Organic Light-emitting Diodes Research Articles

Characterization and Optimization of the Photoluminescent Properties of Imidazo[1,5‐a]quinolines

AbstractIn our previous work we could show, that Imidazo[1,5‐a]quinolines are compounds with interesting optical properties. In this work, we optimized the photoluminescent properties of imidazo[1,5‐a]quinolines by following outlined trends in our previous work. By introducing electron‐rich and electron‐poor residual groups on the imidazole ring, the quantum yield (QY) could be significantly increased. This indicated a clear advantageous substitution pattern for this system. In addition, cyclic voltammetric and UV/vis measurements in solid state lay the foundation for applications, such as organic light‐emitting diodes (OLEDs).

An Azaryl-Ketone-Based Thermally Activated Delayed Fluorophore with Aggregation-Induced Emission for Efficient Organic Light-Emitting Diodes with Slow Efficiency Roll-Offs.

Achieving the concurrent manifestation of thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) within a single molecular system is highly sought after for organic light-emitting diodes (OLEDs), yet remains rare. In this study, we present a novel TADF-AIE dye, named PQMO-PXZ, which has been designed, synthesized, and systematically characterized. Our comprehensive investigation, which includes structural analysis, theoretical calculations, and optical studies, evaluates the potential of PQMO-PXZ for integration into OLEDs. Unlike existing azaryl-ketone-based emitters, PQMO-PXZ exhibits red-shifted emission and enhanced luminescence efficiency, due to its rigid structure and strong intramolecular charge transfer characteristics. Significantly, PQMO-PXZ demonstrates pronounced AIE properties and TADF with a short delayed lifetime. When utilized as the emissive core, OLED devices based on PQMO-PXZ achieve a respectable external quantum efficiency of up to 11.8 % with minimal efficiency roll-off, underscoring PQMO-PXZ's promise as a highly efficient candidate for OLED applications.

Higly Luminescent Aceno[6]helicenones by Intramolecular Radical Cyclization.

Helicenes and helicenoid structures are promising candidates for future applications exploiting circularly polarized light. Ideal candidates should possess near-quantitative photoluminescence quantum yield, high a luminescence dissymmetry factor and an adjustable HOMO-LUMO gap. However, carbo[n]helicenes are poorly luminescent compounds and they absorb light mainly in the ultraviolet region. Here we show that the incorporation of a carbonyl group into helical scaffold significantly improves the fluorescence quantum yield and shifts the absorption to visible region. Although the carbonyl group is commonly considered as detrimental to efficient emission, fluorescence quantum yields up to Φ = 0.43 were recorded. A straightforward synthetic approach to a highly luminescent tetraceno[6]helicenone and an aza analogue has been developed. The key step is a radical cyclization which is achieving dehydrative π-extension. The aza-analogue was incorporated as an emitter in organic light emitting diodes (OLEDs) and showed good performance.

Perspective on the Development and Integration of Hydrogen Sensors for Fuel Cell Control

The measurement of hydrogen concentration in fuel cell systems is an important prerequisite for the development of a control strategy to enhance system performance, reduce purge losses and minimize fuel cell aging effects. In this perspective paper, the working principles of hydrogen sensors are analyzed and their requirements for hydrogen control in fuel cell systems are critically discussed. The wide measurement range, absence of oxygen, high humidity and limited space turn out to be most limiting. A perspective on the development of hydrogen sensors based on palladium as a gas-sensitive metal and based on the organic magnetic field effect in organic light-emitting devices is presented. The design of a test chamber, where the sensor response can easily be analyzed under fuel cell-like conditions is proposed. This allows the generation of practical knowledge for further sensor development. The presented sensors could be integrated into the end plate to measure the hydrogen concentration at the anode in- and outlet. Further miniaturization is necessary to integrate them into the flow field of the fuel cell to avoid fuel starvation in each single cell. Compressed sensing methods are used for more efficient data analysis. By using a dynamical sensor model, control algorithms are applied with high frequency to control the hydrogen concentration, the purge process, and the recirculation pump.

Lateral Diffusion of Holes in Anodic Buffer Layers and Its Application in Organic Light-Emitting Diodes

Lateral Diffusion of Holes in Anodic Buffer Layers and Its Application in Organic Light-Emitting Diodes

TICT, and Deep-Blue Electroluminescence from Acceptor-Donor-Acceptor Molecules.

Donor-acceptor (D-A) materials based on butterfly-shaped molecules could inhibit exciton-migration-induced quenching due to molecular twist. To explore this attribute towards beneficial photophysical properties, three novel bipolar acceptor-donor-acceptor (A-D-A) molecules with triphenyl triazine end capping along with substitution ortho to the Tröger's base (TB) scaffold varying from H, Me, and F were explored. The installation of H/Me/F imparted an electron push-pull effect with concomitant maneuvering of photophysical properties. On increasing solvent polarity, a remarkable bathochromic shift with a significant decrease in emission efficiency was observed due to the twisted intramolecular charge transfer state (TICT). Emission enhancement in the ethylene glycol-water mixture and diminution in the THF-water mixture further confirmed the existence of TICT states in these TBs. The torsional dynamics in the excited state were also evidenced by the time-dependent density-functional theory (TD-DFT) calculations. Owing to the butterfly architecture of the TB that suppressed TICT, TB-Trzs exhibited a significant blue shift, accompanied by a favorable quantum yield in the solid state. Among the three compounds, Me-TB-Trz exhibited deep-blue photoluminescence and was explored as a dopant in organic light-emitting diodes (OLEDs) to obtain deep-blue electroluminescence of brightness 4128 cdm-2 and CIE coordinates of (0.16, 0.09).

The Molecular Design and Electroluminescent Performance of Near-Infrared (NIR) Iridium(III) Complexes Bearing Isoquinoline-, Phthalazine- and Phenazine-Based Ligands.

Near-infrared (NIR) light has characteristics of invisibility to human eyes, less background interference, low light scattering, and strong cell penetration. Therefore, NIR luminescent materials have significant applications in imaging, sensing, energy, information storage and display. The development of NIR luminescent materials thus has emerged as a highly dynamic area of research in the realm of contemporary materials. To date, NIR luminescent materials are roughly divided into inorganic materials and organic materials. Compared with inorganic materials, organic NIR luminescent materials have become a hot research topic in recent years due to their rich sources, easy control of structure, simple preparation process, low cost, and good film-forming properties. Among them, iridium(III) [Ir(III)] complexes exhibit excellent properties such as thermal stability, simple synthesis, easy color modulation, short excited state lifetimes, and high brightness, thus becoming one of the ideal luminescent material systems for preparing high-quality organic light-emitting diodes. Therefore, how to obtain Ir(III) complexes with NIR emission and high efficiency through molecular design is a necessary and promising research topic. This work reviews the research progress of representative NIR Ir(III) complexes bearing isoquinoline-, phenazine-, and phthalazine-based ligands reported in recent years and introduces the design strategies and electroluminescent performances of NIR Ir(III) complexes.

Spiral-Locking Strategy for Efficient Narrowband Multiple Resonance Thermally Activated Delayed Fluorescence Emitters.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are applied in organic light-emitting diodes (OLEDs) due to their high efficiency and color purity. However, the inherent planar structure of MR emitters presents significant challenges, including concentration-induced emission quenching, spectral redshift and broadening. To address these issues, two orthorhombic asymmetric conformational materials, SBNO and SBNOS, have been developed. Both MR-TADF emitters incorporate a sterically hindered spiro-carbon bridge to minimize intermolecular chromophore interactions. Consequently, the spectra of the SBNOS-based devices exhibit only a 4 nm redshift and a 7nm broadening of the full-width at half maximum (FWHM) across a doping ratio range of 1-100 wt%. The steric effect produces pure green OLEDs with a CIE y of 0.69 and enhances performance, achieving a maximum external quantum efficiency (EQEmax) of up to 32.7%. The referent BNO without spiro skeleton suffers from serious spectral redshift and broadening as well as a lower device efficiency. This research demonstrates a promising approach to developing MR-TADF devices that resist redshift and broadening while maintaining high color purity and efficiency.

Peripheral Substitution Engineering of MR-TADF Emitters Embedded With B‒N Covalent Bond Towards Efficient BT.2020 Blue Electroluminescence.

Compared with the classical boron/nitrogen (B/N) doped ones, multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters embedded with B-N covalent bond behave a significantly blue-shifted narrowband TADF, and thus show a greater potential in ultrapure blue organic light-emitting diodes (OLEDs). As a proof of concept, herein a peripheral substitution engineering is demonstrated based on such a B‒N embedded parent core. The simple approach is found to ensure easy synthesis via a one-pot lithium-free borylation-annulation, manipulate the excited states through different electronic coupling between core and substituent, and introduce the steric hindrance to minimize the unwanted spectral broadening. Impressively, ultrapure blue OLEDs are realized to give a high external quantum efficiency of 20.3% together with Commission Internationale de l'Éclairage coordinates of (0.152, 0.046). The performance is well competent with those of B/N doped MR-TADF emitters, clearly highlighting that the B‒N embedded framework is a novel promising paradigm towards efficient BT.2020 blue standard.

Chiral Organometallic Complexes Derived from Helicenic N-Heterocyclic Carbenes (NHCs): Design, Structural Diversity, and Chiroptical and Photophysical Properties.

ConspectusRecently, helicene derivatives have emerged as an important class of molecules with potential applications spanning over asymmetric catalysis, biological activity, magnetism, spin filtering, solar cells, and polymer science. To harness their full potential, especially as emissive components in circularly polarized organic light-emitting diodes (CP-OLEDs), generating structural chemical diversity and understanding the resulting photophysical and chiroptical properties are crucial. In this Account, we shed light on chemical engineering combining helicene and N-heterocyclic carbene (NHC) chemistries to create transition-metal complexes with unique architectures and describe their photophysical and chiroptical attributes. The σ-donating and π-accepting capabilities of the helically chiral π-conjugated NHCs endow the complexes with remarkable structural and electronic features. These characteristics manifest in phenomena such as chirality induction, very long-lived phosphorescence, and strong chiroptical signatures (electronic circular dichroism and circularly polarized luminescence).We describe the different classes of ligands primarily developed in our group by classifying them according to their connection between the helicenic moiety and the imidazole precursor. This connection is essential in determining the degree of π-conjugation and characterizing the emissive state. We comprehensively discuss 6-coordinate, 4-coordinate, and 2-coordinate complexes, delving into their structural nuances and examining how the interplay between metals and auxiliary ligands shapes their photophysical properties, with interpretations enriched by DFT calculations. Helicenes are known to promote intersystem crossing thanks to strong spin-orbit coupling, while metals offer robust frameworks leading to a variety of molecular architectures with specific topologies together with distinct excited-state properties. The electronic configurations and energy levels of the ligand and metal orbitals thus significantly modulate the photophysical and chiroptical behaviors of these complexes. In-depth analysis of chiroptical properties, notably electronic circular dichroism and circularly polarized luminescence, emphasizes the influence of different stereogenic elements on the chiroptical responses across various energy ranges with appealing "match-mismatch" effects. Finally, we describe future prospects of helicene NHCs, particularly in the context of emerging research on cost-effective and abundant transition metals for materials science and for photocatalysis. Indeed, the inherent long-lived MLCT, excited-state delocalization, structural rigidity, and intrinsic chirality of these complexes present intriguing avenues for future investigations.

Controlling Polarization‐Induced Polaron Accumulation in OLEDs via Device Design

AbstractSpontaneous alignment of molecular electric dipole moments, known as spontaneous orientation polarization (SOP), is present in many organic electronic materials. The SOP‐induced field changes the internal voltage distribution within the device, altering polaron injection and accumulation. The important role of SOP in organic light‐emitting devices (OLEDs) has recently been emphasized with clear links between dipole alignment and device efficiency and operational lifetime. Prior work has thus sought to engineer SOP via changes in thin film composition or processing conditions. These efforts have generally been directed at the OLED electron transport layer, which can show strong SOP. This work takes a different approach, focusing instead on how changes in device architecture can be applied to tune the magnitude and dynamics of polaron accumulation in response to SOP. In this work, it is demonstrated that the introduction of SOP into the device emissive layer offers control over the spatial location of accumulation. In parallel, insertion of a spacer layer between the hole transport and emissive layers can be used to tune the polaron accumulation rate and density during device operation. Both of these architectural changes permit an increase in OLED efficiency and a pathway around the deleterious impact of SOP‐induced polaron accumulation.

Precise Regulation of the Reverse Intersystem Crossing Pathway by Hybridized Long-Short Axis Strategy for High-Performance Multi-Resonance TADF Emitters.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules have experienced great success in organic light-emitting diodes (OLEDs) owing to their outstanding quantum efficiencies and narrow full width at half-maximums (FWHMs). However, the reverse intersystem crossing (RISC) rates of MR-TADF emitters are usually small, which will lead to relatively long triplet exciton lifetime and severe efficiency roll-off. Here, we report an effective molecular design strategy to introduce multichannel RISC pathways and thus increase RISC rates without compromising the color fidelity and emission efficiency by the "hybridized long-short axis (HLSA)" strategy. The TPA-CN-BN shows a near-unity photoluminescence quantum yield, rapid RISC rate of 1.4 × 105 s-1, narrow FWHM of 23 nm, and small singlet-triplet energy gap (ΔEST) of 0.06 eV in solution. The non-sensitized OLED based on TPA-CN-BN exhibits a narrowband emission with the FWHM of 31 nm, in company with external quantum efficiency (EQE) of 37.9%. Notably, the device exhibits the low efficiency roll-off as the EQEs maintain 34.8% and 21.8% at 100 and 1000 cd m-2, respectively, representing the best performance for single-host OLEDs based on the BCzBN skeleton. This study provides a fresh and promising approach to realize high-performance OLEDs with high color purity and remarkable device efficiency.

Efficient Triplet Harvesting via Hot Exciton Utilization in Solution Processed OLEDs Employing Tetraphenyl Buta‐1,3‐Diene Based AIE Emitters

AbstractHigh photoluminescence quantum yield (PLQY) especially in solid‐state together with harvesting radiative triplet excitons are essential to achieve efficient organic light‐emitting diodes (OLEDs). Herein, the aggregation induced emission (AIE) active tetraphenyl buta‐1,3‐diene (TPB) moiety has been employed and tactfully modified to obtain orthogonally oriented donor‐acceptor (D‐A‐π‐A‐D) type derivatives (namely, TPB‐CHO‐TPA and TPB‐CN‐TPA) which possess additionally the hybridized local and charge transfer (HLCT) type excited‐state. Moreover, computational studies have revealed the possibility of triplet harvesting through hot exciton mechanism in these designed emitters. These key features along with excellent solubility in most of the organic solvents have encouraged to utilize these as light emitting materials for solution processed non‐doped and doped OLED devices. The optimal OLED device using TPB‐CHO‐TPA exhibits yellow light emission (ELmax = 572 nm and CIEx,y = 0.48, 0.51) having maximum external quantum efficiency (EQEmax) of 7.6% with power and current efficiency of 6.1 lm W−1 and 8.9 cd A−1 where the calculated exciton utilization efficiency (EUE) approaches to 97%, indicating efficient triplet harvesting in electroluminescence process. This work signifies a novel design strategy for AIE‐based HLCT type emitters having efficient hot exciton utilization which can pave the way for future development of TPB based highly efficient OLED devices.

Precise Regulation of the Reverse Intersystem Crossing Pathway by Hybridized Long‐Short Axis Strategy for High‐Performance Multi‐Resonance TADF Emitters

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have experienced great success in organic light‐emitting diodes (OLEDs) owing to their outstanding quantum efficiencies and narrow full width at half‐maximums (FWHMs). However, the reverse intersystem crossing (RISC) rates of MR‐TADF emitters are usually small, which will lead to relatively long triplet exciton lifetime and severe efficiency roll‐off. Here, we report an effective molecular design strategy to introduce multichannel RISC pathways and thus increase RISC rates without compromising the color fidelity and emission efficiency by the “hybridized long‐short axis (HLSA)” strategy. The TPA‐CN‐BN shows a near‐unity photoluminescence quantum yield, rapid RISC rate of 1.4 × 105 s‐1, narrow FWHM of 23 nm, and small singlet‐triplet energy gap (ΔEST) of 0.06 eV in solution. The non‐sensitized OLED based on TPA‐CN‐BN exhibits a narrowband emission with the FWHM of 31 nm, in company with external quantum efficiency (EQE) of 37.9%. Notably, the device exhibits the low efficiency roll‐off as the EQEs maintain 34.8% and 21.8% at 100 and 1000 cd m‐2, respectively, representing the best performance for single‐host OLEDs based on the BCzBN skeleton. This study provides a fresh and promising approach to realize high‐performance OLEDs with high color purity and remarkable device efficiency.

The Combination of a Donor-Acceptor TADF and a MR-TADF Emitting Core Results in Outstanding Electroluminescence Performance.

Here the utility and potential of an emitter design are demonstrated, consisting of a narrowband-emitting multiresonant thermally activated delayed fluorescent (MR-TADF) core that is decorated with a suitably higher energy donor-acceptor TADF moiety. Not only does this D-A TADF group offer additional channels for triplet exciton harvesting and confers faster reverse intersystem crossing (RISC) kinetics but it also acts as a steric shield, insulating the emissive MR-TADF core from aggregation-caused quenching. Two emitters, DtCzBN-CNBT1 and DtCzBN-CNBT2, demonstrate enhanced photophysical properties leading to outstanding performance of the organic light-emitting diodes (OLEDs). DtCzBN-CNBT2, containing a D-A TADF moiety, has a faster kRISC (1.1×105 s-1) and higher photoluminescence quantum yield (ΦPL: 97%) compared to DtCzBN-CNBT1 (0.2×105 s-1, ΦPL: 90%), which contains a D-A moiety that itself is not TADF. The sensitizer-free OLEDs with DtCzBN-CNBT2 achieve a record-high maximum external quantum efficiency (EQEmax) of 40.2% and showed milder efficiency roll-off (EQE1000 of 20.7%) compared to the DtCzBN-CNBT1-based devices (EQEmax of 37.1% and EQE1000 of 11.9%).

Original Blue Light-Emitting Diphenyl Sulfone Derivatives as Potential TADF Emitters for OLEDs

Organic light-emitting diodes (OLEDs) have emerged as one of the dominant technologies in displays due to their high emission efficiency and low power consumption. However, the development of blue color emitters has fallen behind that of red and green emitters, posing challenges in achieving optimal efficiency, stability, and accessibility. In this context, thermally activated delayed fluorescence (TADF) emitters hold promise as a potential solution for cost-effective, exceptionally efficient, and stable blue OLEDs due to their potential high efficiency and stability. TADF is a principle where certain organic materials can efficiently convert both singlet and triplet excitons, theoretically achieving up to 100% internal quantum efficiency. This research focused on diphenyl sulfone derivatives with carbazole groups as TADF compounds. Quantum chemical calculations and photoluminescence properties show the potential TADF properties of the molecules. New materials exhibit glass transition temperatures that would classify them as molecular glasses. Depending on the structure of the molecule, the photoluminescence emission is in the blue or green spectral region. Organic light-emitting diodes were fabricated from neat thin films of emitters by the wet casting method. The best performance in the deep blue emission region was achieved by a device with a turn-on voltage of 4 V and a maximum brightness of 178 cd/m2. In the blue-green emission region, the best performance was observed by an OLED with a turn-on voltage of 3.5 V, reaching a maximum brightness of 660 cd/m2.

Experimental and Theoretical Approaches for Detecting Latent Lateral Leakage Current of Organic Light-Emitting Diodes

Experimental and Theoretical Approaches for Detecting Latent Lateral Leakage Current of Organic Light-Emitting Diodes

Tailoring Ultra-Narrowband Tetraborylated Multiple Resonance Emitter for High-Performance Blue OLED.

Ultra-narrowband multiple resonance (MR) emitters are a key component in the fabrication of highly efficient and stable blue organic light-emitting diodes (OLEDs). To explore the theoretical boundaries of wavelength and full width at half maximum (FWHM) in blue emitters, the currently narrowest boron-based MR emitter is carefully designed by integrating the superior v-DABNA and BBCz-DB structures under the auspices of the ingenious short-range charge-transfer region regulation strategy. The target tetraboron compound TB-PB demonstrates a blue emission with an emission maximum of 473nm, a small FWHM of 12nm and a CIEy coordinate of 0.14. Benefiting from the emitter's high photoluminescence quantum yield (99%), low excited-state energy (2.74eV) and short delayed fluorescence lifetime (0.53µs), the corresponding OLED achieves exceptional efficiencies of 36.4%, 49.1cdA-1, and 51.4lmW-1 with a record-high luminescence of 9.0×105cdm-2, an ultra-narrow FWHM of 15nm and a CIEy coordinate of 0.20. These breakthroughs will accelerate the development of next-generation blue emitters and lead to the advancement of OLED technology.

A 2-D coordination polymer based on 3-(2-pyridyl)-5-(4-chlorophenyl)-1,2,4-triazine ligands and lead(II) bromide: synthesis, structure, luminescent properties, and application in organic light-emitting diodes

The combination of PbBr2 with 3-(2-pyridyl)-5-(4-chlorophenyl)-1,2,4-triazine (PCPT) results in a 2-D Pb(II) coordination polymer, denoted as [Pb9(PCPT)4Br18] n . This compound has been identified and studied through CHN analysis, FT-IR, and 1H NMR spectroscopy, in addition to detailed analysis using single-crystal X-ray diffraction techniques. Considering the primary and secondary Pb–N and Pb–Br bonds, Pb1 and Pb2 exhibit hemidirected pentagonal bipyramidal geometries, whereas Pb3, Pb4, and Pb5 demonstrate holodirected octahedral geometries. The 2-D structure transitions into a 3-D arrangement due to the presence of non-covalent C–H…Br and π…π stacking interactions. The interactions highlighted above enhance the structural stability and support the transfer of charge and energy between metal centers, positioning the polymer as a potential candidate for luminescence applications. Analysis of the electro-optical characteristics of this compound indicates its suitability as a luminescent material for incorporating into organic light-emitting diodes.

Interfacial Exciton-Polaron Quenching in Organic Light-Emitting Diodes

In organic light-emitting diodes (OLEDs), understanding the efficiency loss mechanism known as exciton-polaron quenching (EPQ) has been a longstanding challenge. Traditionally, EPQ was believed to occur mainly the bulk of the OLED emission layer (EML) due to the coexistence of polarons and excitons, limiting our understanding of its full impact. Here, we report a previously overlooked phenomenon termed “interfacial EPQ (Inf. EPQ),” which occurs at the heterointerface between the EML and the adjacent charge transport layer (CTL). We observe the transfer of EML excitons to CTL polarons across this interface, spanning distances of up to 4 nm. Notably, Inf. EPQ exceeds EPQ within the EML bulk, even in devices with a interfacial energy barrier (>0.2 eV). We propose a methodology that enables independent probing of Inf. EPQ, systematic validation of its essential parameters, and the identification of Dexter energy transfer as its governing mechanism. Importantly, our findings highlight Inf. EPQ as a phenomenon across devices with various luminescent mechanisms, wavelengths, and injection levels. By effectively addressing Inf. EPQ, we achieve efficiency enhancements in red, green, and blue phosphorescent OLEDs, by 70%±8%, 47%±5%, and 66%±6%, respectively. Our work advances the physical understanding of exciton and polaron dynamics at organic heterointerfaces, providing applicable solutions to Inf. EPQ-related issues in practical devices. Published by the American Physical Society 2024