The supramolecular organization of the n-type P(NDI2OD-T2) polymer (also known as Polyera Activink N2200), featuring two branched octyldodecyl lateral chains, has been investigated by a combination of quantum-chemical and molecular mechanics techniques coupled to two-dimensional X-ray diffraction simulations. The structures exhibit non fully extended alkyl chains due to a compromise between the size of the monomer unit and the nature and length of the alkyl chains. Interestingly, the supramolecular organization of the polymer chains is only weakly affected by the nature of the stereogenic centers of the branched alkyl chains. The size and shape of the monomer unit and the chemical structure of the side chains appear to be the key elements governing the relationship between the supramolecular organization of P(NDI2OD-T2) polymer chains and their charge transport properties and are as important as the fine-tuning of the electronic properties of the molecular subunits in order to guarantee large electron (and hole) mobility. This study opens new perspectives for the rational design of new n-type and/or p-type polymers for further improvements of organic-based device efficiencies.