Space Charge Limited Current Research Articles

TCNQ molecular semiconductor of the Cu(II)TAAB macrocycle: Optical and electrical properties

The present study reports the doping of a semiconducting molecular material through the formation of hydrogen bonds between the macrocycle Cu(II)(TAAB) and the electronic acceptor TCNQ. According to density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) analysis, the doped compound has the shape of a distorted square pyramid, with four nitrogen atoms in the equatorial position and the apical oxygen atom from the water ligands. These water molecules can generate strong hydrogen bonds with TCNQ and the TAAB metallic complex. Thin films of copper molecular material were obtained through high vacuum evaporation and were structurally characterized by IR spectroscopy, EPR and scanning electron microscopy (SEM). Additionally, the absorption coefficient (α) and photon energy (hν) were calculated from UV–vis spectroscopy and used to determine the optical activation energy in each film, from which its semiconducting behavior was established. An important aspect to consider is that the presence of hydrogen bonds is essential to establish the semiconducting nature of these species; this chemical behavior, as well as the resulting electronic mobility, have been studied by DFT theoretical calculations, which reinforce the experimental conclusion of a relationship between Cu(II)TAAB and TCNQ moieties generated by a weak bond. Finally, I–V characteristics have been obtained from a glass/ITO/doped molecular semiconductor/Ag device using Ag and ITO electrodes. Results for the copper-based device show that, at low voltages, the conduction process is of an ohmic nature while, at higher voltages, space-charge-limited current (SCLC) is found. It is highly probable that the doping effect in TCNQ favors electronic transport due to the formation of conduction channels caused by dopant-favored anisotropy.

Equivalent circuit analysis of Al/rGO-TiO2 metal-semiconductor interface via impedance spectroscopy: Graphene induced improvement in carrier mobility and lifetime

A metal-semiconductor (MS) contact often gives rise to a Schottky barrier junction and is immensely important in electronic devices. Recently, graphene and its nanocomposites have attracted interest for their tremendous potential in schottky barrier diodes (SBDs). To realize a high performance SBD, detail characterization of the MS interface is of utmost importance. In this regard, here we employ impedance spectroscopy (IS) as a simple yet powerful technique for the equivalent circuit analysis and characterization of Al/reduced graphene oxide(rGO)-TiO2 interface. Al/rGO-TiO2 SBDs are fabricated with different weight ratios of rGO (rGO:TiO2 = 0, 1:50, 1: 30, 1:15) in the composite and IS analysis is performed for all the SBDs. Built-in potential, charge carrier density, depletion layer width and barrier height of the diodes are extracted from capacitance-voltage measurements. Moreover, we obtain the charge career lifetime, mobility and diffusion length based on the equivalent circuit model. Current-voltage measurement is also performed and mobility values from IS are further verified by space charge limited current (SCLC) measurements. The best device performance and charge transport properties was exhibited by rGO:TiO2 = 1:15. The carrier mobility increased by almost 2.6 times compared to pure TiO2, while the lifetime and diffusion length improved by 113% and 130% respectively. In short, we demonstrate the equivalent circuit analysis for the investigation of Al/rGO-TiO2 MS interface and successfully implement the IS model to determine charge transport parameters. This study reveals the beneficial impact of graphene on device performance and establish the huge potential of IS technique for characterization of MS devices.

Application of CaCu3Ti4O12 based quadruple perovskites as a promising candidate for optoelectronic devices

We report the synthesis of nanosized (40–50 nm) CaCu3-xMnxTi4-xMnxO12 (x = 0, 0.5 and 1) quadruple perovskite (QP) semiconductor via a modified combustion method for use as Schottky barrier diode (SBD) at the Al/QP junction. The fabricated SBD is analysed on the basis of thermionic emission theory to observe its quality and some important diode parameters. For insight analysis of charge transport mechanism through metal-semiconductor junction, theory of space charge limited currents is applied and discussed in the light of parameters like carrier concentration, mobility-lifetime product and diffusion length. The Mn-doped exhibit better device performance compared to parent material.

Origin of negative resistance in anion migration controlled resistive memory

Resistive random access memory (RRAM) is one of the most promising emerging nonvolatile technologies for the futuristic memory devices. Resistive switching behavior often shows negative resistance (NR), either voltage controlled or current controlled. In this work, the origin of a current compliance dependent voltage controlled NR effect during the resetting of anion migration based RRAM devices is discussed. The N-type voltage controlled NR is a high field driven phenomena. The current conduction within the range of a certain negative voltage is mostly dominated by space charge limited current. But with the higher negative voltage, a field induced tunneling effect is generated in the NR region. The voltage controlled NR is strongly dependent on the compliance current. The area independent behavior indicates the filamentary switching. The peak to valley ratio (PVR) is > 5. The variation of PVR as a function of the conduction band offset is achieved. Compared to other reported works, based on the PVR, it is possible to distinguish the RRAM types. Generally, due to the higher electric field effect on the metallic bridge during RESET, the electrochemical metallization type RRAM shows much higher PVR than the valance change type RRAM.

A thermally cross-linked hole-transporting film with the remarkable solvent resistance for solution-processed OLEDs

Abstract In this work, we report a systematic study of the properties of the thermal cross-linkable triarylamine based hole-transporting material (HTM) N,N′-di-p-tolyl-N,N′-bis(4-vinylphenyl)-[1,1′-biphenyl]-4,4′-diamine (V-p-TPD) and the distinctive nature of its cross-linked thin films fabricated by solution process under mild thermal polymerization without any initiator. The cross-linked V-p-TPD film exhibited remarkable solution resistance to common solute such as chlorobenzene, toluene, chloroform and tetrahydrofuran. The cross-linked V-p-TPD film showed better morphological stability than vacuum-deposited film of its monomer TPD. The space-charge-limited current (SCLC) measurements revealed that the cross-linked V-p-TPD film afforded comparable hole mobility with vacuum-deposited TPD film. Finally, we applied the cross-linked V-p-TPD film in organic light-emitting diodes (OLEDs) to evaluate its performance in all-solution-processed OLEDs.

Fatigue behavior of resistive switching in a BiFeO3 thin film

The change of the resistive switching effect in an Au/BiFeO3/SrRuO3 structure under repetitive switching was studied. The current–voltage characteristics indicated space-charge-limited (SCL) conduction. The transition voltage from ohmic to SCL, voltage of the trap-filled limit, and resistance increased with switching cycle number in the wake-up stage. Such changes were attributed to the increase of trap density caused by the release of polarization domains. Ohmic and SCL currents increased with switching cycle number after fatigue, indicating that charge carrier density increased because of oxygen vacancy generation. The change of transition voltage from ohmic to SCL showed a consistent trend.

Quantitative analysis of charge trapping and classification of sub-gap states in MoS2 TFT by pulse I–V method

The threshold voltage instabilities and huge hysteresis of MoS2 thin film transistors (TFTs) have raised concerns about their practical applicability in next-generation switching devices. These behaviors are associated with charge trapping, which stems from tunneling to the adjacent trap site, interfacial redox reaction and interface and/or bulk trap states. In this report, we present quantitative analysis on the electron charge trapping mechanism of MoS2 TFT by fast pulse I–V method and the space charge limited current (SCLC) measurement. By adopting the fast pulse I–V method, we were able to obtain effective mobility. In addition, the origin of the trap states was identified by disassembling the sub-gap states into interface trap and bulk trap states by simple extraction analysis. These measurement methods and analyses enable not only quantitative extraction of various traps but also an understanding of the charge transport mechanism in MoS2 TFTs. The fast I–V data and SCLC data obtained under various measurement temperatures and ambient show that electron transport to neighboring trap sites by tunneling is the main charge trapping mechanism in thin-MoS2 TFTs. This implies that interfacial traps account for most of the total sub-gap states while the bulk trap contribution is negligible, at approximately 0.40% and 0.26% in air and vacuum ambient, respectively. Thus, control of the interface trap states is crucial to further improve the performance of devices with thin channels.

Conduction behavior of polyaniline/elastomer composites and the influence of carbon black addition

Three different compositions of EPDM, polyaniline (PANI) and carbon black have been studied with respect to conduction properties. It could be concluded that both carbon black and polyaniline influence the low field conductivity. In addition, the influence of different conduction mechanisms was studied. Measurements on the temperature dependence of the electrical conductivity indicated that large amount of polyaniline in EPDM rubber manifested in 1D variable range hopping (1D-VRH) behavior whereas less amount, although still well above the percolation threshold, resulted in behavior more similar to the Arrhenius law. However, further studies are necessary to draw definitive conclusions on the influence of filler content. Studying the field dependent conductivity behavior it could be concluded that there is a Ohmic region at low field and a transition towards space charge limited current (SCLC) at higher field with possible influence of other conduction mechanisms.

Tunable Band Gap and Long Carrier Recombination Lifetime of Stable Mixed CH3NH3PbxSn1–xBr3 Single Crystals

The mixed metal Pb/Sn halide perovskites have drawn significant attentions in perovskite photovoltaics due to their broad absorption spectra and tunable band gaps. To obtain a deeper understanding of these materials properties, single crystals are regarded as the best platform among various building blocks for fundamental study. Here, we report the mixed-metal MAPbxSn1–xBr3 (MA = CH3NH3) perovskite single crystals grown by top seeded solution growth (TSSG) method. Systematical characterizations were applied to investigating their structures and optoelectronic properties. These single crystals kept higher stability even exposed to air over one month than that of MASnBr3. The outstanding electrical properties, such as lower trap-state density and higher carrier mobility, were investigated by space charge-limited current (SCLC) and the Hall Effect measurements. More importantly, these perovskite single crystals exhibited much narrower optical band gap (1.77 eV) and longer carrier lifetime (∼2 μs) than those ...

Improving the Optoelectronic Properties of Mesoporous TiO2 by Cobalt Doping for High-Performance Hysteresis-free Perovskite Solar Cells.

We for the first time report the incorporation of cobalt into a mesoporous TiO2 electrode for application in perovskite solar cells (PSCs). The Co-doped PSC exhibits excellent optoelectronic properties; we explain the improvements by passivation of electronic trap or sub-band-gap states arising due to the oxygen vacancies in pristine TiO2, enabling faster electron transport and collection. A simple postannealing treatment is used to prepare the cobalt-doped mesoporous electrode; UV-visible spectroscopy, X-ray photoemission spectroscopy, space charge-limited current, photoluminescence, and electrochemical impedance measurements confirm the incorporation of cobalt, enhanced conductivity, and the passivation effect induced in the TiO2. An optimized doping concentration of 0.3 mol % results in the maximum power conversion efficiency of 18.16%, 21.7% higher than that of a similar cell with an undoped TiO2 electrode. Also, the device shows negligible hysteresis and higher stability, retaining 80.54% of the initial efficiency after 200 h.

Electrical and noise properties of proton irradiated 4H-SiC Schottky diodes

The current voltage characteristics and the low-frequency noise in high voltage 4H-SiC junction barrier Schottky diodes irradiated with high energy (15 MeV) protons were studied at different temperatures and irradiation doses Φ from 3 × 1012 cm−2 to 1 × 1014 cm−2. Irradiation led to the increase of the base resistance and the appearance of slow relaxation processes at small, V ≤ 0.2 V, and at rather high, V ≥ 2 V, forward voltages. The characteristic times of these relaxation processes ranged from ∼1 μs to 103 s. The exponential part of the current-voltage characteristic was only weakly affected by irradiation. The temperature dependence of the base resistance changed exponentially with temperature with activation energy Ea ∼ 0.6 eV, indicating that the Z1/2 level plays a dominant role in this process. The temperature increase also led to the increase of the ideality factor from 1.05 at 25 °C to 1.1 at 172 °C. At elevated temperatures and high forward voltages V > 2–4 V, the current voltage characteristics tend to be super-linear. It is concluded that at high voltages, the space charge limited current of majority carriers (electrons) and hole injection from the p-n regions play an important role in the formation of the current voltage characteristic. The frequency dependences of noise spectral density S of proton irradiated Schottky diodes have the unusual form of S ∼ 1/f 0.5.

Electronic Conduction Mechanisms in Insulators

The current density–electric field ${(}{J}-\xi {)}$ characteristics of four insulators of dramatically different electrical qualities are assessed in terms of their operative electronic conduction mechanisms. Conduction in the two high-quality insulators is dominated by Ohmic conduction and Fowler–Nordheim tunneling, whereas conduction in the two low-quality insulators involves Ohmic conduction and space-charge limited current (SCLC). Ohmic conduction and SCLC are somewhat puzzling mechanisms for contributing to insulator leakage current since they require the existence of an Ohmic contact at the cathode. Our conventional understanding of an Ohmic contact makes it difficult to ascertain how an Ohmic contact could be formed to a wide bandgap insulator. This Ohmic contact dilemma is resolved by formulating an equivalent circuit appropriate for assessing the ${J}-\xi $ characteristics of an insulator and then recognizing that an insulator Ohmic contact is obtained when the injection-limited current density from the cathode electrode is greater than that of the operative bulk-limited current density, i.e., Ohmic or SCLC for the four insulators under consideration.

XXII Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 12-15 марта 2018 г. Эффект поля в пленках PbSnTe : In с низкой проводимостью в режиме инжекции из контактов и ограничения тока пространственным зарядом

AbstractAt T = 4.2 K, a strong change (up to 4 times) in the space-charge-limited current (SCLC) is observed for the first time at samples based on semi-insulating PbSnTe:In films grown by molecular-beam epitaxy on BaF_2(111) substrates. The obtained results agree with experiments on the effect of treatment of the surface of PbSnTe:In films on the space-charge-limited current with a variation in the current to 10^3 times or more. At a qualitative level, the model is considered which assumes the substantial contribution of localized surface states to space charge formed in the mode of space-charge-limited current due to the injection of charge carriers from contacts.

Highly efficient polyacetylene–based polyelectrolytes as cathode interfacial layers for organic solar cell applications

Abstract For application to inverted organic solar cells (IOSCs), three polyacetylene-based polyelectrolytes, poly(N-alkyl/aryl-2-ethynylpyridinium bromide) (P1-P3), were incorporated as cathode interfacial layers (CILs) between the ZnO and one of the following two photoactive layers: poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) or poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th):PC71BM. This interfacial modification of the IOSCs by the CIL insertion enhanced their performance with an increase in power conversion efficiency (PCE) from 7.46% for a ZnO-based control device to 8.29% for PTB7 and to 9.37% for PTB7-Th. Characterizations of the IOSC devices by using electrical impedance spectroscopy (EIS), space-charge-limited current (SCLC) electron mobility, dark current density-voltage (J-V) curves, and film surface morphology study revealed that the CIL incorporation suppressed the charge recombination and thus facilitated charge extraction, as evidenced by the observed improvement of open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF). The devices with CILs showed higher electron mobility, in line with the higher FF and reduced series resistance. These CILs with ZnO exhibited significantly enhanced PCE and also served as an internal shield against oxygen contamination and humidity, which further increased the device stability.

Observation of space-charge-limited current in AlGaN/GaN ultraviolet light-emitting diodes

AlGaN-containing ultraviolet light-emitting diodes (UV LEDs) emitting at 285 nm show abnormal diode characteristics below the sub-threshold input voltage region, indicating the existence of a space-charge-limited (SCL) current. Distinct transitions from Ohmic to SCL and further diode currents are observed in UV LEDs, unlike GaInN-based LEDs. Under varied temperatures, the transition voltages from Ohmic to SCL currents and the exponents of the power functions in the current–voltage dependency are influenced by the deactivated trap density in the AlGaN film, indicating that the charge injection and compensation limit for the current within the films in the low-voltage regimes for nitride-based UV LEDs.

Electric leakage current density in phase field simulations for nanogenerator concepts

AbstractFor the purpose of energy harvesting on the nanoscale with ferroelectric barium titanate (BaTiO3), our nanogenerator concept transforms parasitic mechanical oscillations into usable electric energy. Experimental difficulties occur in sample preparation, e.g., surface roughness, bonding contact, and leakage currents. Latter can be considered in our finite element phase field model, such that the nanogenerator concept can be optimized. Leakage current density is implemented from different phenomenological approaches. First, Ohm's law represents a linear relation between leakage and electric field. Second, the Space‐Charge‐Limited Current (SCLC) relation assumes a quadratic dependency on the electric field. So far, SCLC is suitable for one‐dimensional problems, however, in two or three dimensions it is not found in literature. Therefore, we discuss a reformulation to bring into account that the electrical field E is a vector out of ℝ3 in our model. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Axial p–n junction and space charge limited current in single GaN nanowire

The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p–n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p–n junction NWs grown by plasma-assisted molecular beam epitaxy. I–V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p–n junction NW. Thanks to an improved contact process, both the electric field induced by the p–n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p–n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116–125 nm. Following our previous work, the acceptor Na doping level was estimated to be 2–3 × 1017 at cm−3 assuming a donor level Nd of 2–3 × 1018 at cm−3. The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

Dc and ac electrical response of MOCVD grown GaN in p-i-n structure, assessed through I–V and admittance measurement

The electrical response of gallium nitride (GaN), produced through metal–organic chemical vapor deposition in a p-i-n structure was investigated through temperature-dependent current–voltage (I–V) and admittance measurement. The I–V curves showed double diode behavior together with several distinct regions in which trap-assisted tunnelling current has been identified at low and moderate forward/reverse direction and space charge limited current (SCLC) at large forward/reverse bias. The value of extracted energy (∼200 meV in forward and ∼70 meV in reverse direction) marked the tunnelling entity as electron and heavy hole in the present structure. These values were also obtained in space charge limited regime and considered as minority carriers which might originate the experimentally observed negative capacitance issue at low frequencies over the junction under both forward and reverse bias directions. Analytically derived expression for the admittance in the revised versions of SCLC model was also applied to explain the inductance effect, yielding good fits to the experimentally measured admittance data.

Effect of titanium chelate as a function of thickness on the electron mobility and electron transport and collection efficiency

Electron extraction layers (EELs) with shallow lowest unoccupied molecular orbitals (LUMO) featuring good energy level alignment with LUMO of acceptor materials are desirable for high-performance polymer solar cells (PSCs). Here, the use of alcohol-soluble titanium chelate, titanium (IV) oxide bis(2, 4-pentanedionate) (TOPD) as EEL layer and to tune the work function (WF) of EEL is proposed. The enhanced photovoltaic performance can be experimentally explained by the decreased barrier potential between photoactive layer and TOPD/ITO composite electrode from the ultraviolet photoelectron spectroscopy (UPS) curve, and the enhancement of electron mobility and electron transport and collection efficiency from space-charge-limited current (SCLC), time of flight (TOF) curve and the photocurrent density versus effective voltage (Jph-Veff) curve. Simultaneously, the weaker bimolecular recombination between photoactive layer and the TOPD layer were confirmed by the short-circuit current (JSC) value as a function of light intensity. Consequently, the PCE of TOPD-based PSCs arrives to 9.16% with 15 nm TOPD layer as EEL, which is higher than the PCE of 8.83% for ZnO-based PSCs.

Cu2ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition Prepared by Selenization of Sputtered Novel Precursors

In this study, we proposed a new method for the synthesis of the target material used in a two stage process for preparation of a high quality CZTSe thin film. The target material consisting of a mixture of CuxSe and ZnxSn1-x alloy was synthesized, providing a quality CZTSe precursor layer for highly efficient CZTSe thin film solar cells. The CZTSe thin film can be obtained by annealing the precursor layers through a 30 min selenization process under a selenium atmosphere at 550 °C. The CZTSe thin films prepared by using the new precursor thin film were investigated and characterized using X-ray diffraction, Raman scattering, and photoluminescence spectroscopy. It was found that diffusion of Sn occurred and formed the CTSe phase and CuxSe phase in the resultant CZTSe thin film. By selective area electron diffraction transmission electron microscopy images, the crystallinity of the CZTSe thin film was verified to be single crystal. By secondary ion mass spectroscopy measurements, it was confirmed that a double-gradient band gap profile across the CZTSe absorber layer was successfully achieved. The CZTSe solar cell with the CZTSe absorber layer consisting of the precursor stack exhibited a high efficiency of 5.46%, high short circuit current (JSC) of 37.47 mA/cm2, open circuit voltage (VOC) of 0.31 V, and fill factor (F.F.) of 47%, at a device area of 0.28 cm2. No crossover of the light and dark current-voltage (I-V) curves of the CZTSe solar cell was observed, and also, no red kink was observed under red light illumination, indicating a low defect concentration in the CZTSe absorber layer. Shunt leakage current with a characteristic metal/CZTSe/metal leakage current model was observed by temperature-dependent I-V curves, which led to the discovery of metal incursion through the CdS buffer layer on the CZTSe absorber layer. This leakage current, also known as space charge-limited current, grew larger as the measurement temperature increased and completely overwhelmed the diode current at a measurement temperature of 200 °C. This is due to interlayer diffusion of metal that increases the shunt leakage current and decreases the efficiency of the CZTSe thin film solar cells.