Electroluminescence Performance Research Articles

Pure room temperature phosphorescence emission in nondoped OLEDs: adjustable oxidation states and excited-state modulation.

Purely organic room temperature phosphorescence (RTP) materials are a new kind of triplet emitter, which can harvest both singlet and triplet excitons in theory, thus showing great application potential for organic light-emitting diodes (OLEDs). However, nondoped OLEDs based on RTP emitters have been rarely explored owing to challenges in realizing efficient phosphorescence in single-component systems. Herein, three donor-acceptor-type luminogens were designed and synthesized in which phenothiazine, with different oxidation degrees, acted as the electron donor and acetophenone as the acceptor. The adjustable oxidation states of phenothiazine enabled the modulation of excited states, facilitating the transition from dual RTP and thermally activated delayed fluorescence emissions to pure RTP. A nondoped OLED device was then fabricated based on the pure RTP emitter, achieving a high exciton utilization efficiency of 86%, clearly demonstrating the enhancement of electroluminescence performance through RTP properties.

Robust Sandwich‐Structured Thermally Activated Delayed Fluorescence Molecules Utilizing 11,12‐Dihydroindolo[2,3‐a]carbazole as Bridge

AbstractConstructing folded molecular structures is emerging as a promising strategy to develop efficient thermally activated delayed fluorescence (TADF) materials. Most folded TADF materials have V‐shaped configurations formed by donors and acceptors linked on carbazole or fluorene bridges. In this work, a facile molecular design strategy is proposed for exploring sandwich‐structured molecules, and a series of novel and robust TADF materials with regular U‐shaped sandwich conformations are constructed by using 11,12‐dihydroindolo[2,3‐a]carbazole as bridge, xanthone as acceptor, and dibenzothiophene, dibenzofuran, 9‐phenylcarbazole and indolo[3,2,1‐JK]carbazole as donors. They hold outstanding thermal stability with ultrahigh decomposition temperatures (556–563 °C), and exhibit fast delayed fluorescence and excellent photoluminescence quantum efficiencies (86 %–97 %). The regular and close stacking of acceptor and donors results in rigidified molecular structures with efficient through‐space interaction, which are conducive to suppressing intramolecular motion and reducing reorganized excited‐state energy. The organic light‐emitting diodes (OLEDs) using them as emitters exhibit excellent electroluminescence performances, with maximum external quantum efficiencies of up to 30.6 %, which is a leading value for the OLEDs based on folded TADF emitters. These results demonstrate the proposed strategy of employing 11,12‐dihydroindolo[2,3‐a]carbazole as bridge for planar donors and acceptors to construct efficient folded TADF materials is applicable.

High‐Efficiency Deep‐Blue Solution‐Processed Organic Light‐Emitting Diodes Using Carbazole Dendrons Modified Hybridized Local and Charge‐Transfer Emitters

In the pursuit of efficient and cost‐effective organic light‐emitting diodes (OLEDs), the development of solution‐processed hybridized local and charge transfer (HLCT) emitters presents a promising approach. HLCT materials uniquely integrate the advantages of both singlet and triplet excitons, surpassing the traditional spin statistical limit of 25% while offering high photoluminescence efficiency and balanced charge transport properties. Herein, we report the synthesis and characterization of two new deep blue, solution‐processable HLCT fluorophores, G1FTPI and G2FTPI. These compounds incorporate fluorenyl carbazole dendron units into the HLCT luminogenic triphenylamine‐phenanthroimidazole (TPI) molecule. Their HLCT and photoluminescence (PL) properties were experimentally and theoretically investigated using solvation effects and density functional theory (DFT) calculations. The molecules exhibit deep blue emission with a high solid‐state fluorescence quantum yield, good solution‐processed film‐forming quality, and high hole mobility values of 2.18 – 2.61 × 10‐6 cm2 V‐1s‐1. Both compounds were successfully employed as non‐doped emissive layers in solution‐processed OLEDs, demonstrating excellent electroluminescent (EL) performance. Notably, the G2FTPI‐based device emitted a deep blue light at 432 nm with CIE coordinates of (0.158, 0.098) and achieved a a maximum current efficiency (CEmax) of 3.13 cd A‐1 and a maximum external quantum efficiency (EQEmax) of 5.30%.

Dual Sulfone-Bridged Triphenylamine Heteroaromatics for High-Performance Blue Organic Electroluminescence.

Polycyclic heteroaromatics (PHAs) are a highly versatile class of functional materials, especially applicable as efficient luminophores in organic light-emitting diodes (OLEDs). Those constructed by tethered phenyl surrounding the main group center attract extensive attention due to their excellent OLED device performance. However, the development of such a class of emitters is often limited to boron, nitrogen-doped π-conjugated heterocycles. Herein, we proposed a novel kind of blue emitter by constructing a donor-acceptor molecular configuration, utilizing a dual sulfone-bridged triphenylamine (BTPO) core and mono/di-diphenylamine (DPA) substituents. The twisted D-A molecular structures and appropriate donor strength facilitate the effective separation of natural transition orbitals, endowing the emitters with charge-transfer dominant hybridized local and charge-transfer characteristics for the excited states. Both BTPO-DPA and BTPO-2DPA own small S1-T1 splitting energy, thus demonstrating blue thermally activated delayed fluorescence. The more symmetrical structure and enhanced CT features brought by additional DPA moiety confer BTPO-2DPA with a shorter delayed fluorescence lifetime, a higher fluorescence quantum yield and narrower emission. Therefore, BTPO-2DPA based OLED devices exhibit superior blue electroluminescence performance, with external quantum efficiencies reaching 12.31 %.

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.

Investigation into characteristics of blue iridium (III) pyrimidine complexes with suppressed shoulder peak emission

Three novel blue-emitting iridium (III) complexes (Ir1, Ir2 and Ir3), embedding pyrimidine group have been synthesized. Density functional theory (DFT) calculations demonstrate that pyrimidine complex series occupy less component of triplet ligand centered (3LC) could restrain the vibronic shoulders comparing to the pyridine series. As for Ir1, the relative intensity of shoulder in electroluminescence (EL) spectrum is ∼0.6 of the intensity for dominate peak, relevant full width at half maximum (FWHM) is merely 47 nm. Such parameter gradually zooms out from trifluoro-to monofluoro-substitution in EL spectra. Furthermore, three complexes could obtain efficient EL performance, in which, devices based on Ir3 achieve the peak external quantum efficiency (EQE) of 28.9 %. In addition, the photoelectric properties of the three complexes display a certain regularity and this work indeed provides a strategy to modulate shoulder peak emission for blue phosphorescence.

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%).

Optimizing QLED Performance and Stability via the Surface Modification of PEDOT:PSS Experimental Insights and DFT Calculations.

The presence of the acidic and weak ionic conductor polystyrenesulfonate (PSS) in poly(3,4-ethylenedioxythiophene:PSS (PEDOT:PSS) leads to degradation and limits the charge transfer within quantum dot light-emitting diodes (QLEDs). Two-step solvent treatment resulted in a 40% reduction of PSS, which could be attributed to ethylene glycol (EG) attenuating the ionic interactions between PSS and PEDOT via interacting with PSS through hydrogen bonding. Methanol dissolved the predominant PSS and EG from the surface. The redshift of the peak representing the symmetrical vibration of Cα═Cβ in the Raman spectrum confirmed the conformation of benzoid structure to quinoid structure after the surface treatment. This conformation was attributed to the extension of the conjugation length and the reduction of the energy barrier within the PEDOT chain. This resulted in the improved conductivity and charge hopping of the PEDOT:PSS, which was also proven using density functional theory (DFT) calculations. Reducing the insulating and acidic PSS improved the electroluminescence performance and extended the operational lifetime of the QLEDs. The tris(dimethylamino)phosphine-based InP QLEDs exhibited an external quantum efficiency (EQE) of 6.4%, that value is comparable to those of tris(trimethylsilyl)phosphine-based QLEDs, and operational lifetime (T50) of 125.6 h.

Challenges and Opportunities of Stretchable Electroluminescent Devices and Displays

AbstractStretchable electroluminescent (EL) devices have been studied for more than 10 years, and their EL and mechanical performance, such as brightness, efficiency, stretchability, and stretching stability show significant improvement. Stretchable display is considered to be the most promising next‐generation display technology, which has broad application prospects in various emerging fields, such as deformable displays, electronic skins, and biomedical devices, and it attracts much attention from worldwide manufacturers. However, no commercial application cases for stretchable displays are reported to date. There are many reasons for this situation, including deficient device performance, immature preparation process, and diversiform device structures and materials. On the other hand, although researchers devise a number of possible applications for stretchable EL devices, it is still controversial what unique and interesting functions stretchable displays can play in these scenarios, and whether they can be replaced by flexible, bendable, and rollable devices with better EL and mechanical performance. Therefore, the future of stretchable displays is full of both opportunities and challenges. In response to the above concerns, this review summarizes the characteristics, problems, and possible solutions of various types of stretchable EL devices, aiming to promote their development and application.

Structure development and photoluminescence properties of high-purity greenish-yellow emitting LaGaO3:Dy3+ phosphors

This study investigates the structure development and photoluminescence properties of high-purity greenish-yellow emitting LaGaO3 phosphors doped with dysprosium ions Dy3+. The phosphors were synthesized using a high-temperature solid-state reaction method. X-ray diffraction (XRD) confirmed the formation of a single-phase LaGaO3:Dy3+ structure and scanning electron microscopy (SEM) revealed uniform particle size and morphology. The phosphor exhibits the capability of being efficiently excited by ultraviolet 350 nm exhibiting the characteristic greenish-yellow emission with two strong peaks at 478 nm (cyan) and 572 nm (yellow), ascribing to the transitions of 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2, respectively. The electronic transition mechanism is direct exchange interaction between activators, contributing to the concentration quenching. The phosphor was able to retain 67 % of its intensity without significant color change at an elevated temperature up to 25 °C to 150 °C. The CIE chromaticity coordinates of the emitted light were calculated and found in the desired greenish-yellow region. The electroluminescence performance of the LaGaO3:0.06Dy3+ phosphor was investigated using 365 nm LED chips to check their applicability in the field of white light-emitting diodes (WLEDs). The synthesized phosphors demonstrated high thermal stability, making them suitable for white-LED applications. The study highlights the potential of LaGaO3:Dy3+ phosphors in enhancing white-LEDs' color quality and efficiency.

Defect passivation with spectral shift for improved efficiency and stability of quasi-2D blue perovskite light emitting diodes

Metal halide perovskites are emerging as promising materials for next-generation light-emitting diodes (LEDs), owing to their cost-effectiveness, facile manufacturing, and excellent optoelectronic properties including a narrow emission-line full width at half maximum, high photoluminescence quantum yield, and spectral stability. However, blue-emission perovskite LEDs (PeLEDs) exhibit perovskite phase segregation and halogen ion migration during operation, attributed to crystal defects, halogen vacancies, and phase instability of mixed-halide perovskites. These defects compromise the spectral stability of blue-emitting PeLEDs, resulting in an undesirable color shift toward longer wavelengths. In this study, we introduce a dynamic treatment with phenylphosphonic dichloride (PPOCl2) to address these challenges and achieve blue-emitting PeLEDs. The chloride in PPOCl2 penetrates the quasi-2D sky-blue perovskite, reducing halide defects. Meanwhile, the phosphonic group forms covalent bonds with undercoordinated Pb on the perovskites, effectively passivating defects. PPOCl2-treated perovskite exhibits a spectral blue shift toward a deep-blue wavelength (≤467 nm), enhancing electroluminescence performance. The quasi-2D PPOCl2-treated PeLEDs achieve a maximum external quantum efficiency of 2.31 % with an emission peak at 488 nm. Dynamic treatment with PPOCl2 shows the potential to improve the performance and stability of blue emission perovskite-based LEDs, providing a spectral shift mechanism to achieve deep-blue emission in PeLEDs.

Ultrafast dynamics and electroluminescence performance of a novel tetraphenylethylene-based aggregation induced emission luminogen

Efficient emission and confined molecular conjugation in aggregated/solid form, are two beneficial characteristics of blue organic light-emitting materials (OLEMs). Aggregation-induced emission molecules (AIEgens) have evolved as auspicious OLEMs that possess these properties. Herein, we discover an innovative sky-blue tetraphenylethene-based AIEgen, 4′,4‴-(1-phenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole-4,5-diyl)bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine) (2TPA-ITPE), with high thermal stability. By employing this AIEgen as a non-doped emitter, the constructed organic light-emitting diode demonstrates a maximum external quantum efficiency of 3.35 % with a power efficiency of 4.1 lm/W and high luminescence of 15896.92 cd/m2 2TPA-ITPE exhibits a weak donor-acceptor character. Moreover, to probe the photo-physical nature of this AIEgen, its excited-state dynamics are examined through ultrafast transient absorption spectroscopy (TAS). The TAS results of this OLEM unveil the generation of a resonant structure species owing to the elongation of C=C double bond. The excited state dynamics reveal that 2TPA-ITPE undergoes three processes after excitation (internal conversion, C=C elongation and vibrational cooling). These findings offer innovative insights into the ultrafast mechanisms underlying excited TPE-based OLEMs.

Deep blue high-efficiency solution-processed triplet-triplet annihilation organic light-emitting diodes using bis(8-carbazol-N-yl)fluorene- and benzonitrile-modified anthracene/chrysene fluorescent emitters

Triplet-triplet annihilation (TTA) emitters can effectively utilize non-radiative triplet excitons through the interaction of low triplet energy excitons to produce high energy singlet excitons, but they are mostly restricted by their multilayered device structure fabricated using layer-by-layer thermal vacuum evaporation. It is a great challenge to develop, for the first time, efficient solution-processed non-doped TTA organic light-emitting diodes (OLEDs). In this study, two solution-processable blue emissive TTA molecules (FAnCN and FCsCN) bearing (anthracen-9-yl)benzonitrile (AnCN) and (chrysen-6-yl)benzonitrile (CsCN) as TTA emissive cores modified with 9,9′-bis(8-(carbazole-N-yl)octyl)fluorene (F) are designed and synthesized, respectively. The experimental and theoretical studies reveal that both molecules exhibit deep blue emissions, amorphous morphology with good thermal stability, high-quality solution-cast thin films, decent hole mobility, high-lying HOMO levels (∼-5.45 eV), and suitable lowest singlet (S1)/triplet (T1) excited states (2T1 > S1) for TTA process. FAnCN and FCsCN are successfully employed as solution-processed non-doped emissive layers (EML) in simple structured TTA OLEDs. These devices show intense blue emissions, low turn-on voltages (∼3.6 V), excellent electroluminescent (EL) performances (EQEmax = 5.47–6.84 % and LEmax = 5.66–5.83 cd/A), and TTA characteristics. Especially, FCsCN-based TTA OLED emits deep blue EL emission peaked at 435 nm with a high EQEmax of 6.84 %. This work not only presents a new strategic design for the preparation of solution-processable TTA emitter, but also further ratifies that the TTA mechanism can also be applicable in solution-processed OLEDs.

Enhancing Carrier Behavior via Controlled Molecular Film Formation Engineering Leads to Significant Improvement in Electroluminescence

The quality of organic thin films critically influences carrier dynamics in organic semiconductors. In neat/doped films, even tiny voids can be penetrated by water or oxygen molecules to create charge‐trap states called water/oxygen‐induced traps that significantly hinder carrier mobility. While the water/oxygen‐induced traps in non‐doped films and crystalline states have been investigated comprehensively, there is a lack of thorough examination regarding their properties in the doped state. Therefore, there is a high demand for a molecular design strategy that effectively modulates the molecular stacking behavior in doped films and practical devices and enhances the quality of these films. Herein, we proposed a versatile molecular design principle that enables the formation of "nano‐cluster" structures on both the surface and interior of doped films in target molecule 10‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐1'‐(4‐fluorophenyl)‐10H‐spiro[acridine‐9,9'‐xanthene] (DspiroO‐F‐TRZ), which is modified with a fluorophenyl group. These "nano‐cluster" structures exhibit more uniform shapes within doped films and effectively reduce defective state densities while enhancing carrier injection and transport properties, ultimately improving device performance. Notably, TADF‐OLED based on DspiroO‐F‐TRZ demonstrates nearly twice as much efficiency as its control counterpart due to contributions from 'nano‐cluster' structure enhancements toward improved electroluminescence performance.

Enhancing Carrier Behavior via Controlled Molecular Film Formation Engineering Leads to Significant Improvement in Electroluminescence.

The quality of organic thin films critically influences carrier dynamics in organic semiconductors. In neat/doped films, even tiny voids can be penetrated by water or oxygen molecules to create charge-trap states called water/oxygen-induced traps that significantly hinder carrier mobility. While the water/oxygen-induced traps in non-doped films and crystalline states have been investigated comprehensively, there is a lack of thorough examination regarding their properties in the doped state. Therefore, there is a high demand for a molecular design strategy that effectively modulates the molecular stacking behavior in doped films and practical devices and enhances the quality of these films. Herein, we proposed a versatile molecular design principle that enables the formation of "nano-cluster" structures on both the surface and interior of doped films in target molecule 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1'-(4-fluorophenyl)-10H-spiro[acridine-9,9'-xanthene] (DspiroO-F-TRZ), which is modified with a fluorophenyl group. These "nano-cluster" structures exhibit more uniform shapes within doped films and effectively reduce defective state densities while enhancing carrier injection and transport properties, ultimately improving device performance. Notably, TADF-OLED based on DspiroO-F-TRZ demonstrates nearly twice as much efficiency as its control counterpart due to contributions from 'nano-cluster' structure enhancements toward improved electroluminescence performance.

Recent Advances in Perovskite-Based Flexible Electroluminescent Devices.

Perovskite-based flexible electroluminescent (EL) devices are emerging as promising candidates in the display field due to their exceptional optoelectronic properties and potential for cost-effective production. However, simultaneously achieving high EL performance, excellent flexibility and stretchability, robust mechanical strength, and diverse applications remains a significant challenge. In this review, we provide a comprehensive overview of the latest developments in perovskite-based flexible EL devices, covering both direct-current (DC) and alternating-current (AC) electroluminescent formats. Our discussion encompasses the materials, working principles, device architectures, failure mechanisms, optimization strategies, and practical applications. Through this review, we aim to deepen our understanding of the current challenges and future directions of perovskite-based flexible light-emitting technologies, hoping to facilitate their potential commercial applications.

Increase the inversion degree in Er-doped MgGa2O4 spinel nanofilms to obtain strong electroluminescence

The Ga2O3/MgO/Er2O3 nanolaminates are fabricated by atomic layer deposition and crystallized into Er-doped MgGa2O4 spinel (MGS:Er) nanofilms after annealing, with their electroluminescence (EL) performance characterized. The annealing above 600 °C achieves the polycrystalline spinel nanofilm, and the crystallization is promoted by the higher annealing temperature and Ga2O3/MgO ratio. The dopant Er3+ ions preferably substitute into the octahedron sites occupied by Ga3+ ions in ordinary spinel and Mg2+ in anti-spinel lattice, while the inversion degree is confirmed to increase with the reduction of Ga2O3/MgO ratio and annealing temperature, resulting the relatively enhanced secondary EL at 1542 nm. This perturbation by Er3+-substitution into anti-spinel sites improves the emission intensity and excitation efficiencies, the main EL emission peaking at 1531 nm from the optimal MGS:Er device exhibits the excitation efficiency reaching 5.8 %, with the enhanced electrical injection realizing the maximum EL intensity above 17.3 mW/cm2. The fluorescence lifetime of these MGS:Er devices is established in the range of 371–760 μs, which decreases mainly with the Er concentrations. The prototype device using the near-stoichiometric Ga2O3/MgO ratio shows the operation time of 1.12 × 105 s. This work explores the fabrication of Si-based spinel nanofilms with designed composition and special microstructure, and their practical application in optoelectronics.

Hybridization of Long‐Range and Short‐Range Charger Transfer Enables Efficient Thermally Activated Delayed Fluorescent and Hyperfluorescent OLEDs

AbstractExploring possible strategy to balance the radiative decay and reverse intersystem crossing (RISC) are of great significance in thermally activated delayed fluorescent (TADF) emitters for enabling both high efficiency and low efficiency roll‐off. Herein, by introducing a multi‐resonance (MR) type acceptor, two TADF emitters featuring hybridized short‐range charge transfer (SRCT) and long‐rang charge transfer (LRCT) are designed. LRCT‐dominate photoluminescent nature is maintained with a short delayed lifetime of 1.5–3.4 µs. While the incorporated SRCT accelerates the radiative decay. Consequently, both emitters demonstrate remarkable electroluminescent performance with maximum EQE (EQEmax) of 40.8% for IT‐BO and 39.0% for IA‐BO, respectively, along with ultra‐high luminance of up to 120 000 cd m−2. More encouragingly, both emitters can enable small efficiency roll‐off with a minimum value of only 4%, highlighting the significance of enhanced kr in suppressing the roll‐off. Moreover, when served as sensitizers, both compounds can also endow excellent performances with EQEmax ≈38%. This work presents an innovative approach by hybridizing SRCT and LRCT for developing practical TADF emitters.

Robust Sandwich-Structured Thermally Activated Delayed Fluorescence Molecules Utilizing 11,12-Dihydroindolo[2,3-a]carbazole as Bridge.

Constructing folded molecular structures is emerging as a promising strategy to develop efficient thermally activated delayed fluorescence (TADF) materials. Most folded TADF materials have V-shaped configurations formed by donors and acceptors linked on carbazole or fluorene bridges. In this work, a facile molecular design strategy is proposed for exploring sandwich-structured molecules, and a series of novel and robust TADF materials with regular U-shaped sandwich conformations are constructed by using 11,12-dihydroindolo[2,3-a]carbazole as bridge, xanthone as acceptor, and dibenzothiophene, dibenzofuran, 9-phenylcarbazole and indolo[3,2,1-JK]carbazole as donors. They hold outstanding thermal stability with ultrahigh decomposition temperatures (556-563 oC), and exhibit fast delayed fluorescence and excellent photoluminescence quantum efficiencies (86%-97%). The regular and close stacking of acceptor and donors results in rigidified molecular structures with efficient through-space interaction, which are conducive to suppressing intramolecular motion and reducing reorganized excited-state energy. The organic light-emitting diodes (OLEDs) using them as emitters exhibit excellent electroluminescence performances, with maximum external quantum efficiencies of up to 30.6%, which is a leading value for the OLEDs based on folded TADF emitters. These results demonstrate the proposed strategy of employing 11,12-dihydroindolo[2,3-a]carbazole as bridge for planar donors and acceptors to construct efficient folded TADF materials is applicable.

Dicyanoimidazole-based organic small molecule fluorophores: Synthesis and luminescent properties

Four new organic D-π-A fluorophores consisting of different electron donors (carbazole and diphenylamine) and acceptors (4,5-dicyanoimidazole and 5,6-dicyanobenzo[d]imidazole), i.e., DCI-Cz, DCI-TPA, DCBI-Cz and DCBI-TPA, were synthesized to obtain deep-blue organic light-emitting diodes (OLEDs). These compounds exhibited good solubility and excellent thermal stability as well as strong deep-blue and blue emission in solution and solid states. With mCP (1,3-di(9H-carbazol-9-yl)benzene) as the host material, the doped devices with a configuration of ITO/PEDOT:PSS (30 nm)/mCP:dopant (x wt%) (25 nm)/TPBi (35 nm)/Liq (2 nm)/Al (150 nm) were fabricated by solution-processing the emitting layers to evaluate their electroluminescence (EL) performances. The devices using the blend of mCP with DCI-TPA as emitting layers exhibited the best electroluminescent performance with a maximum external quantum efficiency (EQEmax) of 4.39 % and a maximum current efficiency of 3.04 cd/A.