CANCELLED: Dr. Martin Brinkmann "Controlled crystallization and orientation of semi-conducting polymers"

Driven by the development of plastic electronics, polymeric semi-conductors (PSCs) have been the center of major research efforts in the last decades. They are used as the active layers in devices such as Organic Field Effect Transistors, Organic Light Emitting Diodes and Organic solar cells. Before integrating new PSCs in devices, it is mandatory to draw proper correlations between their structural/morphological characteristics and their optical, electronic and opto-electronic properties. For this reason, the processing-structure-property nexus is of central importance in plastic electronics. Efforts are still necessary to achieve proper growth control of PSCs and to detetmine their structure. This contribution will focus on recent advances in growth control and crystallization of some key PSCs such as regioregular poly(hexylthiophene). Particular emphasis will be given to the progress made in our group on oriented growth of PSCs using high-Temperature rubbing or epitaxy.

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. Uniquely, the inverse of the lamellar period l scales with the supercooling and can accordingly be controlled by the rubbing temperature TR. The 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).

The large majority of semiconducting polymers based on poly(alkylthiophene)s with linear or branched alkyl side chains are reported to pi-stack in their crystalline phases. In regioregular poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT), however, pi-pi interactions are absent due to the presence of the bulky 2,5-dioctylphenyl side groups. High levels of crystallinity and orientation are created in thin films of PDOPT aligned on substrates of naphthalene by slow directional epitaxial crystallization. Depending on molecular weight, both edge-on and flat-on lamellar crystals are obtained. Electron microscopy imaging reveals a transition from extended to folded chain crystallization for Mn ~ 12.7 kDa. The high orientation and crystallinity levels result in high anisotropy of UV-vis absorption and photoluminescence. The single-crystal-like electron diffraction patterns are used to determine the structure of PDOPT. PDOPT features perfectly planarized chains despite the absence of pi-stacking. The octyl side chains are interdigitated and crystallize in a dense subcell which is compared to that of other semiconducting polythiophenes. This comparison allows to define some correlations between side chain packing and overall crystallinity of the PSC.
 

1.    A. Hamidi Sakr et al., Adv. Funct. Mat. 2016, 26, 408.
2.    M. Brinkmann et al., Macro. Rapid Comm. 2014, 35, 9.
 

invited by Michael Sommer