Toward the high efficiency organic solar cell with a vertical p-n junction structure
ု⋥ p-n ⇇㕙೙ᓮߦࠃࠆ᦭ᯏ⭯⤑ᄥ㓁㔚ᳰߩ㜞ല₸ൻ
Yasuhiro Shirai
ICYS-MANA, National Institute for Materials Science
1. Introduction
Si-based solar cells have been utilized in many occasions as a renewable energy source;
however, it has been impossible to replace other conventional nonrenewable energy sources
with the solar cells because its manufacturing cost is much higher than that of other
nonrenewable energy sources. The organic based solar cells, on the other hand, can possibly
offer special opportunities as a renewable energy source because they can be fabricated over
large area using low cost printing technologies. The progress in the field of the polymer based
solar cells has been promising with the recently reported power-conversion efficiency of ~6%.
Although it is still ineffective to be the technology utilized in commercial products, there could
be plenty of room for the improvement in the power-conversion efficiency of the polymer based
solar cells. In this research, we will present the synthesis methodology to produce high mobility
polymers that can possibly be utilized in polymer-based solar cells with improved
power-conversion efficiency.
N
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O
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S
N
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n
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㪝㫀㪾㪅㪈㩷㪙㪸㫊㫀㪺㩷㫇㫉㫀㫅㪺㫀㫇㫃㪼㩷㫆㪽㩷㫆㫉㪾㪸㫅㫀㪺㩷㪹㪸㫊㪼㪻㩷㫊㫆㫃㪸㫉㩷㪺㪼㫃㫃㫊㪅㩷
It is important to understand how organic or polymer based solar cells work to improve its
power-conversion efficiency. A simple organic solar cell has the planer structures shown in the
Figure 1, and the structure is very similar to that of the Si-based solar cells with the p-n junction
sandwiched with the transparent conductor such as ITO and metal electrodes. The working
principle is also similar to each other. The photo excited species are generated in the p- or n-type
semiconductor materials, and they are separated as holes and electrons at the p-n junction. To
take out energy from a solar cell, the separated holes and electrons have to be collected at the
㧙㧙
electrodes. Thus, the materials need to satisfy the following criteria to be a good solar cell
material: 1) high absorption of light; 2) efficient charge separation; 3) sufficient charge transport.
We have proposed the nanowire based vertical p-n junction structure shown in the figure 2 to
satisfy all these criteria.
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The polymer based organic solar cells with the best power-conversion efficiency reported so
far has the structure named bulk heterojunction, in which photo excited species have chance to
be separated as holes and electrons anywhere in the device because the p-n junction is distribute
all over the mixture of the p- and n-type materials (Fig. 3). The device structure can possibly
account for the two criteria; high absorption of light and efficient charge separation, however,
the charge transport cannot be controlled. In contrast, the vertical p-n junction device structure
can satisfy all these criteria (Fig. 3).
In the construction of the polymer based organic solar cells with the vertical p-n junction
structure (Fig. 2), it is important to have the polymeric nanowires with high carrier mobility
because carriers will need to travel long distance along the nanowires to the electrodes. With the
invention of such high mobility polymer structures, one can possibly fabricate the vertical p-n
junction solar cell structures following a simple fabrication scheme (Fig.4). Here we
demonstrate the synthesis of such high mobility polymeric nanowires through a template
synthesis of PEDOT. The nanowires of PEDOT have been synthesized within the nano-sized
template (Anodic Alumina Oxide, AAO, Fig. 5), and the resulting nanowire’s nanostructure was
characterized using SEM and TEM. The chemical, electrical, and structural properties of the
nanowires were also characterized using EDX, 4-wire conductivity measurement, and Raman
spectroscopy, respectively. These characterizations revealed that the AAO template grown
PEDOT nanowires have improved mobility, being suitable material for the construction of the
polymer based vertical p-n junction organic solar cells.
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2. Experimental
3,4-Ethylenedioxythiophene (EDOT) and anhydrous acetonitrile (CH3CN) were obtained
from Aldrich. Lithium perchlorate (LiClO4) was obtained from Wako. Gold electroplating
solution (Galvanomeister GB3) was purchased from Tanaka Kikinzoku. AAO membrane with a
pore diameter of 0.2 Pm and a thickness of about 60 Pm (Anodisc 13) was obtained from
Whatman. Deionized water was obtained by using a Milli-Q water purification system from
Millipore.
AAO templates with small pore diameters (50 and 100 nm) were fabricated by using a
two-step anodization process.1 An Al sheet (15 u15 u 0.5t, 99.99%) was anodized for about 18 h
at 40 V and 10 qC by using 0.3 M oxalic acid as an electrolyte to fabricate AAO membranes
with a pore diameter of 50 nm. For AAO membranes with 100 nm pores, Al sheets were
anodized for about 18 h at 100 V and 0 qC by using 0.15 M oxalic acid as an electrolyte, and an
anodization current was limited below 350 mA to avoid an excessive heating of anodizing
samples. The anodization process was repeated after the removal of the irregular pores produced
in the initial anodization process to make a well-defined pore structures. Thus, the initially
formed alumina pores were removed by placing the Al sheets with alumina pores in an aqueous
mixture of phosphoric acid and chromic acid at 60qC for about 5 h. The second anodization was
performed for 18 h and 5.5 h for 50 nm and 100 nm pore samples, respectively. The aluminum
㧙㧙
layers left after the anodization process were removed using an acidic copper chloride solution.
The removal of alumina barrier layer and pore-widening were performed by using phosphoric
acid (10 wt %) at 30 qC.
Electropolymerization of EDOT was performed following the method developed by Lee et al.
with the following modifications.2 EDOT was polymerized at a constant current density of 1
mA/cm2 with a galvanostatic mode in a three electrode configuration using a Pt counter
electrode and an Ag/AgCl reference electrode. The concentration of EDOT was 1.0 M, and the
electrolyte solution was 0.1 M LiClO4 in acetonitrile. For a working electrode, one side of an
AAO membrane with nanopores was coated with a thin layer of Au (~300 nm) by using a
sputtering system operating at 200W, and Au plating was performed by galvanostatically
repeating the -1.0 A/dm2 (0.1 s) and +8.0 A/dm2 (0.01 s) to completely fill the bottom of the
nanopores with gold.
SEM, TEM, and Raman spectroscopy characterization of the nanowires were performed by
releasing the nanowires into solution by dissolving the AAO template in 1.0 M KOH, and
casting the nanowire sample solution on to a Si substrate or TEM grid. The samples were
repeatedly centrifuged and rinsed in water to remove the KOH before the casting. Electrical
measurements on the PEDOT nanowires were performed using a 4-wire resistivity measurement
method. The nanowire solution was casted on the Si substrate with patterned 4-wire Au
electrodes fabricated using lithography.
3. Results and Discussion
Synthesis
nanowires
of
were
the
PEDOT
grown
using
nanowires.
a
The
conventional
3-electrode configuration (Fig 6). By applying the
enough potential on the Au electrode at the bottom of
the AAO template, the PEDOT was successfully
oxidized at Au electrode surfaces and formed nanowire
shaped polymeric structures in the AAO template.
AAO template with the three different pore diameters
of 50, 100, and 200 nm were used in this study.
Characterization of polymer nanostructures. It is
important
to
observe
and
understand
the
nanostructures of synthesized PEDOT polymers before
investigating their electrical performance in organic
solar cells because the cell performance will strongly
depend on the nanostructure of the polymers. Thus, the
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㪝㫀㪾㪅㪍 㪚㫆㫅㪽㫀㪾㫌㫉㪸㫋㫀㫆㫅㩷㪽㫆㫉㩷㫋㪿㪼㩷㫋㪼㫄㫇㫃㪸㫋㪼㪻㩷
㪼㫃㪼㪺㫋㫉㫆㪺㪿㪼㫄㫀㪺㪸㫃㩷 㫇㫆㫃㫐㫄㪼㫉㫀㫑㪸㫋㫀㫆㫅㩷 㫆㪽㩷
㫋㪿㪼㩷㪧㪜㪛㪦㪫㩷㫅㪸㫅㫆㫎㫀㫉㪼㫊㪅㩷
template grown polymeric nanowires were investigated using SEM and TEM. The PEDOT
nanowires were released from the AAO template, and the SEM images (Fig.7) on individual
PEDOT nanowires revealed that the nanowire diameter can be controlled by changing the size
of the pore diameters in the AAO template. TEM observation confirmed the formation of the
completely filled nanowires.
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Electrical measurement on the PEDOT nanowires. The conductivity of the PEDOT
nanowires synthesized using three different AAO template sizes (50, 100, 200 nm) was
estimated using a 4-wire resistivity measurement technique (Fig. 8). By applying a small
amount of current going through the nanowire, a voltage drop across the nanowire was
measured using a voltmeter.
Over 100 samples of the
PEDOT nanowires made with
three different AAO templates
were
measured,
and
the
results were summarized in
the Figure 9. It has been
revealed that the PEDOT
nanowire conductivity will
increase as the decrease of the
nanowire
diameters.
This
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result strongly suggests that there are
some chemical and/or structural changes
in
the
nanowires.
The
material’s
conductivity (V) is generally known to be
proportional to the product of the carrier
density (n) and the carrier mobility (P),
and can be expressed as:
V v n˜P
(1)
From this relationship, it is possible to
determine which factor is the dominant
source in the conductivity enhancement in
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the PEDOT nanowires with smaller diameters. To answer this question, we have explored the
carrier density of the nanowires with EDX and the structural properties of nanowires using
Raman spectroscopy methods.
EDX analysis on the chemical composition of the PEDOT nanowires. The electrical
properties and conductivity of a conducting polymer will strongly depend on the extent of the
doping. Thus, it is crucial to determine the amount of the doping materials within the PEDOT
nanowires, and the result is directly related to the carrier density of the material. Because the
electrochemical polymerization was conducted using LiClO4 as an electrolyte, the ClO4 anion
will be incorporated into the polymer matrix as a dopant, and the extent of the doping can be
estimated from the atomic concentration ratio of the sulfur atom in the PEDOT to the chlorine
atom in the ClO4 anion. The estimated doping of the nanowires with the three different
diameters is listed in the table 1. It is clear that the doping of the nanowires remains constant
and has no diameter dependence. This result strongly suggests that the carrier density of the
PEDOT nanowires has no role in the measured conductivity enhancement of the nanowires with
smaller diameters.
Table 1. Doping density estimation using EDX.
Nanowire diameter
Atomic Concentration
ratio: S/Cl
Extent of doping
50 nm
23.3
4.3%
100 nm
21.7
4.6%
200 nm
23.4
4.3%
Raman analysis. According to the equation (1) and the EDX analysis results, the huge
conductivity enhancement in the small diameter PEDOT nanowires is due to the increase in the
carrier mobility in the nanowires. An improvement in the carrier mobility usually reflects the
㧙㧙
enhanced supramolecular structure and/or the formation of better crystal structural order within
the materials. Thus, we have carried out Raman analysis on our PEDOT nanowires to examine
the polymer morphologies. The Raman spectra of PEDOT nanowires recorded using Ar+ 514.53
nm, laser excitation line, are shown in Figure 10. Examination of the 13001600 cm-1 spectral
region shows that in general the peaks are relatively more intense for the 50 nm PEDOT than
for 200 nm PEDOT nanowires. The most intense peak at 1427 cm-1 is assigned to the symmetric
stretching mode of the C=C double bonds, whereas a less intense peak at 1508 cm-1 is
characteristic of the C=C antisymmetric stretching vibration.3 Other weaker peaks at 1361 and
1262 cm-1 are attributed to the stretching mode of single C-C bond and the C-C inter-ring bonds,
respectively.3 Other very weak bands at 600 to 700 cm-1, related to defects in the polymer chains,
and around 1100 cm-1, related to distorted C-C interring bonds of the polymer chains, suggest
that the polymer chains in nanowires present a highly planar structure for all the diameter
PEDOT nanowires. It has been theoretically and experimentally studied that the conjugation
length of the polythiophenes is strongly correlated with the abundance of coplanar segments.
Sauvajol et al. reported using polythiophene films that the symmetric C=C stretching mode is
narrowing when the conjugation length increases.4 Indeed, a careful examination of the intense
line at 1427 cm-1 revealed the line narrowing as the nanowire diameter decreases (Fig. 10). This
result strongly suggests the enhancement in conjugation length in the smaller diameter
nanowires, and this finding supports the mechanism of the conductivity enhancement. The
polymer morphology can be strongly enhanced in the polymerization process within the
nano-sized AAO templates.
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O
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㪈㪉㪇㪇
㪈㪋㪇㪇
㪈㪍㪇㪇
㪄㪈
㪮㪸㫍㪼㫅㫌㫄㪹㪼㫉㩷㩿㪺㫄 㪀
㪝㫀㪾㪅㪈㪇㩷㪩㪸㫄㪸㫅㩷㫊㫇㪼㪺㫋㫉㪸㩷㫆㪽㩷㪧㪜㪛㪦㪫㩷㫅㪸㫅㫆㫎㫀㫉㪼㫊㩷㫊㫐㫅㫋㪿㪼㫊㫀㫑㪼㪻㩷㫀㫅㩷㪘㪘㪦㩷㫋㪼㫄㫇㫃㪸㫋㪼㩷㫎㫀㫋㪿㩷㪌㪇㩷㪸㫅㪻㩷㪉㪇㪇㩷㫅㫄㩷㫇㫆㫉㪼㩷
㫊㫀㫑㪼㫊㪅㩷㪘㩷㪹㫉㫆㪸㪻㩷㪹㪸㫅㪻㩷㪸㫋㩷㪸㫉㫆㫌㫅㪻㩷㪐㪇㪇㩷㫋㫆㩷㪈㪇㪇㪇㩷㪺㫄㪄㪈㩷㫀㫊㩷㪻㫌㪼㩷㫋㫆㩷㪸㩷㪪㫀㩷㫊㫌㪹㫊㫋㫉㪸㫋㪼㪅㩷
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4. Conclusion
We have demonstrated the synthesis of the high mobility polymeric nanowires through a
polymerization using AAO templates. The nanowires of PEDOT have been synthesized within
the nano-sized AAO template (Fig. 5), and the resulting polymer nanostructure was
characterized using SEM and TEM. The chemical, electrical, and structural properties of the
nanowires were also characterized using EDX, 4-wire conductivity measurement, and Raman
spectroscopy, respectively. These characterizations revealed that the AAO template grown
PEDOT nanowires have improved carrier mobility, being suitable material for the construction
of the polymer based vertical p-n junction organic solar cells. We are now taking a next step
toward the realization of the solar cells using the materials and methodologies developed in this
research. This work was supported by the JFE 21st Century Foundation.
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4. Sauvajol, J. L.; Poussigue, G.; Benoit, C.; Lereporte, J. P.; Chorro, C., Synth. Met. 1991, 41,
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