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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NEW SCIENTIFIC OPPORTUNITIES<br />
Development <strong>of</strong> new defect-tolerant inorganic PV materials will require combined experimental<br />
and theoretical ef<strong>for</strong>ts aimed at understanding the factors affecting interactions between a large<br />
variety <strong>of</strong> possible structural defects and charge carriers. This knowledge could lead to the<br />
design and discovery <strong>of</strong> new classes <strong>of</strong> materials satisfying the multiple constraints <strong>of</strong> highvolume,<br />
low-cost PV systems, including utilizing abundant elements, environmentally benign<br />
chemical components, simplicity <strong>of</strong> synthesis and processing, and high PV per<strong>for</strong>mance<br />
efficiency.<br />
Nanoscale building blocks <strong>of</strong>fer many potential advantages <strong>for</strong> solar energy research, such as the<br />
low cost <strong>of</strong> single-crystal synthesis, the above-noted tolerance <strong>for</strong> lattice mismatch in<br />
heterojunctions, and the ability to control three-dimensional architecture through shapecontrolled<br />
growth, microphase separation, and layer-by-layer synthesis. Novel architectures such<br />
as branched nanocrystals, and templated nanowires and nanotubes provide useful building blocks<br />
<strong>for</strong> coupling <strong>of</strong> light and photocatalytic components into functioning photocatalytic assemblies.<br />
The challenge is to design these assemblies in order to drive energetically demanding reactions,<br />
such as water-splitting, by using visible and near-infrared light. It is very important to explore<br />
catalysts that are resistant to poisoning.<br />
The challenge <strong>of</strong> using assembly-disassembly strategies <strong>for</strong> self-repair is the need to understand<br />
the molecular details <strong>of</strong> how to prepare modular artificial photosynthetic systems. These systems<br />
must depend on non-covalent interactions <strong>for</strong> their assembly and disassembly. The disassembly<br />
process must be initiated by recognition <strong>of</strong> specific damage motifs in the overall artificial<br />
photosynthetic system. This requires a design that identifies and anticipates the structural<br />
consequences <strong>of</strong> the principal damage motifs. Once these pathways are identified, the overall<br />
molecular recognition properties <strong>of</strong> each module (which will be based on weak interactions such<br />
as hydrogen-bonding, metal-ligand interactions, and/or π-π stacking <strong>of</strong> chromophores) must be<br />
optimized so that a particular module will tolerate only a narrow range <strong>of</strong> con<strong>for</strong>mations to<br />
recognize its partner modules. Deviations from this narrow range <strong>of</strong> con<strong>for</strong>mations induced by<br />
damage in one or more modules will result in spontaneous disassembly driven by<br />
thermodynamics. Reassembly with intact modules will be driven by having excess intact<br />
modules present in equilibrium with the overall system. This type <strong>of</strong> approach should work<br />
reasonably well <strong>for</strong> artificial photosynthetic systems immobilized at surfaces, where they could<br />
be exposed to a “repair solution” containing the modules needed <strong>for</strong> replacement.<br />
The most challenging and potentially most general approach to self-repair is the design <strong>of</strong> smart<br />
molecules that will (a) seek out damage sites within a modular artificial photosynthetic system,<br />
(b) recognize the damage site, (c) execute a structural repair, and (d) leave the site to seek other<br />
damage. This approach requires building into molecules the self-autonomous features that are<br />
common in biology, but have not yet been developed <strong>for</strong> non-living systems.<br />
RELEVANCE AND POTENTIAL IMPACT<br />
Achieving defect-tolerant or active self-repair devices would enable the practical utilization <strong>of</strong><br />
many types <strong>of</strong> solar energy conversion systems that are currently too unstable to last <strong>for</strong> the<br />
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