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

The recent discovery that thin insulating layers inserted between organic l ayer and metal cathode can significantly improve performance and lifetime o f organic-based light-emitting devices is the reason for detailed studies o f 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 interfa ces prepared in situ have been studied using x-ray photoelectron spectrosco py 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 dec ompose when in contact with polymer film. There is, however, evidence that for coverage higher than 3 Angstrom, molecules are oriented with the fluori ne atom pointing downwards. Deposition of aluminum on such structures cause s CsF to dissociate with cesium n-doping the polymer at the interface and f luorine likely reacting with aluminum to form AlF3. When deposited onto eit her 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 a way from the Al surface. In the case of a clean Al surface, cesium left aft er decomposition resides at the interface, presumably in the form of metal and metal oxide, whereas there is evidence for cesium oxide only for the Cs F/AlxOy interface. Thickness of the converted CsF layer has been found to b e approximately 5 Angstrom in all cases where decomposition takes place. Th e lack of CsF decomposition observed for Au/CsF/PFO and CsF/Au interfaces e mphasizes the crucial role aluminum plays in the whole process. The observe d decomposition of CsF when in contact with Al and following n-doping of th e surface region of PFO likely enhances injection of electrons into the org anic layer, improving device performance. Since CsF dissociation is indepen dent of the underlying material, the Al/CsF/emissive-material structure cou ld be effective for almost all types of polymer/organic-based electronic de vices. (C) 2001 American Institute of Physics.