48
Chiang Mai J. Sci. 2010; 37(1) : 48-54 www.science.cmu.ac.th/journal-science/josci.html
Contributed Paper
Chiang Mai J. Sci. 2010; 37(1)
Niyom Hongsith and Supab Choopun*
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, and ThEP Center, CHE, Bangkok 10400, Thailand.
*Author for correspondence; e-mail: supab99@gmail.com
Received: 1 July 2009
Accepted: 29 July 2009
A BSTRACT
ZnO nanobelts layer of the dye-sensitized solar cell were prepared by RF sputtered
ZnO target onto a copper substrate and characterized by FE-SEM. The structures of solar cells based on ZnO as a photoelectrode, Eosin-Y as a dye sensitizer, iodine/iodide solution as an electrolyte and Pt/TCO as a counterelectrode. The photoelectrochemical characteristics of
ZnO DSSCs were tested under simulated sunlight AM 1.5 came from a solar simulator with the radiant power of 100 mW/cm 2 . It was found that DSSCs based on ZnO nanobelts can generate photocurrent with photoconversion efficiency of 0.6% (J
0.52, FF = 0.61).The higher J
SC
SC
= 2.11 mA cm -2 , V
OC
=
in ZnO nanobelt DSSC sample indicates that larger amount of dye adsorbed on surface of ZnO nanobelts than that of ZnO powder. The J
SC
increases with increasing a thickness of the ZnO photoelectrode for both powder and nanobelt. Moreover, the V
OC
of ZnO DSSCs is independent on morphology and dye adsorption surface area of
ZnO. The obtained photoelectrochemical results can be explained by using energy band diagram.
Keyword: ZnO, nanobelts, sputtering, dye-sensitized solar cell.
1. I NTRODUCTION
Dye sensitization of metal-oxide wideband-gap semiconductors is fast growing field for application of solar cell. Typically, the dye sensitized solar cells (DSSCs) include five major components: (1) Transparent conductive mechanical support, (2) wide-band-gap semiconducting photoelectrode (3) dye sensitizer, (4) redox couple electrolyte, and
(5) Pt counterelectrode. The significant advancement has made by the work of
Gr tzel group [1, 2]. They have employed titanium dioxide nanocrystalline particles as photoelectrode, and ruthenium bipyridyl complexes [Ru(4,4 / -dicarboxylic acid-2-2 / bipyridine)
2
(NCS)
2
or Ru(L)
2
(NCS)
2
] as dye sensitizer. They have obtained the promising results with photoconversion efficiency up to
10.8% as reported several years ago [3].
Nowadays, the DSSC based on TiO
2 investigated.
is widely
ZnO is one promising metal oxide semiconductor that could be used as photoelectrode in DSSC. This is due to its band gap, electron affinity, and electron injection efficiency which are nearly the same as TiO
2
. Recently, the research work on ZnO as an alternative photoelectrode has been intensively performed.
However, the photoconversion efficiencies of

Chiang Mai J. Sci. 2010; 37(1) 49
DSSCs based on ZnO are relatively low. One of the key parameters for the high efficiency of ZnO DSSC is the dye adsorption surface area of the film photoelectrode on conducting glass. It is expected that the larger the surface area the higher the efficiency. Thus, most of the works are recently focused on investigating
ZnO nanostructures such as nanoparticle, nanowires, nanorods which have the large surface-to-volume ratio [4-13].
In this work, we have investigated the effect of the dye adsorption surface area on the photoconversion performance of the
ZnO DSSC. The dye adsorption surface area parameter represented by the different morphology of ZnO photoelectrode which
ZnO powder and nanobelts are used in this investigation. Moreover, the effect of the thickness of ZnO photoelectrode on the photoconversion performance has also been investigated.
2. M ATERIALS AND METHODS
2.1 Preparation of ZnO Nanobelts by rf
Sputtering
From previous reports, ZnO nanobelts were prepared by rf sputtering technique on the copper substrate [14]. ZnO target was homemade by conventional solid-state methods from ZnO powder (Aldrich,
99.9%). For sputter conditions, the base pressure was lower than 10 -5 Torr, the deposition pressure was 40 mTorr under argon atmosphere and the deposition time was 60 min using an RF power of 300 W.
During the deposition of ZnO, the copper substrates had no intentional heating. After finish process, the white thick films were observed on the copper substrate and were characterized by Field Emission Scanning Electron Microscope (FE-SEM) for morphology.
The white ZnO products were separated from a copper substrate and dissolved in a polyethylene glycol (PEG) solution for ZnO nanobelt paste. To form ZnO powder paste,
ZnO powder (Aldrich, 99.9%) was also dissolved in PEG solution.
2.2 Solar Cell Construction and Testing
C
20
H
Two types of ZnO photoelectrode were used in this study: ZnO nanobelt and ZnO powder. A photoelectrode was fabricated by screen painting the ZnO paste on the conductive glass (TCO, F-doped SnO
2
, sheet resistance of 8 Ω / † ), followed by calcinations at 450 o C for 1 h with a temperature rate of 5 o C/min. The film area was 0.5 x 2 cm 2 and the thickness was 50 μ m and 25 μ m. The ZnO nanobelt and powder on conductive glass were soaked in Eosin-Y organic dye solution (0.04 g of Eosin Y,
6
Br
4
Na h. The dye-loaded ZnO as photoelectrode and the Pt counterelectrode (0.5 mM Hydrogen hexachloroplatinate (IV) Hydrate, Cl
6
H
2
Pt.aq, in acetone solution) were assembled into a sealed device using a hot-melted double layer parafilm (50 μ m thick/sheet). The redox electrolyte (0.3 M LiI + 0.03 M I
Polyethylene carbonate) was introduced into the inter-space between the photoelectrode and the counterelectrode through two predrilled holes on the side of the device. The
DSSC structure is F-doped SnO
2
Eosin-Y//I-/Iin Figure 1.
2
O
5
, in acetone 100 cm
//Pt//F-doped SnO
3 ) for 24
2
in
//ZnO//
2
as shown
The photoelectrochemical characteristics of ZnO DSSCs were tested under simulated sunlight AM 1.5 came from a solar simulator with the radiant power of 100 mW/cm 2 . The incident light intensity was calibrated with a standard Si solar cell. J-V characteristic were measured with a dc voltage and current source which interfaced and controlled by a computer. The short current density (J sc
), open circuit voltage (V oc
), fill factor (FF) and the overall photoconversion efficiency ( η ) were determined from the measured J-V curves.

50 Chiang Mai J. Sci. 2010; 37(1)
Figure 1.
Structure of ZnO DSSCs (F-doped SnO
2
//ZnO//Eosin-Y//I /I
3
//Pt//F-doped
SnO
2
).
3. R ESULTS AND DISCUSSION
3.1 Morphology of ZnO Photoelectrode
The morphology of the ZnO powder and ZnO nanobelt were shown as FE-SEM images in Figure 2 (a) and (b), respectively.
ZnO powder showed particle structure with large diameter of 200-500 nm. The crosssectional size of the nanostructures is about
10–50 nm and the length is around several micrometers. The inset in Figure 2 (b) is a high magnification image of ZnO nanostructure and it can be seen that a belt-like structure of
ZnO can be observed. From our previous report [14], ZnO products on copper substrate nanostructure which grew along the <112 0> direction on the (0001) plane.
3.2 Photoelectrochemical Characteristics of ZnO DSSCs
Figure 3 shows J-V characteristic of ZnO
DSSCs with ZnO powder and ZnO nanobelts as photoelectrode and at the thickness of 25 and 50 μ m. All samples can generate
(a) (b)
Figure 2.
FE-SEM images of (a) ZnO powder and (b) ZnO nanobelts with magnification of 5,000.

Chiang Mai J. Sci. 2010; 37(1) 51 photocurrent in the order of mA under solar simulated sunlight as solar cell.
The photoelectrochemical parameters such as short current density (J sc voltage (V
), open circuit oc
), fill factor (FF) and the overall photoconversion efficiency (h) which determined from the measured J-V curves were summarized in Table 1. Clearly, ZnO nanobelt DSSC exhibits higher short current density (J
SC
= 2.11 mA cm -2 ) and finally, higher photoconversion efficiency ( η = 0.67%).
Usually, J sc
is directly related to an amount of dye-sensitizer adsorbed on the ZnO photoelectrode surface, and also a surface area of photoelectrode strongly depends on a size of ZnO. The photoelectrode based on a smaller size of ZnO are expected to exhibit higher surface area than that of based on a larger size. From FE-SEM results, it can be seen that ZnO nanobelt (10-50 nm) has smaller
Figure 3.
J-V characteristic of ZnO DSSCs with ZnO powder and ZnO nanobelts as photoelectrode and at the thickness of 25 and 50 μ m.
Table 1. Summary of the photoelectrochemical parameters such as short current density (J sc open circuit voltage (V oc
),
), fill factor (FF) and the overall photoconversion efficiency ( η ) of
ZnO DSSCs.
DSSCs
Powder 25 μ m
Powder 50 μ m
Nanobelts 25 μ m
Nanobelts 50 μ m
Nanotip array [8]
J
SC
(mA/cm2)
0.97
1.34
1.70
2.11
2.04
V
OC
(V)
0.48
0.48
0.52
0.52
0.66
FF
0.58
0.55
0.61
0.61
0.41
ηηηηη (%)
0.27
0.35
0.54
0.67
0.55

52 Chiang Mai J. Sci. 2010; 37(1) size than that of ZnO power (200-500 nm) and then, ZnO nanobelt has a larger available surface area for dye adsorption than ZnO powder. Therefore, the higher short circuit current density in ZnO nanobelt DSSC samples indicates that larger amount of dye adsorbed on surface of ZnO nanobelt than that of ZnO powder. In addition, J
SC increases with increasing a thickness of the
ZnO photoelectrode for both powder and nanobelt. The J
SC
of ZnO nanobelt DSSC increased from 1.70 to 2.11mA/cm 2 and the
J
SC
ZnO powder DSSC increased from 0.97
to 1.34mA/cm 2 . The increase of J
SC
could be also explained in terms of larger available dye adsorption surface area.
For comparison, the photoelectrochemical results of ZnO DSSC with difference morphology are also shown in Table 1. A.D.
Pasquier and coworker [8], have fabricated
DSSC using well-aligned ZnO nanotip arrays and found that the best performance of their solar cells produced photoconversion efficiency of 0.55%. They have suggested that conversion efficiency of DSSC can be improved by increase nanotip length.
A charge transfer processes of the Eosin-
Y sensitized solar cell are illustrated in Figure
4. When the Eosin-Y sensitizer absorbs a photon and transform to excited state, an electron filled in the lowest unoccupied molecular orbital (LUMO) of Eosin-Y and the electron injects to the conduction band of ZnO as in equation
(1) where D * , and D + are excited dye, and oxidized dye, respectively. After that, injected electron diffuse through FTO and flow through the load via the external circuit and then reach the
Pt counterelectrode. At Pt counterelectrode,
Figure 4.
Schematic energy diagram for ZnO/Eosin Y sensitized solar cell.

Chiang Mai J. Sci. 2010; 37(1) 53 the oxidized redox species ( reduced back to the R through accepting electron. R + is generated from the chemical equation where D , R , and R + is an original dye, redox species, and oxidized redox species, respectively. This equation is usually called dye regeneration process. The oxidized dye is quickly reduced back to its original state by reduce redox species ( R ) in the electrolyte for a complete cycle of electron transfer.
Normally, the maximum open circuit voltage of ZnO DSSC depends on the difference of the energy level between redox potential of electrolyte and Fermi-level of
ZnO as shown in Figure 4. Thus, the V
OC
of
ZnO DSSCs is independent with morphology and dye adsorption surface area of ZnO. This is in agreement with our results that V
OC
of
ZnO DSSC shows close value for both ZnO powder and nanobelt. Moreover, the fill factor also shows close value for both ZnO powder and nanobelt DSSCs.
4. C ONCLUSIONS
R + ) is subsequently
(2)
We presented the effect of the dye adsorption surface area on the photoconversion performance of the ZnO DSSC. It was found that DSSCs based on ZnO nanobelts can generate photocurrent with photoconversion efficiency of 0.6% (J
SC
= 2.11 mA cm
= 0.52, FF = 0.61).The higher J
SC
-2 , V
OC
in ZnO nanobelt DSSC sample indicates that larger amount of dye adsorbed on surface of ZnO nanobelt than that of ZnO powder. The J
SC increases with increasing a thickness of the
ZnO photoelectrode for both powder and nanobelt. Moreover, the V
OC
of ZnO DSSCs is independent on morphology and dye adsorption surface area of ZnO. The obtained photoelectrochemical results can be explained by using energy band diagram.
A CKNOWLEDGEMENTS
We are grateful to the Thailand Research
Fund (TRF) for supporting this work. This work was partially supported by the National
Nanotechnology Center (NANOTEC),
NSTDA, Ministry of Science and Technology,
Thailand, through its program of Center of
Excellence Network. We would like to acknowledge the Institute of Solar Energy
Technology Development (SOLARTEC) and
National Nanotechnology Center (NANOTEC) for supplying materials and measurement facilities. Niyom Hongsith would like to acknowledge the financial support via the
DPST scholarship and the Graduate School,
Chiang Mai University.
R EFERENCES
[1] O’Regan B. and Gr tzel M., A Low Cost,
High-Efficiency Solar Cell based on
Dye-Sensitized Colloidal TiO
Nature , 1991; 353 : 737-740.
2
Films,
[2] Hagfeldt A. and Gr tzel M., Lightinduced Redox Reactions in Nanocrystalline Systems, Chem. Rev.
, 1995; 95:
49-68.
[3] Chiba Y., Islam A., Komiya R., Koide
N. and Han L., Conversion Efficiency of 10.8% by a Dye-Sensitized Solar Cell using a TiO
2
Electrode with High Haze,
App. Phys. Lett.
, 2006; 88: 223505.
[4] Keis K., Bauer C., Boschloo G., Hagfeldt
A., Westermark K., Rensmo H. and
Siegbahn H., Nanostructured ZnO
Electrodes for Dye-Sensitized Solar Cell
Applications, J. Photochem. Photobiol. A ,
2002; 148: 57-64.
[5] Keis K., Magnusson E., Lindstr m H.,
Lindquist S. and Hagfeldt A., A 5%
Efficient Photoelectrochemical Solar Cell based on Nanostructured ZnO
Electrodes, Sol. Energy Mater. Sol. Cells,
2002; 73: 51-58.

54 Chiang Mai J. Sci. 2010; 37(1)
[6] Baxter J.B. and Aydil E.S., Dye-Sensitized
Solar Cells based on Semiconductor
Morphologies with ZnO Nanowires, Sol.
Energy Mater. Sol. Cells , 2006; 90: 607-622.
[7] Chen Z., Tang Y., Zhang L. and Luo L.,
Electrodeposited Nanoporous ZnO
Films Exhibiting Enhanced Performance in Dye-Sensitized Solar Cells, Electrochimica Acta ,2006; 51: 5870-5875.
[8] Pasquier A.D., Chen H. and Lu Y., Dye
Sensitized Solar Cells using Well-Aligned
Zinc Oxide Nanotip Arrays, App. Phys.
Lett.
, 2006; 89 : 253513.
[9] Baxter J.B. and Aydil E.S., Nanowire-
Based Dye-Sensitized Solar Cells, App.
Phys. Lett., 2005; 86 : 053114.
[10] Kashyout A.B., Soliman M., El Gamal
M. and Fathy M., Preparation and
Characterization of Nano Particles ZnO
Films for Dye-Sensitized Solar Cells,
Mater. Chem. and Phys.
, 2005; 90 : 230-233.
[11] Lee W.J., Suzuki A., Imaeda K., Okada
H., Wakahara A. and Yoshida A.,
Fabrication and Characterization of
Eosin-Y-Sensitized ZnO Solar Cell,
Japanese J. Appl. Phys ., 2004 ; 43: 152-155..
[12] Wang Z S, Sayama K. and Sugihara H.,
Efficient Eosin Y Dye-Sensitized Solar
Cell Containing Br-/Br 3 Electrolyte, J.
Phys. Chem. B , 2005; 109 : 22449-22455.
[13] Choopun S., Tubtimtae A., Santhaveesuk
T., Nilphai S., Wongrat E. and Hongsith
N., Zinc oxide nanostructures for applications as ethanol sensors and dyesensitized solar cells, Appl. Surf. Sci.
, 2009; doi:10.1016/j.apsusc.2009.05.139.
[14] Choopun S., Hongsith N., Tanunchai S.,
Chairuangsri T., Mangkorntong P. and
Mangkorntong N., Single-Crystalline
ZnO Nanobelts by RF Sputtering, J. Cryst.
Growth , 2005; 282 : 365-369.