Temperature-dependent Charge Transport Research Articles

Carrier Transport Engineering in Carbon Nanotubes by Chirality-Induced Spin Polarization.

Carbon nanotubes (CNTs), helically wrapped with single-stranded DNA, have recently emerged as a spin-filtering material. The inversion asymmetric helical potential of DNA creates a spin-filtering effect (commonly known as "chirality-induced spin selectivity" or CISS), which polarizes carrier spins in the nanotube. Thus, tuning of the DNA-CNT interaction is expected to affect carrier spins in nanotubes. The CISS effect induces spin polarization, which is coupled with the carrier's momentum direction, and therefore, in one-dimensional systems, such as nanotubes, momentum flip must be accompanied by a simultaneous spin flip. This spin momentum locking can have a profound impact on charge transport in nanotubes as backscattering due to phonons and disorder will be suppressed as these mechanisms are spin-independent. Here, we report DNA-CNT spin filters in which CNTs have been functionalized with two different classes of sequences, exhibiting different degrees of interaction with the CNT. They induce different degrees of spin polarization in the channel, with significant impact on temperature-dependent charge transport and interference phenomena arising from carrier backscattering. This work raises the intriguing possibility of engineering charge transport in nanotubes via CISS-induced spin polarization by tailor-made DNA sequences.

Charge transport in phenazine-fused triphenylene discotic mesogens doped with CdS nanowires

We report the synthesis of oleylamine capped CdS nanowires and we have dispersed a small optimized amount of these NWs in the Colh phase of a recently synthesized phenazine-fused-triphenylene discotic liquid crystal to understand the temperature-dependent charge transport.

Charge transport in pyroprotein-based electronic yarns.

Pyroprotein-based carbon materials produced by heat-treating silk proteins have many potential applications in electronic devices, such as electronic textiles. To further develop potential electronic devices using these pyroproteins, the charge transport mechanism has to be verified. However, the electrical characteristics of the pyroproteins have not been reported yet. In this study, the temperature-dependent charge transport behavior of pyroprotein-based electronic yarns prepared from commercial silks (e-CS yarns) is investigated with respect to various heat treatment temperatures (HTT, 800, 1000, 1200, and 1400 °C). The linear current-voltage properties are shown at a low bias of 100 nA from 9 K to 300 K. The temperature-dependent resistivity of the e-CS yarns can be clearly described by the crossover of 3-dimensional Mott variable range hopping and fluctuation-induced tunneling conduction at the crossover temperature (Tc). These Tc factors are significantly different, due to the structural modulation of the e-CS yarns depending on the HTT, and are characterized by Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. This study is expected to provide a better understanding of the electrical properties of pyroproteins.

Temperature-dependent charge property of silicone rubber/SiC composites under lightning impulse superimposed DC voltage

Field grading materials (FGMs) with nonlinear conductivity, which can form a uniform local field in high-voltage direct current (HVDC) cables, have received considerable attention. This paper focuses on the temperature-dependent charge transport property of an FGM based on high-temperature vulcanized (HTV) silicone rubber (SiR). A nonlinearity in the conductivity is achieved by mixing silicon carbide (SiC) particles with the SiR matrix. The conductivities of the samples are measured at temperatures of 30–90 °C. In addition, the surface potential decay is analyzed. The experimental results show that the nonlinear conductivity is suppressed at high temperatures. Increased temperature can cause a lattice vibration, which scatters carriers and reduces the conductivity. With an increase in the homopolar impulse voltage, all samples experiences a rise in the initial surface potential. The increase in SiC content and the increase in impulse voltage can both accelerate the decay process. The heteropolar impulse voltage not only intensifies the accumulation of surface charge but also slows down the decay process. Neutralization occurs immediately between heterocharges. The positive charges are captured by traps, and the attraction between heterocharges is thought to be the reason for the observed phenomena.

Field and temperature dependent charge transport mechanism in doped Poly(3-methylthiophene) devices

Thermal and electric field activated charge transport mechanism in Silver/Poly(3-methylthiophene)/Stainless Steel device with different charge carrier concentrations is studied. Temperature dependent Current-Voltage (I–V) characteristics of the devices show disorder induced non-linear behaviour Depending upon temperature range, the value of m varies suggesting that different transport mechanisms are active in the different temperature ranges. In the higher temperature range trap controlled space charge limited conduction (TCSCLC) with an exponential distribution of trap states plays a dominant role. In the low temperature range the carriers cannot perform phonon assisted hoping and the mobility varies with the applied electric field and TCSCLC mechanism changes to field dependent mobility. Field dependent charge carrier mobility is found to be more pronounced as carrier density decreases.

Trap-Assisted Switching in Silicon Nanocrystal Based p-i-n Device

An all Si, p-i-n device, consisting of a nanostructured porous Si layer sandwiched between a p-type crystalline Si and an n-type amorphous Si, exhibits current controlled switching. Temperature-dependent charge transport characteristics of the device reveal that trapping and detrapping of injected carriers at the interface of nanocrystalline Si core and the oxide shell, through impact excitation, are responsible for the observed switching. The abundance of surface inhomogeneities in the form of sub-oxides and dangling bonds creates multiple charge trapping centers which, in turn, endow the device with switching characteristics. The density of trap states ( $ \sim 10^{11}$ /cm $^{2}$ ) estimated from the detrapping potential is in very good agreement with the trap density obtained directly from C-V measurements.

Energy barriers at grain boundaries dominate charge carrier transport in an electron-conductive organic semiconductor

Semiconducting organic films that are at the heart of light-emitting diodes, solar cells and transistors frequently contain a large number of morphological defects, most prominently at the interconnects between crystalline regions. These grain boundaries can dominate the overall (opto-)electronic properties of the entire device and their exact morphological and energetic nature is still under current debate. Here, we explore in detail the energetics at the grain boundaries of a novel electron conductive perylene diimide thin film. Via a combination of temperature dependent charge transport measurements and ab-initio simulations at atomistic resolution, we identify that energetic barriers at grain boundaries dominate charge transport in our system. This novel aspect of physics at the grain boundary is distinct from previously identified grain-boundary defects that had been explained by trapping of charges. We furthermore derive molecular design criteria to suppress such energetic barriers at grain boundaries in future, more efficient organic semiconductors.

Electronically Coupled, Two-Dimensional Assembly of Cu1.1S Nanodiscs for Selective Vapor Sensing Applications

We study temperature-dependent charge transport in two-dimensional assemblies of copper sulfide nanodiscs in the covellite crystal phase (Cu1.1S). To enhance interparticle coupling, we cross-link the nanocrystals with the organic π-system Cu-4,4′,4″,4‴-tetraaminophthalocyanine and observe an increase in the conductivity by 6 orders of magnitude. The electrical properties of monolayers of this hybrid ensemble are consistent with a two-dimensional semiconductor and exhibit two abrupt changes at discrete temperatures (120 and 210 K), which may be interpreted as phase changes. X-ray scattering experiments serve to study the importance of electronic conjugation in the organic π-system vs interparticle spacing for efficient charge transport. Applying the hybrid ensemble as a chemiresistor in organic vapor sensing experiments reveals a strong selectivity between polar and nonpolar analytes, which we discuss in light of the role of the organic π-system and its metal center.

Inverse Temperature Dependence of Charge Carrier Hopping in Quantum Dot Solids.

In semiconductors, increasing mobility with decreasing temperature is a signature of charge carrier transport through delocalized bands. Here, we show that this behavior can also occur in nanocrystal solids due to temperature-dependent structural transformations. Using a combination of broadband infrared transient absorption spectroscopy and numerical modeling, we investigate the temperature-dependent charge transport properties of well-ordered PbS quantum dot (QD) solids. Contrary to expectations, we observe that the QD-to-QD charge tunneling rate increases with decreasing temperature, while simultaneously exhibiting thermally activated nearest-neighbor hopping behavior. Using synchrotron grazing-incidence small-angle X-ray scattering, we show that this trend is driven by a temperature-dependent reduction in nearest-neighbor separation that is quantitatively consistent with the measured tunneling rate.

Tunable Optical and Electrical Transport Properties of Size- and Temperature-Controlled Polymorph MoS2 Nanocrystals

The phase transition of chemically synthesized MoS2 nanocrystals (NCs) from the metallic 1T to the semiconducting 2H phase has been investigated in detail. The metallic 1T phase NCs were prepared by the Li+ intercalation–deintercalation exfoliation techniques followed by prolonged sonication. The effect of ex situ thermal annealing on MoS2 polymorphs and their transformation from the 1T to 2H phase has been extensively monitored by the X-ray photoelectron, Raman, and optical absorption spectroscopy techniques. Electrical conductivity measurements have also been carried out to probe the phase transition of the synthesized NCs. The temperature-dependent (10–350 K) electrical charge transport properties of variable-sized NCs have been investigated to probe the scaling of conductivity and activation energy with size, which are yet to be reported experimentally. The charge transport mechanisms through the NC assembly for different temperature regions have been modeled and it is observed that the electron trans...

Temperature-dependent electronic charge transport characteristics at MoS2/p-type Ge heterojunctions

Significant effort has been devoted to constructing two-dimensional transition metal dichalcogenides-based hybrid heterojunctions with enhanced performance or unique functionalities for versatile device applications in emerging electronics and optoelectronics. In this study, we report the temperature-dependent electronic charge transport characteristics in MoS2/p-type Ge heterojunction diodes. From the current-voltage (I-V) characteristics of the heterojunction device, it is observed that different transport phenomena can occur depending upon the temperature and bias voltage. The charge transport is dominated by thermionic emission in the high-temperature regime above 300 K, whereas in low-temperature regime below 300 K, the charge transport mechanism transitions from the thermionic emission to the tunneling mechanism associated with trap sites. In particular, the I-V characteristics in the low-temperature regime show a transition from the direct tunneling at a low bias to the Fowler-Nordheim tunneling mechanism at a high bias. This could be well described by the electrical analyses on temperature-dependent I-V behavior and corresponding energy band diagrams.

(Invited) Dense Layers of (6,5) Nanotubes for Optical and Charge Transport Applications

Large volumes of highly purified dispersions of monochiral (6,5) single-walled carbon nanotubes are readily available through polymer-wrapping and shear-force mixing. They enable the deposition (e.g. by aerosol jet printing or filtration) of semiconducting layers of variable thickness from sparse networks to 300 nm thick films with large optical density. These layers can be applied in lateral and vertical field-effect transistors with excellent device performance but also as electrochromic notch filters or for organic light-emitting diodes in the near-infrared. Here, we present examples of such applications and investigate the impact of novel n-dopants on nanotube networks, temperature-dependent charge transport within monochiral and mixed layers and their optical properties at different charge carrier densities.

Transition from direct to inverted charge transport Marcus regions in molecular junctions via molecular orbital gating.

Solid-state molecular tunnel junctions are often assumed to operate in the Landauer regime, which describes essentially activationless coherent tunnelling processes. In solution, on the other hand, charge transfer is described by Marcus theory, which accounts for thermally activated processes. In practice, however, thermally activated transport phenomena are frequently observed also in solid-state molecular junctions but remain poorly understood. Here, we show experimentally the transition from the Marcus to the inverted Marcus region in a solid-state molecular tunnel junction by means of intra-molecular orbital gating that can be tuned via the chemical structure of the molecule and applied bias. In the inverted Marcus region, charge transport is incoherent, yet virtually independent of temperature. Our experimental results fit well to a theoretical model that combines Landauer and Marcus theories and may have implications for the interpretation of temperature-dependent charge transport measurements in molecular junctions.

Temperature dependent charge transport studies across thermodynamic glass transition in P3HT:PCBM bulk heterojunction: insight from J–V and impedance spectroscopy

Temperature dependent charge transport properties of P3HT:PCBM bulk heterojunction are analysed by dc and ac measurements under dark conditions across a wide temperature range of 110–473 K, which includes the thermodynamic glass transition temperature ( K) of the system. A change from Ohmic conduction to space charge limited current conduction at higher (⩾1.2 V) applied bias voltages above ⩾200 K is observed from J–V characteristics. From capacitance–voltage (C–V) measurement at room temperature, the occurrence of a peak near the built-in voltage is observed below the dielectric relaxation frequency, originating from the competition between drift and diffusion driven motions of charges. Carrier concentration (N) is calculated from C–V measurements taken at different temperatures. Room temperature mobility values at various applied bias voltages are in accordance with that obtained from transient charge extraction by linearly increasing voltage measurement. Sample impedance is measured over five decades of frequency across temperature range by using lock-in detection. This data is used to extract temperature dependence of carrier mobility (μ), and dc conductivity () which is low frequency extrapolation of ac conductivity. An activation energy of ∼126 meV for the carrier hopping process at the metal–semiconductor interface is estimated from temperature dependence of . Above Tg, μ levels off to a constant value, whereas starts to decrease after a transition knee at Tg that can be seen as a combined effect of changes in μ and N. All these observed changes across Tg can be correlated to enhanced polymer motion above the glass transition.

Full quantum treatment of charge dynamics in amorphous molecular semiconductors

We present a treatment of charge dynamics in amorphous molecular semiconductors that accounts for the coupling of charges to all intramolecular phonon modes in a fully quantum mechanical way. Based on ab initio calculations, we derive charge transfer rates that improve on the widely used semiclassical Marcus rate and obtain benchmark results for the mobility and energetic relaxation of electrons and holes in three semiconductors commonly applied in organic light-emitting diodes. Surprisingly, we find very similar results when using the simple Miller-Abrahams rate. We conclude that extracting the disorder strength from temperature-dependent charge transport studies is very possible but extracting the reorganization energy is not.

Ambipolar charge transport in a bis-diketopyrrolopyrrole small molecule semiconductor with tunable energetic disorder.

Energetic disorder and activation energy are important parameters that influence the charge carrier mobility in organic semiconductors. Herein, we investigate temperature-dependent ambipolar charge transport alongside its thermal activation energy in organic field-effect transistors (OFETs) based on a diketopyrrolopyrrole (DPP) based small molecule BTDPP2. The determined energetic disorder in BTDPP2 is analogous to those of highly crystalline molecules, such as pentacene, while lower than those of widely used fullerene derivatives (PCBM) or semi-crystalline polymers, such as P3HT. We demonstrate that the energetic disorder and activation energy in BTDPP2 are both impacted by the crystallinity, which is tuned by thermal annealing; moreover, to a certain extent, these two parameters can reduce with increasing the structural order. Moreover, the energetic disorder tends to decrease when BTDPP2 is subjected to thermal annealing. Through comparing the electron transport in BTDPP2 based OFETs and vertical diodes, in which the electron densities differentiate substantially, the different activation energies are roughly described in terms of achievable carrier densities in these two devices. To the best of our knowledge, this aspect has not been addressed on the electron transport in molecular semi-conductive materials. Our results shine light on fundamental understandings of charge transport properties in solution processed small molecules holding promise for opto-electronic applications.

Understanding the temperature-dependent charge transport, structural variation and photoluminescent properties in methylammonium lead halide perovskite single crystals

Temperature dependent XRD illustrates a tetragonal to orthorhombic phase transition around 150 K in methylammonium lead halide perovskite single crystals.

Temperature dependent charge transport mechanisms in highly crystalline p-PbS cubic nanocrystals grown by chemical bath deposition

Lead sulfide nanocrystal films, comprising of well faceted 350nm cubic-nanocrystals are tailored by ion-flux controlled chemical bath deposition at 50°C to 80°C bath temperatures. These possess compact (111) oriented nanocubic morphology and exhibit significantly improved electrical and opto-electronic properties as necessary for emerging next generation solar energy conversion applications. All films are of p-type nature and concurrently possess high carrier mobility (µ), high hole concentration (p) and low electrical resistivity (ρ) at room temperature. In particular, for films grown at 75°C, high values of µ = 29.3cm2V–1s–1, p = 8.1×1017cm–3 and ρ = 0.26Ωcm are observed on the same nanocubic film. At high temperatures (T > 150K), the charge transport is governed by thermally activated band conduction and nearest-neighbour hopping conduction. However, at lower temperatures (T < 150K), both Mott variable-range hopping (VRH) and Efros–Shklovskii (ES) VRH type hopping are prevalent. PbS films grown at 75°C have the lowest thermal activation energy. The distinct crossover from Mott-VRH to ES-VRH conduction process is observed at ~ 80K. The observed decrease in Mott temperature with increase in bath temperature has been attributed to the lesser defects and enhanced density of states at Fermi level, which suggest the reduction in activation energy of the films beneficial for improved cell performance. Prevalence of conduction mechanisms is correlated with growth process.

Temperature-dependent charge transport mechanisms in carbon sphere/polyaniline composite

Charge transport in the temperature range 80 K &amp;lt; T &amp;lt; 300 K was studied in a composite of carbon spheres (CS), prepared via hydrothermal carbonization of sucrose, and the conducting polymer polyaniline (PANi). PANi was synthesized via the oxidative polymerization of aniline with ammonium peroxydisulfate (APS) in acidic media. The CS/PANi composite was prepared by coating the spheres with a thin polyaniline (PANi) film doped with hydrochloric acid (HCl) in situ during the polymerization process. Temperature dependent conductivity measurements show that three dimensional variable range hopping of electrons between polymeric chains in PANi-filled gaps between CS is the predominant transport mechanism through CS/PANi composites. The high conductivity of the CS/PANi composite makes the material attractive for the fabrication of devices and sensors.

Electric field involved transport at elevated temperature in nanocrystalline silicon carbide nitride (nc-SiCN) thin films for harsh environment applications

The present study represents a systematic temperature dependent charge transport and dielectric properties of nanocrystalline silicon carbide nitride (nc-SiCN) thin films grown on Pt/Ti/SiO2/Si substrate. A large negative temperature coefficient of resistance (TCR) ranging from 6200 to 2300 ppmK-1 in the temperature range 300–773 K, suggests that the nc-SiCN thin films could be useful for futuristic thermal-based sensors. The current density vs. electric field (J-E) characteristics was measured at different temperatures (300–673 K). Detailed J-E analysis revealed an ohmic conduction at the low applied electric field (<65 kV/cm) within the entire temperature range. However, at high electric field (>65 kV/cm), space charge limited conduction (SCLC) mechanism was found to be dominating in low measurement temperature (300–473K), whereas, a transition from SCLC mechanism to Poole-Frenkel mechanism was observed with further increment in the temperature beyond 473 K. The temperature invariant dielectric tunability (nr ∼10%) and low zero electric field leakage current density (J ∼10−7A/cm2) at 673 K temperature, demonstrates the feasibility of nc-SiCN thin films for tunable device applications in the high-temperature and harsh environment.