Reversal of rectification in fullerene-based devices

Model Al/C60/ITO sandwich devices were fabricated, where Al is a magnetron sputtered aluminum layer (top electrode); C60 is a buckminsterfullerene layer routinely deposited by thermal evaporation in vacuum; and ITO is an indium tin oxide layer (bottom electrode). These devices demonstrate weak rectification in the dark and almost negligible photovoltaic activity under illumination, if polyethyleneterephthalate (PET) is used as substrate. If the substrate is glass, inversion of rectification occurs, accompanied by a rather more pronounced photovoltaic effect (in the reverse direction, too) than in their Al/C60/ITO/PET counterparts. Secondary ion mass spectroscopy (SIMS) with depth-profiling suggests that this effect is associated with the differences in the chemical composition of the top Al/C60 interface in devices on PET and on glass. Such differences result from migration of admixtures from the substrate towards the top electrode. Since (photo)electrical properties of the devices reflect the dominating contribution of one of the two interfaces (top Al/C60 and bottom C60/ITO), rectification can be reversed by amending either interface. This fact is to be taken into account when comparing efficiency of multilayer photovoltaic cells (e.g., with a heterojunction) on different substrates.