Space Charge Limited Current Behavior Research Articles

Transformation of MoSe2 to MoSe2-xOyvia controlled oxidation for high-performance resistive switching

The current study shows an improvised resistive switching in oxygenated MoSe2 (MoSe2-xOy) thin film. 2D nanosheets assembled nanoflower morphology of MoSe2 was synthesized with hydrothermal route and then treated with thermal oxidation process at 300⁰C in presence of 0, 0.5, 5 and 20 % of oxygen. The coexistence of MoSe2 and MoOx in each samples was investigated with various spectroscopic techniques. The conversion to MoOx was maximum in 20 % O2 treated sample while other samples were converted partially. Au/MoSe2-xOy/Au structured devices were fabricated on SiO2/p-Si substrate and tested for resistive switching study. Among all four devices, 5 % O2 treated MoSe2-xOy exhibited excellent filamentary type bipolar resistive switching with very low SET and RESET voltages of + 0.84 V and −0.79 V, respectively. The device showed ION/IOFF ratio ∼50 (at read voltage: 0.5 V) with excellent retention (106 s) and endurance (100 cycles) characteristics with minimum cycle-to-cycle variation. The resistive switching mechanism was explained with valance change conducting filament formation/rupture model and the trap-controlled space charge limited current behavior. Rich transport properties of 2D-chalcogenide (MoSe2) and classic oxygen vacancy migrated switching of metal oxide (i.e. MoOx), were simultaneously important for achieving high performance memristive behavior in 5 % O2 treated MoSe2.

Analysis of a Flexible Photoconductor, Manufactured with Organic Semiconductor Films.

This work presents the evaluation of the electrical behavior of a flexible photoconductor with a planar heterojunction architecture made up of organic semiconductor films deposited by high vacuum evaporation. The heterojunction was characterized in its morphology and mechanical properties by scanning electron microscopy and atomic force microscopy. The electrical characterization was carried out through the approximations of ohmic and SCLC (Space-Charge Limited Current) behaviors using experimental J-V (current density-voltage) curves at different voltages and under different light conditions. The optimization of the photoconductor was carried out through annealing and accelerated lighting processes. With these treatments, the Knoop Hardness of the flexible photoconductor has reached a value of 8 with a tensile strength of 5.7 MPa. The ohmic and SCLC approximations demonstrate that the unannealed device has an ohmic behavior, whereas the annealed device has an SCLC behavior, and after the optimization process, an ohmic behavior and a maximum current density of 0.34 mA/mm2 were obtained under blue light. The approximations of the device's electron mobility (μn) and free carrier density (n0) were performed under different light conditions, and the electrical activation energy and electrical gap were obtained for the flexible organic device, resulting in appropriate properties for these applications.

Design of Promising Uranyl(VI) Complexes Thin Films with Potential Applications in Molecular Electronics.

In this work, it is proposed the development of organic semiconductors (OS) based on uranyl(VI) complexes. The above by means of the synthesis and the characterization of the complexes by Infrared spectroscopy, Nuclear magnetic resonance spectroscopy, mass spectrometry, and X-ray diffraction. Films of these complexes were deposited and subsequently, topographic and structural characterization was carried out by Scanning Electron Microscopy, X-ray diffraction, and Atomic Force Microscopy. Additionally, the nanomechanical evaluation was performed to know the stiffness of uranyl films using their modulus of elasticity. Also, the optical characterization took place in the devices and their bandgap value ranges between 2.40 and 2.93 eV being the minor for the film of the uranyl complex with the N on pyridine in position 4 (2 c). Finally, the electrical behavior of the uranyl(VI) films was evaluated, and important differences were obtained: the uranyl complex with the N on pyridine in position 2 (2 a) film is not influenced by changes in lighting and its current density is in the order of 10-3 A/cm2. The film with uranyl complex with the N on pyridine in position 3 (2 b) and 2 c presents a greater current flow under lighting conditions and two orders of magnitude larger than in film 2 a. In these films 2 b and 2 c, ohmic behavior occurs at low voltages, while at high voltages the charge transport changes to space-charge limited current behavior.

Photo-gain optimization in multilayer organic phototransistors by study of space-charge limited current

The photo-gain and sensitivity of solution processed multilayer organic phototransistors are optimized by analyzing the space-charge limited current behavior of the different photoactive layers embedded in hole-only and electron-only diodes.

Carrier conduction mechanisms of mesoporous titania thin films assessed by impedance spectroscopy

The electrical characterization of porous titania thin films remain a subject of study due to the material interest in widespread device applications. Highly ordered mesoporous titania thin films prepared by sol-gel method under different post-synthesis annealing temperatures were prepared and characterized by impedance spectroscopy at different humidity conditions. Slow ionic and fast electronic processes have different responses to the humidity and are carefully studied by assessing their frequency dependence. Specific ionic conduction regimes are identified for different water content that strongly depend on the structural properties of the material such as porosity and pore size. Also, the analysis of the Warburg coefficient describes the response of the mass transport for the different materials characteristics. Highly porous materials obtained for elevated annealing temperatures do not saturate with water even at high humidities and present inferior ionic transport mechanism. By analyzing the impedance response at different bias voltage it was possible to discriminate the electronic conduction for the mesoporous layer and observe the electronic defects by reproducing a space charge limited current behavior.

Electrical conduction mechanisms in the temperature-dependent current-voltage characteristics of poly(3-hexylthiophene)/n-type Si devices

The electrical conduction mechanisms in the temperature-dependent current-voltage characteristics of poly(3-hexylthiophene) (P3HT)/n-type Si devices were investigated. Carrier transport in the low forward-voltage region at room temperature is dominated by thermionic emission (TE). However, at high voltages the current is limited by series resistance and space charge limited current (SCLC) mechanisms. It is shown that the ideality factor increases as temperature decreases, because of a TE-to-SCLC transition. In order to obtain a greater understanding of the transition from TE to SCLC behavior, few-layer black phosphorus (BP) was incorporated into P3HT (i.e., P3HT:BP) and the P3HT:BP/n-type Si device was fabricated. It is suggested that the rectifying behavior is affected by the bulk effects of the P3HT layer. However, the incorporation of BP into P3HT leads to a significant increase in the hole mobility, suppressing the bulk effects of the P3HT layer.

Vertical architecture for enhancement mode power transistors based on GaN nanowires

The demonstration of vertical GaN wrap-around gated field-effect transistors using GaN nanowires is reported. The nanowires with smooth a-plane sidewalls have hexagonal geometry made by top-down etching. A 7-nanowire transistor exhibits enhancement mode operation with threshold voltage of 1.2 V, on/off current ratio as high as 108, and subthreshold slope as small as 68 mV/dec. Although there is space charge limited current behavior at small source-drain voltages (Vds), the drain current (Id) and transconductance (gm) reach up to 314 mA/mm and 125 mS/mm, respectively, when normalized with hexagonal nanowire circumference. The measured breakdown voltage is around 140 V. This vertical approach provides a way to next-generation GaN-based power devices.

Optical and dielectric studies on radio frequency sputtered Gd2O3 doped K0.5Na0.5NbO3 thin films for nonlinear photonic and microwave tunable device applications

(K0.5Na0.5)NbO3+1 wt%Gd2O3 (KNN1G) ferroelectric thin films have been deposited onto quartz and Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. The increase in the refractive index, and decrease in the optical bandgap was observed with the oxygen mixing percentage (OMP), and were in the range of 2.07–2.19, and 4.30–4.20 eV, respectively. The dispersion in the refractive index was analyzed using Wemple-DiDomenico single-oscillator model. The refractive index dispersion parameter for the film deposited under 100% OMP is found to be 6.64 × 10−14 eV m2. The best microwave (5 GHz–15 GHz) dielectric properties were observed for the film deposited under pure oxygen plasma. The film deposited in 100% OMP show the relative low leakage current density 4.06 × 10−6 A/cm2 at 153 kV/cm and the leakage current of the KNN1G films followed the space charge limited current behaviour. Nonlinear optical property of KNN1G thin films is measured by using modified single beam Z-scan technique. The large third order optical nonlinearity was obtained for the film deposited under pure oxygen plasma with nonlinear refractive index n2 = 2.46 × 10−5 cm2/W, nonlinear absorption coefficient β = 5.02 cm/W, real and imaginary parts of nonlinear susceptibility are χR(3) = 3.07 × 10−3 esu, and χI(3) = 3.16 × 10−3 esu, respectively. The obtained results show that the KNN1G thin films have a great potential for nonlinear photonic applications and high frequency applications.

Influence of equilibrium charge reservoir formation on photo generated charge transport in TiO2/organic devices

Charge transport measurements have been performed using the photo induced charge extraction by linearly increasing voltage (photo-CELIV) technique on indium tin oxide/titanium dioxide/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/copper (ITO/TiO2/P3HT:PCBM/Cu) devices. By adjusting the offset voltage such that holes are accumulated at the ITO/TiO2 contact we obtain space charge limited current (SCLC) extraction in the dark. Using photo-generation the current response is limited by SCLC extraction at low carrier concentrations but becomes purely recombination limited at high photo-generated carrier concentration. A 1-D drift diffusion model has been developed to simulate our results and we show that the hole blocking ITO/TiO2 contact is responsible for the SCLC behavior. The highly reduced recombination of charges seen in P3HT:PCBM devices is necessary to obtain the large extraction current transients that are seen in the experimental measurements. By comparing the simulated dark CELIV and photo-CELIV we show that photo-generated extraction is more sensitive towards changes in the surface recombination velocity.

Transparent amorphous memory cell: A bipolar resistive switching in ZnO/Pr0.7Ca0.3MnO3/ITO for invisible electronics application

ZnO/Pr0.7Ca0.3MnO3 (ZnO/PCMO) amorphous thin films were grown on an indium-tin-oxide (ITO)/glass by pulsed laser deposition at room temperature. Interestingly, a stable bipolar resistive switching behavior of the ITO/ZnO/PCMO/ITO cell can be longer than 2.5×103cycles. The on/off ratio of switching behaviors is as high as 104. The structure of ITO/ZnO/PCMO/ITO exhibits a high average transparency of 79.6% in the visible range with a maximum transparency of 84.6% at 590nm wavelength. The conductive mechanism during switching cycling in our structure can be described by a trapped-control space charge limited current behavior. The ZnO/PCMO/ITO/glass structure shows a potential of the transparent memory for future invisible electronics devices.

FABRICATION OF LATERAL ORGANIC SPIN VALVES BASED ON La0.7Sr0.3MnO3 ELECTRODES

We report the successful fabrication of lateral organic spin valves (OSVs) using polycrystalline pentacene as spacer and half-metallic La 0.7 Sr 0.3 MnO 3 (LSMO) as two electrodes. The distance between two electrodes ranges from 30 nm to 100 nm. The current–voltage characteristics follow the power law relation, which are attributed to the space charge limited current behavior. The devices with a spacing of 30 nm exhibits clear spin-valve characteristics with a magnetoresistance (MR) ratio of ~ 2% at 9 K. The MR effects disappear for electrode spacing about 100 nm, suggesting that the spin diffusion length is less than 100 nm.

Dynamic Doping in Bright and Stable Light Emitting Electrochemical Cell

By using fast current density and luminance versus voltage (JL-V) analysis the device operation of sandwiched light emitting electrochemical cells (LECs) during their normal voltage driving operation mode is studied. In LECs the application of a voltage results in the movement of ions changing the state of the device, as a result the JL-V scans need to be performed fast and meet certain conditions to be meaningful. Space-charge limited current behavior is observed once the injection barriers are overcome. The increase of the current density after this point imply that the effective thickness of the devices is reduced which indicates the formation of more conductive regions adjacent to the electrodes. The increase in conductivity is a consequence of compensation of the injected electrons by dissociated ions that effectively increase the carrier concentration similar like in electrochemically doped semiconductors. The extension of the dynamically doped regions can be controlled by applying a pulsed current driving scheme which allows for both sub-second turn-on time and long lifetimes.

Effects of selenophene substitution on the mobility and photovoltaic efficiency of polyquaterthiophene-based organic solar cells

Abstract We investigated thin films comprised of a blend of poly(5,5′-bis(3-dodecylthiophene-2-yl)2–2′-biselenophene)/[6,6] and phenyl C61 butyric acid methyl ester (PDT2Se2/PCBM) for use in bulk heterojunction photovoltaic cells. The charge transport characteristics of PDT2Se2 and its analog, poly(3,3′′′-didodecyl quaterthiophene) (PQT-12), were elucidated through analysis of the space charge limited current behavior at various temperatures. PDT2Se2 showed higher mobilities, lower field activation parameters, and a lower temperature dependence of these parameters than did PQT-12, indicating better charge transport in PDT2Se2. Optimization of the PDT2Se2:PCBM composition ratio produced a bicontinuous donor–acceptor network with domain sizes on the order of 10 nm, which afforded power conversion efficiencies of 1.4%, a short circuit current density of 4.3 mA cm −2 , an open circuit voltage of 0.69 V, and a fill factor of 47%. This performance was much better than the performance achieved previously using PQT-12:PCBM blend systems. Selenophene substitution appears to be an effective strategy for enhancing the photovoltaic effect of thiophene-based polymeric semiconductors for high performance organic solar cells (OSCs).

Direct current and alternating current electrical transport properties of regioregular poly[3-(4-alkoxyphenyl)-thiophenes

In this paper, the direct current and alternating current (ac) electrical transport properties of doped and undoped regioregular poly[3-(4-alkoxyphenyl)-thiophenes], where the alkoxy groups are O-(CH2)n−1CH3 with n=1,4,6, and 8, have been investigated. The films have been synthesized by an experimental procedure based on the oxidation of 3-(4-alkoxylphenyl)-thiophenes with molecular oxygen in presence of VO(acac)2, as the catalyst. Unlike other examples reported in the literature, this approach allows obtaining well structured spin-coated films without the necessity of further processes, such as annealing or exposition to solvent vapors. Direct current-voltage measurements, performed in planar and transverse configuration on 1 μm thick films, show both ohmic and space charge limited current behavior, at low and high applied fields, respectively. Due to the film ordered structure, a significant electrical anisotropy was found. In order to deeply investigate the basic conduction mechanisms, ac measurements have been also carried out in the frequency range between 100 Hz and 100 kHz. Finally, direct current and alternating current conductivity temperature dependence is discussed in the framework of thermally activated hopping and tunneling models.

Analysis of Carrier Injection into Pentacene FET Using Maxwell-Wagner Model

To clarify the carrier transport mechanism of pentacene field-effect transistors (FETs), the FET characteristics was examined in the region where the drain-source voltage V ds is lower than the saturated voltage. The I ds -V ds characteristics shows the ohmic behavior in the low voltage region, whereas it shows the characteristics explained by the Maxwell-Wagner model. This result clearly indicates that carrier injection from source makes a significant contribution to the carrier transport. It was also shown that the change of I ds -V ds characteristics from the ohmic behavior to the Maxwell-Wagner behavior is similar with the I-V characteristics change from the ohmic to the space charge limited current behavior observed in metal-organic film-metal junctions, including metal-pentacene-metal.

The nature of the electrical conduction and light emitting efficiency in organic semiconductors layers: The case of [m-MTDATA] – [NPB] – Alq3 OLED

In this work, a detailed study on the influence of different layers in an Alq3 [aluminium-tris(8-hydroxyquinoline)] based OLED is made. Two hole transport layers have been used: m-MTDATA [4,4′,4″-tris (3-methyl-phenylphenylamino) triphenylamine] and NPB [N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4″-diamine]. It is found that the efficiency (at 520nm) can increase up to one order of magnitude when we consider a three layer device instead of a conventional two layer. The driving voltage decreases in the three layer device, but the output electroluminescence spectra (in green spectral region) tend to be broaden towards high energies. The current–voltage behavior follows a space charge limited current behavior. The influence of the traps states and/or a Poole-Frenkel like field mobility dependence is discussed. The electroluminescence data fit to a large green band (one main green band and a small orange band) and for the three layer device a small blue band appears. Finally, a model involving an individual layer contribution with the corresponding trap levels is proposed.

Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells

Electrical and optical properties of poly(3-hexylthiophene-2.5diyl) (P3HT) used as the main component in a polymer/fullerene solar cell were studied. From the study of space-charge limited current behavior of indium-tin-oxide (ITO)/P3HT/Au hole-only devices, the hole mobility and density were estimated to range from 1.4×10−6 cm2/V s and 5.3×1014 cm−3 at 150 K to 8.5×10−5 cm2/V s and 1.1×1015 cm−3 at 250 K, respectively. The highest occupied to lowest occupied molecular orbital energetic difference was estimated from absorption spectrometry to be about 2.14 eV. Strong quenching of photoluminescence when the polymer was mixed with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), provided evidence of photoinduced charge transfer from P3HT to PCBM. Characterization of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells was done by analyzing the dependence of current density–voltage characteristics on temperature and illumination intensity. The main solar cell characteristics recorded at 300 K under 100 mW/cm2 white-light intensity were: Open-circuit voltage 0.48 V, current density 1.28 mA/cm2, with an efficiency of 0.2%, and fill factor of 30.6%. Open-circuit voltage decreased almost linearly with increasing temperature, while short circuit current density increased with temperature, saturating at around 320 K, and decreased thereafter. Power conversion efficiency and fill factor were maximum around 3 mW/cm2 due to the poor bulk transport properties of the active layer.

Electrical properties of SnSe 2 thin films

SnSe 2 thin films were prepared by thermal evaporation under vacuum at 10 -3 Pa on glass substrates, the films were n-type as detected by thermoelectric power measurements. The electrical resistivity and I– V characteristics were measured for thin films from 300 to 500 K and their thermal energy gap was found to be 1 eV decreasing to 0.96 eV, for films annealed at 500 K for 2 h. The I– V characteristics showed a space charge limited current behaviour and a trap level was detected at 80 meV from the edge of the conduction band which had a density of states N d =2×10 17 cm -3. The I– V and C– V characteristics were measured for the P–Si/n–SnSe 2 heterojunction, the forward current was composite of tunneling and interface recombination currents. The trap density state and depletion width were calculated.

Hysteresis I-V effects in short-channel Silicon MOSFET's

Hysteresis in I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</inf> -V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</inf> characteristics is observed at high drain voltages in short-channel silicon MOSFET's biased into the normally off regime, the degree of which depends on the substrate and gate biases. The MOSFET switches at this hysteresis point from subthreshold to space-charge limited current behavior. It is proposed that this hysteresis effect is due to avalanched holes which accumulate at the gate interface, causing a deformation of the potential distribution in the substrate and the triggering of the device into space-charge limited current behavior.

Electroluminescence in Amorphous Pyrazoline Films Under DC Fields

Abstract We have investigated the electroluminescence, EL, behavior in amorphous films of a pyrazoline derivative under dc fields. The current vs. voltage characteristics indicated a space charge limited current behavior with traps distributed exponentially within the forbidden energy gap. EL was detected in the current range of double carrier injection which ocurred after all traps in the sample were filled up. Spectral studies showed that the light emission zone was loalized around the cathode side of the pyrazoline film. The brightness, measured as a function of current, indicated the EL in the lower current density range to be dominated by delayed luminescence.