Controlling Charge Separation and Recombination Rates in CdSe/ZnS Type I Core-Shell Quantum Dots by Shell Thicknesses

Type I core/shell quantum dots (QDs) have been shown to improve the stability and conversion efficiency of QD-sensitized solar cells compared to core only QDs. To understand how the shell thickness affects the solar cell performance, its effects on interfacial charge separation and recombination kinetics are investigated. These kinetics are measured in CdSe/ZnS type I core/shell QDs adsorbed with anthroquinone molecules (as electron acceptor) by time-resolved transient absorption spectroscopy. We show that the charge separation and recombination rates decrease exponentially with the shell thickness (d), K(d) = k(0)e(-beta d), with exponential decay factors beta of 0.35 +/- 0.03 per angstrom and 0.91 +/- 0.14 per angstrom, respectively. Model calculations show that these trends can be attributed to the exponential decrease of the 1S electron and hole densities at the OD surface with the shell thickness. The much steeper decrease in charge recombination rate results from a larger hole effective mass (than electron) in the ZnS shell. This finding suggests possible ways of optimizing the charge separation yield and lifetime by controlling the thickness and nature of the shell materials.