Recent Advances in Solution‐Processable Dendrimers for Highly Efficient Phosphorescent Organic Light‐Emitting Diodes (PHOLEDs)

Abstract

Phosphorescent dendrimers together with dendritic host materials have inherent advantages in achieving highly efficient phosphorescent organic light‐emitting diodes (PHOLEDs) with low‐cost, solution‐processed device fabrication. Recently, much research effort has been devoted to developing these dendritic materials in the field of electroluminescence (EL). In this Focus Review, the major advances in this line of research are summarized and discussed. The crucial roles played by these dendritic organic electronic materials in coping with the key issues in the field of PHOLEDs have been specially emphasized. These include alleviating the triplet‐triplet annihilation effect through the site‐isolation effect associated with the dendritic structures, promoting charge‐carrier injection/transporting ability and ambipolar properties through functionalized dendrons, fulfilling simple PHOLEDs with high EL efficiencies, obtaining self‐hosting phosphorescent emitters, and avoiding undesired energy transfer process from phosphorescent cores to the dendrons. In addition, the future perspectives and ongoing challenges of this research frontier are also highlighted.

Tailoring the Emission of Fluorinated Bipyridine-Chelated Iridium Complexes

“…Dendrimers can overcome these problems because of their good film formation and generation-by-generation site-specific functionalization. 31−35 In a rigid and structurally well-defined dendritic molecule, the core can be a phosphorescent emitter while the surface is functionalized with host moieties. Thus, the ratio between the hosts and dopants and their positions are accurately controlled in a dendritic structure.…”

“…32,33 There have been many studies on dendrimer-based phosphorescent emitters, especially green and red ones, since the pioneering work of Samuel and Burn et al in 2001. 35−43 So far, very few blue PEs were reported and they all suffered from poor color purity and/or low efficiencies. 44−48 Accordingly, it is worthwhile to develop high-performance, dendrimer-based blue PEs.…”

Blue Light Emitting Polyphenylene Dendrimers with Bipolar Charge Transport Moieties

“…However, blue OLEDs still suffer from much shorter lifetimes than green and red ones [4,5], even though pure blue light emitters have demonstrated over 20% EQE for phosphorescent materials [6,7] and nearly 20% for thermally activated delayed fluorescence (TADF) molecules [8]. Dendrimer-based OLEDs have been investigated in the last two decades, due to their numerous advantages over small molecules and polymers, i.e., cost-effective solution processability, high performance reproducibility thanks to their well-defined structures, in contrast to polymers, and precise functionalization of dendrimers at multiple positions [9][10][11][12][13]. Dendrimers for OLEDs generally include two types: one designed for better charge transport (conjugated scaffold) [14][15][16][17][18][19] and one for surface-to-core energy transfers (non-conjugated scaffold) [20][21][22][23][24].…”

Emission Enhancement by Intramolecular Stacking between Heteroleptic Iridium(III) Complex and Flexibly Bridged Aromatic Pendant Group

“…Cyclometalated iridium­(III) complexes are the most researched class of phosphorescent luminophores considering their extensive use as luminescent probes , and, in particular, as the emitters in organic light-emitting diodes (OLEDs). − The large photoluminescence (PL) quantum yield (Φ PL ) values and triplet harvesting ability of the compounds allow manufacturing OLEDs with external quantum efficiencies (EQE) that are almost reaching the fundamentally attainable limit . Lately the research efforts involving iridium­(III)-based materials have partly shifted toward the development of solution-processable compounds, to reduce the production cost of large-area emissive devices like OLED televisions (TVs) and lighting panels. − Another emerging research direction involves acquisition of emitting layers with anisotropic alignment of molecules, where the fraction of the photons that can leave the device is increased. − In such a way the theoretically attainable EQE of OLEDs can be increased from 30% to ∼40% . This approach has allowed to acquire vacuum-deposited iridium­(III)-based OLEDs with EQE of 38% .…”

“…To attain a sufficient spatial isolation, the mass fraction of the active compounds in the emissive layers of OLEDs is usually limited to ∼10 wt %, as the phosphorescent complexes are dispersed into charge-transporting host materials. , While this approach is highly effective for vacuum-deposited OLEDs, solution-processed devices still strongly suffer from emitter aggregation-induced negative effects, even in the presence of a host material. Encapsulation with massive covalently attached isolating groups is often employed to overcome the emitter aggregation for nonionic iridium­(III) complexes designated for solution-based processing techniques, but the resulting materials often suffer from reduced charge-transfer parameters …”