Band-like charge transport in the HT-HT phase Table : The calculated deformation potential D def,ii (in eV), elastic constant C i (in eV/ Å), effective transport mass m eff (in units of the electron mass m e ), and relaxation time τ ii (in fs) along the three lattice directions (i = a, b, and c) at 300 K. From the results in Table , we see that the band structure's anisotropy also manifests itself in the charge transfer properties. The flat conduction band along the c-axis (ΓZ) leads to a very high effective mass (m eff = 96.12m e ) compared to the other two directions with a modest (ΓX) and high band dispersion (ΓY , cf. Fig. ). The elastic constant along the conjugated backbone in b-direction is one order of magnitude higher than along the directions where the polymer chains are only bound by dispersive forces. Interestingly, the deformation potential, which describes the interaction strength of the charge carrier with the acoustic phonons, is largest for the transport along the a-axis with the tightly stacked thiophene backbones. The van-der-Waals interactions between neighboring chains which mainly govern the crystal structure in aand c-directions are strongest in the direction of the π-π stacking of neighboring thiophene backbones. Thus, the total energy is more prone to structural deformations along the short stacking axis. This, in turn, leads then to a very short relaxation time along a of only a few femtoseconds. In comparison, the relaxation time along the conjugated backbone is three orders of magnitude higher, which shows that charge carriers can move relatively freely through the extended π-electron system along the backbone.