Formation of photo-thermoelectric energy conversion functional films by controlling film structure in nanometer scale.

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In order to exploit solar energy resources, the photo-thermoelectric energy conversion functional films have been investigated. The subject has been composed of two functional parts, I.e., (1) : photo-thermal energy conversion films and (2) : thermoelectric energy conversion films. The aim of the investigation is to improve individual function by forming the suitable mesoscopic structure in the films.(1) Ag/MgO and/or W-N/AlN multi-layer sutuctured films are formed by the sputtering method. With increasing Ag layer thickness, the optical properties change from the surface-plasmon absorption due to fine particle into bulk metallic property via conductive absorption which shows relatively large absorbance in the near infrared region as well as visible light region. The functionally graded Ag/MgO film is formed based on the above results. Then the total solar energy absorbance of 87% is achieved. The same procedure is performed on the W-N/AlN system. The finally obtained functionally graded W-N/AlN film shows a total solar energy absorbance of 93%.(2) Sputtered Fe-Si films are annealed (853K x 2h) in a vacuum. The change of the electrical resistivity (ρ) and the Seebeck coefficient (α) are measured. The film with a composition of FeSiィイD22ィエD2 shows maximum values of ρ and α. The multi-layered FeSiィイD22ィエD2/C films are formed. The value of ρ is successfully reduced but the α is also reduced. The power factor is not improved consequently. The sputtered Co-Sb films are also formed. By annealing, the amorphous-structured films change into polycrystalline structure including high-density voids with nanometer size. Such a nano-void structure may contribute to the reduction of thermal conductivity.It has been presented in this research that the introduction of the suitable mesoscopic structures into the intrinsic film structure is effective to improve the inherent function. The performing the optimization will be a future problem.