Enhanced Charge-Carrier Mobility in High-Pressure-Crystallized Poly(3-hexylthiophene)

Abstract

b S Supporting Information P olymeric organic semiconductors, like many other macromolecular systems, can display the full spectrum of microstructures, from essentially disordered, or amorphous, to highly crystalline. Many electronic properties and processes are critically depending on this molecular and supramolecular order. [1][2][3][4] However, it often is still unclear which particular microstructural aspects contribute to the macroscopic electronic attributes of these materials. In the field of commodity polymers, such as polyethylene (PE), isotactic polypropylene (i-PP), nylons, and polyesters, one structural characteristic that has been found to be of paramount importance for optimizing mechanical functionalities, including Young's modulus and, in particular, tensile strength, is the degree of chain extension and associated lamellar crystal thickness l (see for a schematic Figure 1a). In analogy, in the present study we focused on this specific feature and explored whether l, varied through different processing schemes, influences charge transport in conjugated polymeric matter. Initial indications for such a relation exist. On the basis of a range of poly(3-hexylthiophene)s (P3HT) of relatively low weight-average molecular weight M w (2.4 kg mol -1 < M w < 18 kg mol -1 ), Zhang et al. have, for example, established a correlation between "weight-average contour length L W " and field-effect transistor charge-carrier mobilities μ FET , with μ FET increasing with L W . 5 Considering the relatively low molecular weight of the materials investigated by Zhang et al., one can assume from Brinkmann and Rannou's work that these P3HTs form chain extended crystals, which would imply that L W ∼ l.Here, we focus on P3HTs of larger molecular weights to ensure that the macromolecules are of a length well above the range where they naturally form extended-chain crystals, i.e., in the regime where chain folding sets in, and chain entanglements form in their melt or concentrated solutions (schematically indicated in Figure 1b; see also refs 6-11). Two different P3HTs (weight-average molecular weight M w = 60 and 344 kg mol -1 ) were selected and solidified from the melt at ambient and under elevated pressure. For comparison, both polymers were also cast from solution, as this is the most frequently adopted processing method for this polymer family. In addition, thin films were prepared from solution at ambient from a low-molecular-weight

Structure and morphology control in thin films of regioregular poly(3‐hexylthiophene)

“…From the charge transport viewpoint, one important issue remains the possibility to crystallize high-M w P3ATs in the form of extended-chain crystalline domains, trying to reduce the level of amorphous material and promoting long range crystalline perfection. Some preliminary studies in that direction have been reported recently 95 suggesting that high-pressure crystallization of P3HT may lead to higher charge carrier mobilities. Further improvements can be foreseen with mastering of chemical defects in the chains of P3Ats.…”

“…[92][93][94][95] Correlations between the molecular structure of the monomeric units made of several chemical moities and the crystalline packing in the unit cells (especially the p-p-stacking) need still to be established. In this perspective, P3HT is certainly a case system to test the applicability of experimental methods and insights gained when investigating conventional polymer crystallization, for example, using epitaxial crystallization methods, self-seeding or nucleating agents.…”

Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors

“…The relative contribution of molecularly ordered and disordered regions to transport is manipulated using different thermal treatments, resulting in different degrees of energetic disorder within FETs and thus in a strong modulation of the thermal barriers to electron transport. A transport improvement with lamellar thickening, obtained upon annealing at T2, is generally expected in semicrystalline polymers, 62 but it has been rarely demonstrated, 63,64 especially in solution processed films and FET architectures. Owing to the retention of backbone orientation upon annealing at T2, energetic barriers for charge transfer are strongly reduced exclusively in the direction of chain alignment, where effective interconnectivity of ordered regions can be more easily gained, leading to temperature-independent electron transport near to 300 K. In contrast, in the direction perpendicular to the backbone, thermal charge transport barriers are independent whether the material is annealed at T1 or T2, which is not surprising as in the -stacking direction coherence length is much smaller in any case.…”

Melt‐Processing of Complementary Semiconducting Polymer Blends for High Performance Organic Transistors

“…Arguably, melt‐processing is free from all the drawbacks previously mentioned for solution processing, yet nearly all the benefits are preserved. Stingelin‐Stutzmann and co‐workers have performed extensive studies on melting poly(3‐hexylthiophenes) and small molecule organic semiconductors . The adoption of melt‐processing for organic electronics is however further complicated by the fact that almost all high performance donor–acceptor‐based semiconducting polymers exhibit high melting temperatures (i.e., >250 °C) or decompose before they melt.…”