Data-storage devices based on layer-by-layer self-assembled films of a phthalocyanine derivative

An organic dye, namely nickel phthalocyanine, has been used in data-storage devices. A “high state” has been written by applying a voltage pulse. The state of the device has been “read” by applying a small probe voltage. The dye embedded in an inert polymer matrix retained the high state for more than an hour, which can be refreshed or erased at will. Hysteresis-type behaviour has been observed in the current–voltage characteristics. The space charges at the metal/semiconductor interfaces, stored under the voltage pulse, have been found to control the charge injection and hence the current in these devices. The formation of space charges near the interfaces, and relaxation have been studied in the data-storage devices. The space charges’ slow relaxation process has been shown to result in the memory device applications of the semiconducting dyes.     

基于P3HT/PCBM异质结界面太阳能电池的理论及实验研究

根据整数电荷转移(ICT)模型理论分析基于P3HT为给体PCBM为受体的异质结界面,认为不同等效功函数衬底和电荷传输状态产生不同D/A界面特性。采用增加P3HT缓冲层PCBM缓冲层的方法,制备不同复合层本体异质结结构光伏器件,研究活性膜内组分变化对器件开路电压和短路电流密度的影响。结果表明增加缓冲层使器件的短路电流密度明显提高,从3.96mA/cm2分别增加到4.51mA/cm2和4.70mA/cm2,但对开路电压影响不大。     

A new MNOS memory element—the tunnel diode

MNOS (Metal-Nitride-Oxide-Silicon) memory devices commercially available today consist of transistor arrays where each device represents a memory bit. Typical devices have densities greater than 8 K bits and are generally manufactured on epitaxial based processes for isolation. The state of each bit is determined by its threshold voltage and is sensed by interpreting if the transistor is in the “off” or “on” condition. A new MNOS memory element is described where detection of junction tunnelling current is used as the sense mechanism. Substrate forms the “third” terminal and the element has the possibility of being the basis of a dense array. The technique can be developed in p or n channel and can be used as an add-on to volatile random access memories.     

Rational Design of Small Molecules to Implement Organic Quaternary Memory Devices

Organic small‐molecule‐based devices with multilevel electroresistive memory behaviors have attracted more and more attentions due to their super‐high data‐storage density. However, up to now, only ternary memory molecules have been reported, and ternary storage devices may not be compatible with the binary computing systems perfectly. In this work, a donor–acceptor structured molecule containing three electron acceptors is rationally designed and the field‐induced charge‐transfer processes can occur from the donors. Organic quaternary memory devices based on this molecule are successfully demonstrated for the first time. The switching threshold voltages of the memory device are –2.04, –2.73, and –3.96 V, and the current ratio of the “0,” “1,” “2,” and “3” states is 1:10^1.78:10^3.47:10^5.36, which indicate a low possibility of read and write errors. The results represent a further step in organic high‐density data‐storage devices and will inspire the further study in this field.     

基于CuPc…C60的混合层异质结光伏器件性能研究

制备了基于CuPc…C60混合层异质结有机光伏器件,将其与CuPc-C60双层结构光伏器件进行对比研究。结果表明混合层结构器件性能得到改善,其开路电压、短路电流密度、填充因子和光电转换效率都有提高,分别从CuPc-C60双层结构器件的0.39V、1.92mA/cm2、0.36%、0.48依次提高到CuPc…C60混合层结构器件的0.48V、2.21mA/cm2、0.54%、0.51。根据整数电荷转移模型来分析光伏器件D/A界面及有机材料-ITO衬底界面特性,认为混合层异质结有机光伏器件给体材料HOMO与受体材料LUMO的能级差增加使得器件开路电压提高。混合层异质结有机光伏器件D/A界面面积增加和给体材料HOMO与受体材料LUMO的能级差增加都提高了激子的分离效率,所以器件的短路电流密度增加。     

Organic Memory Device Fabricated Through Solution Processing

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.     

Vertical multijunction solar-cell one-dimensional analysis

The vertical multijunction solar cell is a photovoltaic device which may allow conversion efficiencies higher than conventional planar devices. A one-dimensional model of the device is presented here which allows a simple and straightforward analysis of device performance to be conducted. The analysis covers the derivation of device short-circuit current, saturation current, open-circuit voltage, and maximum power as a function of illumination spectra, device geometry, and device material properties.     

Printed Organic One‐Time Programmable ROM Array Using Anti‐fuse Capacitor

This paper proposes printed organic one‐time programmable read‐only memory (PROM). The organic PROM cell consists of a capacitor and an organic p‐type metal‐oxide semiconductor (PMOS) transistor. Initially, all organic PROM cells with unbroken capacitors store “0.” Some organic PROM cells are programmed to “1” by electrically breaking each capacitor with a high voltage. After the capacitor breaking, the current flowing through the PROM cell significantly increases. The memory data is read out by sensing the current in the PROM cell. 16‐bit organic PROM cell arrays are fabricated with the printed organic PMOS transistor and capacitor process. The organic PROM cells are programmed with –50 V, and they are read out with –20 V. The area of the 16‐bit organic PROM array is 70.6 mm².     

Ultraviolet-illuminated fluoropolymer indium–tin-oxide buffer layers for improved power conversion in organic photovoltaics

We demonstrate that the charge carrier extraction in double heterojunction organic photovoltaic(OPV) devices can be enhanced by inserting an UV-illuminated fluoropolymer polytetrafluoroethylene(PTFE) layer between indium–tin-oxide and the thermal evaporated copper–phthalocyanine(CuPc)/buckyball(C₆₀) organic active layers. In this work, we show that the anode work function influences the photocarrier collection characteristics, where the short-circuit current and open-circuit voltage increase from 1.6 to 4.8 mA/cm² and 0.41 to 0.48 V, respectively after the buffer layer insertion associated primary with the barrier decrease in the ITO/CuPc interface. This result shows the potential of UV-illuminated PTFE as a low-cost stable buffer layer for OPV devices.     

Introducing pinMOS Memory: A Novel,Nonvolatile Organic Memory Device

In recent decades, organic memory devices have been researched intensely and they can, among other application scenarios, play an important role in the vision of an internet of things. Most studies concentrate on storing charges in electronic traps or nanoparticles while memory types where the information is stored in the local charge up of an integrated capacitance and presented by capacitance received far less attention. Here, a new type of programmable organic capacitive memory called p‐i‐n‐metal‐oxide‐semiconductor (pinMOS) memory is demonstrated with the possibility to store multiple states. Another attractive property is that this simple, diode‐based pinMOS memory can be written as well as read electrically and optically. The pinMOS memory device shows excellent repeatability, an endurance of more than 10⁴ write‐read‐erase‐read cycles, and currently already over 24 h retention time. The working mechanism of the pinMOS memory under dynamic and steady‐state operations is investigated to identify further optimization steps. The results reveal that the pinMOS memory principle is promising as a reliable capacitive memory device for future applications in electronic and photonic circuits like in neuromorphic computing or visual memory systems.     

一种有机薄膜器件的制备及电存储特性

研究了一种金属/有机物/金属夹层结构有机薄膜器件的可逆电双稳特性.器件的阳极和阴极分别为真空热蒸发沉积的Ag和Al薄膜,中间介质层为真空热蒸发沉积的2-(hexahydropyrimidin-2-ylidene)-malononitrile(HPYM)有机薄膜.器件起始状态为非导通态,在大气环境下,可用正、反向电场进行信号的写入和擦除,表现为极性记忆特性.通过自然氧化的方法在底电极Al表面形成一层Al2O3薄膜层后,可使器件在不同的正向电压脉冲作用下达到不同的导电态,具有一定的多重态存储特性.同时,研究了不同的电极组合对器件电性能的影响,并通过紫外-可见吸收光谱以及喇曼光谱对器件界面进行表征.     

A Solar Transistor and Photoferroelectric Memory

This study presents a new self‐powered electronic transistor concept “the solar transistor.” The transistor effect is enabled by the functional integration of a ferroelectric‐oxide thin film and an organic bulk heterojunction. The organic heterojunction efficiently harvests photon energy and splits photogenerated excitons into free electron and holes, and the ferroelectric film acts as a switchable electron transport layer with tuneable conduction band offsets that depend on its polarization state. This results in the device photoconductivity modulation. All this (i.e., carrier extraction and poling) is achieved with only two sandwiched electrodes and therefore, with the role of the gating electrode being taken by light. The two‐terminal solar‐powered phototransistor (or solaristor) thus has the added advantages of a compact photodiode architecture in addition to the nonvolatile functionality of a ferroelectric memory that is written by voltage and nondestructively read by light.     

Programmable memory cell using quantum confined Stark effect in multi-quantum well heterojunction bipolar transistor

A three terminal bistable programmable memory cell which can be read either optically or electrically is proposed and demonstrated. The device is based on using Stark effect of the excitonic transitions in a multi-quantum well base region of a heterojunction bipolar transistor. The single device can be flipped (and held) from low transmittance (high voltage) to high transmittance (low voltage) state and vice versa by a varying base current signal.<>     

A flexible polymer memory device

A flexible polymer memory device is demonstrated in a sandwich structure of polypyrrole/P6FBEu/Au. Conductance switching at a voltage of about 4 V, with an ON/OFF current ratio up to 200, was observed in this flexible memory device. At the low-conductivity state, current density–voltage (J–V) characteristics of the device were dominated by a charge injection current. At the high conductivity state, J–V characteristics were dominated by a space-charge-limited current. Both the ON and OFF states are stable up to 10⁶ read cycles at a read voltage of 1 V. The device can be used as a write-once read-many-times (WORM) memory with good electronic stability.     

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

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.     

RF relaxation oscillations in polycrystalline TiO2 thin films

Negative resistance oscillations were studied in polycrystalline TiO₂ thin films in the TiTiO₂Cr device structure. Stable oscillations were readily produced in the 1–3 MHz frequency range and showed lifetimes of more than 8 × 10¹¹ cycles. A simple equivalent circuit model for current controlled negative resistance (CCNR) devices was used to interpret the bias voltage and temperature dependence of amplitude and frequency of negative resistance oscillations. Bias voltage dependence was found not to involve changes in the device parameters. Temperature variations produced changes in the device threshold and minimum (holding) voltages and the “on” and “off” state resistances. The “on” state and “off” state resistances showed thermal activation energies of 0.006 and 0.052 eV respectively. I-V characteristics for these devices are shown to be in agreement with the theory of filamentary double injection space charge limited currents.     

Development of high-quantum-efficiency,lattice-mismatched, 1.0-eV GaInAs solar cells

High-quantum-efficiency, lattice-mismatched, 1.0-eV GaInAs solar cells grown by organometallic vapor phase epitaxy have been developed for ultimate integration into AlGaAs/GaAs/GaInAs 3-junction, 2-terminal monolithic devices. The more standard n/p junction was replaced with an n-i-p structure in the GaInAs cell in order to increase the short-circuit current by overcoming the material deficiencies which arise as a result of accommodating the lattice mismatch. This led to single junction 1.0-eV GaInAs cells with internal quantum efficiencies >90% and short-circuit-current densities that match or closely approach those needed to current match the upper AlGaAs and GaAs cells. A 4.1% (1-sun, air mass 0,25°C) power conversion efficiency was achieved with a developmental structure, indicating the potential of the lattice-mismatched n-i-p 1.0-eV GaInAs cell. An analogous device designed to allow direct monolithic integration with the upper AlGaAs and GaAs cells, with a modified grading layer of AlGaInAs in place of the usual GaInAs, achieved an efficiency of 2.2%, primarily due to a lower open-circuit voltage. The open-circuit voltage is perhaps limited by structural defects revealed in transmission electron micrographs.     

Flash‐Memory Effect for Polyfluorenes with On‐Chain Iridium(III) Complexes

Polyfluorenes containing Ir(III ) complexes in the main chain are demonstrated to have promising application in a polymer memory device. A flash‐memory device is shown whereby a polymer solution is spin‐coated as the active layer and is sandwiched between an aluminum electrode and an indium tin oxide electrode. This device exhibits very good memory performance, such as low reading, writing, and erasing voltages and a high ON/OFF current ratio of more than 10⁵. Both ON and OFF states are stable under a constant voltage stress of ?1.0 V and survive up to 10⁸ read cycles at a read voltage of ?1.0 V. Charge transfer and traps in polymers are probably responsible for the conductance‐switching behavior and the memory effect. The fluorene moieties act as an electron donor and Ir(III ) complex units as the electron acceptor. Furthermore, through the modification of ligand structures of Ir(III ) complex units, the resulting polymers also exhibit excellent memory behavior. Alteration of ligands can change the threshold voltage of the device. Hence, conjugated polymers containing Ir(III ) complexes, which have been successfully applied in light‐emitting devices, show very promising application in polymer memory devices.     

Incorporation of non-conjugated polymer chain in conjugated polymer matrix: A new single step strategy for free standing non-volatile polymer memory

A new single step strategy for polymer memory materials has been explored using free-standing polypyrrole (PPy) film in which non-conjugated polymer chains are incorporated as trap states during synthesis. The PPy film was synthesized by the acidic oxidation of 2,2′:5′,2′′-terpyrrole at the air/water interface. The free-standing PPy films show large hysteresis along with current peaks in opposite directions during current voltage (I–V) characteristics. Hysteric behavior has been utilized to show rewritable memory effect. Furthermore, once electrical state (high or low conduction state) is set, the state is stable for months in ambient condition unless the state is reset by applying a voltage of opposite polarity. Thus the PPy film can be used as read once memory. The memory effect of the film is due to the conformational changes of non-conjugated polymer chains in the PPy matrix. The changes in conformation were confirmed from UV–Vis and FTIR spectra. This new strategy leads free-standing film based all organic polymer memory devices at ultra low cost.     

UV assisted non-volatile memory behaviour using Copper (II) phthalocyanine based organic field-effect transistors

In this report, we have demonstrated the optical non-volatile memory characteristics using CuPc OFET. The memory operation was comprehensively demonstrated with different programming conditions. It was found that the programming of CuPc OFET with an electric pulse at the gate terminal under UV-light photo-illumination compared to other programming conditions, could substantially increase the memory window due to massive charge trapping in the polymer electret layer, which causes shift in the device transfer characteristics from low-conduction state (“OFF state”, or logic 0) to high conduction state (“ON state”, or logic 1) at V_GS = 0V. From device operation at −50V, a memory window of greater than 45V could be achieved by applying a programming voltage of +70 V at the gate terminal under UV-light photo-illumination. Moreover, it was completely erased by applying −100 V at the gate terminal in dark.