Near-UV to red-emitting charged bis-cyclometallated iridium(iii) complexes for light-emitting electrochemical cells

Herein we report a series of charged iridium complexes emitting from near-UV to red using carbene-based N^C: ancillary ligands. Synthesis, photophysical and electrochemical properties of this series are described in detail together with X-ray crystal structures. Density Functional Theory calculations show that the emission originates from the cyclometallated main ligand, in contrast to commonly designed charged complexes using bidentate N^N ancillary ligands, where the emission originates from the ancillary N^N ligand. The radiative process of this series of compounds is characterized by relatively low photoluminescence quantum yields in solution that is ascribed to non-radiative deactivati…

Dynamic doping in planar ionic transition metal complex-based light-emitting electrochemical cells

Using a planar electrode geometry, the operational mechanism of iridium(III) ionic transition metal complex (iTMC)-based light-emitting electrochemical cells (LECs) is studied by a combination of fluorescence miscroscopy and scanning Kelvin probe microscopy (SKPM). Applying a bias to the LECs leads to the quenching of the photoluminescence (PL) in between the electrodes and to a sharp drop of the electrostatic potential in the middle of the device, far away from the contacts. The results shed light on the operational mechanism of iTMC-LECs and demonstrate that these devices work essentially the same as LECs based on conjugated polymers do, i.e., according to an electrochemical doping mechan…

A deep-blue emitting charged bis-cyclometallated iridium(iii) complex for light-emitting electrochemical cells

We report here a new cationic bis-cyclometallated iridium(III) complex, 1, with deep-blue emission at 440 nm and its use in Light-emitting Electrochemical Cells (LECs). The design is based on the 2′,6′-difluoro-2,3′-bipyridine skeleton as the cyclometallating ligand and a bis-imidazolium carbene-type ancillary ligand. Furthermore, bulky tert-butyl substituents are used to limit the intermolecular interactions. LECs have been driven both at constant voltage (6 V) and constant current (2.5 mA cm−2). The performances are significantly improved with the latter method, resulting overall in one of the best reported greenish-blue LECs having fast response (17 s), light intensity over 100 cd m−2 an…

Light-emitting electrochemical cells: recent progress and future prospects

We provide a short review on light-emitting electrochemical cells (LECs), one of the simplest kinds of electroluminescent devices. In their simplest form, they consist of just one active layer containing an emitter and a salt. They operate with low voltages, which allows for high power efficiencies, and air-stable electrodes, which simplifies the encapsulation requirements. The aim of this review is to highlight the recent advances and the main remaining challenges. We describe the current understanding of their peculiar operation mechanism and focus on the major concepts used to improve their performance.

Dynamic doping and degradation in sandwich-type light-emitting electrochemical cells

Photoluminescence spectroscopy has been performed in situ during device operation and after switch-off on ionic transition metal complex (iTMC)-based sandwich-type light-emitting electrochemical cells (LECs). It is demonstrated that the photoluminescence of the LECs decreases with increasing operating time. For operating times up to three hours the decline in photoluminescence is fully recoverable after switching off the bias. These results imply that doping of the iTMC layer is responsible, not only, for the turn-on of LECs but also for their lifetimes.

Simple, Fast, Bright, and Stable Light Sources

In this work we show that solution-processed light-emitting electrochemical cells (LECs) based on only an ionic iridium complex and a small amount of ionic liquid exhibit exceptionally good performances when applying a pulsed current: sub-second turn-on times and almost constant high luminances (>600 cd m(-2) ) and power efficiencies over the first 600 h. This demonstrates the potential of LECs for applications in solid-state signage and lighting.

Solid-State Lighting: Simple, Fast, Bright, and Stable Light Sources (Adv. Mater. 7/2012)

Chloride ion impact on materials for light-emitting electrochemical cells

Small quantities of Cl(-) ions result in dramatic reductions in the performance of ionic transition metal complexes in light-emitting electrochemical cells. Strong ion-pairing between aromatic protons and chloride has been established in both the solid state and solution. X-ray structural determination of 2{[Ir(ppy)2(bpy)][Cl]}·2CH2Cl2·[H3O]·Cl reveals the unusual nature of an impurity encountered in the preparation of [Ir(ppy)2(bpy)][PF6].