Size-dependent energy transfer pathways in CdSe quantum dot–squaraine light-harvesting assemblies: Förster versus Dexter

Energy transfer coupled with electron transfer is a convenient approach to mimic photosynthesis in light energy conversion. Better understanding of mechanistic details of energy transfer processes is important to enhance the performance of dye or quantum dot-sensitized solar cells. Energy transfer through both long-range dipole-based Förster resonance energy transfer (FRET) and short-range Dexter energy transfer (DET) mechanisms have been identified to occur between CdSe quantum dots (QDs) linked to a red-infrared-absorbing squaraine dye through a short thiol functional group (SQSH). Solutions of SQSH linked to CdSe were investigated through steady-state and time-resolved spectroscopy experiments to explore both mechanisms. Photoluminescence studies revealed that smaller QDs had higher energy transfer efficiencies than predicted by FRET, and femtosecond transient absorption experiments revealed faster energy transfer rates in smaller donor QD sizes. These findings supported a DET process dominating at small donor sizes. The presence of both processes illustrates multiple strategies for utilizing energy transfer in light-harvesting assemblies and the required considerations in device design to maximize energy transfer gains through either mechanism.