One way of improving the efficiency of dye-sensitized solar cells is to use two photoelectrodes in a tandem device, one harvesting the high energy photons, and the other harvesting the low energy photons. This enables the photovoltage to be increased, whilst maximizing light harvesting across the solar spectrum. Despite their promise, a tandem cell with a higher efficiency than the state-of-the-art “Grätzel” cell has not yet been achieved. This is because the performances of photocathodes are significantly lower than TiO<sub>2</sub>-based anodes, and the p-type concept has been largely unexplored since the first device was prepared in 1999. The small potential difference between the valence band of the NiO, p-type semiconductor, and the redox potential of the electrolyte and the faster charge-recombination reactions compared to the TiO<sub>2</sub> system limit the efficiency. In recent years, we have made progress by developing new photosensitizers. In parallel we have investigated the charge-transfer processes to determine the mechanism and limitations to efficiency. This has increased our understanding of the redox processes at the dye/electrolyte and NiO/electrolyte interfaces. The fundamental limitation of these devices arises from the NiO material itself and we have re-focussed our efforts on finding a replacement transparent p-type semiconductor. Finally, solid state charge-transport layers have been employed. This presentation will summarise our progress so far and challenges and opportunities for the future.

Materials and interfaces in tandem dye-sensitized solar cells

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ACS Fall 2022

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