SIPS 2018

DETAILLED PROGRAM OVERVIEW

Ionic Liquids and their Role in Creating Efficient Emitter Materials for Light Emitting Electrochemical Cells
Anja-verena Mudring¹;
¹STOCKHOLM UNIVERSITY, Stockholm, Sweden;
PAPER: 398/Molten/Keynote (Oral)
SCHEDULED: 15:55/Tue./Bossa (150/3rd)

ABSTRACT:
Energy consumption is one of the most challenging issues that humankind is facing. Approximately 20% of the world's energy is used for lighting. It is therefore important to reduce the energy consumption of lighting devices and increase their efficiency. For that reason, the old incandescent lamp which has been used for illumination for over 130 years is being phased out in most countries. The most common replacement are CFLs (compact fluorescent lamps), which have certain drawbacks related to the mercury content. LEDs (light emitting diodes) have become competitive for illumination as energy efficient lighting sources. However, it is now realized that both CFLs and LEDs rely on materials like rare earths, gallium and indium that bear a severe supply risk. Thus, there is a significant driving force to look for alternative lighting sources. The discovery of OLEDs (organic light-emitting diodes) marks a significant progress in this direction. However, one of the major drawbacks of OLEDs for lighting applications is their complex device architecture and air-sensitivity which makes them expensive to manufacture and prone to de-composition. The alternative, LECs (light emitting electrochemical cells) can be as simple as being only composed of a light emitting material sandwiched between two electrodes (one reflective electrode: widely the cathode and a second transparent electrode: usually the anode to allow light to exit the device) and LECs are promising as a low cost large area future lighting technology which allows overcoming the problems of OLEDs. Ionic liquids play a key role to enable this still young technology. One of the key challenges is to develop ionic, ionic liquid-based, efficient emitter materials that have a significant lifetime need to be provided for this technology to enter the market. Ionic Ir(III) complexes are the most promising emitters in light emitting electrochemical cells (LECs), especially in the high energy emission range for which it is difficult to find emitters with sufficient efficiencies and lifetimes. To overcome this challenge, the concept of intramolecular π-π-stacking of an ancillary ligand (6-phenyl-2,2'-bipyridine, pbpy) is introduced in the design of a new green emitting iridium ionic transition metal complex with a fluoro-substituted cyclometallated ligand, 2-(4-fluorophenyl)pyridinato (4Fppy). [Ir(4Fppy)2(pbpy)][PF6] has been synthesized, characterized and its photophysical and electrochemical properties have been studied. The complex emits green light with maxima at 561 and 556 nm under UV excitation from powder and thin film, respectively, and displays a high photoluminescence quantum yield (PLQY) of 78.5%. [Ir(4Fppy)2(pbpy)][PF6] based LECs driven under pulsed current conditions showed under an average current density of 100 A m-2 (at 50% duty cycle) a maximum luminance of 1443 cd m-2, resulting in 14.4 cd A-1 and 7.4 lm W-1 current and power efficiencies, respectively. A remarkable long device lifetime of 214 hours was observed. Reducing the average current density to 18.5 A m-2 (at 75% duty cycle) led to an exceptional device performance of 19.3 cd A-1 and 14.4 lm W-1 for current and power efficiencies, an initial maximum luminance of 352 cd m-2 and a lifetime of 617 hours.

References:
J.E. Namanga, N. Gerlitzki, V. Smetana, A.-V. Mudring, Optimizing green light emitting electrochemical cells: Stability improvement without compromising the efficiency , ACS Appl. Mat. Interf. 2018,. DOI: 10.1021/acsami.7b18159.

M. Di Marcantonio, J. E. Namanga, N. Gerlitzki, F. Vollkommer, A.-V. Mudring, G. Bacher, E. Nannen, Bright and Stable Greenish Hybrid Light Emitting Electrochemical Cells, J. Mat. Chem. C, 2017, 5, 12062-12068. DOI: 10.1039/C7TC02976D

J.E. Namanga, N. Gerlitzki, B. Mallick, A.-V. Mudring, Long term stable deep red light-emitting electrochemical cell based on an emissive, rigid cationic Ir(III) complex, J. Mat. Chem. C 2017, 5, 3049-3055, DOI: 10.1039/C6TC04547B.

J.E. Namanga, N. Gerlitzki, V. Smetana, A.-V. Mudring, Scrutinizing design principles towards efficient, long-term stable green light emitting light emitting electrochemical cells, Adv. Func. Mat. 2017, 27, 1605588 (8 pages) DOI: 10.1002/adfm.201605588.