Doping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. By using Density Functional Theory, Silicon Nanocrystals (Si-nc) of different size (diameter ranging from 1.1 nm to 1.8 nm) have been studied localizing impurities at different substitutional sites and calculating the impurity formation energies. Starting from hydrogen terminated silicon Si-nc, we found that codoping is always energetically favored with respect to a single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The formation energy depends on the distance between the two impurities. The codoped Si-nc present bandedge states localized on the impurities which are responsible for a red-shift of the absorption threshold with respect to that of pure undoped Si nanocrystals. Concerning the emission spectra, we find a Stokes shift of the photoluminescence to a lower energy with respect to the absorption edge due to the nanocrystal (nc) structural relaxation after the creation of the electron-hole pair. We have calculated the absorption and emission spectra going beyond a single-particle approach showing the important role played by the many-body effects. The presence of electronic quasi-direct optical transitions between the donor and acceptor states within the band-gap makes it possible to engineer the optical properties of Si-nc.