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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nanostructure design, and development <strong>of</strong> nanoscale pore architectures that steer reaction<br />
intermediates to desired fuel products. Such assemblies could be developed in the <strong>for</strong>m <strong>of</strong><br />
nanoporous membranes, in effect producing an artificial “leaf.”<br />
Multi-junction solar cells convert light to electricity across the solar spectrum, and are the<br />
highest-efficiency solar conversion devices known. In these devices, high quantum efficiency is<br />
achieved only with epitaxially grown single crystal heterojunctions, which are prohibitively<br />
expensive to produce. The analogous nanocrystal heterojunction devices either do not exist, or<br />
have not yet been tested as solar photoconversion devices. However, nanomaterials <strong>of</strong>fer many<br />
potential advantages <strong>for</strong> solar cells, such as the low cost <strong>of</strong> single crystal synthesis, tolerance <strong>for</strong><br />
lattice mismatch in junctions, and the ability to control three-dimensional architecture through<br />
shape-controlled growth, microphase separation, and layer-by-layer synthesis. Novel<br />
architectures such as branched nanocrystals, nanowires, nanoribbons, and nanotubes provide<br />
useful building blocks <strong>for</strong> coupling <strong>of</strong> light-harvesting and photocatalytic components into<br />
functioning photocatalytic assemblies. The challenge is to design these assemblies to drive<br />
energetically demanding reactions, such as water-splitting, by using visible and near-infrared<br />
light.<br />
NEW SCIENTIFIC OPPORTUNITIES<br />
The design and preparation <strong>of</strong> an integrated, molecule-based system that will convert sunlight<br />
into useful fuels is a challenging goal. However, natural photosynthesis has already achieved this<br />
goal within the context <strong>of</strong> the biological world. By understanding the natural process and<br />
exploiting it in artificial constructs, it will be possible to construct artificial photosynthetic<br />
systems maximized <strong>for</strong> production <strong>of</strong> fuels useful to human society.<br />
Understand the Dependence <strong>of</strong> Excitation <strong>Energy</strong> and Charge Flow on Molecular<br />
Structure and Intermolecular Boundaries from the Molecular to the Device Scale<br />
A major scientific challenge is to develop a complete understanding <strong>of</strong> how weak, non-covalent,<br />
associative interactions, such as hydrogen bonds and π-π interactions, promote or inhibit energy<br />
and charge flow across molecular boundaries. This is critical to achieving an integrated artificial<br />
photosynthetic system because <strong>for</strong>mation <strong>of</strong> a functional system by self-assembly <strong>of</strong> building<br />
blocks requires controlled energy and charge flow across the weak associative points <strong>of</strong><br />
molecular contact. Studies are also needed on nanostructured and self-assembling junctions<br />
(e.g., at semiconductor nanocrystal/polymer and polymer/polymer interfaces) to understand the<br />
effects <strong>of</strong> composition, dimensionality, and overall architecture on the dynamics <strong>of</strong> excitons and<br />
charge carriers. In addition, to design better interfacial catalysts <strong>for</strong> water oxidation and fuel<br />
<strong>for</strong>mation, the detailed molecular understanding that is being developed <strong>for</strong> molecular catalysts<br />
needs to be translated to surface-bound and colloidal catalysts. This requires the development <strong>of</strong><br />
time-resolved structure-specific spectroscopic tools (vibrational, X-ray Absorption Fine<br />
Structure [XAFS], etc.) with very high sensitivity to identify transient intermediates and catalyst<br />
structural changes under reaction conditions. Theory and computational tools must also be<br />
developed to assist experimental studies with the goal <strong>of</strong> identifying active sites on surfaces with<br />
atomic precision.<br />
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