Having established a picture of the conjugated polymer solution state we now develop full SAXS models to describe the SAXS profiles observed in this study and fully quantify the shape and structure of the fibrillar aggregates and dispersed polymer chains. Here, we present two SAXS models. The first is a more general model considering two independent populations of semiflexible cylinders with a pseudo-Voigt peak, which we refer to as the 2SFC (two semiflexible cylinder) model for short. The second is a more detailed model that explicitly considers that some population of the semiflexible polymer chains aggregate into fibrils, which we refer to as the FA (fibril aggregation) model for short. While the 2SFC model is more general, the FA model specifically accounts for the assembly of polymers and helps to understand the origin of broad structure factor peaks observed in conjugated polymer SAXS/SANS. A description of the models is discussed below with further details of the model theory found in supplementary section S3.