Low Carrier Research Articles

Synthesis and Characterization of CdS/CISe Quantum Dots with Core–Shell Structure

AbstractQuantum dots have received great interest due to their excellent optoelectronic properties. However, the surface defects of quantum dots affect the carrier transport and ultimately reduce the photovoltaic efficiency. In this paper, a core–shell quantum dot by hot‐injection method is prepared to grow a narrow‐band semiconductor layer (CuInSe2 (CISe) quantumdot) on the surface of a broad‐band core material (cadmium sulfide (CdS) nanocrystal). The composition, structure, optical properties, and decay lifetime of CdS/CISe core–shells are investigated in more detail by X‐ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), UV–vis spectrophotometry, and fluorescence spectroscopy. The CdS/CISe core–shell structure has a broadened absorption range and still shows CISe‐related quantum effects. The increased size of the core–shell and the smaller specific surface area of the CISe shell layer lead to a lower carrier complexation chance, which improves the carrier lifetime.

MIL-53(Fe)@BiOBr/TCN/Ti photoanode assembled visible light responsive photocatalytic fuel cell to enhance rhodamine B degradation and electricity generation

In this research, MIL-53(Fe)@BiOBr/TCN/Ti photoanode was created using the secondary silica sol drop coating method, and assembled with Cu cathode to form photocatalytic fuel cell (PFC) to degrade rhodamine B and generate electricity. The resulting PFC has high visible light response and photocatalytic activity. It has a rhodamine B degradation efficiency of 92.19%, a maximum power density of 6.2 μW cm−2, and a maximum photocurrent density of 0.11 mA cm−2. Its strong photocatalysis capabilities benefits from the wider light absorption range and lower charge carrier recombination rate of the photoanode. The double Z-scheme heterojunction formed among BiOBr, MIL-53(Fe) and TCN in the composite catalyst of the photoanode provide more transfer channels for photogenerated e− and encourage the separation of e−-h+ pairs. ∙OH and ∙O-2 are the main active substances for rhodamine B degradation. This study offers a novel approach to the development of high-efficiency photoanodes for PFC.

Visible light photocatalytic activity of a FeCo metal-organic framework for degradation of acetaminophen and 2,4-dichlorophenoxyacetic acid and a nematode-based ecological assessment

The present work reports the solvothermal synthesis of a bimetallic (FeCo) metal-organic framework (MOF) for the sunlight-driven photocatalytic degradation of acetaminophen (AAP) and 2,4-dichlorophenoxyacetic acid (2,4-D). The FeCo-MOF was confirmed by various analytical techniques. The as-prepared FeCo-MOF displayed improved light absorbance in the visible region and a bandgap energy of 1.73 eV. Photoluminescence spectra showed that the FeCo-MOF exhibited a lower charge carrier recombination rate than bare Fe- or Co-MOFs. The FeCo-MOF achieved a maximum AAP conversion rate (rate constant 0.031/min) of 97.4% in 180 min than bare Fe-MOF (66%) or Co-MOF (73%). A scavenger study showed that the superoxide anion radicals were responsible for the degradations of AAP and 2,4-D. The catalyst proved stable after five recycles with no change in AAP degradation efficiency. On the other hand, under the same conditions, the FeCo-MOF photodegraded 2,4-D by 79.8%. A kinetic study showed that the degradations of AAP and 2,4-D exhibited pseudo-first-order kinetics. In addition, the ecotoxicity results showed that degraded 2,4-D altered the survival, development, reproduction, intracellular redox status, and expression pattern of stress-responsive genes in the Caenorhabditis elegans model.

Polymerization Strategies to Construct a 3D Polymer Passivation Network toward High Performance Perovskite Solar Cells.

The spontaneously formed uncoordinated Pb2+ defects usually make the perovskite films demonstrate strong n-type with relatively lower carrier diffusion length and serious non-radiative recombination energy loss. In this work, we adopt different polymerization strategies to construct three-dimensional passivation frameworks in the perovskite layer. Thanks to the strong C≡N⋅⋅⋅Pb coordination bonding and the penetrating passivation structure, the defect state density is obviously reduced, accompanied by a significant increase in the carrier diffusion length. Additionally, the reduction of iodine vacancies also changed the Fermi level of the perovskite layer from strong n-type to weak n-type, which substantially promotes the energy level alignment and carrier injection efficiency. As a result, the optimized device achieved an efficiency exceeded 24 % (the certified efficiency is 24.16 %) with a high open-circuit voltage of 1.194 V, and the corresponding module achieved an efficiency of 21.55 %.

Significant enhancement of the thermoelectric properties for MnSb2Se4 through Ag doping

MnSb2Se4 is a thermoelectric material that has not been receiving much attention. However, it has the sufficient potential to be exploited with a wide range of applications. It is a narrow band p-type semiconductor with high Seebeck coefficient and low thermal conductivity. The lower carrier concentration prevents it from having better thermoelectric performance. Therefore, we doped the Mn position of MnSb2Se4 with Ag atoms to improve the thermoelectric properties by enhancing the carrier concentration of the material while modulating its thermal conductivity. The final sample Mn0.9Ag0.1Sb2Se4 has a PF of 2.54 μWcm-1K-2 at 773 K. Combined with the reduction of the thermal conductivity κ optimizes the ZT value from 0.17 to 0.65 at 773 K.

Polymerization Strategies to Construct a 3D Polymer Passivation Network toward High Performance Perovskite Solar Cells

AbstractThe spontaneously formed uncoordinated Pb2+ defects usually make the perovskite films demonstrate strong n‐type with relatively lower carrier diffusion length and serious non‐radiative recombination energy loss. In this work, we adopt different polymerization strategies to construct three‐dimensional passivation frameworks in the perovskite layer. Thanks to the strong C≡N⋅⋅⋅Pb coordination bonding and the penetrating passivation structure, the defect state density is obviously reduced, accompanied by a significant increase in the carrier diffusion length. Additionally, the reduction of iodine vacancies also changed the Fermi level of the perovskite layer from strong n‐type to weak n‐type, which substantially promotes the energy level alignment and carrier injection efficiency. As a result, the optimized device achieved an efficiency exceeded 24 % (the certified efficiency is 24.16 %) with a high open‐circuit voltage of 1.194 V, and the corresponding module achieved an efficiency of 21.55 %.

Efficient planar mixed-cation perovskite photovoltaics with low-temperature-processed indium sulfide as electron transport material

Electron transport materials(ETMs) are crucial for efficient transport of photocarriers from the perovskites to the cathode in perovskite solar cells (PSCs). Although titanium dioxide (TiO2) is frequently employed as an ETM, PSC performance and applicability are hindered by its high-temperature manufacturing and limited electron mobility. In this work, a low-temperature solution-processable indium sulfide (In2S3) was incorporated into triple-cation perovskite solar cells to be an efficient ETM for the first time. After solvent-thermal synthesis of In2S3, followed by spin-coatingpreparation of In2S3 ETM, aPSC is constructed. The photoelectrical measurements show that the In2S3-based PSC has lower density of charge traps, higher conductivity, better electron transportation at the In2S3/perovskite interfaces, and a lower transfer resistance and carrier recombination. The PSC-based In2S3 yields a champion efficiency of 18.182%, and the PSC-based TiO2 gets an efficiency of 15.288%. Without encapsulation, the In2S3-based devices demonstrate increased stability in ambient environments. Our ETM layer and perovskite were made at low temperatures, which may open up new possibilities for solution-processed flexible electronics.

Mn- and Yb-Doped BaTiO3-(Na0.5Bi0.5)TiO3 Ferroelectric Relaxor with Low Dielectric Loss

In this work, a Mn-and Yb-doped BaTiO3-(Na0.5Bi0.5)TiO3 ferroelectric relaxor was designed and prepared. The effects of Mn on the microstructures, dielectric and electrical properties of the ceramics were investigated. The X-ray structural analysis shows a perovskite structure. The SEM images show the homogeneous microstructure of ceramics with an average grain size of about 1 μm. The temperature-dependent permittivity shows relaxor characteristics as Mn-doped. Mn at a low level (x ≤ 0.005) is beneficial for low dielectric loss and high resistivity. The maximum resistivity of ≥3 × 1012 Ω cm and minimum dielectric loss of ≤0.06 can be achieved at x ≤ 0.005. The resistivity of the ceramics follows the Arrhenius law with activation energy decreasing from ~1.31 to 1.01 eV as x increases. With lower Mn dopant, oxygen vacancies and charge carrier concentration partially decrease with Mn doping, which is helpful to improve the insulation resistance and decrease the dielectric loss.

Nb, Se-codoped ZnIn2S4/NbSe2 composites with enhanced catalytic activity and photodegradation performance towards tetracycline

Low quantum efficiency and serious photogenerated carrier recombination have been urgent bottleneck problems for photocatalytic materials. Herein, we prepared Nb, Se-codoped ZnIn2S4/NbSe2 composites through a facile solvothermal method. The synergetic effect of codoping and cocatalyst was investigated on the photodegradation performance towards tetracycline under visible-light irradiation. By adjusting the final composition, the comprehensive characterization revealed that the optimum degradation efficiency of NS/ZIS-1.6 catalyst arrived at 75% in 70 min, which was 5.8 times higher than that of pure ZnIn2S4. Deep analysis indicated that the enhanced photocatalytic performance could be attributed to higher light absorption, more efficient electron/hole separation, faster charge transport, and lower carrier recombination. This work may offer novel viewpoint for design of high-performance catalysts towards the visible-light-driven photodegradation system.

Realize high thermoelectric properties in n-type Bi2Te2.7Se0.3 materials via ZrO2 ceramic nanoparticles mediated heterogeneous interface

Bismuth telluride (Bi2Te3)-based thermoelectric materials have been commercially utilized for refrigeration and energy conversion at room/near room temperature. However, n-type Bi2Te3 compounds always exhibit inferior performance compared with their p-type counterparts, which is pronouncedly impeding Bi2Te3-based energy converters from further optimizations. Herein, a promising ZT value (ZT = 1.33) was obtained in n-type Bi2Te2.7Se0.3 (BTS) materials by using an interface engineering strategy to synergistically optimize the electron and phonon transport properties. In detail, ZrO2 ceramic nanoparticles (NPs) were entered into BTS matrix to construct high-density heterogeneous interfaces. The heterogeneous interfaces strongly scatter phonons and lower energy carriers, leading to decreased thermal conductivity and increased Seebeck coefficient, while the electrical conductivity is not sacrificed due to the compromise of the slightly reduced carrier mobility by interfacial scattering and the increased carrier concentration by ZrO2 NPs doping. A planar module consisting of 127 pairs of p-n single legs were assembled. At the temperature difference of 200 K, the output power and energy conversion efficiency reached 9.6 W and 6.2%, respectively. This strategy provides a feasible way to rationally design advanced n-type Bi2Te2.7Se0.3 materials with excellent thermoelectric and mechanical properties induced by high-density heterogeneous interfaces.

Geometric, electronic and transport properties of bulged graphene: A theoretical study.

Out-of-plane deformation in graphene is unavoidable during both synthesis and transfer procedures due to its special flexibility, which distorts the lattice and eventually imposes crucial effects on the physical features of graphene. Nowadays, however, little is known about this phenomenon, especially for zero-dimensional bulges formed in graphene. In this work, employing first-principles-based theoretical calculations, we systematically studied the bulge effect on the geometric, electronic, and transport properties of graphene. We demonstrate that the bulge formation can introduce mechanical strains (lower than 2%) to the graphene's lattice, which leads to a significant charge redistribution throughout the structure. More interestingly, a visible energy band splitting was observed with the occurrence of zero-dimensional bulges in graphene, which can be attributed to the interlayer coupling that stems from the bulged structure. In addition, it finds that the formed bulges in graphene increase the electron states near the Fermi level, which may account for the enhanced carrier concentration. However, the lowered carrier mobility and growing phonon scattering caused by the formed bulges diminish the transport of both electrons and heat in graphene. Finally, we indicate that bulges arising in graphene increase the possibility of intrinsic defect formation. Our work will evoke attention to the out-of-plane deformation in 2D materials and provide new light to tune their physical properties in the future.

Enhanced Optically–Excited THz Wave Emission by GaAs Coated with a Rough ITO Thin Film

In this study, we report enhancement of terahertz (THz) radiation with indium-tin-oxide (ITO) thin-film deposited on semi-insulating gallium arsenide substrate (SI-GaAs). The amplitude of THz emission from both ITO/SI-GaAs and bare SI-GaAs substrate as a function of optical pump (i) incident angle, (ii) polarization angle, and (iii) power were investigated. The enhancement of peak amplitude of a THz pulse transmitted through the ITO/SI-GaAs sample in comparison to bare SI-GaAs substrate varied from 100% to 0% when the pump incidence angle changed from 0° to 50°. The maximum enhancement ratio of peak amplitude for a coated sample relative to the bare substrate is approximately up to 2.5 times at the minimum pump intensity of 3.6 TW/m2 and gradually decreased to one at the maximum pump intensity of 20 TW/m2. From outcomes of these studies, together with data on surface and material characterization of the samples, we show that THz emission originates from the ITO/GaAs interfaces. Further, both interface-field-induced transient current and field-induced optical rectification contribute to the observed THz signal. Observed enhancement was tentatively attributed to surface-plasmon-induced local field enhancement, coupled with constructive interference of forward and retro-reflected backward THz emission from the ITO/GaAs interfaces. The polarity-flip reported previously for very thin Au-coated GaAs was not observed. This was explained by the wide-bandgap, transparency and lower free carriers of ITO. For best results, the incident angle should be in the range of 0 to 30° and the incident polarization should be 0 to 45°. We further predict that the ITO thin film of suitable thickness or with engineered nanostructures, post-annealed under optimum conditions may lead to further enhancement of THz radiation from ITO-coated semiconductor surfaces.

Performance Enhancement of an MoS2-Based Heterojunction Solar Cell with an In2Te3 Back Surface Field: A Numerical Simulation Approach.

Researchers are currently showing interest in molybdenum disulfide (MoS2)-based solar cells due to their remarkable semiconducting characteristics. The incompatibility of the band structures at the BSF/absorber and absorber/buffer interfaces, as well as carrier recombination at the rear and front metal contacts, prevents the expected result from being achieved. The main purpose of this work is to enhance the performance of the newly proposed Al/ITO/TiO2/MoS2/In2Te3/Ni solar cell and investigate the impacts of the In2Te3 BSF and TiO2 buffer layer on the performance parameters of open-circuit voltage (V OC), short-circuit current density (J SC), fill factor (FF), and power conversion efficiency (PCE). This research has been performed by utilizing SCAPS simulation software. The performance parameters such as variation of thickness, carrier concentration, the bulk defect concentration of each layer, interface defect, operating temperature, capacitance-voltage (C-V), surface recombination velocity, and front as well as rear electrodes have been analyzed to achieve a better performance. This device performs exceptionally well at lower carrier concentrations (1 × 1016 cm-3) in a thin (800 nm) MoS2 absorber layer. The PCE, V OC, J SC, and FF values of the Al/ITO/TiO2/MoS2/Ni reference cell have been estimated to be 22.30%, 0.793 V, 30.89 mA/cm2, and 80.62% respectively, while the PCE, V OC, J SC, and FF values have been determined to be 33.32%, 1.084 V, 37.22 mA/cm2, and 82.58% for the Al/ITO/TiO2/MoS2/In2Te3/Ni proposed solar cell by introducing In2Te3 between the absorber (MoS2) and the rear electrode (Ni). The proposed research may give an insight and a feasible way to realize a cost-effective MoS2-based thin-film solar cell.

Metal-free oxidized carbon nitride for efficient sunlight-driven photocleavage of lignin β-O-4 bond

Benefitting from mild reaction conditions and good reusability, photocatalysis has industrial potential for high-value utilization of lignin waste. Commonly used metal sulfide photocatalysts present the risk of contamination of aromatic products by heavy metal ions due to photocorrosion. Here, a metal-free oxidized g-C3N4 photocatalyst was developed to selectively cleave the β-O-4 bond of lignin. Simple high-temperature oxidation treatment reduced carbon nitride size and scavenged the terminal NH2 groups, resulting in higher oxygen adsorption capacity and valence band potential. In the photoelectric effect, oxidized g-C3N4 exhibited lower carrier migration resistance and a longer photogenerated electron lifetime. The high oxygen adsorption and specific surface area led to a 160 % increase in superoxide radicals. Two routes for the photocatalytic cleavage β-O-4 bond in g-C3N4 are proposed. The catalytic activity of oxidized g-C3N4 is 8.5 times higher than that of untreated g-C3N4, the selective photocleavage reactivity attributed mainly to more OOH radicals and higher valence band potential. This work will provide a green and efficient strategy for the high-value conversion of lignin.

Numerical and experimental investigation of the removal of Fe impurity during different DS process under axial magnetic field

The effects of axial magnetic field on the different directional solidification (DS) process were studied by using a transient numerical model of heat and mass transfer and corresponding experiments. The shape of melt-crystal (m-c) interface, melt flow morphology, thermal stress and Fe impurity concentration in the crystal for different DS process have been studied. The simulation results indicate that axial magnetic field provided a slight convex m-c interface shape, better flow pattern, lower thermal stress as well as lower Fe impurity concentration throughout the entire DS process. To further verify the simulation results, multi-crystalline silicon (mc-Si) ingots were prepared and testing were carried out. The experimental results are in good agreement with the simulation results. The experimental results show that a slight convex m-c interface shape was obtained, and the Fe impurity concentration in the middle area of the ingot was lower and higher minority carrier lifetime could be achieved by using the axial magnetic field.

Fluid Hyperthermia and Ultrasonic Studies on Nickel Ferrite Magnetic Nanoparticles

Superparamagnetic nickel ferrite (NiFe2O4) nanoparticles were synthesized by low temperature hydrothermal method using EDTA as templating agent. The synthesized nanoparticles were characterized by FTIR, XRD, FESEM and TEM analysis. The XRD analysis shows the presence of cubic spinel phase with crystallite size of 26.42 nm. The morphology analyzed by FESEM and TEM techniques indicate the presence of cubical shape particles having average particle size of 28.44 nm with no agglomeration. The VSM analysis gives “S” shape curve with zero coercivity and saturation magnetization (Ms) of 30 emu/g indicating the presence of small magnetic particles exhibiting the superparamagnetic behaviour. Nickel ferrite nanofluids were prepared by mixing carrier fluid water with varying amounts of nickel ferrite nanopowder. Zeta potential measurements of dilute solution of nickel ferrite show the formation of stable nickel ferrite nanofluids. Hyperthermia and ultrasonic studies were carried out on various concentrations of nickel ferrite nanofluids. The SAR values and ultrasonic parameters were calculated. Less concentrated nickel ferrite nanofluids quickly attain threshold hyperthermia temperature of 43 °C when exposed to lower applied AC magnetic field of range 5 mT to 20 mT. Ultrasonic studies show at lower concentrations nickel nanoparticles interact with carrier fluid water through cohesive forces mainly hydrogen bonding and keep the particles in an isolated state there by continue to exhibit superparamagnetic characteristics. At higher concentrations, particle-particle interaction predominantly occurs leading to increase in particle size beyond 30 nm, so that the magnetic characteristics of nickel ferrite nanoparticles changes from superparamagnetic to ferromagnetic nature. At low concentrations induction heating occurs through Neel and Brownian relaxation mechanism whereas at high concentrations hysteresis loss mechanism operate to attain the threshold temperature of 43 ºC.

Investigation of III-V GaP solar cell on silicon substrate

The best solar conversion efficiencies have been reached thanks to multijunction solar based on III-V semiconductors on GaAs or Ge substrates. While displaying high conversion efficiencies, these solar cells suffer from the high cost of such substrates. To benefit from both the low cost and technological maturity of silicon cells, III-V tandem cells on silicon seem a good compromise to overpass the theoretical efficiency limit of the Si single cells. To study the GaP/Si interface effect on the solar cell characteristic, a GaP n-i-p solar cell has been grown on silicon substrate. Two types of electrical contacts configurations have been processed: a top-top configuration in which the current does not see the GaP/Si interface and the top-bottom configuration where the electric current crosses the interface. A comparison of dark I-V, I-V under solar illumination, and EQE measurements on both configurations is performed. The top-bottom contacts configuration shows an EQE a little bit lower than the top-top contact one, likely due to lower carrier diffusion length or recombination at the lower interface. However, the result on the EQE of the top-bottom configuration is encouraging for the future development of the GaP-based/Si tandem solar cells, and any other tandem cell on silicon using GaP as an intermediate selective contact.

Scattering Analysis of AlGaN/AlN/GaN Heterostructures with Fe-Doped GaN Buffer.

The results of the study of the influence of Fe segregation into the unintentionally doped GaN channel layer in AlGaN/AlN/GaN heterostructures with Fe-doped GaN buffer layer on the electrical properties of two-dimensional electron gas are presented. A set of several samples was grown by metal-organic vapor-phase epitaxy and characterized by the van der Pauw method. The dependence of concentration and mobility of the two-dimensional electron gas on the channel layer thickness was analyzed theoretically by self-consistent solving of 1D Poisson and Schrödinger equations and scattering rate calculations within the momentum relaxation time approximation. It was found that both concentration and mobility decreases were responsible for the increase in the sheet resistance in the structures with a thinner channel layer, with a drop in mobility being not only due to ionized impurity scattering, but also due to a combined effect of weakening of screening, lower carrier energy and change in form-factors on scattering by interface roughness, dislocations and polar optical phonons.

The effect of CuZn+ZnCu defect complex on Cu2ZnSnS4 thin film solar cell: A density functional theory study

Due to its non-toxic and earth-abundant constituent elements, Cu2ZnSnS4 (CZTS) is a promising material for thin-film solar cells. To date, it is believed that the best-performing CZTS solar cells were fabricated with Cu/(Zn + Sn) = 0.8, Zn/Sn = 1.2 ratios, and efficiency was over 12.6%. Besides, defect also plays an essential role in solar cells' performance. It is known that the most abundant defect complex in this condition is CuZn + ZnCu. This paper shows the first investigation of CuZn + ZnCu defect complex towards CZTS solar cells performance using density functional theory (DFT). The work was carried out using Vienna ab-initio Simulation Package (VASP). First, GGA exchange-correlation functional was applied to perform the first structural relaxation. Then, for better electronic properties, the hybrid functional was used to analyze the density of states calculations with the screening parameter of 0.2 (HSE-06 exchange-correlation functional). The optical properties were also conducted through the HSE-06 exchange-correlation functional. Then, the predicted J-V characteristics were estimated using Spectroscopic Limited Maximum Efficiency (SLME). The result shows that this defect increased the predicted short-circuit current density from 36.593 to 39.392 mA/cm2. As a result, the optimum solar cell efficiency considering SQ Limit achieves 29.68%. Furthermore, this study shows that the defect induced a lower charge carrier effective mass. Therefore, it can be concluded that this defect is one of the reasons for the optimum ratios of Cu/(Sn + Zn) and Zn/Sn being 0.8 and 1.2, respectively.

Seventy-Five Years since the Point-Contact Transistor: Germanium Revisited

The advent of the point-contact transistor is one of the most significant technological achievements in human history with a profound impact on human civilization during the past 75 years. Although the first transistor was made of germanium it was soon replaced by silicon, a material with lower intrinsic carrier mobilities but with a substantially better native oxide. Interestingly, more than two decades ago, germanium was once again considered as a mainstream microelectronic material, since the introduction of high-k dielectrics allowed the consideration of channel materials irrespective of the quality of their native oxide. After about 50 years of limited studies on the defect processes in germanium, the community once again focused on its applicability for mainstream electronic applications. Nevertheless, there are some bottlenecks that need to be overcome, and it was the aim of the present review to discuss the progress in the understanding of the defect processes of Ge.