Carbon black-LaCoO3 composite material as counter electrode for

quasi-solid-state dye-sensitized solar cell

Voranuch Somsongkul, Atchana Wongchaisuwat, Attera Worayingyong and Marisa Arunchaiyaa

Department of Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand

afscimsa@ku.ac.th

Keywords: Dye-sensitized solar cell; counter electrode; carbon black; LaCoO3.

Abstract. Dye-sensitized solar cell (DSSC) is considered as the next generation low cost photovoltaic

device, and one of the important components is the counter electrode which reduces the I3- ions

generated after electron injection from iodide into the oxidized sensitizer. In this work composite

material of carbon black and LaCoO3 synthesized by sol-gel method has been investigated to replace

the platinized counter electrode for quasi-solid-state dye-sensitized solar cells. From cyclic

voltammogram, it was found that carbon black catalyzed with LaCoO3 exhibited increased reduction

current compared with that of carbon black without LaCoO3. This was consistent with increased

surface roughness depicted from scanning electron microscopy (SEM) image. The electrochemical

impedance spectroscopy (EIS) of DSSCs using carbon black-LaCoO3 composite as counter electrode

revealed lower charge-transfer resistance (Rct) than that using carbon black. The highest conversion

efficiency of 5.40% was obtained from DSSC (active area 1 cm2) fabricated with quasi-solid

composite polymer electrolyte using carbon black-LaCoO3 (95:5 ratio by weight) compared to 5.23%

obtained from DSSC equipped with platinum counter electrode. The performances of the

quasi-solid-state DSSCs sensitized with N719 and natural dyes were examined and it was concluded

that the carbon black-LaCoO3 composite was compatible with platinum coated counter electrode for

DSSCs.

Introduction

Dye-sensitized solar cells (DSSCs) gained widespread attention as an alternative energy owing to low

cost materials, easy fabrication and high power conversion efficiency. These cells achieve 11% of

solar conversion efficiencies under AM 1.5 [1]. Counter electrode is an important component which

reduces the I3- ions generated after electron injection from iodide into the oxidized sensitizer.

Although platinum catalyst provides a good performance, the dissolution of the platinum film in the

corrosive electrolyte and the requirement of high-temperature heat treatment necessitate the

development of stable and cost-effective counter electrode materials. Carbon black has a very high

surface area and its electrical conductance provides catalytic activity for the reduction of I3- ions [2-6].

In this work, composite material of carbon black and LaCoO3 synthesized by sol-gel method [7] has

been investigated to replace the platinized counter electrode for quasi-solid-state DSSCs.

Experiment

Carbon black powder (particle size < 150 nm) was purchased from Thai Carbon Black Public Co.,Ltd.

The carbon black paste was prepared by mixing carbon black powder with polytetrafluoroethylene

homopolymer (PTFE) binder and was coated on FTO glass by doctor blading and then sintered at

400 ºC for 30 min to obtain a carbon black electrode. LaCoO3 was synthesized by sol-gel method

reported in the literature [7]. The carbon black-LaCoO3 counter electrode was prepared from LaCoO3

added carbon black paste. Platinum counter electrode was prepared by spin coating H2PtCl6 solution

on FTO glass and sintered at 400 ºC for 30 min. DSSC were fabricated using N719 (Dyesol) dye. The

active area of the cell was 1 cm2. The DSSC devices were tested under the illumination of solar

Materials Science Forum Vols. 663-665 (2011) pp 451-454Online available since 2010/Nov/11 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.663-665.451

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simulator sunlight from the Xenon arc lamp with a light intensity of 100 mW cm-2

. X-ray diffraction

(Philips X’Pert Diffractometer) and scanning electron microscopy (Philips:XL30&EDAX) were used

to characterize the structural properties and surface morphologies of counter electrodes, respectively.

Cyclic voltammetry (CV) was utilized to monitor electrochemical activity of counter electrodes and

the charge-transfer resistance (Rct) was evaluated from the electrochemical impedance spectroscopy

(Autolab PGSTAT 302).

Results and Discussion

The structural properties of counter electrodes were characterized with X-ray diffraction (XRD). The

XRD pattern of carbon black (Figure 1 (a)) showed broad peaks at 23° and 43°, indicating low

crystallinity of carbon black materials [6] and the XRD pattern of LaCoO3 (Figure 1 (e)) showed sharp

and intense peaks at 32.96°, 33.34° and 47.53°, indicating more crystallinity than carbon black [7],

whereas XRD patterns of carbon black-LaCoO3 with various ratio of 95:5, 90:10 and 80:20 by weight

(Figure 1 (b-d)) showed both characteristics which corresponded to carbon black-LaCoO3 ratios. The

surface morphologies of the platinum, carbon black, LaCoO3 and carbon black-LaCoO3 (95:5, 90:10

and 80:20 ratio by weight) layer on FTO glass substrates were shown in the SEM images illustrated in

Figure 2. The platinum film (Figure 2 (a)) showed scale-like structures with polyhedrons granules,

Figure 2 (b) showed a uniform and compact coverage of the carbon black with small spherical

particles, whereas conglomeration of the LaCoO3 particles was observed as shown in Figure 2 (c).

Comparison of SEM images of carbon black-LaCoO3 (Figure 2 (d-f)) with carbon black (Figure 2 (b))

showed that addition of LaCoO3 to carbon black paste provides a more porous film, and the 95:5

carbon black-LaCoO3 gave a more uniform porous film than those of other two ratios.

Figure 1. XRD patterns of counter electrodes: (a) carbon black, (b) carbon black-LaCoO3 (95:5

wt%), (c) carbon black-LaCoO3 (90:10 wt%), (d) carbon black-LaCoO3 (80:20 wt% ) and (e)

LaCoO3.

Figure 3 depicts cyclic voltammograms (CV) of I-/I3

- redox mediator on the platinum, carbon black

and carbon black-LaCoO3 (95:5, 90:10 and 80:20 ratio by weight) counter electrodes. Figure 3 (a)

exhibits an anodic peak at 0.66 V corresponding to oxidation of iodide to triiodide and a cathodic peak

at 0.20 V corresponding to reduction of triiodide to iodide on Pt electrode. The anodic currents were

decreased but the cathodic currents were increased on carbon black and carbon black-LaCoO3

electrodes on comparison with that of platinum electrode. Figure 3 (b) shows the CV of carbon

black-LaCoO3 of 95:5, 90:10 and 80:20 ratio by weight, it can be seen that the cathodic peaks shift to

more negative potentials as the amount of LaCoO3 was increased. The carbon black-LaCoO3 of

95:5 wt% exhibits increased cathodic current and the closest cathodic peak to that of Pt electrode. The

increased cathodic current can be interpreted in terms of increased active surface area of this film and

the less negative cathodic peak can be interpreted in terms of enhanced reduction of I3- at the carbon

black-LaCoO3 (95:5 wt%) working electrodes in comparison with platinum electrode. The results are

consistent with SEM images. The interfacial charge-transfer resistance (Rct) of DSSCs consisting of

various materials as a counter electrode at open-circuit voltage was further studied by electrical

impedance spectroscopy (EIS) and EIS spectra are shown in Figure 4. The smaller semi-circle can be

(a)

(b)

(c)

(d)

(e)

452 Optoelectronic Materials

observed in the case of DSSC assembled with carbon black-LaCoO3 composite as counter electrodes

compared with that of carbon black without LaCoO3. As the ratio of LaCoO3 was increased, the Rct of

DSSC was decreased; this can lead to an increase in photocurrent of DSSC.

Figure 2. Surface morphologies of counter electrodes with magnification of 50,000 times

(a) platinum, (b) carbon black, (c) LaCoO3, (d) carbon black-LaCoO3 (95:5 wt%), (e) carbon

black-LaCoO3 (90:10 wt%) and (f) carbon black-LaCoO3 (80:20 wt% ).

Figure 3. Cyclic voltammograms of (a) platinum, carbon black, and carbon black-LaCoO3

(95:5 wt%) electrodes and (b) Bare FTO, carbon black-LaCoO3 (95:5, 90:10 and 80:20 wt%) in

10 mM KI, 1 mM I2 in acetonitrile containing 0.1 M LiClO4 supporting electrolyte, scan rate of

50 mV/s.

Figure 4. Electrical impedance spectra of DSSCs with different counter electrode materials on FTO.

Table 1 showed the photovoltaic performance of DSSCs fabricated with quasi-solid composite

polymer electrolyte using platinum, carbon black, carbon black-LaCoO3 (95:5, 90:10 and 80:20 wt%)

counter electrodes. When carbon black without LaCoO3 was used as a counter electrode, the

short-circuit current density (Jsc) was decreased compared with that of platinum counter electrode

while the open-circuit voltage (Voc) was increased and the cell efficiency was 4.91%. When

composite material of carbon black-LaCoO3 was employed as a counter electrode, the Jsc was

increased compared with that of carbon black and is comparable with that of platinum counter

electrode. The highest conversion efficiency of 5.40% was obtained from DSSC using 95:5 carbon

black-LaCoO3 counter electrodes and was compatible with 5.23% obtained from DSSC using

platinum counter electrode. The performance of natural dye DSSCs equipped with 95:5 carbon

(a) (b)

(a) (b) (c)

(d) (e) (f)

Materials Science Forum Vols. 663-665 453

black-LaCoO3 counter electrodes were also shown to be comparable with that of platinum counter

electrode as depicted from Table 2. LaCoO3 plays a role in catalytic activity on carbon black-LaCoO3

counter electrode as depicted from increased cathodic current of I-/I3

- redox couple and a decrease in

charge transfer resistance of DSSC revealed by EIS. However the conversion efficiency of DSSCs

was decreased from 5.40% to 4.32% with increasing ratio of LaCoO3 from 95:5 to 80:20, this may be

due to uneven contact of the electrode as revealed by SEM image and the increased crystallinity of the

composite electrode material.

Table 1. Performance of N719 DSSCs equipped with various counter electrodes.

Counter electrode Jsc(mAcm-2

) Voc(mV) FF η (%)

Platinum 9.81 713 0.75 5.23 carbon black 8.52 735 0.78 4.91

carbon black-LaCoO3 (95:5) 9.98 711 0.76 5.40

carbon black-LaCoO3 (90:10) 8.86 728 0.75 4.84

carbon black-LaCoO3 (80:20) 8.90 710 0.68 4.32

Table 2. Performance of natural dye DSSCs equipped with two different counter electrodes.

Dyes Counter electrode Jsc Voc FF η (%)

Roselle Platinum 1.54 478 0.89 0.65

carbon black-LaCoO3(95:5) 1.13 498 0.97 0.55

Carminic

acid Platinum 1.63 409 0.55 0.37

carbon black-LaCoO3(95:5) 1.36 458 0.93 0.57

Conclusion

A low cost quasi-solid-state dye-sensitized solar cells equipped with carbon black-LaCoO3 composite

counter electrode achieved a conversion efficiency of 5.40% which is compatible to that achieved

from platinum counter electrode. This electrode shows low charge-transfer resistance compared to

that of carbon black counter electrode owing to its higher surface area and catalytic activity of

LaCoO3 which enhanced photocurrent of DSSCs. The optimum ratio for the highest conversion

efficiency was 95:5 carbon black-LaCoO3.

Acknowledgement

This work was financially supported by Thailand Graduate Institute of Science and Technology

(TGIST), Kasetsart University Research and Development Institute (KURDI), Graduate School and

Faculty of Science, Kasetsart University.

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Optoelectronic Materials 10.4028/www.scientific.net/MSF.663-665 Carbon Black-LaCoO3 Composite Material as Counter Electrode for Quasi-Solid-State Dye-Sensitized

Solar Cell 10.4028/www.scientific.net/MSF.663-665.451

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