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Keywords:

  • Light-emitting diodes, polymer (PLEDs);
  • Light-emitting materials;
  • Organic electronics;
  • Solution processing

Abstract

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.