Energetics of CdSe Quantum Dots Adsorbed on TiO2

Abstract

Understanding how quantum dot (QD)-sensitized solar cells operate requires accurate determination of the offset between the lowest-unoccupied molecular orbital (LUMO) of the sensitizer quantum dot and the conduction band of the metal oxide electrode. We present detailed optical spectroscopy, low-energy photoelectron spectroscopy, and two-photon photoemission studies of the energetics of size-selected CdSe colloidal QDs deposited on TiO2 electrodes. Our experimental findings show that in contrast to the prediction of simplified models based on bulk band offsets and effective mass considerations, band alignment in this system is strongly modified by the interaction between the QDs and the electrode. In particular, we find relatively small conduction band-LUMO offsets, and near “pinning” of the QD LUMO relative to the conduction band of the TiO2 electrode, which is explained by the strong QD-electrode interaction. That interaction is the origin for the highly efficient QD to electrode charge transfer, and it also bears on the possibility of hot carrier injection in these types of cells.