Μελέτη της ηλεκτρονικής δομής διεπιφανειών οργανικών υμενίων με ανόργανα υποστρώματα με τη χρήση επιφανειακά ευαίσθητων τεχνικών

Abstract

The development of organic-based devices, like transistors and light emitting diodes (FETs, OLEDs), is progressing rapidly over the past few years. A great deal of the physics and chemistry that govern the performance of such devices occur at the interfaces between the organic components and the inorganic electrodes, making the study of the interfacial electronic properties essential. In this work, the electronic structure of the interface formed between a conjugated oligomer (Ooct-OPV5) and inorganic substrates, ιn particular indium-tin oxide (ITO), polycrystalline Au, the Si(111) surface (Si n- and p-doped), and ultrathin (1-5 nm) SiO2 films on Si(111), was studied by X-ray and Ultraviolet photoelectron spectroscopies (XPS, UPS). Ooct-OPV5 is a model for poly(p-phenylenevinylene) (PPV), a polymer that has already been used in OLEDs. ITO is the common anode used in OLEDs because of its transparency and high electrical conductivity. Gold was chosen due to its inert nature and because it is used as a source/drain in FETs. Finally, the study of organic/silicon structures is of great importance for the incorporation of organics in Si-based microelectronic systems. All XPS and UPS measurements were carried out in an ultrahigh vacuum (UHV) apparatus. All substrates were cleaned in-situ by Ar+ sputtering and annealing. High purity oligomer films of up to ~10 nm thickness were produced in-situ by stepwise deposition on the clean substrates. The evolution of the oligomer and substrate-related XPS peaks was followed during Ooct-OPV5 deposition on all substrates. Analysis of these spectra clarified the interfacial chemistry and band bending in the semiconducting materials. UPS spectroscopy is used for the determination of the valence band at the interface and the interfacial dipoles. The interfacial energy band diagrams were deduced in all cases from the combination of experimental results. Based on our experimental data we reached the following conclusions: Dipoles are formed at the interfaces of the oligomer with ITO, Au, Si (p-doped) and SiO2(1-1.8 nm)/Si(111), while the Ooct-OPV5/ Si (n-doped) interface is dipole free. These interface dipoles (eD) are related to the existence of interfacial states and serve for the charge transfer between the materials in contact at the initial stages of the interface formation. In the case of the Ooct-OPV5/ Si (p-doped) interface, eD is related to the interaction of the oligomer molecules with Si surface states. At the Ooct-OPV5/Au and Si interfaces, the charge transfer is completed with a band bending of ~0.20 eV in the oligomer film. The hole injection barriers (eΦbh) or valence band offsets (ΔEV) were also determined in all interfaces. This barrier was measured 1.05 eV at the Ooct-OPV5/ Au interface, and thus Au is inappropriate electrode for hole injection. ITO is also proved a poor hole-injecting electrode (eΦbh=1.45 eV), and thus its surface should be modified by treatments when used as an anode in OLEDs. On the other hand, the valence band offset between the Si substrate and the oligomer is measured ~0.4 eV. Modification of the Si(111) surface with ultra-thin SiO2 layers increases ΔEV by ~0.2 eV.