Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency

Semiconductor quantum dots (QDs) functionalized by metal–organic dyes show great promise in photocatalytic and photovoltaic applications. However, the charge transfer direction and rates—key processes governing the efficiency of energy conversion—are strongly affected by the QD–dye interactions, insights on which are challenging to obtain experimentally. We use density functional theory (DFT) and constrained DFT calculations to investigate a degree of sensitivity of the electronic level alignment and related QD–dye electronic couplings to binding conformations of N719 dye at the surface of the 1.5 nm CdSe QD. Our calculations reveal a lack of direct correlations between the strength of the QD–dye interaction in terms of their binding conformations and the donor–acceptor electronic couplings. While the QD–dye binding conformations are the most stable when the N719 dye is attached to the QD via two carboxylate groups, the strongest electronic coupling between the QD as an electron donor and the dye as an electron acceptor is observed in structures bonded via the isocyanate ligands. Such strong electronic couplings also are responsible for significant stabilization of the dye’s occupied orbitals deep inside in the valence band of the QD making the hole transfer from the photoexcited QD to the dye thermodynamically unfavorable in structures bound via isocyanates. Our results suggest that the most probable binding conformations are those occurring via two carboxylate linkers, which exhibit very weak electronic couplings contributing to the electron transfer from the photoexcited CdSe QD to the N719 dye but provide the most favorable conditions for the hole transfer. Overall, our computational work provides an insightful view about the surface chemistry of CdSe regarding the donor–acceptor interaction, energy level alignment, and charge transfer between CdSe and dye molecule, which can guide the rational design of QD-based materials for energy conversion applications.