A polymer vs solvent diagram of film structures, formed in polystyrene (PS) blends (1:1 w:w PS/PT) with poly(3-alkylthiophenes) PT [regioregular R-P3DDT, R-P3HT and regiorandom P3BT, P3DDT] spin-coated onto oxidized silicon surfaces from various common solvents [p-xylene, toluene, chloroform, chlorobenzene, cyclohexanone] is presented. The structures were determined with microscopic techniques (atomic, AFM and lateral, LFM, force microscopy, fluorescent microscopy FM) and dynamic secondary ion mass spectrometry (dSIMS). The diagram, arranged according to the solubility parameter of the PTs and the solvents, exhibits three main structural classes: dewetting, lamellar, and lateral (quasi-2-dim) morphology. Decrease in PT solubility parameter delta(PT) inhibits dewetting of polymer films. It induces also a transition from lamellar to lateral film structure. Increase in solvent solubility parameter delta(S) has similar effects. Such behavior is related to the stability of transient homogeneous films and multilayers in the course of spin-casting. The role of delta(PT) and delta(S) is elucidated based on the stability analysis performed in terms of spreading coefficient (dependent on delta(PT)) and effective interfacial tension of solvent-rich polymer phase (dependent on delta(S)).