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DOI: 10.1039/C5RA04760A (Paper) RSC Adv., 2015, 5, 56262-56269

Performance evaluation of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based organic solar cells fabricated by spray coating method using chlorine free solvent

Murugesan Vijay Srinivasana, Norio Tsudaa, Paik-Kyun Shinc and Shizuyasu Ochiai*b
aDepartment of Electrical Engineering, Aichi Institute of Technology, Toyota 470-0392, Japan
bProfessor emeritus, Aichi Institute of Technology, Toyota-470-0392, Japan. E-mail: ochiai@aitech.ac.jp
cDepartment of Electrical Engineering, Inha University, Incheon 402-751, Korea

Received 18th March 2015 , Accepted 16th June 2015

First published on 16th June 2015

Abstract

The PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM polymer based solar cells have been successfully fabricated by spin and spray coating technique using chlorine-free solvent (xylene), which are desirable to reduce environmental issues. The surface morphology of fabricated film characterized by AFM reveals that the surface morphology of the film is uniform and smooth when xylene is used as compared with chlorobenzene. The highest power conversion efficiency (PCE) (5.07 ± 0.6) was achieved using spray-coating technique than that of spin coating technique (PCE of 4.47 ± 0.6). The enhancement in the performance of the polymer solar cell could be attributed to the improved charge carrier transportation due to additive. The combination of chlorine-free solvent and spray-coating method minimize the waste material and reduce the environmental problem in large-area production of organic solar cells (OSCs).

1 Introduction

In recent years, researchers are focused on conversion of solar energy into electrical energy because of our energy demand per annum (1.6 × 1010 kW). As, our earth receives 175 peta watts (PW) of energy from the sun, which can provide us 3.8 × 1023 kW per one hour to meet our energy demands. The total energy-consumption of the world is 10 TW, among this solar panels covering only 0.08% with an average 10% power conversion efficiency from our earth surface. Thus, the silicon solar cells have a much better efficiency and higher life span as compared to other the light energy conversion method. However, production cost, recycling and environment pollution is major drawback in silicon solar cell. Therefore, research community extensively focused on developing new materials to achieve a higher energy conversion with low coast production and environmentally compatible using inorganic (silicon) or organic solar cells. Organic photovoltaic has attractive features like low cost, light weight, flexible, large area fabrication, high speed production and solution process compare with inorganic solar cell.1–9 The highest efficiency of OSCs reported around 10–11%, which is still inferior to those silicon solar cells.[thin space (1/6-em)]10–14 In this most high efficient polymer solar cells are made up with chlorinated solvents.15,23 However, chlorine containing solvents are highly toxic and hazardous to environment and this is major problem in the mass production of polymer solar cells even though of the advantage of low-cost and large area solution fabrication.24–35 In this present investigation, we fabricated the high performance assessment of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based organic thin film solar cell device using chlorine-free solvent by spin and spray coating technique. The active layers were prepared using PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM with 1,8-diiodooctane (DIO) in xylene and chlorobenzene. PTB7 and fullerene exhibit poor solubility in commonly used chlorine-free solvents, the addition of additive DIO in the active layer enhances the solubility[thin space (1/6-em)]7,15 and optimizing the desired donor–acceptor phase separation with nano-morphology and it results enhance the photovoltaic performance. To improve the device performance, the solar cell was fabricated with spray coating techniques. The spray coating has an advantage of exhibiting large-area uniformity, preventing of inter layer dissolution and better mixing during spray coating.35 Hence, spray coating technique as a better choice for the chlorine-free solvents than that of spin coating technique for production in high power conversion solar cell.

The polymer–fullerene bulk heterojunction (BHJ) solar cells are widely used and currently attracting a great deal of attention and gaining the power conversion efficiency over 8–10% for single BHJ3–8 and greatly improved PCEs 8–12% for tandem device.1,2,10–14 In this most high efficient polymer solar cells are made up of low band-gap polymers with toxic chlorinated solvents due to their high boiling point and solubility.15–23 The higher boiling solvent evaporates slowly during the device fabrication and it exhibits desired donor–acceptor phase separation along with higher solubility of the solvent which results in satisfactory efficiencies.24–26 At present, processes of polymer solar cell by chlorine-free solvents are very attractive candidates such as toluene, isomers of xylene, and trimethylbenzenes due to their comparable properties with the halogenated analogs. These solvents have the advantage of cost and sustainability points.24,27–31 Unfortunately, some of the low band-gap polymers and fullerene exhibit poor solubility in commonly used chlorine-free solvents. This often results in no uniform phase segregation due to large fullerene/polymer rich grains in the cast films, reveals limiting the device performance.32 This critical issue was solved by adding the more suitable additive with solvent and it enhances the fullerene solubility and exhibit enhanced device performance.26,31,33 Recently, 6–7% power conversion efficiency has been reported for polymer solar cells by spin and blade coated in a halogen-free solvent.24,27 However, spray-coating techniques are being explored for large-area, uniform thin film and wastage of material is low compare with other method. In addition, it can facilitate much more dilute solutions than the other method, allowing the use of various organic materials that have poor solubility in solvents.13,34–43 Therefore, development of polymer solar cell using chlorine-free solvents by spray coating is capable of delivering large-area, uniform films with small amounts of material, which is having the compatibility for roll-to-roll fabrication is crucial.

2 Experimental works

2.1 Materials

PTB7 was purchased from Ossila, England and used as p-type organic donor material. PC71BM was purchased from American Dye Source, Inc and used as n-type organic acceptor material. PEDOT[thin space (1/6-em)]:[thin space (1/6-em)]PSS (Clevios™ PH1000) was purchased from Heraeus, Germany and used as hole transport layer between anode ITO and active layer of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM. Xylene, chlorobenzene, acetone, ethanol and all other chemicals were obtained from Sigma-Aldrich are used as received. Fig. 1 shows that the molecular structures of PTB7 and PC71BM and the energy band diagram of OTFSC device is shown in (Fig. 2).
image file: c5ra04760a-f1.tif
Fig. 1 Molecular structures of PTB7 and PC71BM.

image file: c5ra04760a-f2.tif
Fig. 2 Energy diagram organic thin film solar cell.

2.2 Fabrication and characterization of organic thin film solar cell device

The organic thin film solar cell (OTFSC) device was fabricated using indium doped tin oxide (ITO) conducting glass substrate (Aldrich; 8–12 Ω sq−1). ITO substrate was patterned by longitudinal direction in the width of 5 mm by using an ultra-tape on a glass substrate, substrate was immersed in dilute hydrochloric acid (HCl) for 10 min followed by washed with water for another 10 min. The ITO conducting substrates were cleaned ultrasonically in sequential steps by using a neutral detergent, acetone, ethanol and distilled water for 10 min in each step. The cleaned ITO was exposed to UV ozone for 10 min to achieve hydrophilic state of the ITO surface. PEDOT[thin space (1/6-em)]:[thin space (1/6-em)]PSS was cast by spin coating technique at spin speed of 5000 rpm for 30 s, followed by heat treated about 150 °C in an oven for 10 min. The composite active layers of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM with a relative ratio of 1(9.0 mg)[thin space (1/6-em)]:[thin space (1/6-em)]1.5(13.5 mg) was prepared using different solvents like xylene and chlorobenzene with and without DIO. The active layer was coated by spray coating technique at 25 s with a distance of 15 cm (from nozzle-to-substrate) using spray gun of 2 mm in diameter under N2 pressure 0.05 MPa. To avoid residues of DIO, the spray-coated active layers of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM thin films were dried for 24 h in a physical vapor deposition (PVD) chamber. Three different OTFSC devices were fabricated with (1) PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM W/o DIO in xylene, (2) PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM + 3% DIO in XY and (3) PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM + 3% CB by spray coating method for evaluation photovoltaic performance using chlorine-free solvent presence or absence of DIO. Then, the active layer was coated by a spray coating technique using spray gun of 2 mm in diameter with N2 pressure 0.05 MPa. Finally, aluminum (Al) metal was deposited using shadow mask with diameter of 5 mm width in a high vacuum of 1.3 × 10−6 Pa. The active area of the device was 0.25 cm2. The thickness of the PEDOT[thin space (1/6-em)]:[thin space (1/6-em)]PSS, active layer and Al electrode was measured as ∼45, ∼86 and ∼100 nm, respectively. Fig. 3 shows the OTFSCs device structure and spray coating setup. For comparison, the solar cell was fabricated using conventional spin-coating techniques. For that, the precursor solution was prepared in 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5 ratio of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM with and without DIO using XY and CB. The active layers were coated with a spin speed of 1000 rpm by conventional spin coating method at 2 min (∼84 nm thickness), then cathode Al was deposited on the top of the active layer by physical vacuum chamber using shadow mask.
image file: c5ra04760a-f3.tif
Fig. 3 Schematic representation of OTFSCs and spray coating setup.

2.3 Characterization techniques

Surface morphology of the organic thin films was investigated using Seiko Instrument SPA400-SPI4000 atomic force microscope (AFM) and all AFM images were taken in dynamic force mode at optimal force. The thickness of the active layer and aluminum was measured using a Dektak II profilometer. Performance of the OTFSC devices was examined by measuring photocurrent density–voltage (J–V) characteristics was measured standard solar simulator Newport 66902 150W with an Oriel Cornerstone 130 monochromator and Xe lamp equipped with an Air Mass of 1.5 G (AM 1.5 G) filter was used as the white light source; 100 mW cm−2. A thick black sheet was used as a shadow mask, which is placed in front of the active device area in order to avoid the over-estimation of the current density and PCE. The J–V characteristics were measured using an Advantest-R-6441 A.C. source meter unit. All measurements were carried out in air without any encapsulation.

3 Results and discussion

3.1 Surface morphology and absorption studies

Surface morphology plays a vital role on the photovoltaic characteristics of OTFSC. The surface morphology of OTFSCs is controlled by altering blend ratio of donor and acceptor. However, the polarity and boiling point of solvent are major factor influencing the device performance.14,25–27,40 To understand the surface morphology of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM on photovoltaic performance, the active layer are fabricated with spin and spray coating method using non-chlorinated/chlorinated solvent with and without additives DIO. The effects of additive on the surface morphology of the resulting thin-films of blended PTB7/PC71BM were examined by atomic force microscopy (AFM), which is shown in the Fig. 4. Surface roughness parameters of the PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM prepared in XY and CB with different solvent with and without solvent are presented in Table 1; peak–valley (P–V) and root-mean-square of roughness (RRMS).
image file: c5ra04760a-f4.tif
Fig. 4 AFM image of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based active layer (a and d) W/o DIO in XY, (b and e) 3% DIO in XY, and (c and f) 3% DIO in CB fabricated by spin and spray coating method.
Table 1 Surface characteristic of active layer of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM were fabricated from various solvents without and with DIO by spin and spray coating method
PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM Coating method P–V (nm) RRMS (nm)
W/o DIO-XY Spin 10 2.65
3% DIO-XY Spin 21 1.76
3% DIO-CB Spin 19 1.73
W/o DIO-XY Spray 58 8.2
3% DIO-XY Spray 25 2.27
3% DIO-CB Spray 19 2.67

Fig. 4a clearly shows that there are large domains in the PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM blend film prepared pristine xylene. It implies that molecules of PTB7 and PC71BM might have insufficient mutual solubility and it could not dissolution of the surface aggregates of PC71BM molecule during the thin film formation process. The larger the surface aggregates will diminish exciton migration to the donor/acceptor interface and is not favorable for charge separation, it leads to poor (diminish) the device performance.12–15 Whereas the active layer prepared in XY with additive DIO reveals noticeably smaller micro grains (Fig. 4b) compare with pristine XY. The addition of DIO to the active layer dissolves the surface aggregates of PC71BM molecules in the PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM matrix. The PC71BM is well known for electron-deficient in nature and the iodine atom (DIO) is well known to bear a partial negative charge. This reveals strong interaction between the DIO and PC71BM which enhances the solubility of PC71BM. This leads to greater inter-penetrated blending between the donor and acceptor which could be formed with smaller micro grains of nano-morphology. This might be suggested that better contact between donor and acceptor in the active layer which enhance the charge-carrier-transport between donor and acceptor in the active layer. It also suppresses the recombination charge carrier and improves the device performance,12–15,41 The surface roughness of active layers prepared with 3% DIO in XY and CB show RRMS = 1.76 and 1.73 nm, respectively. It is revels that surface morphology of active layer prepared from non-chlorinated solvent (XY) similar to the chlorinated solvent.

Further to understand the effect of spray coating on surface morphology of active layer prepared with non-chlorinated and chlorinated solvent, we measured the surface morphology of the spray coated film. This result shows that the surface roughness values of active layers fabricated with pristine XY shows higher RRMS = 8.2 nm than and without DIO (RRMS = 2.27 nm). As discussed earlier, addition of additive dissolve the surface aggregates and subsequently increasing the degree of phase separation revel smaller micro grain with nano-morphology, which might improve the device performance. The surface roughness values of active layers prepared in XY (RRMS = 2.27) very close to chlorinated solvent (RRMS = 2.67), it reveals that surface morphology of active layer prepared in XY is similar when compared with chlorinated solvent (CB). The surface morphology of active layer fabricated with XY by spray coating is compared with spin coating method. The surface roughness of active layers fabricated by spray coating exhibit higher surface roughness RRMS = 2.27 nm than spin coating method (RRMS = 1.76 nm). The higher surface roughness of the film might increase the contact area between the active layer and the contact electrode, as well as increase the charge collection. Furthermore, the increased surface roughness may increase internal reflection in the active layer and improve light collection leading to increase in device efficiency.34,36,37,39,41 Surface morphology of blend with and without DIO is further analysed by scanning electron microscopy (SEM) in XY & CB are shown in Fig. 5.


image file: c5ra04760a-f5.tif
Fig. 5 SEM analysis results show the large domain blend prepared without DIO in XY and CB. Whereas in the blend prepared with 3% DIO, the additive dissolves the surface aggregates and reveals a smoother surface.

The absorption spectra of active layers of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM prepared in XY with and without CB are depicted in Fig. 6 shows that strong absorption between 320 and 700 nm. Two broad absorption peaks, around 624 nm and 682 nm, are attributed to the characteristic π–π* transition5 of the PTB7 polymer whereas the broad absorption coverage in the range from 320 to 500 nm is due to absorption by PC71BM.5,28,34 The Fig. 6 indicates that the absorption spectra of the PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM film with additives shows slightly higher absorption in the region of 300–700 nm than PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM film prepared in the absence of additives, which might be due to differences in the degrees of dispersion of PC71BM in PTB7. This implies that the addition of DIO added into XY dissolves the PC71BM molecule reveals greater inter-penetrated blending between the donor and acceptor interface. These results increase the absorption of blend layers and can harvest solar photons more effectively than the film prepared from pristine XY under the same conditions.34 Comparing the absorption spectra of the films prepared with and without solvent additives, the result indicates that there is no significant peak shift observed for the films prepared with organic additives from that of the film prepared with pristine XY. It implies that the thin films prepared with additives have amorphous phase. The absorption spectra of the films prepared with XY additives shows stronger absorption in the range between 450 nm and 550 nm. This slight red shift can be observed for the absorption peak at 645 nm with the decrease of absorption peak at 710 nm as compared to the films prepared CB with additives, indicating that the order of the polymer was more affected by PC71BM.42,43 The absorption spectra slightly increases between 320–550 nm, indicating that PC71BM has stronger absorption in the range between 320 nm and 550 nm attributed due to enhanced solubility of PCBM with better mixing.28,42,43 These results suggest that the absorption properties of active layer film prepared from XY is almost similar to CB and hence the toxic solvents could be avoided. Further, the effects of the organic solvent on the surface morphology of the PC71BM thin films were studied by J–V characteristics measurement.


image file: c5ra04760a-f6.tif
Fig. 6 Absorption spectra of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM films prepared in XY with and without DIO.

3.2 Photocurrent density–voltage (J–V) characteristics

The photovoltaic performance polymer solar cells processed by non-chlorinated solvent are performed by fabrication of spray coating method and its, which is compared with conventional spin coating method. The J–V characteristics and photovoltaic performance of the devices are given in Table 2 by spin and spray coating. Fig. 7 shows the J–V characteristics of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based OTFSC fabricated by spray coating using xylene and chlorobenzene with and without DIO. From this Table 2, the OTFSC prepared in xylene with 3% DIO exhibits higher PCE = 5.07% than without DIO (PCE = 2.90%). This implies that the addition of DIO in active layer reveals increase in the Jsc = 12.07 mA cm−2, FF = 56%, Rsh = 398 Ω cm2 and decreases the Rs = 5.5 Ω cm2, as a result of drastic improvement in the PCE = 5.07% than without DIO, PCE = 2.90%.
image file: c5ra04760a-f7.tif
Fig. 7 J–V characteristics of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based OSC devices with and without DIO using spray coating method.
Table 2 J–V characteristics of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based OTFSC with and without DIO using spin and spray coating method
PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM Coating method Jsc (mA cm−2) Voc (V) FF (%) Rs (Ω cm2) Rsh (Ω cm2) PCE η (%)
W/o DIO-XY Spin 9.33 0.62 41 8.7 155 2.38
3% DIO-XY Spin 13.70 0.71 46 8.6 179 4.47
3% DIO-CB Spin 13.49 0.72 49 8.5 250 4.79
W/o DIO-XY Spray 9.93 0.71 40 6.1 193 2.90
3% DIO-XY Spray 12.09 0.74 56 5.5 397 5.07
3% DIO-CB Spray 12.37 0.75 57 3.9 407 5.31

The AFM studies reveals that the pristine xylene exhibits larger surface aggregates due to improper mixing between donor and acceptor, which will diminish exciton migration and is not favorable for charge separation between donor/acceptor interfaces. The larger surface aggregates may contain a high concentration of grain boundaries and as a result form photo-carrier recombination centers. Furthermore, the large grains in the active layer may cause poor contact with cathode electrode and hole transporting layer and possible for recombination holes and electron as a result lower PCE = 2.90%. Whereas the processing additives have been found to enhance device performance by dissolving the PC71BM aggregates in the PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM matrix. Which facilitates integration of the PC71BM molecules into the PTB7 resulting in a greater donor–acceptor interface and it could be formed with smaller micro grains. The results increases the surface interface between donor and acceptor in the active layer, which will enhance the exciton migration between the donor/acceptor interface and favorable for charge separation. Further, the additive DIO has higher boiling point of 169 °C could influence the formation of nanoscale morphology of molecules due to slow evaporation, result in decrease of series resistance and enhance the charge carrier transport properties, as well as increase of shunt resistance due to suppression of charge carrier recombination in the active layer.7,14,15,34 This reveals that decreases of series resistance Rs = 6.1 Ω resulting in enhanced Jsc of 12.09 mA cm−2, increase of FF = 56% as well as increases the shunt resistance Rsh = 397 Ω observed results improvement for device performance PCE 5.07%. There is an increase in the FF (from 40% to 56%) due to the reduction of series resistance (Rs) and enhancement of shunt resistance (Rsh). The Rs is closely related with active layer morphology, intrinsic resistance and thickness of the active layer. The Rsh is related with the pin holes and traps presents in the morphology cause charge carrier recombination and leakage current. This suggests that the addition of additive dissolves the PC71BM in the active layer and more integration which favors the nano-morphology, results an increase of contact area between donor and acceptor, leads to decrease in the series resistance and enhance the charge carrier transport properties, as well as increase of equivalent parallel resistance due to suppression of charge carrier recombination at the interface result enhancement in the device performance.

The performance of spray coated solar cells device is compared with conventional spin coated device. The J–V characteristics of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based OTFSC prepared in xylene with and without additive DIO by spin coating method are shown in Fig. 8. The spray-coated solar cells shows higher PCE (η = 5.07%) than spin coated device, PCE = 4.47%. The photovoltaic performance of spray-coated device enhanced PCE due to increase in the surface roughness (RRMS = 2.6) than spin coated device (RRMS = 1.7 nm). The higher surface roughness of the film might increase the contact area between the active layer and the contact electrode as well as improve the charge collection. Furthermore, the increased surface roughness may increase internal reflection in the active layer and improve light collection leading to increase in device efficiency.36,37,41 This results shows that spray-coated PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM + 3% DIO based device exhibit higher Voc = 0.74 V, FF = 56%, and significantly improved PCE = 5.07% than spin-coated device. Further, these device performance of chlorinated solvent compared with non-chlorinated solvent exhibits slightly higher PCE = 5.31%, it implies that performance of non-chlorinated closer to chlorinated solvent and it might be considered as an alternate for conventional use of toxic chlorinated solvent. The performance of non-halogenated device compared with chlorobenzene device exhibited slightly higher PCE = 4.79% with additive. This implies that performance of non-chlorinated device closer to chlorinated solvent and it might be attributed due to similar surface morphology of active layer with nano-structure like chlorobenzene. The results show that smoother surface results in better contact between active layer and electrode leads to satisfactory device performance.


image file: c5ra04760a-f8.tif
Fig. 8 J–V characteristics of PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM based OSC devices with and without DIO using spin coating method.

4 Conclusion

PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM + 3% DIO polymer solar cell is successfully fabricated with non-halogenated solvent by spin and spray coating method and its conversion efficiency was examined. The effect of solvents on active layer is studied by surface morphology and J–V measurement. This photovoltaic performance result shows that solar cell prepared by xylene yielding higher PCE = 5.07% with DIO than without DIO (PCE = 2.90%) using spray coated method. The additive dissolve the PC71BM aggregates in the PTB7[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM matrix favoring interblending between donor and acceptor and it could be formed nano-morphology. This surface morphology of the active layer fabricated with xylene forms more uniform surface which enable better contact between donor and acceptor, improving the charge carrier transport results and enhancing the device performance. The device performance of non-chlorinated solvent results is closer to chlorinated solvent, which is considered to be a replacement for conventional use of toxic chlorinated solvent. The spray-coated solar cells compare with conventional spin coating method revels that spray-coated device shows significantly higher PCE = 5.07% than spin coated device (PCE = 4.47%). The spray-coated device exhibit higher performance with more uniform film can be applied for roll-to-roll and it is considered to be an alternative of conventional spin coating. These results indicates that the combination of chlorine-free solvents and spray-coating method minimize the waste material and reduce environmental problem in large-area production of organic solar cell.

Acknowledgements

This research was partly supported by the MEXT Private University Project Grant (no. S1001033), the Japan Science and Technology-Adaptable and Seamless Technology Transfer Program (R&D no. AS232Z02610B), and Joint Research between Aichi Institute of Technology and NDS.

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