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Conjugated random copolymers of benzodithiophene–
benzooxadiazole–diketopyrrolopyrrole with full visible
light absorption for bulk heterojunction solar cells
Jian-Ming Jiang, Hsiu-Cheng Chen, His-Kuei Lin, Chia-Ming Yu, Shang-Che Lan,
Chin-Ming Liu and Kung-Hwa Wei*
We have used Stille coupling polymerization to synthesize a series of new donor–acceptor (D–A)
conjugated random copolymers—PBDTT-BO-DPP—that comprise electron-rich alkylthienyl-substituted
benzodithiophene (BDTT) units in conjugation with electron-deficient 2,1,3-benzooxadiazole (BO) and
diketopyrrolopyrrole (DPP) moieties that have complementary light absorption behavior. These polymers
exhibited excellent thermal stability with thermal degrading temperatures higher than 340 C. Each of
these copolymers exhibited (i) broad visible light absorption from 400 to 900 nm and (ii) a low optical
band gap that is smaller than 1.4 eV and a low-lying highest occupied molecular orbital that is deeper
than 5.22 eV. As a result, bulk heterojunction photovoltaic devices derived from these polymers and
fullerenes provided a high short-circuit current density that is larger than 12 mA cm
2
. In particular, a
photovoltaic device prepared from the PBDTT-BO-DPP (molar ratio, 1 : 0.5 : 0.5)/PC
71
BM (w/w, 1 : 2)
blend system with 1-chloronaphthalene (1 volume%) as an additive exhibited excellent photovoltaic
performance, with a value of V
oc
of 0.73 V, a high short-circuit current density of 17 mA cm
2
,afill
factor of 0.55, and a promising power conversion efficiency of 6.8%, indicating that complementary
light-absorption random polymer structures have great potential for increasing the photocurrent in bulk
heterojunction photovoltaic devices.
Introduction
Polymer solar cells (PSCs) have attracted considerable attention
as promising energy resources because they allow the produc-
tion of low-cost, light-weight, large-area, exible devices
through ink-jet printing and roll-to-roll solution processing.
1
Tremendous efforts have been made toward improving the
power conversion efficiencies (PCEs) of bulk heterojunction
(BHJ) devices that incorporate conjugated polymers and
fullerene derivatives as their electron-donating and -accepting
components, respectively.
2
The PCE of a solar cell device is the
product of its short-circuit current density (J
sc
), open-circuit
voltage (V
oc
), and ll factor (FF). In a device having a BHJ-
structured active layer, the open-circuit voltage is typically
linearly proportional to the difference in energy between the
highest occupied molecular orbital (HOMO) of the polymer and
the lowest unoccupied molecular orbital (LUMO) of the
fullerene; therefore, the value of V
oc
can be increased either by
elevating the LUMO energy level of the fullerene or by
decreasing the HOMO energy level of the polymer.
3
Increases in
FFs can occur mainly through improving active layer
morphologies for balanced charge transport and carrying out
extensive device optimization.
4,5
Whereas, the value of J
sc
is
determined by the amount of absorbed light that results from
the band gap of the polymer, the layer thickness and the
breadth of the absorption as well as the active layer morphology
that dictates the transport of electrons and holes to the cathode
and the anode, respectively. Because conjugated polymers
typically absorb in only a limited region of the solar spectrum,
many PSCs do not exhibit high PCEs. The fabrication of tandem
BHJ solar cells and the use of low-band gap polymers blended
with fullerenes as the active layer are two main approaches that
have been used to enhance the absorption of solar light.
Tandem solar cells usually comprise multiple single-BHJ cells
stacked in series, with each layer featuring a different absorp-
tion band; the resulting combined absorption covers a broader
region of the solar spectrum.
6
The fabrication of a tandem solar
cell, however, is more complicated than that of a single solar
cell because not only proper processing conditions must be
tailored to all cells that are in series but also additional inter-
facial layers between cells are usually required. For single cells,
the development of new broadly absorbing conjugated poly-
mers is a necessary approach toward achieving high-efficiency
PSCs. The donor–acceptor (D–A) approach, in which a perfectly
alternating pattern of covalently bound electron-rich and -poor
chemical units comprises the backbone, is frequently adopted
Department of Materials Science and Engineering, National Chiao Tung University,
1001 Ta Hsueh Road, Hsinchu, 30050, Taiwan, ROC. E-mail: khwei@mail.nctu.edu.
tw; Fax: +886-3-5724727; Tel: +886-3-5731771
Cite this: Polym. Chem., 2013, 4, 5321
Received 25th January 2013
Accepted 27th February 2013
DOI: 10.1039/c3py00132f
www.rsc.org/polymers
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to obtain conjugated polymers exhibiting low band gaps as a
result of the internal charge transfer between the D and A units.
7
Although tuning the band gap of a D–A conjugated polymer
while maintaining a low-lying HOMO for a high value of V
oc
can
be carried out by adopting a weak electron donor conjugated
with a strong electron acceptor, it has limited effect on broad-
ening the absorption of the solar spectrum by the D–A conju-
gated polymer. One approach toward broadening the
absorption of the solar spectrum involves the use of a random
D–A conjugated polymer exhibiting complementary light
absorption from different D units in conjugation with various A
units.
8
For this approach, one architecture is concerned with
using two D units for copolymerizing with one A unit.
8a–d
The
other architecture involves the copolymerization of two
different A units with one D unit to form random polymer
structures.
8e–h
The latter polymer architecture will result in
consistent values of V
oc
for the photovoltaic devices because its
HOMO and LUMO energy levels are largely localized on the D
and A units, respectively, from theoretical studies. These
random copolymers exhibit considerably broadened absorp-
tions and/or two distinct absorption peaks in the short and long
wavelength regions; accordingly, random D–A polymer struc-
tures with complementary light absorption units have great
potential for increasing the photocurrent in PSCs.
Among the various low-band gap conjugated polymers, D–A
conjugated polymers adopting diketopyrrolopyrrole (DPP),
which possesses a lactam structure, as a strong electron-
acceptor unit have emerged as interesting materials in PSC
applications;
9a,b
they possess planar and well-conjugated skel-
etons that give rise to strong p–pinteractions and result in
absorptions in the near-infrared (NIR) region, 600–900 nm, as
well as high carrier mobility.
9c–e
BHJ PSCs based on conjugated
polymers containing DPP units have been reported by several
research groups to exhibit PCEs of 4–6.5%.
9f–j
On the other
hand, benzooxadiazole (BO), which adopts a quinoid structure,
is also a strongly electron-accepting moiety that has been used
in conjugated polymers for PSCs exhibiting PCEs of up to
5–6%,
10,11c
with two strong absorption peaks near 400 and
600 nm, respectively.
Herein, we report the synthesis of new conjugated random
copolymers featuring diketopyrrolopyrrole (DPP) and benzooxa-
diazole (BO) as electron-acceptor units in conjugation with
electron-donating thienyl-substituted benzodithiophene
(BDTT)
11
units that exhibit full-range absorption of visible light
and, thereby, provide high-efficiency BHJ solar cells.
Experimental section
Materials and synthesis
2,6-Bis(trimethyltin)-4,8-bis(5-ethylhexyl-2-thienyl)benzo[1,2-b:4,
5-b0]dithiophene (M1),
11
4,7-bis(5-bromothien-2-yl)-5,6-bis(octy-
loxy)benzo[c][1,2,5]oxadiazole (M2),
10
and 3,6-bis(5-bromothien-
2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (M3)
9a
were prepared according to the reported procedures. [6,6]-
Phenyl-C
61
-butyric acid methyl ester (PC
61
BM) and [6,6]-phenyl-
C
71
-butyric acid methyl ester (PC
71
BM) were purchased from
Nano-C. All other reagents were used as received without further
purication, unless stated otherwise.
General procedure for the synthesis of PBDTT-BO-DPP
through Stille coupling
Alternating polymer PBDTT-BO-DPP (1 : 0.3 : 0.7), P1. A
solution of M1 (100 mg, 0.11 mmol), M2 (23.1 mg, 0.033 mmol),
M3 (52.7 mg, 0.077 mmol), and tri(o-tolyl)phosphine (2.6 mg,
8.0 mol%) in dry chlorobenzene (4 mL) was degassed for
15 min. Tris(dibenzylideneacetone)dipalladium (2.0 mg, 2.0
mol%) was added under N
2
and then the reaction mixture was
heated at 130 C for 48 h. Aer cooling to room temperature, the
solution was added dropwise into MeOH (100 mL). The crude
polymer was collected, dissolved in CHCl
3
, and reprecipitated
in MeOH. The solid was washed with MeOH, acetone, and
CHCl
3
in a Soxhlet apparatus. The CHCl
3
solution was
concentrated and then added dropwise into MeOH. The solids
were collected and dried under vacuum to give PBDTT-BO-DPP
(1 : 0.3 : 0.7), P1 (140 mg, 80%). Anal. calcd: C, 69.70; H, 7.14; N,
2.41. Found: C, 67.85; H, 7.02; N, 2.53%.
Alternating polymer PBDTT-BO-DPP (1 : 0.5 : 0.5), P2. Using
a procedure similar to that described above for P1, a mixture of
M1 (100 mg, 0.11 mmol), M2 (38.4 mg, 0.055 mmol), and M3
(37.5 mg, 0.055 mmol) in dry chlorobenzene (4 mL) was poly-
merized to give P2 (120 mg, 70%). Anal. calcd: C, 69.33; H, 7.18;
N, 2.53. Found: C, 67.98; H, 7.57; N, 2.97%.
Alternating polymer PBDTT-BO-DPP (1 : 0.7 : 0.3), P3. Using
a procedure similar to that described above for P1, a mixture of
M1 (100 mg, 143 mmol), M2 (53.7 mg, 0.077 mmol), and M3
(22.5 mg, 0.033 mmol) in dry chlorobenzene (4 mL) was poly-
merized to give P3 (122 mg, 70%). Anal. calcd: C, 69.18; H, 7.09;
N, 2.52. Found: C, 68.01; H, 7.39; N, 2.85%.
Measurements and Characterization
1
H NMR spectra were recorded using a Varian UNITY 300-MHz
spectrometer. Thermogravimetric analysis (TGA) was per-
formed using a TA Instruments Q500; the thermal stabilities of
the samples were determined under N
2
by measuring their
weight losses while heating at a rate of 20 C min
1
. Size
exclusion chromatography (SEC) was performed using a Waters
chromatography unit interfaced with a Waters 1515 differential
refractometer; polystyrene was the standard; the temperature of
the system was set at 45 C and CHCl
3
was the eluent. UV-Vis
spectra of dilute dichlorobenzene (DCB) solutions (1 10
5
M)
were recorded at approximately 25 C using a Hitachi U-4100
spectrophotometer. Solid lms for UV-Vis analysis were
obtained by spin-coating polymer solutions onto quartz
substrates. Cyclic voltammetry (CV) of the polymer lms was
performed using a BAS 100 electrochemical analyzer operated at
a scan rate of 50 mV s
1
; the solvent was anhydrous MeCN
containing 0.1 M tetrabutylammonium hexauorophosphate
(TBAPF
6
) as the supporting electrolyte. The potentials were
measured against a Ag/Ag
+
(0.01 M AgNO
3
) reference electrode;
ferrocene/ferrocenium ion (Fc/Fc
+
) was used as the internal
standard (0.09 V). The onset potentials were determined from
the intersection of two tangents drawn at the rising and
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background currents of the cyclic voltammogram. HOMO and
LUMO energy levels were estimated relative to the energy level
of the ferrocene reference (4.8 eV below vacuum level). Topo-
graphic and phase images of the polymer:fullerene lms
(surface area: 5 5mm
2
) were recorded using a Digital Nano-
scope III atomic force microscope operated in the tapping mode
under ambient conditions. The thicknesses of the active layers
in the devices were measured using a VeecoDektak 150 surface
proler.
Fabrication and Characterization of Photovoltaic Devices
Indium tin oxide (ITO)-coated glass substrates were cleaned
stepwise in detergent, water, acetone, and isopropyl alcohol
(ultrasonication; 20 min each) and then dried in an oven
for 1 h; subsequently, the substrates were treated with UV
ozone for 30 min prior to use. A thin layer (ca. 20 nm) of poly-
ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS,
Baytron P VP AI 4083) was spin-coated (5000 rpm) onto the ITO
substrates. Aer baking at 140 C for 20 min in air, the
substrates were transferred to a N
2
-lled glove box. The polymer
and PCBM were co-dissolved in DCB at various weight ratios,
but with a xed total concentration (40 mg mL
1
). The blend
solution was stirred continuously for 12 h at 90 C and then
ltered through a PTFE lter (0.2 mm); the photoactive layer was
obtained by spin coating the blend solution onto the ITO/
PEDOT:PSS surface at a rate between 600 and 2500 rpm for 60 s.
The thicknesses of the photoactive layers were approximately
80–105 nm. The devices were nished for measurement
aer thermal deposition of a 30 nm lm of Ca and then a
100 nm lm of Al as the cathode at a pressure of approximately
Scheme 1 Synthesis of the copolymers P1,P2, and P3.
Table 1 Molecular weights and thermal properties of the polymer
Polymer M
wa
M
na
PDI
a
T
db
P1 75.1k 20.3k 3.7 408
P2 87.7k 35.1k 2.5 357
P3 101.5k 37.6k 2.7 345
PBDTTBO 145.8k 52.1k 2.8 333
PBDTTDPP 57.3k 18.5k 3.1 452
a
Values of M
n
,M
w
, and PDI of the polymers were determined through
GPC (in CHCl
3
using polystyrene standards).
b
The 5% weight-loss
temperatures (C) in the air.
Fig. 1 TGA thermograms of the copolymers P1,P2, and P3, recorded at a
heating rate of 20 Cmin
1
under a N
2
atmosphere.
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110
6
mbar. The effective layer area of one cell was 0.1 cm
2
.
The current density–voltage (J–V) characteristics were measured
using a Keithley 2400 source-meter. The photocurrent was
measured under simulated AM 1.5 G illumination at 100 mW
cm
2
using a Xe lamp-based Newport 66902 150-W solar
simulator. A calibrated silicon photodiode with a KG-5 lter was
employed to check the illumination intensity. External
quantum efficiencies (EQEs) were measured using an SRF50
system (Optosolar, Germany). A calibrated mono-silicon diode
exhibiting a response at 300–1000 nm was used as a reference.
For hole mobility measurements, hole-only devices were
fabricated having the structure ITO/PEDOT:PSS/poly-
mer:PCBM/Au. The hole mobility was determined by tting the
dark J–Vcurve to the space-charge-limited current (SCLC)
model.
12
Results and discussion
Synthesis and characterization of the polymers
Scheme 1 outlines the general synthetic strategy that we used to
obtain the random polymers. To ensure good solubility of the
BO derivative M2, we positioned two octyloxy chains on the BO
ring. We synthesized M1,M2, and M3 using the reported
methods.
9a,10,11
Aer Stille coupling using Pd
2
dba
3
as the cata-
lyst in chlorobenzene at 130 C for 48 h, we obtained the poly-
mers P1,P2, and P3 in yields of 70–80%. We determined the
solubilities of these copolymers in various solvents at a
concentration of 5 mg mL
1
.PBDTTBO,P2, and P3 were
completely soluble in CHCl
3
, chlorobenzene, and DCB at room
temperature, whereas P1 and PBDTTDPP were soluble only aer
heating at 50 C. We determined the weight-average molecular
weights (M
w
) of these polymers (Table 1) through gel perme-
ation chromatography (GPC), against polystyrene standards,
with CHCl
3
as the eluent.
Thermal stability
We used TGA to determine the thermal stabilities of the poly-
mers (Fig. 1). In air, the 5% weight-loss temperatures (T
d
)ofP1,
Fig. 2 UV-Vis absorption spectra of the polymers PBDTTBO,PBDTTDPP,P1,P2,
and P3 as (a) dilute solutions (1 10
5
M) in DCB and (b) solid films.
Table 2 Optical properties of the polymers
l
max,abs
(nm) FWHM (nm)
l
onset
(nm)
E
opt
g
(eV)Solution Film Solution Film Film
P1 752 680, 740 184 232 950 1.31
P2 677, 736 664, 737 236 264 920 1.34
P3 640, 722 630, 722 244 258 845 1.46
PBDTTBO 590, 620 590, 630 150 160 695 1.78
PBDTTDPP 764 750 170 212 950 1.31
Fig. 3 Cyclic voltammograms of solid films of the copolymers P1,P2, and P3.
Table 3 Electrochemical properties of the copolymers P1,P2, and P3
E
ox
onseta
(V) E
red
onseta
(V) HOMO
b
(eV) LUMO
b
(eV) E
opt
g
(eV)
P1 0.42 1.27 5.22 3.53 1.69
P2 0.52 1.26 5.32 3.54 1.78
P3 0.56 1.27 5.36 3.53 1.83
a
The potentials were measured against a Ag/Ag
+
(0.01 M AgNO
3
)
reference electrode; ferrocene/ferrocenium ion (Fc/Fc
+
) was used as
the internal standard (0.09 V).
b
HOMO and LUMO energy levels
determined using the equations HOMO ¼(4.8 + E
ox
) eV and LUMO
¼(4.8 + E
red
) eV.
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P2, and P3 were 408, 357, and 345 C, respectively. Thus, they all
exhibited good thermal stability—an important characteristic
for device fabrication and application.
Optical properties
Fig. 2a and b display the absorption spectra of PBDTTBO,
PBDTTDPP, and P1–3in solution (DCB) and in the solid state,
respectively; Table 2 summarizes the optical data, including
the absorption peak wavelengths (l
max,abs
), absorption edge
wavelengths (l
edge,abs
), full widths at half maximum (FWHMs),
and optical band gaps (E
opt
g
). The absorption peaks of
PBDTTBO and PBDTTDPP were located at 590 and 760 nm,
respectively, whereas the lms of the copolymers P1–3exhibi-
ted double absorption peaks in the range 300–1000 nm. The
relative position and intensity of the absorption peaks of the
copolymers P1–3were effectively tuned by their composition;
for example, the main absorption peaks for the PBDTT-BO-DPP
copolymers having compositions of 1 : 0.3 : 0.7, 1 : 0.5 : 0.5,
and 1 : 0.7 : 0.3 were 680/740, 642/737, and 612/721 nm,
respectively; that is, they shied to a shorter wavelength upon
increasing the content of BO units. The absorption spectra of
the lms of each of the copolymers P1–3featured two
absorption bands: one at 300–500 nm, which we assigned to
localized p–p*transitions, and the other, broader band in the
long wavelength region, from 500 to 950 nm, corresponding to
the intramolecular charge transfer (ICT) between the acceptor
BO or DPP units and the donor BDTT units. The absorption
spectra of the three polymers in the solid state were similar to
their corresponding solution spectra, with slight red-shis(ca.
10–40 nm) of their absorption onsets, indicating that some
intermolecular interactions existed in the solid state. The
absorption peak near 600 nm underwent a relative red-shito
750 nm upon increasing the content of DPP units; the
absorption spectra of the polymers were readily tuned by
varying the molar ratio of BO units and DPP units. The
FWHMs of these random PBDTT-BO-DPP copolymers having
compositions of 1 : 0.3 : 0.7, 1 : 0.5 : 0.5, and 1 : 0.7 : 0.3 were
232, 264, and 258 nm, respectively, approximately 60–100 nm
broader than those of PBDTTBO and PBDTTDPP, implying that
the random copolymers would absorb more of the solar
spectrum.
The absorption edges for P1–3(Table 2) correspond to
optical band gaps (E
opt
g
) of 1.31, 1.34 and 1.46 eV, respectively.
Fig. 4 Dark J–Vcurves for the hole-dominated carrier devices incorporating the
polymers blend with PC
71
BM [blend ratio, 1 : 2 (w/w)], and the P2/PC
71
BM blend
prepared in the presence of CN (1 vol%).
Fig. 5 J–Vcharacteristics of PSCs incorporating copolymer:PC
61
BM blends,
copolymer:PC
71
BM blends, and the P2:PC
71
BM blend prepared in the presence of
CN (1 vol%); each blend ratio, 1 : 2 (w/w).
Table 4 Photovoltaic properties of PSCs incorporating PBDTT-BO-DPP copolymers
Polymer/PC
61
BM
(w/w; 1 : 2) V
oc
(V) J
sc
(mA cm
2
) FF (%) PCE (%) Thickness (nm)
P1 0.72 10.8 55 4.3 82
P2 0.74 13.8 50 5.2 95
P3 0.76 10.7 56 4.6 89
Polymer/PC
71
BM (w/w; 1 : 2) V
oc
(V) J
sc
(mA cm
2
) FF (%) PCE (%) Thickness (nm) Mobility (cm
2
V
1
s
1
)
P1 0.70 12.5 62 5.5 85 2.3 10
3
P2 0.72 14.7 56 6.0 88 3.3 10
3
P3 0.75 12 59 5.3 93 3.0 10
3
P2/PC
71
BM (w/w; 1 : 2) (CN, vol%)
P2(0.5) 0.72 15.2 57 6.2 93
P2(1.0) 0.73 17 55 6.8 91 5.1 10
3
P2(1.5) 0.73 15.8 56 6.3 89
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Electrochemical properties
We used CV to examine the electrochemical properties,
including HOMO and LUMO energy levels, of the random BDTT-
based copolymers. Fig. 3 displays the electrochemical properties
of the polymers as solid lms; Table 3 summarizes the relevant
data. Partially reversible n-doping/de-doping processes occurred
for these random polymers in the negative potential range; in
addition, reversible p-doping/de-doping processes occurred in
the positive potential range. The onset oxidation potentials
(E
ox
onset
,vs. Ag/Ag
+
) for the copolymers P1–3were 0.42, 0.52, and
0.56 V, respectively; in the reductive potential region, the onset
reduction potentials (E
red
onset
) were 1.27, 1.26, and 1.27 V,
respectively. On the basis of these onset potentials, we estimated
the HOMO and LUMO energy levels according to the energy level
of the ferrocene reference (4.8 eV below the vacuum level).
13
The
HOMO energy levels of the PBDTT-BO-DPP copolymers with
compositions of 1 : 0.3 : 0.7, 1 : 0.5 : 0.5, and 1 : 0.7 : 0.3 were
5.22, 5.32, and 5.36 eV, respectively, implying that they
varied with respect to the modulated ICT strengths resulting
from the presence of electron-acceptor units with various elec-
tron-withdrawing abilities.
14
The LUMO energy levels of the
copolymers P1–3were all located within a reasonable range
(from 3.53 to 3.54 eV, Fig. 4) and were signicantly greater
than that of PCBM (ca. 4.1 eV); thus, we would expect efficient
charge transfer/dissociation to occur in their corresponding
devices.
15
In addition, the electrochemical band gaps (E
ec
g
) of the
copolymers P1–3, estimated from the differences between the
onset potentials for oxidation and reduction, were in the range
1.69–1.83 eV; that is, they were slightly larger than the corre-
sponding optical band gaps. This discrepancy between the
electrochemical and optical band gaps presumably resulted
from the exciton binding energies of the polymers and/or the
interface barriers for charge injection.
16
Hole mobility
Fig. 4 displays the hole mobilities of the devices incorporating
the polymer/PC
71
BM blends at a blend ratio of 1 : 2 (w/w). The
hole mobilities of the copolymers P1–3blend with PC
71
BM were
2.3 10
3
, 3.3 10
3
, and 3.0 10
3
cm
2
V
1
s
1
, respectively.
When we added a small amount of 1-chloronaphthalene (CN;
1%, by volume relative to DCB) to optimize the miscibility of the
PBDTT-BO-DPP (1 : 0.5 : 0.5)/PC
71
BM blend, the hole mobility
increased to 5.1 10
3
cm
2
V
1
s
1
.
Photovoltaic properties
Next, we investigated the photovoltaic properties of the poly-
mers in BHJ solar cells having the sandwich structure ITO/
PEDOT:PSS/polymer:fullerene (1 : 2, w/w)/Ca/Al, with the pho-
toactive layers having been spin-coated from DCB solutions of
the polymer and fullerene. The optimized weight ratio for the
polymer and fullerene was 1 : 2. In this case, we added a small
amount of CN (0.5–1.5%, by volume relative to DCB) to optimize
the miscibility of the blends. Fig. 5 presents the J–Vcurves of
these PSCs; Table 4 summarizes the data. The devices prepared
from polymer:PC
61
BM blends of the copolymers P1–3exhibited
open-circuit voltages (V
oc
) of 0.72, 0.74, and 0.76 V, respectively;
these values correspond to the difference between the HOMO
energy level of the polymer and the LUMO energy level of
PC
61
BM quite well.
17
We suspect that the PBDTT-BO-DPP
(1 : 0.3 : 0.7) device provided the lowest value of V
oc
because of
its relatively higher-lying HOMO energy level. The J
sc
of the
devices incorporating the copolymers P1–3were 10.8, 13.8, and
10.7 mA cm
2
, respectively. The devices prepared from poly-
mer:PC
71
BM blends of the copolymers P1–3exhibited V
oc
of
0.70, 0.72, and 0.75 V, respectively; their J
sc
were 12.5, 14.7, and
Fig. 6 EQE curves of PSCs incorporating copolymer:PC
71
BM blends and the
P2:PC
71
BM blend prepared in the presence of CN (1 vol%); each blend ratio, 1 : 2
(w/w).
Fig. 7 Topographic AFM images of copolymer:PC
71
BM (1 : 2, w/w) blends
incorporating (a) P1,(b)P2, (c) P3, and (d) P2 processed in the presence of CN
(1 vol%).
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12 mA cm
2
, respectively, providing a PCE of 6.0% for the device
incorporating PBDTT-BO-DPP (1 : 0.5 : 0.5).
Furthermore, we have used a CN additive with different
volume ratios in the solution, from 0.5% to 1.5% to process the
PBDTT-BO-DPP (1 : 0.5 : 0.5)/PC
71
BM active layer for the
devices. When the active layer was processed with 1 vol% CN,
the device incorporating the PBDTT-BO-DPP (1 : 0.5 : 0.5)/
PC
71
BM (1 : 2, w/w) active layer exhibited the highest value of J
sc
that represents an increase of 15% over that in the case without
the CN additive (17 vs. 14.7 mA cm
2
), resulting in an optimal
PCE of 6.8% (see Table 4). Fig. 6 displays EQE curves of the
devices incorporating the polymer:PC
71
BM blends at a weight
ratio of 1 : 2. These devices exhibited signicantly broad EQE
responses that extended from 300 to 950 nm. We attribute these
EQE responses in the visible region to the corresponding
absorbances of the active layers, resulting from both the
intrinsic absorptions of the polymers and the presence of
PC
71
BM, which also absorbs signicantly at 300–500 nm. The
device based on the blend of PBDTT-BO-DPP (1 : 0.5 : 0.5) and
PC
71
BM exhibited the highest EQE response among all of our
studied systems, with a maximum value of 72% at 450 nm,
consistent with its higher photocurrent. The calculated short-
circuit current densities obtained from integrating the EQE
curves of the devices incorporating the copolymer blends of P1–
3with PC
71
BM, and that of P2 processed with CN (1 vol%) as the
additive, were 12.1, 14.1, 11.6, and 16.5 mA cm
2
, values that
agree reasonably with the measured data (AM 1.5G; discrep-
ancy: <5%).
Moreover, when exploring the decisive factors affecting the
efficiencies of PSCs, we must consider not only the absorptions
and energy levels of the polymers but also the surface
morphologies of the polymer blends.
18
Fig. 7 displays the
surface morphologies of our systems, determined using AFM.
We prepared samples of the polymer/fullerene blends using
procedures identical to those employed to fabricate the active
layers of the devices. In each case, we observed quite smooth
surfaces for the fullerene blends of the copolymers P1–3, with
root-mean-square (rms) roughnesses ranging from 1.0 to 1.34
nm. The greater phase segregation and rougher surfaces of the
PBDTT-BO-DPP (1 : 0.3 : 0.7) blends presumably arose because
of poor miscibility with the fullerenes.
Conclusions
We have used Stille copolymerization to prepare a series of new
conjugated random copolymers PBDTT-BO-DPP that absorb the
full spectrum of visible light; they feature random alternating
BDTT units in conjugation with electron-decient BO and DPP
moieties, which have complementary light absorption behavior.
These polymers possess excellent thermal stability, low-lying
HOMO energy levels, and broad absorption bands that extend
from the visible to the NIR—desirable properties that make
these polymers promising materials for solar cell applications.
A device incorporating PBDTT-BO-DPP (1 : 0.5 : 0.5) and
PC
71
BM (blend weight ratio, 1 : 2), with CN (1 vol%) as an
additive exhibited a high value of J
sc
of 17 mA cm
2
and a PCE of
6.8%, indicating that complementary light-absorption random
polymer structures have great potential for increasing the
photocurrent in bulk heterojunction photovoltaic devices.
Acknowledgements
We thank the National Science Council, Taiwan, for nancial
support (NSC 101-3113-P-009-005).
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