Scrutinizing Design Principles toward Efficient, Long‐Term Stable Green Light‐Emitting Electrochemical Cells

Abstract

1 of 8) 1605588be based on non-air sensitive materials allowing for cheap ambient device processing techniques such as spin coating, roll-to-roll, and slot die coating, (iii) even the electrodes can be made out of stable (even noble) metals as the ionic nature of emissive layer omits the need for electrodes made out of metals with low work functions, and (iv) that it is fairly easy to produce large emitting areas. [1][2][3][4][5][6][7][8][9][10] Altogether this makes LECs a very promising low-cost, large area solid state lighting technology solution.LECs based on ionic transition metal complexes (iTMCs) have attracted much research interest, with iridium(III) cationic complexes being the most investigated. [2,11,12] This observation is not surprising considering the overall high photoluminescence quantum yields (PLQY), the relatively short phosphorescent lifetimes for these complexes and the ease with which this class of complexes can be synthesized. [2,11] Equally easily, the emission color of these complexes can be tuned over a wide range through ligand modification. [13][14][15][16] Ir-iTMCs with the generic composition of [Ir(C^N) 2 (N^N)] + [A] − where C^N is a cyclometallated ligand such as 2-phenylpyridine (ppy), N^N a diimine ligand such as 2,2′-bipyridine (bpy) and A − an anion such as hexafluorophosphate [PF 6 ] have been widely studied for LEC device application. Typically, complexes with nonsubstituted C^N ligands like the parent complex [Ir(ppy) 2 (bpy)][PF 6 ] emit in the orange region of the visible spectrum. [11,17,18] In order to obtain emission with shorter wavelengths or higher energy, the phenyl ring of the C^N ligand can be decorated with electron withdrawing groups which results in an energetic stabilization Enhancing the efficiency and lifetime of light emitting electrochemical cells (LEC) is the most important challenge on the way to energy efficient lighting devices of the future. To avail this, emissive Ir(III) complexes with fluoro-substituted cyclometallated ligands and electron donating groups (methyl and tert-butyl)-substituted diimine ancillary (N^N) ligands and their associated LEC devices are studied. Four different complexes of general composition [Ir(4ppy) 2 (N^N)][PF 6 ] (4Fppy = 2-(4-fluorophenyl)pyridine) with the N^N ligand being either 2,2′-bipyridine (1), 4.4′-dimethyl-2,2′-bipyridine (2), 5.5′-dimethyl-2,2′-bipyridine (3), or 4.4′-di-tert-butyl-2,2′-bipyridine (4) are synthesized and characterized. All complexes emit in the green region of light with emission maxima of 529-547 nm and photoluminescence quantum yields in the range of 50.6%-59.9%. LECs for electroluminescence studies are fabricated based on these complexes. The LEC based on (1) driven under pulsed current mode demonstrated the best performance, reaching a maximum luminance of 1605 cd m −2 resulting in 16 cd A −1 and 8.6 lm W −1 for current and power efficiency, respectively, and device lifetime of 668 h. Compared to this, LECs based on (3) and (4) perform lower, with luminance and lifetime of 1314 cd m −2 ,...

The Weak Microcavity as an Enabler for Bright and Fault-tolerant Light-emitting Electrochemical Cells

“…We have in this study focused on the emission from a Super Yellow-based LEC, but the results should be applicable also for LECs based on other emitters. The emission from an optical microcavity is the product of the intrinsic emission spectrum of the emitter and the cavity gain, which implies that broad emitters, such as conjugated polymers 4 , 33 , 45 , 47 , 48 and ionic transition metal complexes 49 – 53 (the two most common emitter materials in LECs), can encompass multiple cavity modes depending on the active-layer thickness and the viewing angle; this feature is clearly displayed in Fig. 2 54 , 55 .…”

Stress Testing Polymer Light‐Emitting Electrochemical Cells: Suppression of Voltage Drift and Black Spot Formation

“…We stress that the T ( LVB ) lifetime can be defined differently for a different material system and under different testing conditions. Some iTMC‐based LECs, for example, exhibit a highly stable average driving voltage under pulsed current operation . The purpose of introducing this new figure‐of‐merit is to quantitatively account for the contribution of voltage drift and black spots to the operational stability of LECs.…”

Efficient and Saturated Red Light‐Emitting Electrochemical Cells Based on Cationic Iridium(III) Complexes with EQE up to 9.4 %

“…Several studies on saturated red LECs, based on iTMCs, have been reported. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] However,m ost of these studies involve LECs that exhibit low or moderate device efficiencies (externalq uantum efficiency (EQE) < 3.3%); saturated red LECs that are moree fficienta re highlyd esired. In iridium iTMCs, the highest occupied molecular orbital (HOMO) is typicallyd istributed on the C^N ligands and metals, whereas the lowest unoccupiedm olecular orbital (LUMO) is locatedo nt he N^N ligands.…”

“…Saturated red‐light sources, which exhibit emission peak wavelengths longer than approximately 640 nm, are essential for full‐color displays because they can generate a large color gamut. Several studies on saturated red LECs, based on iTMCs, have been reported . However, most of these studies involve LECs that exhibit low or moderate device efficiencies (external quantum efficiency (EQE) <3.3 %); saturated red LECs that are more efficient are highly desired.…”