This dissertation is concerned with the material properties of diketopyrrolopyrrole (DPP) based conjugated molecules and their application in bulk heterojunction (BHJ) type solar cells. Relationships between the molecular structures and material properties are investigated, allowing molecules to be tailored to achieve desired characteristics. Density functional theory is used to model the properties of compounds before they are synthesized and identify compounds which are likely to have desirable properties. A variety of materials are synthesized, including dithieno DPP derivatives, a diphenyl DPP derivative and several bis-DPP compounds. Their physical properties are characterized by ultraviolet photoelectron spectroscopy, UV-vis and fluorescence spectroscopy. Properties of films and mixtures with acceptors including fullerene and Vinazene(TM) derivatives are also examined, including their surface morphologies and charge carrier mobilities. The role of cohesive energy densities in film formation is examined. Processing conditions are optimized for the fabrication of BHJ solar cells, yielding devices with good efficiencies. In general, we find that DPP-based conjugated oligomers make an excellent platform for the design of BHJ solar cells.