Green electroluminescence from an ionic iridium complex

Abstract

We report green emission from a single-layer device based on the ionic transition metal complex [Ir(F-mppy)2(dtb-bpy)]+(PF6−), where F-mppy is 2-(4′-fluorophenyl)-5-methylpyridine and dtb-bpy is 4,4′-di-tert-butyl-2,2′bipyridine. External quantum efficiencies of up to 1.1% are achieved with air-stable contacts, and up to 1.8% with a CsF∕Al top contact. Addition of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was found to improve the device response time and cause a bias-dependent shift in the emission spectrum. As a result, electroluminescence was observed at 531 nm (CIE coordinates: 0.3230 and 0.5886), the lowest wavelength reported to date for a device based on ionic transition metal complexes.

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

“…(Figure 24, top). [105,169] Indeed, small amounts of this compound significantly reduced the t on of LECs from 4 h to 25 min, while also increasing luminance levels. These results were attributed to an improvement of ionic conductivity inside the light-emitting layer, because of the higher density of mobile ions.…”

“…[102,103] Therefore, a preliminary photophysical characterization of a given complex in solid-state films can be helpful to predict if it can yield a brightly emitting device. Some main strategies can be used to enhance PLQYs in the solid state and minimize the typical self-quenching effects observed in films: [29] 1) use of inert matrices, such as ionic liquids [104,105] or polymers, [71,106] to disperse the iTMC; ionic liquids improve both PLQY and LEC properties (e.g., turn-on time, luminance, and efficiency), [37] but polymers have often a detrimental effect on device efficiency. [29] 2) dispersion of a low-band-gap iTMC in a wider band-gap iTMC, for example, an orange iTMC in a green emitter; this has been successful to increase the device EQE.…”

“…[76] As discussed in Section 3, there are several reasons that justify their selection over Ru II complexes: 1) high PLQY; [19,86,98,156] 2) easily tunable HOMO-LUMO energy gap by independent chemical modifications of the ligands; [29,37,75,81,82,85,91,99,103,105,111,113,116,118,[157][158][159][160] 3) high stability against degradation processes owing to higher ligand-field splitting. [19,86,97,98,156,[161][162][163][164] Over the last eight years, the use of Ir-iTMCs has brought very significant advances in LEC performances, that is, color, efficiency, stability, and turn-on times.…”

“…[37,82,104,105,126,169,170] All these approaches enhance ionic conductivities, thereby causing a faster build-up of the electrical double layer which induces a faster device t on but, on the other hand, also a quicker propagation of the doped zones leading to more extensive exciton quenching and decrease of the luminance (see Section 2).…”

Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems

“…In particular, the design of efficient blue-emitting charged Ir III complexes with respect to the generation of white light by color-mixing is highly desirable. Here, the major contributions by the groups of Bernhard [73,74,236] and Nazeeruddin [75,237] should be emphasized (Scheme 2). Further color-tuning of charged Ir III complexes for LECs could be realized via tris-cyclometallated complexes with charged groups, e.g., phosphonium salts, in the periphery of the cyclometallating ligand (Scheme 2).…”