The photoelectrochemistry of Si/CH3OH contacts has been investigated under high level injection conditions. Exposure of Si samples having low doping levels (10(13)cm(-3)) and long minority carrier lifetimes (>1 ms) to modest steady-state illumination levels has produced high level injection conditions and minimal electric fields at the solid/liquid contact. Under these conditions, the solid is analogous to a large molecule, with both electrons and holes available for interfacial charge transfer (and recombination) at the solid/liquid contact. In principle, photocurrents of either sign are possible in such samples. Vectorial charge separation with a high quantum yield has been effected by selective removal of one carrier type at the back of the sample, in a fashion analogous to either oxidative or reductive quenching of a molecular excited state. Through specifying the back contact connections, the sign of the photocurrent at the solid/liquid interface, 100 mu m from the initial quenching site, has been manipulated. Charge separation with high quantum yields and with approximate to 10% photoelectrode energy conversion efficiencies under solar simulation has been achieved using this approach.