2 research outputs found
Influence of Multistep Surface Passivation on the Performance of PbS Colloidal Quantum Dot Solar Cells
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
Physical Delithiation of Epitaxial LiCoO<sub>2</sub> Battery Cathodes as a Platform for Surface Electronic Structure Investigation
We report a novel delithiation process for epitaxial
thin films
of LiCoO2(001) cathodes using only physical methods, based
on ion sputtering and annealing cycles. Preferential Li sputtering
followed by annealing produces a surface layer with a Li molar fraction
in the range 0.5 x < 1, characterized by
good crystalline quality. This delithiation procedure allows the unambiguous
identification of the effects of Li extraction without chemical byproducts
and experimental complications caused by electrolyte interaction with
the LiCoO2 surface. An analysis by X-ray photoelectron
spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides
a detailed description of the delithiation process and the role of
O and Co atoms in charge compensation. We observe the simultaneous
formation of Co4+ ions and of holes localized near O atoms
upon Li removal, while the surface shows a (2 × 1) reconstruction.
The delithiation method described here can be applied to other crystalline
battery elements and provide information on their properties that
is otherwise difficult to obtain