The design of small organic molecules for efficient solution-processed organic solar cells is hampered by the absence of relationships that connect molecular structure via processing to blend morphology and power conversion efficiency. Here we study a series of bis-diketopyrrolopyrrole molecules in which we systematically vary the aromatic core, the solubilizing side chains, and the end groups to achieve power conversion efficiencies of 4.4% . By comparing the morphology and performance we attempt to identify and rationalize the structure-property relationships. We find that the tendency to aggregate or crystallize are important factors to control and that these require a subtle balance.