Basic Research Needs for Solar Energy Utilization

Figure 47 Repair of PSII by degrading photo-damaged D1

protein and replacing it with newly synthesized D1 protein

Figure 46 Defect-tolerant solar

cell: dye-sensitized solar cell

using defective nanoparticulate

TiO 2 as electrodes

mismatch strain energy even for highly disparate material

combinations, enabling dense arrays of defect-free

nanostructures. Alternatively, progress in dye-sensitized

nanocrystal-based solar cells provides an excellent example

of how a system that tolerates a lack of perfect structural

order offers a new, potentially disruptive technology that

could have significant impact.

On the other hand, “defect tolerance” in soft materials for
photoconversion encompasses two main ideas: self-repair
and redundant connectivity. Self-repair can be achieved in
several ways: (a) by molecular rearrangement, producing a
new defect-free structure because the repaired structure is
thermodynamically more stable than a grossly damaged one;
(b) using biological structures, including energy-converting
structures, swapping out damaged sub-components often
(such as molecular chromophores), and replacing them with newly manufactured ones; or
(c) leaving damaged components in place and fixing them, rather than replacing or expelling
them, such as in enzymatic repair of damaged DNA. “Redundant connectivity” ensures that
defects do not disproportionately degrade system performance; it is achieved through
multiplicity of equivalent current pathways and is of special importance for nanoscale-material-
based solar cells that operate in a current percolation mode. An example is the nanoparticulate
photoelectrode of the dye-sensitized nanostructured solar cell — sintering redundantly or multi-
dimensionally interconnects particles, as shown in Figure 46.

Within photosynthesis, the most dramatic self-repairing system is the reaction center of
Photosystem II (PSII). PSII catalyzes the light-driven splitting of water and involves highly
oxidative chemistry. The D1-protein binds the majority of the cofactors involved in light-driven
charge transfer reactions of PSII, including the primary electron donor P680 and the Mn-cluster
at which the water-splitting reaction occurs. It seems highly likely that the oxidative damage to
the D1-protein is due to singlet oxygen and/or oxygen radicals formed during the water-splitting
process. The vulnerable D1
protein is removed from the
complex from time to time (about
30–60 minutes in an illuminated
leaf) and replaced by a newly
synthesized D1-protein. Recent
biochemical and molecular
biological studies are starting to
reveal the nature of this process
(see Figure 47), yet the molecular
details of this remarkable repair
mechanism are unknown and are
worthy of more intense research.