Recombination of carriers at a surface or at a grain boundary of a semiconductor is associated with the presence of chemical bonds that are weaker than those in the bulk. Upon strengthening these bonds, by reacting the interface with a strongly bound impurity, the rate of recombination is drastically reduced. For example, a 103-fold increase in EBIC charge collection efficiency for the polycrystalline p–InP/Ti Schottky junction and a corresponding increase in the efficiency of the photoelectrochemical cell polycrystalline p–InP/VCl3–VCl2–HCl/C are observed when silver is chemisorbed on the semiconductor grain boundaries. Chemical control of carrier recombination at semiconductor solution interfaces and grain boundaries results in 12 % efficient monocrystalline and 8 % efficient thin film polycrystalline solar cells, that generate either electrical power or hydrogen.