Charge and energy transfer interplay in hybrid sensitized solar cells mediated by graphene quantum dots

• We report a one pot synthesis metod of GQD with controlled size and optoelectronic properties.
• An improvement of common N3-DSSC characteristics is achieved when GQDs are used as co-sensitiser.
• The role of GQD as cosensitisers in hybrid DSSC was investigated and the interplay between charge and energy transfer phenomena mediated by GQDs was demonstrated.
• The GQDs presence determines an inhibition of the recombination processes at the TiO2/electrolyte interface.

We explored the role of graphene quantum dots (GQDs) as co-sensitizers in hybrid dye sensitized solar cell (DSSC) architectures, focusing on various concurring mechanisms, such as: charge transfer, energy transfer and recombination rate, towards light harvesting improvement. GQDs were prepared by the hydrothermal method that allows the tuning of electronic levels and optical properties by employing appropriate precursors and synthesis conditions. The aim was to realize a type II alignment for TiO2/GQD/dye hybrid configuration, using standard N3 Ru-dye in order to improve charge transfer. When GQDs were used as co-sensitizers together with N3 Ru-dye, an improvement in power conversion efficiency was achieved, as shown by electrical measurements. The experimental analysis indicates that this improvement arises from the interplay of various mechanisms mediated by GQDs: (i) enhancement of charge separation and collection due to the cascaded alignment of the energy levels; (ii) energy transfer from GQDs to N3 Ru-dye due to the overlap between GQD photoluminescence and N3 Ru-dye absorption spectra; and (iii) reduction of the electron recombination to the redox couple due to the inhibition of the back electron transfer to the electrolyte by the GQDs.

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