Nonlinear N,Cl-GQDs as Efficient Energy Transfer Antenna Materials for FRET-Enhanced Solar Energy Conversion

Hybrid solar cells (HSCs) require advanced photoelectrodes to efficiently harvest light across a broad spectrum while minimising charge recombination. Despite their complementary properties, the synergistic integration of nonlinear graphene quantum dots (GQDs) and dye sensitisers remains underexplored. In this study, an efficient hybrid photoelectrode is deposited, combining fluorescent nonlinear GQDs with N3 dye on a plasmonic Au@TiO2 substrate. Hydrothermal synthesis is used to systematically engineer blue, brownish, and bluish-green, fluorescent N-doped, Cl-doped, and N, Cl-codoped GQDs. The N, Cl-GQDs exhibit self-defocusing behaviour, aligning with a negative nonlinear refractive index and displaying distinct nonlinear optical properties. Nonlinear GQDs serve as light-harvesting antennae to optimise charge separation dynamics, with N3 dye molecules acting as energy acceptors in the coupled system. This setup facilitates efficient non-radiative energy transfer, significantly enhancing light-harvesting efficiency and exciton dissociation rates. The spectral overlap between N, Cl-codoped GQDs and N3 dye maximises fluorescence resonance energy transfer (FRET) efficiency and electron injection while suppressing recombination. A FRET efficiency of 52% is achieved for N, Cl-GQDs as antenna material. Under AM 1.5G illumination, the optimised photoelectrode reaches a Voc of 0.74 V, a Jsc of 7.9 mA/cm², a fill factor of 70%, and a PCE of 3.9%, demonstrating an improvement over reference N3-based cells.