This thesis focuses on tuning molecular packing of organic semiconductors through processing or chemical modifications to increase performance and establish structure-property relationships. Chapter 2 utilizes differing processing techniques to alter the molecular packing of bistetracene in the thin film and thorough polymorph characterization to relate the modification of molecular packing to the increase in charge mobility and mechanism. Chapter 3 introduces the oligomer as a model system to resolve issues that would be difficult or impossible using polymeric systems, due to their monodispersity and increased crystallinity allows for more detailed structural characterization. In this chapter we determine a crystal packing and melting point alternation in BTTT monomers, a trend well documented within organic small molecules, yet largely ignored within the organic semiconductor community. A series of BTTT dimers with various side chains lengths were synthesized in Chapter 4 to quantify the effect of side chain length on bimolecular crystal formation with PCBM using smaller side chains, outside the solubility limit of the parent polymer, and discovered and characterized a phase transition from a bimolecular crystal to amorphous blend upon decreasing side chain length, greatly influencing the electronic properties of the blends. Chapter 5 expands on the knowledge of the previous chapter, designing BTTT dimers with variable side chains using side chains that fall on both sides of the phase transition to investigate the influence of side chain position and size on molecular packing and blended morphology. In Chapter 6, using the benchmark BTTT dimer, we explore the effect of dopant chemical structure on morphology and conductivity of blended films. Surprisingly, the doping mechanism differs from that of the parent polymer, and by tuning the dimer/dopant interactions, demonstrate a differing morphology and large variation in conductivity dependent on dopant choice.