Dye-sensitized Solar Cell Based on ZnO Nanorod Arrays by ill20582

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									                     The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
B-024 (O)                                                               21-23 November 2006, Bangkok, Thailand

                               Dye-sensitized Solar Cell Based on ZnO Nanorod Arrays

                                      Patcharee Charoensirithavorn and Susumu Yoshikawa*

                             Institute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011, Japan

Abstract: ZnO nanorod arrays were fabricated on the fluorine-doped SnO2 transparent conducting oxide (FTO) glass substrates and u
sed them as the wide band gap semiconductor in dye-sensitized solar cells. Our objectives are to introduce and demonstrate new possi
bilities in designing the semiconductor morphology. The best performance with this cell structure produced an open circuit voltage (Vo
c) of 0.64 mV, a short circuit current density (Jsc) of 5.37 mA/cm , a fill factor (FF) of 0.49, and conversion efficiency (η) of 1.69 %, pr
                                                                  2

imarily limited by the surface area of the nanorod array.

Keywords: Zinc Oxide, Nanorod Arrays, Dye-sensitized Solar Cell, High Surface Area, One-dimensional

                                                        1. INTRODUCTION

   Solar cell is one of the most promising renewable energy technologies for this century because it has possibility for
solving environmental problems and insufficient energy problem. Dye-sensitized solar cell (DSSC) using inorganic
semiconductor is being studied as a new type of solar cell and expected as a low cost alternative to conventional solid-
state device. One important limiting factor in the DSSC cell performance is electron transport. During its traversal to the
photoelectrode, an electron is estimated to cross 103 to 106 nanoparticles [1]. The disorder structure of the nanoparticles
film leads to enhanced scattering of free electrons, thus reducing electron mobility and causing electron recombination
especially at the grain boundaries between the nanoparticles[2]. The replacing of the nanoparticle film with an array of
oriented single-cryatalline nanorod offers the potential for improved electron transport leading to higher
photoefficiencies. The pathways provided by the nanorods ensure the rapid collection of carriers generated throughout
the device as the nanorod provide a direct path from the point of photogeneration to the conducting substrate. This
greatly reduces the electron recombination losses of the photogenerated charge-carriers due to the fewer grain
boundaries in charge transportation process. Moreover, electron transport in the crystalline rod is expected to be several
orders of magnitude faster than percolation through a random polycrystalline network [3].
   In this study, we have fabricated transparent ZnO nanorod arrays on the fluorine-doped SnO2 transparent conducting
oxide (FTO) glass substrates and used them as the wide band gap semiconductor in dye-sensitized solar cells.

                                                        2. METHODOLOGY

2.1 Synthesis
   Arrays of ZnO nanorods were chemically synthesized on FTO substrate. The procedure consists of two steps. Firstly, zinc acetate
solution was dropped onto substrates by spin coating, and then the substrates were dried and annealed in order to form the
nanocrystal seeds on the substrates. Secondly, vertical ZnO nanorod arrays from the nanocrystal seeds were grown by immersing the
seeded substrates in precursor solution containing Zn(NO3)2 and 0.80 M NaOH at 110oC with different growth time interval.

2.2 Characterization
   The crystalline structure of the samples was evaluated by X-ray diffraction (XRD, RIGAKU RINT 2100). The microstructure of
the prepared materials was analyzed by scanning electron microscopy (SEM, JEOL JSM-6500FE).

2.3 Dye-sensitized solar cell measurement
   The ZnO electrodes were soaked in 0.3 mM of ruthenium (II) dye (known as N719, Solaronix) in a t-butanol/
acetonitrile (1:1, in vol %) solution. The electrodes were washed with acetonitrile, dried, and immediately used for
measuring photovoltaic properties. The electrolyte was composed of 0.6 M dimethylpropylimidazolium iodide, 0.1 M
lithium iodide (LiI), 0.05 M iodide (I2), and 0.5 M 4-tert-butylpyridine in acetonitrile.

                                              3. RESULTS AND DISCUSSION [4]

3.1 Characterization results
   Figure 1 shows typical SEM images of the ZnO nanorods arrays grown on FTO substrate. The low-magnification images (A, C)
show a well-aligned high-density ZnO nanorods growing uniformly in large area on the substrate. From the high magnification image
(D), it can be seen that high-density ZnO nanorods with well-defined hexagonal facets were grown vertically on the substrate. The
cross-sectional view (B) of nanorods arrays demonstrated that the ZnO grew vertically from the substrate. In this work, we could
grow ZnO nanorods in the wafer scale implied that our method is applicable to mass production of well-aligned ZnO nanorod arrays.




Corresponding author: s-yoshi@iae.kyoto-u.ac.jp


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                    The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
B-024 (O)                                                              21-23 November 2006, Bangkok, Thailand



          A                                                      B




          C                                                        D




                     Fig. 1 SEM images of ZnO nanorods grown on FTO substrate (A) tilt view, (B) side view,
                             (C) top view at low magnification, and (D) top view at high magnification

   The crystallinity of the grown ZnO nanorods was investigated using XRD. A typical XRD pattern is shown in Figure 2, the
intensity of the peak assigned to the (002) plane of wurtzite ZnO was markedly strong and the diffraction peaks of other crystal
planes disappeared or very weak revealing that ZnO nanorods were formed through elongation along the c-axis perpendicularly to the
substrate.
                               Intensity




                                                                                             ZnO nanorod arrays
                                                                                             on FTO substrate
                                                                                             FTO substrate

                                           0   10   20     30      40       50      60       70      80


                               Fig. 2 XRD patterns of FTO substrate and ZnO nanorod arrays on FTO

   In addition, we found that the length of ZnO nanorod could be freely modified by controlling the reaction time. The nanorod
lengths increased from 2.6 to 4.0 and 5.1 µm when the reaction times increase from 1 to 2 and 4 h, respectively. By the multiple-step
growth, the nanorod length can be increased up to 10.8 µm when the total reaction time of 18 h.



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                     The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
B-024 (O)                                                               21-23 November 2006, Bangkok, Thailand

3.2 Dye-sensitized solar cell characteristics
   Figure 3 shows the absorption spectra of N719 dye adsorbed onto the ZnO nanorods array with different film thickness. All
samples have the same maximum values around 308, 377, and 511 nm. The intensities of the absorption peaks of N719 were
decreasing gradually with decreasing film thickness, suggesting that fewer dyes have been adsorbed onto the thinner films than onto
the thicker films.

                      Absorbance (a.u.)




                                                                            (d)
                                                                           (c)
                                                                         (b)
                                                                       (a)

                                          300   350      400     450             500       550          600   650    700
                                                                  W avelength (nm)
             Fig. 3 Absorption spectra of adsorbed N719 dye onto ZnO nanorods array films at different nanorods length:
                                           (a) 2.6 µm, (b) 4.0 µm, (c) 5.1 µm, and (d) 10.8 µm.

    In order to examine the device performance, the photocurrent-voltage characteristics of the DSC using ZnO nanorod as the
electrode have been measured under illumination as shown in Figure 5. The fundamental results have been summarized in the Table 1.
The short circuit current density and cell performance significantly increase as the nanorod length increases. A higher amount of the
adsorbed dye on longer nanorods, resulting in improving photon absorption and carrier generation with increased rod length [5].
These results suggest that cell performance is strongly depending on the electrode surface area. The increasing in the nanorod length
provides larger amount of surface area, more adsorbed dyes, and resulting in higher conversion efficiency. The best-worked cell gave
a conversion efficiency (η) of 1.69 %, an open circuit voltage (Voc) of 0.64 mV, a short circuit current density (Jsc) of 5.37 mA/cm2, a
fill factor (FF) of 0.49 in case of the ZnO obtained from the reaction time of 18 h. The use of nanorods with high crystallinity instead
of nanoparticles contributes to the decreased number of grain boundaries, which act as electron traps [6]. This greatly reduces the
electron recombination losses of the photogenerated charge-carriers due to the fewer grain boundaries in charge transportation
process. The nanorod arrays also provide a direct path from the point of photogeneration to the conducting substrate. These pathways
ensure the rapid collection of carriers generated throughout the device. Moreover, electron transport in the crystalline rod is expected
to be several orders of magnitude faster than percolation through a random polycrystalline network [7].

 Table 1 Photoelectric performance parameters of solar cell based on ZnO nanorods arrays at different nanorods lengths
    Samples         Reaction time                     Nanorod length             Jsc             Voc          FF        η
                                                                                       2
                                          (h)             (nm)            (mA/cm )               (V)                   (%)
         A                                 1               2.6              2.88                 0.60         0.54     0.94
         B                                 2               4.0              3.97                 0.59         0.54     1.27
         C                                 4               5.1              4.84                 0.61         0.48     1.42
         D                                18              10.8              5.37                 0.64         0.49     1.69




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                    The 2nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”
B-024 (O)                                                              21-23 November 2006, Bangkok, Thailand


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                                                             5




                             Photocurrent density (mA/cm )
                            2
                                                             4                                 (d)
                                                                                         (c)
                                                                                   (b)
                                                             3
                                                                             (a)
                                                             2


                                                             1


                                                             0
                                                                 0     0.2               0.4         0.6
                                                                                tage ( )
                                                                             V ol     V
             Fig. 5 Photocurrent-Voltage curves of the N719- sensitized ZnO electrodes with different nanorods length:
                                         (a) 2.6 µm, (b) 4.0 µm, (c) 5.1 µm, and (d) 10.8 µm.

                                                                        4. CONCLUSION

   In this study, we have fabricated transparent ZnO nanorod arrays FTO glass substrates and used them as the wide band gap
semiconductor in dye-sensitized solar cells. The nanorods length can be highly controlled by adjusting the reaction time interval. The
short circuit current density and cell performance were mainly determined by the nanorod length. The increasing in the nanorod
length provides larger amount of surface area, more adsorbed dyes, and resulting in higher conversion efficiency. The best
performance with this cell structure produced an open circuit voltage (Voc) of 0.64 mV, a short circuit current density (Jsc) of 5.37
mA/cm2, a fill factor (FF) of 0.49, and a conversion efficiency ( ) of 1.69 %, primarily limited by the surface area of the nanorod
array.

                                                                     5. ACKNOWLEDGMENTS

   The authors would like to express gratitude to Prof. T. Yoko, Institute for Chemical Research, Kyoto University for the use of
XRD equipment. Prof. Mochizuki at AIST for the kind supply of Pt counter electrode. We are also grateful to the Geomatec Co. Ltd.
for providing a part of conducting glass. This work was supported by grant-in-aids from the Ministry of Education, Science Sports,
and Culture of Japan under the 21 COE program, and NEDO under high-performance dye-sensitized solar cell project.

                                                                        6. REFERENCES

[1] Brian, O.R. and Michael, G. (1991) A low-cost, high-efficiency solar-cell based on dye sensitized colloidal TiO2 films, Nature,
     353, pp. 737–740.
[2] Benkstein, K.D., Kopidakis, N., Lagemaat J.V. and Frank, A.J. (2003) Influence of the percolation network geometry on e
    lectron transport in dye-snsitized titanium dioxide solar cells, Journal Physical Chemistry B, 107, (31), pp. 7759–7767.
[3] M i c h a e l , G . ( 2 0 0 0 ) Perspectives for dye-sensitized nanocrystalline solar cells , Progress in Photovoltaics:
    Research and Applications, 8, (1), pp. 27-38.
[4] Patcharee, C., Junji, A. and Susumu, Y., In contribution.
[5] Gopal, K.M., Karthik, S., Maggie, P., Oomman, K.V. and Craig, A.G. (2006) Use of highly-ordered TiO2 nanotube arrays in dye-
    sensitized solar cells, Nano Letters, 6, (2), pp. 215- 218.
[6] Jinting, J., Fumin, W., Seiji, I. and Motonari, A. (2005) Highly Efficient Dye-sensitized solar cells based on single c
    rystalline TiO2 nanorod film, Chemistry Letters, 34, (11), pp.1506-1507.
[7] Matt, L., Lori, E.G., Justin, C.J., Richard, S. and Peidong, Y. (2005) Nanowire dye-sensitized solar cells, Nature, 4, pp. 455-459.




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