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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Figure 46 Defect-tolerant solar<br />
cell: dye-sensitized solar cell<br />
using defective nanoparticulate<br />
TiO2 as electrodes<br />
mismatch strain energy even <strong>for</strong> highly disparate material<br />
combinations, enabling dense arrays <strong>of</strong> defect-free<br />
nanostructures. Alternatively, progress in dye-sensitized<br />
nanocrystal-based solar cells provides an excellent example<br />
<strong>of</strong> how a system that tolerates a lack <strong>of</strong> perfect structural<br />
order <strong>of</strong>fers a new, potentially disruptive technology that<br />
could have significant impact.<br />
On the other hand, “defect tolerance” in s<strong>of</strong>t materials <strong>for</strong><br />
photoconversion encompasses two main ideas: self-repair<br />
and redundant connectivity. Self-repair can be achieved in<br />
several ways: (a) by molecular rearrangement, producing a<br />
new defect-free structure because the repaired structure is<br />
thermodynamically more stable than a grossly damaged one;<br />
(b) using biological structures, including energy-converting<br />
structures, swapping out damaged sub-components <strong>of</strong>ten<br />
(such as molecular chromophores), and replacing them with newly manufactured ones; or<br />
(c) leaving damaged components in place and fixing them, rather than replacing or expelling<br />
them, such as in enzymatic repair <strong>of</strong> damaged DNA. “Redundant connectivity” ensures that<br />
defects do not disproportionately degrade system per<strong>for</strong>mance; it is achieved through<br />
multiplicity <strong>of</strong> equivalent current pathways and is <strong>of</strong> special importance <strong>for</strong> nanoscale-materialbased<br />
solar cells that operate in a current percolation mode. An example is the nanoparticulate<br />
photoelectrode <strong>of</strong> the dye-sensitized nanostructured solar cell — sintering redundantly or multidimensionally<br />
interconnects particles, as shown in Figure 46.<br />
Within photosynthesis, the most dramatic self-repairing system is the reaction center <strong>of</strong><br />
Photosystem II (PSII). PSII catalyzes the light-driven splitting <strong>of</strong> water and involves highly<br />
oxidative chemistry. The D1-protein binds the majority <strong>of</strong> the c<strong>of</strong>actors involved in light-driven<br />
charge transfer reactions <strong>of</strong> PSII, including the primary electron donor P680 and the Mn-cluster<br />
at which the water-splitting reaction occurs. It seems highly likely that the oxidative damage to<br />
the D1-protein is due to singlet oxygen and/or oxygen radicals <strong>for</strong>med during the water-splitting<br />
process. The vulnerable D1<br />
protein is removed from the<br />
complex from time to time (about<br />
30–60 minutes in an illuminated<br />
leaf) and replaced by a newly<br />
synthesized D1-protein. Recent<br />
biochemical and molecular<br />
biological studies are starting to<br />
reveal the nature <strong>of</strong> this process<br />
(see Figure 47), yet the molecular<br />
details <strong>of</strong> this remarkable repair<br />
mechanism are unknown and are<br />
worthy <strong>of</strong> more intense research.<br />
Figure 47 Repair <strong>of</strong> PSII by degrading photo-damaged D1<br />
protein and replacing it with newly synthesized D1 protein<br />
146