Doping and Conduction in Organic Radical Ion Systems

There is a large class of organic compounds which form stable radical ions at room temperature, that is they have a single electron in the highest occupied molecular orbital and carry a negative (e.g., tetracyanoquinodimethane) or a positive charge (e.g., tetracene tetrasulphide). They undergo simple one electron oxidation and reduction reactions and in the solid state these different oxidation states have high resistivities, generally > 1010 ohm cm. However, we have shown that the presence of small amounts (<1%) of other oxidation states as an impurity in an otherwise pure sample can increase the electrical conductivity by 5 or 6 orders of magnitude. The accompanying changes in the Seebeck Coefficient and activation energy for conduction arc of the order of 1 mv/°C and 0.5 ev respectively. These results are explained by assuming a hopping model of conduction and by postulating the existence of a redox equilibrium in the solid state.