Effective surface treatments suppress possible recombination losses and confine photogenerated electrons and holes within the bulk of the silicon wafer, thus maximizing their number and the electrochemical potential that they can deliver to a load. For that to happen, it is necessary to create regions with a high conductivity for one carrier and low for the other, which is the basis for their separation. There is a common thread joining surface passivation and carrier-selective contacts, and the same principles apply to both. One is the manipulation of the concentrations of electrons and holes, which can be achieved by doping or by depositing materials with an appropriate bandgap, work function and conductivity. The other method is to use hydrogen-rich semi-insulators that bond chemically to the silicon atoms. When used as part of a contact structure, they need to be sufficiently thin to permit current flow. Examples of such passivated contacts are dopant diffusions coated with thin insulators or a-Si:H(i), doped polysilicon/SiO, structures, and some transparent conductors.