Manipulation of interface electronic structure by thin metal oxide films
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Manipulation of interface electronic structure by thin metal oxide films Chenggong Wang,1 Irfan1 and Yongli Gao1,2 1
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, U.S.A
2
Institute for Super Microstructure and Ultrafast Process, the Central South University, Changsha, Hunan, The People's Republic of China, 410083, P.R.China
Abstract We have investigated the counter intuitive phenomenon of inserting a metal oxide layer to improve hole injection or extraction in organic semiconductor devices using ultraviolet photoemission, x-ray photoemission, and inverse photoemission spectroscopy (UPS, XPS and IPES). We observed that metal oxides, such as MoO3, substantially increase the work function when deposited on indiumtin-oxide (ITO). The increase lifts up the highest occupied molecular orbital (HOMO) of the hole transport layer, therefore reduces the energy barrier between the HOMO and the Fermi level of the anode. The uplift creates an interface band bending region that results in a drift electric field that encourages the collection of holes at the anode. The optimum thickness for the oxide layer is estimated to be 2 nm. We have also investigated the effects of ambient or O2 exposure of MoO3. We observed that while most of the electronic energy levels of the oxide remained largely intact, the work function reduction was significant. This opens a way for optimal energy level alignment by modifying the work function through exposure. Furthermore, we observed that the work function reduction by exposure could be reversed by proper annealing of the sample in vacuum. The investigations therefore point to manipulate the interface electronic structure and charge injection/extraction by thin metal oxide films. INTRODUCTION The field of inorganic electronic is well established and greatly changed our life. Although the cost of traditional inorganic electronic devices is decreasing, the processing cost associated with it became the most imperative issue in large scale applications, such as solar cell panel production for renewable energy. During the manufacturing of silicon substrates, the environment pollution is a problem which we could not ignore. To address those issues, the development of organic electronic devices based on organic semiconductors (OSCs) has attracted considerable interest from both the scientific and industry communities since its low cost and environment friendly. 1-3 Organic semiconductors have already been successfully demonstrated for their applications including organic photovoltaic cells (OPVs), 4,5 organic light emitting diodes (OLEDs), 6,7 and organic thin film transistors (OTFTs). 8,9 Although we have certain level understanding of organic semiconductors, we still have many questions about the electronic structure, interface processes, and charge transportation. Therefore it is necessary to conduct a systematic study to investigate the characteristic of organic semiconductors. A great deal of effort has been devoted in order to improve the charge transportation
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