Nanostructures for Solar Energy Conversion: Low Cost and High Efficiencies........
Conventional solar cells require relatively pure absorbers to produce electrical current, whereas
nanostructured absorbers can circumvent this limitation by enabling collection of carriers in a
direction orthogonal to that of the incident light. Such systems have produced test devices having
up to 10% efficiency, but typical devices yield 3–5% efficiencies over large areas and have long-
term stability issues. New absorber combinations, control over the nanostructure of such
systems, and a fundamental understanding of the operating principles of such devices are needed
to enable a new generation of systems having two- to five-fold improvement in efficiency, low
cost, and long-term stability.
EXECUTIVE SUMMARY
Although conventional solar cells based on silicon are produced from abundant raw materials,
the high-temperature fabrication routes to single-crystal and polycrystalline silicon are very
energy intensive and expensive. The search for alternative solar cells has therefore focused on
thin films composed of amorphous silicon and on compound semiconductor heterojunction cells
based on semiconductors (e.g., cadmium telluride and copper indium diselenide) that can be
prepared by less energy-intensive and expensive routes. A key problem in optimizing the
cost/efficiency ratio of such devices is that relatively pure materials are needed to ensure that the
photo-excited carriers are efficiently collected in conventional planar solar cell device designs.
The use of nanostructures offers an opportunity to circumvent this key limitation and therefore
introduce a paradigm shift in the fabrication and design of solar energy conversion devices to
produce either electricity or fuels.
The absorber thickness is dictated by the absorption properties of the semiconductor being used;
for example, 100 μm of Si or 1–3 μm of GaAs are required to absorb fully incident sunlight, so
that incident photons are not wasted by virtue of being transmitted through the entire device
assembly. In turn, the absorber must be sufficiently pure that the excited states produced by light
absorption can survive for the required time and distance to be collected in an external circuit
and do not instead recombine to produce heat. The required absorption length therefore dictates
the minimum purity and cost needed to achieve the required carrier collection lengths. The use of
nanostructured and possibly nanoporous systems, however, offers an opportunity to satisfy these
two constraints, by collecting carriers in a direction that is orthogonal (nominally perpendicular)
to the one in which light is absorbed, as illustrated in Figure 32. In this way, such an approach
offers the potential for obtaining high energy conversion efficiency from relatively impure, and
therefore relatively inexpensive, photoconverters.
One important example of such a structure is provided by mesoscopic dye-sensitized solar cells,
which generally involve use of a highly porous film of randomly ordered nanoparticles of a
transparent nanocrystalline oxide, such as TiO 2 , coated with an ultrathin layer of light absorber
(e.g., dye molecules or semiconductor quantum dots). When photo-excited, the absorber injects
electrons into the oxide nanoparticles and creates a positive charge in the absorber. After electron
injection, the positive charge is neutralized by electron transfer to the oxidized dye from a liquid