Increase In Hole Concentration Research Articles

Effect of Ionizing Radiation on the Structure, Electro-Physical and Optical Characteristics of Cd1-xZnxTe Crystals

Based on X-ray studies, conditions for growth of high structural perfect Cd1-хZnxTe (0,02≤х≤0,1) crystalshave been optimized. In crystals obtained, changes in the structure, electrical parameters and optical transmissionat samples irradiation by g-, b- radiations were investigated. At 60Со source gamma-quanta irradiation with a dose of Ф≥ 105 Gy, a slight decrease of samples’ structure and optical transmission, an increase of holes concentration p and a decrease of charge carrier mobility m in p-type crystals was observed. Within 30-40 days, the values of p and m were relaxed to an initial ones. Changes in structural perfection, optical transmission and electricalparameters of samples irradiated by electrons were more significant.

Passivation of Deep-Level Defects by Cesium Fluoride Post-Deposition Treatment for Improved Device Performance of Cu(In,Ga)Se2 Solar Cells.

Heavy-alkali post-deposition treatments (PDTs) utilizing Cs or Rb has become an indispensable step in producing high-performance Cu(In,Ga)Se2 (CIGS) solar cells. However, full understanding of the mechanism behind the improvements of device performance by heavy-alkali treatments, particularly in terms of potential modification of defect characteristics, has not been reached yet. Here, we present an extensive study on the effects of CsF-PDT on material properties of CIGS absorbers and the performance of the final solar devices. Incorporation of an optimized concentration of Cs into CIGS resulted in a significant improvement of the device efficiency from 15.9 to 18.4% mainly due to an increase in the open-circuit voltage by 50 mV. Strong segregation of Cs at the front and rear interfaces as well as along grain boundaries of CIGS was observed via high-resolution chemical analysis such as atomic probe tomography. The study of defect chemistry using photoluminescence and capacitance-based measurements revealed that both deep-level donor-like defects such as VSe and InCu and deep-level acceptor-like defects such as VIn or CuIn are passivated by CsF-PDT, which contribute to an increased hole concentration. Additionally, it was found that CsF-PDT induces a slight change in the energetics of VCu, the most dominant point defect that is responsible for the p-type conductivity of CIGS.

Effect of nitrogen flow rate on the properties of nitrogen-doped Cu<sub>2</sub>O

Cuprous oxide (Cu₂O) has the potential applications in photovoltaic and photocatalytic fields. Nitrogen-doping in Cu₂O can improve the conductivity by increasing hole concentration. However, the chemical states of nitrogen in nitrogen-doped Cu₂O have not been studied thoroughly.A series of nitrogen-doped Cu₂O samples are prepared by increasing nitrogen flow rates and simultaneously keeping the sputtering pressure and other parameters unchanged. The samples are characterized by X-ray diffraction (XRD), step instrument, scanning electron microscope (SEM), energy dispersive spectrometer, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), spectrophotometer and Hall effect et al. The results of Hall effect measurement show that nitrogen-doping can increase the hole concentration by one order of magnitude. The band gap width decreases with the increase of nitrogen flow rate. The nitrogen in Cu₂O is in three forms: β-N (nitrogen atom), α-N₂(molecular nitrogen, —N=N—)and γ-N₂(molecular nitrogen, N≡N). With the increase of nitrogen gas flow rate, the peak of binding energy of β-N increases while that of α-N₂ decreases. The sample prepared at the nitrogen flow rate of 2.0 sccm has the minimum resistivity among all samples.

P -type codoping effect in (Ga,Mn)As: Mn lattice location versus magnetic properties

In the present work, we perform a systematic investigation on p-type codoping in (Ga,Mn)As. Through gradually increasing Zn doping concentration, the hole concentration increases, which should theoretically lead to an increase of the Curie temperature (TC) according to the p-d Zener model. Unexpectedly, although the film keeps its epitaxial structure, both TC and the magnetization decrease. The samples present a phase transition from ferromagnetism to paramagnetism upon increasing hole concentration. In the intermediate regime, we observe a signature of antiferromagnetism. By using channeling Rutherford backscattering spectrometry and particle-induced x-ray emission, the substitutional Mn atoms are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites, which explains the observed behavior. This is also consistent with first-principles calculations, showing that the complex of substitutional Zn and interstitial Mn has the lowest formation energy.

Effects on phase transition and thermoelectric properties in the Pb-doped GeTe-Bi2Te3 alloys with thermal annealing

The phase-transition behavior in GeTe-based alloys has been intensively studied recently for enhancing their thermoelectric performance. In this study, thermal annealing induced phase transition and corresponding effects on the thermoelectric properties of Pb-doped GeTe-Bi2Te3 alloys were systematically studied. By Pb doping, the lattice thermal conductivity of (GexPb1-xTe)0.93(Bi2Te3)0.07 alloys were significantly reduced due to the strong phonon scattering from point defects and stacking faults, resulting in a peak thermoelectric figure of merit ZT of ∼1.47 at 673 K for the (Ge0.84Pb0.16Te)0.93(Bi2Te3)0.07. By thermal annealing, the structure of GeTe-Bi2Te3 alloys can be shifted from pseudo-cubic to rhombohedral phase, and corresponding induced lattice distortion and microstructures can further reduce the lattice thermal conductivity. The thermal annealing can also lead to the increase of hole concentration and carrier mobility of the GeTe-based alloys. Our results proved the significance of thermal annealing treatment for tuning the microstructures and thermoelectric properties in the GeTe based alloys with phase-transition behavior.

Enhanced thermoelectric properties of Bi2Sr2Co2Oy by alkali metal element doping and SiC dispersion

Bi2Sr2Co2Oy (BSCO) is a layered cobalt-based oxide whose thermoelectric properties have been extensively studied recently because its layer structure of CoO2 gives good electrical conductivity. SiC, with larger Seebeck coefficients and higher electrical conductivity, is also widely concerned in the field of thermoelectricity. In this paper, the effects of alkali metal element doping and SiC dispersion on thermoelectric properties of BSCO are studied for two series samples of Bi1·85Na0·15Sr2Co2Oy + m wt% SiC (m = 0, 0.025, 0.05, 0.1, and 0.15) and Bi1·9K0·1Sr2Co2Oy + z wt% SiC (z = 0, 0.025, 0.05, 0.1, and 0.15). After Na+ and K+ replacing Bi3+ sites, the electrical resistivity of BSCO is reduced by the increased hole concentration. On the basis of the optimized Na+ and K+ doping, nano-SiC particles are dispersed on the BSCO matrix, which increases the phonon scattering at the interface and greatly reduces the thermal conductivity. Meanwhile, the Seebeck coefficients of Bi2Sr2Co2Oy materials are basically unchanged under the combined action of cation doping and SiC dispersion. The maximum ZT values of 0.26 and 0.24 at 923 K are obtained for m = 0.025 and z = 0.05 Bi2Sr2Co2Oy-based samples, respectively.

Solution‐processed copper (I) thiocyanate (CuSCN) for highly efficient CdSe/CdTe thin‐film solar cells

AbstractSolution‐processed CuSCN serving as hole transport, electron reflecting layer (HTL, ERL) and Cu dopant source for CdSe/CdTe thin‐film solar has demonstrated high power conversion efficiency (PCE) of ~17%. Two types of solvent, diethyl sulfide (DES) and aqueous ammonia (NH4OH), are explored to deposit CuSCN on CdTe, and both can enhance the performance of CdSe/CdTe solar cells. However, NH4OH solvent is less toxicity, leading to a smoother surface than DES solvent, enabling the deposition of ultra‐thin CuSCN layer and avoiding the high cost of DES. Temperature‐dependent current‐voltage (J‐V‐T) and capacitance‐voltage (C‐V‐T) measurements reveal that the use of CuSCN HTL increases hole concentration in CdTe absorber and significantly reduces back‐contact barrier height. High power conversion efficiency is achievable with the optimal thickness of the CuSCN layer. Our results demonstrate solution‐processed CuSCN HTL for enhancing the efficiency and reducing the cost of CdTe thin‐film solar cells.

Thermoelectric properties of Mn doped BiCuSeO

BiCuSeO is a promising thermoelectric material having earth-abundant non-toxic constituents and favourable thermoelectric properties like ultra-low thermal conductivity. In this study, Mn+2 has been introduced at the Bi+3 site to increase hole concentration as well as Seebeck coefficient, through aliovalent doping and magnetic impurity incorporation respectively. Samples were prepared through two-step solid state synthesis with the composition Bi1-xMnxCuSeO (x = 0.0, 0.04, 0.06, 0.08, 0.10 and 0.12). X-ray diffraction patterns confirmed the tetragonal (space group: P4/nmm) crystal structure of BiCuSeO as well as phase purity of the samples. The Seebeck coefficient and electrical resistivity had a decreasing trend with increasing doping fraction owing to the generation of charge carriers. The samples with x = 0.04 and 0.06 showed temperature independent Seebeck coefficient above 523 K, which is a signature of small polaron hopping. While the Seebeck coefficient of the samples with x = 0.08, 0.10 and 0.12 increased above 523 K due to the combination of localized and extended states. The thermal conductivity was dominated by the lattice part of the thermal conductivity. As a result of moderate Seebeck coefficient and low electrical resistivity, the highest power factor of 0.284 mW m−1-K2 was obtained for the Bi0.92Mn0.08CuSeO at 773 K, leading to a maximum zT of 0.4 at 773.

Effects of Cu–Zn phases on electronic properties in ZnO:Cu films

AbstractTheory predicts Cu‐doped ZnO (ZnO:Cu) has p‐conductivity; however, this has only been demonstrated in a small number of experimental and mechanistic studies. In this paper, ZnO:Cu films were grown in situ with varying Cu content, prepared using radiofrequency atomic source–assisted molecular‐beam vapor deposition. The results indicate that ZnO:Cu films with dopant of Cu2+ only had n‐type behavior. As the Cu content increased, Cu+ was the major dopant and the ZnO:Cu films had p‐type behavior. However, excess Cu dopant resulted in the formation of second phases of Cu2O and Cu–Zn. The formation of a Cu–Zn phase increased the content of Zn vacancy, thus increasing hole concentration. Stronger alloy scattering decreased carrier mobility. Therefore, Cu+ dopant and Zn vacancy give ZnO:Cu films p‐conductivity properties.

Effects of p-layer hole concentration and thickness on performance of p-i-n InGaN homojunction solar cells

In this paper, the effects of p-layer hole concentration and p-layer thickness on the performances of InGaN p-i-n homojunction solar cells with different indium components and their intrinsic mechanisms are investigated by numerical simulations. it is found that the conversion efficiency of solar cells first increases and then decreases slightly with the increase of p-layer hole concentration and p-layer thickness. Moreover, the change of p-layer hole concentration and p-layer thickness will cause great changes of the conversion efficiency of the solar cells, especially as the indium composition increases. In order to better clarify and understand the physical mechanism of this phenomenon, the collection efficiency, <i>I</i>-<i>V</i> characteristic, built-in electric field and carrier transport of solar cells are analyzed in this paper. When the hole concentration is insufficient, the build-in electric filed is not strong enough to separate the most of the electric-hole pairs. This will reduce the collection efficiency. In addition, the lower the hole concentration, the higher the series resistance of solar cells will be and the more the power loss. So a conclusion can be drawn that the lower hole concentration of p-layer would be accompanied by the reduction of collection efficiency and the increase of series resistance, thus resulting in a lower conversion efficiency. With the increase of the hole concentration which is below an optimal value, the built-in electric field reaches the threshold, which can improve the collection efficiency. At the same time, although the series resistance is reduced to a certain extent, it still reduces the effective output power and limits the conversion efficiency. When the hole concentration is higher than the optimal value, the carrier mobility becomes the main factor limiting the conversion efficiency. As for the p-layer thickness, the simulation results indicate that the lateral transport of carriers from the p-layer to the anode electrodes becomes more obstructive with the thinning of p-layer thickness. This is because when the p-layer thickness decreases, thus causing the p-layer sectional area to decrease, the lateral series resistance becomes higher. It is clear that when the p-layer is too thin, the lateral series resistance is one of the main limiting factors affecting the conversion efficiency of solar cells.

Investigation of Capping Layer on Characteristics of Poly-GeSn Junctionless p-Channel Thin Film Transistors

Impact of SiO2 capping layer on the quality of poly-GeSn film and characteristics of bottom-gate junctionless (JL) p-channel thin films transistors (P-TFTs) were studied. From the analysis results of X-ray diffraction (XRD) and Hall-effect measurement, the amorphous GeSn film starts to crystallize at 450°C while the crystallinity enhances at 500°C by employing rapid thermal annealing (RTA) as the film crystallization process. With capping layer on the whole region, the hole concentration becomes too high to turn the JL P-TFTs off. However, by limiting capping layer on source/drain region to increase hole concentration, total series resistance is reduced due to lower sheet resistance and contact resistance, which consequently improves the drive current, ION/IOFF ratio and field-effect mobility. Based on the best process conditions and structure to form the poly-GeSn JL P-TFTs, ION/IOFF ratio up to 5.6 × 102 are achieved while peak field-effect mobility of 20.8 cm2/Vs is obtained. Moreover, the low thermal budget process of 500°C for 30 sec enables next-generation monolithic 3D-ICs.

Thermoelectric properties of Pb and Na dual doped BiCuSeO

BiCuSeO is a promising thermoelectric material not only because of its good thermoelectric properties, but also earth abundant constituents. In this report, Pb and Na have been simultaneously doped at the Bi site of BiCuSeO. Doping Pb is beneficial for the Seebeck coefficient whereas doping Na maintains the hole mobility. Both the dopants increase the carrier concentration and reduce the thermal conductivity by point-defect scattering. The samples with nominal composition Bi0.985-xNa0.015PbxCuSeO (x=0.00, 0.04, 0.06 and 0.08) were prepared using two-step solid-state synthesis. The X-ray diffraction pattern reveals a small amount of Bi2O2.5 phase (<1 vol. %) which is responsible for adversely affecting the electrical conductivity of all the samples. Both the Seebeck coefficient and electrical resistivity decrease with increasing doping fraction due to increasing hole concentration. The highest power factor of 530 μW/mK2 was obtained for Bi0.905Na0.015Pb0.08CuSeO sample at 773 K because of moderate Seebeck coefficient and low electrical resistivity. A low lattice thermal conductivity of 0.37 W/m-K at 773 K was obtained in the Bi0.905Na0.015Pb0.08CuSeO. Due to this low lattice thermal conductivity combined with the high power factor, a zT of 0.63 was obtained for the Bi0.905Na0.015Pb0.08CuSeO sample at 773 K.

Influence of Ba2+ doping on the thermoelectric properties of BiCuSeO fabricated by spark plasma sintering

We studied the influence of Ba2+ doping on the thermoelectric properties of the p-type Bi1–xBaxCuSeO (0 ≤ x ≤ 0.21) fabricated by spark plasma sintering. The substitution of Ba2+ for Bi3+ gradually increased the electrical and thermal conductivities and decreased the Seebeck coefficient, which were due to the increased hole concentration. The largest value of dimensionless figure-of-merit (0.57) was obtained for the Bi0.86Ba0.14CuSeO at 500 °C, which was over three times greater than that of pristine BiCuSeO (0.18) at 500 °C. We believe that the thermoelectric properties of BiCuSeO were substantially enhanced through the partial substitution of Ba2+ for Bi3+.

Behavior of indium alloying with Cu2ZnSn(S,Se)4 and its effect on performances of Cu2ZnSn(S,Se)4-based solar cell

P-type solid solutions of indium (In) in kesterite Cu2ZnSn(S,Se)4 films (CZTSSe(In)) with In contents of 0–19.34 at% were prepared by In alloying with the kesterite Cu2ZnSn(S,Se)4 (CZTSSe) through a solution approach. It is found that the In substitutes for Sn to form InSn acceptor defect in the CZTSSe(In) in the In content range of 0–1.15 at%, and for Cu and Sn to form InCu-InSn donor complex or/and for a small number of Zn to form InZn donor defects in the range of 6.34–19.34 at%. The hole concentration of the CZTSSe(In) increases with increasing In content in the range of 0–1.15 at%, but decreases in the range of 6.34–19.34 at%. The bandgap of the CZTSSe(In) increases from 1.040 eV for CZTSSe to 1.083 eV for CZTSSe(In) with 19.34 at% In. Three solar cells with traditional structure and power conversion efficiency (PCE) of 3.52, 3.24 and 1.67% were fabricated by using the CZTSSe and CZTSSe(In) with In content of 0.67 and 12.27 at% as absorbers, respectively. It is found that the effect of the bandgap on the open-circuit voltage (Voc) is little compared to the hole concentration. The increased hole concentration enhances the Voc but decreases the photogenerated current density (Jph) and reverse saturation current density (J0). The effect of In alloying on the PCE of the CZTSSe(In)-based solar cell is determined by the effects of the hole concentration on Voc, Jph and J0. The influence mechanism of the In alloying on the properties of the CZTSSe and performance of the CZTSSe-based solar cell are discussed in the present work.

Simultaneously enhanced power factor and phonon scattering in Bi0.4Sb1.6Te3 alloy doped with germanium

The thermoelectric properties of Ge-doped Bi0.4Sb1.6−xGexTe3 (x = 0, 0.01, 0.015, 0.02) were investigated in the temperature range from 300 K to 525 K. The results show that Ge doping brings about 13–52% increase in electrical conductivity due to the increase of hole concentration, and 5–11% decrease in thermal conductivity owing to enhanced phonon scattering by the dopant atoms. As a result, a maximum thermoelectric figure of merit (ZT) reaches 1.48 at 350 K for Bi0.4Sb1.59Ge0.01Te3, which is ~25% larger than that of Bi0.4Sb1.6Te3. Present results demonstrate that Ge doping is an effective way to enhance the thermoelectric performance of Bi0.4Sb1.6Te3 alloy.

Enhanced thermoelectric performance of Bi0.4Sb1.6Te3 based composites with CuInTe2 inclusions

Bi0.4Sb1.6Te3 (BST) based composites incorporated with CuInTe2 particles have been prepared and their thermoelectric properties were studied in the temperature range from 300 to 500 K. The results indicate that at 300 K the incorporation of 0.2 wt% CuInTe2 in BST causes 14% increase in power factor (PF) and 38% reduction in lattice conductivity simultaneously. As a result, a maximum ZT = 1.45 at 400 K is achieved in the composite sample with f = 0.2 wt%, which is ∼45% larger than that of BST studied here. The enhancement of PF originates from the increase of both electrical conductivity due to increased hole concentration and thermopower via energy filtering effect. While, the reduction of the lattice thermal conductivity could be ascribed to enhanced phonon scattering at the interfaces and CuInTe2 inclusions in the composite samples. Present results demonstrate that incorporation of a proper amount of CuInTe2 in BST is an effective way to improving thermoelectric performance of BST alloys.

Charge Transport in Thermoelectric SnSe Single Crystals

SnSe has attracted increasing attention as a promising thermoelectric material. In this work, a horizontal vapor transfer method was developed to synthesize high-quality, fully dense, and stoichiometric SnSe single crystals, which enables an evaluation of the transport properties inherent to SnSe along the bc-plane. The electronic transport properties can be well-understood by a single parabolic band model with acoustic phonon scattering, enabling insights into the fundamental material parameters determining the electronic properties. The lattice thermal conductivity (κl) decreases from 2.0 W m–1 K–1 at 300 K to 0.55 W m–1 K–1 at 773 K. It is revealed that an increase in hole concentration, an involvement of low-lying bands for transport, and a further reduction in κl would all enable p-type SnSe to be a promising eco-friendly thermoelectric material. This work not only provides a fundamental understanding of the charge transport but also guides the further improvement of thermoelectric SnSe.

Formation of Acceptor States on the Silicon Hydroxylated Surface Upon NO2 Molecules Adsorption

It is shown that the hypothesis of reactivation of subsurface boron atoms in porous silicon (PS) by NO2 molecules at their adsorption on the surface dangling bonds cannot explain the reversible increase of free holes concentration in n‐type PS and in p–n junctions with oxidized surface. The alternative model of hole conduction emergence in silicon structures upon adsorption of NO2 molecules is proposed. Density functional theory (DFT) calculations show that fixing NO2 molecules on the hydroxyl groups of silicon or its oxide surface can produce acceptor states and, therefore, free holes.

Observation of momentum-dependent charge excitations in hole-doped cuprates using resonant inelastic x-ray scattering at the oxygen K edge

We investigate electronic excitations in La2-x(Br,Sr)xCuO4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the dispersive mode is ascribed to intraband charge excitations which have been observed in electron-doped cuprates.

A study of S doped ZnSb

We report on S-doping of ZnSb for S concentrations ranging from 0.02at% to 2.5at%. There are no previous reports on S-doping. ZnSb is a thermoelectric material with some advantages for the temperature range 400K–600K. The solid solubility of S in ZnSb was estimated to be lower than 0.1% from observations of precipitates by scanning microscopy. Hall and Seebeck measurements were performed as a function of temperature from 6K to 623K. The temperature dependence of the electrical properties suggests that S introduces neutral scattering centers for holes in the p-type material. An increase in hole concentration by S is argued by defect reactions involving Zn vacancies.