Evaluating charge-type of polyelectrolyte as dielectric layer in memristor and synapse emulation.

Based on credible advantages, organic materials have received more and more attention in memristor and synapse emulation. In particular, an implementation of the ionic pathway as a dielectric layer is essential for organic materials used as building blocks of memristor and artificial synaptic devices. Herein, we describe an evaluation of the use of positive and negative polyelectrolytes as dielectric layers for a memristor with calcium ion (Ca2+) doping. The device based on a negative polyelectrolyte shows the potential to obtain an excellent resistive switching performance and synapse functionality, especially in the transformation behaviours from short-term plasticity (STP) to long-term plasticity (LTP) in both the potentiation and depression processes, which were comparable to the perfomrmance obtained with a positive polyelectrolyte. The mechanism of electrical resistance transition and synaptic function can be attributed to the migration of the doped Ca2+ and the ionic functional groups of polyelectrolyte, which result in the formation and vanishing filament-like Ca2+ flux.

Polyelectrolyte Bilayer-Based Transparent and Flexible Memristor for Emulating Synapses.

Memristors will be critical components in the next generation of digital technology and artificial synapses. Researchers are investigating innovative mechanistic understanding of the memristor devices based on low-cost, solution-processable, and organic materials as promising candidates. Here, we demonstrate a novel polyelectrolyte-based memristor device, which is simply prepared by spin-coating poly(acrylic acid) (PAA) and polyethylenimine (PEI) on an indium tin oxide (ITO) substrate followed by a magnetron sputtering of the ITO as the top electrode. The device has a potential to achieve excellent resistive switching (RS) performance and synapse functionality as well as greater flexibility and transmittance when compared to the oxide-based memories. An on/off resistance ratio of 50 can be maintained without degradation for up to 20 000 cycles (flat state) and over 4000 cycles (bending to a 2 mm radius 10 000 times) in the DC sweep mode. Moreover, the device performs various synaptic functions, including spike-timing-dependent plasticity, pulse pair plasticity, and short-term and long-term plasticity in the potentiation and depression processes. The counterions and two oppositely charged polyelectrolyte chains can move in and out of each other depending on the applied electrical potential (pulse), resulting in a change in the potential drop at the interface of the polyelectrolyte bilayer and its electrodes, which can be attributed to the RS mechanism and various synaptic functions. This insight may accelerate the technological deployment of the organic resistive memories.

Rectification Behavior on Polyelectrolyte-Modified Flexible ITO Electrode via Ionic Charge-Selective Electron Transfer.

New types of diodes, such as molecular and ionic diodes, have drawn considerable attention because of their advantages from the viewpoint of potential applications such as the downscaling of electronic devices, ionic circuits, and biological systems. Researchers are motivated to develop a simple, scalable, and promising system that can overcome the existing limitations because this can enable their application in various devices. This study proposes a system that not only integrates the advantages of ionic and single-molecule diodes but also avoids their disadvantages, denoting the rectification behavior due to ionic charge-selective electron transfer between two redox species, i.e., Fe(CN)63- and Ru(NH3)63+, on the polyelectrolyte multilayer (PEM)-modified flexible indium tin oxide (ITO) electrodes. Flexible current rectification devices were easily prepared by sandwiching an electrolytic solution of ionic redox species using bare and PEM-modified plastic ITO electrodes. An ionic bilayer was initially formed via Coulombic interactions on a PEM-modified charge-selective ITO surface. The ionic bilayer was analogous to a conventional single-molecule diode's monolayer with consecutive molecular orbitals of two ionic redox species. Furthermore, the rectification ratio (RR) was increased from ∼6 to ∼10 using a conducting polymer to construct PEMs.

Solution-phase deposition of SnS thin films via thermo-reduction of SnS2.

Here, we demonstrate a novel solution-based route for deposition of tin monosulfide (SnS) thin films, which are emerging, non-toxic absorber materials for low-cost and large-scale PV applications via thermo-reducing Sn(iv) to Sn(ii). Upon optimizing the morphology of the SnS layer via adding a seed layer, the SnS-based hybrid solar cells show promising photocurrent conversion efficiencies.

Sequential chemical bath deposition of Cu(2-x)Se/CdS film by suppressing ion-exchange reaction.

Chemical bath deposition is an attractive technique to form single- and multilayered metal oxide/chalcogenide films on electrode surfaces. However, the occurrence of desorption and/or ion-exchange reaction during subsequent chemical bath deposition has so far limited preparation of multilayered metal oxide/chalcogenide films. In this paper, we report a method to prevent desorption and ion-exchange reaction of metal oxide/chalcogenide on electrode surfaces using a polyelectrolyte multilayer during sequential chemical bath deposition. By controlling the ion permeability of the polyelectrolyte multilayer, Cu(2-x)Se film was successfully deposited on the CdS film. The Cu(2-x)Se/CdS film is confirmed by UV-vis absorption spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, and X-ray powder diffractometer. Furthermore, the Cu(2-x)Se/CdS films were investigated as photoinduced charge transfer devices which showed photocurrents of 0.22 mA/cm(2) under illumination (I = 100 mW/cm(2)).

ZnO-CdS core-shell quantum dots sensitized solar cell: influence of crystalline and amorphous CdS structures in photovoltaic performance.

ZnO nanoparticles (NPs) coated with amorphous and crystalline CdS quantum dots (QDs) were successfully synthesized through chemical bath deposition (CBD) process. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been utilized to characterize the samples morphology and structural properties. The conduction band of CdS QDs is much higher than the ZnO conduction band facilitates electron transfer process through cascade system. The thickness and crystallinity of the CdS QDs coated on ZnO NPs critically controls the electron diffusion length and photovoltaic performance of the solar cell. The red shift from 506 to 524 nm, increased optical absorption in the UV-visible range and electron diffusion length limited by the thickness of the amorphous/crystalline CdS QDs coated on ZnO NPs film, influences the performance of the QDs sensitized solar cell (QDSSCs) under one sun illumination intensity (AM 1.5, 100 mW/cm2). The results discuss the CBD process controlled growth of CdS QDs on ZnO NPs and its influence on the photovoltaic performance of QDSSCs.

PH dependent morphological evolution of beta-Bi2O3/PANI composite for supercapacitor applications.

Crystalline beta-Bi2O3 was synthesized through pH-dependent chemical bath deposition process, altering the morphology and evolution from nanoparticles (approximately 40 nm) at pH 9 to platelets (approximately 40 nm width and 0.8 microm length) at pH 12. In-situ aniline nucleation and growth at less basic condition on the beta-Bi2O3 increased the surface area and specific capacitance of the device. The morphological change of beta-Bi2O3/PANI composite from nanoparticles to platelets like nanostructure facilitated higher specific capacitance from 210 to 430 F/g at a scan rate of 10 mV/s with enhanced ionic diffusion and retention of specific capacitance up to 84% at higher scan rates.

Enhanced photocurrent in the RuL2(NCS)2/ di-(3-aminopropyl)-viologen/Au nanoparticles/ITO system: use of Au nanoparticles for retardation of the back electron transfer reaction.

This paper reports the use of Au nanoparticles (NPs) as electron transfer bridge layers to improve the photocurrent of viologen/Ru complex-based photoelectrochemical cells. The Ru complex/ viologen/Au NPs on electrodes were prepared using self-assembled monolayers. The cell system showed an excellent photocurrent of 25 nA/cm2 under the 1.5 air mass condition (I = 100 mW/cm2), which is five times greater than Au NPs due to the reduced recombination reaction.

Reduced recombination and photodegradation processes of photoelectrochemical cell-based on CdSe nanofibers in the presence of PEDOT:PSS layers.

This paper reports the use of poly(3,4-ethylenedioxythiophen):poly(styrene sulfonate) (PEDOT: PSS) as a protective layer to reduce the photodegradation and recombination processes of CdSe nanofiber films. Due to reduced photodegradation and recombination processes of photoelectrochemical cell-based CdSe nanofiber films, the power conversion efficiency of CdSe nanofibers films was 1.81% in the presence of PEDOT:PSS layers under the 1.5 air mass condition (I = 80 mW/cm2), which is an 82.8% increase compared to films without PEDOT:PSS layers. Furthermore, the CdSe film was highly stable under illumination in the presence of PEDOT:PSS layers.

Modification of ion permeability utilizing pH-dependent selective swelling of polyelectrolyte multilayer films.

The selective swelling behavior of polyelectrolyte multilayer (PEM) films prepared by layer-by-layer (L-b-L) assembly influences the ion-permeability in contrast to surface charge density of the films. The cation terminated polyethylene amine (PEI) and anion terminated polyacrylic acid (PAA) were dissolved in DI water, and the pH was adjusted to 10 and 4, respectively, exemplifies thick denser film with good layering structure. The layered polyelectrolyte films has selective swelling behavior at pH 4 (PEI) or pH 10 (PAA), influences the permeability of both Ru(NH3)6(2+) and Fe(CN)6(3-) rather than surface charge character or film charge density. The swollen top most layer shows "on" character, whereas, shrunken top most layer shows "off" character for the ion-permeability. Such "on-off" character can be used for the pH-dependent switches based on surface morphology.

Heterojunction of hydrophobic poly(1,4-phenylenevinylene) and hydrophilic PEDOT:PSS on hydrophilic CdS nanoparticles.

Heterojunction of hydrophobic poly(1,4-phenylenevinylene) (PPV) on hydrophilic CdS nanoparticles was successfully prepared by the multi-layering of poly(p-xylene tetrahydrothiophenium chloride) (pre-PPV: precursor of PPV polymer) and poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) in an aqueous solution followed by a thermal treatment. CdS nanoparticles thin films were prepared on tin-doped indium oxide (ITO) by a chemical-bath-deposition method. The CdS surface was hydrophilic with low water contact angle of 15 degrees. Positively charged and water-soluble pre-PPV was used to form multilayers with PEDOT:PSS by a layer-by-layer deposition method. Pre-PPV is easily converted to conjugated PPV polymer by a thermal treatment. The CdS nanoparticles-(PPV/PEDOT:PSS) multilayer films constitute efficient acceptor-sensitizer dyad systems, which generate a photocurrent of 2,660 nA/cm2 under the air mass (AM) 1.5 conditions (I=100 mW/cm2) for sample with 4.5 bilayers.

Enhanced performance of blue polymer light-emitting diodes by a self-assembled thin interlayer.

High efficiency and long lifetime, blue polymer light-emitting diodes were obtained by adding a thin interlayer, which was fabricated by a layer-by-layer self-assembly technique between poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonic acid) (PEDOT:PSS) hole transporting and emissive polymer layers in the device configuration of indium tin oxide (ITO)/PEDOT:PSS (65 nm)/interlayer (10-30 nm)/emissive polymer (70 nm)/BaF2 (2 nm)/Ca (50 nm)/Al (300 nm). The interlayer, (PPV/PSS)n, consisted of self-assembled multilayers of the conjugated polymer, poly (p-phenylenevinylene) (PPV), and the polyelectrolyte, poly (styrene sulfonic acid) (PSS). Electroluminescence (EL) characteristics such as luminescence and current efficiency of the devices were enhanced by the addition of the interlayer. Furthermore, the devices with interlayer showed longer lifetime than those without interlayer. The maximum device performance was obtained from the device with (PPV/PSS)3 interlayer.