Showing papers by "Yang Yang published in 2005"

High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends

Abstract: Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.

High-Conductivity Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Film and Its Application in Polymer Optoelectronic Devices†

Abstract: The conductivity of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film can be enhanced by more than two orders of magnitude by adding a compound with two or more polar groups, such as ethylene glycol, meso-erythritol (1,2,3,4-tetrahydroxybutane), or 2-nitroenthanol, to an aqueous solution of PEDOT:PSS. The mechanism for this conductivity enhancement is studied, and a new mechanism proposed. Raman spectroscopy indicates an effect of the liquid additive on the chemical structure of the PEDOT chains, which suggests a conformational change of PEDOT chains in the film. Both coil and linear conformations or an expanded-coil conformation of the PEDOT chains may be present in the untreated PEDOT:PSS film, and the linear or expanded-coil conformations may become dominant in the treated PEDOT:PSS film. This conformational change results in the enhancement of charge-carrier mobility in the film and leads to an enhanced conductivity. The high-conductivity PEDOT:PSS film is ideal as an electrode for polymer optoelectronic devices. Polymer light-emitting diodes and photovoltaic cells fabricated using such high-conductivity PEDOT:PSS films as the anode exhibit a high performance, close to that obtained using indium tin oxide as the anode.

Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene)

Abstract: Regioregular poly(3-hexylthiophene) (RR-P3HT) is a promising candidate for polymer photovoltaic research due to its stability and absorption in the red region. In this manuscript, we report polymer photovoltaic devices based on RR-P3HT:methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) 1:1 weight-ratio blend. We studied the effects of annealing temperature and time on the device performance for devices annealed before and after cathode deposition. Thermal annealing shows significant improvement in the performance for both types of annealing conditions, with postproduction annealing being slightly better. For devices with a 43-nm-thick active layer, maximum power conversion efficiency (PCE) of 3.2% and fill factor up to 67% is achieved under Air Mass 1.5, 100‐mW∕cm2 illumination. We performed atomic force microscopy and ultraviolet-visible absorption spectroscopy on the P3HT:PCBM films to explain the effect of thermal annealing. By keeping the optimized thermal annealing condition and by va...

Polyaniline nanofiber/gold nanoparticle nonvolatile memory

Abstract: A nonvolatile plastic digital memory device based on nanofibers of the conjugated polymer polyaniline decorated with gold nanoparticles is reported. The device has a simple structure consisting of the plastic composite film sandwiched between two electrodes. An external bias is used to program the ON and OFF states of the device that are separated by a 3-orders-of-magnitude difference in conductivity. ON−OFF switching times of less than 25 ns are observed by electrical pulse measurements. The devices possess prolonged retention times of several days after they have been programmed. Write−read−erase cycles are also demonstrated. The switching mechanism is attributed to an electric-field-induced charge transfer from the polyaniline nanofibers to the gold nanoparticles. The active polymer layer is created by growing nanometer size gold particles within 30-nm-diameter polyaniline nanofibers using a redox reaction with chloroauric acid. This device combines two exciting research areasnanoparticles and conducti...

Organic Donor-Acceptor System Exhibiting Electrical Bistability for Use in Memory Devices.

Abstract: An all-organic electrically bistable device and its application in non-volatile memory is reported. The switching mechanism is attributed to electric-field-induced charge transfer from an organic electron donor to an acceptor, so that the device switches from a low- to a high-conductivity state. This device provides a new direction for data-storage technology based on organic composites.

High-performance organic thin-film transistors with metal oxide/metal bilayer electrode

Abstract: We demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFT). The bilayer consists of a transition metal oxide (MoO3,WO3, or V2O5) layer and a metal layer. The metal oxide layer, directly contacting the organic semiconducting layer, serves as the charge-injection layer. The overcoated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an important role in improving the field-effect transistor characteristics of OTFTs. Devices with the bilayer S-D electrodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mobility of 0.4cm2∕Vs and on/off current ratios of 104 were obtained in the pentacene based TFTs using the bilayer S-D electrodes at a gate bias of −40V. The improvement is attributed to the reduction in the contact barrier and the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between ...

Exponential companding technique for PAPR reduction in OFDM systems

Abstract: In this paper, a new nonlinear companding technique, called "exponential companding", is proposed to reduce the high Peak-to-Average Power Ratio (PAPR) of Orthogonal Frequency Division Multiplexing (OFDM) signals. Unlike the /spl mu/-law companding scheme, which enlarges only small signals so that increases the average power, the schemes based on exponential companding technique adjust both large and small signals and can keep the average power at the same level. By transforming the original OFDM signals into uniformly distributed signals (with a specific degree), the exponential companding schemes can effectively reduce PAPR for different modulation formats and sub-carrier sizes. Moreover, many PAPR reduction schemes, such as /spl mu/-law companding scheme, cause spectrum side-lobes generation, but the exponential companding schemes cause less spectrum side-lobes. Computer simulations, which consider a baseband OFDM system with Additive White Gaussian Noise (AWGN) channels and a Solid State Power Amplifier (SSPA), show that the proposed exponential companding schemes can offer better PAPR reduction, Bit Error Rate (BER), and phase error performance than the /spl mu/-law companding scheme.

Ink-Jet Printing, Self-Assembled Polyelectrolytes, and Electroless Plating: Low Cost Fabrication of Circuits on a Flexible Substrate at Room Temperature

Abstract: The driving forces behind the development of flexible electronics are their flexibility, lightweightedness, and potential for low-cost manufacturing. However, because of physical locations, traditional thermal processes cause deformations in the flexible substrate. As a result, the adhesion quality of the printed wires is deteriorated. This article reviews recent developments in printing circuits on a flexible substrate by combining self-assembled polyelectrolytes, ink-jet printing of a catalyst, and electroless plating of metals. The limitations and potential applications of this technology are also discussed. Experiments implementing this technology demonstrated significant results. By a vibration-induced assistance during an ink-jet printing catalyst process, line width and blurring can be controlled to within ′3% variation. Following the IPC 6013 standard for flexible electronics, the results after thermal cycling (288 °C, 6 times) and a hot oil test (260 °C, 3 times) indicated that the metallic circuit had retained excellent adhesion properties and electric characteristics. We also report the first successful demonstration of a metal film in a via-hole inner wall on a flexible substrate. This novel fabrication method is ideal for the realization of large area, flexible electronics and furore multilayer flexible substrate Application, such as flexible display, chip on flexible substrate, etc. particularly where traditional lithographic processes can not be applied.

Efficient photovoltaic energy conversion in tetracene-C 60 based heterojunctions

Abstract: We report organic solar cells fabricated with small-molecule organic semiconductor tetracene∕C60 heterojunction as the photoactive layer. The external power conversion efficiency of the devices under AM 1.5 solar illumination at 100mW∕cm2 (1 sun) is 2.3±0.5% with relatively high open-circuit voltage (Voc=0.58±0.06V) compared to most of the other small-molecular donor-acceptor (D-A) heterojunction solar cells reported so far. Using atomic force microscopy and x-ray diffraction we found that tetracene thin films consist of submicron-sized grains with rough surface and well defined molecular order. Therefore, using high mobility polycrystalline tetracene thin films for D-A heterojunction devices dramatically increases area of tetracene and C60 interface for exciton diffusion to reduce the recombination.

Electric-field-induced charge transfer between gold nanoparticle and capping 2-naphthalenethiol and organic memory cells

Abstract: An electrical transition, induced by a high electric field, was observed in a device consisting of a polystyrene film containing 2-naphthalenethiol-capped Au nanoparticles sandwiched between two Al electrodes. The current through the device changed from a charge-injection-controlled current to a space-charge-limited current. The latter current is higher than the former by more than three orders of magnitude at 2 V. Asymmetrical current-voltage curve was observed along the two polarity directions for the device after the transition. This transition is attributed to an electric-field-induced charge transfer between the Au nanoparticle and the capping 2-naphthalenethiol. The device exhibits good stability in the high conductivity state, so it can be used as a write-once-read-many times electronic memory.

Effective connecting architecture for tandem organic light-emitting devices

Abstract: An effective connecting structure for tandem organic light-emitting devices is reported. The connecting structure consists of a thin metal layer as the common electrode, a hole-injection layer containing MoO3 on one side of the common electrode, and an electron-injection layer involving Cs2CO3 on the other side. Such a connecting structure permits efficient opposite hole and electron injection into two adjacent emitting units and gives tandem devices superior electrical and optical performances. Furthermore, the present connecting structure involves no sputtering or handling of reactive metals during device fabrication and can be prepared purely by thermal evaporation, thus rendering device processing more feasible.

Enzymatic remodeling of heparan sulfate proteoglycans within the tumor microenvironment: growth regulation and the prospect of new cancer therapies.

Abstract: Heparan sulfate proteoglycans (HSPGs), via their interactions with numerous effector molecules such as FGF-2, IL-8, and VEGF, regulate the biological activity of cells by acting as co-receptors that promote signaling. The extent and nature of their role as co-receptors is often misregulated in cancer as manifested by alterations in HSPG structure and expression level. This misregulation of HSPGs can aid in promoting the malignant phenotype. In addition to expression- related changes in HSPGs, recent discoveries indicate that HSPGs localized within the tumor microenvironment can be attacked by enzymes that alter proteoglycan structure resulting in dramatic effects on tumor growth and metastasis. This review focuses on remodeling of HSPGs by three distinct mechanisms that occur in vivo; (i) shedding of proteoglycan extracellular domains from cell surfaces, (ii) fragmentation of heparan sulfate chains by heparanase, and (iii) removal of sulfates from the 6-O position of heparan sulfate chains by extracellular sulfatases. Assessing or monitoring the remodeling of HSPGs has important implications for tumor diagnosis and patient prognosis while therapeutic manipulation of the remodeling process represents an exciting new possibility for treating cancer. J. Cell. Biochem. 96: 897-905, 2005. 2005 Wiley-Liss, Inc.

Influence of composition and heat-treatment on the charge transport properties of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blends

Abstract: Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends have been demonstrated to form highly efficient polymer photovoltaic devices. In this letter, the time-of-flight technique is used to investigate the influence of composition and heat treatment on charge transport properties of P3HT and PCBM blends. The transport of electrons and holes both display a transition from dispersive to nondispersive and return to dispersive again as the percentage of PCBM increases. A balanced mobility of both electron and hole is obtained at a composition of 1:1 weight ratio, and it is nearly independent of the electrical field in the range of our test. The increase in carrier mobility is attributed to the formation of a more-ordered structure in the blend. This structural ordering is further enhanced by slowly evaporating the solvent during film formation which results in additional increase in carrier mobility. However, no such effect is observed in thick films (∼200nm), indicating the...

Capacitance–voltage characterization of polymer light-emitting diodes

Abstract: The capacitance–voltage (C–V) characterization of polymer light-emitting diodes (PLEDs) employing poly[5-(2′-ethylhexyloxy)-2-methoxy-1,4-phenylene vinylene], as the light-emitting layer are reported. Several metals, such as calcium (Ca), aluminum (Al), and gold (Cu) were used as the cathode in order to investigate the influence of the charge injection on the C–V characteristics. Under forward bias, the capacitance increases with majority charge carrier injection into the polymer layer, and, afterwards, decreases upon minority charge carrier injection which results in recombination of electrons and holes in the active polymer layer. The increase in the value of capacitance follows the same dependence as the increase in the value of current density through the device, which suggests that the capacitance depends not only on the amount of charge trapped in the polymer near the interface, but also, and mainly, on the amount of charge injection into the polymer layer from the electrodes. The C–V behavior of PL...

Synthesis of main-chain polyoxometalate-containing hybrid polymers and their applications in photovoltaic cells

Abstract: Hexamolybdate clusters have been embedded through covalent bonds into the main chain of poly(phenylene acetylene)s. These hybrid polymers were synthesized by palladium-catalyzed coupling reactions of a diiodo functionalized cluster with a diethynylbenzene derivative or a diethynyl functionalized cluster with a diiodobenzene derivative. These polymers are soluble in organic solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), and free-standing films can be spin-coated or cast from solutions. While hybrid monomer 2a exhibits a sharp melting transition at 246 °C, polymers 5a and 5b show glass transitions at 125 and 102 °C, respectively. Cyclovoltammetry studies of the hybrid polymers revealed a reversible reduction wave at 1.19 V versus Ag/Ag+, comparable to those of bifunctionalized imido derivatives of hexamolybdates. These polymers show intense absorption in the visible range but with little fluorescence emissions, indicating efficient fluorescence quenching of the embedded polyoxom...

Organic Memory Device Fabricated Through Solution Processing

Abstract: Novel organic memory devices including nonvolatile and write-once-read-many-times memory devices are reported. These devices were fabricated through a simple solution processing technique. Programmable electrical bistability was observed on a device made from a polymer film containing metal nanoparticles capped with saturated alkanethiol and small conjugated organic compounds sandwiched between two metal electrodes. The pristine device, which was in a low-conductivity state, exhibited an abrupt increase of current when the device was scanned up to a few volts. The high-conductivity state can be returned to the low-conductivity state by applying a certain voltage in the reverse direction. The device has a good stability in both states, and the transition from the low- to the high-conductivity state takes place in nanoseconds, so that the device can be used as a low-cost, high-density, high-speed, and nonvolatile memory. The electronic transition is attributed to the electric-field-induced charge transfer between the metal nanoparticles and small conjugated organic molecule. The electrical behavior of the device is strongly dependent on the materials in the polymer film. When gold nanoparticles capped with aromatic thiol were used, the device exhibited a transition from low- to high-conductivity state at the first voltage scan, and the device in the high-conductivity state cannot be returned to the low-conductivity state. This device can be used as a write-once-read-many-times memory device.

White organic light-emitting diodes prepared by a fused organic solid solution method

Abstract: This letter demonstrates a white organic light-emitting diode (OLED) with high color stability fabricated by using a single organic white-emitting layer. The dopants were introduced prior to the device fabrication process through organic solid solution process formed by high-temperature and high-pressure fusion process. A high band gap organic material, α-naphthlyphenylbiphenyl diamine, was adopted and precisely doped with several kinds of fluorescent dyes as the emitting material. The most important benefits of using this fused organic solid-solution technique are the precise control of dopants, ultrauniform mixture of dopants in the host, easy fabrication; and, as a result, the fabricated white OLEDs show extremely little color shift with increasing injection current. The simplified device fabrication process is believed to be beneficial to the white OLED display and lighting industrialization.

Integration of organic light-emitting diode and organic transistor via a tandem structure

Abstract: A high-performance organic active matrix pixel was fabricated by using a metal oxide (V2O5) coupling layer that effectively integrates an organic light-emitting diode (OLED) on top of an organic field-effect transistor (OFET). The field-effect mobility of the OFET approached 0.5cm2V−1s−1 and the ON/OFF current ratio was >103. The brightness of the OLED was on the order of 2000cd∕m2, with an efficiency above 3.3cd∕A. The present work describes in detail a methodology for sizing and stacking an OFET in bottom-emitting active matrix pixel circuits. The confinement of pixel dimension ensures the uniformity of light emission. The material for coupling layer can be tailored to achieve maximum device efficiency. A unique active matrix pixel circuit is proposed that renders both the OFET and OLED their individual performance after integration.

Three-terminal organic memory devices

Abstract: An organic electrical bistable device (OBD) has been reported previously, which has an organic∕metal-nanocluster∕organic structure sandwiched between a top and bottom electrode [L. P. Ma, J. Liu, and Y. Yang, Appl. Phys. Lett. 80, 2997 (2002)]. This device can be switched between a low- (OFF) and a high- (ON) conductivity state by external bias. In this article, we report a three-terminal organic memory device, which is realized by wiring out the metal-nanocluster layer of the OBD as the middle electrode. The ON and OFF states of the device can be read out by measuring the potential of the middle electrode. By controlling the interface formation of the device, a three-terminal OBD with a potential change on the middle electrode of more than three orders in magnitude between the OFF state and ON state (from 0.2mVto0.77V) is achieved. By wiring out the middle electrode, the three-terminal OBD can also be considered as two 2-terminal devices stacked together. By proper interface engineering (to be discussed ...

Analysis of power ramping schemes for UTRA-FDD random access channel

Abstract: The random access channel (RACH) in a universal terrestrial radio access-frequency division duplex (UTRA-FDD) system is a contention-based channel mainly used to carry control information from mobile stations (MS) to base stations (BS) The transmission of a random access request contains two steps: preamble transmission and message transmission In preamble transmission, the power ramping technique is used to favor the delayed preambles by stepping up the transmission power after each unsuccessful access In doing so, the success of transmitting a long-delayed preamble is increased due to the power capture effect This paper analyzes the blocking, throughput, and delay performance of preamble transmission under three power ramping schemes with fixed, linear, and geometric step sizes The interference caused by different power ramping schemes is also compared

Experimental study on thickness-related electrical characteristics in organic/metal-nanocluster/organic systems

Abstract: Organic bistable devices with the trilayer structure, organic/metal-nanocluster/organic, interposed between two electrodes have been systematically studied by varying the thickness of the organic layers and the metal-nanocluster layer. Devices fabricated in this fashion exhibit either electrical bistability or current step, depending on the thickness of the metal-nanocluster layer. Electrical bistable devices have been studied by fixing the metal-nanocluster layer thickness at 20 nm and changing the organic-layer thickness from 20 to 60 nm. Device injection current at the on state shows an exponential decrease with an increasing organic-layer thickness, suggesting that the electron transmission probability of the devices decreases with an increasing thickness of the organic layer. This is in agreement with theoretical calculations based on the single-band Hubbard model. The evolution of the electrical current step is observed for devices fabricated by fixing the organic-layer thickness at 50 nm and changing the metal-nanocluster layer thicknesses (2, 4, and 8 nm). The discontinuous metal-nanocluster layer is believed to lead to the observed current step. When the metal-nanocluster layer is thick enough resonant tunneling occurs between nanoclusters causing positive and negative charges to be stored on the opposite sides of the metal-nanocluster layer inducing electrical bistability. Discussions of the observed phenomena are presented.

Solid-state supercapacitors for electronic device applications

Abstract: We report an all-solid-state supercapacitor with device fabrication by a simple vacuum thermal evaporation method, which allows not only a multilayer stacking structure to further enhance the capacitance, but also permits the supercapacitor to be easily incorporated with other electronic devices, showing interesting characteristics for both fundamental study and practical applications. Discussions about the mechanism of the supercapacitor is given.

Multicode multirate compact assignment of OVSF codes for QoS differentiated terminals

Abstract: Orthogonal variable spreading factor (OVSF) codes are used in both universal terrestrial radio access-frequency division duplex (UTRA-FDD) and time division duplex (UTRA-TDD) of the third-generation (3G) mobile communication systems. They can support multirate transmissions for mobile terminals with multicode transmission capabilities. In this paper, a new OVSF code assignment scheme, namely "multicode multirate compact assignment" (MMCA), is proposed and analyzed. The design of MMCA is based on the concept of "compact index" and takes into consideration mobile terminals with different multicode transmission capabilities and different quality of service (QoS) requirements. Priority differentiation between multirate realtime traffic and best-effort data traffic is also supported in MMCA. Analytical and simulation results show that MMCA is efficient and fair.

Stacked metal cathode for high-contrast-ratio polymeric light-emitting devices

Abstract: An extremely high optical absorbing film made of alternating aluminum–silver layers was used as cathode in polymeric light-emitting devices (PLEDs). Physical properties of the cathodes were characterized by I–V measurement, atomic force microscopy, and x-ray photoemission spectroscopy. As a result of the slow evaporation rate, each pair of the aluminum–silver layer was shown to be in the form of aluminum–aluminum oxide nanoclusters embedded in an amorphous charge conducting network of silver. The nanoclusters helped to absorb and scatter the ambient light effectively. The use of four alternating layers structure in conventional PLEDs demonstrated 126% enhancement of contrast under 1000lx ambient illumination. The I–V characteristics of the black cathode PLEDs remained intact when compared with reference PLEDs. This technology offers precise control of the cathode quality in terms of its reflectivity and conductivity.

Companding technique for PAPR reduction in OFDM systems based on an exponential function

Abstract: In this paper, a new non-linear companding technique, called "exponential companding", is proposed to reduce the high peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. Unlike the mu-law companding scheme, which enlarges only small signals so that increases the average power, the schemes based on exponential companding technique adjust both large and small signals and can keep the average power at the same level. By transforming the original OFDM signals into uniformly distributed signals (with a specific degree), the exponential companding schemes can effectively reduce PAPR for different modulation formats and sub-carrier sizes. Moreover, many PAPR reduction schemes, such as mu-law companding scheme, cause spectrum side-lobes generation, but the exponential companding schemes cause less spectrum side-lobes. Computer simulations, which consider a baseband OFDM system with additive white Gaussian noise (AWGN) channels and a solid state power amplifier (SSPA), show that the proposed exponential companding schemes can offer better PAPR reduction, bit error rate (BER), and phase error performance than the mu-law companding scheme

Organic Solid Solutions: Formation and Applications in Organic Light‐Emitting Diodes

Abstract: To enhance the performance of organic devices, doping and graded mixed-layer structures, formed by co-evaporation methods, have been extensively adopted in the formation of organic thin films. Among the criteria for selecting materials systems, much attention has been paid to the materials' energy-band structure and carrier-transport behavior. As a result, some other important characteristics may have been overlooked, such as material compatibility or solubility. In this paper, we propose a new doping method utilizing fused organic solid solutions (FOSSs) which are prepared via high-pressure and high-temperature processing. By preparing fused solid solutions of organic compounds, the stable materials systems can be selected for device fabrication. Furthermore, by using these FOSSs, doping concentration and uniformity can be precisely controlled using only one thermal source. As an example of application in organic thin films, high-performance organic light-emitting diodes with both single-color and white-light emission have been prepared using this new method. Compared to the traditional co-evaporation method, a FOSS provides us with a more convenient way to optimize the doping system and fabricate relatively complicated organic devices.