The importance of a good solvent of conjugated polymers for preparing dopant solution on the electrical and mechanical properties of sequentially doped conjugated polymer films

Abstract

In this work, we sequentially doped poly(3-hexylthiophene) (P3HT) films with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) from multiple good solvent/orthogonal solvent blends with different ratios. Combining UV-visible (UV-vis) spectroscopy, grazing-incidence wide-angle X-ray diffraction (GIXRD), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM) and four-point probe measurements, we found that while sequentially doping the P3HT film with F4TCNQ from the good solvent/orthogonal solvent blends, the good solvent can dissolve the amorphous P3HT in the film and dope it with F4TCNQ in the solution to form crystalline aggregates. These crystalline aggregates can effectively “bridge” the crystalline domains of the P3HT film to form a percolation crystallization network for the transfer of charge carriers, leading to a higher conductivity of the doped P3HT films compared with those sequentially doped with F4TCNQ from a pure orthogonal solvent. The amount of crystalline aggregates increases with the amount of the good solvent; on the other hand, the good dissolving capacity of the good solvent disrupts some intrinsic crystalline domains in the film, inhibiting the transport of carriers. The combination of the “bridge” effect and the “dissolving” effect results in the increase of conductivity of the doped P3HT film first with the increase in the good solvent ratio and then with a decrease in the conductivity with a further increase in the amount of the good solvent. This work verifies that the sequential doping of the P3HT film is accompanied by solution doping if a good solvent of P3HT is used to prepare the F4TCNQ solution. The choice of a solvent to prepare the dopant solution for the sequential doping process is an important factor that must be considered. Finally, the tensile properties of the doped P3HT film were measured, and the results show that doping can effectively increase the conductivity without significantly damaging the tensile properties of the P3HT films. By carefully regulating the preparation conditions of the P3HT film and the ratio of the good solvent to orthogonal solvent to prepare the F4TCNQ solution, a P3HT film with both good electronic performance and tensile properties can be obtained.