Role of Ligand Exchange at CdSe Quantum Dot Layers for Charge Separation

Abstract

Layers of CdSe quantum dots (QDs) with fixed diameter (4.5 nm) and various ligands were deposited on ITO substrates and investigated by photoluminescence (PL) and surface photovoltage (SPV) techniques. The distance between QDs decreased after washing and ligand exchange. The PL signals in the first excitonic peak as well as in the defect range strongly decreased with successive ligand exchange. In contrast to multilayer deposition, no PL signal related to CdSe QDs has been observed for a monolayer of QDs with pyridine surfactants on ITO. Neither modulated nor transient SPV signals were found for pristine CdSe QD layers. The modulated SPV signals strongly increased with successive ligand exchange. SPV transients became longer with successive ligand exchange. The experiments gave evidence for strong generation of surface defect states with ongoing ligand exchange and for fast initial charge separation of electrons from the first monolayer of CdSe QDs to the ITO surface. SPV transients could be described within a single QD approximation model, in which the number of surface defect states per QD, the energy distributions of defect states, hopping and distant dependent recombination were considered.