New pyridopyrazine skelecton-based electron-transporting materials

2,3-Di(pyridine-2-yl)-7-(4-triphenylsilyl)phenyl)pyrido[2,3-b]pyrazine (DPPP) containing pyridopyrazine was designed and synthesized as a new electron-transporting material for organic light-emitting devices (OLEDs). The obtained material forms homogeneous and stable amorphous film. The new synthesized showed the reversible cathodic reduction for hole blocking material and the low reduction potential for electron transporting material in organic EL devices. The molecule possess excellent thermal stability with glass transition temperature (T(g)) of 115 degrees C in nitrogen. DNTPD (60 nm)/NPD (30 nm)/CBP:Irppy 6% (40 nm)/BAIq (10 nm)/ETL (30 nm)/LiF (0.5 nm)/Al structured device were fabricated using DPPP as electron transport material. The maximum luminance reached at 25000 cd/m2. The current efficiency was 10.9 cd/A even high current.     

Highly efficient blue light-emitting materials based on arylamine substituted DPVBi derivatives

A series of arylamine substituted DPVBi derivatives (1-4) were synthesized via the Horner-Wadsworth-Emmons reaction. Their electroluminescent properties were examined by fabricating a multilayer OLED device with the following structure: ITO/DNTPD (40 nm)/NPB (20 nm)/2% DPVBi derivatives (1-4) doped in MADN (20 nm)/Alq3 (40 nm)/Liq (1.0 nm)/Al. All devices showed efficient blue emission. In particular, a high efficiency blue OLED was fabricated using compound 1 as a dopant in the emitting layer. The maximum luminance, luminous efficiency, power efficiency and CIE coordinates of the blue OLED using compound 1 as a dopant were 16110 cd/m2 at 10 V, 10.1 cd/A at 20 mA/cm2, 4.37 Im/W at 20 mA/cm2, and (x = 0.197, y = 0.358) at 8 V, respectively. Moreover, a device using compound 4 as the dopant exhibited efficient deep blue emission with a luminance, luminous efficiency, power efficiency and CIE coordinates of 7005 cd/m2 at 10 V, 6.25 cd/A at 20 mA/cm2, 2.50 Im/W at 20 mA/cm2 and (x = 0.151, y = 0.143) at 8 V, respectively.     

Fabrication and characterization of organic light-emitting diodes using zinc complexes as hole-blocking layer

2-(2-Hydroxyphenyl)benzoxazole (HPB) was employed as organic ligand and the corresponding zinc complexes (Zn(HPB)2 and Zn(HPB)q) were synthesized. And their EL properties were characterized. The structures of zinc complexes were determined with FT-NMR, FT-IR, UV-Vis, and XPS. The thermal stability showed up to about 300 degrees C under nitrogen flow, which was measured by TGA. The photoluminescence (PL) of zinc complexes were measured from the DMF solution. The PL emitted in blue and yellow region, respectively. The EL devices were fabricated by the vacuum deposition. Two kinds of OLEDs devices were fabricated; ITO/NPB (40 nm)/Zn complexes (60 nm)/LiF/Al and ITO/NPB (40 nm)/Alq3 (60 nm)/Zn complexes (5 nm)/LiF/Al. Both of the EL properties as the emitting and the hole-blocking layer were investigated. The EL emission of Zn(HPB)q exhibited green light centered at 532 nm. The device showed a turn-on voltage at 5 V and a luminance of 6073 cd/m2 at 10 V. Meanwhile, the maximum EL the emission of the Zn(HPB)2 device was found to be at 447 nm. And the device showed a luminance of 2813 cd/m2 at 10 V. The ITO/NPB (40 nm)/Alq3 (60 nm)/Zn(HPB)2 (5 nm)/LiF/Al device showed increased luminance of L=17000 cd/m2 compared to L=12000 cd/m2 for similar device fabricated without the hole-blocking layer. And the turn-on voltage was significantly affected by the existence of the hole-blocking layer.     

Trimethylsilane-containing donor-acceptor-donor type material for red fluorescent organic light-emitting diodes

In this paper, we described a donor-acceptor-donor type red fluorescence material, which have the bulky trimethylsilane groups in the donor moieties. To explore the electroluminescence properties of these materials, multilayered OLEDs were fabricated with a device structure of ITO/2-TNATA (60 nm)/NPB (40 nm)/Red 1 (2%):rubrene (50%):Alq3 (30 nm)/Alq3 (60 nm)/Liq (3 nm)/Al (100 nm). A device using Red 1 as the dopant material showed a maximum luminance of 5138 cd/m2 at 12.0 V, maximum luminous efficiencies of 1.62 cd/A, and maximum power efficiencies of 1.04 lm/W. The Commission Internationale de L'Eclairage coordinates of this device was (0.67, 0.33) at 7.0 V, which indicated stable color chromaticity at various voltages.     

Synthesis, thermal, photoluminescent, and electroluminescent properties of a novel quaternary Eu(III) complex containing a carbazole hole-transporting functional group

A novel quaternary Eu(III) complex containing a carbazole fragment as hole-transporting functional group was synthesized, and its thermal stability, photoluminescent (PL), electroluminescent (EL) properties were studied. Its glass transition temperature (T _g) was 131 °C and 5% weight loss temperature was 325 °C. In studies of its EL properties, two devices with the Eu(III) complex as red light-emitting materials were fabricated and measured. Device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq₃ (30 nm)/LiF (0.7 nm)/Al (100 nm), NPB was N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine as the hole-transporting layer, Alq₃ was tris(8-hydroxyquinoline) aluminum as the electron-transporting layer. Device 1 gave two emission bands of the Eu(III) complex and Alq₃ with the maximum luminance of 437 cd/m² at 17.34 V, and its turn-on voltage was 10 V. In device 2, an electron-transporting/hole-locking layer of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 30 nm) was added between the Eu(III) complex and Alq₃ layers, only a sharp red emission band of the Eu(III) complex was given with the maximum luminance of 186 cd/m² at 20.08 V, and its turn-on voltage was 12 V.     

High efficiency green phosphorescent organic light emitting device with (TCTA/TCTA0.5TPBi0.5/TPBi): Ir(ppy)3 emission layer

A new high efficiency green light emitting phosphorescent device with an emission layer consisting of {4,4',4'-tris(N-carbazolyl)-triphenylamine[TCTA]/TCTA_0.5TPBi_0.5/1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene[TPBi]}:tris(2-phenylpyridine)iridium(III)[Ir(ppy)₃] was fabricated and its electroluminescence characteristics were evaluated in comparison with those of devices with emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/ TPBi):Ir(ppy)₃.The device with the emission layer consisting of (TCTA/TCTA_0.5TPBi_0.5/TPBi):Ir(ppy)₃ showed a luminance of 11,000 cd/m² at an applied voltage of 8 V and maximum current efficiency of 63 cd/A under a luminance of 500 cd/m². The peak wavelength in the electroluminescent spectral and color coordinate on the Commission Internationale de I'Eclairage(CIE) chart were 513 nm and (0.31, 0.62) in this device, respectively. Under a luminance of 10000 cd/m², the current efficiency of this device was 55 cd/A, which is 1.4 and 1.1 times better than those of the devices with the emission layers made of (TCTA_0.5TPBi_0.5):Ir(ppy)₃ and (TCTA/TPBi):Ir(ppy)₃, respectively.     

Synthesis and electroluminescent properties of blue-emitting t-butylated bis(diarylaminoaryl)anthracenes for OLEDs

A new series of blue fluorescent emitters based on t-butylated bis(diarylaminoaryl) anthracenes were synthesized and their electroluminescent properties investigated. Into these blue materials, t-butyl groups were introduced to both prevent molecular aggregation between the blue emitters through steric hindrance and reduce self-quenching. As such, this would contribute to overall improvement in OLED efficiency. To explore the electroluminescent properties of these materials, multilayered OLEDs were fabricated into a device structure of: ITO/NPB(50 nm)/blue emitters doped in ADN(30 nm)/Alq₃(20 nm)/Liq(2 nm)/Al(100 nm). All devices showed efficient blue emissions. In particular, one device exhibited highly efficient sky blue emissions with a maximum luminance of 11,060 cd/m² at 12.0 V and respective luminous and power efficiencies of 6.59 cd/A and 2.58 lm/W at 20 mA/cm². The peak wavelength of the electroluminescence was 468 nm with CIEx,y coordinates of (0.159, 0.198) at 12.0 V. In addition, a deep blue device with CIEx,y coordinates of (0.159, 0.151) at 12.0 V showed a luminous efficiency of 4.2 cd/A and power efficiency of 1.66 lm/W at 20 mA/cm².     

The light-emitting device consisting of organic white-light components

A material compound, 9,9-bis{4′-[2″-(carbazolyl)-vinyl]-phenyl}fluorene (F-CZV), was specially synthesized and used to fabricate the efficient white organic light-emitting devices (WOLEDs). The absorption peaks appear at 340 nm and 346 nm in dilute dichloromethane solution and film, respectively. The photoluminescence peaks appear at 350 nm and 400 nm in the solution and film, respectively. Photoluminescent quantum yield in solution is ca 0.82 by using quinine sulfate as the standard. In an optimized electroluminescent device structure of ITO/NPB (40 nm)/F-CZV (30 nm)/BPhen (40 nm)/Mg:Ag, the saturated white-light emission was observed at Commission International De L’Eclairage (CIE) coordinates of (0.30, 0.33) at 10 V. The El spectrum of the device is close to independent of the applied driving voltage. It’s maximum brightness and current efficiency is 700 cd/m² and 0.41 cd/A, respectively.     

Morphology Engineering for High‐Performance and Multicolored Perovskite Light‐Emitting Diodes with Simple Device Structures

The film morphology is extremely significant for solution processed perovskite devices. Through fine morphology engineering without using any additives or further posttreatments, a full‐coverage and high quantum yield perovskite film has been achieved based on one‐step spin‐coating method. The morphologies and film characteristics of MAPbBr₃ with different MABr:PbBr₂ starting material ratios are in‐depth investigated by scanning electron microscopy, atomic force microscopy, X‐ray diffraction, photoluminescence, and time resolved photoluminescence. High performance organometal halide perovskite light‐emitting didoes (PeLEDs) based on simple device structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/perovskite/TPBi/Ca/Al are demonstrated. The green PeLED based on MAPbBr₃ shows a maximum luminance of 8794 cd m^?2 (at 7.3 V) and maximum current efficiency of 5.1 cd A^?1 (at 5.1 V). Furthermore, a class of hybrid PeLEDs by adjusting the halide ratios of methylammonium lead halide (MAPbX₃, where X is Cl, Br, or I) are also demonstrated at room temperature. These mix‐halogenated PeLEDs show bright luminance (above 100 cd m^?2) with narrow and clean emission bands over the wide color gamut.     

Red-phosphorescent OLEDs employing iridium (III) complexes based on 5-benzoyl-2-phenylpyridine derivatives

A series of red-phosphorescent iridium (III) complexes 1-4 based on 5-benzoyl-2-phenylpyridine derivatives was synthesized. Their photophysical and electrophosphorescent properties were investigated. Multilayered OLEDs were fabricated with a device structure ITO/4,4′,4″-tris(N-(naphtalen-2-yl)-N-phenyl-amino)triphenylamine (60 nm)/4,4′-bis(N-naphtylphenylamino)biphenyl (20 nm)/Ir(III) complexes (8%) doped in 4,4′-N,N′-dicarbazolebiphenyl (30 nm)/2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (10 nm)/tris(8-hydroxyquinolinyl)aluminum(III) (20 nm)/Liq (2 nm)/Al (100 nm). All devices exhibited efficient red emissions. Among those, in a device containing iridium complex 1 dopant, the maximum luminance was 14200 cd/m² at 14.0 V. Also, its luminous, power, and quantum efficiency were 10.40 cd/A, 3.44 lm/W and 9.21% at 20 mA/cm², respectively. The peak wavelength of the electroluminescence was 607 nm, with CIE coordinates of (0.615, 0.383) at 12.0 V, and the device also showed a stable color chromaticity with various voltages.     

Electroluminescence from single-layer thin-film devices based on three binuclear Ru(II) complexes with different length of flexible bridges

The spectroscopic, electrochemical and electroluminescent properties of three binuclear Ru(II) complexes with different length of flexible bridges were investigated. The single-layer electroluminescent devices with configuration of indium-tin oxide/Ru complex (~ 100 nm)/Ga:In were found to give a turn-on voltages as low as 2.3 V, a maximum luminance up to 310 cd/m² at a bias voltage of 5.8 V, and low delay times less than 2 s.     

Fabrication of efficient polymer light-emitting diodes using water/alcohol soluble poly(vinyl alcohol) doped with alkali metal salts as electron-injection layer

An effective electron-injection layer (EIL) is crucial to efficient polymer light-emitting diodes (PLEDs) with high work-function metal as cathode. This work presents the use of water/alcohol soluble poly(vinyl alcohol) (PVA), especially doped with alkali metal salts, as a highly effective EIL to fabricate efficient multilayer PLEDs, allowing the use of stable aluminum as the cathode. Using neat PVA as EIL, the maximum brightness and maximum current efficiency of the device [ITO/PEDOT:PSS/SY/PVA/Al(90 nm)] were significantly enhanced to 5518 cd/m² and 2.64 cd/A (from 395 cd/m² and 0.06 cd/A without the EIL) due to promoted electron-injection and hole-blocking. The device performance is further enhanced by doping the PVA with alkali metal salts (M₂CO₃ or CH₃COOM; M: Na, K, Cs), and the enhancement is increased with increasing dopant concentration. Particularly, the PVA doped with 30 wt% alkali metal carbonates revealed the best performance (20214–25163 cd/m², 5.83–6.83 cd/A). This has been attributed to improved electron-injection from aluminum cathode, which has been confirmed by the corresponding increase in the open-circuit voltages (V _oc) obtained from photovoltaic measurements. Current results indicate that commercially available PVA are promising electron-injection layer for PLEDs when doped with appropriate alkali metal salts.     

Highly efficient red phosphorescent Ir(III) complexes for organic light- emitting diodes based on aryl(6-arylpyridin-3-yl)methanone ligands

A series of phosphorescent Ir(III) complexes 1-4 were synthesized based on aryl(6-arylpyridin-3-yl)methanone ligands, and their photophysical and electroluminescent properties were characterized. Multilayer devices with the configuration, Indium tin oxide/4,4′,4″-tris(N-(naphthalene-2-yl)-N-phenyl-amino)triphenylamine (60 nm)/4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (20 nm)/Ir(III) complexes doped in N,N′-dicarbazolyl-4,4′-biphenyl (30 nm, 8%)/2,9-dimethyl-4,7-diphenyl-phenathroline (10 nm)/tris(8-hydroxyquinoline)-aluminum (20 nm)/lithium quinolate (2 nm)/ Al (100 nm), were fabricated. Among these, the device employing complex 2 as a dopant exhibited efficient red emission with a maximum luminance, luminous efficiency, power efficiency and quantum efficiency of 16200 cd/m² at 14.0 V, 12.20 cd/A at 20 mA/cm², 4.26 lm/W and 9.26% at 20 mA/cm², respectively, with Commission Internationale de l'Énclairage coordinates of (0.63, 0.37) at 12.0 V.     

Pure blue light-emitting fluorene-based conjugated polymer with excellent thermal, photophysical, and electroluminescent properties

A functionalized polyfluorene (FPF) with dendritic carbazole and oxadiazole side chains have been successfully designed, synthesized, and characterized. The weight-average molecular weight (M _w) and number-average molecular weight (M _n) were measured by gel permeation chromatography (GPC) to be 71280 and 31185, respectively. The FPF is thermally stable with high decomposition temperature (T _d = 495 °C) and glass transition temperature (T _g = 160 °C), and show good solubility in organic solvents, such as N,N-dimethyl formamide, tetrahydrofuran, CHCl₃, and toluene. The photoluminescent and electroluminescent (EL) emissions and Commission Internationale de L’Eclairage coordinates indicated pure blue light emission of FPF and its device. The luminance–voltage (L–V) and current density–voltage (J–V) characteristics of the devices based on FPF indicated typical diode characteristics with a maximum luminance of 980 cd/m² at a drive voltage of 13.0 V, a maximum current density of 726 mA/cm², and a turn-on voltage of 8.54 V. The maximum EL efficiency of the device based on FPF was measured to be 0.46 % at the current density of 12.5 mA/cm². These results indicate that the FPF polymer could be a promising candidate for pure blue light-emitting polymer with excellent thermal, photophysical, and electroluminescent properties.     

Transparent organic light-emitting devices with LiF/Yb:Ag cathode

Highly transparent organic light-emitting devices (TOLEDs) based on LiF/Yb:Ag cathode are reported. The device with a structure of Indium tin oxide/N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biph-enyl-4, 4′-diamine (50 nm)/tris (8-hydroxyquinoline) aluminum (50 nm)/LiF(0.5 nm)/Yb:Ag (15 nm, volume ratio 1:1) shows about the same luminances and spectra from both sides. At a current density of 20 mA/cm², the luminance and electroluminescent efficiency of the top/bottom side are 165/167 cd/m² and 0.825/0.835 cd/A, respectively. We attribute the characteristics to the high transmittance and low reflectivity of Yb:Ag cathode. The results suggest that LiF/Yb:Ag electrode can be used as an effective and stable cathode in TOLEDs or top-emitting OLEDs.     

Synthesis and electro-optical properties of carbazole-substituted pyrene derivatives

Mono and dicarbazole-substituted pyrene derivatives, 9H-carbazol-9-ylpyrene (MCzP) and 1,6-di(9H-carbazol-9-yl)pyrene (DCzP), with dual-purpose function as a blue emitting and charge transporting layer in organic light emitting diodes, were synthesized and characterized. These series of molecules consisted of an electron donating (D) carbazole and an electron accepting (A) pyrene in D-A and D-A-D shapes. Non-doped blue electroluminescent devices with the configurations of ITO (150 nm)/alpha-NPD (30 nm)/DCzP (40 nm)/LiF (1 nm)/Al (150 nm) (D1) and ITO (150 nm)/2-TNATA (15 nm)/alpha-NPD (20 nm)/DCzP (40 nm)/BCP (15 nm)/Alq3 (10 nm)/LiF (1 nm)/Al (120 nm) (D2) were fabricated. D1 and D2 devices showed blue emission at 492 nm and 488 nm, and maximum luminance of 840 and 7560 cd/m2 obtained at 13 V and 15 V, respectively.     

Photo/electroluminescence properties of an europium (III) complex doped in 4,4′-N,N′-dicarbazole-biphenyl matrix

The photoluminescence properties of one europium complex Eu(TFNB)₃Phen (TFNB = 4,4,4-trifluoro-1-(naphthyl)-1,3-butanedione, Phen = 1,10-phenanthroline) doped in a hole-transporting material CBP (4,4′-N,N′-dicarbazole-biphenyl) films were studied. A series of organic light-emitting devices (OLEDs) using Eu(TFNB)₃Phen as the emitter were fabricated with a multilayer structure of indium tin oxide, 250 Ω/square)/TPD (N,N′-diphenyl-N,N′-bis(3-methyllphenyl)-(1,1′-biphenyl)-4,4′-diamine, 50 nm)/Eu(TFNB)₃phen (x): CBP (4,4′-N,N′-dicarbazole-biphenyl, 45 nm)/BCP (2,9-dimethyl-4,7-diphenyl-l,10 phenanthroline, 20 nm)/AlQ (tris(8-hydroxy-quinoline) aluminium, 30 nm)/LiF (1 nm)/Al (100 nm), where x is the weight percentage of Eu(TFNB)₃phen doped in the CBP matrix (1-6%). A red emission at 612 nm with a half bandwidth of 3 nm, characteristic of Eu(III) ion, was observed with all devices. The device with a 3% dopant concentration shows the maximum luminance up to 1169 cd/m² (18 V) and the device with a 5% dopant concentration exhibits a current efficiency of 4.46 cd/A and power efficiency of 2.03 lm/W. The mechanism of the electroluminescence was also discussed.     

Novel green-emitting polymer containing fluorene and 1-(2-benzothiazolyl)-3,5-diphenylpyrazoline

Novel electroluminescent (EL) polymer based on fluorene having benzothiazolylpyrazoline unit in the main chain was synthesized. The result polymer possessed satisfactory thermal stability with onset decomposition temperature (T _d) of 401 °C and glass-transition temperature (T _g) of 213 °C. The polymer emits green fluorescence with high photoluminescence (PL) quantum yield of 47%. Polymer light-emitting diode (PLED) was fabricated with the configuration of ITO/PEDT 40 nm/PVK 40 nm/polymer(80 nm)/Ba(4 nm)/Al(160 nm) showed turn-on voltage of 4.5 V, and it can emit green light with maximum brightness of 1726 cd m⁻² with the maximum external quantum efficiency of 1.59%.     

Enhanced performance in organic light-emitting diodes by sputtering TiO2 ultra-thin film as the hole buffer layer

Organic light-emitting diodes were prepared using titanium oxide (TiO₂) ultra-thin film by RF magnetron sputtering as the hole buffer layer. The device configuration is ITO/TiO₂/N-N′-diphenyl-N-N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine/tris(8-quinolinolato)-aluminum/LiF/Al. The maximum luminous efficiency for the 1.2 nm TiO₂ device is increased by approximately 46% (6.0 cd/A), in comparison with that of the control device (4.1 cd/A). The atomic force microscopy shows that with the insertion of TiO₂ buffer layer, the roughness of ITO surface decreases, which is favorable to improve the device luminance and increase the device lifetime. The mechanism behind the enhanced performance is that the TiO₂ layer enhances most of the holes injected from the anode and improves the balance of the hole and electron injections.     

Experimental study of the organic light emitting diode with a p-type silicon anode

We have fabricated and studied an organic light emitting diode (OLED) with a p-type silicon anode and a SiO₂ buffer layer between the anode and the organic layers which emits light from a semitransparent top Yb/Au cathode. The luminance of the OLED is up to 5600 cd/m² at 17 V and 1800 mA/cm², the current efficiency is 0.31 cd/A. Both its luminance and current efficiency are much higher than those of the OLEDs with silicon as the anodes reported previously. The enhancement of the luminance and efficiency can be attributed to an improved balance between the hole- and electron-injection through two efficient ways: 1) restraining the hole-injection by inserting an ultra-thin SiO₂ buffer layer between the Si anode and the organic layers; and 2) enhancing the electron-injection by using a low work function, low optical reflectance and absorption semitransparent Yb/Au cathode.