Effective Carrier Concentration Research Articles

A comparative study of H2O2 production over g-C3N4 photocatalysts made from different sources

The photocatalytic performance of graphitic carbon nitride (g-C3N4) in H2O2 production has been constantly studied, however, whether g-C3N4 made from different raw materials possess distinct photocatalytic ability is not fully explored. In this work, a comparative study of g-C3N4 made from urea, melamine, thiourea, and dicyandiamide respectively under the same thermal process condition was carried out to distinguish their photocatalytic activities. By determining the yield of H2O2 produced under the same light conditions, the ability of the above four g-C3N4 photocatalysts was compared and the amount of H2O2 produced within 5 h was 55.8, 112.7, 101, and 134 µmol/L respectively. Photoelectrochemical and electrochemical impedance spectroscopy measurements indicated that g-C3N4 photocatalysts made from melamine and dicyandiamide with higher H2O2 production had higher photocurrent and lower charge transfer impedance. This might relate to the differences in material crystallinity and the band gap, which influenced their effective carrier concentration.

Boosted charge carrier concentration via luminescent quenching of photoexcited electrons in CdS/FI S-scheme heterostructures for efficient photocatalytic hydrogen production

One of the key strategies to go beyond the limited hydrogen production efficiency during photocatalytic process is to increase effective charge carrier concentration participating hydrogen evolution reaction (HER). Herein, considering organic luminescent materials as “photo-excited electron factory” with large numbers of exited carriers under light irradiation, 2-Fluorene-4,5-diphenylimidazole/CdS (FI/CdS) S-scheme heterojunction was constructed to modulate charge transfer via extracting abundant of photoexcited electrons in FI as luminescent quenching process. The impeded charge recombination in FI/CdS S-scheme heterojunction gave rise to 6 orders of magnitude higher in carriers concentration and 7.8 times higher in H2 evolution rate for CF20 (9.67 × 1026 cm−3, 5.28 mmol h−1 g−1) than that of pure CdS (2.69 × 1020 cm−3, 0.68 mmol h−1 g−1). It demonstrates that quenching photo-excited electrons in luminescent component through synergistic organic-luminescent/semiconductor interaction could be a creative and prominent strategy to enhance photocatalytic performance.

Carrier confinement for mobility enhancement in dual-gate organic thin-film transistors

Mobility of charge carriers in organic thin-film transistors (OTFTs) is one of the most important parameters, which is affected by the disorders in the semiconducting films. In general, realization of higher carrier concentration facilitates improved charge transport against these disorders. Here, we demonstrate in amorphous indacenodithiophen-co-benzothiadia (IDT-BT) based dual-gate organic thin-film transistors (DG-OTFTs) that the mobility can be robustly increased from 0.8 to 1.37 cm2 V−1 s−1, when voltage biases of opposite signs (negative one accumulate holes, positive one deplete holes) are applied on the dual independent gate electrodes. In details, the positive bias can confine the spatial distribution of the accumulated carriers near the negatively biased gate, leading to a higher effective carrier concentration with higher mobility. In addition, the interface confinement effect of carriers was further verified by two-dimensional numerical simulation, and calculations based on the variable range hopping method were performed to accurately reproduce the experimentally observed charge transport behavior in DG-OTFTs.

A strategy of adjusting band alignment to improve photocatalytic degradation and photocatalytic hydrogen evolution of CuSbS2

Photocatalytic technology based on heterojunction structure has been proved to be an effective way to solve environmental pollution and energy crisis. The effective carrier concentration plays an important role in improving the performance of heterojunction photocatalyst. In this work, CuSbS2 (CAS)-CdS-ZnO was prepared by simple 3-steps microwave irradiation method to reveal the influence of energy band structure on photocatalytic performance. Adding CdS could regulate band alignment to translate the “cliff-like” conduction band offset of CAS-ZnO to “spike-like”. That could effectively reduce the carrier recombination rate to improve the photocatalytic degradation activity, which could completely degraded Rhodamine B (RhB) after 80 min under a solar simulation, highest rate constant (0.02028 min−1) and good stability. Furthermore, the transformation of charge transfer mechanism from the Z-type to Ⅱ-type by adding CdS were confirmed by experiments and density functional theory (DFT) calculations. This change could raise the energy level of photoelectrons, resulting in CAS-CdS-ZnO with a certain photocatalytic hydrogen evolution capacity (42.37 μmol g−1 h−1). This work reveals the important role of band structure in photocatalytic properties and applications of heterojunction.

Transport properties of B and N sites vacancy defects graphene/h-BN heterostructures: First-principles study

First-principles calculations based on spin-polarized DFT-D2 approach have been carried out to study the transport properties of graphene/h-BN (G/h-BN) heterostructure (HS), 1 N vacancy defect in G/h-BN (G/h-BN_1 N), nearest neighbour of 1 N & 1B vacancy defects in G/h-BN (G/h-BN_nBN), and alternate zone of 1 N & 1B vacancy defects in G/h-BN (G/h-BN_aBN) materials. Transport properties like electrical conductivity (σ), electronic contribution of thermal conductivity (K), Seebeck coefficient (S) and thermoelectric power factor (P) are estimated through Boltzmann transport equations (BTE) within constant relaxation time approximation (RTA). Quantum ESPRESSO and BoltzTrap packages are used as a computational tool in the calculations. The considered materials are found to be stable van der Waals (vdWs) HS. The σ and K of graphene, h-BN, G/h-BN, G/h-BN_1 N, G/h-BN_nBN and G/h-BN_aBN are found to be increasing monotonically with temperature. The defected materials have slightly less value of σ than that of G/h-BN due to the effect of their effective mass and carrier concentration. At room temperature, K of G/h-BN, G/h-BN_nBN and G/h-BN_aBN retains similar value, while K of G/h-BN_1 N has increased slightly because the increase in Fermi velocity of electrons near Fermi level due to the reduction in effective mass thereby increasing the mean free path. We have analyzed the temperature dependent (constant value of energy) and energy dependent (constant value of temperature) S and P of above-mentioned materials, and found that S and P of defected HS are higher than that of non-defected HS materials due to combine dependence of n, T and m* on the value of S. The estimated S and P values of considered materials are consistent with reported value. Hence, they have budding employment in the thermoelectric devices.

Ultra-low lattice thermal conductivity and high figure-of -merit in Cl and K co-dopped Bi2Se3 prepared by KCl flux

Bismuth telluride (Bi2Te3) is a widely used thermoelectric (TE) material in practical applications. However, it contains expensive and toxic tellurium, a promising TE material should include low-cost and non-toxic element. Bismuth selenide (Bi2Se3) is similar to Bi2Te3 in electronic and crystal structure. The electron effective mass of Bi2Se3 is smaller than that of Bi2Te3, while its electrical transport performance is inferior to that of Bi2Te3. Therefore, in this study, Cl and K co-doped Bi2Se3 thermoelectric material with high density, low thermal conductivity, and excellent thermoelectric performance was prepared using KCl as the solvent. It is found that the effective electron mass and carrier concentration of the doped sample is effectively improved, which significantly increases the doped sample's electrical transport properties and ZT value. Finally, the Bi2Se3(KCl)3.5 sample reached a maximum ZT value of 0.79 at 550 K, and the average ZT in the measured temperature range reached 0.45.

Investigations of methane gas sensor based on biasing operation of n-ZnO nanorods/p-Si assembled diode and Pd functionalized Schottky junctions

In this work, we developed methane gas sensors based on zinc oxide nanorod (n-ZnO-NR) arrayed assembly, synthesized using aqueous phase deposition on p-Si(100) substrates, fabricated as pn junction besides chemical sensitization by Pd (Pdcat/n-ZnO-NR) and Pd-30wt%Ag (Pd–Agcat/n-ZnO-NR) forming Schottky junctions. These sensors are evaluated under reducing methane [CH4] gas with varying concentration (100–10,000 ppm or 0.01–1.0%) mixed with synthetic air under optimum temperature (≤200–230 °C) for thermal activation and UV A (365 nm) light activation supplemented with 50–100 °C, following increased electrical conductivity, bandgap narrowing, decreased barrier height, under biasing operation. The performance metrics include a high response (R; 45–80%), low limit of detection (LOD; 80–270 ppm), fast response-recovery times (<1–67 s), and strong binding constants (0.012–0.021), quantified from saturation current, transient response, and Langmuir adsorption isotherm, respectively. Every factor inducing a change in oxygen content of analyte gas atmosphere above ZnO include: (1) surface chemical reaction with chemisorbed oxygen ions on pristine and Pdcat or Pd-Agcat modified n-ZnO-NR yields electron donors increasing sensor conductance; (2) effective free carrier concentration and amplified interface-dipole allowing reduced barrier height and electron tunneling at high reverse bias prevailing depletion-driven mechanism; (3) faster electron transport shortening the response time due to weakened oxygen ion adsorption removed by analyte gas molecules; and finally, (4) in-depth study of thermal and UV activated processes is supported with space charge model and band-bending theory perspectives.

Цифровая осциллография полимерных мембран с протонной проводимостью

Polyvinyl alcohol-based cast membranes have promising application in creating commonly available fuel cells and fuel synthesizers, as well as inexpensive water sources and microelectromechanical systems. In order to control the properties of such membranes a deep understanding of the charge transfer mechanisms and their relationship with the structure formed at a certain composition and manufacturing technology is required. A method for studying the structure of the proton-conductive polymer membranes using digital oscillography of ion currents excited by the low-frequency rectangular pulses with the amplitudes below the threshold voltage of the ionic conductivity in a dehydrated membrane, and for the analysis of the resulting ion current pulses (spikes) in the frames of the model of a proton pump acting in each layer of the membrane is proposed in this paper. Fast Fourier transform of these oscillograms reveals from 2 to 4 branches or spike sequences corresponding to the phases with different ionic conductivity and makes it possible to determine the thicknesses of both high-conductivity phase layers (7–30 µm) and low-conductivity phase interlayers (1–7 µm) formed in the process of polymerization. The reason of spike merging into bursts is described in terms of successively induced increase in the excited proton density over a threshold value in highly conductive layers. The resonance observed in dry proton membranes at the frequencies of about 2.2 to 3.0 kHz is interpreted as the burst merge point with the further increase in impedance due to proton lagging and respective decrease in the effective thickness of active layers. The effective charge carrier concentrations (as small as 1012 to 1013 cm−3) and the velocity (from 5 to 18 cm/s for the highly conductive phases which turned out to be much higher than those observed in solutions) are estimated. The asymmetry of the cast membranes, which becomes apparent at low frequencies and causes the generation of a direct ion current in response to excitation by a purely alternating current, is studied. It is found that the apparent conductivity determining contribution to the total ohmic resistance is made by a thin interlayer with a very low ion velocity, presumably surface layer, rather than the main layers. The conclusion on the optimization of the production technology and the composition of the proton membranes for various applications is made.

Determination of optical constants of vacuum annealed ZnO thin films using Wemple Di Domenico model, Sellmier's model and Miller's generalized rules

Thin films of zinc oxide (ZnO) were coated by a spin coater on the glass substrate and are annealed at 300 °C and 500 °C under the reduced atmosphere of 10−3 mbar. The structural properties were examined by X-ray diffractogram (XRD), atomic force microscopy (AFM), and optical study of ZnO thin films by UV–Vis spectrophotometer. The XRD study showed that the films had a preferred orientation towards the c-axis, and the AFM study revealed a columnar growth. Optical parameters such as band gap, Urbach energy, electron-phonon interaction strength, refractive index, carrier concentration to the effective mass (N/m*) ratio, carrier concentration, and plasma frequency were extracted from transmittance and absorption curves by using Wemple Di Domenico and Sellmier's models. The non-linear optical parameters such as susceptibility and refractive index are obtained from Miller's generalized rules. The optical bandgap was found to decrease with an increase in dopant concentrations and also with annealing temperature but for 0.5 M bandgap increase at 300°C. The Urbach energy follows the increasing trend for both increase in concentration and annealing temperature. This indicates vacuum annealing induces defect sites in the material under investigation.

Temperature and illumination dependence of silicon heterojunction solar cells with a wide range of wafer resistivities

AbstractRecently, the significant improvements in the surface and contact passivation of silicon (Si) solar cells as well as their bulk quality have shifted their operating point to higher injections. Hence, they are less dependent on wafer doping. This shift opens an opportunity of using high‐resistivity wafers for practical photovoltaic applications, introducing a promising approach to push the cell efficiency towards the intrinsic limit and to improve the module reliability by increasing the cell breakdown voltage. Therefore, insights into the performance of Si solar cells using high‐resistivity wafers at various operating temperatures are of significant interest. In this study, we investigate the temperature‐ and illumination‐dependent performance of Si heterojunction (SHJ) solar cells using a wide range of wafer resistivities (between 3 and 1000 Ω⋅cm). Although a reduction in the passivation quality of the passivating contacts is observed at elevated temperature, the impact on the temperature coefficient of the open‐circuit voltage (TCVoc)—the dominant contributor to the temperature coefficient (TC) of the cell efficiency—is very limited. Their TCVoc are still dominated by the temperature dependence of the effective intrinsic carrier concentration. Furthermore, we also find that the investigated cells are more sensitive to temperature variation at lower illumination intensities. It is noteworthy that the efficiency of the cells fabricated using high‐resistivity wafers is comparable to that of the reference cells at any given temperature, highlighting the potential of using high‐resistivity wafers for solar cells.

Highly stable electrical performances of HfO2-based ferroelectric devices under proton irradiation

The proton irradiation effects on TiN/Zr-doped HfO2 (HZO)/TiN ferroelectric capacitors are investigated in this work. Electrical characterization results show that the change of ferroelectric hysteresis loop and leakage current are negligible under proton irradiation up to fluence 1.0 × 1012 ions/cm2. The decrease of permittivity after irradiation is due to the degradation of the effective carrier concentration in Si substrate induced by displacement damage. The results demonstrate that HZO ferroelectric material is immune to radiation and has the application potential to be used in space system.

Impact of deposition temperature on crystalline quality, oxygen vacancy, defect modulations and hetero-interfacial properties of RF sputtered deposited Ga2O3 thin films on Si substrate

In the current work, thin film (∼55 nm) of n-type Ga2O3 (n-Ga2O3) is deposited on silicon (p-Si) substrate by using radio-frequency (RF) sputtering technique with systematic substrate temperature variations (room temperature to 700 °C). The structural, optical properties and chemical states of elements of the deposited films are observed to depend significantly on the deposition temperatures. The chemical composition and oxidation states, optical properties, defect states and structural quality of the deposited films are investigated in detail by employing x-ray photoelectron spectroscopy, energy dispersive x-ray, spectroscopic ellipsometry, Raman, photoluminescence and atomic force microscopy images. X-ray diffraction result reveals a polycrystalline nature of monoclinic β-phase of Ga2O3 with (403) dominant plane. The work functions are calculated from the ultraviolet photo-electron spectroscopy for all the deposited films and Ga2O3/Si heterojunction properties are investigated by using current–voltage (I–V) and capacitance–voltage (C–V) measurements. Among all the fabricated heterojunctions, 600 °C deposited Ga2O3 film exhibits superior performance in terms of energy bandgap, work function, refractive index, barrier height, rectification ratio and effective carrier concentrations. The current transport mechanism is analysed using the appropriate energy band diagram of Ga2O3 and Si. Therefore, the study suggests that 600 °C deposition temperatures is the optimum temperature for developing a high quality Ga2O3 thin film on Si by using RF sputtering technique and corresponding Ga2O3 thin film/Si heterojunction can be a potential candidate for developing several electronic and optoelectronic devices.

Characteristics of Activation Rate and Damage of Ion-Implanted Phosphorous in 4H-SiC after Different Annealing by Optical Absorption.

We investigated the ellipsometer-based characterization method being used to quickly evaluate the depth of the damage layer in ion-implanted 4H-SiC. This method had the advantages of low cost, convenience, and non-destructiveness. Optical absorption of n-type 4H-SiC substrate, P+ ion-implanted, laser-annealed, and conventional high-temperature annealed wafers were investigated at room temperature. Three peaks were observed in the absorption spectra collected for various samples. The degree of electrical activation after laser annealing or high-temperature annealing was evaluated qualitatively from the absorption peak intensity at 2.67 eV. The circular transmission line method (CTLM) results were consistent with the optical absorption results. However, it was found that the effective carrier concentration after laser annealing was significantly lower than that after high-temperature annealing.

Deep level defect states in β-, α-, and ɛ-Ga2O3 crystals and films: Impact on device performance

A review is given of reported trap states in the bandgaps of different polymorphs of the emerging ultrawide bandgap semiconductor Ga2O3. The commonly observed defect levels span the entire bandgap range in the three stable (β) or meta-stable polymorphs (α and ɛ) and are assigned either to impurities such as Fe or to native defects and their complexes. In the latter case, the defects can occur during crystal growth or by exposure to radiation. Such crystalline defects can adversely affect material properties critical to device operation of transistors and photodetectors, including gain, optical output, threshold voltage by reducing carrier mobility, and effective carrier concentration. The trapping effects lead to degraded device operating speed and are characterized by long recovery transients. There is still significant work to be done to correlate experimental results based on deep level transient spectroscopy and related optical spectroscopy techniques to density functional theory and the dominant impurities present in the various synthesis methods to understand the microscopic nature of defects in Ga2O3.

Relationship between the density of states effective mass and carrier concentration of thermoelectric phosphide Ag6Ge10P12 with strong mechanical robustness

A ternary phosphide Ag6Ge10P12 containing no toxic elements has attracted much attention as an eco-friendly thermoelectric material. This study reveals the relationship between the density of states effective mass mDOS∗ and carrier concentration n for achieving higher thermoelectric performance in Ag6Ge10P12. The Seebeck coefficient S of Ag6Ge10−xGaxP12 (0.0 ≤ x ≤ 0.25) with various carrier concentrations is unexpectedly improved by increasing n. Scrutinizing electrical transport properties, including the S and electrical conductivity σ, and electronic structure indicated that the improved S is owing to the enhanced mDOS∗, which originated from tuning the Fermi level in a valence band with multi-valley and pudding-mold bands. The power factor S2σ is enhanced by both improved S and σ. The total thermal conductivity κtot monotonically decreases with increasing x because of the decrease in the lattice thermal conductivity κL. Combining the improved S2σ and reduced κtot, the maximum ZT value of Ag6Ge9.875Ga0.125P12 at 390 K reaches ∼0.33 with the optimal carrier concentration n ∼7.0 × 1020 cm−3. The present results demonstrate a guideline for enhancing the thermoelectric performance of Ag6Ge10P12 by breaking the trade-off relationship between the S and σ through n. Moreover, Young's modulus E and nanoindentation hardness H of Ag6Ge10-xGaxP12 are greater than 125 GPa and 9 GPa, respectively, comparable to those of thermoelectric Si–Ge alloy. These findings and insights in the present study will serve as a basis for enhancing the thermoelectric performance and fabricating the thermoelectric module for eco-friendly phosphide Ag6Ge10P12.

Ultra-sensitive methanol detection based on S-vacancy-enriched SnS: A combined theoretical and experimental investigation

Theoretical calculations have confirmed that modulation of vacancy defects could enhance the sensing properties of semiconductor gas sensors effectively. In this work, we develop a novel and cost-effective process to prepare S-vacancy-enriched SnS (Vs-SnS) for room temperature methanol sensors and further clarify the enhanced sensing mechanism of Vs-SnS based on first-principles calculation. The Vs-SnS were successful formed via one-step solvothermal process in Sn-rich environment, which could be confirmed by the results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) techniques. Comparative investigations for methanol-sensing properties indicate that the Vs-SnS display an ∼143.9-fold enhancement in gas response at room temperature compared with conventional-SnS (C–SnS). Meanwhile, ultrafast response/recovery and low detection limit are observed for the Vs-SnS sensor (3 s of response time and 7 s of recovery time for 500 ppb methanol). The significant methanol sensing improvement could ascribed to the decrease in effective hole carrier concentration after introducing the S-vacancy as electron donor. The present study demonstrates the considerable effect and potential of vacancy modulation in the field of gas sensor applications, and the Vs-SnS is promising in highly sensitive methanol sensors with low consumption.

Controllability of β-Ga2O3 single crystal conductivity by V doping

Strategy for realizing the tunability of the effective carrier concentration of β-Ga2O3 single crystals using V as a dopant.

On the Application of Lifetime-Equivalent Defect Densities on Solar Cell Level

The concept of lifetime-equivalent defect density based on lifetime measurements as a measure of electrically active defect density is extended to solar cells using Suns-V $_{\mathrm{oc}}$ measurements. It is found to be superior to previous approaches, and offers defect-specific injection-dependent information. However, the correct application of the presented method requires some knowledge of or reasonable assumptions on sample properties and measurement conditions, in particular, effective intrinsic carrier concentration, doping level, and measurement temperature. Furthermore, it is found that samples with high effective excess carrier lifetime can be susceptible to nonequilibrium related issues when the measurement duration is too short compared with lifetime. The use of series-resistance affected current-voltage curves is found to be error-prone.

Achieving high-performance n-type PbTe via synergistically optimizing effective mass and carrier concentration and suppressing lattice thermal conductivity

Introducing resonant levels is a potent strategy to improve the thermoelectric properties of the thermoelectric systems. However, the beneficial effect of resonant levels tends to be weakened with rising temperature for the PbTe system. This motivates us to introduce dynamic doping of Cu into Cr-doped n-type PbTe to achieve the fully optimized carrier transport over a wide temperature range. As a consequence, an extraordinary figure of merit, ZT of ∼ 0.55 at room temperature, a peak ZT of ∼ 1.31 at 773 K and an average ZT of ∼ 1.02 in the range of 323 to 823 K are obtained in the Pb0.975Cr0.025Te-0.2%Cu. Additionally, how co-doped of Cr and Cu contributes to suppressing thermal transports is still ambiguous. Herein, we employ comprehensive electron microscopy investigations to reveal that the synergistic incorporation of Cr and Cu into the PbTe matrix results in a series of phonon scattering sources, including massive point defects, high-density of nanoscale precipitates, microscale secondary phases, and dense grain boundaries, which can significantly reduce the lattice thermal conductivity by enhancing phonon scattering. This work clarifies the fundamental Cr-doping mechanism in PbTe and demonstrates that dynamic Cu-doping engineering is an effective approach to boosting thermoelectric performance, which should be applied in other thermoelectric materials.

Effect of Hole Effective Mass and Carrier Concentration on the Conductivity of a Transparent p‐Type LaCuOS Semiconductor with Good Transmittance in Both Visible and Mid‐Infrared Ranges

The development of advanced transparent electrical devices has created an urgent demand for wide‐bandgap transparent semiconductors. Herein, a simple, low‐cost ink‐based method for preparation of LaCuOS is introduced. High transmittances of above 60% and 65% at visible (380–750 nm) and mid‐infrared (3–5 μm) wavelengths, respectively, are obtained with LaCuOS films prepared using this ink. With these films, the carrier concentration is above 1018 cm−3, while a conductivity of 5 S cm−1 is obtained, electrical properties that are both higher than those of most transparent p‐type semiconductors. From density functional theory calculations, it is surmised that the improved conductivity of the LaCuOS is a result of its high carrier concentration, and its hole effective mass, which is smaller than that of typical transparent p‐type semiconductors.