Excimer Emission in Single Layer Electroluminescent Devices Based on

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Excimer Emission in Single Layer Electroluminescent Devices Based on [Ir(4,5-diphenyl-2-methylthiazolo)2(5-methyl-1,10-phenanthroline)]+ [PF6]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 electrochemical cells based on [Ir(4,5-diphenyl-2-methylthiazolo)2(5-methyl-1,10-phenanthroline)]+ [PF6]- 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 memories,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 conventional 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 electrochemical 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* To whom correspondence should be addressed. E-mail: e.margapoti@ bo.ismn.cnr.it. E-mail: [email protected]. † Istituto per lo Studio dei Materiali Nanostrutturati. ‡ Metallorganica e Analitica “Lamberto Malatesta” dell’Universit degli Studi di Milano and UdR dell’INSTM di Milano.

charge interfacial layer formed at the injecting electrodes results in low voltage device operation.8 However, the detailed mechanism 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/m2 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/m2.21 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-diphenyl-2-methylthiazolo)2(5-methyl-1,10-phenanthroline)]+ [PF6]-.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

10.1021/jp901927e CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009

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Figure 1. (a) Chemical structure of [Ir(dpmt)(Me-phen)]+ [PF6]-. Schematic view of the charge distribution in a [ITO/[Ir(dpmt)(Me-phen)]+ [PF6]+ PMMA(5%)/Al] device: (b) unbiased (V ) 0) and (c) forward biased (V > 0).

found to be irreversible. Once a device has been biased, low energy excimer emission is invariably detected even if the device is left unbiased for long periods of time. Moreover, test measurements performed at later times on previously biased devices reveal that the red-shifted excimer component dominates the EL spectrum at a voltage lower than that previously needed to observe the same spectrum. Both PL lifetime investigations in devices operated at different voltages for different times and morphology studies of biased and unbiased devices support this scenario. II. Experimental Section Synthesis of [Ir(4,5-Diphenyl-2-methylthiazolo)2(5-methyl1,10-phenanthroline)]+ [PF6]-. The complex [Ir(4,5-diphenyl2-methylthiazolo)2(5-methyl-1,10-phenanthroline)]+ [PF6]-, in short [Ir(dpmt)(Me-phen)]+ [PF6]-, was prepared by using a procedure similar to that previously reported for the family of complexes [Ir(phenylpyridine)2(5-X-1,10-phenanthroline)]+ [PF6]-.26 A 2-methoxyethanol/water solution in the ratio of 3:1 v/v of iridium trichloride trihydrate and the stoichiometric amount of 4,5-diphenyl-2-methylthiazole was refluxed for 24 h. The solution was cooled to room temperature affording [Ir(4,5diphenyl-2-methylthiazolo)2Cl]2, which precipitated. A CH2Cl2MeOH solution of [Ir(4,5-dipheny-l-2-methylthiazolo)2Cl]2 and the stoichiometric amount of 5-methyl-1,10-phenanthroline was then refluxed for 5-6 h. The solution was cooled to room temperature, and then an excess of ammonium hexafluorophosphate was added. The solution was then filtered and the supernatant liquid was evaporated to dryness. The obtained solid was then recrystallized by using a CH2Cl2-diethyl ether mixture that afforded [Ir(dpmt)(Mephen)]+ [PF6]-. Details on the full characterization of this complex are reported in ref 25. Device Fabrication. The devices were fabricated by using indium tin oxide (ITO) coated glass as substrates. The ITO layer has a thickness of 150 nm and a surface resistivity of 15 Ω0. Substrates were first cleaned in acetone and then treated by oxygen plasma for 15 min. The organic thin films were prepared by spin coating a solution of the complex [Ir(dpmt)(Mephen)]+ [PF6]- blended with 5% of polymethylmethacrylate (PMMA) in dichloromethane at 4000 rpm on the ITO coated substrates in a nitrogen glovebox with 9.4 V), the intermolecular distance between the charge pairs was modified, yielding different emissive energy bands. In addition, it should be pointed out that maintaining the device unbiased for a long time did not show any spontaneous recovery of the initial emission properties, which in other words means that the effect of excimer formation is irreversible. These results are in agreement with literature reports on EL intensity degradation.31 The behavior of the EL spectra was not mirrored by the PL spectra which, as discussed above, were less broadened and show a very limited red-shift at high voltage. This difference can be explained in terms of different relative mobility of the electrons and holes injected into the active material, which induces the EL to form primarily close to one of the electrode interfaces, while PL originates from the entire film cross section. However, we point out that the region

from where the EL-emission predominantly originates is a thin cross section of the overall film, from where, instead, the PL is generated. In general, the mobility values of electrons and holes in organic materials are different, leading to exciton formation in OLED devices closer to one of the two injecting contacts.32 An electrodynamical model,33,34 confirmed by experimental observations,27 suggested the hole mobility in ionic transition metal complexes to be lower than that of electrons. Thus, we may consider that the EL is generated in the proximity of the ITO interface. Consequently, the EL is dominated by interfacial effects, while the PL, which originates from the entire film thickness, is less sensitive to them. We mention that a red-shift in the PL emission was also observed by Bolink et al.16 in devices based on [Ir(ppy-F2)2 Me4phen]+ [PF6]-. In that case the effect was ascribed to molecular aggregation and correlated to the iTMC concentration in the active thin film. The possibility of excimer formation was excluded because the excited state lifetime of the red-shifted emission was shorter than that of diluted molecules. In our case the ionic metal complex concentration in the film was always extremely high (95:5 ratio between the [Ir(dpmt)(Me-phen)]+ [PF6]- complex and PMMA) and was kept constant in all films used for device fabrication. It is worth pointing out that the decrease of the PL intensity and the red-shift of EL, with respect to PL, could be due to other phenomena such as triplet-triplet annihilation or electric field assisted dissociation of the excitons.28,29 To rule out all these alternative explanations and to confirm the formation of excimers in biased devices we performed time-resolved PL measurements. It is well-known that the excimer lifetime increases significantly with respect to molecular emission.28 In addition, lifetime is a much more sensitive fingerprint of excimer formation than the spectral shift as it is directly related to the excited electronic state deactivation channels. We therefore performed time-resolved PL measurements in a device under different bias conditions and at different times after the initial biasing. In Figure 4 the PL decay recorded from an unbiased device (gray trace, measurement A) is shown. It is important to mention that at this stage the device was never biased and within 48 h from the first measurement no changes in the lifetime were recorded. The device was then biased at 7 V for 1 h while continuously monitoring the PL lifetime. No difference in lifetime was observed. After, the device was left unbiased for 24 h and then measured again at 7 V (blue trace, measurement

Excimer Emission in Electroluminescent Devices

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Figure 5. CLSM images of a [Ir(dpmt)(Me-phen)]+ [PF6]- film before (a) and after (b) the application of a 12 V bias.

Figure 4. PL decay curves measured in ITO/[Ir(dpmt)(Mephen)]+ [PF6]- + PMMA(5%)/Al device. Measurement A (gray trace): The device was not biased (V ) 0) during measurement. Measurement B (blue trace): The device was biased at 7 V. Before the measurement the device was biased for 1 h and left unbiased for 24 h. Measurement C (green trace): The decay curve of the same device at V ) 0 after it was left unbiased for an additional 20 h.

TABLE 1: Photoluminescence Lifetimes of [Ir(dpmt)(Me-phen)]+ [PF6]- Complex in Solution (argon bubbled or in air), Thin Film (in vacuum or in air), and Single Layer Electroluminescent Device samples

τ1 [µs]

τ2 [µs]

τ3 [µs]

τ4 [µs]

solution [Ar (air)] film [vac. (air)] measurement A measurement B measurement C

0.73 (0.023) 0.1 (0.07) 0.13 0.13 0.14

0.92 (0.53) 1.12 1.22 1.23

5.2 3.6

67

B in figure 4). Finally, the PL decay was recorded from the same device at V ) 0, after leaving it unbiased for 20 h (green trace, measurement C in Figure 4). It is worth pointing out that the laser power was kept low enough to avoid any degradation of the material. All the three measurements A, B, and C show a very similar behavior for the fast lifetime components, but very different lifetime values are found for the longer lifetime components. Measurement A, from a never biased device, was fitted with a biexponential curve leading to a fast component, τ1 ) 0.13 µs, and a slower component, τ2 ) 1.12 µs. These values are in agreement with those measured in thin films (see Table 1), indicating that the oxide/organic and metal/organic interfaces have a negligible influence on the [Ir(dpmt)(Mephen)]+ [PF6]- complex excited state lifetime. Measurement B has a longer tail, and can be properly fitted by using three exponentials, which lead to three different lifetimes. The first two lifetimes are almost the same as those of the unbiased device A, τ1 ) 0.13 µs and τ2 ) 1.22 µs, whereas the longer lifetime is τ3 ) 5.2 µs. Finally, measurement C has a tail with much longer lifetime, and requires the use of a fourth exponential. The measured lifetimes are τ1 ) 0.13 µs, τ2 ) 1.23 µs, τ3 ) 3.6 µs, and τ4 ) 67 µs, respectively. All lifetimes derived from measurements A, B, and C are listed in Table 1 and compared with solution and thin-film values. The difference between the lifetimes in measurements A and B confirms that by biasing the device excimers are formed. Indeed, while the first two components, τ1 and τ2, refer to the triplet states of the phosphorescence emission, the longer component, τ3, is assigned to the formation of excimers. Furthermore, after leaving the device unbiased for 20 h, the material does not recover the

original condition, but rather shows an even longer lifetime, τ4, as shown from measurement C. The [Ir(dpmt)(Mephen)]+ [PF6]- complex undergoes a dynamical change of the emitting exciton properties induced by the applied electric field. Already at low bias, the EL showed different wavelength positions compared to the PL. This spectral shift is promoted by the device operation time. Indeed, when we measure the PL and the EL at the time t ) 0 and V ) 7 V, the collected spectra had the same maximum at λPL,EL max (0) ) 586 nm, which coincides with the solution emission maximum. On the contrary, if we measure the EL at the time t ) 1 h and V ) 9 V, the EL is EL (1 h) ) 637 nm. From this shifted toward the red, λmax observation we understand that by varying either the applied voltage or the operation time, excimer formation was favored by the gradual accumulation of charges at the ITO electrode interface. Moreover, if stronger fields are applied, the correlated distance between the electrons and the holes was such that different energy bands associated to different excimer states were formed. These results are in agreement with literature reports of faster EL degradation when higher driving voltage is applied.21 We also performed a study of the morphological changes of the film, when an external electrical field is applied. Materials with MLCT character are known to be sensitive to different polarity media.35,36 Here we demonstrated that the studied material is very sensitive to the application of an external bias. In Figure 5a is shown the CLSM fluorescence image of a device before biasing. The CLSM image of the same film after biasing at 12 V is reported in Figure 5b. It is clear that the film undergoes a rearrangement, which could be due to phase separation or molecular reorganization induced by the applied electric field. The material rearrangement favors the formation of the excimer states identified by optical spectroscopy. IV. Conclusion In conclusion, we have demonstrated excimer formation in single-layer electroluminescent devices of the [Ir(dpmt)(Mephen)]+ [PF6]- complex induced by the applied electric field. Excimer emission, favored by the morphological rearrangement of the material, is reflected in the spectral behavior of EL and in the PL decay times. The strong local electric field generated by the ion concentration at the electrodes interface induces the formation of excimers with emission energy that depends on the applied voltage and on the operation time. These results show that, in addition to oxidation processes, excimer states contribute to the EL emission and degradation in electroluminescent devices based on iTMC. A better understanding of the complex working mechanism of these devices could prompt the design of tailored molecular structures enabling the fabrication of OLECs with improved performances and lifetimes. Acknowledgment. This work was supported by the Italian MUR through projects FIRB-RBNE033KMA, FIRB-

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