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Magneto electroluminescence (MEL) is emerging as a powerful tool to study spin dynamics in organic light emitting diodes (OLEDs). The shape of the MEL response is typically used to draw qualitative inference on the dominant process (singlet fission or triplet fusion) in the device. In this study, we develop a quantitative model for MEL and apply it to devices based on Rubrene, and three solution processable anthradithiophene emitters. The four emitters allow us to systematically vary the film structure between highly textured, poly-crystalline to amorphous. We find significant diversity in the MEL, with the textured films giving highly structured responses. We find that the additional structure does not coincide with energy anti-crossings, but intersections in the singlet character between adjacent states. In all cases the MEL can be adequately described by an extended Merrifield model. Via the inclusion of charge injection, we are able to draw additional information on underlying physics in OLED devices.

Developing a fundamental understanding of charge transport in organic semiconductors has been a decades-long challenge that hinders performance improvement. In particular, recent work suggests that organic semiconductors have unique charge–phonon interactions where phonons temporarily interrupt the band structure causing a transient localization of charge carriers. Despite growing evidence to support this mechanism, further understanding and control will depend on pinpointing the molecular motions that cause substantial change to the band structure. Here we combine experimental and theoretical techniques to demonstrate the phonon energies and associated molecular motions governing the charge–phonon interaction in single crystal tetracene. We investigate phonon properties using polarized Raman spectroscopy, transmission electron microscopy, and density functional theory, and correlate this with the anisotropic mobility. We find that specific phonons disrupt the band orbital in the high-mobility direction, evident in the discrepancy between measured and static calculations of the mobility anisotropy ratio in tetracene. The electronic structure and transport properties of organic semiconductors are known not to follow standard semiconductor models and a complete understanding is still lacking in the literature. The authors experimentally and theoretically demonstrate the role of phonon–charge interactions on the highest occupied molecular orbital and mobility of the organic semiconductor tetracene.

The tunnel-FET (TFET) is a promising candidate for future low-power logic applications, because it enables a sub-60-mV/decadesubthresholdswing. However, themost experimental TFETs are plagued by unwanted trap-assisted tunneling (TAT) and Shockley–Read–Hall (SRH) carrier generation, which degrade the swing and increase the leakage floor, hence forming a major roadblock for TFET adoption. This degradation is attributed to bulk traps, semiconductor/oxideinterface traps, and/or heterojunction interface traps, but it is still unclear which of the three trap types are dominant. In this paper, we focus on TAT and SRH caused by bulk traps. We calibrate SRH and TAT models with the help of In0.53Ga0.47As p+/n+ and p+/i/n+ diodes grown on lattice matched substrates by molecular beam epitaxy (MBE). We then perform calibrated simulations of an In0.53Ga0.47As TFET, which show bulk SRH and TAT are sufficiently low compared with the target OFF-state current and, hence, not a significant issue. Therefore, it is likely that high SRH and TAT commonly observed in experimental homojunction InGaAs TFETs, MBE-grown on lattice matched substrates, are not caused by bulk semiconductor defects, but by semiconductor/oxide interface defects.

Realization of the full promise of solution deposited photovoltaic devices requires processes compatible with high-speed manufacturing. We report the performance and morphology of blade-coated bulk heterojunction devices based on the small molecule donor p-DTS(FBTTh2)2 when treated with a post-deposition solvent vapor annealing (SVA) process. SVA with tetrahydrofuran improves the device performance of blade-coated films more than solvent additive processing (SA) with 1,8-diiodooctane. In spin-coating, SA and SVA achieve similar device performance. Our optimized, blade coated, SVA devices achieve power conversion efficiencies over 8% and maintain high efficiencies in films up to ≈250 nm thickness, providing valuable resilience to small process variations in high-speed manufacturing. Using impedance spectroscopy, we show that this advantageous behavior originates from highly suppressed bimolecular recombination in the SVA-treated films. Electron microscopy and grazing-incidence X-ray scattering experiments show that SA and SVA both produce highly crystalline donor domains, but SVA films have a radically smaller domain size compared to SA films. We attribute the different behavior to variations in initial nucleation density and relative ability of SVA and SA to control subsequent crystal growth.

Spin-dependent nonlinear processes in organic materials such as singlet-fission and triplet-triplet annihilation could increase the performance for photovoltaics, detectors, and light emitting diodes. Rubrene/C60 light emitting diodes exhibit a distinct low voltage (half-bandgap) threshold for emission. Two origins for the low voltage turn-on have been proposed: (i) Auger assisted energy up-conversion, and (ii) triplet-triplet annihilation. We test these proposals by systematically altering the rubrene/C60 interface kinetics by introducing thin interlayers. Quantitative analysis of the unmodified rubrene/C60 device suggests that higher order processes can be ruled out as the origin of the sub-bandgap turn-on. Rather, band-to-band recombination is the most likely radiative recombination process. However, insertion of a bathocuproine layer yields a 3-fold increase in luminance compared to the unmodified device. This indicates that suppression of parasitic interface processes by judicious modification of the interface allows a triplet-triplet annihilation channel to be observed. Conflicting mechanisms have been proposed to explain the sub-bandgap turn-on behaviour of heterojunction organic light-emitting diodes (OLEDs). Here, by probing and modifying the OLED heterojunction, the authors show that band-to-band recombination is the most likely radiative recombination process.

The effective bandgap for heterojunction band-to-band tunneling ( $ {{E}}_{{\text {g,eff}}}$ ) is a crucial design parameter for a heterojunction tunneling FET (TFET). However, there is significant uncertainty on $ {{E}}_{{\text {g,eff}}}$ , especially for In0.53Ga0.47As/ GaAs0.5Sb0.5. This makes the prediction of TFET performance difficult. We calibrate $ {{E}}_{{\text {g,eff}}}$ by fabricating heterojunction p+/i/n+ diodes, comparing the simulated and the measured current–voltage and capacitance–voltage curves, while taking $ {{E}}_{{\text {g,eff}}}$ as a fitting parameter. Our calibration significantly reduces the uncertainty on $ {{E}}_{{\text {g,eff}}}$ compared with the range found in the literature. The comparison with the previous work on highly doped heterojunction diodes suggests that dopant-dependent bandgap narrowing reduces $ {{E}}_{{\text {g,eff}}}$ and therefore significantly impacts the performance of highly doped TFET.

The optical properties of chemical vapor deposition grown graphene were measured with high accuracy by spectroscopy ellipsometry in the energy range of 0.7 eV–9.0 eV, which is spectrally broader compared with those reported in literature. The refractive index (n) and extinction coefficient (k) of graphene were accurately obtained and compared with directly measured transmittance data. The absorption of graphene follows the well-known fine structure constant in the visible range, becomes lower below 1 eV, and shows a strong absorption peak around 4.8 eV. The latter was attributed to the resonant excitons near the van Hove singularity at the M point of the Brillouin zone. A higher energy absorption peak was observed at 6.4 eV resulting from the excitonic effect of the σ-to-σ∗ transition at the Γ point of the Brillouin zone. The multi-layer graphene, fabricated by repeated transfer, exhibit similar optical properties to mono-layer graphene and the 4.8 eV absorption peak exhibits a layer-number dependent peak shift, similar to the exfoliated AB-stacked multi-layer graphene. The broadband optical properties reported in this letter are believed to enhance the understanding of the optical properties of graphene and will benefit the development of graphene-based optoelectronic devices.

It is well known that the morphology of organic semiconductor fi lms plays a signifi cant role in dictating their electrical properties. In particular, anisotropic packing of organic semiconductors typically results in anisotropic charge transport properties, as has been demonstrated for a number of organic semiconductor materials. [ 1–12 ] In single crystal small molecule semiconductors such as rubrene, the anisotropy in charge transport can be directly related to the molecular packing structure and subsequent band structure. [ 13,14 ] In conjugated polymers, orienting the polymer long-axis to be primarily in one-direction has been shown to result in anisotropic charge transport for a number of polymers including poly(3-hexylthiphene) (P3HT), [ 1,11,15 ] poly[9,9′-dioctylfl uorene-co-bithiophene] (F8BT), [ 4,16 ] and Poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno[3,2b ]thiophene] (pBTTT), [ 3,6,12 ]

In this letter, we demonstrate using fast current-voltage measurements, low switching slope of 64 mV/decade over a drain current range between $10^{\mathrm {\mathbf {-3}}}$ and $2 \times 10^{\mathrm {\mathbf {-2}}} \mu $ A/ $\mu $ m in staggered-gap In0.65Ga0.35As/GaAs0.4Sb0.6 tunneling field-effect transistors (TFETs) at $\mathrm{V}_{\mathrm {\mathbf {DS}}}\,=\,0.5$ V. This is achieved through a combination of low damage mesa sidewall etch and improvement in electrical quality of the high- $\kappa $ gate-stack. Benchmarking our results against experimentally demonstrated TFETs, we conclude that, the staggered-gap TFETs are capable of achieving simultaneously high drive current and low switching slope.

Modulated photocurrent spectroscopy was used to investigate the dynamic response of charge carrier transport in thin film CdTe/CdS solar cells. The impact of light bias and temperature over a broad excitation frequency range were measured. The observed features of the data, including a photocurrent ‘phase-lead’ and a ‘phase-lag’ over different regions of the frequency spectrum, were explored in the context of an equivalent circuit model. Comparisons between the model's predicted performance and the measured data suggest that charge carrier recombination at the cell's back metal/semiconductor contact is the main source of photocurrent loss in these devices.

Wave scattering by a potential step is a ubiquitous concept. Thus, it is surprising that theoretical treatments of ballistic transport in nanoscale devices, from quantum point contacts to ballistic transistors, assume no reflection even when the potential step is encountered upon exiting the device. Experiments so far seem to support this even if it is not clear why. Here we report clear evidence of coherent reflection when electron wave exits the channel of a nanoscale transistor and when the electron energy is low. The observed behavior is well described by a simple rectangular potential barrier model which the Schrodinger’s equation can be solved exactly. We can explain why reflection is not observed in most situations but cannot be ignored in some important situations. Our experiment also represents a direct measurement of electron injection velocity - a critical quantity in nanoscale transistors that is widely considered not measurable.

Parameters used to describe the electrical properties of organic field-effect transistors, such as mobility and threshold voltage, are commonly extracted from measured current–voltage characteristics and interpreted by using the classical metal oxide–semiconductor field-effect transistor model. However, in recent reports of devices with ultra-high mobility (>40 cm2 V−1 s−1), the device characteristics deviate from this idealized model and show an abrupt turn-on in the drain current when measured as a function of gate voltage. In order to investigate this phenomenon, here we report on single crystal rubrene transistors intentionally fabricated to exhibit an abrupt turn-on. We disentangle the channel properties from the contact resistance by using impedance spectroscopy and show that the current in such devices is governed by a gate bias dependence of the contact resistance. As a result, extracted mobility values from d.c. current–voltage characterization are overestimated by one order of magnitude or more., Charge mobility, extracted from current–voltage curves, is an important parameter for evaluating the performance of organic field-effect transistors. Bittle et al. show that charge mobility can be overestimated by one order of magnitude due to the gate bias dependence of the charge injection process.

A comprehensive structure and performance study of thin blend films of the small-molecule semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT), with various insulating binder polymers in organic thin-film transistors is reported. The vertically segregated composition profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X-ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X-ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly(α-methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composition profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution-processed bottom-gate/bottom-contact thin-film transistors.

The application of 1H spin diffusion nuclear magnetic resonance (NMR) is expanded to polymer-fullerene blends for bulk heterojunction (BHJ) organic photovoltaics (OPV) by developing a new experimental methodology for measuring the thin films used in poly-3-hexylthiophene–phenyl C61-butyric acid methyl ester (P3HT-PCBM) OPV devices and by creating an analysis framework for estimating domain size distributions. It is shown that variations in common P3HT-PCBM BHJ processing parameters such as spin-coating speed and thermal annealing can significantly affect domain size distributions, which in turn affect power conversion efficiency. 1H spin diffusion NMR analysis reveals that films spin-cast at fast speeds in dichlorobenzene are primarily composed of small (

A novel method of strain-aligning polymer films is introduced and applied to regioregular poly(3-hexylthiophene) (P3HT), showing several important features of charge transport. The polymer backbone is shown to align in the direction of applied strain resulting in a large charge-mobility anisotropy, where the in-plane mobility increases in the applied strain direction and decreases in the perpendicular direction. In the aligned film, the hole mobility is successfully represented by a two-dimensional tensor, suggesting that charge transport parallel to the polymer backbone within a P3HT crystal is strongly favored over the other crystallographic directions. Hole mobility parallel to the backbone is shown to be high for a mixture of plane-on and edge-on packing configurations, as the strain alignment is found to induce a significant face-on orientation of the originally highly edge-on oriented crystalline regions of the film. This alignment approach can achieve an optical dichroic ratio of 4.8 and a charge-mobility anisotropy of 9, providing a simple and effective method to investigate charge-transport mechanisms in polymer semiconductors.

Solution-processable small molecules have tremendous potential in macroelectronics applications by providing both high charge carrier mobility and low cost processing. Fluorinated 5,11-bis(triethylsilylethynl) anthradithiophene enables high performance thin film transistors due, in part, to a self-patterning process where crystals grow from chemically tailored contacts and bridge the transistor channel. This paper outlines a detailed microstructural study that identifies the crystallization mechanisms of the self-patterning. Two crystal habits are observed: we find that crystals on chemically modified electrodes predominantly form (001) oriented platelets while untreated surfaces form a fine mixture of (001) and (111) oriented crystals. For (001) oriented platelets, the (010) fast growth face lies in the plane of the film and allows extended growth from platelets nucleated on the electrode into the transistor channel. The in-plane charge carrier mobility of the (001) platelets is high; for short channel l...

Poly(2,5-Bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes (pBTTTs) are a new class of solution-processable polymer semiconductors with high charge carrier mobilities that rival amorphous silicon. This exceptional performance is thought to originate in the microstructure of pBTTT fi lms, which exhibit high crystallinity and a surface topography of wide terraces. However, the true lateral grain size has not been determined, despite the critical impact grain boundaries can have on the charge transport of polymer semiconductors. Here a strategy for determining the lateral grain structure of pBTTT using dark-fi eld transmission electron microscopy (DF-TEM) and subsequent image analysis is presented. For the fi rst time, it is revealed that the in-plain pBTTT crystal orientation varies smoothly across a length scale signifi cantly less than one micrometer (e.g., with only small angles between adjacent diffracting regions). The pBTTT polymers thus exhibit an in-plane liquid crystalline texture. This microstructure is different from what has been reported for small molecule semiconductors or polymer semiconductors such as poly (3-hexyl thiophene) (P3HT). Even though fi lms processed differently exhibit different apparent domain sizes, they exhibit similar charge carrier hopping activation energies because they possess similar low densities of abrupt grain boundaries.

We have developed compact and physics-based distributed numerical models for cryogenic bulk MOSFET operation down to 20 K to advance simulation and first-pass design of device and circuit operation at low temperatures. To achieve this, we measured and simulated temperature-dependent current-voltage characteristics of 0.16- and 0.18- bulk MOSFETs. Our measurements indicate that these MOSFETs supply approximately 40% more current in the saturation and linear regions of operation when they are cooled from room temperature to 20 K. The threshold voltage monotonically increases as the temperature is lowered, but it saturates below 40 K. The subthreshold slope decreases with the temperature lowering but at a rate that is less than theoretically predicted. The extrapolation of the subthreshold slope indicates a finite value at near absolute zero. We show that the measured behavior can be well corroborated with distributed numerical simulations using the drift-diffusion transport model. In addition, to obtain a compact model for use in low-temperature circuit design, SPICE-type compact models need to be modified to incorporate the subtle temperature effects that are not part of the standard models. To this end, we use the analog behavioral language Verilog-A and the BSIM3 model equation set to include additional temperature dependences into the standard compact models to accurately reproduce measured characteristics.

It has recently been shown that surface energy effects can cause selective segregation at the active layer interfaces of a bulk heterojunction (BHJ) organic photovoltaic device. The active layer interface composition has been suggested to impact device performance. In this study changes in the BHJ vertical composition profile of BHJ active layers cast on two hole transport layers (HTL) with significantly different surface energies (γ) are characterized using spectroscopic ellipsometry and near-edge X-ray absorption fine structure spectroscopy. Changes in the HTL γ are shown to significantly affect the BHJ interfacial segregation at the buried interface near the HTL while the composition near the free surface (air) of the BHJ is unaffected. Despite the significant differences in vertical segregation at the HTL interface, the performances of the resulting organic photovoltaic devices were relatively similar.

SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is experimentally confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by molecular beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band minimum is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band minimum. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect transistor that can potentially achieve high on-currents.

Organic semiconductors are emerging as a viable alternative to amorphous silicon in a range of thin-film transistor devices. With the possibility to formulate these p-type materials as inks and subsequently print into patterned devices, organic-based transistors offer significant commercial advantages for manufacture, with initial applications such as low performance displays and simple logic being envisaged. Previous limitations of both air stability and electrical performance are now being overcome with a range of both small molecule and polymer-based solution-processable materials, which achieve charge carrier mobilities in excess of 0.5 cm 2 V -1 s -1 , a benchmark value for amorphous silicon semiconductors. Polymer semiconductors based on thienothiophene copolymers have achieved amongst the highest charge carrier mobilities in solution-processed transistor devices. In this Progress Report, we evaluate the advances and limitations of this class of polymer in transistor devices.

We present the first characterization of single-crystal devices of a new solution processable material that we have previously demonstrated achieves technologically relevant performance in the polycrystalline thin film state. Our studies include growth and investigation of structural, as well as electronic properties of single crystals of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT). Field-effect transistors fabricated on the surface of diF-TESADT single crystals exhibit excellent electronic properties: mobility as high as 6 cm2/Vs, large current on/off ratios (Ion/Ioff = 1 × 108), small subthreshold slopes (S = 1 V/dec), and extremely small hysteresis in the current−voltage characteristics. These properties, coupled with solution processability, make diF-TESADT attractive for electronic applications and demonstrate the technological potential of soluble oligomers.

We report on a simple a method of inducing self-isolation of the thin film transistors via manipulation of the chemical interactions between the organic molecules and the surfaces where they are deposited. We use pentafluorobenzenethiol (PFBT) treatment of the contacts and investigate the thin film formation of different types of chemical compounds. In particular, three similar organic molecules: triisopropyl-silylethynyl pentacene (TIPS pentacene), triisopropyl-silylethynyl peri-fluoro-pentacene (TIPS PFP) and triisopropyl-silylethynyl cata-fluoro-pentacene (TIPS CFP), were studied and compared to better understand how the molecular, crystal structure and thin film formation influence the electrical behavior of devices fabricated with them. The molecules exhibit significantly different interactions with the chemically treated substrates. The field-effect mobilities of these devices are directly related to their thin film microstructure.

The carrier mobility of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) semiconductors can be substantially enhanced after heating through a thermotropic mesophase transition, which causes a significant improvement in thin film structural order. By directly measuring film structure throughout a heating and cooling cycle, we identify the molecular origin of this mesophase transition as the melting of interdigitated linear alkane side chains, in this case quaterdecyl. The morphology and phase behavior throughout the thermal cycle are controlled by the changing conformation of the side chains. Surprisingly, the melting of the side chains allows increases in the backbone order, p-p stacking, and carrier mobility. Upon cooling, the side chains recrystallize to preserve the excellent mesophase order and enhanced electrical performance.

We present a modular numerical model for organic thin-film field-effect transistors (OTFTs) that allows for an arbitrary density of states to be independently defined for the semiconductor bulk and the semiconductor surface next to the gate insulator. We can derive the surface charge density dependence on the interface field as well as the space-charge-limited current characteristics. Together with a model of the contacts, we arrive at a physical model that is applied to a series of OTFTs in staggered inverted (top contact) geometry with various gate insulator treatments.

Extremely scaled high-k gate dielectrics with high quality electrical interfaces with arsenide (As) and antimonide (Sb) channels are used to demonstrate complimentary ‘all III–V’ Heterojunction Vertical Tunnel FET (HVTFET) with record performance at |V DS |=0.5V. The p-type TFET (PTFET) has I ON =30µA/µm and I ON /I OFF =105, whereas the n-type TFET (NTFET) has I ON =275µA/µm and I ON /I OFF =3×105, respectively. NTFET shows 55mV/decade switching slope (SS) while PTFET shows 115mV/decade SS in pulsed mode measurement. Vertical TFET offers 77% higher effective drive strength than Si-FinFET for given inverter standard cell area. Energy-delay performance of TFET shows gain over CMOS for low V DD logic applications.

We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self assembled monolayer (SAM). The observed shift of the transfer characteristics range from -2 to 50 V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4*10^12/cm^2. In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold region is broadened, indicating that the SAM creates additional trap states, whose density is estimated to be of order 1*10^12/cm^2., Comment: to be published in J. Appl. Phys

The threshold voltage and the turn-on voltage of pentacene thin film transistors (TFT) can be shifted by covering the gate insulator with a self assembled monolayer (SAM) of organosilane molecules. In this article we present experimental evidence identifying the SAM-induced modifications of the surface potential as the main cause for the shifted characteristics. To this end, FETs have been produced both on thin films and on single crystals. In the “flip-crystal” method for fabricating FETs, the single crystals of rubrene were placed onto prefabricated structures comprising a gate electrode, gate insulator and source/drain contacts. Prior to attaching the crystals, the SiO 2 gate insulator was treated with different organosilane molecules that form SAMs on the gate insulator. Depending on the molecule’s dipole moment, shifts of the characteristics by up to 40 V in V g are measured. This behavior can be explained in a simple energy level diagram where the surface potential of the gate insulator is changed by the built in electric dipole field of the self assembled monolayer.

Ambipolar charge injection and transport are a prerequisite for a light-emitting organic fieldeffect transistor (OFET). Organic materials, however, typically show unipolar charge-carrier transport characteristics. Consequently, organic thin-film field-effect transistors based on a single material as active layer can typically either be operated as p- or as n-channel device. In this article we show that by using a heterostructure with pentacene as hole-transport and N,N′-Ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27) as electron-transport material, ambipolar characteristics, i.e., simultaneous p- and n-channel formation, can be observed in a single device. An OFET structure is investigated in which electrons and holes are injected from Mg top and Au bottom contacts into the PTCDI-C13H27 and pentacene layers, respectively. Our device exhibits electron and hole mobilities of 3×10−3 and 1×10−4 cm2/V s, respectively. This device architecture serves as a model structure for ambipolar field-e...

Systematic studies are a prerequisite for a detailed understanding of the internal processes in organic semiconductors and devices, which is of great importance for optimizing organic light-emitting diode performance. Devices based on small molecules are especially well-suited for introducing thin layers (

We report on the contact resistances for pentacene thin film transistors with two different designs: top and bottom contact configurations (referred to as TC and BC TFTs, respectively) for two different contact metals (gold and palladium). The extraction was done based on the dependencies of the channel resistances on the gate length and gate voltage. The extracted gold TC TFT contact resistance depends on VGS, but shows no dependence on the drain bias. The TC TFT contact resistance is comparable to or exceeds the channel resistance for channels shorter than approximately 10 μm. The contact resistance of BC TFTs depends both on gate and drain bias. We propose a circuit simulating the BC TFT contact resistance and verify the circuit applicability by extracting and comparing the TFT channel resistances at different drain voltages. Our results reveal an important role played by contact resistances and provide an accurate model of the contact phenomena suitable for implementation in Spice or other circuit simulators.

The increasing demand for smaller, lighter, cheaper, and more stable power sources for portable electronics, vehicles and aircrafts spurs intensive research of the lithium ion batteries. Rational battery engineering, including design of smart electrode materials, is impossible without in-depth understanding of the chemical and physical processes in galvanic cells at the microscopic, nanoscopic, and eventually, molecular levels. Reaction mechanisms, cathode expansion, formation of cracks and SEI layer, electrolyte decomposition etc., are being extensively studied with a variety of ex and in situ microscopic, spectroscopic and electrochemical techniques. However, a significant drawback of the existing methodologies is their inability to deliver Li+ ions and control their concentration at the nanoscale level. In addition, the classic electrochemical techniques imply existence of liquid-solid interfaces and inevitable formation of SEI layer. This is a significant impediment for studying lithium diffusion and associated morphological changes in cathode materials. We report progress in developing Li+ focused ion beams (FIB) as a novel probe for exploring nanoscale electrochemistry in battery-relevant materials. This work focuses on implantation of lithium ions in crystalline silicon to benchmark this technique and builds on recent, qualitative studies of FIB implantation in Sn microspheres [Takeuchi et al. JES 163 (6), A1010-A1012]. FIB implantation opens the possibility of precise spatial control of nanoscale lithium concentration though variation of the beam positioning and current. Experiments are performed in vacuo without the presence of electrolytes or associated solid-electrolyte interface. The Li+ ion beam operates with energies ranging from 0.5 to 5 keV, currents up to 15 pA, and minimum spot size of 27 nm using a magneto-optical trap ion source (MOTIS). WRM acknowledges support from the National Research Council Research Associateship Program. ES and JG acknowledge support under the Cooperative Research Agreement between the University of Maryland and the National Institute of Standards and Technology Center for Nanoscale Science and Technology, Award 70NANB14H209, through the University of Maryland.

Silicon has emerged as one of the most promising anode materials for high-performance Li-ion batteries because of its exceptional theoretical specific capacity of 4200 mAh g-1, which is more than 10 times the theoretical capacity of graphite (372 mAh/g).1 However, there are critical challenges to using Si as a negative electrode caused by a high volume expansion of up to 400 % on full lithium insertion, pulverization of the material, and growth of a solid electrolyte interphase (SEI) 1. Physico-chemical stability of the SEI at the interface between Si and the liquid electrolyte has become a crucial factor in achieving a long cycle life. In this work, we focus on phase structural transformations during the SEI formation on electron transparent c-Si anodes, which allow correlation of electrochemical properties with the structure on the same electrode feature. We have chosen for this study p-doped thin window -oriented single crystalline planar c-Si membranes as model electrodes suitable for monitoring lithiation processes and characterization by high-resolution analytical scanning/transmission electron microscopy (S/TEM).2 A LiClO4solution in a 1:1 mixture by volume of ethylene carbonate (EC) : diethyl carbonate (DEC) was employed as a nonaqeuous electrolyte due to its low viscosity and anodic stability. Electrochemical measurements were carried out in a three-electrode custom-designed PTFE cell. The working electrode was a 3 mm diameter c-Si membrane that had nine 35 nm-thick windows. The Si membranes were connected to the current collector by a Cu lead. The counter and reference electrodes were made from a lithium metal foil. Cyclic voltammetry of the Si membranes in a 1M electrolyte was carried out from open circuit voltage (OCV) and between 0.01 V and 2.0 V vs. Li/Li+. The sweep rate was 0.1 mV sec-1. Lithiated membranes were held 17 h at 0.15 V. After electrochemical lithiation, the samples were transferred for studies in a TEM. A glove bag purged with an UHP Ar was used to prevent the samples from being exposed to an ambient environment during the transfer. Low-dose bright-field and dark-field TEM, STEM, high-resolution TEM, selected area electron diffraction (SAED), electron energy-loss and energy-dispersive X-ray spectroscopies were applied to characterize the morphology and compositions of the passivation films formed during lithiation/delithiation of the membrane anodes. Soaking of a control Si membrane into the electrolyte for 24 hours without electrochemical processing did not produced the SEI. The positions of the reduction peaks at 1.5 V, 1.3 V and 0.6 V in a cyclic voltammogram (Fig. 1) are in good agreement with the positions of the reduction peaks obtained for a 1 M LiClO4 / EC: dimethyl carbonate (DMC, 1:1 by vol.) with a Pt electrode 3 and can be attributed to decomposition of the electrolyte. It is considered that the SEI consists of an inner “inorganic” layer composed of Li2O, Li2CO3, LiOH, as well as LixSiOy formed from a native oxide and an outer preferentially “organic” layer comprising mostly alkyl carbonates 3-5. Si lithiation with the formation of a Li1.7Si phase occurs at 0.33 V 6 and an increase of the current intensity below 0.3 V can be attributed to lithiation of the Si matrix. The oxidation peaks at 0.7 V and 1.5 V can be attributed to delithiation of Si and SiOx, respectively 7. For the 2nd cycle, the reduction peak at 1.5 V is not as prominent as for the 1st cycle. However, reduction current is still detected suggesting that the SEI continues to grow during the 2nd cycle. TEM studies indicate that dynamically evolving SEI growth profoundly dominates over c-Si lithiation. Observed SEI features include various morphological shapes, such as dendritic sponge aggregates impregnated with a liquid electrolyte (Fig. 2a). By soaking a lithiated Si membrane in DMC, web-like multilayer structures grown on the c-Si surface with random agglomerates of LiClO4nanocrystallites (Fig. 2b) were found. An island-like growth of such loose nonuniform SEI films can cause a rise of the interfacial impedance and low Coulombic efficiency, thus essentially hampering the potential of Si anodes to be practically realized. [1] D. Ma, et al., Nano-Micro Lett. 6, (2014) 347 [2] V.P. Oleshko, et al., Microsc. Microanal. 22(Suppl.3), (2016) 1556 [3] R. Marom, et al., J. Electrochem. Soc., 157(2010) A972 [4] B. Philippe, et al., Chem. Mater. 23(2012) 1107 [5] M. Nie, et al, J. Phys. Chem. C, 117(2013) 13403 [6] J. Wen, R. A. Huggins, J. Solid State Chem., 37(1981) 271 [7] Q. Sun, et al., Appl. Surf. Science, 254 (2008) 3774 Figure 1

Organic semiconductors hold immense promise for the development of a wide range of innovative devices with their excellent electronic and manufacturing characteristics. Of particular interest are nonmagnetic organic semiconductors that show unusual magnetic field effects (MFEs) at small subtesla field strength that can result in substantial changes in their optoelectronic and electronic properties. These unique phenomena provide a tremendous opportunity to significantly impact the functionality of organic-based devices and may enable disruptive electronic and spintronic technologies. Here, we present an approach to vary the MFEs on the electrical resistance of organic-based systems in a simple yet reliable fashion. We experimentally modify the interfacial characteristics by adding a self-assembled monolayer between the metal electrode and the organic semiconductor, thus enabling the tuning of competing MFE mechanisms coexisting in organic semiconductors. This approach offers a robust method for tuning the magnitude and sign of magnetoresistance in organic semiconductors without compromising the ease of processing.

The electrical performance of a tunnel field-effect transistor depends critically on the band offset at their semiconductor heterojunction interface. Historically, it has been difficult to experimentally determine how the electronic bands align at the heterojunction interface. We report here on experimental methods to ascertain a complete energy band alignment of a broken-gap tunnel field-effect transistor based on an InAs/GaSb hetero-junction. By using graphene as an optically transparent electrode in a traditional internal photoemission measurement, both the electron and hole barrier heights at the InAs/GaSb interface can be quantified. For a Al2O3/InAs/GaSb layer structure, the barrier height from the top of InAs and GaSb valence band to the bottom of Al2O3 conduction band is inferred from electron emission whereas hole emissions reveal the barrier height from the top of Al2O3 valence band to the bottom of InAs and GaSb conduction band. Subsequently, the offset parameter at the broken gap InAs/GaSb interface is extracted and thus can be used to facilitate the development of predicted model of electron quantum tunneling efficiency and transistor performance., Comment: 6 figures

Large perturbation transient photovoltage and impedance spectroscopy measurements are used to gain insights into recombination in organic photovoltaic devices. The combination of these two simple optoelectronic techniques enables characterization of recombination order as well as mobile and trapped charge evolution over a large range of carrier densities. The data show that trapped charge is approximately equal to total charge at low carrier densities in the high efficiency devices measured. Between low and high charge carrier density, the order of recombination is observed to vary from monomolecular to bimolecular to higher order. The new techniques and methods presented can be applied to any type of photovoltage device to gain insight into device operation and limitations.

We report on modeling of direct current (DC) characteristics of organic pentacene thin film transistors of different designs. Our model incorporates a gate-voltage dependent mobility and highly nonlinear drain and source contact series resistances. The contact nonlinearities are especially pronounced in bottom source and drain contact thin film transistors. The model successfully reproduced both below- and above-threshold characteristics of top source and drain contact and bottom source and drain contact organic pentacene thin film transistors.

We report on the flicker (1/f ) noise in pentacene organic thin film transistors (TFTs) of different designs. Our studies show that the TFT design affects the noise level and the noise dependence on the gate- and drain-source biases. The measured noise level was the lowest for the TFTs with a top source and drain contacts design. For these devices, the noise dependence at low drain current values resembled that for n-type crystalline Si metal–oxide–semiconductor field-effect transistors. The extracted Hooge parameter α, which allows comparing the noise level in different devices and materials, was 0.045 for the top-contact TFTs. This parameter value is several orders of magnitude lower than that for conducting polymers and only several times higher than that for hydrogenated amorphous Si (α-Si:H) TFTs. The bottom source and drain contacts TFTs had a much higher noise level with a noise dependence on the terminal voltages that differed from the noise voltage dependence for the top-contact TFTs. The Hooge parameter values were in the range of 5–20 for the bottom-contact TFTs. We estimated that the contact noise could be comparable to the channel noise for both top-contact and bottom-contact TFTs.

A rewriteable low-power operation nonvolatile physically flexible memristor device is demonstrated. The active component of the device is inexpensively fabricated at room temperature by spinning a TiO2 sol gel on a commercially available polymer sheet. The device exhibits memory behavior consistent with a memristor, demonstrates an on/off ratio greater than 10 000 : 1, is nonvolatile for over 1.2 times 106 s, requires less than 10 V, and is still operational after being physically flexed more than 4000 times.

We have fabricated organic thin-film transistors (TFT's) using the small-molecule polycyclic aromatic hydrocarbon pentacene as the active material. Devices were fabricated on glass substrates using low-temperature ion-beam deposited silicon dioxide as the gate dielectric, ion-beam deposited palladium for the source and drain contacts, and vacuum-evaporated pentacene to form the active layer. Excellent electrical characteristics were obtained, including carrier mobility as large as 0.6 cm/sup 2//V-s, on/off current ratio as large as 10/sup 8/, and subthreshold slope as low as 0.7 V/dec, all record values for organic transistors fabricated on nonsingle-crystal substrates.