Precise Control of Lamellar Thickness in Highly Oriented Regioregular Poly(3‐Hexylthiophene) Thin Films Prepared by High‐Temperature Rubbing: Correlations with Optical Properties and Charge Transport

Abstract

Precise control of orientation and crystallinity is achieved in regioregular poly(3‐hexylthiophene) (P3HT) thin films by using high‐temperature rubbing, a fast and effective alignment method. Rubbing P3HT films at temperatures TR ≥ 144 °C generates highly oriented crystalline films with a periodic lamellar morphology with a dichroic ratio reaching 25. The crystallinity and the average crystal size along the chain axis direction, lc, are determined by high‐resolution transmission electron microscopy and differential scanning calorimetry. The inverse of the lamellar period l scales with the supercooling and can accordingly be controlled by the rubbing temperature TR. Uniquely, the observed exciton coupling in P3HT crystals is correlated to the length of the average planarized chain segments lc in the crystals. The high alignment and crystallinity observed for TR > 200 °C cannot translate to high hole mobilities parallel to the rubbing because of the adverse effect of amorphous zones interrupting charge transport between crystalline lamellae. Although tie chains bridge successive P3HT crystals through amorphous zones, their twisted conformation restrains interlamellar charge transport. The evolution of charge transport anisotropy is correlated to the evolution of the dominant contact plane from mainly face‐on (TR ≤ 100 °C) to edge‐on (TR ≥ 170 °C).

Bringing Conducting Polymers to High Order: Toward Conductivities beyond 105 S cm−1 and Thermoelectric Power Factors of 2 mW m−1 K−2

“…[26] In semicrystalline polymers such as polyolefins, the thermal resistance of crystalline lamellae along the chain direction W χ is small, whereas the one perpendicular to the chains is significantly larger and comparable to that of the amorphous phase, i.e., W χ ≪ W a . [26] In semicrystalline polymers such as polyolefins, the thermal resistance of crystalline lamellae along the chain direction W χ is small, whereas the one perpendicular to the chains is significantly larger and comparable to that of the amorphous phase, i.e., W χ ≪ W a .…”

“…To answer these questions, we have investigated the possibility to reach high doping levels in films of P3HT and PBTTT oriented by high-temperature rubbing. [18,24,26,27] The choice of the two polymers is guided by their structural differences in terms of nanomorphology in rubbed thin films: P3HT has a periodic semicrystalline lamellar morphology, whereas C 12 -PBTTT shows a liquid-crystal-like structure. [18,19] The choice of FeCl 3 is motivated by the fact that it is a stronger oxidant than F 4 TCNQ.…”

Molecular Orientation of Conjugated Polymer Chains in Nanostructures and Thin Films: Review of Processes and Application to Optoelectronics

“…It is a well-known technique applied in the field of LCDs. 2D-XRD measurements revealed that rubbing P3HT layers generates molecular order whether the films are rubbed with velvet or nylon materials ( Figure 15) [33,[65][66][67][68]. In particular, depending on the temperature at which the rubbing process is performed, the amount of face-on and edge-on crystallites can be controlled to induce the adequate crystallite orientation depending on the application [66].…”

“…2D-XRD measurements revealed that rubbing P3HT layers generates molecular order whether the films are rubbed with velvet or nylon materials ( Figure 15) [33,[65][66][67][68]. In particular, depending on the temperature at which the rubbing process is performed, the amount of face-on and edge-on crystallites can be controlled to induce the adequate crystallite orientation depending on the application [66]. For instance, the vertical hole mobility can be enhanced about twofold with the generation of face-on crystallites and mobility anisotropy can also be generated through rubbing [65][66][67].…”

“…In particular, depending on the temperature at which the rubbing process is performed, the amount of face-on and edge-on crystallites can be controlled to induce the adequate crystallite orientation depending on the application [66]. For instance, the vertical hole mobility can be enhanced about twofold with the generation of face-on crystallites and mobility anisotropy can also be generated through rubbing [65][66][67]. Although most of the studies on rubbing focus on P3HT, this process is not limited to thiophene-based polymers.…”