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
