Basic Research Needs for Solar Energy Utilization - Office of ...

PHOTOELECTROCHEMICAL SOLAR CELLS
Photoelectrochemical (PEC) solar cells are based on hybrid structures of inorganic semiconductors and molecular structures. In one
configuration (called an electrochemical photovoltaic [EPV] cell), a semiconductor is in contact with an electrically conducting liquid
(called an electrolyte) that also contains a chemical species (called a reduction-oxidation or redox couple) that can readily donate
electrons to and accept electrons from an electrode. The semiconductor forms a junction with the liquid by simple immersion and
develops an electric field at its surface. The semiconductor can be n-type or p-type. Upon illumination of the semiconductor, the
photogenerated electrons and holes can separate because of the surface electric field. For n-type semiconductors, the holes move to
the surface and are captured by the redox couple; the electrons move to the back side of the semiconductor, where they leave the cell
via an electrical contact, deliver electrical power to an external load, and then return to the cell at the second electrode. Here, they are
captured by the redox species that initially captured the hole at the semiconductor surface; this process returns the redox species to its
original condition. Thus the redox couple accepts holes at one electrode and accepts electrons at the other electrode — resulting in
charge neutralization and no net change in the redox species. The electrolyte and redox couple just serves to complete the electrical
circuit and to produce the electric field required for charge separation.
In a second configuration, dye molecules that absorb sunlight are adsorbed onto thin films of sintered nanocrystalline particles of TiO2.
The TiO2 does not absorb much of the sunlight because its band gap is too big (3.0 eV); rather, the dye molecules absorb the sunlight
and produce an energetic state (called an excited state). The excited state of the dye molecules results in the injection of electrons into
the TiO2, creating a positively charged dye molecule (the hole); this phenomenon produces the charge separation required for a PV cell.
The TiO2 film is in contact with an electrolyte containing a redox couple. The circuit is completed when the electrons return to the cell,
are captured by a redox species at the second electrode (usually a metal), which then diffuses to the TiO2film, where it donates
electrons to the positively charged dye sitting on the TiO2 surface to neutralize it, returning the dye molecules to their original state.
Organic solar cells also operate with junctions, but the n-type and p-type semiconductors are organic compounds, and the interfacial
junction between the n- and p-type regions does not produce an electric field and serves a different purpose than the inorganic p-n
junctions. Furthermore, when electrons and holes are produced upon light absorption in organic solar cells, the negative electrons and
positive holes become bound to one another through strong attractive electrical forces and form coupled electron-hole pairs, which have
been labeled excitons. These excitons have no net electrical charge and cannot carry current — they must be broken apart, or
dissociated, in order to produce the free electrons and holes required in the cell to produce electrical power. This is the function of the
junction between the n- and p-type organic compounds — when the excitons diffuse to this region of the cell, they split apart and
produce the required free electrons and holes. Also, organic solar cells have electrical contacts with different electronic properties.
Electrochemical Solar Cell Dye-sensitized Nanocrystalline Solar Cell
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