PHOTOELECTROCHEMISTRY

Photoelectrochemical (PEC) systems provide the best-known wet chemical method of convertingsunlight into electrical energy or chemical fuels. PEC systems developed during the mid-1970sand 1980s for capture and conversion of solar energy into electricity and fuels are based on asemiconductor electrode in contact with an electrolyte solution (Memming 2001; Bard et al.2002; Nozik and Memming 1996; Nozik 1978; Grätzel 2001). The solid-liquid configurationoffers the following four advantages: (1) the junction required for efficient charge separation ofphotogenerated electrons and holes is very easily formed by simply immersing thesemiconductor in an appropriate electrolyte solution; (2) the liquid electrolyte offers thecapability of a readily conformable and strain-free junction; (3) a third electrode can be added toPEC cells to provide in-situ chemical storage for 24-hr/day power; and (4) the conversion of light energy directly into fuel eliminates the need for external wires and a separate electrolyzer.The PEC approach to solar energy conversion has achieved high efficiencies for both electricalpower (>15%) and hydrogen generation (>10%). However, photoelectrode lifetime and costissues have restricted commercialization efforts to date .
The electric field formed by the junction at the semiconductor-electrolyte interface plays a largerole in efficiently separating the electron-hole pairs created by light absorption. When electronsor holes cross the interface between the semiconductor and the electrolyte, they can drivechemical oxidation or reduction half-reactions at the semiconductor surface to produce eitherfuels (e.g., hydrogen, reduced carbon, ammonia) or, in a regenerative photovoltaic configuration,DC electrical power. In the latter case, termed an electrochemical photovoltaic (EPV) cell, theoxidation and reduction half-reactions at the two cell electrodes are the inverse of one another,thus producing no net change in the electrolyte. In the former case, the oxidation and reductionhalf-reactions at the two electrodes are different, and their sum produces a net chemical changein the electrolyte (e.g., decomposition of water into H2 and O2).
One critical property of the photoactive material in either a PV or a PEC system is the minoritycarrier diffusion length (i.e., the distance that electrons or holes created upon light absorption cantravel in the structure before they recombine to produce heat). If the minority carrier diffusionlength is too short, photogenerated carriers cannot reach the interface to drive the desiredreactions and produce output power in the form of electricity or fuels. This is the basic limitationof cheap absorbers: because they have a large concentration of impurities and defects, andtherefore have a short minority carrier diffusion length, they generally produce PV cells with lowefficiencies. However, the development of nanoscale materials has rescaled the diffusion lengthrequirements, because the minority carrier diffusion length need only be comparable to thedimensions 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, guaranteeingcomplete filling of the gaps within the porous structures. The goal is to improve the efficiency ofsuch systems by a factor of 2–3, while ensuring stability and robust performance for periods ofyears in sunlight under maximum load conditions.
Another key advantage of PEC cells is that they naturally offer the opportunity to integrate theenergy conversion and storage functions. Photoelectrochemical cells have been shown to directlysplit water into H2 and O2, thereby providing a basis for the renewable, clean production ofhydrogen from sunlight. The materials can be cheap polycrystalline forms, because of therelaxed 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 radiationspectrum; they work best in the ultraviolet (UV) — yielding relatively low efficiencies at thesurface of the Earth. Finding new photoelectrodes, either individually or in combination, that canallow the efficient, integrated conversion of sunlight to chemical fuels is one of the primary aimsof solar energy conversion research. The goal is to identify PEC systems that display the sameefficiency 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 andefficient 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 storageissues, associated with solar energy conversion schemes.
Photoelectrochemical (PEC) solar cells are based on hybrid structures of inorganic semiconductors and molecular structures. In oneconfiguration (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 donateelectrons to and accept electrons from an electrode. The semiconductor forms a junction with the liquid by simple immersion anddevelops an electric field at its surface. The semiconductor can be n-type or p-type. Upon illumination of the semiconductor, thephotogenerated electrons and holes can separate because of the surface electric field. For n-type semiconductors, the holes move tothe surface and are captured by the redox couple; the electrons move to the back side of the semiconductor, where they leave the cellvia an electrical contact, deliver electrical power to an external load, and then return to the cell at the second electrode. Here, they arecaptured by the redox species that initially captured the hole at the semiconductor surface; this process returns the redox species to itsoriginal condition. Thus the redox couple accepts holes at one electrode and accepts electrons at the other electrode — resulting incharge neutralization and no net change in the redox species. The electrolyte and redox couple just serves to complete the electricalcircuit 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 sunlightand produce an energetic state (called an excited state). The excited state of the dye molecules results in the injection of electrons intothe 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 donateselectrons 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 interfacialjunction between the n- and p-type regions does not produce an electric field and serves a different purpose than the inorganic p-njunctions. Furthermore, when electrons and holes are produced upon light absorption in organic solar cells, the negative electrons andpositive holes become bound to one another through strong attractive electrical forces and form coupled electron-hole pairs, which havebeen labeled excitons. These excitons have no net electrical charge and cannot carry current — they must be broken apart, ordissociated, in order to produce the free electrons and holes required in the cell to produce electrical power. This is the function of thejunction between the n- and p-type organic compounds — when the excitons diffuse to this region of the cell, they split apart andproduce the required free electrons and holes. Also, organic solar cells have electrical contacts with different electronic properties.