Abstract

In the p-doping of organic semiconductors with small molecular dopants integer-charge transfer forming ion-pairs (IPAs) and fractional charge transfer through the formation of ground state charge-transfer complexes (CPXs) have been identified as competing fundamental processes. IPAs and CPXs differently affect the performance of doped organic electronic devices, however, the conditions leading to either phenomenon are still to be fully understood. This thesis focuses on the conjugated polymer poly(3-hexylthiophene) (P3HT) p-doped with the strong molecular electron acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ) and its derivatives of lower electron affinity (F2TCNQ, FTCNQ, TCNQ). Under consideration of their different dopant strengths, the role of the critical dopant concentration promoting the one phenomenon over the other is investigated. Cyclic voltammetry is used to determine ionization energy and electron affinity values of the materials involved to gauge their influence on IPA and CPX occurrence, as identified through optical and vibrational spectroscopy. Supported by electrostatic modeling taking into account the width of the Gaussian density of states (DOS) related to the highest occupied molecular orbital in P3HT, DOS broadening upon doping is considered to explain IPA formation with weaker dopants. Grazing incidence x-ray diffraction is employed toassess the interplay between the supramolecular structure and the two doping phenomena, supporting the hypothesis that a CPX polymorph can occurs that effectively prevents IPA formation for a given host-dopant stoichiometry. Conductivity data on doped films highlights the application-related impacts of these findings. Finally, for a series of custom thiophene oligomers of different lengths, instead of P3HT, the common observation of CPX formation being promoted in the molecular doping of (small) conjugated molecules is investigated. The threshold of transition into the doping phenomenology of the polymer limit is observed at a chain length of 10 thiophene units - a parameter to be considered when employing oligothiophene semiconductors in applications demanding molecular doping. Overall, due the multi-technique approach targeting doping phenomena and mechanisms of a prototypical polymer and oligomer equivalents doped with a systematic series of p-dopants, the database presented here provides a consistent and coherent point of reference for assessing the performance and phenomenology encountered with novel dopants.