Low Energy Free-Carrier Generation in Nanoscale Si-Layered Systems: Experimental Evidence

A low energy free-carrier generation appears in nanoscale Si-layered systems. The experimental devices contain a superficial Si nanostratum (3-5 nm thick and tensile strained) formed at the SiO₂ /Si heterointerface during a processing including thermal oxidation, ion implantation and suitable thermal treatment. The nanostratum is composed of transformed c-Si. The investigation in the UV range (400 nm band) has been carried out mainly by spectral response measurements. The low energy generation allows a free-carrier multiplication which can lead to, in suitable conditions, internal quantum efficiencies exceeding unity. The best test collection efficiency we have measured up to now results in 1.35 electrons per incident photon. The new mechanism can be used for the adjustment of the quantum converter to the solar spectrum thanks to the multistage conversion. At least one of the two main limitations of conventional Si-based (bulk or thin film) solar cell performance, i.e. the only one electron-hole pair generation by energetic photons can be then overcome. The thermodynamic limit of conventional photovoltaic conversion is limited at 30%, while in the case of the mechanism reported here, if can be propelled above 60%