The structural, optical, and electrical
transport properties of
nanowires obtained by the deposition of Cu over Sn doped In2O3 and SnO2 nanowires followed by processing
under H2S between 100 and 500 °C have been investigated
for their use in quantum dot sensitized solar cells. We find that
the CuS/Sn:In2O3 nanowires obtained between
100 and 200 °C consist of hexagonal CuS and cubic In2O3 but higher temperatures lead to the formation of Cu0.23In2.59S4 nanowires. Moreover, we
observed the existence of SnO2 quantum dots in tetragonal
Cu2SnS3 nanowires obtained at 400–500
°C which are responsible for ultraviolet emission at 3.65 eV
and a breakdown of the dipole forbidden rule in SnO2.The
CuS/Sn:In2O3 nanowires obtained at lower temperatures
exhibit better rectifying current–voltage characteristics and
higher currents, but we did not observe negative differential resistance,
as expected from a p–n tunnel junction, although this occurred
by bringing Sn:In2O3 nanowires in weak contact
with p-type CuS, similar to a cat’s whisker device. We discuss
the origin of the negative differential resistance which was also
observed in connection with the TiO2 barriers deposited
on the transparent conducting oxide anode and its importance for quantum
dot sensitized solar cells.
SnO2 and Sn:In2O3 nanowires were grown on Si(001), and p-n junctions were fabricated in contact with p-type Cu2S which exhibited rectifying current–voltage characteristics. Core-shell Cu2SnS3/SnO2 and CuInS2/Sn:In2O3 nanowires were obtained by depositing copper and post-growth processing under H2S between 100 and 500 °C. These consist mainly of tetragonal rutile SnO2 and cubic bixbyite In2O3. We observe photoluminescence at 3.65 eV corresponding to band edge emission from SnO2 quantum dots in the Cu2SnS3/SnO2 nanowires due to electrostatic confinement. The Cu2SnS3/SnO2 nanowires assemblies had resistances of 100 Ω similar to CuInS2/In2O3 nanowires which exhibited photoluminescence at 3.0 eV.
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