Cross-cutting Research Challenges.............................................................................
BACKGROUND
Rapid developments in nanoscience and the revolution in biomedical research have combined to
create an unusual opportunity for advances in basic solar energy research. Because the individual
microscopic steps of solar energy use take place on the nanometer scale, the ability to pattern and
control matter on this length scale presents unusual opportunities for researchers to create new
materials for solar energy conversion and use. The emergence of new fundamental physical
properties on this length scale could potentially lead to far more efficient technologies for
conversion of solar energy to electricity and fuels. To take advantage of these new materials and
processes, researchers must create complex arrangements of nanoscale components, like those
that occur throughout biological systems. It may be possible to directly employ living systems to
create fuels; alternatively, biological macromolecules may be used to direct the assembly of
nanoscale artificial building blocks. Artificial approaches to system assembly of nanoscale
components will benefit from incorporating the features — including fault tolerance — of
biological systems.
Low-dimensional Materials and Solar Energy
Specific patterns of matter on small length scales can be used to control the energy distribution,
or density of states, of fundamental excitations associated with light (photons), electrical charge
(electrons), and atomic vibrations such as sound and heat (phonons).
Patterns on the length scale of the wavelength of light can be used to manipulate photons.
Spontaneous absorption and emission rates are directly proportional to the photon density of
states, so this capability can play a wide-ranging role in almost all solar utilization technologies.
Photon management, in this case, leads to systematic manipulation of the wavelengths at which
light is absorbed or emitted.
Patterns of matter on the length scale of the wavelength of electrons allow for the control of the
density of electronic states, as well as the density of phonon states. In such “quantum-confined”
systems, both optical and electrical properties can be widely tuned. Quantum dots and rods may
play a key role as the absorbers of light in future-generation solar converters. The wavelength
range of absorption can be widely adjusted. More importantly, fundamental relaxation processes,
which lead to energy losses in bulk materials, can be adjusted in nanostructures — offering the
promise of more efficient light absorption and energy transfer. Nanowires may serve as the
conduits for the transport of charges. In one-dimensional nanostructures, it is possible to adjust
the length scale for electron scattering. In the harvesting of charges, the use of materials with
ballistic transport of carriers can reduce losses. The phonon scattering rate is enhanced in one-
dimensional superlattices so that the thermal and electrical conductivity can be separately tuned
— which is the key to creating improved thermoelectrics.