Investigation of a solar battery using the environmentally friendly semiconductor : beta-FeSi_2

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Solar batteries are usually made of amorphous Si or poly-Si, but the conversion efficiency is limited. One of the reason is low absorption coefficient of Si. CuInSe2/CdS can give a higher efficiency by a deposited poly-crystalline, but it contains toxic Se and Cd. The purpose of this project is to investigate an environmentally friendly semiconductor with high absorption coefficient, therefore, high efficiency with a thin film.One of the candidate was a semiconducting silicide ; beta-FeSi_2, but there had been no report on a high quality thin film and details of the band structure had not also been clear. We succeeded to grow a high quality beta-FeSi_2 film by multi-layer method and MBE growth, which could give a higher carrier mobility of one order of magnitude than those reported so far. The absorption coefficient was proved to be as high as 10^5/cm. Energy band structure was also made clear: indirect band gap of 0.81eV and direct band gap of 0.97eV. It was found that conduction type was controlled by the depositing Si/Fe ratio.Band gap of 0.81eV is, however, too small to give a high efficiency as a solar battery by itself. Therefore, another semiconducting silicide with a wider band gap was searched. BaSi_2 was another candidate because it was reported that the band gap was either 1.3eV or 0.7eV. Firstly, we investigated poly-crystalline BaSi_2 and found it has an indirect band gap of 1.1eV and direct band gap of 1.25eV, a little bit wider than beta-FeSi_2. We tried to grow BaSi_2 thin film on Si (111) by a MBE method and succeeded to get a high quality thin film for the first time.Solar battery characteristics has not been clarified yet, but these results could indicate that semiconducting silicide is a good candidate of environmentally friendly solar battery materials.