Advances across several science frontiers suggest new approaches that could enable such rational
design, particularly for low-dimensional and tailored multi-component materials. Controlling the
size and dimensionality of the structures on the nanoscale would allow scientists to modify the
density of electronic states, as well as of phonons (Alivisatos 1996; Empedocles and Bawendi
1999; Cahill et al. 2003). In addition, entirely new processes may emerge that expand our view
of how solar energy systems can be designed. One example is the recent discovery of efficient
carrier multiplication in the photo-excitation of semiconducting quantum dots (Schaller and
Klimov 2004). The formation of multiple excitons following the absorption of a single photon
can reduce the loss of energy to heat that usually accompanies carrier relaxation to the band edge
and otherwise places fundamental limits on the efficiency of PV solar energy conversion
(Werner et al. 1994). While the design of structures with optimized properties for the control of
carrier excitation, charge transport, and energy migration remains a challenging problem, recent
advances in the synthesis and assembly of high-quality multi-component and hybrid
nanostructures, in concert with advances in our ability to probe and understand the relationship
between structure and function in model systems, offer a realistic path to achieving this goal.
Research Issues
The ability to synthesize high-quality samples of novel materials forms the foundation for
progress toward the goal of rational design. The promise offered by the control of elementary
processes is suggested particularly in low-dimensional and multi-component materials. It has
long been understood that while the absorption spectra of semiconductor quantum dots are tuned
by the confinement size (Alivisatos 1996; Empedocles and Bawendi 1999), the ligand fields
surrounding the quantum dots also affect absorption spectra and excitation lifetimes (Murray and
Kalyuzhny 2005). Although nanoscale synthesis research efforts are well underway (O’Brien and
Pickett 2001), the ability of scientists to control the composition, shape, morphology, and quality
of nanostructured materials is still inadequate. Synthesis of high-quality, multi-component
nanomaterials is one example. Such multi-component structures could provide heterogeneous
band-gap junction structures that are critical for PV applications, but with controlled excitation
lifetimes. Another opportunity lies in the synthesis of hybrid materials, including those with
controlled interfaces between hard and soft materials, where the advantages of each are exploited
in the resulting hybrid (Wu et al. 2002). The synthesis of a wide variety of novel materials is
essential to enable the rational design of solar energy materials with controlled elementary
processes.
The synthesis of high-quality materials must be coupled to the development and exploitation of
new characterization tools capable of resolving elementary physical processes at appropriate
length and time scales and with sufficient energy resolution. This effort must include (1) the
development of laboratory tools and techniques, such as optical techniques to probe carrier
dynamics on ultra-fast time scales; and (2) the improvement of electron-microscopy techniques
to allow higher resolution and larger working distances for in situ transmission electron
microscopy (TEM) studies. On another scale, the effort requires the development of national
facilities to provide new tools, such as advanced synchrotrons to probe solar material
nanostructures with greater energy and spatial resolution. Given the complexity of the materials
and the underlying physical processes, it is critical to have an array of experimental tools that can
probe the diverse properties that control the functionality of novel materials.