Advanced Functional Materials

Cover image for Vol. 26 Issue 32

17_12/2007Cover Picture: Low-Work-Function Surface Formed by Solution-Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate (Adv. Funct. Mater. 12/2007)

The cover shows the structure of an efficient polymer light emitting diode (PLED) and its energy diagram at the interface between aluminum (Al) and a Cs2CO3 interfacial layer. It reveals the origin of enhanced electron injection from the Al electrode due to the low work function of a thermally evaporated Cs2CO3 layer, as reported on p. 1966 by Jinsong Huang, Zhen Xu, and Yang Yang. Pictures of the white- and red-emitting PLEDs are also shown.

Nanostructured layers of Cs2CO3 are shown to function very effectively as cathodes in organic electronic devices because of their good electron-injection capabilities. Here, we report a comprehensive study of the origin of the low work function of nanostructured layers of Cs2CO3 prepared by solution deposition and thermal evaporation. The nanoscale Cs2CO3 layers are probed by various characterization methods including current–voltage (I–V) measurements, photovoltaic studies, X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), and impedance spectroscopy. It is found that thermally evaporated Cs2CO3 decomposes into CsO2 and cesium suboxides. The cesium suboxides dope CsO2, yielding a heavily doped n-type semiconductor with an intrinsically low work function. As a result, devices fabricated using thermally evaporated Cs2CO3 are relatively insensitive to the choice of the cathode metal. The reaction of thermally evaporated Cs2CO3 with Al can further reduce the work function to 2.1 eV by forming an Al–O–Cs complex. Solution-processed Cs2CO3 also reduces the work function of Au substrates from 5.1 to 3.5 eV. However, devices prepared using solution-processed Cs2CO3 exhibit high efficiency only if a reactive metal such as Al or Ca is used as the cathode metal. A strong chemical reaction occurs between spin-coated Cs2CO3 and thermally evaporated Al. An Al–O—Cs complex is formed as a result of this chemical reaction at the interface, and this layer significantly reduces the work function of the cathode. Finally, impedance spectroscopy results prove that this layer is highly conductive.

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