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Research on dye-sensitized solar cells (DSSCs) with metal-free organic sensitizers have recently received a new impetus from the perspective to create semi-transparent DSSCs [1,2]. We report here the first femtosecond transient absorption (TAS) study of the cyanine derivative VG20 dye. In ethanol, VG20 dye is characterized by a strong absorption band peaking at 827 nm (= 245’000 M-1cm-1), and covering 700-880nm. When grafted on TiO2, aggregation is observed, which can be minimised to some extent upon the addition of CDCA [3]. After in depth optimization of the sensitizing conditions and electrolyte composition, NIR-DSSCs based on VG20 dye achieved a power conversion efficiencies (PCE) of 3.2-3.6% [3].
In order to quantify the characteristic lifetimes for self-quenching (SQ) competing with charge injection, we prepared DSSC’s either with TiO2 as an injecting material and ZrO2 as a non-injecting semi-conductor. For the cell prototypes, the photo-anodes are immersed in a highly concentrated solution of VG20 and 50 mM of CDCA for several hours, leading to a peak optical density ≈0.4 at 850 nm. The devices are then infiltrated by a liquid electrolyte, as used in the best performing dye solar cells.
We conducted intensive broadband transient absorption experiments with a time resolution of 80 fs testing the sample homogeneity and cell-to-cell variability with 5-6 TiO2 DSSCs prepared under similar conditions.
The differences in excited state lifetime and ground state recovery are very significant and larger than variances observed for the TiO2 cells only due to sample homogeneity and cell-to-cell variability. Indeed, the TiO2-based devices are characterised by a ≈1 ps component (30-40%) followed by a 40-50 ps decay component, while the ZrO2 devices only display the latter. Most importantly, long-lived charge injection is observed in TiO2 since the ground state bleach signal does not recover fully (≈10% time zero bleach), on the 2-ns time window probed.
Next experiments, focusing on the effects of CDCA and electrolyte composition will verify to which degree the design-parameter performances correlate with the ultrafast molecular processes.
[1] T. Geiger, I. Schoger, D. Rentsch, A. C. Véron, F. Oswald, T. Meyer, and F. Nüesch, Int. J. Photoenergy, 258984 (2014)
[2] S. Kuster, F. Sauvage, MK Nazeeruddin, M. Grätzel, FA Nüesch, T. Geiger, Dyes and Pigments 87, 30 (2010).
[3] Vittoria Novelli, PhD thesis, Université de Picardie Jules Verne Amiens, (2017)
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