Cyclobutadiene-C60 adducts: N-type materials for organic photovoltaic cells with high VOC

Article abstract of DOI:10.1002/adfm.201203251

New tetraalkylcyclobutadiene-C₆₀ adducts are developed via Diels-Alder cycloaddition of C₆₀ with in situ generated cyclobutadienes. The cofacial π-orbital interactions between the fullerene orbitals and the cyclobutene are shown to decrease the electron affinity and thereby increase the lowest unoccupied molecular orbital (LUMO) energy level of C₆₀ significantly (ca. 100 and 300 meV for mono- and bisadducts, respectively). These variations in LUMO levels of fullerene can be used to generate higher open-circuit voltages (V_OC) in bulk heterojunction polymer solar cells. The tetramethylcyclobutadiene-C₆₀ monoadduct displays an open-circuit voltage (0.61 V) and a power conversion efficiency (2.49%) comparable to the widely used P3HT/PCBM (poly(3-hexylthiophene/([6,6]- phenyl-C61-butyric acid methyl ester) composite (0.58 V and 2.57%, respectively). The role of the cofacial π-orbital interactions between C ₆₀ and the attached cyclobutene group was probed chemically by epoxidation of the cyclobutene moiety and theoretically through density functional theory calculations. The electrochemical, photophysical, and thermal properties of the newly synthesized fullerene derivatives support the proposed effect of functionalization on electron affinities and photovoltaic performance. Copyright

Full text of DOI:10.1002/adfm.201203251

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Cyclobutadiene–C60 Adducts: N-Type Materials for Organic
Photovoltaic Cells with High VOC
Ggoch Ddeul Han, William R. Collins, Trisha L. Andrew, Vladimir Bulovic´,
and Timothy M. Swager*
structures, researchers have developed low
band gap p-type polymers that strongly
absorb high fractions of the solar spec-
trum and assemble into desirable film
morphologies.[6,7] Efforts to improve n-type
acceptor components have included BHJs
with small molecules such as 9,9′-biflu-
orenylidenes, perylenediimides, and vina-
zenes.[8–10] Nevertheless, fullerenes are
presently the most widely used and highest
performing materials in part due to their
high electron affinities and low reorgani-
zation energies for electron transfer.[11]
Tailoring the electronic structure of
fullerene is therefore of interest, and
reactions with organometallic reagents,
transition-metal complexation, and radi-
cals have been investigated to create new
fullerenes for BHJs.[12–14] The reactivity of
fullerene resembles that of an electron-
deficient polyolefin, and as such, a domi-
nant functionalization strategy has been
to use cycloaddition reactions.[15,16] Note-
New tetraalkylcyclobutadiene–C60 adducts are developed via Diels–Alder
cycloaddition of C60 with in situ generated cyclobutadienes. The cofacial
π-orbital interactions between the fullerene orbitals and the cyclobutene are
shown to decrease the electron affinity and thereby increase the lowest unoc-
cupied molecular orbital (LUMO) energy level of C60 significantly (ca. 100 and
300 meV for mono- and bisadducts, respectively). These variations in LUMO
levels of fullerene can be used to generate higher open-circuit voltages (VOC
)
in bulk heterojunction polymer solar cells. The tetramethylcyclobutadiene–C60
monoadduct displays an open-circuit voltage (0.61 V) and a power conversion
efficiency (2.49%) comparable to the widely used P3HT/PCBM (poly(3-hexy-
lthiophene/([6,6]-phenyl-C61-butyric acid methyl ester) composite (0.58 V and
2.57%, respectively). The role of the cofacial π-orbital interactions between
C60 and the attached cyclobutene group was probed chemically by epoxida-
tion of the cyclobutene moiety and theoretically through density functional
theory calculations. The electrochemical, photophysical, and thermal proper-
ties of the newly synthesized fullerene derivatives support the proposed effect
of functionalization on electron affinities and photovoltaic performance.
worthy examples include the synthesis of PCBM ([6,6]-phenyl-
C61-butyric acid methyl ester) via 1,3-dipolar cycloaddition and
indene–C60 adducts from Diels–Alder reactions.[17–19]
1. Introduction
Bulk heterojunction (BHJ) polymer solar cells (PSCs), com-
posed of conjugated polymer donors and small or macromol-
ecule acceptors, are leading compositions in the quest for
practical organic photovoltaic cells.[1,2] The bicontinuous nature
of the phases in BHJ creates a large surface to volume ratio for
efficient exciton dissociation, and facile and low-cost fabrication
methods are compatible with large scale production.[3,4] Despite
extensive efforts to improve the properties of the constituent
materials and morphologies of BHJ systems, competing with
the power conversion efficiency (PCE) of silicon-based solar
cells remains a challenge.[5] In pursuit of ideal BHJ electronic
Less-than-optimal band offsets at the BHJs can lead to energy
loss and low open-circuit voltages in solar cells. In order to
increase power conversion efficiencies, there have been efforts
to increase the highest occupied molecular orbital–lowest unoc-
cupied molecular orbital (HOMOD–LUMOA; A: acceptor, D:
donor) gap which is often considered to be proportional to the
open-circuit voltage.[20] Many polymer donors with alternating
electron-rich and electron-poor units have been designed for
optimized absorption of the solar spectrum. Decreasing the
HOMO energy of the donor polymers can be accomplished by
simple modifications of the electron-rich repeating units, but
this typically leads to an expanded band gap that reduces the
solar absorption efficiency, thereby giving a smaller short-circuit
current density (JSC).[21–23] As a result, an alternative, and per-
haps more predictive, method is to increase the LUMO energy
level of the fullerenes to create increased VOCs. Most C60 func-
tionalization methods, including cycloaddition reactions, break
the full conjugation of the C60 π-system and generally decrease
the electron affinity and raise the LUMO level.[24] The extent
of this change varies depending on the nature of the func-
tional groups attached to the C60 core. The decreased relative
electron affinity of C60 is measured by the change of the onset
G. D. Han, Dr. W. R. Collins, Prof. T. M. Swager
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, MA 02139, USA
E-mail: tswager@mit.edu
Dr. T. L. Andrew, Prof. V. Bulovic´
Department of Electrical Engineering and Computer Science
Massachusetts Institute of Technology
Cambridge, MA 02139, USA
DOI: 10.1002/adfm.201203251
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Scheme 1. AlCl3-Assisted Diels–Alder Reaction of Alkynes
reduction potential or the calculated LUMO level.[25,26] In this
context, there has recently been interest in indene–C60 accep-
tors because these materials exhibit ca. 50–170 meV reduced
electron affinities (higher LUMO) relative to PCBM.[27] It would
appear that the short distance between the C60 π-system and the
π-orbitals of the addend aromatic ring of the indene affects the
LUMO energy.
To expand upon the hypothesis that cofacial π-orbital interac-
tions between C60 and an attached group can effectively raise
the fullerene LUMO levels, we have targeted a new functionali-
zation method that makes use of the well-known zwitterionic
AlCl3–cyclobutadiene adducts. The release of the cyclobuta-
dienes by treatment of AlCl3–cyclobutadiene adducts with weak
Lewis bases has been found to result in Diels–Alder reactions
with alkynes (Scheme 1) [28] to give Dewar benzene products that
have fairly small angles (114.6–114.9°) between two cyclobutene
rings.[29] As a result, we rationalized that Diels–Alder adducts
of similarly generated cyclobutadiene adducts with fullerenes
could give rise to strong cofacial π-orbital interactions.
In this study we report the syntheses of mono- and multiad-
ducts of tetramethylcyclobutadienes (Scheme 2) and tetrabutyl-
cyclobutadienes to fullerenes (Figure 1). The electrochemical,
photophysical, and thermal properties of these new fullerenes
have been studied, and their use in photovoltaic devices with
poly(3-hexylthiophene) (P3HT) has been evaluated in compar-
ison with PCBM. Tetramethylcyclobutadiene–C60 mono-, bis-,
and trisadducts (TMCB-Mono, TMCB-Bis, and TMCB-Tris) all
exhibited higher open-circuit voltages than that of PCBM, and
TMCB-Mono showed comparable power conversion efficiency
(2.49%) to PCBM (2.57%) devices under the identical condi-
tions. Lastly, the π-orbital interactions between cyclobutene
and the C60 cage were probed by removing the appended
double bond by epoxidation. The increased electron affinity of
the cyclobutane–epoxide–C60 was measured by cyclic voltam-
metry and calculated using density functional theory. All of
our results are consistent with the hypothesis that π–π orbital
interactions are an effective means to adjust the fullerene
LUMO levels.
Figure 1. Structures of functionalized fullerenes.
2. Results and Discussion
2.1. Syntheses of Tetraalkylcyclobutadiene–C60 Adducts
It is well-known that aluminum chloride reacts with internal
alkynes to generate cyclobutadiene intermediates with
a
σ-bonded aluminum moiety (Scheme 1).[28] The intermediate
cyclobutadiene, which is liberated by addition of dimethyl-
sulfoxide, is very reactive in [4 + 2] cycloaddition reactions
either as a diene (Scheme 1) or as a reactive alkene.[30] C60 is
a potent dienophile[31] and new n-type C60 derivatives are cre-
ated via Diels–Alder reactions with the in situ generated
tetraalkylcyclobutadienes.
The reactions were run under moisture-free conditions
wherein the reaction of 2-butyne and aluminum chloride
was first combined with C60 and subsequent treatment with
DMSO produced tetramethylcyclobutadiene–C60 mono-, bis-,
and trisadducts (Scheme 2). By varying the molar ratio of the
alkyne, aluminum chloride, and C60, we were able to selectively
increase the yield of each product in the mixture. In many
cases, the syntheses of functionalized fullerenes involve tens
of equivalents of the non-fullerene reagents to ensure a high-
yielding functionalization.[27,32] In contrast, four equivalents of
2-butyne relative to C60 produce monoadduct and bisadduct in
isolated respective yields of 32% and 17% after rigorous purifi-
cation. Generation of four equivalents of cyclobutadiene (8 eq.
of 2-butyne) increases the isolated yield of the pure trisadduct
to 8%. It is also the case that the syntheses of
fullerene derivatives by Diels–Alder reactions
often require long reaction times,[33] and
high temperatures such as refluxing o-dichlo-
robenzene (b.p. 180.5 °C).[32] The cyclobuta-
diene addition was carried out at low temper-
atures (between 0 °C and room temperature)
with reaction times of less than 2 h. After the
reaction, the mixture of fullerene adducts
was separated by HPLC using a 5PBB Cos-
1
mosil column with toluene elution. H and
Scheme 2. Synthesis of TMCB Fullerenes.
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13C nuclear magnetic resonance (NMR) spectroscopy as well
as high-resolution mass spectrometry (HRMS) confirmed the
attachment of the cyclobutene moiety. The similar procedure
provided other tetraalkylcyclobutadiene–C60 adducts starting
with symmetric internal alkynes such as 3-hexyne, 4-octyne,
and 5-decyne (Supporting Information Scheme S1). The longer
alkyl products provide for higher solubility in organic solvents
such as chloroform, dichloromethane, and toluene. Longer
chains disfavor trisadduct formation, presumably due to the
steric strain on the C60 surface. For example, for the functional-
ization starting with 5-decyne, we could not observe the trisad-
duct even when using 16 equivalents of this alkyne. Figure 1
details the representative five new functionalized fullerenes
selected for further investigation and the well-known PCBM.
(a)
PCBM
TMCB-Mono
TMCB-Bis
TMCB-Tris
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Voltage (vs. Ag/Ag+) / V
2.2. Electrochemical Properties
(b)
PCBM
C60 has triply-degenerated low-lying LUMO energy levels and
thereby can be reduced by up to six electrons.[34] A series of cyclic
voltammograms in Figure 2 shows that three or four reversible
redox waves are retained in the functionalized fullerenes under
anhydrous air-free conditions, relative to the ferrocene/ferro-
cenium (Fc/Fc+) internal standard peaks at ca. 200 mV ( 0.5 mV).
The monoadducts (TMCB-Mono and TBCB-Mono) exhibited
four reversible redox peaks (Figure 2a,b), which are all shifted to
negative potentials by ca. 100 mV as compared to PCBM, and the
bisadducts showed an additional ca. 200 mV shift relative to the
monoadducts. The measured half-wave potentials are listed in
Table 1 together with the estimated LUMO energy levels relative
to the vacuum level. The cyclic voltammogram of the trisadduct
contained three major sets of the redox peaks, as well as another
set of peaks, which we assume to be a persistent tetrakisadduct
impurity (Figure 2a). Despite the less defined features, it is clear
that the three main redox waves are further shifted by about 200
mV as compared to the bisadduct. The reversible sets of redox
curves shown in Figure 2a,b confirm that the functionalization
preserves the key electronic accepting characteristics of C60. The
pronounced negative shifts of redox curves and the corresponding
increase of LUMO energy levels relative to PCBM (90 meV
for TMCB-Mono, 260 meV for TMCB-Bis, and 450 meV for
TMCB-Tris) are considerably greater than the indene–C60 mono-
and bisadducts (50 meV and 170 meV, respectively).[27] This
behavior is consistent with our designs that make use of the small
TBCB-Mono
TBCB-Bis
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Voltage (vs. Ag/Ag+) / V
Figure 2. Cyclic voltammograms of a) PCBM and TMCB mono-, bis-,
and trisadducts and b) TBCB mono-, and bisadducts (under N2, 0.1 M
Bu4NPF6 in toluene/acetonitrile (4:1), Pt (WE), Pt wire (CE), Ag/AgNO3
(RE), scan rate 0.1 V/s, Fc/Fc+ internal standard E1/2 at 0.20 V).
angles between fused four-membered rings, which increases the
π-orbital interaction between the pendant cyclobutene moiety
and C60 centered states. It is not straightforward to explain the
difference in LUMO shifts of indene–C60 and cyclobutadiene–
C60. The proximity of the addend π-orbitals to C60 surface, or the
T a b le 1 . HOMO and LUMO energy levels calculated from UV–vis absorption and cyclic voltammetry.
C60 derivative
E1
[V]a)
E2
[V]
E3
[V]
Eonset
[V]b)
LUMO
[eV]c)
HOMO
[eV]e)
Egap
[eV]f)
λonset
[nm]d)
PCBM
723
738
746
697
740
751
1.72
1.68
1.67
1.78
1.68
1.65
−0.90
−0.96
−1.14
−1.37
−0.96
−1.19
−1.31
−1.38
−1.57
−1.79
−1.36
−1.64
−1.85
−1.97
−2.25
−2.57
−1.94
−2.38
−0.77
−0.86
−1.03
−1.22
−0.88
−1.10
−3.83
−3.74
−3.57
−3.38
−3.72
−3.50
−5.55
−5.42
−5.24
−5.16
−5.40
−5.15
TMCB-Mono
TMCB-Bis
TMCB-Tris
TBCB-Mono
TBCB-Bis
a)Half-wave potential, 0.5 (Epa+ Epc); Epa, anodic peak potential; Epc cathodic peak potential; b)Onset reduction potential; c)LUMO (eV) = −e (Eonset + 4.60); d)Onset absorp-
tion wavelength; e)HOMO = LUMO−Egap [eV]; f)Band gap = hc/λonset, converted [J] to [eV]; h, Planks constant; c, speed of light.
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(a)
angle between two rings, of either compound has not been deter-
mined and crystal structures have not been obtained to date. The
different extent of LUMO shifts could be also influenced by other
factors, given two dissimilar addend structures.
PCBM
0.08
TMCB-Mono
TMCB-Bis
TMCB-Tris
0.06
0.04
0.02
0.00
The cyclic voltammograms of TMCB-adducts and TBCB-
adducts are similar in terms of the shape and the positions of
potentials, but the TBCB-adducts exhibit slightly more negative
redox potentials. Considering the higher LUMOs of the TBCB-
series we expect a slightly higher VOC in photovoltaic devices
relative to those obtained for TMCB-adducts.
ε
2.3. Photophysical Properties
400
450
500
550
600
650
700
750
800
The visible and near-infrared spectral absorptions of BHJ devices
are a major determinant of solar cell efficiencies. JSC is dependent
on the number of charge carriers generated and their mobility.[20]
Considerable efforts have focused on creating donor polymers
with band gaps that match the solar spectrum. Although JSC
is generally dominated by the photon collection of the donor
polymer, stronger visible light absorption of the acceptors can
also contribute to light harvesting. Specifically, the weak optical
absorptions of PCBM are considered a limitation and have led to
interest in more costly C70 acceptor materials.[35] In Figure 3, we
show comparative absorption spectra (400–800 nm) of our new
fullerene derivatives relative to PCBM. The cyclobutadiene func-
tionalized fullerenes exhibited the similar or higher absorptivity
relative to PCBM. A more intense absorption was observed for
the higher adducts, which is consistent with desymmetrization of
fullerenes. The weak spikes around 430 nm for TMCB- and TBCB-
Mono are characteristic for 1,2-addition products, which confirms
the structures depicted in Scheme 2. A hypothetical 1,4-adduct,
which would form if C60 acted as a diene and cyclobutadiene
as a dienophile, would exhibit a broad absorption band around
450 nm, instead.[36–39] The absorption peaks around 430 nm
are slightly red-shifted from that of PCBM. Additionally, the bis-
and trisadducts showed broader absorptions as a result of the
fact that they are a mixture of regioisomers. Based upon their
absorption properties these new fullerene derivatives appear to
offer advantages over PCBM in terms of the higher absorptivity
and the red-shifted absorption edge around 700 nm. The absorp-
tion edge wavelengths, λonset, vary from 697 to 751 nm, and
longer wavelengths were observed for bisadducts than monoad-
ducts and PCBM (Table 1). The trisadduct is a 56-π-electron
wavelength / nm
(b)
PCBM
TBCB-Mono
TBCB-Bis
0.08
0.06
0.04
0.02
0.00
ε
400
450
500
550
600
650
700
750
800
wavelength / nm
Figure 3. UV–vis absorption spectra of a) PCBM (2.4 × 10−5 M), TMCB-
Mono (2.5 × 10−5 M), Bis (2.4 × 10−5 M), and Tris (2.0 × 10−5 M), and
b) TBCB-Mono (2.3 × 10−5 M), and Bis (1.3 × 10−5 M) in CHCl3.
temperatures used in polymer solar cell fabrications affect their
performance. To further investigate the thermal properties of
the fullerene adducts and the effects of annealing, we conducted
differential scanning calorimetry (DSC) experiments over the
fullerene and deviates from the trend with a small value of λonset
.
The band gaps and the HOMO energy levels of the fullerenes
were calculated from λonset and the first reduction wave, and are
summarized in Figure 4 and Table 1.
2.4. Thermal Properties
The thermal stability of the molecules was determined by ther-
mogravimetric analysis (TGA), which revealed weight loss of less
than 5% at 200 °C under N2 atmosphere (Supporting Informa-
tion Figure S2). All functionalized fullerenes are less stable than
pristine C60 which has outstanding thermal stability up to 500–
600 °C.[40] C60 derivatives often display a change in morphology
or decomposition at around 150 °C, and the typical annealing
Figure 4. HOMO–LUMO energy diagram of fullerene derivatives. The
energy levels were drawn relative to the vacuum level that is by definition
set to zero.
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The larger grain sizes (root-mean-square roughness) and more
pronounced phase separation were observed in the films with
higher adducts. The P3HT/TMCB-Mono morphology most
closely resembled the roughness of the P3HT/PCBM blend. We
were interested in determining if any of the features observed
for the higher adducts in the DSC and AFM reflected thermal
decomposition or only morphology changes. Indeed our new
fullerene derivatives could be susceptible to retro-cycloaddition
reactions as other fullerene cycloadducts.[41,42] In order to inves-
range from 25 to 200 °C (Supporting Information Figure S3).
In the first cycle, a strong and broad exothermic heat flow was
observed for TMCB-Bis and Tris from 90 to 150 °C and 80 to
190 °C, respectively (Supporting Information Figure S3a).
In contrast, the TMCB-Mono displays a small endothermic
peak which starts to appear at ca. 110 °C, which is likely due to
loss of residual toluene (b.p. 111 °C). The second DSC cycles
were featureless for all of the compounds (Supporting Infor-
mation Figure S3b). The mono- and bisadducts of tetrabutylcy-
clobutadiene exhibited relatively featureless heat flow to their
tetramethyl counterparts (Supporting Information Figure S3c,d).
BHJs formed with P3HT after annealing at 150 °C for 20 min
were studied by atomic force microscopy (AFM) (Figure 5).
1
tigate this, we measured the H NMR spectra of the fullerene
derivatives after three DSC cycles (25–200 °C, 10 °C/min) (Sup-
porting Information Figure S4a,b). The TMCB-Bis gained the
peaks assignable to TMCB-Mono, and TMCB-Tris gained the
peaks indicating the generation of TMCB-Mono and TMCB-
Bis. As a result, it is possible that larger phase separation of
the BHJ layers with the multiadducts is related to conversion
of the fullerene multiadducts into less-functionalized adducts
upon annealing. The 1H NMR spectra of the TMCB-Mono,
TBCB-Mono, and TBCB-Bis after being subjected to three
DSC cycles did not display any detectable changes thereby
reconfirming their superior thermal stability as compared to
the TMCB multiadducts. To determine if crystallization might
also be responsible for the exothermic DSC transitions of the
TMCB multiadducts at high temperatures, we conducted X-ray
diffraction experiments (Supporting Information Figure S5a,b).
TMCB and TBCB monoadducts are a single regioisomer, pre-
sumably fused to C60 in [6,6]-closed fashion as usual Diels–
Alder cycloaddition products of C60,[43–45] and exhibit microcrys-
tallinity. The butyl groups on cyclobutadiene–C60, as expected,
produce a larger lattice spacing (11.2 Å) than methyl groups
(10.1 Å). For multiadducts, the mixture of isomers reduced
the crystallinity of the sample, and TMCB-Bis remained in an
amorphous state even after three DSC cycles. As a result, the
observed DSC exothermic transitions for the multiadducts are
most likely the result of retro-cycloaddition reactions. In an
attempt to produce a more stable analog, the reaction of C60
and naphthocyclobutadiene was conducted. However, the reac-
tion did not proceed, presumably as a result of the fact that the
tetraalkylcyclobutadiene behaves as the diene in the Diels–Alder
reaction and naphthocyclobutadiene would only be expected to
behave as the dienophile.
2.5. Organic Photovoltaic Performances
Figure 6 details current density versus voltage (J–V) character-
istics under illumination of select P3HT/fullerene BHJ solar
cells. Specifically TMCB-Mono, Bis, and Tris are compared to
the well-known P3HT/PCBM system. As predicted based on
the LUMO energy levels, TMCB-Mono gives a higher open-
circuit voltage than PCBM. The P3HT/TMCB-Mono system
also displays a short-circuit current density, fill factor, and PCE
(η) comparable to the P3HT/PCBM device. The J–V curve of
TMCB-Bis displays a higher VOC, consistent with its higher
LUMO level, but a much lower JSC and decreased fill factor lead
to poor efficiency. We expect that the thermal instabilities of the
higher fullerene cyclobutadiene adducts contributes to lower
efficiencies. The performance of the P3HT/TMCB-Tris cell
displays even lower JSC. Field-effect mobilities of electrons are
Figure 5. Tapping-mode atomic force microscopy (1 μm window) phase
and topography images of the films prepared from a) PCBM, b) TMCB-
Mono, c) TMCB-Bis, and d) TMCB-Tris. P3HT was used as a donor for all
devices. PEDOT:PSS was used as the underlayer. All BHJs were annealed
at 150 °C for 20 min. Root-mean-square roughness was measured and
indicated in the topography images.
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PCBM
TMCB-Mono
TMCB-Bis
TMCB-Tris
10
0
Scheme 3. Epoxidation of TMCB-Mono.
(9.72 mA/cm2) compared to P3HT/PCBM (Supporting Informa-
tion Figure S6), but the increased resistance in the blend resulted
in lower fill factor and comparable PCE (2.43%). The decreased
VOC (0.61 V) and JSC (1.37 mA/cm2) of P3HT/TBCB-Bis cell led
to poor PCE (0.23%), thus reconfirming that less desirable per-
formances are obtained from our multiadducts. In addition, the
comparison of the J–V curves from different annealing condi-
tions (90 and 120 °C) reflected the destructive effect of thermal
annealing at high temperatures on the cells.
-10
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage / V
Figure 6. Current density–voltage (J–V) characteristics of bulk hetero-
junction PSCs under AM 1.5 illumination at 100 mW/cm2. ITO (150 nm)/
PEDOT:PSS (40 nm)/P3HT:fullerenes (75 nm)/Ca (25 nm)/Al (80 nm)
used for device fabrication.
2.6. Elimination of π-Orbital Interaction
also decreased for the higher adducts in comparison to TMCB-
Mono and the standard PCBM. 2.53 × 10−3 cm2/V·s for PCBM,
1.08 × 10−3 cm2/V·s for TMCB-Mono, 1.07 × 10−5 cm2/V·s
for TMCB-Bis, and 1.34 × 10−6 cm2/V·s for TMCB-Tris were
obtained. An interesting feature is that the open-circuit voltage
of TMCB-Tris is smaller than that of TMCB-Bis. This discrep-
ancy might result from the excessive decomposition of Tris
upon annealing. In general, the open-circuit voltages of the
BHJ polymer-fullerene films are influenced by intermolecular
interactions in the solid state. Thus, they can exhibit quanti-
tatively non-linear correlation with HOMOD–LUMOA gaps,
predicted by the solution-state measurements. Additionally,
we note that the VOC in organic BHJ solar cells is recombina-
tion limited.[46] Since we observe non-ideal BHJ morphologies
with some of our fullerene adducts, which increases the rate of
bimolecular recombination across the donor-acceptor interface,
we hypothesize that part of the observed discrepancy in VOC
trends is also due to increased recombination. Correspondingly,
the PCE (Table 2) of P3HT/TMCB-Mono (2.49%), comparable
with that of P3HT/PCBM (2.57%), and the smaller values for
P3HT/TMCB-Bis (1.35%) and P3HT/TMCB-Tris (0.65%) were
consistent with the degree of phase separation and root-mean-
square roughness discovered on AFM images.
To chemically probe our hypothesis that large decrease in electron
affinity in the cyclobutadiene-fullerenes is the result of strong
interactions between the cyclobutene double bond and the C60
π-system, we devised a simple experiment to remove the double
bond. Several reactions were attempted, but the most straight-
forward and the highest-yielding reaction was epoxidation with
3-chloroperbenzoic acid (mCPBA) (Scheme 3). The reaction
proceeded with 96% yield, and the product (TMCB-EP) had sig-
nificantly different polarity from the starting material, which facil-
itated purification. The identity of the product was supported by
1H and 13C NMR spectra as well as HRMS. Epoxidation of TBCB-
Mono was unsuccessful presumably as a result of steric hindrance
around the reaction site, and only starting material was recovered.
Cyclic voltammetry of TMCB-Ep (Figure 7) exhibited four
reversible redox peaks like other monoadducts, but half-wave
C60
TMCB-Ep
TMCB-Mono
To obtain the optimized performance of fullerene adducts with
longer alkyl chains, we annealed the devices with TBCB-Mono
and TBCB-Bis at 90 °C (Supporting Information Figure S6).
P3HT/TBCB-Mono blend produced higher VOC (0.64 V) and JSC
T a b le 2 . Characteristics of OPV Devicesa)
TMCB-Mono
7.86
Bis
Tris
3.13
0.65
0.32
0.65
PCBM
8.35
JSC [mA/cm2]
VOC [V]
5.92
0.69
0.33
1.35
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
0.61
0.58
Voltage (vs. Ag/Ag+) / V
FF [%]
0.52
0.53
2.49
2.57
η [%]
Figure 7. Cyclic voltammograms of C60, TMCB-Ep, and TMCB-Mono
(under N2, 0.1 M Bu4NPF6 in toluene/acetonitrile (4:1), Pt (WE), Pt wire
(CE), Ag/AgNO3 (RE), scan rate 0.1 V/s, Fc/Fc+ internal standard E1/2 at
0.20 V).
a)Definitions: short-circuit current density, JSC; open-circuit voltage, VOC; fill factor,
FF; PCE, η.
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T a b le 3 . Electrochemical characteristics and LUMO energy levels of C60,
TMCB-Ep, and TMCB-Mono.
separation and low PCEs. Epoxidation of cyclobutene con-
firmed that strong cofacial π-orbital interactions between C60
π-system and the double bond π-orbital contribute to raising the
LUMO levels. Our results show that n-type materials for bulk
heterojunction polymer solar cells can be chemically modified
to fine-tune their electronic properties and thus the resulting
open-circuit voltages in organic solar cells.
E1
[V]a)
E2
[V]
E3
[V]
Eonset
[V]b)
LUMO
[eV]c)
C60
−0.84
−0.91
−0.97
−1.24
−1.34
−1.38
−1.75
−1.92
−1.95
−0.75
−0.81
−0.86
−3.85
−3.79
−3.74
TMCB-Ep
TMCB-Mono
a)Half-wave potential (V), 0.5 (Epa + Epc); Epa, anodic peak potential; Epc cathodic
peak potential; b)Onset reduction potential; c)LUMO (eV) = −e (Eonset + 4.60).
4. Experimental Section
Materials: C60 was purchased from SES Research, and aluminum
chloride and alkynes were purchased from Sigma–Aldrich and used as
received. Other materials including solvents and electrolyte salt were
commercially available. Anhydrous solvents were obtained from a
solvent purification system (Innovative Technologies).
potentials were located approximately half way between those
of pristine C60 and the TMCB-Mono compound. In Table 3, the
half-wave potentials of TMCB-Ep were summarized. The dif-
ference between LUMO energy levels of C60 and TMCB-Ep is
60 meV, and the gap between those of TMCB-Ep and TMCB-
Mono is 50 meV. From this observation, we conclude that orbital
interactions between the cofacial cyclobutene and C60 π-systems
contribute to the observed LUMO raising. The electron affinity
of TMCB-Ep was still lower than that of C60 and this is presum-
ably the result of the σ-π hyperconjugative interactions.
To further support this assertion, we conducted quantum-
chemical density functional theory (DFT) calculations (B3LYP
functional/6-31G (d,p) basis set) of TMCB-Mono and TMCB-
Ep. The geometry optimized structures (Supporting Informa-
tion Figure S7a,b) had HOMO and LUMO energy levels of
−5.58 and −3.04 eV for TMCB-Mono, and −5.69 and −3.16 eV
for TMCB-Ep (Supporting Information Figure S8a,b). These
results also indicated that epoxidation of the cyclobutene moiety
decreased the LUMO level. The calculated distances between C60
cage and the attached functional groups were 3.05 and 3.10 Å
for TMCB-Mono and TMCB-Ep, sufficiently short to allow for
cofacial π-orbital interactions.[47] It is apparent in the LUMO
representations (side and front views on Supporting Informa-
tion Figure S8a) of TMCB-Mono that the wavefunction on the
double bond of the cyclobutene moiety and that on the adjacent
C60 π-orbital possess the opposite signs. It is believed that the
LUMO level of C60 is raised effectively due to this close π−π
interaction between the two moieties. On the other hand, for
TMCB-Ep (Supporting Information Figure S8b), the hypercon-
jugative interaction between the C–H σ-bonds of methyl groups
and C60 π-system is outstanding for most of the MOs rather than
cofacial π interaction. Therefore, we could theoretically support
the concept of the structural design of cyclobutadiene–C60 for
efficient cofacial π interaction and the experimental results.
Measurements: Reaction mixtures containing multiadducts of
fullerenes were separated by 5PBB Cosmosil column (10 mm ×
250 mm) from Nacalai Tesque, Inc. installed in Agilent Technologies
1
ProsStar 210 High Pressure Liquid Chromatography (HPLC) system. H
and 13C NMR spectra were taken on Varian Inova–500 spectrometers.
Chemical shifts were reported in ppm and referenced to residual
solvent peaks (CDCl3: 7.26 ppm for 1H, 77.20 ppm for 13C). Thermally
assisted direct analysis in real time (TA–DART)[48] high-resolution
mass spectrometry (HRMS) was measured on LCQ DECA (Thermo–
Finnigan LLC) with ID–CUBE source (IonSense, Inc.). Capillary
temperature was 200 °C, and the capillary voltage was set to 15 V in
positive-ion mode. Helium was used as the ionization gas. Bruker
Daltonics Omniflex MALDI–TOF mass spectrometer was also used for
mass determination. The matrix was prepared following a literature
procedure,[49] containing 7,7,8,8-tetracyanquinodimethane in THF
(10 mg/mL) with 1% silver trifluoroacetate as a promoter. Molecules were
dissolved in THF to 0.1 mg/mL concentration and the solution (2 μL)
was mixed with matrix (20 μL). UV–vis absorption spectra were obtained
using Agilent 8453 diode-array spectrophotometer. Electrochemical
measurements were carried out in a glove box under nitrogen, using
an Autolab PGSTAT 10 or PGSTAT 20 potentiostat (Eco Chemie) in a
three-electrode cell configuration. A Pt button (1.6 mm in diameter)
electrode, a Pt wire, and a quasi-internal Ag wire submerged in 0.01
M AgNO3/0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)
in acetonitrile were used as a working electrode, a counter electrode,
and a reference electrode, respectively, in 0.1 M TBAPF6 toluene/
acetonitrile (4:1) solution. The ferrocene/ferrocenium (Fc/Fc+) redox
couple was used as an internal standard, with the half-wave potentials
observed between 0.195–0.203 V vs Ag/Ag+ in toluene/acetonitrile (4:1)
solution. Differential Scanning Calorimetry (DSC) was measured on a TA
Instruments Q1000 DSC at scan rate of 10 °C/min over the range of 25
to 200 °C or 35 to 200 °C. Thermogravimetric Analyses were performed
with a TGA Q50 apparatus(TA Instruments) under nitrogen. Samples
were heated at 10 °C/min from 30 °C to 800 °C. X-ray diffraction was
measured using Cu Kα radiation on an Inel CPS 120 position-sensitive
detector with a XRG 3000 generator using aluminum substrate during
ca. 20 min collection time. Gaussian 03 software package was used for
the structure optimization and the frequency calculation at the DFT level.
B3LYP functional/6-31G (d,p) basis set was used for the computation.
Syntheses of TMCB-Mono and TMCB-Bis: In a flame dried 100 mL
Schlenk flask, AlCl3 (148 mg, 1.11 mmol) was dissolved in dry
o-dichlorobenzene (o-DCB) (10 mL) and stirred at 0 °C under Ar. 2-Butyne
(174 μL, 2.22 mmol) was added to the solution dropwise, and then o-
DCB solution (20 mL) of C60 (400 mg, 0.555 mmol) was added. After 1 h
of stirring to ensure formation of homogeneous phase, DMSO (0.2 mL,
2.33 mmol) was added dropwise and the solution was slowly warmed up
to room temperature. Ethanol (50 mL) was poured, and the precipitate
was filtered and washed with ethanol several times, and then dissolved
in toluene/hexane (1:1, 100 mL). The solution was passed through silica
gel pad (5 cm), and concentrated in vacuo. Monoadduct and Bisadduct
3. Conclusions
Two families of tetraalkylcyclobutadiene–C60 adducts were gen-
erated via Diels–Alder cycloaddition reactions between in situ
generated cyclobutadiene intermediates and C60. The mono-,
bis-, and trisadducts of C60 with tetraalkylcyclobutadiene groups
were formed, and their electrochemical, photophysical, and
thermal properties were compared. The thermal annealing
process in device fabrication was revealed to be destructive for
the new fullerene bis- and trisadducts, leading to large phase
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were separated by 5PBB Cosmosil column in toluene (4 mL/min) with
yields of 31.9% (146.6 mg) and 16.5% (86.0 mg), respectively.
TMCB-Mono 1H NMR (500 MHz, CDCl3, δ): 2.07 (s, 6H), 1.97 (s,
6H). 13C NMR (500 MHz, CDCl3, δ): 154.62 (2C), 154.21 (2C), 147.24
(2C), 146.94 (2C), 146.91 (2C), 146.22 (2C), 146.14 (2C), 146.06 (2C),
145.53 (2C), 145.46 (2C), 145.39 (2C), 145.37 (2C), 145.35 (2C), 145.26
(2C), 144.69 (2C), 144.58 (2C), 143.21 (2C), 143.05 (2C), 142.86 (2C),
142.79 (2C), 142.49 (2C), 142.35 (2C), 142.31 (2C), 142.25 (2C), 142.11
(2C), 140.43 (2C), 140.37 (2C), 139.06 (2C), 137.75 (2C), 129.22 (1C),
128.41 (1C), 74.38 (2C), 60.38 (2C), 53.62 (CH2Cl2), 14.07 (2C), 11.56
(2C).
140.62 (2C), 138.87 (2C), 137.31 (2C), 71.38 (2C), 71.20 (2C), 62.46
(2C), 12.87 (2C), 12.12 (2C).
HRMS (ESI, m/z):[M+H]+ calcd for C68H12O, 845.0961; found, 845.12
Syntheses of TBCB-Mono and TBCB-Bis. In a flame dried 50 mL
Schlenk flask, AlCl3 (150 mg, 1.12 mmol) was dissolved in dry o-DCB
(15 mL) and stirred at 0 °C under Ar. 5-Decyne (0.41 mL, 2.28 mmol) was
added to the solution dropwise, and then o-DCB solution (50 mL) of C60
(200 mg, 0.28 mmol) was added. After 1 h of stirring, DMSO (0.2 mL,
2.33 mmol) was added dropwise and the solution was slowly warmed up
to room temperature. Ethanol (35 mL) was poured, and the precipitate
was filtered and washed with ethanol several times, and then dissolved
in toluene/hexane (1:4, 150 mL). The solution was passed through silica
gel pad (5 cm), and concentrated in vacuo. Monoadduct and bisadduct
were separated by 5PBB Cosmosil column in toluene (4 mL/min) with
yields of 40.1% (111.1 mg) and 24.1% (85.1 mg), respectively. 17.7%
(35.5 mg) of C60 was recovered.
TMCB-Bis 1H NMR (500 MHz, CDCl3, δ): 2.22 (m, 3H), 2.05 (m, 3H),
2.03-2.02 (m, 3H), 1.98 (m, 3H), 1.89 (s, 3H), 1.85-1.84 (m, 3H), 1.75
(m, 3H), 1.66-1.64 (m, 3H). 13C NMR (500 MHz, CDCl3, δ): 157.49 (1C),
157.32 (1C), 155.46 (1C), 155.14 (1C), 155.10 (1C), 154.74 (1C), 154.62
(1C), 154.51 (1C), 154.44 (1C), 149.94 (1C), 149.91 (1C), 149.82 (1C),
149.60 (1C), 148.73 (1C), 148.71 (1C), 146.64 (1C), 148.26 (1C), 148.05
(1C), 147.91 (1C), 147.86 (1C), 147.75 (1C), 147.57 (1C), 147.28 (1C),
146.16 (1C), 145.97 (1C), 145.90 (1C), 145.77 (1C), 145.75 (1C), 145.59
(1C), 145.20 (1C), 145.08 (1C), 145.04 (2C), 144.76 (2C), 144.72 (1C),
144.59 (1C), 144.02 (1C), 143.72 (1C), 143.60 (1C), 142.66 (1C), 142.58
(1C), 142.55 (1C), 141.50 (1C), 141.43 (1C), 141.36 (1C), 141.24 (1C),
140.72 (1C), 139.99 (1C), 139.19 (1C), 138.62 (1C), 138.52 (1C), 138.06
(1C), 137.85 (1C), 137.16 (1C), 137.05 (1C), 129.22 (1C), 128.41 (2C),
125.49 (1C), 60.46 (2C), 60.10 (2C), 60.08 (2C), 60.01 (2C), 13.98 (2C),
13.87 (2C), 11.74 (1C), 11.49 (1C), 11.42 (1C), 11.24 (1C).
TBCB-Mono 1H NMR (500 MHz, CDCl3/CS2 1:1, δ): 2.69-2.63 (m,
4H), 2.47-2.41 (m, 2H), 2.30-2.24 (m, 2H), 2.02-1.98 (m, 2H), 1.91-1.73
(m, 6H), 1.54-1.50 (m, 8H), 1.04-1.01 (t, 12H). 13C NMR (500 MHz,
CDCl3/CS2 1:1, δ): 155.19 (2C), 153.81 (2C), 149.58 (2C), 147.25 (2C),
146.71 (2C), 146.23 (2C), 146.06 (2C), 145.99 (2C), 145.96 (2C), 146.87
(2C), 145.33 (2C), 145.18 (2C), 145.16 (4C), 144.53 (2C), 144.41 (2C),
143.08 (2C), 143.03 (1C), 142.89 (1C), 142.74 (2C), 142.71 (2C), 142.62
(2C), 142.31 (2C), 142.19 (2C), 142.09 (2C), 142.08 (2C), 141.95 (2C),
140.30 (2C), 140.09 (2C), 138.77 (2C), 137.78 (2C), 73.98(2C), 64.00
(2C), 31.46 (2C), 31.43 (2C), 29.51 (2C), 28.90 (2C), 24.34 (2C), 23.83
(2C), 14.31 (2C), 14.28 (2C).
HRMS (ESI, m/z): Monoadduct [M+H]+ calcd for C68H12, 829.1012;
found, 829.04, Bisadduct [M]+ calcd for C76H24, 937.0060; found,
936.99)
TBCB-Bis 1H NMR (500 MHz, CDCl3/CS2 1:1, δ): 2.55-1.34 (m, 48H),
1.01-0.90 (m, 24H). 13C NMR (500 MHz, CDCl3/CS2 1:1, δ): 157.71 (2C),
157.67 (2C), 156.63 (2C), 155.55 (2C), 154.62 (2C), 154.05 (2C), 153.75
(2C), 153.56 (2C), 152.15 (2C), 149.50 (2C), 149.32 (2C), 148.98 (2C),
148.87 (2C), 148.47 (2C), 147.69 (2C), 147.11 (2C), 146.16 (2C), 145.00
(2C), 144.90 (2C), 144.85 (2C), 144.41 (2C), 144.27 (2C), 143.61 (2C),
142.57 (2C), 141.21 (2C), 141.14 (2C), 139.55 (2C), 138.83 (2C), 137.31
(2C), 136.81 (2C), 63.93 (2C), 63.87 (2C), 63.57 (2C), 63.55 (2C), 31.64
(1C), 31.54 (1C), 31.40 (1C), 31.37 (1C), 31.31 (1C), 31.26 (1C), 31.19
(1C), 31.12 (1C), 30.47 (1C), 30.03 (1C), 29.56 (1C), 29.38 (1C), 29.07
(1C), 28.92 (1C), 28.87 (1C), 28.72 (1C), 24.56 (1C), 24.38 (1C), 24.30
(1C), 24.04 (1C), 23.83 (1C), 23.81 (1C), 23.71 (1C), 23.65 (1C), 14.37
(2C), 14.32 (2C), 14.28 (2C), 14.19 (2C).
Synthesis of TMCB-Tris: In a flame dried 200 mL Schlenk flask, AlCl3
(741 mg, 5.56 mmol) was dissolved in dry o-DCB (25 mL) and stirred
at 0 °C under Ar. 2-Butyne (0.88 mL, 11.1 mmol) was added to the
solution dropwise, and then o-DCB solution (50 mL) of C60 (1.0 g,
1.39 mmol) was added. After 1 hour of stirring, DMSO (0.89 mL,
11.4 mmol) was added dropwise and the solution was slowly warmed up
to room temperature. Ethanol (100 mL) was poured, and the precipitate
was filtered and washed with ethanol several times, and then dissolved
in toluene/hexane (1:1, 250 mL). The solution was passed through
silica gel pad (5 cm), and concentrated in vacuo. 115 mg (7.9%) of the
product was obtained after purification by 5PBB Cosmosil column in
toluene (4 mL/min). 1H NMR (500 MHz, CDCl3, δ): 2.16-1.57 (m, 36H).
13C NMR (500 MHz, CDCl3, δ): 157.81 (2C), 157.68 (2C), 157.61 (2C),
155.49 (2C), 155.20 (2C), 155.09 (2C), 154.94 (2C), 154.57 (2C), 152.81
(2C), 151.51 (2C), 151.48 (2C), 151.13 (2C), 151.11 (2C), 149.70 (2C),
149.64 (2C), 149.57 (2C), 149.52 (2C), 149.45 (2C), 149.06 (2C), 148.94
(2C), 148.38 (2C), 148.37 (2C), 144.85 (2C), 144.84 (2C), 144.42 (2C),
144.26 (2C), 144.23 (2C), 115.33 (2C), 115.18 (2C), 114.84 (2C), 59.90
(6C), 59.29 (6C), 13.86 (2C), 13.77 (2C), 11.65 (2C), 11.41 (2C), 11.36
(2C), 11.28 (2C).
HRMS (ESI, m/z): [M+H]+ calcd for C80H36, 997.2890; found, 997.23
HRMS (MALDI–TOF, m/z): [M]+ calcd for C100H72, 1273.6448; found,
1273.62
Device Fabrication: Pre-patterned indium tin oxide (ITO)-coated
glass substrates (Thin Film Devices, Inc.) were sonicated in acetone
(30 min) and isopropanol (30 min) and oxygen plasma-cleaned (3 min)
immediately prior to deposition of the PEDOT:PSS layer. PEDOT:PSS
(2–5 wt% in water, Aldrich) was spin-coated in a nitrogen atmosphere
at 4000 rpm and annealed at 150 °C (using a hotplate) for 15 min under
nitrogen. A 40 nm PEDOT layer was thus obtained. Film thickness was
determined by ellipsometry measurements on separate films prepared
on silicon substrates. For the active layer, a 10 mg/mL solution of
1:1.2 P3HT:fullerene in 1,2-dichlorobenzene (DCB) was employed (in
a representative example, P3HT (1.9 mg) and fullerene (2.3 mg) were
dissolved in DCB (0.2 mL)). This solution (60 μL) was then spin-coated
onto the PEDOT layer at 1000 rpm under nitrogen. The substrate was
taken from the spin chuck and immediately placed under an inverted
Petri dish inside the glovebox for 10 min to encourage solvent annealing
from the small amount of residual DCB on the substrate. Next, the
solar cells were placed on a 150 °C hotplate and annealed for 20 min
under nitrogen. A 70 5 nm active layer was thus obtained. Following
this deposition procedure, the top electrode was deposited by thermal
evaporation of a 25 nm thick film of Ca followed by 80 nm thick film
of Al. The device area, as defined by the anode-cathode overlap, is
1.21 mm2.
HRMS (MALDI–TOF, m/z): [M]+ calcd for C84H36,1045.1871; found,
1045.22
Synthesis of TMCB-Ep: TMCB-Mono (102 mg, 0.123 mmol) was
dissolved in chloroform (15 mL) in a 50 mL round-bottom flask in
which a solution of 3-chloroperbenzoic acid (max. 77% purity, 76 mg,
0.339 mmol) in chloroform (5 mL) was added dropwise at 0 °C. The
solution was slowly warmed up to room temperature during 7 h of
stirring, and a major product spot was detected on a TLC plate. The
reaction mixture was diluted with dichloromethane (75 mL), and
extracted with NaOH aqueous solution several times. The organic layer
was dried with MgSO4 and concentrated in vacuo. 100 mg (96.1%) of
the product was isolated after gradient silica column with 1:13 to 1:1
toluene/hexane eluent.
1H NMR (500 MHz, CDCl3, δ ): 1.98 (s, 6H), 1.87 (s, 6H). 13C NMR
(500 MHz, CDCl3, δ): 153.77 (2C), 153.69 (2C), 147.20 (2C), 146.76 (2C),
146.57 (2C), 146.30 (2C), 146.22 (2C), 146.21 (4C), 145.69 (2C), 145.55
(2C), 145.51 (2C), 145.43 (2C), 145.39 (2C), 144.75 (2C), 144.70 (2C),
143.35 (1C), 143.34 (1C), 142.94 (2C), 142.90 (2C), 142.55 (2C), 142.49
(2C), 142.26 (2C), 142.20 (2C), 142.19 (2C), 141.87 (2C), 140.64 (2C),
Device Characterization: Current density–voltage (J–V) measurements
were recorded by a Keithley 6487 picoammeter both in the dark and
under illumination. The devices were illuminated through the glass
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substrate using an Oriel 91191 150 W full spectrum solar simulator. The
illumination intensity was calibrated to 100 mW/cm2 using an NREL-
certified silicon photodiode. Spectral mismatch was not corrected for in
these measurements.
FET Electron Mobility Measurement: Bottom-gate, bottom-contact FETs
were made on Si substrates with a thermally-grown SiO2 layer (500 nm)
and 10 μm channel lengths (PCBM and TMCB-Mono). Top-gate,
bottom-contact FETs were made on glass substrates with a PMMA layer
(465 nm) top gate and 10 μm channel lengths (TMCB-bis and TMCB-
tris).
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