through the liquid electrolyte to the surface of the silicon electrode. There they react with the four holes associated with the four photoelectrons, the result being two water molecules and an oxygen molecule. Illuminated silicon immediately begins to corrode under contact with the electrolytes. The corrosion consumes material and disrupts the properties of the surfaces and interfaces within the cell. Two types of photochemical systems operate via photocatalysis. One uses semiconductor surfaces as catalysts. In these devices the semiconductor surface absorbs solar energy and acts as an electrode for water splitting. The other methodology uses in-solution metal complexes as catalysts. Photoelectrolytic cells have passed the 10 percent economic efficiency barrier. Corrosion of the semiconductors remains an issue, given their direct contact with water. Research is now ongoing to reach a service life of 10000 hours, a requirement established by the United States Department of Energy. Other photoelectrochemical cells The first photovoltaic cell ever designed was also the first photoelectrochemical cell. It was created in 1839, by Alexandre-Edmond Becquerel, at age 19, in his father's laboratory. The mostly commonly researched modern photoelectrochemical cell in recent decades has been the Grätzel cell, although much attention has recently shifted away from this topic to perovskite solar cells, due to relatively high efficiency of the latter and the similarity in vapor assisted deposition techniques commonly used in their creation. Dye-sensitized solar cells or Grätzel cells use dye-adsorbed highly porous nanocrystalline titanium dioxide (nc-) to produce electrical energy. Materials for photoelectrolytic cells Water-splitting photoelectrochemical (PEC) cells use light energy to decompose water into hydrogen and oxygen within a two-electrode cell. In theory, three arrangements of photo-electrodes in the assembly of PECs exist: photo-anode made of a n-type semiconductor and a metal cathode photo-anode made of a n-type semiconductor and a photo-cathode made of a p-type semiconductor photo-cathode made of a p-type semiconductor and a metal anode There are several requirements for photoelectrode materials in PEC H2 production: light absorbance: determined by band gap and appropriate for solar irradiation spectrum charge transport: photoelectrodes must be conductive (or semi-conductive) to minimize resistive losses suitable band structure: large enough band gap to split water (1.23V) and appropriate positions relative to redox potentials for H2 and O2 catalytic activity: high catalytic activity increases efficiency of the water-splitting reaction stability: materials must be stable to prevent decomposition and loss of function In addition to these requirements, materials must be low-cost