Efficient white organic light-emitting devices using 4,7-diphenyl-1,10-phenanthroline as block layer

A white light-emitting device has been fabricated with a structure of ITO/m-MTDATA (45 nm)/NPB (10 nm)/DPVBi (8 nm)/DPVBi:DCJTB 0.5% (15 nm)/BPhen (x nm)/Alq₃ [(55−x) nm]/LiF (1 nm)/Al, with x=0, 4, and 7. BPhen was used as the hole-blocking layer. This results in a mixture of lights from DPVBi molecules (blue-light) and DCJTB (yellow-light) molecules, producing white light emission. The chromaticity can be readily adjusted by only varying the thickness of the BPhen layer. The CIE coordinates of the device are largely insensitive to the driving voltages. When the thickness of BPhen is 7 nm, the device exhibits peak efficiency of 6.87 cd/A (3.59 lm/W) at the applied voltage of 6 V, the maximum external quantum efficiency η_ext=2.07% corresponding to 6.18 cd/A, and the maximum brightness is 18494 cd/m² at 15 V.     

Combinatorial fabrication and screening of organic light-emitting device arrays

The combinatorial fabrication and screening of 2-dimensional (2-d) small molecular UV-violet organic light-emitting device (OLED) arrays, 1-d blue-to-red arrays, 1-d intense white OLED libraries, 1-d arrays to study Förster energy transfer in guest-host OLEDs, and 2-d arrays to study exciplex emission from OLEDs is described. The results demonstrate the power of combinatorial approaches for screening OLED materials and configurations, and for studying their basic properties.     

Polymer White-Light-Emitting Diodes with High Work Function Cathode Based on a Novel Phosphorescent Chelating Copolymer

Polymer white-light-emitting diodes are fabricated based on the blend of poly[9,9-di-（2-ethylhexyl）-fluorenyl-2, 7- diyl]-end capped with polysilsesquioxane （PFO） and a chelating copolymer of poly[（9,9-bis（3′-（N,N-dimethylamino） propyl）-2, 7-fluorene-alt-2, 7-（9,9-dioctylfluorene） ）-co- [2, 7-（9,9-dioctlyfluorene）-alt-5,5-bis（2-（4-methyl-l-naphtha- lene） pyridine-C^2,N） iridium （III） acethylacetonate]] （PFN-NaIr）. The device with the sole aluminium cathode is able to produce a comparably white electroluminescence efficiency of 1.31 cd/A to that of the device using low work function cathodes （such as Ba, Ca, etc.）. The CIE coordinates of the white light emission consisting of red, green and blue three components are nearly at （0.34, 0.35）. The mechanism of the white light emission from the device with the AI cathode is investigated, which is related to the efficient injection of electrons through the interface of PFN-Nalr/AI.     

Ca/Ag bilayer cathode for transparent white organic light-emitting devices

We have demonstrated that the compositional modification of the Ca/Ag films is principally responsible for a high transmittance (over 70% in the visible range) and low sheet resistance (10-12 Ω/sq). X-ray photoelectron spectroscopy (XPS) sputter depth profiling of Ca/Ag structure reveals the presence of Ca(OH)₂ and Ca metal. A chemical model of the Ca/Ag cathode is proposed. Using transparent ITO anode and Ca (10 nm)/Ag (10 nm) cathode, efficient white organic light-emitting devices (WOLEDs) emitting from both sides have been fabricated. Brightness of 3813 cd/m² and Commission Internationale de l’Eclairage (CIE) coordinates (0.36, 0.34) at 10 V through ITO anode and values of 1216 cd/m² and (0.33, 0.30) through Ca/Ag cathode are reported. A low turn-on voltage of 5.5 V is measured.     

Efficient White Light Emission Using a Single Copolymer with Red and Green Chromophores on a Conjugated Polyfluorene Backbone Hybridized with InGaN-Based Light-Emitting Diodes

We report an efficient white-light emission based on a single copolymer/InGaN hybrid light-emitting diode. The single copolymer consists of a conjugated polyfluorene backbone by incorporating 2,1,3-benzothiadiazole （BT） and 4,7-bis（2-thienyl）-2,1,3-benzothiadiazole （DBT） as green and red light-emitting units, respectively. For the single copolymer/InGaN hybrid device, the Commission Internationale de 1＇Eclairage （CIE） coordinates, color temperature Tc and color rendering index Ra at 20mA are （0.323,0.329）, 5960K and 86, respectively. In comparison with the performance of red eopolymer PFO-DBT15 （DOF：DBT=85：15 with DOF being 9＇9- dioctylfluorene） and green copolymer PFO-BT35 （DOF：BT=-65：35） blend/InGaN hybrid white devices, it is concluded that the chemically doped copolymer hybridized device shows a higher emission intensity and spectral stability at a high driving current than the polymer blend.     

Efficient Top-Emitting Polymer Light-Emitting Diodes Using Chromium as Anode

We demonstrate a high eftlciency top-emitting polymer light-emitting diode （TPLED） with chromium （Cr） taking as the anode. The TPLED structure is Cr/poly-3, 4-ethylenedioxythiophene （PEDOT：PSS）/poly [2-（4-3＇,7＇- dimethyloctyloxy）-phenyl]-p-phenylenevinylene） （P-PP V） /Ba/Ag. The Cr （ 100 nm） anode is prepared by sputterdepositing in a vacuum chamber. It is found that the device emissive properties are affected dramatically by the thickness of both PEDOT：PSS and the Ag cathode. Optimized thicknesses of PEDOT：PSS and Ag layer are 60nm and 15nm, respectively. The diode exhibits excellent electroluminescence （EL） properties, such as a turn-on voltage of 3.32 V, luminous eftlciency of 4.41 cd/A and luminance of 6989cd/m^2 at driving voltage of about 9 V.     

Tuning of the emission color of organic electroluminescent devices by exciplex formation at the organic solid interface

′ ,4^′′-tris(3-methylphenylphenylamino)triphenylamine, 1,3,5-tris[(4-diphenylaminophenyl)phenylamino]benzene, N, N^′-bis(3-methylphenyl)-N, N^′-diphenyl-[1,1^′-biphenyl]-4,4^′-diamine, and 4,4^′,4^′′-tri(N-carbazolyl)triphenylamine, emitted bright light resulting from the exciplex formed at the solid interface between TPOB and the hole-transporting material. The exciplex formation was evidenced by the measurements of the photoluminescence spectra and lifetimes of the mixture of an equimolar amount of TPOB and each of the hole-transporting materials. Tuning of the emission color from greenish blue to orange was attained by varying the ionization potential of the hole-transporting material for the fixed electron-transporting material of TPOB. Received: 27 July 1998/Accepted: 28 July 1998     

Enhancement of Stability of Polymer Light-Emitting Diodes by Post Annealing

We investigate the effect of thermal annealing before and after cathode deposition on the stability of polymer light-emitting diodes （PLEDs） based on green fluorescent polyfluorene derivative. The annealed PLEDs exhibit improved charge transport and red-shift emission compared to the as-fabricated device. The stability of the PLEDs is largely enhanced by post-annealing before and after Ca deposition, which is attributed to the enhanced charge transport and the intimate contact between the cathode and the emissive layer.     

Luminescence mechanisms of green and blue organic light-emitting devices utilizing hole-blocking layers

Optical and electrical measurements on green and blue organic light-emitting devices (OLEDs) with and without hole-blocking layers (HBLs) were performed, and the luminescence mechanisms of green and blue OLEDs utilizing HBLs were investigated by using energy band diagrams and carrier density distributions. The dependence of the electroluminescence efficiencies on the existence of HBLs was described on the basis of a luminescence mechanism. The density distributions of the electrons and the holes in OLEDs under applied electric fields were estimated from the energy band diagrams, taking into account the electronic parameters and the layer thicknesses. The luminescence efficiencies and the color chromaticities were significantly affected by the existence of the HBLs. These analyses can help improve understanding of the luminescence mechanisms at play in and the electroluminescence efficiencies of green and blue OLEDs with HBLs, and the present results provide important information on the optical properties for enhancing the efficiencies of OLEDs operating in the green and the blue regions of the spectra.     

High efficiency small molecule white organic light-emitting devices with a multilayer structure

In this paper, a new white organic light-emitting device (WOLED) with multilayer structure has been fabricated. The structure of devices is ITO/N, N-bis-(1-naphthyl)-N, N^′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB) (40 nm)/NPB: QAD (1%): DCJTB (1%) (10 nm) /DPVBi (10 nm) /2, 9-dimethyl, 4, 7-diphenyl, 1, 10-phenanthroline (BCP) (d nm)/tris-(8-hydroxyquinoline) aluminium (Alq₃)(50-d nm)/LiF (1 nm)/Al (200 nm). In our devices, a red dye 4-(dicyanomethylene)-2-t-butyl-6 (1, 1, 7, 7-tetramethyl julolidyl-9-enyl)-4H-pyran (DCJTB) and a green dye quinacridone (QAD) were co-doped into NPB. The device with 8 nm BCP shows maximum luminance of 12 852 cd/m² at 20 V. The current efficiency and power efficiency reach 9.37 cd/A at 9 V and 3.60 lm/W at 8 V, respectively. The thickness of the blocking layer permit the tuning of the device spectrum to achieve a balanced white emission with Commission International de’Eclairage (CIE) chromaticity coordinates of (0.33,0.33). The CIE coordinates of device change from (0.3278, 0.3043) at 5 V to (0.3251, 0.2967) at 20 V that are well in the white region, which is largely insensitive to the applied bias.     

Influence of Dopant Concentration on Electroluminescent Performance of Organic White-Light-Emitting Device with Double-Emissive-Layered Structure

A novel phosphorescent organic white-light-emitting device （WOLED） with contiguration of ITO/NPB/CBP： TBPe：rubrene/Zn（BTZ）2：Ir（piq）2（acac）/Zn（BTZ）2/Mg：Ag is fabricated successfully, where the phosphorescent dye bis （1-（phenyl）isoquinoline） iridium （Ⅲ） acetylanetonate （Ir（piq）2 （acac）） doped into bis-（2-（2-hydroxyphenyl） benzothiazole）zinc （Zn（BTZ）2） （greenish-blue emitting material with electron transport character） as the red emitting layer, and fluorescent dye 2,5,8,11-tetra-tertbutylperylene （TBPe） and 5,6,11,12-tetraphenyl-naphthacene （rubrene） together doped into 4,4＇-N,N＇-dicarbazole-biphenyl （CBP） （ambipolar conductivity material） as the blue-orange emitting layer, respectively. The two emitting layers are sandwiched between the hole-transport layer N ,N＇-biphenyl-N , N＇-bis （1-naph thyl）-（1,1＇-biphenyl）-4, 4 Cdiamine （NP B） and electron-transport layer （Zn（BTZ）2 ） The optimum device turns on at the driving voltage of 4.5 V. A maximum external quantum efficiency of 1.53%. and brightness 15000 cd/m^2 are presented. The best point of the Commission Internationale de 1＇Eclairage （CIE） coordinates locates at （0.335, 0.338） at about 13 V. Moreover, we also discuss how to achieve the bright pure white light through optimizing the doping concentration of each dye from the viewpoint of energy transfer process.     

Optical properties of white organic light-emitting devices fabricated with three primary-color emitters by using organic molecular-beam deposition

White organic light-emitting devices (WOLEDs) with Mg:Ag/Alq₃/Alq₃:DCJTB/Alq₃/DPVBi/α-NPD/ITO and Mg:Ag/Alq₃/DPVBi:DCJTB/Alq₃/DPVBi/α-NPD/ITO structures were fabricated with three primary-color emitters of red, green, and blue by using organic molecular-beam deposition. Electroluminescence spectra showed that the dominant white peak for the WOLEDs fabricated with host red-luminescence Alq₃ and DPVBi layers did not change regardless of variations in the current. The Commission Inernationale de l'Eclairage (CIE) chromaticity coordinates for the two WOLEDs were stable, and the WOLEDs at 40 mA/cm² with luminances of 690 and 710 cd/cm² showed an optimum white CIE chromaticity of (0.33, 0.33). While the luminance yield of the WOLED fabricated with a host red-luminescent Alq₃ emitting layer below 30 mA/cm³ was larger than that of the WOLED fabricated with a DPVBi layer, above 30 mA/cm², the luminance yield of the WOLED fabricated with the DPVBi layer was higher than that of the WOLED with the Alq₃ layer and became more stable with increasing current density. These results indicate that WOLEDs fabricated with a host red-luminescence DPVBi layer without any quenching behavior hold promise for potential applications in backlight sources in full-color displays.     

Electroluminescence of a Multi-Layered Organic Light-Emitting Diode Utilizing Trans-4-[p-[N-methyl-N-(hydroxymethyl)amino]styryl]-N-Methylphridinium Tetraphenylborate as the Active Layer

Employing an organic dye salt of trans-4-[p-[N-methyl-N-(hydroxymethyl)amino]styryl]-N-methylphridinium tetra\-phenylborate (ASPT) as the active layer, 8-hydrocyquinoline aluminium (Alq₃) as the electron transporting layer and N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine (TPD) as the hole transporting layer, respectively, we fabricate a multi-layered organic light-emitting diode and observe the colour tunable electroluminescence (EL). The dependence of the EL spectra on the applied voltage is investigated in detail, and the recombination mechanism is discussed by considering the variation of the hole-electron recombination region.     

Organic Light Emitting Diodes with an Organic Acceptor/Donor Interface Involved in Hole Injection

Organic light emitting diodes with an interface of organic acceptor 3-, 4-, 9-,10-perylenetetracarboxylic dianhydride （PTCDA） and donor copper phthalocyanine （CuPc） involved in hole injection are fabricated. As compared to the conventional device using a 5 nm CuPc hole injection layer, the device using an interface of 10nm PTCDA and 5 nm CuPc layers shows much lower operating voltage with an increase of about 46% in the maximum power efficiency. The enhanced device performance is attributed to the efficient hole generation at the PTCDA/CuPc interface. This study provides a new way of designing hole injection.     

Organic Light-Emitting Diodes with Magnesium Doped CuPc as an Efficient Electron Injection Layer

Bright organic electroluminescent devices are developed using a metal-doped organic layer intervening between the cathode and the emitting layer. The typical device structure is a glass substrate/indium-tin oxide （ITO）/copper phthalocyanine （CuPc）/N,N＇-bis-（1-naphthl）-diphenyl-1,1＇-biphenyl-4,4＇-diamine （NPB）/Tris（8-quinolinolato） aluminum（Alq3）/Mg-doped CuPc/Ag. At a driving voltage of 11 V, the device with a layer of Mg-doped CuPc （1：2 in weight） shows a brightness of 4312cd/m^2 and a current efficiency of 2.52cd/A, while the reference device exhibits 514 cd/m^2 and 1.25 cd/A.     

Organic Light-Emitting Devices with a LiF Hole Blocking Layer

We introduce a thin LiF layer into tris-8-hydroxyquinoline aluminium （Alq3 ） based bilayer organic light-emitting devices to block hole transport. By varying the thickness and position of this LiF layer in Alq3, we obtain an electroluminescent efficiency increase by a factor of two with respect to the control devices without a LiF blocking layer. By using a 10nm dye doped Alq3 sensor layer, we prove that LiF can block holes and excitons effectively. Experimental results suggest that the thin LiF layer may be a good hole and exciton blocking layer.     

Efficient Solution-Processed Blue Electrophosphorescent Devices Based on a Novel Small-Molecule Host

Efficient blue small molecular phosphorescent fight-emitting diodes with a blue phosphorescent dye bis（3,5- difluoro-2-（2-pyridyl）-phenyl-（2-carboxypride） iridium （Ⅲ） （Flrpic） doped into a novel small-molecule host 9,9- bis[4-（3,6-di-tert-butylcarbazol-9-yl）phenyl] fluorene （TBCPF） as the light-emitting layer have been fabricated by spin-coating. The host TBCPF can form homogeneous amorphous films by spin-coating and has triplet energy higher than that of the blue phosphorescent dye Flrpic. All the devices with different Flrpic concentration in the emitting layer give emission from Flrpic indicating complete energy transfer from TBCPF to Flrpic. The device shows the best performance with a peak brightness of 8050 cd/m^2 at 10.2 V and the maximum current efficiency up to 3.52 cd/A, when the Flrpic doped concentration is as high as 16%.     

Novel transparent Yb-based cathodes for top-emitting organic light emitting devices with high performance

We have studied three kinds of transparent low-work-function Yb-based cathodes for the top-emitting organic light emitting devices (TEOLEDs) with a structure of ITO/NPB/Alq₃/cathodes and compared them with each other. For the Yb/Au cathodes, a series of Yb layers with various thicknesses have been tested and it is found that the Yb layer with a thickness of 4 nm is the optimum one. The Yb:Au (19 nm) and Yb:Ag (19 nm) co-evaporation cathodes possess very high transmittance but relative poor electron injection; whilst the Yb (4 nm)/Au (15 nm) cathode possess a little lower transmittance but much improved electron injection and the TEOLED with this cathode has the highest power efficiency among the TEOLEDs with the three kinds of Yb-based cathodes mentioned above.     

A New Conducting Polymer Electrode for Organic Electroluminescence Devices

Conducting polymer polydimethylsiloxane （PDMS） is studied for the high performance electrode of organic electroluminescence devices. A method to prepare the electrode consisting of a SiC thin film and PDMS is investigated. By using ultra thin SiC films with different thicknesses, the organic electroluminescence devices are obtained in an ultra vacuum system with the model device PDMS/SiC/PPV/Alq3, where PPV is poly para-phenylene vinylene and Alq3 is tris（S-hydroxyquinoline） aluminium. The capacitance voltage （C - V）, capacitance-frequency （C - F）, current-voltage （I - V）, radiation intensity-voltage （R - V） and luminance eFficiency-voltage （E - V） measurements are systematically studied to investigate the conductivity, Fermi alignment and devices properties in organic semiconductors. Scanning Kelvin probe measurement shows that the work function of PDMS/SiC anode with a 2.5-nm SiC over layer can be increased by as much as 0.28eV, compared to the conventional ITO anode. The result is attributed to the charge transfer effect and ohmic contacts at the interface.     

Enhanced luminance of organic light-emitting diodes with metal nanoparticle electron injection layer

Improvement of the performance of organic light-emitting diodes (OLEDs) was achieved by implementing Magnesium-Nickel nanoparticles at the cathode–organic interface using pulsed laser deposition technique. The small geometry of Mg-Ni nanoparticles acts to enhance the localized electric field around them, thus increasing electron injection through tunneling, from the cathode to the organic layer. Improved current and luminance characteristics were demonstrated for both small molecule and polymer-based OLEDs when the nanoparticle layer was incorporated.