It has been recognized that the morphology of the active layer of the polymer-based solar cells has a great influence on the device performance. To push the efficiency to a higher level, morphology design and control by varying processing conditions are crucial. The theme of this dissertation is to characterize and understanding of the morphology of the active layer of polymer-based solar cells and the role that the morphology plays on device performance, so as to develop routes by which the morphology can be optimized. The focus of this dissertation is first on a binary system consisting of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) (Chapter 2). Then composition of the active layer was extended to a more complicated ternary system containing poly(3-hexylthiophene) (P3HT), PCPDTBT and PCBM (from Chapter 3 to 5). Multiple characterization methods were used, including x-ray/neutron scattering or reflectivity techniques in combination with transmission electron microscopy and UV-vis absorption spectroscopy. Degree of ordering, degree of phase separation, molecular orientation and vertical component distribution were determined. Morphology evolution was monitored by conducting in-situ or ex-situ experiments and the driving force for generating the multi-length scale morphology was discussed. It was found that crystallization behaviors of conjugated polymers with or without confinement, with or without the use of processing additives were different. The interaction between polymers and PCBM had great influence on domain size and purity. The key factors for the success of polymer-based solar cells are high crystallinity, separated donor materials, bi-continuous donor and acceptor phases and large interfacial area.