Polymer interfaces studied by photoelectron spectroscopy: Li on polydioctylfluorene and Alq(3)

The behavior of Lithium atoms deposited on the surfaces of ultra-thin spin-coated films of poly(dioctylfluorene), and of condensed molecular solid films of tris(g-hydroxyquinoline) aluminium, have been studied through a combined experimental-theoretical approach. The Li-atoms donate charges to the organic systems, leading to doping-induced electronic states in the otherwise forbidden energy gap. The changes in the electronic structure induced by charge transfer from the Li-atoms are different in the two materials studied, and depend upon the localization of the electronic states to which the electrons are transferred. In the case of the delocalized wave functions of the pi-system of poly(dioctylfluorene), at low doping levels, the added charges lead to the formation of polaron states, while at higher doping concentrations, bipolaron states are formed. In the case of the tris(8-hydroxyquinoline) aluminium, however, up to a level of three added electrons per molecule, the added electrons reside in states localized on each of the three ligands. (C) 2000 Elsevier Science S.A. All rights reserved.