One critical property of the photoactive material in either a PV or a PEC system is the minority
carrier diffusion length (i.e., the distance that electrons or holes created upon light absorption can
travel in the structure before they recombine to produce heat). If the minority carrier diffusion
length is too short, photogenerated carriers cannot reach the interface to drive the desired
reactions and produce output power in the form of electricity or fuels. This is the basic limitation
of cheap absorbers: because they have a large concentration of impurities and defects, and
therefore have a short minority carrier diffusion length, they generally produce PV cells with low
efficiencies. However, the development of nanoscale materials has rescaled the diffusion length
requirements, because the minority carrier diffusion length need only be comparable to the
dimensions of the sub-units of the nanostructured device (typically a few tens of nanometers).
The liquid contact in PEC systems is ideally suited for nanostructured devices, guaranteeing
complete filling of the gaps within the porous structures. The goal is to improve the efficiency of
such systems by a factor of 2–3, while ensuring stability and robust performance for periods of
years in sunlight under maximum load conditions.
Another key advantage of PEC cells is that they naturally offer the opportunity to integrate the
energy conversion and storage functions. Photoelectrochemical cells have been shown to directly
split water into H 2 and O 2 , thereby providing a basis for the renewable, clean production of
hydrogen from sunlight. The materials can be cheap polycrystalline forms, because of the
relaxed requirements on the minority carrier diffusion length. However, the known materials,
which are robust in water splitting, are not responsive to a wide portion of the solar radiation
spectrum; they work best in the ultraviolet (UV) — yielding relatively low efficiencies at the
surface of the Earth. Finding new photoelectrodes, either individually or in combination, that can
allow the efficient, integrated conversion of sunlight to chemical fuels is one of the primary aims
of solar energy conversion research. The goal is to identify PEC systems that display the same
efficiency and stability for visible-light-induced water splitting as those demonstrated for near-
UV-light-induced water splitting. Closing this gap will lead to the development of cheap and
efficient systems that, in an integrated fashion, could produce chemical fuels (e.g., hydrogen)
directly from sunlight and therefore directly address, not only the conversion, but also the storage
issues, associated with solar energy conversion schemes.
These examples illustrate the central role played by nanostructured systems in the renaissance of
PEC systems, which are poised to make an important contribution to solar energy conversion and
storage. Additional basic scientific research will support the development of improvements in the
existing nanostructured cell and photoelectrolysis approaches and will also support the discovery
of disruptive technologies that can dramatically accelerate progress toward high-efficiency solar
photon conversion technologies at low cost.