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 fil ms of tris(g-hydroxyquinoline) aluminium, have been studied through a combi ned experimental-theoretical approach. The Li-atoms donate charges to the o rganic systems, leading to doping-induced electronic states in the otherwis e forbidden energy gap. The changes in the electronic structure induced by charge transfer from the Li-atoms are different in the two materials studie d, and depend upon the localization of the electronic states to which the e lectrons are transferred. In the case of the delocalized wave functions of the pi-system of poly(dioctylfluorene), at low doping levels, the added cha rges lead to the formation of polaron states, while at higher doping concen trations, bipolaron states are formed. In the case of the tris(8-hydroxyqui noline) aluminium, however, up to a level of three added electrons per mole cule, the added electrons reside in states localized on each of the three l igands. (C) 2000 Elsevier Science S.A. All rights reserved.