Silicon-based light emitting diodes on silicon and glass substrates using a low temperature multilayered nanocrystalline structure

Si nanocrystals have been prepared by hydrogenation and subsequent annealing of as-deposited amorphous Si layers on glass and Si substrates. The hydrogenation process has been performed at 350 °C under radio frequency hydrogen plasma. The nanocrystallites were processed by sequential reactive ion etching to allow light emission. Photoluminescence (PL) measurements demonstrate that the nanocrystallites emit light in the range of 500-570 nm. The evolution of nanocrystals has been studied using scanning electron microscopy, while atomic force microscopy and transmission electron microscopy have been utilized to examine the structure of the Si nanocrystals. Multilayer luminescent Si nanocrystals have been fabricated using alternating layers of Si nanocrystals and Si oxy-nitride. Bilayer structures have higher efficiency than a single layer structure, while multilayers with three layers of luminescent nanocrystals and above did not show a higher PL intensity. Transparent light emitting diodes have been realized based on multilayer luminescent Si nanocrystals that displayed bright emission which was visible to the naked eye in a bright room.     

Electroluminescence of organolanthanide based organic light emitting diodes

Recent advances in organolanthanide based organic light emitting diodes have lead to the demonstration of infra-red emitting devices. A silicon based organic light emitting diode exhibiting 1.53 μm electroluminescence at room temperature has also recently been reported. Furthermore, recent work has led to a clearer understanding of the quenching mechanisms in these organolanthanide based devices and suggests that the efficiencies obtained to date can be greatly improved.     

Enhancement of the light extraction efficiency in organic light emitting diodes utilizing a porous alumina film

The light extraction efficiencies of organic light emitting diodes (OLEDs) utilizing various kinds of porous alumina films with different pore diameters were investigated. The OLEDs with the porous alumina film deposited on the glass surface were fabricated to improve their light extraction efficiency. The porous alumina film was fabricated by using a two step anodizing electrochemical procedure. The current densities as functions of the applied voltage do not significantly change, regardless of the existence and the magnitude of the pore diameter in the porous alumina film. The luminance efficiency of the OLEDs increased with increasing pore diameter. The luminance efficiency of the OLEDs utilizing the porous alumina film with a pore diameter of 70 nm was enhanced approximately 9% in comparison with that of the OLEDs without the porous alumina film. These results indicate that highly efficient OLEDs can be fabricated using a porous alumina film with an optimum pore diameter.     

The characteristics of electroplex generated from the organic light emitting diodes

The color stability and purity from OLED is of current interest. Aggregation of dyes alters the device color after fabrication of the devices. The aggregates can be exciplex and electroplex, which is the excited complex that generated after electrically excited state under high electric field. Comparative study of emission spectra of exciplex and electroplex leads us to conclude that the new electroplex states causes the bathochromic shift of the electroluminescence spectrum from the devices with TPD/PBD layers. The photoluminescence maximum obtained from the TPD/PBD layers of the device was 420 nm, and the electroluminescence maximum obtained from the device became 480 nm. The bathochromic shift cannot be attained with photoluminescence study with highly concentrated TPD/PBD mixture. This clearly indicates that the 480 nm spectrum of the devices is not resulted from the exciplex formation with TPD and PBD. We observed the overshoot in EL spectrum from the OLEDs. The most intense overshoot was observed at 460 nm, which is due to the aggregate that are formed after the electric field has been removed from the devices.     

Light extraction efficiency enhancement of GaN-based light emitting diodes on n-GaN layer using a SiO2 photonic quasi-crystal overgrowth

In this paper, GaN-based LEDs with a SiO2 photonic quasi-crystal (PQC) pattern on an n-GaN layer by nano-imprint lithography (NIL) are fabricated and investigated. At a driving current of 20 mA on Transistor Outline (TO)-can package, the better light output power of LED III (d = 1.2 microm) was enhanced by a factor of 1.20. After 1000 h life test (55 degrees C/50 mA) condition, Normalized output power of LED with a SiO2 PQC pattern (LED III (d = 1.2 microm)) on an n-GaN layer only decreased by 5%. This results offer promising potential to enhance the light output power of commercial light-emitting devices using the technique of nano-imprint lithography.     

Gravure printed organic light emitting diodes for lighting applications

A major advantage of polymer based organic light emitting diodes (OLED) is the capability to be manufacturing them with low cost, high-throughput printing techniques. In this paper, we report on double layer gravure printed polymer based OLED light sources with an active area of 0.16 cm² on glass substrate. The devices exhibit brightness of 100 cd/m² and 1000 cd/m² at 4.2 V and 5.4 V, respectively. Furthermore, a large area OLED of 30 cm² in which both polymer layers are gravure printed is demonstrated for lighting applications. Based on the results presented in this paper, the feasibility of the gravure printing technique for the fabrication of large area OLEDs in large-scale production is proved.     

树形分子有机电致发光材料研究

简要介绍了一类新型大分子-树形分子的结构、特性及其合成方法;综述了树形分子材料作为发光材料和载流子传输材料所具有的特点、类型及其在有机电致发光领域的研究进展;展望了此类材料的应用前景.     

Effect of ITO surface treatment on organic light emitting diodes

Performance of organic light emitting diodes with structure of ITO/NPB/NPB:Ir(piq)3/Alq3/Al was studied by inserting MoO3 hole injection layer (HIL) and using differently cleaned indium tin oxides (ITOs). High luminance and quantum efficiencies were obtained using HIL and highly cleaned ITO. High power efficiency was obtained using HIL and conventionally cleaned ITO, while low driving voltage was obtained using highly cleaned ITO without HIL.     

Blue fluorescent organic light emitting diodes with multilayered graphene anode

As an innovative anode for organic light emitting devices (OLEDs), we have investigated graphene films. Graphene has importance due to its huge potential in flexible OLED applications. In this work, graphene films have been catalytically grown and transferred to the glass substrate for OLED fabrications. We have successfully fabricated 2 mm × 2 mm device area blue fluorescent OLEDs with graphene anodes which showed 2.1% of external quantum efficiency at 1000 cd/m². This is the highest value reported among fluorescent OLEDs using graphene anodes. Oxygen plasma treatment on graphene has been found to improve hole injections in low voltage regime, which has been interpreted as oxygen plasma induced work function modification. However, plasma treatment also increases the sheet resistance of graphene, limiting the maximum luminance. In summary, our works demonstrate the practical possibility of graphene as an anode material for OLEDs and suggest a processing route which can be applied to various graphene related devices.     

Highly efficient white phosphorescent organic light emitting diodes using a mixed host structure in deep blue emitting layer

Highly efficient phosphorescent white organic light-emitting diodes (PHWOLEDs) were developed using a deep blue phosphorescent emitter doped into a mixed host of high triplet energy host materials. The deep blue emitting layer was combined with a red:green emitting layer to fabricate PHWOLEDs. A high quantum efficiency of 19.5% with a color coordinate of (0.29,0.38) and 19.8% with a color coordinate of (0.39,0.46) were achieved in the PHWOLEDs using the mixed host emitting layer doped with a deep blue phosphorescent dopant. In addition, a low optimum doping concentration below 5% in red, green and blue dopants was realized in the PHWOLEDs.     

Improved light extraction efficiency of InGaN-based multi-quantum well light emitting diodes by using a single die growth

We have demonstrated that the light extraction efficiency of the InGaN based multi-quantum well light-emitting diodes (LEDs) can be improved by using a single die growth (SDG) method. The SDG was performed by patterning the n-GaN and sapphire substrate with a size of single chip (600 x 250 microm2) by using a laser scriber, followed by the regrowth of the n-GaN and LED structures on the laser patterned n-GaN. We fabricated lateral LED chips having the SDG structures (SDG-LEDs), in which the thickness of the regrown n-GaN was varied from 2 to 6 microm. For comparison, we also fabricated conventional LED chips without the SDG structures. The SDG-LEDs showed lower operating voltage when compared to the conventional LEDs. In addition, the output power of the SDG-LEDs was significantly higher than that of the conventional LEDs. From optical ray tracing simulations, the increase in the thickness and sidewall angle of the regrown n-GaN and LED structures may enhance photon escapes from the tilted facets of the regrown n-GaN, followed by the increase in light output power and extraction efficiency of the SDG-LEDs.     

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.     

Top emitting organic light emitting diodes with opaque metal grid electrodes by transfer technique

The opaque metal grid electrodes are introduced to fabricate top emitting organic light emitting diodes (TOLEDs) through metal transfer technique. To transmit the lights, micrometer-sized patterns of aluminum (Al) were utilized as top cathodes in OLEDs and Al mirrors were also deposited at the other side of transparent substrates to reflect the lights emitted at the bottom sides. Although the only 50% of brightness compared to bottom emitting OLEDs (BOLEDs) could be achieved theoretically, the actual devices showed more than 70% based on the compressive effects during the metal transfer process. Since the resolution of human eyes recognizes these micrometer-sized grid structures as one pixel, TOLEDs can be simply fabricated without significant loss of efficiency.     

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.     

Robust reliability for light emitting diodes using degradation measurements

Taguchi's robust design provides an important paradigm for producing robust products. There are many successful applications of this paradigm, but few have dealt with reliability, i.e. when the quality characteristic is lifetime. In this paper, an actual experiment is presented which was performed to achieve robust reliability of light emitting diodes. Three major factors chosen from many potentially important manufacturing factors and one noise factor were investigated. For light emitting diodes, failure occurs when their luminosity or light intensity fall below a specified level. An interesting feature of this experiment is the periodic monitoring of the luminosity. The paper shows how the luminosity's degradation over time provides a practical way to achieve robust reliability of light emitting diodes which are already highly reliable.     

Multilayer light emitting diodes using a PPV based copolymer

We have investigated the electrical and optical properties of poly((2,5-(dimethoxy) p-phenylene vinylene)-p-phenylene vinylene) (PDMeOPV/PPV) copolymer used as an emitting layer in light emitting diodes. With p-phenylene vinylene (PPV) used as a hole transport layer and polyphenylquinoxaline (PPQ) as an electron transport layer, the emission intensity of the devices has substantially increased without alteration of the transport property. The different conduction mechanisms in the diodes were examined and discussed in terms of the energy band diagrams of the polymer layers. A balance of the injected charge carriers confined in the copolymer could explain the enhancement of the performance of the multilayer diodes.     

Blue light emitting ZnS diodes

Low resistivity n-type ZnS single crystals of about 10³Ω cm and 10²Ω cm were achieved by firing the as-grown high resistivity ZnS single crystals using quartz ampoules and graphite crucible, respectively. Schottky diodes fabricated from sample I gave stable blue emission in reverse bias while sample II gave blue emission in forward bias. The characteristics of these diodes were studied and presented. Carrier concentrations were estimated from the capacitance-voltage measurements of the two types of diodes. Electroluminescence spectra as well as cathodoluminescence spectra of both samples are presented. The blue emission peak was ascribed to the donor-acceptor pairs transition.     

PVK/Alq3 organic light emitting diodes obtained by evaporation

Poly(N-vinycarbazole) (PVK)/tris(8-hydroxy)-quinoline-aluminum (Alq₃) bilayer diodes were deposited by vacuum evaporation onto SnO₂ coated glass substrates. After deposition of an aluminum upper electrode, the diodes were studied by current–voltage (I–V) and electroluminescence–voltage (EL–V) measurements. It is shown that the luminescence of the structures should be attributed to Alq₃. However the presence of evaporated PVK allows us to decrease the barrier height at the interface SnO₂/PVK because of the localized states induced in the PVK band gap during the evaporation which allows multistep tunneling effect. The optimum Alq₃ thickness is shown to be about 75 nm.