Increase In Carrier Mobility Research Articles

Realizing High‐Performance BiSbTe Magnetic Flexible Films via Acceleration Movement and Hopping Migration of Carriers

AbstractImproving the thermoelectric (TE) performance of Bi2Te3‐based flexible films remains a huge challenge. Herein, high‐performance Ni/epoxy/Bi0.5Sb1.5Te3 magnetic flexible TE films are prepared by incorporating Ni nanoparticles (Ni‐NPs). Atomic‐resolution STEM investigation demonstrates that Te vacancies induced by the orientation reaction between Ni‐NPs and Te from Bi0.5Sb1.5Te3 trigger the presence of negatively charged and anti‐site defects and atomic‐sized electric field in the magnetic flexible TE films and further cause the acceleration movement and hopping migration of carriers. The transport measurements indicate an increased carrier concentration due to the anti‐site defects, while the significant increase of carrier mobility originates from the acceleration movement of carriers. The magnetic scattering and hopping migration of carriers are responsible for maintaining large Seebeck coefficient. As compared to epoxy/Bi0.5Sb1.5Te3 flexible TE film, the maximum power factor of the magnetic flexible TE film with 0.1% Ni‐NPs reaches 2.74 mW m−1 K−2 at 300 K and increases by 61%, while the cooling temperature difference increases by 250%.

Elucidating the underlying physics in a two-terminal all-perovskite tandem solar cell: A guideline towards 30% power conversion efficiency

We develop an optoelectronic model for a two-terminal all-perovskite tandem solar cell comprising a top cell, a bottom cell, and a recombination junction in between that connects the two sub-cells in series electrically. In short, the model considers incoherent and coherent light propagation in the glass and thin-film layers respectively, as well as charge carrier transport, generation, and recombination. After calibrating the model to the state of the art two-terminal all-perovskite tandem solar cell with an efficiency of 24.5%, we first study the current matching behavior and S-shaped current–voltage curve of this tandem cell due to the recombination junction. Next, the light interference effect is investigated. Layer thicknesses are adjusted to increase the short circuit current. From a loss analysis, the leading order recombination channels are the surface and Shockley–Read–Hall recombinations. Moreover, we show that the power conversion efficiency of this calibrated tandem model can reach 30.5% by adjusting the layer thicknesses, increasing carrier mobility, and reducing recombination loss. Finally, after switching to bifacial operation, the model predicts a power conversion efficiency of around 35%.

The effect of quasi-free graphene layer on the electrical transport properties of sandwich-like graphene / Co nanoparticles / graphene structure

The paper reports on the charge transport in the sandwich-like graphene / Co nanoparticles / graphene structure (G/CoNPs/G). It is fabricated by combining the electrochemical deposition of Co nanoparticles onto a single layer CVD graphene and their capping with another graphene layer. The isolated semi-spherical, surface oxidized Co NPs of 250 nm in diameter serve as a support for the top graphene layer and provide its suspended state confirmed by atomic force microscopy revealing periodic ripples on its surface. A high optical transparency of 92.4% is found for the G/CoNPs/G sandwich. The observed twofold reduction in the sheet resistance with a ∼ 1.5 times increase in the positive magnetoresistive effect for G/CoNPs/G compared to the pristine graphene is attributed to the suppression of carrier scattering due to the substrate. From the Hall effect measurements, a predominant role of the top graphene layer in the charge transport in G/CoNPs/G is established, with a significantly increased carrier mobility µ∼1800 cm2/(V·s). An elastic intervalley scattering length estimated from the magnetoresistance results is found to be three times increased in G/CoNPs/G, indicating that the top graphene layer is almost unaffected by the substrate. The proposed technology looks promising for possible optoelectronic and magneto-optical applications.

Physical Intuition to Improve Electronic Properties of Thermoelectrics

Thermoelectrics convert heat to electricity and vice versa. They are of technological importance in cooling and energy harvesting. Their performances are defined by figure of merit, zT. Decades of studies have largely focused on the development of novel and advanced materials reaching higher performance in devices. To date, the lack of sufficiently high-performance thermoelectrics, especially among Earth-abundant and lightweight materials, is one of the reasons why there is no broad commercial application of thermoelectric devices yet. This challenge is due to the complex correlations of parameters that make up the zT. Theoretical estimation can reveal the optimal charge carrier concentration, which can provide a good idea of doping compositions. Depending on the material characteristics, decoupling these intercorrelated parameters could be viable. Broadly speaking, increasing carrier mobility, inducing a large fluctuation in density of states (DOS) at the Fermi level, and lowering the lattice thermal conductivity lead to better thermoelectric performance. In this mini review, we provide a broad picture of electronic property optimization for thermoelectric materials. This work will be a useful guide to quickly take readers to the forefront of thermoelectric research.

Light-activated interlayer contraction in two-dimensional perovskites for high-efficiency solar cells.

Understanding and tailoring the physical behaviour of halide perovskites under practical environments is critical for designing efficient and durable optoelectronic devices. Here, we report that continuous light illumination leads to >1% contraction in the out-of-plane direction in two-dimensional hybrid perovskites, which is reversible and strongly dependent on the specific superlattice packing. X-ray photoelectron spectroscopy measurements show that constant light illumination results in the accumulation of positive charges in the terminal iodine atoms, thereby enhancing the bonding character of inter-slab I-I interactions across the organic barrier and activating out-of-plane contraction. Correlated charge transport, structural and photovoltaic measurements confirm that the onset of the light-induced contraction is synchronized to a threefold increase in carrier mobility and conductivity, which is consistent with an increase in the electronic band dispersion predicted by first-principles calculations. Flux-dependent space-charge-limited current measurement reveals that light-induced interlayer contraction activates interlayer charge transport. The enhanced charge transport boosts the photovoltaic efficiency of two-dimensional perovskite solar cells up to 18.3% by increasing the device's fill factor and open-circuit voltage.

Substrate Effects on the Electrical Properties in GaN-Based High Electron Mobility Transistors

We report the electrical characteristics of GaN-based high electron mobility transistors (HEMTs) operated on various substrates/films. For the detailed investigation and comparison of the electrical properties of GaN-based HEMTs according to the substrates/films, GaN-based HEMTs were processed using 4-inch sapphire substrates and separated from their original substrates through the laser lift-off technique. The separated AlGaN/GaN films including processed GaN-based HEMTs were bonded to AlN substrate or plated with a 100 µm-thick Cu at the back-side of the devices since AlN substrate and Cu film exhibit higher thermal conductivity than the sapphire substrate. Compared to the sapphire substrate, DC and RF properties such as drain current, transconductance, cut-off frequency and maximum oscillation frequency were improved, when GaN-based HEMTs were operated on AlN substrate or Cu film. Our systematic study has revealed that the device property improvement results from the diminishment of the self-heating effect, increase in carrier mobility under the gated region, and amelioration of sheet resistance at the access region. C(V) and pulse-mode stress measurements have confirmed that the back-side processing for the device transfer from sapphire substrate onto AlN substrate or Cu film did not induce the critical defects close to the AlGaN/GaN hetero-interface.

Native Atomic Defects Manipulation for Enhancing the Electronic Transport Properties of Epitaxial SnTe Films.

P-type SnTe-based compounds have attracted extensive attention because of their high thermoelectric performance. Previous studies have made tremendous efforts to investigate native atomic defects in SnTe-based compounds, but there has been no direct experimental evidence so far. On the basis of MBE, STM, ARPES, DFT calculations, and transport measurements, this work directly visualizes the dominant native atomic defects and clarifies an alternative optimization mechanism of electronic transport properties via defect engineering in epitaxially grown SnTe (111) films. Our findings prove that positively charged Sn vacancies (VSn) and negatively charged Sn interstitials (Sni) are the leading native atomic defects that dominate electronic transport in SnTe, in contrast to previous studies that only considered VSn. Increasing the substrate temperature (Tsub) and decreasing the Te/Sn flux ratio during film growth reduces the density of VSn while increasing the density of Sni. A high Tsub results in a low hole density and high carrier mobility in SnTe films. The SnTe film grown at Tsub = 593 K and Te/Sn = 2/1 achieves its highest power factor of 1.73 mW m-1 K-2 at 673 K, which is attributed to the optimized hole density of 2.27 × 1020 cm-3 and the increased carrier mobility of 85.6 cm2 V-1 s-1. Our experimental studies on the manipulation of native atomic defects can contribute to an increased understanding of the electronic transport properties of SnTe-based compounds.

Boosting the Optoelectronic Properties of Molybdenum Diselenide by Combining Phase Transition Engineering with Organic Cationic Dye Doping.

Two-dimensional layered transition metal dichalcogenides (TMDs) have been investigated intensively as next-generation semiconducting materials. However, conventional TMD-based devices exhibit large contact resistance at the interface between the TMD and the metal electrode because of Fermi level pinning and the Schottky barrier, which results in poor charge injection. Here, we present enhanced charge transport characteristics in molybdenum diselenide (MoSe2) by means of a sequential engineering process called PESOD-2H/1T (i.e., phase transition engineering combined with surface transfer organic cationic dye doping; 2H and 1T represent the trigonal prismatic and octahedral phases, respectively). Substantial improvements are observed in PESOD-processed MoSe2 phototransistors, specifically, an approximately 40 000-fold increase in effective carrier mobility and a 100 000-fold increase in photoresponsivity, compared with the mobility and photoresponsivity of intact MoSe2 phototransistors. Moreover, the PESOD-processed MoSe2 phototransistor on a flexible substrate maintains its optoelectronic properties under tensile stress, with a bending radius of 5 mm.

Fabrication and characterization of ZnO/Zn2TiO4/ZnAl2O4 composite films by using magnetron sputtering with ceramic targets

Transparent conducting oxides (TCO), including ZnO/Zn2TiO4/ZnAl2O4 composite films, are of considerable importance in optoelectronic applications. We fabricated composite film assisted with DC magnetron sputtering using ceramic target material (ZnO, TiO2, and Al2O3 powders mixture). The influence of processing gases composition (0–50% O2–Ar) on film characteristics, including structural, optical, and electronic properties, is assessed. All films contain hexagonal wurtzite ZnO structure and ZnAl2O4, Zn2TiO4 spinel phases, with (002), (100), and (101) as dominant orientation by increasing oxygen contents. The transmittance of deposited films is above 81% (except for 50% oxygen), and the bandgap lies between 3.73 and 3.28 eV (reduced by oxygen addition). The carrier concentration and mobility increase with oxygen addition up to 25%, and resistivity decreases by increasing oxygen contents. This study shows that using ceramic targets, the compound film with variable composition, structural and optoelectronic properties can be obtained by altering the gases admixture.

Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in GeTe Alloys via Introducing Cu2Te Nanocrystals and Resonant Level Doping.

The binary compound of GeTe emerging as a potential medium-temperature thermoelectric material has drawn a great deal of attention. Here, we achieve ultralow lattice thermal conductivity and high thermoelectric performance in In and a heavy content of Cu codoped GeTe thermoelectrics. In dopants improve the density of state near the surface of Femi of GeTe by introducing resonant levels, producing a sharp increase of the Seebeck coefficient. In and Cu codoping not only optimizes carrier concentration but also substantially increases carrier mobility to a high value of 87 cm2 V-1 s-1 due to the diminution of Ge vacancies. The enhanced Seebeck coefficient coupled with dramatically enhanced carrier mobility results in significant enhancement of PF in Ge1.04-x-yInxCuyTe series. Moreover, we introduce Cu2Te nanocrystals' secondary phase into GeTe by alloying a heavy content of Cu. Cu2Te nanocrystals and a high density of dislocations cause strong phonon scattering, significantly diminishing lattice thermal conductivity. The lattice thermal conductivity reduced as low as 0.31 W m-1 K-1 at 823 K, which is not only lower than the amorphous limit of GeTe but also competitive with those of thermoelectric materials with strong lattice anharmonicity or complex crystal structures. Consequently, a high ZT of 2.0 was achieved for Ge0.9In0.015Cu0.125Te by decoupling electron and phonon transport of GeTe. This work highlights the importance of phonon engineering in advancing high-performance GeTe thermoelectrics.

Ultra-low EQE roll-off and marvelous efficiency perovskite quantum-dots light-emitting-diodes achieved by ligand passivation

Recently, perovskite quantum dots (PQDs) were employed as promising emitters in light-emitting diode devices (LEDs) mainly owing to their marvelous photoelectronics properties. However, these perovskite QLEDs suffered from the inefficient performance, especially their serious efficiency roll-off probably due to the exciton recombination dynamic in the interface of PQDs films and charge imbalance resulting from low electrical transportation. Herein, sodium dodecyl sulfate (SDS) was effectively introduced to synthesize PQDs with many excellent properties and eventually suppressed the efficiency roll-off of QLEDs. SDS-based PQDs films exhibited obviously enhanced radiative recombination, reduced trap density, and increased carrier mobility, especially the transportation of electrons. Therefore, the charge carrier was balanced at high current density together with the remarkably suppressed efficiency roll-off. QLEDs fabricated by SDS-capped PQDs exhibited significantly improved maximum brightness of 193,810 cd/m2, and achieve the external quantum efficiency of 10.13%. More importantly, the EQE roll-off of QLED optimized in the thickness of emitting layer was only 1.5% at the current density of 200 mA/cm2, representing a prominent achievement of our strategy.

Selectively Defect‐Healed Graphene Electrodes for Tungsten Diselenide Thin‐Film Transistors

Abstract2D nanomaterials such as graphene and transition metal dichalcogenides have attracted great interest as future electronic materials, especially for application in next‐generation displays, owing to their extraordinary electrical, mechanical, and optical properties. In order to achieve 2D material‐based practical devices, it is essential to heal the graphene defects which are inevitably generated during chemical vapor deposition‐based large‐area synthesis and transfer process, through doping technology. In this article, a novel approach for selective defect‐healing of graphene with electrodeposited gold nanoparticles is proposed, where the defect‐healed graphene source/drain electrodes are integrated with p‐type tungsten diselenide (WSe2) thin‐film transistors (TFTs), for the first time. This proposed device shows greatly enhanced electrical characteristics (increase of carrier mobility about ≈230.8%) by selective defect‐healing of graphene, and the performance improvement mechanism is systematically investigated in terms of conductivity and Schottky barrier engineering of graphene/WSe2 interface. Also, a long‐term stability of defect‐healed graphene electrode is achieved over a long period of a month, which is enabled by a polymer‐based passivation layer that maintains the doping effects of defect‐engineered graphene. The authors’ findings therefore provide a new strategy for developing graphene electrode‐based high‐performance TFTs, and reveal the enormous potential of graphene as an innovative conductor for prospective displays.

Photodetectors with High Responsivity by Thickness Tunable Mixed Halide Perovskite Nanosheets

Chemical transformation of typically "nonlayered" phases into two-dimensional structures remains a formidable task. Among the thickness tunable CsPbX3 (X = Br, Br/I, I) nanosheets (NSs), CsPbBr1.5I1.5 NSs with a thickness of ∼4.9 nm have structural stability superior to ∼6.8 nm CsPbI3 NSs and better hole mobility than ∼3.7 nm CsPbBr3 NSs. Moving beyond the much-explored CsPbBr3 photodetectors, we demonstrate a sharp improvement of the photodetection of CsPbBr1.5I1.5 NS devices by thickening the NSs to ∼6.1 nm through combining 8-carbon and 18-carbon ligand surfactants. Thereby, the responsivity increases up to one of the highest values of 3313 A W-1 at 1.5 V (and 3946 A W-1 at 2 V) with detectivity of 1.6 × 1011 Jones at 1.5 V, due to the increase in carrier mobility up to 7.9 × 10-4 cm2 V-1 s-1. The better device performance of the NSs than 8.6-13.9 nm nanocubes (NCs) is due to their planarity which facilitates in-plane mobilization of the charge carriers.

ABOUT THE CHARACTERISTICS OF MULTILAYER THIN-FILM STRUCTURES WITH DYES BASED ON TITANIUM DIOXIDE

Polyethylene oxide (PEO) gel polymer electrolytes (GPEs) were prepared using tetrapropylammonium iodide (TPAI). The mass fracti on (TPAI) in the electrolyte was varied to increase the productivity of the solar cell. It increa sed the ionic conductivity of the electrolyte at room temperature from 8.426 ଵ ୫ʝ୦୫כୱ୫ and 373K temperature to 18.117 ଵ ୫ʝ୦୫כୱ୫ . The increase in ionic conductivity with the addition of TPAI salts was associated with an increase in the diffusion coefficient, mobility, and density of charge carriers. Keywords : dye-sensitized solar cell, dye-sensitized solar cells, photoelectric electrode, loop electrode, photoanode, electrolyte, polymer electrolyte gel

Development of High-Performance Thermoelectric Materials by Microstructure Control of P-Type BiSbTe Based Alloys Fabricated by Water Atomization.

Developing inexpensive and rapid fabrication methods for high efficiency thermoelectric alloys is a crucial challenge for the thermoelectric industry, especially for energy conversion applications. Here, we fabricated large amounts of p-type Cu0.07Bi0.5Sb1.5Te3 alloys, using water atomization to control its microstructure and improve thermoelectric performance by optimizing its initial powder size. All the water atomized powders were sieved with different aperture sizes, of 32–75 μm, 75–125 μm, 125–200 μm, and <200 μm, and subsequently consolidated using hot pressing at 490 °C. The grain sizes were found to increase with increasing powder particle size, which also increased carrier mobility due to improved carrier transport. The maximum electrical conductivity of 1457.33 Ω−1 cm−1 was obtained for the 125–200 μm samples due to their large grain sizes and subsequent high mobility. The Seebeck coefficient slightly increased with decreasing particle size due to scattering of carriers at fine grain boundaries. The higher power factor values of 4.20, 4.22 × 10−3 W/mk2 were, respectively, obtained for large powder specimens, such as 125–200 μm and 75–125 μm, due to their higher electrical conductivity. In addition, thermal conductivity increased with increasing particle size due to the improvement in carriers and phonons transport. The 75–125 μm powder specimen exhibited a relatively high thermoelectric figure of merit, ZT of 1.257 due to this higher electric conductivity.

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Boron Strengthened GeTe‐Based Alloys for Robust Thermoelectric Devices with High Output Power Density

AbstractHigh‐performance thermoelectric (TE) devices require not only a high figure of merit (ZT) but also mechanical strength and thermal stability. Here, a simultaneous enhancement of ZT as well as mechanical properties is obtained in GeTe‐based alloys by adding boron. This material is then assembled with n‐type CoSb3 skutterudite into TE modules. The improved ZT values result from the increase in charge carrier mobility due to the reduced interfacial barrier height. A peak ZT of 2.2 at 773 K can be achieved in Ge0.84Pb0.1Sb0.06TeB0.07, which shows a negligible change in the coefficient of thermal expansion upon the phase transition from the rhombohedral to the cubic phase, ensuring good thermal stability of the device. Resulting from the boron‐induced grain refinement and dispersion strengthening, the average compressive strength and Vickers hardness of Ge0.9Sb0.1TeB0.07 can be enhanced to ≈227 MPa and ≈202 Hv, respectively. The improved mechanical properties facilitate the assembly of devices and lower the interfacial contact resistance. As a synergy of increased ZT and mechanical strength, a high output power density of ≈1.76 W cm−2 at ΔT = 425 K and an energy conversion efficiency of 7.4% at ΔT = 477 K can be achieved in the TE modules.

Investigation of electronic transport in InAs/GaAs samples. A study using the metaheuristic self-adaptive differential evolution method

This manuscript presents the study of electronic transport on a set of five multilayer molecular beam epitaxy-grown InAs/GaAs semiconductor samples. We developed an automated switch system to carry out electronic transport measurements of mobility and carrier concentration using the van der Pauw technique. Measurements were carried out as a function of temperature within the range of 260 K to 310 K. To identify which scattering mechanisms most contributed to mobility limitation, It was necessary to use the Self-adaptive Differential Evolution meta-heuristic method. This method allowed the determination of the main scattering mechanisms limiting the electronic mobility and identified as scattering by dislocations and phonons. Dislocations consist of the dominant defects in this lattice mismatch structure. Therefore, to increase carrier mobility, we propose some strategies: a change in the sample growth parameters such as substrate temperature and InAs/GaAs layer thickness. Alternatively, annealing of the samples could also be considered to improve sample mobility.

Study of the Electrical and Dielectric behaviour of selenium doped CCTO ceramics prepared by a facile sol-gel route

A facile sol-gel technique was successfully employed to prepare CaCu3-xSexTi4O12 (x = 0, 0.1, 0.2, 0.3 mM) nanoparticles and after subsequent calcination and sintering at 950 °C and 1050 °C respectively, ceramics of these nanoparticles were obtained. The powder XRD profile reveals that samples have the perovskite phase of CaCu3Ti4O12. SEM micrograph shows the dense microstructure of the ceramics and EDX analysis illustrates the elemental composition of the products. The dielectric behaviour of these ceramics was studied as a function of frequency (1 Hz-1 MHz) with different selenium concentrations at ambient temperature. The results indicate that doping of selenium over the copper (Cu) site enhances the dissipation factor and depresses the dielectric constant (εr) of CaCu3Ti4O12 ceramics. The AC conductivity of the substance was found to decrease with an increase in selenium concentration. The magnitude of high resistivity at high concentrations was mainly caused by the rapid increase in mobility of the charge carriers in the ceramics and follows the positive temperature coefficient of the resistance (PTCR) type behaviour. Electric modulus spectrum of undoped and selenium doped CaCu3Ti4O12 samples indicate that the existence of the non-Debye type of relaxation behaviour. The results suggest that the change in dielectric properties of selenium doped CaCu3Ti4O12 samples may be associated with the lack of oxygen vacancies in the ceramic specimens.

Hydrogen and deuterium termination of diamond for low surface resistance and surface step control

A 2D conductive hole gas in diamond is obtained by exposing diamond to a plasma formed in H2 or D2, which terminates the diamond surface with H or D. The diamond is still insulating until it is activated by exposing it to air or overcoating it by atomic layer deposition, ALD, with Al2O3/SiO2 or evaporated WO3. The carrier density of air activated diamond slowly decreases in time, days to weeks, and the carrier mobility increases. However, the carrier density deceases faster than the mobility increases and the diamond's surface resistance increases in time. The mobility is empirically related to the carrier density by a power law, mobility = Q (carrier density)-0.42, where Q is determined by fitting the equation to the data.The value of Q and the surface resistance of the 2D gas depends upon the removal of impurities such as C, K, and Na in the plasma, as well as the offcut angle from the (100) diamonds. These impurities originate from carbon on the chamber walls, the Mo structure that supports the diamond during plasma exposure, impurities on the diamond, such as H2SO4, after inefficient cleaning, and the carbon from the diamond itself as it is etched in the plasma.Once the impurities are reduced, then the offcut angle of (100) diamond to the [011]-axis generates crystal steps generally parallel to the 01¯1axis on the diamond surface. Resistance along the steps is lower than perpendicular to the steps. With the impurities removed, six (100) diamonds offcut by ~6° to the [011] axis had resistance parallel to the steps, 01¯1,vary from 1.32 to 1.95 kΩ sq.−1 with an average of 1.70 ± 0.25 kΩ sq.−1 and perpendicular to the steps, [011], from 1.93 to 3.20 kΩ sq.−1 with an average of 2.51 ± 0.4 kΩ sq.−1.