Organic solar cells (OSCs), consisted of carbon-based organic semiconductors, either polymers or small molecules, have recently attracted the attention of both academic and industry due to their unique properties such as easy processing, flexibility and scalability. One major limitation toward commercialization is the low power conversion efficiency (PCE) compared to inorganic solar cells. Thus, much research in this field is focused on improving the efficiency. A better understanding to the relationship between the properties of organic semiconductors and the solar device performance is required. In this thesis, perfluorinated-end modified poly(3-hexylthiophene), core-substituted naphthalene diimide, and Zn (II) complexes with azadipyrromethene were investigated. Their properties and applications in organic photovolatic (OPV) are discussed.Previous studies suggested that end-group modification of P3HT affects device efficiency, and that some fluorine in the end group slightly improve the efficiency. In order to further understand how perfluorinated end-groups affect device performance of blends of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl) propyl-1-phenyl [6, 6] C61 (PCBM), we synthesized a series of well-defined P3HT with differing perfluoroalkyl length by Stille coupling of the bromine end of P3HT and stannylated 2-perfluoroalkylthiophene. The reactions occurred quantitatively, confirmed by 1H and 19F NMR spectroscopy, and by MALDI-ToF mass spectroscopy. Electron filtering transmission electron microscopy (EF-TEM) revealed that the polymer/PCBM phase separate on the nanoscale. However, solar cells of the modified P3HTs with PCBM had a lower power conversion efficiency than that of un-modified P3HT:PCBM, suggesting that perfluoroalkyl end-groups are detrimental to solar cell performance.The performance of solution-processed organic photovoltaic is seriously limited by the absorption and energy tuning potential of fullerene-based electron acceptors. Overcoming these limitations requires the development of non-fullerene acceptors. Core-substituted naphthalene diimides (cNDI) are good candidates as non-fullerene acceptors for organic photovoltaic, because they have high electron affinity, excellent electron transport properties, and tunable energy levels. We synthesized several cNDIs with different imide core substituents and different alkylamino substituents (RF1-6). Their optical and electrochemical properties and OPV device properties as electron acceptors were studied. Particularly, RF1 was investigated as electron accepting material for optimization of solar cells. The LUMO energy level of RF1 is -3.7 eV, higher than PCBM (-4.0 eV); correspondingly, a high Voc (~1 V) can be reached from blends of P3HT and RF1. The power conversion efficiency improves from 0.31% (as-casted) or 0.48% (pre-annealed) to 0.96% with a processing 1,8-diiodooctane(DIO) additive at an optimum concentration of 0.2 vol%. The results are explained by changes in morphology observed by atomic force microscopy (AFM) and transmitting electron microscopy (TEM) images. Charge transport properties were estimated by space-charge limited current (SCLC) model, indicating that the electron mobility determines the OSC performance.One reason why efficiency of non-fullerene based solar cell have been relatively low is partly because non-fullerene acceptors are often planar and tend to form unfavorable phase-separated domains when blended with typical donors. We synthesized and characterized a series of new solution-processable azadipyromethene-based complexes, Zn(WS1-5)2. These new complexes have high electron affinity and strong accepting properties, and behave as good electron acceptors in organic solar cells. The best device performance was obtained from Zn(WS3)2 acceptor. The 3D nature of this acceptor prevents crystallization and promotes a favorable nanoscale morphology to give a high PCE of 4.10%. The acceptor also significantly contributed to photocurrent generation by harvesting light between 600 nm and 800 nm. These results demonstrate a new paradigm to designing acceptors with tunable properties that can overcome the limitations of fullerenes.