Surface And Interface Electronic Structure Of The Organic Semiconductors Studied By SRPES

With up-growing development in the organic semiconductor industry, more and more insights have been put into the fundamental physics and chemistry about the operations of the organic microelectronic devices. In those typical layer-based organic devices, including organic light emission diode (OLED), organic field effect transistor (OFET) and organic photovoltaic cell (OPVC), the most focused work is to improve the injection efficiency through the interfaces and the lifetime of the devices. Mostly, the major factor to control carrier injection is the interface barrier height. At metal/organic semiconductor interface, the electron (hole) injection barrier depends on the position of the LUMO (HOMO) with respect to the metal Fermi level under the Schottky-Mott rule. Most of the actual metal/organic or some of the organic/organic interfaces, however, deviate from the rule due to the existence of interface dipoles. Therefore, the studies of energy level alignment and dipoles at organic interfaces would play key role on the optimization of organic electronic devices. It has been demonstrated that photoemission (UPS/XPS) is one of the most successful experimental methods to research the interface energy alignment and chemistry.In this thesis, 3,4,9,10-perylenetetracarboxylic bisimidazole (BZP) and fluorinated BZP (FBZP) were employed as probe molecules. A series of interfaces, including organic/metal, organic/organic and organic/inorganic semiconductors, were investigated systematically to elucidate the interface formation and electronic structures by synchrotron radiation photoemission (SRPES) method. All the molecules or metal atoms were deposited step-wised in situ to substrate under ultra high vacuum system. Particularly, the influence of fluorination on the electronic structures of organic/metal interfaces was discussed. In addition, the surface and interface structure of Rb-doped Alq film, as well as the effects of oxygen and temperature on it, were also assessed.1. Organic/Metal interfaces: BZP/Ag, Ag/BZP, BZP/AlOx The deposition of BZP on polycrystalline Ag surface decreased the work function of the metal by about 0.3eV due to "pillow effect". Interfacial interaction was found to be localized mainly within one monolayer adsorption. At binding energy of 0.8eV and 1.8eV, a polaron and a bi-polaron type gap states were detected respectively, which could be stable at 250℃and disappeared promptly after exposure to O₂. The deposition of hot Ag atoms on BZP film caused distinct interface diffusion and also the formation of two gap states at 0.6 and 1.6eV, which were 0.2eV lower than the binding energy of those at BZP/Ag interface owing to the larger screening effect from Ag atoms to the core hole during the photoemission process. Both BZP/Ag and Ag/BZP interfaces showed nonreactive characteristic regardless of the deposition sequence. As for the adsorption of BZP molecules on the fresh AlOx surface (partially oxidized), obvious chemical interaction was observed between BZP and AlOx. The thickness of the interface was expanded to more than 30A. The surface work function decreased finally only approximate 0.05eV as a consequence of both "pillow effect" and chemical reaction.2. Effect of fluorination: FBZP/Ag, Ag/FBZP, FBZP/AlOxThe substitution of a hydrogen atom by fluorine breaks the original planar structure of BZP and causes molecular distortion to some extent. The strong electrophilic effect of F atom most probably polarizes the surrounding molecules or atoms. At FBZP/Ag interface, the work function of Ag surface ultimately increased about 0.2eV as a result of the co-existence of the electrophilic effect and "pillow effect", which have reverse influence on the work function. Compared with BZP/Ag, the gap states at FBZP/Ag interface had lower binding energies at 0.6eV and 1.7eV respectively, and the influence of fluorination on electron transport had been discussed in 4^th chapter. The interface interaction between FBZP and AlOx was more complex since all factors mentioned above might happen simultaneously resulting in the work function increased by about 0.3eV.3. BZP/NPB, BZP/ITO, BZP/SiO₂During the interface formation of BZP with NPB, band bending from NPB was inferred and caused interface energy alignment to deviate from Schottky-Mott rule. The hole injection barrier from NPB to BZP was found smaller than the electron injection barrier from BZP to NPB, which was consistent with the fact that NPB was a normal hole transfer material. The interface dipole formation at BZP/ITO interface was interpreted as BZP polarization on ITO since no gap state and band bending were detected. The interaction was very weak as BZP was deposited on SiO₂ surface. The energy level alignment at BZP/SiO₂ was in accordance with Schottky-Mott rule showing a typical abrupt interface formed.4. Rb doped Alq:When a small amount of Rb was deposited on Alq film, the active Rb atoms exhibit obvious electron doping effect and arise Fermi level up by 0.2eV with an advent of gap state at 1.2eV. The gap state could be tolerant with the annealing at 110℃while faded away immediately after exposure to O₂, which was ascribed to the oxidation of Rb adsorbate.