Highly Oriented and Crystalline Films of a Phenyl-Substituted Polythiophene Prepared by Epitaxy: Structural Model and Influence of Molecular Weight

Abstract

The large majority of semiconducting polymers based on poly(alkylthiophene)s with either linear or branched alkyl side chains are reported to π-stack in their crystalline phases. In regioregular poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT), however, π−π interactions are absent due to the presence of the bulky 2,5-dioctylphenyl side groups. In this work, high levels of crystallinity and orientation are created in thin films of PDOPT aligned on substrates of naphthalene by slow directional epitaxial crystallization of the side chains. Depending on molecular weight, both edge-on and flat-on lamellar crystals are obtained. As for poly(3-hexylthiophene) (P3HT), electron microscopy imaging reveals a transition from extended to folded chain crystallization in PDOPT for M n ≈ 12.7 kDa. The high orientation and crystallinity result in high anisotropy in UV−vis absorption and photoluminescence with well-defined vibronic structures. The single-crystal-like electron diffraction patterns are further used to refine a structural model of PDOPT (a = 29.09 Å, b = 10.45 Å, c = 7.72 Å, α = β = γ = 90°, and space group P21/c). Uniquely, crystalline PDOPT features perfectly planarized chains despite the absence of π-stacking between polythiophene backbones. The octyl side chains are interdigitated and crystallize in a dense subcell which is compared to that of other semiconducting polythiophenes, e.g. form I poly(3-hexylthiophene) and poly(2,5-bis(3-dodecyl-2-yl)thieno[3,2b]thiophene), to draw a trend between overall crystallinity and density of the side chain packing.

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

“…[32,33] To confirm this stoichiometry, we further analyzed our samples using energy-dispersive X-ray spectroscopy-scanning transmission electron microscopy (EDX-STEM). [24,25,34,35] First, one can see that there is a uniform distribution of both S and Fe atoms in the doped layers, indicating that doping is uniform in the samples.…”

“…Both values are substantially higher than the 18.4 Å 2 for the classical polymethylene subcell, indicating that the layers of side chains can indeed host small molecules such as F 4 TCNQ, FeCl 4 − , or FeCl 2 . [24,25,34,35]…”

Oriented Overgrowths of Poly(l‐Lactide) on Oriented Isotactic Polypropylene: A Sequence of Soft and Hard Epitaxies

“…It is a simple and versatile way for orienting a wide variety of crystalline materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Surface-induced epitaxy is based on favorable crystallographic interactions and defines the crystallographic orientation of the overgrowing polymer. It can control the structure of semi-crystalline polymers in different ways: mutual molecular orientation, crystal structure polymorphs, and face-on or side-on orientation of the backbone for conjugated polymers.…”

“…Oriented overgrowth of a foreign material on oriented crystalline substrates is a process known as epitaxy. It is a simple and versatile way for orienting a wide variety of crystalline materials . Surface‐induced epitaxy is based on favorable crystallographic interactions and defines the crystallographic orientation of the overgrowing polymer.…”

The Role of Tie Chains on the Mechano‐Electrical Properties of Semiconducting Polymer Films

“…Mater. 2020, 6,1901070 . Unstrained P3HT films are semicrystalline; the crystallites are randomly oriented in-plane.…”