Status and Challenges of Blue OLEDs: A Review

Organic light-emitting diodes (OLEDs) have outperformed conventional display technologies in smartphones, smartwatches, tablets, and televisions while gradually growing to cover a sizable fraction of the solid-state lighting industry. Blue emission is a crucial chromatic component for realizing high-quality red, green, blue, and yellow (RGBY) and RGB white display technologies and solid-state lighting sources. For consumer products with desirable lifetimes and efficiency, deep blue emissions with much higher power efficiency and operation time are necessary prerequisites. This article reviews over 700 papers covering various factors, namely, the crucial role of blue emission for full-color displays and solid-state lighting, the performance status of blue OLEDs, and the systematic development of fluorescent, phosphorescent, and thermally activated delayed fluorescence blue emitters. In addition, various challenges concerning deep blue efficiency, lifetime, and approaches to realizing deeper blue emission and higher efficacy for blue OLED devices are also described.

Engineering of Cyano Functionalized Benzo[d]imidazol-2-ylidene Ir(III) Phosphors for Blue Organic Light-Emitting Diodes

In this study, we designed and synthesized three series of blue emitting homoleptic iridium(III) phosphors bearing 4-cyano-3-methyl-1-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (mfcp), 5-cyano-1-methyl-3-phenyl-6-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (ofcp), and 1-(3-(tert-butyl)phenyl)-6-cyano-3-methyl-4-(trifluoromethyl)-benzo[d]imidazol-2-ylidene (5-mfcp) cyclometalates, respectively. These iridium complexes exhibit intense phosphorescence in the high energy region of 435-513 nm in the solution state at RT, to which the relatively large T₁ → S₀ transition dipole moment is beneficial for serving as a pure emitter and an energy donor to the multiresonance thermally activated delayed fluorescence (MR-TADF) terminal emitters via Förster resonance energy transfer (FRET). The resulting OLEDs achieved true blue, narrow bandwidth EL with a max EQE of 16-19% and great suppression of efficiency roll-off with ν-DABNA and t-DABNA. We obtained the FRET efficiency up to 85% using titled Ir(III) phosphors f-Ir(mfcp)₃ and f-Ir(5-mfcp)₃ to achieve true blue narrow bandwidth emission. Importantly, we also provide analysis on the kinetic parameters involved in the energy transfer processes and, accordingly, propose feasible ways to improve the efficiency roll-off caused by the shortened radiative lifetime of hyperphosphorescence.

Bis(2-phenylpyridinato,-C2′,N)[4,4′-bis(4-Fluorophenyl)-6,6′-dimethyl-2,2′-bipyridine] Iridium(III) Hexafluorophosphate

A new bis cyclometallated Ir(III) phosphor, [Ir(ppy)2L]PF6 (ppy = 2-phenylpyridine, L = 4,4′-bis(4-fluorophenyl)-6,6′-dimethyl-2,2′-bipyridine was prepared and structurally characterized in the solid state (X-ray diffraction) and solution (1 and 2D NMR spectroscopy). The compound exhibited yellow photoluminescence (λem = 562 nm). The quantum yield Φ was solvent-dependent (5% in acetonitrile and 19% in dichloromethane solutions, respectively).

Tunable Emissive Ir(III) Benzimidazole-quinoline Hybrids as Promising Theranostic Lead Compounds

Bioactive and luminescent cyclometallated Ir(III) complexes [Ir(ppy)₂ L1]Cl (1) and [Ir(ppy)₂ L2]Cl (2) containing a benzimidazole derivative (L1/L2) as auxiliary mimic of a nucleotide have been synthesised. The emissive properties of both complexes are conditioned by the nature of L1 and L2, rendering an orange and a green emitter respectively. Both are highly emissive with quantum yield increasing in absence of oxygen up to 0.26 (1) and 0.36 (2), suggesting their phosphorescent character. Antiproliferative activity against lung cancer A549 cells increased up to 15 times upon irradiation conditions, reaching IC₅₀ values in the nanomolar range (0.3±0.09 μM (1) and 0.26±0.14 μM (2)) and pointing them as good PSs candidates for photodynamic therapy via ¹ O₂ generation. Cellular biodistribution analysis by fluorescence microscopy suggest the lysosomes as the preferential accumulation organelle. Time-resolved studies showed a greatly increased cellular emission lifetime compared to the solution values, indicating binding to macromolecules or cellular structures and restriction of collision and vibrational quenching.

Cyclometalated Ir(iii) complexes towards blue-emissive dopant for organic light-emitting diodes: fundamentals of photophysics and designing strategies

The fundamental photophysics of cyclometalated Ir(iii) complexes and surveys design strategies for efficient blue phosphorescent Ir(iii) complexes are summarised.

Supramolecularly Caged Green-Emitting Ionic Ir(III)-Based Complex with Fluorinated C^N Ligands and Its Application in Light-Emitting Electrochemical Cells

Ionic Ir(III) complexes are the most promising emitters in light emitting electrochemical cells (LECs), especially in the high energy emission range for which it is difficult to find emitters with sufficient efficiencies and lifetimes. To overcome this challenge, we introduced the concept of intramolecular π-π stacking of an ancillary ligand (6-phenyl-2,2'-bipyridine, pbpy) in the design of a new green-emitting iridium ionic transition metal complex with a fluoro-substituted cyclometallated ligand, 2-(4-fluorophenyl)pyridinato (4Fppy). [Ir(4Fppy)₂(pbpy)][PF₆] has been synthesized and characterized and its photophysical and electrochemical properties have been studied. The complex emits green light with maxima at 561 and 556 nm under UV excitation from powder and thin film, respectively, and displays a high photoluminescence quantum yield (PLQY) of 78.5%. [Ir(4Fppy)₂(pbpy)][PF₆] based LECs driven under pulsed current conditions showed under an average current density of 100 A m^-2 (at 50% duty cycle) a maximum luminance of 1443 cd m^-2, resulting in 14.4 cd A^-1 and 7.4 lm W^-1 current and power efficiencies, respectively. A remarkable long device lifetime of 214 h was observed. Reducing the average current density to 18.5 A m^-2 (at 75% duty cycle) led to an exceptional device performance of 19.3 cd A^-1 and 14.4 lm W^1- for current and power efficiencies, an initial maximum luminance of 352 cd m^-2 and a lifetime of 617 h.

Highly Stable and Efficient Light-Emitting Electrochemical Cells Based on Cationic Iridium Complexes Bearing Arylazole Ancillary Ligands

A series of bis-cyclometalated iridium(III) complexes of general formula [Ir(ppy)₂(N^∧N)][PF₆] (ppy^- = 2-phenylpyridinate; N^∧N = 2-(1H-imidazol-2-yl)pyridine (1), 2-(2-pyridyl)benzimidazole (2), 1-methyl-2-pyridin-2-yl-1H-benzimidazole (3), 2-(4'-thiazolyl)benzimidazole (4), 1-methyl-2-(4'-thiazolyl)benzimidazole (5)) is reported, and their use as electroluminescent materials in light-emitting electrochemical cell (LEC) devices is investigated. [2][PF₆] and [3][PF₆] are orange emitters with intense unstructured emission around 590 nm in acetonitrile solution. [1][PF₆], [4][PF₆], and [5][PF₆] are green weak emitters with structured emission bands peaking around 500 nm. The different photophysical properties are due to the effect that the chemical structure of the ancillary ligand has on the nature of the emitting triplet state. Whereas the benzimidazole unit stabilizes the LUMO and gives rise to a ³MLCT/³LLCT emitting triplet in [2][PF₆] and [3][PF₆], the presence of the thiazolyl ring produces the opposite effect in [4][PF₆] and [5][PF₆] and the emitting state has a predominant ³LC character. Complexes with ³MLCT/³LLCT emitting triplets give rise to LEC devices with luminance values 1 order higher than those of complexes with ³LC emitting states. Protecting the imidazole N-H bond with a methyl group, as in complexes [3][PF₆] and [5][PF₆], shows that the emissive properties become more stable. [3][PF₆] leads to outstanding LECs with simultaneously high luminance (904 cd m^-2), efficiency (9.15 cd A^-1), and stability (lifetime over 2500 h).

Regioisomerism in cationic sulfonyl-substituted [Ir(C^N)2(N^N)]+ complexes: its influence on photophysical properties and LEC performance

In a series of regioisomeric [Ir(C^N)₂(bpy)]⁺ complexes containing methylsulfonyl groups on the cyclometallating ligands, the influence of the substitution position on photophysical, electrochemical and LEC device properties is investigated.

Toward fluorine-free blue-emitting cationic iridium complexes: to generate emission from the cyclometalating ligands with enhanced triplet energy

Generating emission from cyclometalating ligands with enhanced triplet energy is an efficient avenue toward fluorine-free blue-emitting cationic iridium complexes.

Blue-emitting heteroleptic Ir(iii) phosphors with functional 2,3′-bipyridine or 2-(pyrimidin-5-yl)pyridine cyclometalates

Ir(iii) complexes with functional 2-(pyrimidin-5-yl)pyridine cyclometalates display blue electroluminescence with CIE_x,y coordinates occurring at (0.16, 0.17) at 100 cd m⁻².