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Cover Picture: Low‐Work‐Function Surface Formed by Solution‐Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate (Adv. Funct. Mater. 12/2007)
Author(s) -
Huang J.,
Xu Z.,
Yang Y.
Publication year - 2007
Publication title -
advanced functional materials
Language(s) - English
Resource type - Reports
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.200790041
Subject(s) - work function , materials science , x ray photoelectron spectroscopy , cathode , evaporation , layer (electronics) , chemical engineering , caesium , analytical chemistry (journal) , optoelectronics , nanotechnology , inorganic chemistry , chemistry , organic chemistry , physics , engineering , thermodynamics
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 Cs 2 CO 3 interfacial layer. It reveals the origin of enhanced electron injection from the Al electrode due to the low work function of a thermally evaporated Cs 2 CO 3 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 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.