Voltage Sweep Direction Research Articles

The intrinsic origin of hysteresis in MoS2 field effect transistors.

We investigate the hysteresis and gate voltage stress effect in MoS2 field effect transistors (FETs). We observe that both the suspended and the SiO2-supported FETs have large hysteresis in their transfer curves under vacuum which cannot be attributed to the traps at the interface between the MoS2 and the SiO2 or in the SiO2 substrate or the gas adsorption/desorption effect. Our findings indicate that the hysteresis we observe comes from the MoS2 itself, revealing an intrinsic origin of the hysteresis besides some extrinsic factors. The fact that the FETs based on thinner MoS2 have larger hysteresis than that with thicker MoS2 suggests that the surface of MoS2 plays a key role in the hysteresis. The gate voltage sweep range, sweep direction, sweep time and loading history all affect the hysteresis observed in the transfer curves.

Hysteresis Phenomena in mis Structures Based on Graded-Gap MBE Hgcdte with a Two-Layer Plasma-Chemical Insulator SIO2/SI3N4

Capacitance-voltage characteristics (CV characteristics) of MIS structures based on graded-gap MBE Hg 1–x Cd x Te (x = 0.22–0.23) with a two-layer plasma-chemical insulator SiO2/Si3N4 are studied under variation of the voltage sweep direction. Special features are revealed in the CV characteristics at the forward and reverse voltage sweeps for MIS structures based on n-HgCdTe and p-HgCdTe. The results obtained suggest that the electron capture by states of the transition layer between the HgCdTe semiconductor and SiO2 insulator plays an important role in the hysteresis phenomena. It is found that if an insulator is applied, the impurity-defect centers of donor type are formed near the interface, whose density of states in the transition layer is (0.6–2.1)·10 11 сm –2 .

High Efficiency Millimeter-Scale Crystalline Perovskite Solar Cells

Current state-of-the-art photovoltaics utilize high purity single-crystal semiconductors grown by sophisticated, high temperature crystal-growth processes. Alternative emerging solar cell technologies based on solution-processed materials promise to deliver power at lower cost, but so far have had limited success due to the presence of defects in the bulk and at the grain boundaries. Improving the crystal size and quality of solution-processed films could lead to unprecedented improvements in photovoltaic performance. However, large-scale crystal growth is challenging in solution-based deposition where molecular kinetics are limited by the low processing temperature. Here, for the first time we demonstrate organometallic perovskites thin-films composed of extra-ordinarily large crystalline grains (1-2 mm), fabricated using a new processing technique. The increase in the grain-size directly correlates to a dramatic increase in the efficiency, as a direct consequence of increased charge carrier mobility and reduction in defect densities. Our measured efficiency values approach ~18%, which is among the highest reported. These devices are extremely robust, exhibiting no degradation with voltage sweep direction or the rate at which the voltage was scanned. We anticipate that this technique for growing high quality large-area crystals at low temperature with low defect concentration will be universally applicable to a wide variety of other material systems thus providing a solution to a long-standing scientific challenge of overcoming polydispersity, defects and grain boundary recombination in solution-processed thin-films. From the perspective of the global photovoltaics community, these results are expected to lead the field towards the synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency single-junction and hybrid (semiconductor and perovskite) tandem planar cells. In addition, I will also discuss design principles that govern the photostability in these systems.

Environmental effects on the electrical behavior of pentacene thin-film transistors with a poly(methyl methacrylate) gate insulator

The electrical properties of top-contact pentacene thin-film transistors (TFTs) with a poly(methyl methacrylate) (PMMA) gate dielectric were analyzed in air and vacuum environments. Compared to the vacuum case, the pentacene TFT in air exhibited lower drain currents and more pronounced shifts in the threshold voltage upon reversal of the gate voltage sweep direction, together with a decrease in the field-effect mobility. These characteristic variations were explained in terms of two distinctive actions of polar H2O molecules in pentacene TFT. H2O molecules were suggested to diffuse under the source and drain contacts and interrupt the charge injection into the pentacene film, whereas those that permeate at the pentacene/PMMA interface retard hole depletion in and around the TFT channel. The diffusion process was much slower than the permeation process. The degraded TFT characteristics in air could be recovered mostly by storing the device under vacuum, which suggests that the air instability of TFTs is due mainly to the physical adsorption of H2O molecules within the pentacene film.

Subthreshold characteristics of pentacene field-effect transistors influenced by grain boundaries

Grain boundaries in polycrystalline pentacene films significantly affect the electrical characteristics of pentacene field-effect transistors (FETs). Upon reversal of the gate voltage sweep direction, pentacene FETs exhibited hysteretic behaviours in the subthreshold region, which was more pronounced for the FET having smaller pentacene grains. No shift in the flat-band voltage of the metal-insulator-semiconductor capacitor elucidates that the observed hysteresis was mainly caused by the influence of localized trap states existing at pentacene grain boundaries. From the results of continuous on/off switching operation of the pentacene FETs, hole depletion during the off period is found to be limited by pentacene grain boundaries. It is suggested that the polycrystalline nature of a pentacene film plays an important role on the dynamic characteristics of pentacene FETs.

Effect of voltage sweep direction on the performance evaluation of P3HT : PCBM solar cells

ABSTRACTInvestigations on the effect of direction of voltage sweeps, on the current density–voltage (J–V) characteristics in polymer bulk‐heterojunction solar cells, based on the blend of poly(3‐hexylthiophene) (P3HT) and phenyl [6,6] C61 butyric acid methyl ester (PCBM), are reported with time. On the freshly prepared device, the direction of the voltage sweep did not have any effect; however, as the device started degrading, the change in direction of the voltage sweep resulted into different characteristics. Analysis beyond complete degradation, when all the photovoltaic parameters reduced to zero, revealed some interesting results. The J–V characteristics, measured with voltage sweep from −ve to +ve voltage, both in the dark and under illumination, were observed to pass through the second quadrant. On the other hand, with the change in the direction of voltage sweep, viz. from +ve to −ve voltage, the characteristics both in the dark and under illumination passed through the fourth quadrant. These results have been explained on the basis of polarization of the degraded active layer due to applied external voltage. This is an important effect and is observed to depend on the applied voltages during performance evaluation and becomes more prominent with time. This effect puts a question mark on the correctness of the method for calculation of the parameters of a degraded device. Studies on degradation of P3HT : PCBM solar cells showed that both the short circuit current density (Jsc) and the power conversion efficiency (η) decay exponentially, whereas the open circuit voltage (Voc) decays almost linearly with time. Copyright © 2012 John Wiley & Sons, Ltd.

Pre-breakdown negative differential resistance in thin oxide film: Conductive-atomic force microscopy observation and modelling

Negative differential resistance in thin nitrated silicon oxide and silicon oxide films is observed during current voltage measurements using conductive atomic force microscopy in ultra-high vacuum if the voltage sweep direction is reversed just before the dielectric breakdown voltage is reached. Current voltage characteristics are discussed using a trap assisted tunneling model. The resulting concentration of traps needed in order to reproduce the experimental characteristic leads to the consideration of a filamentlike conduction model, rather than a trap assisted tunneling model in which traps are randomly generated. The model reproduces well the effect of the voltage ramp parameters on the experimental current voltage characteristics, including the negative differential resistance. The effect of the growth dynamic parameters on the current voltage characteristics and the occurrence of the pre-breakdown negative differential resistance are discussed with the help of the model.

Effect of the oxygen vacancy gradient in titanium dioxide on the switching direction of bipolar resistive memory

Resistive memory switching behavior depending on voltage sweep direction is studied by intentionally creating oxygen vacancies within titanium dioxide (TiO 2). By inserting a reactive Ti layer on the TiO 2, oxygen deficient TiO 2− x layer is created, which then causes TiO 2− x /TiO 2 which has an oxygen vacancy gradient. This gradient of oxygen vacancy makes it possible to create an insulating TiO 2 layer on the bottom electrode during the first reset with a negative bias at the top electrode. This insulating layer makes counterclockwise directional bipolar switching more stable. On the other hand, under the clockwise directional voltage sweeping, the first set switching is prevented by the insulating TiO 2 layers created during the first and second reset, which leads to a short circuit due to local heating eventually.

Effects of Moisture on Pentacene Field-Effect Transistors with Polyvinylpyrrolidone Gate Insulator

We report the effects of moisture on the electrical characteristics of pentacene field-effect transistors (FETs) with the polyvinylpyrrolidone (PVPy) gate insulator. For the condition of relative humidity below 40%, the pentacene FET exhibited a stable operation without a shift in the threshold voltage upon a gate voltage sweep direction. With increasing the relative humidity above 50%, the threshold voltage shifted critically toward the positive direction, accompanied by significant degradations in the field-effect mobility and the subthreshold slope. These moisture-induced characteristic degradations could be substantially recovered by the low-temperature annealing process under a base pressure of 2 × 10−3 Torr.

Intrinsic doping and gate hysteresis in graphene field effect devices fabricated onSiO2 substrates

We have studied the intrinsic doping level and gate hysteresisof graphene-based field effect transistors (FETs) fabricated overSi/SiO2 substrates. It was found that the high p-doping level of graphene in some as-prepareddevices can be reversed by vacuum degassing at room temperature or abovedepending on the degree of hydrophobicity and/or hydration of the underlyingSiO2 substrate. Charge neutrality point (CNP) hysteresis, consisting of the shift of thecharge neutrality point (or Dirac peak) upon reversal of the gate voltage sweepdirection, was also greatly reduced upon vacuum degassing. However, another type ofhysteresis, consisting of the change in the transconductance upon reversal of thegate voltage sweep direction, persists even after long-term vacuum annealing at200 °C, whenSiO2 surface-bound water is expected to be desorbed. We propose a mechanismfor this transconductance hysteresis that involves water-related defects,formed during the hydration of the near-surface silanol groups in the bulkSiO2, that can act as electron traps.

Hyperfine-Coupling-Programming of Current through Coupled Quantum Dots with Multiple-Sweep Bias Voltage Waveforms

Fine features in the leakage current within the two-electron spin blockade region in a weakly coupled vertical double dot device induced on application of a moderate out-of-dot-plane magnetic field (up to 2.5 T) are studied. The nature of these features is consistent with an electron-spin nuclear-spin (hyperfine) interaction origin, and furthermore, their appearance depends strongly on the bias voltage sweep direction through the spin blockade region. Applying multi-part 10 mHz bias voltage waveforms, we can program the total current response via the hyperfine interaction. We demonstrate the operation of shifting a down- (up-) bias sweep through three up- (down-) bias sweeps which leads to a unique outcome in the measured current.

Influence of the measurement procedure on the field-effect dependent conductivity of ZnO nanorods

The electrical properties of field-effect transistors fabricated on the basis of single ZnO nanorods were analyzed under ambient conditions and in the chamber of a scanning electron microscope under high-vacuum conditions. Under ambient conditions, the threshold voltage and conductivity may depend strongly on the details of the measurement procedure as the chosen gate voltage range and gate voltage sweep direction. Electron irradiation in a scanning electron microscope under high-vacuum conditions at ∼10−5 mbar leads to desorption of oxygen and other electronegative molecules, which can increase the conductivity by more than two orders of magnitude.

Electrical characteristics of organic thin‐film transistors with polyvinylpyrrolidone as a gate insulator

This paper reports the electrical characteristics of polyvinylpyrrolidone (PVPy) and the performance of organic thin‐film transistors (OTFTs) with PVPy as a gate insulator. PVPy shows a dielectric constant of about 3 and contributes to the upright growth of pentacene molecules with 15.3Å interplanar spacing. OTFT with PVPy exhibited a field‐effect mobility of 0.23 cm2/Vs in the saturation regime and a threshold voltage of -12.7 V. It is notable that there was hardly any threshold voltage shift in the gate voltage sweep direction. Based on this reliable evidence, PVPy is proposed as a new gate insulator for reliable and high‐performance OTFTs.

Investigation of the hysteresis phenomenon of an a‐Si:H TFT at an elevated temperature for AMOLED displays

Abstract— The temperature dependence of the hysteresis of an a‐Si:H TFT has been investigated. An a‐Si:H TFT pixel driving scheme has been proposed and investigated. This scheme can eliminate changes in the organic light‐emitting diode (OLED) current caused by hysteresis of an a‐Si:H TFT. The VTH of the a‐Si:H TFT was changed according to the gate‐voltage sweep direction because of the hysteresis of the a‐Si:H TFT. The variation of VTH for a a‐Si:H TFT decreased from 0.41 to 0.17 V at an elevated temperature of 60°C because the sub‐threshold slope (s‐slope) of the a‐Si:H TFT, in the reverse voltage sweep direction, increased more than in the forward voltage sweep direction due to a greater increase in the initial electron trapped charges than the hole charges. Although the OLED current variation caused by hysteresis decreased (∼14%) as the temperature increased, the error in the OLED current needed to be improved in order to drive the pixel circuit of AMOLED displays. The proposed pixel circuit can apply the reset voltage (−10 V) before the data voltage for the present frame that was written to fix the sweep direction of the data voltage. The variation in the OLED current caused by hysteresis of the a‐Si:H TFT was eliminated by the fixed voltage sweep direction in the proposed pixel circuit regardless of operating temperature.

The Hysteresis Phenomenon in a-Si:H TFT and Poly-Si TFT in AMOLED

The hysteresis phenomenon of both a-Si and poly-Si TFT has been studied in order to investigate the hystersis phenomenon effect on residual image of AMOLED display. The transfer characteristics of both a-Si:H TFT and poly-Si TFT were varied according to the gate voltage sweep direction due to the different initial interface trapped charges in the gate insulator. The threshold voltage in a-Si:H TFT was altered by 0.23V due to hysteresis of device according to gate sweep direction while that in poly-Si TFT was varied by 0.21V due to the different gate insulator. In order to suppress hystersis effect on OLED current error we proposed and fabricated a new pixel circuit. By applying the reset voltage in pixel circuit, the data voltage sweep direction could be fixed and hysteresis effect on OLED current error was successfully suppressed. We showed that the OLED current error was successfully suppressed by experiment.

Electrical bistability and memory applications of poly( p-phenylenevinylene) films

Devices based on oriented and unoriented poly( p-phenylenevinylene) films showed presence of electrical bistability. Depending on the voltage sweep direction, current through the device at any voltage had two values. Switching between a low- and a high-conducting state occurred when a suitable voltage pulse was applied. Impedance spectroscopy identified a change in bulk resistance of the device during the transition between the two states. Role of orientation of the polymer film on switching and electrical bistability has been studied. The bistability in these devices had an associated non-volatile memory. We have demonstrated read-only and random access memory applications in such devices using current–voltage and impedance characteristics.

Observation of Current Reversal in the Scanning Tunneling Spectra of Fullerene-like WS2 Nanoparticles

Current-voltage characteristics measured using STM on fullerene-like WS2 nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.

Methods of Measuring Energy Conversion Efficiency in Dye-sensitized Solar Cells

The current–voltage characteristics of dye-sensitized solar cells (DSCs) were measured and compared with those of crystalline silicon solar cells. It was found that the energy conversion efficiency of DSCs is dependent on voltage sweep direction and sampling delay time (Td). Measurement of the transient photocurrent revealed that this dependence is due to the longer time constant of DSCs. This dependence was also confirmed in a simulation of current–voltage curves based on an equivalent circuit model of DSCs. Analysis of the current–voltage characteristics of polymer-based bulk heterojunction solar cells (BHSCs) and simulated measurements showed that the longer time constant is due to slow movement of ions in the electrolyte. To improve accuracy, the I–V measurement should be carried out from short circuit to open circuit with Td of 100 ms or longer.

Measuring methods of cell performance of dye-sensitized solar cells

Cell performance measuring methods for dye-sensitized solar cells (DSCs) were investigated in comparison with those of crystalline silicon solar cells. It was found that cell performances of DSCs varied according to voltage sweep direction and sampling delay time. On the other hand, this dependence was not observed in silicon solar cells. Measurement of the transient photocurrent revealed that the dependence on sweep direction and sampling delay time may be explained by the longer time constant of DSCs. To improve accuracy, measurement should be carried out with a sampling delay time exceeding several seconds. However, it is also found that the average value of the efficiency measured by two sweeping directions is constant when the sampling delay time is longer than 40 ms. These measurement conditions will be useful to measuring cell performance more accurately and rapidly. Additionally, determining cell area using a shading mask with an area smaller than that of TiO2 electrodes will improve accuracy when evaluating solar cell performance.

Conductance switching and data-storage in oriented polymer-based devices: impedance characteristics

We have fabricated and characterized sandwich-type devices based on an oriented polythiophene derivative. Depending on the voltage sweep direction, the devices showed the presence of two conducting states. Switching between the two states and an associated memory have earlier been studied by current–voltage characteristics. In this article, we have studied impedance spectroscopy of the material to model the device. We have studied conductance switching by measuring capacitance of the device, and have probed the high- and low-conducting states of the device. It has been shown that voltage variation of dielectric constant of the material depends on the voltage sweep direction. We have also demonstrated that capacitance of the device can probe the two states for data-storage applications.