Toward the high efficiency organic solar cell with a vertical p-n junction structureု⋥ p-n ⇇㕙೙ᓮߦࠃࠆ᦭ᯏ⭯⤑ᄥ㓁㔚ᳰߩ㜞ല₸ൻYasuhiro 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 othernonrenewable energy sources. The organic based solar cells, on the other hand, can possiblyoffer special opportunities as a renewable energy source because they can be fabricated overlarge area using low cost printing technologies. The progress in the field of the polymer basedsolar 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 basedsolar 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.
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power-conversion efficiency. A simple organic solar cell has the planer structures shown in theFigure 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-typesemiconductor 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 cellmaterial: 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 tosatisfy all these criteria.
㪠㪫㪦㩷㪤㫀㫏㫋㫌㫉㪼㩷㫆㪽㩷 㫇㩷 㪸㫅㪻㩷 㫅㩷 㫋㫐㫇㪼㩷㫄㪸㫋㪼㫉㫀㪸㫃㫊㩷㪠㪫㪦㩷㪠㪫㪦㩷㫇㪄㫅㩷㫁㫌㫅㪺㫋㫀㫆㫅㩷㪙㫌㫃㫂㩷㪿㪼㫋㪼㫉㫆㫁㫌㫅㪺㫋㫀㫆㫅㩷䋨㫅㫆㩷㪺㫆㫅㫋㫉㫆㫃㩷㫆㪽㩷㫄㫆㫉㫇㪿㫆㫃㫆㪾㫐䋩㩷㪤㪼㫋㪸㫃㩷㪧㫃㪸㫅㪸㫉㩷㫁㫌㫅㪺㫋㫀㫆㫅㪘㫃㩷㪟㫀㪾㪿㩷㪭㪼㫉㫋㫀㪺㪸㫃㩷㫇㪄㫅㩷㫁㫌㫅㪺㫋㫀㫆㫅㩷䋨㪺㫆㫅㫋㫉㫆㫃㫃㪼㪻㩷㫄㫆㫉㫇㪿㫆㫃㫆㪾㫐䋩㪝㫀㪾㩷㪉䋮㪭㪼㫉㫋㫀㪺㪸㫃㩷㫇㪄㫅㩷㫁㫌㫅㪺㫋㫀㫆㫅㩷㪣㫆㫎㪿㫆㫃㪼㩷㪤㪼㫋㪸㫃㩷㪼㫃㪼㪺㫋㫉㫆㫅㩷㪙㫌㫃㫂㩷㪿㪼㫋㪼㫉㫆㫁㫌㫅㪺㫋㫀㫆㫅㪚㪸㫉㫉㫀㪼㫉㩷㫋㫉㪸㫅㫊㫇㫆㫉㫋㩷㪚㪸㫉㫉㫀㪼㫉㩷㪾㪼㫅㪼㫉㪸㫋㫀㫆㫅㩷㪣㫆㫎㩷㪟㫀㪾㪿㩷㪝㫀㪾㩷㪊䋮㪚㪸㫉㫉㫀㪼㫉㩷㪾㪼㫅㪼㫉㪸㫋㫀㫆㫅㩷㪸㫅㪻㩷㫋㫉㪸㫅㫊㫇㫆㫉㫋㩷The polymer based organic solar cells with the best power-conversion efficiency reported sofar has the structure named bulk heterojunction, in which photo excited species have chance tobe separated as holes and electrons anywhere in the device because the p-n junction is distributeall 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 theinvention of such high mobility polymer structures, one can possibly fabricate the vertical p-njunction solar cell structures following a simple fabrication scheme (Fig.4). Here wedemonstrate the synthesis of such high mobility polymeric nanowires through a templatesynthesis of PEDOT. The nanowires of PEDOT have been synthesized within the nano-sizedtemplate (Anodic Alumina Oxide, AAO, Fig. 5), and the resulting nanowire’s nanostructure wascharacterized using SEM and TEM. The chemical, electrical, and structural properties of thenanowires 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 thepolymer based vertical p-n junction organic solar cells.

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2. Experimental
3,4-Ethylenedioxythiophene (EDOT) and anhydrous acetonitrile (CH3CN) were obtainedfrom Aldrich. Lithium perchlorate (LiClO4) was obtained from Wako. Gold electroplating
solution (Galvanomeister GB3) was purchased from Tanaka Kikinzoku. AAO membrane with apore diameter of 0.2 Pm and a thickness of about 60 Pm (Anodisc 13) was obtained fromWhatman. Deionized water was obtained by using a Milli-Q water purification system fromMillipore.
AAO templates with small pore diameters (50 and 100 nm) were fabricated by using atwo-step anodization process.1 An Al sheet (15 u15 u 0.5t, 99.99%) was anodized for about 18 hat 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 wereanodized for about 18 h at 100 V and 0 qC by using 0.15 M oxalic acid as an electrolyte, and ananodization 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 producedin the initial anodization process to make a well-defined pore structures. Thus, the initiallyformed 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 wasperformed 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 phosphoricacid (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 1mA/cm2 with a galvanostatic mode in a three electrode configuration using a Pt counterelectrode and an Ag/AgCl reference electrode. The concentration of EDOT was 1.0 M, and theelectrolyte 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 galvanostaticallyrepeating the -1.0 A/dm2 (0.1 s) and +8.0 A/dm2 (0.01 s) to completely fill the bottom of thenanopores with gold.
SEM, TEM, and Raman spectroscopy characterization of the nanowires were performed byreleasing the nanowires into solution by dissolving the AAO template in 1.0 M KOH, andcasting the nanowire sample solution on to a Si substrate or TEM grid. The samples wererepeatedly centrifuged and rinsed in water to remove the KOH before the casting. Electricalmeasurements 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
nanowiresofwerethePEDOT

grown

usingnanowires.
aTheconventional

3-electrode configuration (Fig 6). By applying theenough potential on the Au electrode at the bottom ofthe 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 diametersof 50, 100, and 200 nm were used in this study.
Characterization of polymer nanostructures. It is
importanttoobserveandunderstandthenanostructures 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㧙㧙㪝㫀㪾㪅㪍 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 PEDOTnanowires were released from the AAO template, and the SEM images (Fig.7) on individualPEDOT nanowires revealed that the nanowire diameter can be controlled by changing the sizeof the pore diameters in the AAO template. TEM observation confirmed the formation of thecompletely 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) wasestimated 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 wasmeasured using a voltmeter.
Over 100 samples of thePEDOT nanowires made withthree different AAO templates
weremeasured,and

theresults were summarized inthe Figure 9. It has beenrevealed that the PEDOT
nanowire conductivity will
increase as the decrease of thenanowire

diameters.

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result strongly suggests that there aresome chemical and/or structural changes
inthenanowires.

Thematerial’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 todetermine which factor is the dominantsource in the conductivity enhancement in㪝㫀㪾㪅㪐 㪚㫆㫅㪻㫌㪺㫋㫀㫍㫀㫋㫐 㪻㫀㫊㫋㫉㫀㪹㫌㫋㫀㫆㫅㩷 㫆㪽㩷 㫋㪿㪼㩷 㪧㪜㪛㪦㪫㩷㫅㪸㫅㫆㫎㫀㫉㪼㫊㩷㫊㫐㫅㫋㪿㪼㫊㫀㫑㪼㪻㩷㫌㫊㫀㫅㪾㩷㫋㪿㪼㩷㪘㪘㪦㩷㫋㪼㫄㫇㫃㪸㫋㪼㩷㫎㫀㫋㪿㩷㪌㪇㪃㩷㪈㪇㪇㪃㩷㪸㫅㪻㩷㪉㪇㪇㫅㫄㩷㫊㫀㫑㪼㫊㪅㩷the PEDOT nanowires with smaller diameters. To answer this question, we have explored thecarrier 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 electricalproperties and conductivity of a conducting polymer will strongly depend on the extent of thedoping. 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 theelectrochemical 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 constantand has no diameter dependence. This result strongly suggests that the carrier density of thePEDOT nanowires has no role in the measured conductivity enhancement of the nanowires withsmaller 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 nm21.7

4.6%

200 nm23.4

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 thecarrier 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 13001600 cm-1 spectral
region shows that in general the peaks are relatively more intense for the 50 nm PEDOT thanfor 200 nm PEDOT nanowires. The most intense peak at 1427 cm-1 is assigned to the symmetricstretching mode of the C=C double bonds, whereas a less intense peak at 1508 cm-1 ischaracteristic of the C=C antisymmetric stretching vibration.3 Other weaker peaks at 1361 and1262 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, suggestthat 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 isnarrowing when the conjugation length increases.4 Indeed, a careful examination of the intenseline 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 thenano-sized AAO templates.

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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㧙㧙

4. ConclusionWe have demonstrated the synthesis of the high mobility polymeric nanowires through apolymerization using AAO templates. The nanowires of PEDOT have been synthesized within
the nano-sized AAO template (Fig. 5), and the resulting polymer nanostructure wascharacterized using SEM and TEM. The chemical, electrical, and structural properties of thenanowires 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 constructionof 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 thisresearch. This work was supported by the JFE 21st Century Foundation.
References
1. Masuda, H.; Satoh, M., Japanese Journal of Applied Physics Part 2-Letters 1996, 35,
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3. Sakmeche, N.; Aeiyach, S.; Aaron, J.-J.; Jouini, M.; Lacroix, J. C.; Lacaze, P.-C., Langmuir
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4. Sauvajol, J. L.; Poussigue, G.; Benoit, C.; Lereporte, J. P.; Chorro, C., Synth. Met. 1991, 41,1237-1242.

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