In an attempt to develop a low-cost material for solar cell devices, polycrystalline magnesium silicide (poly-Mg 2 Si) semiconducting layers have been prepared by applying rf magnetron sputtering using a Mg 2 Si target. The optimum substrate temperature for the poly-Mg 2 Si growth was found to be T s ¼ 200 C; the film deposition at higher temperatures leads to desorption of Mg atoms from the growing surface, while the amorphous phase formation occurs at room temperature. The poly-Mg 2 Si layer deposited at T s ¼ 200 C shows the (111) preferential orientation with a uniform grain size of $50 nm. The dielectric function of the poly-Mg 2 Si layer has been determined accurately by spectroscopic ellipsometry. From the analysis, quite high absorption coefficients and an indirect gap of 0.77 eV in the poly-Mg 2 Si layer have been confirmed. The above poly-Mg 2 Si layer shows clear photoconductivity and can be applied as a narrow-gap bottom layer in multi-junction solar cell devices.

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The mapping characterization for the structural and optoelectronic properties of textured SnO 2 :F transparent conductive oxide (TCO) layers has been performed by spectroscopic ellipsometry (SE). From the SE analysis of the free carrier absorption in the SnO 2 :F layer, the optical carrier concentration and mobility are extracted by using the Drude model. As a result, in the textured SnO 2 :F substrate with a size of 9 Â 9 cm 2 , we have confirmed slight non-uniformities in the carrier concentration as well as the layer thickness. Moreover, in order to investigate the effect of the TCO inhomogeneity on hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (c-Si:H) tandem solar cells, we have simulated shortcircuit current density (J sc ) in the a-Si:H and c-Si:H layers by taking the TCO inhomogeneity into account using the optical admittance method. From this procedure, we have quantitatively estimated the variation of J sc by the TCO inhomogeneity assuming a non-textured flat structure.

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