Multi-chain simulations were conducted for systems of 10, 20, 30 and 40mers of P(NDI2OD-T2) in the "good", intermediate, and poor solvents, with flexible (poor solvent only) and regularflexibility (all three solvents) backbones. The P(NDI2OD-T2) system studied in SAXS experiments that showed extended aggregates consisted of approximately 30-monomer chains at a concentration of 5 g/L. Assuming a simulation box roughly three times the polymer contour length to avoid finite-size effects, a system of 30mers at this concentration would contain too many atoms to be easily simulated on the µs time scale. Instead, we focused most of this work on the shorter 20mers. Even shorter (10mer) and longer (30mer, 40mer) chains were also considered for a few select cases. In order to achieve approximately the same behaviour as the experimental 30mer system, we set the concentration of each system so that the ratio of the polymer volume fraction, φ V , to the overlap volume fraction, φ * , was approximately the same for all chain lengths, and close to that in the SAXS experiments. This choice was motivated by recent work that showed the concentration of a polymer solution relative to the polymer overlap concentration to be a key predictor of OSC device performance due to its effect on the extent and type of aggregation. Making the crude approximation of ideal chains gives φ * ∝ 1/N 1/2 for polymer chain length N, and so constant φ V /φ * corresponds to constant φ V N 1/2 . The concentrations that gave the same φ V N 1/2 as the experimental system of 30mers at 5 g/L were 4 g/L for 40mers, 6 g/L for 20mers and 8.5 g/L for 10mers. Unless otherwise stated, the results presented below are for these concentrations. A number of additional systems were studied at different φ V N 1/2 to elucidate the effect of concentration on multi-chain aggregation. As with the single-chain simulations, the chains were first allowed to relax to conformations consistent with a good solvent by initially using purely repulsive WCA nonbonded interactions, before switching to the CG LJ potentials. A detailed description of the simulation procedure is given in the ESI (Section S5.2), with a list of the systems studied and key parameters in Table .