thickness to 1-2 μm. Enhancing charge transport in the two phases could help overcome this
problem by ensuring that electrons and holes reach the contacts before they recombine. Potential
strategies include fabricating ordered arrays of oxide nanopillars to speed up transport toward the
substrate.
Potential molecular strategies to exceed the Shockley-Queisser limit include development of
multijunction structures as well as new light-harvesting (sensitizing) units such as selected
molecular dyes and semiconductor quantum dots that can generate multiple charge carrier pairs
from single high-energy photons. Multilayer and multijunction nanostructured cells can be
fabricated by simple techniques such as screen printing or doctor blading. The short-circuit
photocurrent output of the layers can be readily matched by changing the film thickness and
effective pore size.
The pursuit of high-efficiency cells should also include exploration of photon-energy up-
conversion schemes (e.g., using multi-band-gap nanostructures, metastable electronic states, and
long-lived charge-separated molecular states). An inherent advantage of nanostructured solar
cells is that all of these strategies can be implemented by manipulation of the interface rather
than the bulk.
POTENTIAL IMPACT
Successful research on nanostructured solar cells for renewable energy is particularly relevant to
the solar energy technologies programs in the United States. Since nanostructures will potentially
play a prominent role in many new approaches to photovoltaic conversion, the research is
directly related to the National Nanotechnology Initiative, which crosses many federal agencies.
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