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Dye-Sensitized Solar Cells (DSSCs) belong to the next generation of photovoltaic technologies. They are advantageous, because in DSSCs both photon harvesting and charge separation and collection occur in the active area of the cell. Additionally, the dye wavelength-selective absorption flexibility extends up the near-IR, driving towards transparent applications [1]. We have already introduced and characterized using the fs-Transient Absorption technique, the near-IR cyanine sensitizer VG20 that strongly absorbs at 830 nm (ε=154’000 M-1cm-1) when adsorbed on TiO2 [2]. After optimization and use of the de-aggregating agent CDCA, as well as electrolyte additives to tune the SC energy, VG20 exhibited the record, by the time, product of PCE efficiency (3.1%) and Average Visible Transmission (AVT) of 75% with injection occurring in 2-4 ps. The temporal competition of energy transfer (ET) from monomers to aggregates limits the injection efficiency to 30%. Herein, we report two novel pyrrolopyrrole cyanine dyes (TB207, TB423) designed to strongly absorb in the near-IR and hinder the aggregate formation upon sensitization. Both absorb at ~760 nm (ε~138’000 M-1cm-1) in solution and when incorporated in real DSSCs cover the 650-850 nm region. By the successful combination of fs-fluorescence and fs-transient absorption set-ups in solution, in non-injecting Al2O3 DSSCs and injecting TiO2 DSSCs, it was identified for TB207 that both the injection and ET occur in a distribution of times (2-23 ps) due to the inhomogeneity of the system. The total PCE reached 4.2% for TB207, with AVT 74% and injection efficiency to be 40-60%, while for TB423 the injection efficiency reached 80%. Interestingly, for the latter no pronounced dependence on the aggregate concentrations was recorded indicating possible secondary high yield injection from aggregates. Finally, the next experiments are focusing on new promising dyes designed to overcome limitation issues with target the optimization towards transparent and colourless devices.
[1] F. Grifoni et al., Adv. Energy Mat., 2021, 2101598, 1-47
[2] W. Naim et al., JACS Au, 2021, 2021, 1, 4, 409–426
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