Flexible nonvolatile organic ferroelectric memory transistors fabricated on polydimethylsiloxane elastomer

The flexible organic ferroelectric nonvolatile memory thin film transistors (OFMTs) were fabricated on polydimethylsiloxane (PDMS) elastomer substrates, in which an organic ferroelectric poly(vinylidene-trifluoroethylene) and an organic semiconducting poly(9,9-dioctylfluorene-co-bithiophene) layers were used as gate insulator and active channel, respectively. The carrier mobility, on/off ratio, and subthreshold swing of the OFMTs fabricated on PDMS showed 5 × 10⁻² cm² V⁻¹ s⁻¹, 7.5 × 10³, and 2.5 V/decade, respectively. These obtained values did not markedly change when the substrate was bent with a radius of curvature of 0.6 cm. The memory on/off ratio was initially obtained to be 1.5 × 10³ and maintained to be 20 even after a lapse of 2000 s. The fabricated OFMTs exhibited sufficiently encouraging device characteristics even on the PDMS elastomer to realize mechanically stretchable nonvolatile memory devices.     

Effects of semiconductor/dielectric interfacial properties on the electrical performance of top-gate organic transistors

We investigated the effects of varying the properties of the interface between a semiconductor P3HT layer and a dielectric Cytop™ layer on the performances of the resulting transistor devices by comparing the mobilities of devices prepared with bottom gate/bottom contact or top gate/bottom contact architectures. The reduced channel roughness that arose from the thermal annealing step dramatically enhanced the field-effect mobility, yielding the highest mobility yet obtained for a top-gate transistor: 0.12 cm²/V s. High-performance OFETs may be fabricated by controlling the channel roughness and the properties of the interface between the semiconductor and the gate dielectric.     

Flexible organic transistors on standard printing paper and memory properties induced by floated gate electrode

Integrating electronic devices with unconventional substrate has been a popular research direction. Among these substrates, cellulose fiber paper has advantages in low-cost, recyclable and bio-degradable. We demonstrated directing printing of all contact electrodes on standard untreated Fuji Xerox printer paper without using planarization layer. The screen-printed gate electrodes based on silver nanoparticles can smooth out the paper substrate surface by two orders of magnitude and allow us to use parylene and DNTT as the dielectric and active layer directly. The transistors show average mobility of 0.297 cm² V⁻¹ s⁻¹ and on/off ratio larger than 10⁵. The low leakage current allows us to demonstrate memory properties by employing the floated gate method. The devices show excellent memory retention time for more than 10,000 s. The unique flexibility and combustibility of the organic transistors on paper substrate manifest their applications as next generation of green electronics.     

Effective mobility in amorphous organic transistors: Influence of the width of the density of states

The temperature dependence of poly(3-hexylthiophene-2,5-diyl) (P3HT)/polystyrene (PS) blend organic transistor current/voltage (I–V) characteristics has been experimentally and theoretically studied. The planar transistors, realized by drop casting the P3HT/PS ink, exhibit high mobilities (over 5 × 10⁻³ cm² V⁻¹ s⁻¹) and good overall characteristics. A transistor model accounting for transport mechanisms in disordered organic materials was used to fit the measured characteristics. Using a single set of parameters, the measured effective mobility versus gate bias, either increasing or decreasing with the gate bias depending on temperature, is well reproduced over a wide temperature range (130–343 K). A Gaussian density of states width of 0.045 eV was determined for this P3HT/PS blend. The transistor I–V characteristics are very well described considering disordered material properties within a self-consistent transistor model.     

Enabling high-efficiency organic light-emitting diodes with a cross-linkable electron confining hole transporting material

Wet-process enables flexible, large area-size organic devices to be fabricated cost-effectively via roll-to-roll manufacturing. However, wet-processed devices often show comparatively poor performance due to the lack of solution-process feasible functional materials that exhibit robust mechanical properties. We demonstrate here a cross-linkable material, 3,6-bis(4-vinylphenyl)-9-ethylcarbazole (VPEC), to facilitate the injection of hole and meanwhile effectively confine electron to realize, for examples, high efficiency organic light-emitting diodes, especially at high luminance. The VPEC shows a hole mobility of 1 × 10⁻⁴ cm² V⁻¹ s⁻¹ and a triplet energy of 2.88 eV. Most importantly, the VPEC not only works for devices containing low band-gap red or green emitters, but also for the counterpart with high band-gap blue emitter. With the electron confining hole transporting material, the power efficiency of a studied red device, at 1,000 cd m⁻² for example, is increased from 8.5 to 13.5 lm W⁻¹, an increment of 59%, and the maximum luminance enhanced from 13,000 to 19,000 cd m⁻², an increment of 46%. For a high triplet energy blue emitter containing device, it is increased from 6.9 to 8.9 lm W⁻¹, an increment of 29%, and the maximum luminance enhanced from 9,000 to 11,000 cd m⁻², an increment of 22%.     

Improving solution-processed n-type organic field-effect transistors by transfer-printed metal/semiconductor and semiconductor/semiconductor heterojunctions

Solution-processed n-type organic field effect transistors (OFETs) are in need of proper metal contact for improving injection and mobility, as well as balanced hole mobility for building logic circuit units. We address the two distinct problems by a simple technique of transfer-printing. Transfer-printed Au contacts on a terrylene-based semiconductor (TDI) significantly reduced the inverse subthreshold slope by 5.6 V/dec and enhanced the linear mobility by over 5 times compared to evaporated Au contacts. Hence, devices with a high-work-function metal (Au) are comparable with those with low-work-function metals (Al and Ca), indicating a fundamental advantage of transfer-printed electrodes in electron injection. We also transfer-printed a poly(3-hexylthiophene) (P3HT) layer onto TDI to construct a double-channel ambipolar transistor by a solution process for the first time. The transistor exhibits balanced hole and electron mobility (3.0 × 10⁻³ and 2.8 × 10⁻³ cm² V⁻¹ s⁻¹) even in a coplanar structure with symmetric Au electrodes. The technique is especially useful for reaching intrinsic mobility of new materials, and enables significant enlargement of the material tanks for solution-processed functional heterojunction OFETs.     

Multilevel characteristics and operating mechanisms of nonvolatile memory devices based on a floating gate of graphene oxide sheets sandwiched between two polystyrene layers

Nonvolatile organic memory devices were fabricated utilizing a graphene oxide (GO) layer embedded between two polystyrene (PS) layers. Scanning electron microscope images of GO sheets sandwiched between two PS layers showed that the GO sheets were clearly embedded in the PS layers. Capacitance–voltage (C–V) curves of the Al/PS/GO/PS/n-type Si devices clearly showed hysteresis behaviors with multilevel characteristics. The window margin of the nonvolatile memory devices increased from 1 to 7 V with increasing applied sweep voltages from 6 to 32 V. The cycling retention of the ON/OFF switching for the devices was measured by applying voltages between +15 and −15 V. While the capacitance of the memory devices at an ON state have retained as 230 pF up to 10⁴ cycles, that at an OFF state maintained as 16 pF during three times of repeated measurements. The extrapolation of the retention data for the devices maintained up to 10⁶ cycles. The operating mechanisms of the nonvolatile organic memory devices with a floating gate were described by the C–V results and the energy band diagrams.     

Fabrication of high-mobility organic single-crystal field-effect transistors with amorphous fluoropolymer gate insulators

High-mobility rubrene single-crystal field-effect transistors are built on highly water- and oil-repellent fluoropolymer gate insulators. Roughness is introduced at the surface once to provide good adhesion to metal films and photoresist polymers for stable electrodes. Before constructing interfaces to crystals, smoothness of the fluoropolymer surface is recovered by annealing at a moderate temperature to maximize carrier mobility. Mobility values estimated in the saturation region reproducibly exceeded 15 cm²/V s for all the 10 devices, reaching 30 cm²/V s for the best two devices. The results demonstrate that the water-repellency and smoothness of the dielectric polymers are favorable for the excellent transistor performance.     

Integration of a graphene ink as gate electrode for printed organic complementary thin-film transistors

We demonstrate a new electrode gate based on graphene ink for complementary printed organic metal oxide semiconductor (CMOS) technology on flexible plastic substrates. The goal is to replace the standard silver electrode gate. Devices made with graphene were enhanced and showed a high field-effect mobility of 3 cm² V⁻¹ s⁻¹ for P-type and 0.9 cm² V⁻¹ s⁻¹ for the N-type semiconductors. The improvement is attributed to the increase of the electrical capacitance of the organic dielectric (CYTOP) due to the graphene layer. A seven-stage ring oscillator was made with high oscillation frequencies of 2.1 kHz at 40 V corresponding to a delay/gate value of 34 μs. These performances are promising for use of low cost printed electronic applications.     

Low power flexible organic thin film transistors with amorphous Ba0.7Sr0.3TiO3 gate dielectric grown by pulsed laser deposition at low temperature

We deposited amorphous Ba_0.7Sr_0.3TiO₃ (BST) on silicon and plastic substrate under 110 °C by pulsed laser deposition (PLD) and use it as the dielectric of the organic transistor. Depends on the thickness of BST layer, the highest mobility of the devices can achieve 1.24 cm² V^?1 s^?1 and 1.01 cm² V^?1 s^?1 on the silicon and polyethylene naphthalate (PEN) substrate, respectively. We also studied the upward and downward bending tests on the transistors and the dielectric thin films. We found that the BST dielectric pentacene transistor can maintain the mobility at 0.5 cm² V^?1 s^?1 or higher while the bending radius is around 3 mm in both upward and downward bending. Our finding demonstrates the potential application of PLD growth high-k dielectric in the large area organic electronics devices.     

Addressable growth of oriented organic semiconductor ultra-thin films on hydrophobic surface by direct dip-coating

Oriented organic field-effect transistor (OFET) stripe arrays on hydrophobic substrates were fabricated by fast dip-coating technique. The addressable growth was achieved by decreasing surface energy of the channel areas with respect to the electrodes via hydrophobic treatment. The higher surface energy of the electrodes allows solution to adhere and then organic semiconductors nucleate and bridge the channels after evaporation of the solvent. Area-selective behaviour can be controlled by adjusting surface property of transistor channel, geometry features of the gold electrodes, pulling speed and evaporation atmosphere. The mechanism behind is the competition between receding of the solution and evaporating of the solvent that generate the organic semiconductor films on the substrate. The patterned bottom-contact transistor arrays exhibit carrier mobility of 2.0 × 10⁻³ cm² V⁻¹ s⁻¹, while no field-effect characteristics can be detected for bottom-contact arrays without hydrophobic treatment. Such reliable, fast and solution-based patterned OFET arrays are highly desirable for large-scale and low-cost production.     

Increased photooxidation stability of pentacene derivatives linked with aromatic groups for OTFTs

Pentacene derivatives linked with aromatic groups at the 6,13-positions have been synthesized and characterized for their photooxidation properties. They exhibit high solubility which provides low-cost solution deposition methods. However, most of them are highly susceptible to photooxidation in solution determined with a few minutes of their half-life time under ambient conditions, practically precluding them from solution fabrication applications. Interestingly, their photooxidation stability can be significantly increased by blocking out light. The thin film transistor device for 3,4,5-trifluorophenyl-substituted pentacene (2c) showed the highest mobility of 1.1 × 10⁻² cm² V⁻¹ s⁻¹ with the threshold voltage of 20 V when it was prepared in the dark condition.     

Synergistic effect of polymer and oligomer blends for solution-processable organic thin-film transistors

The thin-film morphologies and thin-film transistor (TFT) characteristics of a series of binary blends of poly(9,9′-dioctylfluorene-alt-bithiophene) (F8T2) and α,ω-dihexylquarterthiophene (DH4T) are reported. The blends of F8T2 and DH4T exhibit good solubility and produce TFT devices with better performances than F8T2 and DH4T devices. The 50% DH4T blend device was found to have a hole mobility of 0.011 cm ² V⁻¹ s⁻¹, which is four times higher than the mobility of the F8T2 device, with a high-on/off ratio of about 10⁵ and a low-off current of 17 pA. The polymer and oligomer domains are phase-separated with large domain size and arranged in characteristic molecular alignments. It was found that carrier transport in the blend systems is mainly controlled by the polymer component, and that the nature of the blended oligomer affects the OTFT performance of the blends.     

In-plane electrical bistability of photochromic diarylethene/Cu composite film

Electrical bistability is an essential property for memory devices. We report here the in-plane electrical bistability of photochromic diarylethene (DAE)/Cu composite film, which is prepared by Cu vapor deposition on the DAE surface with a low glass-transition temperature. The low-current level around 10⁻⁸ A was switched to a high-current level of ca. 10⁻⁴ A at a low threshold voltage (V_th) in the first voltage sweep. Once this switching occurred, the high-current level was kept in the second voltage sweep, and electrical bistability was achieved for the in-plane current. V_th was distributed in a wide range of voltages (0.5–10 V), and the colored sample obtained by the UV irradiation showed a relatively higher V_th than the colorless sample. The highest ON–OFF ratio in current was ca. 10⁶. The origin of the bistability attributed to the electrical breakdown in the insulated lines that was consisted of DAE in Cu film. The in-plane bistability of the DAE/Cu composite film has good retention time (>60 min) and readout-cycle endurance (>10⁶ cycles), indicating that it is suitable for write-once organic semiconductor memory characteristics.     

Low-voltage organic ferroelectric field effect transistors using Langmuir–Schaefer films of poly(vinylidene fluoride-trifluororethylene)

Langmuir–Schaefer transfer was used to fabricate ultrathin films of ferroelectric copolymer, poly(vinylidene fluoride-trifluoroethylene) (70–30 mol%), for non-volatile memory application at low operating voltage. Increasing the number of transferred monolayers up to 10 led to improved film crystallinity in the “in-plane” direction, which reduced surface roughness of the semicrystalline film. Treatment of the substrate surface by plasma results in different film coverage which was subsequently found to be governed by interaction of the deposited film and surface condition. Localized ferroelectric switching was substantially attained using piezo-force tip at 10 V on 10-monolayer films. Integrating this film as a dielectric layer into organic capacitor and field effect transistor yields a reasonably good leakage current (<10^?7 A/cm²) with hysteresis in capacitance and drain current with ON/OFF ratio of 10³ for organic ferroelectric memory application at significantly reduced operating voltage of |15| V.     

High electron mobility triazine for lower driving voltage and higher efficiency organic light emitting devices

The triazine compound 4,4′-bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1′-biphenyl (BTB) was developed for use as an electron transport material in organic light emitting devices (OLEDs). The material demonstrates an electron mobility of ∼7.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at a field of 8.00 × 10⁵ V cm⁻¹, which is 10-fold greater than that of the widely used material tris(8-hydroxyquinoline) aluminum (AlQ₃). OLEDs with a BTB electron transport layer showed a ∼1.7–2.5 V lower driving voltage and a significantly increased efficiency, compared to those with AlQ₃. These results suggest that BTB has a strong potential for use as an OLED electron transport layer material.     

Resistive switching properties of a thin SiO2 layer with CeOx buffer layer on n+ and p+ Si bottom electrodes

Resistive switching properties of a 2-nm-thick SiO₂ with a CeO_x buffer layer on p⁺ and n⁺ Si bottom electrodes were characterized. The distribution of set voltage (V_set) with the p⁺ Si bottom electrode devices reveals a Gaussian distribution centered in 4.5 V, which reflects a stochastic nature of the breakdown of the thin SiO₂. Capacitance–voltage (C–V) measurements indicate the trapping of electrons by positively shifting the C–V curve by 0.2 V during the first switching cycle. On the other hand, devices with the n⁺ Si bottom electrodes showed a broad distribution in V_set with a mean value higher than that of p⁺ Si bottom electrode devices by 0.9 V. Although no charge trapping was observed with n⁺ Si bottom electrode devices, a degradation in interface states was confirmed, causing a tail in the lower side of the V_set distribution. Based on the above measurements, the difference in the V_set can be understood by the work function difference and the contribution of electron trapping.     

The influence of high energy electron irradiation on the Schottky barrier height and the Richardson constant of Ni/4H-SiC Schottky diodes

The influence of high energy electron (HEE) irradiation from a Sr-90 radio-nuclide on n-type Ni/4H–SiC samples of doping density 7.1×10¹⁵ cm⁻³ has been investigated over the temperature range 40–300 K. Current–voltage (I–V), capacitance–voltage (C–V) and deep level transient spectroscopy (DLTS) were used to characterize the devices before and after irradiation at a fluence of 6×10¹⁴ electrons-cm⁻². For both devices, the I–V characteristics were well described by thermionic emission (TE) in the temperature range 120–300 K, but deviated from TE theory at temperature below 120 K. The current flowing through the interface at a bias of 2.0 V from pure thermionic emission to thermionic field emission within the depletion region with the free carrier concentrations of the devices decreased from 7.8×10¹⁵ to 6.8×10¹⁵ cm⁻³ after HEE irradiation. The modified Richardson constants were determined from the Gaussian distribution of the barrier height across the contact and found to be 133 and 163 A cm⁻² K⁻² for as-deposited and irradiated diodes, respectively. Three new defects with energies 0.22, 0.40 and 0.71 eV appeared after HEE irradiation. Richardson constants were significantly less than the theoretical value which was ascribed to a small active device area.     

Ferroelectric transistor memory arrays on flexible foils

In this paper, we successfully fabricated and operated passive matrix P(VDF–TrFE) transistor arrays, i.e. memory arrays in which no pass-transistors or other additional electronic components are used. Because of the smaller cell, a higher integration density is possible. We demonstrate arrays up to a size of 16 × 16, processed on thin (25 μm) poly(ethylene naphthalate) substrates, using Indium–Gallium–Zinc–Oxide (IGZO) as the semiconductor and 200 nm-thick P(VDF–TrFE) as a ferroelectric gate dielectric. The memory transistors have remnant current modulations of ～10⁵ with a retention time of more than 12 days. They can be switched in less than 1 μs at operating voltages of 25 V. Switching speed is strongly decreased with decreasing voltage: at ～10 V the transistors do not switch within 10 s. This difference in switching speed of more than 4 orders in magnitude when changing the electric field by a factor of only 2.5 makes these memories robust towards disturb voltages, and forms the basis of integration of these transistors in passive matrix-addressable transistor arrays that contains only one (memory) transistor per cell. It is shown that with current technology and memory characteristics it is possible to scale up the array size in the future.     

Fabric electrodes coated with polypyrrole nanorods for flexible supercapacitor application prepared via a reactive self-degraded template

Wearable energy storage devices that can be used in the garment industry are strongly required to power E-textiles. In this article, polypyrrole (PPy) nanorods were deposited on cotton fabrics via in situ polymerization of pyrrole in the presence of the fibrillar complex of FeCl₃ and methyl orange as a reactive self-degraded template. The obtained fabrics could be directly used as supercapacitor electrodes, with a maximum specific capacitance of 325 F g⁻¹ and an energy density of 24.7 Wh kg⁻¹ at a current density of 0.6 mA cm⁻². The capacitance remained higher than 200 F g⁻¹ after 500 cycles.