Another critical research need for the design of structures that control carrier excitation, charge
transport, and energy migration is the promotion of theoretical studies that can provide a deeper
understanding of the observed relationships between high-quality materials structure and
function and guide the discovery of new approaches. These studies would involve both greater
computational effort and advanced analytic theories describing the behavior on a range of length
and time scales, as well as within and between various biological, organic, and inorganic
materials.
Impact
Progress in the rational design of structures that control carrier excitation, charge transport, and
energy migration would enable revolutionary advances in solar energy utilization. Such advances
would change the way scientists approach the problems of optimizing efficiency in solar energy
conversion systems. Anticipated advances include a significantly improved use of the solar
spectrum, with reduced loss to dissipative processes. In addition to optimizing elementary
processes for existing solar energy designs, researchers are expected to develop entirely new
approaches that exploit new scientific advances. This approach is illustrated with the recently
discovered phenomenon of carrier multiplication, an advance that enables quantum efficiencies
for carrier generation exceeding 100%. This phenomenon also suggests new approaches to solar-
driven photochemical processes that require multiple oxidation/reduction steps by eliminating
the need to store charge from sequential single-photon/single-electron steps.
INTERFACE SCIENCE OF PHOTO-DRIVEN SYSTEMS
Interfaces are integral to most schemes for solar energy conversion, including solar generation of
electricity and fuels and thermoelectrics. For these technologies, the successful control of the
properties of interfaces between dissimilar materials is essential. Mechanical stability, charge
separation, and charge transfer depend upon detailed atomic configurations, interfacial
chemistry, and electronic coupling. Interfaces can be solid-solid, liquid-solid, and liquid-liquid
and include both bulk-like junctions and junctions between nanomaterials and solids, polymers,
molecules, and solvents. Organic-inorganic material junctions are also expected to play a role in
solar conversion technologies.
The prevalence and importance of interfaces, as well as the serious difficulties that their control
often imposes, can be easily understood by reference to a few of the critical underlying technical
issues. An issue of broad importance is that of electrical contacts. This challenge arises in all PV
approaches to solar energy in which solar energy is removed from the device in the form of
electrical current. A particularly challenging issue concerns transparent conductors (Ohta et al.
2003; Wu 2004), which are vital for most implementations of PV devices (Figure 19). Another
common bottleneck in the exploration of new materials lies in making reliable contacts of low
resistance. Interfaces are also critical with respect to carrier trapping and recombination
processes that often significantly decrease the efficiency of PV devices. The role of interfaces is,
of course, even more critical in the new approaches that use nanostructured materials (O’Brien
and Pickett 2001) because of their enhanced ratio of interface-to-bulk material.