Increase In Charge Carrier Concentration Research Articles

High thermoelectric power factor of n-type Bi2(Se,S)3 via synergetic influence on the charge carrier concentration and energy filtering effect

Excellent thermoelectric performance is sought to address essential energy concerns, including the impact of energy consumption on the environment. In this work, we have synthesized the Bi2Se3 thin film using a simple thermal evaporation route and post‑sulfurized it for different durations at (773 K). The crystal structure change from rhombohedral to orthorhombic with different post‑sulfurization times has been confirmed via x-ray diffraction. The atomic vibrational modes of Bi2(Se,S)3 thin films have been examined by Raman spectroscopy. The surface morphology of Bi2(Se,S)3 thin films have been observed using a scanning electron microscope. The Seebeck coefficient values enhanced from 155.1 to 203.5 μV/K with post‑sulfurization time due to increased charge carrier concentration and energy filtering effect. The electrical conductivity improved from 90.2 to 118.7 S/m of the as-deposited sample to the highest post‑sulfurized sample due to improving charge carrier concentration and changing the crystal structure from rhombohedral to orthorhombic structure. We have achieved the maximum power factor of Bi2(Se,S)3 thin film of the 4.86 μWcm−1 K−2 at room temperature. Obtaining the highest power factor has a straightforward approach to fabricating commercial thermoelectric material.

Highly boosted photocatalytic H2 production from ZnS particles assisted by Cd-Cu co-doping

Cd-Cu co-doping was applied to improve photocatalytic H2 evolution efficiency of the ZnS photocatalyst. This study confirms that an optimal Cd-Cu dopant ratio enhances the photocatalytic activity by improving light absorbance and minimizing charge recombination. Furthermore, the optimum Cd-Cu ratio plays a significant role in inducing a uniform spherical morphology and CdS-CuS crystalline structure. The drawbacks of the ZnS photocatalysts can be overcome by reducing the band gap with an increased charge carrier concentration. In the type-II heterojunction system of the best C2C5 (Cd:Cu = 2:5) photocatalyst sample, electrons are effectively transferred through hierarchical band alignment while blocking the separated holes to reduce charge recombination. The content-optimized C2C5 sample greatly accelerated the charge transfers during the photocatalytic reactions. Thus, ZnS photocatalyst assisted by Cd-Cu co-doping is a promising material for overpowering the inherent problems of the metal sulfides for solar hydrogen productions.

Influence of Surface Resistance of Silicon p-i-n Photodiodes n+-Layer on their Electrical Parameters

Silicon quadrant p-i-n photodiodes of different concentrations of diffused phosphorus in the n+- layer were fabricated. The experimental curve of phosphorus impurity distribution along the depth of the diffusion layer is obtained. The influence of charge carrier concentration in this layer on the dark currents of responsive elements and the guard ring was studied. Volt-ampere characteristics of photodiodes were measured. It was seen that when the surface resistance of the n+-layer decreases, the dark currents decrease, the samples with a surface resistance of 1,9 – 2,4 Ohm / □ have approximately the same level of dark currents of responsive areas, i.e. further increase in charge carrier concentration makes no change. As to dark currents of photodiode guard rings, it was found that for the most part they depend on the state of the crystal periphery but not on the level of doping. The influence of the surface resistance of the n+-layer on interconnection resistance between the responsive areas and the protective ring, and no influence on the capacitance of the photodiodes were revealed.

(Digital Presentation) Plasmonic Enhanced CuBi2O4 Photocathode for Solar Driven Water Splitting

Hydrogen is a fuel for sustainable and renewable energy development that emits no carbon dioxide when compared to traditional fossil fuels. Photoelectrochemical (PEC) water splitting is a sustainable method of converting solar energy that falls on earth into useful chemical fuels. However, the sluggish surface reaction kinetics, high cost, and low natural abundance limit the use of photoelectrodes for practical application. It is critical to developing effective and resilient photoelectrodes that provide high efficiency with good stability. Copper based oxides like CuO, Cu2O, CuBi2O4, CuFeO2, and Cu3VO4 are some of the most well-known low-cost materials with apt band gap and band edges for proton reduction reaction. Among them, CuO is one of the most extensively researched photocathodic materials for PEC water splitting owing to its natural abundance and facile synthesis. However, the photocorrosion problem at CuO limits its use in practical application. The conduction band of CuO is contributed by Cu 3d bands which easily get reduced to Cu0 on photoexcited electron diffuse to conduction band. On the other hand, CuBi2O4 is ternary metal oxide which is more resistant and forbids photocorrosion owing to it conduction band contributed by Bi atom orbitals rather than Cu orbitals1.Metal sulfides like Sb2S3 is good photo-absorber material for solar cells consists of non-toxic and low-cost metal. It has favorable band gap and conduction band and valence band alignment for proton reduction reaction. The type II heterojunction between CuBi2O4 and Sb2S3 can present effective strategy of charge separation and enhanced photoconversion. The plasmon behavior of metal NPs is widely studied in PEC to further improve the photoelectrochemical performance of the photoelectrodes. Plasmonic metal nanoparticles (NPs) absorb light of a threshold wavelength and vibrate emitting out the hot electrons2. Adding Au plasmonic NPs to the photoelectrode might increase the charge carrier concentration, enhance the PEC efficiency, and boost hydrogen production.The objectives of this study are to investigate fabrication of CuBi2O4/Sb2S3 heterojunction and investigate the dual role of plasmonic Au NPs sandwiched between CuBi2O4/Sb2S3 heterojunction. This heterojunction is developed through three synthesis process: synthesis of CuBi2O4 microrods (MRs) through co-precipitation, Sb2S3 plates via a hybrid chemical bath deposition, and Au NPs via seed-assisted synthesis. XRD, SEM, TEM, and XPS are used to examine the CuBi2O4/Au/Sb2S3, CuBi2O4/Sb2S3, and CuBi2O4/Au heterostructures that were synthesized. At 0 VRHE, the photocurrent of CuBi2O4/Au/Sb2S3 (-3.12 mA.cm-2) was > 200 percent greater than that of pristine CuBi2O4 (-1.45 mA.cm-2), Sb2S3 (-1.02 mA.cm-2), and CuBi2O4/Sb2S3 (-2.15 mA.cm-2). Impedance spectroscopy revealed that CuBi2O4/Au/Sb2S3 has the lowest charge transfer resistance value when compared to the pristines. Stability tests revealed the superior stability of CuBi2O4/Au/Sb2S3 compared to their counterparts3. The increase in photoelectrode PEC activity can be attributed to: (i) effective heterojunction formation for facile charge transfer, (ii) increased light absorption, (iii) incorporation of plasmonic Au NPs increased charge carrier concentration, and (iv) Au NPs also served as a relay for charge transfer between CuBi2O4 and Sb2S3 due to their metallic properties, which increased photoelectrode conductivity. The synergistic effect of fabricated CuBi2O4/Sb2S3 heterojunction and dual role of Au NPs sandwiched resulted in improved PEC conversion and boosted hydrogen evolution. The findings of this study will pave the way for photoelectrodes to be sandwiched between plasmonic NPs through a heterojunction interface that boosts hydrogen production. References (1) Li, C.; He, J.; Xiao, Y.; Li, Y.; Delaunay, J. J. Earth-Abundant Cu-Based Metal Oxide Photocathodes for Photoelectrochemical Water Splitting. Energy Environ. Sci. 2020, 13 (10), 3269–3306. https://doi.org/10.1039/d0ee02397c.(2) Subramanyam, P.; Meena, B.; Biju, V.; Misawa, H.; Challapalli, S. Emerging Materials for Plasmon-Assisted Photoelectrochemical Water Splitting. J. Photochem. Photobiol. C Photochem. Rev. 2022, 51 (November 2021), 100472. https://doi.org/10.1016/j.jphotochemrev.2021.100472.(3) Kumar, M.; Ghosh, C. C.; Meena, B.; Ma, T.; Subrahmanyam, C. Plasmonic Au Nanoparticle Sandwiched CuBi2O4 /Sb2S3 Photocathode with Multi-Mediated Electron Transfer for Efficient Solar Water Splitting. Sustain. Energy Fuels 2022. https://doi.org/10.1039/D2SE00600F. Figure 1

Structural and Photocatalytic Studies on Oxygen Hyperstoichiometric Titanium-Substituted Strontium Ferrite Nanoparticles

Doping of ferrites is an important domain of research for their application as photocatalysts. In the present work, the effect of Ti4+ substitution on the structural and photocatalytic properties of strontium ferrite nanoparticles (NPs) is studied. Ternary doped Sr1−xTixFe2O4+δ ferrite NPs (x = 0.0–1.0) were synthesized by sol–gel methodology. Tetravalent Ti4+ ions caused oxygen hyperstoichiometry and enhancement in the surface area from 44.3 m2/g for SrFe2O4 NPs to 77.6 m2/g for Sr0.4Ti0.6Fe2O4+δ NPs. The average diameter of NPs ranged between 25–35 nm as revealed by TEM analysis. The presence of two sextets in the Mössbauer spectrum of pristine SrFe2O4 and Ti4+-substituted ferrite NPs and a paramagnetic doublet in the TiFe2O5 confirmed their phase purity. The photocatalytic potential of pure and Ti4+-substituted ferrite NPs was studied using nitroaromatic compounds, viz. pendimethalin, p-nitrophenol and Martius yellow, as model pollutants. Doped ferrite NPs with a composition of Sr0.4Ti0.6Fe2O4+δ NPs showed the highest degradation efficiency ranging from 87.2% to 94.4%. The increased photocatalytic potential was ascribed to the lowering of band gap (Eg) from 2.45 eV to 2.18 eV, a fourfold decrease in photoluminescence intensity, increased charge carrier concentration (4.90 × 1015 cm−3 to 6.96 × 1015 cm−3), and decreased barrier height from 1.20 to 1.02 eV. O2●− radicals appeared to be the main reactive oxygen species involved in photodegradation. The apparent rate constant values using the Langmuir–Hinshelwood kinetic model were 1.9 × 10−2 min−1, 2.3 × 10−2 min−1 and 1.3 × 10−2 min−1 for p-nitrophenol, pendimethalin and Martius yellow, respectively. Thus, tuning the Ti4+ content in strontium ferrite NPs proved to be an effective strategy in improving their photocatalytic potential for the degradation of nitroaromatic pollutants.

The effects of Ba0.85Ca0.15Zr0.1Ti0.9O3 addition on the phase, microstructure, and thermoelectric properties of Ca3Co4O9 ceramics

ABSTRACT In this work, the influences of Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) addition on phase, microstructure, and thermoelectric properties of Ca3Co4O9 (CCO) were investigated. (1-x)CCO-(x)BCZT ceramics where x= 0, 0.003, 0.005, and 0.010 were fabricated successfully via a conventional solid-state sintering at 1,223 K for 24 h. The substitution of BCZT introduced the chemical defects (, , ) in CCO ceramic, which increased charge carrier concentration and enhanced the electrical conductivity. The presence of Ca3Co2O6 phase and Co3+ improved the Seebeck coefficients of CCO ceramic. The thermal conductivity of CCO ceramic decreased when BCZT was added. The addition of BCZT at x = 0.010 promoted the highest thermoelectric power factor (PF~235 μW/mK2), and the highest figure of merit (ZT~0.5) at 800 K, which presents this ceramic an alternative p-type oxide thermoelectric for a high-temperature thermoelectric device.

Bismuth Vacancy-Mediated Quantum Dot Precipitation to Trigger Efficient Piezocatalytic Activity of Bi2WO6 Nanosheets.

Point defects in piezoelectric semiconductors play a significant role in regulating the piezocatalytic performance. However, the role of metal vacancies in piezocatalysis has been less explored than that of oxygen vacancies. Herein, Bi2WO6 (BWO) nanosheets with tunable Bi defects were synthesized using an ion exchange method. High-resolution transmission electron microscopy directly revealed the existence of Bi vacancies in the lattice of BWO nanosheets and the precipitation of Bi quasiparticles. The BWO nanosheets with the highest concentration of Bi vacancies exhibited an excellent decomposition efficiency (7.83 × 10-2 min-1) over rhodamine B under ultrasound. The phenomenon is mainly attributed to the increased charge carrier concentration as a consequence of defect energy levels. In addition, the significant enhancement of light absorption capacity caused by the surface plasmon resonance effect of quasiparticles indicates that Bi ions escape from the lattice and combine with free electrons around BWO to form Bi quantum dots, which function as electron traps to facilitate the separation of charge carriers during the piezocatalytic process. This work systematically reveals the essential affiliation of metal vacancies and surface metal clusters in piezocatalysts and verifies the significance of vacancy engineering in piezocatalytic application.

Robust Ni-doped Ruddlesden-popper (La,Sr)Fe1-xNixO4+δ oxide as solid oxide cells fuel electrode for CO2 electrolysis

Ruddlesden-popper (La, Sr)FeO 4+δ perovskite oxide with excellent redox stability shows insufficient electrochemical catalytic activity for CO 2 reduction because of low conductivity and oxygen vacancy concentration. In this work, Ni modified (La, Sr)Fe 1-x Ni x O 4+δ cathode was developed to improve the conductivity and catalytic activity for CO 2 electrolysis. The introduction of the Ni element significantly increases the conductivity of (La, Sr)FeO 4+δ perovskite oxide in both air and 50%CO 2 /CO due to the increasing charge carrier's concentration. Furthermore, the symmetric cell with (La, Sr)Fe 0·9 Ni 0·1 O 4+δ (RPLSFNi0.1) electrode exhibits the lowest polarization resistance in 50%CO 2 /CO, suggesting that the RPLSFNi0.1 electrode has the best catalytic activity for CO 2 electrolysis. Moreover, the addition of Sm 0.2 Ce 0·8 O 2-δ (SDC) in RPLSFNi0.1 electrode further enhances the electrochemical performance, and the current density of −1170 mA cm −2 is obtained at 850 °C and 1.5 V. In addition, the electrolysis cell exhibits excellent reversible cycling operating stability between 0.6 V at fuel cell mode and 1.2 V at electrolysis mode, indicating that RPLSFNi0.1 is a robust cathode material for solid oxide cells (SOCs) fuel electrode. • Ni doping increases the conductivity of (La, Sr)FeO 4+δ perovskite oxide in both air and 50%CO 2 /CO. • The substitution of Fe with Ni favors the formation of oxygen vacancy. • The current density of electrolysis cell with RPLSFNi0.1-SDC electrode attains −1170 mA cm −2 @ 1.5 V.

Положения экстремумов зонного спектра носителей заряда в тонких пленках висмута

Many studies report an increase in charge carrier concentration in thin bismuth films as their thickness decreases at low temperatures. These results are obtained on the basis of data on resistivity, magnetoresistance, Hall coefficient, and thermal EMF measured in these films. The calculation is most often carried out within the framework of the two-band approximation assuming the quadraticity of the dispersion law of electrons and holes. Using these approximations, it is possible to estimate the change in the position of energy extremes relative to the chemical potential in these films based on concentration values of charge carriers. This article presents all these calculations and analyses the extrema movement of the charge carrier band spectrum in bismuth films with a change in their thickness.

Phenomenological analysis of the low temperature

The low-temperature phase diagram of a high-temperature cuprate superconductor consists of the superconducting, pseudogap and normal phases is analyzed in yttrium barium copper oxide systems by a phenomenological scheme using the shallow valence band electronic structure. The superconducting and pseudogap phases, which involve condensates, are assumed to be equilibrium systems of the condensate and normal subphases. The condition for phase transitions is given by equality of the total electronic energy gains. Using the mean strength of the pairing energy reproduces the experimental data well and the strength of the pairing energy decreases with increasing charge carrier concentration from the under-doped regime to the over-doped regime. We propose that this result can be explained by the screening effect.

PbS-Pb-CuxS Composites for Thermoelectric Application.

Composite materials offer numerous advantages in a wide range of applications, including thermoelectrics. Here, semiconductor-metal composites are produced by just blending nanoparticles of a sulfide semiconductor obtained in aqueous solution and at room temperature with a metallic Cu powder. The obtained blend is annealed in a reducing atmosphere and afterward consolidated into dense polycrystalline pellets through spark plasma sintering (SPS). We observe that, during the annealing process, the presence of metallic copper activates a partial reduction of the PbS, resulting in the formation of PbS-Pb-CuxS composites. The presence of metallic lead during the SPS process habilitates the liquid-phase sintering of the composite. Besides, by comparing the transport properties of PbS, the PbS-Pb-CuxS composites, and PbS-CuxS composites obtained by blending PbS and CuxS nanoparticles, we demonstrate that the presence of metallic lead decisively contributes to a strong increase of the charge carrier concentration through spillover of charge carriers enabled by the low work function of lead. The increase in charge carrier concentration translates into much higher electrical conductivities and moderately lower Seebeck coefficients. These properties translate into power factors up to 2.1 mW m-1 K-2 at ambient temperature, well above those of PbS and PbS + CuxS. Additionally, the presence of multiple phases in the final composite results in a notable decrease in the lattice thermal conductivity. Overall, the introduction of metallic copper in the initial blend results in a significant improvement of the thermoelectric performance of PbS, reaching a dimensionless thermoelectric figure of merit ZT = 1.1 at 750 K, which represents about a 400% increase over bare PbS. Besides, an average ZTave = 0.72 in the temperature range 320-773 K is demonstrated.

Mo-doped ZnV2O6/reduced graphene oxide photoanodes for solar hydrogen production

We report the fabrication and characterization of molybdenum (Mo)-doped ZnV2O6/reduced graphene oxide (rGO) composite and its use as photoanode for photoelectrochemical (PEC) hydrogen production. Compared to pure ZnV2O6, Mo ions act as electron donor in the ZnV2O6:Mo lattice increasing charge carrier concentration and subsequently mobility in the bulk by the polaron transport. We measured the hole transfer efficiency for the pure and Mo-doped ZnV2O6 electrodes and revealing a substantial increase from 16 to 25%. The mechanism of enhanced photoactivity of Mo-doped ZnV2O6 was studied by density functional theory calculations. Moreover, electrochemical impedance spectroscopy measurements show that graphene modification improves carrier separation and transfer across the electrode/electrolyte interface. Therefore, the combination of the two strategies triggers a synergistic enhancement in PEC performance in terms of incident photon-to-current efficiency, which is 17% at 370 nm, being 4.5- and 3.6-times greater than those of pristine ZnV2O6 and ZnV2O6:Mo photoanodes, respectively. With photocurrent onset potentials of 0.6 V and photocurrent densities of 2.07 mA/cm2 at 1.23 V vs. RHE, ZnV2O6:Mo/rGO photoanodes are of interest for the design of high performance PEC visible-light-induced water-splitting devices.

Synthesis, Bottom up Assembly and Thermoelectric Properties of Sb-Doped PbS Nanocrystal Building Blocks.

The precise engineering of thermoelectric materials using nanocrystals as their building blocks has proven to be an excellent strategy to increase energy conversion efficiency. Here we present a synthetic route to produce Sb-doped PbS colloidal nanoparticles. These nanoparticles are then consolidated into nanocrystalline PbS:Sb using spark plasma sintering. We demonstrate that the introduction of Sb significantly influences the size, geometry, crystal lattice and especially the carrier concentration of PbS. The increase of charge carrier concentration achieved with the introduction of Sb translates into an increase of the electrical and thermal conductivities and a decrease of the Seebeck coefficient. Overall, PbS:Sb nanomaterial were characterized by two-fold higher thermoelectric figures of merit than undoped PbS.

Supramolecular Functionalization for Improving Thermoelectric Properties of Single-Walled Carbon Nanotubes-Small Organic Molecule Hybrids.

Single-walled carbon nanotube (SWCNTs-P)-small organic molecule hybrid materials are promising candidates for achieving high thermoelectric (TE) performance. In this study, we synthesized rod-coil amphiphilic molecules, that is, tri(ethylene oxide) chain-attached bis(bithiophenyl)-terphenyl derivatives (1 and 2). Supramolecular functionalization of SWCNTs-P with 1 or 2 induced charge-transfer interactions between them. Improved TE properties of the supramolecular hybrids (SWCNTs-1 and SWCNTs-2) are attributed to increased charge-carrier concentration (electrical conductivity), interfacial phonon scattering (thermal conductivity), and energy difference between the transport and Fermi levels (ETr - EF; Seebeck coefficient). SWCNTs-2 exhibited a ZT of 0.42 × 10-2 at 300 K, which is 350% larger than that of SWCNTs-P. Furthermore, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-doped SWCNTs-2 showed the highest ZT value of 1.96 × 10-2 at 300 K among SWCNTs-P/small organic molecule hybrids known until now. These results demonstrated that the supramolecular functionalization of SWCNTs-P with small organic molecules could be useful for enhancement of TE performance and applications in wearable/flexible thermoelectrics.

Modifying field-effect transistor response in a conjugated polymer upon the addition of radical dopants

As open-shell moieties are increasingly integrated into organic electronic devices, there remains a need to establish the interactions that occur between these oxidation–reduction-active (redox-active) radical species and commonly-used conjugated polymers. In this report, we show that the addition of the stable radical galvinoxyl to the conjugated polymer poly(3-hexylthiophene) (P3HT) alters the thin film transistor response from semiconducting to conducting as well as modestly enhances the electrical conductivity. This interaction is not seen with other radical species. While an increase in charge carrier concentration is observed, the interaction does not seem to be otherwise consistent with a simple charge-transfer doping mechanism, due to the mismatched reduction and oxidation potentials of the two species. Additionally, no freeze-out of charge carriers is observed at lower temperatures. It is also not due to parallel conduction through the radical fraction of the bulk composite, as the radical species is non-conductive. Hole mobility is enhanced at lower concentrations of the radical, but it decreases at higher concentrations due to the reduced fraction of conductive material in the polymer bulk. Despite the increase in mobility at lower concentrations, the activation energy for charge transport is increased by the presence of the radical. This suggests that the radical is likely filling trap states within the P3HT for the composite thin film while not affecting their energetic distribution.

PVP Treatment Induced Gradient Oxygen Doping in In2S3 Nanosheet to Boost Solar Water Oxidation of WO3 Nanoarray Photoanode

AbstractThe photoelectrochemical performance of the WO3 photoanode is limited by the severe charge recombination in the bulk phase and at the WO3/electrolyte interface. Herein, In2S3 nanosheets are integrated onto the surface of the WO3 nanowall array photoanode, followed by a facile polyvinylpyrrolidone (PVP) solution treatment. The PVP treatment results in sulfur vacancies and a gradient oxygen doping into In2S3 from surface to interior, which induces the formation of a gradient energy band distribution. The gradient band structured In2S3 and type II band alignment at the WO3/In2S3 interface simultaneously create a channel that favors photogenerated electrons to migrate from the surface to the conductive substrate, thereby suppressing bulk carrier recombination. Meanwhile, the sulfur vacancies and oxygen doping contribute to increased charge carrier concentration, prolonged carrier lifetime, more active sites, and small interfacial transfer impedance. As a consequence, the PVP treated WO3/In2S3 heterostructure photoanode exhibits a significantly enhanced photocurrent of 1.61 mA cm−2 at 1.23 V versus reversible hydrogen electrode (RHE) and negative onset potential of 0.02 V versus RHE.

Synthesis of Silicon and Higher Manganese Silicide Bulk Nano-composites and Their Thermoelectric Properties

In this work, silicon and higher manganese silicide (HMS) bulk nanocomposites were synthesized, Si100-x(HMS)x, where x is 10–50, and their thermoelectric properties were studied. The powders of Si and Mn in a stoichiometric ratio were mixed by a ball milling, followed by hot-pressing to obtain the Si + HMS bulk nanocomposite samples. Every bulk sample was highly dense, with a density of over 90% of its theoretical value. The x-ray diffraction analysis revealed that only the Si and HMS phases, without any impurity phase, were obtained. The fractured surface morphology showed that the composites consisted of micro-sized Si matrix grains with the HMS nanodots homogenously distributed within the grains. The thermoelectric studies showed that thermal conductivity decreased with a higher concentration of HMS, mainly due to the reduction of lattice thermal conductivity. It was believed to be the presence of the HMS nanodots which effectively scattered phonon transport. Furthermore, the addition of HMS increased charge carrier concentration. Hence, the Seebeck coefficient decreased, and the electrical conductivity increased with increasing HMS content. However, the presence of the HMS nanodots also contributed to strong scattering of charge carriers. The charge mobility was reduced significantly for the composite samples compared to the bulk Si sample. Therefore, the electrical conductivity is relatively low compared to the reported values in the literature. The dimensionless figure-of-merit (ZT) was improved in the nanocomposite samples compared to the bulk Si sample, and the values were largest for the sample with x = 50.

Ferroelectric order versus metallicity in Sr1−xCaxTiO3−δ ( x=0.009 )

We report on a thermal-expansion study of the ferroelectric phase transition in insulating Sr$_{1-x}$Ca$_x$TiO$_3$ ($x=0.009$) and its evolution upon increasing charge-carrier concentration up to $n\simeq 60 \times 10^{19}$cm$^{-3}$. Although electric polarization is screened by mobile charge carriers, we find clear signatures of the ferroelectric phase transition in the thermal-expansion coefficient $\alpha$ of the weakly doped metallic samples. Upon increasing $n$, the transition temperature $T_\mathrm{C}(n)$ and the magnitude of the anomalies in $\alpha$ rapidly decrease up to a threshold carrier density $n^\star$ above which broadened anomalies remain present. There is no indication for a sign change of $\alpha$ as is expected for a pressure-dependent quantum phase transition with $n$ as the control parameter. Thus, the ferroelectriclike transition is either continuously fading away or it transforms to another low-temperature phase above $n^\star$, but this change hardly affects the temperature-dependent $\alpha(T)$ data.

Improved TCO characteristics of ZnO:Si films via utilization of Si4+ ionized donor states and its application in n-SZO/p-Si heterojunction solar cells

Utilizing Si4+ ionized states providing two free electrons per atom, ZnO:Si (SZO) thin films have been grown as a superior transparent conducting oxide (TCO) material ideal for Si-based electronic devices. Energetically favoured substitutional doping of Zn2+ by Si3+ and Si4+ states facilitates increase in concentration (ne) and mobility (μ) of the charge carriers. At increased dopant incorporation at the interstitial sites Si2+ and Si1+ states are proportionally contributed which are not electronically active and act as scattering centres for the existing free electrons. Further, the formation of non-conducting SiOx bonds and their segregation at grain boundaries restrict increase in charge carrier concentration and reduce their mobility. The ideal dopant incorporation in ZnO:Si network accommodates ~1.6 ± 0.07 at.% Si. A minimal resistivity, (ρ)min ~7.16 × 10−5 Ωcm corresponding to the highest mobility, (μ)max ~85 V−1cm2s−1 and superior carrier density, (ne)max ~1.0 × 1021 cm−3 signify a highly degenerate semiconductor characteristic of the SZO films with reduced effective mass of electron, m* ~0.15 m0. Significantly high magnitude of Figure-of-Merit (FoM: Φ) ~9.7 × 10−2 Ω−1, even at moderately low growth temperature ~150 °C demonstrates excellent TCO characteristics of SZO films. The n-SZO/p-Si solar cells reveal increased VOC ~312 mV and JSC ~27.9 mA/cm2 via moderate widening in optical band gap and a significant increase in carrier concentration in the SZO component, respectively. The n-SZO/p-Si heterojunction solar cells with a FF ~0.499 and photovoltaic conversion efficiency η ~4.34% has been obtained with improved SZO layer grown by optimized doping via consumption of Si4+ donor states.

On the electric conduction of ZrO2 in the steady stage of flash sintering

The electric conductivity of zirconia in the steady stage of flash sintering is measured under various current densities. We find that the steady stage conductivity (σs) is higher than the normal conductivity (σn) measured at non-flashing conditions, even Joule heat is considered. The higher conductivity is ascribed to the increase in charge carrier concentration. The difference between σs and σn is then used to estimate the oxygen vacancy concentration. It is found that the concentration increases with increasing current density at beginning and then decreases with further increasing current density. A speculative explanation is proposed to account for this phenomenon.