The passivating effect of cadmium in PbS/CdS colloidal quantum dots probed by nm-scale depth profiling

Abstract

Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.

Surface and Interface Chemistry in Colloidal Quantum Dots for Solar Applications Studied by X‐Ray Photoelectron Spectroscopy

“…However, the basic features of the PbS oxidation reaction determined by Cant et al . are confirmed in our studies on PbS CQDs …”

“…Each sample is labelled with its effective CdS shell thickness, calculated from these ratios. (C) Schematic drawing showing the ligands attached to the 0.15 nm CdS shell PbS sample, with ligands present in the relative amounts determined by XPS …”

“…Comparison with the data (for example the data presented in Figure A) gives information on the shell thickness and whether alloying between the core and shell is present. In the second method the core diameter, shell elements to core elements ratio, and other material parameters are used to calculate the shell thickness for each sampling depth …”

“…We have carried out a detailed investigation of the structure of these shells using depth‐profiling XPS. In addition, the effect of Cd in both stabilizing the CQDs against oxidation and modifying the electronic structure was also investigated …”

“…Shell thicknesses were calculated using the depth‐profiling XPS data and the methods developed by Shard et al . Only in the case where more than one monolayer of CdS was present on the surface was evidence for alloying into the PbS core found …”