Morphological stabilization by supramolecular perfluorophenyl-C 60 interactions leading to efficient and thermally stable organic photovoltaics

Article abstract of DOI:10.1002/adfm.201300437

A new PC₆₁BM-based fullerene, [6,6]-phenyl-C₆₁ butyric acid pentafluorophenyl ester (PC₆₁BP^F) is designed and synthesized. This new n-type material can replace PC₆₁BM to form a P3HT:PC₆₁BP<

Full text of DOI:10.1002/adfm.201300437

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Morphological Stabilization by Supramolecular
Perfluorophenyl-C60 Interactions Leading to Efficient and
Thermally Stable Organic Photovoltaics
Ming-Hung Liao, Che-En Tsai, Yu-Ying Lai, Fong-Yi Cao, Jhong-Sian Wu,
Chien-Lung Wang, Chain-Shu Hsu, Ian Liau, and Yen-Ju Cheng*
as the electron acceptor are the most
A new PC61BM-based fullerene, [6,6]-phenyl-C61 butyric acid pentafluoro-
phenyl ester (PC61BPF) is designed and synthesized. This new n-type mate-
rial can replace PC61BM to form a P3HT:PC61BPF binary blend or serve as
an additive to form a P3HT:PC61BM:PC61BPF ternary blend. Supramolecular
attraction between the pentafluorophenyl group of PC61BPF and the C60 cores
of PC61BPF/PC61BM can effectively suppress the PC61BPF/PC61BM materials
from severe aggregation. By doping only 8.3 wt% PC61BPF, device PC61BPF651
exhibits a PCE of 3.88% and decreases slightly to 3.68% after heating for
25 h, preserving 95% of its original value. When PC61BP with non-fluorinated
phenyl group is used to substitute PC61BPF, the stabilizing ability disap-
pears completely. The efficiencies of PC61BP651 and PC61BP321 devices
significantly decay to 0.44% and 0.11%, respectively, after 25 h isothermal
heating. Most significantly, this strategy is demonstrated to be effective for a
blend system incorporating a low band-gap polymer. By adding only 10 wt%
PC61BPF, the PDTBCDTB:PC71BM-based device exhibits thermally stable mor-
phology and device characteristics. These findings demonstrate that smart
utilization of supramolecular interactions is an effective and practical strategy
to control morphological evolution.
widely investigated system with power
conversion efficiencies approaching 5%
(Scheme 1).[1g,2a,3] Thermal annealing
of P3HT:PC61BM composite is an effec-
tive way to induce maximum interfacial
area with higher degree of P3HT crystal-
linity for efficient charge generation and
charge transport.[3b,4] Unfortunately, when
thermal treatment at elevated tempera-
ture was applied persistently,[5] the kineti-
cally trapped state of the morphology
shifted toward the more thermodynami-
cally stable state. The P3HT continued to
undergo higher extent of crystallization,
while the spherical PC61BM molecules
with high molecular mobility gradually
diffused out of the polymer matrix and
aggregated into larger clusters or single
crystals.[5d,5e,6] Such progressive phase-
segregation eventually leads to microm-
eter-sized P3HT:PC61BM domains with
dramatic reduction of the interfacial area.
Because a photovoltaic device is expected
be exposed under long-term sunlight irradiation, the accumu-
lated heat may raise the operational temperature that conse-
quently destroys the optimal morphology and deteriorate the
device performance.[7] Several elegant strategies have been
developed to stabilize the active layer morphology, such as
reduction of crystallinity of polythiophene[8] or fullerene deriva-
tives,[9] increase of Tg of the PPV derivatives,[10] introduction of
compatibilizer to impart secondary interactions between the
donor and acceptor constitutes,[11] and in situ chemical cross-
linking between components in active layers.[12]
Researchers have shown that PC61BM molecules can dif-
fuse rapidly within crystalline P3HT upon thermal treatment,
this phenomenon is even more pronounced in an amorphous
polymer matrix such as MDMO-PPV.[5c,5d] Restricting the
molecular movement of PC61BM to prevent them from severe
diffusion would be a straightforward solution to maintain the
optimal morphology. Very recently, we developed a PC61BM-
based derivative, [6,6]-phenyl-C61 butyric acid styryl ester
(PCBS), containing a polymerizable styrene group.[12f ] The
PC61BM:P3HT blend system was added with a small amount
of PCBS that can undergo in situ thermal polymerization
to suppress the thermal-driven aggregation of the fullerene
1. Introduction
Polymeric solar cells (PSCs) are a promising alternative for
clean and renewable energy due to their potential to be fabri-
cated onto large area, light-weight, and flexible substrates by
solution processing at a lower cost.[1] Although the concept
of bulk heterojunction (BHJ) offers the most straightforward
strategy to maximize internal donor–acceptor (D–A) interfacial
area for efficient charge separation, tailoring the morphology of
the blend in a BHJ device to achieve optimized performance is
of pivotal importance and remains rather challenging.[2] Active
layers containing poly(3-hexylthiophene) (P3HT) as the electron
donor and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM)
M.-H. Liao, C.-E. Tsai, Dr. Y.-Y. Lai, F.-Y. Cao,
Dr. J.-S. Wu. Prof. C.-L. Wang, Prof. C.-S. Hsu,
Prof. I. Liau, Prof. Y.-J. Cheng
Department of Applied Chemistry
National Chiao Tung University
1001 Ta Hsueh Road Hsin-Chu, 30010, Taiwan
E-mail: yjcheng@mail.nctu.edu.tw
DOI: 10.1002/adfm.201300437
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fullerene assembles into an interlocked
one-dimensional zigzag array in the solid
state.[17] Inspired by this work, we envision
that the C6F5–C60 physical interaction could
be smartly utilized to substitute the function
of chemical polymerization for achieving
morphological stabilization. To demonstrate
this strategy, we designed and synthesized a
new PC61BM-based fullerene, [6,6]-phenyl-
C61 butyric acid pentafluorophenyl ester
(PC61BPF, Scheme 1). The structural differ-
ence between PC61BM and PC61BPF is that
the methyl group in PC61BM is replaced by
a pentafluorophenyl group in PC61BPF. This
small modification not only makes PC61BPF
maintain intrinsic properties of PC61BM but
also imparts C6F5–C60 interaction between
PC61BPF molecules. For comparison, we
also synthesized a reference compound,
[6,6]-phenyl-C61 butyric acid phenyl ester
(PC61BP), which has an identical chemical
structure to PC61BPF except that the pen-
tafluorophenyl in PC61BPF is changed to a
Scheme 1. Chemical structures of PC61BM, PC61BPF, PC61BP, P3HT, and PDTBCDTBT
copolymer used in this research.
non-fluorinated phenyl analogue (Scheme 1).
materials, thereby the optimized morphology can be pre-
served through the covalent fixation. Although the concept of
covalent locking is proved to be effective, the success of this
strategy is critically dependent on manipulating the tempera-
ture window to control the occurrence of polymerization.
Initiation of thermal-induced polymerization must not occur
before the optimized morphology is developed. Undesired
polymerization at the early stage will sterically interrupt the
molecular assembly of P3HT and PC61BM during the morpho-
logical evolution.
The PC61BPF can be used as an acceptor to form a
P3HT:PC61BPF binary blend, or be added into a P3HT:PC61BM
system to form a P3HT:PC61BM:PC61BPF ternary blend.
Regardless of using binary or ternary blends, the devices incor-
porating PC61BPF exhibited stable or enhanced device charac-
teristics during long-term thermal treatment. In sharp contrast,
the devices employing non-fluorinated PC61BP do not show the
ability of morphological stabilization upon thermal heating. We
have successfully demonstrated that the smartly utilization of
supramolecular physical interaction is a simple and feasible
approach to control nano-morphology leading to efficient and
stable organic photovoltaics.
Beyond thermal-induced polymerization in bulk, mor-
phological engineering involving noncovalent self-assembly
emerges as a new exploratory direction for modulating mor-
phology. Introducing small amount of additives into active
layers to assist physically the formation of suitable nano-mor-
phology has been extensively investigated.[13] Despite of sub-
stantial success of this strategy in improving efficiency, under-
standing the mechanism of morphological evolution is still
lacking. Hydrogen bonding approach is widely used for self-
assembly.[14] However, the complementary groups with high
polarity for H-bonding formation are known to have negative
effect on charge transport. Therefore, so far this strategy has
not been successful.[15] It was documented that a 1:1 mixture
of benzene/hexafluorobenzene adopts a face-to-face stacking
with an alternating sequence.[16] This stacking arrangement
is due to complementary attractions between benzene and
hexafluorobenzene. The stabilization energy of this attrac-
tion is estimated to be 4.3 kcal mol−1 and the cofacial distance
between two planar structures is about 3.7 Å.[16e] Such physical
interactions have been observed in various aromatic/perfluoro-
aromatic systems that create a variety of fascinating molecular
assemblies.[16b–16h] In a very similar manner, perfluorophenyl
motif has a favorable interaction with the surface of a fullerene
that actually contains 20 hexagonal rings with more localized
electrons. It has been demonstrated that through such a face-
to-face C6F5–fullerene interaction, 1,4-bis(pentafluorobenzenyl)
2. Results and Discussion
2.1. Synthesis and Thermal Properties of the Pentafluorophenol
and Phenol-Functionalized PC61BM Derivatives
PC61BPF and PC61BP were easily prepared by the esterifi-
cation of [6,6]-phenyl-C61 butyric acid (PC61BA) with pen-
tafluorophenol and phenol, respectively, in the presence of
1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and
4-dimethylaminopyridine (DMAP) in 42% yield (Scheme 2).
Thermogravimetric analysis (TGA) measurement and dif-
ferential scanning calorimetry (DSC) were used to measure
the thermal properties of PC61BPF molecule (Figures S1,S2,
Supporting Information). Both of PC61BPF and PC61BM
exhibited a melting point which is indicative of their crys-
talline nature. PC61BPF exhibits almost identical absorption
spectrum and similar cyclic voltammetry curve to PC61BM
(Figures S3,S4, Supporting Information), indicating that
pentafluorophenol group does not have influence on these
intrinsic properties.
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Furthermore, the device PC61BPF651 with
the lowest PC61BPF content showed a much
improved PCE of 3.88%. To systematically
evaluate the PC61BPF's effect on morpho-
logical evolution under influence of thermal
treatment, we fabricated a series of analo-
gous devices PC61BPF110, PC61BPF651,
PC61BPF321, PC61BPF211, and PC61BPF101
where the corresponding active layers,
initially thermally annealed at 140 °C for
15 min, were further subjected to isothermal
heating at 150 °C for 5, 10, 15, 20, and 25 h
prior to the deposition of top electrodes. The
J–V curves of all the devices are shown in the
Figure 1, and their corresponding photovol-
taic parameters (PCE, Voc, Jsc, and FF) as a
function of heating time at 150 °C are sum-
marized in Table 1 and plotted in Figure 2
(see Tables S1–S4, Supporting Information,
Scheme 2. The synthetic route for PC61BP and PC61BPF.
for more detailed data). It was found that
the performance of PC61BPF110 reference
2.2. Performance and Morphological Stability
device decreases as the heating time increases. The PCE of the
device dropped dramatically from 4.08% to 0.69% after 25 h
isothermal heating. The decreased efficiency is mainly a result
of the decrease of Jsc value. Encouragingly, the efficiency of the
PC61BPF101 devices does not decrease at all as the heating time
increases. In contrast, the PCE values were gradually improved
from 2.5% to 3.34% after 25 h heating. In a similar manner,
the PC61BPF211 devices exhibited steady improvement of PCE
values from 3.2% to 3.43%, while the PC61BPF321 devices
also showed improved performance from 3.18% to 3.7% after
25 h isothermal heating. Interestingly, the device PC61BPF651
showed slight decrease of PCE values from 3.88% to 3.68%.
These results indicated that the morphological evolution of
the active layers is highly dependent on the relative blending
ratio of PC61BM and PC61BPF. In the P3HT:PC61BM system
(PC61BPF110), the optimized morphology for high performance
can be rapidly achieved by moderate thermal annealing (i.e.,
140 °C for 15 min). However, when heating is applied con-
stantly, this optimized morphology is also easily deteriorated
due to the continuous phase separation toward the thermody-
namically stable state.
In this research, we employed a conventional device con-
figuration (ITO/PEDOT:PSS/active layer/Ca/Al) where all
the spin-coated active layer blends were thermally annealed
at 140 °C for 15 min. This is a standard condition used to
develop suitable morphology for traditional P3HT:PC61BM
system. To evaluate the multiple functions of PC61BPF to
serve as an acceptor material, PC61BPF was incorporated into
P3HT:PC61BM blend to form a ternary system. For simplicity,
PC61BPFxyz abbreviation is used to denote a ternary blend
system where x, y, and z represent the relative weight ratio of
P3HT:PC61BM:PC61BPF, respectively. For comparison, we first
fabricated a PC61BPF110 reference device with a P3HT:PC61BM:
PC61BPF (1:1:0 in wt%) blend that delivered a PCE of 4.08%
under AM 1.5G illumination at 100 mW cm−2. We then fab-
ricated a PC61BPF101 device using a P3HT:PC61BM:PC61BPF
blend (1:0:1 in wt%) as the active layer where PC61BM is com-
pletely replaced by PC61BPF. Under AM 1.5G illumination at
100 mW cm−2, PC61BPF101 device showed a relatively lower
PCE of 2.50%. This primary result implies that the electron
mobility of PC61BPF might be lower than that of PC61BM, or
the morphology of P3HT:PC61BPF blend is not optimized for
charge transport. To potentially improve the electron-trans-
porting properties in the composite, we incorporated PC61BM
into the P3HT:PC61BPF blend to dilute the PC61BPF content
and thus form a ternary blend system. Considering that the
structures of PC61BM and PC61BPF are very similar, PC61BPF
should be physically compatible with PC61BM in mixed solid
state. We fixed the blending weight ratio of P3HT to the total
n-type materials (i.e., PC61BM plus PC61BPF) as 1:1 but the
relative content between PC61BM and PC61BPF can be adjust-
able. Device PC61BPF211 (P3HT:PC61BM:PC61BPF = 2:1:1 in
wt%), device PC61BPF321 (P3HT:PC61BM:PC61BPF = 3:2:1 in
wt%) and device PC61BPF651 (P3HT:PC61BM:PC61BPF = 6:5:1
in wt%) were therefore fabricated. Under the identical condi-
tions, the efficiencies were indeed improved to 3.18% for device
PC61BPF321 and 3.20% for device PC61BPF211, respectively.
In contrast, the presence of PC61BPF greatly suppresses the
tendency of the fullerene fraction (i.e., PC61BM and PC61BPF)
toward aggregation. To demonstrate the existence of C6F5–C60
interaction, we used PC61BP with non-fluorinated phenyl group
to substitute PC61BPF and fabricated PC61BP651 and PC61BP321
devices under otherwise identical conditions. The J–V curves of
all the devices are shown in Figure 3, and their corresponding
photovoltaic parameters as a function of heating time at 150 °C
were plotted in Figure 4 and summarized in Table 1 (see Tables
S5,S6, Supporting Information, for more detailed data). Ini-
tially, device PC61BP651 yielded a PCE of 3.45%. Unlike the
case of PC61BPF651, the performance of PC61BP651 degraded
significantly to 0.63% only after 5 h heating at 150 °C, and con-
tinued to decrease to 0.44% after 25 h heating. Similarly, device
PC61BP321 delivered a PCE of 2.23% in the first place but
dropped to a PCE of 0.11% with a very poor Jsc of 0.73 mA cm−2
after 25 h thermal treatment. The decay of Jsc and Voc values is
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Figure 1. J–V characteristics of PSCs based on a) PC61BPF651, b) PC61BPF321, c) PC61BPF211, and d) PC61BPF101 blends before and after isothermal
heating at 150 °C for various periods of time.
the major reason accounting for the low PCEs. These behav-
iors clearly demonstrate that the presence of pentafluorophenyl
group in PC61BPF to exert C6F5–C60 interactions plays a deci-
sive role in controlling and stabilizing the morphology against
thermal heating.
2.3. Optical Microscopy and Atomic Force Microscopy
Optical microscopy (OM) was used to investigate the morpho-
logical alteration of the blends in size of micrometer under
the influence of heating (Figure 5). Thermal annealing of the
P3HT:PC61BM blend at 150 °C for 25 h induced a severe mac-
rographic alteration, forming the needle-shaped PC61BM crys-
tals of hundreds micrometers in length.[5e,5f,12f ] In contrast, the
morphologies of the PC61BPF321, PC61BPF211, and PC61BPF101
blend remain almost unchanged before and after thermal
annealing, demonstrating the physical C6H5–C60 interactions
of PC61BPF show excellent ability to prevent the morpholo-
gies from undergoing phase separation. However, PC61BPF651
blend with the lowest PC61BPF content shows less ability to
lock the morphology. The OM image of PC61BPF651 exhibited
a slight macrographic alteration with observable needle-shaped
crystals in some local areas after 25 h heating. This morpholog-
ical change is also consistent with the slight reduction in PCE
of the PC61BPF651 device. It should be emphasized that the OM
image of PC61BP651 blend showed extensive phase separation
after 25 h heating, meaning that PC61BP without pentafluoro-
phenyl group has no effect on morphological stabilization.
Atomic force microscopy (AFM) was also used to observe
the evolution of the surface morphology before and after 25 h
heating at 150 °C (Figure S5, Supporting Information). The sur-
face roughness of height image of PC61BPF651 blend increased
from 1.01 nm to 4.92 nm after 25 h heating, indicating some
degree of morphology alternation, which is also consistent with
Table 1. Photovoltaic parameters of PSCs (ITO/PEDOT:PSS/active
layer/Ca/Al) before and after isothermal heating at 150 °C for 25 h.
Devices
Time
[h]
Voc
[V]
Jsc
FF
[%]
PCE
[%]
[mA cm−2
]
PC61BPF110
PC61BPF110
PC61BPF651
PC61BPF651
PC61BPF321
PC61BPF321
PC61BPF211
PC61BPF211
PC61BPF101
PC61BPF101
PC61BP651
PC61BP651
PC61BP321
PC61BP321
0
25
0
0.60
0.62
0.60
0.62
0.62
0.60
0.62
0.60
0.58
0.62
0.60
0.60
0.58
0.50
−10.26
−2.27
−9.87
−9.06
−8.19
−9.24
−8.20
−9.05
−8.15
−8.81
−8.92
−1.78
−6.62
−0.73
66
4.08
0.69
3.88
3.68
3.18
3.70
3.20
3.43
2.50
3.34
3.45
0.44
2.23
0.11
49
65.60
65.47
62.35
66.66
60.72
63.15
52.94
61.05
64.40
41.68
58.05
30.06
25
0
25
0
25
0
25
0
25
0
25
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Figure 2. Photovoltaic parameters of PSCs based on the PC61BPF651, PC61BPF321, PC61BPF211, and PC61BPF101 blends as a function of heating time
at 150 °C. a) PCE, b) open-circuit voltage (Voc), c) short-circuit current (Jsc), and d) fill factor (FF).
the optical microscopy image of PC61BPF651 thin film. How-
ever, other ternary PC61BPF321, PC61BPF211, and PC61BPF101
systems with higher content of PC61BPF showed marginal
increases of roughness after thermal treatment. This obser-
vation suggests that these morphologies are generally stable,
and the slightly increased roughness might be indicative of
the increased P3HT crystallinity. Nevertheless, the PC61BP651
blend without C6F5–C60 interaction showed a significant
increase of roughness from 4.60 to 7.71 nm.
2.4. X-Ray Diffraction Measurements
To gain more insight into the morphological evolution, three
representative PC61BPF110, PC61BPF101, and PC61BPF321
blending systems were selected for wide angle X-ray diffrac-
tion (WAXRD) investigation. After taking the first XRD meas-
urement of the as-cast films, each pristine film was annealed
at 150 °C for 25 h followed by the second measurement to
monitor the thermal-induced structural changes in the films,
as shown in Figure 6. Three diffraction peaks located at 2θ =
5.3° (d-spacing: 1.67 nm), 10.7° (d-spacing: 8.3 Å) and 15.8°
(d-spacing: 5.6 Å) were observed for all the as-cast films of
PC61BPF110, PC61BPF101, and PC61BPF321 blends. These three
diffractions with scattering vector (q) ratio of 1:2:3 are an indi-
cation of the presence of a long-range ordered lamellar struc-
ture. The d-spacing of the first diffraction peak fits closely with
the a dimension of the monoclinic unit cell of the crystalline
Figure 3. J–V characteristics of PSCs based on a) PC61BP651, b)
PC61BP321 blends before and after isothermal heating at 150 °C for var-
P3HT.[18] Thus, the three diffraction peaks can be indexed as the
ious periods of time.
(100), (200), and (300) diffractions of P3HT. For comparison,
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Figure 4. Photovoltaic parameters of PSCs based on the PC61BP651 and PC61BP321 blends as a function of heating time at 150 °C. a) PCE, b) open-
circuit voltage (Voc), c) short-circuit current (Jsc), and d) fill factor (FF).
Figure 5. OM images of the blends before and after isothermal heating at 150 °C for 25 h for a) PC61BPF110, b) PC61BPF651, c) PC61BPF321,
d)PC61BPF211, e) PC61BPF101, and f) PC61BP651.
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process also led to the emergence of diffraction peaks near 2θ =
9° and from 17 to 23°. By matching with the diffraction pattern
of the pristine PC61BM, the additional peaks were identified
as the formation of crystalline domain of PC61BM (Figure S5,
Supporting Information). In contrast, upon thermal treat-
ments, PC61BPF101 and PC61BPF321 blends (Figure 6b,c) only
exhibited more pronounced P3HT patterns without showing
distinguishable crystalline diffractions of either PC61BM or
PC61BPF. Based on the XRD analysis, several points should
be noted. First, thermal treatment gradually induces higher
degree of P3HT crystallinity regardless of the composition of
n-type materials. Second, in the presence of PC61BPF, thermal-
induced aggregation of PC61BPF in the PC61BPF101 binary
blend, or PC61BPF/PC61BM in the PC61BPF321 ternary blend is
suppressed, which is in contrast to that from the PC61BPF110
system showing severe P3HT:PC61BM phase separation. The
XRD data again confirm the effectiveness of the PC61BPF in
hindering the crystallization of fullerene-based materials in the
active layers.
2.5. The Carrier Mobilities in the Blends
To quantify the influence of the PC61BPF on the charge
transporting characteristics in the active layers, we fabri-
cated unipolar devices to independently evaluate the hole
(Figure 7) and electron mobilities (Figure 8) of PC61BPF651,
PC61BPF321, PC61BPF211, and PC61BPF101 blends before
and after 25 h heating at 150 °C by space-charge-limited
current (SCLC) technique (Table 2). It was found that all
hole mobilities of PC61BPF651, PC61BPF321, PC61BPF211,
and PC61BPF101 were increased to some degree after 25 h
heating. These results suggest that the thermal annealing
drives P3HT to align and form higher crystallinity. How-
ever, the electron mobilities of PC61BPF321 and PC61BPF211
blends were increased slightly after 25 h heating. The
PC61BPF101 device containing 100% PC61BPF as the n-type
material exhibited improved electron mobility by 1.8 times
after 25 h thermal annealing.
2.6. Theoretical Calculations
Figure 6. 1D WAXD patterns of a) PC61BPF110, b) PC61BPF101, and
c) PC61BPF321 drop-cast films. The patterns of each film were measured
before and after thermal annealing at 150 °C for 25 h.
In order to investigate the extra stabilization brought by the
pentafluorophenyl group of PC61BPF, quantum-chemical cal-
culations were carried out. Geometry optimizations were per-
formed with the Gaussian09 suite at the wB97XD/6−311G(d,p)
level of theory. Specifically, the wB97XD functional, which
contains empirical dispersion-energy correction, has been
demonstrated to be effective in describing the weak intermo-
lecular interaction.[21] With consideration of reducing large
consumption of computational time, pristine C60 and methyl
perfluorobenzoate (BPF) were used as simplified model com-
pounds for studying the interactions between fullerenes
and fluorinated aromatic esters. The computational results
reveal that C60 can interact with BPF to form a stable complex
C60BPF. As illustrated in Figure 9, the optimized structure of
C60BPF exhibits that BPF is situated approximately above the
5–6 and 6–6 edges of C60 and the average distance between
the XRD measurement of pure PC61BM as well as PC61BPF cast
films was also performed, respectively (Figure S6, Supporting
Information). The multiple diffraction peaks observed in the
patterns indicate that both PC61BM and PC61BPF are crystalline
in their pristine films. These characteristic peaks of crystalline
PC61BM and PC61BPF were unobservable in the as-cast sam-
ples, which indicate that the presence of P3HT chains strongly
hinders the crystallization of the PC61BM and PC61BPF during
the solvent evaporation process.[19] After thermal annealing
of the PC61BPF110 blend at 150 °C for 25 h, the intensities of
(100), (200), and (300) diffractions of P3HT were enhanced
significantly (Figure 6a).[20] More importantly, the annealing
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to elucidate the stabilization energy brought
by the fluorinated phenyl group.[22] For more
information about ETS-NOCV analysis, one
can consult the publications listed in refer-
ence [21]. ETS-NOCV analysis was conducted
with the ADF suite using BP86-BJDAMP/
TZP. The incorporation of BJ damping into
the BP86 functional provides dispersion-
energy correction and has been demon-
strated to give better results for medium to
long-range intermolecular interactions.[23]
As listed in Table 3, the bonding energy
(ΔE) calculated with BP86-BJDAMP/TZP
is slightly greater than that obtained with
wB97XD/6−311G(d,p). The bonding energy
ΔE consists of two contributions: the prep-
aration energy ΔEprep and the interaction
energy ΔEint. ΔEint can be further decom-
posed into four chemically meaningful com-
ponents, electrostatic interaction (ΔVelst),
Pauli repulsion (ΔEpauli), orbital interac-
tion (ΔEoi), and dispersion (ΔEdis). A more
detailed description of these interactions is
summarized in supporting information. For
C60BPF, 59% of the total stabilization energy
comes from the dispersion-energy term
(ΔEdis) and the electrostatic interaction and
the orbital interaction have contributions
Figure 7. Hole-only devices for a) PC61BPF651, b) PC61BPF321, c) PC61BPF211, and
d) PC61BPF101 blends before and after thermal annealing at 150 °C for 25 h.
C60 and BPF is ≈3.21 Å. The strength of this intermolecular
interaction can be further quantified by the bonding energy
(association energy), –10.58 kcal mol−1, of the resultant com-
plex. Subsequently, computational energy decomposition anal-
ysis (ETS-NOCV)[21] of the C60/BPF interaction was performed
of 23% and 18%, respectively. It is evident that the additional
stabilization energy comes mostly from the dispersion energy,
which is the van der Waals interaction between C60 and BPF.
However, electrostatic and orbital interactions still have notice-
able contributions.
2.7. Morphological Evolutions
To carefully analyze the influence of PC61BPF
in the active layers, we plotted the initial
PCEs and the PCEs after heating for 25 h
as a function of the PC61BPF content in the
active layers (Figure 10). Based on the anal-
yses, we propose mechanisms for the mor-
phological evolution of P3HT:PC61BM and
P3HT:PC61BM:PC61BPF blends, respectively
in Figure 11. Upon thermal annealing of the
as-cast P3HT:PC61BM thin films at 140 °C
for 15 min, the PC61BM molecules, origi-
nally dispersed and intercalated in the P3HT
domains (Figure 11a), can freely diffuse out
of these mixed regions to aggregate into pure
PC61BM clusters. This evolution of PC61BM
simultaneously induces more ordered packing
of P3HT, leading to an optimized morphology
for efficient exciton dissociation and charge
transport (Figure 11b). However, prolonged
thermal annealing results in overgrowth of the
Figure 8. Electron-only devices for a) PC61BPF651, b) PC61BPF321, c) PC61BPF211, and
PC61BM aggregates that reduce the percolating
electron transport pathways within the mixed
d) PC61BPF101 blends before and after thermal annealing at 150 °C for 25 h.
©
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Table 2. Carrier mobilities of the blends extracted from space-charge limited current method before and after thermal annealing at 150 °C for 25 h.
PC61BPF651
before
PC61BPF651
after
PC61BPF321
before
PC61BPF321
after
PC61BPF211
before
PC61BPF211
after
PC61BPF101
before
PC61BPF101
after
2.35 × 10−5
9.16 × 10−6
0.39
2 × 10−5
3.44 × 10−5
1.72
1.78 × 10−5
5.64 × 10−6
0.23
2.47 × 10−5
2.26 × 10−5
1.27
3.96 × 10−5
1.27 × 10−5
0.32
4.11 × 10−5
3.27 × 10−5
0.80
1.59 × 10−5
9.53 × 10−6
0.60
2.90 × 10−5
4.94 × 10−5
1.70
Electron mobility, μe
[cm2 s−1 V−1
]
Hole mobility, μh,
[cm2 s−1 V−1
]
Ratio μh /μe
Figure 9. Side view of the optimized structure of C60BPF.
regions and thus adversely enhances charge recombination losses
(Figure 13c). Although the device PC61BPF110 without PC61BPF
achieved the highest PCE value of 4.08% at the beginning, its
morphological stability against heating turns out to be the worst.
For the as-cast PC61BPF101 (50 wt%), PC61BPF211 (25 wt%),
and PC61BPF321 (16.6 wt%) thin films (Figure 11a′), aggrega-
tion of the fullerene molecules dispersed in the mixed regions
is partially restricted by the intermolecular C6F5–C60 interac-
tions. Pre-annealing at 140 °C for 15 min is therefore not suffi-
cient to achieve optimized morphology (Figure 11b′). By further
heating at 150 °C during 25 h, the fullerene molecules in the
mixed regions might gradually overcome the supramolecular
attractions and slowly aggregate into fullerene clusters with
concomitant enhancement of P3HT crystallinity (Figure 11c’).
As a result, we observed marginal enhancement of electron
mobilities (i.e., PC61BPF321, PC61BPF211 and PC61BPF101)
and appreciable improvement of hole mobilities of the blends.
In short, the morphologies of the PC61BPF-containing blends
are not degraded upon thermal heating but gradually evolved
toward the optimal state.
Figure 10. The PCE values as a funtion of the wt% content of PC61BPF
in the active layers. (0 wt% for PC61BPF110, 8.3 wt% for PC61BPF651,
16.6 wt% for PC61BPF321, 25 wt% for PC61BPF211, 50 wt% for PC61BPF101)
before and after thermal annealing at 150 °C for 25 h.
cess. Even after 150 °C heating for 25 h, more than 95% of its
initial PCE can be retained.
2.8. PDTBCDTBT:PC71BM Blending System
Recent research on morphological stability of BHJ solar cells
is mainly focused on the typical P3HT:PC61BM system. The
use of other low band-gap polymers has not been extensively
investigated. After successfully demonstrating that PC61BPF
can effectively stabilize the morphology of the P3HT:PC61BM-
based system, it is envisioned that this strategy could be
a general approach applicable to other blending systems
using a low band-gap polymer with better molecular prop-
However, when the content of PC61BPF in the n-type mate-
rials is only 8.3 wt% (i.e., PC61BPF651), the suitable morphology
can be achieved more easily at an early stage, delivering a PCE
of 3.88% (Figure 10). Despite constant heat drove the original
morphology away from its optimized state, only a small amount
of PC61BPF can effectively mitigate this negative evolution pro-
erties.
A
crystalline p-type polymer poly(dithienobenzo-
(PDTBCDTBT)
carbazole-alt-dithienylbenzothiadiazole)
is selected to test this idea (Scheme 1).[24] A device (ITO/
PEDOT:PSS/PDTBCDTBT:PC71BM (2:3 in wt%)/Ca/Al),
where the active layer was thermally annealed at 100 °C for
10 min, achieved a high efficiency of 5.34%. Unfortunately,
Table 3. BP86-BJDAMP/TZP ETS-NOCV analysis of the interaction between C60 and BPF in kcal mol−1. Percentages of total stabilization are given in
parentheses.
ΔE
ΔEprep
ΔEint
ΔEpauli
ΔVelst
ΔEoi
ΔEdis
C60BPF
−9.12
0.32
−9.45
12.91
−5.20 (23%)
−4.05 (18%)
−13.10 (59%)
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Figure 11. Morphological evolutions of P3HT:PC61BM blend and P3HT:PC61BM:PC61BPF (3:2:1 in wt%) blend. a–c) Unstable morphology of a
P3HT:PC61BM binary blend upon thermal heating, and a′–c′) utilization of C6F5–C60 interactions to stabilize the morphology against thermal heating.
after isothermal heating of the active layer at 150 °C for 24 h,
the efficiency dramatically degraded to an extremely low value
of 0.01%. A device using ITO/PEDOT:PSS/PDTBCDTBT:PC71
3. Conclusions
In this study, we have easily synthesized a novel PC61BM-based
fullerene, [6,6]-phenyl-C61 butyric acid pentafluorophenyl
ester (PC61BPF). The merit of PC61BPF molecular design is
to impart complementary attraction between the pentafluo-
rophenyl group of PC61BPF and the C60 cores of PC61BPF/
PC61BM. Such a supramolecular interaction can effectively
suppress the n-type PC61BPF/PC61BM materials from exten-
sive thermal-driven aggregation to overcome the morpho-
logical instability. Therefore, prolonged thermal annealing at
elevated temperatures gradually improves the polymer hole
mobility while maintaining sufficient fullerene percolation
within the polymer matrix for electron transport. The extent
of the physical interaction in bulk can be adjustable by simply
varying the content of PC61BPF. By doping only 8.3 wt%
PC61BPF, the device PC61BPF651 yielded a PCE of 3.68% after
25 h heating at 150 °C, preserving 95% of its original value.
In sharp contrast, the PCE of the device using a traditional
P3HT:PC61BM blend decayed drastically to 0.69% over 25 h
heating. When PC61BP with non-fluorinated phenyl group
was used to substitute the fluorinated PC61BPF, the feature of
morphological stabilization disappeared completely. Most sig-
nificantly, PC61BPF is also capable of mitigating morphological
evolution of another blend system incorporating a conjugated
polymer PDTBCDTBT to stabilize the device performance. We
envision that this strategy will inspire new design of additive
materials that can control BHJ morphology through supramo-
lecular interactions to realize highly efficient and thermally
stable solar cells.
BM:PC61BPF (4:5:1 in wt%)/Ca/Al configuration was fabricated
by adding 10 wt% PC61BPF into the active layer. Encouragingly,
under otherwise identical conditions, this device delivered an
initial efficiency of 5.24% (Table 4), indicating that the incorpo-
ration of PC61BPF into this system does not alter the device per-
formance. Most importantly, after thermal annealing at 150 °C
for 25 h, the PC61BPF-incorporated device still achieved a com-
parable PCE of 4.24%, that is about 81% of its original value.
This example demonstrates the general capability of PC61BPF to
thermally stabilize the morphology and preserve the efficiency.
It should be mentioned that we cannot rule out the possibility
of the interaction between the pentafluorophenyl moieties and
the electron-rich segments of the polymer. Even though this
interaction exists, it is still beneficial for the morphological
stability, considering that such interactions should prevent the
phase separation between the polymer and fullerene.
This experiment also demonstrates that PC61BPF is effec-
tive not only for PC61BM-based system but also for PC71BM-
based system. To the best of our knowledge, this is the first
non-P3HT blend system that shows excellent morphological
stability against thermal heating.
Table 4. Photovoltaic parameters of PSCs (ITO/PEDOT:PSS/active
layer/Ca/Al) before and after isothermal heating at 150 °C.
Devices
Time
[h]
Voc
[V]
Jsc
FF
[%]
PCE
[%]
[mA cm−2
]
PDTBCDTBT:PC71BM
PDTBCDTBT:PC71BM
0
25
0
0.80
1.34
0.78
−10.28
−0.05
64.9
20.4
65.8
5.34
0.01
5.24
4. Experimental Section
PDTBCDTBT:PC71BM:
−10.21
PC61BPF (4:5:1 in wt%)
General Measurement and Characterization: All chemicals were
purchased from Aldrich and Acros, unless otherwise specified. 1H and
13C spectra were recorded on Varian Unity-300 and 400 spectrometers.
Differential scanning calorimetry (DSC) was measured on TA Q200
PDTBCDTBT:PC71BM:
PC61BPF (4:5:1 in wt%)
25
0.78
−8.62
63.0
4.24
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Instrument under nitrogen atmosphere at a heating rate of 10 °C min-1
and thermogravimetric analysis (TGA) was recorded on a Perkin-Elmer
Acknowledgement
The authors thank the National Science Council and the “ATU Program”
of the Ministry of Education, and Center for Interdisciplinary Science
(CIS) of the National Chiao Tung University, Taiwan, for financial
support. The authors are grateful to the National Center for High-
performance Computing (NCHC) in Taiwan for computer time and
facilities. The authors also thank Prof. Yuan-Pern Lee for the support of
the AFM measurements.
Pyris system under nitrogen atmosphere at
a heating rate of
20 °C min−1. Absorption spectra were collected on a Hitachi U-4100
spectrophotometer. Electrochemical cyclic voltammetry (CV) was
conducted on a CH Instruments electrochemical analyzer at a scanning
rate of 100 mV s−1. A carbon glass was used as the working electrode,
Pt wire was used as the counter electrode, and Ag/Ag+ electrode
(0.01 M AgNO3, 0.1 M TBAP in acetonitrile) was used as the reference
electrode, while 0.1
M tetrabutylammonium hexafluorophosphate in
Received: February 3, 2013
Revised: June 10, 2013
Published online: October 11, 2013
o-dichlorobenzene/acetonitrile (4:1) was the electrolyte. The LUMO
energy levels were obtained from the equation LUMO = −(Eroedn + 4.75) eV,
on
where E is the onset reduction potential (in volts) versus Ag/Ag+. The
red
AFM images under tapping mode were taken on Agilent 5500 system.
The optical microscopy images were measured by Zeiss Axiophot.
XRD
Measurement:
Blends
of
PC61BPF110,
PC61BPF101
and PC61BPF321, PC61BM, and PC61BPF were dissolved in
1,2-dichlorobenzene, and drop-cast on glass substrates. The cast
films were first characterized with a powder diffractometer (Bruker
D8 Advance operated at 40 kV and 40 mA; Cu Kα: λ = 1.5418 Å). The
samples were scanned in reflection across a 2θ ranging from 4° to 35°
with a step size of 0.07° and a counting period of 1 s step-1 to obtain
the 1D WAXD patterns of the as-cast films. The tested films were then
annealed at 150 °C for 25 h in a glove box and characterized again with
the diffractometer to generate the 1D WAXD patterns for the annealed
samples.
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a Keitheley 2400 Source
Measurement Unit. For SCLC device fabrication, the top electrode was
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Supporting Information
Synthetic procedures, DSC measurement of PC61BPF, absorption
spectra, cyclic voltammetry, TGA measurement of PC61BPF, 1D WAXD of
thin films, detailed photovoltaic parameters, computational details, and
1H and 13C NMR spectra of PC61BPF. Supporting Information is available
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