Methods for passivating crystalline grains in an active layer for an optoelectronic device and optoelectronic devices having active layers with passivated crystalline grains are disclosed. Crystalline grains of an active layer material and/or window layer material are formed within the nanotubes of an insulating nanotube template. The dimensions of the nanotubes correspond to the dimensions of a crystalline grain formed by the deposition technique used to form the grains. A majority of the surface area of these grains is in contact with the wall of the nanotube template rather than with other grains.