In this dissertation we report the synthesis and photovoltaic characterization of a number of semiconducting polymers and colloidal inorganic nanomaterials and their implementation into organic solar cells with different architectures (Schottky single layer, bilayer heterojunction, and bulk heterojunction), with research emphasis on the mechanisms underlying material and device optimization, which sheds light on future material design for high efficiency solar cells and other organic electronic devices, such as organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). In the first part, the synthesis, characterization, and photovoltaic applications of a new conjugated copolymer (C12DPP-Pi-BT) are reported. The energy levels of C12DPP-Pi-BT were designed to be intermediate to those of popular electron donor and acceptor photovoltaic materials, P3HT and PCBM. The unique ambipolar nature of C12DPP-Pi-BT was then explored in two different photovoltaic systems where C12DPP-Pi-BT serves as either an electron donor or an acceptor when paired with PCBM or P3HT to form junctions with large built-in potentials. Optical, electrical, and structural characterization have been carried out to understand the photoinduced charge separation, charge carrier transport and recombination mechanism in different device configurations. The influence of polymers' molecular weight and processing condition on device performance has also been explored. In addition, preliminary studies of OLED and OFET application of the C12DPP-Pi-BT have been carried out. In the second part, the synthesis, surface ligand treatment and photovoltaic application of inorganic PbSe and CdSe nanocrystals have been investigated. In Chapter 3, photoluminescence quenching, current-voltage characterization and electrochemical measurements have been used to study the mechanism of photoinduced charge transfer between PbSe and P3HT, which confirmed material incompatibility and suggested new directions for the design of inorganic material as electron acceptor. In Chapter 4, the photovoltaic application of thiocyanate capped CdSe nanocrystals in combination with P3HT in bilayer hybrid devices has been explored. Important factors such as nanocrystal size and bilayer interfacial mixing on the device performance have been investigated and discussed. Bilayer solar cells with ligand exchanged CdSe nanocrystals and P3HT achieved 1.3% power conversion efficiency with good tunability in performance parameters and promising optimization potential.