Quantitative Measurements of the Temperature-Dependent Microscopic and Macroscopic Dynamics of a Molecular Dopant in a Conjugated Polymer

Understanding the nature of dopant dynamics in the solid state is critical for improving the longevity and stability of organic electronic devices and for optimizing the doping-induced solubility control (DISC) patterning method. In this work, we use quasi-elastic neutron scattering (QENS) and fluorescence quenching techniques to develop a comprehensive picture of both the microscopic and macroscopic dynamics of the soluble p-type molecular dopant tetrafluoromethyloxycarbonyltricyanoquinodimethane (F4MCTCNQ) in the conductive polymer poly(3-hexylthiophene-2,5-diyl) (P3HT). Specifically, fast dynamics (ps-ns) of the dopant, such as the methyl and the methoxycarbonyl group rotations, are observed in QENS experiments. From confocal fluorescence microscope experiments, longer-range/slower dopant diffusion (ms-days) is captured. However, in order to fit these data, it is necessary to incorporate a Langmuir isotherm equilibrium between the neutral and ionized dopant molecules. Ionized F4MCTCNQ is strongly favored by the equilibrium, but it diffuses 3 orders of magnitude slower than neutral species. Moreover, the macroscopic diffusion is found to depend mostly on the minority concentration of neutral dopant molecules in the film. Finally, the global diffusion coefficient of the monoester-substituted dopant F4MCTCNQ is shown to be more than an order of magnitude smaller than that of the widely used dopant F4TCNQ