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

Abstract

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(3hexylthiophene-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 2,3,5,6tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ), 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.

Quantifying Doping Efficiency to Probe the Effects of Nanoscale Morphology and Solvent Swelling in Molecular Doping of Conjugated Polymers

“…However, the relative ratio of these two peaks changes as the doping ratio varies. We interpret this result as indicating that the crystalline regions of the P3HTs are composed of two competing species with constant lamellar spacing whose relative fraction depends on the doping level . In the case of P­(g 3 2T-TT), the pristine film has a lamellar peak at q z = 0.474 Å –1 , while the peak fitting of the doped films is again indicative of two peaks contributing to the lamellar feature.…”

Disorder-to-order transition of regiorandom P3HT upon electrochemical doping

“…The F 4 TCNQ doped film exhibits absorption peaks appearing around 800 and 400 nm appeared that indicate doping. 34–36 Overall, the GIWAXS diffractograms show minimal differences between the electrochemically doped RRa P3HT film and the RRa P3HT thin film immersed in the electrolyte with no applied bias, indicating the absence of induced ordering under constant biasing when compared to F 4 TCNQ doped films. The spectroelectrochemistry results, however, demonstrate irreversible electrochemical doping of RRa P3HT.…”