Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

Abstract

Artificial solids composed of semiconductor quantum dots (QDs) are being developed for large-area electronic and optoelectronic applications, but these materials often have defect-induced in-gap states (IGS) of unknown chemical origin.Here we performed scanning probe based spectroscopic analysis and density functional theory calculations to determine the nature of such states and their electronic structure. We found that IGS near the valence band occur frequently in the QDs except when treated with reducing agents. Calculations on various possible defects and chemical spectroscopy revealed that molecular oxygen is most likely at the origin of these IGS. We expect this impurity-induced deep IGS to be a common occurrence in ionic semiconductors, where the intrinsic vacancy defects either do not produce IGS or produce shallow states near band edges. Ionic QDs with surface passivation to block impurity adsorption are thus ideal for highefficiency optoelectronic device applications.

Towards Photo-Switchable Transport in Quantum Dot Solids

“…It is noteworthy that according to Figure 3, holes and electrons both contribute significantly to the conductivity. Typically, trap states in PbS QD films are invoked by surface defects due to adsorbed oxygen or water, which exclusively leads to unipolar hole transport [25]. Therefore, the observation of ambipolar transport supports the picture of shallow traps for electrons and holes at the PbS/DTCP interface as expected for the open form of DTCP according to Scheme 1.…”

Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy

“…The observation of two distinct distributions may explain why some groups reported states close to the valence level and others associated them with the conduction level. [9][10][11][12][13][14]28] The surface chemistry of PbS CQDs is more complex than simple schemes such as the one in Figure 1a suggest. Even assynthesized CQDs do not only exhibit two differently polarized crystal facets ((001) and (111)), but are also covered with hydroxyl and oleate ions additional to oleic acid.…”

“…Such defect states were observed for PbS before by various techniques. Reported were especially a state 0.2 eV above the valence level of 1,2-ethanedithiol or 1,3-mercaptopropionic acid (EDT, MPA) capped CQDs via Kelvin probe force microscopy (KPFM) and scanning tunneling spectroscopy (STS), [9][10][11][12] Additionally, a quasimetallic midgap band ≈0.4 eV below the conduction …”

Advancing colloidal quantum dot photovoltaic technology

“…This was likely due to the exposure of the films to oxygen and water, which shifted the Fermi level and lowered the measured shortcircuit current (J SC ) by 23%. A recent study identified adsorbed oxygen molecules as the mostly likely origin of ingap states measured to lie~200 MeV above the valence band edge in a PbS CQD film [137]. Diffusive transport has been a limiting factor in CQD solar cell performance.…”