Publication date: 1st April 2013
Efficient electron transfer (ET) at the quantum dot-oxide interface is a key step in the operation of quantum dot sensitized oxide solar cells. Here we unravel the influence of QD-oxide coupling strength on ET rates. The electronic coupling strength was controlled through the nature and length of the molecular bridge anchoring donor and acceptor phases; ET rates are monitored by transient Terahertz (THz) spectroscopy. THz spectroscopy allows an unambiguous characterization of the ET kinetics due to the fact that the arrival of the electron in the oxide is measured directly, in real-time, through the THz photoconductivity.
Figure 1 summarizes how ET rates between the QD and oxide can be tuned through bridge length for two types of bridges based on n-methylene (black) and n-phenylene (red) moieties, respectively. For both bridges, ET rates decrease exponentially with molecular length,indicating that ET through the molecular bridge occurs as a pure tunnelling process with characteristic decay factors of ~0.85 Å-1 and ~0.29 Å-1. These figures imply taller barrier potentials for ET through methylene groups when compared with phenylene ones. Our results are in good agreement with previous reports based on conductance measurements through single molecule junctions (STM probes) and through self-assembled monolayers (SAMs sandwiched in a capacitor). Notably, odd even effects are clearly resolved for the methylene bridges and characterized by identical decay factors, suggesting that odd-even effects arise from subtle differences in the bonding bridge geometry.
Figure 1: Electron transfer rate constant vs nominal length of molecular bridges consisting of HS-[CH2]n-COOH (black) and HS-[C6H4]n-COOH (blue). The potential energy difference driving ET is 1.1eV, which is given by the LUMO-CB energy difference between 3nm CdSe QDs sensitizing SnO2.
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