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

We explored the role of graphene quantumdots (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. GQDswere 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 GQDswere used as cosensitizers together with N3 Ru-dye, an improvement in power conversion efficiency was achieved, as shown by electricalmeasurements. 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 betweenGQDphotoluminescence 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. (C) 2014 Elsevier Ltd. All rights reserved.