24P002-O
The Characteristic Analysis of the Sputter-deposited Pt Thin film for theDye-sensitized solar Cell
色素増感太陽電池のスパッタ蒸着による白金薄膜の特性分析
Hyunwoong Seo1,2, Min-Kyu Son2, Jin-Kyoung Kim2 and Hee-Je Kim2
徐 鉉雄, 孫 珉圭，金 珍暻，金 熙濟
1Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka, 819-0395, Japan
Department of Electrical Engineering, Pusan National University, Geumjeong, Busan, 609-735, Republic of Korea
1
九州大学 システム情報科学府 〒819-0395 福岡市 西区 元岡 744
2
釜山大学校 電子電気工学科 〒609-735 釜山市 金井區 長箭二洞 山30

2Dye-sensitized solar cells consist of the photo electrode of TiO 2/dye layer, the platinized counter electrodeand the redox electrolyte. After light absorption, the electrons from excited dye are transferred to theexternal load through the TiO2 network. The oxidized dyes are recovered by the electron injection from anelectrolyte. In the operation, Pt catalyst plays an important role in the smooth electron supply to electrolyte
for a high performance. In this work, the Pt thin films by various fabrication methods were characterized.
I-V data, electrochemical impedance, and cyclic voltammetry were analyzed for the verification.

1. Introduction
Dye-sensitized solar cells (DSCs) have been
considered as a possible alternative to current
silicon based photovoltaic devices because of
their advantages of high efficiency, simple
fabrication process and low production cost [1,2].
Typical DSCs consist of a dye-sensitized
nano-porous TiO2 film interpenetrated by a redox
electrolyte. After light absorption, electrons fromexcited dye molecules are transferred to theconduction band of TiO 2, and diffuse through theTiO2 network to the contact electrode. The
oxidized dye molecules are recovered by theelectron supply from electrolyte. Especially, thecounter electrode, as one important component inDSCs, are usually constructed of conductive glass
substrates coated with Pt films due to its superior
catalytic activity for I-/I3- redox couple. Pt counterelectrode is fabricated by various methods such assputter, electroplating, thermal depositions and soon [3-5]. In this work, the property of Pt thinfilms according to the fabrication methods wasinvestigated. In order to verify each property, theeffect in terms of the photovoltaic performance,
the electrochemical impedance and the cyclic
voltammetry was examined.
2. Experimental
DSCs used in this work were fabricated asfollows. FTO substrates were used as thetransparent conductive oxide (TCO) to make thephoto and counter electrodes. The substrates werecleaned by sonicating in ethyl alcohol. And anano-porous TiO2 with a thickness of about 50 μm

was pasted onto the FTO substrate. The film wassintered at 450°C for 30min and the sintered photoelectrode was soaked in a 0.2mM N719 dye
solution for 24h. For the counter electrode, Pt thinfilms were deposited by three methods; sputtering,solution, and paste depositions. For the sputter
deposition, Pt thin film was deposited by radio
frequency sputtering at a power of 150W and aworking pressure of 2.4×10-3 Torr. For the solution
deposition, 10mM H2PtCl6 solution in theisopropanol was spin-coated on the FTO substratewith the rotating speed of 3,000 rpm and sintered at450oC for 30 min. For the paste deposition, Pt
precursor paste was deposited by the doctor blade
method and sintered at 400oC for 30 min. Depositedfilms were morphologically characterized by thefield emission scanning electron microscopy(FE-SEM). After that, the photo and counterelectrodes were sealed and completed by injecting aredox electrolyte.
The photovoltaic performance was measured
under 1 sun. The irradiated cell area was 0.25cm2.
The I-V data were used to calculate the short-circuitcurrent density (JSC), open-circuit voltage (VOC), fill
factor (FF) and the overall efficiency (η). The
electrochemical impedance spectroscope (EIS) andthe cyclic voltammogram (CV) were analyzed bythe electrochemical analysis instrument. In order tocharacterize Pt counter electrode, CV was operated
at the scan rate of 10mV/s and the potential region
from -1V to 1V. EIS spectra were measured over afrequency range from 10mHz to 1MHz. The
applied bias voltage and AC amplitude are set at
VOC of DSC and 10mV, respectively.

3. Results and discussion

Fig.1. FE-SEM images of Pt thin films deposited by (a)sputter, (b) solution, (c) paste deposition

Figure 1 shows FE-SEM images of Pt thin films
by various deposition methods. The thickest film
was coated by doctor blade method. Therefore,
FTO structure was not shown unlike other twofilms. However, different thickness was notaffected because the performance of DSC didn't
dependent on the thickness of Pt thin film [3]. And
the sputter deposition made the roughest film andthe lowest transmittance while other two films weretransparent.

Fig.2. Cyclic voltammetry of Pt films

Figure 2 shows CV of different Pt films. In theCV, the peak values of negative and positive pairsmean the activity of I3-+2e-→3I- and 3I-→I3-+2ereactions, respectively. That is, the catalytic
reaction was the most active in the case of thesputter deposition and the solution deposition had
the worst catalytic reaction.

Fig.3. Electrochemical impedances according to Pt films

And the charge transportation at the counterelectrode was compared by EIS analysis. Figure 3
shows a Nyquist diagram of EIS. Two semicircles
are attributed to the charge transportation at thecounter electrode and the electron transfer at theTiO2/dye/electrolyte interface in the high andmiddle frequency ranges, respectively [6]. In figure
3, the second semicircles were similar each otherbecause the experimental condition was identical
except for the fabrication of Pt thin film. However,the first semicircles related to the chargetransportation at the counter electrode were muchdifferent. The results were almost same with theabove-mentioned CV analysis. The chargetransportation was the best on the sputtered Pt film
and the worst on the solution coated Pt film.
The sputtered Pt thin film had the most active
catalytic reaction and the best charge transportation
based on the CV and EIS. Finally, the photovoltaic
performance was measured. Table I shows theirperformance. As follow above results, DSC withthe sputtered Pt film had the best performance
while DSC with the solution deposited film had theworst performance. That is, their different catalytic
activity made different performance. And theirdifferent catalytic activity came from the roughnessof surface because the rough film has wider surface
for the active catalytic reaction.
Table I. The performance of DSCs with different Pt films

VOC(V) JSC(mA/cm2)

FF

η(%)

Sputtering

0.7312.96

0.65

5.47

Pt Solution

0.73

11.58

0.43

3.65

Pt Paste

0.73

12.99

0.50

4.72

4. Conclusion
In this work, the different platinized counterelectrodes were fabricated by various deposition
methods such as sputter, solution, and paste
deposition. The CV and EIS analysis were verified
their catalytic activities and charge transportation at
the counter electrode, respectively. As a result, thesputtered Pt film had the most active catalytic
reaction and the best charge transportation. And thegood catalytic activity and charge transportation
made the photovoltaic performance enhanced.
Consequently, the performance of DSC withsputtered Pt counter electrode was the best based on
above-mentioned properties.
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