Organic semiconducting polymers have garnered interest for their numerous applications in lightweight, flexible and solution-processable devices including organic field effect transistors (OFETs), organic light-emitting diodes, organic photovoltaics, biomedical devices and sensors. In addition to tuning the chemical structure of the materials, the role of morphology has been identified as a key parameter in determining device performance. Use of conjugated polymers in optoelectronic devices requires thin films to be cast from appropriate solutions. The solubility of the polymer is, however, restricted by the strong π-π interaction and the large chain rigidity that greatly lowers the entropy of mixing. Conjugated polymers are seldom molecularly dispersed in solution even through attaching flexible short side chains to the conjugated backbone. Abundant evidence have demonstrated that the conjugated segments tend to form submicrometer aggregate domains in the solutions. The internal structure of these aggregates and the nature of the interaction leading to the aggregation has however not been addressed unequivocally. Furthermore, detailed correlations between molecular properties, solution aggregation structures, and the ultimate film performances of organic semiconductors are still largely unknown. Our study indicated that polymer chain conformation (in solution, during film formation and post treatment by solvent annealing) is important in determining the final film morphology. First, the photophysics and solution aggregation behavior of different conjugated polymers, which strongly depends on polymer molecular weight, conjugated length, solvent quality and temperature for a specific polymer system are investigated. Moreover, their determinant effects for the final morphology in the cast films will be discussed in detail.