26 August 2008 Mechanism of LiF interlayer for electron injection
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Despite its wide application in devices, the mechanism of improvement induced by the LiF insertion layer remains controversial and to be fully resolved. We report our study of the interface formation when gold or Al is deposited onto 5 Å LiF covered Alq using X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). We found that initial Au deposition produced a small shift of energy levels toward higher binding energy, which was reversed by subsequent Au coverages. The energy level positions finally reach those of the pristine Alq, resulting in a flat-band situation in the interface region. This is in sharp contrast to the Al/LiF/Alq interface, where ~1 eV downward shift of the energy levels substantially reduces the electron injection barrier. The observation of the overall flat-band condition in the interface region explains well why for thin LiF interlayer, the metal overlayer material is critical for the improvement of charge injection. As we observed here for Au, the low reactivity of the deposited metal atoms do not result in substantial n-doping of the Alq in the interface region, in contrast to more reactive metals like Al and Mg that can cause substantial n-doping of Alq, signified by the ~1 eV energy level shifts toward higher BE and emergence of the gap state, and reduce the electron injection barrier as a result.
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Kiwan Park, Kiwan Park, Huanjun Ding, Huanjun Ding, Franky So, Franky So, Yongli Gao, Yongli Gao, } "Mechanism of LiF interlayer for electron injection", Proc. SPIE 7051, Organic Light Emitting Materials and Devices XII, 70511V (26 August 2008); doi: 10.1117/12.798983; https://doi.org/10.1117/12.798983


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