The experimental SAXS results showed that P(NDI2OD-T2) aggregates in extremely poor solvents may be significantly larger than in better solvents, and have a rod-like structure. The conformation of aggregates in solution was characterised by their radius of gyration, R g . Over time, as the average aggregate size increased, we observed a corresponding increase in the root-mean-squared (RMS) R g (Fig. ). Separating this into the R g of aggregates of a specific size showed that as the aggregate size (N agg , number of chains) increased, the RMS R g of the aggregate also increased beyond that of a single chain much more rapidly than would be expected if stacking in a perfect π-stacking arrangement (Fig. ). Examining the evolution of the aggregate structure over time (see e.g. the structures inset in Fig. ) shows an aggregation mechanism in which chains initially collide with random backbone orientations, before 'zipping' up to form a more rod-like structure. Although the aggregates are still relatively small, this behaviour is indicative of the formation of extended aggregates in which chains are not fully overlapping. Due to computational constraints, it is challenging to model a system large enough to form aggregates with R g much larger than observed here. However, the trend towards more extended structures suggests that the formation of larger, extended aggregates is expected. poor intermediate "good" stacked scaling Fig. RMS radius of gyration as a function of aggregate size for 20mers in varying solvent qualities, averaged over the entire simulation. The horizontal solid black line indicate the RMS R g for a single 20mer in the "good" solvent conditions, calculated as described in Fig. . The dotted black line is an approximation of the radius of gyration of an aggregate with fully overlapping chains that form a rectangular block, calculated as