Aggregates Promote Efficient Charge Transfer Doping of Poly(3-hexylthiophene)

The doping efficiencies of regioregular (r-Re) and regiorandom (r-Ra) poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) were studied in solution using electron paramagnetic resonance (EPR), (19)F NMR, optical absorption, and Raman spectroscopy. EPR spectra of doped r-Re P3HT possess significantly larger amounts of paramagnetic species than r-Ra P3HT with similar F4-TCNQ loadings (∼0.1% up to 50%), which is confirmed by corresponding optical absorption spectra. (19)F NMR also show a rapid disappearance of the pristine F4-TCNQ signal when small amounts of r-Re P3HT are added due to minority paramagnetic species acting as efficient spin relaxation channels. Raman spectra of both P3HT variants indicate strong interactions with F4-TCNQ, however, the presence of free charges is only detected in r-Re samples owing to its ability to aggregate and adopt ordered conformations allowing for delocalization of hole charges after initial contact with the dopant.