Efficient white organic light-emitting device based on a thin layer of hole-transporting host with rubrene dopant

A white organic light-emitting device (WOLED) in which a yellow fluorescent dye, rubrene (5,6,11,12-tetraphenylnaphthacene), is doped into a thin layer of traditional hole-transporting material NPB {4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl} is fabricated. The device has a simple structure of indium tin oxide (ITO)/CuPc/NPB/NPB: 0.7 wt.% rubrene/TPBI/lithium fluoride (LiF)/Al, where CuPc (copper phthalocyanine) and TPBI {2,2′,2″-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole]} are used as the hole-injecting layer and electron-transporting and hole-blocking layer, respectively. The device exhibits peak efficiency of 3.7 cd/A (2.1 lm/W) at 5.5 V and maximum brightness of 8200 cd/m² at 20 V. The Commission Internationale de l’Eclairage (CIE) coordinates of (0.291, 0.303) are determined at 6 V. When the bias increased from 6 V to 14 V, the colour coordinates shifted only by 2%, which is presumably related closely to the thickness of the doped NPB layer. Besides, the electroluminescent (EL) efficiency can also be improved by decreasing the thickness of the doped NPB layer. The mechanisms of generating stable white colour and improving EL performances are also discussed.     

White organic light-emitting diodes based on emission from DPVBi-doped 4,48-bis (2,28-diphenylvinyl)-1, 18-biphenyl

Organiclight-emitting diodes (OLEDs) have beenin-vestigated for many years on account of their highlumi-nance,low driven voltages ,wide visual range,flexiblesubstratesinflat-panel ,full color displays and backlightapplications .For high brightness and eff…     

Effect of lithium and silver diffusion in single-stack and tandem OLED devices

A study of metal (Li, Ag) diffusion has been carried out in an archetypal OLED device based on N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine|tris(8-hydroxyquinolinato) aluminum|4,7-diphenyl-1,10-phenanthroline (NPB|Alq₃|Bphen). Using single-stack and two-stack tandem OLED structures with variations of layer thicknesses and metal layer placements, we have found that Ag vapor-deposited on Alq₃ layer can diffuse or penetrate deep into Alq₃, up to ∼2,000 Å, causing luminescence quenching. This diffusion can be substantially prevented by a thin layer of Li or Bphen deposited on Alq₃ prior to the deposition of Ag. In contrast, Li diffusion in either Alq₃ or Bphen is limited to about 50–100 Å. Li appears to be able to diffuse into Bphen irrespective of the order of Li and Bphen depositions.     

Mechanical modeling of flexible OLED devices

General expressions are deduced for the stresses developed in the individual thin layers of a multi-layer structure as a result of bending to a specified radius. These are appropriate for analysing flexible organic light emitting diode (FOLED) devices on flexible substrates. Residual stress (caused internally by temperature change and differential thermal expansion) after material deposition and return to ambient temperatures is not considered. The reduced elastic modulus of the typical small molecule OLED materials: N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPD) and tris-(8-hydroxyquinoline)aluminum (Alq₃) are measured as thin-films using nano-indentation techniques. A conventional device: polyethylene terephthalate (PET)/Buffer layer (BL)/ITO/OLED/Al is considered from a modeling standpoint, as a preliminary to actual fabrication and subsequent comparative testing of OLED performance on rigid and flexible supports.     

Organic alternating current electroluminescence device based on 4,4′-bis(N-phenyl-1-naphthylamino) biphenyl/1,4,5,8,9,11-hexaazatriphenylene charge generation unit

An organic alternating current electroluminescence (OACEL) device based on 4,4′-bis(N-phenyl-1-naphthylamino) biphenyl (NPB)/1,4,5,8,9,11-hexaazatriphenylene (HAT-CN)/tris(8-hydroxy-quin-olinato) aluminum (Alq₃) doped with cesium carbonate (Cs₂CO₃) internal charge generation unit is demonstrated. Maximum luminance of 299 cd/m² is observed for Alq₃ doped with 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H-(1) benzopyropyrano (6,7-8-I,j)quinolizin-11-one (C545T) fluorescent emission layer when driven with a peak–peak voltage of 80 V at 120 kHz. The key charge-generation role of NPB/HAT-CN interface is studied experimentally. Furthermore, influence of evaporation sequence of this internal charge generation unit on OACEL performance is investigated. This work demonstrated that the undoped charge generation unit – NPB/HATCN, can also be a good candidate for charge generation unit of OACEL device.     

Improved property in organic light-emitting diode utilizing two Al/Alq3 layers

We reported on the fabrication of organic light-emitting devices (OLEDs) utilizing the two Al/Alq₃ layers and two electrodes. This novel green device with structure of Al(110 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃)(65 nm)/Al(110 nm)/Alq₃(50 nm)/N,N′-dipheny1-N, N′-bis-(3-methy1phyeny1)-1, 1′-bipheny1-4, 4′-diamine (TPD)(60 nm)/ITO(60 nm)/Glass. TPD were used as holes transporting layer (HTL), and Alq₃ was used as electron transporting layer (ETL), at the same time, Alq₃ was also used as emitting layer (EL), Al and ITO were used as cathode and anode, respectively. The results showed that the device containing the two Al/Alq₃ layers and two electrodes had a higher brightness and electroluminescent efficiency than the device without this layer. At current density of 14 mA/cm², the brightness of the device with the two Al/Alq₃ layers reach 3693 cd/m², which is higher than the 2537 cd/m² of the Al/Alq₃/TPD:Alq₃/ITO/Glass device and the 1504.0 cd/m² of the Al/Alq₃/TPD/ITO/Glass. Turn-on voltage of the device with two Al/Alq₃ layers was 7 V, which is lower than the others.     

Low sublimation temperature cesium pivalate complex as an efficient electron injection material for organic light-emitting diode devices

Cesium pivalate ((CH₃)₃CCOOCs) has been synthesized and applied as an electron injection material for organic light-emitting diodes, which showed low sublimation temperature of 180 °C. Typical bilayer structure of ITO/NPB (60 nm)/Alq₃ (50 nm)/EIL/Al was used to evaluate the electron injection efficacy of (CH₃)₃CCOOCs, the results showed (CH₃)₃CCOOCs/Al exhibits better electron injection than LiF/Al cathode and the power efficiency was improved by about 19% at current density of 50 mA/cm². More interestingly, in the typical three layer OLED structure ITO/2-TNATA (60 nm)/NPB (10 nm)/Alq₃:2% C545T (40 nm)/MADN (15 nm)/(CH₃)₃CCOOCs (2 nm)/Al, the maximum current efficiency is up to 20 cd/A with Commission Internationale d’Eclairage (CIE_x,_y) color coordinates of (x = 0.30, y = 0.65) at current density of 140 mA/cm², which indicates that the non-aromatic alkali metal complex can also have good match with the chemically stable compound and exhibit good electron injection properties.     

Silicon dioxide deposited by ECR-PECVD for low-temperature Si devices processing

Silicon dioxide films have been deposited at temperatures less than 270 °C in an electron cyclotron resonance (ECR) plasma reactor from a gas phase combination of O₂, SiH₄ and He. The physical characterization of the material was carried out through pinhole density analysis as a function of substrate temperature for different μ-wave power (E_w). Higher E_w at room deposition temperature (RT) shows low defects densities (<7 pinhole/mm²) ensuring low-temperatures process integration on large area. From FTIR analysis and Thermal Desorption Spectroscopy we also evaluated very low hydrogen content if compared to conventional rf-PECVD SiO₂ deposited at 350 °C. Electrical properties have been measured in MOS devices, depositing SiO₂ at RT. No significant charge injection up to fields 6–7 MV/cm and average breakdown electric field >10 MV/cm are observed from ramps I–V. Moreover, from high frequency and quasi-static C–V characteristics we studied interface quality as function of annealing time and annealing temperature in N₂. We found that even for low annealing temperature (200 °C) is possible to reduce considerably the interface state density down to 5 × 10¹¹ cm⁻² eV⁻¹. These results show that a complete low-temperatures process can be achieved for the integration of SiO₂ as gate insulator in polysilicon TFTs on plastic substrates.     

利用石墨烯掺杂在NPB中的OLED性能研究

采用NPB掺杂石墨烯作为空穴传输层,制备有机电致发光器件(OLED),器件结构为ITO/NPB:Graphene(20wt.%)(50nm)/Alq3(80nm)/LiF(0.5nm)/Al(120nm)。将其与标准器件ITO/NPB(50nm)/Alq3(80nm)/LiF(0.5nm)/Al(120nm)作性能比较,研究石墨烯对OLED性能的影响。结果表明,在NPB中掺杂石墨烯薄层的器件,在同等条件下性能最佳,当电流密度为90mA/cm2时器件电流效率达到最大值3.40cd/A,与标准器件最高效率相比增大1.49倍;亮度在15V时达到最大值10 070cd/m2,比标准器件最大亮度增大5.16倍。     

A DCM‐Type Red‐Fluorescent Dopant for High‐Performance Organic Electroluminescent Devices

2‐(2‐tert‐Butyl‐6‐((E)‐2‐(2,6,6‐trimethyl‐2,4,5,6‐tetrahydro‐1H‐pyrrolo[3,2,1‐ij]quinolin‐8‐yl)vinyl)‐4H‐pyran‐4‐ylidene)malononitrile (DCQTB) is designed and synthesized in high yield for application as the red‐light‐emitting dopant in organic light‐emitting diodes (OLEDs). Compared with 4‐(dicyanomethylene)‐2‐tert‐butyl‐6‐(1,1,7,7,‐tetramethyljulolidyl‐9‐enyl)‐4H‐pyran (DCJTB), one of the most efficient red‐emitting dopants, DCQTB exhibits red‐shifted fluorescence but blue‐shifted absorption. The unique characteristics of DCQTB with respect to DCJTB are utilized to achieve a red OLED with improved color purity and luminous efficiency. As a result, the device that uses DCQTB as dopant, with the configuration: indium tin oxide (ITO)/N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB; 60 nm)/tris(8‐quinolinolato) aluminum (Alq₃):dopant (2.3 wt %) (7 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 12 nm)/Alq₃(45 nm)/LiF(0.3 nm):Al (300 nm), shows a larger maximum luminance (L_max = 6021 cd m^–2 at 17 V), higher maximum efficiency (η_max = 4.41 cd A^–1 at 11.5 V (235.5 cd m^–2)), and better chromaticity coordinates (Commission Internationale de l'Eclairage, CIE, (x,y) = (0.65,0.35)) than a DCJTB‐based device with the same structure (L_max = 3453 cd m^–2 at 15.5 V, η_max = 3.01 cd A^–1 at 10 V (17.69 cd m^–2), and CIE (x,y) = (0.62,0.38)). The possible reasons for the red‐shifted emission but blue‐shifted absorption of DCQTB relative to DCJTB are also discussed.     

Bulk and interface properties of molybdenum trioxide-doped hole transporting layer in organic light-emitting diodes

Effects of doping molybdenum trioxide (MoO₃) in N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) are studied at various thicknesses of doped layer (25–500 Å) by measuring the current–voltage characteristics, the capacitance–voltage characteristics and the operating lifetime. We formed charge transfer complex of NPB and MoO₃ by co-evaporation of both materials to achieve higher charge density, lower operating voltage, and better reliability of devices. These improved performances may be attributed to both bulk and interface properties of the doped layer. The authors demonstrated that the interface effects play more important role in lowering the operating voltage and increasing the lifetime.     

Versatile hole injection of VO2: Energy level alignment at N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine/VO2/fluorine-doped tin oxide

Energy level alignments at the interface of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB)/VO₂/fluorine-doped tin oxide (FTO) were studied by photoemission spectroscopy. The overall hole injection barrier between FTO and NPB was reduced from 1.38 to 0.59 eV with the insertion of a VO₂ hole injection layer. This could allow direct hole injection from FTO to NPB through a shallow valence band of VO₂. Surprisingly, VO₂ can also act as a charge generation layer due to its small band gap of 0.80 eV. That is, its conduction band is quite close to the Fermi level, and thus electrons can be extracted from the highest occupied molecular orbital (HOMO) of NPB, which is equivalent to hole injection into the NPB HOMO.     

Pure‐Red Dye for Organic Electroluminescent Devices: Bis‐Condensed DCM Derivatives

In this paper, the bis‐condensed 4‐(dicyanomethylene)‐2‐methyl‐6‐[p‐(dimethylamino)styryl]‐4H‐pyran ( DCM) derivatives are introduced as a new class of red dye for organic light‐emitting devices (OLEDs). They showed more red‐shifted emission than the mono‐substituted DCM derivatives and the emission maxima increased as the electron‐donating ability of the aromatic donor group increased. On the basis of these results, red light‐emitting devices were fabricated with bis‐condensed DCM derivatives as red dopants. For a device of configuration ITO/TPD/Alq₃ + DADB (5.2 wt.‐%)/Alq₃/Al (where ITO is indium tin oxide, TPD is N,N′‐diphenyl‐N,N′‐bis(3‐methylphenyl)‐1,1′‐biphenyl‐4,4′‐diamine, Alq₃ is tris(8‐hydroxyquinoline) aluminum, and DADB is [2,6‐bis[2‐[5‐(dibutylamino)phenyl]vinyl]‐4H‐pyran‐4‐ylidene]propanedinitrile), pure red emission was observed with Commission Internationale de l’Eclairage (CIE 1931) coordinates of (0.658, 0.337) at 25 mA/cm².     

Blue organic light-emitting diode with improved color purity using 5-naphthyl-spiro[fluorene-7,9′-benzofluorene]

Novel spiro-type blue host material, 5-naphthyl-spiro[fluorene-7,9′-benzofluorene] (BH-1SN) and dopant material, 5-diphenyl amine-spiro[fluorene-7,9′-benzofluorene] (BH-1DPA) were successfully synthesized, and a blue OLED was made from them. The structure of the blue device is ITO/DNTPD/α-NPD/BH-1SN:5% dopant/Alq₃ or ET4/Al–LiF. Here, α-NPD is used as the hole transport layer, DNTPD as the hole injection layer, BH-1DPA or BD-1 as the blue dopant materials, Alq₃ or ET4 as the transporting layer and Al as the cathode. The blue devices doped with 5% BH-1DPA and BD-1 show blue EL emissions at 444 and 448 nm at 7 V, respectively, and a high efficiency of 3.4 cd/A at 5 V for the device was obtained from BH-1SN:5% BD-1/ET4. The CIE coordinates of the blue emission are 0.15, 0.08 at an applied voltage of 7 V for the device obtained from BH-1SN/5% BH-1DPA/Alq₃.     

Origin of Enhanced Hole Injection in Inverted Organic Devices with Electron Accepting Interlayer

Conventional organic light emitting devices have a bottom buffer interlayer placed underneath the hole transporting layer (HTL) to improve hole injection from the indium tin oxide (ITO) electrode. In this work, a substantial enhancement in hole injection efficiency is demonstrated when an electron accepting interlayer is evaporated on top of the HTL in an inverted device along with a top hole injection anode compared with the conventional device with a bottom hole injection anode. Current–voltage and space‐charge‐limited dark injection (DI‐SCLC) measurements were used to characterize the conventional and inverted N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)(1,1′biphenyl)‐4,4′diamine (NPB) hole‐only devices with either molybdenum trioxide (MoO₃) or 1,4,5,8,9,11‐hexaazatriphenylene hexacarbonitrile (HAT‐CN) as the interlayer. Both normal and inverted devices with HAT‐CN showed significantly higher injection efficiencies compared to similar devices with MoO₃, with the inverted device with HAT‐CN as the interlayer showing a hole injection efficiency close to 100%. The results from doping NPB with MoO₃ or HAT‐CN confirmed that the injection efficiency enhancements in the inverted devices were due to the enhanced charge transfer at the electron acceptor/NPB interface.     

Effects of quantization on random telegraph signals observed in deep-submicron MOSFETs

Random telegraph signals (RTS) have been investigated in the drain to source voltage of W_eff×L_eff=1.37×0.17 μm² medium-doped drain (MDD) n-type MOSFETs. The emission (τ_e) and capture (τ_c) times of the probed trap were studied as a function of gate voltage as well as substrate voltage. The small size and high doping density of the n-MOSFETs studied create a strong electric field in the MOSFET inversion layer, which makes the surface conduction band split into discrete energy levels. Therefore, modified expressions of τ_e and τ_c including the influence of bulk bias (V_SB), which changes the degree of quantization, are presented. The trap position in the oxide with respect to the Si–SiO₂ interface, and the trap energy, were calculated from the gate voltage dependence of the emission and capture times under different bulk bias conditions. The behavior of the emission and capture times predicted by the two-dimensional (2D) surface quantization effects is in qualitative agreement with the experimental results. The RTS amplitude (ΔV_DS/V_DS) shows a positive dependence on V_SB. The coefficient α for screened oxide charge scattering was calculated at different gate voltages and bulk bias from the RTS amplitude. In addition, the theoretical calculation of the scattering coefficient α, using a 2D surface mobility fluctuation model, was presented, which shows a good agreement with the experimental data.     

High efficiency fluorescent white organic light-emitting diodes with red,green and blue separately monochromatic emission layers

Highly efficient fluorescent white organic light-emitting diodes (WOLEDs) have been fabricated by using three red, green and blue, separately monochromatic emission layers. The red and blue emissive layers are based on 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) doped N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) and p-bis(p-N,N-diphenyl-amino-styryl) benzene (DSA-ph) doped 2-methyl-9,10-di(2-naphthyl) anthracene (MADN), respectively; and the green emissive layer is based on tris(8-hydroxyquionline)aluminum(Alq₃) doped with 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,1[H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-1]-one (C545T), which is sandwiched between the red and the blue emissive layers. It can be seen that the devices show stable white emission with Commission International de L’Eclairage coordinates of (0.41, 0.41) and color rendering index (CRI) of 84 in a wide range of bias voltages. The maximum power efficiency, current efficiency and quantum efficiency reach 15.9 lm/W, 20.8 cd/A and 8.4%, respectively. The power efficiency at brightness of 500 cd/m² still arrives at 7.9 lm/W, and the half-lifetime under the initial luminance of 500 cd/m² is over 3500 h.     

Fabrication and characterization of air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors

We present our first application of the neutral cluster beam deposition (NCBD) method to fabricate bilayer heterojunction-based organic light-emitting field-effect transistors (OLEFETs) by superimposing two layers of α,ω-dihexylsexithiophene (DH6T) and N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) successively. Based upon well-balanced ambipolarity (hole and electron field-effect mobilities of 2.22 × 10⁻² and 2.78 × 10⁻² cm²/Vs), the air-stable OLEFETs have demonstrated good field-effect characteristics, stress-free operational stability and electroluminescence under ambient condition.     

A host material containing tetraphenylsilane for phosphorescent OLEDs with high efficiency and operational stability

A host material containing tetraphenylsilane moiety, 9-(4-triphenylsilanyl-(1,1′,4,1′′)-terphenyl-4′′-yl)-9H-carbazole (TSTC), was synthesized for green phosphorescent organic light emitting diodes. The tetraphenylsilane moiety was introduced to provide high triplet energy level, thermal and chemical stability, and glassy properties leading to high efficiency and operational stability of the devices. Ir(ppy)₃ based OLEDs using the TSTC host and DTBT (2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine) hole blocking layer (HBL) resulted in the maximum external quantum efficiency of 19.8% and the power efficiency of 59.4 lm/W. High operational stability with a half lifetime of 160,000 h at an initial luminance of 100 cd/m² was achieved from an electrophosphorescent device using TSTC host and BAlq HBL.     

Four-wavelength white organic light-emitting diodes using 4,4′-bis-[carbazoyl-(9)]-stilbene as a deep blue emissive layer

White organic light-emitting diodes (WOLEDs) with four wavelengths were fabricated by using three doped layers, which were obtained by separating recombination zones into three emitter layers. Among these emitters, blue emissions with two wavelengths (456 and 487 nm) were occurred in the 4,4′-bis(carbazoyl-(9))-stilbene (BCS) host doped with a perylene dye. Also, a green emission was originated from the tris(8-quinolinolato)aluminum (III) (Alq₃) host doped with a green fluorescent of 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano [6,7,8-ij]-quinolizin-11-one (C545T) dye. Finally, an orange emission was obtained from the N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) host doped with a 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) dye. The white light could be emitted by simultaneously controlling the emitter thickness and concentration of fluorescent dyes in each emissive layer, resulting in partial excitations among those three emitter layers. Electroluminescent spectra of the device obtained in this study were not sensitive to driving voltage of the device. Also, the maximum luminance for the white OLED with the CIE coordinate of (0.34, 0.34) was 56,300 cd/m² at the applied bias voltage of 11.6 V. Also, its external quantum and the power efficiency at about 100 cd/m² were 1.68% and 2.41 lm/W, respectively.