Investing in an alternative renewable and clean energy source at lower prices is in urgent need and has been recognized by several government agencies including the Department of Energy and the National Science Foundation. One such energy source which has the potential to meet all of these requirements is organic solar cells. The use of small molecules in organic solar cells is desirable because they often exhibit long range ordering and can be readily synthesized and functionalized with high purity. My thesis focuses on how conjugation length and alkyl chain length of diketopyrrolopyrrole-based materials influence on the optical and charge transport properties, molecular packing, thin film morphology, and the overall device performance. For all studies, we use [6,6]-phenyl C71 butyric acid methyl ester as an electron acceptor. We use a combination of techniques such as atomic force microscopy to probe surface morphology and donor-acceptor domain sizes, differential scanning calorimeter to measure the glass transition temperature, x-ray diffraction to gauge crystallinity, ultraviolet photoelectron spectroscopy to measure HOMO-LUMO levels, and single charge carrier diodes to study hole and electron mobilities. The results from these studies will provide design guidelines for new generation of diketopyrrolopyrrole-based materials for applications in organic solar cells.