Strong Luminescent Iridium Complexes with CˆN=N Structure in Ligands and Their Potential in Efficient and Thermally Stable Phosphorescent OLEDs

Abstract

Organic luminescent materials are an important class of materials for a multitude of optoelectronic applications, such as organic light-emitting diodes (OLEDs), [1][2][3] luminescence-based sensors, [4] and photocatalysts. [5] According to the nature of emission, organic luminophors can be categorized as fluorescent emitters, which emit light from their singlet excited states, and phosphorescent emitters, which conduct radiative decay from their triplet excited states. It has been widely accepted that for excitons generated by carrier injection, which is exactly the case in OLEDs, the ratio of singlet and triplet exciton is 1:3. Due to this intrinsic factor, although abnormal cases exist, [6,7] the internal quantum efficiency (IQE) of OLEDs employing fluorescent emitters is generally limited to 25%, while IQE of OLEDs utilizing phosphorescent emitters (PHOLEDs) can reach almost 100%. [3] Hence, due to this higher efficiency and motivated by the potential applications in flat panel displays (FPDs), solid-state lighting (SSL), and backlights for liquid-crystal displays (LCDs) based on OLED technology, PHOLED materials and devices have been intensively studied since the seminal work in 1998.[3] Great success has been achieved in device efficiency, [8][9][10][11] color tuning, [12][13][14] and also white-light emission. [15][16][17] In contrast, lifetimes of PHOLEDs have been less explored, and need to be further improved. [18][19][20] For SSL applications, OLEDs performances are far from satisfied. [15,21,22] It is believed that the OLED must have an efficiency of 50 lm W À1 or more and an operation lifetime of 20 kh or longer tailored to the SSL application. Thus, the design of good emitters for this demanding device performance is one of the challenging tasks.From the practical point of view, the primary criteria for phosphorescent emitters are strong phosphorescence coupling with good thermal stability. Due to the spin conservation rule, phosphorescence is generally limited to metal complexes, where spin-orbital coupling effects exist. Usually, phosphorescence is characterized by its 3 MLCT (metal ligand charge transfer) nature, or has mixed 3 (p-p*) and 3 MLCT features. Orthometalated iridium complexes are the most intensively studied materials for usage in PHOLEDs, due to several reasons: i) Benefiting from its large atomic number, Ir is a heavy metal that can offer good spin-orbital coupling leading to strong phosphorescence properties; ii) Ir has large d-orbital splitting, thus precluding the d-d transition that may deactivate the MLCT process, which is one of the important processes for room temperature phosphorescence; iii) the stable oxidation state 3þ of Ir(III) can form neutral complexes, allowing sublimation under vacuum. Regarding the thermal stability of phosphorescent emitters, forming homoleptic complexes other than their heteroleptic counterpart is one of the strategies.[23] Besides metal, organic ligands drastically influence the molecular frontier orbitals of a complex, such as the highest occupi...

Influences of fluorination on homoleptic iridium complexes with C∧N=N type ligand to material properties, ligand orientation and OLED performances

“…Previously, we and other groups have reported the phosphorescent materials bearing C^N=N structure in the ligand [20][21][22][23]. Because of the small steric hindrance for N and Ir(III) chelating attributed to the adjacent sp 2 N, that possesses a pair of lone electrons and without H attachment, that is, C^N=N structure, the Ir-N bonding is likely to be strong, leading to robust iridium complex with strong phosphorescence.…”

“…Because of the small steric hindrance for N and Ir(III) chelating attributed to the adjacent sp 2 N, that possesses a pair of lone electrons and without H attachment, that is, C^N=N structure, the Ir-N bonding is likely to be strong, leading to robust iridium complex with strong phosphorescence. Based on them, excellent PhOLED performances have been demonstrated [20,21,24]. However, the emissions of these materials are almost all in long wavelength (above 540 nm).…”

Molecular hosts for triplet emitters in organic light-emitting diodes and the corresponding working principle

“…Moreover, it was demonstrated by H. H. Liao et al that the morphology of the phosphor-doped film was significant for the engenderment of Dexter energy transfer from the host to the dopant, and it could occur in a system without triplet confinement if the triplet exciton lifetime of phosphor was short [22]. In addition, we have studied new green [19] and red [46] Ir complexes in CBP host. By inserting a buffer layer between the HTL and EML, PhOLEDs with high efficiency and low roll-off have been achieved [19].…”

“…Dicarbazole-containing CBP (7) is the most widely used host material for triplet emitters, which can efficiently host green to red phosphorescent emitters with triplet energies smaller than that of CBP (7) (Figure 6) (E T = 2.6 eV [17]) [46,[60][61][62]. Using CBP as the host, and employing a high electron transport material as ETL, Ir(ppy) 3 based device achieved efficiency of 133 lm/W (29%) [25], which is the highest efficiency for Ir(ppy) 3 to date.…”

High Efficiency Red Phosphorescent Organic Light-Emitting Diodes with Simple Structure

“…Various red emitting Ir(III) phosphorescent complexes are summarized in Table 1 (Lamansky et al, 2001;Tsuboyama et al, 2003;Duan et al, 2003;H.-K. Kim et al, 2007;Ohmori et al, 2007;J. Huang et al, 2007;Tsuzuki and Tokito, 2008;Mi et al, 2009;Pode et al, 2010;Tsujimoto et al, 2010). The wide band gap host and narrow band gap (E g ) guest red light emitting system has a significant difference in HOMO (highest occupied molecular orbital) and/or LUMO (lowest unoccupied molecular orbital) levels between the guest and host materials.…”