Temperature Dependence Of Transport Properties Research Articles

Temperature Dependence of Transport Properties of Spiro-MeOTAD as a Hole Transport Material in Solid-State Dye-Sensitized Solar Cells

The internal transport and recombination parameters of solid-state dye-sensitized solar cells (ssDSCs) using the amorphous organic semiconductor 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) as a hole transport material (HTM) are investigated using electrical impedance spectroscopy. Devices were fabricated using flat and nanostructured TiO2 and compared to systems using nanostructured ZrO2 to differentiate between the transport processes within the different components of the ssDSC. The effect of chemically p-doping the HTM on its transport was investigated, and its temperature dependence was examined and analyzed using the Arrhenius equation. Using this approach the activation energy of the hole hopping transport within the undoped spiro-MeOTAD film was determined to be 0.34 ± 0.02 and 0.40 ± 0.02 eV for the mesoporous TiO2 and ZrO2 systems, respectively.

Thermoelectric transport and Hall measurements of low defect Sb2Te3 thin films grown by atomic layer deposition

Sb2Te3 has recently been an object of intensive research since its promising applicability in thermoelectric, in phase-change memory devices and as a topological insulator. In this work, we report highly textured Sb2Te3 thin films, grown by atomic layer deposition on Si/SiO2 wafers based on the reaction of SbCl3 and (Et3Si)2Te. The low deposition temperature at 80 °C allows the pre-patterning of the Sb2Te3 by standard lithography processes. A platform to characterize the Seebeck coefficient S, the electrical conductivity σ as well as the Hall coefficient RH on the same film has been developed. Comparing all temperature-dependent transport properties, three different conductive regions in the temperature range of 50–400 K are found. Room temperature values of S = 146 × 10–6 VK−1, σ = 104 Sm−1 and mobility µ = 270.5 × 10−4 m2 V−1 s−1 are determined. The low carrier concentration in the range of n = 2.4 × 1018 cm−3 at 300 K quantifies the low defect content of the Sb2Te3 thin films.

Li-Ion Cell Operation at Low Temperatures

Substantially reduced energy and power capabilities of lithium-ion cell operating at low temperatures pose a technical barrier for market penetration of hybrid electric vehicles and pure electric vehicles. The present work delineates Li-ion cell behaviors at low temperatures by a combined experimental and modeling approach. An electrochemical-thermal coupled model, incorporating concentration- and temperature-dependent transport and kinetic properties, is applied and validated against 2.2Ah 18650 cylindrical cells over a wide range of temperatures (−20°C to 45°C) and discharge rates. Simulation and experimental results demonstrate the dramatic effects of cell self-heating upon electrochemical performance. A nonisothermal Ragone plot accounting for these important thermal effects is proposed for the first time for Li-ion cells and more generally for thermally coupled batteries. Detailed resistance analysis indicates that performance limits at −20°C depend on not only discharge rates but also thermal conditions. Optimization of cell design parameters and material properties is performed for 1 C rate discharge starting from −20°C, where the principal performance limitations are found to be Li+ diffusion in the electrolyte and solid-state Li diffusion in graphite particles, instead of charge-transfer kinetic or ohmic resistance.

Fabrication and Characterization of Nanostructured Bulk Skutterudites

ABSTRACTLatest nanotechnology concepts applied in thermoelectric (TE) research have opened many new avenues to improve the ZT value. Low dimensional structures can improve the ZT value as compared to bulk materials by substantial reduction in the lattice thermal conductivity, κL. However, the materials were not feasible for the industrial scale production of macroscopic devices because of complicated and costly manufacturing processes involved. Bulk nanostructured (NS) TEs are normally fabricated using a bulk process rather than a nano-fabrication process, which has the important advantage of producing in large quantities and in a form that is compatible with commercially available TE devices.We developed fabrication strategies for bulk nanostructured skutterudite materials based on FexCo1-xSb3. The process is based on precipitation of a precursor material with the desired metal atom composition, which is then exposed to thermochemical processing of calcination followed by reduction. The resultant material thus formed maintains nanostructured particles which are then compacted using Spark Plasma Sintering (SPS) by utilizing previously optimized process parameters. Microstructure, crystallinity, phase composition, thermal stability and temperature dependent transport property evaluation has been performed for compacted NS FexCo1-xSb3. Evaluation results are presented in detail, suggesting the feasibility of devised strategy for bulk quantities of doped TE nanopowder fabrication.

Transport properties of Cu5SmSe4

The electrical conductivity, Hall coefficient, thermoelectric power, and thermal conductivity of Cu5SmSe4 have been measured at temperatures from 80 to 400 K, and its linear thermal expansion coefficient has been determined from X-ray diffraction data. The observed negative a-axis linear thermal expansion coefficient and anomalies in temperature-dependent transport properties of this compound are tentatively attributed to a transition of the samarium atoms from a divalent to a trivalent state.

A VOF Method for Isobaric Expandable Flows

AbstractIn this paper a method is described to simulate isobaric multiphase flows at low Mach numbers with steep temperature gradients for fluids with non‐negligible thermal expansivity. Governing equations and solution procedure are outlined. Further, a test case is shown in order to verify the model. Single phase natural convection flows with large temperature differences and either constant or temperature dependent transport properties were simulated to prove the solution of coupled Navier‐Stokes and energy equations. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Universal scaling of resistivity in bilayer graphene

We report the temperature dependent electrical transport properties of gated bilayer graphene devices. We see a clear evidence of insulating behavior due to electron-hole charge puddles. The electrical resistivity increases while the mobility decreases with decreasing temperature, a characteristic due to carrier inhomogeneity in graphene. The theoretical fittings using an empirical formula of single electron tunneling indicate that electrical resistivity follows a universal curve with a scaling parameter. The scaling parameter is determined to be a measure of the fluctuations in the electron-hole puddle distribution.

Strain-induced suppression of weak localization in CVD-grown graphene

We investigate the magnetic-field- and temperature-dependent transport properties of CVD-grown graphene transferred to a flexible substrate (Kapton) and subjected to externally applied strain. In zero magnetic field, a logarithmic temperature-dependent conductivity correction, resulting from strong electron–electron interaction, becomes weaker with the application of strains as large as 0.6% because of an increased rate of chiral-symmetry-breaking scattering. With the application of a perpendicular magnetic field, we also observe positive magnetoconductance at low temperature (T = 5 K) due to weak localization. This magnetoconductance is suppressed with increasing strain, concomitant with a rapid decrease of the intervalley scattering rate (). Our results are in good agreement with theoretical expectations and are consistent with a strain-induced decoupling between graphene and its underlying Kapton substrate.

Temperature dependence of transport properties in a suspended carbon nanotube

The temperature dependence of the conductance of a suspended carbon nanotube, as probed by means of a scanning tunneling microscope (STM) tip, is analyzed. Employing the theory of coupling between electronic and vibronic degrees of freedom, local linear and nonlinear transport features are studied as a function of temperature. It is shown that the thermal activation of excited vibronic modes leads to a reduction of the STM signal.

Structural and transport properties of Sn–Mg alloys

In this work, the structural and transport properties of Mg-doped Sn-based alloys have been investigated. The temperature-dependent transport and structural properties of Sn–Mg alloys were investigated for five different samples (Pure Sn, Sn-1.0 wt% Mg, Sn-2.0 wt% Mg, Sn-6.0 wt% Mg and Pure Mg). Scanning electron microscopy (SEM), X-ray diffraction and energy dispersive X-ray analysis measurements were carried out in order to clarify the structural properties of the samples. It was found that the samples had tetragonal crystal symmetry, except for pure Mg which had hexagonal crystal symmetry. We also found that the cell parameters changed slightly with the addition of Mg element. The SEM micrographs of the samples showed that they had smooth surfaces with a clear grain boundary. The electrical and thermal conductivity of the samples were measured by four-point probe and the radial heat flow method, respectively. The electrical resistivity of the samples increased almost linearly with the increasing temperature. The thermal conductivity values ranged between 0.60 and 1.00 W/Km, while they decreased slightly with temperature and increased with Mg composition. The thermal conductivity values of the alloys were in between the values of pure Sn and Mg. The thermal conductivity results of the alloys were compared with other available results, and a good agreement was seen between the results. In addition, the temperature coefficients of electrical resistivity and thermal conductivity were determined; these were independent of the composition of the alloying elements.

Physical properties of transparent perovskite oxides (Ba,La)SnO3with high electrical mobility at room temperature

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys. Express. 5, 061102 (2012)]. Following that work, we report various physical properties of (Ba,La)SnO3 single crystals and films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We find that almost doping-independent mobility of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4 ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest value. We attribute the high mobility to several physical properties of (Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding, small disorder effects due to the doping away from the SnO2 conduction channel, and reduced carrier scattering due to the high dielectric constant. The observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly attributed to additional carrier-scatterings which are presumably created by the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the in-gap states only slightly increased, thus maintaining optical transparency in the visible region. Based on these, we suggest that the doped BaSnO3 system holds great potential for realizing all perovskite-based, transparent high-frequency high-power functional devices as well as highly mobile two-dimensional electron gas via interface control of heterostructured films.

Temperature-dependent electron transport in ZnO micro/nanowires

Temperature dependent electron transport properties of individual ZnO mirco/nanowires from 293 K to 473 K were investigated by a four-probe method, and the size dependence of activation energy was studied. The conductivity increased with the increasing temperature, which was attributed to the thermal activations of donors: shallow donors and deep donors. The activation energy related to shallow donors was effectively independent of the radius, suggesting that it was associated with the surface conduction channel of ZnO wire, while a nearly inverse dependence of the activation energy of deep donors on the wire diameter may result from the dielectric confinement effect.

입구유동 가진이 있는 층류 후향계단 유동에서 열전달에 대한 프란틀수 효과해석

Key Words: Reattachment Length(재부착길이), Thermal Boundary Layer Thickness(열경계층두께), InletPulsation(입구가진), Nusselt Number(누설트수), Prandtl Number(프란틀수)초록: 입구유동가진이있는층류후향계단유동에서수의변화에따른열전달특성변화를조사하기위하여비정상수치해석을수행하였다. 입구는가진주파수와가진진폭의변화에따른교란이주어졌고,온도함수의물성치가적용되었다. 열전달변화에대한다양한특징들이열경계층변화에의해서설명되었다. 물성치가일정한경우와비교하였을때, 물성치가온도의함수성이있는경우인조건에서열전달은감소하였고, 반대로인조건에서열전달은증가하였다. 또한가진진폭이증가함에따라계단후류바닥면에서열전달도증가하였다. 그렇지만가진주파수변화의경우인조건에서열전달이크게증가하는특정주파수영역이존재하였다. 특히, 열전달의증가는재부착길이의rms값의변동정도와밀접한관련이있음을보여주었다.Abstract: The wall heat transfer of backward-facing step laminar flows with different Prandtl numbers and apulsating inlet is investigated by unsteady simulations. The inlet is perturbed by the variation of frequencyand amplitude. Temperature-dependent transport properties are adopted. Various characteristics of the wall heattransfer are explained by the variation of the thermal boundary layer. For Pr < 1, the wall heat transfer oftemperature-dependent properties is decreased compared to that of constant properties, whereas it increases forPr < 1. In addition, the wall heat transfer increases depending on the pulsating amplitude. However, theresults of frequency variation for St < 0.2 show that the heat transfer is strongly enhanced at a specificfrequency. In particular, the increase in the wall heat transfer is strongly related to the root mean square ofthe fluctuations of the reattachment length.

Interplay between phonon and impurity scattering in two-dimensional hole transport

We investigate temperature-dependent transport properties of two-dimensional $p$-GaAs systems taking into account both hole-phonon and hole-impurity scattering effects. By analyzing the hole mobility data of $p$-GaAs in the temperature range $10\phantom{\rule{4.pt}{0ex}}\text{K}&lt;T&lt;100$ K, we estimate the value of the appropriate deformation potential for hole-phonon coupling. Due to the interplay between hole-phonon and hole-impurity scattering the calculated temperature-dependent resistivity shows interesting nonmonotonic behavior. In particular, we find that there is a temperature range (typically $2\phantom{\rule{4.pt}{0ex}}\text{K}&lt;T&lt;10$ K) in which the calculated resistivity becomes independent of temperature due to a subtle cancellation between the temperature-dependent resistive scattering contributions arising from impurities and phonons. This resistivity saturation regime appears at low carrier densities when the increasing resistivity due to phonon scattering compensates for the decreasing resistivity due to the nondegeneracy effect. This temperature-independent flat resistivity regime is experimentally accessible and may have already been observed in a recent experiment.

Current transport in nonpolar a-plane InN/GaN heterostructures Schottky junction

The temperature dependent current transport properties of nonpolar a-plane (112¯0) InN/GaN heterostructure Schottky junction were investigated. The barrier height (ϕb) and ideally factor (η) estimated from the thermionic emission (TE) model were found to be temperature dependent in nature. The conventional Richardson plot of the ln(Is/T2) versus 1/kT has two regions: the first region (150–300 K) and the second region (350–500 K). The values of Richardson constant (A*) obtained from this plot are found to be lower than the theoretical value of n-type GaN. The variation in the barrier heights was explained by a double Gaussian distribution with mean barrier height values (ϕb¯) of 1.17 and 0.69 eV with standard deviation (σs) of 0.17 and 0.098 V, respectively. The modified Richardson plot in the temperature range 350–500 K gives the Richardson constant which is close to the theoretical value of n-type GaN. Hence, the current mechanism is explained by TE by assuming the Gaussian distribution of barrier height. At low temperature 150–300 K, the absence of temperature dependent tunneling parameters indicates the tunneling assisted current transport mechanism.

CsSnI3: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions

CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.

The effect of position and length of alkyl substituents in pyridinium based ionic liquids on temperature dependent transport properties

Abstract The effect of position and length of alkyl substituent in pyridinium based ionic liquids (IL) has been studied at various temperatures. The IL investigated contained the bis(trifluoromethanesulfonyl)imide (NTf 2 ) anion combined with 1-alkyl-3,5-dimethylpyridinium or 1-alkyl-2,3-dimethylmpyridinium cations, respectively. Data for the conductivity 0–80 °C, the viscosity 10–70 °C, density 0–80 °C and thermal expansion coefficients 0–80°C of the ionic liquids [B23MMPYR][NTf 2 ], [B35MMPYR][NTf 2 ], [O23MMPYR][NTf 2 ], [O35MMPYR][NTf 2 ] are reported. The viscosity and conductivity data obtained could be well described with the empirical Vogel–Fulcher–Tammann equation. Based on the molar conductivities and viscosities measured from 10 to 70 °C, a significant influence of the position of the methyl substituents in the cation on the transport properties has been found. Walden plots of these data demonstrate that the investigated compound can be classified as high ionicity ionic liquids. IL containing 1-alkyl-2,3-dimethylpyrindium cations were found to be closer to the “ideal” KCl line suggesting less ion association compared to their 1-butyl-3,5-dimethylpyridinium homologues.

Off-diagonal carrier-phonon coupling and polaron transport

A method extending the Munn-Silbey approach has been applied to obtain the temperature dependence of transport properties of a generalized Holstein model incorporating simultaneous diagonal and off-diagonal carrier-phonon coupling. The Hamiltonian is partially diagonalized by a canonical transformation, and optimal transformation coefficients are determined in a self-consistent manner. Effects of off-diagonal coupling on the optimal transformation coefficients and diffusion coefficients have been discussed in details. The off-diagonal coupling has been revealed as a localization factor as well as a transport mechanism. Moreover, momentum-space variation of the transformation coefficients are found to be responsible for enhanced transport due to off-diagonal coupling.

Magnon excitation and temperature dependent transport properties in magnetic tunnel junctions with Heusler compound electrodes

Magnetic tunnel junctions were prepared with the Heusler compounds Co2FeAl, Co2FeSi, and Co2MnSi as the soft magnetic electrode. The Co2MnSi electrodes had a multilayer design that used either the Co2FeAl or the Co2FeSi compound as a buffer material. Pinned Co-Fe was used as the hard reference electrode. The electronic transport characteristics were analyzed by tunneling spectroscopy. The dependence of sample properties on the buffer material was of interest, especially the gap in the minority density of states of the Heusler electrode. The temperature dependence of the transport properties was also investigated.

One dimensional ternary Cu–Bi–S based semiconductor nanowires: synthesis, optical and electrical properties

Ternary Cu–Bi–S based compounds have been thought to be alternative materials for well-known CuInS2 because of their abundance. Cu–Bi–S based nanomaterials have been less studied. We here report the synthesis, optical and electrical properties of single crystal Cu4Bi4S9 nanowires. High-quality Cu4Bi4S9 nanowires were synthesized through a modified solvothermal route by controlling the reaction sources and temperature. The optical bandgap for Cu4Bi4S9 nanowires were determined by using UV-vis-NIR and cyclic voltammetry techniques. Single nanowire devices were fabricated by using lithographic techniques. The devices exhibit photoconductive response with high external quantum efficiency (2.9 × 108%). Temperature-dependent electrical transport properties were also investigated. We observed that the transport properties of Cu4Bi4S9 nanowire show typical semiconductor behaviour in the temperature region 10–140 K and metal-like character in the temperature region of 150–300 K. The carrier transport in Cu4Bi4S9 nanowires can be described by the small polaron model in temperature region of 60–140 K and the variable range hopping mechanism in temperature region of 10–50 K. We further studied the properties of Cu4Bi4S9 nanowires in field-emission devices. The devices exhibit a relatively low turn-on field (6.9 V μm−1). The potential applications of Cu4Bi4S9 nanowires as field emitting materials and light absorbers in detectors are indicated.