Effect of carbon chain length in the substituent of PCBM-like molecules on their photovoltaic properties

Article abstract of DOI:10.1002/adfm.200902447

A series of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)-like fullerene derivatives with the butyl chain in PCBM changing from 3 to 7 carbon atoms, respectively (F1-F5), are designed and synthesized to investigate the relationship between

Full text of DOI:10.1002/adfm.200902447

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Effect of Carbon Chain Length in the Substituent of
PCBM-like Molecules on Their Photovoltaic Properties
By Guangjin Zhao, Youjun He, Zheng Xu, Jianhui Hou,* Maojie Zhang,
Jie Min, Hsiang-Yu Chen, Mingfu Ye, Ziruo Hong, Yang Yang,* and
Yongfang Li*
1. Introduction
A series of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-like fullerene
Polymer solar cells (PSCs) with a bulk-
derivatives with the butyl chain in PCBM changing from 3 to 7 carbon atoms,
respectively (F1–F5), are designed and synthesized to investigate the
relationship between photovoltaic properties and the molecular structure of
fullerene derivative acceptors. F2 with a butyl chain is PCBM itself for
comparison. Electrochemical, optical, electron mobility, morphology, and
photovoltaic properties of the molecules are characterized, and the effect of
the alkyl chain length on their properties is investigated. Although there is
little difference in the absorption spectra and LUMO energy levels of F1–F5,
an interesting effect of the alkyl chain length on the photovoltaic properties is
observed. For the polymer solar cells (PSCs) based on P3HT as donor and F1–
F5, respectively, as acceptors, the photovoltaic behavior of the P3HT/F1 and
P3HT/F4 systems are similar to or a little better than that of the P3HT/PCBM
device with power conversion efficiencies (PCEs) above 3.5%, while the
performances of P3HT/F3 and P3HT/F5-based solar cells are poorer, with
PCE values below 3.0%. The phenomenon is explained by the effect of the
alkyl chain length on the absorption spectra, fluorescence quenching degree,
electron mobility, and morphology of the P3HT/F1–F5 (1:1, w/w) blend films.
heterojunction (BHJ) active layer in the
structure have attracted great attention in
recent years, because of their advantages of
low cost, light weightedness, and flexibil-
ity.[1–4] Various combinations of donor and
acceptor materials have been used in the
BHJ active layer of the PSCs. One of the
most representative BHJ PSCs is the device
based on a blend of poly(3-hexylthiophene)
(P3HT) as an electron donor and a soluble
C60 derivative, [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM) as an electron accep-
tor. The power conversion efficiency (PCE)
of the PSCs based on P3HT:PCBM reached
over 4% by thermal treatment,[5a] solvent[5b]
and vapor[6] annealing, as well as mixture
solvent treatment.[7] To further improve the
PCE, new conjugated polymer donors and
fullerene derivative acceptors are needed
for a higher short circuit current (Jsc) and a
higher open circuit voltage (Voc). In recent
years, various new types of low bandgap polymer donors have
beendeveloped, andthePCEsofthePSCsbasedonthesepolymers
reach 5%–7%.[2,8–12]
In comparison with the significant publications on the new
polymer donors, the attention on new fullerene derivative
acceptors is much less. Recent reports on the new fullerene
derivatives include PCBM derivatives with electron-donating
groups on the phenyl ring[13,14] or with other groups that replace
thephenylring,[15] aPCBMbisadduct[16] orPCBMmultiadduct,[17]
penta(organo)[60]fullerenes,[18] silymethyl[60]fullerene,[19] endo-
hedral fullerenes,[20] and other C60 derivatives.[21] Unfortunately,
among these new fullerene derivatives, most show poorer
photovoltaic properties, and only a few show similar or a slightly
better photovoltaic performance than PCBM.[15c,16,20,21b] It should
be noticed that a new fullerene derivative, an indene-C60 bisadduct
(ICBA), was reported recently, and the PSC based on P3HT/ICBA
showed a high Voc of 0.84 Vand a higher PCE of 5.44%,[22] which is
significantly improved in comparison with that of the PSCs based
on P3HT/PCBM. Up to now, the most important fullerene
derivative acceptors are still PC60BM and PC70BM (the corre-
sponding PCBM derivative of C70). It is also interesting to note that
a slight structural modification of PCBM can influence the
[*] Prof. Y. F. Li, G. J. Zhao, Y. J. He, M. J. Zhang, J. Min, M. F. Ye
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Organic Solids
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: liyf@iccas.ac.cn
Dr. J. H. Hou, Dr. Z. Xu
Solarmer Energy Inc.
El Monte, California 91731 (USA)
E-mail: jianhuih@solarmer.com
Prof. Y. Yang, Dr. H. Y. Chen, Dr. Z. R. Hong
Department of Materials Science and Engineering & California
Nanosystems Institute
University of California at Los Angeles
Los Angeles, California 90095 (USA)
E-mail: yangy@ucla.edu
G. J. Zhao, Y. J. He, M. J. Zhang, M. F. Ye
Graduate University of Chinese Academy of Sciences
Beijing 100039 (P. R. China)
DOI: 10.1002/adfm.200902447
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Scheme 1. Molecular structures of the five fullerene derivatives F1–F5.
photovoltaic performance significantly. Therefore, it is essential to
understand the effect of the side chain structure of the fullerene
derivatives on their photovoltaic properties.
ꢀ0.80, ꢀ0.80, ꢀ0.79, ꢀ0.81, and ꢀ0.80 V, respectively. The LUMO
energy levels of the fullerene derivatives were calculated from the
onset reduction potentials (wred) according to the following
[23]
Here we designed and synthesized a series of PCBM-like C60
derivatives, F1, F2, F3, F4, and F5 (Scheme 1), by changing the
butyl carbon chain length of PCBM (F2) from 3 to 7 carbon atoms,
respectively, to investigate the effect of the carbon chain length of
PCBM on the physical properties and photovoltaic performance of
thePCBMderivatives. Amongthemolecules, F2isPCBMitself for
comparison. The molecules of F1–F5 possess almost the same
lowest unoccupied molecular orbital (LUMO) energy levels and
absorption characteristics. But, interestingly, the PSCs based on
P3HT as donor and F1–F5 as acceptor showed different
photovoltaic performances depending on the PCBM derivatives.
The PSCs with F1, F2, or F4 as the acceptor displayed higher
photovoltaic performance with PCE above 3.5%, while those with
F3 or F5 as acceptors showed a relatively lower PCE below 3.0%. In
comparison with PCBM (F2), F1 with a side-chain one carbon
shorter, and F4 with a side-chain two carbons longer, display
similar or a slightly better photovoltaic performance than PCBM,
while F3 with a side-chain one carbon longer and F5 with a side-
chain three carbons longer show a poorer photovoltaic perfor-
mance. The absorption and photoluminescence spectra, electron
mobilities, and the morphology of the P3HT/F1–F5 blend films
were analyzed to explain the phenomenon.
equation:
E
¼ ꢀ(wred þ 4.71) (eV), where the units of wred
LUMO
are V vs. Ag/Agþ. The LUMO energy levels of F1–F5 are ꢀ3.91,
Figure 1. Absorption spectra of F1–F5 solutions in toluene.
2. Results and Discussion
2.1. Absorption Spectra and Electrochemical Properties of the
Fullerene Derivatives
Figure 1 shows the UV-vis absorption spectra of the fullerene
derivatives (F1–F5) in toluene solution. The absorption spectra of
F1–F5 derivatives are nearly the same: there are two absorption
peaks located in 283 and 332 nm, respectively. The absorption
spectra indicate that the alkyl chain length in the side chain of
PCBM affects the absorption spectra very little.
The electrochemical property is one of the most important
properties of fullerenes. Electronic energy levels (especially the
LUMO level) of the fullerene derivatives are crucial for their
application in PSCs as acceptor, which can be measured by
electrochemical cyclic voltammetry. Therefore, we measured the
cyclic voltammograms of F1–F5, as shown in Figure 2. It can be
seen that the five cyclic voltammograms exhibit similar three
Figure 2. Cyclic voltammograms of F1, F2, F3, F4, and F5 in a mixed
solution of ortho-dichlorobenzene/acetonitrile (5:1, v/v) with 0.1 M
reversible reduction/reoxidation processes over a negative
potential range. The onset reduction potentials of F1–F5 are
Bu4NPF6 at 100 mV sꢀ1
.
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Table 1. Photovoltaic performance of the PSCs based on a blend of P3HT
.
ꢀ3.91, ꢀ3.92, ꢀ3.90, and ꢀ3.91 eV, respectively. Obviously, there
is little change of the LUMO energy levels of the five fullerene
derivatives, which means that the influence of the alkyl chain
length in the side chain of PCBM on the LUMO energy level of the
molecules is very weak.
and F1–F5 (1:1, w/w) under the illumination of AM1.5G, 100 mW cmꢀ2
Active layer
Voc [V]
Isc [mA cmꢀ2
]
FF [%]
PCE [%]
P3HT/F1
P3HT/F2
P3HT/F3
P3HT/F4
P3HT/F5
0.564
0.571
0.535
0.596
0.540
10.8
9.6
8.1
9.9
9.3
60.3
64.6
53.2
61.5
56.4
3.7
3.5
2.3
3.6
2.8
2.2. Photovoltaic Properties
experiments more than 10 times, the tendency of the effect of the
alkyl chain length of the PCBM-like C60 derivative on the
photovoltaic properties is the same and reproducible.
Figure 4 shows the external quantum efficiencies (EQEs) of the
PSCsbasedontheblendofP3HT/F1–F5films(1:1, w/w).TheEQE
values of the devices are consistent with their Isc values and the
PSCs with F3 and F5 as acceptor show lower EQE values than that
with PCBM (F2) as acceptor.
The motivation for the design and synthesis of these fullerene
derivatives is to look for new fullerene derivatives as acceptors for
highly efficient PSCs and to investigate the effect of the side chain
structure of PCBM on its photovoltaic properties. We fabricated
PSCs based on P3HT/F1–F5 blend films with the structure of
indium tin oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly-
(styrene sulfonate) (PEDOT:PSS)/P3HT:F1–F5 (1:1, w/w)/Ca/Al,
where the polymer P3HT was used as the electron donor and
fullerene derivatives F1–F5 were used as electron acceptors. For
comparison, the experimental conditions of the PSCs are the same
except for the different acceptors of F1–F5. Figure 3 shows the I–V
curves of the PSCs under the illumination of AM1.5G,
100 mWcmꢀ2. The photovoltaic performance data of the PSCs,
including the open circuit voltage (Voc), short circuit current (Isc),
fill factor (FF), and PCE values, are summarized in Table 1 for a
clear comparison. It can be seen from Table 1 that the PSCs based
on P3HT/F1, P3HT/F2(PCBM), and P3HT/F4 blend films
demonstrate a similar performance with PCE values of 3.7%,
3.5%, and 3.6%, respectively. While those for P3HT/F3 and P3HT/
F5-based solar cells show poorer PCEs of 2.3% and 2.8%,
respectively. In comparison with the PSC with PCBM (F2) as the
acceptor, the PSCs with F1 and F4 as the acceptor show similar or a
slightly higher efficiency, which is higher than 3.5%, however, the
PSCs with F3 and F5 as the acceptor show poorer efficiency, which
islowerthan3.0%.ThehigherPCEofthedeviceswithF1andF4as
acceptor has benefited from a slightly higher Isc, and the poorer
PCE of the PSCs with F3 and F5 as acceptor has suffered from a
lower Voc, lower FF, and a little lower Isc. We repeated the
2.3. Optical Properties of Absorption and Photoluminescence
Spectra for P3HT/F1–F5 Blend Films
The absorption spectra of P3HTand F1–F5 blend films (1:1, w/w)
are shown in Figure 5a. There are three vibronic absorption
shoulders for P3HT/F1–F5 blend films in visible region deriving
from the contribution of the absorption for P3HT. The absorption
intensities of the blend films differ in the order of P3HT/
F1 > P3HT/F4 > P3HT/F2 > P3HT/F5 > P3HT/F3 which is con-
sistent with the tendency of Isc for the five PSCs based on the
P3HT/F1–F5 systems. As shown in Figure 5a, the five blend films
show one absorption peak at ꢁ332 nm in ultraviolet region which
is from the contribution of fullerenes derivatives F1–F5. And the
P3HT/F3 and P3HT/F5, especially for P3HT/F3, show weaker
absorption intensities than those of other three blend films. We
find the densities of F3 and F5 are lower than those of the other
three acceptors F1, F2, and F4. Therefore, the reason for the weak
Figure 3. I–V curves of the PSCs based on the blend of P3HT as donor and
F1–F5 as acceptors (1:1, w/w) under the illumination of AM1.5G,
Figure 4. EQEs of the PSCs based on the blend of P3HT/F1–F5 films
(1:1, w/w).
100 mW cmꢀ2
.
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Figure 5. a) Absorption and b) photoluminescence spectra of P3HT/F1–
F5 blend films.
Figure 6. J–V characteristics of the electron-only diodes (a) and corre-
sponding fits to the SCLC model (b) of P3HT/F1–F5 blend films (1:1, w/w).
The numbers shown in plots are the film thickness of samples.
absorption intensities at ꢁ332 nm for P3HT/F3 and P3HT/F5 may
be the relatively large size of the molecular structure of F3 and F5,
which induces a the low content of F3 and F5 in the unit thickness
of the blend films.
2.4. Electron Mobility
Photoluminescence (PL) quenching for the BHJ blend films
results from the charge separation of excitons at the donor/
acceptor interface, and the degree of the PL quenching reflects the
efficiency of the exciton charge separation which influences the Isc
value of the PSCs based on the blend films. In order to understand
the origin of the effect of the alkyl chain length of the PCBM-like
molecules F1–F5 on the photovoltaic performance of the PSCs
with the molecules as acceptor, we measured the PL spectra of the
P3HT/F1–F5films,asshowninFigure5b.Alltheblendfilmsshow
obvious PL peaks of P3HT at ꢁ650 nm, which indicates that the
P3HT aggregates to some extent and the charge separation
efficiency of the excitons in the blend films is lower than 100%.[24]
Among the five blend films, the P3HT/F3 blend film displays the
highest PL intensity, which means that the degree of PL quenching
of the P3HT/F3 blend film is lower than those of other blend films.
Probably, the domain size of the P3HTaggregation is larger, or the
electron transfer speed at the interface of P3HT/F3 is lower, in the
P3HT/F3 blendfilm, whichresultsinalowerPLquenchingdegree
and lower Isc of the corresponding PSC device.
Charge carrier mobility is one of the major concerns in designing
organic photovoltaic materials and in fabricating PSCs. A high
charge carrier mobility is preferred for efficient transportation and
photocurrent collection of the photo-induced charge carriers. We
measured the electron mobilities of P3HT/F1–F5 blend films by
the space-charge limited current (SCLC) method[25] with the
electron-only devices, to investigate the effect of the alkyl chain
length of F1–F5 on the electron mobility. For the electron-only
devices, SCLC is described by:
8
V2
L3
J ¼ "r"0me
(1)
9
where J is the current density, er is the dielectric constant of the
fullerene derivatives F1–F5, e0 is the permittivity of the vacuum, L
is the thickness of the blend film, V ¼ Vappl – Vbi, Vappl is the
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Figure 7. AFM (5 mm ꢂ 5 mm) topography images of a) P3HT/F1, b) P3HT/F2, c) P3HT/F3, d) P3HT/F4, and e) P3HT/F5 blend films.
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Scheme 2. Synthetic route of the fullerene derivatives: i) Methanol, sulfuric acid, reflux for 4–8 h; ii) 4-methylbenzenesulfonohydrazide, methanol, reflux
for 4–8 h; iii) C60, pyridine, NaOMe, ortho-dichlorobenzene, 65 8C for 24 h; then, ortho-dichlorobenzene, reflux for 7 h.
applied potential, and Vbi is the built-in potential which results
from the difference in the work function of the anode and the
cathode (in this device structure, Vbi ¼ 0 V).
nearly the same absorption spectra and electrochemical proper-
ties, and their LUMO energy levels are almost the same as that of
PCBM (F2) at approx. ꢀ3.9 eV, which indicates that the alkyl chain
length influences theabsorptionandelectronicenergylevels ofthe
C60 derivatives very little. The photovoltaic properties of the C60
derivatives were investigated by fabricating the PSC devices with
P3HTas donor and F1–F5 as acceptor with the device structure of
ITO/PEDOT:PSS/P3HT:F1–F5 (1:1, w/w)/Ca/Al. The PCEs of the
PSCsbasedonP3HT/F1–F5are3.7%, 3.5%, 2.3%, 3.6%, and 2.8%
respectively. Obviously, there is a significant and interesting
influence of the alkyl chain length in the side chain of the PCBM-
like molecules on the photovoltaic performance of the PSCdevices
based on P3HT/F1–F5, F1 with a one carbon shorter alkyl chain
and F4 with a two carbons longer alkyl chain show similar or a little
higher efficiency than PCBM, while F3 with a one carbon longer
alkyl chain and F5 with a three carbons longer alkyl chain display
lower efficiency than PCBM. The better photovoltaic performance
of F1 may be a benefit of its strong absorption intensity, higher
electronmobility,andoptimummorphologyoftheP3HT/F1blend
film. While the poorer photovoltaic performance of F3 couldsuffer
from its weaker absorption intensity, lower electron mobility, and
poorer morphology of the P3HT/F3 film. The results indicate that
thealkylchainlengthonthesidechainofthePCBM-likemolecules
significantly influence their absorption intensity, electron mobi-
lity, morphology of the films blended with P3HT, and the P3HT/
fullerene derivative interface structure, so that it influences the
photovoltaic performance of the fullerene derivatives.
Figure 6 shows the J–V curves of the P3HT/F1–F5 blend films.
The values of electron mobility were calculated from the plots of
ln(J) versus ln(V). The electron mobilities obtained for the P3HT/
F1–F5 blend films are 2.2 ꢂ 10ꢀ3
,
1.3 ꢂ 10ꢀ3
,
8.4 ꢂ 10ꢀ5
,
3.9 ꢂ 10ꢀ4, and 2.8 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1, respectively. The electron
mobility of 1.3 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1 for P3HT/F2(PCBM) is close to
that reported in the literature (2.0 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1).[26a] F1
shows a higher electron mobility than PCBM, and F3 displays the
lowest electron mobility, which agrees with the higher Isc value of
the PSCwith F1 as acceptor and the lower Isc value of the PSCwith
F3 as acceptor.
2.5. Morphology
The morphology of the photoactive layer is very important for the
photovoltaic performance of PSCs.[5] We used atomic force
microscopy (AFM) to investigate the morphology of the P3HT/F1–
F5 (1:1, w/w) blend films. The AFM topography images are shown
in Figure 7. We can see intuitively that there is obviously phase
separation for all the blend films with bright islands for P3HTand
dark valleys for the C60 derivatives. There are large fullerene
derivative aggregates being confined within the P3HT matrix,
which implies that an interpenetrating network was formed in the
blend films, which is beneficial for forming an efficient exciton
dissociation interface and bicontinuous charge transport chan-
nels. The surface rms (root-mean-square) roughness of the P3HT/
F1–F5 films is 9.2, 15.8, 23.3, 14.2, and 13.6 nm respectively. The
rough surface is a signature of P3HT self-organization, which
enhances ordered structural formation in the thin film.[5b] But too
large a rms roughness of 23.3 nm for the P3HT/F3 film could be
one reason for the lower photovoltaic performance.
4. Experimental
Materials: 4-Phenylbutanoic acid, 5-phenylhexanoic acid, 6-phenylhep-
tanoic acid, 7-phenyloctanoic acid, and 4-methylbenzenesulfonohydrazide
were purchased from Alfa Co. C60 was obtained from Yongxin Co. (China).
P3HT (4002E) was bought from Rieke Metals and used as received. PCBM
(purity > 99.0%) was purchased from American Dye Inc. and used as
received. Other reagents and solvents were obtained from the Beijing
Chemical Co. as analytical grade quality and used as received.
Synthesis: The fullerene derivatives of F1–F5 were synthesized by the
same method as that for the synthesis of PCBM.[26] The synthetic route is
shown in Scheme 2.
3. Conclusions
A series of PCBM-like C60 derivatives with different alkyl chain
lengths of their side chain, F1–F5, have been synthesized to
investigate the effect of the alkyl chain length on the photovoltaic
properties of the fullerene acceptors. The C60 derivatives show
3-Phenylbutanoic Acid Methyl Ester, 2: 4-Phenylbutanoic acid (0.1 mol)
was added to 100 mL of methanol, and then 6 drops of sulfuric acid was
added to the solution. The mixture was refluxed for 12 h and then cooled to
room temperature. After evaporating the solvent, the crude product was
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purified by silica gel chromatography eluted with a mixture of petroleum
and ethyl acetate (5:1) to obtain 2. 1H NMR (300 MHz, CDCl3, d): 7.95 (d,
2H), 7.55 (m, 1H), 7.44 (t, 2H), 3.66 (s, 3H), 2.98 (t, 2H), 2.44 (t, 2H).
EIMS (m/z): 192.
3-Phenylbutanoic Acid Methyl Ester Hydrazone, 3: 4-Phenylbutanoic acid
methyl ester (0.1 mol) and 4-methylbenzenesulfonohydrazide (0.11 mol)
were added to 100 mL of methanol. The mixture was then refluxed for 12 h
and cooled to room temperature. After evaporating the solvent, 3 was
obtained by purification of the crude product by silica gel chromatography
eluted with a mixture of petroleum and ethyl acetate (5:1). 1H NMR
(300 MHz, CDCl3, d): 9.75 (s, 1H), 7.95 (d, 2H), 7.78 (d, 1H), 7.55 (m, 2H),
7.44 (m, 2H), 7.30 (m, 1H), 7.17 (m, 1H), 3.64 (m, 3H), 2.95 (t, 2H), 2.61
(t, 2H), 2.44 (t, 3H). EIMS (m/z): 506.
33.95, 30.18, 22.73. MALDI-TOF MS (m/z): 911.3. Anal. calcd for
C
72H14O2: C 94.95, H 1.54, O 3.52; found: C 94.87, H 1.48, O 3.48.
F3: 1H NMR (400 MHz, CS2/CDCl3 3:1, d): 7.83 (d, 2H), 7.47 (t, 2H),
7.41 (t, 1H), 3.57 (s, 3H), 2.83 (t, 2H), 2.32 (t, 2H), 1.80 (m, 4H). 13C NMR
(400 MHz, CS2/CDCl3 3:1, d): 169.16, 144.90, 144.02, 142.00, 141.37,
141.26, 140.98, 140.87, 140.65, 140.27, 139.96, 139.15, 138.44, 138.32,
137.24, 136.97, 134.23, 133.82, 133.07, 128.36, 125.11, 124.94, 124.76,
48.34, 47.97, 30.28, 26.29, 22.93, 21.44. MALDI-TOF MS (m/z): 924.3.
Anal. calcd for C73H16O2: C 94.81, H 1.73, O 3.46; found: C 94.76, H 1.65,
O 3.37.
F4: 1H NMR (400 MHz, CS2/CDCl3 3:1, d): 7.88 (d, 2H), 7.47 (t, 2H),
7.41 (t, 1H), 3.57 (s, 3H), 2.82 (t, 2H), 2.26 (t. 2H), 1.84 (m, 2H), 1.66 (m,
2H), 1.46 (m, 2H). 13C NMR (400 MHz, CS2/CDCl3 3:1, d): 173.97, 150.53,
146.85, 146.37, 146.15, 145.78, 145.46, 144.65, 143.98, 143.82, 142.75,
142.48, 139.78, 134.62, 134.00, 132.83, 132.50, 132.16, 130.23, 129.91,
129.58, 81.80, 54.15, 53.56, 36.11, 35.86, 31.98, 31.43, 28.85, 27.11.
MALDI-TOF: 938.4. Anal. calcd for C74H18O2: C 94.67, H 1.92, O 3.41;
found: C 94.56, H 1.87, O 3.36.
5-Phenylhexanoic Acid Methyl Ester, 4: Compound 4 was prepared by the
same method as the synthesis of compound 2, except 5-phenylhexanoic
acid was used instead of 3-phenylbutanoic acid. 1H NMR (300 MHz,
CDCl3, d): 7.95 (d, 2H), 7.55 (m, 1H), 7.44 (t, 2H), 3.66 (s, 3H), 2.98 (t, 2H),
2.35 (t, 2H), 1.68 (m, 4H). EIMS (m/z): 220.
5-Phenylhexanoic Acid Methyl Ester Hydrazone, 5: Compound 5 was
prepared by the same method as for the synthesis of compound 3, except
compound 4 was used instead of compound 2. 1H NMR (300 MHz, CDCl3,
d): 8.35 (s, 1H), 7.96 (d, 2H), 7.63 (d, 2H), 7.55 (m, 2H), 7.44 (m, 1H), 7.30
(m, 1H), 7.17 (m, 1H), 3.67 (m, 3H), 2.61 (t, 2H), 2.44 (t, 5H), 1.45 (2H),
1.23 (t, 2H).
6-Phenylheptanoic Acid Methyl Ester, 6: Compound 6 was prepared by
the same method as that for the synthesis of compound 2, except 6-
phenylheptanoic acid was used instead of 3-phenylbutanoic acid. 1H NMR
(300 MHz, CDCl3, d): 7.95 (d, 2H), 7.55 (m, 1H), 7.44 (t, 2H), 3.66 (s, 3H),
2.98 (t, 2H), 2.35 (t, 2H), 1.68 (m, 4H), 1.40 (t, 2H). EIMS (m/z): 235.
6-Phenylheptanoic Acid Methyl Ester Hydrazone, 7: Compound 7 was
prepared by the same method as that for the synthesis of compound 3,
except compound 6 was used instead of compound 2. 1H NMR (300 MHz,
CDCl3, d)& 8.01 (s, 1H), 7.90 (d, 2H), 7.77 (d, 2H), 7.63 (m, 2H), 7.45 (m,
1H), 7.33 (m, 1H), 7.03 (m, 1H), 3.67 (m, 3H), 2.61 (t, 2H), 2.44 (t, 5H),
1.45 (2H), 1.23 (t, 2H).
F5: 1H NMR (400 MHz, CS2/CDCl3 3:1, d): 7.88 (d, 2H), 7.52 (t, 2H),
7.45 (t, 1H), 3.63 (s, 3H), 2.87 (t, 2H), 2.31 (t. 2H), 1.64 (m, 2H), 1.53 (m,
2H), 1.46–1.28 (m, 4H). 13C NMR (400 MHz, CS2/CDCl3 3:1, d): 173.88,
150.37, 146.66, 146.17, 145.96, 145.59, 145.27, 144.45, 143.78, 143.64,
142.56, 142.30, 139.18, 134.41, 133.80, 132.57, 132.24, 131.91, 129.94,
129.61, 129.29, 99.98, 81.67, 54.08, 53.26, 36.04, 35.69, 31.81, 31.47,
31.20, 28.81, 26.95. MALDI-TOF MS (m/z): 952.3. Anal. calcd for
C75H20O2: C 94.54, H 2.10, O 3.36; found: C 94.46, H 1.98, O 3.31.
Instrument and Measurements: 1H NMR spectra were measured on a
Bruker DMX-400 spectrometer, chemical shifts were reported in ppm
relative to the singlet of CDCl3 at 7.26 ppm, and splitting patterns were
designated as s (singlet), d (doublet), t (triplet), m (multiplet), and br
(broaden). Absorption spectra were taken on a Hitachi U-3010 UV-vis
spectrophotometer. Photoluminescence spectra were conducted on a
Hitachi F4500 spectrophotometer. Electrochemical cyclic voltammetry was
conducted on a Zahner IM6e Electrochemical Workstation with a Pt disk, a
Pt wire, and a Ag/Agþ electrode as working electrode, counter electrode,
and reference electrode, respectively, in a 0.1 mol Lꢀ1 tetrabutylammonium
hexafluorophosphate (Bu4NPF6) ortho-dichlorobenzene/acetonitrile solu-
tion mixture (5:1, v/v). The AFM measurement of the surface morphology
of samples was conducted on a Nanoscope III (DI, USA) in contact mode
with 5 mm scanners.
Device Fabrication and Characterization: The PSCs were fabricated in the
configuration of the traditional sandwich structure with an ITO glass
positive electrode and a metal negative electrode. Patterned ITO glass with
a sheet resistance of 10 V sqꢀ1 was purchased from CSG HOLDING Co.,
LTD (China). The ITO glass was cleaned by sequential ultrasonic treatment
in detergent, deionized water, acetone, and isopropyl alcohol, and then
treated in an ultraviolet-ozone chamber (Ultraviolet Ozone Cleaner, Jelight
Company, USA) for 20 min PEDOT:PSS (Clevious P VP AI 4083 H. C. Stark,
Germany) was filtered through a 0.45 mm filter and spin coated at 4000 rpm
for 60 s on the ITO electrode. Subsequently, the PEDOT:PSS film was baked
at 150 8C for 20 min in air. The thickness of the film was around 30 nm. The
blend solution of P3HT and acceptors (F1–F5) in ortho-dichlorobenzene
(DCB) (1:1, w/w, 36 mg mLꢀ1) was spin-coated on top of the PEDOT:PSS
layer at 800 rpm for 30 s. The wet polymer/acceptor blend films were then
put into glass Petri dishes to undergo the solvent annealing process [5b].
The thickness of the photoactive layer was in the range of 200–250 nm as
measured using an Ambios Technology XP-2 profilometer. The negative
electrode consisted of Ca (ꢁ20 nm) capped with Al (ꢁ100 nm) that was
thermally evaporated under a shadow mask at a base pressure of ꢁ10ꢀ4 Pa.
The device active area was ꢁ4–6 mm2 for all the PSCs discussed in this
work. The current–voltage (I–V) measurement of the devices was
conducted on a computer-controlled Keithley 236 Source Measure Unit.
Device characterization was done in a glovebox under simulated AM1.5G
irradiation (100 mW cmꢀ2) using a xenon-lamp-based solar simulator
(from Newport Co., LTD). The EQE was measured using a Stanford
Research Systems model SR830 DSP lock-in amplifier coupled with a
WDG3 monochromator and 500 W xenon lamp. The light intensity at each
wavelength was calibrated with a standard single-crystal Si photovoltaic
7-Phenyloctanoic Acid Methyl Ester, 8: Compound 8 was prepared by the
same method as that for the synthesis of compound 2, except 7-
1
phenyloctanoic acid was used instead of 3-phenylbutanoic acid. H NMR
(300 MHz, CDCl3, d): 7.95 (d, 2H), 7.56 (m, 1H), 7.43 (t, 2H), 3.66 (s, 3H),
2.98 (t, 2H), 2.33 (t, 2H), 1.68–1.62 (m, 6H), 1.37 (t, 2H). EIMS (m/z): 249.
7-Phenyloctanoic Acid Methyl Ester Hydrazone, 9: Compound 9 was
prepared by the same method as that for the synthesis of compound 2,
except compound 8 was used instead of compound 2. 1H NMR (300 MHz,
CDCl3, d): 8.01 (s, 1H), 7.920 (d, 2H), 7.77 (d, 2H), 7.65 (m, 2H), 7.43 (m,
1H), 7.31 (m, 1H), 7.03 (m, 1H), 3.67 (m, 3H), 2.61 (t, 2H), 2.44 (t, 5H),
1.45 (2H), 1.23 (t, 4H).
The C60 Derivatives: Under protection of nitrogen, compound 3 or 5 or 7
or 9 (4 mmol) was added to a 250 mL three-necked flask. Dry pyridine
(30 mL) and NaOMe (225 mg) were then added. The mixture was stirred
for 15 min at room temperature. A solution of 1.44 g of C60 in 100 mL of
ortho-dichlorobenzene was then added to the above mixture. The mixture
was heated to 65 8C and stirred for 24 h at this temperature. After
evaporating the solvent, the crude product was dissolved in 100 mL of
ortho-dichlorobenzene, and then was refluxed for 7 h. The solvents were
then evaporated, the mixture was separated and purified by silica gel
chromatography with toluene to get the C60 derivatives.
F1: 1H NMR (400 MHz, CS2/CDCl3 3:1, d): 7.83 (d, 2H), 7.47 (t, 2H),
7.41 (t, 1H), 3.60 (s, 3H), 2.79 (t, 2H), 2.40 (t, 2H). 13C NMR (400 MHz,
CS2/CDCl3 3:1, d): 172.02, 147.30, 145.77, 145.23, 145.11, 145.01, 144.82,
144.71, 144.54, 143.79, 143.10, 143.05, 143.00, 142.27, 142.18, 142.08,
141.11, 138.25, 136.11, 132.16, 128.70, 51.74, 50.85, 31.65, 30.14, 29.84.
MALDI-TOF MS (m/z): 896.3. Anal. calcd for C71H12O2: C 95.09, H 1.34, O
3.57; found: C 94.89, H 1.32, O 3.51.
F2: 1H NMR (400 MHz, CS2/CDCl3 3:1, d): 7.83 (d, 2H), 7.47 (t, 2H),
7.41 (t, 1H), 3.57 (s, 3H), 2.83 (t, 2H), 2.32 (t, 2H), 2.02 (m, 2H). 13C NMR
(400 MHz, CS2/CDCl3 3:1, d): 173.13, 148.85, 147.90, 146.02, 145.39,
145.25, 144.99, 144.87, 144.69, 144.28, 143.97, 143.21, 143.15, 142.33,
141.24, 138.26, 137.86, 136.86, 132.27, 128.71, 128.50, 52.02, 51.74, 34.10,
Adv. Funct. Mater. 2010, 20, 1480–1487
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www.afm-journal.de
www.MaterialsViews.com
cell. The measurement of electron mobilities was conducted in the dark by
the space-charge limited current (SCLC) method on a computer-controlled
Keithley 236 Source Measure Unit, for the electron-only devices. For the
electron-only devices, Al film (60 nm) was thermally evaporated onto a
glass slide. The P3HT:fullerene derivative solutions were spin-coated onto
the Al film/glass substrate, and then the Al (100 nm) electrode was
thermally evaporated onto the blend film through a shadow mask.
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