Polymer electronics has become one of the thriving research fields due to their potential to have much lower cost than their inorganic counterparts. Among the categories, polymer solar cells have drawn much attention with the raising interest in green and renewable energy sources. In recent years, different processing methods such as using high boiling point solvent to purposely dry the polymer film slowly and adding a very small amount of additives into polymer solutions have significantly improved the performance of polymer solar cells. In this dissertation, a brief introduction of polymer electronics is given, followed by a detailed study on the function of additives in a polythiophene/fullerene system. In the following chapter, changing the crystallinity of a low bandgap polymer through molecular design is achieved and studied. In the last chapter, a polymer/nanocrystal system showing rather high photoconductive gain is observed and its optical response is studied.