Donor-acceptor (donor) polymers with differently conjugated side groups at the acceptor units for photovoltaics

Article abstract of DOI:10.1002/pola.26526

Four new donor-acceptor (donor) [D-A(D)], PBDT-PTQ, PBDT-PTTQ, PBDT-TQ, and PBDT-TTQ, bearing the same backbone of alternative benzodithiophene (BDT) and quinoxaline units but with phenylene thienyl, phenylene di-thienyl, thienyl and di-thienyl groups (ot

Full text of DOI:10.1002/pola.26526

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Donor–Acceptor (Donor) Polymers with Differently Conjugated Side
Groups at the Acceptor Units for Photovoltaics
Hongmei Qin, Lisheng Li, Tianxiang Liang, Xiaobin Peng, Junbiao Peng, Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices,
South China University of Technology, Guangzhou 510640, China
Correspondence to: X. Peng (E-mail: chxbpeng@scut.edu.cn)
Received 6 November 2012; accepted 7 December 2012; published online 5 January 2013
DOI: 10.1002/pola.26526
ABSTRACT: Four new donor–acceptor (donor) [D–A(D)], PBDT-
PTQ, PBDT-PTTQ, PBDT-TQ, and PBDT-TTQ, bearing the same
backbone of alternative benzodithiophene (BDT) and quinoxa-
line units but with phenylene thienyl, phenylene di-thienyl,
thienyl and di-thienyl groups (other donors), respectively, at
the acceptor quinoxaline units, were designed and synthesized
to investigate the impacts of the conjugated side chains at the
acceptor units on the photovoltaic properties of polymers. The
power conversion efficiencies (PCEs) of the polymer solar cells
(PSCs) based on PBDT-TQ:[6,6]-phenyl-C-70-butyric acid methyl
ester (PC70BM) and PBDT-PTQ:PC70BM reach to 4.39 and
3.58%, respectively, which are 43 and 17% higher, respectively,
than that of a reported alkylphenyl substituted polymer with the
same main chain. However, the PCEs based on PBDT-TTQ and
PBDT-PTTQ, in which an additional thiophene is added at a side
chain of PBDT-TQ and PBDT-PTQ, respectively, decline. The
mechanism how the conjugated side chains affect the perform-
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2013 Wiley Periodicals,
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KEYWORDS: benzodithiophene; polymer; quinoxaline; side
groups; solar cell
INTRODUCTION Bulk-heterojunction (BHJ) polymer solar
cells (PSCs) are attracting strong interest in recent years due
to the advantages of low cost, light weight, easy fabrication,
and capability to fabricate flexible devices. To obtain active
layer materials with ideal performance, tremendous efforts
have been devoted to tune their properties through molecu-
lar designs1–5; and, the power conversion efficiencies (PCEs)
of the BHJ PSCs acquired significant improvement in the past
decade due to the introduction of donor–acceptor (D–A)
molecular structures,6–10 in which the intrinsically internal
charge transfer (ICT) could lead to more desirable double-
bond characteristics between the repeating donor and the
acceptor units so that the conjugated backbone could adopt
a more planar configuration to let the p-electrons be delocal-
ized more efficiently along the conjugated backbone.11–13
occupied molecular orbital (HOMO) energy levels. Very
recently, thienyl groups were attached to benzodithiophenes
(BDTs) moieties of D–A backbones to form D(D)–A poly-
mers.18–20 Since the polymers before and after the introduc-
tion of thienyl groups exhibited similar HOMO levels and
band gaps, the improved VOC and JSC of the devices could not
be simply attributed to lower HOMO levels or smaller band
gaps induced by the thienyl groups, and it is still unknown
how the 2D conjugated structures influence the photovoltaic
properties of the D–A polymers.
Although the photovoltaic properties of D-p-A and D(D)–A
polymers have been reported, those of D–A (D) polymers, in
which donor side groups are attached to the acceptor units
of D–A polymer main chains, have not been investigated.
Especially, the systematic studies how the type and length of
aromatic side groups impact the performance of PSCs are
still to be explored. Quinoxaline-based D–A copolymers were
successfully developed and exhibited promising photovoltaic
performance since the electron-deficient N-heterocycle of
quinoxaline could effectively tune the energy levels and
lower the band gaps of the resulting copolymers.10,21–29 BDT
As it is still very challenging to optimize absorptions, energy
levels, and charge mobilities of PSCs simultaneously for lin-
ear D–A polymers, two-dimensional (2D) conjugated poly-
mers are attracting attentions for modulating the electronic
structures of polymers in recent years.14,15 Rather than
inserting electron-deficient moieties into the backbones,
Huang et al.16,17 reported a molecular engineering strategy
of attaching strong electron-withdrawing groups at the end
of the side chains to synthesize 2D like D-p-A conjugated
polymers to lower the band gaps and deeper the highest
was also successfully employed for OPV studies and a copol-
27,30–36
ymer based on BDT
and phenyl-substituted quinoxa-
line showed a PCE up to 3.06%.27 In this study, four D–A(D)
polymers based on BDT and quinoxaline with an identical
Additional Supporting Information may be found in the online version of this article.
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conjugated backbone but four different side groups attached
at 2 and 3 positions of their acceptor quinoxaline units were
designed and synthesized to systematically investigate the
impacts of differently conjugated side chains at the acceptor
units of D–A polymers on their optical, electrical, hole mobil-
ity, and photovoltaic properties.
The current–density–voltage (J–V) characteristics were meas-
ured with a Keithley 236 digital source meter under AM 1.5
solar simulator (Oriel model 91192) at 100 mW/cm2. The
spectral response was measured with a commercial photo
modulation spectroscopic setup (Oriel). A calibrated Si pho-
todiode was used to determine the photosensitivity.
Atomic Force Microscopy
EXPERIMENTAL
The film morphology was measured using an atomic force
microscope (DI Multimode Nanoscope IIIa) using the tapping
mode. The bend Films were prepared by spin-coating on the
top surꢀface of a PEDOT:PSS substrate, followed by annealing
at 110 C for 10 min.
Materials
All reagents, unless otherwise specified, were obtained from
Aldrich, Acros, and TCI Chemical Co. and used as received.
Toluene and tetrahydrofuran (THF) were distilled from so-
dium metal at the presence of benzophenone and degassed
prior to use. All manipulations involving air-sensitive
reagents were performed under an atmosphere of dry Ar.
Hole Mobility Measurement
Hole-only devices, consisting of thin layers of poly-
mer:PC70BM blend (1:1) were fabricated. The device struc-
ture was ITO/PEDOT/active layer/MoO3/Al. The mobility
was determined by fitting the dark current to the model of a
single carrier space charge limited current.
Instrumentation
1
The H NMR spectra were collected on a VARIAN INOVA-400
spectrometer operating at 400 MHz for 1H in deuterated
chloroform solution with tetramethylsilane as the reference.
Thermogravimetric analyses (TGA) were carried out with a
Netzsch TG 209 under N2 flow at a heating rate of 10 ꢀC/
min. UV–visible (UV–vis) absorption spectra were recorded
on a SHIMADZU UV-2450 UV–vis spectrophotometer. Cyclic
voltammetry (CV) was carried out on a CHI660C electro-
chemical workstation with a platinum electrode coated with
a thin polymer film, a Pt wire and a saturated calomel elec-
trode (SCE) were used as the working electrode, the counter
electrode and the reference electrode, respectively, at scan
rate of 50 mV/s in a nitrogen-saturated solution of 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in ace-
tonitrile (CH3CN). Molecular weights of the polymers were
obtained on a Waters GPC 2410 using a calibration curve of
polystyrene standards with THF as the eluent.
RESULTS AND DISCUSSION
Synthesis
The molecular structures of the four polymers and the gen-
eral synthetic route are shown in Schemes 1 and 2, respec-
tively. The four 5,8-bis(5-bromothiophen-2-yl)quinoxaline
derivatives were synthesized by closed loop reaction
between 3,6-bis(5-bromothiophen-2-yl)benzene-1,2-diamine
and 1,2-dione derivatives using acetic acid as solvent and
being heated at 60 ꢀC for 10 h under N2 atmosphere. The
four target polymers were prepared through Stille polycon-
densation reaction with a yield of 60-80% (Supporting Infor-
mation). Molecular weights of the polymers and polydisper-
sity were estimated by gel permeation chromatography
method using monodispersed polystyrene as the standard
and THF as the eluent. The number-average molecular
weight (Mn) of PBDT-PTQ, PBDT-PTTQ, PBDT-TQ, and PBDT-
TTQ are 10.1, 71.3, 12.3, and 22.7 K, respectively.
Fabrication and Characterization of Photovoltaic Devices
BHJ PSCs were fabricated through solution processing and
the structure of the solar cells was Glass/ITO/poly(3,4-ethyl-
enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/
active layer/Ca/Al. The ITO-coated glass substrates were
cleaned with acetone, detergent, distilled water, and isopro-
pyl alcohol in an ultrasonic bath and then dried overnight in
an oven at >70 ꢀC. After oxygen plasma treatment of the
ITO substrates for 5 min, a thin layer of PEDOT:PSS was
spin-cast on precleaned ITO-coated glass from a PEDOT:PSS
aqueꢀous solution at 3000 rpm for 30 s and then dried at
130 C for 20 min to give a film with a thickness of 40 nm.
The active layer was deposited on top of the PEDOT:PSS
layer by casting from relevant solution according to the solu-
bility of materials. Because of the solubility problem, PBDT-
PTTQ was dissolved in hot chlorobenzene (CB) (70 ꢀC) and
then spin-coated on the PEDOT:PSS layer. All blended ratios
of polymer:PC70BM were 1:1 by weight and the thicknesses
of blend layers were about 80–90 nm measured by profilom-
eter. The devices were put on the heater at 110 ꢀC for 10
min after spin coated the active layer. Finally, Ca (3 nm)/Al
(80 nm) used as top electrodes was evaporated under high
vacuum (<3 ꢁ 10ꢂ6 mbar). The effective area was 0.16 cm2
defined by masks for all the solar cell devices.
Solubility
PBDT-TQ and PBDT-PTQ could be easily dissolved in CHCl3,
o-dichlorobenzene (DCB) and CB, but PBDT-TTQ showed lim-
ited solubility in CB and DCB although it also could be easily
dissolved in CHCl3. In the case of PBDT-PTTQ, it showed
poor solubility in CHCl3, DCB and CB. The solubility of the
polymers decreases as the enlargement of the conjugated
side chains from PBDT-TQ to PBDT-TTQ and to PBDT-PTTQ.
The solubility of PBDT-PTQ is much better than PBDT-TTQ,
which could be attributed to the larger steric hindrance
between the quinoxaline and the phenylene in PBDT-PTQ
than that between the quinoxaline and its adjacent thiophene
in PBDT-TTQ. These results demonstrate that the differently
conjugated side chains affect the solubility dramatically even
though the back bones of the four polymers are the same.
Thermal Stability
TGA of the polymers shown in Figure 1 reveals a 5% weight
loss at around 350 ꢀC for all the polymers, which is similar
with the reported BDT-based polymers and is acceptable for
organic electronic device applications.37
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SCHEME 1 Chemical structures of the polymers.
Optical Properties
note that the absorption peak assigned to the p–p* transition
blue-shifts from 424 nm of PBDT-TQ (one group at one side
chain) to 415 nm of PBDT-PTQ and PBDT-TTQ (two groups
at one side chain) and to 398 nm of PBDT-PTTQ (three
groups at one side chain), suggesting that the p–p* transition
of the main chain decreases as the enlargement of the conju-
gated side chains and the p-electrons are delocalized the
most effectively in PBDT-TQ.
The normalized UV–vis absorption spectra of the four poly-
mers in dilute THF and thin films spin-coated on quartz sub-
strates are shown in Figure 2, and the detailed absorption
data, including the absorption peaks of the solutions and
films, the absorption edges (onset wavelength of the absorp-
tion peak, konset) of films and the optical bandgaps deduced
from the absorption edges are summarized in Table 1. All
the four polymers show two absorption peaks at around 400
and 600 nm, which is a typical feature for D–A copolymers.
The absorption peak around 400 nm originates from the p–
p* transition of the polymer main chain, and that around
600 nm could be attributed to the ICT between BDT donor
units and quinoxaline acceptor units. The four polymers
show the similar absorption peaks due to the same back-
bone structure, but the wavelengths around 400 nm and the
relative intensities at about 600 nm are apparently different,
which are induced by the different side chains. It is worth to
The ICT absorption peaks around 600 nm give further evi-
dences how the conjugated side chains affect the optical
properties of the polymers. Compared with those of PBDT-TQ
and PBDT-PTQ, the peak intensities of PBDT-TTQ and PBDT-
PTTQ, in which an additional thiophene is added to each
side chain of PBDT-TQ and PBDT-PTQ, respectively, decrease
significantly, respectively, indicating that the enlargement of
the conjugated side chains by adding additional thiophenes
to the acceptor units is not favorable for the ICT. Further-
more, compared with that of PBDT-TQ, the peak intensities
SCHEME 2 Synthetic route of PBDT-TQ, PBDT-TTQ, PBDT-PTQ and PBDT-PTTQ.
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TABLE 1 Optical Properties of the Four Polymers in Solid Films
In THF
In Film
Polymer
kmax (nm)
kmax (nm) konset (nm) Eogpt (eV)a
PBDT-PTQ
415 586 415 612 765
1.62
1.62
1.61
1.62
PBDT-PTTQ 398 575 417 614 764
PBDT-TQ
PBDT-TTQ
a
424 569 431 610 772
416 591 415 606 767
Estimated from the onsets of the absorption spectra of the copoly-
mers’ films.
cantly, respectively, suggesting that the phenyl group con-
nected to quinoxaline shows different impacts to the ICT
from the thiophene group. The differences among the
absorption spectra of the four polymers are attributed to
that the electron pull ability of a quinoxaline decreases as
the enlargement of its conjugated side chains.38
FIGURE 1 TGA plots of the polymers with a heating rate of 10
ꢀC/min under an inert atmosphere.
The absorption spectra of the four polymers in film are
shown in Figure 2(b). Except for the 431 nm peak of PBDT-
TQ, the absorption peaks related to the p–p* transition of
the main chain of the other three polymers in films are ca.
415 nm and those related to the ICT are all around 610 nm.
The ICT bands of the polymer films all red-shift in compari-
son with those in solutions, which is a common phenomenon
of self-aggregation of the conjugated polymers in films. The
onsets of the absorption of PBDT-PTQ PBDT-PTTQ, PBDT-TQ,
and PBDT-TTQ are at 765, 764, 772, and 767 nm, respec-
tively, from which the optical bandgaps (Eogpt) of the poly-
of PBDT-PTQ and PBDT-TTQ, in which a thiophene group
were connected to quinoxaline by a phenylene and a thienyl
linkage, respectively, increase slightly and decreased signifi-
mers are calculated and listed in Table 1 according to Egopt
1240/konset
¼
.
Quantum Chemistry Calculations
The optimum geometries of the monomers of the four poly-
mers were investigated at the DFT B3LYP/6-31G* level using
the Gaussian 09 program suite (Supporting information).39
DFT calculations show that the dihedral angles between
FIGURE 2 vis absorption spectra of polymers in (a) THF and
FIGURE 3 Cyclic voltammograms of the polymer films coated
(b) films.
on platinum electrodes in 0.1 mol/L Bu4NPF6 CH3CN solutions.
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TABLE 2 Electrochemical Properties of the Four Polymers in
each side chain, show almost the same oxidation potential,
but the oxidation potentials increase and decrease when the
number of the side group(s) increase and decrease in PBDT-
PTTQ and PBDT-TQ, respectively, which is in line with the
changes of their absorption peaks assigned to the p–p* tran-
sitions. The HOMO, lowest unoccupied molecular orbital
(LUMO) energy levels, and the electrochemical band gaps
Solid Films
ox
onset
onset
re
E
(V)
E
(V)
EHOMO
ELUMO
Eegc
Polymer
vs. SCE
vs. SCE
(eV)
(eV)
(eV)
PBDT-PTQ
PBDT-PTTQ
PBDT-TQ
0.81
0.88
0.69
0.78
ꢂ0.66
ꢂ0.68
ꢂ0.68
ꢂ0.69
ꢂ5.11
ꢂ5.18
ꢂ4.99
ꢂ5.08
ꢂ3.64
ꢂ3.62
ꢂ3.62
ꢂ3.61
1.47
1.56
1.37
1.47
(Eegc) of the polymers are estimated from the onset oxidation
red
potentials (Eoonxset) and onset reduction potentials (E
)
onset
PBDT-TTQ
ferro-
onset
based on the following equations: HOMO ¼ ꢂe[E
ꢂ E
ox
cene
1/2
re
onset
þ 4.8] (eV) and LUMO ¼ ꢂe[E
ꢂ Eferrocene
þ
1/2
4.8] (eV), which are summarized at Table 2. The LUMO
energy levels of the four polymers are almost the same of
ꢂ3.62 eV, but the HOMO energy levels are slightly changed
and are ꢂ4.99, ꢂ5.11, ꢂ5.08, and ꢂ5.18 eV for PBDT-TQ,
PBDT-PTQ, PBDT-TTQ and PBDT-PTTQ, respectively. The elec-
trochemically estimated band gaps (Eegc) of all the four poly-
mers are in the range of 1.37-1.56 eV, which are slightly
lower than the corresponding optical band gaps (Eogpt) but
within the range of error (0.05-0.25 eV).
thiophene and quinoxaline in both PBDT-TQ and PBDT-TTQ,
between the phenylene and quinoxaline in both PBDT-PTQ
and PBDT-PTTQ, between the phenylene and thienyl group
in PBDT-PTQ and PBDT-PTTQ and between the thiophene
and thiophene in both PBDT-TTQ and PBDT-PTTQ are ca.
24ꢀ, 43ꢀ, 24ꢀ, and 19ꢀ, respectively. These results indicate
that both phenyl and thienyl substitutions conjugate with
the quinoxaline moieties but the steric hindrance between
the phenylene and quinoxaline is larger than that between
the thiophene and quinoxaline. The smaller dihedral angles
between thiophene and quinoxaline in both PBDT-TQ and
PBDT-TTQ could increase the p-electron conjugation between
the side chains and the main chain, and thus enable the
p-electrons of the side chains to be pulled more efficiently to
the main chain and therefore affect the p-electron distribu-
tion more dramatically. On the other hand, the larger dihe-
dral angles between phenylene and quinoxaline in both
PBDT-PTQ and PBDT-PTTQ decrease the p-electron conjuga-
tion between the side chains and the main chain to some
extent, but help to prevent the molecules from self-aggregat-
ing, which is consistent with the solubility of the polymers
and also demonstrated by the atomic force microscopy
(AFM) studies reported below.
Photovoltaic Properties
BHJ PSCs based on the blends of the polymers and PC70BM
were fabricated to investigate the impacts of the conjugated
side groups of the four polymers on the PSC performance.
The structure of fabricated devices is ITO/PEDOT:PSS/active
layer/Ca/Al, where the active layer is composed of one of
the four polymers as the donor and a fullerene derivative
PC70BM as the acceptor. Because of the different solubility of
the four polymers, different solvents were used to dissolve
the active layers as list in Table 3 and PBDT-PTTQ was dis-
ꢀ
solved in hot CB at 70 C. After some optimization, we found
that using Ca/Al as the cathode and postdeposition anneal-
ing at 110 ꢀC for 10 min gave us the best device perform-
ance. Figure 4 shows the I-V curves of the optimized devices
based on each polymer under the illumination of AM1.5, 100
mW/cm2. The open-circuit voltages (VOC), short-circuit
Electrochemical Characterization
The electrochemical behaviors of the four polymers were
investigated by CV in a solution of Bu4NPF6 (0.1 M) in aceto-
nitrile at a scan rate of 50 mV/s at room temperature under
the protection of argon. As shown in Figure 3, these poly-
mers exhibit both reversible p-doping/dedoping (oxidation/
re-reduction) and n-doping/dedoping (reduction/re-oxida-
tion) processes, indicating their good stability in charged
states. Their reduction processes all start at about ꢂ0.68 V
versus SCE, but their oxidation processes start at 0.69, 0.81,
0.78, and 0.88 V versus SCE for PBDT-TQ, PBDT-PTQ, PBDT-
TTQ, and PBDT-PTTQ, respectively. As shown in Figure 3,
PBDT-PTQ and PBDT-TTQ, in which two conjugated groups at
TABLE 3 Photovoltaic Performance of the PSCs
JSC
VOC FF PCE l
Polymer
Solvent
DCB
(mW/cm2) (V) (%) (%) (cm2/V/S)
PBDT-TQ
12.5
10.53
8.13
0.65 54 4.39 2.4 ꢁ 10ꢂ5
0.66 52 3.58 1.2 ꢁ 10ꢂ5
0.72 45 2.62 4.6 ꢁ 10ꢂ6
0.62 51 1.52 6.9 ꢁ 10ꢂ6
PBDT-PTQ DCB
PBDT-PTTQ CB
FIGURE 4 Current density–voltage (J–V) curves of poly-
mer:PC70BM-based solar cell devices under AM1.5G illumina-
tion (100 mW/cm2).
PBDT-TTQ DCB/CHCl3 4.80
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FIGURE 5 Tapping mode AFM images of polymer:PC70BM films (a) PBDT-PTQ, (b) PBDT-PTTQ, (c) PBDT-TQ, and (d) PBDT-TTQ.
currents (JSC), fill factors, and PCEs of the PSCs are summar-
ized in Table 3. The four polymers exhibit the similar VOC
from 0.62 to 0.72 eV, the devices based on PBDT-TQ exhibit
the highest JSC up to 12.5 mA/cm2 and the best PCE up to
4.39%, and those based on PBDT-PTQ showed a JSC to 10.53
mA/cm2 and a PCE to 3.58%. The PCEs of PBDT-TQ and
PBDT-PTQ are 43 and 17%, respectively, higher than that of
the devices based on a similar polymer with the same main
chain but a phenyl group as the side chain reported by Hou
and coworkers27, suggesting that the performance of a poly-
mer could be also increased by changing the side chains at
the acceptor moieties. However, the devices based on PBDT-
TTQ and PBDT-PTTQ, in which an additional thienyl group is
added to each side chain of PBDT-TQ and PBDT-PTQ, respec-
tively, show decreased JSC to 4.80 and 8.13 mA/cm2 and
reduced PCEs to 1.52 and 2.62%, respectively, indicating that
the performance could be declined when the conjugated side
chains are too enlarged.
of PBDT-TQ are delocalized the most effectively as shown by
the absorption spectra, and the decrease of the electron-pull
ability of quinoxaline and the severe self-aggregation induced
by the enlargement of the conjugated side chains could be
among the reasons for the reduced performance of PBDT-
PTQ, PBDT-PTTQ and PBDT-TTQ-based solar cells after addi-
tional phenyl or thienyl groups are added to PBDT-TQ, which
is further confirmed by morphology studies and the solva-
tion investigation.
AFM
The surface morphology of the polymer:PC70BM blends was
analyzed by tapping mode AFM. As shown in Figure 5 and
Supporting Information Figure S4, the surfaces of PBDT-
PTQ:PC70BM and PBDT-TQ:PC70BM blend films are quite
smooth and without large phase separation, indicating good
miscibility between the two polymers and PC70BM. However,
the surface of PBDT-TTQ:PC70BM blend film is rougher and
very coarse phase separation could be observed [Fig. 6(d)],
which is not favorable for the charge separation from the
polymer to PC70BM and thus limits the efficiencies of the
One of the reasons that the PBDT-TQ-based devices show the
best performance could be attributed to that the p-electrons
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in line with the conclusion drawn from the absorption spec-
troscopy and CV.
CONCLUSIONS
In summary, we have designed and synthesized a series of
D–A(D) polymers based on BDT and quinoxaline with differ-
ently conjugated side-chains at the acceptor units for appli-
cation in PSCs. The devices based on PBDT-TQ and PBDT-
PTQ show PCEs up to 4.37 and 3.58%, respectively, which
are 43 and 17% higher, respectively, than that of a reported
alkylphenyl substituted polymer, indicating that the replace-
ment of an alkylphenyl group by an alkylthienyl group is an
effective way to improve the photovoltaic performance. How-
ever, adding an additional thienyl groups to each side chain
of PBDT-TQ and PBDT-PTQ decreases the photovoltaic per-
formance due to the reduced p–p* transition and ICT or the
intensified self-aggregation induced by the enlarged conju-
gated side chains. Our studies demonstrated that the elec-
tron pull properties of the acceptor units or self-aggregation
properties of 2D D–A polymers could be affected by the dif-
ferently conjugated side chains and therefore can provide
useful guidelines to design 2D D–A conjugated polymers for
organic semiconductors.
FIGURE 6 EQE spectra of the four polymer:PC70BM devices.
resulted devices. For the film of PBDT-PTTQ:PC70BM, its
phase separation falls in between those blends of PBDT-
PTQ:PC70BM and PBDT-TQ:PC70BM and that of PBDT-
TTQ:PC70BM.
External Quantum Efficiency
ACKNOWLEDGMENTS
Figure 6 shows the external quantum efficiency (EQE) curves
of the PSCs based on the four polymer:PC70BM blends under
the illumination of monochromatic light. The EQE curves of
the devices indicate that the PSCs exhibit good response to
the sunlight at the range from 350 to 750 nm. Comparing
the EQE plots with the absorption spectra of the polymers,
we can see that PC70BM absorption contributes substantially
to the photocurrent by filling the valley of the polymer
absorption. The EQE values of the PSCs based on PBDT-TQ,
PBDT-PTQ, and PBDT-PTTQ are the largest, the second and
the third, respectively, in the whole range from 350 to 800
nm, which is consistent with conclusions drawn from the
absorption spectra that the enlargement of the conjugation
side chains by adding additional thiophenes to the acceptor
units is not favorable for the p–p* transition and the ICT.
The PBDT-TTQ-based devices show much lower EQEs than
those of the other three polymers in the whole region, which
is ascribed to two reasons: one reason is that the p–p* tran-
sition and the ICT are not as efficient as in PBDT-TQ revealed
by the absorption spectra. But the main reason is that the
conjugated bi-thiophene side chains of PBDT-TTQ leads to
such a severe self-aggregation that it shows poor miscibility
with PC70BM with very coarse phase separation and there-
fore lots of absorbed light could not be converted into
photocurrent.
This work was financially supported by the grants from the
National Natural Science Foundation of China (51073060,
50990065, and 51010003), International Science & Technology
Cooperation Program of China (2010DFA52150).
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