Electroluminescence Efficiency Research Articles

Electroluminescent Clusters

AbstractOptoelectronic cluster materials emerge rapidly in recent years especially for light‐emitting devices, owing to their 100 % exciton harvesting and unique organic–inorganic hybrid structures with tunable excited‐state characteristics for thermally activated delayed fluorescence and/or phosphorescence and inheritable photo‐ and thermo‐stability. However, for efficient electroluminescence, excited‐state compositions of cluster emitters should be tuned through ligand engineering to enhance ligand‐centered radiative components and reduce cluster‐centered quenching states. Nonetheless, the balance of optoelectronic properties requires delicate and controllable ligand functionalization. On the other hand, in addition to balancing carrier fluxes, it showed that device engineering, especially host matrixes and interfacial optimization, can not only alleviate triplet quenching, but also modify processing and passivate defects. As consequence, the record external quantum efficiencies of cluster light‐emitting diodes already reached ≈30 %. Herein, we overview recent progress of electroluminescent cluster materials and discuss their structure–property relationships, which would inspire the continuous efforts making cluster light‐emitting diodes competent as the new generation of displays and lighting sources.

Electroluminescent Clusters.

Optoelectronic cluster materials emerge rapidly in recent years especially for light-emitting devices, owing to their 100 % exciton harvesting and unique organic-inorganic hybrid structures with tunable excited-state characteristics for thermally activated delayed fluorescence and/or phosphorescence and inheritable photo- and thermo-stability. However, for efficient electroluminescence, excited-state compositions of cluster emitters should be tuned through ligand engineering to enhance ligand-centered radiative components and reduce cluster-centered quenching states. Nonetheless, the balance of optoelectronic properties requires delicate and controllable ligand functionalization. On the other hand, in addition to balancing carrier fluxes, it showed that device engineering, especially host matrixes and interfacial optimization, can not only alleviate triplet quenching, but also modify processing and passivate defects. As consequence, the record external quantum efficiencies of cluster light-emitting diodes already reached ≈30 %. Herein, we overview recent progress of electroluminescent cluster materials and discuss their structure-property relationships, which would inspire the continuous efforts making cluster light-emitting diodes competent as the new generation of displays and lighting sources.

Carbonyl- and Nitrogen-Embedded Multi-Resonance Emitter with Ultra-Pure Green Emission and High Electroluminescence Efficiencies.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with narrow emission spectra have garnered significant attention in future organic light-emitting diode (OLED) displays. However, current C=O/N-embedded MR-TADF systems still lack satisfactory performance in terms of electroluminescence bandwidths and external quantum efficiencies (EQEs). In this study, a C=O/N-embedded green MR-TADF emitter, featuring two acridone units incorporated in a sterically protected 11-ring fused core skeleton, is successfully synthesized through finely controlling the reaction selectivity. The superior combination of multiple intramolecular fusion and steric wrapping strategies in the design of the emitter not only imparts an extremely narrow emission spectrum and a high fluorescence quantum yield to the emitter but also mitigates aggregation-induced spectral broadening and fluorescence quenching. Therefore, the emitter exhibits leading green OLED performance among C=O/N-based MR-TADF systems, achieving an EQE of up to 37.2 %, a full width at half maximum of merely 0.11 eV (24 nm), and a Commission Internationale de l'Éclairage coordinate of (0.20, 0.73). This study marks a significant advance in the realization of ideal C=O/N-based MR-TADF emitters and holds profound implications for the design and synthesis of other MR-TADF systems.

Regulating excited states by varying different acceptors of D-π-A emitters for efficient non-doped blue electroluminescence with high luminance and low efficiency roll-off.

Chromophores with hybridized local and charge-transfer (HLCT) excited state are promising for the realization of high performance blue organic light-emitting diodes (OLEDs). The rational manipulation of HLCT excited state for efficient emitters remains challenging. Herein, we present three donor-π-acceptor (D-π-A) molecules (mPAN, mPANPH, and mPNAPH) with phenanthro[9,10-d]imidazole (PI) and pyridinyl as donor and π-bridge respectively. Changes in various kinds of polycyclic aromatic derivative acceptors (anthracene, 9-phenylanthracene, and 1-phenylnaphthalene) could manipulate the excited states and optoelectronic properties. Theoretical calculations reveal that the S1 state of mPNAPH exhibits HLCT nature while the other two molecules show local excited (LE) state dominated feature. The photophysical properties also demonstrate this characteristic. Therefore, compared with mPAN and mPANPH, mPNAPH has higher photoluminescence quantum yield (PLQY) whether in solutions or neat films.Ultimately, the non-doped devices based on these emitters show high luminance larger than 35000 cd m-2, and high maximum external quantum efficiencies (EQEmaxs) larger than 5% with low efficiency roll-off. In particular, the mPNAPH-based device displays an excellent performance of pure blue emission at 456 nm with Commission Internationale de L'Eclairage coordinate of (0.15, 0.16) and EQEmax of 6.13% that benefited from the HLCT state and high-lying reverse intersystem crossing process.

Cost-Effective Approach for Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters.

Multiresonant thermally activated delayed fluorescence (MR-TADF) emitters have recently attracted great interest for application in organic light-emitting diodes due to their remarkable electroluminescent efficiency and narrow emission spectra. It is therefore essential to establish computational methodologies that can accurately model the excited states of these materials at manageable computational costs. With regard to MR-TADF design and their associated photophysics, previous works have highlighted the importance of wave function-based methods, at much higher computational costs, over the traditional time-dependent density functional theory approach. Herein, we employ two independent techniques built on different quantum mechanical frameworks, highly correlated wave function-based STEOM-DLPNO-CCSD and range-separated double hybrid density functional, TD-B2PLYP, to investigate their performance in predicting the excited state energies in MR-TADF emitters. We demonstrate a remarkable mean absolute deviation (MAD) of ∼0.06 eV in predicting ΔEST compared to experimental measurements across a large pool of chemically diverse MR-TADF molecules. Furthermore, both methods yield superior MAD in estimating S1 and T1 energies over earlier reported SCS-CC2 computed values [J. Chem. Theory Comput. 2022, 18, 4903]. The short-range charge-transfer nature of low-lying excited states and narrow fwhm values, hallmarks of this class of emitters, are precisely captured by both approaches. Finally, we show the transferability and robustness of these methods in estimating rates of radiative and nonradiative events with adequate agreement against experimental measurements. Implementing these cost-effective computational approaches is poised to streamline the identification and evaluation of potential MR-TADF emitters, significantly reducing the reliance on costly laboratory synthesis and characterization processes.

Hot luminescence from single-molecule chromophores electrically and mechanically self-decoupled by tripodal scaffolds

Control over the electrical contact to an individual molecule is one of the biggest challenges in molecular optoelectronics. The mounting of individual chromophores on extended tripodal scaffolds enables both efficient electrical and mechanical decoupling of individual chromophores from metallic leads. Core-substituted naphthalene diimides fixed perpendicular to a gold substrate by a covalently attached extended tripod display high stability with well-defined and efficient electroluminescence down to the single-molecule level. The molecularly controlled spatial arrangement balances the electric conduction for electroluminescence and the insulation to avoid non-radiative carrier recombination, enabling the spectrally and spatially resolved electroluminescence of individual self-decoupled chromophores in a scanning tunneling microscope. Hot luminescence bands are even visible in single self-decoupled chromophores, documenting the mechanical decoupling between the vibrons of the chromophore and the substrate.

Highly Bright and Stable Lead-Free Double Perovskite White Light-Emitting Diodes.

Lead-free double perovskites (DPs) are emerging highly stable emitters with efficient broadband self-trapped exciton (STE) photoluminescence (PL), but their low electroluminescent (EL) efficiency is a critical shortcoming. This work promotes the external quantum efficiency (EQE) and luminance of DP-based white light-emitting diode (wLED) with a normal device structure to 0.76% and 2793cd m-2 via two modifications: This work prevents the formation of adverse metallic silver, spatially confined STE, and lowers local site symmetry in Cs2 Na0.4 Ag0.6 In0.97 Bi0.03 Cl6 DP by terbium doping; and this work develops a guest-host strategy to improve film morphology, reduce defect density, and increase carrier mobility. These alterations cause substantial increase in STE radiative recombination and charge injection efficiency of perovskite layer. Finally, pure white EL with ideal color coordinates of (0.328, 0.329) and a record-breaking optoelectronic performance is achieved by introducing additional green carbon dots in LED to fill the deficient green component.

Constructing Multiresonance Thermally Activated Delayed Fluorescence Emitters for Organic LEDs: A Computational Investigation.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials have acquired substantial attention due to their high electroluminescence efficiency with narrow emission spectra. However, the existing MR-TADF emitters suffer from substantial efficiency roll-off due to insufficient rate constants of the reverse intersystem crossing (kRISC) process compared to the traditional TADF materials. Herein, we employ the DLPNO-STEOM-CCSD method, which is computationally less expensive than the wave function-based EOM-CCSD method, to evaluate the electronic and photophysical properties of MR-TADF materials accurately. The predicted singlet-triplet energy gap (ΔEST), one of the critical parameters governing the TADF efficiency, exhibits remarkable agreement with the experimental measurement, with a standard deviation value of 0.026 eV (obtained based on five experimentally synthesized MR-TADF systems). The proposed technique was utilized to determine the suitability of 15 newly designed MR-TADF emitters via their computed radiative and nonradiative rates, luminescence efficiencies, and exciton characteristics. Moreover, most conceived molecules exhibit blue emission with decent to strong oscillator strengths, making them potential candidates for practical light-emitting applications. The proposed computational route will undoubtedly accelerate the designing and prescreening of potential MR-TADF emitters before their expensive laboratory synthesis and characterization.

Synergistic insights into pyrazinophenazine based hybrid materials for advancing optoelectronics

The crucial point of present organic light emitting diode (OLED) research is the improvement of device stability and electroluminescent efficiency. Recently, cheaply available organic compound based thermally activated delayed fluorescent (TADF) emitters are promising to resolve current issues of OLEDs. In this context, we have designed and successfully synthesized a two novel hybrid molecules 3-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-12,13- diphenyldibenzo[a,c]pyrazino[2,3-i]phenazine (Ac-DibzPyrQx) and 3-(4-(3,6-di‑tert‑butyl‑9H-carbazol-9-yl)phenyl)-12,13 diphenyldibenzo[a,c]pyrazino [2,3-i]phenazine (tCz-DibzPyrQx), comprising electron-donating (9,9-dimethyl-9,10-dihydroacridine, 3,6-di‑tert‑butyl‑9H-carbazole) and electron accepting Pyrazinophenazine groups. The incorporation of highly planar and rigid pyrazinophenazine electron-accepting moieties holds significant importance due to their unique properties. Their planar structure facilitates strong π-π stacking interactions and efficient charge transfer within the molecular framework, leading to improved exciton formation and enhanced intersystem crossing (ISC) rates, which are critical for TADF processes. The two different electron-donating groups with pyrazinophenazine were synthesized with the view to tune the photophysical and electrochemical properties of the hybrids. These compounds showed high thermal and morphological stability, as well as appropriate frontier molecular orbital (FMO) energy levels. These synthesized molecules show decomposition temperatures (262 °C and 282 °C) and exhibited good glass transition temperatures (328 °C and 300 °C), indicating their significant stability and potential utility as TADF emitters.

Tailored Near‐Infrared Luminescent Materials with Fast Reverse Intersystem Crossing for Efficient Deep‐Red/Near‐Infrared Organic Light‐Emitting Diodes with High Efficiencies and Small Roll‐Offs

AbstractRobust organic luminescent materials radiating deep‐red/near‐infrared (DR/NIR) light are highly desired in organic light‐emitting diodes (OLEDs). However, most reported DR/NIR materials show only moderate electroluminescence (EL) efficiencies accompanied by rock‐ribbed efficiency roll‐offs, which immensely limit their practical use in OLEDs. In this work, two tailor‐made NIR luminescent materials with thermally activated delayed fluorescence (TADF) property are developed based on rigid and strong electron‐accepting core of 2,7‐bis(4‐cyanophenyl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile and electron donors of triphenylamine or N,N‐di([1,1′‐biphenyl]‐4‐yl)phenylamine. They exhibit high photoluminescence quantum yields, fast reverse intersystem crossing processes and large horizontal dipole ratios. Their nondoped OLEDs show NIR lights with maximum external quantum efficiencies (ηext,maxS) of 0.59% at 856 nm and 1.14% at 850 nm. And their doped OLEDs radiate DR to NIR lights (646–737 nm) and attain superb ηext,maxs of up to 30.3% at 646 nm with small efficiency roll‐offs. The state‐of‐the‐art EL performances of both NIR TADF materials demonstrate their high potential as emitters in OLEDs and the insights gained in this work will enlighten the further development of efficient NIR luminescent materials.

Anomalous efficiency elevation of quantum-dot light-emitting diodes induced by operational degradation

Quantum-dot light-emitting diodes promise a new generation of high-performance and solution-processed electroluminescent light sources. Understanding the operational degradation mechanisms of quantum-dot light-emitting diodes is crucial for their practical applications. Here, we show that quantum-dot light-emitting diodes may exhibit an anomalous degradation pattern characterized by a continuous increase in electroluminescent efficiency upon electrical stressing, which deviates from the typical decrease in electroluminescent efficiency observed in other light-emitting diodes. Various in-situ/operando characterizations were performed to investigate the evolutions of charge dynamics during the efficiency elevation, and the alterations in electric potential landscapes in the active devices. Furthermore, we carried out selective peel-off-and-rebuild experiments and depth-profiling analyses to pinpoint the critical degradation site and reveal the underlying microscopic mechanism. The results indicate that the operation-induced efficiency increase results from the degradation of electron-injection capability at the electron-transport layer/cathode interface, which in turn leads to gradually improved charge balance. Our work provides new insights into the degradation of red quantum-dot light-emitting diodes and has far-reaching implications for the design of charge-injection interfaces in solution-processed light-emitting diodes.

Benzonitrile/pyridylbenzoimidazole hybrid electron-transport material for efficient phosphorescence and TADF OLEDs

A new electron-transport material (ETM) of iTPyBI-CN is developed through non-catalytic C-N coupling reaction from 2,4,6-trifluorobenzonitrile and 2-pyridyl-1H-benzo[d]imidazole. Compared to the commercial ETM of 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBI), the donor–acceptor type iTPyBI-CN exhibits a more twisted geometry for comparable triplet energy, higher LUMO and slightly deeper HOMO levels. In both phosphorescence and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs), devices based on iTPyBI-CN as ETM demonstrates better electroluminescence (EL) efficiencies than TPBI, with maximum external quantum efficiency (EQE) of 20.2, 22.0, 16.3 and 16.1 % versus 16.4, 18.9, 11.0 and 3.0 % for various phosphorescent and TADF OLEDs.

Improving Electroluminescence of Two‐Coordinate Au(I) Complexes: Insights into Steric and Electronic Control

AbstractThis research elucidates the effects of structural modulations on electroluminescent Au(I) complexes, shedding light on factors governing radiative and nonradiative processes. A series of Au(I) complexes, fortified with ortho‐substituents in carbene and amido ligands, are subjected to rigorous structural, photophysical, and quantum chemical investigations, which unveil distinct structural and electronic effects exerted by the ligands. The investigations reveal that nonradiative processes are governed primarily by the energy‐gap law. Radiative processes are observed to have a weak correlation with the mutual interactions of the molecular orbitals of carbene and amido ligands. Rather, it is discovered that an accumulation of the negative charge in the Au 5d orbital in the excited state decelerates radiative processes. The effectiveness of these findings is substantiated through the larger external quantum efficiency of electroluminescence devices employing the Au(I) complex, in comparison to those based on the archetypical Au(I) complex and the organic thermally activated delayed fluorescent molecule. These compelling revelations underscore the untapped potential of Au(I) complexes in the advancement of electroluminescence technology and advocate for continued investigations into the intriguing domain of ligand structural control.

Effects of host-molecule transport polarities on the transient electroluminescence decay and efficiency roll-off of doped organic light-emitting diodes

Effects of host-molecule transport polarities on the transient electroluminescence decay and efficiency roll-off of doped organic light-emitting diodes

Phase Regulation of Layered Perovskites toward High‐Performance Light‐Emitting Diodes

AbstractAs a promising family of semiconductor materials for high‐quality displays, quasi‐2D perovskites have gained a significant amount of attention in the field of light‐emitting diodes for their superior optoelectronic properties compared with their 3D counterparts. The intrinsic multiple‐quantum‐well characteristics induced by reducing the crystal dimensionality enable perovskite emitters to possess high photoluminescence quantum yield and good film morphology. It is demonstrated that the n‐value distribution of quasi‐2D perovskites, where n refers to the number of inorganic MX6 octahedron layers, has a strong impact on the electroluminescence efficiency and stability of perovskite light‐emitting diodes (PeLEDs). In this review, the crystal structure of quasi‐2D perovskites, the formation origin of different n‐value distributions, and their optoelectronic characteristics are first introduced. Then, the mainstream strategies for regulating the phase distribution of layered perovskites are systematically reviewed to suggest their great significance in boosting the performances of PeLEDs. Lastly, the current deficiencies and potentially valuable research directions of phase distribution management for efficient and stable quasi‐2D PeLEDs are provided.

Achieving Long-Wavelength Electroluminescence Using Two-Coordinate Gold(I) Complexes: Overcoming the Energy Gap Law.

Two-coordinate coinage metal complexes have emerged as promising emitters for highly efficient organic light-emitting devices (OLEDs). However, achieving efficient long-wavelength electroluminescence emission from these complexes remains as a daunting challenge. To address this challenge, molecular design strategies aimed at bolstering the photoluminescence quantum yield (Φ) of Au(I) complex emitters in low-energy emission regions are investigated. By varying amido ligands, a series of two-coordinate Au(I) complexes is developed that exhibit photoluminescence peak wavelengths over a broad range of 533-750nm. These complexes, in particular, maintain Φ values up to 10% even in the near-infrared emission region, overcoming the constraints imposed by an energy gap. Quantum chemical calculations and photophysical analyses reveal the action of radiative control, which serves to overcome the energy gap law, becomes more pronounced as the overlap between hole and electron distributions (Sr (r)) in the excited state increases. It is further elucidated that Sr (r) increases with the distance between the hole-distribution centroid and the nitrogen atom in an amido ligand. Finally, multilayer OLEDs involving the Au(I) complex emitters exhibit performances beyond the borderline of the electroluminescence wavelength-external quantum efficiency space set by previous devices of coinage metal complexes.

Large-scale ultraviolet electroluminescence from twin wollastonite in silicon-photonic device

In the past decades, ultraviolet (UV) spectroscopy has been enormously utilized in academic and industrial applications. In spite of that, lower optoelectrical performance compared to their visible light counterparts resulted in limitations for expanding the potential market. Understanding the current issue, we herein propose a highly efficient large-scale emission UV electroluminescent (EL) device that is prepared with a cost-effective process. The UV-EL device consists of a multi-dielectric layer that sandwiches the active layer in a silicon substrate. The UV light is generated from the f-d transitions of Ce3+ ions occupied in the wide bandgap polymorphic twinning wollastonite CaSiO3 host lattice in a process of so-called impact ionization. This work provides proof that the multi-dielectric layer in this structure plays an important role in preventing the leakage current and enhancing the light emission and expectantly will pave the future and more advanced development for efficient UV light realization in silicon photonics.

High efficiency double light-emitting layer sky blue phosphor organic light-emitting diode with slow emission efficiency attenuation

In this study, by doping the iridium (III) double ((4,6-difluorobenzene) -pyridine-N, C2’Pyridinformate (FIrpic) into 4,4′, 4″ three (carbazole) trioxilamine (TcTa) and 9 (4 tert-butyl-benzene) −3,6 double (triphenylsilyl) -9H carbazole (CzSi) layers as light emitting layers (EMLs) to achieve sky blue phosphor organic electroluminescence (EL) devices with very low efficiency roll-off. The dependence of the EL performance on the doping concentration of the light-emitting molecules and the different EML thicknesses are studied in detail. Experimental data show that the thin-film-doped TcTa layer is very important for the efficiency of the device. Finally, the optimal device achieves extremely high power efficiency, current efficiency, and external quantum efficiency (EQE), reaching 74.82 lm/W, 66.68 cd/A, and 31.9 %, respectively. At a certain luminance of 2000 cd/m2 (3.4 V), the same device retained power efficiency with current efficiency and external quantum efficiency of 57.71 lm/W, 62.46 cd/A and 29.3 %, respectively, based on theoretical discussion, the improved EL efficiency was attributed to the energy levels of the host and luminescent molecules, which helps to balance the charge carrier in the emitter molecule distribution and expand the recombination region.

Asymmetric Ternary Spiro Phosphine Oxide Hosts for Yellow Thermally Activated Delayed Fluorescence Diodes with External Quantum Efficiency Beyond 30%

AbstractYellow thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs) are indispensable for lighting, navigation, caution, and so on; however, their development is far behind those of three‐elementary‐color and white analogs, partially because of the lack of host specificity. In this contribution, spiro[bithoxanthene] (ST) and spiro[thioxanthene‐xanthene] (STX) are used as spiro skeletons to form asymmetric ternary structures through introducing two electron‐donating tert‐butyl carbazole (tBCz) and one electron‐withdrawing diphenyl phosphine oxide (DPPO) groups on different rings. The resulting hosts named STSPOtBCz2 and STXSPOtBCz2 not only inherit the steric hindrance of their spiro skeletons, but also achieve the ambipolar characteristics. Their suitable triplet energy levels (T1) of ≈2.5 eV support efficient energy transfer to a conventional yellow TADF emitter 4CzTPNBu with the T1 of ≈2.2 eV. It is shown that compared to STSPOtBCz2 with a symmetric core, the accumulated asymmetry of the spiro skeleton and the kind, number, and position of substitution groups in STXSPOtBCz2 further suppresses the emission quenching, resulting in the state‐of‐the‐art photoluminescent and electroluminescent quantum efficiencies of 93% and 30.6%, respectively. The maximum power efficiency beyond 90 lm W−1 further demonstrates the great potential of multi‐asymmetric host systems for energy‐saving practical application.

Indication of Intramolecular Triplet–Triplet Annihilation Upconversion in Organic Light‐Emitting Diodes

AbstractIt is well known that triplet–triplet annihilation upconversion (TTU) can enhance electroluminescence (EL) efficiency by converting two low‐energy triplet excitons into one high‐energy singlet exciton. However, conventional intermolecular TTU requires high doping concentration to activate, limiting device architecture. Here, six dimer molecules comprised of two anthracene units connected by a spacer unit are investigated, which have a possibility of the intramolecular TTU process. As a result, two dimer molecules show high TTU efficiencies close to the theoretical maximum even in dispersed, diluted conditions. By comparing these molecular conformations, a correlation is found between the dimer structures of two anthracene units and the TTU activity in the diluted condition, i.e., the 3D molecular structure with the exciton utilizing efficiency.