Reflective Anode Research Articles

Robust Laminated Anode with an Ultrathin Titanium Nitride Layer for High-Efficiency Top-Emitting Organic Light-Emitting Diodes.

The effective reflective anode remains a highly desirable component for the fabrication of reliable top-emitting organic light-emitting diodes (TE-OLEDs) which have the potential to be integrated with complementary metal-oxide-semiconductor (CMOS) circuits for microdisplays. This work demonstrates a novel laminated anode consisting of a Cr/Al/Cr multilayer stack. Furthermore, we implement an ultra-thin titanium nitride (TiN) layer as a protective layer on the top of the Cr/Al/Cr composite anode, which creates a considerably reflective surface in the visible range, and meanwhile improves the chemical stability of the electrode against the atmosphere or alkali environment. Based on [2-(2-pyridinyl-N)phenyl-C](acetylacetonate)iridium(III) as green emitter and Mg/Ag as transparent cathode, our TE-OLED using the TiN-coated anode achieves the maximum current efficiency of 71.2 cd/A and the maximum power efficiency of 66.7 lm/W, which are 81% and 90% higher than those of the reference device without TiN, respectively. The good device performance shows that the Cr/Al/Cr/TiN could function as a promising reflective anode for the high-resolution microdisplays on CMOS circuits.

Field emission properties of tetrapod-shaped ZnO/TiN point light source with semicircle reflecting anode

A novel point light source composed of tetrapod-shaped ZnO/TiN (T-ZnO/TiN) cathode and semi-circular groove reflective anode is demonstrated. The T-ZnO/TiN cathode is fabricated on nickel wire by spraying T-ZnO NRs powder and subsequent sputtering of TiN film. Semicircular reflective anodes with different opening sizes were prepared by wire electro-discharge machine. The field-emission properties of point light sources could be dramatically enhanced under semicircle reflecting anode by decreasing the threshold field from 2.61 to 2.10 V/μm. The point light source of D1500 has an extremely high emission current density of 132.73 mA/cm2 at 3.53 V/μm. The value of Jmax is about 2.7 times to that of D3000 (∼47.89 mA/m2 at 4.18 V/μm). Experiments and ANSYS electric field simulation show that the combination of low work function hybrid cathode and semicircular reflective anode structure leads to the increase of local field concentration and the enhancement of field emission characteristics. As far as we know, this point light source with semicircular reflective anode is prepared and applied for the first time. The results show that the point light source structure proposed here can not only effectively reduce the threshold field, but also improve the emission current density and luminescence uniformity.

The Selection of Anode and Cathode Materials for Top Emission Organic Light-Emitting Diodes

In this study top emission organic light-emitting diodes (TE-OLED) were successfully fabricated on flexible PET/multilayer disordered silver nanonetwork (MDSN) substrate with conventional LiF/Al as a semitransparent cathode and Ag as a reflective anode. The effects of the hole injection layer, anode buffer layer, and an electron injection layer on the luminescence characteristics of TE-OLEDs were investigated. At first, the thickness of cathode Al is adjusted based upon the overall transmittance of the top-emission cathode and its conductivity. There is a high energy barrier for the hole between the work function of the anode Ag (4.2 eV) and the NPB highest occupied molecular orbital (HOMO) energy level (5.5 eV), which is not favorable for hole injection. This study tested four types of hole injection layer (HIL) materials. Finally, MoO3 was selected as an optimal HIL material, and the optimum thickness was adjusted, enabling the hole to be injected smoothly from the anode to the NPB and then to an emitting layer. The TE-OLED luminance reached 268 cd/m2. There is a high energy barrier between the work function of Ag and MoO3 HOMO (5.3 eV)—about 1.1 eV—which is still not conducive to the hole injection, so a thin layer of high work function metal Au (work function 5.1 eV) was added to the top of the anode silver, which more matches with the MoO3 energy level. It can make the hole easier to inject from the anode to MoO3 (HIL) and protect the silver from oxidation. At 8 V, the luminance is increased to 413.7 cd/m2, and the current efficiency is 0.81 cd/A. The luminance is significantly improved. An electron transport/hole blocking layer TPBi (10 nm) was added to enhance the electron transport capability and effectively block the holes with higher electron mobility and a higher HOMO energy level of TPBi. So that more holes can remain in the emitting layer and increase the chance of the electron-hole recombination to improve the luminance and current efficiency of TE-OLED. At 8 V, the luminance and current efficiency can reach 611.4 cd/m2 and 0.95 cd/A, respectively.

Improving the Thermal Stability of Top‐Emitting Organic Light‐Emitting Diodes by Modification of the Anode Interface

AbstractTop‐emitting organic light‐emitting diodes (OLEDs) are of interest for numerous applications, in particular for displays with high fill factors. To maximize efficiency and luminance, molecular p‐doping of the hole transport layer (p‐HTL) and a highly reflective anode contact, for example, made from silver, are used. Atomic layer deposition (ALD) is attractive for thin film encapsulation of OLEDs but generally requires a minimum process temperature of 80 °C. Here it is reported that the interface between the p‐HTL and the silver anode of top‐emitting OLEDs degrades during an 80 °C ALD encapsulation process, causing an over fourfold reduction in OLED current and luminance. To understand the underlying mechanism of device degradation, single charge carrier devices are investigated before and after annealing. A spectroscopic study of p‐HTLs indicates that degradation is due to the interaction between diffusing silver ions and the p‐type molecular dopant. To improve the stability of the interface, either an ultrathin MoO3 buffer layer or a bilayer HTL is inserted at the anode/organic interface. Both approaches effectively suppress degradation. This work shows a route to successful encapsulation of top‐emitting OLEDs using ALD without sacrificing device performance.

P‐229: Late‐News‐Poster: Suppression of Viewing‐Angle Dependence of Top‐Emitting OLEDs by Applying Nanostructured Reflective Anode

To improve the viewing angle dependence of TEOLEDs, we have modified the surface morphology of highly reflective anode by using nano‐island structure. This nano‐island structure is simply fabricated by depositing a specific inorganic material and applying an appropriate environmental condition of a certain temperature and humidity. The resultant TEOLEDs with nano‐structured reflective anode showed stable white angle difference (WAD) with Δ u'v' of 0.005.

Production and Heat Properties of an X-ray Reflective Anode Based on a Diamond Heat Buffer Layer.

This paper introduces an X-ray reflective anode with a diamond heat buffer layer, so as to improve heat dissipation of micro-focus X-ray sources. This also aids in avoiding the destruction of the anode target surface caused by the accumulation of heat generated by the electron beam bombardment in the focal spot area. In addition to the description of the production process of the new reflective anode, this study focuses more on the research of the thermal conductivity and compounding ability. This paper also introduces a method that combines finite element analysis (FEA) in conjunction with thermal conductivity experiments, and subsequently demonstrates the credibility of this method. It was found that due to diamonds having a high thermal conductivity and melting point, high heat flux produced in the micro-focus spot region of the anode could be conducted and removed rapidly, which ensured the thermal stability of the anode. Experiments with the power parameters of the radiation source were also completed and showed an improvement in the power limit twice that of the original.

Suppression of viewing angle dependence of TEOLED devices with internal nano-wrinkle structure

Abstract In order to suppress the viewing angle dependence of top emission organic emitting diodes (TEOLEDs) with strong microcavity effect, we introduced randomly distributed nano-wrinkle structure beneath highly reflective anode. By applying such nano-wrinkle pattern, viewing angle dependence and color shift could be improved compared with that without nano-structure [e.g. Δ (u’, v’): 0.077 → 0.025]. However, current efficiency reduced more than half. To compensate this problem, we additionally introduced index matching epoxy filler between encapsulation glass and TEOLED device. By this approach, current efficiency drop of device with nano-wrinkle structure could be improved. For example, the efficiency dropped by 7.7% when we filled the epoxy filler between capping layer and glass lid while, it decreased by 30.8% if we don't use epoxy filler. We found that index matching helps to extract more light confined inside TEOLED, so device performance degradation could be minimized. In addition, when we attached a circular polarizer film to these devices, depolarization was observed in most cases, but we were able to achieve a perfect black state from the devices with nano-wrinkle structure underneath reflective anode.

Study on High Thermal Conductivity of X-ray Anode with Composite Diamond Substrate

The thermal loading capacity of the anode of a micro-focus X-ray source is an important factor to limit the output power. In this paper, a composite reflective anode model with a thick film diamond layer as the target substrate of X-ray source was proposed. With the advantage of high thermal conductivity of the diamond film, the heat accumulated in a micro-focus area can be quickly output, to improve the thermal loading capacity of the anode. A composite anode with diamond substrate was modeled and simulated with the finite element analysis method. By studying the principle of high thermal conductivity of composite anode in the numerical simulation and comparing the composite anode with copper substrate anode, the essential causes of the high thermal conductivity of the anode with diamond substrate were identified. The relevant technical parameters were obtained in the simulation and comparison, which can be used for the design of anode of micro-focus X-ray sources.

Highly efficient top-emission organic light-emitting diode on an oxidized aluminum anode

In today's manufacturing of organic light-emitting diode on silicon for microdisplay technologies, a top-emitting OLED (TEOLED) is required to be fabricated on top of an active-matrix circuitry located on the silicon backplane. This requires a highly reflective anode to enhance the luminance output. However, during the production process of a TEOLED, the hole injection efficiency and electrical conductivity may be suppressed by environmental exposure, in particular, moisture and oxygen. Given this, aluminum is an unfavorable reflective anode due to the primary concern of its native insulating oxide layer. The native oxide tends to grow during the patterning of the metal anode. In this paper, we have discovered that, by utilizing an Al2O3/MoO3 heterojunction anode structure, a highly efficient device can be made to achieve a current efficiency of 94 cd/A at a luminance of 1000 cd/m2. X-ray/ultraviolet photoelectron spectroscopy measurements show the formation of molybdenum gap states and favorable energy level alignment for hole injection.

Optical modeling of the emission zone profile and optimal emitter position based on the internal field profile of the air mode in organic light-emitting diodes.

We propose a theoretical formulation to calculate the internal profile of the air mode in the organic light-emitting diode (OLED) on the combination of the transfer matrix method and source-term method. The spatial distributions of the air mode are calculated in a top-emitting OLED with respect to the light polarization, extraction angle, dipole orientation, and dipole position. Air modes are also calculated on the basis of the previously used external source model, where the input optical wave is injected from the air into the OLED multilayer. Comparison of the calculated air modes between two models checks the validity of the external source model. In addition, we propose an improved formula to determine the optimal emitter positions that maximize the two-beam interference of the micro-cavity effect. In the improved formula, a non-ideal reflection phase shift at a reflective metal anode is treated as the skin depth of the air mode. Finally, the effect of the dipole orientation on the air mode is investigated. Compared with the air mode emitted by the horizontally oriented dipole, the air mode generated by the vertically oriented dipole has relatively small intensity and shows the opposite dependence of the emitter position variation. The calculation results of the internal profile of the air mode within the emission layer are matched with the profile of the emission zone obtained by output radiant flux on the basis of the currently used point dipole model.

P‐176: Distinquished Poster Paper: The Suppression of Viewing Angle Dependence of Top Emission Organic Light Emitting Diodes Having Strong Microcavity Characteristics by Applying Concave Patterned Anode

In order to obtain the stable viewing angle characteristics of top emission organic light emitting diodes (TEOLEDs) having strong microcavity characteristics, we fabricated nano‐porous film on the glass substrate before depositing highly reflective anode. And then, we could obtain the concave patterned anode by depositing anode on the nano‐porous film and fabricated TEOLED composed of concave patterned layers. From this approach, we could successfully obtain not only the stable color shift and luminance distribution with viewing angle but also high efficiency caused by uneven morphology. In addition, we found that the driving voltage of TEOLED could be reduced due to increased surface area effect of the concave patterned anode, so that the power efficiency was enhanced by about 15% in comparison with reference device.

The suppression of viewing angle dependence of top emission organic light emitting diodes having strong microcavity characteristics by applying concave patterned anode

AbstractIn order to obtain the stable viewing angle characteristics of top emission organic light emitting diodes (TEOLEDs) having strong microcavity characteristics, we fabricated nano‐porous film on the glass substrate before depositing highly reflective anode. And then, we could obtain the concave patterned anode by depositing anode on the nano‐porous film and fabricated TEOLED composed of concave patterned layers. From this approach, we could successfully obtain not only the stable color shift and luminance distribution with viewing angle but also high efficiency caused by uneven morphology. In addition, we found that the driving voltage of TEOLED could be reduced due to increased surface area effect of the concave patterned anode, so that the power efficiency was enhanced by about 15% in comparison with reference device.

Fabrication of a compact glass-sealed x-ray tube with carbon nanotube cold cathode for high-resolution imaging

A glass-sealed x-ray tube with field emission electron sources has been fabricated using carbon nanotubes (CNTs) grown on a silicon substrate by direct current plasma-enhanced chemical vapor deposition. Here, the authors report on the fabrication of CNT-based emitters, the field emission characteristics of these emitters, and the properties of the glass-sealed x-ray tube. The field emission produced a current of 5 mA with an electron transmission rate of 91.1% in a high-vacuum chamber. The glass-sealed x-ray tube had a conventional design and comprised a reflection anode, an evaporation getter, and a vacuum-sealed glass tube without additional focusing electrode requirements for ease of commercialization. Using this x-ray tube, the authors obtained x-ray images of objects, including a human finger and a commercial universal serial bus (USB) flash drive. The x-ray image allowed a 100 μm metal wire to be distinguished in the USB flash drive. The x-ray images were obtained at a dose rate of 1944 mrad/h, which represents a relatively low dose. The grown CNT emitters showed stable field emission after electrical aging. These CNT-based glass-sealed x-ray tubes are expected to become a platform for the fabrication of medical and dental x-ray systems.

(Invited) Foldable Circuits and Organic Electronic Devices Based on Solution-Processed Graphene Composites and Inkjet Printing

A facile and cost-effective strategy for foldable and flexible electronics is solution process, such as inkjet printing, of graphene-based materials, which will be also useful for application with large area or ultra-thin form factor. In this presentation, graphene ink and graphene/silver nanowire (AgNW) paste were formulated and printed on the surface-treated flexible/foldable substrates, including plastic or functionalized paper. Circuits of the graphene nano-plates, prepared on the paper substrate by simple transfer printing, showed a very small change in the conductance under various folding angles, retaining more than 90% of initial conductance after repeated cycles [1]. Moreover, compared with electrodes formed by neat graphene, graphene/AgNW circuit showed superior performance both for sheet resistivity as well as folding stability. For the inside (−180◦) and outside (+180◦) folding angle condition, the circuits revealed much smaller change in conductance, maintaining the electronic path on foldable paper. Due to the lack of transparency for most paper-based substrate, top-emitting organic light emitting diode (TOLED) was employed, where its light emitting behavior and efficiency were investigated in terms of the multilayer surface composition on paper and functionality. TOLED on paper was prepared by thermal evaporation or printing of several organic layers onto the reflective anode, where the interconnecting graphene/AgNW circuit supplied the bias voltage and current. Electrical and luminous properties of TOLED were dependent upon the applied bias voltage driven by graphene/AgNW electrodes [2].Solution-phase direct exfoliation of neat graphene is favorable for maintain higher conductivity of printed patterns [3]. We have dispersed graphene nano-platelets in methanol, with 0.5wt% polymeric surfactant (such as triton X-100). Improved uniformity and printability of graphene-based ink materials and their patterns were evaluated using the commercial inkjet/simple nozzle printing. Tiny droplets of graphene or graphene-AgNW composite solution ink could be jetted onto a fast-moving substrate to produce 100μm-width pristine line patterns. Pattern uniformity and resolution were further controllable by the adjusting of the surface tension via ink formulation as well as modification of functional surface of paper substrates. Organic thin film transistor with various patterns/channel length of graphene-base electrodes were fabricated for the evaluation of printed graphene circuits.

Towards the design of efficient quantum dot light-emitting diodes by controlling the exciton lifetime.

Time-resolved photoluminescence and electroluminescence measurements were used to explore the emission characteristics of excitons in quantum dot light-emitting diodes (QD-LEDs). It is found that the lifetime of excitons in the QDs can be varied by adjusting the distance between the excitons and metal Al mirror, which is due to the effect of local density of optical states (LDOS) on the exciton decay rate. QD-LEDs with different hole transport layer (HTL) thickness, i.e., different distance between QDs and Al reflective anode, have been fabricated and it is found that the HTL thickness affects the device efficiency performance greatly, and the optimal HTL thickness for the red QD-LED (emission peak is at 621 nm) is 80 nm. These results shed light on the factors affecting the efficiency and efficiency roll-off in QD-LEDs, thus may provide a clue for high performance QD-LEDs.

Efficiency Improvement of Organic Photovoltaic Using Ozone and PEDOT:PSS Modifications on Ag Anode

We demonstrated a fabrication process using UV-Ozone treatment to obtain a qualified silver oxide (AgOx) film to be anode, and then spinning the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) ( PSS) layer on the Ag/AgOx anode to smooth the its surface, resulting to efficiency improvement of the organic photovoltaic (OPV). The basic OPV structure mainly contained with a high reflective Ag anode, a semitransparent Ag cathode and a planar heterojunction small molecular active layer. Surface morphologies, roughness, reflectivity and workfunction of Ag/AgOx anode with various UV-Ozone treatment times were investigated to affect the device performance. With 60 sec UV-Ozone treatment on Ag/AgOx anode, OPV showed a good power conversion efficiency (PCE) of 0.96%. The further PSS lamination on 60 sec Ag/AgOx anode, OPV exhibited an advanced PCE up to 1.42%.

Foldable Organic Devices Based on the Graphene/Metal Grid Composite Electrodes

Foldable electrodes with solution-processed graphene/silver nanowire (AgNW) composite circuit were formed on the surface-treated commercial paper substrates. Compared with neat graphene or AgNW electrodes, graphene/AgNW circuit showed both folding stability as well as sufficiently low sheet resistance. For the inside (−180°) and outside (+180°) folding angle condition, the circuits revealed a small change in conductance, maintaining the electronic path on foldable paper. Top emitting OLED (TOLED) on paper was prepared by thermal evaporation of several organic layers onto the reflective anode, where the interconnecting graphene/AgNW circuit supplied the bias voltage and current. Luminous efficiency behavior of TOLED on paper showed stable light emitting characteristics on paper, while the current density and brightness was reduced by repeated folding of driving circuit.

A portable x-ray source with a nanostructured Pt-coated silicon field emission cathode for absorption imaging of low-Z materials

We report the design, fabrication, and characterization of a portable x-ray generator for imaging of low-atomic number materials such as biological soft tissue. The system uses a self-aligned, gated, Pt-coated silicon field emitter cathode with two arrays of 62 500 nano-sharp tips arranged in a square grid with 10 μm emitter pitch, and a natural convection-cooled reflection anode composed of a Cu bar coated with a thin Mo film. Characterization of the field emitter array demonstrated continuous emission of 1 mA electron current (16 mA cm − 2) with >95% current transmission at a 150 V gate-emitter bias voltage for over 20 h with no degradation. The emission of the x-ray source was characterized across a range of anode bias voltages to maximize the fraction of photons from the characteristic K-shell peaks of the Mo film to produce a quasi-monochromatic photon beam, which enables capturing high-contrast images of low-atomic number materials. The x-ray source operating at the optimum anode bias voltage, i.e. 35 kV, was used to image ex vivo and nonorganic samples in x-ray fluoroscopic mode while varying the tube current; the images resolve feature sizes as small as ~160 µm.

Pure and Stable Top-Emitting White Organic Light-Emitting Diodes Utilizing Heterojunction Blue Emission Layers and Wide-Angle Interference

In top-emitting white organic light-emitting diodes (TWOLEDs), it is usually difficult to realize a good chromaticity due to the strong suppression on the blue emission induced by the microcavity effect. In our work, the blue emission layer (EML) is located near the hole transport layer and the reflective anode to strengthen the wide-angle interference on the blue emission and enhance the output of light. Then we utilize the dual blue EMLs based on an electron-rich heterojunction to constrain most of the excitons in the blue EMLs. With the above two strategies, the intensity of the blue emission is significantly enhanced accompanying the chromaticity improvement in white emission. Some key factors including exciton distribution, energy transfer, and carrier trapping are analyzed to design the structure of the EMLs to acquire the pure and stable white emission. The excellent color stability with a Commission International de L'Eclairage (CIE) coordinate drift of only (0.009, 0.001) in the luminance range of 10-10(4) cd/m(2) is obtained in our optimized TWOLED. The TWOLED also shows the high performances with a maximum luminance of 15360 cd/m(2), the CIE coordinates of (0.33, 0.41), and a current efficiency of 13.3 cd/A.

Low-Bremsstrahlung X-Ray Source Using a Low-Voltage High-Current-Density Nanostructured Field Emission Cathode and a Transmission Anode for Markerless Soft Tissue Imaging

We report the design, fabrication, and proof-of-concept characterization of an X-ray generator for improved X-ray absorption imaging that uses a nanostructured field emission cathode as the electron source and a microstructured transmission anode as the X-ray generating structure. Field emission cathodes consume less power, respond faster, and tolerate lower vacuum than the thermionic cathodes used in conventional X-ray generators. The use of a transmission anode, instead of a conventional reflection anode, allows filtering of the background radiation (bremsstrahlung) while allowing efficient generation of X-ray at lower voltages by exciting atomic shell transitions, resulting in emission of X-ray with narrow spectral linewidth for sharper imaging of biological tissue. The fabricated field emission cathode contains arrays of self-aligned and gated silicon field emitters. The field emission cathodes turn on at bias voltages as low as 25 V, and their gates transmit almost 100% of the electrons to the anode. The cathodes produce per-emitter electron currents in excess of $2~\mu $ A (current density >2 A/cm $^{2}$ ) at a bias voltage of 80 V. A desktop rig is built to generate X-ray with a field emission cathode and transmission anode. Using the facility, we obtained X-ray absorption images of several objects. The images clearly show details under $500~\mu $ m in size, as well as soft tissue and fine bone structures without using contrast agents. [2014-0087]