Thin interfacial layers in polymer-based electronics.

摘要

Electronic devices based on organic materials are presently being developed. In these devices, the cathode interface and the nature of the charge carrying species are often of crucial importance. In that context, the behavior of lithium atoms deposited on the surfaces of ultra-thin spin-coated films of poly(dioctyl-fluorene), PFO, and of condensed molecular solid films of tris(8-hydroxyquinoline) aluminum, Alq_3, have been studied. 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 π-system of PFO, 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 Alq_3, however, up to a level of three added electrons per molecule, the added electrons reside in states localized on each of the three ligands. The role of thin (～5 A) interfacial layers of LiF or CsF between poly(9,9-dioctylfluorene) polymer film and an aluminum electrode was studied as well. LiF-deposition on poly(9,9-dioctyl-fluorene) did not cause doping of the polymer films, nor did the LiF dissociate at the interface. No significant shifts in the binding energy of the core levels nor any changes in the work function occurred. Al-deposition on LiF/PFO films did not cause dissociation of LiF, unlike the case for Alq_3. We found that CsF does not dissociate when deposited on a polymer film. However, deposition of aluminum on CsF will cause dissociation with cesium n-doping the PFO film at the interface and fluorine likely reacting with aluminum to form A1F_3. When deposited onto either of sputter-cleaned Al surface or Al with native oxide layer (Al_xO_y), CsF also was found to dissociate at the interface. CsF does not dissociate for Au/CsF/PFO and CsF/Au interfaces, which emphasizes the crucial role aluminum plays in the process. The observed decomposition of CsF occurring upon Al deposition and following Cs-doping of the surface region of PFO likely enhances injection of electrons into 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.