Low‐Work‐Function Surface Formed by Solution‐Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate

Abstract Nanostructured layers of Cs 2 CO 3 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 Cs 2 CO 3 prepared by solution deposition and thermal evaporation. The nanoscale Cs 2 CO 3 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 Cs 2 CO 3 decomposes into CsO 2 and cesium suboxides. The cesium suboxides dope CsO 2 , yielding a heavily doped n‐type semiconductor with an intrinsically low work function. As a result, devices fabricated using thermally evaporated Cs 2 CO 3 are relatively insensitive to the choice of the cathode metal. The reaction of thermally evaporated Cs 2 CO 3 with Al can further reduce the work function to 2.1 eV by forming an Al–O–Cs complex. Solution‐processed Cs 2 CO 3 also reduces the work function of Au substrates from 5.1 to 3.5 eV. However, devices prepared using solution‐processed Cs 2 CO 3 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 Cs 2 CO 3 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.