range of time and length scales spanned by these phenomena poses a significant theoretical
challenge. The ability to compute the properties of systems with 1,000 to 10,000 atoms will
permit modeling of complex problems, such as the electrochemical behavior of molecule-
nanocrystal systems, catalytic hydrogen production, and biological light-harvesting systems. In
addition, there is a significant need to create “inverse tools,” which receive a wide range of
simultaneous desired properties as inputs and yield materials arrangements as outputs.
Specific areas requiring improved understanding to enable significant progress in any approach
to solar energy include photon management, carrier excitation, charge transport, energy
migration, and interface science. The challenges in each of these areas are outlined below.
ENHANCED PHOTON MANAGEMENT
Two primary steps are common to all solar energy architectures: the guidance of sunlight to a
target and the absorption of this radiation. Solar energy converters must first be able to harvest
sunlight and channel the photons with minimal energy loss to an appropriate receiver. Optimal
use of the sunlight then requires a match of the solar spectrum to the absorption spectrum of the
solar converter. The design of new solar energy systems — whether photovoltaic (PV),
photocatalytic, or based on other approaches — must consequently be matched to high efficiency
in these primary processes of photon collection and absorption.
Recent scientific advances suggest several promising new approaches to these challenges, which
we collectively term “photon management.” Progress in materials and nanoscale optics has the
potential to greatly impact the design of light-collection technologies. It has long been known
that sharp features can concentrate electric fields. The corresponding physical phenomenon for
electric fields of the rapidly varying optical radiation can be achieved with elements structured
on the nanometer length scale (Figure 17). Such schemes have led to electric fields sufficient to
allow enhancement of normally weak Raman cross-sections to a level at which spectroscopy of
individual molecules has been demonstrated (Nie and Emery 1997). A related technique uses
surface plasmons to channel and concentrate electric fields (Hutter and Fendler 2004). Such
approaches, in which electric fields can be significantly enhanced, might also enable nonlinear
(or multi-step) optical processes to play a role in future solar energy conversion schemes. Other
approaches for photon management include the creation of novel photon up-converters and
down-converters to achieve better matching of the solar spectrum with the electronic excitations
in the solid (Trupke et al. 2002). For example, an up-conversion design might involve the use of
intermediate band states in semiconductors that allow the sequential absorption processes of
lower-energy photons to produce a higher-energy excitation (Cuadra et al. 2004).
Photon management might also take a biomimetic approach, for which the light-harvesting
techniques found in natural systems provide guidance (Scholes 2003; Diner and Rappaport 2002;
Chitnis 2001). Other areas in which new approaches might emerge include the use of materials
exhibiting a negative index of refraction (Pendry 2000) or the use of highly scattering materials
to localize photons (Ziegler 2003).