Crystal Structure Control of the Energetics of Chemical Doping in Rub-Aligned P3HT Films

Semiconducting polymers have received increased attention in recent years as low-cost, electrically conductive layers. To show reasonable electrical conductivity, however, semiconducting polymers must be doped, a process that requires oxidation or reduction of the conjugated backbone and structural rearrangement to accommodate a charge-balancing counterion into the polymer network. Here, we aim to understand how this structural rearrangement contributes to the energetics of doping. We utilize the fact that rub-aligned poly-(3-hexylthiophene-2,5-diyl) (P3HT) films contain two polymorphs, one with a crystal structure that is less dense than the structure observed in unaligned films and the other with a more compact, denser structure. The two structures are dominantly face-on and edge-on with respect to the substrate, respectively, so their diffraction is well separated in q-space and thus doping induced structural changes can be monitored separately for each population. When films are doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the less-dense face-on oriented P3HT polymorph undergoes doping induced structural change more easily than the denser edge-on oriented polymorph. This finding suggests that rearrangement of the polymer crystal structure to accommodate the dopant counterion is a significant energetic term in the doping process and that doping of semiconducting polymers can be facilitated by designing new polymers where dopant counterions can be accommodated in the polymer lattice with reduced structural change.