  Because of its low-cost, silicon is the standard material for photovoltaic
conversion. Yet, its band-edge absorption spectrum is narrower than the
spectrum of the solar radiation, which reduces its conversion efficiency. In
this paper, it is shown that the spectrum of absorbance of silicon can be
extended to longer wavelengths by proximity to a two-dimensional (2D)
semiconductor. Photo-induced Hall effect, together with standard absorption
spectroscopy, was employed to estimate the increase of efficiency of absorbance
of a 2D-platinum-diselenide/intrinsic-silicon bilayer. The bilayer shows a
significantly higher absorption in the infrared as compared to the single
films. Moreover, an overall increase of absorption efficiency by a factor
twenty was measured in the entire spectrum of light of a halogen lamp. X-ray
Photoelectron Spectroscopy (XPS) confirms that a reduction of the band-gap
occurs in the silicon substrate at the interface between the two
semiconductors. The results are interpreted in the framework of band-gap
narrowing due to hole-plasma confinement in the Si, induced by
electron-confinement in the 2D film. Possible application of the effect in
photo-voltaic cells is discussed.
