Synthesis and characterization of pyrido[3,4-b]pyrazine-based low-bandgap copolymers for bulk heterojunction solar cells

Article abstract of DOI:10.1021/ma100522a

We have used Stille polycondensation to prepare a series of low-bandgap copolymers, P1-P4, by conjugating the electron-accepting pyrido[3,4-b]pyrazine (PP) moieties with the electron-rich benzo[1,2-b:3,4-b′]dithiophene (BDT) or cyclopentadithiophene (CPDT

Full text of DOI:10.1021/ma100522a

6270 Macromolecules 2010, 43, 6270–6277
DOI: 10.1021/ma100522a
Synthesis and Characterization of Pyrido[3,4-b]pyrazine-Based
Low-Bandgap Copolymers for Bulk Heterojunction Solar Cells
Mao-Chuan Yuan, Mao-Yuan Chiu, Chien-Ming Chiang, and Kung-Hwa Wei*
Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu,
Taiwan, ROC
Received March 8, 2010; Revised Manuscript Received June 16, 2010
ABSTRACT: We have used Stille polycondensation to prepare a series of low-bandgap copolymers,
P1-P4, by conjugating the electron-accepting pyrido[3,4-b]pyrazine (PP) moieties with the electron-rich
benzo[1,2-b:3,4-b0]dithiophene (BDT) or cyclopentadithiophene (CPDT) units. P1 and P3 are based on PP
and BDT units while P2 and P4 are based on PP and CPDT units. All of these polymers exhibited excellent
thermal stability and sufficient energy offsets for efficient charge transfer and dissociation, as determined
through thermogravimetric analyses and cyclic voltammetry, respectively. The bandgaps of the polymers
could be tuned in the range 1.46-1.60 eV by using the two different donors, which have different electron-
donating abilities. The three-component copolymers, P3 and P4, incorporating the thiophene and bithio-
phene segments, respectively, absorbed broadly, covering the solar spectrum from 350 to 800 nm. The
morphologies of the blends of P3 and P4 with [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) were more
homogeneous than those of P1 and P2; in addition, devices incorporating the P3 and P4 blends exhibited
superior performance. The best device performance resulted from an active layer containing the P4:PC70BM
blend; the short-circuit current was 10.85 mA cm-2 and the power conversion efficiency was 3.15%.
Introduction
These PP-containing polymers have displayed higher electron-
accepting abilities relative to that of quinoxaline-containing
In recent years, conjugated polymers possessing extended
delocalized π-electron systems have been studied extensively for
their application in bulk heterojunction (BHJ) solar cells, which
generally feature a polymeric donor and a fullerene-based accep-
tor.1-4 Regioregular polythiophene derivatives have been widely
investigated for BHJs because of their highly crystallizable state,
leading to good light harvesting in the visible spectrum and
excellent carrier mobility. Power conversion efficiencies (PCEs)
of up to 5% have been achieved when using poly(3-hexylthio-
phene)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM)
composites as the photoactive layer.5-8 Nevertheless, the effici-
ency of this system is difficult to improve because of the limited
absorption range of P3HT, which absorbs photons only at wave-
lengths less than 650 nm and merely ca. 22% of the absorbed
photons from sunlight.9 In attempts to harvest more photons and
further improve the efficiencies of polymer solar cells (PSCs),
several conjugated polymers featuring electron donor-acceptor
(D-A) units in main chain-conjugated configurations10-14 and
side chain-attached architectures15-19 have been developed
because of their tunable electronic properties, ambipolar charge
transport abilities, and enlarged spectral absorption ranges. Low-
bandgap D-A polymers have attracted considerable attention
because their absorption ranges are broader than that of P3HT;
for example, the presence of intramolecular charge transfer (ICT)
bands arising from push/pull interactions between the D and
A units can extend the absorption into the near-infrared (NIR)
region of the solar spectrum.20-26 The properties of D-A
polymers can be tuned by varying the structures of the D and
A units.
polymers because the former feature more electron-withdrawing
nitrogen atoms in the fused ring.27-29 The poly(pyridopyrazine-
phenylene) polymer has been used in NIR light-emitting diodes;30
another polymer, in which the PP moieties were conjugated with
thiophene/fluorene units, has been used in field-effect transistors,
with hole mobilities of up to 4.4 ꢀ 10-3 cm2 V-1 s-1 31
. To date,
only a few PSCs prepared from PP-containing polymers have
been investigated;32-35 theyhave exhibited moderate PCEs (up to
1.1%)34 because of their low molecular weights and/or poor
solubility.
In this study, we used Stille polycondensation to prepare four
PP-containing low-bandgap polymers of high molecular weight
and exhibiting good solubility. The presence of octyloxyphenyl
groups attached to the PP moieties increased the solubility and,
thereby, extended the conjugation length of the synthesized
polymers. Furthermore, because several low-bandgap polymers
incorporating benzo[1,2-b:3,4-b0]dithiophene (BDT)21,36,37 and
cyclopentadithiophene (CPDT)38-41 units in their backbones
exhibit high electron-donating ability and good performance
when applied in PSCs, we incorporated these two electron-rich
donors into our polymer backbones by conjugating them with
electron-withdrawing PP moieties. Using this approach allowed
us to fine-tune the absorption spectra and energy levels because
of the modulated ICT strength between the PP moieties and the
BDT/CPDT units, thereby leading to PSCs exhibiting varied
photovoltaic performance. Because our synthesized polymers, P1
and P2 absorb too little in the visible range, where the radiated
intensity of photons in the solar spectrum was quite high, due
to their low-bandgap characteristics; therefore, we synthesized
the three-component random copolymers P3 and P4, through
further incorporation of thiophene/bithiophene segments, to
extend the light-harvesting range. P3 and P4 displayed very
broad absorption ranges, covering the solar spectrum from the
Several D-A conjugated polymers based on electron-accepting
pyrido[3,4-b]pyrazine(PP) moieties have been described previously.
*Corresponding author. E-mail: khwei@mail.nctu.edu.tw.
pubs.acs.org/Macromolecules
Published on Web 07/12/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 15, 2010 6271
visible to a significant portion of the NIR region. Thus, we
expected the performance of the devices prepared from P3 and P4
to be significantly improved.
10 min prior to use. A thin layer (ca. 20 nm) of poly(ethylene-
dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron
P VP AI 4083) was spin-coated at 5000 rpm onto the ITO sub-
strates. After baking at 140 °C for 20 min in air, the substrates
were transferred into a N2-filled glovebox. The copolymers
P1-P4 were codissolved with PC70BM in DCB (weight ratio,
1:4; total concentration, 25 mg mL-1) and stirred continuously
for 12 h at 50 °C. The photoactive layer was obtained by spin-
coating the blend solution onto the ITO/PEDOT:PSS surface at
800 rpm for 60 s without further special treatment. The thick-
nesses of the photoactive layers were ca. 75-95 nm. Finally, an
Al layer (100 nm) was thermally evaporated through a shadow
mask under a vacuum of less than 1 ꢀ 10-6 Torr. The effective
layer area of one cell was 0.04 cm2. The current density-voltage
(J-V) characteristics were measured using a Keithley 236
source-meter. The photocurrent was measured under simulated
AM 1.5 G irradiation at 100 mW cm-2 using a Xe lamp-based
Newport 66902 150W solar simulator. The spectrum of the solar
simulator was calibrated using a PV-measurement (PVM-154)
monosilicon solar cell (NREL calibrated) to minimize spectral
mismatch; a silicon photodiode (Hamamatsu S1133) was em-
ployed to check the uniformity of the exposed area. External
quantum efficiency (EQE) was measured using a system estab-
lished by Optosolar, Inc. Monochromatic light was created from
a 500-W Xe lamp source passing through a monochromator.
The photocurrent of the device was detected using a lock-in
amplifier under short-circuit conditions by illuminating the
monochromatic incident beam. A calibrated mono silicon diode
exhibiting a response at 350-1000 nm was used as a reference.
5,8-Dibromo-2,3-bis(3-octyloxyphenyl)pyrido[3,4-b]pyrazine
(M1). A mixture of 1 (0.91 g, 1.95 mmol), 2 (0.52 g, 1.95 mmol),
and acetic acid (20 mL) was heated at 50 °C for 4 h under N2. The
mixture was poured into water (100 mL) and the resulting
precipitate collected by filtration. The crude product was puri-
fied through column chromatography (EtOAc/hexane, 1:10)
and then recrystallized twice from ethanol to afford M1 (0.91
g, 67%) as a yellow solid. 1H NMR (300 MHz, CDCl3): δ
0.87-0.91 (m, 6H), 1.30-1.44 (m, 20H), 1.69-1.78 (m, 4H),
3.83-3.89 (m, 4H), 6.95-6.99 (m, 2H), 7.15-7.28 (m, 6H), 8.76
(s, 1H). 13C NMR (75 MHz, CDCl3): δ 14.1, 22.7, 26.0, 29.0,
29.2, 29.3, 31.8, 68.1, 115.6, 115.7, 117.0, 117.3, 120.1, 122.4,
122.5, 129.5, 135.8, 138.3, 138.4, 142.4, 146.2, 147.2, 156.1,
158.2, 159.1, 159.2. MS (m/z): [M]þ calcd for C35H43Br2N3O2,
697.1; found, 697. Anal. Calcd for C35H43Br2N3O2: C, 60.27; H,
6.21; N, 6.02. Found: C, 60.32; H, 6.61; N, 6.05.
Experimental Section
Materials. 1-Bromo-3-octyloxybenzene (1),42 1,2-bis(3-octy-
loxyphenyl)ethane-1,2-dione (2),42 3,4-diamino-2,5-dibromo-
pyridine (3),27 benzo[1,2-b:4,5-b0]dithiophene-4,8-dione (4),21
1,5-bis(trimethylstannyl)-4,8-dioctylbenzo[1,2-b:4,5-b0]dithio-
phene (M2),21 4,4-bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-
4H-cyclopenta[2,1-b:3,4-b0]-dithiophene (M3),38 and 5,50-di-
bromo[2,20]-bithiophene (M5)44 were prepared according to
reported procedures. [6,6]-Phenyl-C70-butyric acid methyl ester
(PC70BM) was purchased from Nano-C. 2,5-Dibromothio-
phene (M4) and all other reagents were used as received without
further purification, unless stated otherwise.
Measurements and Characterization. 1H and 13C NMR spec-
tra were recorded using a Varian UNITY 300 MHz spectro-
meter. Mass spectra were obtained using a JEOL JMS-HX 110
spectrometer. Differential scanning calorimetry (DSC) was
performed using a Perkin-Elmer Pyris 1 unit operated at heat-
ing and cooling rates of 20 and 40 °C min-1, respectively; the
glass transition temperatures (Tg) were determined from the
second heating scan. Thermogravimetric analysis (TGA) was
undertaken using a TA Instruments Q500; the thermal stabilities
of the samples were determined under a N2 atmosphere 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
and THF was the eluant. UV-Vis spectra of dilute chloroform
solutions (1 ꢀ 10-5 M) were measured using a Hitachi U-4100
spectrophotometer. Solid films for UV-Vis analysis were ob-
tained by spin-coating dichlorobenzene (DCB) solutions of the
copolymers (10 mg mL-1) onto quartz substrates. Cyclic vol-
tammetry (CV) was performed using a BAS 100 electrochemical
analyzer operated at a scan rate of 50 mV s-1; the solvent was
anhydrous acetonitrile containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte.
The potentials were measured against a Ag/Agþ (0.01 M AgNO3)
reference electrode; ferrocene/ferrocenium ion (Fc/Fcþ) was
used as the internal standard. The onset potentials were deter-
mined from the intersection of two tangents drawn at the rising
and background currents of the cyclic voltammogram. Topo-
graphic images of the copolymer:PC70BM films (surface area:
5 ꢀ 5 μm2) were obtained through atomic force microscopy
(AFM) in the tapping mode under ambient conditions using
a Digital Nanoscope IIIa instrument. Transmission electron
microscopy (TEM) images of the copolymer:PC70BM films
were recorded using a FEI T12 TEM operating at 120 KeV.
The thickness of the active layer was measured using a Veeco
Dektak 150 surface profiler. Hole-only mobility measurements
were performed using the device structure of ITO/PEDOT:PSS/
P1-P4:PC70BM (1:4, w/w)/Au. The hole mobilities of the
blends were determined by fitting the dark J-V curve into
the space-charge-limited current (SCLC) model,17,43 based on
the equation
Alternating Copolymer P1. A mixture of M1 (100 mg, 0.143
mmol), M2 (111 mg, 0.143 mmol), and tri(o-tolyl)phosphine
(3.6 mg, 8.0 mol %) was dissolved in dry chlorobenzene (CB)
(3 mL) and degassed for 15 min. Tris(dibenzylideneacetone)-
dipalladium (2.6 mg, 2.0 mol %) was added under N2 and then
the reaction mixture was heated at 130 °C for 36 h. After cooling
to room temperature, the solution was added dropwise into
methanol (100 mL). The crude polymer was collected, dissolved
in chlorobenzene, and reprecipitated in methanol. The solid was
washed with methanol, acetone, and chloroform in a Soxhlet
apparatus. The chloroform solution was concentrated and then
added dropwise into methanol. Finally, the polymer was col-
1
lected and dried under vacuum to give P1 (113 mg, 80%). H
V2
h L3
NMR (300 MHz, CDCl3): δ 0.76-2.10 (m, 60H), 3.67-4.41 (m,
8H), 6.84-7.80 (m, 10H), 8.84 (br, 1H). Anal. Calcd: C, 74.42;
H, 8.29; N, 4.27. Found: C, 73.56; H, 8.02; N, 4.23.
9
J ¼ ε0εrμ
8
Here ε0 is the permittivity of free space, εr is the dielectric
constant of the material, μh is the hole mobility, V is the vol-
tage drop across the device, and L is the thickness of the active
layer.
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
Alternating Copolymer P2. Using a polymerization procedure
similar to that described above for P1, a mixture of M1 (120 mg,
172 mmol) and M3 (125 mg, 172 mmol) in dry CB (3 mL) was
copolymerized to give P2 (110 mg, 68%). 1H NMR (300 MHz,
CDCl3): δ 0.62-0.67 (m, 12H), 0.75-1.01 (m, 22H), 1.15-1.45
(m, 22H), 1.66-2.08 (m, 8H), 3.97 (br, 4H), 7.07 (br, 2H),
7.36-7.48 (m, 6H), 7.88 (s, 1H), 8.67 (s, 1H), 9.18 (s, 1H). Anal.
Calcd: C, 76.63; H, 8.68; N, 4.47. Found: C, 72.26; H, 8.08;
N, 4.11.

6272 Macromolecules, Vol. 43, No. 15, 2010
Scheme 1. Synthetic Routes of the Monomers
Yuan et al.
Scheme 2. Synthetic Routes of the Copolymers.a
Alternating Copolymer P3. Using a polymerization procedure
similar to that described above for P1, a mixture of M1 (60.0 mg,
0.086 mmol), M2 (133 mg, 0.172 mmol), and M4 (20.8 mg,
0.086 mmol) in dry CB (2.8 mL) was copolymerized to give P3
(106 mg, 82%). 1H NMR (300 MHz, CDCl3): δ 0.77-0.95
(m, 46H), 1.18-1.48 (m, 146H), 3.94-4.32 (m, 12H), 7.05-7.47
(m, 14H), 8.98 (br, 1H). Anal. Calcd: C, 72.32; H, 7.94; N, 2.78.
Found: C, 72.00; H, 7.88; N, 2.87.
Alternating Copolymer P4. Using a polymerization procedure
similar to that described above for P1, a mixture of M1 (72.1 mg,
0.103 mmol), M3 (151 mg, 0.206 mmol), and M5 (33.5 mg, 0.103
mmol) in dry CB (2.8 mL) was copolymerized to give P4
(108 mg, 70%). 1H NMR (300 MHz, CDCl3): δ 0.66-1.15
(m, 68H), 1.32-1.53 (m, 18H), 1.75-1.98 (m, 12H), 3.97-4.10
(m, 4H), 6.95-7.17 (m, 8H), 7.24-7.55 (m, 6H), 7.82 (br, 1H),
8.62 (s, 1H), 9.16 (s, 1H). Anal. Calcd: C, 74.20; H, 8.10; N, 2.79.
Found: C, 73.57; H, 7.97; N, 2.81.
Results and Discussion
Synthesis and Characterization of the Copolymers. As
indicated in Scheme 1, we prepared the symmetric 1,2-dike-
tone 2 presenting two 3-octyloxyphenyl groups through a
Grignard reaction between 3-octyloxyphenylmagnesium
bromide (1) and oxalyl chloride in the presence of CuBr
and LiBr. Condensation of 2 with the diamine 3 afforded the
monomer M1; the distannyl monomer M2 was prepared
through dilithiation of compound 4 with n-BuLi, followed by
quenching with trimethylstannyl chloride. With the mono-
mers M1-M5 inhand, we prepared the PP-based alternating/
random copolymers P1-P4 through Stille polycondensa-
tions using various feed molar ratios of the corresponding
monomers (Scheme 2). P1 and P3, which featured electron-
donating BDT units, had weight-average molecular weights
(Mw) of 398 and 563 kg mol-1, respectively, and polydisper-
sities of 2.65 and 2.54, respectively, as determined through
gel permeation chromatography (GPC) using polystyrene
standards. The donor moieties of the polymers P2 and P4
were highly coplanar fused CPDT units; their values of Mw
were 101 and 89 kg mol-1, respectively, with polydispersities
of 1.65 and 1.79, respectively. The relatively high molecu-
lar weights of P1 and P3, relative to those of P2 and P4,
presumably resulted from the high reactivity of the BDT
moieties in their polymerization reactions. The solubility
of the copolymers in various solvents was determined at a
concentration of 5 mg/mL. We found that P1 and P3 were
completely soluble in THF, CHCl3, CB, or DCB at room
temperature and became soluble in toluene at 60 °C; P2
and P4 were completely soluble in THF, CHCl3, CB, DCB,
or toluene at room temperature. We also synthesized
a Reagents: (i) Pd2dba3, tri(o-tolyl)phosphine, chlorobenzene.
nonsubstituted polymers through reacting the PP unit with-
out any alkyl chain with the BDT or CPDT units, and both of
them have insoluble precipitation in toluene, THF, CB or
DCB. We characterized the synthesized monomers and
polymers using 1H and 13C NMR spectroscopy, mass spec-
trometry, and elemental analysis. We estimated the copoly-
mers’ composition using the ratio of the integrated value of
the proton peak on the PP unit (-N-CH-) to that of the
proton peak on the thiophene/bithiophene units (thiophene-
H) from their NMR spectra; the output composition (m:n)
for P3 and P4 was 0.97:1 and 1:0.98, respectively, and they
are very close to the feed ratios.

Article
Macromolecules, Vol. 43, No. 15, 2010 6273
Figure 1. DSC traces of the copolymers P2 and P4, recorded at a
heating rate of 20 °C min-1 under a N2 atmosphere.
Figure 3. UV-vis absorption spectra of the copolymers P1-P4 (a) in
dilute chloroform solutions (1 ꢀ 10-5 M) (b) as solid films.
the PP acceptor moieties and the BDT/CPDT donor units.
The absorption maximum of P2 was red-shifted by ca. 108
nm, relative to that of P1, because of the stronger ICT effect
in P2 than that in P1, indicating that the fused CPDT unit
has stronger electron-donating ability than does the BDT
unit. The three-component copolymer P3 exhibited two
combined, but distinct, absorption bands of almost equal
intensity, with absorption maxima at 468 and 616 nm,
respectively. We attribute the higher-energy band to the
absorption of the BDT-thiophene segment, consistent with
reports of related polymers in the literature,36 and the lower-
energy band to ICT of the BDT-PP segment. The spectrum
of P3 exhibited an absorption maximum having a hypso-
chromic shift of ca. 50 nm relative to that of P1 because the
incorporation of the higher-energy BDT-thiophene segment
shifted the absorption to the shorter wavelength region. P4
exhibited similar optical properties as those of P3, with
absorption maxima at 505 and 676 nm that we attribute to
the CPDT-bithiophene and CPDT-PP segments, respec-
tively. The absorption spectra of P1-P4 in the solid state
were similar to their corresponding solution spectra, with
slight red shifts of ca. 6-18 nm of their absorption maxima,
indicating there some intermolecular interactions occurred
in their solid states. Figure 3 reveals that P3 and P4 absorbed
broadly in the region 350-800 nm, extending from the visible
to a portion of the NIR region of the solar spectrum; such an
absorption is a desirable property for generating higher
photocurrents in solar cells.
Figure 2. TGA thermograms of the copolymers P1-P4, recorded at a
heating rate of 20 °C min-1 under a N2 atmosphere.
Table 1. Polymerization Data and Thermal Properties of the
Copolymers
feed ratio
Mw Mn
Tg
Td
copolymer M1 M2 M3 M4 M5 (104) (104) PDI (°C) (°C)
P1
P2
P3
P4
0.5 0.5
0.5
0.25 0.5
0.25
39.8 15.0 2.65 n.d.a 332
10.1 6.1 1.65 68 412
56.3 22.1 2.54 n.d.a 330
0.25 8.9 5.0 1.79 77 410
0.5
0.5
0.25
a Glass transition temperature was not detectable.
We investigated their thermal behavior through DSC
analysis, which revealed no obvious thermal transitions for
P1 and P3 in the temperature range from 40 to 300 °C; in
contrast, P2 and P4 exhibited distinct glass transition tem-
peratures (Tg) at 68 and 77 °C, respectively (Figure 1). Thus,
P1-P4 displayed amorphous properties. We attribute the
slightly higher value of Tg obtained for P4 to the presence in
its backbone of the more rigid bithiophene segment, which
presumably enhanced the chain rigidity45 while reducing the
number of the alkyl groups in the polymer chain. Figure 2
displays the TGA curves of these polymers. The 5% weight
loss temperature (Td) of each polymer was greater than
330 °C; such good thermal stability is an important prere-
quisite for a polymer’s application to PSCs. Table 1 sum-
marizes the molecular weights, polydispersities, and thermal
properties of the polymers.
Photophysical Properties. Figure 3 presents absorption
spectra of the copolymers P1-P4, recorded for both dilute
(1 ꢀ 10-5 M) CHCl3 solutions and solid films; Table 2
summarizes the spectral data. In solution, P1 and P2 dis-
played absorption peaks close to 400 nm that we assign to
their localized π-π* transition bands and at 668 and 776 nm,
respectively, that we attribute to ICT interactions between
The optical bandgaps (Egopt) of P1-P4, estimated from the
onsets of absorption in their solid films, were 1.58, 1.46, 1.60,
and 1.49 eV, respectively. Generally, the incorporation of
electron donating groups reduces the optical bandgaps by
raising the energy levels of the highest occupied molecular
orbital (HOMO).46 We suspect that the optical bandgaps of
P2 and P4 were lower than those of P1 and P3 because the
incorporation of the more strongly electron-donating and
highly coplanar CPDT units resulted in stronger ICT inter-
actions when conjugated with the PP acceptors.12,47

6274 Macromolecules, Vol. 43, No. 15, 2010
Table 2. Optical and Redox Properties of the Copolymers
Yuan et al.
absorption, λmax (nm)
solution film
Eogpt (eV)a
E
(V)
E
(V)
onset
HOMO (eV)b
LUMO (eV)b
Eegc (eV)c
ox
onset
red
P1
P2
P3
P4
668
776
468, 616
505, 676
680
779
494, 634
529, 690
1.58
1.46
1.60
1.49
0.42
1.48
5.22
5.18
5.20
5.13
3.32
3.31
3.28
3.30
1.90
1.87
1.92
1.83
0.38
0.40
0.33
1.49
1.52
1.50
a Estimated from the onset wavelength absorptions of the solid films. b Calculated from the corresponding onset potentials. c Calculated form the
difference between the onset potentials for oxidation and reduction.
Figure 5. Dark J-V curves for the hole-dominated carrier devices
incorporating the P1-P4:PC70BM blends, each prepared at a blend
ratio of 1:4 (w/w).
Figure 4. Cyclic voltammograms of the copolymers P1-P4 as solid
films.
Electrochemical Properties. We used CV to investigate the
redox behavior of the copolymers and obtain their HOMO
and lowest occupied molecular orbital (LUMO) energy
levels. Figure 4 displays the electrochemical behavior of the
copolymers as solid films; Table 2 summarizes the relevant
data. All synthesized polymers exhibited partially reversible
oxidations, except for P3, which underwent an irreversible
oxidation. The onset potentials of P1 and P3 were 0.42 and
0.40 V, respectively, arising essentially from the oxidation of
the electron-donating BDT units,36 although the cyclic vol-
tammograms revealed indistinct oxidation peaks of the BDT
units intermixed in the hybrid oxidation region. P2 and P4
exhibited oxidation onsets of 0.38 and 0.33 V, respectively,
resulting from the oxidation of the CPDT units.40 In their re-
duction traces, P1-P4 exhibited reversible reduction peaks,
with similar onset potentials between -1.48 and -1.52 V,
which we assign to reductions of the electron-accepting PP
moieties; these values are comparable with those reported
previously for other PP-containing copolymers.27 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 vacuum level).23,48,55
Accordingly, the HOMO energy levels of P1-P4 were
5.22, 5.18, 5.20, and 5.13 eV, respectively, implying that they
varied with respect to the modulated ICT strengths resulting
from the presence of the electron donors exhibiting various
electron-donating abilities.12,20,47 The LUMO energy levels
of P1-P4 were all located within a reasonable range (3.28-
3.32 eV) and were significantly greater than that of PC70BM
(ca. 4.2 eV);13,20 thus, we would expect efficient charge
transfer/dissociation to occur in their devices.13,49-51 In
addition, the electrochemical bandgaps (Eegc) of P1-P4,
estimated from the difference between the onset potentials
for oxidation and reduction, were in the range 1.83-1.92 eV;
i.e., they were slightly larger than their corresponding optical
bandgaps. This discrepancy between the electrochemical and
optical bandgaps presumably resulted from the exciton
binding energies of the polymers31 and/or the interface
barrier for charge injection.52
Figure 6. J-V characteristics of PSCs incorporating P1-P4:PC70BM
blends, each prepared at a blend ratio of 1:4 (w/w).
blend ratio of 1:4 (w/w). The hole mobilities of P2 and
P4 blends were 1.67 ꢀ 10-4 and 2.36 ꢀ 10-4 cm2 V-1 s-1
,
respectively while that of P1 and P3 blends are 7.81 ꢀ 10-5
and 1.37 ꢀ 10-4 cm2 V-1 s-1, respectively. The fact that the
hole mobility of the P4:PC70BM blend being higher than that
of P1-P3 blends is probably due to the more homogeneous
morphology of the P4 blend.
Photovoltaic Properties. Next, we investigated the photo-
voltaic properties of the polymers in bulk heterojunction
solar cells having the sandwich structure ITO/PEDOT:PSS/
polymer:PC70BM (1:4, w/w)/Al, where the photoactive
layers had been spin-coated from dichlorobenzene solutions.
Figure 6 presents the J-V curves of these PSCs; Table 3
summarizes the data. The devices prepared from P1-P4:
PC70BM blends exhibited open circuit voltages (Voc) of 0.74,
0.66, 0.71, and 0.63 V, respectively; each value is related to
the difference between the HOMO energy level of the poly-
mer and the LUMO energy level of PC70BM.51 We suspect
that the P4 blend provided the lowest value of Voc because of
its relatively higher-lying HOMO energy level. The short-
circuit current densities (Jsc) of the devices incorporating the
P3 and P4 blends (6.40 and 10.85 mA cm-2, respectively)
Hole Mobility. Figure 5 displays the hole mobilities of the
devices incorporating the P1-P4:PC70BM blends with a
were significantly greater than those of P1 (4.69 mA cm-2
)

Article
Macromolecules, Vol. 43, No. 15, 2010 6275
Table 3. Photovoltaic Properties of Polymer Solar Cells Incorporating
P1-P4:PC70BM Blends Prepared at 1:4 Weight Ratios
polymer thickness (nm) Voc (V) Jsc (mA cm-2
)
FF PCE (%)
P1
P2
P3
P4
95
80
92
75
0.74
0.66
0.71
0.63
4.69
6.26
6.41
10.85
34.5
44.1
51.7
46.1
1.20
1.82
2.35
3.15
Figure 8. J-V characteristics of PSCs incorporating P4:PC70BM
blends prepared at various weight ratios (w/w).
Table 4. Photovoltaic Properties of Polymer Solar Cells Incorporating
P4:PC70BM Blends Prepared at Various Weight Ratios
P4:PC70BM
(w/w)
thickness
(nm)
Voc (V) Jsc (mA cm-2
)
FF PCE (%)
1:1
1:2
1:3
1:4
84
80
78
75
0.63
0.63
0.62
0.63
7.01
9.16
9.92
10.85
33.3
42.1
43.6
46.1
1.47
2.43
2.68
3.15
Figure 7. EQE curves of PSCs incorporating P1-P4:PC70BM blends,
each prepared at a blend ratio of 1:4 (w/w).
and P2 (6.26 mA cm-2). Because the light-harvesting cov-
erages of the former pair were significantly broader than
those of the latter pair, more available photos from the solar
radiation could be absorbed by P3 and P4, thereby leading to
higher photocurrents. The device incorporating the P3 blend
exhibited a value of Voc of 0.71 V, a value of Jsc of 6.40 mA
cm-2, a fill factor (FF) of 0.517, and a PCE of 2.35%; the
device featuring the P4 blend displayed even greater perfor-
mance, with values of Voc, Jsc, FF, and PCE of 0.63 V, 10.85
mA cm-2, 0.461, and 3.15%, respectively. Figure 7 displays
the external quantum efficiency (EQE) curves of the devices
incorporating the P1-P4:PC70BM blends at weight ratio
of 1:4. These devices exhibited a significantly broad EQE
responses extending from the visible to the NIR, with a
maximum intensity ranging between 460 and 480 nm. We
attribute their higher EQE responses in the visible region to
the corresponding higher absorbance of the blend, resulted
from both the intrinsic absorption of the polymer and the
presence of a high content of PC70BM, which also absorbs
significantly at 400-500 nm. In contrast, these devices dis-
played relatively lower EQE responses at wavelengths above
700 nm because of the moderate absorbance of the polymer
blend. The device of P4 blend exhibited a higher EQE
response than that of other polymer blends, with a maximum
of 54% at 480 nm, corresponding to its higher photocurrent.
The theoretical short-circuit current density obtained from
integrating the EQE curve of the P4 blend is 9.90 mA/cm2,
that is reasonably agreed to the measured Jsc of 10.85 mA/
cm2 AM 1.5G. For the P1-P3 blends, the discrepancy
between the theoretical and the measured Jsc is around
10%. To optimize the device performance of the most
efficient polymer P4, devices of various compositions incor-
porating the P4:PC70BM blend were investigated. Figure 8
presents the J-V curves of the PSCs; Table 4 summarizes the
data. The power conversion efficiencies of P4 devices in-
creased with the weight ratio of PC70BM owing to the
enhanced photocurrent and fill factor. A high percentage
(80%) of PC70BM was required in the P4 blend to obtain
such high efficiency. Similar phenomena have been observed
in other amorphous polymer:PC70BM systems.2,13,42
absorption and energy levels of polymers but also the surface
morphologies of the polymer blends.53,54 Figure 9 displays
the surface morphologies determined through AFM mea-
surements. Samples of the P1-P4:PC70BM blends were
prepared using procedures identical to those used to fabri-
cate the active layers of the devices. In each case, we observed
coarse phase separation in the images of the polymer blends,
except for the P4 blend, which displayed a moderately
homogeneous morphology. The P1 and P2 blends feature
somewhat larger domains than those of the P3 and P4
blends; the root-mean-square roughness of the former pair
were 4.06 and 8.05 nm, respectively, significantly larger than
those of the latter pair (3.50 and 1.53 nm, respectively). The
greater phase segregation and rougher surfaces of the P1 and
P2 blends presumably arose because of poor miscibility with
PCp70BM;a result of the greater numbers of alkyl groups
on the repeat units in these polymer chains.55,56 This phe-
nomenon was further confirmed by their respective TEM
images. Figure 10 shows the TEM images of P1-P4:
PC70BM blends, where the aggregated PC70BM size in the
P2 blend is in the range of 200-500 nm and is significantly
larger than that in the P4 blend (ca. 70 nm), indicating the
relatively better miscibility between P4 and PC70BM because
of the smaller number of alkyl chains. A similar phenomenon
was also observed for comparing P1 with P3 blends; the
aggregated PC70BM size in P1 was larger than that in P3.
Accordingly, we suspect that the miscibility of the P3 and P4
blends increased after the incorporation of their correspond-
ing thiophene and bithiophene segments because of the
reduced number of alkyl groups in the backbone. The
AFM and TEM images clearly reveal that the P4 blend
was more homogeneous and smoother than the other three;
therefore, its device provided the highest photocurrent. We
suggested that the lower power conversion efficiencies of P1
and P2 based heterojunction devices as compared to that of
P3 and P4 based devices result from not only their narrower
absorption region but more likely from the phase separation
within the blends, as demonstrated by relatively larger
PC70BM aggregations in the P1 and P2 blends. Even though
a few BDT37 or CPDT39 based polymers have demonstrated
higher devices’ power conversion efficiencies, we have used a
Moreover, when exploring the decisive factors affecting
the efficiencies of PSCs we must consider not only the

6276 Macromolecules, Vol. 43, No. 15, 2010
Yuan et al.
Figure 9. Topographic AFM images of copolymer:PC70BM (1:4, w/w) blends incorporating (a) P1, (b) P2, (c) P3, and (d) P4.
Figure 10. TEM images of copolymer:PC70BM (1:4, w/w) blends incorporating (a) P1, (b) P2, (c) P3, and (d) P4.
different acceptor, PP, for our synthesized polymers; the
differences in the intrinsic physical/electronic properties and
the blend morphology between our PP based polymers and
the BDT or CPDT based polymers result in different device’s
power conversion efficiencies.
polymers. Moreover, P3 and P4 absorbed broadly, covering the
solar spectrum from the visible to the NIR, because of the
presence of the thiophene/bithiophene segments in their polymer
backbones; accordingly, their corresponding devices functioned
with improved photocurrents. Furthermore, among our four
tested polymer blends, the P4 blend possessed the most homo-
geneous surface, presumably contributing to its more efficient
carrier transport and dissociation. As a result, the best device
performance was obtained when using the P4:PC70BM blend,
characterized by a high short-circuit current of 10.85 mA cm-2
and a resulting PCE of 3.15%.
Conclusion
We have prepared a series of new pyrido[3,4-b]pyrazine (PP)-
based low-bandgap copolymers conjugated with electron-donat-
ing benzo[1,2-b:3,4-b0]dithiophene (BDT) and/or cyclopenta-
dithiophene (CPDT) units. The absorption spectra, bandgaps,
and energy levels of the polymers can be tuned by using the two
different donors, which exhibit different electron-donating abili-
ties. Because the CPDT unit exhibited higher electron-donating
ability relative to that of the BDT unit, its presence resulted in
stronger ICT interactions when conjugated with PP moieties.
Therefore, the absorptions of the CPDT-containing polymers were
significantly red-shifted relative to those of the BDT-containing
Acknowledgment. We thank the National Science Council
for financial support through project NSC 98-2120-M-009-006.
Supporting Information Available: Figures showing 1H
NMR spectra of the synthesized polymers. This material is
available free of charge via the Internet at http://pubs.acs.org.

Article
Macromolecules, Vol. 43, No. 15, 2010 6277
€
(28) Jonforsen, M.; Johansson, T.; Inganas, O.; Andersson, M. R.
References and Notes
Macromolecules 2002, 35, 1638.
(29) Jonforsen, M.; Johansson, T.; Spjuth, L.; Inganas, O.; Andersson,
(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789.
(2) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.;
Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem.,
Int. Ed. 2003, 42, 3371.
(3) Hoppe, H.; Niggemann, M.; Winder, C.; Kraut, J.; Hiesgen, R.;
Hinsch, A.; Meissner, D.; Sariciftci, N. S. Adv. Funct. Mater. 2004,
14, 1005.
€
M. R. Synth. Met. 2002, 131, 53.
(30) Thompson, B. C.; Madrigal, L. G.; Pinto, M. R.; Kang, T.-S.;
Schanze, K. S.; Reynolds, J. R. J. Polym. Sci., Part A: Polym.
Chem. 2005, 43, 1417.
(31) Wu, P.-T.; Kim, F. S.; Champion, R. D.; Jenekhe, S. A. Macro-
molecules 2008, 41, 7021.
(32) Jonforsen, M.; Ahmad, I.; Johansson, T.; Larsson, J.; Roman,
€
L. S.; Svensson, M.; Inganas, O.; Andersson, M. R. Synth. Met.
2001, 119, 185.
(33) Zhang, F.; Jonforsen, M.; Johansson, D. M.; Andersson, M. R.;
€
(4) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107,
1324.
(5) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct.
Mater. 2005, 15, 1617.
(6) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.;
Yang, Y. Nat. Mater. 2005, 4, 864.
(7) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.;
Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha,
C.-S.; Ree, M. Nat. Mater. 2006, 5, 197.
(8) Chiu, M.-Y.; Jeng, U.-S.; Su, C.-H.; Liang, K. S.; Wei, K.-H. Adv.
Mater. 2008, 20, 2573.
(9) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007,
91, 954.
(10) Huo, L. J.; Tan, Z.; Wang, X.; Zhou, Y.; Han, M. F.; Li, Y. F.
J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4038.
(11) Xin, H.; Guo, X.; Kim, F. S.; Ren, G.; Watson, M. D.; Jenekhe,
S. A. J. Mater. Chem. 2009, 19, 5303.
€
Inganas, O. Synth. Met. 2003, 138, 555.
(34) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
^
Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc,
M. J. Am. Chem. Soc. 2008, 130, 732.
(35) Wu, P.-T.; Bull, T.; Kim, F. S.; Luscombe, C. K.; Jenekhe, S. A.
Macromolecules 2009, 42, 671.
(36) Hou, J.; Park, M.-H.; Zhang, S.; Yao, Y.; Chen, L.-M.; Li, J.-H.;
Yang, Y. Macromolecules 2008, 41, 6012.
(37) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.;
Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649.
(38) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Muhlbacher, D.;
€
Scharber, M.; Brabec, C. J. Macromolecules 2007, 40, 1981.
(39) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim,
J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008,
130, 3619.
(40) Bijleveld, J. C.; Shahid, M.; Gilot, J.; Wienk, M. M.; Janssen,
R. A. J. Adv. Funct. Mater. 2009, 19, 3262.
(41) Li, K.-C.; Huang, J.-H.; Hsu, Y.-C.; Huang, P.-J.; Chu, C.-W.; Lin,
J.-T.; Ho, K.-C.; Wei, K.-H.; Lin, H.-C. Macromolecules 2009, 42,
3681.
(12) Zhou, E.; Yamakawa, S.; Tajima, K.; Yang, C.; Hashimoto, K.
Chem. Mater. 2009, 21, 4055.
(13) Lindgren, L. J.; Zhang, F.; Andersson, M.; Barrau, S.; Hellstrom,
€
S.; Mammo, W.; Perzon, E.; Inganas, O.; Andersson, M. R. Chem.
€
Mater. 2009, 21, 3491.
(14) Qin, R.; Li, W.; Li, C.; Du, C.; Veit, C.; Schleiermacher, H.-F.;
€
Andersson, M.; Bo, Z.; Liu, Z.; Inganas, O.; Wuerfel, U.; Zhang, F.
J. Am. Chem. Soc. 2009, 131, 14612.
(15) Chang, Y.-T.; Hsu, S.-L.; Su, M.-H.; Wei, K.-H. Adv. Funct.
(42) Gadisa, A.; Mammo, W.; Andersson, L. M.; Admassie, S.; Zhang,
€
F.; Andersson, M. R.; Inganas, O. Adv. Funct. Mater. 2007, 17,
3836.
(43) Melzer, C.; Koop, E. J.; Mihailetchi, V. D.; Blom, P. W. M. Adv.
Mater. 2007, 17, 3326.
(16) Chang, Y.-T.; Hsu, S.-L.; Chen, G.-Y.; Su, M.-H.; Singh, T. A.;
Diau, E. W.-G.; Wei, K.-H. Adv. Funct. Mater. 2008, 18, 2356.
(17) Chang, Y.-T.; Hsu, S.-L.; Su, M.-H.; Wei, K.-H. Adv. Mater. 2009,
21, 2093.
(18) Huang, F.; Chen, K.-S.; Yip, H.-L.; Hau, S. K.; Acton, O.; Zhang,
Y.; Luo, J.; Jen, A. K.-Y. J. Am. Chem. Soc. 2009, 131, 13886.
(19) Yuan, M.-C.; Su, M.-H.; Chiu, M.-Y.; Wei, K.-H. J. Polym. Sci.,
Part A: Polym. Chem. 2010, 48, 1298.
(20) Huo, L.; Hou, J.; Chen, H.-Y.; Zhang, S.; Jiang, Y.; Chen, T. L.;
Yang, Y. Macromolecules 2009, 42, 6564.
(21) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L.
J. Am. Chem. Soc. 2009, 131, 56.
(22) Mondal, R.; Miyaki, N.; Becerril, H. A.; Norton, J. E.; Parmer, J.;
ꢀ
Mayer, A. C.; Tang, M. L.; Bredas, J.-L.; McGehee, M. D.; Bao, Z.
Chem. Mater. 2009, 21, 3618.
(23) Zhou, E.; Wei, Q.; Yamakawa, S.; Zhang, Y.; Tajima, K.; Yang,
C.; Hashimoto, K. Macromolecules 2010, 43, 821.
(24) Zhou, H.; Yang, L.; Xiao, S.; Liu, S.; You, W. Macromolecules
2010, 43, 811.
Funct. Mater. 2004, 14, 865.
(44) Chaloner, P. A.; Gunatunga, S. R.; Hitchcock, P. B. J. Chem. Soc.,
Perkin Trans. 2 1997, 1597.
(45) Jung, Y. K.; Kim, H.; Park, J.-H.; Lee, J.; Lee, S. K.; Lee, Y. S.;
Shim, H.-K. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3573.
(46) Ajayaghosh, A. Chem. Soc. Rev. 2003, 32, 181.
(47) Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Macromolecules 2006,
39, 8712.
€
(48) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler,
H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551.
ꢀ
(49) Bredas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. Rev.
2004, 104, 4971.
ꢀ
(50) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47,
58–77.
(51) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf,
€
C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789.
(52) Pal, B.; Yen, W.-C.; Yang, J.-S.; Chao, C.-Y.; Hung, Y.-C.; Lin,
S.-T.; Chuang, C.-H.; Chen, C.-W.; Su, W.-F. Macromolecules
2008, 41, 6664.
(25) Chen, G.-Y.; Chiang, C.-M.; Kekuda, D.; Lan, S.-C.; Chu, C.-W.;
Wei, K.-H. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1669.
(26) Yamamoto, T.; Zhou, Z.-H.; Kanbara, T.; Shimura, M.; Kizu, K.;
Maruyma, T.; Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.;
Tomaru, S.; Kurihara, T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am.
Chem. Soc. 1996, 118, 10389.
(53) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. Adv.
Funct. Mater. 2007, 17, 1636.
(54) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45.
(55) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L.
J. Am. Chem. Soc. 2009, 131, 7792.
(56) Lai, M.-H.; Chueh, C.-C.; Chen, W.-C.; Wu, J.-L.; Chen, F.-C.
J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 973.
(27) Lee, B.-L.; Yamamoto, T. Macromolecules 1999, 32, 1375.