Wide band gap molecules such as 4,4′ -N,N′ -dicarbazole-biphenyl (CBP), which has a deep ionization potential of 6.0 eV, are known to form charge transfer complexes with very high electron affinity TMOs such as MoO 3. For OLEDs in display applications with n-channel field-effect transistors, it is necessary to have the cathode in contact with the driving element of the circuit 7, thus requiring an inverted top emission device in which the structure is flipped so that the top anode is transparent and the bottom cathode is reflective. In an inverted OLED structure 7, 8, 9, 10, inverted organic photovoltaic cells (OPVs) 11, 12, 13, 14, and organic field-effect transistors (OFETs) 15 the TMO is deposited on top of the organic semiconductor. In device applications, these TMOs are involved in charge exchange between substrate and adsorbed organic molecules, resulting in favorable energy level alignment and improved performance. TMOs such as MoO 3 4, V 2O 5 5, WO 3 6, and many others have also been shown to reduce the energy offset between electrode and organic materials by acting as a surface modification layer, or electrode buffer layer. Transition metal oxides (TMOs) are often used in organic electronic devices due to their excellent hole injection characteristics, by acting as a p-dopant 1, 2, 3. For the interface electronic structure, energy level alignment is observed in agreement with the universal energy level alignment rule of molecules on metal oxides, despite deposition order inversion. For the interface chemical structure, new carbon and molybdenum core-level states are observed, as a result of interfacial electron transfer from organic semiconductor to MoO 3. For the interface physical structure, it is found that MoO 3 diffuses into the underlying organic layer, exhibiting a trend of increasing diffusion with decreasing molecular molar mass. Ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy are used to measure the evolution of the physical, chemical and electronic structure of the interfaces at various stages of MoO 3 deposition on these organic semiconductor surfaces. Eight organic hole transport materials have been used in this study. ![]() We have systematically studied interface structure formed by vapor-phase deposition of typical transition metal oxide MoO 3 on organic semiconductors.
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