Highly efficient full-fluorescence organic light-emitting diodes with exciplex cohosts

Full-fluorescence organic light-emitting diodes (FOLEDs) with low cost and high efficiency are imperious demands for commercial process in flat panel display and lighting products. We fabricated a series of FOLEDs employing C545T and DCJTB as doped dyes and different exciplex blends as cohosts. The results proved that reverse intersystem crossing (RISC) efficiency of exciplex cohost has a significant effect on the device performance. Devices with TAPC:PIM-TRZ as cohost which possessed the highest RISC efficiency showed the best results. The green FOLEDs exhibited the maximum external quantum efficiencies (EQEs) approaching to 20%, the red FOLEDs exhibited EQEs over 10% and all the EQE roll-offs are less than 10% at 1000 cd m⁻², which are among the best reported results so far, suggesting these exciplex cohosts are promising for FOLEDs.     

Nanofabrication of gallium nitride photonic crystal light-emitting diodes

We describe a comparison of nanofabrication technologies for the fabrication of 2D photonic crystal structures on GaN/InGaN blue LEDs. Such devices exhibit enhanced brightness and the possibility of controlling the angular emission profile of emitted light. This paper describes three nano lithography techniques for patterning photonic crystal structures on the emitting faces of LEDs: direct-write electron beam lithography, hard stamp nanoimprint lithography and soft-stamp nanoimprint lithography with disposable embossing masters. In each case we describe variations on the technique as well as its advantages and disadvantages. Complete process details have been given for all three techniques. In addition, we show how high performance GaN dry etch techniques, coupled with optical process monitoring can transfer resist patterns into underlying GaN material with high fidelity.     

Advantage of InGaN-based light-emitting diodes with trapezoidal electron blocking layer

In this study, a trapezoidal-shaped electron blocking layer is proposed to improve efficiency droop of InGaN/GaN multiple quantum well light-emitting diodes. The energy band diagram, carrier distribution profile, electrostatic field, and electron current leakage are systematically investigated between two light-emitting diodes with different electron blocking layer structures. The simulation results show that, when traditional AlGaN electron blocking layer is replaced by trapezoidal-shaped electron blocking layer, the electron current leakage is dramatically reduced and the hole injection efficiency in markedly enhanced due to the better polarization match, the quantum-confined Stark effect is mitigated and the radiative recombination rate is increased in the active region subsequently, which are responsible for the alleviation of efficiency droop. The optical performance of light-emitting diodes with trapezoidal-shaped electron blocking layer is significantly improved when compared with its counterpart with traditional AlGaN electron blocking layer.     

Highly efficient and foldable top-emission organic light-emitting diodes based on Ag-nanoparticles modified graphite electrode

Top-emission flexible organic light-emitting devices (TE-FOLEDs) are highly suitable for next generation display due to their numerous assets including top-emitting configuration and mechanical flexibility. One major challenge in TE-FOLEDs is to prepare a deformable and reflective bottom electrode capable of effective carrier injection. In this paper, a new strategy for efficient and foldable TE-FOLEDs is demonstrated. It is based on a highly conductive Ag-nanoparticles (Ag-NPs) modified graphite that is used as a flexible bottom electrode. The good reflectance to full-color emission (>59% over the whole visible wavelength range), ultralow sheet resistance (<5 Ω/sq), and high tolerance to mechanical bending (almost unchanged in resistance after bending 1000 times with an angle of ±90°) of the modified graphite synergistically constitute a breakthrough in the domain of TE-FOLEDs. The maximum current efficiencies reach 15.0, 50.2, 16.8 cd/A for red, green, blue emissions, respectively. Colorimetric gamut increased by 99.6% compared to bottom emission structure with the corresponding Commission Internationale de L'Eclairage (CIE) coordinates of the red/green/blue (R/G/B) devices. In particular, the TE-FOLEDs incorporating highly flexible graphite electrodes offer great mechanical durability and the initial brightness of 5000 cd/m can be maintained over 90% after bending for 1000 bending cycles. This approach is expected to open a new avenue for developing foldable displays.     

Flexible perovskite light-emitting diodes for display applications and beyond

The flexible perovskite light-emitting diodes (FPeLEDs), which can be expediently integrated to portable and wearable devices, have shown great potential in various applications. The FPeLEDs inherit the unique optical properties of metal halide perovskites, such as tunable bandgap, narrow emission linewidth, high photoluminescence quantum yield, and particularly, the soft nature of lattice. At present, substantial efforts have been made for FPeLEDs with encouraging external quantum efficiency (EQE) of 24.5%. Herein, we summarize the recent progress in FPeLEDs, focusing on the strategy developed for perovskite emission layers and flexible electrodes to facilitate the optoelectrical and mechanical performance. In addition, we present relevant applications of FPeLEDs in displays and beyond. Finally, perspective toward the future development and applications of flexible PeLEDs are also discussed.     

Effect of AlInGaN barrier layers with various TMGa flows on optoelectronic characteristics of near UV light-emitting diodes grown by atmospheric pressure metalorganic vapor phase epitaxy

Near ultraviolet light-emitting diodes (LEDs) with quaternary AlInGaN quantum barriers (QBs) are grown by atmospheric pressure metalorganic vapor phase epitaxy. The indium mole fraction of AlInGaN QB could be enhanced as we increased the TMG flow rate. Both the wavelength shift in EL spectra and forward voltage at 20 mA current injection were reduced by using AlInGaN QB. Under 100 mA current injection, the LED output power with Al_0.089In_0.035Ga_0.876N QB can be enhanced by 15.9%, compared to LED with GaN QB. It should be attributed to a reduction of lattice mismatch induced polarization mismatch in the active layer.     

Highly efficient solution-processable europium-complex based organic light-emitting diodes

The development of solution-processable europium-complex based organic light-emitting diodes (OLED) has been limited by their low efficiency. In this paper, we show that it is possible to produce a highly efficient, solution-processable, europium-complex based OLED with an external quantum efficiency of 4.3% at a brightness of 100 Cd/m² using off-the-shelf materials and without any specific optical design for improved light extraction. This is achieved by optimizing the device structure and the host matrix used. To our knowledge, this is the highest efficiency reported for solution-processable europium-complex based OLED devices, and the efficiency roll-off has been reduced compared with other reported europium-complex based devices. Our approach should be applicable to a wide range of solution-processable lanthanide complexes.     

Highly efficient solution-processed small-molecule white organic light-emitting diodes

Solution-processed small-molecule white organic light-emitting diodes (WOLEDs) were fabricated with a co-host of hole-transporter 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TCTA) and electron-transporter 2,7-Bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13). By doping 15 wt% FIrpic or F₃Irpic and 0.5 wt% Ir(MDQ)₂(acac) in to the TCTA/SPPO13 host, highly efficient white OLEDs have been achieved which exhibit nearly identical emission spectra at different luminance. The F₃Irpic and Ir(MDQ)₂(acac)-based WOLED shows maximum efficiencies of 40.9 cd/A, 36.7 lm/W and 16.9%, and even high efficiencies of 30.1 cd/A and 12.3% at the practical luminance of 1000 cd/m², which are among the highest efficiencies of the solution-processed small-molecule WOLEDs. These results demonstrate a convenient way to realize solution-processed WOLEDs with high efficiency and high spectral stability through full small-molecule materials system.     

Highly efficient phosphorescent organic light-emitting diodes using a beryllium metal–chelate complex as electron-transporting host material

A classical fluorescent metal–chelate complex bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp₂) has been used as an efficient electron-transporting host material to construct highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) with an orange-emitting phosphorescent guest bis(7,8-benzoquinolinato) iridium (III) (N,N′-diisopropyl-benzamidine) ((bzq)₂Ir(dipba)). Due to the well-matched energy levels of Bepp₂ with the corresponding hole-/electron- transporting (HT/ET) materials and the high-efficiency and complete energy transfer of this host–guest system, the Bepp₂-based PHOLEDs exhibit rather low driving voltage (2.8 V) and high peak EL efficiencies of over 70 cd A⁻¹ for luminous efficiency, 55 lm W⁻¹ for power efficiency, and 23% for external quantum efficiency, a performance significantly better than that using CBP as the host.     

Improved quantum dot light-emitting diodes with a cathode interfacial layer

Colloidal quantum dot light-emitting diodes (QLEDs) are reported with improved external quantum efficiencies (EQE) and efficiency roll-off under high current densities by introducing a thermally-evaporated organic cathode interfacial material (CIM) Phen-NaDPO. QLEDs with this new CIM modified Al cathode were fabricated, giving an upwards of 25% enhancement in the EQE relative to the bare Al device. Ultraviolet photoemission spectroscopy (UPS) suggests that this material can effectively lower the work function of Al, therefore facilitating the electron injection in QLEDs. Furthermore, Phen-NaDPO was introduced into the LiF/Al device to afford better balanced hole/electron injection in the emitting layer. Consequently, the QLEDs with the organic CIM/LiF/Al cathode further increased EQE and current efficiency by 44% and 52%, respectively, with higher luminance and lower efficiency roll-off under high current densities.     

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.     

Highly efficient thienylquinoline-based phosphorescent iridium(III) complexes for red and white organic light-emitting diodes

Highly efficient 2-(thiophen-2-yl)quinoline-based phosphorescent iridium(III) complexes bearing 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine or picolinic acid as ancillary ligands are designed and synthetised. The variation of ancillary ligands is attempted to finely tune the photophysical properties of these complexes, especially the solution phosphorescent quantum yields (ΦPL), full width at half maximum (FWHM), etc. The picolinic acid-based complex displays the slightly red-shifted dual-peak emission compared to triazolpyridine-based one. The complexes show bright emission with broad FWHM up to 83 nm, and the emissions are in red region with the very high absolute ΦPL up to 0.76 in solution. Moreover, high-performance red and three-color-based white organic light-emitting diodes (OLEDs) with excellent color stability have been fabricated. The maximum external quantum efficiencies of red and white OLEDs can reach 16.2% and 15.1%, respectively. The maximum current efficiency and power efficiency of white OLED are as high as 35.5 cd A⁻¹ and 34.0 lm W⁻¹, respectively. Especially, the designed white OLED exhibits excellent spectral stability under wide operating voltage range, and the 1931 Commission Internationale de L'Eclairage of white OLED only changes from (0.43, 0.42) to (0.44, 0.44), the color rendering index is in a narrow range of 75–77.     

Highly efficient silicon light emitting diodes produced by doping engineering

This paper reviews our recent progress on silicon (Si) pn junction light emitting diodes with locally doping engineered carrier potentials.Boron implanted Si diodes with dislocation loops have electrol...     

Enhanced performance of spectra stable blue perovskite light-emitting diodes through Poly(9-vinylcarbazole) interlayer incorporation

Since the emergence of organic-inorganic hybrid perovskites, the development of perovskite light-emitting diodes (PeLEDs) with green/red emission have made great progress, and the corresponding external quantum efficiency (EQE) has exceeded 20%. However, the research progress of blue-emitting PeLED still has certain challenges. In this article, a multi-cation per-bromine perovskite film is prepared by introducing polymer molecules poly(9-vinylcarbazole) (PVK) in an anti-solvent (chloroform). When the concentration of PVK is optimized to 0.1 mg/mL, a smooth, dense, high-quality film with photoluminescence quantum efficiency (PLQY) up to 20.70% is obtained. The introduction of PVK can assist the formation of perovskite films for interface modification via surface defect passivation. The optimized blue PeLED has a maximum brightness of 3136 cd/m² and a maximum EQE of 3.49% at 488 nm. More importantly, the optimized blue PeLED has excellent color stability under high applied voltage up to 12 V or continuous operation.     

Highly efficient inverted blue light-emitting diodes by thermal annealing and interfacial modification

Inverted polymer light-emitting diodes (IPLED) were fabricated by using polyfluorene (PF-FSO10) as the emissive layer, polyethyleneimine ethoxylate (PEIE) as the interlayer, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as the electron-transport layer with the configuration of ITO/ZnO/PEIE/TPBI/PF-FSO10/MoO₃/Al. Thermal annealing PF-FSO10 made the hole transporting ability decreased, the electron transporting ability enhanced, and thus, jointly promoted the carrier balances in the emissive layer. Inserting a TPBI layer between the PEIE and PF-FSO10 much enhanced the electron transportation and reduced the exciton quench on the ZnO interface. Therefore, the inverted blue light-emitting diode was obtained with a luminous efficiency of 6.8 cd A⁻¹ (an external quantum efficiency of 7.1%), and the CIE color coordinates of (0.15, 0.15), which was among the highest efficiency values in solution-processed inverted blue fluorescent light-emitting diodes.     

Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers

Solution-processed blue quantum dot light-emitting diodes (QLEDs) suffer from low device efficiency, whereas the balance of electron and hole injection is critical for obtaining high efficiency. Herein, synergistical double hole transport layers (D-HTLs) are employed, which use poly(9-vinylcarbazole) (PVK) stacked on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB). The fabrication of D-HTLs is achieved by using dimethyl formamide (DMF) as the solvent for PVK, with which the underlying TFB layer almost remains unwashed and undamaged during the spin-coating process of PVK layer. TFB/PVK D-HTLs form the stepwise energy level for hole injection, which reduces the hole injection barrier and favors the carrier balance in the emission layer (EML). The optimized blue QLED with TFB/PVK D-HTLs shows a maximum external quantum efficiency (EQE) of 13.7%, which is 3-fold enhancement compared to that of the control device with single TFB HTL. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the stepwise D-HTLs based QLEDs. This work manifests the positive effect on performance boost by selecting appropriate solvents towards stepwise D-HTLs formation and paves the way to fabricate highly efficient all-solution processed light emitting diodes.     

Controlling charge balance using non-conjugated polymer interlayer in quantum dot light-emitting diodes

The balance of electron–hole charge carriers in quantum dot (QD) light-emitting diodes (QLEDs) is an important factor to achieve high efficiency. However, poor interfacial properties between QDs and their adjacent layers are likely to deteriorate the electron–hole charge balance, resulting in the poor performance of a QLED. In this paper, we report an enhanced efficiency in red-emitting inverted QLEDs by modifying the interface properties between QDs and ZnO electron transport layer (ETL) using a thin layer of non-conjugated polymer, poly(4-vinylpyridine) (PVPy). Based on the precise control of the electrical properties with PVPy, the maximum efficiency of the QLED is enhanced by 30% compared to the device without a PVPy layer. In particular, the efficiency at low current density region is significantly increased. We investigate the effect of the PVPy interlayer on the performance of QLEDs and find that this thin layer not only shifts the energy levels of the underlying ZnO ETL, but also effectively blocks the leakage current at the ETL/QD interface.     

Unveiling photophysical properties of phenanthro[9,10-d]imidazole derivatives for organic light-emitting diodes

Recently, two phenanthro[9,10-d]imidazole derivatives exhibited excellent advantages in organic light-emitting devices (i.e. high luminous efficiency, high carrier mobility, and low turn-on voltage). However, the relationship between their photophysical properties and the structural characters or intermolecular interactions remain elusive, which is considerable importance to further performance improvement. Currently, density functional theory (DFT) and time-dependent DFT (TD-DFT) have become powerful tools to rationalize photophysical properties and to design new materials with improvement performance. The simulated electron absorption and emission wavelengths of compounds 1 and 2 are in good agreement with the experimental ones. For the studied compounds, the involvement of tert-butyl moiety has negligible effect on energy level and distribution of frontier molecular orbitals (FMOs), whereas greatly affects electron transition of deep energy level and charge transport property. Synergy of π-π and CH···π intermolecular interactions is responsible for the bipolar carrier transport, while CH···π for hole transport. The incorporation of NH₂ on phenanthro[9,10-d]imidazole and NO₂ on diphenylamino part is an effective way to tune FMOs energy level and intramolecular charge transfer, leading to the substantial enhancement of the second-order nonlinear optical (NLO) response. Our work is also important for understanding photophysical properties and designing photoelectric materials of phenanthro[9,10-d]imidazole derivatives.     

Highly efficient and stable blue quantum-dot light-emitting diodes based on polyfluorenes with carbazole pendent groups as hole-transporting materials

Highly efficient and stable blue quantum-dot light-emitting diodes (QD-LEDs) have been realized by using poly (9,9-bis(N-(2′-ethylhexyl)-carbazole-3-yl)-2,7-fluorene) (PFCz) as hole-transporting layers (HTLs). Due to the carbazole units as substituents at the 9-position of polyfluorene, PFCz shows higher hole mobility and better electrochemical stability than poly (N-vinlycarbazole) (PVK). As a result, the maximum current efficiency (CE) and external quantum efficiency (EQE) of the blue QD-LEDs increased from 4.32 cd A⁻¹ to 7.9% for PVK HTL to 7.38 cd A⁻¹ and 12.61% for PFCz HTL, respectively. Furthermore, the PFCz-based blue QD-LED exhibited lower turn-on voltage and longer device lifetime than the PVK-based device. The improvement performance of blue QD-LED should be attributed to the conjugated fluorene backbone and the substituents of the carbazole active sites, thus enhancing hole mobility and electrochemical stability. This result demonstrates that polyfluorenes with pendent carbazole groups is a promising hole-transporting materials for improving performance of blue QD-LEDs.     

Fluorene derivatives for highly efficient non-doped single-layer blue organic light-emitting diodes

Diphenylamino- and triazole-endcapped fluorene derivatives which show a wide energy band gap, a high fluorescence quantum yield and high stability have been synthesized and characterized. Single-layer electroluminescent devices of these fluorene derivatives exhibited efficient deep blue to greenish blue emission at low driving voltage. The single-layer OLED of PhN-OF(1)-TAZ shows a maximum current efficiency of 1.54 cd/A at 20 mA cm⁻² with external quantum efficiency (EQE) of 2.0% and CIE coordinates of (0.153, 0.088) in deep blue region, while the single-layer device of oligothienylfluorene PhN-OFOT-TAZ shows a maximum brightness of 7524 cd/m² and a maximum current efficiency of 2.9 cd/A with CIE coordinates of (0.20, 0.40) in greenish blue.