Chemical Doping of Well-Dispersed P3HT Nanowire Networks

Chemical doping is one of the most widely used methods to enhance the optical and electronic properties of conjugated polymers (CPs). Solution-based chemical doping is well adopted due to its low-cost, easy operation, and compatibility with printing methods for device fabrications. Sequential doping is the preferred solution doping method because of better CP film quality preservation and enhanced electrical conductivity. In this work, uniformly distributed poly(3-hexylthiophene) (P3HT) nanowire (NW) networks thin-film is prepared. The networks’ optical, electronic, and morphological property changes resulting from sequential doping by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) were investigated by UV-Vis spectroscopy, atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), and electrical conductivity measurements. Higher dopant solution concentration (~1 mg/mL) causes the formation of “dot-like” features on the film surface. KPFM measurements clearly differentiate these features from P3HT NWs, indicating the accumulation of dopant molecules on the surface at the excess dopant loading. At a moderate F4TCNQ dopant solution concentration of 250 μg/mL, the P3HT NW networks show little morphological differences from its pristine state, but its absorption band corresponding to doping products can be clearly observed. At even lower doping levels, a significant increase of the electrical conductivity by at least three to four magnitudes can be observed while no morphology changes can be observed from AFM measurements. Our results demonstrate that AFM and KPFM can be used to distinguish undoped, moderately doped, and excessively doped P3HT NW networks and offer direct insights into the location of dopant molecules in the doped systems.