Color switching behaviors of organic bistable light-emitting devices fabricated utilizing an aluminum-nanoparticle-embedded tris(8-hydroxyquinoline)aluminum layer and double emitting layers

Organic bistable light-emitting devices (OBLEDs) with an aluminum (Al)-nanoparticle-embedded tris(8-hydroxyquinoline)aluminum (Alq₃) layer and double emitting layers (EMLs) were fabricated to investigate their color switching behaviors. Scanning electron microscopy images showed that Al nanoparticles were formed on the Alq₃ layer. The Al nanoparticles in the Alq₃ layer improved the storage margin of the organic bistable devices (OBDs), and the double EMLs changed the emission color of the organic light-emitting devices (OLEDs) according to the variations of the ON and the OFF states of the OBDs. The variations of the ON and the OFF states of the OBDs could be clearly distinguished by the color switching of the OLED. The luminances of the OBLEDs with double EMLs in the ON and the OFF states were 641.80 and 22.25 cd/m², respectively, and their CIE coordinates at 20 V were (0.42, 0.46) and (0.51, 0.47), respectively, which corresponded to the ON and the OFF states of the OBLEDs.     

Uniform bipolar organic memory using vapor phase polymerized poly (3,4-ethylenedioxythiophene)

Organic memory device has emerged as an excellent candidate for the next generation storage devices due to its high performance and low production cost. In this paper, we report the fabrication and electrical characterization of an organic memory device made of vapor-phase polymerized PEDOT thin films that are highly uniform and free of PSS and free of unreacted reactants. The PEDOT memory device exhibited a typical bipolar resistive switching with a high ON/OFF current ratio of at least 10³, which was maintained for more than 10³ dc sweeping cycles. The device performance was stable for more than 10⁵ s. Moreover, the device containing 64 cells has very high cell to cell uniformity as demonstrated by (1) at least 93% of the cells displaying the ON/OFF current ratio of at least 10³ and (2) the deviation of the set and reset voltages from the average values being less than 0.5 V and 0.4 V, respectively. The maximum current before switching in the reset process was found to increase linearly with increase in the compliance current applied during the set process.     

Electrical characterization of poly(amide-imide) for application in organic field effect devices

The admittance spectra and current–voltage (I–V) characteristics are reported of metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) capacitors employing cross-linked poly(amide–imide) (c-PAI) as the insulator and poly(3-hexylthiophene) (P3HT) as the active semiconductor. The capacitance of the MIM devices are constant in the frequency range from 10 Hz to 100 kHz, with tan δ values as low as 7 × 10⁻³ over most of the range. Except at the lowest voltages, the I–V characteristics are well-described by the Schottky equation for thermal emission of electrons from the electrodes into the insulator. The admittance spectra of the MIS devices displayed a classic Maxwell–Wagner frequency response from which the transverse bulk hole mobility was estimated to be ∼2 × 10⁻⁵ cm² V⁻¹s⁻¹ or ∼5 × 10⁻⁸ cm² V⁻¹s⁻¹ depending on whether or not the surface of the insulator had been treated with hexamethyldisilazane (HMDS) prior to deposition of the P3HT. From the maximum loss observed in admittance-voltage plots, the interface trap density was estimated to be ∼5 × 10¹⁰ cm⁻² eV⁻¹ or ∼9 × 10¹⁰ cm⁻² eV⁻¹ again depending whether or not the insulator was treated with HMDS. We conclude, therefore, that HMDS plays a useful role in promoting order in the P3HT film as well as reducing the density of interface trap states. Although interposing the P3HT layer between the insulator and the gold electrode degrades the insulating properties of the c-PAI, nevertheless, they remain sufficiently good for use in organic electronic devices.     

Chemical linkage functions of poly(ether imide)s on the resistive switching memory effects

A series of aromatic poly(ether imide)s, AZTA-PEIs containing triphenylamine and 1,2,4-triazole moieties are prepared and characterized. All the polymers with inherent viscosity from 0.58 to 1.1 dL/g show glass transition temperatures in the range of 250–278 °C. Resistive switching memory devices are constructed based on the processable poly(ether imide) (AZTA-PEIa). The device can be switched from the initial OFF state to the ON state under either positive or negative electrical sweep at about ±3.2 V. The ON state is nonvolatile and can maintain the high conducting state even turning off the electrical power and applying a reverse bias. The device fulfills the requirements of a write-once read-many times memory (WORM) with a high ON/OFF current ratio up to 10⁵ and a long retention time in both ON and OFF states. The bistable switching effects of the polymer result from the conformation-coupled charge transfer from electron donors (triazole-substituted triphenylamine moieties) to electron acceptors (phthalimide moieties). By comparing with the memory behaviors of analogue polymers, the functions of ether and imide in the chemical polymer structure on the memory behaviors are discussed.     

Fabrication of poly(3-hexylthiophene) nanowires for high-mobility transistors

Presented here is a novel and efficient method used to improve carrier mobilities of poly(3-hexylthiophene) (P3HT)-based organic field effect transistors by means of nanowire formation. The treatment, termed solvation, consists of depositing a small quantity of a solvent directly on top of a previously deposited semiconducting film, and allowing the solvent to evaporate slowly. Such treatment results in an increase of the saturation mobility by more than one order of magnitude, from 1.3 × 10⁻³ up to 3.4 × 10⁻² cm²/Vs, while devices preserve their high ON/OFF ratio of ∼10⁴. The atomic force and scanning electron microscopy studies show that solvated P3HT layers develop a network of nanowires, which exhibit increased degree of structural order, as demonstrated by micro Raman spectroscopy. The time-of-flight photoconductivity studies suggest that higher hole mobility stems from a reduced energy disorder of the transporting states in these structures.     

Unipolar nonvolatile memory devices with composites of poly(9-vinylcarbazole) and titanium dioxide nanoparticles

Organic-based devices with an 8 × 8 array structure using titanium dioxide nanoparticles (TiO₂ NPs) embedded in poly(9-vinylcarbazole) (PVK) film exhibited bistable resistance states and a unipolar nonvolatile memory effect. TiO₂ NPs were a key factor for realizing the bistability and the concentration of TiO₂ NPs influenced ON/OFF ratio. From electrical measurements, switching mechanism of PVK:TiO₂ NPs devices was closely associated with filamentary conduction model and it was found that the OFF state was dominated by thermally activated transport while the ON state followed tunneling transport. PVK:TiO₂ NPs memory devices in 8 × 8 array structure showed a uniform cell-to-cell switching, stable switching endurance, and a high retention time longer than 10⁴ s.     

Polymer composite of poly(vinyl phenol)-reduced graphene oxide reduced by vitamin C in low energy consuming write-once–read-many times memory devices

We report the application of reduced graphene oxide, using vitamin C as reducing agent, to make a composite with poly(vinyl phenol) as the active layer of write-once–read-many times memory devices. These devices present a high ON/OFF current ratio of 10⁵ when read at 1 V, retain the information for a long time maintaining the ON/OFF current ratio constant, and require low energy for performing at 5 V the memory write (less than 10^?8 J cm^?2 device active area) and read operations.     

Organic memory cells based on the switching by nanoparticles containing thin films

We report an electrical bistable switching property in two terminal organic memory cells fabricated with a sandwich structure of Alq₃/Liq/Alq₃, Alq₃/10% doped Liq, and mixed compounds of Alq₃, PVP, and Zn nitrate hexahydrate nanoparticles (NPs) as the active component between two external electrodes. The conductance switching to two states such as ON/OFF states in the ITO/Alq₃/Liq/Alq₃/Al devices shows their conductance difference by ON/OFF ratio of several orders (10³) in magnitude and outstanding stability having a tendency to remain in that states for an extended period of times (∼24 h). Also, the high and low conductivity states of the memory cells can be exactly obtained by applying a negative voltage pulse to write or a positive voltage pulse to erase, respectively. The memory cells have been maintained during numerous (10⁶) writing-erasing cycles in ambient conditions without serious degradation of the device performance.     

Bipolar resistive switching of chromium oxide for resistive random access memory

This study investigates the resistance switching characteristics of Cr₂O₃-based resistance random access memory (RRAM) with Pt/Cr₂O₃/TiN and Pt/Cr₂O₃/Pt structures. Only devices with Pt/Cr₂O₃/TiN structure exhibit bipolar switching behavior after the forming process because TiN was able to work as an effective oxygen reservoir but Pt was not. Oxygen migration between Cr₂O₃ and TiN was observed clearly before and after resistance switching from Auger electron spectroscopy (AES) analysis. Both low resistance state, ON state, and high resistance state, OFF state, of Pt/Cr₂O₃/TiN structures are stable and reproducible during a successive resistive switching. The resistance ratio of ON and OFF state is over 10², on top of that, the retention properties of both states are very stable after 10⁴ s with a voltage of −0.2 V.     

An optical programming/electrical erasing memory device: Organic thin film transistors incorporating core/shell CdSe@ZnSe quantum dots and poly(3-hexylthiophene)

An optical programming/electrical erasing memory device was fabricated by adopting organic thin film transistors incorporating core/shell CdSe@ZnSe quantum dots (QDs) and poly(3-hexylthiophene) (P3HT) as active layers. After illumination, the presence of quantum well-structured core/shell CdSe@ZnSe QDs within the P3HT film enhanced the maximum ON/OFF ratio substantially to 2700; this value was maintained for 8000 s without noticeable decay. The ON state current could be erased effectively when using a single pulse of the gate voltage (?10 V). This fabrication approach opens up the possibility of improving the memory performance of polymeric materials prepared at low cost using simple processes.     

Novel Digital Nonvolatile Memory Devices Based on Semiconducting Polymer Thin Films

Recent increases in the demand for mobile devices have stimulated the development of nonvolatile memory devices with high performance. In this Communication, we describe the fabrication of low‐cost, high‐performance, digital nonvolatile memory devices based on semiconducting polymers, poly(o‐anthranilic acid) and poly(o‐anthranilic acid‐co‐aniline). These memory devices have ground‐breaking and novel current–voltage switching characteristics. The devices are switchable in a very low voltage range (which is much less than those of all other devices reported so far) with a very high ON/OFF current ratio (which is on the order of 10⁵). The low critical voltages have the advantage for nonvolatile memory device applications of low operation voltages and hence low power consumption. With this very low power consumption, the devices demonstrate in air ambient to have very stable ON‐ and OFF‐states without any degradation for a very long time (which has been confirmed up to one year so far) and to be repeatedly written, read and erased. Our study proposes that the ON/OFF switching of the devices is mainly governed by a filament mechanism. The high ON/OFF switching ratio and stability of these devices, as well as their repeatable writing, reading and erasing capability with low power consumption, opens up the possibility of the mass production of high performance digital nonvolatile polymer memory devices with low cost. Further, these devices promise to revolutionize microelectronics by providing extremely inexpensive, lightweight, and versatile components that can be printed onto plastics, glasses or metal foils.     

Indium-tin-oxide,free, flexible,nonvolatile memory devices based on graphene quantum dots sandwiched between polymethylsilsesquioxane layers

Indium-tin-oxide (ITO) free, nonvolatile memory (NVM) devices based on graphene quantum dots (GQDs) sandwiched between polymethylsilsesquioxane (PMSSQ) layers were fabricated directly on polyethylene terephthalate (PET) substrates by using a solution process technique. Current-voltage (I-V) curves for the silver nanowire/PMSSQ/GQD/PMSSQ/poly(3,4-ethylenethiophene):poly(styrene sulfonate)/PET devices at 300 K showed a current bistability. The ON/OFF ratio of the current bistability for the NVM devices was as large as 1 × 10⁴, and the cycling endurance time of the ON/OFF switching for the NVM devices was above 1 × 10⁴ s. The Schottky emission, Poole-Frenkel emission, trapped-charge limited-current, and space-charge-limited current were dominantly attributed to the conduction mechanisms for the fabricated NVM devices based on the obtained I-V characteristics, and energy band diagrams illustrating the “writing” and the “erasing” processes of the devices.     

Highly-reproducible nonvolatile memristive devices based on polyvinylpyrrolidone: Graphene quantum-dot nanocomposites

The properties of nonvolatile memristive devices (NMD) fabricated utilizing organic/inorganic hybrid nanocomposites were investigated due to their superior advantages such as mechanical flexibility, low cost, low-power consumption, simple technological process in fabrication and high reproducibility. The current-voltage (I-V) curves for the Al/polyvinylpyrrolidone (PVP): graphene quantum-dot (GQD)/indium-tin-oxide (ITO) memristive devices showed current bistability characteristics at 300 K. The window margins corresponding to the high-conductivity (ON) state and the low-conductivity (OFF) state of the devices increased with increasing concentration of the GQDs. The ON/OFF ratio of the optimized device was 1 × 10⁴, which was the largest memory margin among the devices fabricated in this research. The endurance number of ON/OFF switching was above 1 × 10² cycles, and the retention time was relatively constant, maintaining a value above 10⁴ s. The devices showed high reproducibility with the writing voltage being distributed between −0.5 and −1.5 V and the erasing voltage being distributed between 2 and 3 V. The ON state currents remained between 0.02 and 0.03 A, and the OFF state currents stayed between 10⁻⁶ and 10⁻⁴ A. The carrier transport mechanisms are illustrated by using both the results obtained by fitting the I-V curves and the energy band diagrams of the devices.     

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSS_ADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSS_IMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSS_IMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSS_IMM electrode out-performed the PEDOT:PSS_ADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm⁻² to 7.15 mA cm⁻². The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.     

Tristable switching of the electrical conductivity through graphene quantum dots sandwiched in multi-stacked poly(methyl methacrylate) layers

Tristable switching nonvolatile memory (NVM) devices based on graphene quantum dots (GQDs) sandwiched between multi-stacked poly (methyl methacrylate) (PMMA) layers were fabricated on indium-tin-oxide (ITO)-coated glass substrates by using a solution-processed method. Current-voltage (I-V) curves at 300 K for the silver nanowire/PMMA/GQD/PMMA/GQD/PMMA/ITO/glass devices showed tristable switching currents with high-resistance, intermediate-resistance, and low-resistance states. The device's cycling endurance of the three resistance states remained stable with a distinguishable value for each resistance state over 1000 cycles, and the obtained retention results showed well-distinguished resistance states without degradation for up to 1 × 10⁴ s. Schottky emission, Poole-Frenkel emission, trapped-charge limited-current, and ohmic conduction were proposed as the dominant conduction mechanisms for the fabricated NVM devices based on the obtained I-V characteristics. The described energy-band diagrams confirm the proposed conduction band mechanisms.     

Organic low voltage rewritable memory device based on PEDOT:PSS/f-MWCNTs thin film

We report on devices constructed using a small quantity (?0.01 wt.%) of functionalized multiwalled carbon nanotubes (f-MWCNTs) embedded in a conducting polymer (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS) matrix and aluminum top electrodes, prepared on indium-tin-oxide (ITO) substrates. Our ITO/(PEDOT:PSS + f-MWCNTs)/Al devices show current bistability. The low resistance ON-state, as well as the high resistance OFF state, retain the information for hours and are stable after hundreds of write–read–erase–read (WRER) cycles, being potentially interesting for erasable and rewritable volatile memory device applications. Moreover, the operation voltages used for performing these WRER cycles are very low. The threshold voltage for OFF to ON switching can be adjusted changing the f-MWCNTs concentration. Our results suggest that the nanotubes are necessary for the production of an inhomogeneous electric field playing a role in the electroforming (dielectric breakdown) of the aluminum oxide layer at the Al₂O₃/(PEDOT:PSS) interface.     

Characterization of PI:PCBM organic nonvolatile resistive memory devices under thermal stress

In this study, we fabricated nonvolatile organic memory devices using a mixture of polyimide (PI) and 6-phenyl-C61 butyric acid methyl ester (PCBM) (denoted as PI:PCBM) as an active memory material with Al/PI:PCBM/Al structure. Upon increasing the temperature from room temperature to 470 K, we demonstrated the good nonvolatile memory properties of our devices in terms of the distribution of ON and OFF state currents, the threshold voltage from OFF state to ON state transition, the retention, and the endurance. Our organic memory devices exhibited an excellent ON/OFF ratio (I_ON/I_OFF > 10³) through more than 200 ON/OFF switching cycles and maintained ON/OFF states for longer than 10⁴ s without showing any serious degradation under measurement temperatures up to 470 K. We also confirmed the structural robustness under thermal stress through transmission electron microscopy cross-sectional images of the active layer after a retention test at 470 K for 10⁴ s. This study demonstrates that the operation of PI:PCBM organic memory devices could be controlled at high temperatures and that the structure of our memory devices was maintained during thermal stress. These results may enable the use of nonvolatile organic memory devices in high temperature environments.     

Layered (C6H5CH2NH3)2CuBr4 Perovskite for Multilevel Storage Resistive Switching Memory

Although there have been attempts to use non‐lead based halide perovskite materials as insulating layers for resistive switching memory, the ratio of low resistance state (LRS) to high resistance state (HRS) ( = ON/OFF ratio) and/or endurance is reported to be mostly lower than 10³. Resistive switching memory characteristics of layered (BzA)₂CuBr₄ (BzA = C₆H₅CH₂NH₃) perovskite with high ON/OFF ratio and long endurance are reported here. The X‐ray diffraction (XRD) pattern of the deposited (BzA)₂CuBr₄ layer shows highly oriented (00l) planes perpendicular to a Pt substrate. An Ag/PMMA/(BzA)₂CuBr₄/Pt device shows bipolar switching behavior. A forming step at around +0.5 V is observed before the repeated bipolar switching at the SET voltage of +0.2 V and RESET voltage of ‐0.3 V. The ON/OFF ratio as high as =10⁸ is monitored along with an endurance of ≈2000 cycles and retention time over 1000 s. The high ON/OFF ratio enables multilevel storage characteristics as confirmed by changing the compliance currents. Ohmic conduction at the LRS and Schottky emission at HRS are involved in electrochemical metallization process. The bipolar resistive switching property is retained after storing the device at ambient condition under relative humidity of about 50% for 2 weeks, which indicates that (BzA)₂CuBr₄ is stable memory material.     

Effects of incorporation of copper sulfide nanocrystals on the performance of P3HT: PCBM based inverted solar cells

It has been reported that performance of bulk heterojunction organic solar cells can be improved by incorporation of an additive like metal and semiconducting nanoparticles in the active layer. Here in, we have synthesized Cu₂S nanocrystals (NCs) by chemical route and studied its dispersion in poly (3-hexylthiophene) [6, 6]-phenyl C61-butyric acid methyl ester (P3HT: PCBM) matrix. Variation in the performance parameters with change in the concentration of Cu₂S NCs into the P3HT: PCBM matrix has also been studied and it was found that the inverted geometry device with concentration of 20 wt% of Cu₂S NCs and having the structure ITO/ZnO (NPs)/P3HT: PCBM:Cu₂S NCs/MoO₃/Al has shown maximum efficiency of 3.39% which is more than 100% increase in comparison with devices without Cu₂S NCs. Photoluminescence measurements studies unveiled that the incorporation of Cu₂S NCs into a P3HT: PCBM matrix has helped in quenching photoluminescence which suggests more effective exciton dissociation at the interfaces between the P3HT and PCBM domains. The Nyquist plots obtained from impedance spectroscopy at 1 V bias in the dark has suggested the effective lifetime and global mobilities for P3HT: PCBM as 0.267 ms and 1.17 × 10⁻³ cm²/V-S and for P3HT: PCBM:Cu₂S NCs (20 wt%) systems as 0.156 ms and 2.02 × 10⁻³ cm²/V-S respectively. Based on observed photoluminescence quenching, calculated effective lifetime and global mobility, we have tried to explain the possible reason for improvement in the efficiency with the very well dispersion of Cu₂S NCs into the P3HT: PCBM matrix.     

Inkjet printed silver source/drain electrodes for low-cost polymer thin film transistors

Silver tracks for source/drain (S/D) electrodes in low-cost polymer thin film transistors (TFTs) have been realized through inkjet printing technique, using heavily n-doped silicon wafer with thermally grown silicon dioxide as the substrate and poly(3-hexylthiophene) (P3HT) as the channel material. Spin coating a layer of poly-4-vinylphenol (PVPh) onto the substrate was found to enhance the silver track uniformity and lower the cure temperature (from 300 to 210 °C). The surface roughness of the PVPh film was optimized to improve the device performance. The fabricated P3HT TFT with a channel length of 20 μm exhibited a saturation mobility of 3.5 × 10⁻2 cm²/V/s which was three times higher than that obtained in P3HT TFTs with gold S/D electrodes.