Temperature-Dependent Hole Transfer from Photoexcited Quantum Dots to Molecular Species: Evidence for Trap-Mediated Transfer.

The effect of temperature on the rate of hole transfer from photoexcited quantum dots (QDs) is investigated by measuring the driving force dependence of the charge transfer rate for different sized QDs across a range of temperatures from 78 to 300 K. Spherical CdSe/CdS core/shell QDs were used with a series of ferrocene-derived molecular hole acceptors with an 800 meV range in electrochemical potential. Time-resolved photoluminescence measurements and photoluminescence quantum yield measurements in an integrating sphere were both performed from 78 to 300 K to obtain temperature-dependent rates for a series of driving forces as dictated by the nature of the molecular acceptor. For both QD sizes studied and all ligands, the Arrhenius plot of hole transfer exhibited an activated (linear) regime at higher temperatures and a temperature-independent regime at low temperatures. The extracted activation energies in the high-temperature regime were consistent across all ligands for a given QD size. This observation is not consistent with direct charge transfer from the QD valence band to the ferrocene acceptor. Instead, a model in which charge transfer is mediated by a shallow and reversible trap more accurately fits the experimental results. Implications for this observed trap-mediated transfer are discussed including as a strategy to more efficiently extract charge from QDs.