Improved performance of the single-layer and double-layer organic light emitting diodes by nickel oxide coated indium tin oxide anode

The electrical and optical properties of the NiO films deposited under various conditions were first characterized. An ultra-thin layer of nickel oxide (NiO) was then deposited on the indium-tin oxide (ITO) anode to enhance the hole injection in the organic light-emitting diode (OLED) devices. A very low turn-on voltage (3 V) was actually observed for the device with the ITO/NiO anode in the conventional double layer heterojunction OLEDs. The enhancement of hole injection by the ITO/NiO anode was further verified by the hole-only device and by the device with a patterned NiO layer on the ITO anode. The luminance and the current density of the single-layer OLED device were also significantly improved by using the ITO/NiO anode to enhance the hole injection. Although the luminescence efficiency was low, the reasons of low efficiency were studied and the improvement method was proposed. Our results suggest that the NiO/ITO anode is an excellent choice to enhance the hole injection in OLED devices.     

Recombination zone in white organic light emitting diodes with blue and orange emitting layers

White fluorescent OLED devices with a 10 nm thick blue-emitting layer and a 31 nm thick orange-emitting layer have been fabricated, where the blue-emitting layer is stacked on a hole transport layer. An interlayer was inserted between the two emitting layers. The thickness of the interlayer was changed among 0.3, 0.4, and 1.0 nm. White emission with CIE coordinates close to (0.33, 0.33) was observed from all the OLEDs. OLED with 0.3 nm thick interlayer gives the highest maximum luminous efficiency (11 cd/A), power efficiency (9 lm/W), and external quantum efficiency (5.02%). The external quantum efficiency becomes low with increasing the interlayer thickness from 0 nm to 1.0 nm. When the location of the blue- and orange-emitting layers is reversed, white emission was not obtained because of too weak blue emission. It is suggested that the electron–hole recombination zone decreases nearly exponentially with a distance from the hole transport layer.     

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.     

Enhancement of the lifetime in organic light-emitting devices fabricated utilizing wide-bandgap-impurity-doped emitting layers

The degradation behaviors of the electrical and the optical properties of organic light-emitting devices (OLEDs) fabricated with an emitting layer (EML) doped with or without a wide-bandgap-impurity were investigated. The OLEDs with a wide-bandgap-doped Alq₃ EML were more stable than those with an undoped Alq₃ EML. The existence of the doped wide-bandgap-impurity in the EML decreased the trap-charge density in the EML, resulting in an increase in the number of electrons in the Alq₃ EML. That increases in the number of electron in the Alq₃ EML for the OLEDs with a wide-bandgap-impurity decreased the staying time of the holes in the Alq₃ EML, resulting in an enhanced lifetime for the OLEDs. These results indicate that OLEDs with a wide-bandgap-impurity-doped EML hold promise for potential applications in long-lifetime OLED displays.     

Red phosphorescent organic light-emitting diodes based on the simple structure

We demonstrated that the simple layered red phosphorescent organic light-emitting diodes (OLEDs) are possible to have high efficiency, low driving voltage, stable roll-off efficiency, and pure emission color without hole injection and transport layers. We fabricated the OLEDs with a structure of ITO/CBP doped with Ir(pq)2(acac)/BPhen/Liq/Al, where the doping concentration of red dopant, Ir(pq)2(acac), was varied from 4% to 20%. As a result, the quantum efficiencies of 13.4, 11.2, 16.7, 10.8 and 9.8% were observed in devices with doping concentrations of 4, 8, 12, 16 and 20%, respectively. Despite of absence of the hole injection and transport layers, these efficiencies are superior to efficiencies of device with hole transporting layer due to direct hole injection from anode to dopant in emission layer.     

Organic light-emitting devices with a mixed layer acting as a hole transport and as an emitting/electron transport layer

The electrical and the optical properties of organic light-emitting devices (OLEDs) with a mixed layer acting as a hole transport layer and as an emitting layer/electron transport layer were investigated. The OLEDs with a mixed layer showed the highest efficiency, and the emitting color of the OLEDs was pure yellow. The enhancement of the luminous efficiency in the OLEDs with a mixed layer was attributed to a decrease in hole mobility.     

Enhance current injection of organic light emitting diodes by inserting an organic superlattice

Poor electron injection is a great concern for organic light emitting diodes (OLEDs). In order to improve the electron mobility, inserting organic superlattice structures in the electron transport layer was investigated in conventional OLEDs configuration. The superlattices are composed of alternating tris(8-hydroxyquinoline aluminium (Alq₃) and copper phthalocyanine (CuPc) thin films, which are used as electron and hole injection layers. Experimental results show superlattices with a 6-nm period have the largest injected current. Reduction of turn-on voltage and resistance of superlattice OLEDs were also observed. After thermal annealing, the current-voltage characteristic changes and shows the possibility of layer intermixing in organic superlattices.     

Enhancement of the efficiency and the color stabilization in green organic light-emitting devices with multiple heterostructures acting as a hole transport layer

The electrical and the optical properties of organic light-emitting devices (OLEDs), with and without multiple heterostructures consisting of N, N-bis-(1-naphthyl)-N, N-diphenyl-1,1-biphenyl-4,4-diamine (NPB)/5,6,11,12-tetraphenylnaphthacene (rubrene) acting as a hole transport layer (HTL), were investigated. The OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures acting as an HTL showed the highest luminances and efficiencies. While the electroluminescence (EL) peak corresponding to the rubrene layer did not appear for the OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures, only the EL peak related to the tris (8-hydroxyquinoline) aluminum layer was observed. The Commission Internationale de l’Eclairage chromaticity coordinates of the OLEDs with 3 periods of NPB/mixed rubrene:NPB multiple heterostructures at 14 V were (0.321, 0.531), indicative of a deep stabilized green color.     

Micropatterning of emitting layers by microcontact printing and application to organic light-emitting diodes

The microcontact printing (μCP) technique, which is a simple and low damage fabrication technique for thin films, was successfully applied to fabricate patterned emitting layers such as polyfluorene (PF). We fabricated micropatterns by transferring dried and uniform thin films, and observed strong electroluminescence (EL) from the fabricated organic light-emitting diodes (OLEDs) with the patterned emitting layers. The performance of the fabricated device was superior to that of a conventionally fabricated device. This demonstrates the well-controlled interfaces achieved by μCP. Furthermore, we succeeded in fabricating OLEDs with multiple emitting layers. These results show that this technique is promising for application to cost-effective, high luminance and multicolored OLED displays.     

Vapor deposition polymerization of vinyl compounds and fabrication of OLED having double emissive layers

Physical vapor deposition polymerization was achieved by evaporating vinyl monomers followed by heat treatment. This technique was utilized for preparing a hole-transport layer (HTL) and emissive layers (EMLs) of organic light emitting diodes (OLEDs). Adivinyl derivative of tetraphenyldiaminobiphenyl, DvTPD, was used both for the HTL and for the host material of red phosphorescent EML. A vinyl derivative of bis(N-carbazolyl)benzene, vmCP, was used for the host material of blue phosphorescent EML. The ELMs were doped with red or blue-emitting phosphorescent iridium complexes modified with styril units. After characterizing the devices that have single EML of red or blue emission, an OLED having multilayered ELM was prepared by stacking these layers to achieve quasi-white emission. It was found that the deposition polymerization was effective to improve the emission efficiency. The improvement appeared to be related to the proper carrier balance in the EML.     

Bi-layer gravure printed nanoscale thick organic layers for organic light emitting diode

The gravure printed single layer structure and bi-layer structure of MEH-PPV/rubrene organic light emitting diodes (OLEDs) were investigated in this work. Typically, the formation of bi-layers in polymer light emitting diodes (PLEDs) is challenging. The brightness and efficiency polymer light emitting materials were enhanced by the gravure printed bi-layer structure in this work. The layer structure of the OLED devices was glass/ITO/PEDOT:PSS/active layer/LiF/Al. The active layers were made using two different processes-one was a gravure printed single organic layer made of a blended mixture of MEH-PPV and rubrene, and the other was a gravure printed bi-layer of MEH-PPV and rubrene. The gravure printed bi-layer devices exhibited a higher brightness and efficiency than the blended devices. The efficiency of the bi-layer MEH-PPV/rubrene structure was improved by a factor of 1.6 approximately 3.2, and the brightness was improved by a factor of 1.9 approximately 2.0 compared to the blended single layer structure. This work demonstrated that organic bi-layers could be formed using gravure printing technology and the bi-layer structure exhibited a higher efficiency than the blended single layer structure.     

Study of GaP single crystal layers grown on GaN by MOCVD

The performance of GaN based devices could possibly be improved by utilizing the good p-type properties of GaP layer and it provides the possibility of the integration of InAlGaN and AlGaInP materials to produce new devices, if high quality GaP compounds can be grown on III-nitride compounds. In this paper, the growth of GaP layers on GaN by metalorganic chemical vapor deposition (MOCVD) has been investigated. The results show that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature GaP growth. Using a 40 nm thick GaP buffer layer, a single crystal GaP layer, whose full-width at half-maximum of the (1 1 1) plane measured by double crystal X-ray diffraction is 580″, can be grown on GaN. The V/III ratio plays an important role in the GaP layer growth and an appropriate V/III ratio can improve the quality of GaP layer. The GaP:Mg layer with hole carrier concentration of 4.2 × 10¹⁸ cm⁻³ has been obtained.     

Effect of inorganic nanoparticle addition to the hole-collecting buffer layers in polymer solar cells

We investigated the influence of nickel oxide (NiO) nanoparticles that are incorporated into the hole-collecting buffer layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] on the performance of polymer:fullerene solar cells. To understand the optimum composition of NiO nanoparticles, the composition of NiO nanoparticles was varied from 0 wt% to 23 wt%. Results showed that the optical transmittance was gradually decreased as the NiO content increased. However, the device performance (short circuit current density, fill factor, series resistance, power conversion efficiency) exhibited a two stage trend in a boundary of approximately 9 wt% NiO content. This trend was in good agreement with the trend of sheet resistance in the presence of slight discrepancy owing to the different charge transport geometry.     

Effects of metallic absorption and the corrugated layer on the optical extraction efficiency of organic light-emitting diodes

The absorption of a metallic cathode in OLEDs is analyzed by using FDTD calculation. As the light propagates parallel to the layer, the intensity of E(z) polarization decreases rapidly. The intensity at 2.0 microm from the dipole is less than a quarter of that at 0.5 microm. The strong absorption by a cathode can be a critical factor when considering the increase of optical extraction by means of bending the optical layers. The calculation indicates that the corrugation of layers helps the guided light escape the guiding layer, but also increases the absorption into a metallic cathode. The final optical output power of the corrugated OLED can be smaller than that of the flat OLED. On the contrary, the corrugated structure with a non-absorptive cathode increases the optical extraction by nearly two times.     

Organic light emitting devices with doped electron transport and hole blocking layers

This study reports on heterostructure OLEDs with n-type molecularly doped electron transport layer and hole blocking layer. The influence of doping on the operating voltage and on light emission performances was investigated. The n-type doping molecule is 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) dispersed into either an 8-(hydroquinoline) aluminum (Alq) electron transport layer (ETL) or a 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine BCP) hole blocking layer (HBL). The typical device structure is glass substrate/indium tin oxide/PEDOT/TPD–F4-TCNQ/Alq–DCM/BCP/Alq/Mg–Ag where Poly(3,4)ethylenedioxythiophene/Polystyrenesulphonate (PEDOT/PSS) is a hole injecting layer, TPD–F4-TCNQ is a hole transport layer (HTL) made of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) doped with 2 wt.% of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) and Alq–DCM is the emitting layer (EML) made of Alq doped with 2 wt.% of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) orange dye. The modified cathode consists in a combination of a BCP HBL and an Alq ETL where BCP or/and Alq were doped with PBD. Lowest operating voltage (3 V for a luminance of 10 Cd/m²) and brightest devices (6000 Cd/m²) were obtained with a hole blocking bilayer made of BCP doped with 28 wt.% deposited onto an undoped BCP (each one being 5 nm thick). Adding an undoped Alq layer improved the device current efficiency (4 Cd/A) but is detrimental to the operating voltage (6 V for a luminance of 10 Cd/m²). In the absence of real n-type doping with organic molecules, our results point out that the design of molecular doped injection layer at the cathode will need for a compromise between high luminance and efficiency on one hand and low operating voltage on the other hand.     

Electroluminescence and impedance analyses of organic light emitting diodes using anhydride materials as cathode interfacial layers

Pyromellitic dianhydride (PMDA) and trimellitic anhydride (TMA) were tried as cathode interfacial layers between tris-(8-hydroxyquinoline) aluminum (Alq₃) and Al in organic light emitting diodes (OLEDs). Both ultra-thin anhydride cathode interfacial layers improved the electroluminescence characteristics of OLEDs compared to those without any interfacial layer, and the PMDA interfacial layer showed the most significant enhancement of the device performance. According to impedance measurements and equivalent circuit analysis, the PMDA interfacial layer decreased the impedance, probably due to the increase in the injection efficiency of electrons from the Al cathode.     

磁控溅射法制备TiO2空穴缓冲层的有机发光器件

采用磁控溅射方法在ITO表面制备了不同厚度的TiO2超薄膜用做有机发光二极管（OLEDs）的空穴缓冲层,使OLEDs（ITO／TiO2／TPD／Alq3／Al）的发光性能得到很大改善。研究TiO2缓冲层厚度对器件性能影响的结果表明,当TiO2缓冲层厚度为1nm,电流密度为100mA／cm^2时,器件的发光效率为2cd／A,比未加缓冲层器件的发光效率增加了近一倍。这是由于加入适当厚度的TiO2缓冲层限制了空穴的注入并且提高了空穴与电子注入之间的平衡。     

Preparation of NiO Buffer Layer by Direct Oxygenation of Ni Tape

Recently, Nickel-based alloys are widely used for textured substrates tapes. However, the deposition of oxide buffer layers on nickel tapes is required to stop the diffusion of nickel from the tape to superconducting layer and to improve the mismatch between the substrate and superconducting film. Biaxially textured NiO buffer layer is easily formed on cubic textured nickel tape by the technique called surface oxidation epitaxy. In this work, we developed a direct oxygenating method to make the NiO buffer layer on nickel tape. The nickel tape was directly oxygenated in different atmospheres. It is found that in inert atmosphere, a high quality NiO layer can be formed on the surface of the tape. The oxygenating conditions, including atmospheres and temperatures, and their influence on the structure of the NiO buffer layer, were studied in detail and discussed.     

NiO as an inorganic hole-transporting layer in quantum-dot light-emitting devices

We demonstrate a hybrid inorganic/organic light-emitting device composed of a CdSe/ZnS core/shell semiconductor quantum-dot emissive layer sandwiched between p-type NiO and tris-(8-hydroxyquinoline) aluminum (Alq3), as hole and electron transporting layers, respectively. A maximum external electroluminescence quantum efficiency of 0.18% is achieved by tuning the resistivity of the NiO layer to balance the electron and hole densities at quantum-dot sites.     

Infrared Organic Light‐Emitting Diodes with Carbon Nanotube Emitters

While organic light‐emitting diodes (OLEDs) covering all colors of the visible spectrum are widespread, suitable organic emitter materials in the near‐infrared (nIR) beyond 800 nm are still lacking. Here, the first OLED based on single‐walled carbon nanotubes (SWCNTs) as the emitter is demonstrated. By using a multilayer stacked architecture with matching charge blocking and charge‐transport layers, narrow‐band electroluminescence at wavelengths between 1000 and 1200 nm is achieved, with spectral features characteristic of excitonic and trionic emission of the employed (6,5) SWCNTs. Here, the OLED performance is investigated in detail and it is found that local conduction hot‐spots lead to pronounced trion emission. Analysis of the emissive dipole orientation shows a strong horizontal alignment of the SWCNTs with an average inclination angle of 12.9° with respect to the plane, leading to an exceptionally high outcoupling efficiency of 49%. The SWCNT‐based OLEDs represent a highly attractive platform for emission across the entire nIR.