Relating Polymer Doping and Nanostructure Formation

It is well known that conjugated polymers like P3HT form nanocrystals in poor solvent solutions and that these same polymers “crash out” of solutions containing p-type dopants. We examine the relationship between polymer doping and nanostructure formation using a combination of small angle neutron scattering (SANS) and inelastic neutron scattering (INS). We use SANS to monitor the formation of P3HT nanocrystals in solution and examine how the presence of the p-type dopant (F4TCNQ) changes the crystal size, structure, and formation kinetics of P3HT nanocrystals. Next we use INS to determine P3HT configurations both inside and outside of crystalline domains for both doped and undoped polymers. These results demonstrate that only a small percentage of the volume is occupied by the ideal structure determined by x-ray crystallography and that the side chains are much more disorded than presented in literature. In addition, we fit the P3HT/F4TCNQ configuration using DFT modeling. We determine that the charging of the polymer backbone leads to increased side chain stiffness, which has significant implications for design of organic electronic devices based on thiophenes.