Bottom Cathode Research Articles

46.4: Effects of Electron Injection Layer on Storage and Operational Stability of Air‐Stable OLEDs

Air‐stable OLEDs are necessary for expanding the availability of OLEDs. We examined the configuration of the electron injection layer (EIL) suitable for air‐stable OLEDs. An OLED with high operational stability was demonstrated by employing a newly developed EIL that requires no chemical n‐doping for electron injection from the bottom cathode.

Top illuminated organic photodetectors with dielectric/metal/dielectric transparent anode

The top illuminated organic photodetectors (OPDs) with a Dielectric/Metal/Dielectric (DMD) transparent anode are fabricated. The transparent electrode is composed of molybdenum trioxide (MoO3)/silver (Ag)/MoO3 layers and zinc oxide (ZnO)/aluminum (Al) is used for bottom cathode. The optimized DMD electrode has an optical transmittance of 85.7% at the wavelength of 546nm and sheet resistance of ∼6Ω/sq. The fabricated OPDs exhibit a high detectivity and wide range linearity.

High-performance inverted polymer solar cells based on thin copper film

We report the fabrication of cost-effective indium-free polymer solar cells (PSCs) with an inverted structure that incorporates an ultrathin copper (Cu) film as a bottom cathode via thermal evaporation. The average optical transmittance of the 15-nm Cu coated glass substrate in the visible region of the spectrum was found to be around 80% with a highest value of 84.5%. The Cu electrode was modified by an interfacial layer of an alcohol-/water-soluble conjugated polymer, poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) interlayer to ensure a very smooth surface. Upon the use of the PFN interfacial layer, the work function of Cu was decreased from 4.68 to 4.31 eV, which can form an Ohmic contact with photoactive layer and facilitate electrode transport and extraction. As a result, a power conversion efficiency of 3.6% was achieved when poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] and a [6,6]-phenyl C71-butyric acid methyl ester blend were utilized as the photoactive layers, demonstrating that the thermally evaporated Cu thin-film electrode can be a promising candidate to replace indium tin oxide for highly efficient PSCs.

Mass and heat transfer at an array of horizontal cylinders placed at the bottom of a square agitated vessel

Rates of liquid–solid mass transfer at horizontal array of cylinders resting on the base of a square agitated vessel were studied. An electrochemical technique which involves measuring the limiting current of the cathodic reduction of K3Fe(CN)6 in a large excess of NaOH supporting electrolyte was used. The mass transfer data were correlated by dimensionless mass transfer equations. Drag reducing polymers were found to reduce the rate of mass transfer at the tube array by an amount ranging from 2.4 to 21.8% depending on the operating conditions. The volumetric mass transfer coefficient (kA) at the tube array was found to be higher than that of the flat tank bottom cathode by a factor ranging from 4.24 to 9.33. The importance of the present study in the design and operation of stirred square tank reactor with a cooling system at the bottom was noted. Also the importance of the present results in designing semi-continuous tubular dialyzers was highlighted. The possibility of using the outer tube surface as a catalyst support and the inner surface as a cooler for conducting diffusion controlled reactions which need critical temperature control such as immobilized enzyme catalyzed biochemical reactions was discussed.

Efficient organic light-emitting diodes fabricated on cellulose nanocrystal substrates

Organic light-emitting diodes (OLEDs) fabricated on recyclable and biodegradable substrates are a step towards the realization of a sustainable OLED technology. We report on efficient OLEDs with an inverted top-emitting architecture on recyclable cellulose nanocrystal (CNC) substrates. The OLEDs have a bottom cathode of Al/LiF deposited on a 400 nm thick N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) layer and a top anode of Au/MoO3. They achieve a maximum luminance of 74 591 cd/m2 with a current efficacy of 53.7 cd/A at a luminance of 100 cd/m2 and 41.7 cd/A at 1000 cd/m2. It is shown that the α-NPD layer on the CNC substrate is necessary for achieving high performance OLEDs. The electroluminescent spectra of the OLEDs as a function of viewing angle are presented and show that the OLED spectra are subject to microcavity effects.

Highly efficient and air-stable inverted organic light-emitting diode composed of inert materials

The feasibility of a highly efficient and air-stable organic light-emitting diode (OLED) was examined. A phosphorescent OLED not containing an air-sensitive material was fabricated by employing an inverted structure with an air-stable electron injection layer. Efficient electron injection from the bottom cathode to the emitting layer was demonstrated from the fact that the device characteristics of the inverted OLED were almost the same as those of a conventional OLED. No dark spot formation was observed after 250 days in the inverted OLED encapsulated by a barrier film with a water vapor transmission rate of 10−4 g m−2 day−1.

Efficient inverted polymer solar cells based on ultrathin aluminum interlayer modified aluminum-doped zinc oxide electrode

A convenient and economical design for inverted polymer solar cells has been developed by introducing an ultrathin aluminium (Al) interlayer to modify aluminum-doped zinc oxide (AZO) electrode as the bottom cathode. An ultrathin interlayer of Al could lower the work function of AZO electrode. Power conversion efficiency (PCE) of 3.84% was obtained for poly(3-hexyl-thiophene):phenyl-C61-butyric acid methyl ester based device and that of poly(3-hexyl-thiophene):indene-C60 bisadduct based device with such electrodes could reach to 5.52%, which was much higher than the structurally identical device based on ITO/Al(1 nm) electrode and almost the same as that of the conventional device. This indicates that the ultrathin Al modified AZO composite electrode is a strong competitor for the cathode in inverted polymer solar cells especially the large scale and low costs devices.

Fabrication and characterization of a GaN/(4H)SiC vertical pn power diode using direct and interfaced epitaxial-growth approaches

We report the fabrication and characterization of a vertical pn power diode which is realized using two separate epitaxial-growth mechanisms: (a) p-GaN over p-(4H)SiC, and (b) p-GaN over n-(4H)SiC with AlN as the interface layer. In all of the cases, n+-doped (4H)SiC serves as the cathode substrate. Pd(200 Å)/Au(10000 Å) is used for the anode contact while Ni(1000 Å) is used for the bottom cathode contact. The measured forward drop of the pn diode with AlN as the interface material is found to be around 5.1 V; whereas, it is 3 V for the other sample structure. The measured reverse-blocking voltage is found to be greater than 200 V.

Electron injection and transport for high-performance inverted organic light-emitting diodes

Flexible organic light-emitting diodes (OLEDs) are considered the next-generation displays. To realize stable and high-performance flexible OLEDs, inverted OLEDs (iOLEDs) integrated into oxide thin-film transistors are desired for active-matrix OLEDs. Unfortunately, however, the efficiency of iOLEDs has not been as good as that of the conventional OLEDs up to recent years, due to the difficulties in electron injection from the bottom cathodes to the organic layers in iOLEDs. In this paper, electron injection and transport are described to explain how to enhance the efficiency of iOLEDs. Various methods of improving the electron injection from the bottom cathodes to the organic layers are reviewed, especially focusing on the n-doping technology. The importance of the n-electron-transporting layer (ETL)/undoped ETL homo- and hetero-junction for efficient iOLEDs is also explained. Finally, the p–n junction is introduced as a new kind of electron injection layer that works very well independently of the cathode materials.

Synthesis and characterization of Ni-Mo filler brazing alloy for Mo-W joining for microwave tube technology

A brazing process based on Ni-Mo alloy was developed to join porous tungsten cathode bottom and dense molybdenum cathode body for microwave tubes manufacture. The Ni-Mo alloy was obtained by mixing and milling powders in the eutectic composition, and applied on the surface of the components. The brazing was made at 1400 °C by using induction heating in hydrogen for 5 minutes. Alumina surfaces were coated with the binder and analyzed by Energy Dispersive X-rays Fluorescence. The brazed samples were analyzed by Scanning Electron Microscopy coupled to Energy Dispersive Spectroscopy. Stress-strain tests were performed to determine the mechanical behavior of the joining. The quality of the brazing was evaluated by assuring the presence of a "meniscus" formed by the Ni-Mo alloy on the border of the tungsten and molybdenum joint, the absence of microstructural defects in the interface between the tungsten and molybdenum alloys, and the adhesion of the brazed components.

Top-illuminated organic solar cells fabricated by vacuum-free and all-solution processes

Solution-processable silver nanoparticles were adopted as the bottom cathode of top-illuminated organic solar cells based on a blend of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester as the bulk-heterojunction layer. The developed device, with the top transparent anode based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and silver grids, shows a power conversion efficiency of 1.27% with a fill factor of 53.8% compared to 1.53% efficiency and 53.9% fill factor from the control devices fabricated by a vacuum-evaporated bottom cathode. The newly developed device demonstrated the feasibility of this alternative approach to developing an all-solution and vacuum-free route for fabricating large-area devices using wet coating processes in atmospheric pressure.

Highly efficient inverted top-emitting green phosphorescent organic light-emitting diodes on glass and flexible substrates

Green phosphorescent inverted top-emitting organic light-emitting diodes with high current efficacy and luminance are demonstrated on glass and polyethersulfone (PES) substrates coated with polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS). The bottom cathode is an aluminum/lithium fluoride bilayer that injects electrons efficiently into an electron transport layer of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TpPyPB). The cathode is found to be highly sensitive to the exposure of trace amounts of O2 and H2O. A high current efficacy of 96.3 cd/A is achieved at a luminance of 1387 cd/m2 when an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) is deposited on the anode.

Research on the Panel Fabrication for the Field Emission Display with Double Strip Emitters

With high effective screen-printing process, the field emission display panel with double stripe emitters was fabricated and their good field emission property was also characterized. Using sintered silver slurry with better electric conduction property as material, the cathode electrode including two parts was formed. The top cathode electrode was made up of two silver stripes, which the carbon nanotube material would reside on its surface. The bottom cathode electrode was separated by the cathode separate layer with the top cathode electrode, but the electric-connection would be performed between them through the preformed round holes. Using carbon nanotube as cathode, the field emission display panel with double stripe emitters was demonstrated.

High-Efficiency Inverted Polymer Solar Cells with Double Interlayer

We have studied the performance of normal and inverted bulk-heterojunction solar cells with an active layer composed of a blend of poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PDTS-BTD) and {6,6}-phenyl-C71 butyric acid methyl ester (PC(71)BM). For inverted cells, a thin layer of ZnO nanoparticles and MoO(3) were used as interlayers for the bottom cathode and the top anode respectively. To enhance the device performance, a thin film of 4,4',4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (MTDATA) was used along with MoO(3) as an anode interlayer to improve the hole extraction from the photoactive layer to the anode. The inverted polymer solar cells with double interlayer exhibit a higher power conversion efficiency of 6.45% compared to the conventional cell of 4.91% due to efficient charge extraction and favorable vertical morphology of active layer blend. Our ultraviolet photoemission spectroscopy results indicate that the formation of band bending due to interlayer leads to the enhancement in hole extraction.

Interlayer for Modified Cathode in Highly Efficient Inverted ITO‐Free Organic Solar Cells

Inverted polymer solar cells with a bottom metal cathode modified by a conjugated polymer interlayer show considerable improvement of photocurrent and fill factor, which is due to hole blocking at the interlayer, and a modified surface energy which affects the nanostructure in the TQ1/[70]PCBM blend.

Polarizer-free, high-contrast inverted top-emitting organic light emitting diodes: effect of the electrode structure

Circular polarizer (CP)-free, high-contrast inverted top-emitting organic light-emitting diodes (ITOLEDs) are demonstrated by using a semitransparent Ag electrode capped with a single dielectric layer as a top anode and Cs(2)CO(3) (1.5 nm)/Al (1.5 nm)/Cr (100nm) as a partially absorbing bottom cathode. Low luminous reflectance is achieved by combining the broadband absorption of Cr, the weak but inherent cavity structure, and the optimal thickness of the capping layer yielding a high transmittance of a top electrode. With the optimized organic capping layer, contrast-enhanced ITOLEDs exhibit a luminous reflectance as low as 3.6% with a large thickness margin. Their luminous efficiency is shown to be comparable to or even higher than that of CP-based conventional OLEDs.

Solution-processable small molecules as efficient universal bipolar host for blue, green and red phosphorescent inverted OLEDs

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode are of great interest for large-size active-matrix displays due to their easy integration with n-type thin film transistors (TFTs) based on low-cost and highly-uniform amorphous silicon and oxide. In this work, a solution-processable electron transporting material 2,7-bis(diphenylphosphoryl)-9,9′-spirobi[fluorene] (SPPO13) is employed to blend with a solution-processable hole transporting material 4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) to be used as a universal bipolar co-host for blue, green and red phosphors, and for the first time, phosphorescent IOLEDs are fabricated by solution-processing small molecules. High efficiency and reduced efficiency roll-off are achieved in the solution-processed IOLEDs, which mainly contribute to the high quality of the solution-processed small molecule films as well as the balanced charge injection in the co-host system. Importantly, the solution process is advantageous over vacuum evaporation to deposit multi-component small molecule films, and can be expected to reduce manufacturing costs. Our results demonstrate a promising approach to fabricate low-cost and high-performance IOLEDs for n-type TFT-based displays.

A Fabrication of the High Emitter-Isolating-Layer for Printed Cold Cathode in Field Emission Display Panel

Using black insulation slurry as manufacture material, the high emitter-isolating-layer was fabricated on the cathode plate. The high emitter-isolating-layer which possessed good insulation performance was composed of many parallel emitter-isolation-stripes, which the bottom cathode electrode would be exposed in the center of the emitter-isolation-stripe. The carbon nanotube paste was prepared onto the cathode plate to form the field emitters. And the field emission display panel with high emitter-isolating-layer was demonstrated, which the better field emission property including the large emission current and stable electron emission was confirmed.

A Fabrication of the High Emitter-Isolating-Layer for Printed Cold Cathode in Field Emission Display Panel

Using black insulation slurry as manufacture material, the high emitter-isolating-layer was fabricated on the cathode plate. The high emitter-isolating-layer which possessed good insulation performance was composed of many parallel emitter-isolation-stripes, which the bottom cathode electrode would be exposed in the center of the emitter-isolation-stripe. The carbon nanotube paste was prepared onto the cathode plate to form the field emitters. And the field emission display panel with high emitter-isolating-layer was demonstrated, which the better field emission property including the large emission current and stable electron emission was confirmed.

All-solution-processed ITO-free polymer solar cells fabricated on copper sheets

We report on cost-effective and ITO-free polymer solar cells (PSCs) fabricated on the copper (Cu) sheets consisting mainly of a Cu electrode and a polyimide substrate. The Cu electrode was used as a bottom cathode in inverted PSCs, and each layer above the Cu sheet was all-solution-processed. The work-functions of Cu electrodes were engineered with an interfacial dipole layer to enhance the device-performance. Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (hcPEDOT:PSS) was used as a transparent top anode, and a photoactive layer was based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The fabricated PSCs exhibited a power conversion efficiency (PCE) of ∼2.5%, demonstrating PSCs fabricated on Cu sheets have the great potential for cost-effective power generators. This study could be an important approach to the development of all-solution-processed and roll-to-roll PSCs using low-cost Cu electrodes as a cathode.