Charge Carrier Concentration Research Articles

Demonstration of Si-doped Al-rich regrown Al(Ga)N films on AlN/sapphire with >1015/cm3 carrier concentration using CCS-MOCVD reactor

Thin Si-doped Al-rich (xAl > 0.85) regrown Al(Ga)N layers were deposited on AlN on sapphire template using metal-organic chemical vapor deposition (MOCVD) techniques. The optimization of the deposition conditions, such as temperature (1150 °C), V/III ratio (750), deposition rate (0.7 Å/s), and Si concentration (6 × 1019/cm3), resulted in a high charge carrier concentration (> 1015 cm−3) in the Si-doped Al-rich Al(Ga)N films. A pulsed deposition condition with pulsed triethylgallium and a continuous flow of trimethylaluminum and ammonia was employed to achieve a controllable Al composition xAl > 0.95 and to prevent unintended Ga incorporation in the AlGaN material deposited using the close-coupled showerhead reactor. Also, the effect of unintentional Si incorporation on free charge carrier concentration at the regrowth interface was studied by varying the thickness of the regrown Al(Ga)N layer from 65 to <300 nm. A maximum charge carrier concentration of 4.8 × 1016 and 7.5 × 1015/cm3 was achieved for Al0.97Ga0.03N and AlN films with thickness <300 nm compared to previously reported n-Al(Ga)N films with thickness ≥400 nm deposited using MOCVD technique.

Thermally Induced Enhancement of Photoresponse in Radio Frequency‐Sputtered CdS Thin‐Film Photodetectors

This work focuses on understanding the defect‐related electronic transport in cadmium sulfide (CdS) thin films, and finds thermal treatment as an efficient tool to tailor its intrinsic defect charge carrier concentration for optimum visible‐light photodetection performance. The radio frequency (RF)‐sputtered CdS thin‐films show a substantial decrease in measured dark‐current by three orders of magnitude (μA to nA) with an increase in substrate deposition temperature (Ts) from room‐temperature (RT) to a maximum of 400 °C. With increase in Ts, the current conduction behavior changes from Ohmic (at RT) to Schottky‐behavior (Ts ≥ 100–400 °C). The decrease in dark‐current and the crossover from Ohmic to Schottky electronic transport behavior, pointed to a decrease in defect‐density charge carrier concentration, with increased Ts. Additionally, post‐deposition thermal annealing of CdS thin films also is found to result in a similar decrease in dark‐current (μA to nA). The photo‐to‐dark‐current ratio of CdS thin‐film visible‐light photodetectors increased by two‐orders of magnitude, and its dynamic response time decreased by an order of magnitude via. thermal engineering. The thermal‐annealing treatment possibly reduced the defect‐related trap‐sites, which enables a reliable and faster photo‐switching response for CdS thin‐film‐based visible‐light photodetectors.

Novel synthesis of reduced PdO-WO3 hybrids for enhanced photoelectrochemical water splitting

In this study, a novel synthesis approach for WO3 nanoparticles and their hybrids is proposed, followed by an investigation of their photoelectrochemical performance for water splitting applications. The surface modification of WO3 nanoparticles is carried out by immersing them in a Pd precursor solution, followed by calcination to obtain PdO-WO3 hybrid nanoparticles. Subsequently, the PdO-WO3 is reduced in a hydrogen (H2) environment to prepare PdOx-WO3-y hybrid material. XPS analysis revealed that H2 reduction converts residual metallic Pd to PdH and simultaneously generates oxygen vacancy defects in WO3 hybrid nanoparticles, as confirmed by EPR analysis. Additionally, TPR analysis further confirms the H2-induced reduction in the PdO-WO3 hybrid at a lowered temperature of 150 °C. The optical band gap energy of PdOx-WO3-y hybrid nanoparticles is slightly reduced (by 0.06 eV) compared to that of the PdO-WO3 reference sample. Photoelectrochemical measurements were conducted to assess the impact of these tailored modifications on the performance of conventional PdO-WO3 nanoparticles. The reduced PdOx-WO3-y catalyst exhibits an approximatly ∼22 % improvement in incident photon-to-current conversion efficiency (IPCE), over the unreduced PdO-WO3. The PEC efficiency improvement was attributed to the composite electronic structure changes introduced during the annealing process. Mott-Schottky measurements confirm that the reductive treatment consolidates the material’s two distinct flat band potentials (Efb) into with a single intermediate energy level and increases the charge carrier concentration. This study presents a straightforward method to generate oxygen vacancy defects in hybrid metal oxides and provides a convenient method for band gap engineering for photoelectrocatalytic applications.

Cu-Bi2S3 nanorods promote reactive oxygen species production for photodynamic therapy of prostate cancer

Photodynamic therapy (PDT) has emerged as a promising cancer treatment approach due to its non-invasive and specifically targeted nature. However, the effectiveness of PDT is hindered by the complex synthesis of conventional photosensitizers and inadequate reactive oxygen species (ROS) generation. Here, we synthesize a copper-doped Bi2S3 (Cu-Bi2S3) nanorod to investigate its PDT potential against PCa. Compared with bulk Bi2S3 (58%), Cu-Bi2S3 nanorod caused 87% of PC3 cells to die under light. The dispersed Cu in the Bi2S3 bulk phase effectively inhibits the recombination of photogenerated electron-hole pairs, ultimately providing a high concentration of charge carriers. DFT calculations show that Cu doping causes the d-band center of Cu-Bi2S3, promoting the adsorption and activation of O2 on Cu-Bi2S3 to enhance ROS generation. This work offers a viable solution to the pressing scientific challenge of ROS generation, a key aspect of enhancing the efficacy of PDT for cancer treatment.

Study of the Photovoltaic Parameters of Inorganic Solar Cells Based on Cu<i>2</i>O and CuO

A theoretical study of the photovoltaic parameters of inorganic solar cells based on ZnO/Cu2O and ZnO/CuO heterojunctions was carried out to improve the energy conversion efficiency. The influence of the thickness, charge carrier concentration and band gap of Cu2O and CuO films, as well as ZnO, on the photovoltaic parameters of solar cells has been studied. The simulation results showed that the efficiency of solar cells is significantly affected by the contact potential difference, the diffusion length of minority charge carriers, the amount of generated photocurrent and the recombination rate. The maximum efficiency of a solar cell based on ZnO/Cu2O was obtained equal to 10,63%, which is achieved with a band gap, thickness and charge carrier concentration in Cu2O equal to 1.9 eV, 5 μm and 1015 cm–3 and band gap, thickness and the concentration of charge carriers in ZnO is equal to 3,4 eV, 20 nm and 1019 cm–3, as well as the displacement of the edges of the conduction bands is 0.8 eV. For a solar cell based on ZnO/CuO, a maximum efficiency of 18.27% was obtained with a band gap, thickness and charge carrier concentration in CuO equal to 1.4 eV, 3 μm and 1017 cm–3, as well as a displacement of the conduction band edges of 0.03 eV. The obtained results of modeling solar cells can be used in the design and manufacture of inexpensive and efficient photovoltaic structures.

Effective Steady‐State Recombination Decay Times in Comparison to Time‐Resolved Photoluminescence Decay Times in Halide Perovskite Solar Cells

One of the key topics in perovskite solar cells is the reduction of charge carrier recombination, with the aim of increasing power conversion efficiency. The recombination lifetime is a commonly used tool, as it directly affects the current–voltage curve via the diffusion length. The lifetime is often estimated using time‐domain measurement methods such as time‐resolved photoluminescence. However, two obstacles emerge when applying the transiently measured decay times to the steady‐state theory. In general, the decay time depends on the charge carrier concentration, and it is often not clear under which conditions the transient measurement must be conducted to be comparable with the steady‐state performance of the device. Furthermore, diffusion and capacitive effects due to charge injection and extraction can influence transient techniques and cause the measured decay time to deviate from the sought‐after recombination lifetime. Voltage‐dependent steady‐state photoluminescence measurements can be used to estimate the internal voltage during device operation and allow the extraction of collection efficiencies and effective steady‐state decay times that are independent of transport and capacitive effects. Here, the differences between the steady‐state and transient decay times are identified and discussed, and the losses in the current–voltage curve caused by extraction issues are quantified.

Harnessing the Potential of Morphologically Tailored ZnSn(OH)6 Nanograss Photoanode for Solar‐Driven Water Splitting

AbstractHydrothermal growth of ZnSn(OH)6 nanograss on ZnO‐coated FTO substrates is demonstrated. Effect of surfactants addition (polyvinylpyrrolidone and polyethylene glycol), precursor concentration, and reaction temperature on surface morphology of ZnSn(OH)6 nanograss are investigated. Addition of surfactant in precursor solution results in growth of approximately 20–30 nm thick nanograss morphology with varying orientation. The nanograss grow longer by increasing the concentration of precursors solution. The grown ZnSn(OH)6, exhibits XRD peaks corresponding to cubic phase ZnSn(OH)6. Optical bandgap of the grown nanograss are calculated in the range from ∼3.0 to 3.5 eV. The nanograss photoanode grown with PEG surfactant (at 200 °C) has shown the highest photocurrent of 2.2 mA/cm2 at RHE 1.23VRHE and photoconversion efficiency of 1.4% at 0.46 VRHE. The longer nanograss has shown lower charge transfer resistance and higher charge carrier concentration, which is favorable for enhancing the PEC performance.

Influence of platinum content on semiconductor and electrochemical properties of materials based on titanium suboxides

The effect of platinum content on the surface morphology, phase composition and electrochemical properties of materials based on titanium suboxides was investigated. Titanium suboxides are formed at the stage of coating reduction under cathodic polarization, which allows the formation of a porous developed surface for electrodeposition of catalytic platinum layers. The main allotropic modification of TiOx in the studied composite materials is anatase, which contains particles of elemental titanium and platinum. The investigated materials are highly doped semiconductors with a high concentration of charge carriers. With an increase in the platinum content, an extreme dependence of the potential of flat zones is observed, which is associated with the formation of a composite material. The electrocatalytic activity was studied in the reactions of oxygen and hydrogen evolution. The overvoltage of oxygen production on platinum-containing materials is much lower than on the TiOx electrode. The slope of the linear dependence of the potential on the current density decreases with an increase in platinum content. The Tafel slope for the studied materials in the hydrogen evolution reaction was 160 mVdec–1 for coatings containing 0.25 mgcm–2 Pt; whereas it was equal to 42 and 43 mVdec–1 for electrodes with 0.5 and 2 mgcm–2 Pt, respectively.

Mutual Connection Between Concentration of Charge Carriers and Seebeck Coefficient in Si0.96Ge0.04 Thermoelectric by Annealing in the Range (523-673)K

Mutual Connection Between Concentration of Charge Carriers and Seebeck Coefficient in Si0.96Ge0.04 Thermoelectric by Annealing in the Range (523-673)K

Enhanced optoelectronic properties of spray deposited antimony-doped tin oxide thin films

Antimony-doped tin oxide (Sb:SnO2) thin films with varying Sb concentrations were deposited using the spray pyrolysis technique. A comprehensive analysis of the films' elemental, structural, and optoelectronic properties is presented. All films possess a polycrystalline single-phase nature and have a tetragonal rutile structure. As Sb concentration increases, the unit cell shrinks continuously, suggesting proper substitution of Sn4+ by Sb5+. This substitution causes a continuous increase in the concentration of charge carriers and a reduction in electron mobility. X-ray photoelectron spectroscopy investigations revealed the presence of asymmetric Sn 3d core-level peaks distinguished by peak splitting, which increases with increasing carrier concentration. Utilizing the plasmon absorption model, an effective mass value of (0.51 ± 0.07)me for the conduction electron at the fermi level is obtained. With increasing Sb concentration, the optical energy gap increases gradually from 3.84 eV for undoped SnO2 to 4.35 eV for 5.0 at. % Sb. After considering the Urbach tailing phenomenon as well as the Moss–Burstein effect, this increase was attributed to the increase in Moss–Burstein energy due to increased charge carrier concentration. Our study revealed that the film doped with 2.0 at. % Sb has the best optoelectronic properties, with a figure of merit of 4.18 (Ω/cm2)−1.

High-temperature transport properties of a two-dimensional weakly doped parabolic semiconductor

A version of the Mexican-hat Hamiltonian is used to study high-temperature transport properties of a two-dimensional weakly doped semiconductor with electron-hole symmetric bands. For a finite doping level and a temperature-dependent band gap, we find a closed analytical form of the temperature-dependent chemical potential. The effective concentrations of charge carriers participating in transport coefficients are analyzed in the space spanned by the total electron concentration and temperature. It is shown that these concentrations are the sum of a residual contribution and two thermally activated contributions, with a complicated dependence on temperature. The analytical expression for the Hall coefficient RHis also found. It is argued that it is a non-monotonic function of the doping level with the maximum at the doping nmax that is a linear function of temperature at high enough temperatures. The analysis of the real part of the interband conductivity shows that it is inversely proportional to incoming photon energy at low temperatures and that it is nearly constant over a wide energy range at high temperatures. This results are expected to be of significant importance in understanding transport and optical properties of weakly doped two-dimensional semiconductors with nearly symmetric parabolic bands.&#xD.

Na+ conductivity of (100−x)Na3PS4−xCa3N2 glass and glass−ceramics

AbstractThe ionic conductivity of Na3PS4 can be improved via the aliovalent substitution of P and S as well as multivalence cation substitution of Na. Although the enhanced ionic conductivities of tetragonal and cubic Na3PS4 have been theoretically and experimentally studied, the dynamics of sodium ions in these systems have been rarely reported. In this study, (100−x)Na3PS4−xCa3N2 (0 ≤ x ≤ 10) glass samples were synthesizing using high‐energy planetary ball‐milling method. These samples were subjected to heat treatment at 280°C for 1 h in an argon atmosphere to obtain glass–ceramic samples. The structures of these samples were characterized via Raman spectroscopy and X‐ray diffraction analysis. The lattice parameter and cell volume of the glass–ceramic samples strongly depended on the x value in (100−x)Na3PS4−xCa3N2 (0 ≤ x ≤ 10). Their ionic conductivities were determined via alternating current electrochemical impedance spectroscopy (EIS). 92Na3PS4−8Ca3N2 glass–ceramics exhibited an ionic conductivity of 8.14 × 10−4 Scm−1 at 25°C. Charge carrier concentration and hopping rate were extracted from the EIS data to determine the ionic conductivities of the samples. The activation energies of DC conductivity and mobile ion formation and migration were also derived from the EIS data to analyze the effect of Ca3N2 doping on the ionic conductivity of the synthesized samples.

2D g-C3N5 p-Doping of Donor Material for High-Efficiency Organic Solar Cells.

Molecular doping of organic semiconductor is a great strategy for significantly regulating the electronic band structure of organic semiconductor while increasing charge mobility and carrier concentration. Here, a facile strategy is presented by introducing 2D g-C3N5 as a p-dopant into PM6, improving the charge mobility and hole carrier concentration of PM6. Moreover, the electron transfer between PM6 and g-C3N5 can effectively downshift the Fermi energy level and highest occupied molecular orbital (HOMO) energy level of PM6, which leads to the increase the built-in electric field of organic solar cells (OSCs). The addition of g-C3N5 also effectively enhances the crystallization of active layer, thereby improving the stability of OSCs. As a result, a champion bulk-heterojunction (BHJ) and layer-by-layer (LbL) structure OSCs are successfully achieved featuring a high-power conversion efficiency of 18.10%/18.25%, simultaneously having excellent device stability. This work shows that introducing a low concentration dopant into organic donor is an effective method for improving the electrical performance of organic donor and the efficiency of OSCs.

Modulation of electronic and thermal properties of boron phosphide nanotubes under electric and magnetic fields

This work theoretically investigates the thermoelectric properties of boron phosphide nanotubes (BPNTs) using the tight-binding model, Green function method, and Kubo formalism, focusing on a zigzag BPNT with indices (20, 0). The tight binding parameters obtained by matching its band structure with calculated density functional theory band structure. The study examines the effects of transverse electric fields and axial magnetic fields on various physical properties, such as band structure, density of states (DOS), heat capacity, magnetic susceptibility, and other thermoelectric properties. BPNTs consistently show semiconducting properties with a nearly 1 eV direct band gap. The electronic properties of BPNTs are significantly affected by applied electric field, which at very strong strengths can induce a semiconducting to metallic phase transition. In contrast, the magnetic field leads to the splitting of energy bands, especially around the Fermi level. The DOS also changes with the electric field, including variations in the position, intensity, and number of DOS peaks. The thermal properties and thermoelectric performance of BPNTs are temperature-dependent. Increasing of excited electrons thermal energy cause more occupation of high energy levels in the conduction bands. The electric field further enhances the thermal properties of BPNTs by modifying their electronic properties and reducing the band gap. Stronger electric fields cause a noticeable enhancement in the BPNTs thermal properties because it is increasing the concentration of excited charge carriers. This aspect is crucial for improving the thermoelectric efficiency of BPNTs, making them more competitive for practical applications.

Colloidal Ag2SbBiSe4 nanocrystals as n‑type thermoelectric materials

Materials with low intrinsic thermal conductivity are essential for the development of high-performance thermoelectric devices. At the same time, the solution processing of these materials may enable the cost-effective production of the devices. Herein, we detail a high-yield and scalable colloidal synthesis route to produce Ag2SbBiSe4 nanocrystals (NCs) using amine-thiol-Se chemistry. The quaternary chalcogenide material is consolidated by a rapid hot-press maintaining the cubic crystalline structure. Transport measurements confirm that n-type Ag2SbBiSe4 exhibits an inherently ultralow lattice thermal conductivity of ca. 0.34 W m−1K−1 at 760 K. Moreover, a modulation doping strategy based on the blending of semiconductor Ag2SbBiSe4 and metallic Sn NCs is demonstrated to control the charge carrier concentration in the final composite material. The introduction of Sn nanodomains additionally blocks phonon propagation thus contributing to reducing the thermal conductivity of the final material. Ultimately, a peak thermoelectric figure of merit value of 0.64 at 760 K is achieved for n-type Ag2SbBiSe4-Sn nanocomposites that also demonstrate a notable Vickers hardness of 185 HV.

Effect of succinonitrile on the structural, ion conductivity, and dielectric properties of PVDF‐HFP based solid polymer electrolytes

AbstractA detailed account of the poly(vinylidene difluoride‐co‐hexafluoropropylene) (PVDF‐HFP)/succinonitrile (SN)/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) based solid polymer electrolytes (SPEs) of various compositions is reported. SN is incorporated as plasticizer, while LiTFSI is employed as the source of lithium ions. Fourier transfer infrared (FTIR) spectroscopy reveals the formation of polar and electroactive γ crystal phase by PVDF in pristine PVDF‐HFP and the prepared SPE films. FTIR further confirms the interaction between the host polymer and the salt and between the salt and the plasticizer. The interaction between the salt and plasticizer suggests that SN not only offers the plasticization effect but also facilitates salt dissociation. The ion conductivity increases with increasing salt content up to 20 wt% but beyond that concentration a decline in ion conductivity is observed, which could be attributed to the ion pair association or cluster formation due to the excess salt concentration that reduces the availability of free ions and ultimately leads to the reduction in ion conductivity. After including SN in the SPE formulation, electrochemical impedance spectroscopy reveals a substantial increase in room temperature ion conductivity of the SPE from ~1.08 × 10−5 S cm−1 for pristine SPE with 20 wt% salt to ~2.7 × 10−4 S cm−1 for plasticized SPE with 40 wt% SN and 20 wt% salt. This is attributed to the plasticizing effect of SN that enhances ion mobility through the matrix and to the increased charge carrier concentration (SN facilities salt dissociation). The fabricated SPEs exhibit a significant enhancement in ion conductivity with increasing temperature and the maximum ion conductivity of 1.1 × 10−3 S cm−1 is attained at 393 K by plasticized SPE with 40 wt% SN content. The ion conductivity of all the prepared SPEs follows the temperature dependent Arrhenius behavior, suggesting the thermally activated ion hopping mechanism of ionic conduction. Further, the plasticized SPEs display near unity ion transference number indicating the predominance of the ionic mode of conduction. The dielectric and electric modulus analysis reveal a faster relaxation phenomenon and hence higher ion conductivity of the prepared SPEs with increasing salt and plasticizer content.

Charge transport in two-dimensional system with massless Mexican-hat-like bands

We study the single-particle, DC and optical properties of a two-dimensional (2D) system with massless Mexican-hat-like (W-shaped) valence bands. A proposed 2 × 2 Hamiltonian matrix gives the W-bands and two ground state phases: a gapped phase and a nodal-loop phase. The intraband properties include the single particle density of states, the effective concentration of charge carriers (the Drude weight), the Hall coefficient and electron mobility, electronic heat capacity and thermopower. The real part of the interband conductivity is calculated for the gapped phase as it depends on doping and temperature. As for the undoped nodal-loop phase only the zero-frequency interband conductivity is finite. Among several results two most decisively stand out: By measuring the Hall coefficient as a function of doping at zero-temperature we can answer whether the bands are W-shaped. By measuring the square-root singularity in the finite-temperature optical conductivity, we identify the gapped phase.

Ultrahigh Electrical Conductivity in p-Type CsPbI2Br Perovskite Thin Films by Modulation Doping.

The widespread adoption of halide perovskites for application in thermoelectric devices, DC power generators, and lasers is hindered by their low charge carrier concentration. In particular, increasing their charge carrier concentration is considered the main challenge to serve as a promising room-temperature thermoelectric material. Efforts have been devoted to enhancing the charge carrier concentration by doping and composition engineering. However, the coupling between charge carrier concentration and mobility, along with the poor stability of these materials, impedes their development for thermoelectric applications. Herein, we demonstrate the successful increase in the charge carrier concentration of CsPbI2Br by forming a heterojunction structure with Cu2S via a facile spin-coating method. The excellent band alignment between two materials combined with a charge-transfer mechanism realizes the modulation doping, resulting in 8 orders of magnitude increase in carrier concentration from 1012 to 1020 cm-3 without detrimental effect on the carrier mobility of CsPbI2Br. The thermoelectric power factor of the heterostructured CsPbI2Br reached 6.6 μW/m·K2, which is 330 times higher than that of pristine CsPbI2Br. Furthermore, these films showed higher humidity stability than the control films. This study offers a promising avenue for increasing the charge carrier concentration of halide perovskites, thereby enhancing their potential for various applications.

High sensitivity and selectivity of h-BN/WO3 n-n heterojunction to triethylamine at low-temperature

Due to the unique properties of heterojunction interfaces, heterojunction materials have broad application prospects in gas sensors. In this work, a facile and economical two-step synthesis method was employed to fabricate h-BN/WO3 heterojunctions, exhibiting excellent performance in triethylamine (TEA) detection. The results indicate that compared to pure WO3 sensors, h-BN/WO3 sensors exhibit superior TEA sensing capabilities, with an excellent response of 281.45 to 20 ppm TEA at 100 °C, which is 3.4 times higher. Moreover, h-BN/WO3 sensors demonstrate favorable response times, low detection limits, and good stability. These significant enhancements are attributed to the increase in oxygen vacancies and the establishment of heterojunctions between h-BN and WO3. Heterojunctions can regulate the concentration and transport rate of charge carriers, as well as the interface potential barrier, thereby affecting the gas sensing processes. This work may promote the further development of sensing materials and the practical application of WO3 sensors in TEA detection.

Electronic and thermal transport engineering of a highly earth-abundant and low-cost Sn-based thermoelectric material

Tin monosulfide (SnS), composed of abundant, low-cost, and environmentally friendly elements, has garnered significant attention in the field of thermoelectrics due to its analogous electron and phonon transport properties to SnSe. However, undoped SnS exhibits a limited charge carrier concentration, resulting in poor electrical conductivity. In this study, a dual doping approach was employed, introducing both cationic (Na and Pb) and anionic (Se and Te) dopants into polycrystalline SnS. This approach yielded a notable enhancement in electrical properties and a reduction in thermal conductivity. As a result of these modifications, the figure of merit (zT) reached 0.38 at approximately 525 K. Specifically, the power factor increased to 2.25 μWcm−1K−2, while the total thermal conductivity was reduced to 0.3 Wm−1K−1. These findings highlight the potential of synergistic cation and anion doping strategies to significantly improve the thermoelectric performance of SnS at relatively lower temperatures, offering a pathway for further exploration of cost-effective thermoelectric materials based on SnS.