The
hole density of individual copper sulfide nanocrystals (Cu2–x
S NCs) is determined from the stoichiometric
mismatch (x) between copper and sulfide atoms. Consequently,
the electronic properties of the material vary over a range of x. To exploit Cu2–x
S
NCs in devices, assemblies of NCs are typically required. Herein,
we investigate the influence of x, referred to as
the stoichiometric doping effect, on the structural, optical, electrical,
and thermoelectric properties of electronically coupled Cu2–x
S NC assemblies. The doping process is done by immersing
the solid NC assemblies into a solution containing a Cu(I) complex
for different durations (0–10 min). As Cu+ gradually
occupied the copper-deficient sites of Cu2–x
S NCs, x could be controlled from 0.9 to less
than 0.1. Consequently, the near-infrared (NIR) absorbance of Cu2–x
S NC assemblies changes systematically
with x. With increasing x, electrical
conductivity increased and the Seebeck coefficient decreased systematically,
leading to the maximal thermoelectric power factor from a film of
Cu2–x
S NCs at an optimal doping
condition yielding x = 0.1. The physical characteristics
of the Cu2–x
S NC assemblies investigated
herein will provide guidelines for exploiting this emerging class
of nanocrystal system based on doping.
The charge carrier density of copper sulfide nanocrystals (Cu2-xS NCs) is sensitive to variations in the atomic composition, which determines the nature of sulfur bonding (sulfur-to-sulfur bonding or copper-to-sulfur bonding)...
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