(Invited) Light Absorption Enhancement Using Graphene Quantum Dots in Dye-Sensitized Solar Cells and Photoelectrochemical Water Splitting
Authors: Zhang Yuegang , Subramanian Alagesan 
Published: February 2020

Graphene quantum dots (GQDs) have received great interest in the past few years due to its unique properties such as the quantum confinement and edge effects when their sizes are down to 10 nm. These new physical properties can induce size-dependent bandgap, and unique optical and electronic properties, which make them excellent materials for photovoltaics and photoelectochemical (PEC) devices.
In a dye-sensitized solar cell (DSSC), light harvesting capability of dyes could be enhanced by Förster resonance energy transfer (FRET) phenomena, wherein energy is transferred from an excited donor fluorophore to a suitable acceptor dye molecule. The strict requirement that the overlap of the emission spectrum of donor and absorption spectrum of acceptor, which also requires proper band edge alignment between the them, significantly limits the variety of donors and acceptors suitable for the FRET-based DSSCs. In our DSSC work, graphene quantum dots were co-sensitized with N719 dyes and we successfully explored that the GQDs is a suitable donor material for the N719 dyes. From the emission and time decay spectra analysis, the FRET efficiency was measured to be 27 %. The power conversion efficiency (ƞ) of the standard DSSC is 6.12 % with a short-circuit current density (Jsc
) of 12.74 mAcm-2 and open-circuit voltage (Voc
) of 760 mV. The co-sensitization of GQD greatly enhances the ƞ up to 7.96 % with a Jsc
of 16.54 mAcm-2 and Voc
of 770 mV. The co-sensitization of GQDs improved both the Jsc
and ƞvalues by ~ 30 %. The FRET enhanced the overall light absorption capacity of the N719 dye and hence photocurrent generation of the DSSC. Moreover, the co-sensitization of GQDs increase the TiO2/dye/electrolyte interface resistance, which suppresses the charge recombination and leads to increased Voc
value.
For PEC water splitting, higher light absorption can also be achieved by utilizing light-matter interplaymechanisms, such as, surface plasmon resonance (SPR), plasmonicresonance energy transfer (PRET) and exciton−plasmon interactions (EPI). The optical properties of the semiconductor and metal nanoparticles (SNPs-MNPs) are strongly modified viaEPI comparing to single NPs. Therefore, we have utilized the unique combination of GQDs and AuNPs (SNPs-MNPs) to effectively enhance PEC performance of TiO2 nanorods (TNRs). The AuNP-GQD decorated TNR photoelectrode has achieved a superior PEC performance with a photocurrent density of 1.75 mAcm-2 at 1.23 V vs RHE, which is one of higher value obtained in PEC measurement using TiO2 nanorods due to an efficient utilization of solar light. The experimental analysis has indicated that the AuNPs has contributed to the PRET / SPR-mediated hot electron injection into TiO2 and the GQDs contributed to the improved electron injection into the TiO2. Moreover, the significantly improved PEC water splitting efficiency is attributed to the synergistic effect due to EPI and/or FRET between AuNPs and GQDs in addition to the good catalytic property of the AuNPs and the GQDs.