Interfaces are important in all solar cells, but they are especially significant in organic-based photovoltaic (OPV) cells such as organic semiconductor PV cells and dye-sensitized solar cells. In OPV cells, charge generation, charge separation and charge recombination processes often take place at interfaces, while in conventional PV cells these crucial processes occur mainly in the bulk. Interfacial exciton dissociation can lead to photovoltages that exceed the built-in potential of the cell, thus Voc is not necessarily a measure of the band bending. Photovoltages up to ~1V have been achieved without band bending. Most OPV cells are majority carrier devices, and thus are fundamentally different than conventional p-n junction cells. The interfacial potential induced by the photogeneration of charge carriers may, at high light intensities, overwhelm the equilibrium potential and hinder charge separation. It is often advantageous to increase the exciton-dissociating surface area by structuring the interface, however this also increases the area over which carriers can recombine. In dye-sensitized solar cells, the surface area is so high, and recombination so rapid, that only a single redox couple with ultra-slow kinetics is viable. We describe a method of passivating the interfacial recombination sites in these cells that permits for the first time the use of kinetically fast redox couples and may facilitate the development of solid-state dye cells. Finally, we describe a UV treatment of dye cells that alters the interfacial energetics and dramatically increases the efficiency in some cases.