Light-emitting and electrochemically active polymers for optoelectronic applications
Ludvig Edman, Department of Physics

Polymers with an alternation of single- and double-bonds along the main chain (so called "conjugated polymers") are semiconductors in the pristine state, but they can be transformed into very good conductors with conductivities approaching that of copper via chemical or electrochemical doping. (This unexpected discovery was awarded the Nobel prize in chemistry year 2000.) In addition, many conjugated polymers exhibit electroluminescence, meaning that when they are stimulated by an electric current they emit light with a colour that reflects upon the conjugated-polymer band gap (Eg). The PhD. thesis work will be aimed at the development of the light-emitting electrochemical cell (LEC), which is a device that exploits both the conducting and electroluminescent properties of conjugated polymers. An LEC typically consists of a mixture of a conjugated polymer, an ion-conducting polymer and a salt positioned between two electrodes. When a voltage equal to the band gap of the conjugated polymer (V = Eg/e) is applied between the electrodes, the conjugated polymer becomes n-type doped at the negative electrode and p-type doped at the positive electrode, and after some time a p-n junction is formed. Subsequently injected electrons and holes migrate through high-conductivity doped regions before meeting at the p-n junction, where they form electron-hole pairs which quickly decays via the emission of light. The current generation of LECs exhibit a number of appealing properties, since they can emit all visible colours, they are very efficient in transforming current into light, stable metals can be employed as electrode materials, etc. Unfortunately, they also exhibit two serious drawbacks: the emission lifetime is non-adequate, and the turn-on time is relatively slow (of the order of seconds). The PhD. thesis work will be split into two parts: First, a careful characterization and evaluation of new types of materials and preparation methods with appropriate techniques will be performed. Second, the most promising materials and preparation method from an improved lifetime and turn-on time perspective will be used for the fabrication, testing and evaluation of actual LEC devices.