV) determined by the redox onset
(−0.48 and −0.5 eV). Both can provide sufficient driving
force (＞0.3 eV) for the exciton dissociation compared
to the LUMO energy level (−4.0 eV) of PC61BM. These
results indicate that the HOMO and LUMO energy lev-
els of our new polymers match well with fullerene de-
rivatives for photovoltaic application.
Photophysical and electrochemical properties
The photophysical properties of the polymers were
investigated by UV-Vis spectroscopy as shown in Fig-
ure 2, and the key parameters are summarized in Table
2. Both polymers absorb solar photons in a broad range
extending to the near infrared region. Especially the
PBDSe-DPP extends its absorption to the near infrared
range approaching 800 nm. Both polymers show two
distinct absorption bands in solution and film states. The
first absorption band is located in the violet region with
the absorption peak (λs,max) at 409 nm and 374 nm for
PBDSe-DPP and PBDSe-ID respectively, which can be
identified with a delocalized excitonic π-π* transition.
The energy level alignment diagram of the two
polymers along with the PC61BM, PEDOT:PSS and
electrodes is shown in Figure 3.
Theoretical calculations
In general, polymer coplanarity is considered im-
portant to influence the intermolecular packing, the de-
vice fill factor (FF) and the photovoltaic performance. A
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New Semiconducting Polymer: Synthesis, Characterization and Photovoltaics
Table 2 Optical and electrochemical properties of polymers
λmax /nm
Solution
λonset /nm
Solution
Polymer
Egopt a/eV
HOMOb/eV
LUMOb/eV
EgEC /eV
Film
695
674
Film
634
776
PBDSe-DPP
PBDSe-ID
683
668
622
731
1.46
1.68
−5.15
−5.31
−3.59
−3.61
1.56
1.70
a Egopt s were estimated from the onset of UV-vis spectra of films. The HOMO/LUMO(elec) energy level is calculated by equation
b
HOMO/LUMO＝−(Eox/Ered−0.70) −4.80 eV.
Table 3 Photovoltaic properties of different polymers blended with PC61BM (w/w＝1∶4), when additive DIO or DCB is used or not
Polymer
Voc/V
Jsc/(mA•cm−2)
FF/%
PCE/%
Rs/(Ω•cm2)
Rsh/(kΩ•cm2)
PBDSe-DPP
PBDSe-DPPa
PBDSe-DPPb
PBDSe-ID
0.70
0.70
0.69
0.75
0.74
0.73
5.65
7.11
9.25
2.84
5.27
6.23
66.8
65.7
58.3
67.8
41.6
55.6
2.64
3.27
3.72
1.44
1.62
2.53
98.7
89.8
51.4
364
86.2
217
22.4
16.3
11.2
30.4
6.71
13.0
PBDSe-IDa
PBDSe-IDb
a with additive 2% DIO. b with additive 10% DCB.
Figure 3 Energy diagram for HOMO-LUMO energy levels of
the two polymers.
Figure 4 Characterizations of planarity, key dihedral angles and
the HOMO/LUMO levels of the polymers by DFT calculations.
suitable band gap and finely tuned energy levels are
beneficial for the device short circuit current (JSC) and
VOC.[47,50] To get insight into the planarity of the mo-
lecular architecture and the energy levels, we performed
the density functional theory (DFT) calculations at the
B3LYP/6-31GD level on model compounds using the
Gaussian 09 program suite. Solvent effect was taken
into account by using a proper dielectric constant of 2.4.
To simplify the calculation, two repeating unit of each
polymer was subject to calculation, with alkyl chains
replaced by CH3 groups. The key dihedral angles be-
tween the donor and accepter unit of the backbone were
measured to compare the distortions. The two polymers
show good backbone planarity as demonstrated by DFT
calculations. As shown in Figure 4, the key dihedral
angels of D1, i.e. 1.38° for selenophene-thiophene in
PBDSe-DPP and 22.46° for selenophene-benzene in
PBDSe-ID are determined respectively. The smaller
dihedral angles of PBDSe-DPP imply that the planarity
is better performed. The theoretically calculated HOMO
level and band gap are −4.82 eV and 1.65 eV for
PBDSe-DPP and −5.18 eV and 1.54 eV for PBDSe-ID.
These calculations are consistent with the results ob-
tained from CV and optical measurements.
Photovoltaic property and film morphology
The solar cells employing PBDSe-DPP or PBDSe-
ID as the electron donor and PC61BM as the electron
acceptor were fabricated, with device configuration of
ITO/PEDOT:PSS/Polymer:PC61BM/LiF/Al. The current
density-voltage (J-V) curves of the solar cells are pre-
sented in Figure 5. The specific parameters of the pho-
tovoltaic performances are summarized in Table 3. The
best power conversion efficiencies (PCEs) were ob-
tained for both polymer-based devices when the ratio
1∶4 (w/w) of polymer:PC61BM was adopted. Although
the original solar cells present moderate PCEs, rela-
tively high VOCs (0.7 and 0.75 V) and FF (0.67 and 0.68)
have been accomplished for the two polymers, which is
consistent with the results and analysis of the cyclic
voltammetry measurement. PBDSe-DPP delivers higher
JSC than PBDSe-ID due to its broader absorption ex-
tending into the infrared regime. To improve the device
performance, additives were applied in the solvent sys-
tems. Significant improvement of the device perform-
ance is observed when additive dichlorobenzene (DCB)
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or 1,8-diiodooctane (DIO) is applied. Notably, the JSC
shows an enhancement of 64% (from 5.65 to 9.25
mA/cm2) when DCB is used in the PBDSe-DPP:
PC61BM system, with steady VOC and only slightly de-
creased FF, offering the highest PCE of 3.72% (Figure
5). Huge improvement is also observed when DIO is
used as the additive, which deliveries an improved PCE
of 3.27%. The more balanced parameter values of the
device with additive DCB provide the best performance
(Table 3). For the PBDSe-ID:PC61BM system, even
larger improvement (~120%) of JSC is observed with
additive DCB, while keeping VOC steady. However,
greatly decreased FFs are observed for devices treated
with additive DCB or DIO, offering moderate PCEs of
2.53% and 1.62%, respectively (Table 3). Relatively
high VOCs were obtained for both PBDSe-DPP and
PBDSe-ID based devices (Table 3), due to their deep
HOMO levels. Specifically, the PBDSe-ID provided a
slightly higher VOC than PBDSe-DPP, which could be
attributed to the deeper HOMO of PBDSe-ID. However,
moderate overall JSCs were obtained for both polymer
based devices, and consequently a moderate overall de-
vice performance compared with other ideal polymer
based devices. This could result from the fact that the
intermolecular arrangements and resulting energy land-
scapes at the donor-acceptor interface are unfavourable
for charge separation.[51] The external quantum effi-
ciency (EQE) spectra of the solar cells based on both
polymers are depicted in Figure 6. Both polymer-based
devices showed broad photon response extending into
the near infrared region, which is consistent with the
absorption spectra. And from the EQE spectra, rela-
tively low (~15% lower) integrating JSCs are obtained
for both polymer-based devices, compared to their
measured JSC values, which could result from the deg-
radation of the polymer/device in atmosphere in the
measurement.
Figure 6 EQE curves of the photovoltaic devices based on
PBDSe-DPP and PBDSe-ID with 10% DCB additive.
atomic force microscopy (AFM), as shown in Figure 7
and Figure S2. The root mean square (RMS) roughness
of 1.52 nm was determined for the PBDSe-DPP:
PC61BM film. Smoother surface morphologies with
RMS 1.26 and 1.0 nm were observed when additives
DIO and DCB were used, respectively, which is consis-
tent with the corresponding results of the device per-
formance. Again obvious improvement of the film
morphology was observed when additives were used in
the PBDSe-ID:PC61BM blend films (Figure 7d, 7e and
7f). Much smaller domain size and lower RMS were
achieved when additives were applied. To sum up,
overall better surface morphology and smaller domain
size are obtained for the PBDSe-DPP:PC61BM blend
compared to the PBDSe-ID:PC61BM blend, and both
blend systems can be effectively improved with additive
DIO/DCB, which could have contributed to the device
performances. The overall better film morphology of the
PBDSe-DPP:PC61BM blend could partially account for
its better device performance, compared to the
PBDSe-ID:PC61BM device. As more direct information
to evaluate the blend quality, the series resistance (Rs)
and shunt resistance (Rsh) of the devices are summarized
in Table 3. For the PBDSe-DPP system, both Rs and Rsh
decrease when additive DIO or DCB is used, which
could mainly account for the increased JSC and de-
creased FF respectively, and are consistent with the de-
vice performances. For the PBDSe-ID system, de-
creased Rs and Rsh are observed as well for both additive
systems. However less enhancement of PCE is obtained
compared to the PBDSe-DPP system, due to the lower
JSC and drastically decreased FF which could mainly be
caused by the much faster dropped Rsh.
In addition, GIXD was used to study the molecular
arrangement in blend film of the two polymers. How-
ever, disordered packing and random orientation nature
in blend film can be concluded from the observed weak
diffractions for both polymers (Figure S3), which is
consistent with the observed relatively low JSCs. And it
could result in the relatively moderate charge carrier
mobility of 1.5×10−4 cm2•V−1•s−1 and 1.3×10−4
cm2•V−1•s−1 for PBDSe-DPP and PBDSe-ID respec-
Figure 5 J-V curves of polymer:PC61BM-based devices, with
and without 10% DCB additive, processed with chloroform.
To further understand the varying performance of
the devices, the surface morphology of the PBDSe-DPP:
PC61BM and PBDSe-ID:PC61BM blend films (with and
without additive DCB/DIO) were investigated by
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Chin. J. Chem. 2015, 33, 909—916

New Semiconducting Polymer: Synthesis, Characterization and Photovoltaics
tively, determined by fitting the slope of the J1/2-V
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To summarize, based on the side-chain free donor
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desired band-gaps and suitable HOMO/LUMO levels.
Relatively high VOC (0.7 and 0.75 V) and FF (＞65%)
were demonstrated for both polymer based solar cells,
and decent PCEs of 3.72% and 2.53% were achieved by
utilizing additive with only simple optimizing consid-
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This work was supported by the National High
Technology Research and Development Program of
China (863 Program) (Grant No. 2011AA050520), the
National Natural Science Foundation of China (Grant
no. 61176054), the China Postdoctoral Science Founda-
tion (Grant No. 2014M550302 and 1302015A), the
Project Sponsored by the Scientific Research Founda-
tion for the Returned Overseas Chinese Scholars (State
Education Ministry), and the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institu-
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(Pan, B.; Fan, Y.)
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Chin. J. Chem. 2015, 33, 909—916