A simple and effective modification of PCBM for use as an electron acceptor in efficient bulk heterojunction solar cells

Article abstract of DOI:10.1002/adfm.201001807

[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and poly(3- hexylthiophene) (P3HT) are the most widely used acceptor and donor materials, respectively, in polymer solar cells (PSCs). However, the low LUMO (lowest unoccupied molecular orbital) energy level of PCBM limits the open circuit voltage (V_oc) of the PSCs based on P3HT. Herein a simple, low-cost and effective approach of modifying PCBM and improving its absorption is reported which can be extended to all fullerene derivatives with an ester structure. In particular, PCBM is hydrolyzed to carboxylic acid and then converted to the corresponding carbonyl chloride. The latter is condensed with 4-nitro-4′-hydroxy-α-cyanostilbene to afford the modified fullerene F. It is more soluble than PCBM in common organic solvents due to the increase of the organic moiety. Both solutions and thin films of F show stronger absorption than PCBM in the range of 250-900 nm. The electrochemical properties and electronic energy levels of F and PCBM are measured by cyclic voltammetry. The LUMO energy level of F is 0.25 eV higher than that of PCBM. The PSCs based on P3HT with F as an acceptor shows a higher V_oc of 0.86 V and a short circuit current (J_sc) of 8.5 mA cm^-2, resulting in a power conversion efficiency (PCE) of 4.23%, while the PSC based on P3HT:PCBM shows a PCE of about 2.93% under the same conditions. The results indicate that the modified PCBM, i.e., F, is an excellent acceptor for PSC based on bulk heterojunction active layers. A maximum overall PCE of 5.25% is achieved with the PSC based on the P3HT:F blend deposited from a mixture of solvents (chloroform/acetone) and subsequent thermal annealing at 120 °C. A simple, low-cost and effective approach of modifying PCBM and improving its absorption is reported. The modified PCBM, F, is an excellent acceptor for BHJ solar cells. The solar cell based on P3HT:F shows a power conversion efficiency (PCE) of 4.23%, while that based on P3HT:PCBM shows a PCE of about 2.93%. A maximum PCE of 5.25% is achieved with the P3HT:F blend deposited from a mixture of solvents and subsequent thermal annealing.

Full text of DOI:10.1002/adfm.201001807

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A Simple and Effective Modification of PCBM for Use
as an Electron Acceptor in Efficient Bulk Heterojunction
Solar Cells
John A. Mikroyannidis,* Antonis N. Kabanakis, S. S. Sharma,
and Ganesh D. Sharma*
to their potential for fabrication onto large
[
(
6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene)
P3HT) are the most widely used acceptor and donor materials, respectively, in
areas of lightweight flexible substrates
by low-cost solution processing. To maxi-
mize the donor-acceptor heterojunction
interfacial area for efficient exciton disso-
ciation, mainstream PSC devices adopt the
concept of a bulk heterojunction (BHJ),
where an active layer contains a p-type
donor and an n-type acceptor to form an
polymer solar cells (PSCs). However, the low LUMO (lowest unoccupied molec-
ular orbital) energy level of PCBM limits the open circuit voltage (V ) of the
oc
PSCs based on P3HT. Herein a simple, low-cost and effective approach of modi-
fying PCBM and improving its absorption is reported which can be extended to
all fullerene derivatives with an ester structure. In particular, PCBM is hydrolyzed
to carboxylic acid and then converted to the corresponding carbonyl chloride.
The latter is condensed with 4-nitro-4’-hydroxy-α-cyanostilbene to afford the
modified fullerene F. It is more soluble than PCBM in common organic solvents
due to the increase of the organic moiety. Both solutions and thin films of F
show stronger absorption than PCBM in the range of 250–900 nm. The electro-
chemical properties and electronic energy levels of F and PCBM are measured
by cyclic voltammetry. The LUMO energy level of F is 0.25 eV higher than that
[1]
interpenetrating nanoscale network.
A conventional BHJ PSC with an active
layer sandwiched by a low-work-function
aluminum cathode and a hole-conducting
poly(3,4-ethylenedioxylenethiophene):p
oly(styrenesulfonic acid) (PEDOT:PSS)
layer on top of an indium tin oxide (ITO)
substrate is the most widely used and
investigated device configuration. Broad
visible–NIR absorption, higher charge car-
rier mobility, and suitable electronic energy
levels of both the donor and acceptor mate-
of PCBM. The PSCs based on P3HT with F as an acceptor shows a higher V
of 0.86 V and a short circuit current (J ) of 8.5 mA cm , resulting in a power
oc
−2
sc
conversion efficiency (PCE) of 4.23%, while the PSC based on P3HT:PCBM
shows a PCE of about 2.93% under the same conditions. The results indicate
that the modified PCBM, i.e., F, is an excellent acceptor for PSC based on bulk
heterojunction active layers. A maximum overall PCE of 5.25% is achieved with
the PSC based on the P3HT:F blend deposited from a mixture of solvents (chlo-
roform/acetone) and subsequent thermal annealing at 120 °C.
[2]
rials are crucial for high-efficiency PSCs.
Poly(3-hexylthiophene) (P3HT) is the most
representative conjugated polymer donor
material, and [6,6]-phenyl-C-61-butyric acid
methyl ester (PCBM) is the most impor-
tant acceptor material. Power conversion
efficiency (PCE) of 4–5% has been reached
for the PSCs based on the P3HT/PCBM
system by device optimization.[3–5] In order to further improve
the PCE of the PSCs, much research work has been devoted
to finding new conjugated polymer donor materials aiming at
broader absorption, lower band gap, higher hole mobility, and
1
. Introduction
Polymer solar cells (PSCs) have emerged as a promising alter-
native technique for producing clean and renewable energy due
Prof. J. A. Mikroyannidis, A. N. Kabanakis
Chemical Technology Laboratory
Department of Chemistry
Prof. G. D. Sharma
R & D Centre for Engineering and Scicence
Jaipur Engineering College
Kukas, Jaipur (Raj.), India
University of Patras
GR-26500 Patras, Greece
E-mail: mikroyan@chemistry.upatras.gr
S. S. Sharma
Department of Physics
Prof. G. D. Sharma
Physics Department
Government Engineering College for Women
Ajmer, Rajasthan, India
Molecular Electronic and Optoelectronic Device Laboratory
JNV University
Jodhpur (Raj.) 342005, India
Email: sharma_in@yahoo.com
S. S. Sharma
Thin Film and Membrane Science Laboratory
University of Rajasthan
Jaipur, Rajasthan, India
DOI: 10.1002/adfm.201001807
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suitable electronic energy levels, and some donor–acceptor
difficulties. Chemically modified fullerenes for BHJ solar cells
[
27]
(D–A) copolymers show higher photovoltaic (PV) efficiency
have been recently reviewed. The synthesis of new fullerene
derivatives with stronger visible absorption and higher LUMO
energy levels than PCBM is currently a challenge for all those
chemists engaged in the chemical modification of fullerenes
for PV applications.
than P3HT. Nevertheless, the research efforts toward new C60
derivative acceptor materials to replace PCBM have not been
very successful until now.
PCBM has been prepared by Hummelen and Wudl in 1995.[6]
It offers the advantages of good solubility in organic solvents
such as chloroform, chlorobenzene, dichlorobenzene, etc.,
In the present investigation we report a simple synthesis of a
modified fullerene derivative, F, in three steps. It included the
hydrolysis of PCBM to carboxylic acid, which was subsequently
converted to the corresponding carbonyl chloride. Finally, the
latter reacted with 4-nitro-4’-hydroxy-α-cyanostilbene (NHCS) to
afford F. NHCS is a low cost product which has been prepared
in our laboratory from one-step reaction from inexpensive and
widely used starting materials. Details for the synthesis, charac-
terization and utilization of NHCS in dye-sensitized solar cells
[
1a,1e,3–5,7]
higher electron mobility, and higher electron affinity.
However, weak absorption in the visible region and a low LUMO
energy level are its weak points. Weak absorption in the visible
region limits its contribution to light harvesting in the PV con-
version. The low LUMO energy level of the acceptor material
results in lower open circuit voltage (V ) of the PSCs, since
oc
the Voc of the PSCs is proportional to the difference between
the LUMO energy level of the acceptor and the HOMO energy
level of the donor.[ Therefore, it is very important to design
and synthesize new soluble fullerene derivatives with stronger
visible absorption and higher LUMO energy levels than PCBM.
[28]
have been reported in our previous publication. The modified
fullerene F possesses the ester structure like PCBM. However,
the ester methyl group of PCBM has been replaced in F by the
large 4-nitro-α-cyanostilbene moiety. This replacement aimed
to increase the absorption, enhance the solubility and suppress
the crystallization. Interestingly, F shows higher solubility in
common organic solvents and stronger absorption than PCBM
in the range of 250–900 nm. It is worth noticing that the present
modification process can be readily applied to all fullerene deriv-
atives bearing the ester structure, such as PC BM, ThCBM
8]
Although many C60 derivatives[
9–16]
and a new C84 derivative
[17]
have been synthesized and used as acceptors in PSCs, most
of the device performance is poorer than or similar to that of
PCBM. [6,6]-phenyl-C-71-butyric acid methyl ester (PC BM),
70
which has a broader absorption in the visible region than
[18–22]
PCBM, shows improved PV performance over PCBM,
70
PCBM bis-adduct (bisPCBM) shows ca. 0.1 eV higher LUMO
energy level than PCBM, and the PCE of the PSC based on
P3HT and bis-PCBM reached 4.5%, an increase of 18% over
that of PCBM, which results from a higher Voc of 0.73 V owing
(which contains thiophenyl instead of phenyl) and PCBM- or
ThCBM-bisadducts. The LUMO energy level of F is 0.25 eV
higher than that of PCBM. The PSC based on the P3HT with F
as electron acceptor displayed higher Voc of 0.86 V and a PCE of
4.23%, when the blend was cast from chloroform solvent, while
the device based on P3HT:PCBM displays PCE of 2.93% under
the same conditions. This indicates that F is superior to PCBM
when it is used as electron acceptor along with conjugated poly-
mers as electron donor in PSCs. We have also fabricated PSCs
with the BHJ active layer (P3HT:F) deposited from a mixture of
solvents (chloroform/acetone) and subsequent thermal annealing
and achieved PCE of 4.62% and 5.25%, respectively. The higher
PCE for the device with the BHJ deposited from the mixture
of solvents and further improvement with subsequent thermal
annealing has been attributed to the increase in the crystallinity
of the P3HT. The interpenetrating network structure forms the
in situ self organized P3HT crystallites in the BHJ and produces
controlled bi-continuous percolation pathways that enhance both
hole and electron mobilities, affording higher photocurrent.
[
23]
to the higher LUMO energy level of the acceptor. Recently,
a novel endohedral fullerene, Lu N@C -PCBH, has been
3
80
reported, which also possesses a higher LUMO energy level
than PCBM.[ The P3HT-based PSCs with Lu N@C -PCBH
24]
3
80
as acceptor showed higher Voc (0.26 V higher) than the device
with PCBM as acceptor. Nevertheless, the high cost of PC BM
70
and Lu N@C -PCBH can limit their future commercial appli-
3
80
cation in PSCs. A remarkable bis-adduct fullerene derivative,
ICBA, formed by two indene units covalently connected to the
fullerene sphere of C has recently been reported by Hou and
6
0
[25]
Li. Interestingly, the presence of two aryl groups improves
the visible absorption compared to the parent PCBM, as well
as its solubility (>90 mg mL in chloroform) and the LUMO
energy level, which is 0.17 eV higher than PCBM. The PV
devices formed with P3HT as the semiconducting polymer
revealed PCE values of 5.44% under illumination of AM1.5,
−1
−2
1
00 mW cm , thus surpassing PCBM which afforded an effi-
2
. Results and Discussion
ciency of 3.88% under the same experimental conditions. More
recently, device optimization of BHJ PSCs based on P3HT as
donor and ICBA as acceptor has been performed by the same
research group. In particular, the optimized PSC with the
P3HT:ICBA weight ratio of 1:1, solvent annealing and pre-
thermal annealing at 150 °C for 10 min, has exhibited a high
2.1. Synthesis and Characterization
The synthetic route to modify PCBM is shown in Scheme 1.
Starting from PCBM, the carboxylic acid (PCBA) was obtained
−2
[29]
PCE of 6.48% with Voc of 0.84 V, Jsc of 10.61 mA cm , and FF
by treatment with hydrochloric acid and acetic acid in toluene.
−2
of 72.7%, under the illumination of AM 1.5G, 100 mW cm .
These values are the highest reported in the literatures so far
for P3HT based PSCs.[
It was further reacted with thionyl chloride in CS solution to
2
[29]
form the corresponding carbonyl chloride (PCBC). The latter
26]
[28]
reacted with an equivalent amount of NHCS in pyridine to
A major drawback in the synthesis of bis-adducts of fuller-
enes is that the products formed consist of a mixture of regioi-
somers which are not separated because of the experimental
afford the target F as a greenish-brown solid in 82% yield. In
this reaction, pyridine acted as both reaction medium and acid
scavenger. The reaction product was repeatedly washed with
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1
in the H NMR spectrum of F, the signals of
the aliphatic segments remained the same
with those of PCBM. However, the reso-
nances of F assigned to the aromatic were
significantly differentiated from the corre-
sponding of PCBM. In particular, F showed
an upfield signal at 8.17 ppm assigned to the
phenylene ortho to nitro. The other aromatic
of F as well as the cyanovinylene resonated at
the region of 7.84–7.06 ppm. These changes
1
in the IR and H NMR spectra supported the
successful modification of PCBM.
2
.2. Photophysical Properties
For PV applications the absorption, especially
in the visible region, is a significant prop-
erty. For this reason, PC BM is superior to
Scheme 1. Synthesis of F.
70
PCBM, when it is used as electron acceptor
in PV devices. The absorption of F was investigated in both
methanol to dissolve any possible un-reactive NHCS and finally
purified by silica column chromatography. Since the organic
moiety of F was increased by this modification, the modified
fullerene displayed higher solubility than PCBM in common
organic solvents, such as chloroform and tetrahydrofuran
−5
dilute (10 M) chloroform solution and thin film. The thin film
was prepared from chloroform solution of F by spin coating on
quartz substrate. Figure 2 shows the UV–vis absorption spectra
of PCBM and F in chloroform solution for the spectrum range
of 250–900 nm (top) and 400–800 nm (bottom). The small peak
−1
(
THF)(≈40 mg mL ). The solubility of PCBM in these solvents
−1
is ≈25 mg mL .
Structural characterization of F was accomplished by Fourier-
1
transform infrared (FTIR) and H NMR spectroscopy. Figure 1
presents the IR spectra of PCBM and F. Both spectra showed
−1
absorption bands around 523, 574, 1182, and 1426 cm asso-
ciated with the fullerene. Besides, F displayed characteristic
−1
absorptions at 2200 (cyano) and 1516, 1340 cm (nitro). In
addition, the carbonyl stretching vibration of the ester shifted
−1
−1
from 1736 cm in PCBM to 1702 cm in F. On the other hand
Figure 2. UV–vis absorption spectra of PCBM and F in chloroform solution
Figure 1. FTIR spectra of PCBM (top) and F (bottom).
for the spectrum range of 250–900 nm (top) and 400–800 nm (bottom).
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groups in the molecule of F, which increase its electron affinity
(
acceptor strength), thus leading to higher shift value than that
[
24]
reported for PCBM bis-adduct (bisPCBM). The higher LUMO
energy level of F is desirable for its application as acceptor in
polymer BHJ PV devices to get higher voltage.
The onset oxidation potentials (E ) for PCBM and F are
ox
+
1
.5 V and 1.2 V versus Ag/Ag , respectively. This indicates
that the onset oxidation potential for F shifts by −0.3 V as com-
pared to PCBM. The HOMO energy levels of PCBM and F were
estimated from their onset oxidation potentials, which were
obtained from the cyclic voltammograms of Figure 4. From the
onset oxidation potentials, the HOMO energy levels of PCBM
Figure 3. UV–vis absorption spectra of PCBM and F in thin film.
[32]
approximately at 700 nm is assigned to the [6,6]-addition in
and F were calculated according to the equation: HOMO =
C . It can be seen from these figure that the absorption of F
–q (Eox + 4.7), where E is the onset oxidation potential (in V)
versus Ag/Ag . The HOMO energy levels of PCBM and F cal-
culated from cyclic voltammogram are −6.2 eV and −5.9 eV,
respectively. The results indicate that the HOMO energy level
of F shifted upwards in comparison to that of PCBM.
ox
6
0
+
is stronger than that of PCBM in all spectrum region. It should
be noted that certain modified fullerenes, such as an indene-
C60 bis-adduct,[ had displayed in solution stronger absorption
than PCBM only in the visible region (400–800 nm), while it was
weaker in the UV region (below 300 nm). Interestingly, the thin
film absorption of F was also much stronger than that of PCBM
in all spectrum region (Figure 3). The thin film absorption coef-
25]
2
.4. Optical Properties of Blends
4
−1
−1
4
−1
−1
ficient ranged from 5.3 × 10
M
cm to 2.5 × 10
M
cm for
F and from 2.4 × 104
−1
cm−1 to 1.2 × 10 M cm−1 for PCBM,
4
−1
The absorption spectra of P3HT:PCBM and P3HT:F thin films
deposited from chloroform solution are shown in Figure 5a.
It can be seen from this figure that the absorption spectra of
the blends show the combination of individual components.
The absorption peak around 525 nm corresponds to the P3HT
component, which is associated with interchain π to π∗ transi-
M
for the wavelength region of 380–500 nm. These results sup-
port that F is a better electron acceptor than PCBM from the
viewpoint of light absorption.
2
.3. Electrochemical Properties PCBM and F
[33]
tion. The absorption shoulder at around 615 nm is related
to the interchain interaction and the height of this shoulder
[
33,34]
Electrochemistry is one of the most important properties of
fullerene derivatives.[ Therefore, we studied the electrochem-
ical properties of PCBM and F by CV (Figure 4). The LUMO
levels of PCBM and F were estimated from their onset reduc-
indicates the ordering of the chain packing.
The optical
30]
absorption spectra of the P3HT:F blend show broader band as
compared to P3HT:PCBM, which is attributed to the absorption
of F in the lower wavelength region. Upon the addition of ace-
tone to the blend solution in chloroform, the absorption band is
red shifted and the intensity is increased (Figure 5b). A further
red shift in the absorption band has also been observed for ther-
mally annealed blends. A high degree of P3HT crystallinity was
indicated by a red shift in the absorption band, clear vibronic
shoulder appeared at long wavelengths (between 610 and
625 nm) as the enhanced interchain π– π ∗ stacking, and
increased absorption intensity. During the spin coating process,
the blend solutions with ordered precursors solidified into film
structures as the majority of solvent evaporated from the blend
solution. The blend film from the mixed solvents showed higher
[
31]
tion potentials indicated in cyclic voltammograms. The onset
reduction potentials (Ered) of PCBM and F were −0.75 V and
+
−
0.95 V versus Ag/Ag , respectively. From the onset reduction
potentials, the LUMO energy levels of PCBM and F were cal-
[
32]
culated according to the equation: LUMO = –q (Ered + 4.7),
where q is electronic charge and Ered is the onset reduction
+
potential (in V) versus Ag/Ag . The LUMO levels of PCBM and
F estimated from the cyclic voltammogram are −3.95 eV and
−3.75 eV, respectively. The LUMO level of F is raised by 0.20 eV
in comparison with that of PCBM. This shift is attributed to the
presence of the strong electron withdrawing nitro and cyano
−1
Figure 4. Cyclic voltammograms of a) PCBM and b) F at scan rate 100 mV s .
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Figure 5. Absorption spectra of a) P3HT:PCBM (1:1 w/w) and P3HT:F (1:1 w/w) blends in thin films deposited from chloroform solution; b) as-cast
and subsequent thermally annealed P3HT:F blend film deposited from mixed solvents.
crystallinity than the film cast from a good solvent. Figure 5b
clearly shows that the crystallinity of the blend film was enhanced
by thermal annealing. Thermal annealing treatment allowed the
film to crystallize more extensively, as indicated by the increased
absorption intensity and the prominent vibronic features. The
improved P3HT crystallinity, which is facilitated through self
organized chain stacking, can enhance not only the hole trans-
port but also the light harvesting capabilities, thus improving the
photocurrent in the PV device.[
device based on P3HT:F is attributed to the higher LUMO
energy level of F, because it is well known that the Voc of PSCs is
proportional to the difference between the HOMO of the donor
(P3HT) and the LUMO of the acceptor. The increase in the Jsc
for the device based on P3HT:F in comparison to P3HT:PCBM
is attributed to the broader absorption of the P3HT:F blend,
which causes an enhancement on the photogenerated excitons
in the blend, resulting slightly higher photocurrent.
[36]
35]
The values of incident photon to current efficiency (IPCE)
have been estimated using following expression
2
.5. Photovoltaic Properties of P3HT:PCBM and P3HT:F
I PC E(%) = 1240Jsc /8Pin
−
2
PCBM and modified PCBM, i.e., F, were used as acceptors to
fabricate BHJ PSCs with P3HT as donor, and the weight ratio of
the donor to acceptor was 1:1. The current–voltage (J–V) charac-
teristics of the ITO/PEDOT:PSS/P3HT:PCBM or F/Al devices
under illumination intensity of 100 mW cm−2 are shown in
Figure 6a. The PV parameters, i.e., short circuit current (Jsc),
where Pin (W m ) and λ (nm) are the illumination intensity
and wavelength of the monochromatic light, respectively. The
IPCE spectra of the devices based on the blends are shown in
Figure 6b. It can be seen from this figure that the IPCE spectra
of the devices resemble the absorption spectra of the blend used
in the device, indicating both components used in the blend
contribute to the photocurrent. It is noted that the IPCE in the
wavelength region 340–500 nm is higher for the device based
on P3HT:F than that for P3HT:PCBM which could be ascribed
to the contribution of the absorption of F in this wavelength
open circuit voltage (V ), fill factor (FF) and PCE of the devices
oc
are listed in the Table 1. It can be seen from this table that the
Voc is increased from 0.68 V for PCBM-based device to 0.86 V
for the F-based device. The higher value of Voc for the PSC
-
0.487
-9.13
-8.92
-8.7
-8.53
-8.4
-9.6
-9.49
-9.36
3
3
3
3
3
05
18.7
26.8
37.5
42.3
-
-
-
0.42
0.34
0.26
6
2370 32.7
30
40
55
39.5
(
-9
a
.2
)
43.2 (b)46
-
-
-
0.195
0.131
0.068
0
-9.04
-8.92
-8.8
40.8
28.6
22
46
40.2
36.6
35.5
35.6
36.6
38.8
45.2
48.4
52
56.3
59.4
61.2
61.5
62.8
63.3
62.5
60
P3HT:PCBM
P3HT :F
4
-8.28
P3HT:PCBM
P3HT:F
3
74.5
-8.27
-8.15
-7.84
60
3
4
90
00
-8.66
-8.47
-8.28
-8
-7.77
-7.6
20
0.079
0.157
0.237
0.303
420
444
460
488
500
510
17.7
17.1
19.3
29.2
36.4
42.7
49.8
2
-7.55
-7.26
-6.88
-6.48
5
0
0
.36
.4
.436
0
0
-6.12
-7.39
-7.2
0
-5.9
-5.4
521.1
0
0
.49
.55
-6.94
-6.62
-5.86
-5.14
-4.58
-4.2
-3.37
-0.8
0
53240 56.9
-4.54
-2-2.25
0
1.47
2.49
-44.6
543
552
560
58.5
60.7
61.9
62.2
61
58.2
52.6
41.4
33.6
23
0
0
.632
.68
0.709
0.728
0.767
0.833
0.849
0.895
0.926
570
58830
601
610
55.1
46.7
41.4
27.4
18.7
13.1
8.4
621.1
630
641
2.05
3.71
2
0
-6
6
6
6
6
6
7
7
7
7
7
7
50
63
73
15.6
11
7.1
89 24 10 5.7
4
7.8
-
8
05
17
35
49
64
72
3.1
3.7
2.8
3.1
2.5
5
3.6
3.2
2.8
2.8
3.2
-
10
0
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
300
400
500
600
700
800
2.5
Voltage (V)
Wavelength (nm)
Figure 6. a) Current–voltage characteristics and b) IPCE spectra of the PSCs based on P3HT as donor and PCBM or F as acceptor with weight ratio
:1 deposited from chloroform solvent.
1
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T a b le 1 . Photovoltaic parameters of the devices based on P3HT:PCBM
and P3HT:F under illumination intensity of 100 mW cm , white light.
components occur within a single step, and the two processes
can, therefore, interfere with each other. It is difficult to effec-
tively control both polymer (donor) crystallization and acceptor
−2
Active layer
Short circuit Open circuit
Fill factor
(FF)
Power conversion
efficiency (PCE)
[%]
diffusion to ensure the presence of bi-continuous pathways for
current (Jsc) voltage (Voc)
[41b]
charge carriers and an appropriate interface area.
We have
−2
[
mA cm
]
[V]
prepared BHJ active layer, formed from a two-step process,
consisting first of the crystallization of P3HT, through the addi-
tion of the marginal solvent, i.e., acetone, to the blend solu-
tion in chloroform, followed by the nanoscale phase separation
achieved through thermal annealing at 120 °C. The PSCs were
fabricated using P3HT:F active layer spin cast from the mixed
solvents having structure ITO/PEDOT:PSS/P3HT:F/Al. Since F
is a modified form of PCBM, this concept may also be feasible
for P3HT:PCBM blend. The performance and the characteris-
tics of these devices, with the additional thermal annealing step,
were measured and compared. The J–V characteristics and the
corresponding incident photon to current efficiency (IPCE) are
shown in Figure 7a and b, respectively. The PV parameters for
the devices are summarized in Table 1. For the device based
on mixed solvents without thermal annealing, the perform-
ance reached a PCE of 4.75%. Upon thermal annealing at
120 °C, the device PCE has been enhanced up to 5.25%. The
IPCE spectra of the devices (Figure 7b) also confirmed the
increase in the Jsc as indicated by the magnitude and broad
shape of the IPCE spectra. The IPCE spectral curve of the device
fabricated from the mixed solvents display broader band region
than the device formed from the single solvent. The value of
IPCE has been further increased when the device was fabricated
with the thermally annealed blend. The red shift of the IPCE
spectra, which agreed well with the red shift observed in the
absorption spectra of the blend films, was interpreted as arising
from the enhanced P3HT crystallinity and resulting improved
hole mobility. The enhancement in the value of IPCE and PCE
of devices based on mixed solvents and additional thermal
annealing is mainly due to the increase in the Jsc photocurrent.
P3HT:PCBMa)
8.0
0.68
0.86
0.82
0.81
0.54
0.58
0.60
0.63
2.93
4.23
4.62
5.25
P3HT:Fa)
8.5
P3HT:Fb)
9.4
P3HT:Fc)
10.3
a) cast from chloroform b) cast from chloroform/acetone c) cast from chloroform/
acetone and subsequent thermal annealing at 120 °C.
region. The IPCE results indicate that the F acceptor is benefi-
cial to the solar light harvest and affords higher photocurrent.
The solvent annealing and thermal annealing is very impor-
tant for the PSCs based on the P3HT:F blend. The annealing
process controls the morphology of the active layer and enhances
the electron and hole mobilities. In most of the organic solar
cells based on BHJ active layers, the electron mobility is higher
than the hole mobility, which increases the recombination loss
of the charge carriers.[ Furthermore, the nanoscale mor-
phology can deteriorate if the acceptor (PCBM) domains grow
beyond the length scale of exciton diffusion, thereby prohibiting
efficient charge transfer.[ Thermal annealing at relatively high
temperature also has risks, such as oxidation and degradation
of the active layer used in the device.[38] In addition to thermal
annealing, solvent annealing can improve the nanoscale mor-
phology of the BHJ layer by promoting polymer self organization
35]
37]
[
3,39]
via control over the evaporation rate of the residual solvent.
Aside from the thermal and solvent annealing processes,
annealing free approaches have been also developed. P3HT
nano-fiber formation has been promoted through the cooling of
a saturated P3HT solution.[ Mixed solvents approaches allow
40]
[
41]
control over the active layer morphologies in blend films.
A drop in V occurred for the device fabricated with mixed sol-
oc
However, it is difficult to obtain optimized morphologies of
blend films through the above mentioned treatments, because
polymer (donor) crystallization and phase separation of the two
vents and also with an additional thermal annealing. It is widely
accepted that the V for organic BHJ solar cells is determined by
oc
the energy difference between the HOMO level of donor and the
-
-
-
-
-
0.514
0.419
0.301
0.216
0.102
0
-10.75
-11.46
-11.3
-11.08
305
26.8
26.8
37.5
42.3
46
6
-10.6
-10.29
3
330
340
1
2300 37.5
42.3
46
46
40.2
36.6
-10.06 (a-10
)
.92
As cast
(
b)
-9.83
-9.64
-9.52
-9.2
-9.02
-8.75
-8.44
-8.21
-8
-10.75
As cast
355
374.5
390
46
4
2
0
-10.53
-10.45
-10.14
-10.06
-9.83
-9.6
40.2
36.6
36.6
38
0.082
0.162
0.207
Thermally
annealed
Thermally Annealed
40080 35.5
420
444
460
488
500
510
35.6
36.6
38.8
45.2
50.6
56.3
62.7
64.7
67.1
68
0
.28
.349
.4
.45
.517
.56
39.3
43.7
52.9
65.1
72.9
77.1
80.3
82.4
82.8
83.2
83.2
82.6
80
0
0
-9.44
-9.14
-8.9
-8.59
-8.05
-7.6
-6.36
-4.04
0
0
0
0
0
-7.67
-7.39
6
0
0
521.1
.619 -2 -6.8
5
5
5
5
5
5
6
32
43
52
60
70
0.66
-6.23
-4.8
-2.88
0
2
4.4
.714
.76
0
0
.828
.871
.8 -4
68.7
68.7
68.2
66
0
0
1.06
4.46
40
88
01
-
-
6
8
610
21.1
62.4
55.3
47.5
27.4
77.1
72.3
62.3
51.2
39.8
26.5
16.5
12.9
11.1
9.3
6
630
641
6
6
6
6
6
7
5020 18.7
63
73
82
94
05
13.1
11.3
10.4
8.8
8
-
10
-12
717
35
0
6.4
5.6
8
6.5
6.5
6
7
-
0.6
-0.4
-0.2
0
0.2
Voltage (V)
0.4
0.6
0.8
1
749 3004.8
400
500
600
700
800
764
4.4
4.8
772
6
Wavelength (nm)
Figure 7. a ) J–V characteristics under illumination and b) IPCE spectra of the devices based on the as-cast and thermally annealed P3HT:F blend films
deposited from mixed solvents.
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LUMO level of acceptor.[42] It can be seen from the absorption
spectra of the P3HT:F blend deposited from the mixed solvent
and additional thermal annealing that the onset absorption edge
shifted towards the longer wavelength region, indicating reduc-
tion in the optical band gap of P3HT. Therefore, we assume that
the reduction in the band gap of P3HT, results in an upward shift
in the HOMO level of P3HT, due to the extension of conjugation
length and the interchain overlap,[ as indicated in the absorp-
tion spectra of the P3HT:F blend. Therefore, the devices con-
taining the P3HT:F blend films with higher crystallinity resulting
from the mixed solvents and thermal annealing, showed lower
Voc than the devices with films which have lower crystallinity.
The crystallinity of the blend films was also evaluated by X-ray
diffraction (XRD) measurements on the spin coated blend films.
Figure 8 shows the XRD patterns of the P3HT:F blend films pre-
pared under different conditions (i.e., as-cast from chloroform,
as-cast from mixed solvents and additional thermal treatment
of film deposited from mixed solvents). It can be seen from this
figure that the P3HT:F blend film fabricated from chloroform
shows a diffraction peak around 2θ = 5.8° which corresponds
to the P3HT. When the film was deposited from the mixed sol-
vents, the height of diffraction peak observed around 2θ = 5.8°
is increased and it is further increased when the film is ther-
mally annealed. The observed increase in the intensity of dif-
fraction peak also suggests an increase of the P3HT crystallinity,
when the film was deposited from mixed solvents and additional
thermal annealing. The structure of P3HT in the blend films
deposited from mixed solvents included crystalline nanowires,
annealing at 120 °C, to get information about the average surface
roughness of the film, using atomic force microscopy (AFM).
The AFM images of the films cast under different conditions
are shown in Figure 9. The average surface roughness of the
43]
[44]
nanocrystals and amorphous regions.
In the absence of
thermal annealing P3HT and F were homogeneously mixed.
During the thermal annealing process, F diffused and aggre-
gated to form de-mixed domains, which organized the percola-
tion pathways for electron transport. We assume that the blend
film deposited from the mixed solvents and subsequent thermal
annealing permitted fine tuning of morphology of the interpen-
etrating nanowire, with evenly distributed F domains, for the for-
mation of bi-continuous percolation pathways and a large D–A
interface area between the P3HT and F components. This leads to
an enhancement in the photocurrent and results in higher PCE.
We have also investigated the morphologies of P3HT:F fi lm
cast in different conditions, i.e., cast from chloroform, chloro-
form/acetone and chloroform/acetone and subsequent thermal
Figure 8. XRD patterns of P3HT:F blend films cast from chloroform, chlo-
roform/acetone and thermally annealed at 120 °C.
Figure 9. AFM images of P3HT:F blends as-cast in different conditions
(3 μm × 3 μm).
7
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0
.1 3.00E-06 1.62E-05 5.89E-05
0.1 2.51E-05 6.31E-05 1.51E-04
0. 76 -05 3.02E-04 1.20E-03
0.35 4.68E-04 3.80E-03 2.04E-02
0.468 2.88E-03 2.29E-02 1.62E-01
0
.217 1.74E-05 1.20E-04 8.32E-04
2 E
1.E7.+02
1
.E+01
0
0
0
.35 1.51E-04 1.20E-03 9.77E-03
.43 5.01E-04 3.24E-03 3.16E-02
.57 3.09E-03 1.95E-02(a1.5
)
1E-01
(b)
0.57 1.20E-02 1.17E-0
1
6.17E-01
0
0
1
1
1
.719 1.48E-02 6.17E-02 4.47E-01
.916
0.777 2.29E-01 1.70E+00 5.13E+00
1
4.
.
9
E
0E
+
-
0
02
0
2.00E-01 1.12E+00
1 +01
9 9
.E2.0
1.01 E+00 6.03E+00 1.58E+01
.276 1.12E-01 4.07E-01 2.09E+00
.615 1.70E-01 6.03E-01 2.88E+00
.853 2.19E-01 7.41E-01 3.39E+00
1.329 4.57E+00 1.00E+01 2.63E+01
1.74 7.08E+00 1.58E+01 3.72E+01
1.98 8.13E+00 1.95E+01 4.57E+01
2.15 2.63E-01 9.33E-01 4.27E+00
2.15 8.91E+00 2.19E+01 5.01E+01
1
.E-01
1.E+00
2
.294 3.09E-01 1.07E+00 4.68E+00
2.38 1.00E+01 2.63E+01 5.75E+01
2.45 3.31E-01 1.20E+00 5.25E+00
2.59 1.26E+01 3.02E+01 6.76E+01
1
1
1
1
1
.E-02
.E-03
.E-04
.E-05
.E-06
1.E-01
Chloroform
1.E-02
1.E-03
1.E-04
1.E-05
Acetone/chloroform
Thermally annealed
Chloroform
Acetone/chloroform
Thermally annealed
0
0.5
1
1.5
2
2.5
3
0
0.5
1
1.5
2
2.5
3
(
Vappl -V ) (V)
(Vappl - V ) (V)
bi
bi
Figure 10. Current–voltage characteristics of the a) hole-only b) electron-only devices fabricated with P3HT:F active layers from chloroform solvent, chlo-
−2
rofrm/acetone solvent, and subsequent thermal annealing, where J is the current density (mA cm ) and L is the thickness of the active layer (cm).
blend films are 1.2 nm and 1.87 nm for the blend cast from the
chloroform and chloroform/acetone solvent, respectively. The
average surface roughness further increases to 2.6 nm, when
the P3HT:F blend cast from the chloroform/acetone solvent
is thermally annealed at 120 °C. The surface domain size has
also been increased upon thermal annealing. This indicates
that both increased domain size and average surface roughness
results in an improvement in the crystallinity of the blend. This
is responsible for the enhancement in the PCE due to the more
efficient photoinduced charge transfer at the D–A interfaces
and the more balanced charge transport.
from the chloroform only. When the blend film is deposited
from mixed solvents, the hole mobility has been increased to
−4
2
−1 −1
6.4 × 10 cm V s , two orders of magnitude higher than
−6
2
−1 −1
the mobility of 9.6 × 10 cm V
s
in the blend film cast
from chloroform solvent, and one order of magnitude higher
than the mobility measured in the film cast from chloroform
only subjected to thermal annealing, which showed a mobility
−5
2
−1 −1
of 6.7 × 10 cm V s . The increase in the hole mobility is
attributed to the crystalline nature of the blend as indicated
from the XRD data. The electron mobility was also enhanced
−
5
2
−1 −1
by a factor of twenty, i.e., 3.5 × 10 cm V
s
and 7.4 ×
−4
2
−1 −1
For the efficient polymer BHJ solar cells, the mobility
of electrons and holes in the polymer blends is an impor-
tant parameter that must be well controlled because a bal-
10 cm V
s
for the blend cast from chloroform solvent
and chloroform/acetone solvent, respectively. The electron
mobility has been slightly improved when the blend deposited
from chloroform/acetone solvent was subsequently thermally
annealed at 120 °C. The ratio between the electron and hole
mobilities approached towards unity, i.e., 1.15 and 0.94 for
blend deposited from chloroform/acetone solvent and subse-
quent thermal annealing, respectively, resulting in highly bal-
anced charge transport in the BHJ PV devices based on these
blends. A well controlled morphology, through the enhance-
ment in the crystallinity of P3HT, led to balance charge trans-
port in the active blend layer, and enhanced Jsc and FF. The
hole mobility in the device fabricated from chloroform solvent
was insufficient to ensure efficient charge transport and the
anced charge carrier transport is an essential prerequisite for
increasing PCE and FF.[
35,45]
The space charge limited cur-
rent (SCLC) method was employed to estimate the hole and
electron mobilities of the devices prepared with various thin
film active layers.[ SCLC J–V characteristics were obtained
in dark from the hole only devices (ITO/PEDOT:PSS/P3HT:F/
Au) and the electron only devices (Al/P3HT:F/Al ) as a func-
tion of the applied bias voltage, which was corrected for the
built in potential V = Vappl – Vbi and shown in Figure 10. The
hole and electron mobilities were estimated from the J–V char-
acteristics at low voltage region, where the current is described
45]
2
3
by the Mott–Gurnay square law: JSCLC = (9/8)ε ε μ(V /L ),
o
r
where ε ε is the permittivity of the polymer, μ is the charge
o
r
T a b le 2 . Electron and hole mobilities of P3HT:F blend films cast under
carrier mobility, and L is the film thickness. The hole and elec-
tron mobilities of P3HT:F blend films cast from chloroform
only, mixed solvents and subjected to thermal annealing are
listed in Table 2. The highly ordered π– π ∗ conjugation crystal-
line nature of the blend as observed in the optical absorption
different conditions, estimated using the Mott-Gurney law.
μe [cm2 V−1 s−1 ]
3.5 × 10−5
μh [cm2 V−1 s−1 )
9.6 × 10−6
μe/μh
3.64
1.15
0.94
Cast from chloroform
7.4 × 10−
4
6.4 × 10−4
1.2 × 10−3
Cast from chloroform/acetone
spectra and XRD pattern, respectively, is known to improve
1.13 × 10−
3
the charge carrier mobility in the blend films.[3,35,46] It can be
Cast from chloroform/
acetone and subsequent thermal
annealing at 120 °C
seen from Table 2 that both electron and hole mobilities for
the blend cast from mixed solvents is higher than that for cast
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1H NMR (CDCl3) ppm: 8.17 (m, 2H, phenylene ortho to nitro);
.84–7.06 (m, 11H, other phenylene; 1H, cyanovinylene); 2.83
hole and electron mobilities were less balanced than those
measured for the films as-cast from chloroform/acetone sol-
vent and subsequent thermal annealing.
7
(
t, 2H, CH (CH ) COO); 2.32 (t, 2H, (CH ) CH COO); 2.02 (m, 2H,
2 2 2 2 2 2
CH CH CH COO). Anal. Calcd. for C H N O : C, 90.20; H, 1.76; N,
2
2
2
86 20
2
4
2
.45. Found: C, 89.63; H, 1.38; N, 2.21.
Characterization Methods: IR spectra were recorded on a Perkin-
1
Elmer 16PC FTIR spectrometer with KBr pellets. H NMR (400 MHz)
spectra were obtained using a Brucker spectrometer. Chemical shifts
(δ values) are given in parts per million with tetramethylsilane as an
internal standard. UV–vis spectra were recorded on a Beckman DU-640
spectrometer with spectrograde chloroform. Elemental analyses were
carried out with a Carlo Erba model EA1108 analyzer.
3
. Conclusions
Starting from PCBM, a new C60 derivative was successfully
synthesized by a reaction of three steps. F showed higher
solubility than PCBM being readily soluble in common
organic solvents. Both solutions and thin films of F dis-
played stronger absorption than PCBM in the region of
The electrochemical properties of PCBM and F were studied by cyclic
voltammetry (CV) (CH Instruments). A glassy carbon coated with PCBM
or F, a Pt plate and an Ag/Ag electrode were used as the working-,
+
2
50–900 nm. We have fabricated PSCs with the P3HT:F
blend and compared the PV performance with that of the
device based on P3HT:PCBM blend. The Voc and PCE of
the P3HT based PSC with F as electron acceptor cast from
chloroform solvent, reached 0.86 V and 4.23%, respectively,
under the illumination intensity of 100 mW cm , which is
significantly improved as compared to the Voc of 0.68 V and
PCE of 2.93% of the device with PCBM as electron acceptor
under the same experimental conditions. The increase in
the Voc has been attributed to the higher LUMO level for
counter- and reference-electrode respectively. The electrolyte solution was
0
.1 M tetrabutylammonium hexafluorophosphate (Bu NPF ) in anhydrous
4 6
−1
acetonitrile. The potential scan rate was 100 mV s . The films for the
electrochemical measurements were coated from chloroform solutions.
The crystallinity of the films was studied using the X-ray diffraction
−
2
(XRD) technique (panalytical make USA) having CuKα, as radiation
source of wavelength λ = 1.5405 Å with the films coated on the quartz
substrates. AFM images were recorded using digital instrument
nanoscope in trapping mode.
Device Fabrication and Characterization: The blend solutions of
P3HT:PCBM or P3HT:F (1:1 w/w) were prepared using chloroform
F relative to PCBM. The J has also been improved up
sc
−1
in concentration 10 mg mL and stirred for 2 h. For the preparation
−
2
to 8.5 mA cm for the device based on F as compared to
of mixed solventss, 0.05 mL of acetone was added to 1 mL P3HT:F
solution in chloroform and then stirred for another 2 h. The PSCs were
fabricated according to the following procedure. First, the indium doped
tin oxide (ITO) coated glass substrate was cleaned with detergent, then
ultrasonicated in distilled water and isopropyl alcohol and finally dried
overnight in an oven at 80 °C. To supplement this bottom electrode, a
hole transport layer of PEDOT:PSS (BAYTRON, conductive grade) was
spin cast from aqueous solution on the ITO coated glass substrate,
and was subsequently dried at 80 °C for 20 min. The photoactive layer
of the blend (P3HT:PCBM or P3HT:F) was deposited by spin coating
the prepared blend solutions on the top of the PEDOT:PSS layer. The
thickness of the photoactive layer is approximately 80–85 nm. Finally,
an aluminium (Al) electrode (thickness about 100 nm) was deposited
−
2
PCBM, which is 8.0 mA cm , ascribed to the light absorp-
tion by the F in visible region.
We have also investigated the effect of mixed solvents and
subsequent thermal annealing on the PV performance of the
PSCs based on P3HT:F blend. The PCE of the PSCs based
on P3HT:F blend deposited from acetone/chloroform sol-
vent and subsequent thermal annealing is 4.62% and 5.25%,
respectively. This increase in the PCE has been attributed to
the improved P3HT crystallinity and nanoscale morphology,
resulting in balanced charge transport in the blend due to
the increase in both electron and hole mobilities. These
results indicate that the modified PCBM, i.e., F is an excel-
lent electron acceptor for PSCs and could be a promising
acceptor instead of PCBM for further improving the PCE of
the high performance PSCs based on low band gap conju-
gated polymer or small molecule donor by increasing both
Voc and Jsc.
−5
by thermal evaporation under high vacuum (1 × 10 mbar). The
2
effective area of the devices is 10 mm . For thermal annealing, the
blend films were placed on a hot plate and annealed at temperature of
1
20 °C for 2 min, before the deposition of Al electrode. The hole only
devices, i.e., ITO/PEDOT:PSS/blend/Au, used to extract hole mobility
in the blend films, were fabricated as described above, except that the
top electrode was replaced with Au (60 nm). Electron only devices
having structure Al/blend/Al, were also fabricated by spin coating the
active layer on glass/Al substrates, followed by the deposition of Al
cathode electrode.
The current–voltage (J–V) measurement of the devices was carried
out on a computer controlled Keithley 238 source meter. A xenon lamp
4
. Experimental Section
(
500 W) was used as white light source and the optical power at the
Reagents and Solvents: 4-nitro-4’-hydroxy-α-cyanostilbene (NHCS) has
−2
sample was 100 mW cm (the equivalent of one sun at 1.5 AM). The
AM 1.5 spectrum was obtained from the light source with the use of air
mass 1.5 filter.
been synthesized as a dark green solid in 60% yield from the reaction
of 4-hydroxybenzaldehyde with 4-nitrobenzylcyanide (mol ratio 1:1) in
ethanol in the presence of sodium hydroxide.[28] PCBM was purchased
from Aldrich.
Synthesis of F: A flask was charged with a solution of PCBC (0.0892 g,
0
.0974 mmol) in pyridine (15 mL). NHCS (0.0259 g, 0.0974 mmol)
Acknowledgements
dissolved in pyridine (10 mL) was added portionwise to the stirred solution
at 0 °C. The mixture was stirred and heated at 80 °C for 2 h under N . It
We are thankful to Prof. Y. K. Vijay of Thin film and Membrane
Science Laboratory, University of Rajasthan Jaipur (Raj.) for allowing
us to undertake the device fabrication and characterization in his
laboratory.
2
was subsequently concentrated under reduced pressure and methanol was
added to the concentrate and the obtained suspension was centrifuged to
isolate F. The addition of methanol to the solid isolated and centrifugation
of the resulting suspension was repeated two times. The crude reaction
product was purified by silica gel column chromatography eluting with
toluene/THF (1:1) to afford F as a greenish-brown solid (0.0914 g, 82%).
Received: August 31, 2010
Revised: October 8, 2010
Published online: December 13, 2010
−1
FTIR (KBr, cm ): 2200 (cyano); 1702 (ester carbonyl); 1516, 1340 (nitro).
7
54 wileyonlinelibrary.com
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