Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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Advances across several science frontiers suggest new approaches that could enable such rational<br />
design, particularly <strong>for</strong> low-dimensional and tailored multi-component materials. Controlling the<br />
size and dimensionality <strong>of</strong> the structures on the nanoscale would allow scientists to modify the<br />
density <strong>of</strong> electronic states, as well as <strong>of</strong> phonons (Alivisatos 1996; Empedocles and Bawendi<br />
1999; Cahill et al. 2003). In addition, entirely new processes may emerge that expand our view<br />
<strong>of</strong> how solar energy systems can be designed. One example is the recent discovery <strong>of</strong> efficient<br />
carrier multiplication in the photo-excitation <strong>of</strong> semiconducting quantum dots (Schaller and<br />
Klimov 2004). The <strong>for</strong>mation <strong>of</strong> multiple excitons following the absorption <strong>of</strong> a single photon<br />
can reduce the loss <strong>of</strong> energy to heat that usually accompanies carrier relaxation to the band edge<br />
and otherwise places fundamental limits on the efficiency <strong>of</strong> PV solar energy conversion<br />
(Werner et al. 1994). While the design <strong>of</strong> structures with optimized properties <strong>for</strong> the control <strong>of</strong><br />
carrier excitation, charge transport, and energy migration remains a challenging problem, recent<br />
advances in the synthesis and assembly <strong>of</strong> high-quality multi-component and hybrid<br />
nanostructures, in concert with advances in our ability to probe and understand the relationship<br />
between structure and function in model systems, <strong>of</strong>fer a realistic path to achieving this goal.<br />
<strong>Research</strong> Issues<br />
The ability to synthesize high-quality samples <strong>of</strong> novel materials <strong>for</strong>ms the foundation <strong>for</strong><br />
progress toward the goal <strong>of</strong> rational design. The promise <strong>of</strong>fered by the control <strong>of</strong> elementary<br />
processes is suggested particularly in low-dimensional and multi-component materials. It has<br />
long been understood that while the absorption spectra <strong>of</strong> semiconductor quantum dots are tuned<br />
by the confinement size (Alivisatos 1996; Empedocles and Bawendi 1999), the ligand fields<br />
surrounding the quantum dots also affect absorption spectra and excitation lifetimes (Murray and<br />
Kalyuzhny 2005). Although nanoscale synthesis research ef<strong>for</strong>ts are well underway (O’Brien and<br />
Pickett 2001), the ability <strong>of</strong> scientists to control the composition, shape, morphology, and quality<br />
<strong>of</strong> nanostructured materials is still inadequate. Synthesis <strong>of</strong> high-quality, multi-component<br />
nanomaterials is one example. Such multi-component structures could provide heterogeneous<br />
band-gap junction structures that are critical <strong>for</strong> PV applications, but with controlled excitation<br />
lifetimes. Another opportunity lies in the synthesis <strong>of</strong> hybrid materials, including those with<br />
controlled interfaces between hard and s<strong>of</strong>t materials, where the advantages <strong>of</strong> each are exploited<br />
in the resulting hybrid (Wu et al. 2002). The synthesis <strong>of</strong> a wide variety <strong>of</strong> novel materials is<br />
essential to enable the rational design <strong>of</strong> solar energy materials with controlled elementary<br />
processes.<br />
The synthesis <strong>of</strong> high-quality materials must be coupled to the development and exploitation <strong>of</strong><br />
new characterization tools capable <strong>of</strong> resolving elementary physical processes at appropriate<br />
length and time scales and with sufficient energy resolution. This ef<strong>for</strong>t must include (1) the<br />
development <strong>of</strong> laboratory tools and techniques, such as optical techniques to probe carrier<br />
dynamics on ultra-fast time scales; and (2) the improvement <strong>of</strong> electron-microscopy techniques<br />
to allow higher resolution and larger working distances <strong>for</strong> in situ transmission electron<br />
microscopy (TEM) studies. On another scale, the ef<strong>for</strong>t requires the development <strong>of</strong> national<br />
facilities to provide new tools, such as advanced synchrotrons to probe solar material<br />
nanostructures with greater energy and spatial resolution. Given the complexity <strong>of</strong> the materials<br />
and the underlying physical processes, it is critical to have an array <strong>of</strong> experimental tools that can<br />
probe the diverse properties that control the functionality <strong>of</strong> novel materials.<br />
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