Abstract
Monocrystalline 
(pyrite, 
) photoelectrodes with high photocurrent quantum efficiency (> 90%) have been obtained by improvement of the solid‐state and interfacial chemistry. During intensive illumination (4–5 W/cm2), photocurrent densities between 1 and 2 A/cm2 have been observed for single crystals with high electron mobility
in presence of the 
redox couple. Under illumination, a charge of 623.000 C/cm2 was passed without evidence of photocorrosion. The influence of etching treatments, various redox systems, and organic electrolytes on the photochemistry of 
was investigated. The formation and the dynamics of a thin oxidation layer that forms at the surface of the electrode in the presence of an acid electrolyte were studied using light reflection techniques and ESCA. 
has a valence energy band with strong d‐characterlike Mo‐ and W‐dichalcogenides, that is significant for its stabilization. An unresolved problem with this photoelectrode concerning applications in solar cells is the small photopotential which up to now does not exceed 200 mV (500 mV is theoretically possible). A strong pinning of the Fermi level by surface states is evident from photoelectrochemical measurements. In addition, it is expected that the distance between the conduction band and the Fermi level in our sample will be too large. A low effective carrier density
was measured, resulting in an extended space‐charge layer, which has to be compared with the high absorption coefficient (
for 
).