Influence of Shell Thickness and Surface Passivation
on PbS/CdS Core/Shell Colloidal Quantum Dot Solar Cells
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Abstract
Cation-exchange
has been used to synthesize PbS/CdS core/shell
colloidal quantum dots from PbS starting cores. These were then incorporated
as the active material in solar cell test devices using a solution-based,
air-ambient, layer-by-layer spin coating process. We show that core/shell
colloidal quantum dots can replace their unshelled counterparts with
a similar band gap as the active layer in a solar cell device, leading
to an improvement in open circuit voltage from 0.42 to 0.66 V. This
improvement is attributed to a reduction in recombination as a result
of the passivating shell. However, this increase comes at the expense
of short circuit current by creating a barrier for transport. To overcome
this, we first optimize the shell thickness by varying the conditions
for cation-exchange to form the thinnest shell layer possible that
provides sufficient surface passivation. Next, ligand exchange with
a combination of halide and bifunctional organic molecules is used
in conjunction with the core/shell strategy. Power conversion efficiencies
of 5.6 ± 0.4% have been achieved with a simple heterojunction
device architecture
