Application of carbon materials as counter electrodes of dye-sensitized solar cells

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Abstract

Hard carbon spherule (HCS) has been investigated as a counter electrode for dye-sensitized solar cells. The overall conversion efficiency of the cell reached 5.7%, which is comparable to 6.5% of the counter electrode of platinum-sputtered fluorine-doped tin oxide used in Grätzel-type solar cells under the same experimental condition. It is found that the photovoltaic performance was strongly affected by the specific surface areas of the carbon materials.

Introduction

Dye-sensitized solar cells (DSSCs) have attracted much attention due to its moderate light-to-electricity conversion efficiency (10.4%), easy fabrication and low cost [1], [2]. Counter electrode serves to transfer electrons from external circuit to triiodide and iodine in the redox electrolyte. Currently, a layer of platinum coated on transparent conducting oxide (TCO) substrate is widely used as counter electrode in DSSCs. This is not economy way for mass production. Carbonaceous materials are quite attractive to replace platinum due to their high electronic conductivity, corrosion resistance towards I2, high reactivity for triiodide reduction and low cost [3], [4], [5]. Under simulated sunlight (AM 1.5 at 100 mW cm−2), graphite, carbon nanotubes and activated carbon showed conversion efficiency of 6.67% (active area 0.40 cm2), 4.5% (0.25 cm2) and 3.9% (0.05 cm2), respectively [6], [7], [8], [9], [10]. It seems that high crystallinity of carbon is favorable. In this letter, a disordered carbon material (named hard carbon spherule, HCS) with different specific surface area was tested as counter electrodes for DSSCs [11], [12], [13]. It shows a conversion efficiency of 5.7%. In this case, the efficiency is depending on the surface area.

Section snippets

Carbon counter electrode preparation

HCS was prepared following the method reported in the literature [13]. In brief, aqueous sugar solution of 1.5 M was filled in a stainless steel autoclave with a filling ratio of 90%. After 5 h hydrothermal treatment at 190 °C, the obtained black powder was further carbonized at 1000 °C in a tube furnace in argon atmosphere, the obtained sample is named as HCS-1. HCS-1 was activated by heating at 900 °C under water vapor flow for 5 h to give HCS-2 with higher the surface area. Both HCS-1 and HCS-2

Redox behaviors

Fig. 2 compares cyclic voltammograms of I2/I system for the HCS-1, HCS-2, graphite and Pt plate electrodes at [I]/[I2] = 9/1, the same electrolyte of the DSSCs. Two pairs of redox peaks were observed in all cases. The relative negative pair is assigned to the redox reaction (1) and the positive one is assigned to redox reaction (2) [15].I3+2e=3I3I2+2e=2I3

It can be observed that HCS-2 shows high redox reactivity towards I3 and I2, even higher than Pt. Fig. 3 illustrates the dependence of

Conclusion

In summary, the disordered microporous hard carbon spherules showed high electrocatalytic activity in iodide/triiodide redox reaction, and thus is suitable for fabrication of counter electrode on dye-sensitized solar cells.

Acknowledgements

We express our thanks to Professor K. Tennakone for his help discussion and suggestions to this work. We gratefully acknowledge the support of the National Natural Science Foundation of China (Contract No. 20543003, Contract No. 20673141), the National 973 program of China (2006CB202606), the “100-talent” project of the Chinese Academy of Sciences and the Young Scholar Research Foundation of Jilin University, China (No. 2003QN017).

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