High‐Performance,Low‐Operating‐Voltage Organic Field‐Effect Transistors with Low Pinch‐Off Voltages

Organic field‐effect transistors suffer from ultra‐high operating voltages in addition to their relative low mobility. A general approach to low‐operating‐voltage organic field‐effect transistors (OFETs) using donor/acceptor buffer layers is demonstrated. P‐type OFETs with acceptor molecule buffer layers show reduced operating voltages (from 60–100 V to 10–20 V), with mobility up to 0.19 cm² V^?1 s^?1 and an on/off ratio of 3 × 10⁶. The subthreshold slopes of the devices are greatly reduced from 5–12 V/decade to 1.68–3 V/decade. This favorable combination of properties means that such OFETs can be operated successfully at voltages below 20 V (|V_DS| ≤ 20 V, |V_GS| ≤ 20 V). This method also works for n‐type semiconductors. The reduced operating voltage and low pinch‐off voltage contribute to the improved ordering of the polycrystalline films, reduced grain boundary resistance, and steeper subthreshold slopes.     

The Mechanism of Charge Generation in Charge‐Generation Units Composed of p‐Doped Hole‐Transporting Layer/HATCN/n‐Doped Electron‐Transporting Layers

The rate‐limiting step of charge generation in charge‐generation units (CGUs) composed of a p‐doped hole‐transporting layer (p‐HTL), 1,4,5,8,9,11‐hexaazatriphenylene hexacarbonitrile (HATCN) and n‐doped electron‐transporting layer (n‐ETL), where 1,1‐bis‐(4‐bis(4‐methyl‐phenyl)‐amino‐phenyl)‐cyclohexane (TAPC) was used as the HTL is reported. Energy level alignment determined by the capacitance–voltage (C–V) measurements and the current density–voltage characteristics of the structure clearly show that the electron injection at the HATCN/n‐ETL junction limits the charge generation in the CGUs rather than charge generation itself at the p‐HTL/HATCN junction. Consequently, the CGUs with 30 mol% Rb₂CO₃‐doped 4,7‐diphenyl‐1,10‐phenanthroline (BPhen) formed with the HATCN layer generates charges very efficiently and the excess voltage required to generate the current density of ±10 mA cm^?2 is around 0.17 V, which is extremely small compared with the literature values reported to date.     

Shedding Light on the Operation of Polymer Light‐Emitting Electrochemical Cells Using Impedance Spectroscopy

A combination of impedance spectroscopy, device characterization, and modeling is used to pinpoint key processes in the operation of polymer light‐emitting electrochemical cells (LECs). At low applied voltage, electric double layers with a thickness of ≈2–3 nm are shown to exist at the electrode interfaces. At voltages exceeding the bandgap potential of the conjugated polymer (V ≥ 2.5 V for superyellow), a light‐emitting p–n junction forms in situ, with a steady‐state structure that is found to depend strongly on the applied voltage. This is exemplified by that the effective p–n junction thickness (d_pn) for a device with an interelectrode gap of 90 nm decreases from ≈23 nm at 2.5 V to ≈6 nm at 3.9 V. The current increases with decreasing d_pn in a concerted manner, while the brightness reaches its peak at V = 3.4 V when d_pn ≈ 10 nm. The existence of an optimum d_pn for high brightness in LECs is attributed to an offset between an increase in the exciton formation rate with decreasing d_pn, due to an increasing current, and a simultaneous decrease in the exciton radiative decay rate, when an increasing fraction of excitons diffuses away from the p–n junction into the surrounding non‐radiative doping regions.     

Effect of Doping Concentration on Microstructure of Conjugated Polymers and Characteristics in N‐Type Polymer Field‐Effect Transistors

Despite extensive progress in organic field‐effect transistors, there are still far fewer reliable, high‐mobility n‐type polymers than p‐type polymers. It is demonstrated that by using dopants at a critical doping molar ratio (MR), performance of n‐type polymer poly[[N,N9‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,59‐(2,29‐bithiophene)] (P(NDI2DO‐T2)) field‐effect transistors (FETs) can be significantly improved and simultaneously optimized in mobility, on–off ratio, crystallinity, injection, and reliability. In particular, when using the organic dopant bis(cyclopentadienyl)–cobalt(II) (cobaltocene, CoCp₂) at a low concentration (0.05 wt%), the FET mobility is increased from 0.34 to 0.72 cm² V^–1 s^–1, and the threshold voltage was decreased from 32.7 to 8.8 V. The relationship between the MR of dopants and electrical characteristics as well as the evolution in polymer crystallinity revealed by synchrotron X‐ray diffractions are systematically investigated. Deviating from previous discoveries, it is found that mobility increases first and then decreases drastically beyond a critical value of MR. Meanwhile, the intensity and width of the main peak of in‐plane X‐ray diffraction start to decrease at the same critical MR. Thus, the mobility decrease is correlated with the disturbed in‐plane crystallinity of the conjugated polymer, for both organic and inorganic dopants. The method provides a simple and efficient approach to employing dopants to optimize the electrical performance and microstructure of P(NDI2DO‐T2).     

Native Defect‐Induced Hysteresis Behavior in Organolead Iodide Perovskite Solar Cells

Despite the high power conversion efficiency and ease of fabrication, planar‐junction organolead halide perovskite solar cells often exhibit anomalous hysteretic current–voltage (I–V) characteristics. In this work, the origin of the I–V hysteresis is studied by fine‐tuning the precursor ratio of methylammonium lead iodide and thus varying the native defects in the material. It is shown that the perovskites synthesized from “PbI₂ excess,” “methylammonium iodide excess,” and “stoichiometric” precursors exhibit identical film morphology but different I–V hysteresis in a planar solar cell configuration. Through a comparative analysis on the temperature‐dependent continuous and stepwise‐stabilized I–V responses of the three devices, a model involving transport and trapping of the ionic native defects is proposed. The active energy of the transport process is estimated to be between 0.10 and 0.18 eV, most likely associated with the vacancy‐mediated iodide ion migration. The lower activation energy of the “PbI₂ excess” and “Stoichiometric” samples indicates that the presence of methylammonium vacancies may provide a favorable pathway for the migration of iodide ions due to reduced steric hindrance. Furthermore, the slow trapping and release processes of iodide ions at the TiO₂/perovskite interface are accounted for the long time scale current decay (or raise) following a voltage change.     

Current‐voltage characteristics of high‐concentration,photovoltaic arrays

The current–voltage ( I–V ) characteristics of photovoltaic (PV) systems have always been a good indicator of the overall performance of a system. The aim of this paper is to give an overview and elucidate the use of the I–V characteristics of concentrator PV (CPV) modules and arrays as an important diagnostic tool to identify factors that lower a system's performance and the types of mismatch that exist between series‐connected single‐junction cells within a module. Possible causes for mismatch between cells include factors such as; misalignment of optical elements and cells, nonuniform cell material parameters, uneven cell illumination due to dew, dust or degradation of the secondary and main optical elements. The different types of mismatch typically found in CPV are categorized and their effects on the resultant module I–V curves are discussed and shown. The effect of bypass diodes on the module's I–V curves is also illustrated. This paper also reports on, and interprets I–V measurements that were recorded for a commercially available point‐focus concentrator module under various real outdoor conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

Anti‐Ferromagnet Controlled Tunneling Magnetoresistance

The requirement for high‐density memory integration advances the development of newly structured spintronic devices, which have reduced stray fields and are insensitive to magnetic field perturbations. This could be visualized in magnetic tunnel junctions incorporating anti‐ferromagnetic instead of ferromagnetic electrodes. Here, room‐temperature anti‐ferromangnet (AFM)‐controlled tunneling anisotropic magnetoresistance in a novel perpendicular junction is reported, where the IrMn AFM stays immediately at both sides of AlO_x tunnel barrier as the functional layers. Bi‐stable resistance states governed by the relative arrangement of uncompensated anti‐ferromagnetic IrMn moments are obtained here, rather than the traditional spin‐valve signal observed in ferromagnet‐based tunnel junctions. The experimental observation of room‐temperature tunneling magnetoresistance controlled directly by AFM is practically significant and may pave the way for new‐generation memories based on AFM spintronics.     

Bacteriorhodopsin‐Monolayer‐Based Planar Metal–Insulator–Metal Junctions via Biomimetic Vesicle Fusion: Preparation,Characterization, and Bio‐optoelectronic Characteristics

A reliable and reproducible method for preparing bacteriorhodopsin (bR)‐containing metal–biomolecule–monolayer‐metal planar junctions via vesicle fusion tactics and soft deposition of Au top electrodes is reported. Optimum monolayer and junction preparations, including contact effects, are discussed. The electron‐transport characteristics of bR‐containing membranes are studied systematically by incorporating native bR or artificial bR pigments derived from synthetic retinal analogues, into single solid‐supported lipid bilayers. Current–voltage (I–V) measurements at ambient conditions show that a single layer of such bR‐containing artificial lipid bilayers pass current in solid electrode/bilayer/solid electrode structures. The current is passed only if retinal or its analogue is present in the protein. Furthermore, the preparations show photoconductivity as long as the retinal can isomerize following light absorption. Optical characterization suggests that the junction photocurrents might be associated with a photochemically induced M‐like intermediate of bR. I–V measurements along with theoretical estimates reveal that electron transfer through the protein is over four orders of magnitude more efficient than what would be estimated for direct tunneling through 5 nm of water‐free peptides. Our results furthermore suggest that the light‐driven proton‐pumping activity of the sandwiched solid‐state bR monolayer contributes negligibly to the steady‐state light currents that are observed, and that the orientation of bR does not significantly affect the observed I–V characteristics.     

Current transport studies of amorphous n/p junctions and its application in a‐Si:H/HIT‐type tandem cells

This paper presents an understanding of the fundamental carrier transport mechanism in hydrogenated amorphous silicon (a‐Si:H)‐based n/p junctions. These n/p junctions are, then, used as tunneling and recombination junctions (TRJ) in tandem solar cells, which were constructed by stacking the a‐Si:H‐based solar cell on the heterojunction with intrinsic thin layer (HIT) cell. First, the effect of activation energy (E_a) and Urbach parameter (E_u) of n‐type hydrogenated amorphous silicon (a‐Si:H(n)) on current transport in an a‐Si:H‐based n/p TRJ has been investigated. The photoluminescence spectra and temperature‐dependent current–voltage characteristics in dark condition indicates that the tunneling is the dominant carrier transport mechanism in our a‐Si:H‐based n/p‐type TRJ. The fabrication of a tandem cell structure consists of an a‐Si:H‐based top cell and an HIT‐type bottom cell with the a‐Si:H‐based n/p junction developed as a TRJ in between. The development of a‐Si:H‐based n/p junction as a TRJ leads to an improved a‐Si:H/HIT‐type tandem cell with a better open circuit voltage (V_oc), fill factor (FF), and efficiency. The improvements in the cell performance was attributed to the wider band‐tail states in the a‐Si:H(n) layer that helps to an enhanced tunneling and recombination process in the TRJ. The best photovoltage parameters of the tandem cell were found to be V_oc = 1430 mV, short circuit current density = 10.51 mA/cm², FF = 0.65, and efficiency = 9.75%. Copyright © 2015 John Wiley & Sons, Ltd.     

Influences of magnetic fields on current–voltage characteristics of gold-DNA-gold structure with variable gaps

Achieving highly sensitive magnetic sensors by means of Metal-DNA-Metal (MDM) structure is a key issue. DNA, being a genetic information carrier in living cells reveals tunable semiconducting response in the presence of external electric and magnetic fields, which is promising for molecular electronics. The influence of magnetic fields up to 1200 mT on the current–voltage (I–V) behavior of Gold-DNA-Gold (GDG) structure having variable gap sizes from 20–50 μm are reported in this work. These structures were fabricated using UV lithography, DC magnetron sputtering and thermal evaporation techniques. DNA strands were extracted from Boesenbergia rotunda plant via standard protocol. The acquired I–V characteristics display the semiconducting diode nature of DNA in GDG structures. The potential barrier for all the structures exhibit an increasing trend with the increase of externally imposed magnetic field irrespective of variable gap sizes. Furthermore, the potential barrier in GDG junction at higher magnetic field strengths (>1000 mT) is found to be considerably enhanced. This enhancement in the junction barrier height at elevated magnetic fields is attributed to the reduction of carrier mobility and augmentation of resistance. The achieved admirable features of magnetic sensitivity suggest the viability of using these GDG sandwiches as a prospective magnetic sensor.     

Estimation of multi‐junction solar cell parameters

The paper deals with the parameter estimation of InGaP/GaAs/Ge multi‐junction solar cell and is based on minimizing the difference between the measured I–V and the theoretical I–V characteristics—the objective function. The parameter estimation was first performed on a multi‐junction solar cell represented by a single‐diode model containing eight parameters: five conventional parameters and three additional parameters for the negative diode breakdown voltage. An extended model is also presented for detailed analysis of the multi‐junction cell containing three subcells connected in series. In this model, each subcell is represented by eight parameters, and therefore a total of 24 parameters describe the cell. The parameter estimation procedure requires derivatives of the first and the second order of an objective function, filtering of noisy measurements, iteration algorithm, guessing of initial parameters, zero finding, and stopping criteria. The paper presents a mathematical method and a procedure for extracting solar cell parameters based on I–V measured data. The parameters' values may be used for analysis of the current mismatch of the subcells, the power loss, the output power of the multi‐junction cell for different environmental conditions, and to some extent, for cell fabrication. Copyright © 2012 John Wiley & Sons, Ltd.     

Self‐consistent electrical parameter extraction from bias dependent spectral response measurements of III‐V multi‐junction solar cells

Spectral response of multi‐junction solar cell is complicated because of the interplay between external measurement conditions such as bias light intensity, monochromatic light intensity, bias voltage, and intrinsic electrical properties of series interconnected subcells. In this paper, we report an experimental study on the bias voltage‐dependent spectral response (SR) for multi‐junction solar cell. A self‐consistent iteration loop was developed from a nonlinear least square Powell hybrid algorithm that was used for curve fitting the experimental SR versus bias voltage data of each subcell. We demonstrated for the first time that this approach enabled us to derive the electrical parameters such as dark saturation currents (J₀), shunt resistance (R_sh), series resistance (R_s), and spectra response (J_photo) for each subcell of a Ga_0.99In_0.01As/Ge dual junction solar cell with stable convergence. The accuracies of the fitting results were confirmed by the agreement between the J–V curves calculated on the basis of these parameters and the experimental J–V curve of multi‐junction solar cell measured under AM1.5 and 1 sun condition. Copyright © 2013 John Wiley & Sons, Ltd.     

Field‐Induced n‐Doping of Black Phosphorus for CMOS Compatible 2D Logic Electronics with High Electron Mobility

Black phosphorus (BP) has been considered as a promising two‐dimensional (2D) semiconductor beyond graphene owning to its tunable direct bandgap and high carrier mobility. However, the hole‐transport‐dominated characteristic limits the application of BP in versatile electronics. Here, we report a stable and complementary metal oxide semiconductor (COMS) compatible electron doping method for BP, which is realized with the strong field‐induced effect from the K⁺ center of the silicon nitride (Si_xN_y). An obvious change from pristine p‐type BP to n type is observed after the deposit of the Si_xN_y on the BP surface. This electron doping can be kept stable for over 1 month and capable of improving the electron mobility of BP towards as high as ~176 cm² V^–1 s^–1. Moreover, high‐performance in‐plane BP p‐n diode and further logic inverter were realized by utilizing the n‐doping approach. The BP p‐n diode exhibits a high rectifying ratio of ~10⁴. And, a successful transfer of the output voltage from “High” to “Low” with very few voltage loss at various working frequencies were also demonstrated with the constructed BP inverter. Our findings paves the way for the success of COMS compatible technique for BP‐based nanoelectronics.     

Quantitative Analysis of Current–Voltage Characteristics of Semiconducting Nanowires: Decoupling of Contact Effects

A metal‐semiconductor‐metal (M‐S‐M) model for quantitative analysis of current–voltage (I–V) characteristics of semiconducting nanowires is described and applied to fit experimental I–V curves of Bi₂S₃ nanowire transistors. The I–V characteristics of semiconducting nanowires are found to depend sensitively on the contacts, in particular on the Schottky barrier height and contact area, and the M‐S‐M model is shown to be able to reproduce all experimentally observed I–V characteristics using only few fitting variables. A procedure for decoupling contact effects from that of the intrinsic parameters of the semiconducting nanowires, such as conductivity, carrier mobility and doping concentration is proposed, demonstrated using experimental I–V curves obtained from Bi₂S₃ nanowires and compared with the field‐effect based method.     

Interface States of Fe3O4/Si Interfacial Structure and Effect of Magnetic Field

Electronic transport across Fe₃O₄/Si interfacial structure has been studied with and without the application of magnetic fields along the interfacial plane, up to 8 kG. Current–voltage (I–V) and capacitance–voltage (C–V) characteristics across the junction have been recorded for various bias voltages, frequency and magnetic field. The interfacial parameters, such as, ideality factor (n), barrier height (? _B0), series resistance (R _S) and donor concentration (N _D) etc. have been estimated from the characteristics. The interface state density (N _SS) and their energy distribution have been estimated by using the interfacial parameters. It has been observed that the N _SS decreases as the energy increases from the conduction band edge towards the valence band. A magnetoresistance (MR) of ～40% has been estimated from the I–V–H data along with its variation with magnetic field. The change of interface state density with the magnetic field shows a similar variation as MR versus H. From the observed variations, the interface states seem to be related to electronic spins. The possibility of an interfacial magnetic silicide or magnetic ions in the interfacial region has been invoked for the observed interface states.     

Schottky‐Barrier‐Controllable Graphene Electrode to Boost Rectification in Organic Vertical P–N Junction Photodiodes

Monolayer graphene is used as an electrode to develop novel electronic device architectures that exploit the unique, atomically thin structure of the material with a low density of states at its charge neutrality point. For example, a single semiconductor layer stacked onto graphene can provide a semiconductor–electrode junction with a tunable injection barrier, which is the basis for a primitive transistor architecture known as the Schottky barrier field‐effect transistor. This work demonstrates the next level of complexity in a vertical graphene–semiconductor architecture. Specifically, an organic vertical p‐n junction (p‐type pentacene/n‐type N,N′‐dioctyl‐3,4,9,10‐perylenedicarboximide (PTCDI‐C₈)) on top of a graphene electrode constituting a novel gate‐tunable photodiode device structure is fabricated. The model device confirms that controlling the Schottky barrier height at the pentacene–graphene junction can (i) suppress the dark current density and (ii) enhance the photocurrent of the device, both of which are critical to improve the performance of a photodiode.     

Low‐Operating‐Voltage Organic Transistors Made of Bifunctional Self‐Assembled Monolayers

Self‐assembled monolayers (SAMs) are molecular assemblies that spontaneously form on an appropriate substrate dipped into a solution of an active surfactant in an organic solvent. Organic field‐effect transistors are described, built on an SAM made of bifunctional molecules comprising a short alkyl chain linked to an oligothiophene moiety that acts as the active semiconductor. The SAM is deposited on a thin oxide layer (alumina or silica) that serves as a gate insulator. Platinum–titanium source and drain electrodes (either top‐ or bottom‐contact configuration) are patterned by using electron‐beam (e‐beam) lithography, with a channel length ranging between 20 and 1000 nm. In most cases, ill‐defined current–voltage (I–V) curves are recorded, attributed to a poor electrical contact between platinum and the oligothiophene moiety. However, a few devices offer well‐defined curves with a clear saturation, thus allowing an estimation of the mobility: 0.0035 cm² V^–1 s^–1 for quaterthiophene and 8 × 10^–4 cm² V^–1 s^–1 for terthiophene. In the first case, the on–off ratio reaches 1800 at a gate voltage of –2 V. Interestingly, the device operates at room temperature and very low bias, which may open the way to applications where low consumption is required.     

Voltage‐Induced Coercivity Reduction in Nanoporous Alloy Films: A Boost toward Energy‐Efficient Magnetic Actuation

Magnetic data storage and magnetically actuated devices are conventionally controlled by magnetic fields generated using electric currents. This involves significant power dissipation by Joule heating effect. To optimize energy efficiency, manipulation of magnetic information with lower magnetic fields (i.e., lower electric currents) is desirable. This can be accomplished by reducing the coercivity of the actuated material. Here, a drastic reduction of coercivity is observed at room temperature in thick (≈600 nm), nanoporous, electrodeposited Cu–Ni films by simply subjecting them to the action of an electric field. The effect is due to voltage‐induced changes in the magnetic anisotropy. The large surface‐area‐to‐volume ratio and the ultranarrow pore walls of the system allow the whole film, and not only the topmost surface, to effectively contribute to the observed magnetoelectric effect. This waives the stringent “ultrathin‐film requirement” from previous studies, where small voltage‐driven coercivity variations were reported. This observation expands the already wide range of applications of nanoporous materials (hitherto in areas like energy storage or catalysis) and it opens new paradigms in the fields of spintronics, computation, and magnetic actuation in general.     

Diode and MOSFET Properties of Trench‐Gate‐Type Super‐Barrier Rectifier with P‐Body Implantation Condition for Power System Application

In this paper, we investigate the electrical characteristics of two trench‐gate‐type super‐barrier rectifiers (TSBRs) under different p‐body implantation conditions (low and high). Also, design considerations for the TSBRs are discussed in this paper. The TSBRs’ electrical properties depend strongly on their respective p‐body implantation conditions. In the case of the TSBR with a low p‐body implantation condition, it exhibits MOSFET‐like properties, such as a low forward voltage (V_F) drop, high reverse leakage current, and a low peak reverse recovery current owing to a majority carrier operation. However, in the case of the TSBR with a high p‐body implantation condition, it exhibits pn junction diode–like properties, such as a high V_F, low reverse leakage current, and high peak reverse recovery current owing to a minority carrier operation. As a result, the TSBR with a low p‐body implantation condition is capable of operating as a MOSFET, and the TSBR with a high p‐body implantation condition is capable of operating as either a pn junction diode or a MOSFET, but not both at the same time.     

Influence of Hole‐Transport Layers and Donor Materials on Open‐Circuit Voltage and Shape of I–V Curves of Organic Solar Cells

The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p‐type materials with HOMOs varying between –5.0 and –5.6 eV as hole‐transport layer (HTL) and as donor in vacuum‐evaporated multilayer p‐i‐metal small‐molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift‐diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open‐circuit voltage (V_oc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V_oc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which V_oc is independent of the HTL.