**3. Platinum nanoparticle modified microcrystalline silicon thin films**

Hydrogenated microcrystalline silicon (c-Si:H) thin films are promising new materials for low-cost solar cells. The microcrystalline Si thin film approach has several advantages, including minimal use of semiconductor resources, large-area fabrication using low-cost chemical vapor deposition (CVD) methods, and no photodegradation of the solar cell's characteristics (Matsumura, 2001, Meier et al., 1994, Yamamoto et al., 1994). We applied microcrystalline Si thin films to solar hydrogen production by the photodecomposition of hydrogen iodide (Yae et al., 2007a, 2007b) and solar water splitting(Yae et al., 2007b). Figure 13 schematically shows a cross-section of the microcrystalline silicon thin-film photoelectrode. Photoelectrochemical solar cells require neither a p-type semiconductor layer nor a transparent conducting layer, which is necessary to fabricate solid-state solar cells.
