Space Charge Limited Current Research Articles

Corrections to “Electronic Conduction Mechanisms in Insulators” [Jan 18 223-230

In the above article, space-charge-limited current (SCLC) was modeled as involving an exponential trap distribution associated with the existence of conduction band tail states. This is no longer believed to be a viable SCLC mechanism; the poor quality of the insulators exhibiting SCLC makes it highly unlikely that the electron quasi-Fermi level can ever be modulated close enough to the conduction band edge that band tail states play any role in establishing SCLC. Instead, SCLC is now being ascribed to a Gaussian distribution of deep traps.

Molecular organized isoindigo based small molecules with terminal thermally cleavable protecting group

The potential of a new series of H-bonded isoindigo (ID) based small molecules as charge transporting materials was examined. The soluble precursor units contained two terminal isoindigo units bearing tert-butoxycarbonyl (Boc) solubilizing substituents. Thiophene, thienothiophene and/or diketo pyrrolopyrrole are used as π-conjugated spacers between the isoindigo units. The protected Boc-ID units are converted to the hydrogen-substituted counterparts by annealing the spin-coated precursor films up to 220 °C. The solid state molecular ordering after Boc thermal decarboxylation was investigated using thermogravimetric analysis, UV–vis spectroscopy and X-ray diffraction analysis. The thermogravimetric analysis confirmed the full removal of Boc groups. It was revealed that the formation of terminal intermolecular hydrogen bonds gave the ability to the deprotected materials to change their molecular ordering to edge-on or face-on orientation based on the spacer size and the π-conjugated length. The [-NH----OC] H-bonding dramatically improved the strength of the intermolecular interactions and induced a significant shift on the wavelength of annealed films. The hole mobilities of isoindigo based organic materials measured by the space charge-limited current (SCLC) method and showed a tenfold improvement after thermal decarboxylation.

Electronic Tunability and Mobility Anisotropy of Quasi-2D Perovskite Single Crystals with Varied Spacer Cations.

The organic-inorganic hybrid quasi-two-dimensional (quasi-2D) perovskites have attracted increasing attention for solar cell applications due to their improved moisture stability and excellent optoelectronic properties. To achieve a profound understanding and delicate control on the component of perovskite, it is indispensable to know how different spacer cations affect their intrinsic properties. Here, we synthesized a series of quasi-2D perovskite single crystals with different organic spacer cations and conducted a systematic investigation to correlate the optoelectronic behaviors to the spacer cations. We revealed the coupling effect between the π-electron (PEA+) and p orbital of I- from the inorganic framework, which changed the electronic configuration of resultant crystals leading to different band gap and carrier behavior. In particular, the anisotropic carrier mobility was probed quantitatively by space-charge limited current (SCLC) measurement, where PEA2MA2Pb3I10 exhibited the highest in-plane mobility due to the reduced exciton binding energy and the lowest out-of-plane mobility because of the widest organic barrier.

Boosting the efficiency of commercial available carbon-based perovskite solar cells using Zinc-doped TiO2 nanorod arrays as electron transport layer

Recently, the rapid development of efficient organic-inorganic halide perovskite solar cells (PSCs) has been witnessed extensively. In this study, the TiO2 nanorod arrays (NAs) with different molar ratios of zinc doping were employed as electron transport layer in carbon-based perovskite solar cells for enhancing photovoltaic performance. Zinc-doped TiO2 NAs as electron transport and TiO2 compacted electron blocking layer were simultaneously grown on the fluorine-doped tin oxide glass. Zn-doped TiO2 NAs show the vertical growth on FTO surface with good crystallinity which provides the direct pathway for the perovskite penetration and the better carrier transfer performance. The surface morphology, crystal structure, absorption and electrical conductivity of Zn-doped TiO2 NAs are investigated methodically. The reduced contact angle shows the enhanced affinity of Zn-doped TiO2 with perovskite. Study shows that Zn-doped TiO2 based PSC device has higher shunt resistance and low series resistance and longer carrier lifetime when compared with pristine sample. The space-charge-limited current (SCLC) measurement demonstrates that 2.5 mol% Zn-doped TiO2 NAs ETL exhibits higher electron mobility than that of pristine TiO2 NAs. The increased in Jsc of the Zn-doped TiO2 NAs based cells is mainly ascribed to the reduced trap-state density and enhanced electrical conductivity. As compared with the flat band potential (VFB) of pristine sample, the VFB of 2.5 mol% Zn-doped TiO2 NAs has a negative shift of 0.33V, thus resulting in the positive shift of conduction band and better band alignment with perovskite layer. The power conversion efficiency was improved from 12.99% to 14.45% with the optimized zinc doping concentration of 2.5 mol%. Furthermore, the optimized cell without encapsulation still maintains 85% of original efficiency via 30 days stability test.

New Extraction Technique of In-Gap Electronic-State Spectrum Based on Time-Resolved Charge Extraction.

The in-gap electronic state (trap state) is an important factor that determines the photovoltaic performance of solar cells. In this article, we put forward a new technique for extracting the density of trap state (DOST) distribution based on the time-resolved charge extraction (TRCE) experiment result. Based on strict derivation, we find that when the TRCE result is linear, the extracted DOST distribution is exponential type and vice versa. Compared to the approach given by Wang et al., the method introduced in this paper is more accurate and reliable. Compared to the approach based on the space charge-limited current (SCLC) experiment result, our method needs less computation.

Conductive filament evolution dynamics revealed by cryogenic (1.5 K) multilevel switching of CMOS-compatible Al2O3/TiO2 resistive memories

Non-volatile resistive switching devices are considered as prime candidates for next-generation memory applications operating at room temperature and above, such as resistive random-access memories or brain-inspired in-memory computing. However, their operability in cryogenic conditions remains to be mastered to adopt these devices as building blocks enabling large-scale quantum technologies via quantum–classical electronics co-integration. This study demonstrates multilevel switching at 1.5 K of Al2O3/TiO2-x resistive memory devices fabricated with complementary metal-oxide-semiconducto-compatible processes and materials. The I–V characteristics exhibit a negative differential resistance (NDR) effect due to a Joule-heating-induced metal-insulator transition of the Ti4O7 conductive filament. Carrier transport analysis of all multilevel switching I–V curves show that while the insulating regime follows the space charge limited current (SCLC) model for all resistance states, the conduction in the metallic regime is dominated by SCLC and trap-assisted tunneling for low- and high-resistance states respectively. A non-monotonic conductance evolution is observed in the insulating regime, as opposed to the continuous and gradual conductance increase and decrease obtained in the metallic regime during the multilevel SET and RESET operations. Cryogenic transport analysis coupled to an analytical model accounting for the metal-insulator-transition-induced NDR effects and the resistance states of the device provide new insights on the conductive filament evolution dynamics and resistive switching mechanisms. Our findings suggest that the non-monotonic conductance evolution in the insulating regime is due to the combined effects of longitudinal and radial variations of the Ti4O7 conductive filament during the switching. This behavior results from the interplay between temperature- and field-dependent geometrical and physical characteristics of the filament.

Resistive-switching tunability with size-dependent all-inorganic zero-dimensional tetrahedrite quantum dots

All-inorganic zero-dimensional (0D) tetrahedrite (Cu12Sb4S13, CAS) quantum dots (QDs) have attracted extensive attention due to their excellent optical properties, bandgap tunability, and carrier mobility. In this paper, various sized CAS QDs (5.1, 6.7, and 7.9 nm) are applied as a switching layer with the structure F:SnO2 (FTO)/CAS QDs/Au, and in doing so, the nonvolatile resistive-switching behavior of electronics based on CAS QDs is reported. The SET/RESET voltage tunability with size dependency is observed for memory devices based on CAS QDs for the first time. Results suggest that differently sized CAS QDs result in different band structures and the regulation of the SET/RESET voltage occurs simply and effectively due to the uniform size of the CAS QDs. Moreover, the presented memory devices have reliable bipolar resistive-switching properties, a resistance (ON/OFF) ratio larger than 104, high reproducibility, and good data retention ability. After 1.4 ×106 s of stability testing and 104 cycles of quick read tests, the change rate of the ON/OFF ratio is smaller than 0.1%. Furthermore, resistive-witching stability can be improved by ensuring a uniform particle size for the CAS QDs. The theoretical calculations suggest that the space-charge-limited currents (SCLCs), which are functioned by Cu 3d, Cu 3p and S 3p to act as electron self-trapping centers due to their quantum confinement and form conduction pathways under an electric field, are responsible for the resistive-switching effect. This paper demonstrates that CAS QDs are promising as a novel resistive-switching material in memory devices and can be used to facilitate the application of next-generation nonvolatile memory.

Leakage Current Reduction in β-Ga2O3 Schottky Barrier Diodes by CF4 Plasma Treatment

This letter reports on the suppression of reverse leakage current (Ir) in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Schottky barrier diodes (SBDs) through Schottky barrier modification by a low power CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -plasma treatment prior to Schottky metal deposition. Revealed by an X-ray photoelectron spectroscopy (XPS) analysis, the fluorine-plasma treatment brought an incorporation of fluorine ions and depletion of silicon donors in the near surface region of the β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and thus raised its surface potential by around 0.14 eV. Furthermore, insulating GaFx was likely created at the Schottky interface. Attributed to the fluorine-plasma modified Schottky barrier, a reduced Ir by around four orders of magnitude and enhanced blocking voltage (Vblock) from 150 V to 470 V at Ir = 100μ A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have been achieved without degrading the forward characteristics. Different from the untreated device whose Ir was purely governed by the thermionic field emission (TFE), the fluorine-plasma-treated SBD showed a greatly suppressed TFE-current until a space-chargelimited current (SCLC) started to dominate at around -500 V.

Manganese doped eco-friendly CuInSe2 colloidal quantum dots for boosting near-infrared photodetection performance

CuInSe2 (CISe) colloidal quantum dots (QDs) display promising applications in photodetection especially within near-infrared (NIR) regions due to their high extinction coefficient and environmental-friendly. However, the high trap density and poor crystal quality introduced by the ternary structure result in low photodetection of CISe QDs devices. Herein, we dope transition metal manganese ions (Mn2+) into CISe QDs to tackle the above problems. Structural characterization results demonstrate the crystal quality of CISe QDs is improved by doping Mn2+ during the synthesis of QDs. The transient absorption (TA) spectroscopic study together with the space-charge-limited current (SCLC) measurements show the charge carrier lifetime of Mn-CISe QDs is much longer than that of the CISe QDs, due to the Mn2+ doping state serve as hole capturer forming a charge-compensated pair with the Cu2+ defect that makes the long-lived Cu2+ radiative recombination dominate. Furthermore, Mn2+ doping concurrently modifies the conduction band minimum and valence band maximum level of the QDs verified by the ultraviolet photoelectron spectroscopy (UPS), which determines the driving force for charge carrier transfer to acceptors. The optimal Mn2+ doping level (0.01 Mn:Cu feed ratio) balanced the above two factors in the QDs. The detector based on such Mn-CISe QDs exhibits responsivity of 30 mA/W and specific detectivity of 4.2 × 1012 Jones at near-infrared wavelength, the response speed of 0.76 µs, and suppressed dark current density of 1.6 × 10−10 A cm−2.

Electrical and photocatalytic properties of composites of manganese and titanium oxides

In this study, mesoporous manganese oxide-titanium dioxide (Mn2O3/TiO2) nanocomposites have been synthesized following sol-gel method. Different molar ratios of Mn/Ti were evaluated and MT2 (5:1) showed the best photoactivity as well as carrier transport properties. The Langmuir–Hinshelwood model was delineated to calculate the reaction rate which was found to be highest for MT2 (min-1). Several parameters including pore size distribution, pore volume, adsorption isotherms have been analyzed as per Brunauer–Emmett–Teller (BET) method. That was attributed to the high surface area (121.81 m2/g). The space charge limited current (SCLC) theory was employed to calculate the mobility and transit time. MT2 showed the highest carrier mobility (326×10–7 m2 V-1s-1) and lowest transit time (0.08×10–5 s) which are desirable for efficient photoactivity.

Nanoscale mobility mapping in semiconducting polymer films

Local electrical properties of thin films of the polymer PTB7 are studied by conductive atomic force microscopy (C-AFM). Non-uniform nanoscale current distribution in the neat PTB7 film is revealed and connected with the existence of ordered PTB7 crystallites. The shape of local I-V curves is explained by the presence of space charge limited current. We modify an existing semi-empirical model for estimation of the nanoscale hole mobility from our experimental C-AFM measurements. The procedure of nanoscale charge mobility estimation was described and applied to the PTB7 films. The calculated average C-AFM hole mobility is in good agreement with macroscopic values reported for this material. Mapping of nanoscale hole mobility was achieved using the described procedure. Local mobility values, influenced by nanoscale structure, vary more than two times in value and have a root-mean-square value 0.22 × 10−8 m2/(Vs), which is almost 20% from average hole mobility.

Structural and Electrical Investigation of Porous GaAs Layers on Different Crystallographically Oriented GaAs Substrates

In this paper, electrochemical anodization was used to fabricate porous layers on n-type GaAs substrates with different orientations, i.e., (511)A, (100) and (111)B, under the same experimental conditions. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and x-ray diffraction (XRD) were used to characterize the formed porous layers, with respect to the substrate orientation. Results obtained from SEM and AFM data are evidence of the formation of GaAs nanocrystallites and reveal that pores grow along definite crystallographic directions. XRD analysis confirm the single crystalline structure of porous layers and a relative peak shift as compared to GaAs, indicating a change in the lattice constant due to strain effects. The photoluminescence spectra of the porous layers showed red PL bands attributed to quantum confinement effects in GaAs nanocrystallites. The investigated current–voltage (I–V) in the forward bias region demonstrates that the transport mechanism in the Au/porous-GaAs/GaAs structures was governed by space-charge-limited current (SCLC). In fact, non-saturating behavior was observed in the reverse bias I–V characteristics due to both the effective barrier height dependence on the electric field and the thermionic field emission.

Modelling the photocatalytic behaviour of p-n nickel-titanium oxide nanocomposite

NiO-TiO2 (TN) nanocomposite is synthesized from an assembly of p-type nickel oxide (NiO) and n-type titanium oxide (TiO2) in an ultrasound assisted operation. Transmission electron microscopy (TEM) demonstrates the existence of hexagonal particles in the nanocomposite. Dynamic light scattering (DLS) measurements show stable TN nanoparticles (NPs) at a negative zeta potential (−18.5 ± 0.8 mV). The nanocomposite is tested for its catalytic activity towards degrading malachite green (MG), a known water toxicant and methylene blue (MB) under ultraviolet (UV) irradiation. Response surface methodology (RSM) is employed to optimise the influence of significant variables such as initial dye concentration, catalyst dose and time. Their mutual interactions are mapped by response surface and contour plots and correlated with degradation process by a designed model. The best photocatalytic efficiency (87%) is observed at an optimised concentration of 5 ppm dye with 10 ppm catalyst. The presence of inorganic ions and organic matter hardly affected the aggregate size of TN but caused a decline in photoactivity. The catalyst is found effective even in real water system (Hooghly River). A (Fluorine doped tin oxide) FTO/TN/Al heterojunction is fabricated. TN showed enhanced carrier mobility (1.03.10−4 m2V-1s-1) and low transit time (1.76.10-6 s) evaluated using space charge limited current (SCLC) theory. The nanocomposite appears suitable for energy preservation and environmental applications.

Transparent Bipolar Thin Film Transistors Based on Topological Insulator of Magneto-Resistive Effect.

Thin film transistors using SiOC insulation film generated a very large IDS current with tunneling phenomena due to the occurrence of space charge limit current due to magnetic resistance effect. When SiOC gate insulation barrier transistor characteristics were investigated, transmission characteristics of bi-directional were observed, low current at levels ~10-7 A, but switching characteristics were superior to or greater than 10-10 A. SiOC Thin Film Insulation Characteristics are a good material for observing magnetic resistance effects, SiOC Transistors without channels clearly demonstrate the difference in spatial charges due to electromagnetic phenomena, and even in low-current insulation, nano-current flows at high speed. The current due to magnetic resistance is bi-directional and has an alternating current that flows in both directions at 0 V.

Sonochemical synthesis of nanospherical TiO2 within graphene oxide nanosheets and its application as a photocatalyst and a Schottky diode

The hybrid of graphene oxide (GO) nanosheets with well embedded nanospherical TiO2 was prepared through mechanical mixing using ultrasound. This modified TiO2-GO (TGO) composite exhibits reduced recombination of charge carriers. Benefiting from unique morphology and surface area (92 m2g−1), the TGO composite showed remarkable catalytic activity for the photodegradation of congo red (CR) and methyl orange (MO). The rate constant values were 0.0652 min−1 (pseudo first order) for CR (5 ppm) and 0.0631 min−1 for MO (5 ppm). The stability and aggregation behavior of TGO was examined using the dynamic light scattering (DLS) technique. The key parameter affecting photocatalytic performance is electron-hole separation. To illustrate this issue, a Schottky diode has been fabricated with TGO composite, in contact with aluminium. Important diode parameters i.e. ideality factor, series resistance, barrier height have been estimated from forward current-voltage (I-V) characteristics. Space charge limited current (SCLC) theory was employed to provide insight into the carrier transport properties of TGO and enhanced carrier mobility (2.11.10−6 m2V−1s−1) and low transit time (0.237.10−6 s) under illumination was obtained. The composite appears suitable for energy and photocatalytic applications.

Space Charge-Limited Current Transport Mechanism in Crossbar Junction Embedding Molecular Spin Crossovers.

Spin crossover complexes are among the most studied classes of molecular switches and have attracted considerable attention for their potential technological use as active units in multifunctional devices. A fundamental step toward their practical implementation is the integration in macroscopic devices adopting hybrid vertical architectures. First, the physical properties of technological interest shown by these materials in the bulk phase have to be retained once they are deposited on a solid surface. Herein, we describe the study of a hybrid molecular inorganic junction embedding the spin crossover complex [Fe(qnal)2] (qnal = quinoline-naphthaldehyde) as an active switchable thin film sandwiched within energy-optimized metallic electrodes. In these junctions, developed and characterized with the support of state of the art techniques including synchrotron Mössbauer source (SMS) spectroscopy and focused-ion beam scanning transmission electron microscopy, we observed that the spin state conversion of the Fe(II)-based spin crossover film is associated with a transition from a space charge-limited current (SCLC) transport mechanism with shallow traps to a SCLC mechanism characterized by the presence of an exponential distribution of traps concomitant with the spin transition temperature.

Organic heterojunctions of phthalocyanine-reduced graphene oxide above percolation threshold for photovoltaic application

In this article, non-fullerene, cost effective, and highly soluble zinc tetra-tert-butyl phthalocyanine-reduced graphene oxide (ZnTTBPc-RGO) heterojunction have been used as an active layer in organic photovoltaic devices. The RGO content in the active material is varied from 0.25–0.5 volume fractions which are above the percolation threshold of 0.167 volume fraction of RGO. Temperature dependent current density–voltage characteristics show a transition from Ohmic conduction to space charge limited current (SCLC) conduction at higher applied bias voltages (≥1 V). This enables us to extract charge carrier mobility values of the heterojunction at different temperatures using a trap-free SCLC model. A donor–acceptor solution processable photovoltaic device with 0.33 vol.% RGO content of active layer show the highest power conversion efficiency of 1.02%, highest recombination resistance of 48.65 kΩ, and maximum carrier mobility of 4.32 × 10−4 cm2V−1s−1 among the three devices studied. 80 h exposure to ambient conditions see a ∼40% decay of power conversion efficiency. Quantitative analysis on the density of trap states indicates that the best power conversion efficiency and the highest charge carrier mobility correlate well with a minimum of the trap density and a maximum of recombination resistance for these devices.

Extraction current transients for mobility determination—A comparative study

In this work, we measure the hole mobility in the model polymer system poly(3-hexylthiophene-2,5-diyl) by using different measurement techniques. Our main purpose is to determine how the recently developed charge extraction by a linearly increasing voltage technique for metal–insulator–metal devices (MIM-CELIV) compares to other commonly used methods, such as space charge limited currents and time-of-flight. Our findings suggest that the MIM-CELIV technique gives a slightly lower mobility than the other techniques, which is understandable given that the method measures the mobility of relaxed charge carriers in the dark unlike, for example, time-of-flight where charge carriers are photogenerated. In addition, we scrutinize the accuracy and reliability of the techniques used, showing that differences in mobility smaller than a factor of two are not detectable unless statistics from many devices are available.

Space-charge limited current with a finite injection velocity revisited

The problem of space-charge limited current flow between two electrodes with a finite particle injection velocity is revisited theoretically, and numerically with particle-in-cell (PIC) simulations. Using an Eulerian approach, we obtain a complete analytical solution for both the space-charge limited (SCL) current (which agrees with previously derived expressions), as well as the spatial profiles of all physical quantities. These analytical solutions are unique in that they apply to both fluid, and kinetic particle descriptions of the system, and we demonstrate how they can be used to verify the accuracy and correctness of certain aspects of numerical simulation codes. For injection currents above the SCL limit, the PIC simulations show that the system becomes unstable, and self-excited space charge oscillations are generated with a period of the order of the particle transit time between the electrodes. In this oscillatory regime, the maximum time-averaged current that can be transmitted depends on the ratio of the particle injection energy to the applied potential difference across the electrodes, and is always less than the SCL current. By considering a finite particle injection temperature, these oscillations can be damped for sufficiently high temperatures, and we show that a new non-oscillatory steady-state solution can be obtained.

Temperature dependent I–V characterization of Coronene/p-Si based heterojunctions: Space charge limited current, Schottky emission at high voltages, thermionic emission and pool-Frenkel emission at low voltages

A Heterojunction of Au/Coronene/p-Si/Al was fabricated by thermal evaporation technique. The features of current-voltage (I–V) were examined under dark condition at several temperatures in 292–373 K range. Extensive analysis of I–V curves in forward bias demonstrated that the mechanisms of thermionic emission and space charge limited currents represent the dominant mechanisms in the low and high range of used voltages, respectively. The reverse I–V curves were interpreted via Poole– Frenkel and Schottky effects. Some specific parameters were estimated as diode ideality factor, series resistance and shunt resistance. The photovoltaic manner of Coronene/p-Si heterojunction was shown under illumination. The cell parameters were evaluated from the analysis; open circuit voltage, short circuit current and fill factor.