Basic Research Needs for Solar Energy Utilization

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 TiO 2.

The TiO 2 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 TiO 2 , creating a positively charged dye molecule (the hole); this phenomenon produces the charge separation required for a PV cell.

The TiO 2 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 TiO 2 film, where it donates

electrons to the positively charged dye sitting on the TiO 2 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