Achieving Defect-tolerant and Self-repairing Solar Conversion Systems
No molecule-based solar energy conversion system, including photosynthesis, or system made
from amorphous silicon or many other thin-film inorganic materials, currently maintains its
performance in sunlight for 20–30 years. Defect formation mechanisms in photovoltaic
materials, as well as self-repair and photo-protection mechanisms in photosynthesis, must be
understood and implemented in real-world systems. Achieving defect-tolerant or active self-
repair devices would enable the practical utilization of many types of solar energy conversion
systems that currently are too unstable to last for the required 20–30 years of operation to
produce cost-effective solar electricity or fuels.
EXECUTIVE SUMMARY
Insensitivity of performance to manufacturing and usage-induced defects over a pre-specified
range is an inherent requirement of system design, specification, affordability, and performance
for extended periods of time in real-world systems. By way of contrast, biological systems, such
as photosynthesis, have built-in repair mechanisms that can restore useful function following
damage to the system. Identifying and implementing fault-tolerant and/or self-repair paradigms
in inorganic and organic systems is a “grand-challenge”-level basic science enterprise that would
revolutionize not only the solar energy conversion field but a wide variety of other application
areas as well. To ensure that complex systems designed for solar fuels production maintain their
efficiency over long lifetimes, the following research directions must be addressed:
(1) understand the factors affecting interactions between a large variety of possible structural
defects and charge carriers in inorganic PV materials; (2) understand repair and photoprotection
mechanisms in natural photosynthesis; (3) control three-dimensional architectures in nanoscale
materials to promote redundancy and distributed function as a strategy to tolerate defects;
(4) explore assembly-disassembly strategies as a mode of self-repair; and (5) develop active
repair molecules that specifically identify and target defects and repair them.
SUMMARY OF RESEARCH DIRECTION
The current highest efficiency (~35%) solar cells are the nearly perfect epitaxially grown
compound semiconductor multi-junction structures that collect light across the solar spectrum
but are both sensitive to defects and prohibitively expensive to mass-produce. There is thus a
critical need to discover, design, and synthesize new materials and structures with solar energy
conversion properties that are intrinsically insensitive to defects (point and line defects, grain
boundaries, impurity/composition, disorder, morphology, and interface defects) and thus relax
the strict requirements on manufacturing of nearly perfect material structures. Since absorption
of solar photons and separating the resulting electron-hole pair (usually exciton) is the
fundamental basis for conversion to electricity, designing materials and structures that inherently
permit reduction of defects or allow charge separation and transport mechanisms tolerant of
defects is the fundamental challenge faced in realizing high-performance and affordably
manufacturable photovoltaic (PV) solar cells. An example of the defect-reduction approach is the
use of selective growth on nanoscale spatial templates, which allows relief of the lattice