High Carrier Mobility Polythiophene Thin Films: Structure Determination by Experiment and Theory

Abstract

Polymer semiconductors such as the alkyl-substituted polythiophenes have long been recognized as solution-processable materials for device applications, but the carrier mobility of polymers is typically lower than insoluble organic small molecules such as pentacene. The lower mobility is generally attributed to less structural order; specifically the smaller and less-aligned crystals typical of polymer semiconductors should exhibit reduced intermolecular p-orbital overlap at grain boundaries.[1] The more disordered nature of polymer semiconductors has made it challenging to determine the details of their thin-film crystal structure.[2] Structure measurements are further complicated by the small volume of the thin, 20-50 nm, polymer semiconductor films used in organic field effect transistors. [3,4] X-ray diffraction (XRD) usually provides only primary index peaks, most often the <h00> and <0l0> series, that reveal crystal structure, orientation, and size, but exact atomic positions can rarely be determined. Moreover, it is clear that the thin-film crystal structure can differ from single crystal or powder structure.[5]Alkyl-substituted polythiophenes feature a backbone of sequentially bonded thiophene rings with linear alkane chains attached to their sides. In thin films, they self-assemble into lamellae; comparing typical unit-cell dimensions to molecular dimensions leads to a generally accepted layer-packing motif where planar backbones p stack in aromatic lamellae vertically segregated from lamellae of side chains. [6][7][8] Two critical but poorly characterized aspects of the crystal structure are the conjugated-plane tilt and the alkane side-chain configuration. In the lamellar motif, the conjugated planes must be roughly vertical (orthogonal to the lamella) so that they can p stack face-to-face within a quasi two-dimensional sheet. However, a variety of conjugated-plane tilts and a rich possibility of side-chain configurations-distinguished by varying tilt and degree of interdigitation-are consistent with this layer-packing motif and available diffraction data. Greater structural detail will support the development of new synthetic and processing strategies, because carrier transport critically depends on the intermolecular overlap of carrier band orbitals, which is controlled by the conjugated-plane spacing and tilt. Recently, poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophenes) (pBTTT) have been reported to exhibit hole mobility comparable to that of many small molecule semiconductors [9] and rivaling what is achievable in amorphous silicon. The structure of this material is indicated schematically in the insets of Figures 1 and 3a. The high performance of the pBTTTs was attributed to greater structural order than typical polymer semiconductors, and XRD indicated large and well-oriented crystals.[9] Here we exploit this crystallinity to study structural detail within ca. 25 nm thick films of a pBTTT with tetradecyl side chains (pBTTT-C 14 ). From first-principles energy minimization using density f...

Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy

“…41,42 Alternatively introducing a non-or less-conjugated unit as a component of the polymer backbone has also been shown to increase the polymer IP. [43][44][45] On the basis of these strategies, we have prepared a series of polythiophenes with IPs ranging from 4.8 to 5.6 eV.…”

Synthesis and Electropolymerization of Hexadecyl Functionalized Bithiophene and Thieno[3,2-b]thiophene End-Capped with EDOT and EDTT Units

“…Figure 1 shows the centrosymmetric molecule of compound 1, in which the are known to be important in facilitating the growth of ordered crystalline microstructures. 38 The sidechain attachment density of a polymer is known to be related to the chemical structure of the monomer, therefore it is an intrinsic property of the polymer and is independent of processing history, M W , or sidechain length. 37 Although the structure of 1 provides only some indication of how the conjugated chains may align in the polymer, the likelihood of the local geometry of the structure (EDOT-thienothiophene segments) persisting in the polymer is considered to be high.…”

Liquid Crystalline Polymers for Efficient Bilayer-Bulk-Heterojunction Solar Cells

“…In this context, a new class of polythiophene derivatives, including poly(2,5-bis(3-alkylthiophen-2-yl)thieno [3,2-b]thiophene) (PBTTT), 19 has recently been synthesized to contain chemical moieties with strong π-π intermolecular interactions for close packing of the macromolecule chains into large crystalline domains upon crystallization from a liquidcrystal phase. [20][21][22] Due to the increased structure order and crystallinity, thin-film transistors made from PBTTT have been demonstrated to exhibit a hole mobility of 0.2-0.6 cm 2 /V s, [23][24][25][26] which is higher than that of P3HT and is compared favorably to that of amorphous silicon. However, the use of such a highmobility semiconducting polymer in bulk-heterojunction solar cells, though very promising, hardly has been investigated while its use in a single-layer bulk-heterojunction solar cell was briefly discussed in a recent paper.…”