Molecular Engineering of Ionic Transition Metal Complexes and Counterions for Efficient Flexible Green Light-Emitting Electrochemical Cells

Abstract

Light-emitting electrochemical cells (LECs) based on ionic transition metal complexes (iTMCs) represent a cost-effective solidstate lighting technology compatible with large-area and industrial-scale manufacturing. To improve the current LEC performance and compete with rivaling light-emitting diode (LED) devices, it is pivotal to design efficient iTMCs/counterion couples that combine high photoluminescence efficiency with optimized ionic and electron carrier transport. Despite the continuous proposal of novel iTMCs, the investigated counterions are typically limited to the traditional ones, including PF 6 − and BF 4 − . In this work, we introduce both rigid and flexible LEC architectures based on a novel single active layer of [Ir-(ppy) 2 (phtz)] − [Et 3 NH] + + X (ppy = 2-phenylpyridine, phtz = 5-phenyl-1H-tetrazole and X = lithium bis(trifluoromethane)sulfoneimide (LiTFSI), tetrabutylammonium perchlorate (TBAP), or sodium perchlorate (NaClO 4)) sandwiched between a FTOcoated glass or ITO-coated polyethylene terephthalate (PET) anode and Ga:In cathode. Our new Ir-cyclometaled complex with a tetrazole ligand, without salt additives or polymers, shows a bright green electroluminescence emission at 508 nm. The LECs based on the synthesized iTMC and TBAP additive show a current efficiency as high as 1.44 cd/A, a luminance of 503.82 cd/m 2 , and an external quantum efficiency of 1.73% at 3.7 V. By using a dual salt additive made of TBAP:LiTFSI (1:1), the LECs further improve the performance of the single salt-based devices, exhibiting a current efficiency of 1.72 cd/A, a luminance of 603.14 cd/m 2 , and an external quantum efficiency of 2.06% at 3.6 V. Such improvement of the LEC performance is attributed to the combination of the TBAP anion−iTMC cation size matching and the peculiar electrical properties of the LiTFSI-based solid electrolytes (i.e., high TFSI − mobility), leading to a compact space charge region near the electrodes and low turn-on voltage, respectively.

High-Efficiency Deep-Red Light-Emitting Electrochemical Cell Based on a Trinuclear Ruthenium(II)–Silver(I) Complex

“…Although in these efforts the turn-on times are reduced, in some of them, other important metrics such as lifetime, luminance, and external quantum efficiencies (EQE) were debilitated. ,,,,− Therefore, further studies on iTMCs containing ionically charged ligands that could improve both response time and other EL metrics are necessary to point out. Surprisingly, our previous studies also indicated that the type of counterions with emitter not only increased the electroluminescence properties but also shifted the electroluminescence maxima. , …”

Versatile Electroluminescence Color-Tuning Strategy of an Efficient Light-Emitting Electrochemical Cell (LEC) by an Ionic Additive

“…Upon the increase in voltage, the device starts to turn on and light is detected, indicating that the additive-free LEC shows higher injection barriers in comparison with the additive-based LECs. As a general strategy, the introduction of salt additives inserts ionic species with smaller sizes and increased mobility in contrast to iTMC ionic species. , As a consequence, the presence of extra salt ions contributes to the quick creation of a compact space charge zone around the electrodes. At the electrode/emissive layer interface, the creation of a local electric field balances the carrier injection, minimizes the excessive increase of the space charge area, and hence improves the efficiency of LEC.…”

“…Interestingly, the LEC based on TBAP demonstrated a significant improvement in device performance as well color changing with an irradiance of 220.8 μW/cm 2 , which showed a 32.7% increase compared to the one exhibited by additive-free LECs, corresponding to a luminance ( L ) of 355.9 cd/m 2 , twice more than a device without additives. This is probably due to the size matching of the tetrabutylammonium cation to the [CLO 4 – ] and adjustment of the charge carrier’s balance. , In the case of the LiTFSI additive, the high intrinsic conductivity of LiTFSI and the high mobility of [TFSI – ] enhanced the LEC performance . The efficacy and luminance increased up to 1.06 cd/A and 212.9 cd m –2 , respectively, with 1.28% external quantum efficiency (EQE) at the applied voltage of 2.4 V. Furthermore, we explored the turn-on time ( T on ), the time required to reach the maximum luminance ( L Max ) value at 5 V, and the lifetime ( T 1/2 ), the time required to reach half of the maximum luminance, of our LEC devices.…”

“…However, metal-free organic phosphorescent materials have been extensively explored for several applications. Among all these phosphorescent materials, iridium cyclometalated complexes are one of the most commonly used classes of emitters in ss-LEDs such as organic light-emitting diodes (OLEDs) − and light-emitting electrochemical cells (LECs). , The phosphorescent nature of the Ir complexes makes it possible for singlet and triplet excitons to be used effectively, enabling the internal quantum efficiency (IQE) to consequently reach 100% and leading to high-performance OLEDs. Their unique properties, especially tunable emission color that covers approximately the entire visible spectrum, and high luminescence efficiencies have drawn tremendous attention in recent years and have prompted remarkable works in the LEC field.…”

“…However, all ruthenium complexes, which are commonly used in LEC because of their limited ligand-field splitting energies, only gave emission in the red-orange region, and this restricts the emission colors in this big family. Through switching the iTMC from a second row to a third row of transition metal (e.g., iridium), a number of emission properties and system stability can be improved, and consequently, higher ligand-field splitting energies can result to higher color tunability. ,− Generally, the range of electroluminescence (EL) emission of Ir-cyclometalated complexes is extensively distributed in all three fundamental colors: blue, − green, ,− and red. , Meanwhile, LECs based on the yellow-emitting iridium complex (≈560 nm) were reported in 2004 to extend the emission color of LECs. The next crucial point is that electrically neutral complexes are used for OLED devices, whereas ionic species are targeted to LEC technology, 10 and by adjusting the energy gap between the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO), the emission color tuning is produced .…”