We also used simulated X-ray scattering, as a powerful tool in calculating structural factors of polymer models , to further verify our bulk models (see SI section 3 for details). Figures and illustrate similar features in the scattering patterns as obtained from our models and GIWAXS 8 experiment. In both cases, the presence of "backbone" reflections (00l), at ≈ 0.4 and 0.8 Å -1 , which is a feature associated with the polymer repeat unit length (≈1.6 nm), and the broad peak around 1.5 Å -1 attributed to diffraction from disordered side chains 13 can be seen. Counting the number of π-stacking interactions per monomer (using the definition proposed in ref. and with criteria explain in SI section 2), we find that there is less than 0.005 IDT-IDT π-π interaction per IDT while about 10% of BT fragments are in π-π interactions in all models (see Figure ). The lack of π-π interactions for IDT is most likely due to the steric hinderance of bulky sidechains around it. The simultaneous evidence of BT-BT and lack of IDT-IDT π-π interactions suggests that polymer chains are not aligned in parallel, as is usually the case for stacked chains in semi-crystalline polymers. Thus, we calculated the relative angle between local director vectors θ ij of chains which have BT pairs in π-π interactions (see Figure where θij is illustrated for two interacting chains). Figure shows the total θij distribution for all BT pairs in π-π interactions in IDTBT16-5, IDTBT16-10, and IDTBT16-20, during 100 ns of simulation (through 4 annealing cycles). As shown, a clear θij = 90 ° preferential angle is observed. This suggests that bulky sidechains attached to IDT force chains to form BT-BT ππ interactions with a local relative perpendicular orientation. Note that these BT crossings have been previously observed for IDTBT and similar SCPs with low degree of energetic disorder . Furthermore, TEM results suggest an avoidance of the "ordered" region overlap with relative angle under 20 °, and a more uniform distribution until 90 °-with a small peak at 90 °. Although θij (as calculated here) is the relative angle between BT molecules and represents a very local orientation of chains as compared to TEM results, which give relative angle between a few nm regions, the two observations are in agreement that π-π interactions in IDTBT do not lead to parallel chain stacking as often observed for semi-crystalline polymers.