Figure 18 Concentration of sunlight provides significant advantages for many schemes of solar energy
conversion. While traditional methods using mirrors have been developed, new approaches suitable for
large areas are critical. The figure depicts a concentrator that makes use of photonic crystal filters (top of
collector) and quantum dots or dye molecules (solution between plates) to achieve large-area light
collection. (Source: FULLSPECTRUM undated)
CONTROL OF CARRIER EXCITATION, CHARGE TRANSPORT, AND ENERGY
MIGRATION
Many of the limitations in solar energy use are imposed by the efficiency limits of elementary
physical processes. Most broadly, these processes include (1) the absorption of a photon from the
solar spectrum, (2) the transport of the charge carriers or energy from the absorption site to a site
where it can be used (e.g., a heterojunction where excitons are split into separate electrons and
holes), and (3) the conversion of the excitation energy to drive the desired process
(e.g., electrical current or a chemical reaction). Complex interactions determine the efficiency of
these steps. Scattering processes can limit carrier transport, but on certain length scales, such
transport can also be ballistic. Excitons created by absorption of light can diffuse, but they are
also subject to radiative relaxation processes. Heterogeneous media, interfaces, defects, and
disordered homogeneous materials can also trap excitons and carriers. Further, it may be possible
to control the flow of energy among various competing paths, as natural systems that harvest
solar energy through photosynthesis do. Current research frequently takes an empirical approach,
surveying available materials and selecting those with the best parameters. However,
revolutionary progress in solar energy utilization would be possible if we had a sufficient
understanding of the relationship between materials structure and function. Such an
understanding would enable the rational design of structures that would provide detailed control
of the elementary processes of carrier excitation, charge transport, and energy migration.