interface for charge separation. For instance, a self-assembling, thin inorganic charge-mediating
layer with appropriate electronic levels, covering the nanostructured surface, could allow for
vectorial charge transfer from the sensitizer to the conduction band of the semiconducting oxide,
while blocking the back electron transfer to the oxidized sensitizer or to the hole-carrying species
in the pore structure. Development of ionic or electronic conductors with high charge mobility
will be required to transmit the holes rapidly to the collecting electrode. Designing and
developing novel materials and fabrication methodologies that are compatible for high-
throughput, low-cost fabrication would also be useful.
Enhance Understanding of Nanostructured Photoelectrochemical Systems and
Use the Knowledge to Establish and Control the Factors Governing the Efficiency
and Stability of Nanostructured Sensitized Solar Cells
Theoretical and experimental studies should be aimed at understanding the phenomena
determining light absorption, charge or exciton generation, charge separation, transport,
recombination, and, ultimately, cell efficiency. Methodologies should be developed for forming
ordered nanostructured, inorganic electrodes for optimum charge separation, and the effect of the
resulting structures on the photoconversion properties should be investigated. Near-infrared
absorbing molecular and quantum-confined sensitizers should be developed and studied. The
stability of these sensitizers to ultraviolet and visible photolysis, moisture, and oxygen should
also be studied. If the sensitizers degrade, approaches to suppress the photodegradation processes
should be explored. Light management strategies should be investigated and exploited. Self-
assembling molecular, supermolecular, and inorganic interface layers should be developed and
studied. Factors determining the transport and recombination dynamics should be studied.
Incorporate Novel Third-Generation Concepts to Achieve Efficiencies Beyond the
Shockley-Queisser Limit of 32%
To exceed the Shockley-Queisser limit, special light-harvesting (sensitizing) units, such as
selected molecular sensitizers, and quantum-scaled structures (i.e., quantum dots) capable of
generating charge carriers at a quantum yield greater than 100% must also be identified through
detailed study. To fully utilize the usable regions of the solar spectrum, photon-energy up-
conversion schemes (using, e.g., multijunction band-gap nanostructures and rare earth metal
compounds) must be developed and understood. It will be important to understand the charge-
carrier extraction and transport dynamics to enable the multiple excitons from quantum dots and
charge carriers from oxidized sensitizers to escape recombination or deactivation and reach the
collecting electrodes. It will be critical to identify and elucidate systems with long-lived
intermediate charge-separated states and low-energy absorption bands. It will also be necessary
to develop the basic knowledge base to create and eventually control the energetics for matching
electronic band gaps and for tailoring the requisite optical properties or potential sensitizers.