Charge Carrier Transport Properties Research Articles

Evaluation of the effect of oxygen addition on charge carrier transport properties in single-crystal diamond growth

The effects of oxygen addition during single-crystal diamond growth by microwave plasma CVD method on crystal surface morphology and charge carrier transport properties were evaluated. Diamond single-crystals were homoepitaxially grown on (001) surface of high-pressure and high-temperature (HPHT) synthesized type IIa single-crystal diamond substrates with a fixed methane concentration of 1 % and oxygen concentrations of 0–0.8 %. Inverted pyramid-shaped pits were generated as the oxygen concentration increased. The free exciton recombination emission intensity in the cathodoluminescence spectrum reached a maximum at oxygen concentration of 0.5 %. The samples with oxygen concentrations of 0.5 % and 0.8 % showed excellent charge collection efficiency and energy resolution. On the other hand, the mobility-lifetime (μτ) product was only (7.1 ± 1.7) × 10−5 cm2/V, which is approximately 1/4 of that of the sample with 0.2 % methane and no oxygen addition.

Odd-Even Alkyl Chain Effects on the Structure and Charge Carrier Transport of Two-Dimensional Sn-Based Perovskite Semiconductors.

Oscillations in the chemical or physical properties of materials, composed of an odd or even number of connected repeating methylene units, are a well-known phenomenon in organic chemistry and materials science. So far, such behavior has not been reported for the important class of materials, perovskite semiconductors. This work reports a distinct odd-even oscillation of the molecular structure and charge carrier transport properties of phenylalkylammonium two-dimensional (2D) Sn-based perovskites in which the alkyl chains in the phenylalkylammonium cations contain varying odd and even carbon numbers. Density functional theory calculations and grazing-incidence wide-angle X-ray scattering characterization reveal that perovskites with organic ligands containing an alkyl chain with an odd number of carbon atoms display a disordered crystal lattice and tilted inorganic octahedra accompanied by reduced mobilities. In contrast, perovskites with cations of an even number of carbon atoms in the alkyl chain form more ordered crystal structures, resulting in improved charge carrier mobilities. Our findings disclose the importance of minor changes in the molecular conformation of organic cations have an effect on morphology, photophysical properties, and charge carrier transport of 2D layered perovskites, showcasing alkyl chain engineering of organic cations to control key properties, of layered perovskite semiconductors.

Machine learning accelerates the screening of efficient metal-oxide catalysts for photocatalytic water splitting

Photocatalytic water splitting represents a sustainable avenue for hydrogen production, yet the traditional trial-and-error approach for identifying efficient photocatalysts among numerous candidates is cumbersome. In this paper, we introduce a machine learning (ML)-driven approach for rapid screening of promising metal oxide photocatalysts for hydrogen generation. This methodology comprises three key steps: the construction of a comprehensive material library, ML-accelerated material screening, and Density Functional Theory (DFT) calculations for verification. Through this process, we successfully identify ten metal oxides with good energy levels and charge carrier transport properties from an initial library of 860 candidates. Notably, CsYO2 emerges as a standout candidate, exhibiting excellent photocatalytic water-splitting performance on its (001) crystal face, with Y atoms identified as active sites through DFT calculations. This study demonstrates the efficacy of applying machine learning to the precise and rapid screening of vast material databases, paving the way for discovering novel and efficient photocatalysts.

Enhanced Photoelectrocatalytic Activity of CuO/CuNb2O6 Heterojunction Photocathodes for Efficient Solar Water Splitting

AbstractCuNb2O6 (CNO), a novel ternary metal oxide, has garnered much attention since it has a modest bandgap and potentially high current density. However, the reported photocurrent density so far is still much lower than the theoretical value. In this study, a CuO/CNO photoelectrode heterojunction was synthesized for the first time by using a solution‐based spray pyrolysis method. The properties of the heterojunction were investigated through various characterization techniques, including Mott‐Schottky analysis, time‐resolved photoluminescence, surface photovoltage analysis, chopper linear sweep voltammetry, and electrochemical impedance spectroscopy. The optimized maximum photocurrent density of the CuO/CNO heterojunction was ~0.75 mA/cm2 at 0.4 V vs. reversible hydrogen electrode (RHE), which is 2.8 times higher compared to the bare CNO photocathode. The improvement is attributed to the enhanced transport properties of charge carriers and promoted better separation of photo‐generated electrons and holes in the photoelectrode by the heterostructure. By the generation of heterojunctions, this work provides a new foundation for the design and construction of photocathodes with high charge separation efficiencies. This is also useful for future studies on water decomposition using heterojunctions as photoelectrode materials.

Enhanced Interface Charge Carrier Transport of SnO2/CeO2 via Oxygen Vacancy Synergized Heterojunction for Triethylamine Sensing Property.

Efficient charge carrier transport characteristics are critical to achieving the excellent performance of metal-oxide semiconductor gas sensors. Herein, SnO2/CeO2 heterojunction layered nanosheets with abundant oxygen vacancies were successfully synthesized through a simple solvothermal assisted high-temperature calcination method. The synergistic effect of oxygen vacancies and heterojunctions promoting the charge carrier transport properties at the SnO2/CeO2 interface for the enhanced sensing properties of triethylamine (TEA) was highlighted. As a result, the optimized SnO2/CeO2 exhibits improved gas sensing performance at 173 °C to 50 ppm of TEA. These include high response (205), excellent selectivity, low detection limit, and good long-term stability. This enhanced gas sensing property of SnO2/CeO2 is mainly attributed to the fact that the heterojunction and oxygen vacancies act as dual active sites synergistically inducing electron transfer, thereby effectively modulating the transport properties of the interfacial charge carriers, and thus facilitate the surface reactions efficiently. In this work, the dual-engineering strategy of synergistic interaction of heterojunction and oxygen vacancies can provide new perspectives for the design of advanced gas sensing materials.

Changes in the Crystal Lattice Parameters of Bismuth Films on Substrates with Different Thermal Expansion

AbstractDue to the sensitivity of the electronic structure of semi‐metals to small distortions of the crystal lattice, the study of the electrical and galvanomagnetic properties of bismuth films requires taking into account the deformation that occurs in the film‐substrate system due to the difference in the thermal expansion of the film and substrate materials. The magnitude of these deformations plays an important role in analyzing the temperature dependencies of the transport properties of charge carriers. The paper presents an experimental study of the magnitude of deformation of bismuth films on various substrates at 300 and 77 K using X‐ray diffraction. Changes in the lattice constant of crystallites, the trigonal axis of which is perpendicular to the film plane, depending on the substrate material, are obtained. A comparison between the assessment of the deformation of these crystallites in the film plane based on Hooke's law and the difference in the coefficients thermal expansion of the film and substrate materials is made.

Fostering Charge Carrier Transport and Absorber Growth Properties in CZTSSe Thin Films with an ALD-SnO2 Capping Layer.

The present study demonstrates that precursor passivation is an effective approach for improving the crystallization process and controlling the detrimental defect density in high-efficiency Cu2ZnSn(S,Se)4 (CZTSSe) thin films. It is achieved by applying the atomic layer deposition (ALD) of the tin oxide (ALD-SnO2) capping layer onto the precursor (Cu-Zn-Sn) thin films. The ALD-SnO2 capping layer was observed to facilitate the homogeneous growth of crystalline grains and mitigate defects prior to sulfo-selenization in CZTSSe thin films. Particularly, the CuZn and SnZn defects and deep defects associated with Sn were effectively mitigated due to the reduction of Sn2+ and the increase in Sn4+ levels in the kesterite CZTSSe film after introducing ALD-SnO2 on the precursor films. Subsequently, devices integrating the ALD-SnO2 layer exhibited significantly reduced recombination and efficient charge transport at the heterojunction interface and within the bulk CZTSSe absorber bulk properties. Finally, the CZTSSe device showed improved power conversion efficiency (PCE) from 8.46% to 10.1%. The incorporation of ALD-SnO2 revealed reduced defect sites, grain boundaries, and surface roughness, improving the performance. This study offers a systematic examination of the correlation between the incorporation of the ALD-SnO2 layer and the improved PCE of CZTSSe thin film solar cells (TFSCs), in addition to innovative approaches for improving absorber quality and defect control to advance the performance of kesterite CZTSSe devices.

Effect of surface states and in-plane deformation on transport properties of charge carriers in bismuth films

Since the energy spectrum of bismuth charge carriers is highly sensitive to mechanical deformations, studying their influence, leading to an increase or decrease in the overlap of actual extrema, on the transport properties of charge carriers in ultrathin bismuth films makes it possible to investigate the combination of manifestations of the quantum size effect and metallic surface states. In this work, the temperature dependencies of the electrical and galvanomagnetic properties of thin bismuth films (10–1250 nm) are studied under conditions of in-plane tensile and compressive deformation. Deformations are created as a result of the difference in the coefficients of thermal expansion of the film and substrate materials. Silicon with an oxidized surface, mica, and polished cleavage (111) CaF2 are used in order to create either tensile and compressive deformations. The presence of film deformations is confirmed by XRD. The magnitude of deformation is calculated based on the CTEs of the materials. The absence of mechanical stress relaxation or the formation of additional defects during temperature measurements is confirmed by the absence of hysteresis of properties in the range of 77–300 K. Based on the experimental results, the mobilities and concentration of charge carriers are calculated within the framework of the two-band model. As the film thickness decreases to less than 100 nm, an increase in the charge carrier concentration is observed, which is associated with metallic surface states. It is shown that the magnitude of the effect of deformation on the concentration of charge carriers in films of all thicknesses remains unchanged.

Elimination of Intragrain Defect to Enhance the Performance of FAPbI3 Perovskite Solar Cells by Ionic Liquid.

Ionic liquids have been widely used to improve the efficiency and stability of perovskite solar cells (PSCs), and are generally believed to passivate defects on the grain boundaries of perovskites. However, few studies have focused on the relevant effects of ionic liquids on intragrain defects in perovskites which have been shown to be critical for the performance of PSCs. In this work, the effect of ionic liquid 1-hexyl-3-methylimidazolium iodide (HMII) on intragrain defects of formamidinium lead iodide (FAPbI3) perovskite is investigated. Abundant {111}c intragrain planar defects in pure FAPbI3 grains are found to be significantly reduced by the addition of the ionic liquid HMII, shown by using ultra-low-dose selected area electron diffraction. As a result, longer charge carrier lifetimes, higher photoluminescence quantum yield, better charge carrier transport properties, lower Urbach energy, and current-voltage hysteresis are achieved, and the champion power conversion efficiency of 24.09% is demonstrated. These observations suggest that ionic liquids significantly improve device performance resulting from the elimination of {111}c intragrain planar defects.

Exploring the ferroelectric photocatalytic potential of R3c-structured InVO3: A computational study

Ferroelectric oxides with large band gaps or band edge positions that are difficult to cross the water oxidation-reduction potential are not conducive to photocatalytic hydrogen production. This study systematically investigates the potential application of R3c-structured InVO3 in the field of photocatalysis through density functional theory calculations. The mechanical and phonon frequency stability of InVO3 is verified through stability analysis, followed by the confirmation of its excellent ferroelectric properties through first-principles calculations. The calculated optical properties of InVO3 are explored to reveal its strong absorption of visible light and high photoelectric conversion efficiency. InVO3 exhibits excellent charge carrier transport properties through calculations of electron mobility, promising efficient energy conversion in photocatalytic water splitting. The band edge positions of InVO3 can cross the water oxidation-reduction potential, providing theoretical support for its application in the field of photocatalysis. R3c-structured InVO3 is a potential high-performance ferroelectric photocatalytic material.

Self-Assembly Behavior, Aggregation Structure, and the Charge Carrier Transport Properties of S-Heterocyclic Annulated Perylene Diimide Derivatives.

The construction of high-performance n-type semiconductors is crucial for the advancement of organic electronics. As an attractive n-type semiconductor, molecular systems based on perylene diimide derivatives (PDIs) have been extensively investigated over recent years. Owing to the fascinating aggregated structure and high performance, S-heterocyclic annulated PDIs (SPDIs) are receiving increasing attention. However, the relationship between the structure and the electrical properties of SPDIs has not been deeply revealed, restricting the progress of PDI-based organic electronics. Here, we developed two novel SPDIs with linear and dendronized substituents in the imide position, named linear SPDI and dendronized SPDI, respectively. A series of structural and property characterizations indicated that linear SPDI formed a long-range-ordered crystalline structure based on helical supramolecular columns, while dendronized SPDI, with longer alkyl side chains, formed a 3D-ordered crystalline structure at a low temperature, which transformed into a hexagonal columnar liquid crystal structure at a high temperature. Moreover, no significant charge carrier transport signal was examined for linear SPDI, while dendronized SPDI had a charge carrier mobility of 3.5 × 10-3 cm2 V-1 s-1 and 2.1 × 10-3 cm2 V-1 s-1 in the crystalline and liquid crystalline state, respectively. These findings highlight the importance of the structure-function relationship in PDIs, and also offer useful roadmaps for the design of high-performance organic electronics for down-to-earth applications.

The design and preparation of NDI modified MOFs for high efficiency removal of MB and Cr(VI)

The naphthalene tetracarboxdiimide (NDI) modified NH2-UiO-66(Zr) and NH2-MIL-101(Cr) (NDI/UZr and NDI/MCr for short) metal organic frameworks (MOFs) materials with strong covalent bond OC–N–CO were designed and prepared by solvothermal method. The comprehensive physical and chemical properties of the materials were characterized, and the adsorption and photocatalytic properties of the materials were evaluated by using methylene blue (MB) and Cr(VI) in single and mixture system. When the initial concentration of MB was 50 mg·L−1, the photocatalytic degradation rate constant of NDI/MCr(2:1) was 10.3 and 13.8 times higher than that of NH2-MIL-101(Cr) and NDI/UZr(2:1), respectively. Additionally, NDI/MCr(2:1) exhibited 1.6 times higher reaction rate constant than that of NDI/UZr(2:1) for the removal of 10 mg∙L−1 Cr(VI). The enhanced adsorption and photocatalytic ability over NDI/MOFs were attributed to the MOFs larger specific surface area and wider spectral response, and NDI showed high electron affinity, superior charge carrier transport properties. Free radical and electron capture experiments indicated that h+, •O2−, and e− were the main active species in the NDI/MOFs system. Among them h+ and •O2− played an essential role in the removal of MB, while e− and •O2− contributed to the photocatalytic reduction of Cr(VI). The mechanism of the photocatalytic process was discussed in detail.

Integration of One-Dimensional (1D) Lead-Free Perovskite Microbelts onto Silicon for Ultraviolet-Visible-Near-Infrared (UV-vis-NIR) Heterojunction Photodetectors.

Lead-free perovskites are considered to be candidates for next-generation photodetectors, because of their excellent charge carrier transport properties and low toxicity. However, their application in integrated circuits is hindered by their inadequate performance and size restrictions. To aim at the development of lead-free perovskite-integrated optoelectronic devices, a CsAg2I3/silicon (CAI/Si) heterojunction is presented in this work by using a spatial confinement growth method, where the in-plane growth of CAI microbelts with high-quality single-crystal characteristics is primarily dependent on the concentration of surrounding precursor solution. The fabricated photodetectors based on the CAI/Si heterojunctions exhibit a broad-spectrum detection capability in the ultraviolet-visible-near-infrared (UV-vis-NIR) range. In addition, the photodetectors show good photoelectric detection performance, including a maximum responsivity of 48.5 mA/W and detectivity of 1.13 × 1011 Jones, respectively. Besides, the photodetectors have a rapid response of 6.5/224 μs and good air stability for over 2 months. This work contributes a new idea to design next-generation optoelectronic devices with high integration density.

Photoinduced conductance and carrier switching in homoannulene ester derivatives: A theoretical exploration

Although difficult, recently a neutral homoannulene ester has been derived, which shows photoswitching between two isomers. On calculating the charge-carrier transport properties, we have found a substantial change in conductance as well as current values as we move from one photoswitchable state to another. Furthermore, the homoannulene ester shows carrier switching on DCV (dicyanovinylene, a strong electron withdrawing group) substitution. Additionally, DCV substitution enhances the conductance switching ratio to a huge extent at zero-bias and this large value is also retained at higher biases. This approach can be successfully used in organic field-effect transistors (OFETs) and other optoelectronic devices, where change in current is the essential requirement.

Electron effective mass in GaN revisited: New insights from terahertz and mid-infrared optical Hall effect

Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38–340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 ± 0.02) m0 to (0.34 ± 0.01) m0 at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 ± 0.002) m0. A possible explanation for the different findings from THz OHE and MIR OHE is proposed.

Morphology-dependent terahertz electrical conductivity of liquid-exfoliated graphite

Liquid-phase exfoliation (LPE) is a promising and scalable technique to produce low-cost dispersible nanosheets of graphene and nano-graphite for electronic, optoelectronics, and photonics applications. Fundamental information about how LPE affects the electrical properties is lacking. Here, a relationship is provided between the morphology of nano-graphite flakes resulting from LPE and cascade centrifugation to the charge-carrier transport properties. A range of process parameters, such as centrifuge force and exfoliation solvent, are employed, leading to a range of flake sizes. Morphology is characterized by scanning electron microscopy, atomic force microscopy and optical profilometry. Raman spectroscopy is used to confirm morphology, crystallite size, and chemical properties. Terahertz time-domain spectroscopy with a Drude-Smith conduction model provides the charge-carrier concentration and scattering times from AC conductivity. Carrier concentration increases with a reduction in flake area, potentially resulting from the introduction of electronic defect states at the edge of the nano-crystallites. Meanwhile, the carrier scattering time decreases with decreased flake size, similarly due to this self-doping that increases the carrier-carrier scattering. The approach and results serve as a foundation for understanding the processing-dependent electrical characteristics of LPE flakes and nanosheets.

Ambipolar Charge Carrier Transport Properties at the S-Benzyl-l-cysteine-Induced 2D/3D Halide Perovskite Interface

Ambipolar Charge Carrier Transport Properties at the S-Benzyl-<scp>l</scp>-cysteine-Induced 2D/3D Halide Perovskite Interface

High-performance flexible organic gas sensor via alkyl side chain engineering of polyalkylthiophene

Poly(3-alkylthiophene) (P3AT) is a representative p-type organic semiconductor with a distinctive molecular structure that incorporates a π-electron system, promoting charge carrier mobility and providing exceptional electrical conductivity. The alkyl side chain linked to the π-electron backbone was found to have a significant effect on charge carrier transport and mechanical properties. This study presents a comprehensive comparison of the gas-sensing performances of various gases (NO2, SO2, and CO2) and the mechanical properties of gas sensors fabricated from P3AT with alkyl chain lengths of 6, 8, 10, and 12 (P3HT, P3OT, P3DT, and P3DDT). Our investigations revealed that as the alkyl side chain length increased, the gas sensor performance significantly improved owing to the increase in free volume, concurrently promoting improved flexibility. The P3DDT sensor exhibited a sensitivity approximately twice that of the P3HT sensor, along with superior mechanical flexibility owing to its free volume.

Charge Carriers Localization Effect Revealed through Terahertz Spectroscopy of MXene: Ti3C2Tx.

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti3C2Tx) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti3C2Tx. Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries.

Structure, Composition and Morphology of Self-Assembled 2D Nanostructures Based on SnO2 Nanoparticles Observed in Unannealed Mn Doped Hydrated Form of Tin Oxide (II) or (IV) Synthesized by Co-precipitation Method

Background: 2D nanostructures are greatly interested in different technological applications, particularly optoelectronics. Tin oxide 2D nanostructures have shown great transparency and ideal charge carrier transport properties. Objective: The current study aims to evaluate the main characteristics of 2D-nanostructures observed during the synthesis of hydrated forms of tin oxide (II) or (IV) doped with Mn. Methods: A chemical co-precipitation method was used for the synthesis of the hydrated forms of tin oxide (II) or (IV) with different conditions on time (1 and 1.5 h) and temperature (60ºC and 90ºC), using MnCl2 as the manganese source. Results: X-ray diffraction and XPS results revealed the formation of the hydroromarchite phase (Sn6O4(OH)4) as the main product of the synthesis reaction. Scanning electron microscopy images were used to identify and measure, in a first approach, the 2D nanostructures observed as a result of the synthesis. Morphological characterization using different transmission electron microscopy techniques revealed the presence of nanoparticles that were observed to self-assemble to form the 2D nanostructures observed (nanorods and nanosheets). Nonetheless, selected-area electron diffraction suggested the presence of the cassiterite phase (SnO2) in the nanoparticles forming the 2D nanostructures. Furthermore, chemical analyses using energy-dispersive X-ray spectroscopy supported the observations made by the diffraction studies regarding the presence of cassiterite phase (SnO2) in the 2D nanostructures. The number of 2D nanostructures observed in the analyzed samples increased as the Mn concentration increased in the synthesis reaction. Conclusions: The addition of Mn as an intended doping element increased the crystallite size and the polycrystallinity of the synthesized hydrated forms of tin oxide (II) or (IV). Additionally, it also promoted the formation of 2D nanostructures made of SnO2 nanoparticles.