High Efficiency and Small Roll‐Off Electrophosphorescence from a New Iridium Complex with Well‐Matched Energy Levels

Abstract

It is well-known that, under electrical excitation, singlet and triplet excitons will be formed in organic light-emitting diodes (OLEDs) in the ratio of approximately 1 to 3 in the organic layers, where recombination of opposite charges occurs. The harvesting of triplet excitons for emission (termed as phosphorescence) cannot be obtained in common organic compounds because of the spin-forbidden rule. Thus, metal complexes, possessing the spin-orbit coupling effect caused by the heavy metal, are commonly used as phosphorescent emitters. Hence, intensive efforts have been made towards the development of metal complexes with different colors.[ [9] and 11 % (or 11.2 lm W -1 ) for red [3] have been reported in the literature. On the other hand, PhOLEDs, in spite of having the ability of harvesting triplet energy that can result in high efficiency, are usually characterized by high rolloff in efficiency with increasing current density.[2,9-11] Typically, the external quantum efficiency (EQE) declines sharply at current densities above 20 mA cm -2 , which is the practical driving condition for passive-matrix OLEDs (PMOLEDs). Furthermore, high driving voltages are usually observed in these devices, which can result in low power efficiencies. [12,13] As with the issues of generally high roll-off and driving voltage in PhOLEDs, it has been found that the roll-off was attributed to the triplet-triplet (T-T) annihilation [14,15] and/or the field-induced exciton dissociation [16,17] when there was a high density of triplet excitons with long lifetimes. The high driving voltage is supposed to be caused by two factors: one is the deep and strong charge traps caused by the doped iridium complex in the emitting layer; [12,13] the second is related to the highest occupied molecular orbital (HOMO), which transfers holes, and the lowest unoccupied molecular orbital (LUMO), which transfers electrons, of materials used. In PhOLEDs, to match the triplet energy level of the dopant for exothermic energy transfer, high bandgap host materials are usually required. So the difference in HOMO levels and/or LUMO levels between the carrier transporting and the emitting layers is usually large, making charge injection into the emitting layer energetically unfavorable and thus leading to high driving voltage.[18]To achieve high efficiency PhOLEDs with the characteristics of small roll-off and low driving voltage, it is believed that both high-efficiency phosphorescent materials and the practical construction of devices are needed. Of primary importance in device architectures is that the hole/electron injection barrier from the hole/electron transport layer to the emitting layer should be small, which will favor charge injection and avoid high driving voltage. Secondly, the density of triplet excitons should be low enough for the purpose of minimizing T-T exciton annealing. This criterion can be achieved by evenly distributing excitons in the bulk emission layer rather than having excitons located only at the narrow interface sites. Finally, the...

Adduct effects on structure and luminescence in a series of new dithiocarbamatogold(I) complexes

“…During the last decade, research on luminescent metal complexes has become important because of their possible application as dopant emitters in organic light emitting device technology [1][2][3][4][5][6]. In particular, interest in various gold(I) complexes has been stimulated by their tendency to give supramolecular structures such as pairs, rings, chains, and layers through closed shell d 10 aurophilic interactions and their resultant luminescent and anticancer properties [7][8][9][10][11][12][13][14].…”

Layered Double Hydroxide Materials: Assembly and Photofunctionality

“…Transition-metal-based photoactive complexes have been extensively investigated in optoelectronics due to their high chemical stability and excellent luminescence properties [134]. Ruthenium-based complexes, tris(1,10-phenanthroline-4,7-diphenylsulfonate)ruthenium(II) (denoted as Ru(dpds) 3 ), have been studied by [76] assembly with LDH nanosheet to obtain orderly UTFs [83].…”

Synthesis, Crystal Structure and Luminescence Properties of a Cyclometalated Ir(III) Complex of 3,4-Diphenylcinnoline

“…Mi et al reported efficient and thermally stable OLEDs using diphenylphthalazine iridium(III) [8]. Based on this work, it has been found that the ligands with an sp 2 -hybridized N atom adjacent to the chelating N atom, such as phenylpyridazine and phenylphthalazine derivatives [1], are beneficial for the iridium(III) complexes due to the shorter bond length and the stronger bonding strength between the chelating N atom and the Ir atom, compared with analogs which have a C atom instead of the non-chelating N atom. In one of our studies, cinnoline derivatives as another type of isomers of benzopyridazine besides phthalazine were now used to synthesize a novel cyclometalated Ir(III) complex, [(dpci-H) 2 Ir(dafo)](PF 6 ) (dpci-H = deprotonated 3,4-diphenylcinnoline, dpci; dafo = 4,5-diazafluoren-9-one).…”

“…Cyclometalated Ir(III) complexes belong to the family of organometallic triplet emitters, which are of great interest because of their application in electroluminescent devices and sensors [1,2]. Owing to the strong spin-orbital mixing of heavy metal ions in phosphorescent complexes, both singlet and triplet excitons can be fully utilized, creating the possibility for electrophosphorescent dye-doped devices to reach an internal quantum efficiency of 100 % [3].…”