that optically excited dye molecules adsorbed at the surface of the crystal inject charge carriers. The underlying phenomenon is called sensitized photoconductivity. It occurs when photo-exciting a dye molecule with appropriate oxidation/reduction potential adsorbed at the surface or incorporated in the bulk. This effect revolutionized electrophotography, which is the technological basis of today's office copying machines. It is also the basis of organic solar cells (OSCs), in which the active element is an electron donor, and an electron acceptor material is combined in a bilayer or a bulk heterojunction. Doping with strong electron donor or acceptors can render organic solids conductive even in the absence of light. Examples are doped polyacetylene and doped light-emitting diodes. Today organic semiconductors are used as active elements in organic light-emitting diodes (OLEDs), organic solar cells (OSCs) and organic field-effect transistors (OFETs). Materials Amorphous molecular films Amorphous molecular films are produced by evaporation or spin-coating. They have been investigated for device applications such as OLEDs, OFETs, and OSCs. Illustrative materials are tris(8-hydroxyquinolinato)aluminium, C60, phenyl-C61-butyric acid methyl ester (PCBM), pentacene, carbazoles, and phthalocyanine. Molecularly doped polymers Molecularly doped polymers are prepared by spreading a film of an electrically inert polymer, e.g. polycarbonate, doped with typically 30% of charge transporting molecules, on a base electrode. Typical materials are the triphenylenes. They have been investigated for use as photoreceptors in electrophotography. This requires films have a thickness of several micrometers that can be prepared using the doctor-blade technique. Molecular crystals In the early days of fundamental research into organic semiconductors the prototypical materials were free-standing single crystals of the acene family, e.g. anthracene and tetracene. The advantage of employing molecular crystals instead of amorphous film is that their charge carrier mobilities are much larger. This is of particular advantage for OFET applications. Examples are thin films of crystalline rubrene prepared by hot wall epitaxy. Neat polymer films They are usually processed from solution employing variable deposition techniques including simple spin-coating, ink-jet deposition or industrial reel-to-reel coating which allows preparing thin films on a flexible substrate. The materials of choice are conjugated polymers such as poly-thiophene, poly-phenylenevinylene, and copolymers of alternating donor and acceptor units such as members of the poly(carbazole-dithiophene-benzothiadiazole (PCDTBT) family. For solar cell applications they can be blended with C60 or PCBM as electron acceptors. Aromatic short peptides self-assemblies Aromatic short peptides self-assemblies are a kind of promising candidate for bioinspired and durable nanoscale semiconductors. The