Efficient Electroluminescent Devices Research Articles

High-performance fluorescent organic electroluminescent devices benefit from sensitization of thermally activated delayed fluorescence

Efficient fluorescent electroluminescent devices were achieved by exploiting a thermally activated fluorescent material (3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone (DTCBPy) as a sensitizer.

Efficient red electroluminescent devices with very low operation voltage by utilizing hole and electron transport materials as the host

In this work, we report high performance red electroluminescent (EL) devices with very low operation voltage based on iridium complex iridium(Ш)bis(2-phenylquinoly-N,C2′)-dipivaloylmethane (PQ2Ir(dpm)) by utilizing hole transport material di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) and electron transport material 1,3,5-tris(6-(3-(pyridine-3-yl)phenyl)pyridine-2-yl) (Tm3PyP26PyB) as host materials. A series of simplified devices with single or double light-emitting layer(s) (EML(s)) were fabricated and examined. The advantages of this design strategy can be summarized as follows: (i) easier injection and transport of holes and electrons, (ii) broadening recombination zone, and (iii) balanced carriers' distribution. Finally, efficient red EL device with highest current efficiency, power efficiency, external quantum efficiency (EQE) and brightness up to 40.59 cd/A, 47.20 lm/W, 14.7% and 34280 cd/m2, respectively, was realized. Excitedly, the operation voltage was only 2.4 V at the brightness of 1 cd/m2. Even at high brightness of 1000 cd/m2 (3.5 V), current efficiency as high as 36.34 cd/A (EQE = 13.1%) can still be retained by the same device. Devices with such low operation voltage are of great significance for the industrialization of organic light-emitting diodes (OLEDs).

Asymmetric Thermally Activated Delayed Fluorescence Materials With Aggregation-Induced Emission for High-Efficiency Organic Light-Emitting Diodes.

The exploitation of thermally activated delayed fluorescence (TADF) emitters with aggregation-induced emission is highly prerequisite for the construction of highly efficient electroluminescent devices in materials science. Herein, two asymmetric TADF emitters of SFCOCz and SFCODPAC with charming aggregation-induced emission are expediently designed and prepared based on highly twisted strong electron-withdrawing acceptor (A) of sulfurafluorene (SF)-modified ketone (CO) and arylamine donor (D) in D1−A–D2 architecture by simple synthetic procedure in high yields. High photoluminescence quantum yields up to 73% and small singlet–triplet splitting of 0.03 eV; short exciton lifetimes are obtained in the resultant molecules. Strikingly, efficient non-doped and doped TADF organic light-emitting diodes (OLEDs) facilitated by these emitters show high luminance of 5,598 and 11,595 cd m−2, current efficiencies (CEs) of 16.8 and 35.6 cd/A, power efficiencies (PEs) of 9.1 and 29.8 lm/W, and external quantum efficiencies (EQEs) of 7.5 and 15.9%, respectively. This work furnishes a concrete instance in exploring efficient TADF emitter, which is highly conducive and encouraging in stimulating the development of TADF OLEDs with high brightness and excellent efficiencies simultaneously.

Blue thermally activated delayed fluorescence emitters based on a constructing strategy with diversed donors and oxadiazole acceptor and their efficient electroluminescent devices

A series of highly efficient blue emission compounds with D-A-D molecular structures were designed and synthesized, in which oxadiazole group was used as electronic acceptor (A), and carbazole/acridine groups as electronic donor (D). It is found that the compounds containing acridine both had small energy splitting between singlet excited state (S1) and triplet excited state (T1) (that is, ΔEST) and thus showed distinct thermally activated delayed fluorescence (TADF) characteristics. Moreover, they both had high photoluminescent quantum yields (PLQY) even up to 93%. In comparison, the compound only containing carbazole just showed prompt emission but no delayed fluorescence. The blue organic light-emitting diodes (OLEDs) using the TADF compounds as emitters achieved a high maximum external quantum efficiencies (EQE) up to 22.3% with Commission Internationale de L'e'clairage (CIE) coordinates of (0.16, 0.24). Triplet excitons were efficiently utilized in the device. The further optimized OLEDs achieved more outstanding comprehensive performance.

Highly efficient white electroluminescent devices with hybrid double emitting layers of quantum dots and phosphorescent molecules.

Hybrid quantum dot light-emitting diodes (QLEDs) with double emitting layers (EMLs) were developed to achieve highly efficient white emission. The first emitting layer comprised blue and green quantum dots that were deposited by spin-coating, and the second emitting layer comprised red phosphorescent organic molecules that were deposited by thermal evaporation without any buffer layer. These unique trichromatic devices showed three distinct electroluminescent (EL) peaks with similar intensities at 15 V and the variation of the EL spectra with applied voltage was investigated systematically. These hybrid QLEDs for white emission led to high device performance with a maximum luminance of 20 453 cd m-2 and an external quantum efficiency of 9.19%. These results indicate that the unique design of double EMLs is promising for achieving bright and efficient white devices with a high color rendering index.

Organic-Inorganic Hybrid Passivation Enables Perovskite QLEDs with an EQE of 16.48.

Perovskite quantum dots (QDs) with high photoluminescence quantum yields (PLQYs) and narrow emission peak hold promise for next-generation flexible and high-definition displays. However, perovskite QD films often suffer from low PLQYs due to the dynamic characteristics between the QD's surface and organic ligands and inefficient electrical transportation resulting from long hydrocarbon organic ligands as highly insulating barrier, which impair the ensuing device performance. Here, a general organic-inorganic hybrid ligand (OIHL) strategy is reported on to passivate perovskite QDs for highly efficient electroluminescent devices. Films based on QDs through OIHLs exhibit enhanced radiative recombination and effective electrical transportation properties compared to the primal QDs. After the OIHL passivation, QD-based light-emitting diodes (QLEDs) exhibit a maximum peak external quantum efficiency (EQE) of 16.48%, which is the most efficient electroluminescent device in the field of perovskite-based LEDs up to date. The proposed OIHL passivation strategy positions perovskite QDs as an extremely promising prospect in future applications of high-definition displays, high-quality lightings, as well as solar cells.

Highly efficient green organic light-emitting devices based on terbium complex by employing hole block material as host

In this work, efficient green electroluminescent (EL) devices with simplified device structure were prepared by doping trivalent terbium complex Tb(PMIP)3 into hole block material TmPyPB. The high triplet energy of TmPyPB helps to confine excitons within light-emitting layer, while the electron transport characteristic of TmPyPB facilitates the balance of carriers on Tb(PMIP)3 molecules. By optimizing the doping concentration of Tb(PMIP)3 and the thickness of each functional layer, highly efficient green EL device with the structure of ITO/MoO3 (3 nm) /TAPC (50 nm) /Tb(PMIP)3 (30 wt%):TmPyPB (25 nm) /TmPyPB (60 nm) /LiF (1 nm) /Al (100 nm) displayed pure Tb3+ characteristic emission with maximum current efficiency, power efficiency and brightness up to 47.24 cd/A (external quantum efficiency (EQE) of 14.4%), 43.63 lm/W and 1694 cd/m2, respectively. At certain brightness of 100 cd/m2, the device still maintained a current efficiency of 19.96 cd/A (EQE=6.1%). Such a device design strategy helps to improve the EL performances of Tb(PMIP)3 and to simplify device fabrication processes, thus reduce the fabrication cost.

2.2: Alcohol‐soluble quantum dots for photoluminescent and electroluminescent applications

Traditional solution synthesized colloidal quantum dots (QDs) capped by long‐chain lipophilic ligands can be well dispersed in non‐polar organic solvents, however they are not suitable for the integration in thin film devices as well as polymer matrix for further applications. We have developed general and versatile ligand exchange routes to replace the long‐chain methylterminated oleylamine (OLA) on QDs' surface with short ‐chain hydroxyl‐terminated 6‐mercaptohexanol (MCH). Such ligand exchange strategy not only altered the solubility of the QDs from conventional non‐polar solvents to polar alcoholic solvents, which meet the requirement for green manufacturing, but also enhanced their solution processability, which is a crucial factor for the industrial application in display and lighting. High efficient electroluminescent device was realized using the alcohol‐soluble QDs, the improvement of device performance is ascribed to decreased energy barriers and increased charge injection rate between charge transport layer and the emitting layer by the substitution of short‐chain ligands. Furthermore, the alcohol‐soluble QDs can be well embedded into silica via a controlled sol‐gel process for on‐chip type LED and a luminous efficiency up to 80.25 lm/W was achieved.

A dibenzo[a,c]phenazine-11,12-dicarbonitrile (DBPzDCN) acceptor based thermally activated delayed fluorescent compound for efficient near-infrared electroluminescent devices

A high external quantum efficiency of 7.68% was achieved in a near-infrared organic light-emitting diode with a novel TADF molecule.

Novel hole transport materials based on triarylamine/naphtho[2,1-b]benzofunan for efficient green electroluminescent device

Two hole transport materials, N-([1,1'-biphenyl]-4-yl)-9,9-dimethyl-N-(4-(naphtho[2,1-b]benzofuran-6-yl)phenyl)-9H-fluoren-2-amine (DFA) and N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9-dimethyl-7-(naphtho[2,1-b]benzofuran-6-yl)-N-phenyl-9H-fluoren-2-amine (TFA) were designed, synthesized and fully characterized. The photophysical and thermal properties of these two compounds were investigated by UV–vis absorption spectra, photoluminescence spectra, thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC), which indicated that DFA and TFA would be efficient hole transport materials due to their proper HOMO energy levels and excellent thermal stability. Then, green OLEDs with DFA and TFA as hole transport layer, respectively, were fabricated, NPB-based OLEDs was also prepared for comparison. It turned out that DFA- and TFA-based devices exhibited higher efficiencies than that of NPB-based device, and TFA-based device showed the best performances with current efficiency, power efficiency and external quantum efficiency of 41.68 cd/A, 32.04 lm/W and 12.04%, respectively.

A novel electroluminescent device based on a reduced graphene oxide wrapped phosphor (ZnS:Cu,Al) and hexagonal-boron nitride for high-performance luminescence.

Reduced graphene oxide (rGO) has recently emerged as a very promising family of exotic carbon material with augmented performance in electronic and optoelectronic devices. Herein, we report an efficient and novel inorganic electroluminescent device geometry, where a new phosphor composite, reduced graphene oxide wrapped ZnS:Cu,Al, acts as an active emitting layer and an exfoliated hexagonal boron nitride (h-BN) as a dielectric layer. The roles of rGO in the active layer as a conductive support and local electric field enhancing agent are attributed to its wrinkles being unraveled compared with other carbon exotic nano-forms such as carbon nanotubes, graphite, charcoal and activated carbon, which significantly improves the brightness of the device (∼50 cd m-2 for 0.50 wt% rGO/ZnS:Cu,Al at 10 kHz and 110 V with an external quantum efficiency of ∼6.3% ± 0.1% and current efficiency of ∼0.81 ± 0.09 cd A-1). This new and facile strategy to construct the luminescent devices could be a paradigm shift towards cost effective, highly stable in air (for several days) and energy efficient next generation display devices.

Optimization of processing parameters for designing an efficient AC driven powder electroluminescent device

This paper reports various critical processing parameters for the development of efficient ZnS doped with Cu, Al phosphors optimized to exhibit AC electroluminescence. Different firing atmospheres and temperatures were used for the synthesis of electroluminescent phosphor materials. As synthesized ZnS: Cu, Al phosphor powders were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Photoluminescence (PL) and Electroluminescence (EL) spectroscopy studies. Average crystallite size calculated from XRD lies in a range of 44–49µm and average grain size as revealed by SEM lies in the range 3–5µm. The results showed that the phosphor powders have hexagonal phase and exhibit near band edge luminescence in the blue region. This is one of very few reports that depict the observation of EL from hexagonal ZnS:Cu,Al phosphor system. Polycrystalline powders emit PL in a broad band with a maximum at 2.53–2.74eV above the valence band. The PL spectrum of the ZnS:Cu,Al phosphor showed emission peaks at 453nm, 467nm, 485nm and 490nm that could be attributed to the generation of deep level energy traps. The prototype of an alternating current driven thick (40±2µm) film EL device has been fabricated and the threshold voltage was observed to be around 95–120V at a frequency of 1kHz. The critical processing parameters for designing an efficient EL device have been highlighted and discussed thoroughly in the current paper.

Efficient green electroluminescent devices based on iridium complex with wide energy gap complexes as sensitizers

In this work, electroluminescent (EL) performances of a green iridium complex (tfmppy)2Ir(tpip) were significantly improved by utilizing wide energy gap iridium complexes FK306 and FIrpic as sensitizers. Due to the low-lying energy levels, the co-doped FK306 or FIrpic molecules function as electron trappers, which are helpful in balancing holes and electrons on (tfmppy)2Ir(tpip) molecules and in broadening exciton recombination zone. Consequently, the co-doped devices displayed high EL efficiencies and slow efficiency roll-off. Compared with FIrpic, FK306 acts as a more effective sensitizer because of its relatively lower energy levels. Consequently, highly efficient green EL device with maximum current efficiency, power efficiency and brightness up to 102.29 cd/A (external quantum efficiency (EQE) of 25.3%), 88.67 lm/W and 96,268 cd/m2, respectively, was realized by optimizing the co-doping concentration of FK306. Even at the practical brightness of 1000 cd/m2, EL current efficiency up to 92.93 cd/A (EQE = 23%) can still be retained.

Highly efficient pure red organic light-emitting devices based on tris(1-phenyl-isoquinoline) iridium(III) with another wide gap iridium(III) complex as sensitizer

In this work, we report the realization of highly efficient pure red organic co-doped electroluminescent device with tris(1-phenyl-isoquinoline) iridium(III) (Ir(piq)3) and iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) as emitter and sensitizer, respectively. By selecting 4,4′,4″-Tri(9-carbazoyl)triphenylamine and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine as host materials, a series of devices with single or double light-emitting layer(s) were fabricated and investigated. The well-known wide gap blue emitter FIrpic, which possesses low-lying energy levels, was co-doped minutely into electron dominant light-emitting layer. Compared with reference devices, co-doped devices displayed significant improvement of device performances attributed to improved carriers' balance and broadening recombination zone. Finally, the 0.3 wt% co-doped double light-emitting layers device obtained the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 28,031 cd/m2, 14.89 cd/A, 12.99 lm/W and 9.0%, respectively. Even at the certain brightness of 1000 cd/m2, EL efficiency as high as 12.06 cd/A can be retained by the same device.

Correction: Superior upconversion fluorescence dopants for highly efficient deep-blue electroluminescent devices.

Correction for ‘Superior upconversion fluorescence dopants for highly efficient deep-blue electroluminescent devices’ by Yi-Hsiang Chen et al., Chem. Sci., 2016, DOI: ; 10.1039/c6sc00100a.

Superior upconversion fluorescence dopants for highly efficient deep-blue electroluminescent devices.

In this study, we revealed a new approach for the development of new triplet-triplet annihilation (TTA) materials with highly efficient deep-blue fluorescence via the incorporation of a styrylpyrene core and an electron-donating group. The resulting deep-blue emitters (PCzSP, DFASP, and DPASP) exhibit intramolecular charge transfer emissions with remarkably high emission quantum yields. The electroluminescent devices based on these three fluorophores as dopants using CBP as a host exhibit very high device efficiencies; in particular, the DPASP-doped device reveals an extremely high EQE of 12%, reaching the limit of a TTA-based device. The EL characteristics of DPASP-doped CBP-based devices at various doping concentrations (0-5%) suggest that the dopant DPASP is responsible for the TTA-type delayed fluorescence in the device; no delayed fluorescence was observed for the device using CBP as the host emitter. Moreover, when using DMPPP with ambipolar characteristics as the host, the deep-blue DPASP-doped device also gives outstanding performance with an EQE of nearly 11% with an extremely small efficiency roll-off, which was ascribed to the excellent charge balance in the emitting layer of the EL device. The TTA process of the SP-based dopants accounts significantly for the superior efficiencies of the EL devices.

Dianthranide derivative for efficient deep blue-emitting organic electroluminescence devices

We report that fluorescent organic light-emitting diodes (FOLEDs) with the emitting layer including a dianthranide derivative material produced highly efficient and bright emission in the deep blue region. It is found that this material works as an excellent host for creating high-performance FOLEDs with Commission International de L’Eclairage (CIE) coordinates (x = 0.147, y = 0.125), high current efficiency (CE) of 6.4 cd A−1 and external quantum efficiency (EQE) of 7.40% at 20 mA cm−2.

Efficient single light-emitting layer pure blue phosphorescent organic light-emitting devices with wide gap host and matched interlayer

In this work, we report the highly efficient pure blue electroluminescent (EL) device based on bis[(3,5-difluoro-4-cyanophenyl)pyridine]picolinate iridium(III) (FCNIrpic) doped 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) film. The matched energy levels of FCNIrpic and CzSi are helpful in facilitating the trapping of carriers, while the high triplet energy of CzSi can well avoid the undesired reverse energy transfer. More importantly, the injection of holes was further accelerated by inserting 5nm 4,4′,4″-Tri(9-carbazoyl)triphenylamine (TcTa) film between hole transport layer and lighting-emitting layer (EML) as interlayer. Consequently, EL performances were significantly enhanced attributed to wider recombination zone and better balance of holes and electrons. Interestingly, single-EML device displayed higher performances than those of double-EMLs device. Finally, pure blue EL device with the structure of ITO/MoO3 (3nm)/TAPC (40nm)/TcTa (5nm)/FCNIrpic (20%): CzSi (30nm)/TmPyPB (40nm)/LiF (1nm)/Al (100nm) realized the maximum brightness, current efficiency, power efficiency and external quantum efficiency up to 12,505cd/m2, 36.20cd/A, 28.42lm/W and 16.9%, respectively. Even at the high brightness of 1000cd/m2, current efficiency and external quantum efficiency up to 17.40cd/A and 8.1%, respectively, can be retained by the same device.

Progress in small-molecule luminescent materials for organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have been extensively studied since the first efficient device based on small molecular luminescent materials was reported by Tang. Organic electroluminescent material, one of the centerpieces of OLEDs, has been the focus of studies by many material scientists. To obtain high luminosity and to keep material costs low, a few remarkable design concepts have been developed. Aggregation-induced emission (AIE) materials were invented to overcome the common fluorescence-quenching problem, and cross-dipole stacking of fluorescent molecules was shown to be an effective method to get high solid-state luminescence. To exceed the limit of internal quantum efficiency of conventional fluorescent materials, phosphorescent materials were successfully applied in highly efficient electroluminescent devices. Most recently, delayed fluorescent materials via reverse-intersystem crossing (RISC) from triplet to singlet and the “hot exciton” materials based on hybridized local and charge-transfer (HLCT) states were developed to be a new generation of low-cost luminescent materials as efficient as phosphorescent materials. In terms of the device-fabrication process, solution-processible small molecular luminescent materials possess the advantages of high purity (vs. polymers) and low procession cost (vs. vacuum deposition), which are garnering them increasing attention. Herein, we review the progress of the development of small-molecule luminescent materials with different design concepts and features, and also briefly examine future development tendencies of luminescent materials.

The locally twisted thiophene bridged phenanthroimidazole derivatives as dual-functional emitters for efficient non-doped electroluminescent devices

A series of locally twisted dual-functional materials namely PIPT, PITT and PIFT have been designed and synthesized by introducing different polyaromatic hydrocarbon groups to a phenanthroimidazole backbone through a thiophene bridge. In these molecules, the thiophene bridge and phenanthroimidazole platform are nearly coplanar and this endows these materials with relatively shallow HOMO levels (−5.35 to −5.21eV). On the other hand, the bulky polyaromatic hydrocarbon units introduce non-planar twisty structures which reduce molecular aggregations. These three materials show color-tunable emission (emission peak from 468 to 532nm in film) and high thermal stability (Tg>160°C). Simple trilayer devices using these three phenanthroimidazole derivatives as non-doped emitting layers exhibit low turn-on voltages (2.3–2.7V) and high maximum efficiencies of 3.74, 6.15 and 6.89cd/A for PIPT, PITT and PIFT, respectively. Above all, owing to their shallow HOMO levels for enabling efficient hole-injection, even simpler bilayer devices employing these materials as hole-transporting emitters show low turn-on voltages (2.6–2.8V) and high efficiencies of 5.77cd/A for PIPT, 6.03cd/A for PITT and 6.04cd/A for PIFT, respectively. These comparable performances with those of the trilayer configurations show the efficient hole-injection/transport ability of these three newly developed emitters.