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

本研究的目的是通过使用非晶硅（a-Si）和相关半导体如晶体硅（c-Si）开发新型太阳能电池，并通过太阳光分解水以进行有效的太阳能到化学转化。单个<oc>p-i-n结a-Si太阳能电池的开路光电压（V_）仅为0.8V，远低于电解水所需的电压（约1.8V）。因此，我们制作了堆叠的串联型a-Si电极，其中a-Si电池的光电压被串联添加，并研究了这种类型的电极固有的问题，特别是关于由于a-Si电池之间的界面引起的反向光电压的能量损失。我们成功地用具有以下结构的电极进行了水的持久太阳能分解：铟镓合金（欧姆接触）/p型c-Si/n型a-Si/p-i-n结a-Si/p-i-n结a-Si/薄Pt层/水性电解质，并且获得了3%的太阳能到化学转化效率。这是太阳光有效分解水的第一个结果。作为另一种方法，我们研究了一种新型的太阳能电池，这种太阳能电池的半导体表面涂有我们最近发现的几纳米宽的超细金属岛。我们寻找尽可能均匀地存款尺寸和空间分布的超细金属岛的方法，并尝试使用离子团簇束沉积方法，制备金属胶体溶液并滴加它，以及电化学存款金属。对于由具有铂岛的c-Si制成的湿型太阳能电池，获得的V_<oc>为0.64 - 0.68V，太阳能转换效率为11 - 14%。对于固态太阳能电池，V_<oc>为0.60 V，转换效率为10%。0.68V的V_<oc>值比常规p-n结c-Si太阳能电池的V_值（0.59V）高得多，清楚地表明了本类型太阳能电池的优点。

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.