A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the reflection spectra of freestanding SiNW arrays is presented in this Letter. At a fixed nanowire diameter, three different wire lengths reproducibly formed a cross point in their reflectance curve plots. The cross point wavelength corresponded to the optical bandgap, as verified by the classical Tauc’s model. The optical bandgap of the SiNW arrays ( in average diameter) was measured to be , which is larger than the bandgap of bulk Si. Further decreasing the wire diameter to caused an increase of the bandgap to , which is closer to the optimal bandgap () required to achieve the highest conversion efficiency in single-junction photovoltaic devices. Our method suggests that the multijunction tandem structure can be realized via control of the diameter of SiNW arrays.
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