Quantum efficiencies exceeding unity in amorphized silicon layers

There are two main limitations to the performance of Si solar cells: the fundamental (band-to-band) absorption threshold; and the single electron-hole pair generation by more or less energetic photons. An investigation in the UV and visible light range has been carried out on solar cells with surface layers totally amorphized by implantation. An additional photocurrent has been observed over the whole range of measured wavelengths from λ=400 to 1300 nm in comparison with a single-crystal Si reference solar cell of good quality. This considerable improvement is due to two phenomena: an internal quantum efficiency largely above unity in the UV and visible; and a near-infrared improvement. Contrary to single-crystal impact ionization which is limited by the semiconductor band-gap, the observed effect in amorphized materials is of a sub-gap nature. Unfortunately, this phenomenon is unstable and disappears with integrated light intensity