Effect of the alkyl chain length of C70-PCBX acceptors on the device performance of P3HT:C70-PCBX polymer solar cells

Article abstract of DOI:10.1039/c0jm02459g

A series of soluble C₇₀-derivatives (C₇₀-PCBX) was synthesized by varying the alkyl chain length of the adduct (X = R₁, R₃, R₅, R₇, R₉) attached to the C₇₀-polyhedron.

Full text of DOI:10.1039/c0jm02459g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Effect of the alkyl chain length of C70-PCBX acceptors on the device
performance of P3HT : C70-PCBX polymer solar cells†
Won Suk Shin, Jong-Cheol Lee, Jae-Ryoung Kim, Hye Young Lee, Sang Kyu Lee, Sung Cheol Yoon
*
and Sang-Jin Moon
Received 29th July 2010, Accepted 3rd October 2010
DOI: 10.1039/c0jm02459g
A series of soluble C70-derivatives (C70-PCBX) was synthesized by varying the alkyl chain length of the
adduct (X ¼ R1, R3, R5, R7, R9) attached to the C70-polyhedron. After blending with P3HT, bulk
heterojunction polymer solar cells were fabricated, and their performances were evaluated. As the alkyl
chain length increased, phase segregation between the P3HT and C70-PCBX domains in the
P3HT : C70-PCBX blend active layer increased, which enhanced the device performance to a moderate
degree. The performance enhancement may have been due to the increased solubility and self-
aggregation of C70-PCBX as a result of the longer alkyl chains, as was predicted by molecular
simulations. The nanomorphologies of the P3HT : C70-PCBX active layers offered an explanation for
the trend in solar cell performance: as the series progressed from C70-PCBR1 to C70-PCBR7, the
open circuit voltage and the short circuit current gradually increased, followed by a slight drop for
C70-PCBR9. An optimal overall power conversion efficiency was observed for C70-PCBR7 with
a P3HT : C70-PCBR7 blend ratio of 1 : 0.8 to 1 : 0.9.
although most have not shown properties superior to those of
Introduction
C60- and C70-PCBM.5,6 Only a few novel materials, such as
endohedral or bis-indene fullerenes, have shown good photo-
voltaic properties.7 The search has continued for new types of
electron acceptor materials, such as perylene and graphene,8 for
use in solution-based organic solar cells. However, new materials
that perform better than fullerene derivatives have not yet been
identified.
In addition to the known advantages of organic solar cells, which
include low manufacturing costs, light weight, thinness, flexi-
bility and possible transparency, solution-based polymer solar
cells provide manufacturing flexibility; for example, such solar
cells can be fabricated using ink-jet, spray, brush, doctor blading,
or roll-to-roll printing technologies.1 A dramatic increase in the
energy conversion efficiencies of solution-based polymer solar
cells may be achieved by the introduction of soluble fullerene
derivatives,2 and the success of this method subsequently
attracted the interest of the solar industry. Recently, the energy
conversion efficiency for a mixture of poly-3-hexylthiophene
(P3HT) and phenyl-C61-butyric acid methyl ester (C60-PCBM)
reached nearly 5%.3
In bulk-heterojunction polymer solar cells, securing percola-
tion pathways for hole and electron transfer is a key issue. Two
different approaches can be taken, depending on the properties
of the donor polymers. In crystalline polymers, particularly in
the case of regioregular P3HT, the optimal domain size of the
polymer and fullerene derivatives, in terms of charge percolation,
is achieved through thermal9 or solvent annealing.10 Most of the
recently developed high-efficiency low band gap polymers have
been amorphous, and these amorphous polymers cannot be
optimized through thermal or solvent annealing. In the case of
these amorphous polymers, a small quantity of high boiling point
ingredients is added to the active material blend solution as the
only effective method for improving the percolation pathways
via selective solvation of the fullerene derivatives.11,12 Unfortu-
nately, the high boiling additive must be removed prior to
packaging the device because it may negatively impact perfor-
mance during long-term operation. More reliable methods for
controlling the nanomorphology of the active layers are required
in order to successfully establish bulk-heterojunction organic
solar cells.
The C70 analog (C70-PCBM) of C60-PCBM is a key material
that provides an appropriate match for low band gap polymers.
Because many low band gap polymers do not have high
absorption coefficients at short wavelengths, the strong absorp-
tion of visible light due to the asymmetric structure (contrasting
with the symmetric C60) makes C70-derivatives good counter-
parts for low band gap polymers. By virtue of the introduction of
the C70-PCBM counterpart, many recently developed low band
gap polymers have broken the energy conversion efficiency
record and reached efficiencies of nearly 7%.4
Following the success of C60- and C70-PCBMs, many new
soluble C60-, C70- and other derivatives have been synthesized,
In this work, we introduced alkyl groups with various lengths
to the terminal ester of C70-PCBX ([6,6]-phenyl C71-butyric acid
alkyl ester). This region was anticipated to directly interact with
neighboring molecules. We investigated the physico-chemical
and photovoltaic properties of the blend using the P3HT
polymer as a donor.
Energy Materials Research Center, Korea Research Institute of Chemical
Technology (KRICT), 100 Jang-dong, Yuseong-gu, Daejeon, 305-600,
Korea. E-mail: moonsj@krict.re.kr; Fax: +82 42 860 7590; Tel: +82 42
860 7517
† Electronic supplementary information (ESI) available: NMR
spectroscopic spectra, Table S1 and Fig. S1–S3. See DOI:
10.1039/c0jm02459g
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ꢀ
1
vacuo at 70 C for 12 h. Yield: 1.4 g (65%). H NMR (CDCl3,
300 MHz); d (ppm): 7.92–7.88 (m, 2H), 7.55–7.42 (m, 3H), 4.04–
4.00 (m, 2H), 2.51–2.41 (m, 4H), 2.21–2.12 (m, 2H), 1.70–1.61
(m, 2H), 0.95–0.89 (m, 3H). 13C NMR (CDCl3, 75 MHz);
d (ppm): 173.1, 126.4–155.9, 71.8, 69.7, 66.2, 35.9, 34.1,
34.0, 22.0, 21.7, 10.5. MALDI-TOF: M+ ¼ 1059 m/z (calcd.
1058.1 g molꢁ1).
Experimental
Materials
Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C71-butyric
acid methyl ester (C70-PCBR1) were purchased from Rieke Metal
Ltd. and Nano-C, respectively, and were used without further
treatment.
The C70-PCBR3–R9 were synthesized via UV irradiation of
[5,6]-isomers in an ODCB solution, which were prepared by
a modified procedure described in the literature (Scheme 1).13,14
1H NMR and 13C NMR spectra were obtained with a Bruker
DPX-300 instrument as indicated, at 298 K using TMS as an
internal standard. MALDI-TOF data was acquired on
a Voyager-DE STR instrument in Reflector Mode with a laser
intensity of 2000. 200 shots were averaged to give the resulting
spectra.
[6,6]-Phenyl C71-butyric acid pentyl ester 3b. Yield: 1.3 g (62%).
1H NMR (CDCl3, 300 MHz); d (ppm): 7.90–7.77 (m, 2H), 7.55–
7.41 (m, 3H), 4.12–4.04 (m, 2H), 2.48–2.41 (m, 4H), 2.19–2.07
(m, 2H), 1.62–1.60 (m, 2H), 1.32–1.25 (m, 4H), 0.94–0.89
(m, 3H). 13C NMR (CDCl3, 75 MHz); d (ppm): 173.1, 128.2–
155.9, 71.8, 69.7, 64.8, 35.9, 34.1, 34.0, 28.3, 28.1, 22.3, 21.7,
14.0. MALDI-TOF: M+ ¼ 1087 m/z (calcd. 1086.4 g molꢁ1).
[6,6]-Phenyl C71-butyric acid heptyl ester 3c. Yield: 1.3 g (61%).
1H NMR (CDCl3, 300 MHz); d (ppm): 7.92–7.89 (m, 2H), 7.55–
7.43 (m, 3H), 4.03 (m, 2H), 2.51–2.42 (m, 4H), 2.35–2.21 (m, 2H),
1.67–1.65 (m, 2H), 1.35–1.22 (m, 8H), 0.90–0.84 (m, 3H). 13C
NMR (CDCl3, 75 MHz); d (ppm): 173.1, 128.2–155.9, 71.8, 69.7,
64.8, 35.9, 34.1, 34.0, 31.7, 28.9, 28.6, 25.9, 22.6, 21.7, 14.1.
MALDI-TOF: M+ ¼ 1116 m/z (calcd. 1114.4 g molꢁ1).
Synthesis
General procedure for the synthesis of [6,6]-phenyl C71-butyric
acid alkyl esters (3a–3d).
[6,6]-Phenyl C71-butyric acid propyl ester 3a. A mixture of
propyl 5-phenyl-5-(2-tosylhydrazono)pentanoate 2a (0.89 g,
2.22 mmol) and sodium methoxide (0.12 g, 2.20 mmol) were
suspended in dry pyridine (30 mL) under an atmosphere of dry
nitrogen and the mixture was stirred at RT for 30 min. To the
mixture a solution of C70 (1.7 g, 2.00 mmol) in ODCB (75 mL)
[6,6]-Phenyl C71-butyric acid nonyl ester 3d. Yield: 1.2 g (55%).
1H NMR (CDCl3, 300 MHz); d (ppm): 7.90–7.77 (m, 2H), 7.53–
7.41 (m, 3H), 4.12–4.05 (m, 2H), 2.48–2.42 (m, 4H), 2.18–2.05
(m, 2H), 1.61–1.59 (m, 2H), 1.30–1.23 (m, 12H), 0.91–0.85
(m, 3H). 13C NMR (CDCl3, 75 MHz); d (ppm): 173.1, 128.5–
155.9, 71.8, 69.7, 64.8, 35.8, 34.1, 34.0, 31.8, 29.5, 29.2, 28.6,
25.9, 22.6, 21.7, 14.1. MALDI-TOF: M+ ¼ 1143 m/z (calcd.
1142.5 g molꢁ1).
ꢀ
was added. The resulting mixture was heated to 75–90 C and
allowed to react for 12 h in the dark under an atmosphere of N2.
After cooling down to RT the reaction mixture was irradiated
with a 400 W sodium lamp for 2 h. The volatile components were
evaporated in vacuo. The crude product was purified using
column chromatography (Silica gel/ODCB). After elution of the
recovered C70, the fraction containing mono-adduct [70] PCBM
isomers was collected. The solvent was removed in vacuo and the
remaining solid was transferred to a centrifugal bottle using
a minimal amount of toluene and subsequently precipitated with
MeOH. The residue was washed with MeOH (2x) and dried in
Photovoltaic device fabrication
For the fabrication of active layers composed of poly(3-hexy-
lthiophene) (P3HT) and [6,6]- phenyl C71-butyric acid alkyl ester
(C70-PCBX), blend solutions with weight ratios of 1 : 0.7–1 : 1
P3HT : C70-PCBX were prepared using chlorobenzene as
Scheme 1 Synthetic route to fullerene derivatives.
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a solvent. The solutions were stirred overnight and spun-cast
onto a quartz plate to measure the optical absorption and pho-
toluminescence (PL) spectral properties.
acetonitrile containing 0.1
M
tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) as
a
supporting electrolyte.
Bu4NPF6 was recrystallized sequentially from ethanol and ethyl
acetate, then dried under vacuum prior to use. Other chemicals
and acetonitrile (Aldrich, >99.8%, anhydrous, sealed under
N2 gas) were used as received.
The polymer solar cells were fabricated using a typical sand-
wich structure of ITO/PEDOT:PSS/active layer/LiF/Al. The ITO
coated glass substrates were cleaned by a routine cleaning
procedure that included sonication in detergent, distilled water,
acetone, and 2-propanol. The PEDOT:PSS (Clevios P, H. C.
Starck) layer with thickness of 50 nm was spin-coated onto
a cleaned ITO substrate after exposing the ITO surface to ozone
for 10 min to improve the wetting properties of the PEDOT:PSS.
Results and discussion
Characterization of C70 derivatives
The C70-PCBR3–R9 were synthesized via UV irradiation of
[5,6]-isomers in an ODCB solution, which were prepared by
a modified procedure described in the literature (Scheme 1).13,14
Due to the 1,3-dipolar addition to C70, C70-PCBR3–R9 consisted
ꢀ
The PEDOT:PSS layer was then baked on a hotplate at 140 C
for 10 min and transported to a glovebox with an argon atmo-
sphere. Again, the blend solution of P3HT and C70-PCBX in
chlorobenzene (weight ratios of 1 : 0.7–1 : 1) was spin-coated on
top of the PEDOT:PSS layer to constitute the active layer. The
thickness of the active layer was fixed at 90 nm by varying the
spin speed, as measured using a profilometer with an accuracy of
1 nm (KLA Tencor Alpha-step IQ). The device structure was
completed by thermally depositing the cathode electrode con-
sisting of LiF (0.6 nm) and Al (110 nm) onto the photoactive
layer through a shadow mask with an aperture size of 9 mm2
under 3 ꢂ 10ꢁ6 Torr vacuum. Finally, fabricated devices were
annealed at 150 ꢀC for 30 min to improve the morphology of the
active layer and the performance of the devices.
1
of a mixture of regioisomers. The H-NMR spectrum showed
distinct signals corresponding to the a-protons of the ester group
between d ¼ 3.80 and d ¼ 4.17 ppm, which indicated the presence
of three isomers having the following content ratios; R3
¼
¼
10 : 80 : 10, R5 ¼ 14 : 71 : 15, R7 ¼ 8 : 81 : 11, and R9
9 : 81 : 10. In this study, a mixture of the C70-PCBR3–R9
regioisomers was used.
Fig. 1 shows the cyclic voltammograms of C70-PCBR1–R7.
The first reduction potentials of C70-PCBX in solution were
observed at almost the same value, ꢁ0.89 V, as summarized in
Table 1. This very small change in the first reduction potential
was reasonable because the extended alkyl chain did not affect
the conjugation length of the C70-PCBX materials. Because the
open circuit voltage (Voc) of the bulk-heterojunction polymer cell
is determined from the difference between the HOMO level of the
donor polymer and the LUMO level of the acceptor material,15
the Voc of solar cells comprising a blend of P3HT and C70-PCBX
derivatives would be nearly the same if the LUMO levels also
showed a similar trend in the film state.
Characterization
Current density–voltage (J–V) characteristics of all polymer solar
cells were measured under the illumination of simulated solar
light with 100 mW cmꢁ2 (AM 1.5 G) using an Oriel 1000 W
solar simulator. Electric data were recorded using a Keithley
236 source-measure unit, and all characterizations were carried
out in an ambient environment. The illumination intensity used
was confirmed by a standard KG5 filtered Si photovoltaic cell
from PV Measurements Inc., which was calibrated at the
National Renewable Energy Laboratory (NREL).
To characterize the synthesized fullerene derivatives, we
measured the solubility of derivatives in chlorobenzene and the
work function of the derivative films, as listed in Table 1.
During the film drying step, the solubility is one of the most
important factors (or parameters) for the formation (or self-
organization) of a nanoscale morphology in the blended film.
The solubility test was performed through the modified
method of Troshin6 as shown in ESI.† The solubility of the
C70-PCBX series in chlorobenzene moderately increased from
The incident photon-to-current conversion efficiency (IPCE)
was measured as a function of wavelength from 360 to 800 nm
(PV Measurements Inc.) using a halogen lamp as a light source.
The light intensity at each wavelength was calibrated using
a reference silicon photodiode.
The work function measurement of C70-PCBXs was con-
ducted with a UV intensity of 10 nW on a low-energy photo-
electron spectrometer (Riken-Keiki, AC-2). The samples were
prepared by spin-coating a 0.2 wt% C70-PCBX solution dissolved
in chlorobenzene onto pre-cleaned glass substrates at 400 rpm in
a glove box. The surface morphology of P3HT : C70-PCBX ¼
1 : 0.9 films was imaged by AFM (DI3100, Veeco) operating in
tapping mode. X-ray diffraction (XRD) patterns of
P3HT : C70-PCBX ¼ 1 : 0.9 films were obtained using a Rigaku
Rotaflex with a setting of 30 kV, 100 mA, 0.01 degree steps, and
a 0.2 degree minꢁ1 scan speed. Cyclic voltammograms (CV) were
measured using a computer-controlled potentiostat/galvanostat
(model 273A from EG&G Princeton Applied Research). A Pt
disk (diameter ¼ 2 mm) was used as a working electrode along
with a Pt wire as a counter, and an Ag/AgNO3 was used as
a reference electrode. For the measurements, C70-PCBXs were
dissolved in a 4 : 1 volume mixture of o-dichlorobenzene and
Fig. 1 Cyclic voltammograms of fullerene derivatives.
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Table 1 Physico-chemical properties of fullerene derivatives
the HOMO levels. The smaller work function of C70-PCBR1,
measured by the surface analyzer, implies a closer molecular
packing compared to the case of C70-PCBR7.
Solubilitya/
mg mlꢁ1
Work
E1/2 b/V E1/2 b/V E1/2 b/V Functionc/eV
ꢁ1
ꢁ2
ꢁ3
C70-PCBR1 63
ꢁ0.90
ꢁ0.89
ꢁ0.89
ꢁ0.89
ꢁ1.28
ꢁ1.27
ꢁ1.27
ꢁ1.21
ꢁ1.67
ꢁ1.67
ꢁ1.67
ꢁ1.63
ꢁ5.92
ꢁ5.97
ꢁ5.98
ꢁ6.04
ꢁ6.00
Photovoltaic properties of the devices
C70-PCBR3 71
C70-PCBR5 74
C70-PCBR7 88
C70-PCBR9 89
The polymer solar cells were fabricated with a typical sandwich
structure of ITO/PEDOT:PSS/active layer/LiF/Al, and post-
d
d
d
—
—
—
ꢀ
a
b
annealing was performed at 150 C for 30 min to improve the
Solubility was determined in chlorobenzene solution. Half way
potentials were measured in the solution state with 0.1 M Bu4NPF6 in
o-dichlorobenzene : acetonitrile (4 : 1) solution with the setting
morphology of the active layer and the performance of the
devices.18 For each of the C70-PCBX acceptors, the active layer
composition of the P3HT : C70-PCBX blend devices was varied
from 1 : 0.7 to 1 : 1 wt ratio. We fabricated more than 24 devices
in more than 6 independent batches for each composition, and
the photovoltaic (PV) performance of devices fabricated from
C70-PCBX comprising various alkyl lengths are presented in
a statistical manner in Fig. 2 and summarized in Tables 2 and S1,
ESI.†
c
ferrocene-ferrocenium ¼ +0.193 V. Work functions were measured
using the surface analyzer on thin films of C70-PCBXs spin-coated
from chlorobenzene solution. Half way potentials of C70-PCBR9
could not be obtained because of its broad cyclovoltammogram.
d
63 mg mLꢁ1 for C70-PCBR1 to 89 mg mLꢁ1 for C70-PCBR9.
The work functions were measured using a surface analyzer on
the films of C70-PCBXs spin-coated from a chlorobenzene
solution. The work function gradually increased from
C70-PCBR1 to C70-PCBR7. The work function of the
C70-PCBX derivative film corresponded to the HOMO level of
each C70-PCBX in the solid state. Theoretically, if the C70-
PCBX molecules are well separated from each other, as in the
solution state, the HOMO level should be the same for
different alkyl chain lengths. However, when the C70-PCBX
molecules are sufficiently closely spaced to form aggregates or
nanocrystals, the HOMO–LUMO gap of the C70-PCBX
cluster would be smaller than that of the separate molecules
due to dipole–dipole interactions among the closely packed
C70-polyhedra. Closer interactions among the C70-polyhedra
could lower the gap between the HOMO and LUMO levels,
resulting in a decrease in the LUMO levels and an increase in
The results showed that the short circuit current (Jsc) and
open-circuit voltage (Voc) significantly increased on going from
C70-PCBR1 to the longer alkyl-substituted C70-derivatives
throughout the range of the P3HT : C70-PCBX blend ratios; the
optimum Jsc appeared in the cases of C70-PCBR5 and
C70-PCBR7, and the maximum Voc was obtained for C70-PCBR7.
However, the fill factor (FF) gradually decreased as the alkyl
chain length increased, and this trend became more distinct for
higher content ratios of C70-PCBX derivatives. Meanwhile, the
overall FFs generally increased as the C70-PCBX content
increased in the active layer. As a result, the power conversion
efficiency (PCE) is increased for longer alkyl chain lengths up to
C70-PCBR7, for all blend ratios, and the highest PCE reached
values up to 4.3% in the P3HT : C70-PCBR7 blend at a ratio
of 1 : 0.8.
Fig. 2 Statistical box graphs for Jsc, Voc, FF, and efficiency for five C70 derivatives and four different C70-PCBX : P3HT ratios. Each statistical box
represents the data over 24 devices from more than 6 different batches.
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Table 2 Mean photovoltaic performance of P3HT : C70-PCBX ¼ 1 : 0.9
devicesa and roughness of P3HT : C70-PCBX ¼ 1 : 0.9 films
C70-PCBX Jsc/mA cmꢁ2 Voc/V FF PCE (%) Rab/nm Rqc/nm
C70-PCBR1 9.59
C70-PCBR3 10.43
C70-PCBR5 10.57
C70-PCBR7 10.46
C70-PCBR9 10.22
0.61
0.62
0.64
0.65
0.64
0.59 3.50
0.56 3.59
0.57 3.84
0.58 3.95
0.54 3.51
0.77
1.01
1.17
1.34
1.62
1.00
1.30
1.47
1.76
2.09
a
Each device performance represents the mean value of over 24 devices.
Mean roughness and from AFM after annealed at 150 ꢀC for 30 min.
Root mean roughness from AFM after annealed at 150 ꢀC for 30 min.
b
c
Fig. 4 UV-Visible spectra of P3HT : C70-PCBX ¼ 1 : 0.9 films spin-
coated on quartz form chlorobenzene solution, (a) before and (b) after
thermal annealing at 150 ꢀC for 30 min.
Fig. 3 Representative IPCE spectra for P3HT : C70-PCBX ¼ 1 : 0.9
blend solar cells.
Fig. 3 shows the incident photon-to-current efficiency (IPCE)
spectra for P3HT : C70-PCBX blend solar cells. The overall
spectral shapes were identical, irrespective of the alkyl chain
length of C70-PCBX. Due to the mismatch between the IPCE and
the photon flux under AM 1.5 illumination, an approximate
mismatch of 11% was present between the convolution and the
solar simulator data.
Characterization of active layers
The active layers with different C70-PCBX acceptors were char-
acterized by performing UV-Vis spectroscopic analysis, XRD,
and AFM. Fig. 4a and 4b show the UV-Vis absorption spectra of
P3HT : C70-PCBX ¼ 1 : 0.9 blend films spin-coated from
a chlorobenzene solution (4a) and annealed under an inert
atmosphere for 30 min (4b). After the annealing process, the
vibronic absorptions in the 500–600 nm region became stronger
than those of the pristine film, and they increased, with small
variations, as the alkyl chain length increased. This phenomenon
is attributed to the presence of highly crystallized P3HT
domains.16 The increased crystallinity of P3HT for longer alkyl
chain lengths of C70-PCBX was also confirmed by the XRD peak
of the annealed P3HT : C70-PCBX ¼ 1 : 0.9 film near 2q ¼ 5.4ꢀ
(Fig. 5). The diffraction peak at 2q ¼ 5.4ꢀ corresponded to
Fig. 5 XRD data of P3HT : C70-PCBX ¼ 1 : 0.9 blend films annealed at
150 ꢀC for 30 min.
crystallinity of the P3HT polymer in P3HT : C70-PCBX ¼
1 : 0.9 film.
AFM images were recorded of the active layer surface of
P3HT : C70-PCBX ¼ 1 : 0.9 devices, which were fabricated using
normal device fabrication procedures with an Al cathode and
annealed at 150 ꢀC for 30 min, then the Al part was removed by
immersing in a 1N HCl solution.18 The RMS roughness of the
interface between the Al cathode and the active layer gradually
increased from 0.48 nm in C70-PCBR1 to 2.09 nm in C70-PCBR9
ꢀ
a distance of 16 A and was correlated with the length of the fully
extended hexyl side chain of P3HT (with a length of about
17
13 A). The increased intensity indicates an increase in the
ꢀ
(see Fig.
6
and Table 2), which suggested the higher
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Fig. 6 AFM phase images (top) and topological images (bottom) of P3HT : C70-PCBX ¼ 1 : 0.9 blend films annealed at 150 ꢀC for 30 min.
self-organization of P3HT in the longer alkyl chain acceptor. The
AFM phase images shown in Fig. 6 demonstrated that
P3HT-rich and C70-PCBX-rich domains in all films were well
separated, and this clear phase separation was helpful for charge
percolation.
The cyclic voltammograms of C70-PCBX in the
ODCB : CH3CN (4 : 1) solution indicated that the first reduction
potentials, the LUMO levels of the C70-PCBX acceptors, yielded
almost the same value, ꢁ0.89 V, setting the ferrocene-ferroce-
nium oxidation potential at +0.193 V as a reference.
The UV-Vis, XRD, and AFM data all suggested the same
result, that the phase segregation in the P3HT : C70-PCBX
blended active layer was enhanced by the increase in alkyl length
of the C70-PCBX molecules after annealing. This trend was
correlated with the measured solubilities of C70-PCBX in chlo-
robenzene, given in Table 1. According to Heeger et al.,11 addi-
tions of a small quantity of a high boiling point component to the
blend of the low band gap polymer PCPDTBT/PCBM solution
yielded a more segregated PCPDTBT/PCBM film during the film
forming and spin-coating processes, which resulted in better
charge percolation properties. This phenomenon was caused by
the selective solubilization of the fullerene component by the high
boiling point additives. In this work, although the additive was
not used in the blend solution, better phase segregation appeared
to have been obtained using the longer alkyl substituted
C70-PCBX compounds. C70-PCBR1 had higher solubility than
P3HT.6 Elongation of the alkyl chain enhanced the solubility of
C70-PCBR3–R9 in chlorobenzene, and the solubility gap with
P3HT becomes wider. Therefore, the P3HT polymer solidified at
an early stage of the film forming process, and much of the
C70-PCBX derivative was retained in the reduced chlorobenzene
solution in a concentrated form due to the high solubility of
C70-PCBX in chlorobenzene. Finally, the P3HT polymer and
C70-PCBX in blended solutions had a higher tendency to self-
organize, and a highly concentrated C70-PCBX solution thereby
solidified, resulting in percolation pathways.
However, the PV devices fabricated using this C70-PCBX
series revealed a gradual increase in Voc from C70-PCBR1 to
C70-PCBR7, where P3HT was used as a common donor polymer.
This suggested that the electronic properties of C70-PCBX in
a solid form inside the active layer were apparently different from
those in solution.
It seems natural to assume that the larger Voc for C70-PCBR7
was related to the higher LUMO level due to the relatively looser
packing of C70-PCBR7 molecules compared to the cases of
shorter alkyl chains. In addition, the fullerene derivatives with
longer alkyl chains are possibly more hindered, resulting in
reduced electronic coupling with the P3HT polymer. According
to Vandewal et al., as shown in eqn (1), reductions in the elec-
tronic coupling between the polymer and the fullerene would
suppress the charge-transfer band oscillator strength and would
reduce J0, and thus increase Voc.19
ꢀ
ꢁ
kT
q
Jsc
Jo
Voc
¼
ln
þ 1
(1)
Here, k is the Boltzmann constant, T is the absolute room
temperature, q is the elementary electron charge and J0 is the
dark saturation current.
With a further increase in the alkyl chain length to C70-PCBR9,
all solar cell parameters are lower than C70-PCBR7, even with the
highest P3HT crystallization. No clear morphological differences
were observed in the AFM images (Fig. 6). This charge deteri-
oration may have been caused by excessive intermolecular
distance within or between C70-PCBR9 aggregates resulting from
alkyl chains that were too long.
The overall spectral shapes of the IPCE spectra were identical,
irrespective of the alkyl chain length of C70-PCBX, as shown in
Fig. 3. This suggested that the photocurrent difference at each
wavelength as a function of the alkyl chain length was not caused
by differences in the photo-absorption, but was mainly due to
differences in charge percolation. Therefore, the reason for
a higher Jsc near C70-PCBR5 and C70-PCBR7 may have origi-
nated from the optimal domain sizes of the P3HT : C70-PCBX
layer.
Mixing properties from the results of a computational calculation
To understand the intermixing behavior between P3HT and
C70-PCBX acceptors, we adopted the Blends (Accelrysꢀ)
program,20 which combines a modified Flory-Huggins model21
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J. Mater. Chem., 2011, 21, 960–967 | 965

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and molecular simulation techniques. These calculations esti-
mated the compatibility of binary mixtures.
Conclusions
Extended alkyl chains of the C70-derivatives increased the phase
segregation and interface roughness in bulk heterojunction
polymer solar cells with P3HT as a donor polymer. These effects
were caused by the enhanced solubility and increased molecular
binding energy of C70-PCBX itself with longer alkyl chains. A
moderate phase segregation was thought to enhance the nano-
morphology in the active layer of the device, resulting in
improved charge percolation, as evidenced by the higher Jsc and
Voc of the C70-PCBR5–R7 samples. However, in the case of an
excessively long alkyl chain, such as that in C70-PCBR9, all solar
cell parameters deteriorated, possibly due to poorer charge
transport across the excessively long intermolecular distance.
Through preparation of a series of C70-PCBX acceptor materials,
we have tested the P3HT polymer. A more meaningful applica-
tion would be to introduce new amorphous low band gap poly-
mers, which may require effective phase segregation without
further treatments, and they may require complementary
absorption of shorter wavelengths by the C70 moiety.
The chi-parameter, c, is the interaction parameter defined as
shown in eqn (2). The traditional Flory-Huggins model describes
each component as occupying a lattice site. For a lattice with
a coordination number Z, the mixing energy is represented by
eqn (3).
Emix
c ¼
(2)
RT
1
2
Emix
¼
ZðEbs þ Esb ꢁ Ebb ꢁ EssÞ
(3)
where Emix is the mixing energy, T is the absolute temperature,
R is the gas constant, and Eij is the binding energy between a unit
of component i and a unit of component j.
To perform a Blends calculation, the 3-hexylthiophene unit
was used as a base and the C70-PCBX molecules were used as
screens. Geometry-optimized structures of each 3-hexylth-
iophene repeating unit and C70-PCBX were optimized using
a density function theory (DFT) quantum mechanical molecular
modeling program (DMol3 in the Materials Studio 5.0 package,
Accelrysꢀ)22 with symmetry restrictions, as shown in the
supplementary data, Fig. S3.† In the previous section, we
assumed that the aggregation and/or segregation of the active
material was affected by the interactions with the solvent during
the film-forming process. However, in the Blends calculation,
only the interactions between the active components were
considered, and the solvent effect was ignored. The Blends
calculation results obtained using a Dreiding force field are
summarized in Table 3.
Acknowledgements
This work was supported by the Converging Research Center
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (2010K000970) and by the New and Renewable
Energy of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant funded by the Korea government
Ministry of Knowledge Economy (2008-N-PV08-02).
Notes and references
The positive values of c for blending between P3HT and
C70-PCBXs indicated that both molecules preferred to be sur-
rounded by similar components, rather than intermixing. The c
parameter gradually increased in going from C70-PCBR1 to
C70-PCBR7, then decreased for C70-PCBR9. This calculation
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the binding energy of C70-PCBX itself (Ess) rather than in the
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C70-PCBR9 42.14 24.96
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