Temperature-Dependent Dynamics of Molecular Dopant in 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 dopinginduced solubility control (DISC) patterning method. In this work, we use quasi-elastic neutron scattering (QENS) to study the dynamics of the soluble p-type molecular dopant tetrafluoromethyloxycarbonyltricyanoquinodimethane (F4MCTCNQ) in the semiconductive 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.

Methods
QENS measurements were performed on the backscattering spectrometer called BASIS at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) with an energy resolution of 3.5 μeV (full width at half-maximum, fwhm), a dynamic range of ±170 μeV, and a scattering wavevector Q range from 0.3 to 2.0 Å−1. The elastic scans were obtained from 50 to 375 K at a heating rate of 1 K/min, and the high statistical QENS spectra were recorded at 273, 323, and 373 K. The spectra collected at 50 K were used as the instrument resolution function, and all the data were corrected for detector efficiency using a vanadium standard.

To prepare the blend sample for QENS measurements, solutions of F4MCTCNQ and d-P3HT in deuterated chlorobenzene were first mixed to achieve a 17% mole fraction of dopant to polymer. The blend solution was then drop-cast onto a clean glass slide.  The sample was placed into a vacuum chamber to completely evaporate the solvent. The resulting blend layer was scraped from the glass slide using a clean razor blade. Both blend and neat samples were ground to a power using a mortar and pestle for 15 min and then weighed prior to being loaded into a 0.5 mm thick flat aluminum container for further analysis.