Understanding the chain conformation of conjugated polymers in casting solutions and its impact on the crystalline morphology of solution-cast thin films is crucial for many electronic applications. Using small-angle neutron scattering, we show that well-dissolved poly(3-hexyl thiophene) (P3HT) chains in good solvent (chloroform) form long rectangular parallelepipeds (RPs) via nucleation and growth processes upon increasing the volume fraction of poor solvent (hexane) above a certain critical point. The growth of the RPs is due to the π–π stacking of the P3HT main backbone occurring along the long axis of the RPs. P3HT solutions prepared with different poor solvent volume fractions were drop-cast onto Si-wafers to prepare thin films, which were examined using 2D grazing-incidence X-ray scattering and 1D X-ray diffraction. The results indicate that the RPs grown in solution preferentially orient on the substrate with their two longer axes parallel to the surface after solvent evaporation, and give rise to much improved crystallinity and crystal orientation compared to the disordered chains.