Increase In Hole Concentration Research Articles

Epitaxial analysis of GaInP/AlGaInP red light-emitting diodes with ternary AlGaP quantum barriers for quantum efficiency enhancement

We have numerically analyzed Gallium Indium Phosphide (GaInP)/Aluminum Gallium Indium Phosphide (AlGaInP) red light-emitting diodes (LEDs) by utilizing ternary Aluminum Gallium Phosphide (AlGaP) quantum barriers instead of the conventional quaternary AlGaInP quantum barriers. The simulation findings show that the use of AlGaP quantum barriers in AlGaInP red light-emitting diodes leads to a significant increase in both electron and hole concentration in the quantum wells which in turn enhances the rate of radiative recombination compared to conventional (reference) red light-emitting diodes. Furthermore, the proposed LEDs have a substantial decline in efficiency droop from 38% to 32%.

Optimizing Energy Levels and Improving Film Compactness in PbS Quantum Dot Solar Cells by Silver Doping.

PbS quantum dot (QD) solar cells harvest near-infrared solar radiation. Their conventional hole transport layer has limited hole collection efficiency due to energy level mismatch and poor film quality. Here, how to resolve these two issues by using Ag-doped PbS QDs are demonstrated. On the one hand, Ag doping relieves the compressive stress during layer deposition and thus improves film compactness and homogeneity to suppress leakage currents. On the other hand, Ag doping increases hole concentration, which aligns energy levels and increases hole mobility to boost hole collection. Increased hole concentration also broadens the depletion region of the active layer, decreasing interface charge accumulation and promoting carrier extraction efficiency. A champion power conversion efficiency of 12.42% is achieved by optimizing the hole transport layer in PbS QD solar cells, compared to 9.38% for control devices. Doping can be combined with compressive strain relief to optimize carrier concentration and energy levels in QDs, and even introduce other novel phenomena such as improved film quality.

Lasing Threshold Reduction of AlGaN‐Based Ultraviolet‐C Laser Diodes Using Strain Relaxed Lower Cladding Layer

The dependence of the performance of ultraviolet‐C (UV‐C) laser diodes on the strain relaxation of lower cladding layer (LCL) is numerically investigated. As the strain relaxation increases from 0% to 100%, the threshold current density decreases from 25 to 12.7 kA cm−2, and the slope efficiency increases from 0.039 to 0.087 W A−1, thanks to an increase in internal quantum efficiency (IQE). The improvement of IQE is mainly due to the enhanced hole injection resulting from the increased hole concentration in the distributed polarization doping layers and the reduced hole blocking barrier height of the upper waveguide layer with increasing the strain relaxation. Additionally, the effective screening of the polarization field within the quantum well ameliorates the quantum‐confined Stark effect, further improving the IQE. Our study reveals that strain relaxed LCL is imperative to achieve low‐threshold UV‐C laser diodes.

Low Thermal Conductivity and High Thermoelectric Performance of Nb-Doped Quarternary Mixed Crystal Nb0.05W0.95-xMox(Se1-xSx)2.

Transition-metal dichalcogenide WSe2 has attracted increasing interest due to its large thermopower (S), low-cost, and environment-friendly constituents. However, its thermoelectric figure of merit, ZT, of WSe2 is limited due to its large lattice thermal conductivity (κL) and low electrical conductivity. In view of WSe2 and MoS2 having the same crystal structure, here we designed and prepared Nb-doped quarternary mixed crystal (MC) Nb0.05W0.95-xMox(Se1-xSx)2 (0 ≤ x ≤ 0.095). The results indicate that the κL of the MC can reach as low as 0.12 W m K-1 at 850 K, being 93% smaller than that of WSe2. Our analysis reveals that its low κL originates chiefly from intense scattering of both high-frequency phonons from point defects (mainly alloying elements) and mid/low-frequency phonons from MoS2 inclusions residual within MC. In addition, the alloying of WSe2 with MoS2 causes a 5-fold increase in cation vacancies (VW‴'), leading to a large increase in hole concentration and electrical conductivity, which gives rise to a ∼7.5 times increase in power factor (reaching 4.2 μ W cm-1 K-2 at 850 K). As a result, a record high ZTmax = 0.63 is achieved at 850 K for the MC sample with x = 0.076, which is 20 times larger than that of WSe2, demonstrating that MC Nb0.05W0.95-xMox(Se1-xSx)2 is a promising thermoelectric material.

Enhancing Thermoelectric Performance in P-Type Sb2Te3-Based Compounds Through Nb─Ag Co-Doping with Donor-Like Effect.

P-type Sb2Te3 has been recognized as a potential thermoelectric material for applications in low-medium temperature ranges. However, its inherent high carrier concentration and lattice thermal conductivity led to a relatively low ZT value, particularly around room temperature. This study addresses these limitations by leveraging high-energy ball milling and rapid hot-pressing techniques to substantially enhance the Seebeck coefficient and power factor of Sb2Te3, yielding a remarkable ZT value of 0.55 at 323K due to the donor-like effect. Furthermore, the incorporation of Nb─Ag co-doping increases hole concentration, effectively suppressing intrinsic excitations ≈548K while maintaining the favorable power factor. Simultaneously, the lattice thermal conductivity can be significantly reduced upon doping. As a result, the ZT values of Sb2Te3-based materials attain an impressive range of 0.5-0.6 at 323K, representing an almost 100% improvement compared to previous research endeavors. Finally, the ZT value of Sb1.97Nb0.03Ag0.005Te3 escalates to 0.92 at 548K with a record average ZT value (ZTavg) of 0.75 within the temperature range of 323-573K. These achievements hold promising implications for advancing the viability of V-VI commercialized materials for low-medium temperature application.

O, N co-doped CuCrO2 as efficient hole transport layer for high-performance ultraviolet photodetectors

The optoelectronic properties and film-formation characteristics of CuCrO2 are optimized through oxygen (O) and nitrogen (N) co-doping using the space-limited domain annealing method in ambient air. After O, N co-doping at 100 °C, the conductivity of CuCrO2 is enhanced due to a decrease in trap-state density and an increase in hole concentration, thereby facilitating carrier transport. With a work function (WF) of 5.24 eV and an electron affinity (χ) of 2.29 eV, the CuCrO2 films serves as both a hole transport layer (HTL) and an electron-blocking layer, playing a pivotal role in enhancing photogenerated holes extraction while suppressing nonradiative charge recombination at the heterojunction interface. Consequently, the responsivity and detectivity of the device with FTO/TiO2 NRs/O, N co-doped CuCrO2/Au structure reach 188 mA W−1 and 3.14 × 1013 cm Hz1/2 W−1 in self-powered mode, respectively. Simultaneously, the rise/decay time is reduced to just 31.2/32.1 ns. Furthermore, after applying a bias of − 2 V, the photodetector demonstrates significantly improved performance with a responsivity of up to 5 × 105 mA/W (365 nm, 0.97 mW cm−2) and an apparent quantum efficiency of ∼30,000%. These results highlight the promising potential of utilizing O, N co-doped CuCrO2 HTL for constructing high-performance optoelectronic devices.

Investigation of low-temperature thermoelectric properties of Si0.8Ge0.2 alloy irradiated by high energy electron beam

In this work, electron beam (EB) irradiation has been significantly used to alter the structure and transport properties of Si0.8Ge0.2 alloys to improve thermoelectric behaviour, doses ranging from (0–40 kGy). Compared to the pristine sample the resistivity of the EB irradiated sample increases, this may be due to the increase in lattice distortion induced by the EB irradiation, which leads to a decrease in mobility of the charge carriers. Further, the resistivity decreases for higher EB irradiation samples (20 kGy and 40 kGy), this may be due to an increase in hole concentration (p) induced by EB irradiation, which dominates the mobility (μ). The Seebeck coefficient value reveals the p-type nature of the alloy, which increased with EB irradiation. The occurrence of metallicity and increase in the Seebeck coefficient is understood due to the bombardment-induced defects. The alloy exhibits the maximum power factor of 287.76 μW/mK2 at 320 K under a 40 kGy EB dosage.

Constructing built-in electric field in 3D core-shell p-n heterostructure of NiFe-LDH nanosheets modified Cr-doped Ni3S2 nanofibers for efficient and stable oxygen evolution reaction

Designing and creating effective, dependable, and conveniently accessible electrocatalysts for the oxygen evolution process is necessary to increase the efficiency of hydrogen production by water electrolysis. In this study, utilizing hydrothermal and electrodeposition methods, we have successfully synthesized a unique 3D Cr-Ni3S2 @NiFe-LDH/NF core-shell p-n heterostructure with a built-in electric field for use as an oxygen evolution reaction (OER) electrocatalyst. By constructing a built-in electric field into the p-n junction, the electronic state of the NiFe-LDH surface can be modified, resulting in an increase in hole concentration and enhanced adsorption of hydroxyl groups from the electrolyte. This facilitates OER reaction kinetics and improves electrocatalytic performance. The Cr-Ni3S2 @NiFe-LDH/NF-120 electrocatalyst exhibits a low overpotential of only 190 mV at a current density of 10 mA cm−2, with a Tafel slope of 33.4 mV dec−1. Remarkably, the overpotential is low, remaining at just 270 mV even at 100 mA cm−2. By operating continuously for 40 h at room temperature in a 1.0 M KOH solution while maintaining a current density of 100 mA cm−2, the exceptional stability was demonstrated. Our research provides a solid framework for the creation of extremely stable and effective 3D core-shell heterojunction electrocatalysts, which can greatly improve OER performance.

Recombination mechanism of heavily Be-doped GaAsN by time-resolved photoluminescence

The optical properties of GaAsN system alloys have not been clarified, particularly for the localized level around the bottom of the conduction band induced by nitrogen atoms. Herein, the recombination mechanism is systematically investigated for heavily Be-doped p-type GaAsN using both continuous-wave (CW) and time-resolved (TR) photoluminescence (PL) characteristics, which is expected to be applied to devices such as a p+-n+ tunnel diode inserted into a multijunction solar cell composed of GaAs system alloys and as the base layer of a heterojunction bipolar transistor. The S-shape characteristic weakened with increasing hole concentration (p) in the CW-PL spectra of Be-doped GaAsN. Both short and long lifetimes were evaluated using TR-PL decay curves. Specifically, the long lifetime was distributed between 0.7 and 1 ns independent of temperature and p. This long lifetime corresponds to radiative recombination lifetime from a localized level, supporting that a localized level is formed in Be-doped GaAsN despite high p on the order of 1019 cm−3. Electrons are tightly bound at a localized level, equivalent to this long lifetime, whereas the electron lifetime decreases with increasing p, resulting in the S-shape characteristic vanishing in the temperature dependence of the CW-PL spectra for ultraheavily Be-doped GaAsN with p of 5 × 1019 cm−3. Moreover, this S-shape characteristic vanished in the temperature dependence of TR-PL spectra for moderately Be-doped GaAsN with p of 8 × 1018 cm−3, indicating that the density of states is limited for a localized level.

High‐Performance Silver‐Doped Porous CuBi2O4 Photocathode Integrated with NiO Hole‐Selective Layer for Improved Photoelectrochemical Water Splitting

AbstractCuBi2O4(CBO) has received considerable attention owing to its ideal optical bandgap and positive photocurrent onset potential. However, CBO photocathodes exhibit poor charge carrier separation and transfer across the conducting substrate interface. Herein, a systematic incorporation of Ag+‐cations into nanosized porous CBO network (ACBO) using a simple pulsed‐electrodeposition method. In ACBO photocathode, the Ag+‐ions replace the Bi3+‐ions, thereby building an increased hole concentration, which further signifies the photogenerated electron‐hole separation. Additionally, introducing a low‐cost NiO hole‐selective layer between ACBO and the conducting substrate enables a back‐interface‐aided hole‐extraction and electron blocking, resulting in an improved charge transfer across the back interface. Compared with an unmodified CBO, the NiO/ACBO photocathode exhibits a three‐fold enhanced photocurrent performance. This enhances the photocurrent originating from the incorporation of a substantial amount of Ag+‐ions into the CBO structure, leading to an increased acceptor density as well as the formation of an appropriate hole‐selective layer across the back‐contact. The absorption percentage, time‐resolved photoluminescence, and photoelectrochemical impedance spectroscopy measurements unveil the potential light harvesting, charge separation, and transfer characteristics of the NACBO photoelectrode, respectively. Through this systematic study, an efficient and simple strategy is determined for developing ternary metal oxide‐based photocathode/photoanode systems for sustainable energy applications.

Performance Improvement of AlGaN‐Based Deep‐Ultraviolet Light‐Emitting Diodes with Multigradient Electron Blocking Layer and Triangular Last Quantum Barrier

The performance improvements of AlGaN‐based deep ultraviolet light‐emitting diodes (DUV‐LEDs) with multigradient electron blocking layer (EBL) and triangular last quantum barrier (LQB) are investigated. The results show that the maximum internal quantum efficiency (IQE) is improved by 44.9% and light output power (LOP) is improved by 58.1% at 200 A cm−2, exhibiting a tremendous improvement compared to the conventional structure. These improvements are mainly attributed to the fact that both multigradient EBL and triangular LQB can generate negative polarization charges in the graded composition layer, which leads to a significant increase in hole concentration compared to the conventional EBL and LQB structures. Meanwhile, the multigradient EBL can transform the conventional triangular barrier into an arc‐shaped barrier, increasing the electron potential barrier height and decreasing the hole potential barrier height. This unique design suppresses electron leakage and enhances hole injection, leading to a significant enhancement of the IQE and alleviation of the efficiency droop.

Simultaneous Enhancement of the Power Factor and Phonon Blocking in Nb-Doped WSe2

Transition-metal dichalcogenide WSe2 is a potentially good thermoelectric (TE) material due to its high thermopower (S). However, the low electrical conductivity (σ), power factor (PF), and relatively large lattice thermal conductivity (κL) of pristine WSe2 degenerate its TE performance. Here, we show that through proper substitution of Nb for W in WSe2, its PF can be increased by ∼10 times, reaching 5.44 μW cm-1 K-2 (at 850 K); simultaneously, κL lowers from 1.70 to 0.80 W m-1 K-1. Experiments reveal that the increase of PF originates from both increased hole concentration due to the replacement of W4+ by Nb3+ and elevated thermopower (S) caused by the enhanced density of states effective mass, while the reduced κL comes mainly from phonon scattering at point defects NbW. As a result, a record high figure of merit ZTmax ∼0.42 is achieved at 850 K for the doped sample W0.95Nb0.05Se2, which is ∼13 times larger than that of pristine WSe2, demonstrating that Nb doping at the W site is an effective approach to improve the TE performance of WSe2.

Operando NMR electrochemical gating studies of ion dynamics in PEDOT:PSS

Although organic mixed ionic-electronic conductors are widely proposed for use in bioelectronics, energy generation/storage and neuromorphic computing, our fundamental understanding of the charge-compensating interactions between the ionic and electronic carriers and the dynamics of ions remains poor, particularly for hydrated devices and on electrochemical cycling. Here we show that operando 23Na and 1H nuclear magnetic resonance (NMR) spectroscopy can quantify cation and water movement during the doping/dedoping of films comprising the widely used mixed conductor poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS). A distinct 23Na quadrupolar splitting is observed due to the partial ordering of the PSS chains within the PEDOT:PSS-rich domains, with respect to the substrate. Operando 23Na NMR studies reveal a close-to-linear correlation between the quadrupolar splitting and the charge stored, which is quantitatively explained by a model in which the holes on the PEDOT backbone are bound to the PSS SO3- groups; an increase in hole concentration during doping inversely correlates with the number of Na+ ions bound to the PSS chains within the PEDOT-rich ordered domains, leading to a decrease in ions within the ordered regions and a decrease in quadrupolar splitting. The Na+-to-electron coupling efficiency, measured via 23Na NMR intensity changes, is close to 100% when using a 1 M NaCl electrolyte. Operando 1H NMR spectroscopy confirms that the Na+ ions injected into/extracted from the wet films are hydrated. These findings shed light on the working principles of organic mixed conductors and demonstrate the utility of operando NMR spectroscopy in revealing structure-property relationships in electroactive polymers.

Evaluating the Rate-determining Steps of Oxygen Permeation in Ba<sub>0.5</sub>La<sub>0.5</sub>FeO<sub>3−</sub><i><sub>δ</sub></i> Perovskite

Perovskite oxides obtained from Ba1−xLaxFeO3−δ (BLF) are considered beneficial materials for electrodes of solid oxide fuel cells and oxygen permeation membranes because of their high oxygen permeability, which is a criterion of oxide ion (O2−)-electronic mixed conductivity. In this paper, the prime focus was to understand the oxygen permeation mechanism through surface exchange and bulk diffusion of the Ba0.5La0.5FeO3−δ (BLF55) sample. The permeated oxygen flux displayed higher than that of the typical mixed conductor La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), which was explored simultaneously with corresponding oxygen chemical potentials employing an especial experimental setup. This study found that the surface exchange reaction on the oxygen-lean side was the rate-determining step (RDS) of the oxygen permeation below 800 °C, resulting from lower hole concentration on the oxygen-lean side surface. Enhancing the charge transfer from the surface oxygen by increasing hole concentration is a prime important strategy to improve the surface exchange reaction.

Band Structure, Phonon Spectrum and Thermoelectric Properties of Ag3CuS2.

Sulfides and selenides of copper and silver have been intensively studied, particularly as potentially efficient thermoelectrics. Ag3CuS2 (jalpaite) is a related material. However very little is known about its physical properties. It has been found that the compound undergoes several structural phase transitions, having the tetrahedral structural modification I41/amd at room temperature. In this work, its band structure, phonon spectrum and thermoelectric properties were studied theoretically and experimentally. Seebeck coefficient, electrical conductivity and thermal conductivity were measured in a broad temperature range from room temperature to 600 K. These are the first experimental data on transport properties of jalpaite. Ab initio calculations of the band structure and Seebeck coefficient were carried out taking into account energy dependence of the relaxation time typical for the scattering of charge carriers by phonons. The results of the calculations qualitatively agree with the experiment and yield large values of the Seebeck coefficient characteristic for lightly doped semiconductor. The influence of intrinsic defects (vacancies) on the transport properties was studied. It was shown that the formation of silver vacancies is the most probable and leads to an increase of hole concentration. Using the temperature dependent effective potential method, the phonon spectrum and thermal conductivity at room temperature were calculated. The measurements yield low lattice thermal conductivity value of 0.5 W/(m K) at 300 K, which is associated with the complex crystal structure of the material. The calculated room temperature values of the lattice thermal conductivity were also small (0.14-0.2 W/(m K)).

Thermoelectric properties of solid state reaction-prepared ZnO based alloys with various CuO doping in (CuO)x(ZnO)1−x

Thermoelectric (TE) materials based on ZnO and CuO materials in the form (CuO)x (ZnO)1−x were prepared via a solid state reaction method. The structure of the samples was analyzed using X-ray diffraction (XRD), where observed to be polycrystalline and monoclinic in structure. The nanostructure of the samples was characterized by scanning electron microscopy (SEM). By increasing CuO concentration, the surface morphologies of the samples were changed from a layered shape and well-packed structures to a uniform large grain distribution. Increases in CuO caused an increase in hole concentration (p) and electrical conductivity (σ), whereas, a comparatively decrease in Seebeck coefficient (S). A higher power factor (S2σ) of 12.92 μW/mK2 was obtained at about 575 K for a CuO0·75ZnO0.25 sample (x = 0.75).

Enhancement of H2S sensing performance of rGO decorated CuO thin films: experimental and DFT studies

We demonstrate a highly selective and sensitive Cupric oxide (CuO) thin film-based low concentration Hydrogen sulfide (H2S) sensor. The sensitivity was improved around three times by decorating with reduced graphene oxide (rGO) nanosheets. CuO thin films were deposited by Chemical Vapor Deposition followed by inter-digital electrode fabrication by a thermal evaporations system. The crystal structure of CuO was confirmed by x-ray diffraction. The sensing response of pristine CuO was found around 54% at 100 °C to 100 ppm of H2S. In contrast, the sensing response was enhanced to 167% by decorating with rGO of 1.5 mg ml−1 concentration solution. The sensing was improved due to the formation of heterojunctions between the rGO and CuO. The developed sensor was examined under various gas environments and found to be highly selective towards H2S gas. The improvement in sensing response has been attributed to increased hole concentration in CuO in the presence of rGO due to the Fermi level alignment and increased absorption of H2S molecules at the rGO/CuO heterojunction. Further, electronic structure calculations show the physisorption behavior of H2S molecules on the different adsorption sites. Detailed insight into the gas sensing mechanism is discussed based on experimental results and electronic structure calculations.

Enhanced thermoelectric properties of Li and Mg co−substituted Bi2Sr2Co2O fabricated by combined conventional sintering and spark plasma sintering

• Highly dense Bi 2-2 x Li x Mg x Sr 2 Co 2 O y is prepared by the combined (CS + SPS) process. • Li/Mg co−doping increases electrical conductivity due to increased hole concentration. • Combined (CS + SPS) process is effective way for enhancing thermoelectric properties. Bi 2 Sr 2 Co 2 O y is a promising thermoelectric material for high–temperature applications and is normally fabricated by conventional solid–state sintering (CS). However, the CS processed Bi 2 Sr 2 Co 2 O y suffers from a low electrical conductivity due to a low bulk density, which is major hurdle for its widespread thermoelectric applications. In this work, we propose an effective way to solve this problem by conducting an additional spark plasma sintering (SPS) after the CS process. In addition, to further enhance the electrical transport properties of Bi 2 Sr 2 Co 2 O y , we increase the electrical conductivity through the co−substitution of Li and Mg for Bi in Bi 2 Sr 2 Co 2 O y . We believe that the combined process of CS and SPS as well as the co−substitution of Li and Mg are highly effective ways for improving the electrical transport properties of Bi 2 Sr 2 Co 2 O y .

Room-temperature DC-sputtered p-type CuO accumulation-mode thin-film transistors gated by HfO2

CuO grown by room-temperature direct current reactive magnetron sputtering is introduced to realize p-type thin-film transistors (TFTs) with a high-k HfO2 gate dielectric fabricated by atomic layer deposition. The devices work in an accumulation mode (AM) with two apparent threshold voltages corresponding to the formation of a buried channel and an accumulation layer, respectively. A CuO AM TFT with a channel length of 25 μm exhibit a competitive on-off ratio (Ion/Ioff) of 1.3 × 102, a subthreshold swing (SS) of 1.04 V dec−1, and a field-effect mobility (μFE) of 1.1 × 10−3 cm2 V−1 s−1 at room temperature. By measuring a CuO metal oxide semiconductor (MOS) capacitor at room temperature, a high acceptor doping density (NA) of ∼5 × 1017 cm−3, a high positive effective fixed surface charge density (Qf) of ∼9 × 1012 cm−2, and a low interfacial trap charge density (Dit) of ∼6 × 1010 eV−1 cm−2 at the HfO2/CuO interface are estimated. The μFE extracted from the accumulation regime appears lower than the Hall mobility measured for a similarly processed CuO layer on glass due to the increased hole concentration in CuO TFTs, compared to a Hall concentration of ∼1014 cm−3, following the MOS process. SS appears limited by the decreased channel to gate capacitance (Ccg) related to the buried channel in AM TFTs, parasitic capacitance to ground, and potentially very high interfacial traps at the non-passivated CuO/air interface.

Intrinsic ferromagnetic Janus Cr2PAs monolayer with controllable magnetic anisotropy

Two-dimensional materials with intrinsic ferromagnetism, high Curie temperature and sizable magnetic anisotropy are promising candidates for the development of next-generation information technology. Based on first-principles calculations, we propose a new tetragonal Janus Cr2PAs monolayer, which is not only dynamically and thermally stable, but also shows ferromagnetism with a band gap of 0.718 eV. The predicted Curie temperature of Janus Cr2PAs monolayer is 794 K. Janus Cr2PAs monolayer exhibits an in-plane magnetic anisotropy of −0.260 meV/unit cell. Remarkably, the magnetization easy axis of Janus Cr2PAs monolayer changes from in-plane [100] to out-of-plane [001] direction with a critical hole doping at −0.6 e/unit cell. The perpendicular magnetic anisotropy increases with the further increase of hole concentration, it reaches up to 0.301 meV/As at concentration of −0.8 e/unit cell. The high Curie temperature and controllable magnetic anisotropy of Janus Cr2PAs monolayer make it a promising candidate for practical spintronic applications.