Showing papers by "Yang Yang published in 2006"

Transition metal oxides as the buffer layer for polymer photovoltaic cells

Abstract: Polymer-based photovoltaic cells have been fabricated by inserting a thin, transparent, transition metal oxide layer between the transparent anode (indium tin oxide) and the polymer layer. Two different transition metal oxides, namely vanadium oxide and molybdenum oxide, were used and the device performance was compared. The surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy. The effect of the thickness of the oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100mW∕cm2.

Patterning organic single-crystal transistor arrays.

Abstract: Organic flexible electronics are being developed for computer displays, radio frequency identification tags, sensors and devices that have not been dreamt of yet. Practical applications so far are few, as their electrical performance is poor compared with conventional electronics. In terms of charge carrier mobility, however, field-effect transistors made of organic single crystals have a very high performance. The obstacle to the use of single-crystal devices is that they have to be individually hand-made. The report of a method of fabricating large arrays of high performance transistor devices by direct patterning of single crystals onto clean silicon surfaces or flexible plastics may help to change that. The new method retains the high performance of field-effect transistors even after significant bending. Field-effect transistors made of organic single crystals are ideal for studying the charge transport characteristics of organic semiconductor materials1. Their outstanding device performance2,3,4,5,6,7,8, relative to that of transistors made of organic thin films, makes them also attractive candidates for electronic applications such as active matrix displays and sensor arrays. These applications require minimal cross-talk between neighbouring devices. In the case of thin film systems, simple patterning of the active semiconductor layer9,10 minimizes cross-talk. But when using organic single crystals, the only approach currently available for creating arrays of separate devices is manual selection and placing of individual crystals—a process prohibitive for producing devices at high density and with reasonable throughput. In contrast, inorganic crystals have been grown in extended arrays11,12,13, and efficient and large-area fabrication of silicon crystalline islands with high mobilities for electronic applications has been reported14,15. Here we describe a method for effectively fabricating large arrays of single crystals of a wide range of organic semiconductor materials directly onto transistor source–drain electrodes. We find that film domains of octadecyltriethoxysilane microcontact-printed onto either clean Si/SiO2 surfaces or flexible plastic provide control over the nucleation of vapour-grown organic single crystals. This allows us to fabricate large arrays of high-performance organic single-crystal field-effect transistors with mobilities as high as 2.4 cm2 V-1 s-1 and on/off ratios greater than 107, and devices on flexible substrates that retain their performance after significant bending. These results suggest that our fabrication approach constitutes a promising step that might ultimately allow us to utilize high-performance organic single-crystal field-effect transistors for large-area electronics applications.

Efficient inverted polymer solar cells

Abstract: We investigate the effect of interfacial buffer layers—vanadium oxide (V2O5) and cesium carbonate (Cs2CO3)—on the performance of polymer solar cells based on regioregular poly-(3-hexylthiophene) and [6,6]-phenyl C60 butyric acid methyl ester blend. The polarity of solar cells can be controlled by the relative positions of these two interfacial layers. Efficient inverted polymer solar cells were fabricated with the structure of indium tin oxide (ITO)/Cs2CO3/polymer blend/vanadium oxide (V2O5)/aluminum (Al). Short-circuit current of 8.42mA∕cm2, open-circuit voltage of 0.56V, and power conversion efficiency of 2.25% under a AM1.5G 130mW∕cm2 condition were achieved. The interfacial layers were also used to fabricate polymer solar cells using ITO and a thin gold (Au) layer as the transparent electrodes. The thickness of V2O5 layer (10nm) makes it an effective protective layer for the active layer so that ITO can be used for both the electrodes, enabling highly efficient transparent polymer solar cells (i.e., p...

A Theory of Network Localization

Abstract: In this paper, we provide a theoretical foundation for the problem of network localization in which some nodes know their locations and other nodes determine their locations by measuring the distances to their neighbors. We construct grounded graphs to model network localization and apply graph rigidity theory to test the conditions for unique localizability and to construct uniquely localizable networks. We further study the computational complexity of network localization and investigate a subclass of grounded graphs where localization can be computed efficiently. We conclude with a discussion of localization in sensor networks where the sensors are placed randomly

Accurate Measurement and Characterization of Organic Solar Cells

Abstract: Methods to accurately measure the current–voltage characteristics of organic solar cells under standard reporting conditions are presented. Four types of organic test cells and two types of silicon reference cells (unfiltered and with a KG5 color filter) are selected to calculate spectral-mismatch factors for different test-cell/reference-cell combinations. The test devices include both polymer/fullerene-based bulk-heterojunction solar cells and small-molecule-based heterojunction solar cells. The spectral responsivities of test cells are measured as per American Society for Testing and Materials Standard E1021, and their dependence on light-bias intensity is reported. The current–voltage curves are measured under 100 mW cm–2 standard AM 1.5 G (AM: air mass) spectrum (International Electrotechnical Commission 69094-1) generated from a source set with a reference cell and corrected for spectral error.

Electrical Switching and Bistability in Organic/Polymeric Thin Films and Memory Devices

Abstract: Recently, films created by incorporating metallic nanoparticles into organic or polymeric materials have demonstrated electrical bistability, as well as the memory effect, when subjected to an electrical bias. Organic and polymeric digital memory devices based on this bistable electronic behavior have emerged as a viable technology in the field of organic electronics. These devices exhibit fast response speeds and can form multiple-layer stacking structures, demonstrating that organic memory devices possess a high potential to become flexible, ultrafast, and ultrahigh-density memory devices. This behavior is believed to be related to charge storage in the organic or polymer film, where devices are able to exhibit two different states of conductivity often separated by several orders of magnitude. By defining the two states as “1” and “0”, it is now possible to create digital memory devices with this technology. This article reviews electrically bistable devices developed in our laboratory. Our research has stimulated strong interest in this area worldwide. The research by other laboratories is reviewed as well.

Digital memory device based on tobacco mosaic virus conjugated with nanoparticles

Abstract: Nanostructured viruses are attractive for use as templates for ordering quantum dots to make self-assembled building blocks for next-generation electronic devices. So far, only a few types of electronic devices have been fabricated from biomolecules due to the lack of charge transport through biomolecular junctions. Here, we show a novel electronic memory effect by incorporating platinum nanoparticles into tobacco mosaic virus. The memory effect is based on conductance switching, which leads to the occurrence of bistable states with an on/off ratio larger than three orders of magnitude. The mechanism of this process is attributed to charge trapping in the nanoparticles for data storage and a tunnelling process in the high conductance state. Such hybrid bio-inorganic nanostructures show promise for applications in future nanoelectronics.

Achieving High-Efficiency Polymer White-Light-Emitting Devices

Abstract: The external electroluminescence (EL) quantum efficiency (QEEL) of a polymer light-emitting diode (PLED) can be affected by the following four factors: a) charge balance, b) the efficiency of producing singlet excitons, c) photoluminescence quantum efficiency (QEPL), and d) the output coupling effect. The QEPL can approach unity and the efficiency of producing singlet excitons can be high in long-chain polymers. Therefore, the dominating factor for achieving high efficiency for a given polymer is the balance and confinement of electrons and holes. Unfortunately, most conjugated polymers have unbalanced charge-transport properties as the hole mobility is much larger than the electron mobility. In this manuscript, we report a general method to significantly increase the efficiency of PLEDs by controlling the charge injection and distribution through material processing and interface engineering in the device. By blending high-bandgap and low-bandgap polymers in proper ratios, we were able to introduce charge traps in the light-emitting polymer (LEP) layer. Similarly, by introducing an electron-injection/hole-blocking layer, we were able to enhance the minority carrier (electron) injection and confine holes to the emissive layer. Efficient and balanced charge injection, as well as charge confinement, are attained simultaneously, and as a result high-efficiency devices can be achieved. This is a simple yet powerful concept in enhancing the overall efficiency of PLEDs. To illustrated our concept, we have blended 0.25–2 % of poly[2-methoxy-5-(2′ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with poly(9,9-dioctylfluorene) (PFO) as the active polymer layer for PLEDs. A Cs2CO3 electron-injection (and hole-blocking) layer is used at the cathode interface. The emission from the device covers colors from white to yellow, depending on the blend ratio, with the highest peak efficiency being 16 lm W. To the best of our knowledge, this is the highest reported efficiency for a white-light emitting PLED. There are several benefits to using a polymer blend: 1) the low-bandgap LEP behaves as a dopant for energy transfer from the higher-bandgap LEP, 2) the low-bandgap LEP behaves as a charge-trapping site to trap (and confine) the injected charges, which is particularly important in the low-voltage regime where only one type of charge is often present, and 3) the trapped electrons in the low-bandgap LEP will eventually help with the injection of holes and lead to self-balanced charge injection. When this LEP blend system is coupled with an electron-injection (and hole-blocking) layer of Ca(acac)2 [4] (acac: acetylacetonate) or Cs2CO3 [5] at the cathode interface, holes are blocked within the LEP layer as well. As a result, both electrons and holes are effectively confined in the LEP layer rather than being extracted directly at the electrodes. Hence, efficient recombination occurs due to the overlapping distribution of electrons and holes (through formation of excitons). All of these factors can help to increase the efficiency of PLED devices. The schematic profile of the energy structure is shown in Figure 1.

Effect of self-organization in polymer/fullerene bulk heterojunctions on solar cell performance

Abstract: The authors investigate the effect of self-organization by controlling the growth rate on the performance of polymer/fullerene bulk-heterojunction solar cells. The effect of growth rate on the morphology of the active layer is studied by atomic force microscopy technique. The electrical characterization by dark current and photocurrent measurements is performed. The hole mobility in the polymer increases by about two orders in magnitude and the carrier transport becomes highly balanced. Increased exciton generation rate, more efficient electron-hole pair dissociation, higher carrier mobility, and balanced carrier transport in the active layer explain the enhancement in the short-circuit current and fill factor.

High‐Performance Organic Single‐Crystal Transistors on Flexible Substrates

Abstract: The electronic properties of organic single crystals have been intensely studied for well over 40 years. Until recently, organic single-crystal field-effect transistors have generated results that are comparable to and sometimes better in performance than hydrogenated amorphous silicon. Organic thin-film transistors are being actively pursued for a broad area of electronic applications, but their charge-carrier mobilities are limited by structural imperfections (i.e., grain boundaries) and impurities. Organic single crystals, on the other hand, have been limited to charge-transport studies mainly because the fabrication of single-crystal transistors poses a technological challenge. Novel methods for fabricating single-crystal devices include the flip-crystal technique, elastomeric stamp platforms, and freestanding devices, where the source–drain electrodes, dielectric, and gate are all fabricated onto the crystal surface. For the most part, a relatively thick and rigid single crystal is employed (5–500 lm thick). Because the fragility makes them difficult to handle, their use has been restricted to simple and basic devices and wide-ranging applications in sensors or plastic transistors for flexible electronics have not yet been possible. Thus, there is a strong need for the development of mechanically flexible, nondestructive, single-crystal devices with prospective applications in organic electronics while maintaining the intrinsic properties and characteristics of organic single crystals. We demonstrate field-effect transistors fabricated from thin and conformable organic single crystals. We report on proofof-concept “flexible” organic single-crystal field-effect transistors with performance exceeding those of previously reported organic thin-film flexible devices. Rubrene single-crystal devices constructed on low-cost flexible substrates (Fig. 1b) yielded mobilities as high as 4.6 cm V s and on/off ratios of approximately 10.

Regioregular copolymers of 3-alkoxythiophene and their photovoltaic application.

Abstract: Low band gap conjugated polymers with proper energy levels for charge transfer are required to achieve high-efficiency polymer solar cells. We report the synthesis and characterization of two new regioregular copolymers that are based on 3-alkoxythiophene monomers: poly(3-octylthiophene-2,5-diyl-co-3-decyloxythiophene-2,5-diyl) (POT-co-DOT) and poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5',5' '-diyl} (PF-co-DTB). Compared to the alkyl substituents, the alkoxy side chains on the thiophene units can effectively lower the band gap of copolymers and enhance the charge transfer to electron acceptors such as (6,6)-phenyl C(61)-butyric acid methyl ester (PCBM). The chemical structure and regioregularity of the copolymers were confirmed by NMR. Both copolymers are readily soluble in organic solvents and form high-quality thin films. Electrochemical and photophysical studies reveal band gaps of 1.64 eV for POT-co-DOT and 1.78 eV for PF-co-DTB. Bulk heterojunction photovoltaic devices were fabricated using blends of these copolymers with PCBM as the active layer, ITO-glass as the anode, and aluminum as the cathode. Power conversion efficiency of 1.6% was obtained under simulated solar light AM 1.5 G (100 mW/cm(2)) from a solar cell with an active layer containing 20 wt % PF-co-DTB and 80 wt % PCBM. Regioregular poly(3-decyloxythiophene-2,5-diyl) (P3DOT) was also studied for comparison purposes.

Modeling optical effects and thickness dependent current in polymer bulk-heterojunction solar cells

Abstract: Device characteristics of polymer based bulk-heterojunction photovoltaic cells incorporating poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] and methanofullerene ([6,6]-phenyl C61-butyric acid methyl ester) as the active materials are examined as a function of active layer thickness. The dependence of short circuit current on optical effects and its oscillatory variation on the polymer layer thickness is explained by solving the short circuit current using the drift-diffusion equations, where the light intensity calculated from the optical transfer matrix theory is used as the input for optical carrier generation. Furthermore, the effects of polymer layer thickness on other device operation parameters such as open-circuit voltage, fill factor, and series resistivity are measured. Considering the variation of above mentioned parameters, an optimized power conversion efficiency as high as 1.8% (under simulated air mass 1.5 global conditions) was achieved for a device with a polymer layer thickn...

Efficient light harvesting in multiple-device stacked structure for polymer solar cells

Abstract: We demonstrate a multiple-device stacked structure of polymer solar cells for efficient light harvesting. Two polymer photovoltaic cells are stacked together and connected in series or in parallel to achieve a tandem structure. In this two-cell structure, a multilayer semitransparent electrode, made of lithium fluoride (LiF)/aluminum (Al)/gold (Au), is used as the top contact in the bottom cell to efficiently transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent cathode. Upon stacking, the open-circuit voltage and the short-circuit current are almost doubled compared to a single cell.

Polymer memory device based on conjugated polymer and gold nanoparticles

Abstract: Electrical bistability is demonstrated in a polymer memory device with an active layer consisting of conjugated poly3-hexylthiophene and gold nanoparticles capped with 1-dodecanethiol sandwiched between two metal electrodes. The device was fabricated through a simple solution processing technique and exhibited a remarkable electrical bistable behavior. Above a threshold voltage the pristine device, which was in a low conductivity state, exhibited an increase in conductivity by more than three orders of magnitude. The device could be returned to the low conductivity state by applying a voltage in the reverse direction. The electronic transition is attributed to an electric-field-induced charge transfer between the two components in the system. The conduction mechanism changed from a charge-injection-controlled current in the low conductivity state to a charge-transport-controlled current in the high conductivity state. In the high conductivity state the conduction was dominated by a field-enhanced thermal excitation of trapped charges at room temperature, while it is dominated by charge tunneling at low temperatures. The device exhibited excellent stability in both the conductivity states and could be cycled between the two states for numerous times. The device exhibits tremendous potential for its application as fast, stable, low-cost, high storage density nonvolatile electronic memory. © 2006 American Institute of Physics. DOI: 10.1063/1.2337252

Tuning acceptor energy level for efficient charge collection in copper-phthalocyanine-based organic solar cells

Abstract: We compare organic donor/acceptor heterojunction photovoltaic devices based on copper phthalocyanine (CuPc) as the donor and [6,6]-phenyl C60 butyric acid methyl ester (PCBM) or buckminsterfullerene (C60) as the acceptor. The effect of the two acceptors on open-circuit voltage and short-circuit current density is studied. Improved interface properties, optimal energy level matching, and more balanced charge transport are believed to be the reasons for better performance of CuPc/PCBM heterojunction device, in terms of photocurrent and fill factor. The energy conversion efficiency of the devices under AM1.5G solar illumination at 130mW∕cm2 is improved to 1.18%, up from 0.74%.

Conducting Polymer as Transparent Electric Glue

Abstract: Flexible electronic devices, including organic electronic devices and devices fabricated using nanomaterials or biomaterials, have numerous applications, and progress in the field has been rapid. They have several merits, including high mechanical flexibility and low fabrication cost. The conventional way to fabricate these flexible devices is through a bottom-up process, but it is not the most efficient way. Roll-to-roll coating is the most efficient fabrication method, which greatly lowers fabrication cost. However, an efficient roll-to-roll fabrication process has not yet been demonstrated for these materials. The challenge lies in laminating two films electrically and mechanically. It requires conductive glue to adhere the films, and a lamination process free of solvent. This cannot be achieved using conventional glues. Some methods have been demonstrated for laminating two organic films, but these methods are either only suitable for special materials or are impractical. Here, we report three simple approaches for developing solvent-free electric glue using a conducting polymer. This electric glue exhibits a conductivity of 10 S cm, and could effectively laminate various materials electrically and mechanically. The organic electronic devices fabricated through a lamination process using this electric glue exhibit high performance. Many applications have been discovered for conducting polymers, but electric glue from conducting polymers has not been developed so far. The potential for developing electric glue using conducting polymers is high, since most types of glue are made of polymers. Besides application in a roll-toroll fabrication process, electric glue using conducting polymers may replace conventional toxic lead solders in flexible electric circuits. It will have better compatibility with organic devices and the plastic substrates. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has emerged as the most important conducting polymer because of its availability in aqueous solution, high transparency in the visible range, and excellent thermal stability. It has been extensively used in organic electronic devices, particularly in optoelectronic devices. Recently, it has been discovered that the conductivity of a PEDOT:PSS film could be enhanced by about two orders of magnitude by adding an organic compound with multiple polar groups into the PEDOT:PSS aqueous solution or by treating the PEDOT:PSS film with an organic compound having multiple polar groups. When the organic compound with multiple polar groups is solid at room temperature, it needs to be heated at a temperature higher than its melting point for conductivity enhancement. We have discovered a new application for high-conductivity PEDOT:PSS films when an appropriate organic compound is added into its aqueous solution or when the film is treated with an appropriate compound, such as D-sorbitol. The highconductivity PEDOT:PSS film can adhere various films together, so that the PEDOT:PSS film can serve as an electric glue with high transparency. This adhesive function was not observed with an untreated PEDOT:PSS film. When epoxy, which is a widely used glue, was blended into PEDOT:PSS, the PEDOT:PSS film either lost its conductivity or did not act as a glue. Three methods to prepare such an adhesive PEDOT:PSS film with high conductivity are presented in the Experimental section. The PEDOT:PSS films modified by the three approaches all have the same function, but the method of thermally depositing D-sorbitol on the PEDOT:PSS film is the most controllable. It is represented as PEDOT:PSS(D-sorbitol) and is the main discussion topic of this paper. The PEDOT:PSS(D-sorbitol) film is able to laminate two substrates well, and the lamination process is free of solvent. One substrate can be a flexible substrate, such as plastic or plastic coated with indium tin oxide (plastic/ITO), and the other substrate can be flexible or rigid, such as plastic, plastic/ ITO, glass, or glass/ITO. The PEDOT:PSS(D-sorbitol) can be formed on either of the two laminated substrates. The plastic substrates used in our experiments were poly(ethylene terephthalate) (PET). Moreover, lamination can also take place between a plastic or glass substrate (with or without ITO coating) coated with a PEDOT:PSS(D-sorbitol) film and a film of a polymeric semiconductor, such as poly(2-methoxy-5-(2′ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) or poly(3-hexylthiophene) (P3HT). Figure 1a–c shows the laminated structures. The laminated structure of Figure 1c has been fabricated by laminating PET/ITO/PEDOT:PSS(D-sorbitol) with PET/Al/MEH-PPV with D-sorbitol in contact with MEH-PPV. The fabrication process is presented in Scheme 1 and details are given in the Experimental section. This structure is described by PET/ITO/PEDOT:PSS(D-sorbitol)// MEH-PPV/Al/PET. (The double slash (//) is used to indicate the lamination of two parts.) The laminated structures presented in Figure 1a and b have been fabricated through a similar process, but a PET/ITO substrate and glass substrate were used in place of the PET/Al/MEH-PPV in Figure 1a and b, respectively. C O M M U N IC A IO N S

Efficient Blue‐Light‐Emitting Electroluminescent Devices with a Robust Fluorophore: 7,8,10‐Triphenylfluoranthene

Abstract: Silicon semiconductor technology has driven the profusion of information technology into every aspect of modern life. An obvious example of this is the emergence of portable electronic devices, such as cellular phones (mobile phones), personal digital assistants, palmtop computers, etc., that are rapidly becoming essential. These share a common Achilles heel, namely, battery life. The most obvious way to attack this problem is through replacement of the power hungry back-lit liquid-crystal displays (LCDs) that reside in all lightweight devices. It is this impetus that has brought organic light-emitting devices (OLEDs) to the forefront of modern materials science. The ability to mass produce thin, efficient, bright displays from organic polymers and small molecules that can supplant modern LCDs depends almost entirely on the ability to create new materials that can undergo efficient electroluminescence at a variety of wavelengths. This has lead to many publications and patents but to date has failed to produce an efficient, cheap, and robust blue-light emitter. It is, of course, not an easy task to find a small molecule that possesses not only a very large bandgap but also stability to the harsh electrochemical environment of OLEDs and a very large quantum yield in the solid state. One class of molecules in particular, fluorenes—especially spirofluorenes—has received much attention because of the outstanding properties in this area, but lengthy syntheses and low-yielding steps, such as boronic acid/ester formations, are less than ideal for singlelayer/host materials. In an effort to redirect some of the explorations, we have investigated a close cousin of fluorenes—fluoranthenes—for applications as blue-light emitters in OLEDs (Scheme 1). In particular, we have studied 7,8,10triphenylfluoranthene (TPF), a highly luminescent solid-state blue-light-emitting small molecule, which can be obtained in two steps from commercial starting materials. After the elucidation of the structure of fluoranthene at the turn of the 20th century, the chemistry of fluoranthenes evolved rapidly. Studies of the interesting photophysical properties followed, but interest in fluoranthenes faded fast. Of particular note is the synthesis of fluoranthene derivatives by a double Knoevenagel condensation between 2-propanone and acenaphthenequinone, which allows functionalization at the carbon 7 and 10 positions. A subsequent Diels–Alder addition allows further functionalization at the 8 and 9 positions. Finally, starting from bromoacenapthenequinone, the carbon 3 position is open to functionalization. With these synthetic tools in hand, we set about finding a fluoranthene derivative that would not crystallize and would remain highly blue-luminescent in the solid state. The most obvious candidate was a perphenylated derivative, which, due to steric hindrance, would keep the phenyl rings out of plane, thus presenting a ball-like surface to resist crystallization and reduce facial contacts that can lead to excimer quenching and bathochromic shifts in emission. To our surprise, only the 7,8,10-triphenyl derivative (2, Scheme 2) exhibited strong luminescence; the 7,8,9,10-tetraphenyl derivative is essentially non-fluorescent (in the solid state), and the 3,7,8,9-tetraphenyl derivative suffers from a large bathochromic shift in the solid state. The introduction of other functionality (e.g., esters, carboxylic acids, and halides) led to green/yellow emission and/or solubility problems. As with the other derivatives, 7,8,10-triphenylfluoranthene was synthesized via the Knoevenagel/Diels–Alder method (Scheme 2) from acenapthenequinone, diphenylacetone, and phenylacetylene, using only ethanol (EtOH) and (optionally) xylenes for solvents; all are inexpensive and readily available. Purification is uncompliC O M M U N IC A IO N S

Effects of C70 derivative in low band gap polymer photovoltaic devices: Spectral complementation and morphology optimization

Abstract: Efficient polymer solar cells based on a low band gap copolymer poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1,3-benzothiadiazole]-5′,5″-diyl} and (6,6)-phenyl-C71-butyric acid methyl ester (C70-PCBM) were demonstrated with 2.4% power conversion efficiency under air mass 1.5G, 100mW∕cm2 illumination. The broad absorption peak of C70-PCBM in 440–530nm complements the absorption valley (regions between two absorption peaks at 416 and 584nm) of the polymer. The external quantum efficiency measurement further demonstrates that this increased absorption contributes significantly to the generation of photocurrent. Morphology studies on the blend films indicated that excellent miscibility between polymer and C70-PCBM favors exciton separation. The linear relationship between light intensity and short circuit current density shows efficient and balanced charge transport resulting in increased photocurrent and fill factor.

Improving the power efficiency of white light-emitting diode by doping electron transport material

Abstract: Highly efficient white light emission was realized via the partial energy transfer from blue host polyfluorene (PF) to orange light emission dopant rubrene A more balanced charge transport was achieved by adding an electron transport material, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), into the PF-rubrene system to enhance the electron transportation Efficiency improvement by as much as a factor of 2 has been observed through the addition of PBD These devices can easily reach high luminance at low driving voltages, thus achieving high power efficiency at high luminance (148, 135, and 120lm∕W at the luminances of 1000, 2000, and 4000cd∕m2, respectively) Therefore, this performance is an important approach toward solid-state lighting application The enhancement is mainly attributed to three factors: increased electron transport property of the host material, increased photoluminescence quantum efficiency, and the shifting of emission zone away from cathode contact The reported

Nanoparticle-induced negative differential resistance and memory effect in polymer bistable light-emitting device

Abstract: Recently, electrical bistability was demonstrated in polymer thin films incorporated with metal nanoparticles [J. Ouyang, C. W. Chu, C. R. Szmanda, L. P. Ma, and Y. Yang, Nat. Mater. 3, 918 (2004)]. In this letter, we show the evidence that electrons are the dominant charge carriers in these bistable devices. Direct integration of bistable polymer layer with a light-emitting polymer layer shows a unique light-emitting property modulated by the electrical bistability. A unique negative differential resistance induced by the charged gold nanoparticles is observed due to the charge trapping effect from the nanoparticles when interfaced with the light-emitting layer.

Activation of glycogen synthase kinase-3 induces Alzheimer-like tau hyperphosphorylation in rat hippocampus slices in culture

Abstract: Formation of neurofibrillary tangle from hyperphosphorylated tau is one of the hallmark lesions seen in Alzheimer's disease (AD) brain, and neuronal deregulation of glycogen synthase kinase-3 (GSK-3) activity plays key role in tau hyperphosphorylation. In the present study, the role of GSK-3 on tau phosphorylation in hippocampus slice culture was examined by incubating the slice with wortmannin (WT), an inhibitor of phosphatidylinositol 3-kinase (PI3K) and GF-109203X (GFX), an inhibitor of protein kinase C (PKC). It was found that treatment of the slices with GFX or WT separately induced tau hyperphosphorylation both at Ser396/Ser404 (PHF-1) and Ser199/Ser202 (Tau-1) sites. The phosphorylation rate of tau at PHF-1 and Tau-1 epitopes was further increased when GFX and WT were used in combination, and at this condition, AD-like tau accumulation was observed. GSK-3 activity was significantly increased with a concurrently decreased level of inactivated form of GSK-3. Lithium chloride (LiCl), a GSK-3 inhibitor, prevented tau from WT- and GFX-induced hyperphosphorylation. It suggests that GSK-3 is regulated through PI3K and PKC pathway, and activation of GSK-3 not only induces hyperphosphorylation of tau but also leads to accumulation of tau in cultured rat brain slice.

Highly efficient 7,8,10-triphenylfluoranthene-doped blue organic light-emitting diodes for display application

Abstract: We have demonstrated an organic light-emitting diode based on blue-fluorescent dopant 7,8,10-triphenylfluoranthene in a host of dipyrenylfluorene derivatives. The device shows pure blue emission with a peak wavelength of 456 nm and Commission International de L’Eclairage coordinate at (0.164, 0.188). An electroluminescence efficiency as high as 3.33cd∕A and external quantum efficiency of 2.48% can be achieved. Comparison of the photoluminescence and electroluminescence spectra reveals a nearly identical exciton relaxation and efficient energy transfer from the host to the dopant.

SHORT: Shortest Hop Routing Tree for Wireless Sensor Networks

Abstract: For time-sensitive applications requiring frequent data collections from a remote wireless sensor network, it is a challenging task to design an efficient routing scheme that can minimize delay and also offer good performance in energy efficiency, network lifetime and throughput. In this paper, we propose a new routing scheme, called Shortest Hop Routing Tree (SHORT), to achieve those design objectives through effectively generating simultaneous communication pairs and identifying the shortest hop (closest neighbor) for packet relay. Compared with the existing popular schemes such as PEGASIS, BINARY and PEDAP-PA, SHORT offers the best "energy x delay" performance and has the capability to achieve a very good balance among different performance metrics.

Emerging memory devices

Abstract: Each memory device presented has its unique range of advantages and challenges. DRAM and FLASH have radically different characteristics; hence, they are used for different applications. Accordingly, the search for memory devices beyond CMOS comes with an important caveat: different memory for different applications. FENA's research path will continue to focus on improving our presented memory devices, and integrating with logic elements, while exploring other emerging memory devices based on nanomaterials, nanostructures, and the next generation of low-cost assembly techniques

Expression of Toll-like receptors and their association with cytokine responses in peripheral blood mononuclear cells of children with acute rotavirus diarrhoea.

Abstract: To understand virus and host interactions and host responses to rotavirus infection in children, we analysed by real-time polymerase chain reaction (PCR) the expression of mRNA for five Toll-like receptors (TLRs) (TLR2, TLR3, TLR4, TLR7 and TLR8) and four T helper (Th)1 and Th2 cytokines [interleukin (IL)-2, IL-12, interferon (IFN)-gamma and IL-4) in peripheral blood mononuclear cells (PBMC) of children with acute rotavirus diarrhoea. We observed significantly higher expression of genes encoding TLR2, TLR3, TLR4, TLR7 and TLR8 in PBMC of 41% (31/75) patients within 3 days of illness onset than those in healthy children. After 3 days of illness onset, only TLR3 and TLR8 mRNA expressions were still significantly (P<0.05) increased in 59% (44/75) children with diarrhoea. We also observed significantly (P<0.05) elevated expression of IL-12p40 and IFN-gamma in PBMC of patients during the entire period of illness and the first 3 days of illness, respectively. We further demonstrated a weak but significant association between elevated levels of gene expression of four TLRs (TLR2, TLR3, TLR4 and TLR8) and IFN-gamma. Our results suggest that multiple TLRs may modulate the immune response in the acute phase of rotavirus infection and play a role in the activation of IFN-gamma.

Generalized pairwise complementary codes with set-wise uniform interference-free windows

Abstract: This paper introduces an approach to generate generalized pairwise complementary (GPC) codes, which offer a uniform interference free windows (IFWs) across the entire code set. The GPC codes work in pairs and can fit extremely power efficient quadrature carrier modems. The characteristic features of the GPC codes include: the set size is 2K, the processing gain is 4NK, and the IFW's width is 8N identically for all codes in a set, where K is the times to perform Walsh-Hadamard expansions and N is element code length of seed complementary codes. Therefore, by using different N, the IFW width of a GPC code set can be adjusted with its set size unchanged. Each GPC code set consists of two code groups, with each having K codes, and they have sparsely and uniformly distributed autocorrelation side lobes and cross-correlation levels outside the IFWs.

Organic single-crystal complementary inverter

Abstract: The authors demonstrate the operation of an organic single-crystal complementary circuit in the form of a simple inverter. The device is constructed from a high mobility p-type organic single-crystal transistor of tetramethylpentacene (TMPC) and a n-type single-crystal transistor of N,N′-di[2,4-difluorophenyl]-3,4,9,10-perylenetetracarboxylic diimide (PTCDI). Field-effect mobilities of up to 1.0cm2∕Vs are reported for TMPC devices, while a mobility of 0.006cm2∕Vs is reported for a n-type PTCDI single-crystal device. Considering that organic single-crystal inverters have not yet been explored, they are representative of potential candidates for use in high-performance complementary circuits.

Understanding and harnessing biomimetic molecular machines for NEMS actuation materials

Abstract: This paper describes the design, assembly, fabrication, and evaluation of artificial molecular machines with the goal of implementing their internal nanoscale movements within nanoelectromechanical systems in an efficient manner. These machines, a unique class of switchable molecular compounds in the shape of bistable [2]rotaxanes, exhibit internal relative mechanical motions of their ring and dumbbell components as a result of optical, chemical, or electrical signals. As such, they hold promise as nanoactuation materials. Although micromechanical devices that utilize the force produced by switchable [3]rotaxane molecules have been demonstrated, the current prototypical devices require a mechanism that minimizes the degradation associated with the molecules in order for bistable rotaxanes to become practical actuators. We propose a modified design in which electricity, instead of chemicals, is employed to stimulate the relative movements of the components in bistable [3]rotaxanes. As an initial step toward the assembly of a wholly electrically powered actuator based on molecular motion, closely packed Langmuir-Blodgett films of an amphiphilic, bistable [2]rotaxane have been characterized and an in situ Fourier transform infrared spectroscopic technique has been developed to monitor molecular signatures in device settings. Note to Practitioners-Biological molecular components, such as myosin and actin in skeletal muscle, organize to perform complex mechanical tasks. These components execute nanometer-scale interactions, but produce macroscopic effects. Inspired by this concept, we are developing a new class of mechanical nanodevices that employ a group of artificial molecular machines called bistable rotaxanes. In this paper, a series of experiments has been conducted to study the molecular properties of bistable rotaxanes in thin films and on solid-state nanodevices. Our results have shed light on the optimization of future molecular machine-based systems particularly with respect to their implementation and manufacture.

Cooperative doping effects of Ti and C on critical current density and irreversibility field of MgB2

Abstract: Carbon and titanium concurrently doped MgB2 alloys with a composition of Mg1−xTixB2−yCy have been prepared by an in situ solid state reaction method. The combination effects of carbon and Ti doping on structure, microstructure, irreversibility field (Hirr), and critical current density (Jc) of MgB2 have been investigated. It is found that doping Ti in MgB2 does not reduce the solubility of carbon in MgB2, but, on the contrary, it eliminates the porosity present in the carbon-doped MgB2, resulting in an improved intergrain connectivity. As a result, Jc and Hirr of MgB2 have been significantly improved, sharply contrasted to the situation of C-only-doped or Ti-only-doped MgB2 bulks. Our results clearly reveal that carbon and Ti codoping are largely cooperative in improving the performance of MgB2 in the high magnetic fields (>3T) and at high temperature (∼20K).

Radar Waveform Design using Minimum Mean-Square Error and Mutual Information

Abstract: This paper addresses the problem of radar waveform design for target identification and classification. Both the ordinary radar with a single transmitter and receiver and the recently proposed multiple-input multiple-output (MIMO) radar are considered. A random target impulse response is used to model the scattering characteristics of the extended (nonpoint) target, and two radar waveform design problems with constraints on waveform power have been investigated. The first one is to design waveforms that maximize the conditional mutual information (MI) between the random target impulse response and the reflected waveforms given the knowledge of transmitted waveforms. The second one is to find transmitted waveforms that minimize the mean-square error (MSE) in estimating the target impulse response. Our analysis indicates that under the same total power constraint, these two criteria lead to the same solution for a matrix which specifies the essential part of the optimum waveform design. The solution employs water-filling to allocate the limited power appropriately. We also present an asymptotic formulation which requires less knowledge of the statistical model of the target