Improved Stability of Solid State Light Emitting Electrochemical Cells Consisting of Ruthenium and Iridium Complexes
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0965-S03-04
Improved Stability of Solid State Light Emitting Electrochemical Cells Consisting of Ruthenium and Iridium Complexes Henk Jan Bolink1, Luca Cappelli1, Eugenio Coronado1, Rubén D. Costa1, Michael Graetzel2, MdK Nazeeruddin2, Pedro M. Viruela1, and Enrique Ortí1 1 Instituto de Ciencia Molecular, Universidad de Valencia, P.O. Box 22085, Valencia, E-46071, Spain 2 Laboratory for Photonics and Interfaces, Ecole Polytechnique Fédérale de Lausanne, Lausanne, CH-1015, Switzerland
ABSTRACT Two charged organometallic complexes containing bulky hydrophobic ligands based on ruthenium (II) and iridium (III) were synthesized and their performance in solid state light emitting electrochemical cells is described. The complexes were chosen as due to their large ligands a diminished susceptibility towards the formation of destructive complexes during device operation is expected. The LEC device performances reveal the longest living devices reported so far under dc bias. Quantum chemical calculations confirm that the major effect of the bulky diphenylphenanthroline ligands is of steric origin and not related with changes in the molecular electronic structure of the complexes.
INTRODUCTION One of the latest types of organic light emitting devices (OLEDs) makes use of ionic charges to facilitate electronic charge injection from the metal electrodes into the organic or inorganic molecular semiconductor[1]. These ionic charges allow for electronic charge injection into the light emitting film independently of the metallic electrode employed, opening the road for un-encapsulated stable devices. Additionally, these devices have a larger tolerance to the thickness of the emitting layer, which facilitates the production process. The first example of such light emitting electrochemical cells (LECs) was reported when an ionic salt was added to a polyphenylenevinylene light emitting polymer[2]. More recently, the focus has shifted to organometalic compounds that yield single-component solid-state light emitting devices. The compound most widely used in these single-component devices is tris-bipyridine ruthenium, Ru(bpy)32+, balanced by a large negative counter ion such as hexafluorophosphate[3,4]. Nevertheless, the LECs still have a low stability, ranging from several hours to days, and a limited amount of colors available. Making use of heteroleptic iridium and ruthenium pyridyl complexes the range of available emission colors has been widened[5-7]. Also significant research has been done towards the identification of the stability limiting factors of LECs using Ru(bpy)32+ as the emitting entity[8-10]. One of the reasons for the low stability of Ru(bpy)32+ devices seems to be the creation of quenching molecules during device operation such as Ru(bpy)2(H2O)22+ [9] or dimers of the Ru complex[11], both occurring due to the presence of small amounts of water in the device. Considering the charged nature of the emitting complexes used
in the LEC devices, and hence the almost unavoidable presence of traces of water in the luminesce
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