Hybrid interfaces of poly(9,9-dioctylfluorene) employing thin insulating layers of CsF: A photoelectron spectroscopy study

摘要

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 ( -5 A) 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 A, 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 A1F3. When deposited onto either a sputter-cleaned Al surface or Al with native oxide layer (Al;Oy), 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 Lhere is evidence for cesium oxide only for the CsF/AlxOy interface. Thickness of the converted CsF layer has been found to be approximately 5 A 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 p.1ays 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 polymerlorganic-based electronic devices.