Benzodithiophene-thiophene-based photovoltaic polymers with different side-chains

Article abstract of DOI:10.1002/pola.27516

A series of benzodithiophene-thiophene-based alternating copolymers were synthesized with different side-chains, and their photovoltaic characteristics were examined. The choice of solubilizing side-chains influences significantly the chain conformation, frontier orbital energy levels, intermolecular organization, and the resulting optical, morphological, and photovoltaic properties. The incorporation of an e-withdrawing carbonyl group in the side-chain decreased the highest occupied molecular orbital (HOMO, ca. -5.4 eV) level and improved the chain planarity through intrachain hydrogen bonding. The shortest π-π stacking distance (3.72 ?) was also measured for the alkylcarbonyl-substituted BDTCOT:PC₇₁BM blended film by two dimensional grazing incidence X-ray scattering. With compared to other polymers, the BDTCOT:PC₇₁BM device showed a substantially improved open-circuit voltage and short-circuit current density, leading to a 4.66% power conversion efficiency. The side-chains need to be designed to be multifunctional to induce a deep HOMO level and chain planarity (for interchain ordering) as well as good solution processability.

Full text of DOI:10.1002/pola.27516

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Benzodithiophene-Thiophene-Based Photovoltaic Polymers with Different
Side-Chains
Thanh Luan Nguyen,1,2 Seyeong Song,3 Seo-Jin Ko,3 Hyosung Choi,3 Ji-Eun Jeong,1,2
Taehyo Kim,3 Sungu Hwang,4 Jin Young Kim,3 Han Young Woo1,2
1Department of Nanofusion Engineering, Pusan National University, Miryang 627-706, Korea
2Department of Cogno-Mechatronics Engineering, Pusan National University, Miryang 627-706, Korea
3School of Energy and Chemical Engineering and KIER-UNIST Advanced Center for Energy, Ulsan National Institute of Science
and Technology (UNIST), Ulsan 689-798, Korea
4Department of Nanomechatronics Engineering, Pusan National University, Miryang 627-706, Korea
Correspondence to: H. Y. Woo (E-mail: hywoo@pusan.ac.kr) or J. Y. Kim (E-mail: jykim@unist.ac.kr).
Received 6 November 2014; accepted 17 December 2014; published online 00 Month 2015
DOI: 10.1002/pola.27516
ABSTRACT: A series of benzodithiophene-thiophene-based alter-
nating copolymers were synthesized with different side-chains,
and their photovoltaic characteristics were examined. The
choice of solubilizing side-chains influences significantly the
chain conformation, frontier orbital energy levels, intermolecu-
lar organization, and the resulting optical, morphological, and
photovoltaic properties. The incorporation of an e-withdrawing
carbonyl group in the side-chain decreased the highest occu-
pied molecular orbital (HOMO, ca. 25.4 eV) level and improved
the chain planarity through intrachain hydrogen bonding. The
two dimensional grazing incidence X-ray scattering. With com-
pared to other polymers, the BDTCOT:PC71BM device showed
a substantially improved open-circuit voltage and short-circuit
current density, leading to a 4.66% power conversion effi-
ciency. The side-chains need to be designed to be multifunc-
tional to induce a deep HOMO level and chain planarity (for
interchain ordering) as well as good solution processability.
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shortest p–p stacking distance (3.72 A) was also measured for
KEYWORDS: benzodithiophene; conjugated polymers; copoly-
the alkylcarbonyl-substituted BDTCOT:PC71BM blended film by
merization; organic photovoltaics; renewable resources
To further optimize the PSCs, the molecular design of pho-
tovoltaic polymers should consider the low bandgap, deep
highest occupied molecular orbital (HOMO) level, chain pla-
narity and interchain interactions with a crystalline inter-
molecular organization for efficient charge carrier transport
with minimal electron–hole recombination. These are
closely related to the photovoltaic parameters, that is, open-
circuit voltage (VOC), short-circuit current density (JSC), and
fill factor (FF). The strong crystalline organization and deep
HOMO level of photovoltaic polymers can also improve the
thermal and oxidational stability of PSC devices. Crystalline
low bandgap polymers may be a promising candidate to
satisfy the above requirements. The low bandgap induces
broad absorption from visible to near-infrared (NIR) wave-
lengths of the solar spectrum19–21 and the coplanarity of a
polymer chain with tight solid-state p–p stacking can
improve both polaron and exciton delocalization as well as
their transport characteristics, thereby increasing the JSC
and FF.22,23
INTRODUCTION Polymer solar cells (PSCs) have been investi-
gated widely for portable and flexible (or even wearable) pho-
tovoltaic devices via a roll-to-roll process with low cost ambient
solution-processing technologies.1 Despite the many advantages
of PSCs, that is, chemically tunable optoelectronic properties,
light weight, mechanical flexibility and large-area solution proc-
essing, low power conversion efficiency (PCE) and poor opera-
tional stability have been
a major impediment to real
commercialization.2,3 Over the past decade, a remarkable pro-
gress has been achieved by developing ideal photovoltaic poly-
mers and optimization of the device architectures, leading to a
more than 9% PCE in a single cell architecture.4,5 A variety of
conjugated photovoltaic polymers have been designed and syn-
thesized based on electron-rich and poor units, such as carba-
zole, cyclopenta[2,1-b:3,4-b0]dithiophene, dithieno[3,2-b:20,30-
d]silole, benzo[1,2-b:4,5-b0]dithiophene, thienothiophene, naph-
thalene, dithieno[2,3-b:7,6-b]carbazole, naphtho[2,3-b:6,7-
b0]dithiophene, benzo[1,2-b:4,5-b0]difuran, thienopyrrolodione
and diketopyrrolopyrrole, and so forth.6–18
Additional Supporting Information may be found in the online version of this article.
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Generally, a conjugated polymer is comprised of the follow-
ing three components: conjugated backbone, side-chains, and
substituents.24,25 The conjugated backbone dictates most of
the optical, electrochemical, electronic, and electrical proper-
ties. On the other hand, the side-chains play a critical role in
improving the molecular weight, solubility and processability
of conjugated polymers. Finally, substituents are often used
to fine-tune the properties of conjugated polymers, particu-
larly the electronic properties (energy levels, bandgap,
mobility, etc.).26 To induce a crystalline morphology of the
photovoltaic active layer, the solubilizing side-chain and the
chemical structure of the main backbone should be consid-
ered carefully. Alkyl substituents on the conjugated chain
improve the solubility but often increase the HOMO level
and disturb the intermolecular ordering with an increased
torsional angle of the main chain.27,28 An effective approach
to planarize a polymer chain with strong interchain organiza-
tion (without losing its solution processability) is to create
intra- and/or interchain noncovalent coulomb interactions
via hydrogen bonds and dipole–dipole interactions, and so
forth.29–31 Noncovalent coulomb interactions (via SAO, SAF,
and CHAN) have often been emphasized in controlling the
chain conformation and intermolecular stacking orientation,
and so forth.4,32,33 The choice of solubilizing side-chains
should consider the solution processability without disturb-
ing the crystalline ordering of the photovoltaic polymers.
ical Laboratory, South Korea. The number- and weight-
average molecular weights of polymers were determined by
gel permeation chromatography (GPC) with chloroform as
eluent on an Agilent GPC 1200 series, relative to polystyrene
standards. UV–vis spectra were obtained with a JASCO V-630
spectrophotometer. Cyclic voltammetry (CV) experiments
were performed with Versa STAT 3 analyzer. All CV measure-
ments were carried out in 0.1 M tetrabutylammonium tetra-
fluoroborate (Bu4NBF4) in acetonitrile (scan rate, 50 mVs21
)
with a conventional three-electrode configuration, employing
a platinum wire as the counter electrode, a platinum elec-
trode coated with a thin polymer film as the working elec-
trode, and the Ag/AgNO3 reference electrode. Thermo
gravimetric analysis (TGA) and differential scanning calorim-
etry (DSC) measurements were performed using the 2050
TGA V5.4A and DSC Q200 V24.4 instruments under a nitro-
gen atmosphere at a heating and cooling rate of 10 ꢀC/min.
Atomic force microscopy (AFM) images were obtained using
a Veeco AFM microscope in a tapping mode. Grazing inci-
dence wide-angle X-ray scattering (GIWAXS) measurements
were carried out at a PLS-II 9A U-SAXS beam line of Pohang
Accelerator Laboratory, South Korea.
Solar-Cell Fabrication and Characterization
ITO-coated glasses were cleaned with detergent, rinsed with
distilled water, ultra-sonicated in acetone and isopropyl alcohol,
ꢀ
and subsequently dried in an oven overnight at 100 C. After
In this study, a series of benzodithiophene (BDT)-thiophene
(T)-based alternating copolymers were synthesized by modi-
fying the side-chains and their structure–property relation-
ships were examined in terms of the polymer main-chain
conformation, frontier orbital levels, film morphology and the
resulting photovoltaic properties. The 3-position of the thio-
phene moiety was substituted with hydrogen (BDTT), n-octyl
(BDTOT), and octylcarbonyl groups (BDTCOT). A conforma-
tional lock with chain planarity is expected by incorporating
the octylcarbonyl group through hydrogen bonding between
the electronegative carbonyl oxygen and hydrogen in the
neighboring BDT moiety.34 In addition, e-withdrawing car-
bonyl functionality decreases the HOMO level of the resulting
polymer (BDTOT: 25.28 eV, BDTCOT: 25.4 eV), which not
only increases the VOC of the PSCs, but also improves the envi-
ronmental or oxidational stability by minimizing p-doping by
ambient oxygen. This article discusses the effects of the side-
chain on the optical, electrochemical, and morphological prop-
erties, as well as the resulting photovoltaic characteristics.
UV/ozone treatment for 20 min, poly(3,4-ethylenedioxythiophe-
ne):polystyrene sulfonic acid (PEDOT:PSS, Clevios P VP.AI4083,
Germany) solutiꢀon was spin cast at 5000 rpm for 40 s, dried for
10 min at 140 C, and transferred into a nitrogen filled glove
box. Solutions containing a mixture of polymer:[6,6]-phenyl-C71
butyric acid methyl ester (PC71BM) in o-dichlorobenzene (DCB,
12 mg/mL) were spin cast on top of the PEDOT:PSS layer. The
devices were pumped down in vacuum (<1026 Torr) and a
100 nm thick Al layer was deposited on top of the active layer
by thermal evaporation (active area: 13 mm2). A shadow mask
(13 mm2) was used for the measurement of device characteris-
tics. Current density–voltage (J–V curve) characteristics were
measured using a Keithley 2635A source measurement unit
under an AM 1.5G illumination at 100 mWcm22. External quan-
tum efficiency (EQE) was measured (PV measurements) by
applying monochromatic light from a xenon lamp under ambi-
ent conditions. The monochromatic power was calibrated rela-
tive to a standard Si photodiode.
Synthesis
3-Cyanothiophene (3a)
EXPERIMENTAL
A mixture of 3-bromothiophene (2.00 g, 12.3 mmol) and
copper (I) cyanide (CuCN) (1.7 g, 18.9 mmol) in dry N,N-
dimethylformamide (DMF, 10 mL) was stirred under reflux
for 4 h. After cooling, the resulting solution was added to a
solution of ion (III) chloride (FeCl3) (8.50 g) in 15 mL aq.
General
All reagents and monomer 1 were purchased from Sigma-
Aldrich and used without further purification. Monomer 2
was prepared by following the procedures reported previ-
ously.35–37 Microwave reactor (Biotage InitiatorTM) was used
for polymerization reactions. 1H and 13C NMR spectra were
recorded on a JEOL (JNM-AL300) FT NMR system operating
at 300 and 75 MHz, respectively. Elemental analysis was per-
formed at Korea Basic Science Institute, Seoul Branch Analyt-
ꢀ
hydrochloric acid (HCl, 2 M) and further reacted at 70 C for
45 min. After cooling to room temperature, the reaction mix-
ture was extracted with dichloromethane (CH2Cl2) three
times. The extracted organic layer was washed with 6 M HCl,
water, saturated sodium bicarbonate (NaHCO3) solution and
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water successively. The organic layer was dried over anhy-
drous magnesium sulfate. The crude product was then puri-
fied by column chromatography on silica gel (eluent,
hexane:CH2Cl2 5 50:50 by volume) to afford a light yellow
liquid (72% yield).
reduced pressure and the residue was purified by column
chromatography on silica gel (eluent, hexane:CH2Cl25 80:20
by volume) to give a pure product as a light yellow liquid
(80% yield).
1H NMR (300 MHz, CDCl3, d, ppm): 7.28 (s, 1H), 2.85
(t, J 5 7.2 Hz, 2H), 1.68 (m, 2H), 1.25 (br, 10H), 0.86
(t, J 5 6.8 Hz, 3H). 13C NMR (75 MHz, CDCl3, d, ppm): 194.0,
139.9, 131.3, 116.5, 111.8, 41.8, 31.8, 29.4, 29.2, 29.1, 23.9,
22.6, 14.1. Found: C, 41.11; H, 4.7; S, 8.27. Calc. for
1H NMR (300 MHz, CDCl3, d, ppm): 7.88 (dd, J 5 3Hz, J 5 1.2
Hz, 1H), 7.36 (dd, J 5 5.8 Hz, J 5 3.0 Hz, 1H), 7.24 (dd, J 5
5.8 Hz, J 5 1.2 Hz, 1H); 13C NMR (75 MHz, CDCl3, d, ppm):
135.5, 128.6, 127.2, 115.2, 110.6.
C13H18Br2OS: C, 40.86; H, 4.75; S, 8.39%.
Thiophen-3-yl-octyl Ketone (3b)
Synthesis of Copolymers
In glove box, 2,6-bis(trimethyltin)-4,8-bis(alkoxy)-
A Gringard reagent, 2 M octylmagnesium bromide in hexane
(6.9 mL, 13.8 mmol) was added dropwise to compound 3a (1 g,
9.2 mmol) in dry ether (15 mL). After the resulting mixture was
refluxed overnight, water (10 mL) was added and further
stirred for 15 min. After cooling to room temperature, the mix-
ture was extracted with CH2Cl2. The organic layer was dried
over anhydrous magnesium sulfate and concentrated by rotary
evaporation. The crude solid was then purified by column chro-
matography on silica gel (eluent, hexane:CH2Cl2 5 80:20 by vol-
ume) to give a brown liquid (50% yield).
a
benzo[1,2-b:4,5-b0]dithiophene (4) (0.3 mmol), dibromo-
monomer (1–3) (0.3 mmol), tris(dibenzylideneacetone)di-
palladium(0) (2 mol %), tri(o-tolyl)phosphine (8 mol %)
and 2 mL chlorobenzene was added in a 5 mL microwave
vial. The reaction mixture was heated at 80 ꢀC (10 min),
at 100 ꢀC (10 min) and at 140 ꢀC (40 min) in a micro-
wave reactor. The solution was cooled down to room tem-
perature and 2-(tributylstannyl)thiophene (0.05 g, 0.14
mmol) was added for end-capping and further reacted at
140 ꢀC for 20 min. The reaction solution was cooled
down again and 2-bromothiophene (0.09 g, 0.53 mmol)
was added by syringe. The mixture was heated at 140 ꢀC
for another 20 min in a microwave reactor. After the reac-
tion was finished, the crude polymer was precipitated into
a mixture of methanol and HCl (36%) (350 mL:10 mL)
and further purified by Soxhlet extraction with acetone,
hexane and chloroform. The dissolved portion in chloro-
form was concentrated under reduced pressure and pre-
cipitated into cold methanol. The polymer was filtered and
dried under vacuum.
1H NMR (300 MHz, CDCl3, d, ppm): 8.02 (dd, J 5 3.0 Hz, J 5 1.2
Hz, 1H), 7.52 (dd, J 5 5.8 Hz, J 5 1.2 Hz, 1H), 7.28 (dd, J 5 5.8
Hz, J 5 3.0 Hz, 1H), 2.84 (t, J 5 7.2 Hz, 2H), 1.68 (m, 2H), 1.25
(br, 10H), 0.86 (t, J 5 6.8 Hz, 3H); 13C NMR (75 MHz, CDCl3, d,
ppm): 194.9, 142.5, 131.6, 127.0, 126.2, 40.0, 31.8, 29.4, 29.3,
29.1, 24.4, 22.6, 14.0. Found: C, 69.18; H, 9.09; S, 14.34. Calc.
for C13H20OS: C, 69.59; H, 8.98; S, 14.29%.
2,5-Dibromothiophen-3-yl-octyl Ketone (3)
N-bromosuccinimide (NBS) (0.95 g, 5.3 mmol) was added to
compound 3b (1 g, 4.5 mmol) in anhydrous DMF (20 mL)
ꢀ
and the mixture was stirred overnight at 50 C. After cooled
BDTT
to room temperature, the reaction solution was poured into
water. The mixture was extracted with dichloromethane
three times and the combined organic layer was dried over
anhydrous sodium sulfate. Solvent was removed under
Yield: 50%.1H NMR (300 MHz, CDCl3, d, ppm) 7.50–6.80 (br,
3H), 4.04 (br, 4H), 2.81 (br, 2H), 1.89 (br, 6H), 1.80–1.30 (br,
30H), 0.88 (br, 9H).
SCHEME 1 Synthesis of the monomers and copolymers. Reagents and reaction conditions: (i) CuCN, DMF, FeCl3, HCl; (ii)
C8H17MgBr, diethyl ether; (iii) NBS, DMF; (iv) Pd2(dba)3, tri(o-tolyl)phosphine, chlorobenzene.
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TABLE 1 Summary of Photophysical, Electrochemical, and Thermal Properties of Polymers
Polymer
Mn (kDa)a
PDI (Mw/Mn)
kabs [sol.] (nm)
konset [film] (nm)
Eopt (eV)b
HOMO (eV)c
LUMO (eV)d
Td(ꢀC)e
g
BDTT
8.2
34
32
2.5
1.5
1.9
498
494
535
595
600
630
2.08
2.07
1.97
25.31
25.28
25.40
23.23
23.21
23.43
341
333
339
BDTOT
BDTCOT
a
d
Number-average molecular weight (Mn) determined by GPC in
LUMO level was estimated from the HOMO value and optical bandgap
in film.
chloroform.
b
e
Optical bandgap in film.
HOMO level was measured from the onset of oxidation by cyclic
Decomposition temperature (Td) was determined by TGA with 5%
c
weight-loss.
voltammetry.
BDTOT
prepared using a slight modification of previously reported
procedures.35–37 The detailed synthetic procedure for mono-
mer 3 is described in the experimental section. Briefly,
3-cyanothiophene (3a) was prepared in 72% yield from 3-
bromothiophene with CuCN in the presence of FeCl3 in 2 M
aqueous HCl. Compound 3a was further reacted with a Gri-
gnard reagent, octylmagnesium bromide, in dry ether to
yield thiophen-3-yl-octyl ketone (3b, 50% yield), which was
brominated with N-bromosuccinimide in DMF to form mono-
mer 3 (80% yield). Three different types of BDT-T copoly-
mers were synthesized in 50–84% yield via a microwave-
assisted Stille coupling reaction of the dibromo monomers
(1–3) and bis(trimethylstannyl)benzodithiophene (4) with
Yield: 84%. 1H NMR (300 MHz, CDCl3, d, ppm): 7.20–6.80
(br, 4H), 4.04 (br, 4H), 2.00–0.82 (br, 34H).
BDTCOT
Yield: 71%. 1H NMR (300 MHz, CDCl3, d, ppm): 7.79 (br,
1H), 7.50–7.00 (br, 2H), 4.29 (br, 4H), 2.83 (br, 2H), 1.93 (br,
6H), 1.80–1.00 (br, 30H), 0.88 (br, 9H).
RESULTS AND DISCUSSION
Synthesis and Characterization
Scheme 1 shows the synthetic routes to the BDT-T alternat-
ing copolymers. The intermediates and monomers 1–3 were
the
catalysts,
tris(dibenzylideneacetone)-dipalladium(0)
FIGURE 1 (a) Torsional profiles and (b) energy minimum conformations (red: oxygen, yellow: sulfur). [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
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31G** level).38,39 For a simple calculation, the ethyl moiety
was considered to be an alkyl side-chain in place of octyl
and ethylhexyl substituents. Figure 1 presents the torsional
profiles [Fig. 1(a)] and energy minimized conformational
structures [Fig. 1(b)]. Side-chain substitution increases the
torsional angle between the BDT and T moieties compared
to unsubstituted BDTT (dihedral angle: ꢁ16ꢀ) due to the
increased steric hindrance. The introduction of an ethyl side-
chain on the thiophene unit increases the dihedral angle
(ꢁ40ꢀ). Interestingly, BDTCOT has a smaller torsion angle
(ꢁ30o) compared to BDTOT, probably due to a non-covalent
hydrogen bonding interaction between the carbonyl oxygen
and hydrogen in the neighboring BDT moiety. According to
the torsional profiles for BDTOT, there are two minimum
energy conformations (ca. 40ꢀ and 140ꢀ) for the BDT-T link-
age. On the other hand, BDTCOT shows one main conforma-
tion with the minimum energy, indicating that O---H
hydrogen bonding plays a key role in fixing the conforma-
tion. Intra- and/or intermolecular hydrogen bonding interac-
tions may also increase the interchain organization, leading
to a semicrystalline film morphology.
Optical Properties
Figure 2(a) shows the UV–vis absorption spectra of the copoly-
mers in toluene and in thin film. The absorption maxima for
BDTT, BDTOT and BDTCOT in toluene were observed at
kabs 5 498, 494, and 535 nm, respectively. By attaching the e-
withdrawing carbonyl-containing side-chain (BDTCOT), the
spectrum showed a substantial red-shift compared to those of
FIGURE 2 (a) UV–vis absorption spectra in toluene (open) and in
film (solid). (b) Emission spectra of polymers in toluene (open)
and in tetrahydrofuran (solid).
(2 mol %) and tri(o-tolyl)phosphine (8 mol %) in 1 mL
chlorobenzene. The resulting polymers were purified by
Soxhlet extraction with acetone, hexane and chloroform, and
the portion dissolved in chloroform was re-precipitated into
methanol. The number-average molecular weight (Mn) was
determined to be 8.2 (polydispersity index, PDI 5 2.5), 34
(1.5), and 32 kDa (1.9) for BDTT, BDTOT and BDTCOT,
respectively (Table 1). The relatively low Mn of BDTT is
probably due to the poor solubility without the solubilizing
alkyl chains on the thiophene unit. With the exception of
BDTT, all the polymers (with alkylated thiophene) had a
moderate or high molecular weight and were readily soluble
in common organic solvents (such as chloroform, toluene,
and chlorobenzene). The thermal properties of the polymers
were analyzed by TGA and DSC under nitrogen (Supporting
Information Fig. S1). All polymers showed a similar decom-
position temperature (Td) of ꢁ335 ꢀC with 5% weight loss
and no thermal transitions were measured in the DSC meas-
ꢀ
urements in the range, 30–300 C.
Conformational Analysis by Density Functional Theory
Conformational analysis was performed for the BDT-T series
copolymers by density functional theory (DFT, B3LYP/6–
FIGURE 3 (a) Cyclic voltammogram and (b) energy band dia-
gram of polymers.
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pared to those of BDTT and BDTOT, indicating the stronger
intramolecular charge transfer (ICT) nature of BDTCOT. The
kPL for BDTCOT was red-shifted by 32 nm by changing the
solvent from toluene to THF. This also supports the ICT-
related polar properties of BDTCOT with the electron-
withdrawing carbonyl moiety in its side-chains.
Electrochemical Properties
Different alkyl substituents on the same conjugated back-
bone also influenced the electrochemical properties. CV was
performed in 0.1 M tetrabutylammonium tetrafluoroborate
(Bu4NBF4) in acetonitrile with a scan rate of 50 mVs21 (Fig.
3). The HOMO energy level was estimated from the tangen-
onset
ox
tial onset potential of oxidation (E
) against an internal
standard of ferrocene (Fc).40,41 The LUMO level was calcu-
lated from the HOMO value and the corresponding optical
bandgap (ELUMO5 EHOMO 1 Eg). The calculated HOMO energy
levels of BDTT, BDTOT, and BDTCOT were 25.31, 25.28, and
25.40 eV, respectively. The e-withdrawing carbonyl group
clearly decreased the frontier orbital energy levels. The rela-
tively deep HOMO energy level of BDTCOT is desirable for
achieving a high VOC when the PSC is fabricated with fuller-
ene derivatives. Table 1 lists the physical, optical, thermal,
and electrochemical properties of the polymers.
Photovoltaic Properties
To examine the photovoltaic properties of the BDT-T series
polymers, conventional single cell devices were fabricated
with a configuration of ITO/PEDOT:PSS/polymer:PC71BM/Al.
Bulk heterojunction (BHJ) solar cell devices were prepared
by blending polymers with PC71BM in o-dichlorobenzene
(DCB). To optimize the device fabrication conditions, the PSC
devices were tested under a range of conditions, such as the
different blend ratio, thermal treatment, and processing addi-
tives (Supporting Information Fig. S2 and Table S1).
FIGURE 4 (a) J–V characteristics and (b) EQE spectra of poly-
mer:PC71BM photovoltaic devices.
BDTT and BDTOT. Interestingly, the absorption spectrum of
BDTCOT was red-shifted by ꢁ40 nm compared to those of
BDTT and BDTOT. This can be explained by the increased
effective conjugation originating from the higher electron
accepting ability of the octylcarbonyl group. From solution to
film, the maximum absorption for BDTT, BDTOT, and BDTCOT
was red-shifted to kabs5 541, 515, and 540 nm, respectively,
because of the pronounced interchain packing with enhanced
effective p-conjugation. The optical bandgap for BDTT, BDTOT,
and BDTCOT was estimated to be 2.08, 2.07, and 1.97 eV, from
the onset wavelength in the film.
The best performing devices were obtained using the blend,
polymer:PC71BM5 1:1 (by weight) in DCB (12 mg/mL). Fig-
ure 4(a,b) shows the current density–voltage (J–V) character-
istics and EQE spectra of the devices. Table 2 lists the
detailed photovoltaic parameters. The BDTT-based device
showed a VOC of 0.75 V, JSC of 5.95 mA/cm2, FF of 0.56, and
a resulting PCE of 2.52%. BDTOT showed a slightly improved
PCE of 2.68%. The measured VOC follows the HOMO energy
levels of the polymers, showing 0.68, 0.75, and 0.81 V for
BDTOT, BDTT, and BDTCOT, respectively. The octylcarbonyl
side-chain clearly down-shifted the HOMO level and
increased the VOC. The best photovoltaic performance was
obtained for BDTCOT:PC71BM: VOC of 0.81 V, JSC of 9.22 mA/
cm2, FF of 0.62, and a resulting PCE of 4.66%.
The emission spectra of the polymers were also measured in
toluene and tetrahydrofuran (THF). The respective emission
maxima (kPL) for BDTT, BDTOT, and BDTCOT were observed
at 570, 548, and 634 nm in toluene, and 572, 575, and
666 nm in THF. As shown in Figure 2(b), the kPL of BDTCOT
was red-shifted substantially in both toluene and THF com-
TABLE 2 Summary of Photovoltaic Parameters
JSC [cal.]
The film morphology was analyzed by AFM and GIWAXS to
further understand the different photovoltaic characteristics
of the three polymers. Figure 5 shows AFM topography
images of the blended films with PC71BM. The BDTT:PC71BM
film showed an undesirable phase separation that formed a
larger domain size with a root mean square (RMS) rough-
ness of ꢁ4.1 nm (compared to the other polymer blends,
rms 5 ꢁ1 nm), leading to reduced JSC. This might be due to
Polymer JSC (mA/cm2) VOC (V)
FF
PCE (%) (mA/cm2)a
BDTT
5.95
6.84
0.75
0.68
0.81
0.56 2.52
0.57 2.68
0.62 4.66
5.56
6.20
9.53
BDTOT
BDTCOT 9.22
a
JSC [cal.], calculated JSC from a EQE curve.
6
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FIGURE 5 AFM topography images for blend films (1.5 lm 3 1.5 lm).
the poor solubility of BDTT, showing interchain aggregation
without solubilizing side-chains on thiophene. We also meas-
ured the photoluminescence (PL) spectra of pristine and
blended polymer films to compare the PL quenching and
exciton separation. The generated excitons from absorbed
photons are dissociated into free charge carriers in an active
layer, inducing PL quenching that gives an indication of an
effective charge transfer in a donor and acceptor blended
system. The pristine and blended polymer films were pre-
pared under the same condition for device fabrication. As
shown in Supporting Information Figure S3, PL spectra were
measured by exciting at 470 nm, showing the PL maxima at
630, 630, and 690 nm for pristine BDTT, BDTOT, and
BDTCOT, respectively. Upon blending with PC71BM, a clear
PL quenching was measured for all blended films. The PL
quenching efficiency was roughly estimated to be 87.5, 94,
and 95.9% for the BDTT:PC71BM, BDTOT:PC71BM, and
BDTCOT:PC71BM, respectively. The relatively small PL
quenching for BDTT:PC71BM must be related to the film
morphology where the larger domains were observed due to
the poor solubility of BDTT with interchain aggregation and
less interfaces with PC71BM.
The molecular arrangements and packing characteristics of the
pristine and blend films were also investigated by GIWAXS. Fig-
ure 6 and Supporting Information Figure S4 shows the GIWAXS
diffraction profiles for the pristine and polymer:PC71BM blend
films in the xy (in-plane) and z (out-of-plane) directions. The
interlamellar spacing for the pristine films was calculated to be
17, 24.2, and 26.2 Å for the BDTT, BDTOT, and BDTCOT, respec-
tively, from the (100) peak in the in-plane direction. In particular,
the interlamellar distance of BDTT was substantially small (17
Å) compared to the other structures (24–26 Å) due to the
absence of alkyl side-chains on the thiophene moiety. All the
polymers showed a pronounced (010) peak in the out-of-plane
direction, suggesting a favorable face-on orientation. The meas-
ured p–p stacking distance for the pristine BDTT, BDTOT, and
BDTCOT films was 3.93, 3.81, and 3.74 Å, respectively. The inter-
chain hydrogen bonding interactions in BDTCOT may induce
tighter p–p interchain packing compared to BDTT and BDTOT.
Interestingly, BDTT showed the shortest interlamellar spacing
(17 Å) but the longest p–p stacking distance, suggesting that the
absence of alkyl substituents on thiophene induces tight lamellar
packing with poor short-range ordering. The blended films with
PC71BM showed a similar interlamellar distance to those of the
pristine films (17 Å for BDTT:PC71BM, 24–25 Å for other poy-
mers:PC71BM), indicating negligible intercalation of PC71BM in
the polymer domain. This is probably due to the close distance
between the side-chains of neighboring repeat units in the main
backbone, where the spacing is not enough to allow intercalation
of PC71BMs.42,43
FIGURE 6 2D-GIWAXS images of pristine and blend films.
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CONCLUSIONS
A series of benzodithiophene (BDT) and thiophene (T)-based
photovoltaic copolymers with varying solubilizing side-
chains were synthesized and their optical, electrochemical,
morphological, and photovoltaic properties were studied.
The side-chains of the photovoltaic polymers need to be
designed carefully to induce a deep HOMO level and chain
planarity to improve the interchain ordering and charge car-
rier transport as well as the good solution processability.
With regard to the above concerns, it is a useful strategy to
induce intrachain hydrogen bonding and/or dipole-dipole
coulomb interactions between the moieties in the main chain
and alkyl side-chains. The e-withdrawing octylcarbonyl side-
chain was incorporated onto the BDT-T copolymeric back-
bone (BDTCOT) to induce a deep frontier orbitals and intra-
chain hydrogen bonding between the carbonyl oxygen and
hydrogen in the neighboring BDT. BDTCOT showed good sol-
ubility in common organic media and a smaller torsion angle
between the BDT and thiophene moieties compared to that
in octyl-substituted BDTOT. Among the three polymers,
BDTCOT showed the deepest HOMO (25.4 eV) and the
smallest p–p stacking distance (3.72 Å), due to the e-
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hydrogen bonding interactions. The BDTCOT:PC71BM device
exhibited promising photovoltaic characteristics with a JSC of
9.22 mA/cm2, a high VOC of 0.81 V and a resulting PCE of
4.66%. This side-chain design strategy can possibly be
extended to other efficient low bandgap polymeric structures
to further improve the photovoltaic properties by forming a
deeper HOMO, broader absorption and semi-crystalline inter-
chain packing without damage in the solution processability.
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ACKNOWLEDGMENTS
T. L. Nguyen and S. Song contributed equally to this work. This
work was supported by the National Research Foundation
(NRF) of Korea (2012R1A1A2005855, 2009-0093020) and the
New & Renewable Energy Core Technology Program of the
Korea Institute of Energy Technology Evaluation and Planning
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