Stable Single-Layer Light-Emitting Electrochemical Cell Using 4,7-Diphenyl-1,10-phenanthroline-bis(2-phenylpyridine)iridium(III) Hexafluorophosphate Henk J. Bolink,* ,† Luca Cappelli, † Eugenio Coronado, † Michael Gra ¨ tzel, ‡ Enrique Ortı ´, † Rube ´ n D. Costa, † Pedro M. Viruela, † and Md. K. Nazeeruddin ‡ Instituto de Ciencia Molecular, UniVersidad de Valencia, P.O. Box 22085, ES-46071 Valencia, Spain, and Laboratory for Photonics and Interfaces, Ecole Polytechnique Fe ´ de ´ rale de Lausanne, CH-1015 Lausanne, Switzerland Received September 5, 2006; E-mail: henk.bolink@uv.es Light-emitting electrochemical cells (LEECs) are single-layer electroluminescent devices consisting of a luminescent material in combination with ionic charges. 1,2 The main characteristic of these devices is the insensitivity to the workfunction of the electrodes employed. This is due to the generation of a strong interfacial elec- tric field caused by the displacement of the mobile ionic species toward the charged electrodes when applying an external electric field over the device. Additionally, these devices have a large tol- erance to the thickness of the emitting layer, which simplifies the production process. First examples of these devices were based on conjugated polymers to which inorganic salts were added. 1 More recently, the focus has shifted to organometallic compounds that yield single-component solid-state light-emitting devices. The major- ity of the devices is based on charged organometallic complexes using iridium(III) and ruthenium(II) as the metal core. 3-7 The com- pound most widely used in these single-component devices is tris- (bipyridine)ruthenium(II), Ru(bpy) 3 2+ , balanced by a large negative counterion such as hexafluorophosphate. 2,8,9 The devices are inter- esting candidates for use in thin-film lighting applications as they operate at very low voltages, yielding high power efficient devices, and are easy to produce. There remain, however, a number of bottlenecks that impede their integration in products. These include a limited temporal stability and the lack of deep-blue light-emitting complexes. Using various chemical approaches, the range of avail- able colors has increased recently up to a blueish-green device. 4 A more serious obstacle for implementation of the devices is their limited lifetimes. Detailed studies performed on Ru(bpy) 3 2+ -based devices revealed that the device stability can be related to the for- mation of small amounts of quenching molecules during device operation. 10,11 In a recent paper, we showed that by using more bulky ligands the lifetime of a ruthenium(II)-complex-based device can be greatly enhanced. 12 One explanation for this increase in lifetime is the protection of the metal-ligand bond from chemical reactions that may lead to possible quenching molecules as un- wanted products. Thus we showed that it is possible to significantly increase the lifetime of devices based on charged ruthenium organometallic complexes. However, to obtain devices emitting yellow, green, and blue light, other metals such as iridium have to be used as the core of the light emitting complexes. The lifetime of the devices using these complexes is generally low, ranging from minutes to a few hours. It is therefore of great interest to verify if the approach of introducing bulky shielding ligands in iridium com- plexes can also increase the device lifetime. In this Communication, a device is described based on the heteroleptic iridium(III) com- plex: 4,7-diphenyl-1,10-phenanthroline-bis(2-phenylpyridine)irid- ium(III) hexafluorophosphate (abbreviated as [Ir(ppy) 2 dpp]PF 6 ), which shows a significant increase in lifetime compared to previous charged iridium-based electroluminescent devices. The title complex was chosen as it resembles most of the other iridium complexes used in LEEC devices, namely, a complex based on two cyclometalating C ∧ N ligands and one neutral diimine N ∧ N ligand resulting in a 1+ overall charge, which is countered by one negative ion, hexafluorophosphate. The complex was synthesized by reacting 1 equiv of the dimeric iridium(III) complex [Ir(ppy) 2 - (Cl)] 2 with 2.5 equiv of 4.7-diphenyl-1,10-phenanthroline in dichlo- romethane under nitrogen. The photoluminescent quantum yield was determined to be 53% using [Ru(dpp) 3 ]Cl 2 as the standard (see Supporting Information). 13 Solid films of [Ir(ppy) 2 dpp]PF 6 were prepared by spin coating from acetonitrile solutions. Neat films were obtained without adding inactive polymers such as polymethylmethacrylate (PMMA). The thickness of the films ranged between 100 and 200 nm as determined using a profilometer. Devices were prepared by depositing gold, silver, or aluminum electrodes on top of the spin- coated films, which were thermally evaporated under vacuum (<2 × 10 -6 mbar) to a thickness of 100 nm. Structured ITO-containing glass plates were used as the substrates. Device preparation and characterization were performed in inert atmosphere (<0.1 ppm H 2 O and <0.1 ppm O 2 ). Upon applying a bias of 3 V to an ITO/[Ir(ppy) 2 dpp]PF 6 /Au device, light emission, slowly increasing in intensity with time, is observed (Figure 1). The electroluminescence spectrum of the [Ir- (ppy) 2 dpp]PF 6 device is broad with a maximum at 600 nm, very similar to the photoluminescence spectra obtained in solution (see Supporting Information for PL spectra). The time-delayed response of the current density and the luminance is one of the striking features of the operation of an electrochemical cell and reflects the mechanism of device operation. The slow response is due to the low migration rate of the PF 6 - ions through the solid film and can be enhanced by changing this counterion with smaller ones such as BF 4 - . 14 For comparison, the current density and the luminance versus time are shown in Figure 1 for a similar device under the same external electric field but making use of the complex 3,4,7,8-tetra- methyl-1,10-phenanthroline-bis(2-phenylpyridine)-iridium(III) hexaflu- orophosphate (abbreviated as [Ir(ppy) 2 phen]PF 6 ). From this com- parison it is obvious that the lifetime of the ITO/[Ir(ppy) 2 dpp]PF 6 / Au device is drastically improved with respect to the ITO/ [Ir(ppy) 2 phen]PF 6 /Au device. The time to half of the maximum light intensity is approximately 65 h, in the same range as the previously reported 115 h for a ruthenium-based device. 12 It is, to our knowledge, the longest stability observed up to now for solid- state LEEC devices using charged iridium organometallic com- plexes under dc driving. † University of Valencia. ‡ Ecole Polytechnique Fe ´de ´rale de Lausanne. Published on Web 10/27/2006 14786 9 J. AM. CHEM. SOC. 2006, 128, 14786-14787 10.1021/ja066416f CCC: $33.50 © 2006 American Chemical Society