Electrical bistability and memory applications of poly(p-phenylenevinylene) films

Devices based on oriented and unoriented poly(p-phenylenevinylene) films showed presence of electrical bistability. Depending on the voltage sweep direction, current through the device at any voltage had two values. Switching between a low- and a high-conducting state occurred when a suitable voltage pulse was applied. Impedance spectroscopy identified a change in bulk resistance of the device during the transition between the two states. Role of orientation of the polymer film on switching and electrical bistability has been studied. The bistability in these devices had an associated non-volatile memory. We have demonstrated read-only and random access memory applications in such devices using current–voltage and impedance characteristics.     

Desktop inkjet printer as a tool to print conducting polymers

In order to exploit mechanical flexibility of organic-based electronic devices, conducting polymer anodes, such as polyaniline or poly(3,4-ethylenedioxy)-thiophene-poly(styrene sulfonate) (PEDOT-PSS), have been extensively studied. Along with the use of solution based processing techniques, conducting polymers can simplify the device fabrication procedure and yield themselves easily to printing techniques. In this paper, we present the results of utilizing desktop inkjet printer as a tool for direct printing and patterning of conducting polymer. Design of printable patterns and adjustment of printing parameters can be performed using any software such as Power Point. PEDOT-PSS suspension can be loaded into an inkjet cartridge and deposited on a given substrate in any designed pattern. The gray-scale color scheme can be employed to control the layer thickness and sheet resistivity of the inkjet printed layers. These layers are then used as anodes in organic light-emitting devices (OLEDs).     

Effects of Electrodes on the Filament Formation in HfO2- Based Resistive Random Access Memory

研究了Cu/HfO2/ITO和TiN/HfO2/ITO 2种结构阻变存储器件的电阻转变特性。2种结构均表现出稳定的、可重复的双极电阻转变行为。Cu/HfO2/ITO器件的电阻转变机制是Cu导电细丝的形成,而对于TiN/HfO2/ITO器件,在TiN顶电极和HfO2薄膜中会形成界面层,因此氧空位导电细丝的形成与断裂,是其主要的电阻转变机制。     

An alternative method to build organic photodiodes

We demonstrate a new method to build inverted organic photodiodes with highly conducting polymer anodes. In the inverted design the cathode is deposited first, followed by the deposition of the active material and finally the anode. A cathode of bismuth was vacuum evaporated followed by the evaporation of a capping layer of C₆₀. A semiconducting polymer, poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophene) (PEOPT) was then spin-coated on the cathode, followed by a surface energy modification step and a subsequent spin-coating of the anode, a solution of glycerol and the conducting polymer poly(3,4-ethylenedioxythiophene) with the polyelectrolyte poly(4-styrenesulfonate) (PEDOT-PSS). As the pristine PEOPT was too hydrophobic to spin-coat the aqueous based PEDOT-PSS, the semiconducting polymer was silanized with 3-glycidoxypropyltrimethoxysilane to increase its surface energy before spin-coating of the PEDOT-PSS solution. The device geometry was proven successful by current–voltage measurements of devices in dark and under illumination.     

Charge carrier mobility in substituted polythiophene-based diodes

We have investigated the transport properties of the semiconducting polymer poly(3-(2′-methoxy-5′-octylphenyl)thiophene) (POMeOPT). We have measured the current–voltage (C–V) characteristics of single polymer layer devices in two regimes contact limited current and bulk-limited current. The passage from one regime to the other was done upon insertion of a conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT–PSS) between the metallic electrode and the semiconducting polymer. With PEDOT–PSS as electrode, the polymer gave space–charge limited current (SCLC) with the mobility dependent on electric field. Fitting the data, we were able to obtain important parameters, such as the zero-field mobility and the characteristic field. We have compared our results with the well-studied polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)–1,4-phenylene vinylene) (MEH–PPV) in similar experiments earlier reported.     

Dual-type electrochromic devices based on conducting copolymers of thiophene-functionalized monomers

Dual polymer electrochromic devices (ECDs) composed of electrochemically deposited conducting copolymers of thiophene-functionalized monomers, 2-[(3-thienylcarbonyl)oxy] ethyl 3-thiophene carboxylate (TOET), 2,3-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOPT), and 3-[(3-thienylcarbonyl)oxy]-2,2-bis-[(3-thienylcarbonyl)oxy]propyl 3-thiophene carboxylate (TOTPT), and polyethylene dioxythiophene (PEDOT) as the counter part were constructed with tetrabutylammonium tetrafluoroborate (TBAFB) doped gelled poly(methyl methacrylate) (PMMA) and evaluated. An electrochromic device with the following configuration was assembled — indium tin oxide (ITO)-coated glass /conducting copolymer || gel electrolyte || PEDOT/ITO. Spectroelectrochemistry, electrochromic switching, stability and open-circuit memory of the devices were examined.     

Application of polyaniline (emeraldine base,EB) in polymer light-emitting devices

We report the fabrication of several multilayer light-emitting-diode (LED) devices based on a novel conjugated polymer, poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) (PPV-PPy V), involving the use of polyaniline (emeraldine base, EB) as an insulating layer between the emissive polymer layer and the electrodes. In all the above devices with various configurations (‘3-layers’, ‘4-layers’-1’, ‘4-layers-2’ and ‘5-layers’), only the symmetrically configured a.c. light-emitting (SCALE) (‘5-layers’) device shows the capability of operating in both forward and reverse bias modes and in an a.c. mode. The SCALE devices have a typical turn-on voltage of about 4–6 V and work well under both forward and reverse bias modes. It is important to note that the total resistance (R= V/I+) of the four devices at any given applied potential decreases as the number of insulating polymer layers increases, suggesting that the nature of the electrode/polymer interface plays a critical role in determining the characteristics of the devices.     

Diodes based on blends of molecular switches and conjugated polymers

Here we report polymer diodes based on a conjugated polymer host and a dispersed molecular switch. In this case, the molecular switch is a photochromic (PC) molecule that can be reversibly switched between low and high energy gap states, triggered by exposure to ultra-violet and visible light, respectively. While dispersed inside the conjugated polymer bulk and switched to its low energy gap state, the PC molecules act as traps for holes. Solid-state blends of this PC material and conjugated polymers have been demonstrated in diodes. The state of the PC molecule controls the current density versus voltage (JV) characteristics of the resulting diode. Both poly(2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) host materials have been studied. The two conjugated polymers resulted in differing JV switching characteristics. A more pronounced JV switch is observed with MEH-PPV than with P3HT. We postulate that the PC material, while switched to its low energy gap state, act as traps in both the conjugated polymers but at different trap depth energies.     

Memory applications of a thiophene-based conjugated polymer by photoluminescence measurements

Data-storage and memory applications of conjugated polymers and organic semiconductors are generally probed by device current or dielectric properties to determine the storage and switching properties. In this article, we use photoluminescence as an alternate method of probing the states of the devices. We have fabricated such devices based on a polythiophene derivative, and recorded photoluminescence (PL) spectra during and after applying bias. The presence of localized charges on the backbone of the polymer has been found to modulate the PL intensity. Since the relaxation of the space charges is slow, we have shown that PL intensity can be used as a probe for memory applications to read the state of the device. PL as a probe for memory applications of polymeric materials offers an intrinsic advantage that the state can be read without affecting the device properties.     

Electrical bistability of composite film comprising hyper-branched polymer and gold nanoparticle

A hyper-branched polymer (HPS) with dithiocarbamate (DC) end groups and gold nanoparticle stabilized by HPS (HPSAu) were synthesized. We fabricated a single-layered organic electrical bistable device (OBD). It exhibited a unipolar switching behavior with a negative differential resistance (NDR). When a certain voltage was applied to the pristine state, the device changed irreversibly and began exhibiting bistable conductive states; the equivalent circuit of this device is a parallel circuit comprising one resistance and one capacitor.     

Electrical and optical properties of molecularly doped conducting polymers

Novel electrical and optical characteristics have been observed in conducting polymers doped with molecular dopants such as fullerenes (C₆₀, C₇₀ etc.), photochromic dyes and another (guest) conducting polymer. Highly effective photo-induced charge transfer results in various effects such as photoluminescence quenching, photoconductivity enhancement, electroluminescence quenching and persistent photoconductivity which have been observed in fullerene-doped conducting polymers. Unique photo-sensitive characteristics have also been found in conducting polymer/ fullerene systems. Photoluminescence and photoconductivity in conducting polymers doped with photochromic dyes have been changed dramatically by photo-induced isomerization of photochromic dyes and memory effects in both dark conductivity and photoconductivity have been observed. We also present here some results on conducting polymer-conducting polymer mixtures and discuss general features of such composites. These characteristics can be explained by the asymmetry of the relative electronic energy states of conducting polymer and molecular dopants on account of self-trapping effects. Superconductivity evolved upon alkali metal doping of C₆₀-conducting polymer composites, and has been confirmed by low-field microwave absorption (LFMA) and SQUID magnetometry measurements. The origin of superconducting phases in this doubly doped conducting polymer is discussed.     

Conducting polymer artificial muscles

The application of conducting polymers for the direct conversion of electrical energy to mechanical energy in electromechanical actuators is analyzed using theoretical and experimental results. Basic principles of operation, predicted performance advantages and disadvantages, comparisons with natural muscle, evaluations of initial device demonstrations, and methods for improving device performance are provided. The very high predicted work densities per cycle, force generation capabilities, and power densities provide major advantages compared with piezoelectric polymers – as do the low operation voltages. These advantages are countered by cycle life and energy conversion efficiency limitations, as well as the need to use microelectrodes in order to achieve very high rates. Hydrostatic devices and extensional devices that provide either in-phase or out-of-phase electrode deformations are considered. Special types of conducting polymer actuators are also proposed, including photo-powered, chemically powered and self-powered actuators, which provide novel methods for assembling complex microstructures. Novel methods are described for actuator fabrication, such as ‘redox poling’, wherein anode, cathode and separating electrolyte layers are generated from a film in a single redox step. New actuator compositions are also proposed for obtaining improved performance, such as conjugated carbon phases having conjugation in either two or three dimensions. Finally, conducting polymer actuators based on double-layer charging are proposed which are predicted to provide increased energy efficiency and cycle life compared with the faradaic actuators.     

Impact of electrode metals on a pentacene-based write-once read-many memory device

We report a pentacene-based bistable memory device using Fe as the top electrode and compare it to the Al/pentacene/Al devices. The device displays stable switching from the low-current OFF state to the high-current ON state and long retention time. Our results suggest that Fe has the advantage over Al as the top electrode because it lowers the switching threshold voltage. The Fe devices exhibit similar temperature-dependent behaviors with the reported Al/pentacene/Al devices [D. Tondelier, K. Lmimouni, D. Vuillaume, Appl. Phys. Lett. 85 (2004) 5763]. The device is promising as a write-once read-many (WORM) memory.     

Electrical characterization of Au/Pd/n-GaN/Pd/Au device structure in the radio frequency range by simulation study

GaN based devices are highly promising optoelectronic devices for many years due to their useful applications in photovoltaic energy conversation, fiber optic communication and atmosphere monitoring. GaN based devices such as Schottky barrier, metal–semiconductor–metal and p-i-n structure have been fabricated and characterized so far. A proper understanding of the impedance or admittance spectroscopy result is crucial since it is a powerful tool to calculate the physical and electronic parameters of a device. In this study, temperature dependent dark current–voltage (I–V) and dark impedance spectra of n type GaN based metal–semiconductor–metal device have been studied with current–voltage and impedance spectra by simulation. All current–voltage characteristics exhibited good rectification behavior. The forward and reverse bias capacitance–voltage (C–V) characteristics of the Au/Pd/n-GaN/Pd/Au device were simulated at 2 MHz probing frequency. Nyquist plots of simulated device at 295 K are shown that two-barrier heights can be observable above critical threshold bias point. Frequency depended inverse dielectric loss tangent spectra of Au/Pd/n-GaN/Pd/Au device at different DC bias voltages also shows significant two peaks. This indicates that our simulation is extremely useful for determination of potential barrier numbers.     

Switching characteristics of copper-doped GexTe1−x solid electrolyte films incorporated by nitrogen for programmable metallization cell memory applications

The switching characteristics of PMC devices (device diameter =0.5 μm) with copper-doped Ge_xTe_1−xN electrolyte films were investigated as a function of the Te composition of the electrolyte films. Nitrogen doped in order to increase the crystallization temperature of GeTe chalcogenide films was incorporated into the Ge lattice alone, and copper in Ge_xTe_1−xN films was incorporated into the Te lattice. The copper concentration in copper-doped Ge_xTe_1−xN layers is directly related to the Te concentration in GeTeN films. PMC devices with copper-doped Ge₇₅Te₂₅N electrolytes were swept at a threshold voltage of 1.0 V and showed stable switching characteristics with a switching time of 1 μs with a set voltage of 2.5 V and a reset voltage of −4.0 V.     

Polyaniline thin film electrochromic devices

Solid-state electrochromic devices were fabricated from polyaniline and conducting polymer electrolytes prepared by mixing protonic acids or alkali metal salts with either poly(ethyleneoxide) or poly(ethyleneimine). The devices switch rapidly from colorless to green between −3 and +3 V and appear to be stable after several thousand switching cycles.     

新型超窄脉宽脉冲电源的设计与实现

脉冲电源的脉宽及占空比对电化学加工的精度有着重要的影响。传统的脉冲电源一般采用开关斩波形式,对开关器件的开关速度要求很严格,且对开关器件的驱动也有很高的要求。对于电化学微细精加工,常用的开关器件很难达到超窄脉宽的要求。利用DDS信号电路产生基本信号,结合脉冲调理电路,设计一种新型超窄脉宽脉冲电源。实践表明:该超窄脉宽脉冲电源的脉宽最低可达10 ns,占空比可调,输出信号稳定性强,被成功应用于光学码板的加工。     

Organic light emitting diodes fabricated with single wall carbon nanotubes dispersed in a hole conducting buffer: the role of carbon nanotubes in a hole conducting polymer

In order to investigate the role of single wall carbon nanotubes (SWNTs) in a hole conducting polymer, organic light emitting diodes (OLEDs) were fabricated with a conjugated emissive copolymer, poly(3,6-N-2-ethylhexyl carbazolyl cyanoterephthalidence) (PECCP) and SWNTs dispersed in a hole conducting buffer polymer, polyethylene dioxythiophene (PEDOT). Devices made with SWNTs dispersed in PEDOT and devices made without SWNTs in the PEDOT emit green light at 2.37 eV as expected for PECCP. However, we observed that the device made with SWNTs in the buffer layer shows a significant decrease in the electroluminescence (EL) as compared to that of the device without the SWNTs. In contrast, the photoluminescence (PL) from the same organic layer combination, excited from the PECCP side and measured through the PEDOT and the indium tin oxide glass, shows very little difference between the films with and without the SWNTs. The current–voltage (I–V) characteristics of OLEDs with SWNTs show a lower I–V power dependence (I–V²) near 1–2 V than that of the device without SWNTs which has a power dependence of I–V⁵. The EL and the I–V data together with the PL suggest an electronic interaction between the SWNTs and the host polymeric material, PEDOT. We propose that this electronic interaction originates from the hole trapping nature of SWNTs in a hole conducting polymer.     

零压软开关逆变式弧焊电源的工作状态分析与试验研究

通过对零压软开关逆变式弧焊电源的工作状态进行分析和试验研究,确定了影响电路软开关特性的主要参数,在此基础上,研制出额定额输出电流为２５０Ａ的手工电弧焊机,在较宽负载范围内实现了功率器件的零压软开关。     

Towards all-polymer field-effect transistors with solution processable materials

We have fabricated polymer field-effect transistors (FET) from solution processable polymers. Starting with an inorganic structure using only an organic semiconductor (regio-regular poly(3-hexylthiophene)), the transistor performance was studied as the inorganic materials were replaced with polymeric alternatives one at a time. We see a gradual increase in subthreshold swing and off-currents and an increased threshold voltage when substituting the inorganic materials with polymer materials. The small reduction in transistor performance when going from inorganic substrate and insulator to polymeric materials indicates that it is possible to make flexible polymer devices from solution processed materials suitable for roll-to-roll processing. The all-polymer FET was realized using two different conducting polymers, polyaniline for the source and drain electrodes and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), PEDOT:PSS, for the gate electrode.