The interplay between microphase separation and crystallization in the semicrystalline PS-b-PEO system is investigated in the context of solution-based thin film formation. For this end, the film morphology is studied depending on the PEO volume fraction in a broad f(vol)(PEO) range from 0.17 to 0.8. Topographical two-dimensional AFM measurements are extended to the third dimension to gain information about the vertical film structure by successive removal of the surface layer by controlled O-2 plasma etching. The depth profiling reveals a dynamic film formation process where competing microphase separation and crystallization forces create a two-layered BCP film composed of a buried microphase-separated layer featuring cylinder morphology under a fine-structured crystalline lamellae surface layer. An explanation is presented considering the thermal properties and solvent interactions. Phase-selective TiO2 nanoparticles addressing the PEO block are further used to demonstrate the accessibility of the buried microphase-separated layer and thus that we were able to extend the usable range of the phase diagram of PS-b-PEO thin films to the PEO-rich regime. Lastly, the impact of a second film-air interface is shown, which is formed in free-standing BCP membranes.