New rare-earth quinolinate complexes for organic light-emitting devices H. Camargo a , T.B. Paolini b , E. Niyama a , H.F. Brito b , M. Cremona a, ⁎ a Physics Department, Pontifical Catholic University of Rio de Janeiro, 22453-900 Rio de Janeiro, Brazil b Chemistry Institute, Department of Fundamental Chemistry, University of São Paulo, USP, 05599-970 São Paulo, Brazil abstract article info Available online 6 November 2012 Keywords: Rare-earth ions Tetrakis complexes Photoluminescence Electroluminescence OLEDs 8-Hydroxyquinoline Because of its thermal and morphological stability and optical and electrical properties, tris(8-hydroxyquinoline) aluminum (Alq 3 ) is one of the most widely used electron transporting materials in organic light-emitting devices (OLEDs). The search for substitutes for this compound constitutes an important field of research in organic elec- tronics. We report on a study of a new rare-earth tetrakis 8-hydroxyquinoline complex. Synthesis of tris com- plexes with rare-earth metals and 8-hydroxyquinoline resulted in unstable compounds. However, the inclusion of an additional quinoline group stabilized these compounds. Li[RE(q) 4 ] (where RE=La 3+ , Lu 3+ and Y 3+ and q=8-hydroxyquinoline) were synthesized and then used as the electron-transporting and emitting layer in OLEDs. Thin films were deposited in a high-vacuum environment by thermal evaporation on quartz and silicon substrates. Optical characterization of the RE complexes revealed emission in the 510–525 nm range, the same as that observed for Alq 3 , while absorption was observed at wavelengths of 382 nm for the Y/La complexes and 388 nm for the Lu complex. The OLEDs were fabricated with an indium tin oxide layer (ITO) as the anode, (N,N′-bis (1-naphtyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine) NPB as the hole-transporting layer (25 nm), Li[RE(q) 4 ] as the electron-transporting and emitting layer (40 nm) and aluminum as the cathode (120 nm). The electroluminescence (EL) spectra showed a broad band from 520 to 540 nm and green-colored emission associated with the 8-hydroxyquinoline ligand. There was an interesting dependence of the maximum energy peak position and half-width of the emission band in the EL spectra on the atomic radius of the RE ion used. The best luminance for the OLEDs produced in this study was achieved with the Li[RE(q) 4 ] compound. The optical and electrical prop- erties of this OLED were comparable to those of similar devices based on Alq 3 . © 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent decades, interest in organic and organometallic compounds has been increasing as a result of their application in organic light emitting devices (OLEDs) [1–5], solar cells [6] and transistors [7]. The development of electronic devices based on these materials and structures has made technological advances possible in many fields, including telecommunica- tions [8], optoelectronics [9] and energy generation [10]. In this context, the study of new materials that can be used to produce more efficient light sources than those based on inorganic materials acquires particular importance [11]. Specifically, the study of new materials with properties similar to or better than those of tris(8-hydroxyquinoline) aluminum (Alq 3 ), which is widely used as the electron-transporting and emitting layer in OLEDs [12], can be expected to lead to improvements in the effi- ciency of OLEDs and to allow the color of their emissions to be tuned. There are many reports in the literature of complexes of transition metals (Cd 2+ and Pt 2+ ) with 8-hydroxyquinoline being used as chemosensors [13] or sensitizers [14], as well as studies of the use of rare-earth complexes (Yb 3+ , Er 3+ and Nd 3+ ) with 8-hydroxyquinoline as the emission layer in infrared (IR) OLEDs [15,16]. Similarly, there is an abundance of works on trivalent [2,17,18] and tetrakis [1,19] rare-earth (RE) complexes with organic ligands that exhibit intense lumi- nescence and an inner quantum yield close to 100% because of the higher-efficiency energy transfer from the ligand's triplet state to the rare-earth ions. As the 4f electrons are shielded by the outer 5s 2 and 5p 6 electrons, these complexes are only moderately influenced by the chemical environment and are consequently characterized by almost monochromatic emission and narrow emission peaks (atomic behavior) shifted by only a small amount [20–22]. Complexed rare-earth ions with different atomic radii would be expected to have similar physical and chemical properties and different spectroscopic characteristics if the same organic ligands were used [23]. However, different rare-earth ions (e.g., Eu 3+ or Tb 3+) complexed with the same ligands are known to exhibit different optical properties. The majority of rare-earth compounds reported in the literature are of the “tris” type with three ligand molecules around the central ion and water molecules in the first coordination sphere, which can act as emis- sion quenchers. Hence, substitution of these water molecules by other organic ligands is essential to improve the emission efficiency of the complex. When one more ligand molecule is added to the tris complex the anhydrous tetrakis complex is formed, with the general formula C[RE(ligand) 4 ], where C is the countercation (an alkali metal or Thin Solid Films 528 (2013) 36–41 ⁎ Corresponding author. E-mail address: cremona@fis.puc-rio.br (M. Cremona). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.09.085 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf