White organic light-emitting diodes using a quantum dot as a color changing material

White organic light-emitting diodes (WOLEDs) were fabricated by using a blue emitting layer combined with quantum dot (QD) based color converting materials. Orange emitting QD was used as a color converting material and effective color conversion was realized. A white color coordinate of (0.32, 0.41) was obtained with a current efficiency of 8.3 cd/A at 1000 cd/m².     

The influence of type-I and type-II triplet multiple quantum well structure on white organic light-emitting diodes

We demonstrate high-efficient white organic light-emitting diodes (WOLEDs) based on triplet multiple quantum well (MQW) structure and focus on the influence on WOLEDs through employing different potential barrier materials to form type-I and type-II MQWs, respectively. It is found that type-I MQW structure WOLEDs based on 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene as potential barrier layer (PBL) offers high electroluminescent (EL) performance. That is to say, maximum current efficiency and power efficiency are achieved at about 1,000 cd/m² with 16.4 cd/A and 8.3 lm/W, which increase by 53.3% and 50.9% over traditional three-layer structure WOLEDs, respectively, and a maximum luminance of 17,700 cd/m² is earned simultaneously. The achievement of high EL performance would be attributed to uniform distribution and better confinement of carriers within the emitting layer (EML). However, when 4,7-diphenyl-1,10-phenanthroline or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline is used as PBL to form type-II MQW structure, poor EL performance is obtained. We attribute that to improper energy level alignment between the interface of EML/PBL, which leads to incomplete confinement and low recombination efficiency of carriers, a more detailed mechanism was argued.     

Novel white-light-emitting polyfluorenes with benzothiadiazole and Ir complex on the backbone

Novel white-emitting polyfluorenes were synthesized by mixing fluorescence and phosphorescence emission. Benzothiadiazole(BT) and iridium(III)bis(2-(1-naphthalene)pyridine-C^2′,N)-2,2,6,6-tetramethyl-3,5-heptanedione[(1-npy)₂Ir(tmd)] units were incorporated into polyfluorene backbone as green and red chromophores by Suzuki polycondensation. The device from PFG03-IrR07 shows a maximum luminous efficiency (LE) of 5.3 cd/A, a maximum luminance of 9900 cd/m² at a current density of 453 mA/cm² and a CIE coordinate of (0.32, 0.34) with the configuration: ITO/PEDOT:PSS/PVK/emissive layer/CsF/Al. Besides, the EL efficiencies decline slightly with increasing the current density. All emissions located very close to the equi-energy white point (0.33, 0.33) when applied voltage change from 9 to 14 V. Furthermore, the white emission of devices based on these materials shows very good color quality, with high color rendering index range between 84 and 89. Our results indicate that, by incorporation of singlet and triplet species into polymer backbone, the obtained white-emitting materials and devices are promising candidates for display and solid-state-lighting purpose.     

Bipolar Hosts Based on a Rigid 9,10-Dihydroanthracene Scaffold for Full-Color Electrophosphorescent Devices

Two bipolar hosts, CzDPO and CzDOxa , comprising a hole-transporting carbazole and an electron-transporting diphenyl phosphine oxide or oxadiazole with a saturated linker (9,10-dihydroanthracene) have been synthesized and characterized. The saturated linker limited the effective extension of π conjugation, leading to high triplet energies (E_T=2.97–2.98 eV). The diastereomers with a rigid configuration provided an amorphous thin film with high thermal (T_d=415–444 °C) and morphological stability (T_g=188–224 °C). The high triplet energies and bipolar carrier transport characteristics (ambipolariy) of CzDPO and CzDOxa can facilitate exothermic energy transfer to the dopants and balance carrier injection/transport in the emission layers. As a result, they were utilized as universal hosts for various phosphorescent OLEDs (from blue to red), showing average external quantum efficiencies (η_ext=8.2–13.1 %) and low efficiency roll-off. In addition, we also fabricated dual-emitter white organic light-emitting diodes with a co-doped single emissive layer. WOLED hosted by CzDOxa exhibited satisfactory device efficiencies (14.4 %, 25.7 cd A⁻¹, 27 lm W⁻¹) with highly stable chromaticity (CIE_x=0.26–0.28 and CIE_y=0.38) at brightnesses from 560 to 15200 cd m⁻².     

Synthesis and optoelectronic properties of thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine)

This paper describes the synthesis of a new thermally cross-linkable hole-transporting poly(fluorene-co-triphenylamine) (PFTV) by Suzuki coupling reaction and its application in polymer light-emitting diodes (PLEDs). The characteristics of PFTV were analyzed by ¹H NMR, differential scanning calorimetry, optical spectroscopy, cyclic voltammetry, and atomic force microscopy. Its HOMO level lies between those of PEDOT:PSS and poly(9,9-dioctylfluorene), forming a stepwise energy ladder to facilitate hole-injection. Multilayer device with thermally cross-linked PFTV as hole-transporting layer (ITO/PEDOT:PSS/HTL/PFO/LiF/Ca/Al) was readily fabricated by successive spin-coating processes, its maximum luminance efficiency (2.27 cd/A) was significantly higher than that without PFTV layer (0.50 cd/A). In addition, the PFTV was successfully applied as host for red-emitting Ir(piq)₂acac to obtain a device with moderate performance (5300 cd/m² and 2.64 cd/A). The PFTV is a promising hole-transporting material for the fabrication of multilayer PLEDs by wet processes as well as a potential host for phosphorescent PLEDs.     

Electrophosphorescence from iridium complex-doped mesogen-jacketed polymers

A mesogen-jacketed polymer P-Ct {poly{2,5-bis[(5-tert-butylphenyl)-1,3,4-oxadiazole]styrene}} has been investigated as a host material for IrMDPP [Ir(III)bis(5-methyl-2,3-diphenylpyrazine) (acetyl acetonate)] doped polymer electrophosphorescent device. It was found that the device with P-Ct was more efficient than that with PVK. The maximum luminance and external luminous efficiency can reach 3702 cd/m² and 0.83 cd/A, respectively, which were higher than those of device with PVK (1999 cd/m² and 0.68 cd/A), which can be partly explained by the more balanced carrier injection and transportation and longer lifetime of excitons in P-Ct-TPD-IrMDPP. It was also found that as the IrMDPP content increased in P-Ct-TPD, the EL spectra color shifts from green-yellow to yellow-orange and were different from PL spectra, which was partly due to the dominating role of direct charge-trapping and recombination in the EL process over the energy-transfer routes.     

White Electroluminescence from a Single-Polymer System with Simultaneous Three-color Emission

A series of polymers were synthesized by incorporating low contents of fluorenone (FO) and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) into the main chain of poly(9,9-dioctylfluorene). White-light emission was obtained from a single polymer by adjusting the FO and DBT contents. All polymers showed good thermal stability with 5% weight loss up to 410 °C and good solubility in common organic solvents. Electroluminescence devices with indium tin oxide/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/emission layer/Ca/Al structure were found to emit white light with Commission Internationale de l’Eclairage coordinate of (0.37, 0.34). These devices exhibited a maxium brightness of 3414 cd/m² and a maximum current efficiency of 2.79 cd/A.     

Alternating-current white a-C:H thin-film light-emitting diodes with composition-graded carrier injection layers

The optoelectronic characteristics of hydrogenated intrinsic amorphous carbon (i-a-C:H) alternating-current white thin-film light-emitting diodes (ACW-TFLEDs) with composition-graded (CG) hydrogenated intrinsic amorphous silicon carbide (i-a-SiC:H, CG C) layers had been obviously improved with additionally incorporated CG hydrogenated intrinsic amorphous silicon germanium (i-a-SiGe:H, CG Ge) carrier injection layers. For an ACW-TFLED with CG Ge carrier injection layers, the electroluminescence (EL) threshold voltage and brightness were improved from 9 V 344 cd/m² of a CG C one to 7.5 V, 1000 cd/m² under direct-current forward bias, and from 9 V, 200 cd/m² of a CG C one to 7.8 V, 560 cd/m² under direct-current reverse bias, respectively, at an injection current density of 300 mA/cm² for brightness measurement. The enhancement of EL performance with CG Ge carrier injection layers was due to the increased carrier injection efficiency and reduced contact resistance resulting from the lower barrier and partially formed polycrystalline Ge layer between the Al (electrode)/Ge interface. Moreover, the EL intensity of an obtained ACW-TFLED increased with the frequency up to 20 kHz and then decreased rapidly and became very weak as the frequency was increased to about 100 kHz. This frequency-dependent EL behavior would be qualitatively explained with the mobility of charge carriers.     

Full‐visible spectrum emitting Zn–In–S:Cu/ZnS nanocrystals based on varying Cu concentrations and its application in white LEDs

A series of high‐quality Cu‐doped Zn–In–S nanocrystals (d‐NCs) were prepared by a conventional hot injection process. The full‐visible spectrum emission from 480 to 648 nm can be easily achieved by adjusting the Cu doping concentration in the Zn–In–S system, but not by varying the ratio of In/Zn in the alloyed host material. After wrapping the ZnS shell around the Zn–In–S:Cu d‐NCs core, the resultant Zn–In–S:Cu/ZnS core/shell d‐NCs not only exhibited an enhanced prominent photoluminescent quantum yield (PLQY) up to 65% but also possessed the excellent thermal, photochemical stability, and longer PL lifetime. Furthermore, high color rendition white light was generated from a single color converter Zn–In–S:Cu/ZnS core/shell NCs‐assisted white light‐emitting diodes (LEDs). Under operation of 38 mA forward bias current, the fabricated white LEDs emitted bright natural white light with a luminous efficiency of 62 lm/W, and the correlated color temperature of 5658 K. Simultaneously, the good color stability was accompanied by the CIE color coordinates of (0.3287, 0.3527) under different forward bias currents.     

Blue light-emitting diodes from 2-(10-naphthylanthracene)-spiro[fluorene-7,9′-benzofluorene] host material

A novel, spiro-type host material 2-(10-naphthylanthracene)-spiro[fluorene-7,9′-benzofluorene] was prepared by reacting 2-bromo-spiro[fluorene-7,9′-benzofluorene] with 9-(2-naphthylanthracene)-10-boronic acid via the Suzuki reaction. 2-4′-(Phenyl-4-vinylbenzeneamine)phenyl-spiro[fluorene-7,9′-benzofluorene], 4-[2-naphthyl-4′(phenyl-4-vinylbenzeneamine)]phenyl and diphenyl-[4-(2-[1,1;4,1]terphenyl-4-yl-vinyl)-phenyl]-amine were used as dopant materials. Devices with the configuration of ITO/N,N′-bis[4-(di-m-tolylamino)phenyl]-N,N′-diphenylbiphenyl-4,4′-diamine)/bis[N-(1-naphthyl)-N-phenyl]benzidine/2-(10-naphthylanthracene)-spiro[fluorene-7,9-benzofluorene]:5% dopant/aluminum tris(8-hydroxyquinoline)/Al-LiF showed a maximum power efficiency of 3.7 cd/A at 17.93 mA/cm² and a maximum luminance of 5018 cd/m² at 10 V with a turn-on voltage of 4.5 V.     

Synthesis and electrophosphorescent device study of carbazole containing dendronized styrenic polymers

Styrenic polymers P1(G0-CZ) and P2(G2-CZ) with carbazoles and carbazole containing dendrons as side chains were efficiently synthesized via “graft-to” approach by using copper-catalyzed azide/alkyne cycloaddition (CuAAC) reaction. The new polymers showed wide band gaps and had good thermal stabilities. Two new polymers were studied as electrophosphorescent host materials in OLED devices. Electrophosphorescent devices with the configuration of ITO/PEDOT:PSS/polymers:Ir(ppy)₃/TPBI/LiF/Al were fabricated. The polymer P1(G0-CZ) based device showed a maximum current efficiency of 21.4 cd/A, a power efficiency of 12.7 lm/W and an external quantum efficiency of 6.02%. The effect of host polymer structures on the aggregation of transition metal complexes Ir(ppy)₃ in active layer was also investigated.     

Synthesis and optoelectronic properties of thermally cross-linkable fluorene derivative containing hole-transporting triphenylamine terminals

This paper describes the synthesis of a solution-processable and thermally cross-linkable 2,7-bis-[4-bis(4-vinylphenyl)aminophenyl]-9,9-dihexylfluorene (VTF) and its application as hole-transporting layer in multilayer polymer light-emitting diodes (PLEDs). The thermal, photophysical, and electrochemical properties of VTF were investigated by differential scanning calorimetry, thermogravimetric analysis, optical spectroscopy, and cyclic voltammetry. The VTF is readily cross-linked via vinyl groups by heating at 180 °C for 30 min to obtain homogeneous film with excellent solvent resistance. Multilayer PLEDs (ITO/PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al) were readily fabricated by spin-coating process using cross-linked VTF as hole-transporting layer (HTL). The maximum brightness (13,640 cd/m²) and current efficiency (0.69 cd/A) were superior to those without HTL (ITO/PEDOT:PSS/MEH-PPV/Ca/Al: 7810 cd/m², 0.28 cd/A). In addition, the cured-VTF could replace conventional hole-injection layer (PEDOT:PSS) to reveal comparable performance (8240 cd/m², 0.44 cd/A). Current results indicate that the VTF with four thermally cross-linkable terminal vinyl groups is a promising optoelectronic material, which is readily processed by wet processes.     

Functional derivatives of (bi)phenyl-substituted carbazoles as building blocks for electro-active polymers

(Bi)phenyl-substituted carbazoles containing reactive functional groups were synthesized by the multi-step synthetic rout. The monomers were examined by various techniques including thermogravimetry, differential scanning calorimetry, UV and fluorescence spectrometry as well as electron photoemission technique. These derivatives were also tested as hole transporting materials in bilayer OLEDs with Alq₃ as the emitter. The devices exhibited promising overall performance with a turn-on voltage of ～3 V, a maximal photometric efficiency of 5.1 cd/A and maximum brightness of 12,200–15,600 cd/m².     

Hole-limiting conductive vinyl copolymers for AlQ3-based OLED applications

A series of soluble conductive vinyl copolymers containing a hole-transporting N-(4-methoxyphenyl)-N-phenylnaphthalen-1-amine (MeONPA) moiety and an electron-transporting/hole-blocking 2,5-diphenyl-1,3,4-oxadiazole (OXA) moiety at different composition ratios were synthesized and characterized. The copolymers were applied as the hole-transporting layer (HTL) for a series of heterojunction Organic Light-emitting Diodes (OLEDs) employing the commonly used green emitter tris(8-hydroxyquinolinato)aluminum (AlQ₃) as the electron-transporting layer. AlQ₃ is known to have inferior electron mobility compared to most typical hole-transporting materials. As a result, oxidative degradation of the AlQ₃ emitters caused by the excessive holes accumulated at the interface led to deterioration of the device over time. From the measurement of hole current only devices using electron blocking gold as cathode (ITO/PEDOT:PSS/copolymer/Au), it was found that the hole current for the copolymers reduced as the OXA composition increased. Optimum performance for the AlQ₃-based OLED (ITO/PEDOT:PSS/copolymer/AlQ₃/Ca/Al) was achieved for a 82/18 (molar ratio) (MeONPA/OXA) copolymer. The maximum current efficiency and luminance were 4.2 cd/A and ca 24,000 cd/m² respectively for the charge-balanced copolymer compared to 3.5 cd/A and 6600 cd/m² for similar device employing a homopolymer P(MeONPA) as the HTL.     

New tetraphenylethylene-containing conjugated polymers: Facile synthesis,aggregation-induced emission enhanced characteristics and application as explosive chemsensors and PLEDs

In this paper, tetraphenylethylene (TPE) units, one of the typical aggregation-induced emission (AIE) moieties, are utilized to construct a new functional polyfluorene (PF) P1, which exhibited the exciting property of the aggregation-induced emission enhancement (AIEE), instead of the aggregation-caused quenching (ACQ) of normal PFs, and could probe the explosive with high sensitivity both in the nanoparticles and solid state. Three other TPE-containing polymers, P2–P4, were also successfully prepared, and demonstrated good performance as explosive chemosensors and light-emitting materials. P3, bearing carbazole as hole-transporting units showed the best performance with a maximum luminance efficiency of 1.17 cd/A and a maximum brightness of 3609 cd/m² at 12.9 V in its light-emitting diode device.     

High-efficiency phosphor-in-glass with ultra-high color rendering indexing for white laser diode lighting

Stable color converters exhibiting high color rendering index have drawn researchers’ attention for their applications in high-quality white laser lighting. In this study, we develop the multi-color phosphor-in-glass (PIG) with the weight ratio of green-emitting (Y₃Al_3.08Ga_1.92O₁₂:Ce³⁺) to red-emitting (CaAlSiN₃:Eu²⁺) phosphor powders (10/1–18/1) by low temperature co-sintering method. The obtained composite material displays an outstanding optical and thermal performance, including a high internal quantum efficiency of 84.2%, a high transparency of 45% in the visible region and a low thermal quenching (it remains 86% at 448 K). By integrating 450 nm blue laser diodes with optimized multi-color PIG, the white light with a maximum luminous flux of 258 lm and a luminous efficiency of 137 lm/W is achieved for the first time. Additionally, considering the white balance, by tailoring the weight ratio of green-emitting to red-emitting phosphor and the thickness of PIG, the 14/1 PIG at fixed thickness of 0.75 mm produces pure white light with ultra-high color rendering index of 95.2 and a high luminous efficiency of 120.9 lm/W under power density of 2.39 W/mm² irradiation. The above superior characteristics imply that the multi-color PIG is an ideal candidate for high-quality white laser lighting applications.     

Electroluminescent Characteristics of DBPPV–ZnO Nanocomposite Polymer Light Emitting Devices

We have demonstrated that fabrication and characterization of nanocomposite polymer light emitting devices with metal Zinc Oxide (ZnO) nanoparticles and 2,3-dibutoxy-1,4-poly(phenylenevinylene) (DBPPV). The current and luminance characteristics of devices with ZnO nanoparticles are much better than those of device with pure DBPPV. Optimized maximum luminance efficiencies of DBPPV–ZnO (3:1 wt%) before annealing (1.78 cd/A) and after annealing (2.45 cd/A) having a brightness 643 and 776 cd/m² at a current density of 36.16 and 31.67 mA/cm² are observed, respectively. Current density–voltage and brightness–voltage characteristics indicate that addition of ZnO nanoparticles can facilitate electrical injection and charge transport. The thermal annealing is thought to result in the formation of an interfacial layer between emissive polymer film and cathode.     

Broadband excited Na3Tb(PO4)2:Ce3+/Eu2+ green/yellow-emitting phosphors with high color purity for LED-based application

To enhance the display quality of light-emitting diodes (LEDs), it is of great significance to exploit green/yellow-emitting phosphors with narrow emission band, high quantum yield, and excellent color purity to satisfy the application. Herein, orthophosphate-based green/yellow-emitting Na₃Tb(PO₄)₂:Ce³⁺/Eu²⁺ (NTPO:Ce³⁺/Eu²⁺) phosphors have been successfully synthesized by a facile solid-state reaction method. The absorption band of NTPO samples was extended to the near-ultraviolet region and the absorption efficiency was significantly improved owing to a highly efficient energy transfer from Ce³⁺/Eu²⁺ ion to Tb³⁺ ion in NTPO host certified by time-resolved PL spectra. Upon 300 nm excitation, the NTPO:Ce³⁺ is characterized by ultra-narrow-band green emission of Tb³⁺ with an absolute quantum yield of 94.5%. Unexpectedly, NTPO:Eu²⁺ emits bright yellow light with a color purity of 73% as a result of the blending of green light emission from Tb³⁺ and red light emission from Eu³⁺. The thermal stability has been improved by controlling the stoichiometric ratio of Na⁺. The prototype white LED used yellow-emitting NTPO:Eu²⁺ phosphor has higher color-rendering index (Ra = 83.5), lower correlated color temperature (CCT = 5206 K), and closer CIE color coordinates (0.338, 0.3187) to the standard white point at (0.333, 0.333) than that used green-emitting NTPO:Ce³⁺ phosphor, indicating the addition of the yellow light component improved the Ra of the trichromatic (RGB) materials.     

Synthesis and optoelectronic properties of luminescent copolyfluorenes slightly doped with thiophene chromophore

This paper describes the synthesis of new copolyfluorenes (P05-P5) slightly doped with 2,5-bis(2-phenyl-2-cyanovinyl)thiophene (GM, <3.4 mol%) and their application in electroluminescent (EL) devices. In film state, EL spectra of the copolyfluorenes are very different from photoluminescence (PL) spectra, which have been ascribed to charge trapping in GM and energy transfer from fluorene segments to GM chromophores. The maximum brightness and current efficiency of EL device from P05 (5230 cd/m², 0.65 cd/A) are significantly enhanced when compared with those from poly(9,9-dihexylfluorene) (PF) (1310 cd/m², 0.18 cd/A). The EL device using blend of P5 and PF (w/w = 10/1) as emitting layer exhibits near-white emission with CIE coordinate being (0.26, 0.32). The results demonstrate that the copolyfluorenes slightly doped with GM chromophore are promising emitting materials for optoelectronic devices.     

Vanadium nitride and carbon nanofiber composite membrane as an interlayer for extended life cycle lithium-sulphur batteries

In this study, carbon nanofibre (CNF) and vanadium-nitride-modified CNF (VNCNF) were fabricated by the electrospinning method, followed by carbonisation. The fabricated VNCNF and CNF were sandwiched between the cathode and separator to be used as interlayers. The lithium-sulphur (Li–S) cell employing the VNCNF interlayer exhibited high initial charge and discharge capacities of 1452 and 1480 mAh g^?1, corresponding a coulombic efficiency higher than 100% at a rate of 0.5 C, whereas the cell with the CNF interlayer delivered the initial charge and discharge capacities of 858 and 772 mAh g^?1, respectively, corresponding to a coulombic efficiency of 89.97%. Even after 400 cycles, the cell with the VNCNF interlayer retained a remarkable high capacity of 923 mAh g^?1, whereas the cell with the CNF interlayer showed only 586 mAh g^?1 at a rate of 0.5 C. Furthermore, the cell with the VNCNF interlayer exhibited an excellent rate capability of up to 2 C, which was much higher than those of the cells with CNF and without an interlayer. The better performance of Li–S cell with the VNCNF interlayer is attributed to the strong adsorption ability of VNCNF for polysulphide by suppressing the shuttle effect. In addition, the catalytic effect of VN in CNF accelerated the capturing and utilisation of the dissolved active materials, thereby enhancing the Red-Ox kinetics and capacity.