an improved photovoltaic cell , and method for its manufacture , comprising front and back electrically conductive layers between which is sandwiched a primary light - absorbing layer of amorphous semiconducting material displaying low photogenerated carrier mobility below about 50 cm 2 / v second , the constitution of which primary layer is fabricated to provide an affinity gradient reducing back - diffusion of a preselected species of photogenerated carrier to thereby forestall non - productive recombination with carriers of opposite type .