Decomposition of Water by Use of a Wet-Type Solar Cell Equipped with a Stacked Multilayer Amorphous Silicon Electrode

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The present research started with the aim of developing a solar cell of a new type by use of amorphous silicon (a-Si) and related semiconductors such as crystalline silicon (c-Si) and decomposing water by solar light for the efficient solar to chemical conversion. The open-circuit photovoltage (V_<oc>) of a single p-i-n junction a-Si solar cell is only 0.8 V, much lower than the voltage needed for the electrolysis of water (about 1.8 V). Thus, we fabricated stacked tandem-type a-Si electrodes in which the photovoltages of the a-Si cells were added in series, and investigated problems inherent to the electrodes of this type, especially on the energy loss due to the inverse photovoltages arising from the interfaces between a-Si cells. We succeeded in the durable solar decomposition of water with an electrode having a structure: indium-gallium alloy (ohmic contact)/p-type c-Si/n-type a-Si/p-i-n junction a-Si/p-i-n junction a-Si/thin Pt layer/aqueous electrolyte, and obtained a solar to chemical conversion efficiency of 3%. This is the first result for the efficient decomposition of water by solar light. As another approach, we studied on solar cells of a new type having semiconductors coated with ultrafine metal islands several nanometers wide, which we recently discovered. We searched for methods to deposit ultrafine metal islands as uniformly in size and spatial distribution as possible, and tried to use the ion-cluster-beam deposition method, to prepare a metal-colloid solution and drop it, and to deposit metals electrochemically. For the wet-type solar cells made of c-Si with platinum islands, the obtained V_<oc> was 0.64 - 0.68 V and the solar conversion efficiency was 11 - 14%. For the solid-state solar cells, the V_<oc> was 0.60 V and the conversion efficiency was 10%. The V_<oc> value of 0.68 V is much higher than that for the conventional p-n junction c-Si solar cell (0.59 V), clearly indicating the merit of solar cells of the present type.