Dynamic Trap Formation and Elimination in Colloidal Quantum Dots

Abstract

Using first-principles simulations on PbS and CdSe colloidal quantum dots, we find that surface defects form in response to electronic doping and charging of the nanoparticles. We show that electronic trap states in nanocrystals are dynamic entities, in contrast with the conventional picture wherein traps are viewed as stable electronic states that can be filled or emptied, but not created or destroyed. These traps arise from the formation or breaking of atomic dimers at the nanoparticle surface. The dimers' energy levels can reside within the bandgap, in which case a trap is formed. Fortunately, we are also able to identify a number of shallow-electron-affinity cations that stabilize the surface, working to counter dynamic trap formation and allowing for trap-free doping.

A Theoretical-Experimental Comparison of CdSe Quantum Dot Optical Properties

“…We obtained a Cd character and a Se character for the states from -16 eV to -14 eV and from -10 eV to -5 eV, respectively. Similar results have been obtained by Deglmann et al and Voznyy et al [23,4,5]. On the other hand, a Cd-character can be associated to the energy states located from -3 eV to 3 eV.…”

“…During the last years, many experimental and theoretical results have been published reporting on intrinsic and extrinsic electronic and optical properties and their dependence on QD size, shape, method of preparation, among others [1,2,3,4,5,6]. CdSe QDs are of such great importance because they can be used in so many applications like bioimaging, lasing, diodes, photovoltaics, etc [2].…”

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

“…Packing additional ligands onto the CQD surface or growing a complete shell to passivate the core more fully can be expected to improve overall charge carrier lifetimes 38,43,44 . In addition, unintentionally introduced impurities presented at the time of synthesis or deposition represent a little-explored potential source of midgap trap states [45][46][47][48] . Distinguishing the nature of trap states in these studies may be done spectroscopically 49 and with transmission electron microscopy 50 .…”

Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance

“…[ 2,11,25,26 ] It can contribute to excessive n-type doping due to changes in stoichiometry and lead to self-compensation via surface reconstruction and trap formation. [ 40,41 ] This overtreatment results in devices with poor effi ciency due to decreased V oc and fi ll factor (see Figure S2, Supporting Information). The optimal I:Pb ratio in XPS is found to be 5% which corresponds to ≈15-25% of available surface S sites.…”