First-principles calculations carried out for compounds based on Si implanted with different species, as Ti or chalcogens (S, Se, Te), show them as solid candidates to intermediate band (IB) photovoltaic materials. This DFT study predicts electronic structures, formation energies, relaxed atomic structures, optoelectronic properties, diffusion paths, for supercells containing up to several hundreds of atoms. The knowledge of Si-based devices is a relevant factor to facilitate the creation of an IB solar cell. Crystalline samples with a concentration of Ti several orders of magnitude above the solubility limit have been already grown. Formation energy calculations agree with the experiment in showing mainly interstitial implantation. Calculated electronic structure presents an IB, which is in agreement with electrical measurements and models, and is expected to cause an increase of the absorption coefficient across the solar spectrum. Chalcogen-implanted Si is an efficient IR absorber when implantation is carried out at ultra-high concentrations. Substitutional implantation produces a filled band inside Si band-gap and our calculations predict that plausible co-doping with IIIA atoms (as Al, B) would allow to obtain an IB fulfilling all the needed requirements.