Electrical Properties of Intermediate Band (IB) Silicon Solar Cells Obtained by Titanium Ion Implantation

Intermediate band silicon solar cells have been fabricated by Titanium ion implantation and laser annealing. A two-layer heterogeneous system, formed by the implanted layer and by the unimplanted substrate is obtained. In this work we present electrical characterization results which evidence the formation of the intermediate band on silicon when ion implantation dose is beyond the Mott limit. Clear differences have been observed between samples implanted with doses under or over the Mott limit. Samples implanted under the Mott limit have capacitance values much lower than the non-implanted ones as corresponds to a highly doped semiconductor Schottky junction. However, when the Mott limit is surpassed the samples have much higher capacitance, revealing that the intermediate band is formed. The capacitance increase is due to the big amount of charge trapped at the intermediate band, even at low temperatures. Titanium deep levels have been measured by Admittance Spectroscopy. These deep levels are located at energies which vary from 0.20 to 0.28 eV bellow the conduction band for implantation doses in the range 10^13-10^14 at/cm^2. For doses over the Mott limit the implanted atoms become non recombinant. Admittance measurements are the first experimental demonstration the Intermediate Band is formation. Capacitance voltage transient technique measurements prove that the fabricated devices consist of two-layers, in which the implanted layer and the substrate behave as an n^+/n junction