Operational mechanism of light-emitting devices based on Ru(II) complexes: Evidence for electrochemical junction formation

Abstract

In this work, the operational mechanism of single-layer light-emitting electrochemical cells (LECs) based on the small molecule tris(2,2’ bipyridyl) ruthenium(II) [Ru(II)] was investigated using capacitance and resistance measurements. The current–voltage and capacitance–voltage characteristics of such devices suggest that an electrochemical junction is formed during operation with a high electric field across the junction. A similar mechanism has been proposed for polymer LECs. In the case of Ru(II) devices, electrically conducting regions adjacent to the electrodes are the result of mixed-valent states that form due to oxidation and reduction of the complex. The junction thickness is a function of the type of counterions used and the operating voltage. Thinner junctions were observed for devices with high ionic conductivity and at higher operating voltages. Transient capacitance and resistance measurements show that the junction formation is faster in devices with higher ion mobility and during higher operating voltages. In addition, the capacitance and resistance exhibit a relaxation time after the device is turned off. This relaxation shows that the electrochemical junction stays present in a device for some time (several seconds to minutes) once a device is turned off. The electrochemical junction disappears as the counterions relax back. Furthermore, a theoretical model is presented, which shows that due to the concentration gradient of mixed-valent states during operation, an electric field has to be present in the device. The model also shows that there can be no local charge neutrality in the bulk of the device during operation.

Luminescent Ionic Transition‐Metal Complexes for Light‐Emitting Electrochemical Cells

“…First attempts were made using impedance spectroscopy, a technique that applies a small AC perturbation on a DC voltage to get information on the resistance and capacitance of the active material, and supported the ECD model. [57,58] In fact, the standard tools for studying conventional light-emitting devices, which entail the measurement of luminance (L) and current density (J) as a function of voltage (V) JL-V measurements, are difficult to use in LECs because the application of a voltage triggers the movement of ionic charges and physically modifies the device itself. Therefore, to obtain reliable operative parameters of a LEC for a given ionic distribution, it is crucial to avoid ionic movement during the JL-V scans.…”

The dynamic behavior of thin-film ionic transition metal complex-based light-emitting electrochemical cells

“…42 Lately, van Reenen et al demonstrated a junction width to active layer thickness ratio of 0.3-0.8 dependent on the semiconducting polymer used. 48 In case of an iTMC-LEC comprising an iridium(III) complex similar to the one used in this work, Lenes et al recently highlighted a strong decrease in the effective thickness from 120 nm to 26 nm at a low operating voltage of 3.5 V. 38 In general, thinner junctions were observed at higher operating voltages 47,52 and for devices with higher ionic conductivity, respectively. 52 …”

“…It is to be noticed that the thickness of the intrinsic region was found to be strongly dependent on the operating voltage. 41,47,52 The transient impedance and phase angle of the iTMC-LEC as a function of frequency recorded at a bias voltage of 3 V are illustrated in Figure 2. Applying the bias to the device leads to a continuous decrease in the impedance with operating time (Figure 2(a)).…”

“…It was applied to study degradation in these devices 49,50 to make statements about the impact of ion mobilities, 51 to prove electrochemical junction formation and to highlight influencing variables on the junction width, and to estimate the transient thickness of the p-i-n junction during operation. 38,52 One of the latest publications in the field of LECs used IS to demonstrate that pLECs and iTMCLECs are essentially behaving as one class of device. 53 In this paper, the results from an impedance spectroscopy study on stacked iTMC-LECs based on the ionic iridium(III) model compound bis-2-phenylpyridine 6-phenyl-2,2 0 -bipyridine iridium(III) hexafluoridophosphate ([Ir(ppy) 2 (pbpy)][PF 6 ]) are highlighted.…”

On-demand photochemical stabilization of doping in light-emitting electrochemical cells

“…[28,29] The p-n junction is built at a temperature at which the ions are mobile, and the device is thereafter cooled to a temperature at which the ions effectively become immobile and the p-n junction as a consequence is stabilized. [30][31][32][33][34][35][36] Lonergan and co-workers stabilized a p-n junction at the interface between a cationic and an anionic conjugated polyelectrolyte by solvent-induced removal of the mobile counter-ions, [37] while Bazan et al utilized in-situ boron-fluoride chemistry on a similar device structure for the same purpose. [38] Finally, Leger and Bartholomew [39] conceptualized the more generic "chemical stabilization" approach, which utilizes designed dopant counter-ions that can be physically immobilized via cross-linking polymerization reactions that presumably are initiated by electrochemical means.…”