Influence of Multistep Surface Passivation on the Performance of PbS Colloidal Quantum Dot Solar Cells

Abstract

The performance of devices containing colloidal quantum dot (CQD) films is strongly dependent on the surface chemistry of the CQDs they contain. Multistep surface treatments, which combine two or more strategies, are important for creating films with high carrier mobility that are well passivated against trap states and oxidation. Here, we examine the effect of a number of these surface treatments on PbS CQD films, including cation exchange to form PbS/CdS core/shell CQDs, and solid-state ligand-exchange treatments with Cl, Br, I, and 1,2-ethanedithiol (EDT) ligands. Using laboratory-based and synchrotron-radiation-excited X-ray photoelectron spectroscopy (XPS), we examine the compositions of the surface layer before and after treatment, and correlate this with the performance data and stability in air. We find that halide ion treatments may etch the CQD surfaces, with detrimental effects on the air stability and solar cell device performance caused by a reduction in the proportion of passivated surface sites. We show that films made up of PbS/CdS CQDs are particularly prone to this, suggesting Cd is more easily etched from the surface than Pb. However, by choosing a less aggressive ligand treatment, a good coverage of passivators on the surface can be achieved. We show that halide anions bind preferentially to surface Pb (rather than Cd). By isolating the part of XPS signal originating from the topmost surface layer of the CQD, we show that air stability is correlated with the total number of passivating agents (halide + EDT + Cd) at the surface.

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 …”

“…This is due to a less aggressive, reduced of Br treatment (one cycle) compared to the Br only case (three cycles). However, the amount of Br present on the CQD surface is approximately equal for both the Br only and Br+EDT treatments, showing that a good passivation can be achieved without etching the surface …”

“…The beige bars represent the percentage of the surface Pb cation exchanged for Cd, red represents Br bound to Pb, blue represents a thiol from EDT ligands bound to the surface, and grey represents the percentage of the available surface sites that remains as PbS (non‐stoichiometric). Schematic diagrams of the CQDs at each stage are shown on the right (note the number of organic ligands shown is under‐representative) …”

“…The surface compositions were compared to data from solar cell devices incorporating these films, and a study of the air stability of the films (using XPS and optical absorption) . An inverse correlation was found between both good performance and air stability and the amount of unpassivated Pb and S sites present on the surface after treatment.…”

Efficiently Passivated PbSe Quantum Dot Solids for Infrared Photovoltaics

“…The magnified spectra of Cl 2p and Cd 3d (top panels in Figure c and d) reveal that the elements of Cl and Cd can only be found in the QD film (PbSe-IBrCd) treated with hybrid ligands. The Cl 2p peaks are deconvolved to be at 198.7 eV (bottom panel in Figure c), suggesting formation of a Cd–Cl bond rather than a Pb–Cl bond, because the binding energy of the Pb–Cl peak always appears at ∼198.0 eV . This is consistent with the fact that Cl has a stronger interaction strength with Cd than with Pb due to the smaller ion radius of Cd 2+ .…”