Synthesis and characterization of fluorene and cyclopentadithiophene-based copolymers exhibiting broad absorption for photovoltaic devices

Article abstract of DOI:10.1002/pola.24544

In this study, two low bandgap copolymers composed of fluorene (Fl), cyclopentadithiophene (CDT), and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) were synthesized, and their optical, electrochemical, and photovoltaic (PV) characteristics were investigated for applications in PV devices. The feed ratio of the Fl and CDT moieties was modulated to tune the electronic structures and resulting optical properties of the polymers. In the copolymeric structures, the Fl-CDT unit absorbs the short-wavelength UV/vis regions, and the CDT-DBT (or Fl-DBT) unit with strong intramolecular charge transfer characteristics covers the long-wavelength visible regions. P1 exhibited a wide UV absorption spectrum covering the UV and entire visible region in the range of 300-800 nm, and P2 showed absorption covering from 300 to 700 nm. UV/vis and electrochemical studies confirmed the desirable highest occupied molecular orbital/lowest unoccupied molecular orbital levels of the copolymers with bandgaps of 1.62-1.86 eV, enabling efficient electron transfer and a high open-circuit voltage when blending them with fullerene derivatives. When the polymers were blended with [6,6]phenyl-C₆₁-butyric acid methyl ester, P1 exhibited the best device performance with an open-circuit voltage of 0.66 V, short-circuit current of 4.92 mA cm^-2, and power conversion efficiency of 1.13% under Air Mass 1.5 Global (AM 1.5G, 100 mW cm^-2) illumination.

Full text of DOI:10.1002/pola.24544

Synthesis and Characterization of Fluorene and
Cyclopentadithiophene-Based Copolymers Exhibiting
Broad Absorption for Photovoltaic Devices
1
2
3
4
2
IN HWAN JUNG, SUN HEE KIM, EUNJAE JEONG, RENQIANG YANG, KWANGHEE LEE,
3
1
HAN YOUNG WOO, HONG-KU SHIM
1
Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
2
Department of Materials Science and Engineering, Gwangju Institute of Science and Technology,
Gwangju 500-712, Republic of Korea
3
Department of Nanofusion Technology, Department of Cogno-Mechatronics Engineering (WCU),
Pusan National University, Miryang 627-706, Republic of Korea
4
Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT), Chinese Academy of Sciences,
Qingdao, Shandong Province 266-101, People’s Republic of China
Received 2 November 2010; accepted 7 December 2010
DOI: 10.1002/pola.24544
Published online 10 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: In this study, two low bandgap copolymers com-
posed of fluorene (Fl), cyclopentadithiophene (CDT), and 4,7-
bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) were synthesized, and
their optical, electrochemical, and photovoltaic (PV) characteris-
tics were investigated for applications in PV devices. The feed
ratio of the Fl and CDT moieties was modulated to tune the elec-
tronic structures and resulting optical properties of the poly-
mers. In the copolymeric structures, the Fl-CDT unit absorbs the
short-wavelength UV/vis regions, and the CDT-DBT (or Fl-DBT)
unit with strong intramolecular charge transfer characteristics
covers the long-wavelength visible regions. P1 exhibited a wide
UV absorption spectrum covering the UV and entire visible
region in the range of 300–800 nm, and P2 showed absorption
covering from 300 to 700 nm. UV/vis and electrochemical stud-
ies confirmed the desirable highest occupied molecular orbital/
lowest unoccupied molecular orbital levels of the copolymers
with bandgaps of 1.62–1.86 eV, enabling efficient electron trans-
fer and a high open-circuit voltage when blending them with
fullerene derivatives. When the polymers were blended with
[6,6]phenyl-C61-butyric acid methyl ester, P1 exhibited the
best device performance with an open-circuit voltage of 0.66 V,
ꢀ
2
short-circuit current of 4.92 mA cm , and power conversion
efficiency of 1.13% under Air Mass 1.5 Global (AM 1.5G,
ꢀ2
100 mW cm ) illumination. VC 2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 49: 1248–1255, 2011
KEYWORDS: conducting
polymers;
conjugated
polymers;
copolymerization
INTRODUCTION The interest in photovoltaic (PV) cells has
increased because of the growing demand for renewable and
green energy sources. Currently, silicon-based PVs are the
dominant technology owing to their high power conversion
efficiency (PCE), long-term stability, and so forth. However,
further growth and expansion of the commercial PVs market
are limited because of the high production cost. Polymeric
solar cells (PSCs) have several advantages over silicon-based
ones, such as low cost, light weight, easy fabrication by solu-
As an active semiconducting material in PSCs, extensive
studies have been carried out with regioregular poly(3-
hexylthiophene) (P3HT), including detailed investigations on
charge carrier generation and separation, regioregularity of
alkyl substituents and molecular ordering, substituent
effects, polymer molecular weights and polydispersity effects,
morphology control by heat and solvent treatments, and
3
so forth. However, P3HT has a relatively large bandgap and
restricted light absorption because of the mismatch of its
1
4
tion processing, mechanical flexibility, and so forth. The
bandgap with the maximum photon flux of solar radiation.
bulk heterojunction (BHJ) concept of conjugated polymers
As a result, various types of low bandgap polymers have
been synthesized and developed to use a larger portion
of the solar energy. Recently, Ding and coworkers reported
(
as an electron donor) and fullerene derivatives (as an elec-
5
tron acceptor) has significantly improved the PCE of the
PSCs by forming large interfacial areas between the donors
and acceptors, leading to efficient photoinduced charge
an efficient PV device with a PCE value of 5.4% using a
low bandgap polymer, poly[2,6-(4,4-dihexyl-4H-cyclepenta[2,
2
0
0
generation and ultrafast charge transfer in devices.
1-b:3,4-b ]dithiophene)-alt-5,5 -(3,6-bis(4-hexylthienyl-2-yl)-s-
Correspondence to: H. Y. Woo (E-mail: hywoo@pusan.ac.kr) or H.-K. Shim (E-mail: hkshim@kaist.ac.kr)
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1248–1255 (2011) V
2011 Wiley Periodicals, Inc.
C
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WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
tetrazine)], and You et al. reported a highly efficient PSC
device with 6.3% PCE using a low bandgap polymer with
deep lowest unoccupied molecular orbital (LUMO) level.
tetrahydrofuran (THF), which were used after drying over
sodium/benzophenone. The CDT-, Fl-, and DBT-based mono-
mers, 4,4-bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-cyclo-
6
0
penta[2,1-b:3,4-b ]dithiophene (1), 4,4-bis(2-ethylhexyl)-2,
However, the absorption of the low bandgap polymers is
shifted to the long-wavelength region, which means that the
short-wavelength UV–vis region of the solar spectrum cannot
be used for energy conversion. Therefore, it is important to
design polymers exhibiting strong and wide absorption cover-
ing solar radiation from the UV/vis to near-IR regions. The
most important strategy for tuning the bandgap of conjugated
structures is to use intramolecular charge transfer (ICT) from
an electron-rich unit to an electron-poor moiety in electron
0
6
-dibromo-4H-cyclopenta[2,1-b:3,4-b ]dithiophene (5), 2,7-
dibromo-9,9-dioctylfluorene (2), 2,7-bis(4,4,5,5-tetramethyl-
,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (4), and 4,7-bis
5-bromo-2-thienyl)-2,1,3-benzothiadiazole (3), were prepared
1
(
1
0–12
according to the previous literature methods.
Measurements
1
13
H and C NMR spectra were recorded on a Bruker AVANCE
7
400 spectrometer, with tetramethylsilane as an internal
reference. Elemental analysis was performed by the KAIST
Research Supporting Team (EA 1110 Fisons analyzer). The
polymerization reaction was done by using a microwave
rich–poor alternating copolymers. Recently, many researchers
have focused on developing low bandgap polymers by using
ICT interactions. However, most previous studies reported the
structures with single ICT interaction, which often induces
long-wavelength absorption but loses short-wavelength
absorption (ca. 300–500 nm). The incorporation of more than
two ICT pairs (or ICT interactions) in the polymeric structure
may induce the broader absorption for high exciton genera-
tion and short-circuit current. To achieve wide absorption
TM
reactor (Biotage Initiator ). The number– and weight–aver-
age molecular weights of the polymers were determined by
gel permeation chromatography (GPC) using THF as an eluent
on a Waters GPC-150C instrument, relative to polystyrene as a
standard. Mass spectra were obtained with an Autospec ultra
spectrometer. Thermogravimetric analysis (TGA) measurement
of the polymers was performed using a TA Q500 instrument,
and the differential scanning calorimetric (DSC) measurement
was made using a TA Q100 instrument under a nitrogen
0
polymers, 4H-cyclopenta[2,1-b;3,4-b ]-dithiophene (CDT), fluo-
rene (Fl) as an electron-sufficient unit, and 4,7-bis(2-thienyl)-
2,1,3-benzothiadiazole (DBT) as an electron-deficient moiety
were incorporated into the same polymer backbone. The CDT
and Fl moieties are currently the preferred electron-rich moi-
ety for PV applications owing to their good electron-donating
properties, rigid coplanar structure with easy p–p intermolec-
ular interactions, and the possibility of side-chain manipula-
ꢁ
ꢀ1
atmosphere at a heating rate of 10 C min . UV–vis spectra
were measured on a Jasco V-530 UV–vis spectrometer. The
polymer films for UV–vis measurements were prepared by
spin coating with a polymer solution (1 wt %) in chloroform.
Cyclic voltammetry (CV) was performed on an AUTOLAB/PG-
STAT12 model system with a three-electrode cell in a 0.1 N
8
tion for processibility. The DBT moiety and its derivatives are
a useful building block that can produce a low bandgap poly-
mer suitable for electroluminescent and PV devices on
ꢀ
1
Bu NBF solution in acetonitrile at a scan rate of 50 mV s
.
4
4
9
A film of each polymer was coated onto a Pt wire electrode
by dipping the electrode into a polymer solution in chloro-
form. All measurements were calibrated against an internal
account of their good electron-withdrawing properties. New
electron rich–poor ICT copolymers for PSC devices were
designed through the random incorporation of the CDT-Fl unit
and CDT-DBT or Fl-DBT units. The feed ratio of the CDT and
Fl moieties was modulated to tune the absorption of the poly-
mers. In copolymeric structures, the CDT-Fl unit and the CDT-
DBT (or Fl-DBT) unit absorb the short-wavelength UV/vis
regions and long-wavelength visible regions, respectively.
standard of ferrocene (F ), the ionization potential (IP) value
c
þ
of which is ꢀ4.8 eV for the F /Fc redox system.
c
PSC Device Fabrication
PSCs with a device configuration of glass/indium tin oxide
(
(
ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
PEDOT:PSS)/polymer:PCBM/Al were prepared. Prior to
This article reports the synthesis and PV characteristics of two
ICT polymers, P1 and P2, showing broad absorption covering the
entire visible region. P1 contained 50 mol % CDT-Fl and 50 mol
device fabrication, the ITO substrates were cleaned with
detergent and ultrasonicated in deionized water, acetone, and
isopropanol, and then dried overnight in an oven. The sub-
strate was spin coated with a PEDOT:PSS solution (Baytron P
%
CDT-DBT units, and P2 contained 50 mol % Fl-CDT and 50
mol % Fl-DBT moieties. In the film with both structures, the intra-
molecular (and/or intermolecular) CDT-DBT and Fl-DBT charge
transfer interactions as well as the CDT-Fl (or Fl-CDT) interac-
tions are expected to exhibit wide absorption spectra. BHJ-type
PSCs were fabricated by blending [6,6]phenyl-C -butyric acid
ꢁ
VP AI 4083, Bayer AG), dried at 140 C for 10 min under air,
and then transferred to a glove box for spin casting of the
polymer layer. The solution containing a blended mixture of
polymer and PCBM in chlorobenzene (1 wt %) was spin
casted onto the above substrate, and the casted film was dried
61
methyl ester (PCBM) with the synthesized polymers, and their
optical and PV current density–voltage (J–V) characteristics were
investigated in detail for PSC device applications.
ꢁ
at 70 C for 10 min. Subsequently, the device was pumped
ꢀ
6
down under vacuum (<10 Torr), and an Al electrode was
deposited with a thickness of ꢂ100 nm. The active area of
EXPERIMENTAL
2
the devices was 14.5 mm , which is defined by the cathode
Materials
area. Current density–voltage (J–V) characteristics of the devi-
ces under nitrogen were measured using a Keithley 238
Source Measure unit. The PV properties were characterized
All chemical reagents purchased commercially were used
without further purification except for diethyl ether and
BROAD ABSORPTION POLYMERS BASED ON Fl AND CDT, JUNG ET AL.
1249

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic routes to polymers.
under an Air Mass 1.5 Global (AM 1.5G) solar simulator with
transferred to the above solution via cannula. After the mix-
ture was refluxed for 24 h, the degassed 2 mL aqueous potas-
sium carbonate and 4 mL of toluene were added again. The
reaction mixture was further reacted under reflux for 24 h,
showing increased viscosity. The reaction mixture was cooled
down to room temperature, and the mixture was slowly
added into a vigorously stirred mixture of 220 mL of metha-
nol and 10 mL of 1 N HCl. The precipitated polymer was col-
lected by filtration, dissolved in chloroform, and reprecipitated
into methanol three times. The resulting polymer was purified
by Soxhlet extraction with acetone/methanol for 2 days to
remove oligomers and the catalyst residue. The final product
ꢀ
2
irradiation intensity of 100 mW cm
.
Synthesis of Polymers
Synthesis of P1
4
,4-Bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-cyclo-
0
penta[2,1-b:3,4-b ]dithiophene (1) (0.168 g, 0.231 mmol),
2
,7-dibromo-9,9-dioctylfluorene (2) (0.063 g, 0.115 mmol),
and 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (3)
0.053 g, 0.115 mmol) in a 10-mL flask was transferred into
(
a dry box, and tris(dibenzylideneacetone)dipalladium (9.00
mg, 0.010 mmol) and tri-o-tolylphosphine (12.0 mg, 0.040
mmol) were added. The mixture was dissolved in chloroben-
zene (4 mL), purged with nitrogen for 1 h, and reacted in a
ꢁ
was obtained after drying under vacuum at 60 C, yielding
0.130 g (58.0%).
ꢁ
microwave reactor at 170 C for 1 h. The reaction mixture
1
H NMR (400 MHz, CDCl ): d (ppm) 8.15 (s, 2H), 7.93 (s,
3
was cooled down to room temperature, and the mixture was
slowly added into a vigorously stirred mixture of 220 mL of
methanol and 10 mL of 1 N HCl. The precipitated polymer
was collected by filtration and purified by dissolution in
chloroform and reprecipitation into methanol several times.
The resulting polymer was also purified by Soxhlet extrac-
tion with acetone/methanol for 2 days to remove oligomers
and the catalyst residue. The final product was obtained af-
2H), 7.49–7.69 (m, 14H), 7.26 (s, 2H), 2.06 (b, 12H), 1.10 (b,
60H), 0.62 (m, 30H).
RESULTS AND DISCUSSION
Synthesis and Characterization
Monomer 1 was synthesized by the stannylation of the 4,4-
0
bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b ]dithiophene with
ꢁ
ter drying in vacuo at 60 C, yielding 0.085 g (49.0%).
trimethyltin chloride after lithiation with n-butyllithium
ꢁ
1
at ꢀ78 C in dry THF. The other CDT-based monomer 5
H NMR (400 MHz, CDCl ): d (ppm) 8.03 (s, 2H), 7.83 (d,
3
was synthesized from the 4,4-bis(2-ethylhexyl)-4H-cyclo-
2
1
H), 7.44–7.63 (m, 6H), 7.13–7.22 (m, 6H), 1.94 (s, 12H),
.05 (b, 60H), 0.62 (m, 30H).
0
penta[2,1-b:3,4-b ]dithiophene by bromination with NBS. The
Fl-based monomer 4 was prepared by boronation with 2-iso-
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane after lithia-
tion of compound 2 in anhydrous THF. The copolymer P1
was synthesized in 49% yield by Stille polymerization of
compounds 1 (2 equiv), 2 (1 equiv), and 3 (1 equiv). P2
was prepared in a similar manner in 58% yield by Suzuki
polymerization of compounds 4 (2 equiv), 5 (1 equiv), and 3
(1 equiv). P1 is a CDT-based copolymer containing 25 mol %
of Fl and 25 mol % of DBT moieties, and P2 is a Fl-based co-
polymer containing 25 mol % of CDT and 25 mol % of DBT
units. New low bandgap copolymers containing two types of
Synthesis of P2
,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-
2
fluorene (4) (0.193 g, 0.300 mmol), 4,4-bis(2-ethylhexyl)-2,6-
0
dibromo-4H-cyclopenta[2,1-b:3,4-b ]dithiophene (5) (0.084 g,
0.150 mmol), and 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothia-
diazole (3) (0.068 g, 0.150 mmol) in a 10-mL flask was trans-
ferred into a dry box, and then 7.00 mg of tetrakis(triphenyl-
phosphine)palladium (2 mol %) was added. Subsequently, the
degassed 2 M aqueous potassium carbonate (4 mL) and
AliquatVR 336 (12.2 mg, 0.030 mmol) in 8 mL toluene were
1250
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ARTICLE
Table 2. The polymer film was prepared by spin casting a 1
wt % solution in chloroform. The absorption maxima of P1
were observed at 447 and 594 nm in chloroform and 452
and 626 nm in the film state. P2 shows the maximum
absorption at 468 nm in chloroform and 477 nm in the film
state. Although P1 and P2 contain the same monomers, the
two polymers show different UV absorption spectra due to
the different chemical connectivity and ICT interactions in
the main backbone. P1 exhibits wide absorption with two
peaks covering the entire visible region (300–800 nm),
whereas P2 shows an absorption spectrum covering 300–
700 nm. As shown in Scheme 1, the CDT-Fl and CDT-DBT
linkages are randomly incorporated in the main chain of P1.
A strong ICT interaction is expected between the electron-
sufficient CDT and electron-deficient DBT moieties, which is
responsible for the long-wavelength absorption in the range
of 550–800 nm. The CDT-Fl linkage covers the short-wave-
length absorption in the range of 300–550 nm. The CDT-Fl
linkage may not lead to a strong ICT and significant red-shift
in the absorption spectrum because both moieties are typical
electron donors. Similarly, the electron rich–poor ICT and
electron rich–rich interactions in P2 are expected between
the Fl and DBT moieties and between the Fl and CDT units,
respectively. The electron–rich Fl and electron–poor DBT
linkage induces strong charge transfer and broad absorption
with kabs ¼ ꢂ470 nm. According to the absorption spectra
of P1 and P2, the ICT interaction in the CDT-DBT units in P1
FIGURE 1 TGA thermograms of the polymers.
electron rich–poor ICT structures for PSC devices were real-
ized by the alternating incorporation of CDT, Fl, and DBT
moieties. In the final polymers, the electron rich–poor (with
strong ICT) interaction in the CDT-DBT moieties and the
electron rich–rich (with weak ICT) interaction in the CDT-Fl
units are expected for P1. Similarly, P2 has two kinds of ICT
interactions in the Fl-DBT and Fl-CDT moieties, which indu-
ces wide absorption. The detailed synthetic routes and struc-
tures of the final polymers are shown in Scheme 1.
The number–average molecular weights (M ) of the synthe-
n
sized copolymers, P1 and P2, were determined to be 9600 g
ꢀ
1
ꢀ1
mol
(PDI ¼ 1.81) and 12,900 g mol
(PDI ¼ 2.12),
respectively, by GPC relative to a polystyrene standard. The
polymers were readily soluble in common organic solvents
(
i.e., toluene, chloroform, and chlorobenzene). The thermal
properties of the polymers were evaluated by TGA and DSC
ꢁ
ꢀ1
under nitrogen at a heating rate 10 C min . The polymers
exhibited good thermal stability, showing ꢃ5% weight loss
ꢁ
on heating to approximately 400 C (Fig. 1). The glass transi-
tion temperature (T
g
) of P1 and P2 was approximately
ꢁ
90 C. The physical properties of the polymers are summar-
ized in Table 1.
Optical Properties
The UV–vis absorption spectra of the polymers in chloro-
form and in film are shown in Figure 2 and summarized in
TABLE 1 Physical Properties of the Polymers
Polydispersity
a
b
M
n
M
w
Index (PDI)
(M /M
ꢀ
1
ꢀ1
c
d
Polymers (g mol
)
(g mol
)
w
n
)
T
d
T
g
ꢁ ꢁ
397 C 94.5 C
P1
P2
a
9,600
17,300
27,300
1.81
2.12
ꢁ ꢁ
414 C 85.7 C
12,900
Number–average molecular weight.
Weight–average molecular weight.
b
c
Decomposition temperature determined by TGA (based on 5% weight
loss) under nitrogen.
d
FIGURE 2 UV–visible absorption spectra of the polymers in
Glass transition temperature determined by a second heating DSC
ꢁ
ꢀ1
curve under nitrogen at a heating rate of 10 C min
.
chloroform (a) and in film (b).
BROAD ABSORPTION POLYMERS BASED ON Fl AND CDT, JUNG ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Optical and Electrochemical Properties of the Polymers
P-Doping
þ
Film
(V vs. Ag/Ag )
e
a
Solution
k
abs
E
g
E
HOMO
E
LUMO
b
c
d
Polymer
k
abs (nm)
(nm)
(eV)
E
ox/onset
(eV)
(eV)
P1
P2
a
447, 594
468
452, 626
477
1.62
1.86
0.43
0.51
ꢀ5.13
ꢀ5.21
ꢀ3.51
ꢀ3.35
Calculated from the HOMO energy level and the optical bandgap in film.
Dilute solution in chloroform.
b
c
Spin-cast film on quartz plate from 1 wt % chloroform solution for 30 s at 1500 rpm.
Bandgap calculated from the absorption edge in solid film.
d
e
HOMO level of the polymer film was determined by measuring Eonset of the first oxidation potential rela-
þ
tive to ferrocene (F
c
), the ionization potential (IP) value of which is ꢀ4.8 eV for the F
c
/F
c
redox system.
is more pronounced with a significant red-shift in absorption
when compared with P2, highlighting the stronger electron
donor ability of the CDT structure than Fl. In addition, the
UV absorption maxima of the polymers in the film state are
red-shifted, and the peak bandwidth is broader than those in
solution because of the increased intermolecular interactions
including enhanced p–p stacking between the neighboring
molecules in the film state. The optical bandgap of the poly-
mers, which was estimated by measuring the absorption
edge in the UV–vis spectra of the film, was found to be 1.62
eV for P1 and 1.86 eV for P2. These values were much
smaller than the bandgap (ꢂ2.23 eV) of a CDT-based
absorption of the polymeric film. By combining EHOMO and
Eg, the LUMO energy levels (ELUMO) of P1 and P2 were
determined to be ꢀ3.51 eV and ꢀ3.35 eV, respectively.
Photovoltaic Properties
To examine the PV properties of the polymers, the BHJ PV
devices were fabricated with the following configuration:
ITO/PEDOT:PSS (30 nm)/polymer:PCBM (80 nm)/Al (100
nm). The PV spectra and device characteristics were meas-
ured under a nitrogen atmosphere. The J–V characteristics of
ꢀ
2
the devices (under AM 1.5G condition, 100 W cm ) as a
function of blend ratio of the (P1 and P2):PCBM mixtures
are shown in Figure 4 and summarized in Table 3. The
open-circuit voltage (Voc) of the polymers is closely related
to the energy difference between the HOMO of the polymer
1
3
copolymer containing electron-rich thiophene derivatives.
This clearly shows that the ICT interactions between the
electron-rich CDT (or Fl) moiety and electron-poor DBT
moiety can decrease the bandgap of the polymers. P1 is
expected to absorb sun light more effectively and show
better PV properties than P2, considering that the maximum
1
6
and the LUMO of PCBM. The Voc for the P1:PCBM blend
and the P2:PCBM mixture was measured to be ꢂ0.64 V and
ꢂ0.82 V, respectively. As shown in Table 2, the HOMO of P1
(ꢀ5.13 eV) is ꢂ0.08 eV higher than that of P2 (ꢀ5.21 eV),
1
4
photon flux of the solar spectrum is around 1.6–1.8 eV.
which is consistent with the measured V values of the devi-
oc
Electrochemical Properties
ces. The Voc changed little at different polymer:PCBM mixing
ratios, which suggests that the devices are well fabricated.
P1 showed the highest PCE value of 1.13% in the device
The electrochemical properties of the copolymers were
examined by CV to determine the highest occupied molecular
orbital (HOMO) and LUMO. The CV measurements were
carried out in a 0.1 M tetrabutylammonium tetrafluoroborate
(
s
Bu
4
NBF
4
) solution in acetonitrile at a scan rate of 50 mV
ꢀ
1
under nitrogen. A platinum (Pt) electrode coated with
the polymer film by dip coating was used as a working elec-
trode, and the Ag/AgNO electrode was used as a reference
3
electrode. The HOMO and LUMO levels of the copolymers
were calculated from the oxidation onset potential relative to
ferrocene (as an internal standard) and the optical bandgap
in film, considering that the IP of the ferrocene/ferrocenium
þ
15
(
Fc/Fc ) redox system is ꢀ4.8 eV. The electrochemical data
for P1 and P2 are summarized in Table 2, and their CVs are
shown in Figure 3. The oxidation potentials of P1 and P2
were found to be 0.43 V and 0.51 V, respectively, which cor-
respond to a HOMO energy level (EHOMO) of ꢀ5.13 eV and
ꢀ
5.21 eV, respectively. The reduction potential could not be
obtained in the cathodic scan of the polymers. Therefore, the
LUMO of the structures was estimated from their EHOMO and
optical bandgap (E ) derived from the spectral edge in the
FIGURE 3 Cyclic voltammograms of the copolymers.
g
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ARTICLE
TABLE 3 Summary of Photovoltaic Properties
Polymer:PCBM
w/w)
V
oc
J
sc
Fill
PCE
(%)
ꢀ
2
(
(V)
(mA cm
)
Factor
P1
P2
1:1
0.64
0.66
0.62
0.63
0.81
0.80
0.84
ꢀ4.40
ꢀ4.92
ꢀ4.32
ꢀ4.08
ꢀ2.12
ꢀ2.11
ꢀ0.97
0.32
0.35
0.33
0.31
0.39
0.39
0.46
0.90
1.13
0.89
0.80
0.66
0.65
0.37
1
1
1
:2
:3
:4
1:1
1
1
:2
:3
FF ¼ 0.39) at a weight ratio of P2:PCBM ¼ 1:1. Similar
device properties were observed at 1:2 blend ratio. At higher
blend ratios (polymer:PCBM ¼ 1:3 and 1:4 w/w), both poly-
mers exhibited reduced J values, which is probably due to
sc
the increased aggregation of PCBM that decreases donor–
acceptor interfacial areas and prevents efficient charge
1
7
carrier separation. The carrier mobility of the polymers
FIGURE 4 Photovoltaic J–V characteristics of (a) P1:PCBM and
b) P2:PCBM mixtures with different blend ratios under AM
.5G illumination. (c) External quantum efficiency (EQE) of PSC
devices with an active layer of the polymer (P1 and P2):PCBM
w/w ¼ 1:2).
(
1
(
with a weight ratio of P1:PCBM ¼ 1:2, at which the open-cir-
cuit voltage (Voc), short-circuit current density (Jsc), and fill
factor (FF) of the device were determined to be 0.66 V,
ꢀ
2
ꢀ
4.92 mA cm , and 0.35, respectively. P2 showed the best
FIGURE 5 AFM topographic images of P1:PCBM (top) and
P2:PCBM (bottom) blended films at 1:2 w/w blend ratio.
ꢀ
2
PCE value of 0.66% (V ¼ 0.81 V, Jsc ¼ ꢀ2.12 mA cm
oc
,
BROAD ABSORPTION POLYMERS BASED ON Fl AND CDT, JUNG ET AL.
1253

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
was measured with the blended films (polymer:PCBM ¼ 1:2
MEST, Korea, and a grant from the Fundamental R&D Program
for Core Technology of Materials (no. M2009010025) funded
by the Ministry of Knowledge Economy (MKE), Republic of
Korea (H. Y. Woo).
w/w) using the space-charge-limited current method
1
8
reported elsewhere.
The measured hole mobility of
ꢀ
5
2
ꢀ1 ꢀ1
P1:PCBM and P2:PCBM was 6.2 ꢄ 10 cm
V
s
and
ꢀ
5
2
ꢀ1 ꢀ1
9
.8 ꢄ 10 cm V
s , respectively. The polymers showed
similar hole mobility. This relatively poor mobility is closely
related to the amorphous morphology of the polymers due
to the random incorporation of the Fl, CDT, and DBT units in
the main chain.
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This study was supported by KISCO (H.-K. Shim). This study
was supported by the NRF grant (2009-00605) funded by the
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