Studies Of The Electronic Structure At The FePc/MoO_x(x≤3) Interface And The Organic Molecular Orientation Modulated By The Magnetic Field

Recently, the studies related to the organic/inorganic interfaces have attracted large interests since the interfacial interactions play a vital role in determining the charge transfer properties and the performance of the organic semiconductors (OSC) devices. Transition metal oxides (TMO) with high work functions (WF) and deep conduction band (CB) levels are usually used as buffer layers to assistant the charge transfer in the OSC devices. Moreover, MPc/TMO interfaces have wide range applications in organic light emitting diodes (OLED), organic photovoltaic devices (OPV), organic spin valves (OSV), organic field effect transistors (OFET) etc. Accordingly, a detailed and thorough study on the MPc/TMO interface becomes particularly important, which can give a useful guide in optimizing the OSC devices.In this work, we systematically investigated the interfacial electronic structure and energy alignment of the FePc/Mo03/ITO sample using the synchrotron radiation photoemission spectroscopy (SRPES), near edge X ray absorption fine structure (NEXAFS) spectra combined with conventional techniques like laser Raman spectroscopy, atomic force microscopy (AFM) etc. The interface interaction and charge transfer about the FePc/MoOx interface have been explained unambiguously and the main results are summarized as follows:First, the WF of MoO3depends on the thickness of the MoO3films. It was found the WF of MoO3increased with the gradual deposition of MoO3until the films reached a thickness of3nm at which the WF realized the maximum of6.85eV and didn’t vary with the further deposition of MoO3. So the3nm MoO3films is recognized as the favorable hole buffer layer with an optimum thickness. After the thermal annealing of the3nm MoO3film (200℃), a large amount of oxygen vacancies appeared at the surface of MoO3, and some gap states arose in the band gap, which changed the electronic structure of MoO3. The energy level diagram showed that the energetic distance between the highest occupied molecular orbitals (HOMO) of FePc and the CB minimum (CBM) of MoOx was decreased. This means the charge transfer from the HOMO levels of FePc to the CBM of MoOx becomes much easier. Principally, the gap states open up a new path for the hole transfer from the ITO anode to the HOMO levels of FePc via MoOx, and this is theoretically favorable for the improvement of the charge transfer efficiency. Second, we studied the reduction of MoO3at different annealing temperatures in detail. The X-ray photoemission spectroscopy (XPS) suggested that the reduction of MoO3was strengthened gradually with the annealing temperature increasing within400℃, over which the MoO3film remained stable and no further reduction was detected any more upon temperature. The AFM results showed that the MoOx particles had much smaller size than MoO3, which would supply more adsorption sites for FePc.The XPS spectra showed the MoOx adsorbed with1ML FePc had much higher reduction extent than the bare MoOx, which suggests the FePc promotes the reduction of MoOx, and some strong interactions exist at the FePc/MoOx interface that are caused by the O atoms of MoOx binding with FePc molecules. The NEXAFS spectra revealed that the intensity of the peak associated with the terminal oxygen and the asymmetrical bridging oxygen species was declined, and the energy separation was enhanced, implying a large variation of the coordination field around oxygen atoms at the FePc/MoOx interface. So we can conclude that the strong interaction at the FePc/MoOx interface is mainly caused by the Fe-0coordination, which provides a convenient route for charge transfer from FePc to MoOx.The Ⅰ-Ⅴ curves suggests there is a much larger electric current in the FePc/MoOx/ITO sample than in the FePc/MoO3/ITO sample at the same voltage. This means the MoOx is more favorable for charge transfer than the stoichiometric MoO3.Third, a high magnetic field of8.5T was used to modulate the organic molecular orientation at the substrates in order to change the interfacial electronic structure. It was found the molecules grown on the Si substrate had10°orientation change after applied to the magnetic field than without the magnetic field when the substrates were placed vertically to the magnetic field. However, for the Cu substrate, the change was14°. When the Si substrate has an angle of20°with the direction of the magnetic field, the molecular orientation changed13°relative to the case of the substrate vertical to the direction of the magnetic field, while for the Cu substrate, the change was7°. So we conclude that the magnetic field is effective to modify the molecular orientation during the FePc deposition. When the sample was put in the magnetic field for longer time after the growth, the FePc orientation angle also changed indicating the magnetic field effect on the molecular orientation of the deposited FePc films.