Efficiency Roll-off Research Articles

Minimizing Efficiency Roll-Off in Organic Emitters via Enhancing Radiative Process and Reducing Binding Energy: A Theory Insight.

Organic solid-state lasers have received increasing attention due to their great potential for realizing organic continuous-wave or electrically driven lasers. Moreover, they exhibit significant promise for optoelectronic devices due to their chemically tunable optoelectronic properties and cost-effective self-assembly traits. Recently, a great progress has been made in organic solid-state lasers via spatially separated charge injection and lasing. However, making directly electrically driven organic semiconductor lasers is very challenging. It is difficult because of a number of excitonic losses caused by the spin-forbidden nature as well as serious efficiency roll-off at a high current density. Here, a multifunction gain material, functioning both as a thermally activated delayed fluorescence (TADF) emitter with exceptional optical gain and as a source of phosphorescence, was theoretically investigated. The new molecule we designed exhibits a reduction of triplet accumulation through an effective exciton radiative process (5-fold boost in figure of merit) and significantly decreased exciton binding energy (dipole moment from 5.77 to 14.03 D), which benefit amplified spontaneous emission and lasing emission. Our work provides theoretical insights into organic solid-state lasers and may contribute to the development of new and efficient laser-gaining molecules.

Quantifying Efficiency Roll-Off Factors in Quantum-Dot Light-Emitting Diodes.

The application of quantum-dot light-emitting diodes (QLEDs) is hindered by efficiency roll-off at high current densities. Factors contributing to this roll-off include Auger recombination, electric field-induced quenching, Joule heating, and electron leakage into the hole transport layer. However, a method to quantitatively attribute the contribution of each factor to roll-off has not yet been available, leaving the primary cause of roll-off unidentified. This work addresses this gap using electrically pumped transient absorption spectroscopy, which measures the accumulated electrons and electric field in quantum dots (QDs). This study also introduces a method to quantify electron leakage in QLEDs using this spectroscopic technique. Based on the spectroscopic experimental results, the contribution of each factor to roll-off is quantified. A green QLED with a peak external quantum efficiency (EQE) of 26.8% is studied as an example. The EQE declines to 20.5% at a current density of 354mA cm-2, where field-induced quenching accounts for 5% of the efficiency roll-off, and electron leakage contributes 95%. Contributions from Auger recombination and heat-induced quenching are negligible. This work demonstrates strong correlations between roll-off and electron leakage amplitude using statistical data obtained in multiple QLEDs, confirming that electron leakage is the primary factor in EQE roll-off.

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.

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.

High-Performance perovskite quantum dots light-emitting diodes with hole transport layer engineering and synergetic outcoupling enhancement

Perovskite quantum dot light-emitting diodes (PeQLEDs) are frequently considered as the most promising alternatives to organic light-emitting diodes (OLEDs). However, the efficiency of PeQLEDs remains inferior to OLEDs due to suboptimal charge carrier transport and intrinsic light outcoupling efficiency (ηout). Herein, a combination of hole transport layer (HTL) engineering and substrate engineering is demonstrated to improve the charge injection and ηout of PeQLEDs. To replace the conventional PEDOT:PSS, a novel HTL bilayer based on modified PEDOT:PSS and PVK with a lower refractive index and fewer trap densities at the HTL-QDs interface is developed. Additionally, this study identifies the ideal thickness of ITO for achieving optimal ηout of PeQLEDs through optical simulations and experimental validation. Based on the synergistic use of HTL bilayer and 70-nm-thick ITO, the PeQLEDs achieved an optimal external quantum efficiency of 17.96 % at a luminance of 1763 cd/m2 and 15.19 % at 8300 cd/m2 without using any external outcoupling structure, indicating a low efficiency roll-off.

Lattice reconstruction for mixed-halide blue perovskite light-emitting diodes with high brightness, outstanding color stability and low efficiency roll-off

We report a simple, effective, and universal lattice reconstruction approach to improve the quality of perovskite films by using nonpolar solvents with high Gutmann donor numbers (DNs). We find that high-DN nonpolar solvents, for instance, ethyl acetate, can interact with perovskite precursors. Such a solvent can make the perovskite lattice more ordered and “harder” and promote the formation of heterostructures with low-dimensional perovskite impurities and residual solvent molecules. As a result, the lattice-reconstructed perovskite films exhibit reduced defect densities and suppressed ion migration. The resultant mixed-halide blue perovskite light-emitting diodes (PeLEDs) show greatly enhanced tolerance to high driving current densities and voltages, demonstrating high brightness, outstanding color stability and low efficiency roll-off. Our work provides a deep understanding of the interactions between nonpolar solvents and perovskites and offers useful guidelines for further development of high-power PeLEDs.

Efficient Deep-Blue Organic Light-Emitting Diodes Employing Doublet Sensitization.

Fast and efficient exciton utilization is a crucial solution and highly desirable for achieving high-performance blue organic light-emitting diodes (OLEDs). However, the rate and efficiency of exciton utilization in traditional OLEDs, which employ fully closed-shell materials as emitters, are inevitably limited by spin statistical limitations and transition prohibition. Herein, a new sensitization strategy, namely doublet-sensitized fluorescence (DSF), is proposed to realize high-performance deep-blue electroluminescence. In the DSF-OLED, a doublet-emitting cerium(III) complex, Ce-2, is utilized as sensitizer for multi-resonance thermally activated delayed fluorescence emitter ν-DABNA. Experimental results reveal that holes and electrons predominantly recombine on Ce-2 to form doublet excitons, which subsequently transfer energy to the singlet state of ν-DABNA via exceptionally fast (over 108s-1) and efficient (≈100%) Förster resonance energy transfer for deep-blue emission. Due to the circumvention of spin-flip in the DSF mechanism, near-unit exciton utilization efficiency and remarkably short exciton residence time of 1.36µs are achieved in the proof-of-concept deep-blue DSF-OLED, which achieves a Commission Internationale de l'Eclairage coordinate of (0.13, 0.14), a high external quantum efficiency of 30.0%, and small efficiency roll-off of 14.7% at a luminance of 1000cdm-2. The DSF device exhibits significantly improved operational stability compared with unsensitized reference device.

Suppression of efficiency roll-off in solution-processed OLEDs using pyrimidine-based iridium complexes modified with sterically hindered groups

Three bis-cyclometalated iridium complexes Ir1-Ir3 were successfully synthesized through mild reaction conditions. All complexes exhibit emission peaks at 507-509 nm, with excellent photoluminescence quantum efficiencies (PLQY) ranging from 69% to 97%. Self-quenching is significantly reduced because the steric spacers lead to minimum bimolecular interactions. Accordingly, the electroluminescence device based on complex Ir3 exhibits a maximum luminous efficiency of 39.6 cd A−1, a high EQE of 13.1% with an efficiency roll-off of 4.2% at 1000 cd m−2, which is much smaller than that (10.8%) of the Ir1-based device. Moreover, its devices exhibit excellent stability even at elevated doping concentrations, suggesting a notable reduction in intermolecular interactions caused by the steric pinene spacer.

Ultralow Roll-off Thermally Activated Delayed Fluorescent Light-Emitting Diodes Based on Furo[2,3-b]quinoxaline Emitters.

Herein, a Y-type compound (67dMeOTPA-FQ) and a T-type compound (58dMeOTPA-FQ) based on furo[2,3-b]quinoxaline were synthesized. The theory calculation shows the S1 and T1 of both compounds own a charge-transfer feature while their T2 states have a local excitation feature. The calculated kRISC(T2-S1) is one to 2 orders of magnitude larger than kRISC(T1-S1). Thus, the nonadiabatic spin-vibronic mechanism involved in the T2 state is suggested to be responsible for the thermally activated delayed fluorescence (TADF) feature. Meanwhile, when 2-methyl-9,10-bis(naphthalen-2-yl)anthracene is selected as host, the maximum luminance of the device based on 67dMeOTPA-FQ is up to 104215 cd·m-2, and the external quantum efficiency (EQE) keeps in the 8.2-8.0% range with the luminance changed from 55.0 cd·m-2 to 90000 cd·m-2, only 2.4% efficiency roll-off. As for 58dMeOTPA-FQ, a slightly lower EQE of 7.1-6.7% with the luminance range of 1-40000 cd·m-2 was achieved for orange-red emission. Both the reverse intersystem crossing (RISC) and triplet-triplet annihilation mechanisms are supposed to concurrently contribute to the utilization of triplet excitons and suppress the notorious efficiency roll-off observed in TADF-based devices.

Room-temperature efficient and tunable interlayer exciton emissions in WS2/WSe2 heterobilayers at high generation rates.

Transition metal dichalcogenide (TMDC) heterobilayers (HBs) have been intensively investigated lately because they offer novel platforms for the exploration of interlayer excitons (IXs). However, the potentials of IXs in TMDC HBs have not been fully studied as efficient and tunable emitters for both photoluminescence (PL) and electroluminescence (EL) at room temperature (RT). Also, the efficiencies of the PL and EL of IXs have not been carefully quantified. In this work, we demonstrate that IX in WS2/WSe2 HBs could serve as promising emitters at high generation rates due to its immunity to efficiency roll-off. Furthermore, by applying gate voltages to balance the electron and hole concentrations and to reinforce the built-in electric fields, high PL quantum yield (QY) and EL external quantum efficiency (EQE) of ∼0.48% and ∼0.11% were achieved at RT, respectively, with generation rates exceeding 1021 cm-2·s-1, which confirms the capabilities of IXs as efficient NIR light emitters by surpassing most of the intralayer emissions from TMDCs.

Highly Efficient, Low-Roll-Off, Red, "Hot Exciton" Organic Light-Emitting Diodes Promoted by the Heavy-Atom Effect of Selenium.

It is important to attain red hot exciton materials applicable in highly efficient organic light-emitting diodes with low-efficiency roll-off, but their development is restricted by the energy gap law. Herein, the sulfur atom was replaced by a heavier selenium atom based on benzothiadiazole to obtain a new benzoselenadiazole acceptor with a heavy atom effect and stronger electron-withdrawing ability. Two novel red hot exciton materials named BSe-DtBuTPA and BSe-2PhCz-d24 were designed and synthesized based on the benzoselenadiazole unit. Benefiting from the heavy-atom effect of selenium and the small ΔES1T2, both emitters exhibited ultrafast high-lying reverse intersystem crossing rate constants (7.00 × 107 and 1.17 × 107 s-1). The devices based on BSe-DtBuTPA and BSe-2PhCz-d24 demonstrated maximum external quantum efficiencies of 4.81 and 7.15% with emission peaks at 653 and 596 nm, respectively. The device based on deep-red BSe-DtBuTPA exhibited negligible efficiency roll-off of 18.5% at 10000 cd/m2.

Origin of the Efficiency Roll-off in Quantum Dot Light-Emitting Diodes: An Electrically Excited Transient Absorption Spectroscopy Study.

In situ characterizations of charge injection dynamics, equilibrated concentration, and electric field distributions shed light on the critical mechanisms of quantum dot light-emitting diodes (QD-LEDs). In this work, we developed electrically excited transient absorption spectroscopy, which can provide the above key information, to investigate the efficiency roll-off of QD-LEDs. We found that the average electron populations per QD are low when QD-LEDs exhibit efficiency roll-off, excluding Auger recombination as the main cause. We also revealed that the weak electrical field inside the QD layer under forward biases has a negligible impact on the efficiency. Interestingly, we found that as the voltage increases the electron concentration in the QD layer saturates at very low levels. When combined with the concomitant efficiency roll-off, we propose electron leakage is the main loss at elevated driving voltages. We further demonstrate that increasing the electron confinement potential with the ZnS shell enables us to efficiently mitigate the efficiency roll-off.

Molecular Engineering Towards Efficient Aggregation-Induced Delayed Fluorescence Luminogens as Emitters and Sensitizers for High-Performance Organic Light-Emitting Diodes.

Exploring efficient thermally-activated delayed fluorescence materials having maximum external quantum efficiencies (ηext,maxs) exceeding 30 % for organic light-emitting diodes (OLEDs) still remains challenging because it generally requires efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (ΦPL), and large optical out-coupling efficiency (Φout) simultaneously. Herein, two green aggregation-induced delayed fluorescence (AIDF) luminogens, named XTCz-2 and XTCz-3, are designed and constructed by using xanthone (XT) as electron acceptor and phenylcarbazole-substituted carbazole as donors. XTCz-2 and XTCz-3 exhibit distinguished advantages of high thermal stability (439-560 °C), excellent ΦPLs (84-88 %) and fast RISC rates (1.9×105-4.2×105 s-1), and prefer horizontal dipole orientation and thus have high Φouts. Consequently, they can achieve the state-of-the-art electroluminescence (EL) performances with ηext,maxs of up to 35.0 %. Moreover, XTCz-3 is selected as a sensitizer for sky-blue multi-resonance delayed fluorescence emitter in hyperfluorescence OLEDs, providing narrow EL spectra and excellent ηext,maxs of up to 33.8 % with low efficiency roll-offs. The splendid comprehensive performances demonstrate the significant application potential of these AIDF luminogens as both light-emitting materials and sensitizers for OLEDs.

Isomer engineering of triptycene-fused multi-resonance TADF emitters: Implications for controlling blue electroluminescence performance

Concurrently achieving high efficiency, narrowband emission, and low efficiency roll-off remains a challenging task for realizing stable blue organic light-emitting diodes (OLEDs) with high color purity. To tackle this issue, herein, two isomeric pairs of B,N-doped multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, namely, 2,3-Tp-BuDABNA-R and 3,4-Tp-BuDABNA-R (R=H, 3,5-Xyl), featuring triptycene (Tp) moieties fused into the 2,3- and 3,4-positions of the B,N core, respectively, are presented. Both isomeric forms exhibit blue emissions with near-unity photoluminescence (PL) quantum yields in their doped host films. Notably, the 3,4-Tp-fused emitter shows 6-fold faster reverse intersystem crossing (RISC) compared to the 2,3-Tp-fused emitter, albeit with PL spectral broadening. Along with a smaller activation energy of RISC, the stronger spin–orbit coupling between the singlet (S1) and triplet (T2) excited states of the 3,4-Tp-fused emitter is suggested to facilitate the RISC process. Furthermore, a high-frequency vibration in the Tp-fused benzene ring, accompanied by a large reorganization energy, is found to mainly contribute to the spectral broadening of the 3,4-Tp-fused emitter. Blue TADF-OLEDs based on each isomeric emitter similarly achieve a high maximum external quantum efficiency of ∼ 26%. Remarkably, the devices with the 3,4-isomer exhibit significantly reduced efficiency roll-offs compared to the 2,3-isomer, demonstrating that isomer engineering of MR emitters can have a distinct impact on controlling device performance.

Efficient Narrowband Multiple Resonance Thermally Activated Delayed Fluorescent Emitters with Alleviated Efficiency Roll-Off via Steric Shielding and Heavy Atom Effect

Efficient Narrowband Multiple Resonance Thermally Activated Delayed Fluorescent Emitters with Alleviated Efficiency Roll-Off via Steric Shielding and Heavy Atom Effect

White Organic Light-Emitting Diodes with External Quantum Efficiency of 20.77% at 10000cdm-2 and Ultra-High Color Stability by Controlling Exciton Distribution.

Although brightness and efficiency have been continually improved, the inability to achieve superior efficiency, color stability, and low-efficiency roll-off simultaneously in white organic light-emitting diodes (OLEDs) remains a knotty problem restricting the commercial application. In this paper, emission balance for two different horizontal orientation emitting molecules is maintained by using hole transport materials and bipolar host materials to control carriers' recombination and exciton diffusion. Impressively, the obtained devices exhibit extremely stable white emission with small chromaticity coordinates variation of (0.0023, 0.0078) over a wide brightness range from 1000 to 50000cdm-2. Meanwhile, the optimal white OLED realizes the power efficiency, current efficiency, and external quantum efficiency up to 70.68lmW-1, 85.53cdA-1, and 24.33%, respectively at the practical brightness of 1000cdm-2. Owing to reduced heterogeneous interfaces and broadening recombination region, this device exhibits a high EQE over 20% under high luminance of 10000cdm-2, demonstrating slight efficiency roll-off. The operating mechanism of the device is analyzed by versatile experimental and theoretical evidences, which concludes precise manipulation of charges and excitons is the key points to achieve these excellent performances. This work provides an effective strategy for the design of high-performance white OLEDs.

Thermally activated delayed fluorescence dendrimers with strategic dendrons to construct highly efficient solution-processed OLEDs

For the promising merits of the precise molecular structures and the satisfactory dissolvability, Thermally activated delayed fluorescence (TADF) dendrimers are turning into the attractive candidates for fully solution-processed organic light-emitting diodes (OLEDs). However, the rational design of TADF dendrimers with strategic dendrons for improving the efficiency and decreasing efficiency roll-off is still challenging. Here, two TADF dendrimers, (4S,5R)-2,4,5,6-tetrakis(4-((6-((9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)oxy)hexyl)oxy)-9H-carbazol-9-yl)isophthalonitrile (G-O-mCP) and (4S,5R)-2,4,5,6-tetrakis(4-((6-(3,5-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)phenoxy)hexyl)oxy)-9H-carbazol-9-yl)isophthalonitrile (G-O-DPBi) were developed and synthesized by integrating the peripheral strategic dendrons with the emissive core through nonconjugated alkoxy chains. Impressively, the peripheral dendron of G-O-mCP possessing the superior hole-transporting capability belongs to the electron-donor category and the peripheral dendron of G-O-DPBi with the admirable electron-transporting capability belongs to the electron-acceptor category. The elaborate regulation of the different dendrons with solubilizing groups is beneficial for restraining the aggregation-caused quenching (ACQ) effect and balancing the carrier transport capability. Consequently, the fully solution-processed device based on G-O-mCP realized the maximum external quantum efficiency (EQE) of 19.98 %. In addition, the formation of the interfacial exciplex endows the solution-processed device with a extremely low turn-on voltage (Von) of 2.7 V and a high power efficiency (PE) of 43.80 lm/W, which is among the highest power efficiency of the reported TADF-OLEDs.

Peripheral carbazole units-decorated MR emitter containing B−N covalent bond for highly efficient green OLEDs with low roll-off

Boron−nitrogen doped multiple resonance (BN-MR) emitters, characterized by B−N covalent bonds, offer distinctive advantages as pivotal building blocks for facile access to novel MR emitters featuring narrowband spectra and high efficiency. However, there remains a scarcity of exploration concerning synthetic methods and structural derivations to expand the library of novel BN-MR emitters. Herein, we present the synthesis of a BN-MR emitter, tCz[B−N]N, through a one-pot borylation reaction directed by the amine group, achieving an impressive yield of 94%. The emitter is decorated by incorporating two 3,6-di-t-butylcarbazole (tCz) units into a B−N covalent bond doped BN-MR parent molecule via para-C−π−D and para-N−π−D conjugations. This peripheral decoration strategy enhances the reverse intersystem crossing process and shifts the emission band towards the pure green region, peaking at 526 nm with a narrowband full-width at half maximum (FWHM) of 41 nm. Consequently, organic light emitting diodes (OLEDs) employing this emitter achieved a maximum external quantum efficiency (EQEmax) value of 27.7%, with minimal efficiency roll-off. Even at a practical luminance of 1000 cd∙m−2, the device maintains a high EQE value of 24.6%.

Twisted Structure and Multiple Charge-Transfer Channels Endow Thermally Activated Delayed Fluorescence Devices with Small Efficiency Roll-Off and Low Concentration Dependence.

Despite the rapid development of thermally activated delayed fluorescent (TADF) materials, developing organic light-emitting diodes (OLEDs) with small efficiency roll-off remains a formidable challenge. Herein, we have designed a TADF molecule (mClSFO) based on the spiro fluorene skeleton. The highly twisted structure and multiple charge-transfer channels effectively suppress aggregation-caused quenching (ACQ) and endow mClSFO with excellent exciton dynamic properties to reduce efficiency roll-off. Fast radiative rate (kr) and rapid reverse intersystem crossing (RISC) rate (kRISC) of 1.6 × 107 s-1 and 1.07 × 106 s-1, respectively, are obtained in mClSFO. As a result, OLEDs based on mClSFO obtain impressive maximum external quantum efficiency (EQEmax) exceeding 20% across a wide doping concentration range of 10-60 wt%. 30 wt% doped OLED exhibits an EQEmax of 23.1% with a small efficiency roll-off, maintaining an EQE of 18.6% at 1000 cd m-2. The small efficiency roll-off and low concentration dependence observed in the TADF emitter underscore its significant potential.

Crafting host materials for narrowband blue OLEDs with low efficiency roll-off by the medium-ring strategy

Crafting host materials for narrowband blue OLEDs with low efficiency roll-off by the medium-ring strategy