Increase In Carrier Concentration Research Articles

Random band-edge model description of thermoelectricity in high-mobility disordered semiconductors: Application to the amorphous oxide In-Ga-Zn-O

Unraveling the dominant charge transport mechanism in high-mobility amorphous oxide semiconductors is still a matter of controversy. In the present study we extended the random band-edge model suggested before for the charge transport and Hall-effect mobility in such disordered materials [Fishchuk et al., Phys. Rev. B 93, 195204 (2016)], and also describe the field-effect-modulated thermoelectricity in amorphous In-Ga-Zn-O ($a$-IGZO) films under the same premises. The model is based on the concept of charge transport through the extended states and assumes that the transport is limited by the spatial variation of the position of the band edge due to the disorder potential, rather than by localized states. The theoretical model is formulated using the effective medium approximation framework and describes well basic features of the Seebeck coefficient in disordered materials as a function of energy disorder, carrier concentration, and temperature. Carrier concentration dependencies of power factor and thermoelectric figure of merit have been also considered for such systems. Besides, our calculations reveal a remarkable turnover effect from a negative to a positive temperature dependence of Seebeck coefficient upon increasing carrier concentration. The suggested unified model provides a good quantitative description of available experimental data on the Seebeck coefficient and the charge mobilities measured in the same $a$-IGZO transistor as a function of the gate voltage and temperature by considering the same charge transport mechanisms. This promotes a deeper understanding and a more credible and accurate description of the transport process in $a$-IGZO films.

Understanding the dopability of p-type Mg2(Si,Sn) by relating hybrid-density functional calculation results to experimental data

It is crucial to reach a sufficiently high carrier concentration in order to optimize the thermoelectric (TE) material in the development of Mg2 X (X= Si, Ge, and Sn)-based TE generators. While n-type Mg2(Si,Sn) has excellent TE properties, p-type shows suboptimal TE performance because of insufficient carrier concentration, in particular for Mg2Si and Si-rich Mg2(Si,Sn). A systematic investigation of Li-doped Mg2Si1-x Sn x has been performed as Li, in contrast to other typical dopants, has a high solubility in the material system and has been shown to yield the highest reported carrier concentrations. We observe that the carrier concentration increases with Li content, but the dopant efficiency decreases. With respect to the Si:Sn ratio, we find a clear increase in maximum achievable carrier concentration and dopant efficiency with increasing Sn content. The trends can be understood by employing defect formation energies obtained within the hybrid-density functional theory (DFT) for the binaries. Further, we use a linear interpolation of the hybrid-DFT results from the binaries to the ternary Mg2(Si,Sn) compositions and a simple single parabolic band model to predict the maximal achievable carrier concentration for the solid solutions, providing a simple guideline for experimental work. Finally, we show that the approach is transferable to other material classes. This work highlights that, besides dopant solubility, the interplay between intrinsic and extrinsic defects determines the achievable carrier concentration.

Mass Production of Pt Single-Atom-Decorated Bismuth Sulfide for n-Type Environmentally Friendly Thermoelectrics.

Single-atom materials are widely explored in catalysis, batteries, sensors, etc. However, limited by mass production and centimeter-scale assembly, they are rarely studied in thermoelectrics. Herein, we demonstrate a solvothermal synthesis assisted by a syringe-pump method to yield Bi2S3-supported Pt single-atom materials (Bi2S3-Pt1) at a 10 g scale. Different from Ptn clusters, Pt1 single atoms can increase carrier concentration at a high doping efficiency and provide a unique atomic environment to enhance carrier mobility, and an enlarged effective mass leads to an enhanced Seebeck coefficient. As a result, a high power factor (348 μW m-1 K-2) is achieved at 823 K. Benefiting from the scattering of phonons by Pt1 atomic sites, a minimum thermal conductivity of 0.37 W m-1 K-1 is achieved. Consequently, the Bi2S3-0.5 wt % Pt1 realizes a record-high zT of ∼0.75 at 823 K, being among the best in the state-of-the-art n-type environmentally friendly metal sulfides. The enhancement of the carrier mobility and suppression of the thermal conduction by the unique Pt1 single atoms will inspire various fields, as exemplified by electronic devices and thermal management.

Role of V doping in core–shell heterostructured Bi2Te3/Sb2Te3 for hydrogen evolution reaction

Non-noble metal-based materials as low-cost hydrogen evolution reaction (HER) catalysts are key materials for sustainable hydrogen energy production. Bismuth and antimony chalcogenides are among the hopeful candidates to achieve this goal. In this work, a V-doped Sb2Te3 encapsulated Bi2Te3 core-shell electrocatalyst (Bi2Te3/Vx-Sb2Te3) has been synthesized by a two-step solvothermal method. V doping adjusts the electronic structure of catalyst, dramatically enhances electric double layer capacitance (Cdl) of the catalyst, decreases charge transfer resistance (Rct) of the catalyst and increases carrier concentration of the catalyst. Therefore, the V doping method increases the active sites on the surface of the material, and promotes the charge transfer and electron transport in the HER process. In addition, V doping can also adjust the hydrophilicity of the material surface, promote the release of hydrogen, and quickly re-expose the active sites. Bi2Te3/Vx-Sb2Te3 electrocatalysts exhibit brilliant HER activity and high stability in both acidic and alkaline electrolytes. This study uses the strategy of V doping to control the electronic structure of materials, which will provide suggestions for the design and preparation for other high-activity catalysts.

Significantly enhanced thermoelectric figure of merit of n-type Mg3Sb2-based Zintl phase compounds via co-doped of Mg and Sb site

N-type Mg3Sb2 thermoelectric material has a complex crystal structure and electronic structure, which is a new thermoelectric material with excellent application prospects. The state density and band structure of Mg24Sb16 and Bi/Se/Pr–Se/Nd–Se doped Mg24Sb16 are first-principles calculated. Se/Pr–Se/Nd–Se doped increases the density of states around the Fermi level. Bi-doped does not impact the band structure because the electronic structure between Sb and Bi are similar. The band gap of Pr–Se/Nd–Se doped becomes narrower, which is more conducive to carrier transport and increases carrier concentration than that of single-doped Se. The experimental results show that Pr and Nd replace Mg to provide more electrons for the system. The carrier concentration of the co-doped sample increases significantly, thus improving the power factor and optimizing the samples' electrical properties. The highest ZT value of the Mg3.2Sb1.5Bi0.49Se0.01 sample is 1.21. Pr–Se/Nd–Se co-doped ZT values are 1.67 and 1.74, respectively, 38% and 43% higher than the Mg3.2Sb1.5Bi0.49Se0.01 sample. Multi-element doped is an effective strategy to improve the thermoelectric properties of materials.

Numerical simulation of quantum dots as a buffer layer in CIGS solar cells: a comparative study

Quantum bandgap buffer layers can improve sunlight absorption in the short wavelength region, hence improving the performance of CIGS solar cells. In this study, we use numerical modelling to determine the impact of various buffer layers' electrical characteristics on the performance of CIGS thin film photovoltaic devices, particularly, carrier concentration and the quantum effect. As well Ag2S buffer layer has been experimentally examined to fulfilment its effect in term of bulk and quantum bandgap. Experimental results depicted that, Ag2S QDs has polycrystalline nature of films, with smooth surface roughness, and average diameter 4 nm. Meanwhile, a simulation revealed that the Fermi level of the (n-buffer layer) material shifts closer to the conduction band with an increase in carrier concentration. The findings indicate that, a buffer layer with a wider bandgap and carrier concentration is an essential demand for achieving a device with a higher conversion efficiency and a broader bandgap-CBO window. It was attributed to beneficial synergistic effects of high carrier concentration and narrower depletion region, which enable carriers to overcome high CBO barrier. Most importantly, modelling results indicate that the optic-electrical characteristics of the buffer layer are critical in determining the progress of a CIGS solar cell.

Preparation and Investigation of Thermoelectric Properties of Cu3SbS4-Cu3SbSe4 Solid Solutions

Cu-based ternary chalcogenides composed of earth-abundant and nontoxic elements are considered promising thermoelectric materials, although there are few reports on them. In our previous studies, optimal mechanical alloying (MA) and hot pressing (HP) were used to synthesize Cu3SbS4 (famatinite) and Cu3SbSe4 (permingeatite). In this study, solid solutions of Cu3SbS4-ySey (y = 0–4) were prepared using MA and HP. X-ray diffraction and Rietveld refinement confirmed the formation of Cu3SbS4-Cu3SbSe4 solid solutions. The variation in lattice constants with Se content followed Vegard’s law. As the Se content increased, the carrier concentration and mobility increased. All the specimens exhibited p-type conduction, showing positive Hall and Seebeck coefficients. As the Se content increased, the Seebeck coefficient decreased due to an increase in carrier concentration, while the power factor of the solid solutions was not enhanced compared with that of the end compounds. Cu3SbS2.4Se1.6 exhibited the lowest lattice thermal conductivities of 0.84 Wm-1K-1 at 323 K and 0.56 Wm-1K-1 at 623 K because of phonon scattering (alloying scattering) caused by the formation of the solid solutions. In addition, the thermal conductivity was less than 0.8 Wm-1K-1 at 623 K. This was attributed to the phonon scattering (boundary scattering) caused by several grain boundaries introduced during MA. As the Se content increased, the dimensionless figure of merit gradually increased from 0.14 to 0.39 at 623 K.

Microscopic theory of exciton and trion polaritons in doped monolayers of transition metal dichalcogenides

We study a doped transition metal dichalcogenide (TMDC) monolayer in an optical microcavity. Using the microscopic theory, we simulate spectra of quasiparticles emerging due to the interaction of material excitations and a high-finesse optical mode, providing a comprehensive analysis of optical spectra as a function of Fermi energy and predicting several modes in the strong light-matter coupling regime. In addition to exciton-polaritons and trion-polaritons, we report polaritonic modes that become bright due to the interaction of excitons with free carriers. At large doping, we reveal strongly coupled modes corresponding to excited trions that hybridize with a cavity mode. We also demonstrate that the increase of carrier concentration can change the nature of the system’s ground state from the dark to the bright one. Our results offer a unified description of polaritonic modes in a wide range of free electron densities.

Enhancements of thermoelectric performance in n-type Bi2Te3-based nanocomposites through incorporating 2D Mxenes

Bi2Te2.7Se0.3 compound has been considered as an efficient n-type room-temperature thermoelectric (TE) material. However, the large-scale applications for low-quality energy harvesting were limited due to its low energy-conversion efficiency. We demonstrate that TE performance of Bi2Te2.7Se0.3 system is optimized by 2D Ti3C2Tx additive. Here, a 43% reduction of electrical resistivity is obtained for the nanocomposites at 380 K, originating from the increased carrier concentration. Consequently, the g = 0.1 sample shows a maximum power factor of 1.49 Wmm−1K−2. Meanwhile, the lattice thermal conductivity for nanocomposite samples is reduced from 0.77 to 0.41 Wm−1K−1 at 380 K, due to the enhanced phonon scattering induced by the interfaces between Ti3C2Tx nanosheets and Bi2Te2.7Se0.3 matrix. Therefore, a peak ZT of 0.68 is achieved at 380 K for Bi2Te2.7Se0.3/0.1 wt% Ti3C2Tx, which is enhanced by 48% compared with pristine sample. This work provides a new route for optimizing TE performance of Bi2Te2.7Se0.3 materials.

Optical/electrical properties of RENiO3 (RE = Pr, Nd, Sm, Gd, Dy, Ho, Er, Y and Lu) with intrinsic point defects: A first-principles study

Defects in rare-earth nickelates (RENiO3) are closely related to their metal-to-insulator phase transitions and optical/electrical properties, therefore play an important part in the applications of RENiO3. In this work, the intrinsic point defects of RENiO3 (RE ​= ​Pr, Nd, Sm, Gd, Dy, Ho, Er, Y and Lu) with stoichiometric and non-stoichiometric ratios were studied through first-principles calculations. In stoichiometric systems, the defect formation energy has the order of RE Frenkel ​> ​Cation antisite defect ​≈ ​Ni Frenkel ​> ​O Frenkel ​> ​Schottky. Among non-stoichiometric RENiO3, the defects are prone to form in RE2O3 excessive environment as compared to those in the Ni2O3 excessive situation. In terms of electronic properties, oxygen vacancies reduce the band gap of RENiO3 and introduce a new occupied state of Ni–O. At 300 ​K, the conductivity of RENiO3 increases with the increase of carrier concentration. For the optical properties, the O vacancies block the infrared light and increase the transmission of visible light. The intrinsic point defects discussed in this study can not only explain the difference in defect type, but also provide a way to optimize performance of RENiO3.

Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping.

Vanadium dioxide (VO2) featuring a distinct thermally triggered phase transition is regarded as the most attractive thermochromic material for smart window applications. However, the high transition temperature (∼67 °C) and moderate luminous transmittance (<50%) of the pristine VO2 circumvent room temperature applications. In this work, epitaxial cobalt-doped VO2 thin films were fabricated to tailor the electric and optical properties on a c-plane sapphire substrate. At the highest doping concentration of 10%, the transition temperature of VO2 is reduced to 44 °C, accompanied by a high luminous transmittance of 79% for single-element Co-doped VO2. The roles of cobalt doping and detailed band variation are fully explained experimentally and by modeling (DFT calculation), respectively. Furthermore, the dramatically increased carrier concentration in cobalt-doped VO2 underscores the promising future of cobalt-doped VO2 unveiled by temperature-dependent Hall effect measurement.

Epitaxial growth of metastable c-plane rhombohedral indium tin oxide using mist chemical vapor deposition

Corundum structured metastable rhombohedral indium tin oxide (rh-ITO), which is known to grow as thin films, has attracted significant research interest; however, its properties, based on the Sn concentration, have not been evaluated in detail thus far. Herein, rh-ITO thin films comprising different Sn concentrations were grown on c-plane α-Al2O3 substrates with α-Ga2O3 buffer layers using the mist chemical vapor deposition method, and their properties were investigated. X-ray diffraction analysis revealed that c-plane rh-ITO thin films grew epitaxially at Sn concentrations of 0–10 at.%. The lowest thin film resistivity is determined to be approximately 3.3 × 10−4 Ω cm, which is sufficiently low for a transparent conductive oxide. The carrier concentration and mobility exhibited by the rh-ITO films were consistent with those of typical single-crystalline ITO films, which exhibited the most stable phase. The rh-ITO thin films exhibited high transmittance in the visible light region, and the absorption edge was blue-shifted with the increasing carrier concentration. This study demonstrates that rh-ITO can be used as a new transparent conductive oxide material for diverse applications.

Study on the mechanism of Y doping effect for improving the properties of ZnO thermoelectric ceramics

The oxygenated ZnO samples have been prepared by powder metallurgy, and ZnO is doped with high-valence Y element as electron donors. The study found that the electrical conductivity increased significantly, which is mainly due to the increase of carrier concentration upon Y doping. Y doping would widen the band gap, introduce impurity levels, and enhance the density of states near the Fermi level. The Seebeck coefficient decreases due to the increase of carrier concentration, and the power factor shows an upward trend. Due to the enhancement of phonon scattering after Y doping, the lattice thermal conductivity decreases. Finally, ZT was promoted from 0.009 of the pristine sample to 0.427 of the Y doped at 873 K.

Modulation of optical, photophysical and electrical properties of poly(3-hexylthiophene) via Gd:CdS nanoparticles

Herein, the impact of Gadolinium (Gd) doped cadmium sulphide (CdS) nanoparticles on the structural, optical, photophysical and electrical properties of poly(3-hexylthiophene) (P3HT) thin films have been investigated. The structural, morphological and vibrational properties of fabricated films were probed by measuring X-ray diffraction spectra, scanning electron microscope images and FT-Raman spectra, respectively. The absorption spectra of P3HT thin films was noted to be red shifted and energy bandgap slightly decreases with incorporation of Gd:CdS nanoparticles. The extinction coefficient (k), refractive index (n) and dielectric constants (εr, εi) of pure and hybrid films were extracted from spectroscopic ellipsometer data fitted by new amorphous dispersion model (for P3HT) and Adachi-New Forouhi dispersion model (for CdS) and noted that the optical constants decreases with increase of Gd:CdS nanoparticles concentrations. The photoluminescence intensity of P3HT/Gd:CdS films was quenched as compare to pristine P3HT, also an additional bands observed in hybrid films corresponds to emission from Gd:CdS nanoparticles. The average fluorescence life times of pristine P3HT thin films was evaluated to be 154 ps which drop to 109 ps for hybrid film, ascribes to formation of additional decay paths created due to Gd:CdS doping. The electrical resistivity (ρ) and mobility (μ) of P3HT films were measured from Hall Effect systems and noted to be decreases for hybrid films whereas charge carrier concentration (nC) and sheet resistance (RS) increases.

Ionic conductivity enhancement for PVA/ 20wt.% CuSO4 gel polymer electrolyte by using glycerin

Gel polymer electrolyte based on PVA+20wt.%CuSO4 was plasticized with (1,2 and 3) mL glycerin and prepared by casting method. Ionic conductivity for gel electrolytes enhancement occurs primarily as a result of increased carrier concentration and secondarily as a result of increased carrier mobility. Using X-ray diffraction (XRD) characterization, they observed that the glycerin-free electrolyte matrix has a regular arrangement with the crystalline phase. After adding glycerin, the observed orderly organization and crystalline phase fully transformed to amorphous. Fourier transform infrared spectroscopy (FTIR) result shows that the stretching vibration region of the O–H at 3261 cm−1 The hydrogen bonds formed by the hydroxyl groups of both PVA and glycerol structures explanation for the hydrophilic behavior of glycerol and the increased area. The new intense and strong peak that emerged at 2359.62 cm-1 is related to the aromatic Cu-O stretching of the salt CuSO4, The interaction of the PVA with the electrolyte elements increases its intensity with increasing glycerol.

Development of directly grown‐graphene–silicon Schottky barrier solar cell using co‐doping technique

Low-cost, highly efficient, and low-power devices are essential for energy harvesting applications. Despite numerous studies having been conducted on transferable graphene solar cells, large-area growth and long-term stability continue to remain elusive, which limits the practicability of graphene-based photovoltaic devices. Plasma-enhanced chemical vapor deposition is a promising, efficient, and facile method for synthesizing graphene over a large area without the use of a metal catalyst, but defects formed during graphene growth adversely affect the power conversion efficiency (PCE) of graphene/Si solar cells in which the graphene is used. In this work, we successfully employed acid-based graphene-Si macromolecular photovoltaic devices chemically doped with polymeric perfluorinated sulfonic acid (PFSA) macromolecules and nitric acid (HNO3). We achieved an improved PCE of about 9.27% with PFSA doping compared with the value (7.64%) without PFSA doping, and it increased substantially to 10.44% after co-doping with HNO3. PFSA macromolecular doping also led to a substantial increase in the carrier concentration, which helped to reduce the sheet resistance of graphene and improved the work function of the device. In particular, the synergistic effect of co-doping with HNO3 helped to improve the number of active sites and charge separation. Along with the increase in carrier concentration, doping with PFSA reduced the number of defects, resulting in the graphene having a smooth and uniform surface, which effectively increased the open-circuit voltage (Voc) from 0.500 to 0.521 V. We surmise that nonvolatile PFSA macromolecular and volatile HNO3 co-doping has high potential for use in the fabrication of low-cost directly grown-graphene-based photovoltaic devices. Highlights Plasma-enhanced chemical vapor deposition technique used for the direct growth of graphene without metal catalyst. Naturally grown oxide thickness optimized. Nonvolatile macromolecular dopant per-fluorinated polymeric sulfonic acid (PFSA) and volatile nitric acid (HNO3) were used for direct growth graphene doping. The power conversion efficiency of ~9.27% with PFSA doping from 7.64%; further, its substantial increase up to 10.44% after co-doping with HNO3 were achieved with long-term stability.

Effects of dynamic disorder on thermoelectric transport in soft materials

A model is developed that accounts for the effects of thermal disorder (both static and dynamic) in predicting the thermoelectric (TE) performance of weakly bonded semiconductors. With dynamic disorder included, the model is found to fit well with experimental results found in the literature for the density-of-states and the energy-dependent carrier mobility, which are key for assessing TE properties. The model is then used to analyze the concentration-dependent TE properties of the prototypical small molecular semiconductor rubrene. At low (e.g., intrinsic) carrier concentrations, where Fermi level pinning occurs, dynamic disorder is found to reduce electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF) to values that are much lower than those traditionally predicted by static disorder models. As carrier concentration (p) increases, S exhibits nonlinear behavior, increasing well above the conventional S vs log(p) relationship before reaching a peak value (Speak∼1550μV/K). A critical carrier concentration (pcrit.≈4.299×10−4 molar ratio) is observed near Speak at which thermoelectric transport transitions from trap-limited behavior at low concentrations to conventional band behavior at high concentrations. Above this value, σ and PF are reduced compared to the perfect crystal and static-only conditions, causing a drop in the maximum PF by factors of 3 and 2.3, respectively. This PF reduction, while not as large as the PF reduction that occurs for low carrier concentration, is found to occur in a high concentration regime (p&amp;gt;pcrit.) that contains the PF maximum and has remained inaccessible to experimentalists due to dopant limitations that are worsened in the presence of dynamic disorder.

Growth of Aluminum Doped Zinc Oxide Nanostructure Thin Films by Nonconventional Sol‐Gel Method

AbstractThe present work demonstrates the growth of undoped and Al doped ZnO (AZO) thin films by using nonconventional sol‐gel method. The structural, morphological, optical, and electrical properties of obtained thin films are studied. X‐ray diffraction (XRD) analysis confirms hexagonal wurtzite structures for both undoped and doped thin films. The crystallite size is found between 15 and 30 nm. Scanning electron microscope (SEM) images show that the films have approximately uniform morphologies, consisting in several flower‐like aggregates with nanosized multipetals. From the optical properties, it is found that with the increase in the doping concentration, there is a decrease in the absorption coefficient in the visible wavelength range. It is also noticed that the increase in doping concentration causes an increase in bandgap (Eg) and a decrease in extinction coefficient. Urbach energy decreases with increase in the doping concentration. In the Hall effect study, it is observed that the conductivity and carrier concentration increase sharply with increasing the aluminum quantity. The causes and significance of these observations are discussed.

Investigations on the Thermoelectric Transport Properties in the Hole‐doped La2CuO4

AbstractCompared with the traditional thermoelectric semiconductors, the research progress of thermoelectric oxides with high‐temperature thermal stability is far behind. In this work, a layered copper oxide La2CuO4 was synthesized by combining the high‐energy ball milling, solid state reaction and spark plasma sintering. By introducing metal elements (Ba, Sr, Pb) at La site, the increased carrier concentration with optimized electrical transport properties were achieved in La2CuO4‐based thermoelectric oxides. Results showed that Ba has the best doping effect among the three dopants, with an optimal carrier concentration of ∼1020 cm−3. Furthermore, the figure of merit (ZT) value was enhanced over the entire temperature range with a maximum ZTave of ∼0.03 at 300–923 K, which is nearly two orders of magnitude higher than that of printine La2CuO4. Our research has successfully developed the thermoelectric properties of La2CuO4‐based oxide, which has the great potential to be a reliable candidate as the thermocouple for high‐temperature detection in the key parts of aircrafts.

Mono-crystalline Ge1-xSnxSe micro-sheets with hexagonal morphologies for Visible-NIR photodetectors: Increased carrier concentration, narrowed band gap and improved performances

GeSe is a narrow band gap semiconductor with feasible application as infrared photoelectric devices. However, challenges are still maintained to improve their carrier concentration and conductivity for broader scope of photodetector applications. Aiming at such scope, the Ge1-xSnxSe (0 ​< ​x ​< ​0.2) were synthesized via a facile “one-pot” wet chemical method and a lattice site substitution strategy. Compared to the pure GeSe micro-sheets the synthesized Ge1-xSnxSe micro-sheets possessed a smaller band gap (0.914 ​eV) and a better electrical conductivity (2.80 ​× ​10−5 ​Ω−1 cm−1), where the former exhibit greatly improved photo-current. The Ge1-xSnxSe micro-sheet was then assembled into a photodetector device to demonstrate its remarkable photo responsivities in visible to NIR spectral region (3614 A/W at 532 ​nm and 86 A/W at 1319 ​nm). The photo-detection range broadening and the responsivity enhancement can be attributed the carrier concentration increment as well as the band gap narrowing. Therefore, the Ge1-xSnxSe micro-sheets can be considered as an excellent candidate that can be applied as high performance photodetectors in the visible to near infrared region.