Molecular Doping of Polar Conjugated Polymers

Sammanfattning: Organic semiconductors may enable a wide range of flexible electronics, such as flexible displays or solar cells. Conjugated polymers constitute one of the most widely studied class of organic semiconductors. Molecular doping is an important tool to adjust their electrical conductivity through introduction of negative or positive charges by addition of molecular dopants. However, solution-processing of conjugated polymers with molecular dopants is limited due to precipitation and a low number of charges formed per dopant molecule as a result of dopant aggregation or inappropriate energy levels. Thus, large amounts of dopant are required that compromise the nanostructure of conjugated polymers and in turn reduce the electrical conductivity, which is furthermore sensitive to elevated temperatures or the exposure to air in case of n-doping. Therefore, ways that mitigate processing issues and increase the efficiency and stability of molecular doping are highly desired. This thesis explores several concepts that may allow to improve the efficiency of molecular doping. A reduction of the required amount of dopant molecules is achieved by enhancing the compatibility of conjugated polymer:dopant pairs, which results in increased numbers of charges that are created per dopant molecule. In particular, polar side chains on conjugated polymers permit processing of polymer:dopant pairs from the same solution and largely suppress dopant aggregation resulting in improved electrical conductivity for p- and n-doping. Additionally, both the thermal stability of p-doped and air stability of n-doped films are found to benefit from polar side chains. Further, a low ionisation energy of conjugated polymers gives rise to dianion formation of common p-dopants. This double doping results in formation of two charges per dopant molecule and, thus, allows doubling of the doping efficiency. The concepts presented in this thesis provide several important design rules to guide the development of more efficient and stable molecularly doped conjugated polymers.