Charge Carrier Transport Properties Research Articles

Band versus Polaron: Charge Transport in Antimony Chalcogenides.

Antimony sulfide (Sb2S3) and selenide (Sb2Se3) are emerging earth-abundant absorbers for photovoltaic applications. Solar cell performance depends strongly on charge-carrier transport properties, but these remain poorly understood in Sb2X3 (X = S, Se). Here we report band-like transport in Sb2X3, determined by investigating the electron–lattice interaction and theoretical limits of carrier mobility using first-principles density functional theory and Boltzmann transport calculations. We demonstrate that transport in Sb2X3 is governed by large polarons with moderate Fröhlich coupling constants (α ≈ 2), large polaron radii (extending over several unit cells), and high carrier mobility (an isotropic average of >10 cm2 V–1 s–1 for both electrons and holes). The room-temperature mobility is intrinsically limited by scattering from polar phonon modes and is further reduced in highly defective samples. Our study confirms that the performance of Sb2X3 solar cells is not limited by intrinsic self-trapping.

Avoid Pitfalls in Identifying Perovskite Grain Size.

Perovskite grain size has been used as one of the indicators to evaluate the morphological quality of perovskite films. Large grain size is customarily regarded as an indicator of high photovoltaic performance because it is thought to result in superior charge carrier transport properties due to less carrier scattering by the grain boundaries (GBs). Consequently, the characterization of perovskite grain size has become routine in perovskite solar cell research, and large grain size is in general pursued. However, grain size estimation relying on the prevailing methods, such as scanning electron microscopy (SEM), can be viewed only as "apparent grain size", which is incomplete or possibly wrong for evaluating GB distribution. To avoid the pitfalls and advocate for accurate methodologies in identifying perovskite grain size, this Viewpoint highlights the pitfalls by demonstrating specific examples and then provides an appropriate platform to accurately evaluate perovskite grain information.

A Unified Distribution Form of the Density of States in Disordered Organic Semiconductors

An accurate density of states (DOS) is essential for understanding charge carrier transport properties and then better developing the electronic and optoelectronic devices. Currently, Gaussian and exponential forms are the two most general DOS, which can be unified in the tail state of the state. However, Gaussian DOS at high concentrations is not applicable, as well as exponential DOS at low concentrations. To avoid the disadvantage of Gaussian and exponential form, based on the Poisson flow, a unified distribution form of the DOS have been developed to discuss the charge carrier transport. The proposed DOS is more consistent with the actual situation and exhibits better rationality under the whole range of carrier density. Otherwise, we for the first time introduced the entropy to analyze the disorder parameters. Based on the proposed DOS, the concentration and temperature dependence of mobility has been discussed in detail.

Effect of rare earth doping on electronic and gas-sensing properties of SnO2 nanostructures

Tin dioxide (SnO2) and rare earth (Y, La, Pr, Tb, and Er)-doped SnO2 materials were synthesized by a solvothermal method and used for gas sensors. The effect of rare-earth (RE) doping on structural, electronic, and gas-sensing properties of SnO2 has been investigated. According to a comparative study on the gas-sensing properties of SnO2 and RE-doped SnO2 gas sensors to various testing gases, the RE-doped SnO2 sensors showed enhanced sensitivities to different testing gases. Especially, the Pr-doped SnO2 sensor exhibited outstanding sensing properties to SO2, including a high response of 19.5–50 ppm SO2, excellent selectivity, repaid response and recovery rates, and superior long-term stability. According to the structural analyses, DFT calculation, and the electrochemical measurement of the SnO2 and Pr-SnO2 materials, the improved electron excitation efficiency endowed the Pr-SnO2 with a high density of free electrons that can be trapped by atmospheric oxygen species and participated in SO2-sensing reactions. Moreover, after the Pr doping of SnO2, the enhanced charge carrier transport properties, including prolonged electron lifetime, improved electron diffusion coefficient, and increased effective diffusion length, were conducive to improving the SO2-sensing property.

Excellent Long‐Range Charge‐Carrier Mobility in 2D Perovskites

AbstractThe use of layered, 2D perovskites can improve the stability of metal halide perovskite thin films and devices. However, the charge carrier transport properties in layered perovskites are still not fully understood. Here, the sum of the electron and hole mobilities (Σμ) in thin films of the 2D perovskite PEA2PbI4, through transient electronically contacted nanosecond‐to‐millisecond photoconductivity measurements, which are sensitive to long‐time, long‐range (micrometer length scale) transport processes is investigated. After careful analysis, accounting for both early‐time recombination and the evolution of the exciton‐to‐free‐carrier population, a long‐range mobility of 8.0 +/− 0.6 cm2 (V s)–1, which is ten times greater than the long‐range mobility of a comparable 3D material FA0.9Cs0.1PbI3 is determined. These values are compared to ultra‐fast transient time‐resolved THz photoconductivity measurements, which are sensitive to early‐time, shorter‐range (tens of nm length scale) mobilities. Mobilities of 8 and 45 cm2 (V s)–1 in the case of the PEA2PbI4 and FA0.9Cs0.1PbI3, respectively, are obtained. This previously unreported concurrence between the long‐range and short‐range mobility in a 2D material indicates that the polycrystalline thin films already have single‐crystal‐like qualities. Hence, their fundamental charge carrier transport properties should aid device performance.

BiOX/Bi/BiOX (X = Cl, Br) Double‐Side Nanosheet Arrays: Synthesis, Structures, and Photo(electro)catalytic Applications

AbstractArray‐structured photocatalysts, featuring unique transport properties of charge carriers and special texture structures, have captured widespread interest in photocatalytic and/or photoelectrocatalytic applications. However, the fabrication of arrays usually suffers from complicated synthetic routes and the indispensable use of substrates. Herein, a novel BiOX/Bi/BiOX (X = Cl, Br) double‐side nanosheet array catalyst, with vertically aligned BiOX nanosheets symmetrically grown on the two sides of horizontal Bi nanoplates, is first constructed by a facile solution‐phase solvothermal route in the absence of any substrates. Both l‐cysteine and ethylene glycol are found to play critical roles in the formation of Bi nanoplates and the growth of BiOX nanosheets. Thanks to the synergism of double‐side metal‐semiconductor array, BiOX/Bi/BiOX presents significantly boosted performance for photocatalytic selective oxidation of benzyl alcohol to benzaldehyde, far surpassing Bi/BiOX and pristine BiOX. Additionally, BiOX/Bi/BiOX also exhibits superior photoelectrocatalytic performance with excellent hydrogen and oxygen evolution reaction activity. The electrochemical analysis and photoluminescence results reveal that the middle Bi nanoplate can function as a fast transport channel for photogenerated electrons, significantly accelerating the separation of photogenerated carriers. This work provides a general substrate‐free strategy to construct array catalysts with double‐side structure and reveals the outstanding advantages toward improved photocatalysis.

Space–Charge Profiles and Carrier Transport Properties in Dopant‐Free GaN‐Based p‐n Junction Formed by Distributed Polarization Doping

Herein, the operation of dopant‐free GaN‐based p‐n junctions formed by distributed polarization doping (DPD) is experimentally demonstrated and their space charge profiles and carrier transport properties are investigated. The device exhibits ideal space charge profiles explained by polarization effects and demonstrates the excellent controllability of DPD. In addition, it shows rectification and electroluminescence under forward‐biased conditions. The carrier transport properties could be explained by the conventional recombination/diffusion model used for impurity‐doped p‐n junctions. Repeatable breakdowns are also observed in all devices and the temperature‐dependent breakdown voltages reveal that the breakdowns are caused by avalanche multiplication, which is also the same as those reported in impurity‐doped GaN p‐n diodes. These results indicate that DPD is a promising doping technology for GaN‐based power devices overcoming any issues associated with conventional impurity doping.

Tunneling effect in phosphorene through double barriers

We study the transport properties of charge carriers in phosphorene with a mass term through double barriers. The solutions of the energy spectrum are obtained and the dependence of the eigenvalues on the barrier potentials and wave vectors in the x-direction is numerically computed. Using the boundary conditions together with the matrix transfer method, we determine transmission and the conductance of our system. These two quantities are analyzed by studying their main characteristics as a function of the physical parameters along the armchair direction. Our results show the highly anisotropic character of phosphorene and the signature of Klein tunneling at normal incidence. Moreover, it is found that the transmission and conductance display oscillatory behaviors in terms of the barrier width under suitable conditions.

Truxenone Triimide: Two‐Dimensional Molecular Arrangements of Triangular Molecules for Air Stable n‐Type Semiconductors

AbstractThe preparation of a new class of electron deficient organic compounds is of great interest for the development of high‐performance n‐type organic semiconductors. Herein, the facile synthetic protocol for the isolation of the first examples of truxenone triimide (TTI) is reported. The electrochemical analysis of the resulting TTIs confirms that the multi‐imidization helps in lowering the LUMO energy level, tailoring prerequisite optoelectronic and/or semiconducting properties. Their microscopic charge carrier transport properties and dynamics are screened by flash‐photolysis time‐resolved microwave conductivity measurements. Compound 2D exhibited an electron mobility of 0.27 cm2 V−1 s−1 under ambient conditions.

Nonplanar Rhombus and Kagome 2D Covalent Organic Frameworks from Distorted Aromatics for Electrical Conduction.

Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of promising 2D materials with high crystallinity and tunable structures. However, the low electrical conductivity impedes their applications in electronics and optoelectronics. Integrating large π-conjugated building blocks into 2D lattices to enhance efficient π-stacking and chemical doping is an effective way to improve the conductivity of 2D COFs. Herein, two nonplanar 2D COFs with kagome (DHP-COF) and rhombus (c-HBC-COF) lattices have been designed and synthesized from distorted aromatics with different π-conjugated structures (flexible and rigid structure, respectively). DHP-COF shows a highly distorted 2D lattice that hampers stacking, consequently limiting its charge carrier transport properties. Conversely, c-HBC-COF, with distorted although concave-convex self-complementary nodes, shows a less distorted 2D lattice that does not interfere with interlayer π-stacking. Employing time- and frequency-resolved terahertz spectroscopy, we unveil a high charge-carrier mobility up to 44 cm2 V-1 s-1, among the highest reported for 2D COFs.

Electron transport properties in degenerate magnesium tin oxynitride (Mg1−xSn1+xN2−2yO2y) with average wurtzite structure

MgSnN2 with an average wurtzite structure (wz-MgSnN2) has recently emerged as a pseudo-III-nitride semiconductor, studied for applications in tandem solar cells, green light-emitting diodes, and other optoelectronic devices. This compound has only been researched recently, and, therefore, its charge-carrier transport properties are poorly understood. Understanding these properties is essential for optoelectronic applications. In this study, we grew wz-Mg1−xSn1+xN2 biaxially oriented polycrystalline films with x = −0.08 to 0.29 by reactive sputtering and investigated the charge-carrier transport properties using both direct current and optical techniques. We regarded the wz-Mg1−xSn1+xN2 films as magnesium tin oxynitride films (wz-MTNO) because a certain amount of oxygen was unintentionally incorporated into the sputtered wz-Mg1−xSn1+xN2 films. The wz-MTNO layers were n-type degenerate semiconductors with an electron density (ne) of the order of 1020 cm−3. In films with ne > 8 × 1020 cm−3, optically extracted resistivities (ρopt) obtained via a Drude-fit analysis of the infrared transmittance and reflectance spectra were almost identical to the direct-current resistivities (ρdc), indicating that the contribution of grain boundary scattering to the electron transport was negligible. However, the contribution of grain boundary scattering became unignorable with decreasing ne. The Drude-fit analysis also allowed the determination of the conduction-band effective mass (mc*) for the first time. A band edge mass of mc*/m0 ≈ 0.2 (m0 denotes the free-electron mass) was obtained in the wz-MTNO layers with |x| < 0.1. As x was increased from −0.18 to 0.29, mc*/m0 substantially increased from 0.18 to 0.56, indicating that the conduction-band dispersion decreased. That is, the conduction-band dispersion may be affected by the cation composition x. The findings of this study will provide important information to establish this material as a practical nitride semiconductor.

Investigation of anisotropic mechanical, electronic, and charge carrier transport properties of germanium-pnictogen monolayers

Recently, novel two-dimensional (2D) GeP and GeAs systems have been fabricated by mechanical exfoliation and utilized in various applications. These developments have brought the 2D germanium-pnictogens, C2/m-GeX (X = N, P, As, Sb, and Bi) structures into the limelight. In this study, we systematically investigate the structural, mechanical, electronic, and charge carrier transport properties of GeX monolayers by using first-principles methods. Our results show that the considered systems are dynamically stable and possess anisotropic physical properties. Examined structures are found to be flexible, and their mechanical strength and stiffness decrease down the group-V, in line with the trends of the bond strength, cohesive energy, charge transfer, and electron localization function. Additionally, the zigzag in-plane direction is mechanically superior to the armchair direction. The electronic band structure calculations based on HSE06 hybrid functional with the inclusion of spin–orbit coupling indicate that GeX monolayers are either direct or quasi-direct semiconductors with band gaps lying within the infrared and visible spectrum. The estimated charge carrier mobilities are highly anisotropic and also differ significantly depending on the structure and carrier type. These unique properties render GeX monolayers as suitable 2D materials for flexible nanoelectronic applications.

Densification and charge transport characterization of composite cathodes with single-crystalline LiNi0.8Co0.15Al0.05O2 for solid-state batteries

Solid-state batteries (SSBs) hold great promise for high-performance and safe energy storage solutions. Several challenges, however, including the standardization of cell/electrode formats and interface instabilities between electrode components, remain to be overcome. Here, the densification effect of composite cathodes on the electrochemical properties in SSBs was studied. By varying the pressure during uniaxial pressing, composite cathode pellets consisting of single-crystalline LiNi0.8Co0.15Al0.05O2 and Li6PS5Cl particles are fabricated without conductive agents, and the microstructural evolution, charge transport kinetics, and electrochemical performance are examined. The densification process results in a close connection between the component particles while reducing the pore spaces within the composite cathodes. The cathode compacted with a nominal pressure of 1500 MPa shows excellent capacity retention (≥ 99%) with discharge capacities of ∼160 mAh g−1 (areal capacity of ∼3.68 mAh cm−2) at 0.1 C (∼0.46 mA cm−2) over 100 cycles and improved rate capability. These improvements are attributed to the enhanced effective conductivities and diffusion coefficients of Li+ ion and electron, based on impedance and DC polarization measurements. These findings will enhance an understanding of the correlation between the microstructure and charge carrier transport properties of composite cathodes for SSBs based on sulfide-based SEs.

Charge-Carrier Transport in Quasi-2D Ruddlesden-Popper Perovskite Solar Cells.

In recent years, 2D Ruddlesden-Popper (2DRP) perovskite materials have been explored as emerging semiconductor materials in solar cells owing to their excellent stability and structural diversity. Although 2DRP perovskites have achieved photovoltaic efficiencies exceeding 19%, their widespread use is hindered by their inferior charge-carrier transport properties in the presence of diverse organic spacer cations, compared to that of traditional 3D perovskites. Hence, a systematic understanding of the carrier transport mechanism in 2D perovskites is critical for the development of high-performance 2D perovskite solar cells (PSCs). Here, the recent advances in the carrier behavior of 2DRP PSCs are summarized, and guidelines for successfully enhancing carrier transport are provided. First, the composition and crystal structure of 2DRP perovskite materials that affect carrier transport are discussed. Then, the features of 2DRP perovskite films (phase separation, grain orientation, crystallinity kinetics, etc.), which are closely related to carrier transport, are evaluated. Next, the principal direction of carrier transport guiding the selection of the transport layer is revealed. Finally, an outlook is proposed and strategies for enhancing carrier transport in high-performance PSCs are rationalized.

Polypyrrole and a polypyrrole/nickel oxide composite – single-walled carbon nanotube enhanced photocatalytic activity under visible light

A novel NiO/PPy/SWCNT composite for removal of organic dyes with an emphasis on the effect of photocatalytic charge carrier transport and photoluminescence properties.

First-principles study of strain-tunable charge carrier transport properties and optical properties of CrI<sub>3</sub> monolayer

Because the single-layer CrI<sub>3</sub> is a half semiconductor with indirect band gap and magnetic anisotropy, it has received much attention in the spintronic, magneto-electronic and magnetic storage applications. However, the knowledge of the dependence of carrier mobility and optical property on strain is still rather limited. The uniaxial and biaxial strain dependence of electronic, transport, optical and magnetic properties of single-layer CrI<sub>3</sub> are systematically investigated by using first-principles calculations, and the results are compared with experimental results. The electronic structures under different strains are first calculated by using the accurate HSE06 functional, then the carrier mobility is estimated by the deformation potential theory and the dielectric function is obtained to estimate the optical absorption especially in the visible light range. Finally, the magnetic anisotropy energy used to estimate the magneto-electronic properties is studied by the Perdew-Bueke-Ernzerhof functional including the spin-orbit coupling. It is found that the ferromagnetic CrI<sub>3</sub> is an indirect and half semiconductor with band gap 2.024 eV,<inline-formula><tex-math id="M1">\begin{document}$ \Delta {\text{CBM}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M1.png"/></alternatives></inline-formula>= 1.592 eV, <inline-formula><tex-math id="M2">\begin{document}$ \Delta {\text{VBM}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M2.png"/></alternatives></inline-formula>= 0.238 eV and can be driven into AF-Néel antiferromagnetic phase by applying –6% to –8% (compressive) biaxial stain, exhibiting excellent agreement with the results from the literature. It is found that of single-layer CrI<sub>3</sub> has very low carrier mobility with a value within 10 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> due to the large effective mass and small in-plane stiffness can be remarkably increased by increasing biaxial compression strain attributed to the reduced effective mass. A high electron mobility 174 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> is obtained in the zigzag direction by applying a –8% biaxial strain reaching the level of monolayer MoS<sub>2</sub>. The calculated imaginary component of dielectric function along the <i>x </i>(<i>y</i>) direction having two peaks (I, II) in the visible light range is obviously different from that along the <i>z</i> direction, indicating that the single-layer CrI<sub>3</sub> has optical anisotropy, demonstrating the good agreement with results from the literature. It is found that the imaginary part of dielectric function shows that an obvious redshift and peak (I, II) values strongly increase with the increase of compressive strain (biaxial), showing good agreement with the calculated electronic structures and indicating that monolayer CrI<sub>3</sub> possesses high optical adsorption of visible light under a compressive biaxial strain. Furthermore, it is found that the magnetic anisotropy energy of monolayer CrI<sub>3</sub> mainly stemming from the orbital magnetic moment of Cr ions remarkably increases from 0.7365 to 1.08 meV/Cr with g compressive strain increasing. These results indicate that the optoelectronic property of single-layer CrI<sub>3</sub> can be greatly improved by applying biaxial compressive strain and the single-layer CrI<sub>3</sub> is a promising material for applications in microelectronic, optoelectronic and magnetic storage.

Defect-Interceded Cascading Energy Transfer and Underlying Charge Transfer in Europium-Doped CsPbCl3 Nanocrystals.

Rare-earth ion (RE3+) doping in cesium lead chloride (CsPbCl3) has unlocked novel prospects to explore changes in optical, magnetic, and charge carrier transport properties. This leads to a huge advancement in optoelectronic applications, yet deep understanding of the photophysics governing the energy transfer processes is lacking and demands vital attention. Herein, we probe into the mechanistic transfer processes from the band edge of the host (CsPbCl3) to the dopant europium ion (Eu3+) with the aid of femtosecond fluorescence upconversion and transient absorption (TA) spectroscopy. The upconversion measurement portrays a defect-mediated cascading energy transfer from CsPbCl3 to Eu3+ and further cross-relaxation among Eu3+ states. Moreover, TA studies reveal that there is charge transfer from the band edge of CsPbCl3 to doping-induced shallow defect states. Furthermore, two-photon absorption study establishes no compromise in the transfer mechanism even upon bandgap excitation. This work validates that Eu-CsPbCl3 is an apt entrant for optoelectronic applications.

Harnessing Intramolecular Chalcogen-Chalcogen Bonding in Merocyanines for Utilization in High-Efficiency Photon-to-Current Conversion Optoelectronics.

A novel series of donor (D)-π-acceptor (A) merocyanine molecules harnessed with intramolecular chalcogen bonding (ChaB) is designed, synthesized, and characterized. ChaB comprises periodic chalcogen atoms, S, Se, and Te, and a neighboring oxygen atom of a carbonyl moiety. Compared to the D-π-A merocyanine dye with nontraditional intramolecular hydrogen bonding, the novel molecules with an intramolecular ChaB exhibit remarkably smaller absorption spectral widths and higher absorption coefficients attributed to their cyanine-like characteristics approaching the resonance parameter (c2) ∼0.5; furthermore, they exhibit better thermal stabilities and electrical charge-carrier transport properties in films. These novel D-π-A merocyanines harnessed with intramolecular ChaB networks are successfully utilized in high-performance color-selective organic photon-to-current conversion optoelectronic devices with excellent thermal stabilities. This study reports that the unique intramolecular ChaB plays an essential role in locking the molecular conformation of merocyanine molecules and enhancing the optical, thermal, and optoelectronic properties of high-performance and high-efficiency organic photon-to-current conversion devices.

Study of charge carrier transport properties and lifetimes in HR GaAs:Cr with Timepix3

The response of a Timepix3 (256 × 256 pixels, pixel pitch 55 μm) detector with a 500 μm thick HR GaAs:Cr sensor was studied in proton beams of 125 MeV at the Danish Centre for Particle Therapy in Aarhus, Denmark and in a 120 GeV/c pion beam at the Super-Proton Synchrotron (SPS) at CERN. The sensor was biased at different voltages and irradiated at different angles. The readout chip was configured to operate in electron and hole collection modes. Measurements at grazing angles allowed to see elongated tracks with well-defined impact and exit points, so that charge carrier production depths could be determined in each pixel. We extracted the charge collection efficiencies and the charge carrier drift times as a function of the distance to the pixel plane. It was found that measured proton tracks are shorter in hole collection than in the case of electron collection, which is explained by the shorter lifetime of holes. At an angle of 60 degrees with respect to the sensor normal, the average track length in hole collection was ∼700 μm and 950 μm in electron collection mode. To understand the experimental findings, models describing the properties of HR GaAs:Cr were implemented into the Allpix2 simulation framework. We added previously presented experimental results describing the dependence of the electron drift velocity on the electric field and validated the response by comparing measurement and simulation for various X- and gamma-ray sources in the energy range of 10–60 keV. By comparison of the experimental and the simulated results, the mobility μ h and the lifetime of holes τ h were estimated as μ h = (320 ± 10) cm2/V/s and τ h = (4.5 ± 0.5) ns.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.