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In our previous work [Synth.Met.200(2015)99], we demonstrated dye-based tandem photovoltaic cells with vertically stacked Schottky junctions that generate open circuit voltage Uoc = 1.0 V. In present work, using the same concept we reached Uoc = 1.92 V. Such a progress is basically associated with optimization of the anode material in both subcells. Two axially substituted phthalocyanine complexes were used as absorbers: boron chloride subphthalocyanine SubPc in the bottom subcell and indium chloride phthalocyanine InClPc in the top subcell. Inorganic semiconducting oxide MoOx was introduced twice. First, MoOx layer was deposited over the ITO anode in the bottom subcell. This largely increases Uoc as compared to the archetypal Schottky-type cell ITO/SubPc/Al. For the bottom subcell with bilayer LiF/Ag cathode Uoc = 1.23 V. Second, MoOx was used in the composite intermediate layer (LiF/Ag/MoOx) that assembles subcells in stack, thus acting as anode buffer in the top subcell.

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.

We report on fabrication and testing of an all-organic-dye-based photovoltaic cell using a tandem architecture. Proceeding from the well-known concept of stacked molecular p/n-junctions, we developed a device with two stacked Schottky barrier subcells. Both subcells are based on p-type organic dyes: boron subphthalocyanine chloride (SubPc/Al barrier) for the front cell and vanadyl phthalocyanine (PcVO/Mg:Ag barrier) for the back one. These two dyes are complementary to each other in terms of optical activity, so the newly formed tandem cell absorbs a broad range of wavelengths from 300 to 1100 nm. Introduction of ultrathin intermediate layer of a third conductive dye, vanadyl naphthalocyanine (NcVO), at the Al/PcVO interface makes possible a serial connection of subcells, resulting in summation of open circuit voltages.