2012 12th IEEE International Conference on Nanotechnology (IEEE-NANO)

The International Conference Centre Birmingham

20-23 August 20112, Birmingham, United Kingdom

Dye-sensitized solar cells using graph ene-based counter electrode

Kai-Hsiang Hung, Yi-Sheng Li, Hong-Wen Wang*

Department of Chemistry, Center for Nanotechnology, Chung-Yuan Christian University,

Chungli, 320, Taiwan, R.O.C.

*Corresponding author: hongwen@cycu.edu.tw, Tel: 886-3-2653310, Fax: 886-3-2653399

Abstract Graphene film was fabricated using graphene oxide (GO) prepared from a modified Hummers'

method. The reduction of GO (rGO) was accomplished by coating the GO paste on a piece of

glass and immersing into a Ti+3 solution. The freeze drying (FD) process employed for drying

GO before its forming of paste and coating into a film exhibits a profound effect on the

quality of final graphene counter electrode. The graphene electrode obtained using freeze

drying process exhibits a porous structure and is an excellent replacement for the Pt counter

electrode of dye-sensitized solar cells (DSSCs). The conversion efficiency for the

FD-rGO-based DSSC reaches 6.21 %, which is much higher than that of the normal Pt-based

DSSC 5.62% using standard P25 materials. The much more reaction sites on the porous

FD-rGO-film electrode than the smooth Pt electrode are considered to be the major reason for

this high performance.

Keywords: Counter electrode, Graphene, DSSC

I. Introduction Graphene is a two-dimensional single layer of carbon atoms materials with the

hexagonal packed structure [1,2]. It was successfully synthesized and isolated using

mechanical exfoliation of graphite by Geim et al. in 2004 [3]. Currently, there are many ways

to synthesize graphene materials such as chemical vapor deposition (CVD) on Ni or Cu

surface [4,5], high temperature annealing of SiC [6] and oxidation and exfoliation of graphite

oxide, followed by the chemical reduction [7]. Among these methods, the oxidation and

reduction method becomes one of the most developed methods in the literature. Many

applications using graphene as an electrode for devices or additives for composites have been

reported [8-11].

The platinum (Pt) counter electrode in dye-sensitized solar cells acts two roles, which are

collecting electrons from external circuit and catalyze the reduction process of iodide liquid

electrolyte to ensure the continuity of electron transfer [12]. The Pt metal exhibits not only

high catalytic activity but also highly resistant to the corrosive liquid electrolyte [12-15].

However, Pt is an expensive and precious metal and the alternative materials for Pt are

required. Graphite [16], amorphous carbon (carbon black, nano- or micro-carbon [17-19],

carbon nanotubes (single layer, double layers and multilayers) [20-22] have been utilized for

this purpose. Graphene has been considered an ideal candidate for the counter electrode due to

their high conductivity, high catalytic activity and high surface area after reduction at high

temperature. Hong et al. [23] reported that the efficiency of 4.5% for solar cell using

graphene/(PEDOT:PSS) as the counter electrode, about 70% of the Pt-based solar cells (6.3%).

Joseph et al. [24] studied the functionalized grapheme thin film/PEO for counter electrode and

achieved 5% efficiency, 90% of 5.5% Pt-based counter electrode.

In the present study, the graphene materials were synthesized using a modified Hummers'

chemical oxidation and a Ti(III) reduction process [25-26], which were subsequently

employed as the counter electrode for DSSCs. Freeze drying process on GO after oxidation

shows that the efficiency of graphene-based DSSCs exceeds those of Pt-based DSSCs.

II. Experimental Procedure

Synthesis of GO and it's paste:

A modified Hummers' method was employed to synthesize the graphene oxide (GO). 1 g

graphite flask (Alfa, 7�10 !lm, purity :2:99%), 0.75 g Na2S04 (MERCK) and 30 ml

concentrated H2S04 (95-97%) were mixed and ice cooled for 30 min. 2.25g KMn04 (99%)

was added gradually into this ice cold solution and stirred for 2 h. After stirring at room

temperature for 48 h, 300 ml de-ionized water was added and stirred for additional 30 min.

Then, 30 ml H202 (35%) were added and stirred for another 1 h. The residuals were collected

using centrifugation and re-washed using de-ionized water twice and ethanol once. Two GO

pastes were made. One GO was freeze dried and the other was not. Both GO residuals were

then redispersed in ethanol (2: 1) and stirred into a paste. Those GO pastes were then

employed to form the coatings on a piece of glass.

Preparation of the electrodes

The GO films were formed by coating the GO pastes on a piece of glass (Green cross,

26x76mm, 1.0-1.2mm thick) manually using a glass rod. A polyimide (PI) double-side sticker

was attached on the glass. The GO films were actually formed on the PI materials. These

GO/PI/glass assembly electrode was then dried at 80°C before the chemical reduction process.

The dried GO/PI/glass electrodes were then immersed into a Ti (III) solution at 120°C for 30

min to reduce the GO [28], which was a modification from ref [26-27]. The electrode without

freeze drying process was assigned as rGO-film and the one with freeze drying was assigned

as FD-rGO-film.

Preparation of the DSSC and its characterization

The rGO-film and FD-rGO-film were used as the counter electrodes to replace the Pt

electrode in DSSCs, as shown in Fig. 1. Standard Ti02 photoelectrodes were fabricated using

a 15 nm thick anatase scattering layer and 10 Ilm thick anatase P25 absorbing layer. An

active area of 0.7 cm x 0.7 cm was selected from the Ti02 electrodes and immersed in a

5xl0-3M solution of the dye [RuL2(NCS)2]TBA2, where TBA is tetra-n-butylammonium

(N719 dye, Everlight Chemical, Taiwan) for overnight. The acting area was covered by the

graphene electrode or N-doped graphene electrode. Pt-based counter electrode was also

prepared for comparison. The iodide/triiodide (r/I3 -) electrolyte (Tripod-Technology

E-l_MPN) IIlL was dropped and absorbed into the clamped DSSC assembly. The

photovoltaic characteristics of DSSC devices were measured from an illuminated area of 0.5

cm x 0.5 cm by an electrochemical analyzer (CHI C611B, CH Instruments Co., U.S.A) under

a standard AM 1.5 sunlight illumination (XES-ISIS, San-Ei, Japan) with 100 mW/cm2

light

source. The electrical impedance spectra were also measured in the range of 0.01 Hz to 100

kHz using the same equipment and setup. Surface morphology and thickness of the films

were measured by field emission scanning electron microscope (FESEM, JEOL 7600F). The

incident photon-to-current conversion efficiency (IPCE) was measured within the range of

350-800 nm using an IPCE system (Enlitech QE-MINI, Taiwan, R.O.C.) that was specifically

designed for DSSCs.

III. Results and Discussion

Characterization of the graphene electrode

Fig.l shows the FESEM for rGO-film and FD-rGO-film. The morphology of rGO-film

electrode was quite smooth and without pores and voids. Freeze drying rendered the

FD-rGO-film a very porous electrode. Despite the distinguished difference of morphologies

between rGO-film and FD-rGO-film, both film electrode exhibited very similar surface

conductivities, being around 100/sq. Fig. 2 shows the TEM of graphene film obtained after

the reduction of rGO residuals using Ti(III) solution. The TEM image demonstrated that the

graphene film had nanosheet morphology with some corrugations and wrinkles. This is the

feature structure of graphene nanosheets. The corresponding SAED pattern demonstrated the

typical graphene six-fold-symmetry diffraction pattern.

The performance of DSSCs

The current density (J)-voltage (V) curves for the studied DSSCs were shown in Fig. 3.

It demonstrated that FD-rGO-film electrode exhibited the highest performance among three

studied DSSCs, being 6.21 %. FD-rGO-film even has 20% higher surface resistance than

that of rGO-film electrode. The freeze dry provided a porous graphene electrode for DSSC,

which the Voc was exceptional higher than that of normal Pt counter electrode. The EIS

shown in Fig. 4 exhibited three semi-circles. The first semi-circle, Rct!, was an indication of

electron transition resistance in the interface of counter-electrode/electrolyte. The second

semi-circle, Ret2, was the electron transition resistance between Ti02/Dye/electrolyte

interfaces. The third semi-circle was the Nernst diffusion of electrolyte. From Fig. 4, it was

clear that the Retl of Pt counter electrode was around 10Q/sq, and it' s Rct2 was 45Q/sq.

However, when the rGO-film or FD-rGO-film were used as the counter electrode, their Retl

were vanished, while their Ret2 increased to 91Q/sq and 77 Q/sq, respectively. We would like

to discuss this after the presence of Bode plot.

Electron transfer process on graphene

As shown in Fig. 5, the Bode plot is an indicator of the electron transfer frequency.

X-axis represents the electron transfer in different interface. The high frequency band

(�10000Hz) is the electron transfer frequency for the Pt counter-electrode/electrolyte interface.

The low frequency band (1 O� 100 Hz) is the electron transfer frequency for

Ti02/dye/electrolyte interface. From Bode plot, it is clear that the high frequency band is

disappeared when the graphene counter electrode is employed. Instead, a medium frequency

band (1 OO� 1000 Hz) emerged for the graphene counter electrode. This medium frequency

band was likely due to the electron transfer in the electrolyte/graphene interface whose

frequency was very close to those of Ti02/dye/electrolyte and merged with the low frequency

band. When EIS diagram for graphene-based electrode was illustrated, the resistance of

Ti02/dye/electrolyte and counter-electrode/electrolyte overlapped. Therefore, the Ret2 was

largely increased but Ret! disappeared. It implied that the catalytic power of graphene counter

electrode was lower than that of Pt counter electrode. Similar conclusions were also reported

by Wu et al. [27] in comparison of the Pt counter electrode to the thermal reduced graphene

electrode. The electron transfer frequency for the graphene electrode/electrolyte interface was

much lower than that of Pt electrode/electrolyte. The reason why the performance of DSSC

based on FD-rGO-film electrode higher than that of Pt counter electrode could be explained

as the mechanism shown in Fig. 6. The Pt counter electrode was fabricated by sputtering. The

thickness of Pt counter electrode was estimated to be a few tens nanometer and was in FCC

crystal structure. The stacking of Pt atoms was dense and the penetration and diffusion of

liquid iodide couple was limited. The catalytic reduction reaction can only take place on the

surface of Pt electrode. However, the frequency of this electron transfer process was rapid. i.e.

The interface resistance (Ret!) of this electron transfer was low. On the contrary, the porous

FD-rGO-film electrode provides many catalytic reduction sites for the electron transfer,

though slow, their contribution to the current in total exceeds the current of Pt electrode. The

easy penetration, diffusion and huge reduction sites in FD-rGO-film electrode makes this

electrode a higher performance than that of Pt. The dense rGO-film electrode did not provide

such huge reduction sites for electron transfer due to smooth and dense morphology; even its

surface conductivity was better than that of FD-rGO-film electrode.

IV. Conclusion A facile process has been employed to synthesize the graphene films, which were

subsequently used for the counter electrodes of DSSCs. The obtained graphene films

exhibited good conductivity and adhesion on polyimide double-side tape on a glass substrate.

After freeze drying, the graphene films acts an excellent counter electrode for DSSCs, even

much better than conventional Pt-counter electrode. It is considered that the porous structure

and the deep penetration of electrolyte in freeze-dried rGO film electrode give many more

reduction sites for 13- than those of conventional Pt electrode. The graphene film electrode

was an excellent alternative material for replacing Pt counter electrode in DSSC devices.

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Fig. 1 FESEM of (a) (b) rGO-film, and (c) (d) FD-rGO-film

200nm Fig. 2 TEM for graphene powder obtained after reducing GO in Ti(III) solution.

12 r-----------------__ _

10

8

6

4

--Blank 2

- rGO-film(100/sq)

o - FD-rGO-film(120/sq)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Potential (Voltage)

ID Voc(mV) Blank 781

rGO-film 751

FD-rGO-film 836

11.16

9.71

11.88

11(%) 5.62

1.61

6.21

FF Resistance(!!/sq)

0.64 7.2

0.23 10

0.63 12

Fig. 3 J-V curve and performance data for DSSCs based on the counter electrodes of Pt,

rGO-film and FD-rGO-film, respectively.

35 �------------�----------------------� 30

25

� 20 ] � 15 N

I

10

5

• Blank - rGO-film - - -- - -... FD-rGO-film) - .........

, '" ... -I. ...

. ,.. ... . . . ... . . ...

- ---• • • . .-•

• ... -

•

.. • j

•

"

... � ... .... --V"-

, O+-�������������������� o 10 20 30 40 50 60 70 80 90 100 110 120 130

Z' (ohm)

Fig. 4 EIS for the DSSCs based on Pt , rGO-film and FD-rGO-film counter electrode.

• Blank

30 • rGO-film • FD-rGO-film)

O+-���������������--��� 1 10 100 1000 10000 100000

Frequence (Hz)

Fig. 5 Blode plot for the DSSCs based on Pt, rGO-film and FD-rGO-film counter electrode.

• • • • • • • • • • • •

•

• • • • • • • • • • • • • · ... . . � .:s ...

· · r ..... ... • • .ae • • • • • • • • • • •

:: ..... a.1 •••• :.:. '.1 •• •••• :.:. '.1 •

• •• • •• • • • • • • • •

Fig. 6 Proposed reaction mechanism for the electron transfer on the Pt counter electrode (left)

and that of FD-rGO-film counter electrode (right).

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