Excimer Emission in Single Layer Electroluminescent Devices Based on [Ir(4,5-diphenyl-2-methylthiazolo) 2 (5-methyl-1,10-phenanthroline)] + [PF 6 ] - E. Margapoti,* ,† V. Shukla, † A. Valore, ‡ A. Sharma, † C. Dragonetti, ‡ C. C. Kitts, † D. Roberto, ‡ M. Murgia, † R. Ugo, ‡ and M. Muccini* ,† CNR, Istituto per lo Studio dei Materiali Nanostrutturati, Via P. Gobetti 101, I-40129 Bologna, Italy, and Dipartimento di Chimica Inorganica, Metallorganica e Analitica “Lamberto Malatesta” dell’UniVersitá degli Studi di Milano and UdR dell’INSTM di Milano, Via Venezian 21, I-20133 Milano, Italy ReceiVed: March 2, 2009; ReVised Manuscript ReceiVed: May 22, 2009 A correlated photoluminescence (PL) and electroluminescence (EL) investigation of light-emitting electro- chemical cells based on [Ir(4,5-diphenyl-2-methylthiazolo) 2 (5-methyl-1,10-phenanthroline)] + [PF 6 ] - reveals the excimer nature of the EL emission. Excimers are formed when the device is biased and the EL emission energy is a fingerprint of the excimer intermolecular interactions induced by the local electric field at the applied voltage. Imaging measurements provide evidence that the molecular rearrangement into aggregates favors the formation of excimer states, which are irreversibly formed once the device is biased and are the preferential emitting states even in devices left unbiased for a long period of time. PL lifetime measurements in working devices provide unambiguous evidence of the excimer character of EL. These results show that excimers play a role in the mechanism of operation and performance degradation of organic light-emitting electrochemical cells (OLEC) based on ionic transition metal complexes (iTMC). I. Introduction The recent advancements of organic materials science have enabled the development of a variety of devices such as organic light-emitting diodes (OLEDs), 1,2 solar cells, 3,4 organic memo- ries, 5 and field-effect transistors (OFETs) 6,7 with constantly improved performances. Interest in organic electronics stems from the desire to produce low-cost, large-area, lightweight, and flexible devices which are able to integrate functionalities that are currently accomplished by using more expensive conven- tional semiconductors and components. In particular, OLED devices hold the promise of a massive marketing in the field of flat panel displays and lighting technologies. However, device brightness and lifetime matching the market requirements are achieved by using complex fabrication processes that increase production costs. Multilayer structures are engineered, which are composed of up to seven different organic materials, each of them selected to perform a specific function in the device. Recently, it has been demonstrated that ionic transition metal complexes (iTMCs) allow for the fabrication of single-layer electroluminescent devices, named organic light-emitting elec- trochemical cells (OLECs). 8-19 These can be produced by using much simpler fabrication processes with respect to OLEDs and have the potential to afford high brightness and large area devices. The combination of ionic and electronic processes enables efficient charge injection and recombination without the inclusion of interfacial injection and charge transport layers in the device structure. The working mechanism of OLECs relies on the movement of counterions when an electric field is applied between the cathode and the anode. This changes the charge density and the electric field distribution across the film. The efficient electron and hole injection prompted by the counter- charge interfacial layer formed at the injecting electrodes results in low voltage device operation. 8 However, the detailed mech- anism of electroluminescence (EL) generation and degradation in this class of devices is not fully understood yet and represents one of the most important limiting factors for the developments of OLEC devices with improved brightness and lifetime comparable to that of OLEDs. The longest lifetime reported to date for an electroluminescent device based on iridium ionic metal complexes is about 3000 h at an average luminance of 200 Cd/m 2 and is achieved through the control of the supramolecular interactions. 20,21 At higher brightness the device lifetime dramatically decreases and is reported to be of a few hours for a brightness of 2770 Cd/m 2 . 21 The effect of oxygen and water on the degradation of the EL emission has been largely demonstrated by a number of investigations. 20-24 Different mechanisms might be responsible for the observed degradation, among which the formation of an oxo-bridged dimer has been identified as an effective quencher of the luminescence in devices based on [Ru(bpy) 3 ] 2+ , where bpy is bipyridine. 22 For this reason, in order to go one step further in the understanding of the complex mechanism of EL emission and degradation in electrochemical cells beyond the effect induced by oxygen and water, we have performed an extended investigation of the EL and photoluminescence (PL) properties in samples systematically prepared and measured in the absence of oxygen and water. Here we report the first observation of excimer emission in single-layer electroluminescent devices based on [Ir(4,5-diphen- yl-2-methylthiazolo) 2 (5-methyl-1,10-phenanthroline)] + [PF 6 ] - . 25 Through a correlated investigation of the PL and EL properties, recorded at different bias voltages, we show that the energy of the emitting excimer state is determined by the applied electric field. By increasing the bias voltage the EL spectrum is dominated by red-shifted components, which are ascribed to excimer states with increased intermolecular interactions induced by the enhanced local electric field. The formation of excimer states in biased devices is * To whom correspondence should be addressed. E-mail: e.margapoti@ bo.ismn.cnr.it. E-mail: m.muccini@bo.ismn.cnr.it. † Istituto per lo Studio dei Materiali Nanostrutturati. ‡ Metallorganica e Analitica “Lamberto Malatesta” dell’Universit degli Studi di Milano and UdR dell’INSTM di Milano. J. Phys. Chem. C 2009, 113, 12517–12522 12517 10.1021/jp901927e CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009