A possible approach is to utilize transition-metal compounds with similar properties to those of noble metals (Shi et al., 2004). Compared with metallic materials, transition-metal compounds have a great potential to be used as cost-effective CEs in DSCs due to their unique properties including a broad variety of low cost materials, a good plasticity, a simple fabrication, high catalytic activity, selectivity, and good thermal stability under rigorous conditions (Wu et al., 2011a). Recently, several kinds of transition-metal compounds CEs, e. g., nitrided Ni particle film, TiN nanotube arrays, MoC have been reported in DSCs which have a conversion efficiency superior to Pt.
Jiang et al (Jiang et al., 2009) used TiN nanotube arrays as CE in DSC for the first time, and the resulting DSC had photovoltaic performances comparable to those using the conventional TCO/Pt counter electrodes, which should be attributed to the obviously lower charge-transfer resistances at the CE/electrolyte interfaces and ohmic internal resistances. The exciting photovoltaic performances comparable to Pt CE inspire the researches on the transition-metal compounds used as the new kind of CEs. They also investigated the surface-nitrided nickel film as a low cost CE material, and the resulting DSCs presented an excellent photovoltaic performance competing with that with the conventional Pt CE (Jiang et al., 2010). Molybdenum and tungsten carbides embedded in ordered mesoporous carbon materials (MoC-OMC, WC-OMC) as well as Mo2C and WC were prepared respectively by Wu et al (Wu et al., 2011a). They demonstrated that DSCs equipped with optimized MoC – OMC, WC-OMC, Mo2C, and WC showed higher power conversion efficiency than those devices with a Pt CE. Very recently, they have developed another kind of CE, tungsten oxides, based on their excellent catalytic activity (Wu et al., 2011b). They found that WO2 nanorods showed excellent catalytic activity for triiodide reduction, and the DSC based on a WO2 CE reached a high energy conversion efficiency of 7.25%, close to that of the DSC using Pt CE (7.57%). Their results exhibit that tungsten oxides are promising alternative catalysts to replace the expensive Pt in DSCs system. Wang et al (Wang et al., 2009b) have demonstrated, for the first time, that CoS is very effective in catalyzing the reduction of triiodide to iodide in a DSC, superseding the performance of Pt as an electrocatalyst. They deposited the CoS layer on a flexible ITO/polyethylene naphthalate films. CoS based flexible and transparent CEs not only matched the performance of Pt as a triiodide reduction catalyst in DSCs, but also showed excellent stability in ionic liquids-based DSCs under prolonged light soaking at 60 °C. Clearly, the CoS has an advantage for large scale application as being a much more abundant, transparent and cheaper CE.