Figure 35 p-n Photoelectrolysis cell
(photochemical diode). The n-type anode and p-
type cathode are connected in a bi-layered
monolithic structure through a contact that
produces electron-hole recombination to permit
charge balance. (Source: Nozik 1977)can be used to reduce the light scattering in the electrolyte caused by bubble formation when
gases are evolved.
Multiple-band-gap Systems. A photoelectrolysis system could couple a p-type and an n-type
semiconductor to drive photoreductions and photo-oxidations, respectively, where band gaps in
the range of 1.0–1.2 eV for each electrode would be optimum; for a single-band-gap device, the
optimum is in the range of 1.6–2.0 eV (see Figure 35). This approach provides extra voltage or
driving force for photoelectrolysis, but it
lowers the quantum yield by a factor of two.
However, the two smaller-band-gap
semiconductors can extend the utilization of
the solar spectrum into the near infrared.
Tandem configurations could be used, in
which different band gap p-type and n-type
electrodes are stacked so that the light
impinges first on the higher-gap semi-
conductor, where the high-energy photons
are absorbed and converted to photo-
products. The lower-gap semiconductor
then absorbs the light passing through the
higher-gap semiconductor to perform the
complementary photoelectrolysis reaction.
Material and device properties in multiple-
band-gap systems require discovery of at
least two semiconductors that must be
configured to match the currents in the two
electrodes to achieve optimum device
efficiency.
SCIENTIFIC CHALLENGES
A high-throughput search for photoelectrolysis electrodes will produce libraries of new
candidates that may also be useful for other scientific problems of relevance to energy
conversion, such as fuel cell materials and catalysts. The coupling of computational science with
experimental search techniques represents a new approach to exploration and discovery of new
photoelectrodes with specific and unique properties.
POTENTIAL IMPACT
Successful research on new highly efficient, stable, and cost-effective photoelectrodes for
photoelectrolysis represents a major advance in the critically important goal to produce hydrogen
from solar energy and water, and hence, would help create the hydrogen economy. Storage of
solar energy as useful fuels is needed to level out the demand cycle with the availability of
sunlight.