Tandem solar cell concepts are currently successfully applied in conventional pn-junction solar cells with up to 40 % conversion efficiencies. Several tandem concepts have so far been introduced in the area of dye-sensitised solar cells, such as stacking of preassembled dye-sensitised solar cells, the combination of complete dye-sensitised solar cells with CdTe inorganic solar cells as well as the combination of dye-sensitised photocathodes with dye-sensitised photoanodes (pnDSSCs). The latter is of particular interest as it requires only marginally higher fabrication costs, compared to other concepts that involve stacking of complete preassembled cells, which de facto results in a doubling of manufacturing costs.
In this work we will present a more detailed analysis of dye-sensitised p-type nano-structured nickel(II)oxide electrodes as a model system for dye-sensitised photocathodes. We introduce a novel preparation method to fabricate nanostructured NiO electrodes based on a preformed NiO nanopowder.. Film preparation conditions strongly influence the nanostructure and photovoltaic performance of dye-sensitised nanostructured NiO photocathodes. Under optimised conditions NiO based cells show photon-to-electron conversion efficiencies beyond 50%, suggesting that there are no fundamental limitations to achieving photocathodes that can match the photocurrent outputs of conventional dye-sensitised photoanodes.
Several aspects of this model system will impede its future use in tandem dye-sensitised solar cells. Nevertheless this work shows that there should be no fundamental limitations for the development of novel efficient dye-sensitised tandem solar cells. Further advancements will require the identification of nanostructured p-type semiconductor materials with optimised band positions for their use in conjunction with dye-sensitised TiO₂ photoanodes.