Charge-transfer complexes: new perspectives on an old class of compounds

The discovery of the organic metal TTF–TCNQ in 1973 led to an explosion of research conducted on organic charge-transfer complexes. While these materials have been studied intensely for several decades, the research was mostly aimed at the discovery of materials with high room-temperature conductivity or high-temperature superconductivity. Recently, attention has turned to technologically-relevant properties of charge-transfer complexes, such as ambipolar transport, metallicity, photoconductivity, ferroelectricity or magnetoresistance. This manuscript reviews the growth, structure and properties of charge-transfer complexes and underlines recent progress in their application in organic devices. Their prospects in future applications are discussed, as well as the challenges yet to be overcome to understand the fundamental parameters governing their operation.