The recent discovery that thin insulating layers inserted between organic layer and metal cathode can significantly improve performance and lifetime of organic-based light-emitting devices is the reason for detailed studies of related interfaces. In this work we investigate the role of thin (similar to5 Angstrom) interfacial layer of CsF between poly(9,9-dioctylfluorene) (PFO) polymer film and aluminum electrode. A number of sandwich-type interfaces prepared in situ have been studied using x-ray photoelectron spectroscopy including CsF/poly(9,9-dioctylfluorene). Al/CsF/poly(9,9-dioctylfluorene), Al/CsF/Au, CsF/Al, CsF/AlxOy, and CsF/Au. We found that CsF does not decompose when in contact with polymer film. There is, however, evidence that for coverage higher than 3 Angstrom, molecules are oriented with the fluorine atom pointing downwards. Deposition of aluminum on such structures causes CsF to dissociate with cesium n-doping the polymer at the interface and fluorine likely reacting with aluminum to form AlF3. When deposited onto either a sputter-cleaned Al surface or Al with native oxide layer (AlxOy), CsF also was found to dissociate at the interface but remain in the CsF form away from the Al surface. In the case of a clean Al surface, cesium left after decomposition resides at the interface, presumably in the form of metal and metal oxide, whereas there is evidence for cesium oxide only for the CsF/AlxOy interface. Thickness of the converted CsF layer has been found to be approximately 5 Angstrom in all cases where decomposition takes place. The lack of CsF decomposition observed for Au/CsF/PFO and CsF/Au interfaces emphasizes the crucial role aluminum plays in the whole process. The observed decomposition of CsF when in contact with Al and following n-doping of the surface region of PFO likely enhances injection of electrons into the organic layer, improving device performance. Since CsF dissociation is independent of the underlying material, the Al/CsF/emissive-material structure could be effective for almost all types of polymer/organic-based electronic devices.