Polymer-based solar cells are very promising candidates towards cheap solar energy, since they can be solution processed and light weight. The best polymer solar cells currently achieve an efficiency of about 8%, which is not competitive with their thin film inorganic counterparts yet. On the other hand, reducing the manufacturing cost and improving the stability of polymer solar cells are also curial for future commercialization of polymer solar cells. These further developments can be facilitated on more detailed design strategies that can only be established through the elucidation of the fundamentals on conjugate polymers, interface, and device structures. In this thesis, quantitatively investigations of side chains and substituents to construct ideal conjugated polymers for organic solar cells have been presented. The side chain of a conjugated polymer significantly impacts the photovoltaic properties of the corresponding bulk heterojunction (BHJ) solar cell. In addition to side chains, substituents can further tune energy levels, band gaps, and even morphology. A proper combination of side chains and fluorine substituents on the conjugated backbone is a viable approach to high efficient BHJ devices. Moreover, the poly(3-methylthiophene) (P3MT) interfacial layer successfully serves as the hole transport layer for solutioniv processed BHJ polymer solar cells with efficiency as high as 5%, which largely extends the lifetime of polymer solar cells. In addition, solution-processed flexible polymer BHJ solar cells based on silver nanowires (Ag NWs) have been successfully fabricated with recoverable efficiency of 2.5%, which indicates that Ag NW electrodes can serve as a low cost, flexible alternative to indium tin oxide (ITO), and thereby improve the economic viability of polymer solar cells. Finally, a conceptually new approach, parallel bulk heterojunction (PBHJ) has been demonstrated in this thesis. The PBHJ solar cell maintains the low cost manufacturing of single junction BHJ cells, while inherits the major benefit of incorporating multiple polymers in tandem cells. Very respectable power conversion efficiency (PCE) over 7% has been obtained in the PBHJ device, which is among the best performances for polymer solar cells.