The high cost of production of solar panels has prevented the widespread adoption of solar energy. A possible solution is to pursue solution-based solar cell technologies, since they can enable a low-cost and high-throughput manufacturing process. Both organic semiconductors and inorganic nanocrystals have emerged as promising solution-processable materials for solar cells. In this dissertation, I present my work on the investigation of both classes of materials for solar cell applications. Organic photovoltaics consist of donor and acceptor organic semiconductors. The mechanism of charge transfer between the donor poly(3-hexylthiophene) (P3HT) and acceptor C60 was studied by incorporating an inter-layer into the bilayer solar cell. Charge transfer was shown to take place in a two-step process whereby energy transfer of the photo-generated excitons in P3HT to C60 is followed by a backward charge transfer step to P3HT. Novel ways to process these materials are also investigated. Solar cells from P3HT and a fullerene derivative, phenyl C61-butyric acid methyl ester (PCBM) were fabricated by spray-deposition. Good power conversion efficiencies above 2 % were demonstrated, indicating the viability of spray deposition as a fabrication method. In a separate effort, a novel fluorinated resorcinarene photoresist was used to photolithographically pattern solar cells based on a blend of P3HT and PCBM for highvoltage applications. A 15 mm array of 300 solar cells connected in series achieved an open circuit voltage (VOC) of 90 volts. Three new classes of materials for organic solar cell acceptors are presented, namely pentacenes, hexacenes, and anthradithiophenes. Solar cells based on P3HT and pentacenes gave efficiencies as high as 1.2 %. The hexacenes have the lowest bandgap, enabling hexacene-based solar cells to have photocurrent response up to 800 nm. The anthradithiophene-based solar cells achieved the highest VOC approaching 1.1 Volts, and decent efficiencies of around 0.8 %. Finally, a facile alcothermal method for the synthesis of dispersible CuO and Cu2O nanocrystals is presented. A bilayer CuO / PCBM solar cell demonstrated an efficiency of 0.04 %, indicating the potential of these materials for light harvesting applications.