Effective Density Of States Research Articles

Characterization and analysis of thermoelectric transport using SPB model in nanostructured aluminum doped zinc tellurium

Low-temperature electronic and thermal transport measurements are carried out on nanostructured Zn1−xAlxTe (0 ⩽ x ⩽ 0.15) fabricated using hydrothermal synthesis followed by evacuated-and-encapsulated sintering. A single parabolic band with acoustic phonon scattering is used to analyze thermoelectric transport data. It is found that reduced Fermi energy gets closer to the valence band edge and density of states effective mass, effective density of states, and Hall factor decrease with increasing x in doped samples. The chemical carrier concentration, carrier density independent mobility, β, and theoretical zT values increase with increasing x in doped samples. The nanostructured Zn1−xAlxTe exhibits significant reduction of thermal conductivity at 300 K (1.82–3.71 W m−1 K−1) as compared to bulk ZnTe (18 W m−1 K−1). The point-defect scattering and phonon-grain scattering play an important role in reducing the lattice thermal conductivity. In addition, partial substitution of Al3+ for Zn2+ significantly improves both the power factor and zT values.

Study of the mobility activation in ZnSe thin films deposited using inert gas condensation

Abstract ZnSe thin films were synthesized on glass substrates using the inert gas condensation technique at substrate temperature ranging from 25 °C to 100 °C. The hexagonal structure and average crystallite size (6.1–8.4 nm) were determined from X-ray diffraction data. The transient photoconductivity was investigated using white light of intensity 8450 lx to deduce the effective density of states ( N eff in the order of 1.02 × 10 10 –13.90 × 10 10 cm −3 ), the frequency factor ( S in the range 2.5 × 10 5 –24.6 × 10 5 s −1 ) and the trap depth ( E ranging between 0.37–0.64 eV) of these films. The trap depth study revealed three different types of levels with quasi-continuous distribution below the conduction band. An increase in the photoconductivity was observed as a result of the formation of potential barriers ( V b ) and of the increase of carrier mobility at the crystallite boundaries. The study of the dependence of various mobility activation parameters on the deposition temperature and the crystallite size has provided better understanding of the mobility activation mechanism.

Bulk and interface recombination in planar lead halide perovskite solar cells: A Drift-Diffusion study

A theoretical approach based on Drift-Diffusion equations is presented to study planar mixed lead halide perovskite solar cells. Updated physical parameters such as permittivity, mobility, effective density of states and doping density is employed in simulations. Current-voltage curve data for two experimental sample is imported and through fitting with the model, density of bulk and interface defects is calculated. We obtain the bulk defect density around 1016cm−3 and surface recombination velocities in the range of 10cm/s. These values which are in good agreement with experimental measurements and considerably deviated from previous theoretical studies, verify the model and adopted constants. Shockley-Queisser limit is also presented as the ideal device and the effect of bulk and interface defects are presented as loss factors that cause departure from this limit. Our simulations conclude that the overall efficiency of perovskite solar cells is mainly governed by the open-circuit voltage and also identify the interface defects as the major loss factor in these devices.

Numerical modeling of thermal behavior and structural optimization of a-Si:H solar cells at high temperatures

A large amount of solar energy is collected in tropical zones of the earth, where the temperature of photo-voltaic (PV) modules can exceed to more than $$70\,^{\circ }\text {C}$$70źC. At such high temperatures, the performance of most solar cells is greatly degraded, since the performance parameters of the cells are mainly decreasing functions of temperature. Despite the high importance of solar energy conversion in hot places, little attention has been paid to the investigation of high-temperature effects on the conversion efficiency of the cells. In this paper, the performance of a wide selection of solar cells of different structures made from different materials is analyzed at high temperatures in the range of 50---75 $$^{\circ }\text {C}$$źC, which is different from the standard test condition usually used in the assessment of solar cells. An accurate model for thermal behavior of the cell is suggested, in which almost all important parameters affecting each layer on the cell's thermal behavior are included, such as mobility, thermal velocity of carriers, bandgap, Urbach energy of band tails, electron affinity, relative permittivity, and effective density of states in the valence and conduction bands. The effects of possible arrangements of different layers and their materials and structural parameters, as well as light-induced defects and sunlight intensity, are also studied in the analysis. A relation is proposed between the optimal thickness of the absorber layer and the working temperature. Finally, an optimal structure of the solar cell at high temperatures is suggested.

Structure Making and Breaking Effects of Cations in Aqueous Solution: Nitrous Oxide Pump-Probe Measurements.

Ultrafast IR pump-probe responses resonant with the ν3 asymmetric stretch of nitrous oxide (N2O) at ∼2230 cm-1 are reported for 2 M aqueous salt solutions of MgCl2, CaCl2, NaCl, KCl, and CsCl at room temperature. The solvated cations of these chloride solutions span the range from strongly to weakly hydrating ions, and correspondingly are often categorized as structure makers and structure breakers, respectively. The observed salt dependent trends of the N2O ν3 vibrational energy relaxation (VER) and rotational reorientation anisotropy (R(t)) decays are consistent with the categorization of these cations as structure breakers or makers, and show evidence of effects on the water hydrogen bonding network beyond the first solvation shell of these ions. This N2O mode is resonant with the H2O bend-libration band region. The corresponding FTIR is fitted well by a two Gaussian plus sloping continuum baseline model that allows a framework for characterizing the salt perturbations of the solvent spectral density in the ν3 resonant region. Both coupling strengths and density of states effects appear to contribute the systematic cation dependent T1 effects reported here. R(t) decays follow bulk viscosity values. These results are contrasted with previous IR pump-probe studies predominantly based on the relaxation dynamics of the OH/OD vibrational stretch of HOD hydrogen bonded to anions in salt solutions.

An analytical model to explore open-circuit voltage of a-Si:H/c-Si heterojunction solar cells

The effect of the parameters on the open-circuit voltage, V OC of a-Si:H/c-Si heterojunction solar cells was explored by an analytical model. The analytical results show that V OC increases linearly with the logarithm of illumination intensity under usual illumination. There are two critical values of the interface state density (D it) for the open-circuit voltage (V OC), D it crit,1 and D it crit,2 (a few 1010 cm−2∙eV−1). V OC decreases remarkably when D it is higher than D it crit,1 . To achieve high V OC, the interface states should reduce down to a few 1010 cm−2·eV−1. Due to the difference between the effective density of states in the conduction and valence band edges of c-Si, the open-circuit voltage of a-Si:H/c-Si heterojunction cells fabricated on n-type c-Si wafers is about 22 mV higher than that fabricated on p-type c-Si wafers at the same case. V OC decreases with decreasing the a-Si:H doping concentration at low doping level since the electric field over the c-Si depletion region is reduced at low doping level. Therefore, the a-Si:H layer should be doped higher than a critical value of 5×1018 cm−3 to achieve high V OC.

Enhancement of Intrinsic Carrier Concentration in the Active Layer of Solar Cell Using Indium Nitride Quantum Dot

This paper presents the improvement of intrinsic carrier concentrations in the active layer of solar cell structure using Indium Nitride quantum dot as the active layer material. We have analyzed effective density of states in conduction band and valance band of the solar cell numerically using Si, Ge and InN quantum dot in the active layer of the solar cell structure in order to improve the intrinsic carrier concentration within the active layer of the solar cell. Then obtained numerical results were compared. From the comparison results it has been revealed that the application of InN quantum dot in the active layer of the device structure improves the effective density of states both in conduction band and in the valance band. Consiquently the intrinsic carrier concentration has been improved significently by using InN quantum dot in the solart cell structure.

What parameters can be reliably deduced from the current-voltage characteristics of an organic bulk-heterojunction solar cell?

Through the analysis of scales and simplification of the drift-diffusion device model, we have obtained a quantitative description of the mechanisms underlying the current-voltage (j–V) characteristics of organic bulk-heterojunction solar cells. The mechanisms have been resolved into the competition between the photogeneration, recombination, and extraction/injection rates, which determines the bulk charge carrier concentration; and the combined effect of the built-in field and the boundary layers in shaping the electric potential distribution, which determines the bulk field. The relationships between the j–V characteristics and standard model parameters have been captured with analytical expressions and verified through 1-D numerical simulations. We have determined that while the charge carrier generation rate can be reliably extracted with the device model from j–V measurements alone, the effective density of states and built-in potential, and the mobility and recombination prefactor are clustered pairs that can only be decoupled through other characterization techniques.

Transport Property Measurements in Doped Bi2Te3 Single Crystals Obtained via Zone Melting Method

Single crystals of Se- and Fe-doped Bi2Te3 have been synthesized via the zone melting method. Energy-dispersive x-ray and x-ray powder diffraction analyses have been carried out to identify the constituent elements and determine the lattice parameters of the grown crystals. Surface topological features of the as-grown single crystals have been studied. The transport properties of doped stoichiometric Bi2Te3 single crystals have been studied by measuring the thermoelectric power and electrical conductivity in the temperature range from 303 K to 473 K. The thermoelectric power, S, effective mass, scattering parameter, and Fermi energy have been calculated from thermoelectric power measurements. The temperature dependence of the electrical conductivity, σ, shows that the dopants in the crystals are thermally activated. All the crystals exhibit semiconducting behavior as confirmed by the temperature dependence of σ and S. The effective mass of electrons and the effective density of states have been determined and are reported for Bi2Te3−x Se x (0 ≤ x ≤ 0.3) and Bi2−y Fe y Te3 (0 ≤ y ≤ 0.3).

Structural, Electrical, and Thermal Properties Study of CVT Grown CuAlS_2 Single Crystals

CuAlS_2 single crystals were grown by the chemical vapour transport (CVT) technique using iodine as a transporting agent in a close-spaced geometry. The crystal structure, lattice constants, Miller indices, density and %d errors of as grown CuAlS_2 were determined from X-ray diffraction (XRD). The crystalline nature was confirmed by the selected area electron diffraction (SAED) pattern of transmission electron microscopy (TEM). The Scanning Electron Microscopy (SEM) images of the as grown surfaces of the single crystals showed the layer growth mechanism. The variation of resistivity with temperature was studied along the perpendicular to the c-axis using the four probes technique. The activation energy values were evaluated from the slopes of the resistivity variation with temperature plot. The Hall measurement data at ambient temperature were used to determine the resistivity (ρ), Hall coefficient (R_H), carrier concentration (η), Hall mobility (μ), and conductivity type of the grown crystals. The hot probe method was used to confirm the conductivity type. Values of the Fermi energy (E_f ), constant values (A), scattering parameter (s), Seebeck coefficient (S), effective density of states (N_A) and effective mass (m_e∗) were determined by the data of the Seebeck coefficient variation with temperature. The I-V characteristics study was carried out in dark as well as under white and UV illumination. The high pressure electrical resistance measurements have been performed on as grown single crystals up to a pressure of 5 GPa. It was observed that the resistance decreases with pressure up to some point, then afterward increases gradually with pressure. Thermogravimetric (TG), differential thermal analysis (DTA), and differential thermogravimetric (DTG) analysis were carried out on the as grown CuAlS_2 single crystals in an inert nitrogen atmosphere in the temperature range from room temperature to 1223 K. The kinetic parameters were determined employing nonmechanistic equations using the Broido and Coats-Redfern (C-R) relations. All the obtained results are discussed in this paper.

A New Model for the 1/f Noise of Polycrystalline Silicon Thin-Film Transistors

An alternative model for the 1/f noise of polycrystalline Si thin-film transistors (poly-Si TFTs) is developed. The model adequately incorporates the grain boundary (GB) effect, different from previous work, however, the current noise is attributed to fluctuations in both carrier number and the GB barrier caused by carrier trapping/detrapping between the channel inversion carriers and the intragrain traps within the GB depletion region. The large dispersion of the trapping time constants in the 1/f noise behavior is attributed to the variation in carrier tunneling distance. The model fits the noise data very well in the low-drain current region. Based on the model, the effective density of states of the intragrain traps is obtained, providing a feasible method to evaluate the grain quality of poly-Si TFTs.

Condensation of bosons with Rashba-Dresselhaus spin-orbit coupling

Cold atomic Bose-Einstein systems in the presence of simulated Rashba- Dresselhaus spin-orbit coupling exhibit novel physical features. With pure in-plane Rashba coupling the system is predicted in Bogoliubov-Hartree-Fock to have a stable Bose condensate below a critical temperature, even though the effective density of states is two-dimensional. In addition the system has a normal state at all temperatures. We review here the new physics when the system has such spin-orbit coupling, and discuss the nature of the finite temperature condensation phase transition from the normal to condensed phases.

Search for Organic Thermoelectric Materials with High Mobility: The Case of 2,7-Dialkyl[1]benzothieno[3,2-b][1]benzothiophene Derivatives

Control of doping is crucial for enhancing the thermoelectric efficiency of a material. However, doping of organic semiconductors often reduces their mobilities, making it challenging to improve the thermoelectric performance. Targeting on this problem, we propose a simple model to quantitatively obtain the optimal doping level and the peak value of thermoelectric figure of merit (zT) from the intrinsic carrier mobility, the lattice thermal conductivity, and the effective density of states. The model reveals that high intrinsic mobility and low lattice thermal conductivity give rise to a low optimal doping level and a high maximum zT. To demonstrate how the model works, we investigate, from first-principles calculations, the thermoelectric properties of a novel class of excellent hole transport organic materials, 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene derivatives (Cn-BTBTs). The first-principles calculations show that BTBTs exhibit high mobilities, extremely low thermal conductivities (∼0.2 W m...

Study on intrinsic carrier concentration of direct bandgap Ge1-xSnx

Indirect bandgap Ge can be turned to a direct bandgap semiconductor by the alloy-modified technique, which can be applied to advanced photonic devices and electronic devices. Based on 8 bands Kronig-Penny Hamilton, this paper focuses on the physical parameters of direct bandgap Ge1-xSnx, such as conduction band effective density of states, valence band effective density of states and the intrinsic carrier concentration, and aims to provide valuable references for understanding the direct bandgap modified Ge materials and device physics as well as their applications. Results show that: conduction band effective density of states in direct bandgap Ge1-xSnx alloy decreases obviously with increasing Sn fraction, while its valence band effective density of states almost does not change with increasing Sn fraction. Compared with bulk Ge, the conduction band effective density of states and valence band effective density of states in direct bandgap Ge1-xSnx alloy are lower by two and one orders of magnitude respectively; the intrinsic carrier concentration in direct bandgap Ge1-xSnx alloy increases with increasing Sn fraction, and its value is an order of magnitude higher than that of bulk Ge.

Carrier mobility enhancement in poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) having undergone rapid thermal annealing

The conjugated polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is subjected to non-adiabatic rapid thermal processing and exhibits an increase in conductivity through the film. Electrical measurements on an ITO/PEDOT:PSS/Al diode structure display a current-voltage relationship that correlates to space charge limited conduction with the presence of an exponential trap distribution, which is commonly seen in other organic media. With careful application of this current transport theory to the obtained experimental results, the root cause of the conductivity enhancement can be attributed solely to an increase in the charge mobility of carriers in the PEDOT:PSS film. In comparison to an untreated PEDOT:PSS film, processing at 200 °C for 30 s results in a 35% increase in carrier mobility to 0.0128 cm2 V−1 s−1. Values for other material characteristics of PEDOT:PSS can also be extracted from this electrical analysis, and additionally are found to be unchanged with processing. Hole concentration, effective density of states, and total trap density are found to be 7.4 × 1014 cm−3, 1.5 × 1018 cm−3, and 3.7 × 1017 cm−3, respectively.

Analysis of the series resistance in pin-type thin-film silicon solar cells

The series resistance of microcrystalline hydrogenated silicon thin-film pin-type solar cells is investigated using illumination dependent current/voltage characteristics. We present a simple analytical model describing the total series resistance of low-mobility pin-type solar cells. The model thus provides insight into the influence of the material properties of the intrinsic layer on the series resistance. Our model allows us to separate the voltage dependent internal resistance of the intrinsic layer from the residual, external resistance. We verified our model over a wide range of parameters relevant to thin-film silicon devices by comparison to numerical simulations. Finally, we demonstrate that our model can consistently describe the series resistance of experimental a μc-Si:H pin-type solar cell. Furthermore, the fitting of the model with experimental data yields the external series resistance and information of the carrier mobilities and effective density of states in the bands of the intrinsic layer in the device.

An approach to high efficiencies using GaAs/GaInNAs multiple quantum well and superlattice solar cell

A new type of photovoltaic device where GaAs/GaInNAs multiple quantum wells (MQW) or superlattice (SL) are inserted in the i-region of a GaAs p-i-n solar cell (SC) is presented. The results suggest the device can reach record efficiencies for single-junction solar cells. A theoretical model is developed to study the performance of this device. The conversion efficiency as a function of wells width and depth is modeled for MQW solar cells. It is shown that the MQW solar cells reach high conversion efficiency values. A study of the SL solar cell viability is also presented. The conditions for resonant tunneling are established by the matrix transfer method for a superlattice with variable quantum wells width. The effective density of states and the absorption coefficient for SL structure are calculated in order to determinate the J-V characteristic. The influence of superlattice length on the conversion efficiency is researched, showing a better performance when width and cluster numbers are increased. The SL solar cell conversion efficiency is compared with the maximum conversion efficiency obtained for the MQW solar cell and shows an efficiency enhancement.

Quantum-Fluctuation Effects in the Transport Properties of Ultrathin Films of Disordered Superconductors above the Paramagnetic Limit

We study the transport in ultrathin disordered film near the quantum critical point induced by the Zeeman field. We calculate corrections to the normal state conductivity due to quantum pairing fluctuations. The fluctuation-induced transport is mediated by virtual rather than real quasiparticle excitations. We find that at zero temperature, where the corrections come from purely quantum fluctuations, the Aslamazov-Larkin paraconductivity term, the Maki-Thompson interference contribution, and the density of states effects are all of the same order. The total correction leads to the negative magnetoresistance. This result is in qualitative agreement with the recent transport observations in the parallel magnetic field of the homogeneously disordered amorphous films and superconducting two-dimensional electron gas realized at the oxide interfaces.

Einstein Relation of Two Dimensional Strained Si/Si1-x Gex MOSFETs in Nondegenerate Regime and Degenerate Regime

The carrier propagation in quasi two dimensional strained silicon is quantum confined in the direction normal to the channel; while has the analog energy spectrum in the other two Cartesian direction. In this paper, we report the Einstein relation that relates the diffusivity to the carrier mobility of two dimensional strained silicon for both the nondegenerate regime and degenerate regime. The germanium fraction, x in the relaxed Si 1-x Ge x is taken to be 20%. The Fermi Dirac distribution function in the form of Fermi integral order zero, is employed in developing the carrier statistic and the intrinsic velocity. It was demonstrated that the Einstein Relation is related to the channel width in nondegenerate regime but in degenerate regime, it is related to the channel width, carrier concentration and the effective density of state. Also, the intrinsic velocity shows the carrier concentration dependence behavior in the degenerate regime but is dependence of the temperature in the nondegenerate regime.

Conduction mechanism of resistive switching films in MgO memory devices

In this work, nonpolar resistance switching behavior was demonstrated in Pt/MgO/Pt structure. The resistance ratio of high resistance state (HRS) and low resistance state (LRS) is about on the order of 105 for the compliance current (Icomp) of 1 mA at 300 K. Using enough Icomp (≥0.5 mA) during SET processes, the LRS resistances reach a minimum of about 102–103 Ω and the RESET currents reach a maximum of about 10−4–10−3 A. Experimental results indicate that the conduction mechanism in MgO films is dominated by the hopping conduction and the Ohmic conduction in HRS and LRS, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in MgO films were obtained.