Conjugated polymers and organic electronics

Plastics, unlike metals, are reputed not to conduct current. In fact, they are used to insulate the copper wires of ordinary electrical cables.

In the late 1970s, A.J. Heeger, A.G. MacDiarmid and H. Shirakawa, winners of the 2000 Nobel Prize in Chemistry, showed that, after certain modifications, a plastic can become an electrical conductor, i.e. a "synthetic metal". To achieve this, the polymer must be conjugated. To achieve this, the polymer's main chain must contain alternating single and multiple bonds; in addition, it must be "doped", i.e. electrons must be removed (by oxidation) or added (by reduction). These missing electrons (usually called "holes") or extra electrons constitute charge carriers that can move along the polymer chain, making it electrically conductive.

Following the pioneering work on polyacetylene (1977), research focused on the development of new families of air-stable, electronically conductive polymers, with the aim of increasing the conductivity of these materials, obtained mainly as black, insoluble films or powders. Research focused on :

• on the one hand, the engineering and synthesis of these polymers to control their electrical and optical properties, and their processing;

• understanding transport mechanisms in conjugated polymers in the broadest sense.

Of these conductive polymers, only those from the poly(3,4-ethylenedioxythiophene) (PEDOT) family are currently finding applications in organic electronics, either as a layer for injecting holes into components, or as an active layer for organic electrochromism.

Since 1990, new fields of application have emerged, such as the possibility of using oligomers or conjugated polymers in their undoped (semiconducting) state as active layers in components for organic electronics: organic light-emitting diodes (OLED or PLED, respectively), organic field-effect transistors (OFET or PFET, respectively), organic photovoltaic cells and electrically pumped organic lasers.

A.J. Heeger, A.G. MacDiarmid and H. Shirakawa have made conjugated polymers and conjugated organic materials more generally (small molecules and polymers) a major field of research for chemists, physicists and technologists alike. These materials are the source of intense scientific activity, at both fundamental and application levels. Today, conjugated materials are present in our everyday environment as active layers in displays (cell phone, tablet and TV screens) and lighting devices using OLED technology.

This article begins with a brief history of the development of research interests and directions in the field of conjugated polymers...