Carbon Nanostructures as Low Cost Counter Electrode for Dye-Sensitized Solar Cells

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In the last two decades, dye sensitized solar cells (DSSCs) have gained extensive attention as a low cost alternative to conventional Si solar cells (Oregan & Gratzel 1991; Fan et al. 2008; Xie et al. 2009; Alibabaei et al. 2010; Gajjela et al. 2010; Xie et al. 2010; Yum et al. 2010). A typical DSSC is made of a TiO2 photoanode and a Pt counter electrode separated by an electrolyte comprising an iodide/triiodide (I/I3) redox couple. The photoanode is usually prepared from TiO2 nanoparticles on a transparent conducting oxide (TCO), while the counter electrode is a thin layer of Pt deposited on another TCO substrate. The dye molecules are adsorbed onto TiO2 surface. When exposed to sunlight, photoelectrons are generated and injected into the photoanode. Afterward, the electrons travel to counter electrode through an outside load. The oxidized dye molecules then retake electrons from I ions and oxidize I into I3. Meanwhile, the I3 is reduced into I by taking electrons from counter electrode. Pt counter electrode has been extensively used as an efficient electrocatalyst for reduction of I3 ions in DSSCs (Gratzel 2003; Sun et al. 2010). However, Pt is an expensive metal and can also be corroded by I/I3 redox couple (Kay & Gratzel 1996). Recently, various carbonaceous materials including graphite, carbon black, and carbon nanotubes have been studied as a low cost replacement for Pt as an electrocatalyst for reduction of I3 ions (Kay & Gratzel 1996; Burnside et al. 2000; Imoto et al. 2003; Imoto et al. 2003; Suzuki et al. 2003; Murakami et al. 2006; Ramasamy et al. 2007; Fan et al. 2008; Hinsch et al. 2008; Joshi et al. 2009; Lee et al. 2009; Skupien et al. 2009; Calandra et al. 2010). The carbonaceous materials are plentiful, inexpensive, and also exhibit high resistivity to corrosion (Ramasamy et al. 2007). Replacement of Pt with carbon-based materials can also speed up DSSC commercialization (Burnside et al. 2000; Hinsch et al. 2008; Han et al. 2009; Skupien et al. 2009; Joshi et al. 2010). In this chapter, we review some carbon nanostructures including carbon nanoparticles and electrospun carbon nanofibers that have been successfully used as a low cost alternative to Pt in DSSCs. The carbon nanoparticleand carbon nanofiber-based DSSCs showed comparable performance as that of Pt-based devices in terms of short circuit current density (Jsc) and open circuit voltage (Voc). Electrochemical impedance spectroscopy (EIS) measurements indicated that the carbon nanoparticle and carbon nanofiber counter electrodes showed lower charge transfer resistance (Rct), suggesting that carbon nanoparticle and carbon nanofiber counter electrodes are an efficient electrocatalyst for DSSCs. In addition, the series resistance of carbon-based counter electrodes was found to be a little