Effect of side chain conjugation lengths on photovoltaic performance of two-dimensional conjugated copolymers that contain diketopyrrolopyrrole and thiophene with side chains

Article abstract of DOI:10.1002/pola.27767

Two new two-dimensional conjugated copolymers that contain diketopyrrolopyrrole and thiophene with different π conjugation lengths as side chains, called PDPPMTD and PDPPBTD, were designed and synthesized for use in polymer solar cells (PSCs). The resulti

Full text of DOI:10.1002/pola.27767

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Effect of Side Chain Conjugation Lengths on Photovoltaic Performance of
Two-Dimensional Conjugated Copolymers That Contain
Diketopyrrolopyrrole and Thiophene With Side Chains
You-Ren Liu,1 Li-Hsin Chan,2 Horng-Yi Tang1
1Department of Applied Chemistry, National Chi Nan University, Nantou, Taiwan, 54561, Republic of China
2Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou, Taiwan, 54561, Republic
of China
Correspondence to: L.-H. Chan (E-mail: lhchan@ncnu.edu.tw)
Received 30 March 2015; accepted 6 July 2015; published online 00 Month 2015
DOI: 10.1002/pola.27767
ABSTRACT: Two new two-dimensional conjugated copolymers
that contain diketopyrrolopyrrole and thiophene with different
p conjugation lengths as side chains, called PDPPMTD and
PDPPBTD, were designed and synthesized for use in polymer
solar cells (PSCs). The resulting copolymers in the thin film
state displayed broad absorption in the visible range with an
absorption edge at over 1000 nm, and both had relatively low-
lying HOMO levels, at 25.20 and 25.18 eV for PDPPMTD and
PDPPBTD, respectively. The power conversion efficiency (PCE)
of the PSC that was based on PDPPBTD/PC61BM (w/w 5 1:2)
reached 4.10 % with a Jsc of 14.5 mA/cm2, a Voc of 0.59 V and
an FF of 48%, while PDPPMTD/PC61BM (w/w 5 1:2) had a PCE
of 2.96% with a Jsc of 12.6 mA/cm2, a Voc of 0.60 V, and an FF
of 39%. These results indicate that subtle tuning of the chemi-
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cal structure can significantly influence Jsc and FF.
2015
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2015, 00, 000–000
KEYWORDS: diketopyrrolopyrrole; donor–acceptor copolymer;
polymer solar cell; thiophene with side chains; two-dimen-
sional conjugated copolymer
must have a broad absorption spectrum that matches the
solar spectrum as closely as possible, a deep highest occu-
pied molecular orbital (HOMO), excellent carrier mobility,
and compatible with the fullerene derivative. For most of the
semiconducting polymers that have been developed, in
which the push-pull effect is utilized for band gap engineer-
ing have become the most useful class of materials for use
in donor-acceptor (D-A) type BHJ PSCs because they gener-
ally have a narrow band gap and their energy levels [both
HOMO and lowest unoccupied molecular orbital (LUMO)]
can be easily tuned via appropriate molecular design. How-
ever, most polymers that can be used in PSCs have strong
absorption ability below 700 nm in the visible region. Few
examples exhibit good absorption to the major component of
the solar spectrum in the red or near infrared region.
Accordingly, a narrow bandgap material that can absorb light
in the longer wavelength region is sought to fabricate highly
efficient PSCs with less energy loss and enhanced photocur-
rents.15–18 Of wide range of narrow bandgap materials that
have been developed for BHJ PSCs, the diketopyrrolopyrrole
(DPP)-based polymers are of particular interest because of
their excellent optical and electrical characteristics.19–23 The
INTRODUCTION In the last few years, polymer solar cells
(PSCs) based on bulk heterojunction (BHJ) concept that con-
sisted of a blend with a donor polymer and a acceptor fuller-
ene derivative, [6,6]-phenyl C61 butyric acid methyl ester
(PC61BM) or [6,6]-phenyl C71 butyric acid methyl ester
(PC71BM), have attracted considerable interest because these
materials are flexible, lightweight, and potentially low-cost,
and so suitable for roll-to-roll production, and the cells in
which they are used represent a renewable alternative to
fossil energy.1–6 Substantial progress has been made in poly-
mer/fullerene hybrid systems that are based on the concept
of BHJ, and power conversion efficiencies (PCEs) over 8%
have been obtained.7–9 Much attention has been paid to put
these BHJ PSCs into large-scale manufacturing (such as roll-
to-roll production). Notably, only a few of polymers with a
high PCE achieve the requirements for roll-to-roll manufac-
turing.10–14 Therefore, there is still a plenty of room to
develop new polymers with a high PCE for commercializa-
tion. The development of materials with a high PCE is chal-
lenging for material chemists who must incorporate several
required characteristics into a specific molecular design and
synthetic sequences. To achieve a high efficiency, a polymer
Additional Supporting Information may be found in the online version of this article.
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electron-deficient character of DPP has been utilized to syn-
thesize large amounts of D-A type materials in which the
DPP moiety was directly copolymerized with aromatic
electron-rich units. The combination of DPP and unsubsti-
tuted thiophene units were particularly advantageous for
organic electronic applications. To date, BHJ PSCs that con-
tain DPP moiety have now exceeded the PCEs up to 8% and
mobilities of organic thin film transistors have been achieved
synthetic routes of (E)-4-(5-(2-(2,5-dibromothiophen-3-yl)vi-
nyl)thiophen-2-yl)-N,N-diphenylaniline (MTD), including the
intermediates and reactants, N,N-diphenyl-4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)aniline (BDA), 5-bromothiophene-
2-carbaldehyde (BTC), 5-(4-(diphenylamino)phenyl)thiophene-2-
carbaldehyde (DTC), and diethyl(2,5-dibromothiophen-3-yl)
methylphosphonate (BTP), were previously established and
these compounds were synthesized in our laboratory.40,41
[6,6]-Phenyl C61 butyric acid methyl ester (PC61BM), [6,6]-phe-
nyl C71 butyric acid methyl ester (PC71BM) and poly(3,4-ethyle-
nedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were
purchased from UR and H. C. Starck, respectively, and used as
received.
about 1.5 cm2 V21 s21 24–27
However, most of the above
.
mentioned DPP-based copolymers are linear D-A type mate-
rials and only limited number of studies have examined the
devices performance on DPP-based copolymers with two-
dimensional (2D) configurations.16,28,29
50-Bromo-2,20-Bithiophene-5-Carbaldehyde (BBC)
To an ice-cooled solution of 2,20-bithiophene-5-carbaldehyde
(0.78 g, 4 mmol) in DMF (10 mL), N-bromosuccinimide
(0.72 g, 4 mmol) was added portion-wise, and the mixture
thus formed was stirred overnight at room temperature. This
mixture was poured into iced water, extracted twice using
dichloromethane, dried over anhydrous MgSO4 and evapo-
rated to full dryness. Further purification was conducted on
a silica column using hexane:ethyl acetate (20:1) as the elu-
ent to yield BBC as a yellowish solid (0.78 g, 71%). 1H
NMR(CD2Cl2, 300 MHz, d/ppm): 9.83 (s, 1H), 7.65 (d, 1H, J
5 3.9 Hz), 7.20 (d, 1H, 3.9 Hz), 7.12 (d, 1H, J 5 4.2 Hz),
7.04 (d, 1H, J 5 3.9 Hz). 13C NMR (CDCl3, 75 MHz, d/ppm):
182.54, 145.83, 141.99, 137.42, 137.23, 131.20, 126.21,
124.38, 114.17.
Recently, researchers have developed a new class of 2D con-
jugated polymers with side chains or conjugated side groups
that broaden the absorption region of the polymers; do not
reduce the high hole mobility of the polymers, and provide a
relatively low-lying HOMO energy level. These polymers are
of interest for their high-performance when they use in
PSCs.30–36 Herein, two 2D conjugated D-A copolymers that
contained DPP and thiophene with different p conjugation
lengths as side chains, called PDPPMTD and PDPPBTD,
were synthesized via Suzuki cross-coupling polymerization,
exhibiting favorable light harvesting, charge transporting
properties, and photovoltaic properties. The DPP derivative
was employed as an electron-withdrawing unit while the thi-
ophene with side chain was used as an electron-donating
unit in construction of 2D D-A type copolymers. Therefore,
the resulting copolymers exhibit electron D-A architectures,
with small bandgaps and wide absorption bands. The D-A
strategy has been established as one of the commonly useful
strategy for preparing narrow bandgap polymers by taking
the advantage of the efficient intramolecular charge transfer
(ICT) from the electron-donating to the electron-withdrawing
moieties. In this work, the influence of the number of thio-
phenes on the side chains of the resulted copolymers on
their photophysical and electrochemical characteristics are
investigated. Additionally, the morphological and photovoltaic
properties of the polymer/fullerene blend films are studied
in detail. This work demonstrates that subtle tuning of the
chemical structure can considerably affect both Jsc and FF.
50-(4-(Diphenylamino)phenyl)-2,20-Bithiophene-5-
Carbaldehyde (DBC)
Under an atmosphere of nitrogen, a mixture of BDA (3.71 g,
10 mmol), BBC (2.73 g, 10 mmol), and Pd(PPh3)4 (231 mg,
0.2 mmol) was added into a two-neck flask. Next, a mixed
solution of 90 mL of degassed toluene and 30 mL of
degassed aqueous K2CO3 (2 M) were added into the above
mixture. The mixture was heated to 1208C and stirred vigo-
rously for 48 h. After the reaction mixture was cooled to
room temperature, and then it was extracted using ethyl ace-
tate and water, dried over anhydrous MgSO4 and evaporated
to full dryness. Further purification was conducted on a
silica column using EA:hexanes (1:10) as the eluent to yield
DBC as a yellow solid (3.32 g, 76%). 1H NMR(CDCl3, 300
MHz, d/ppm): 9.85 (s, 1H), 7.67 (d, 1H, J 5 3.9 Hz), 7.46 (d,
2H, J 5 8.7 Hz), 7.32–7.23 (m, 6H), 7.18–7.04 (m, 9H). 13C
NMR (CDCl3, 75 MHz, d/ppm): 182.48, 148.17, 147.59,
146.40, 141.37, 137.63, 134.18, 129.54, 127.38, 127.20,
126.74, 123.87, 123.59, 123.27.
EXPERIMENTAL
Chemicals
All chemicals, unless otherwise noted, were obtained from
commercial sources (including Alfa Aesar, Aldrich, and TCI
Chemical Co.), and they were utilized as received. Diethyl
ether, dichloromethane, ethyl acetate, chlorobenzene, N,N-
dimethylmethanamide (DMF), tetrahydrofuran (THF), o-
dichlorobenzene (o-DCB) and toluene, were freshly distilled
and dried before use. 2,5-Bis(2-ethylhexyl)-3,6-dithiophen-2-
yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione,37 2,5-bis(2-eth-
ylheyl)-3,6-bis[5-(4,4,5,5-tetramethyl-1,3,2-dioxoborolan-2-yl)
thiophene-2-yl]-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione
(DPPB),38 and 2,20-bithiophene-5-carbaldehyde39 were pre-
pared according to corresponding literature procedures. The
(E)-4-(50-(2-(2,5-Dibromothiophen-3-yl)vinyl)-2,20-
Bithiophen-5-yl)-N,N-Diphenylaniline (BTD)
In an atmosphere of nitrogen, CH3ONa (0.54 g, 10 mmol)
was added in portions to a solution of BTP (1.96 g, 5 mmol)
in 25 mL DMF in an ice bath and stirred for 20 min. Then a
solution of DBC (1.50 g, 3.42 mmol) in 30 mL DMF was
added. The reaction mixture was allowed to warm up slowly
to room temperature and then stirred for 5 h. The reacting
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system was poured into ice water, extracted with ethyl ace-
tate, washed with brine, dried over magnesium sulfate and
distilled under reduced pressure. Further purification was
conducted on a silica column (dichloromethane/hexanes,
1:10 as eluent) to afford BTD as a yellow solid (1.41 g,
61%). 1H NMR(CD2Cl2, 300 MHz, d/ppm): 7.57 (d, 2H, J 5
8.7 Hz), 7.42–7.37 (m, 5H), 7.27–7.14 (m, 11H), 7.11–7.05
(m, 2H), 6.86 (d, 1H, J 5 15.9 Hz).13C NMR (CDCl3, 75 MHz,
d/ppm): 147.61, 147.51, 143.65, 140.67, 138.89, 137.40,
135.67, 129.48, 127.94, 127.27, 126.51, 125.02, 124.75,
124.29, 123.91, 123.61, 123.36, 123.07, 119.41, 112.11,
109.99. Anal. Calcd for C32H21Br2NS3: C,56.90; H, 3.13; N,
2.07; S, 14.24. Found: C, 56.71; H, 2.85; N, 2.33; S, 14.23.
cation. Fibrous polymer PDPPMTD was obtained as a dark
green solid with isolated yields of 40% after drying under a
vacuum for 24h. Gel permeation chromatography (GPC) THF:
Weight-average molecular weight (Mw) 5 59.8 kg/mol and
polydispersity index (PDI, Mw/Mn) 5 1.75. 1H NMR (CD2Cl2,
300 MHz, d/ppm): 9.10–8.86 (br, 2H), 7.75–6.65 (m, 21H),
4.20–3.80 (br, 4H), 2.01–0.55 (m, 30H). Anal. Calcd for
C58H57N3O2S4: C,72.69; H, 6.21; N, 4.38; S, 13.38. Found: C,
72.00; H, 6.75; N, 4.35; S, 12.78.
Synthesis of PDPPBTD
PDPPBTD (dark green solid) was prepared using the same
procedures as described for PDPPMTD using BTD (676 mg,
1.0 mmol) and DPPB (777 mg, 1.0 mmol). Yield: 42%. GPC
(THF): Mw 5 57.6 kg/mol and PDI 5 1.55. 1H NMR (CDCl3,
300 MHz, d/ppm): 9.11–8.88 (br, 2H), 7.60–6.62 (m, 23H),
4.20–3.81 (br, 2H), 2.02–0.52 (m, 30H). Anal. Calcd for
2,5-Bis(2-Ethylheyl)-3,6-bis[5-(4,4,5,5-Tetramethyl-1,3,2-
Dioxoborolan-2-yl)thiophene-2-yl]-2,5-
Dihydropyrrolo[3,4-c]pyrrole-1,4-Dione (DPPB)
DPPB was synthesized according to the methods in the liter-
ature.38 Under an atmosphere of nitrogen, a solution of diiso-
propylamine (0.20 mL, 1.41 mmol) in 6 mL of dry THF was
put into a two-neck flask. Then, the n-Butyllithium in n-hex-
ane (0.50 mL, 1.25 mmol) was added slowly at 08C to the
flask. The freshly prepared LDA solution was added at
2258C within 20 min to 4 mL of a THF solution of 2,5-bis(2-
ethylhexyl)-3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyr-
role-1,4-dione (264 mg, 0.50 mmol) and 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxoborolane (280 mg, 1.50
mmol). After stirring at 08C for 1 h, the reaction was
quenched by adding 5 mL of 1 M HCl solution. Following
extraction with chloroform, the organic layer was dried over
magnesium sulfate and evaporated to full dryness. After that,
the residue was dissolved using 5 mL of dichloromethane,
and poured into a stirred acetone (300 mL) solution. The
obtained precipitate was then filtered off and washed with
acetone to form DPPB as a pink solid (0.27 g, 70%). 1H
NMR (CDCl3, 300 MHz, d/ppm): 8.92 (d, 2H, J 5 3.9 Hz),
7.70 (d, 2H, J 5 3.9 Hz), 4.03 (m, 4H), 1.82 (m, 2H), 1.36-
1.24 (m, 40H), 0.86 (m, 12H). 13C NMR (CDCl3, 75 MHz, d/
ppm): 161.90, 140.65, 137.82, 136.34, 135.78, 108.86, 84.76,
46.05, 39.17, 30.36, 28.83, 24.91, 23.74, 23.18, 14.21, 10.63.
Anal. Calcd for C42H62B2N2O6S2: C,64.95; H, 8.05; N, 3.61; S,
8.26. Found: C, 64.87; H, 8.16; N, 3.56; S, 8.06.
C62H59N3O2S5: C,71.57; H, 5.91; N, 4.04; S, 15.41. Found: C,
71.82; H, 6.29; N, 4.16; S, 14.43.
Characterization of Copolymers
General
A Varian Unity Inova 300WB NMR spectrometer was used to
record 1H and 13C NMR spectra. Elemental analyses were
conducted using an Elementar Vario EL III elemental ana-
lyzer. GPC (Waters GPC-1515 with Refractive Index Detector
2414) was performed with THF as the eluent and polysty-
rene as the standard to determine the molecular weights of
copolymers. Thermogravimetric analysis (TGA) measure-
ments were conducted on a Perkin-Elmer TGA-7 analyzer
with a scanning rate of 10 deg/min under an atmosphere of
nitrogen to determine the thermal decomposition tempera-
tures (Td) of copolymers. Hewlett-Packard 8453 and Hitachi
F-7000 spectrophotometers were used to obtain the UV–visi-
ble absorption and fluorescence spectra of the copolymers,
respectively. Cyclic voltammetry (CV) measurements were
performed by using an electro-chemical analyzer CHI 612D
(with a scanning rate of 50 mV s21) to determine the Redox
potentials of the polymers. The measurements were per-
formed in an anhydrous N2-saturated solution of 0.1 M
Bu4NClO4 in MeCN with a platinum (Pt) plate coated with a
thin polymer film as the working electrode, a Pt wire as the
counter electrode, and an Ag/Ag1 (0.10 M AgNO3 in MeCN)
as the reference electrode. The CV curves was calibrated
against Ferrecene/Ferrocene1. Atomic force microscope
(AFM) topographic images of the polymer–fullerene blend
films were conducted on a Seiko SII SPA400 in tapping
mode. Veeco Dektak 150 surface profiler was used to deter-
mine the thickness of the active layer of the device. Trans-
mission electron microscopy (TEM) images of the active
layers were performed using a JEOL JEM-1200EX II instru-
ment at an accelerating voltage of 120 kV. Powder X-ray dif-
fraction (XRD) measurements were performed in a Philips
X’Pert diffractometer equipped with an X’Celerator detector.
The sample was irradiated with a beam of monochromatic
Cu Ka having a wavelength of 0.154 nm. To determine the
hole and electron mobilities of the blend films, hole-only
(ITO/PEDOT:PSS/blend film/Au) and electron-only (ITO/
Synthesis of PDPPMTD
In an atmosphere of nitrogen, MTD (594 mg, 1.0 mmol) and
DPPB (777 mg, 1.0 mmol) were added into a two-neck flask.
Next, 30 mL of toluene and 4 mL of aqueous K3PO4 (1.6 M)
were added into the mixture. The reaction mixture was sub-
jected to three freeze-pump-thaw cycles degassing process in
order to remove O2. Next, Pd2(dba)3 (22.9 mg, 0.025 mol)
and [(t-Bu)3P]HBF4 (14.5 mg, 0.05 mol) were added to the
reaction mixture and heated at 1208C for 72 h. After the
reaction mixture was cooled to room temperature, the reac-
tion system was poured into a 400 mL solution of methanol
and deionized water (3:1). The precipitate was filtrated with
a filter, then dissolved in chloroform, and reprecipitated from
water first and then from methanol. Soxhlet extractions were
conducted using acetone and ethyl acetate for further purifi-
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blend film/LiF/Al) devices were fabricated. The carrier
mobility was estimated using the well-known Mott-Gurney
space-charge-limited-current (SCLC) equation,42,43
Table S1 and Fig. S1 for the solubility tests). This high solubil-
ity may arise from the slightly twisted conformation that is
caused by the introduction of bulky aromatic side chains,
which increase the steric hindrance of the polymer main
chains. Table 1 presents the number-average (Mn) and
weight-average (Mw) molecular weights of these polymers, as
determined by GPC with THF as the eluent and polystyrene as
the standard. The number-average molecular weights (Mn) of
PDPPMTD and PDPPBTD were found to be 59.8 and 57.6 kg
mol21 respectively, and the respective polydispersity indices
were 1.75 and 1.55.
9
V2
L3
J5 ee0l
8
where J denotes the current density; e is the relative permit-
tivity of a blend film; e0 is the permittivity of a vacuum; V is
the applied voltage, and L is the thickness of the blend film.
Fabrication and Characterization of PSCs
All PSCs were prepared using the following fabrication pro-
cedure.44,45 Glass substrates that had been coated with
indium tin oxide (ITO) (with patterned lithographically)
were cleaned using the following procedures: cleansed with
detergent, ultrasonicated in acetone and isopropyl alcohol,
dried on a hot plate at 1208C for 5 min, and treated with
oxygen plasma for 5 min. Then the hole-transporting mate-
rial PEDOT:poly(styrenesulfonate) (PEDOT:PSS, Clevios P-VP
AI4083) was spin-cast on top of ITO-coated glass at 4000 rpm
and dried on a hot plate at 1208C for 10 min. Subsequently,
the film-based photo-active layer which blended with fuller-
ene derivatives (both PC61BM and PC71BM) and copolymer at
various weight ratios (1:1 to 1:3, w/w) in o-DCB was spin-
cast on the top of the PEDOT:PSS layer at 600 rpm for 60s
and then annealed at 1408C for 10 min. Following deposition
of the Ca-based (30 nm)/Al (100 nm) cathode, the current
density-voltage (I-V) curves of the PSCs (device area was 4
mm2) were examined under illumination by 100 mW cm22
solar light from an AM 1.5G solar simulator (Oriel solar simu-
lator) with current and voltage sources programmable elec-
trometer (Keithley 2400). The calibration of illumination
Intensity was using a standard Si-photodiode detector that
was contained a KG-5 filter. To measure the incident photon-
to-electron conversion efficiency (IPCE), a xenon lamp con-
necting to a monochromator was used as the light source and
its output was monitored using a photodiode with a lock-in
amplifier. All electrical measurements were made in air.
The operational stability of an optoelectronic device is
directly related to the thermal stability of the conjugated
polymers within it. Therefore, a high Td is preferred for the
application of a conjugated polymer to PSCs.40 As indicated
in Table 1, PDPPMTD and PDPPBTD had Td values (at
which a 5% weight loss occurred) of 375 and 3948C, respec-
tively, under an atmosphere of nitrogen. These copolymers
clearly exhibited favorable thermal stability, which sufficed
for their application to use in PSCs. Powder XRD measure-
ments were carried out to evaluate the crystallinity of these
copolymers (See Fig. S2 in Supporting Information). The XRD
patterns of PDPPMTD and PDPPBTD are similar, which
implies that these two copolymers are amorphous. However,
a broad reflection with high intensity at 2h 5 24.78 (corre-
sponding to a p-p stack d spacing of 3.6 Å) were observed in
both XRD profiles, this results suggest that p-p stacking
interaction can be obtained in both copolymers even though
they are structurally disordered.29 Their ability to form the
aggregates is beneficial for allowing locally efficient ICT.
Optical Properties of the Conjugated Copolymers
Figure 1 displays the absorption spectra of PDPPMTD and
PDPPBTD in toluene solutions and solid films. Table 2 sum-
marizes the corresponding absorption data of these copoly-
mers. In solution, both copolymers exhibited broad
absorption with two obvious absorption peaks and an
absorption edge at over 1000 nm. The first absorption band
in the range 3002500 nm originated from the n-p* transi-
tion of the conjugated side chain and the p-p* transition of
the copolymer main chain. The second absorption band in
the range 500–1000 nm is attributed to the ICT interactions
between the donors (thiophene derivatives units) and
acceptors (DPP units) along the main chain.46,47 The absorp-
tion peaks of these copolymers in thin film state are red-
shifted from those of the same copolymers in solution and
the absorption band is remarkably broadened, as revealed
by the full width at half-maximum. This phenomenon arises
from the p-p stacking interaction between the polymer
chains. PDPPBTD exhibits a broad absorption and slight
red-shift in the first absorption band at short wavelengths
both in solution and in the thin film state, as compared to
PDPPMTD—an effect that is attributed to the longer effec-
tive conjugation in the side chain and the reduction of the
twist angle between the conjugated side chain and the poly-
mer backbone. This finding implies the presence of an addi-
tional thiophene moiety in the side chain that increases the
RESULTS AND DISCUSSION
Characterization of Copolymers
Scheme 1 describes the synthesis of monomers that are used
herein and the designed copolymers. The synthetic routes of
MTD, including the intermediates and reactants were synthe-
sized as in our previous report.40,41 Another monomer, BTD,
was obtained from a Wittig-Horner-Emmons reaction of BTP
with DBC, which had been easily prepared by reacting BDA
with BBC in the presence of Pd(PPh3)4 as a catalyst in a
Suzuki cross-coupling reaction. PDPPMTD was also synthe-
sized by the Suzuki cross-coupling polymerization of MTD
and DPPB, but the catalyst was Pd2(dba)3 to increase the
degree of polymerization. PDPPBTD was synthesized using
the same method, by coupling BTD and DPPB. The copoly-
mers thus obtained were purified by repeated precipitation
and Soxhlet extractions. The resulting copolymers were solu-
ble in some organic solvents such as THF, chloroform, tolu-
ene, chlorobenzene and o-DCB (See Supporting Information
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SCHEME 1 Synthetic routes for monomers and copolymers PDPPMTD and PDPPBTD.
conjugation length as well as the absorption range. If this
interpretation is correct, it suggests that increasing the conju-
gated length in the side chain results in the capture of sunlight
at shorter wavelengths and contributes to the performance of
the resultant PSC devices. Generally, conjugated polymers with
relatively broad absorption are favorable for harvesting solar
light. This broad absorption characteristic is expected to
improve the absorption efficiency of the active layer and,
thereby, increase the induced photocurrent in PSCs.40
from the CV curves of the copolymers. The potentials were
performed in a 0.1 M electrolyte of tetrabutylammonium hex-
afluorophosphate in acetonitrile using a scan rate of 100 mV
TABLE 1 Molecular Weights and Thermal Properties
of Copolymers
a
a
b
Mn
Mw
PDI
Td
Copolymers
(kg/mol)
(kg/mol)
(Mw/Mn)
(8C)
PDPPMTD
PDPPBTD
59.8
57.6
104.8
107.1
1.75
1.55
394
375
Electrochemical Properties of Conjugated Copolymers
The electrochemical properties of all of the copolymers herein
were elucidated by CV and the HOMO and LUMO energy levels
of these conjugated copolymers were estimated. Figure 2
shows the oxidation and reduction behaviors that are evident
a
Mn, Mw and PDI of the polymers were determined by GPC using poly-
styrene standards in THF.
b
Temperature of 5% weight loss.
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FIGURE 1 UV–vis absorption spectra of copolymers in dilute tol-
uene solutions and as thin films. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 2 Cyclic voltammograms of copolymer films on plati-
num plates in acetonitrile solution of 0.1 mol L21 Bu4NClO4.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
S21, with an Ag/Ag1 reference electrode, a Pt plate coated
polymer as working electrode, and a Pt wire as counter elec-
trode. The HOMO values of PDPPMTD and PDPPBTD were
calculated from the potentials at the onset of oxidation, taking
the ferrocene as the reference level (4.8 eV) below the vac-
uum level, according to the following equation.
PDPPBTD was slightly raised by the addition of an electron-
donating thiophene moiety into the conjugated side chain
that was not present PDPPMTD. Table 2 presents the elec-
trochemical properties of the copolymers. All of the copoly-
mers exhibited favorable stability in air with a HOMO energy
level below the oxidation-in-air threshold around 25.2 eV,
and this relatively low HOMO level of the copolymers sup-
ports a high open-circuit voltage (Voc) for a PSC device in
which they are used.48–50
ox
on
ferrocene
on
HOMO 5 – eðE 2E
14:8ÞðeVÞ
Therefore, the HOMO levels of PDPPMTD and PDPPBTD
were 25.20 and 25.18 eV, respectively. Since the reduction
potentials of PDPPMTD and PDPPBTD were indeterminate,
the LUMO levels were estimated from the obtained HOMO
levels and the optical bandgap (Egopt) values were obtained
from the absorption edges of UV-visible spectra using the fol-
lowing equation,
The charge carrier mobility of the active layer in a PSC is
one of the essential factors that govern the performance of
the device, because it is directly related to exciton dissocia-
tion, charge transport, and recombination.16 To probe the
effects of the conjugated side chains on the charge transport
of the copolymers, the SCLC method was performed to mea-
sure the mobility of the copolymers by using hole-only
devices with an ITO/PEDOT:PSS/blend film/Au structure
and electron-only devices with an ITO/blend film/LiF/Al
structure, at an optimized copolymer:PC61BM blend ratio
under the conditions used to fabricate the solar cell devices
(See PV Properties of PSCs for a detailed discussion). As dis-
played in Figure 3, the hole mobilities (lSCLC,h) of PDPPMTD
and PDPPBTD are 1.56 3 1025 and 2.11 3 1025 cm2 V21
S21, respectively, while the electron mobilities (lSCLC,e) of
PDPPMTD and PDPPBTD are 2.08 3 1025 and 2.20 3
1025 cm2 V21 S21, respectively. The PDPPBTD blend film
exhibits higher carrier mobilities and a better charge balance
than does the PDPPMTD blend film. These results suggest
that the conjugated lengths of the side chains have an impor-
tant role in charge transport. This effect is attributed to the
increased orbital overlap and reduced torsion angles of
PDPPBTD relative to those of PDPPMTD. Thus the
PDPPBTD-based solar cell is expected to have high Jsc and
FF values owing to the balanced charge carrier transport in
the active layer.16 The effects of treatment with 1,8-diiodooc-
tane (DIO) on the charge-transport properties of the
LUMO5Egopt1HOMOðeVÞ
The Egopt values of these copolymers that were calculated
from the absorption edges of UV-visible spectra in the thin
film state were approximately 1.29 eV. Accordingly, the
LUMO levels of PDPPMTD and PDPPBTD were 23.91 and
23.89 eV, respectively. Although the optical bandgaps of
these copolymers were almost identical, the HOMO level of
TABLE 2 Photophysical and Electrochemical Properties of
Copolymers
In solutiona
In Filmb
(nm)
(nm)
HOMO/
LUMOc (eV) dEgopt (V)
abs
abs
Copolymers
k
k
max
max
PDPPMTD
PDPPBTD
397, 761
415, 756
401, 764 25.20/–3.91
420, 778 25.18/–3.89
1.29
1.29
a
Measured in toluene solution.
Casted from o-DCB solution.
b
c
The energy levels were calculated according to: HOMO
5 –
e(Eoonx 2E
1 4.8) (eV), LUMO 5 Egopt 1 HOMO (eV).
ferrocene
on
d
Optical band gap was estimated from the wavelength of the optical
edge of the copolymer film.
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TABLE 3 Carrier Mobilities of Copolymer/PC61BM Blend Films
Copolymer/PC61BM
lSCLC,h
(cm2V21S21
lSCLC,e
(cm2V21S21
)
(w/w 5 1:2)
)
PDPPMTD
PDPPBTD
PDPPBTDa
1.56 3 1025
2.08 3 1025
1.85 3 1025
2.11 3 1025
2.20 3 1025
2.41 3 1025
a
Processed with DIO (3 vol %).
that were based on the blends of copolymer/fullerenes
(PC61BM and PC71BM), measured under simulated AM 1.5 G
illumination (100 mW/cm2). Table 4 presents the detailed
photovoltaic characteristics of these devices, including Voc,
the short-circuit current density (Jsc), the fill factor (FF), and
PCE. The PDPPMTD-based devices exhibited a Voc in the
range of 0.56–0.60 V with various PC61BM contents in the
blends (devices i-iii), and the device with the weight ratio of
1:2 showed the highest Jsc, as shown in Figure 4 and Table
4. The variation in PCE among the three devices is ascribed
to the morphologies of the blend films, which are responsi-
ble for the variation in the Voc and Jsc values of the corre-
sponding devices (See active layer morphology investigation
for a detailed discussion).
Larger Voc and higher Jsc values resulted in a higher PCE
value among three PSCs, so the PDPPMTD/PC61BM-based
(w/w 5 1:2) PSC had the highest PCE of 2.96%, with a Jsc of
12.6 mA/cm2, a Voc of 0.60 V, and an FF of 39%. Solar cell
efficiency was further optimized by using a 1:2 blending
ratio for PDPPMTD with PC71BM as the acceptor. Replacing
PC61BM with PC71BM as an acceptor in the active layer nor-
mally increases the photocurrent, as the latter has higher
absorption in the visible region.45,51 When PC71BM was used
instead of PC61BM as the electron acceptor in the devices,
however, the PDPPMTD/PC71BM-based (w/w 5 1:2) devices
(device iv) exhibited a slightly lower Jsc value and a PCE of
2.16% was achieved. We suspect that the variation in Jsc
FIGURE 3 Current and voltage (I-V) curves of (a) hole- and (b)
electron-only devices that contain copolymer and PC61BM.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
PDPPBTD blend film were also examined. The hole and elec-
tron mobilities of the DIO-treated PDPPBTD blend film are
1.85 3 1025 and 2.41 3 1025 cm2 V21 S21, respectively.
The DIO-treated PDPPBTD blend film has much higher elec-
tron mobility than the untreated blend film, indicating that
DIO treatment improved the morphology of the blend film
(See active layer morphology investigation for a detailed dis-
cussion). Table 3 summarizes the carrier mobilities of these
blend films.
PV Properties of PSCs
PSCs were fabricated from copolymer as the donor and
PC61BM or PC71BM as the acceptor; they had a traditional
device structure of ITO/PEDOT:PSS/copolymer:PC61BM or
PC71BM/Ca/Al and were utilized to investigate the photovol-
taic properties of these two copolymers. At first, the active
layer was spin-cast using o-DCB as the solvent, and the con-
centration was 24 mg/mL. Copolymer/PC61BM weight ratios
of 1:1, 1:2, and 1:3, were used. The optimal copolymer/
PC61BM weight ratio for both PDPPMTD- and PDPPBTD-
based devices was found to be 1:2. Figure 4 plots typical
photocurrent density-voltage (I-V) characteristics of the PSCs
FIGURE 4 Current density-potential characteristics of illumi-
nated (AM 1.5 G, 100 mW/cm2) BHJ solar cells. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 4 Photovoltaic Performance of BHJ Solar Cells
No.
Active Layer(w/w ratio)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
i
PDPPMTD: PC61BM (1:1)
PDPPMTD: PC61BM (1:2)
PDPPMTD: PC61BM (1:3)
PDPPMTD: PC71BM (1:2)
PDPPBTD: PC61BM (1:2)
PDPPBTD: PC71BM (1:2)
PDPPBTD: PC61BM (1:2)a
PDPPBTD: PC61BM (1:2)b
0.56
0.60
0.58
0.60
0.59
0.58
0.59
0.60
7.5
39
39
38
33
48
47
48
44
1.64 [1.61]c
2.96 [2.85]
2.07 [1.95]
2.16 [2.05]
4.10 [3.97]
2.59 [2.38]
3.03 [2.91]
2.53 [2.30]
ii
12.6
9.4
iii
iv
v
10.9
14.5
9.5
vi
vii
viii
10.7
9.6
a
c
Processed with DIO (3 vol %).
The values in square brackets indicate the average values of PCEs.
b
Spin-cast from dichlorobenzene:chloroform (4:1 by volume).
values at the same blend ratio with different acceptors arose
mainly from the variation in the morphologies of the active
layers. The PDPPBTD:PC61BM blend ratio was optimized
and the best PCE of 4.10% was achieved with a Jsc of 14.5
mA/cm2, a Voc of 0.59 V, and an FF of 48%, with the optimal
blend weight ratio of 1:2 (device v). We attribute the fact
that PDPPBTD had the highest value of PCE to its extended
p-conjugated area in the side chain and the fact that its
absorption region between 3002500 nm was slightly higher
than that of PDPPMTD. In addition, the PCE declined from
4.10% to 2.59% when the acceptor was replaced by PC71BM
(device vi). A similar effect has been observed in the
PDPPMTD/PC71BM-based device, for which a slightly lower
Jsc was ascribed to the morphologies of the blended active
layers. Numerous groups have recently demonstrated that
device performance can be substantially enhanced by treat-
ment with some additive, such as DIO, which has been pro-
ven to change efficiently the interpenetrating nanoscale
morphology of a BHJ cell.16,52,53 To exploit this effect, 3 vol
% DIO (3 vol % relative to DCB) was added to improve the
morphology of the polymer blend herein, especially that
used for the best PDPPBTD/PC61BM-based (w/w 5 1:2)
device without DIO treatment. The DIO-treated PDPPBTD-
based device (device vii) exhibited a PCE of 3.03%, with a Jsc
of 10.7 mA/cm2, a Voc of 0.59 V, and an FF of 48%. These
Voc and FF values are almost the same as those of the
untreated device, but the Jsc value is lower. The charge
imbalance that is caused by the hole and electron mobilities
of the blend film after the DIO treatment is likely to affect
device performance. Furthermore, the co-solvent system has
been proved to improve the film morphology in a BHJ cell,
retarding the aggregation of the conjugated polymer and
PCBM, to prevent the formation of oversized polymer or
PCBM aggregates. The most commonly used co-solvents are
chloroform (CF) and o-DCB. When the low boiling point of
pure CF is applied to process the blend films, the morphol-
ogy of blend films that are formed is always poor for the
rapid drying caused by pure CF.16,54 However, most polymers
exhibit high solubility in pure CF solvent. Consequently,
incorporating some solvent with a low boiling point, such as
CF, as a co-solvent can improve the film morphology. To
exploit this merit, the co-solvent system (o-DCB:CF 5 4:1 by
volume) was utilized to improve the PCE of the best
PDPPBTD-based device herein. Also, as displayed in Figure
4 and Table 4, the device (device viii) that was cast from the
co-solvent system exhibited a PCE of 2.53%, with a Jsc of 9.6
mA/cm2, a Voc of 0.60 V, and an FF of 44%. These Jsc and FF
values were lower than those of the device that was cast
from pure o-DCB, as a result of the morphological changes
that were induced by processing with the co-solvent. The
fact that PDPPBTD/PC61BM-based devices have higher Jsc
and FF than PDPPMTD/PC61BM-based devices under the
same optimized conditions (device v and device ii), appa-
rently arises from the greater light-harvesting ability and
charge mobility of PDPPBTD. Notably, all of the PDPPBTD-
based PSCs have higher FF values than the PDPPMTD-based
PSCs. To confirm the improvement in Jsc, external quantum
efficiency (EQE) curves of the PSCs that were fabricated
under the same optimal conditions were measured and are
plotted in Figure 5. Two PSCs exhibited a broad response
range, from 300 to 1000 nm, revealing that these PSC
devices respond favorably to sunlight. Clearly, the
PDPPBTD/PC61BM-based PSC has a broader absorption
region and higher EQE value in sunlight throughout this
FIGURE 5 EQE curves of photovoltaic cells with (copoly-
mer:PC61BM 5 1:2) as active layer. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 6 AFM topographic images of (a) PDPPMTD/PC61BM (w/w 5 1:1), (b) PDPPMTD/PC61BM (w/w 5 1:2), (c) PDPPMTD/
PC61BM (w/w 5 1:3), (d) PDPPMTD/PC71BM (w/w 5 1:2), (e) PDPPBTD/PC61BM (w/w 5 1:2), (f) PDPPBTD/PC71BM (w/w 5 1:2), (g)
DIO-treated PDPPBTD/PC61BM (w/w 5 1:2), and (h) PDPPBTD/PC61BM (w/w 5 1:2) cast using co-solvent, in tapping mode. Area of
all images is 300 nm 3 300 nm. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
range than does the PDPPMTD/PC61BM-based PSC. These
results demonstrate that increasing the conjugated length of
the side chain considerably improved the photovoltaic prop-
erties of the polymer herein. Furthermore, the Jsc values that
were calculated from the EQE curves under standard AM
1.5G conditions match closely those obtained from the I-V
measurements. Notably, the PDPPBTD blend with PC61BM
displayed a slightly higher intensity of EQE than did the
PDPPMTD/PC61BM blends, which finding is consistent with
the UV-Vis spectra of the polymer films. The enhanced light
absorption is also consistent with the increased EQE, indicat-
ing its contribution to the increase in Jsc.16
had the lowest surface roughness of the blend films, indicat-
ing that the ideal morphology consists of a nanoscale inter-
penetrating network between donor and acceptor that
provides a large interface area for exciton dissociation and
continuous percolating paths for the transport of holes and
electrons to the corresponding electrodes.56 Clearly, the suit-
able morphology of the blend films is essential to obtain bet-
ter device performance of the PSCs. When the acceptor
PC61BM was replaced with PC71BM, as presented in Figure
6(d), the PDPPMTD/PC71BM (w/w 5 1:2) blend film
showed clearer grain aggregation and homogeneous disper-
sion (RMS roughness 5 0.65 nm) than did the PDPPMTD/
PC61BM blend film. Obviously, suitable domain sizes and
phase separation can optimize photovoltaic performance.
The PDPPBTD/PC61BM (w/w 5 1:2) blend film [Fig. 6(e)]
was observed to have a similar bicontinuous interpenetrating
network. The PDPPBTD/PC61BM (w/w 5 1:2) blend film
displayed lower RMS roughness than the PDPPMTD/
PC61BM (w/w 5 1:2) blend film (0.47 and 0.68 nm for
PDPPBTD and PDPPMTD, respectively.), suggesting that the
introduction of an additional thiophene unit into each side
chain improved compatibility with PC61BM. Notably, the Jsc
and FF values of PDPPBTD-based devices are higher not
only because of the film morphology but also because of the
charge mobility, as evidenced by SCLC measurements. Hence,
the introduction of more thiophene rings into the side chains
enhanced the compatibility with PC61BM as well as its light
harvesting ability and charge mobility. This fact may explain
why PDPPBTD outperformed PDPPMTD. However, clearer
grain aggregates were observed in the PDPPBTD-based
blend film when the acceptor PC61BM was replaced with
PC71BM, with an RMS roughness of 0.49 nm, as shown in
Active Layer Morphology Investigation
Optimal phase segregation and the formation of a bicontinu-
ous interpenetrating network between the polymer donor
and the PC61BM or PC71BM acceptor are essential to achiev-
ing high device performance, and strongly affect the charge
separation and exciton dissociation.16,55 Tapping-mode
atomic force microscopy (AFM) was utilized to gain further
insight into the morphology of the active layer. Figure 6
shows AFM images of blend films that are identical to those
that were used to fabricate the active layers in the devices
that are listed in Table 4. Thin films of PDPPMTD/PC61BM
blends with various weight ratios (w/w 5 1:121:3) were
observed to have rather smooth surfaces [root-mean-square
(RMS) roughness 5 0.2420.73 nm], implying that all blend
films exhibited satisfactory miscibility. Additionally, as dis-
played in Figure 6(a–c), the PDPPMTD/PC61BM blend film
with a weight ratio of 1:2 exhibited a bicontinuous inter-
penetrating network unlike the other blend films, although
the PDPPMTD/PC61BM blend film with a weight ratio of 1:3
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FIGURE 7 TEM images of (a) PDPPMTD/PC61BM (w/w 5 1:1), (b) PDPPMTD/PC61BM (w/w 5 1:2), (c) PDPPMTD/PC61BM (w/w 5
1:3), (d) PDPPMTD/PC71BM (w/w 5 1:2), (e) PDPPBTD/PC61BM (w/w 5 1:2), (f) PDPPBTD/PC71BM (w/w 5 1:2), (g) DIO-treated
PDPPBTD/PC61BM (w/w 5 1:2), and (h) PDPPBTD/PC61BM (w/w 5 1:2) cast using co-solvent.
Figure 6(f); therefore the PDPPBTD/PC71BM-based device
had the lower Jsc. The lower Jsc of the PDPPBTD/PC71BM-
based device can be explained by the large number of carrier
recombinations on account of the clear grain-aggregation in
the blend film. To elucidate the effect of DIO treatment, the
morphology of the film was also examined, as presented in
Figure 6(g). When a small amount of DIO (3 vol % relative
to DCB) was added in the processing of the PDPPBTD/
PC61BM (w/w 5 1:2) blends, the surface of the active layer
became very smooth, with an RMS roughness of 0.28 nm,
revealing improved aggregation of the polymer and PC61BM
in the active layer. However, treatment with DIO reduced Jsc.
We suspect that this reduction arose mainly from the fact
that DIO treatment increased the imbalance in charge carrier
transport in the active layer (Table 3), reducing the Jsc value.
The morphology of the PDPPBTD/PC61BM blend film that
was cast from CF and o-DCB solution (co-solvents) was also
studied using AFM, as shown in Figure 6(h). Extensive phase
separation and relatively high levels of surface roughness
(RMS roughness 5 2.41 nm) were observed following cast-
ing from the co-solvents, and these observations may explain
why the device that was cast from o-DCB solution outper-
formed that cast from the co-solvent. As reported above, the
fabrication of a PSC device can be further improved by using
DIO as an additive or by using a co-solvent during the proc-
essing of the active layer. In our work, we use these two
methods to optimize the device performance of PDPPBTD.
No further improvement was observed when these two
methods were used and the use of a single solvent without
an additive is favored to simplify the device fabrication
process.
Thin film of PDPPMTD/PC61BM with a weight ratio of 1:1
displayed a fine and homogeneous morphology, as shown in
Figure 7(a), which indicated good miscibility between
PDPPMTD and PC61BM. In addition, the PDPPMTD/PC61BM
(w/w 5 1:2) blend film exhibited an interconnected network
structure, as presented in Figure 7(b), suggesting that a
well-organized bicontinuous network was obtained in this
blend film. In contrast, the PDPPMTD/PC61BM blend film
with a weight ratio of 1:3 [Fig. 7(c)] showed much larger p-
n junction domains which are not favorable for efficient exci-
ton dissociation, leading to relatively low in Jsc.57 When the
acceptor PC61BM was replaced with PC71BM, as shown in
Figure 7(d), the PDPPMTD/PC71BM (w/w 5 1:2) blend film
displayed a few vividly aggregated PC71BM domains. The
morphology of PDPPBTD/PC61BM (w/w 5 1:2) blend film
[Fig. 7(e)] showed similar features to that of PDPPMTD/
PC61BM (w/w 5 1:2) blend film, thus we suggest that the
difference in devices performance apparently arose mainly
from the light harvesting ability and carrier mobility. Fur-
thermore,
a few ambiguously PC71BM aggregates were
observed in the PDPPBTD-based blend film when the
acceptor PC61BM was replaced with PC71BM [Fig. 7(f)]. Upon
addition of 3 vol % DIO in the processing of the PDPPBTD/
PC61BM (w/w 5 1:2) blend film, moderately homogeneous
morphology was obtained, as presented in Figure 7(g). For
the PDPPBTD/PC61BM (w/w 5 1:2) thin film cast from the
co-solvents [Fig. 7(h)], large diffused PC61BM domains with
spots were observed. Although the distinct fibrillar structure
for each of the blends was not observed in the TEM
images,58,59 the obtained TEM results that were correlated
well with the AFM images.
TEM was carried out to investigate the space image of the
separated morphology of the copolymer2fullerene blends.
Figure 7(a–h) display TEM images of the copolymer2fuller-
ene blend films that are identical to those that were used to
fabricate the active layers in the devices. The polymer and
fullerene domains appear as bright and dark regions, respec-
tively, owing to their different degrees of electron scatting.42
CONCLUSION
In summary, two new two-dimensional copolymers that con-
tain DPP and thiophene with different p conjugation lengths
as side chains, called PDPPMTD and PDPPBTD, were suc-
cessfully designed and synthesized by Suzuki cross-coupling
polymerization reactions. The synthesized copolymers
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12 F. C. Krebs, M. Jorgensen, Adv. Opt. Mater. 2014, 2, 465–
477.
exhibited broad absorptions, favorable thermal properties,
and molecular energy levels that make them promising
materials for use in PSCs. For PSC devices that are based on
PDPPMTD and PDPPBTD, PCEs in the range 1.6424.10% in
BHJ with PC61BM or PC71BM were achieved. More impor-
tantly, incorporating additional thiophene rings into the side
chains of PDPPBTD broadened the absorption spectrum and
increased the charge mobility over those for PDPPMTD. The
subtle tuning of the chemical structure of the side chains
appeared significantly to influence the Jsc and FF. The device
that was based on the PDPPBTD/PC61BM (w/w 5 1:2)
blend film had the best PCE of 4.10% with a Jsc of 14.5 mA/
cm2, a Voc of 0.59 V, and an FF of 48%. The highest PCE of
the device that was based on the PDPPMTD/PC61BM (w/w
5 1:2) blend film was 2.96%, with a Jsc of 12.6 mA/cm2, a
Voc of 0.60 V, and an FF of 39%. The increased Jsc and FF of
the PDPPBTD-based PSC apparently arose from the
enhanced light harvesting ability, carrier mobility and mor-
phology, as evidenced by EQE, SCLC, AFM and TEM measure-
ments. The findings herein provide significant insight into,
and a feasible method for the development of, a new genera-
tion of low-bandgap materials. The photovoltaic properties
of these 2D conjugated copolymers can be tuned by varying
their conjugated side chains.
ꢀ
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ACKNOWLEDGMENTS
The financial support of the National Science Council of Taiwan
(ROC) under contract no. NSC 102-2113-M-260-006, and the
Ministry of Science and Technology, Taiwan (ROC), under
contract no. MOST 103-2113-M-260-005 is gratefully
acknowledged.
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