For semiflexible polymers, a class that describes many organic semiconductors, the folding of a single polymer chain is expected to depend on the chain stiffness. By comparing coarse-grained simulations of P(NDI2OD-T2) with a backbone parameterised to match the flexibility of the all-atom model with those of a much more flexible equivalent, we found more rapid folding of the flexible chain. In both cases, the folding rate also displayed a chainlength dependence, increasing with increasing chain length as has previously been reported. By comparing the approximate time scales characterising single-chain folding and multi-chain aggregation in the poor solvent, we determined approximate concentrations at which each of these processes are expected to dominate. A theory relating this critical concentration to the chain length was developed, and the simulations were found to agree well with the predictions. The critical concentration depended both on backbone flexibility, with a more flexible backbone ex-pected to result in predominantly single-chain folding at higher concentrations than a more rigid one, and chain length, with longer chains transitioning from single-chain folding to multichain aggregation at higher concentrations due to their more rapid folding. In comparing the simulated solution-phase behaviour of flexible and regular P(NDI2OD-T2) chains, this proposed dependence was observed, with the more flexible chains giving more compact structures and less multi-chain aggregation. This finding rationalises apparent discrepancies between experimental measurements of the P(NDI2OD-T2):toluene system and emphasises the importance of both concentration and chain length on predicting solution-phase behaviour.