Organic Bistable Memory Devices Research Articles

Low-voltage, solution-processed, two-terminal organic bistable memory device

An low-voltage, solution-based spin cast-processed two-terminal bi-stable memory device was fabricated by using organic-inorganic-organic tri-layer structure (MEHPPV/ZnO/MEHPPV) onto indium tin oxide coated glass. Tri-layer device structure displayed the significant switching characteristics. The absence of degradation after 1 h retention test at 120 °C was noticed. The consistent memory behavior was noticed even after 50 cycles. Electron tunneling mechanism was utilized for describing switching mechanism of the device. The result emphasizes that the class of memory devices have been fabricated with organic-inorganic-organic nanocomposites, which has led to considerable research and development efforts to the scene of latent applications.

Carrier mechanism of ZnO nanoparticles-embedded PMMA nanocomposite organic bistable memory device

ZnO nanoparticles embedded polymethylmethacrylate (ZnO-PMMA) organic bistable memory device was fabricated by sol-gel spin coating technique. ZnO-PMMA thin films were deposited on ITO coated glass substrates (Al/ZnO-PMMA/ITO). XRD and SAED patterns of ZnO-PMMA nanocomposite thin films showed the polycrystalline behavior. FTIR peaks of ZnO-PMMA nanocomposite thin films were observed at 540 cm−1 for Zn–O bond and at 1766 cm−1 for ester carbonyl CO stretch vibration of PMMA. The bandgap of ZnO-PMMA thin films was observed to be lower (3.07 eV) than the pure ZnO (3.20 eV), while the PL peak was observed at a wavelength of 403 nm. Current-voltage (I–V) measurements of the device, Al/ZnO-PMMA/ITO showed a nonvolatile electrical bistable behavior at room temperature. The ON to OFF current ratio of the bistable device was found to be ~104, which confirmed the write and the read sequence and discussed the carrier transport mechanism.

Tunable Switching Characteristics of Low Operating Voltage Organic Bistable Memory Devices Based on Gold Nanoparticles and Copper Phthalocyanine Thin Films

The influence of the top contact electrode on the switching characteristics of a low operating voltage organic bistable memory device, using copper phthalocyanine and gold nanoparticle thin films, was investigated using Au, Al, and Hg electrodes. While the ON/OFF ratio higher than 105 was achieved for all the devices, the nature of the memory behavior was found to be dependent on the top electrodes. Thermally evaporated Au and Al electrodes resulted in write-once–read-many times (WORM) behavior, whereas Hg drop soft contact led to write–read–erase–read (rewritable) characteristics, with the device retaining the ON state in the former and returning to the OFF state in the latter. The switching voltage was found to be influenced by the top electrode with the devices switching to the ON state at around 2 V for Hg and close to 1 V for Au and Al electrodes. Additionally, though the ON state conduction mechanism was dominated by Fowler–Nordheim (FN) tunneling through AuNP trap states in all the devices, the dyn...

Effect of device structure on the resistive switching characteristics of organic polymers fabricated through all printed technology

Developments in organic bi-stable non-volatile memory devices have shown that organic materials are essential for the next generation of electrical memory unit owing to their low cost, high flexibility and large scalability. This study depicts an important aspect of organic memory devices by observing the effect of changing device structure on its switching characteristics. Memory devices with a bilayer and bulk-heterojunction structure were fabricated through an all printed technology by utilizing two organic polymers such as MEH: PPV and PMMA. Silver (Ag) was selected as the top and bottom electrode due to its high conductivity and easy processing. Though identical polymers were used in both device structures, but interestingly change in structure caused change in properties. It was observed that bilayer structure had much higher switching ratio and stability against various biasing cycles as compared to its bulk-heterojunction counterpart. Superior switching characteristics of bilayer structure were due to the presence of a well-defined interface between both polymers. Bulk-heterojunction device suffers the drawback of phase separation in a single organic layer between the two polymers.

Organic bistable memory devices based on MoO3 nanoparticle embedded Alq3 structures

Organic bistable memory devices were fabricated by embedding a thin layer of molybdenum trioxide (MoO3) between two tris-(8-hydroxyquinoline)aluminum (Alq3) layers. The device exhibited excellent switching characteristics with an ON/OFF current ratio of 1.15 × 103 at a read voltage of 1 V. The device showed repeatable write–erase capability and good stability in both the conductance states. These conductance states are non-volatile in nature and can be obtained by applying appropriate voltage pulses. The effect of MoO3 layer thickness and its location in the Alq3 matrix on characteristics of the memory device was investigated. The field emission scanning electron microscopy (FE-SEM) images of the MoO3 layer revealed the presence of isolated nanoparticles. Based on the experimental results, a mechanism has been proposed for explaining the conductance switching of fabricated devices.

Organic Bistable Switching Memory Devices with MeH-PPV and Graphene Oxide Composite

We have reported about bipolar resistive switching effect on Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]:Graphene oxide composite films, which are sandwiched between aluminum and indium tin oxide electrodes. In this case, I-V sweep curve showed a hysteretic behavior, which varied according to the polarity of the applied voltage bias. The device exhibited excellent switching characteristics, with the ON/OFF ratio being approximately two orders in magnitude. The device had good endurance (105 cycles without degradation) and long retention time (5 × 103 s) at room temperature. The bistable switching behavior varied according to the trapping and de-trapping of charges on GO sites; the carrier transport was described using the space-charge-limited current (SCLC) model.

Fabrication of bistable switching device using CdS nanorods embedded in PMMA (polymethylmethacrylate) nanocomposite

An organic bistable memory device was fabricated using CdS NR’s/PMMA (polymethylmethacrylate) nanocomposite sandwiched between ITO and Al thin film. XRD analysis confirmed that CdS NR’s reveals wurtzite hexagonal phase. FESEM and TEM images showed the monodispersed rod-like morphology. The nanorods were uniformly dispersed in the PMMA (polymethylmethacrylate) layer. EDX spectrum confirms that the product consists of only Cd and S elements. No other impurities were found. UV–visible absorption of CdS NR’s showed a slight blue shift. The photoluminescence spectra of CdS NR’s spectra showed surface defects which may be due to sulfur vacancies in surface of nanorods. The CdS NR’s polymer nanocomposite showed better thermal stability. The current–voltage (C–V) measurement of ITO/CdS NR’s/Al showed a clockwise hysteresis with flatband shift voltage in C–V curve for the device.

Charge Carrier Transport Mechanism Based on Stable Low Voltage Organic Bistable Memory Device.

A solution processed two terminal organic bistable memory device was fabricated utilizing films of polymethyl methacrylate PMMA/ZnO/PMMA on top of ITO coated glass. Electrical characterization of the device structure showed that the two terminal device exhibited favorable switching characteristics with an ON/OFF ratio greater than 1 x 10(4) when the voltage was swept between - 2 V and +3 V. The device maintained its state after removal of the bias voltage. The device did not show degradation after a 1-h retention test at 120 degrees C. The memory functionality was consistent even after fifty cycles of operation. The charge transport switching mechanism is discussed on the basis of carrier transport mechanism and our analysis of the data shows that the charge carrier trans- port mechanism of the device during the writing process can be explained by thermionic emission (TE) and space-charge-limited-current (SCLC) mechanism models while erasing process could be explained by the FN tunneling mechanism. This demonstration provides a class of memory devices with the potential for low-cost, low-power consumption applications, such as a digital memory cell.

ORGANIC MEMORY DEVICES WITH NATURAL RUBBER/ FULLERENE COMPOSITES

ABSTRACT The application of natural rubber (NR) as cost-effective component for an organic bistable memory device is demonstrated for the first time. An organic bistable memory device based on Cis 1–4 polyisoprene (NR) and fullerene (C60) is a trilayered structure consisting of NR/C60 composite sandwiching a C60 layer and thermally deposited aluminum electrodes on both sides (Al/NR:C60/C60/NR:C60/Al). Investigations on current-voltage (I-V) characteristics of the device revealed promising device performance with a high ON/OFF current ratio and nonvolatile stability in both of its conducting states, indicating the strong potential of the NR:C60 composite as a nonvolatile memory material. Further, attempts are made to fit the current response of the device using existing charge transport models and explain the origin of memory switching in present device.

A transparent and flexible organic bistable memory device using parylene with embedded gold nanoparticles

In this work, we demonstrate the fabrication of a transparent and flexible memory device in the simple structure of metal/dielectric/metal (MIM). Here, the MIM structure consists of gold electrode/200 nm Parylene-C/20 nm gold nanoparticles/100 nm Parylene-C/indium-tin-oxide (ITO) coated polyethylene terephthalate (PET). The use of parylene as the dielectric layer is important to ensure that there is no thermal stress induced on the flexible ITO/PET substrate compare to other reported works using various organic dielectrics that require high temperature curing. In addition, parylene deposition does not disturb the drop-casted gold nanoparticles. Hence, the use of parylene will be the right step forward in the fabrication of mechanically flexible and optically transparent devices. Current versus voltage (I–V) plot shows the presence of hysteresis suggesting the charge storage capability as a memory device. In the I–V plot, three distinct regions based on the slope have been identified and the transport mechanisms are discussed and explained. The fabricated device shows similar behavior as write-once-read-many memory device and can be programmed with either positive or negative bias voltage. However, the memory device shows unstable current state when being bent under different curvature diameters.

Effect of the ZnS shell layer on the charge storage capabilities of organic bistable memory devices fabricated utilizing CuInS2–ZnS core–shell quantum dots embedded in a poly(methylmethacrylate) layer

The electrical characteristics of organic bistable memory devices (OBDs) fabricated utilizing CuInS2 (CIS) core or CIS–ZnS core–shell quantum dots (QDs) embedded in a poly(methylmethacrylate) (PMMA) layer on indium–tin-oxide (ITO) coated glass substrates were investigated. X-ray photoelectron spectroscopy spectra demonstrated that the stoichiometries of the QDs embedded in a PMMA layer were CIS or CIS–ZnS QDs. Current–voltage measurements on Al/CIS or CIS–ZnS QDs embedded in PMMA layer/ITO glass devices at 300K showed current bistabilities. The maximum ON/OFF current ratios of the OBDs with CIS or CIS–ZnS QDs were approximately 1×103 and 1×105, respectively. The retention number of ON and OFF states was measured by 1×105. The memory mechanisms of the OBDs with CIS or CIS–ZnS QDs are described on the basis of the experimental results.

Solution based-spin cast processed organic bistable memory device

A two terminal organic bistable memory was fabricated by solution based spin casting of thin films of poly-[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEHPPV):ZnO nanoparticles onto ITO coated glass. The morphology of the thin films were characterized by Raman and FTIR spectroscopies. The electrical characterization showed that the two-terminal device exhibited excellent switching characteristics with ON/OFF ratio greater than 1×104 when the voltage was swept between −1V and +1V. The device maintained its state even after removal of the bias voltage. The device exhibited stable performance at various performance tests. Two distinct capacitance states with hysteresis were observed and we suggest that the switching mechanism involved could be attributed to trapping and detrapping of electrons.

A facile route for the preparation of organic bistable memory devices based on size-controlled conducting polypyrrole nanoparticles

Organic bistable memory devices (OBDs) with size-controlled polypyrrole (PPy) nanoparticles are developed by employing conducting PPy nanoparticle-embedded poly(vinyl alcohol) (PVA) as an active layer in OBDs. Monodisperse polypyrrole (PPy) nanoparticles with different diameters (20, 60, and 100nm) are fabricated via dispersion polymerization in order to evaluate size-dependent bistable memory characteristics. Stable multilevel switching and a high on/off ratio over 100 were demonstrated with 20nm of PPy in the OBDs.

Organic bistable devices utilizing carbon nanotubes embedded in poly(methyl methacrylate)

The electrical and memory behavior of organic bistable memory devices in the form of metal-embedded insulator-metal (MIM) structure are described. The devices utilize layer-by-layer (LbL) deposited single walled carbon nanotubes (SWCNTs) as charge traps embedded between two polymethylmethacrylate (PMMA) insulating layers. The stack was sandwiched between two aluminium electrodes to form an Al/PMMA/SWCNTs/PMMA/Al structure. The current-voltage (I-V) characteristics of the devices exhibit electrical bistability and non-volatile memory characteristics in terms of switching between high conductive (ON) and low conductive (OFF) states. The different conductive states were programmed by application of a positive and negative voltage pulse for the ON and OFF states, respectively. A maximum ON/OFF ratio of 2 × 105 is achieved at low reading voltage of 1 V. Space-charge-limited-current (SCLC) conduction model was used to describe the carriers transport and the electrical bistability in the devices, which was attributed to the trapping and detrapping of electrons inside the SWCNTs.

Electrical bistable properties of copper phthalocyanine at different deposition rates

Organic bistable memory device is a next-generation of the electrical memory unit. In this paper, we report about the influence of structural properties on electrical bistable behavior of copper phthalocyanine organic memory device. Copper phthalocyanine (CuPc) layer was prepared by thermal evaporation technique at different deposition rates. When the deposition rate is increased, the film crystalline decreases and the surface morphology gradually changes from large flat grain to fine grain structure. Structural parameters such as the crystalline size of CuPc films and dislocation density can be calculated from XRD spectra. Moreover, the effect of deposition rate of CuPC layer on the bistable properties can be performed by current–voltage characteristics, retention measurement, impedance spectroscopy and temperature dependence measurement. The conduction mechanism in both ON and OFF states of the bistable device was analyzed by theoretical model, which can be proposed as a possible trap center of the carrier trapping and de-trapping process by structural defects in CuPc layer. Furthermore, the reliability issue such as cycling endurance and data retention is presented.

Fabrication of stable low voltage organic bistable memory device

Organic bistable memory device was fabricated by spin coating of polymethyl methacrylate (PMMA):ZnO nanoparticles onto ITO coated glass. The thin films were characterized by Raman, SEM, AFM and FTIR spectroscopies. The electrical characterization showed that the two-terminal device exhibited excellent switching characteristics with ON/OFF ratio greater than 2 × 10 3 when the voltage was swept between −2 V and +2 V. The device maintained its state even after removal of the bias voltage. Device did not show degradation after 1-h retention test at 120 °C. The memory functionality was consistent even after multiple cycles of operation and the device is reproducible. The switching mechanism is discussed on the basis of carrier transport mechanism. This demonstration provides a class of memory devices with the potential for extremely low-cost, low-power consumption applications, such as digital memory cell.

Luminance Control of Organic Light-Emitting Diodes Using an Organic Bistable Memory Device

The luminance control of organic light-emitting diodes (OLEDs) using an organic bistable memory device was investigated. A quantum dot embedded organic bistable memory device was connected to the OLED instead of a common thin film transistor to manage the luminance of the OLEDs. Multilevel luminance control of the OLEDs could be enabled by simply controlling the setting voltage of the organic bistable memory device. High luminance was obtained at the writing voltage corresponding to the high current state, while low luminance was obtained at the erasing voltage corresponding to the low current state.

Unipolar Bistable Switching of Organic Non-Volatile Memory Devices with Poly(styrene-co-styrenesulfonic acid Na)

We demonstrated unipolar organic bistable memory devices with 8 x 8 cross-bar array type structure. The active material for the organic non-volatile memory devices is poly(styrene-co-styrenesulfonic acid Na) (PSSANa). From the electrical measurements of the PSSANa organic memory devices, we observed rewritable unipolar switching behaviors with a stable endurance and narrow cumulative probability. Also the PSSANa memory devices exhibited a uniform cell-to-cell switching with a high ON/OFF ratio of approximately 10(5) and good retention time of approximately 10(4) seconds without significant degradation.

Synthesis of titania embedded silica hollow nanospheres via sonication mediated etching and re-deposition

Titania embedded silica hollow nanospheres were synthesized from sonication-mediated etching and re-deposition of silica/titania core/shell nanospheres. The designed structure of the hollow nanospheres was proved to be a key factor for the charge trapping/detrapping and resulting bistability in non-volatile organic bistable memory devices (OBDs).

Organic Light-Emitting Bistable Memory Devices with Self-Assembled Organic Nanoparticles as a Charge Trapping Center

Organic bistable memory devices were demonstrated by applying poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiopen) (PEDOT:PSS) as a self-organized charge trapping center inside a poly(9,9-dioctylfluorene) (PFO). PEDOT:PSS was introduced as nanoparticles by the chemical surface energy difference between PFO and PEDOT:PSS. The PEDOT:PSS nanoparticles played a role of the charge trapping center and an average on/off ratio around 100 was observed at a reading voltage of -2 V. The combination of PEDOT:PSS nanoparticle and LiF/Al cathode resulted in organic light-emitting bistable devices with a stable bistability and light-emitting performances.