Hole Carrier Research Articles

Vacancy Manipulation Induced Optimal Carrier Concentration, Band Convergence and Low Lattice Thermal Conductivity in Nano‐Crystalline SnTe Yielding Superior Thermoelectric Performance

AbstractSynergetic optimization of electrical and thermal transport properties is achieved for SnTe‐based nano‐crystalline materials. Gd doping is able to suppress the Sn vacancy, which is confirmed by positron annihilation measurements and corresponding theoretical calculations. Hence, the optimal hole carrier concentration is obtained, leading to the improvement of electrical transport performance and simultaneous decrease of electronic thermal conductivity. In addition, the incremental density of states effective mass m* in SnTe is realized by the promotion of the band convergence via Gd doping, which is further confirmed by the band structure calculation. Hence, the enhancement of the Seebeck coefficient is also achieved, leading to a high power factor of 2922 µW m−1 K−2 for Sn0.96Gd0.04Te at 900 K. Meanwhile, substantial suppression of the lattice thermal conductivity is observed in Gd‐doped SnTe, which is originated from enhanced phonon scattering by multiple processes including mass and strain fluctuations due to the Gd doping, scattering of grain boundaries, nano‐pores, and secondary phases induced by Gd doping. With the decreased phonon mean free path and reduced average phonon group velocity, a rather low lattice thermal conductivity is achieved. As a result, the synergetic optimization of the electric and thermal transport properties contributes to a rather high ZT value of ≈1.5 at 900 K, leading to the superior thermoelectric performance of SnTe‐based nanoscale polycrystalline materials.

α-In2O3 monolayer: A promising material as field-effect phototransistor and out-of-plane piezoelectric device

Two-dimensional (2D) semiconductors have opened promising avenues for designing advanced functional devices due to their novel electronic features. In this work, the 2D α-In2O3 monolayer is systematically studied using first-principles calculations. Our calculations show that the α-In2O3 monolayer exhibits a stable configuration with good oxidation resistance. The electronic structure calculations identify that α-In2O3 monolayer is a semiconductor with a direct bandgap of 1.41 eV, which makes the α-In2O3 monolayer to be as a promising candidate in modern bottom/top gate technologies. Interestingly, the partial charge density shows that the valence band maximum and conduction band minimum locate at the different atomic region, and the electron carrier mobility of the α-In2O3 monolayer is much larger than hole carrier mobility, indicating that the electrons and holes can be effectively separated. Additionally, the α-In2O3 monolayer has excellent optical absorption coefficient among the infrared and visible range. Hence, the α-In2O3 monolayer can be used as field-effect phototransistor. Moreover, the asymmetric atomic-layer configuration and strong inner covalent bond of the α-In2O3 monolayer induce a superior out-of-plane piezoelectric response (e31 = 1.81 × 10-10C·m−1, d31 = -0.89 pm·V−1), make α-In2O3 monolayer as a promising material as out-of-plane piezoelectric device.

Addition of en-APTAS to a Polythiophene Film for Enhanced NO2 Gas Sensing

With the aim of enhancing gas-sensor performance, we introduced N-[3-(trimethoxysilyl)propyl]ethylenediamine (en-APTAS) into poly(3-hexylthiophene) (P3HT) thin films. The P3HT films with en-APTAS as an additive showed superior gas-sensing performance compared with pristine P3HT films. The sensitivity of the gas sensor with en-APTAS was especially enhanced for NO2 gas because the en-APTAS has two amine groups that act as adsorption sites for NO2 gas molecules as a consequence of their strong binding energy. The P3HT film with 0.5 vol % en-APTAS fabricated on a OH-functionalized substrate exhibited the greatest responsivity (ΔID/I0 = 0.83) and highest response, recovery rates (ΔR/Δt = 0.26 and 0.004, respectively), and sensitivity (2.9%/ppm) among the investigated films. The superior performance of this film is attributed to the en-APTAS coupling-agent additive in the P3HT active layer concomitantly modifying the OH-functionalized substrate via a one-step spin-coating procedure. The increase in the abundance of adsorption sites in the channel region can induce more p-doping by NO2, leading to an increase of the hole carrier density and, thus, an increase of the current level in a p-type transistor gas sensor.

Chemical Conformation Induced Transport Carrier Switching in Molecular Junction based on Carboxylic-Terminated Thiol Molecules

The paper demonstrates the effect of the chemical conformation of the -COOH group on the transport characteristic including conductance, rectification, and length effect in molecular junctions (MJs) formed by self-assembled monolayers of carboxylic-terminated thiol molecules. For an alkyl chain shorter than C11, the transport mechanism was attributed to a direct off-resonant tunneling of a hole carrier, located at the Au-S interface, whereas a hopping mechanism was assigned to the alkyl chain longer than the C11 chain located at the -COOH group. The hopping mechanism may be operated by electron transport associated with the breaking of the -OH bonding likely driven by a voltage. Importantly, at the C11 alkyl chain, we observed that the transport carrier operating in MJs could change from a hole carrier into an electron carrier. The result strongly proves that the chemical conformation should be considered in analyzing molecular electronics and provides a basis for the rational design of molecular electronic devices.

Ordered phase transformation and Cu doping effects in room-temperature ferromagnetic Sr3YCo4O10.5+δ.

Sr3YCo4O10.5+δ (314-SYCO), with an unusual ordered structure and a high Curie temperature (Tc ≈ 335K), is attracting increasing attention. Herein, to improve the electrical performance of 314-SYCO, Cu-doped Sr3YCo4-x Cu x O10.5+δ (x = 0-0.8) ceramics were prepared using a solid-state reaction method. Systematic research was conducted on both the ordered phase transformation and the effects of Cu doping on the microstructure, electrical transport characteristics, and magnetic properties. For x = 0-0.4, the (103) and (215) planes were observed and combined with Rietveld refinement results for the X-ray diffraction data, confirming the formation of ordered tetragonal Sr3YCo4-x Cu x O10.5+δ . This phase was formed with a mass gain of ∼0.8% and heat released at ∼1,042°C. With increasing Cu content, the concentration of hole carriers also increased, leading to a substantial reduction in electrical resistivity. The electrical resistivity decreased by 92-99% at 300K. The polycrystalline materials have semiconducting behaviour with a three-dimensional Mott variable-range hopping mechanism. For the magnetic properties, a Hopkinson peak was observed at 319K, and the Tc was approximately 321K for x = 0. The magnetisation and Tc decreased with increasing Cu content, and a G-type antiferromagnetic-to-ferromagnetic phase transition occurred due to the spin state change for some Co3+ ions from high/intermediate spin to low/intermediate spin. These results lay the groundwork for refinement of the sintering procedure and doping parameters to enhance the performance of 314-SYCO in the context of current applications such as microwave absorbers and solid oxide fuel cell cathodes.

Defect-State Transition Associated Hole Dynamics in Cuprous Iodide Nanosheets

The cuprous iodide of zinc-blende structure (γ-CuI) is a p-type semiconductor with a wide band gap of 3.1 eV. It is considered promising as the third generation of semiconductors, in applications of hole transport layer and transparent electrode due to the high hole density and carrier mobility, but its carrier transport characteristics have not yet been understood in detail. We synthesized γ-CuI nanosheets with large sizes and measured the carrier dynamics by femtosecond transient absorption (TA) spectroscopy. We observed signals of defect trapping photogenerated carriers during band-edge transitions, as well the weak band filling and photoinduced absorption directly associated with defect states. The species and lifetimes of defect assisted transition are identified and quantified based on TA measurements with various excitation fluences, and the changes of spectra and kinetics of defect band filling are correlated to the trend of density of Cu and I vacancies in samples with different thicknesses, based on the observations on a single nanosheet. We extract a diffusion coefficient of ∼71 cm2 s–1 and derive a diffusion length longer than the sample thickness, which possibly makes a remarkable contribution to the generation of a photocurrent. Our findings are expected to offer help for harnessing the defects in γ-CuI and improving its performances in optoelectronic applications.

Copper in compensated p- and n-type Czochralski silicon: Diffusivity, influence on the majority charge carrier density and mobility

Copper contamination risks are present throughout the whole crystalline silicon solar cell process flow, namely during ingot growth processes using recycled feedstock, wafer sawing and metallisation. Copper is a fast-diffusing impurity which can modify the carrier transport and recombination properties. The aim of this study is to present new insights into the properties of copper-contaminated highly doped and compensated silicon, with a focus on the copper diffusivity and the influence of this impurity on the majority carrier mobility and density. For this, compensated Czochralski silicon ingots were grown from electronic grade feedstock doped with boron and phosphorus, and the wafers intentionally contaminated with copper. Its concentrations were measured by Glow Discharge Mass Spectrometry, while the electrical properties were extracted from Hall Effect measurements. The dopant concentrations were used to compute the effective diffusivity of interstitial copper and important changes in the copper diffusivity are expected in the vicinity of the p-type/n-type transition. For the n-type compensated wafers, a strong migration of copper atoms towards the surface and a reduced copper bulk content were highlighted. Furthermore, the presence of precipitates was experimentally revealed. Both the equilibrium electron density and electron mobility were not influenced by the contamination. In p-type compensated material, copper atoms did not accumulate at the surface, probably due to both the limited copper precipitation in p-type silicon and the lower copper diffusivity. In addition, the equilibrium hole density was not influenced. However, the hole carrier mobility was affected, likely associated to the higher bulk copper content. • Highly doped compensated silicon was intentionally contaminated with copper. • Electrical properties were obtained from Hall Effect measurements. • Large differences in copper diffusivity close to the p-to n-type transition. • Strong migration of copper towards the surface in n-type compensated wafers. • Equilibrium electron density and mobility are not influenced by the contamination.

Enhancement of superconductivity in multilayer FeSe film by Nb coating

Recently, the superconductivity of FeSe has been greatly enhanced in one-unit-cell films and ultrathin surface doped films grown on STO substrates, which lead to extensive research on the interfacial effect and charge carriers doping in FeSe films. Herein, by using ion beam sputtering deposition, transition metal (Nb) is successfully coated on the multilayer FeSe films and the superconductivity of FeSe is significantly enhanced compared to pristine FeSe. The superconducting transition temperature (Tc) varies with the thickness of Nb coatings and the highest Tc up to 13.0 K is obtained in the FeSe film with Nb coating about 10 nm. X-ray diffraction analysis (XRD) demonstrates Nb incorporates in FeSe lattice. In particular, the existence of Nb5+ in FeSe films is confirmed by the X-ray photoelectron spectroscopy (XPS). The salient change of Nb binding energy measured by XPS demonstrate the binding and hole-carriers doping mechanisms between Nb5+ and FeSe layers. In particular, the positive Hall coefficients demonstrate the hole carriers dominant in Nb-doped FeSe films. This work demonstrates the doping characteristics of Fe site in FeSe, providing an avenue for exploring diverse superconductivity of FeSe by transition metal doping.

Improve the efficiency of organic light-emitting diodes using a novel hole injection layer with triple-layer gradually doped structure

In this study, we reported the fabrication of an organic light-emitting diodes (OLED) using the metal oxide Nb-doped ZnO (NZO) and the common hole transport layer of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) to form a novel hole injection layer with gradually-doped triple-layer structure (HITL). The comparison between the stepwise doped double-layer and uniformly doped device showed that the HITL structure could effectively improve the current efficiency of the OLED device. The results of both current density-voltage measurement by hole-only device (HOD) and the UV photoelectron spectroscopy (UPS) confirmed that the highest occupied molecular orbital (HOMO) energy level could help to improve the hole injection performance. In addition, the non-destructive analysis method of capacitance-voltage was used to observe the movement of charge carriers during the operation of the device. The comparison study indicated that by the stepwise doped double-layer injection layer, the influence of the doping gradient on the carrier injection was confirmed, and a smaller doping gradient was found to show the better performance of carrier injection. Based on the results obtained from this study, the maximum luminance and current efficiency of OLED device with HITL was found to be enhanced by approximately 2.48 times and 32.6% respectively when compared with the device without hole injection layer. Various measurements carried out in this study showed that the HITL structure could effectively improve the characteristics of hole carriers, and thereby the better charge balance effect and a more stable current efficiency performance could be achieved by reducing the current efficiency roll-off effect. • In this study, we demonstrated the performances of Alq 3 -based OLED using a novel hole injection layer with triple-layer gradually-doped structure (HITL). • The HITL structure can effectively improve the characteristics of hole carriers, thereby obtaining a better charge balance effect, reducing the efficiency roll-off effect, and obtaining a more stable efficiency performance. • Compared with the double-layer stepwise doped injection layer, the influence of the doping gradient on the carrier transport is confirmed, and a smaller doping gradient has a better performance of carrier injection and transport.

Effect of carrier (hole) temperature on performance of optical amplifiers quantum dot structure

Abstract Carriers temperature in quantum dot (QD) optical amplifiers has been modelled theoretically taken into account hole contribution, which is not considered early. The contributions of wetting layer (WL), first excited state (ES1), and second excited state (ES2) have also been considered. Effect of WL−ES2 recombination time of both electrons and holes, carrier heating (CH) time of electrons and holes, in addition to electron hole recombination time are examined. The results show that there is a gap between electron and hole heating temperature due to the difference between there recovery times. It is found that fast hole (long electron) recombination time yield a high carrier temperature.

Highly Improved Efficiency and Stability of 2D Perovskite Solar Cells via Bifunctional Inorganic Salt KPF6 Modified NiOx Hole Transport Layer

In general, improving the electrical conductivity of bare NiO x film and alleviating the undesired energy level mismatch by interface engineering are operative to enhance the property of 2D perovskite solar cells (2D PSCs) based on NiO x . Herein, the NiO x /2D perovskite interface is modified by inorganic salt KPF6 to improve the interfacial contact and conductivity of NiO x . It is revealed that KPF6 not only improves the conductivity of NiO x through the formation of NiF coordination bond with NiO x by PF6‐ but also promotes the energy level matching between NiO x and the 2D perovskite layer. Thus, KPF6‐modified NiO x greatly enhances the extraction ability of hole carriers in 2D perovskite layers and improves the crystallinity of 2D perovskite films. Accordingly, the NiO x /KPF6‐based 2D PSCs attain an optimal power conversion efficiency (PCE) of 14.34%. Meanwhile, the KPF6 modified device without sealed maintains 93% of its original PCE for 600 h on a 60 °C hot plate filled with nitrogen and 72% after aging for 600 h with a humidity of 35 ± 10% in the atmosphere, respectively. This study demonstrates the effectiveness of using inorganic salts to modify NiO x hole transport layers to augment the optoelectronic properties and stability of 2D PSCs.

Theoretical study on the photocatalytic potential of BSe nanotubes for water splitting under visible light

Photocatalytic splitting of water has been a useful tool in recent years for solving the world's energy and environmental problems. However, the wide band gap and low carrier mobility of traditional photocatalysts are the reasons that limit their wide application. In this work, the structure, and optical properties of BSe nanotubes are studied for the first time based on density functional theory.The results show that single-walled BSe nanotubes (SW BSe NTs) not only have a stable structure but can also extend the ultraviolet light absorption range of the BSe monolayer to the visible light range. Significantly, the SW BSe NT has excellent photoreduction ability, low electron-hole recombination rate, and high hole carrier mobility (∼103 cm2V−1s−1). Under uniaxial strain, the SW BSe NT exhibits a direct-indirect band gap transition. Furthermore, the electronic structure of double-walled BSe nanotubes facilitates charge separation and has a wider range of light absorption. Unluckily, its lower band edge position prevents it from having an overall photocatalytic decomposition capability. Our results show that SW BSe NTs have potential applications in visible light-driven photocatalytic water splitting.

Hall Current Effect of Magnetic-Optical-Elastic-Thermal-Diffusive Non-Local Semiconductor Model during Electrons-Holes Excitation Processes

This paper investigates a theoretical model for the interaction between electrons and holes (E/H) in elastic non-local semiconductors. When the medium is activated by photo-energy because of high temperatures, an optical-elastic-thermal-diffusion (OETD) process occurs and is described by this mathematical-physical model. A study is conducted on the impact of the Hall current brought on by the collapse of a strong magnetic field on the exterior of the non-local semiconductor medium. A Hall effect is brought on by the magnetic field’s effect on the density of magnetic flux. The Laplace transform with initial conditions of the dimensionless main physical fields in one dimension (1D) is used to demonstrate this. Mathematically, in the Laplace domain, the generic linear solutions for the strain and temperature distributions, as well as charge carrier holes and electrons, are derived. The key physical fields’ complete solutions in the time domain are obtained by numerically simulating a few thermal, mechanical, and optical conditions at the free surface of the semiconductor using the Laplace inverse approximation technique. For silicon material, the photo-thermoelasticity theory’s Hall current effect, non-local parameter, and effects of thermal relaxation durations are graphically displayed and analyzed.

Growth of delafossite CuAlO2 single crystals in a reactive crucible

Delafossite oxide CuAlO2 has received great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully synthesized with a reactive crucible melting method. The crystals are characterized by x-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, transport measurement, and magnetic susceptibility measurement. The CuAlO2 single crystals show semiconducting behavior with hole carriers, which is consistent with other crystals grown by the conventional slow-cooling method. This growth method we reported here eliminates the process of removing the remaining flux, allowing easy access to the high-quality single crystals. This new approach to growing high-quality delafossite oxide CuAlO2 with a few mm size is important for new technologies that demand p-type semiconductor-based device fabrication.

Violation of Luttinger's theorem in the simplest doped Mott insulator: Falicov-Kimball model in the strong-correlation limit

The Luttinger's theorem has long been taken as the key feature of Landau's Fermi liquid, which signals the presence of quasiparticles. Here, by the unbiased Monte Carlo method, violation of Luttinger's theorem is clearly revealed in the Falicov-Kimball (FK) model, indicating the robust correlation-driven non-Fermi liquid characteristic under any electron density. Introducing hole carriers to the half-filled FK leads to Mott insulator-metal transition, where the Mott quantum criticality manifests unconventional scaling behavior in transport properties. Further insight on the violation of the Luttinger's theorem is examined by combining Hubbard-I approximation with a composite fermion picture, which emphasizes the importance of a mixed excitation of the itinerant electron and the composite fermion. Interestingly, when compared FK model with a binary disorder system, it suggests that the two-peak band structure discovered by Monte Carlo and Hubbard-I approaches is underlying the violation of Luttinger's theorem.

Wannier Diagrams for Semiconductor Artificial Graphene

Quantum transport has been simulated in hexagonal semiconductor lattices of antidots with a period of 80 nm and short-range disorder. Wannier diagrams, i.e., DoS(n, B) maps of the density of states, where n is the electron density and B is the magnetic field strength, have been calculated for several potential modulation amplitudes comparable to or much larger than the Fermi energy. Deep dips in the maps of the density of states have the form of rays with positive, zero, and negative slopes. In addition to the fan of the rays separating the first and second, as well as the second and third Landau levels, the maps include rays that are parallel to them and are shifted in n and B by integers of the characteristic electron density n0 and the characteristic magnetic field strength B0, respectively. It has been shown that the sign and magnitude of the slope of the rays in the density of states correspond to the centers of the plateaus of quantized Hall resistances Rxy. The lattice is brightly manifested in the Rxy(n, B) maps as the replicas of the first and second plateaus in Rxy and as oscillations of Rxy between negative and positive values at a fixed magnetic field or a fixed electron density, which indicates the interchange between the hole and electron charge carriers.

Realizing high thermoelectric performance of Cu and Ce co-doped p-type polycrystalline SnSe via inducing nanoprecipitation arrays

Thermoelectric (TE) performance of polycrystalline stannous selenide (SnSe) has been remarkably promoted by the strategies of energy band, defect engineering, etc. However, due to the intrinsic insufficiencies of phonon scattering and carrier concentration, it is hard to simultaneously realize the regulations of electrical and thermal transport properties by one simple approach. Herein, we develop Cu and Ce co-doping strategy that can not only greatly reduce lattice thermal conductivity but also improve the electrical transport properties. In this strategy, the incorporated Cu and Ce atoms could induce high-density SnSe2 nanoprecipitation arrays on the surface of SnSe microplate, and produce dopant atom point defects and dislocations in its interior, which form multi-scale phonon scattering synergy, thereby presenting an ultralow thermal conductivity of 0.275 W·m-1·K-1 at 786 K. Meanwhile, density functional theory (DFT) calculations, carrier concentration, and mobility testing reveal that more extra hole carriers and lower conducting carrier scattering generate after Cu and Ce co-doping, thereby improving the electrical conductivity. The co-doped Sn0.98Cu0.01Ce0.01Se bulk exhibits an excellent ZT value up to ~1.2 at 786 K and a high average ZT value of 0.67 from 300 to 786 K. This work provides a simple and convenient strategy of enhancing the TE performance of polycrystalline SnSe.

Sensitive and selective ozone sensor based on CuCo2O4 synthesized by a facile solution combustion method

In this study, p-type spinel CuCo2O4 nanomaterials with abundant oxygen vacancies were synthesized by a facile one-step solution combustion method, and their sensing characteristics have been investigated. The p-type CuCo2O4 sensor shows a high response of 11 to 1 ppm O3 and low responses (< 2) to various interference gases (100 ppm NH3, NO2, SO2, and 500 ppm ethanol, methanol and acetone) at 90 °C in dry air, indicating a superior selectivity. The sensor also possesses a wide O3 detection range from 10 to 3000 ppb and good repeatability. More importantly, the relative humidity (RH) could promote the electrical response to O3, and the response to 1 ppm O3 only deviates from 24 to 27 as the RH increases from 50% to 90%, benefitting for practical applications in ambient humid air atmospheres. The humidity enhanced response for p-type CuCo2O4 could be ascribed to the competition of hole carrier reduction, O2− active sites reduction and O3 decomposition promotion induced by the hydroxyl groups.

Tuning Threshold Voltage of Electrolyte-Gated Transistors by Binary Ion Doping.

Electrolyte-gated transistors (EGTs) operating at low voltages have attracted significant attention in widespread applications, including neuromorphic devices, nonvolatile memories, chemical/biosensors, and printed electronics. To increase the practicality of the EGTs in electronic circuits, systematic control of threshold voltage (Vth), which determines the power consumption and noise margin of the circuits, is essential. In this study, we present a simple strategy for systematically tuning Vth to almost half of the operating potential range of the EGT by controlling the electrochemical doping of electrolyte ions into organic p-type semiconductors. The type of anion in the ionogel determines Vth as well as other transistor characteristics, such as the subthreshold swing and mobility, because the positive hole carriers are the majority carriers. More importantly, Vth can be finely controlled by binary anion doping using ionogels with two anions with varying molar fractions at a fixed cation. In addition, the binary anion doping successfully controls the inversion characteristics of ion-gated inverters. As unlimited combinations of ion pairs are possible for ionogels, this study opens a route for controlling the device characteristics to expand the practicality and applicability of ionogel-based EGTs for next-generation ionic/electronic devices.

Disorders and oxygen vacancies in p‐type Sn2B2O7 (B = Nb, Ta): Role of the B‐site element

AbstractSn2Nb2−xTaxO7 (x = 0.0–2.0) with pyrochlore structure is a promising material for p‐type oxide semiconductors. A systematic study of its Nb/Ta ratio indicated that the hole–generation efficiency of the Nb end (Sn2Nb2O7) was an order of magnitude lower than that of the Ta end (Sn2Ta2O7). Although this occurs due to differences in oxygen‐vacancy formation, the origins of the hole–generation efficiencies remain unclear due to limited information on local and global crystal‐structure disorders in pyrochlore Sn2Nb2O7 and Sn2Ta2O7. In this study, the crystal structures of Sn2B2O7 (B = Nb, Ta), composed of BO6 octahedra and Sn4O tetrahedra, were investigated using X‐ray absorption spectroscopy and X‐ray diffraction. A detailed investigation of the local and global crystal structures indicated a larger amount of disorder in the Sn4O tetrahedra in Sn2Nb2O7 compared to Sn2Ta2O7; disorder in the BO6 octahedra occurred only in Sn2Ta2O7. This study indicates that an appropriate selection of the B‐site element is vital for suppressing defect and disorder formation in Sn4O tetrahedra and subsequently improving the hole–carrier–generation efficiency.