Phosphorescent Pt(II) and Pd(II) Complexes for Efficient, High‐Color‐Quality, and Stable OLEDs

Abstract

Phosphorescent organic light-emitting diodes (OLEDs) are leading candidates for next-generation displays and solid-state lighting technologies. Much of the academic and commercial pursuits in phosphorescent OLEDs have been dominated by Ir(III) complexes. Over the past decade recent developments have enabled square planar Pt(II) and Pd(II) complexes to meet or exceed the performance of Ir complexes in many aspects. In particular, the development of N-heterocyclic carbene-based emitters and tetradentate cyclometalated Pt and Pd complexes have significantly improved the emission efficiency and reduced their radiative lifetimes making them competitive with the best reported Ir complexes. Furthermore, their unique and diverse molecular design possibilities have enabled exciting photophysical attributes including narrower emission spectra, excimer -based white emission, and thermally activated delayed fluorescence. These developments have enabled the fabrication of efficient and "pure" blue OLEDs, single-doped white devices with EQEs of over 25% and high CRI, and device operational lifetimes which show early promise that square planar metal complexes can be stable enough for commercialization. These accomplishments have brought Pt complexes to the forefront of academic research. The molecular design strategies, photophysical characteristics, and device performance resulting from the major advancements in emissive Pt and Pd square planar complexes are discussed.

Homoleptic Iridium(III) Compounds Bearing Bulky Bipyridine Ligand for Potential Application to Organic Light‐emitting Diodes

“…As shown in Figure 1, both compounds displayed strong absorption (ε > 50 000/M/cm) at 260-350 nm, which can be ascribed to a spin-allowed ligand-centered (LC) π-π* transition. The weak absorption (5000 < ε <10 000/M/cm) around 370 nm is mainly due to singlet metal-to-ligand charge transfer ( 1 MLCT), and additional weak and broad absorption at longer wavelength (430 nm) is attributable to 3 LC mixed with 3 …”

“…Both ligands A and B were obtained in moderate-to-good yields (54-74%) ( Figures S1 and S2). The procedure to produce the Ir(C^N) 3 compounds uses reactive [Ir(COD) 2 ]BF 4 and reacts it with the corresponding bipyridine ligand under mild conditions. These reaction conditions usually result in facial iridium compounds.…”

“…11 Our group has also reported the effect of concentration on the emission efficiency of Ir(ppy) 3 and Ir(disppy) 3 derivatives, where ppy = phenylpyridine and disppy = trimethylsilylphenylpyridine. 12 The photoluminescence efficiency (η PL ) of Ir(disppy) 3 with a bulky trimethylsilyl ppy ligand group did not change significantly upon increasing the concentration (43.97% at 50 wt %), whereas that of Ir(ppy) 3 decreased remarkably by more than 50% as the doping concentration increased. In addition, Park et al recently reported that concentration quenching was observed by increasing the concentration of Ir(ppy) 2 (DBM), DBM = 1,3-diphenyl-1,3-propanedione.…”

“…In contrast to the well-established development of green and red triplet emitters for WOLED application, the development of blue emitters with high efficiency and long device operation lifetimes remains challenging. 3 One of the advantages of organic lighting is the high color rendering index (CRI). Based on our previous report, a blue emitter with an emission maximum (λ max ) below 470 nm as well a high quantum efficiency is required to achieve WOLEDs with high CRI (>90).…”

Aromatic‐Imide‐Based Thermally Activated Delayed Fluorescence Materials for Highly Efficient Organic Light‐Emitting Diodes

“…2 and 21.1 %, respectively. Significantprogresshasbeenmadeinthepastthreedecades in the development of organic light-emitting diodes (OLEDs), [1] which have aw ide range of applications,f or example,i nl ighting, [2] flat-panel displays, [3] and flexible devices. [5] Although phosphorescent OLEDs could harvest both singlet and triplet excitons (100 %) for emission by utilizing precious-metal-based phosphorescent materials to increase spin-orbit coupling of singlet (S 1 )and triplet (T 1 ) states, [6] theuse of expensive metals has limited their practical application to acertain extent. [5] Although phosphorescent OLEDs could harvest both singlet and triplet excitons (100 %) for emission by utilizing precious-metal-based phosphorescent materials to increase spin-orbit coupling of singlet (S 1 )and triplet (T 1 ) states, [6] theuse of expensive metals has limited their practical application to acertain extent.…”