Direct observation of the evolution of occupied and unoccupied energy levels of two silole derivatives at their interfaces with magnesium

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The electronic structures of 2,5-bis(6′-(2′,2″-bipyridyl))-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy) and 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PPSPP) at their interfaces with Mg were investigated using ultraviolet, inverse, and x-ray photoemission spectroscopies. PyPySPyPy and PPSPP have been used as both electron injection/transport layers and emitters in high-efficiency organic light-emitting diodes (OLEDs). Deposition of either PyPySPyPy or PPSPP onto Mg results in the appearance of two energy levels within the energy gap of the organic. Upon deposition of Mg onto PyPySPyPy there is a shift of the occupied energy level structure to higher binding energy, away from the Fermi level, and appearance of two energy levels within the energy gap of PyPySPyPy. The lowest unoccupied molecular orbital is also shifted to higher binding energy. Upon deposition of Mg onto PPSPP there is also a rigid shift of the occupied energy level structure to higher binding energy, away from the Fermi level, but there are no apparent energy levels created within the energy gap of PPSPP. The different chemical reactivity of the two silole derivatives with magnesium is shown to have pronounced effects on the formation of cathode contacts in OLED structures.