Parameterization of CuIn1−xGaxSe2 (x = 0, 0.5, and 1) energy bands

Parameterization of the electronic band structure of CuIn_1−xGa_xSe₂ (x = 0, 0.5, and 1) demonstrates that the energy dispersions of the three uppermost valence bands [E_j(k); j = v1, v2, and v3] are strongly anisotropic and non-parabolic even very close to the Γ-point valence-band maximum E_v1(0). Also the lowest conduction band E_c1(k) is anisotropic and non-parabolic for energies ~ 0.05 eV above the band-gap energy. Since the electrical conductivity depends directly on the energy dispersion, future electron and hole transport simulations of CuIn_1−xGa_xSe₂ need to go beyond the parabolic approximation of the bands. We therefore present a parameterization of the energy bands, the k-dependency of the effective electron and hole masses m_j(k), and also an average energy-dependent approximation of the masses m_j(E).     

Preparation and characterization of CuIn1 − xGaxSe2 − ySy thin film solar cells by rapid thermal processing

This paper describes the synthesis and characterization of CuIn_1 − xGa_xSe_2 − yS_y (CIGSeS) thin-film solar cells prepared by rapid thermal processing (RTP). An efficiency of 12.78% has been achieved on ~ 2 µm thick absorber. Materials characterization of these films was done by SEM, EDS, XRD, and AES. J-V curves were obtained at different temperatures. It was found that the open circuit voltage increases as temperature decreases while the short circuit current stays constant. Dependence of the open circuit voltage and fill factor on temperature has been estimated. Bandgap value calculated from the intercept of the linear extrapolation was 1.1-1.2 eV. Capacitance-voltage analysis gave a carrier density of 4.0 × 10¹⁵ cm^− 3.     

Sub-Gap Modulated Photo Current Spectroscopy performed on Cu(Inx,Ga1 − x)(Sey,S1 − y)2 based solar cells

Sub-Gap Modulated Photo Current Spectroscopy (SGMPCS) is an excellent tool in order to investigate the band gap defect density of the absorber layer, directly on Cu(In_x,Ga_1 − x)(Se_y,S_1 − y)₂ (CIGSS) based solar cells. This technique is essentially sensitive to defect states located in the absorber layer, which has the lowest band gap of the heterojunction solar cell. It allows the determination of the σ · N(E) product, where σ is the defect Optical Cross Section (OCS) and N(E) is its Density Of States (DOS).We have developed an analytical model, allowing to derive the above product from the imaginary part of the ac photocurrent of the solar cell, under reverse applied dc bias. We have then applied this model to study the defect density of the co-evaporated CIGS (i.e. y = 1) absorber layer of a heterojunction solar cell. Two different defect distributions have been exhibited by SGMPCS, the properties of which vary with thermal annealing.Correlation with Admittance Spectroscopy allows us to derive an estimation of the defect OCS.     

One-step synthesis and double emission (Ca1 + x − yEuy)Ga2S4 + x phosphor for white LEDs

(Ca_{1 + x − y}Eu_y)Ga₂S_4 + x phosphors have been synthesized one step by solid state reaction. The photoluminescence excitation and emission spectra of phosphors have been studied; the influence of host composition and Eu²⁺ concentration on emission spectra has also been investigated. The emission spectrum consists of yellow emission at 550 nm and red emission at 650 nm. It also indicates that the excitation spectrum is a broadband and can be well matched with the emission of GaN chip. Combined these phosphors with 460 nm-emitting GaN chips, White LEDs have been fabricated. Their electroluminescence spectra have been measured under 20 mA forward-bias current. Their CIE chromaticity coordinates and color temperature indicate that (Ca_{1 + x − y}Eu_y)Ga₂S_4 + x phosphors are promising phosphors for GaN-based white LEDs.     

Steady state and transient photoconductivity measurements in a-Se90 − xSb10Inx thin films

Amorphous thin films of Se_90 − xSb₁₀In_x (0 ≤ x ≤ 15) have been prepared by electron beam evaporation method. The steady state and transient photoconductivity measurements on the thin films of Se_90 − xSb₁₀In_x (0 ≤ x ≤ 15) were carried out at different levels of light intensities (500 lx-5000 lx) at room temperature (301 K). The plot of photocurrent (I_ph) versus light intensity (F) follows a power law I_ph ∝ F^γ. The value of exponent γ lies between 0.5 and 1.0, which indicates there exists a continuous distribution of localized states in the mobility gap of Se_90 − xSb₁₀In_x (0 ≤ x ≤ 15) thin films. For transient photoconductivity, when the samples were illuminated with light, the photocurrent reaches the maximum value during the first 5 s of exposure time and thereafter, it starts decreasing and becomes stable after 15 min of exposure. This kind of phenomenon is termed as photo-degradation of photocurrent. The results have been explained on the basis of charged defect model and the intercluster interaction model. The magnitude of photocurrent of the system a-Se₇₅Sb₁₀In₁₅ is higher than the parent system a-Se₉₀Sb₁₀. The photosensitivity shows a minimum value at 5 atomic percentage of indium (In) concentration, which is explained based on chemically ordered network model and the topological model.     

Influence of laser-irradiation on the optical constants Se75S25 − xCdx thin films

Amorphous thin films of glassy alloys of Se₇₅S_25 − xCd_x (x = 2, 4 and 6) were prepared by thermal evaporation onto chemically cleaned glass substrates. Optical absorption and reflection measurements were carried out on as-deposited and laser-irradiated thin films in the wavelength region of 500-1000 nm. Analysis of the optical absorption data shows that the rule of no-direct transitions predominates. The laser-irradiated Se₇₅S_25 − xCd_x films showed an increase in the optical band gap and absorption coefficient with increasing the time of laser-irradiation. The results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. The value of refractive index increases decreases with increasing photon energy and also by increasing the time of laser-irradiation. With the large absorption coefficient and change in the optical band gap and refractive index by the influence of laser-irradiation, these materials may be suitable for optical disc application.     

CuAl1 − xInxSe2 solid solutions: Dielectric function and inter-band optical transitions

The dielectric function of bulk CuAl_1 − xIn_xSe₂ with composition x varying from x = 0.07 to x = 0.6 were studied over the photon energy region 1.0-6.0 eV at room temperature by spectroscopic ellipsometry. Information on the inter-band optical transitions was obtained from the results of the standard critical point analysis of the obtained dielectric function. With increasing Indium content, all spectral features of the obtained dielectric functions were found to gradually shift towards lower energies. The details of this shift for each critical point retrieved from the obtained dielectric function were disclosed. A compositional dependence of the optical transitions in Γ point of the Brillouin zone was verified to be strong. Such dependence for N and T points turned out to be weak by comparison. The later fact was accounted for a small compositional shift of the conduction band states in N and T points as compared to Γ point.     

Quantum-confinement effect on recombination dynamics and carrier localization in cubic InN and InxGa1 − xN quantum boxes

In this study, the quantum confinement effect on recombination dynamics and carrier localization in cubic InN (c-InN) and cubic In_xGa_1 − xN (c-In_xGa_1 − xN) low dimensional structures are theoretically examined. The small InN and In-rich In_xGa_1 − xN low dimensional structures show a strong quantum confinement effect, which results in ground states away from the band edge and discrete eigen-states. Depending on composition, temperature, and size of the InN and In_xGa_1 − xN low dimensional structures, quantum confinement effect can affect the exciton dimensions (D). In InN quantum cubes, the strong quantum confinement effect leads to temperature-dependent radiative lifetimes showing a large size variation. The nearly-temperature-independent and shorter radiative lifetimes in small InN and In-rich In_xGa_1 − xN low dimensional structures suggest that the strong quantum confinement leads to 0 D carrier confinement, stronger carrier localization, and high recombination efficiency. Reported radiative lifetimes were found to match very well with our simulation results of In-rich quantum cubes, which indicates that spontaneous emissions come from the radiative recombination of localized excitons in In-rich In_xGa_1 − xN clusters. The simulation results could provide important information for the designs and interpretations of c-InN and c-In_xGa_1 − xN devices.     

Synthesis and magnetic properties of layered MnPSxSe3−x (0 < x < 3) and corresponding intercalation compounds of 2,2′-bipyridine

In this work, we synthesize a series of new MnPS_xSe_3−x (0 < x < 3) compounds by high temperature solid-state reaction and also obtain the corresponding intercalation compounds (Mn_1−yPS_xSe_3−x(bipy)_4y, x = 1.2, 1.8 and 2.4) via the intercalation of 2,2′-bipyridine with MnPS_xSe_3−x. XRD results confirm that MnPS_xSe_3−x compounds show the layered structure and can be regarded as the solid solution of MnPS₃ and MnPSe₃. Magnetic measurements indicate that MnPS_xSe_3−x compounds exhibit paramagnetism with negative Weiss constant in the paramagnetic temperature region, and an antiferromagnetic phase transition occurs at the Neel temperature. It is found that the magnetic properties of MnPS_xSe_3−x slab are dramatically changed after the intercalation of 2,2′-bipyridine, which is close related to the relative ratio of S and Se atom as well as the intralayered Mn²⁺ vacancies of MnPS_xSe_3−x slab.     

Synthesis of Zn1 − xCdxO bramble-like nanostructures

Ternary Zn_1 − xCd_xO bramble-like nanostructures with a Cd incorporation of about 6.7 at.% were produced onto Au-catalyzed Si substrate by thermal evaporation of Zn and Cd. The X-ray diffraction (XRD) analysis showed that the existence of lattice expansion in the c-axis orientation. The ultra-violet (UV) near-band-edge (NBE) emission of the Zn_1 − xCd_xO nanobrambles was red-shifted from 369 nm (3.37 eV) to 397 nm (3.13 eV) due to Cd substitution. The oxygen partial pressure was deemed as the critical experimental parameter for the formation of the bramble-like Zn_1 − xCd_xO nanostructures.     

Calorimetric and optical study of amorphous Se85 − xTe15Bix glassy alloy

Bulk samples of Se_85 − xTe₁₅Bi_x (where x = 0, 1, 2, 3, 4, 5) glassy alloys are obtained by melt quenching technique. Differential scanning calorimetric (DSC) technique has been applied to determine the thermal properties of Se-rich Se_85 − xTe₁₅Bi_x glassy alloys in the glass transition and crystallization regions at four heating rates (5, 10, 15, 20 K min^− 1). The glass transition temperature (T_g) and peak crystallization temperature(T_p) are found to shift to a higher temperature with increasing heating rate. With Bi addition, the value of (T_g)increases. (T_p) is found to increase as Bi is introduced to the Se-Te host, however further increase in Bi concentration is responsible for the reduction of. Thin film of bulk samples are deposited on glass substrate using thermal evaporation technique under vacuum for optical characterization. Optical band gap is estimated using Tauc's extrapolation and is found to decrease from 1.46 to 1.24 eV with the Bi addition.     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.     

Study of flash evaporated CuIn1 − xGaxTe2 (x = 0, 0.5 and 1) thin films

CuIn_1 − xGa_xTe₂ thin films with x = 0, 0.5 and 1, have been prepared by flash evaporation technique. These semiconducting layers present a chalcopyrite structure. The optical measurements have been carried out in the wavelength range 200-3000 nm. The linear dependence of the lattice parameters as a function of Ga content obeying Vegard's law was observed. The films have high absorption coefficients (4 · 10⁴ cm^− 1) and optical band gaps ranging from 1.06 eV for CuInTe₂ to 1.21 eV for CuGaTe₂. The fundamental transition energies of the CuIn_1 − xGa_xTe₂ thin films can be fitted by a parabolic equation namely E_g1(x) = 1.06 + 0.237x − 0.082x². The second transition energies of the CuInTe₂ and CuGaTe₂ films were estimated to be: E_g2 = 1.21 eV and E_g2 = 1.39 eV respectively. This variation of the energy gap with x has allowed the achievement of absorber layers with large gaps.     

Performance study of InAs/GaAs quantum dot covered by graded InxGa1 − xAs layer

InAs/GaAs quantum dots (QDs) with graded In_xGa_1 − xAs strained-reducing layer (SRL) are grown by metal-organic chemical vapor deposition, the effects of Indium (In) composition and thickness in In_xGa_1 − xAs on QD morphological characteristics and optical properties are investigated. Compared with In_xGa_1 − xAs SRL with fixed In content, gradient In_xGa_1−xAs SRL can further improve the growth quality of InAs QDs, enhance luminescence intensity and extend emission spectrum toward longer wavelength.     

Effect of GaAs substrate misorientation on InxGa1 − xAs crystalline quality and photovoltaic performance

This paper presents the quality of In_xGa_1 − xAs (0 < x < 0.2) layers grown on GaAs substrate with different miscut angle (2° and 15°) by metal organic chemical vapor deposition. The crystalline quality of In_xGa_1 − xAs layers was found to strongly depend on indium content and substrate misorientation. The In_0.16Ga_0.84As solar cells with PN structure were grown on a 2°- and 15°-off GaAs substrates. It was found that the photovoltaic performance of In_0.16Ga_0.84As solar cell grown on 2°-off GaAs substrate was better than that of In_0.16Ga_0.84As grown on a 15°-off GaAs substrate, because the In_xGa_1 − xAs films grown on 15°-off GaAs substrate shows a highly strain relaxation in active layer of solar cell, which causes the high dislocation density at the initial active layer/In_xGa_1 − xAs graded layer interface.     

Impurity induced band-gap narrowing in p-type CuIn1 − xGax(S,Se)2

Green's functions modelling of the impurity induced effects in p-type CuIn_1 − xGa_xS₂ and CuIn_1 − xGa_xSe₂ (x = 0.0, 0.5, and 1.0) reveals that: (i) the critical active acceptor concentration for the metal non-metal transition occurs at N_c ≈ 10¹⁷-10¹⁸ cm^− 3 for impurities with ionization energy of E_A ≈ 30-60 meV. (ii) For acceptor concentrations N_A > N_c, the hole gas of the metallic phase affects the band-edge energies and narrows the energy gap E_g = E_g⁰ − ΔE_g. The energy shift of the valence-band maximum ΔE_v1 is roughly twice as large as the shift of the conduction-band minimum ΔE_c1. (iii) ΔE_v1 depends strongly on the non-parabolicity of the valence bands. (iv) Sulfur based compounds and Ga-rich alloys have the largest shifts of their band edges. (v) A high active acceptor concentrations of N_A = 10²⁰ cm^− 3 implies a band-gap narrowing in the order of ΔE_g ≈ 0.2 eV, thus E_g = E_g⁰ − 0.2 eV, and an optical band gap of E_g^opt ≈ E_g⁰ − 0.1 eV.     

Towards an electronic model for CuIn1 − xGaxSe2 solar cells

We model some aspects of highly efficient CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3 as well as wide band gap cells with x = 1 and ask for the dominant recombination mechanism which limits the V_oc of these devices. For CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3, interface recombination combined with Fermi-level pinning is a possible but unlikely recombination mechanism. We argue that these cells are rather limited by recombination in the quasi-neutral region (QNR) including the back contact. Using the expression for the QNR recombination rate we calculate the derivative of the collection function in the absorber at the space charge region edge which is in reasonable agreement with the experiment. It turns out that the diffusion length must approximate the absorber thickness. Based on this information, we draw a band diagram for a CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3 and plot the simulated collection function. For cells with x = 1 (Cu-poor CuGaSe₂), the experimental activation energy of the recombination rate mostly equals the absorber band gap, i.e. E_a ≈ E_g,a = 1.67 eV. As the experimental interface band gap is smaller than E_a, interface recombination must be ruled out. Thus, the carrier lifetime in the Cu-poor CuGaSe₂ absorber should be so small that bulk recombination is more efficient than interface recombination. From this consideration, we postulate an electron lifetime value of 10⁻¹² s for CuGaSe₂.     

Compositional dependence on the optical properties of amorphous As2 − xS3 − xSbx thin films

In this paper, we report results of the optical properties of thermally deposited As_2 − xS_3 − xSb_x thin films with x = 0.02, 0.07, 0.1 and 0.15. We have characterized the deposited films by Fourier Transform Infrared, Raman and X-ray photoelectron spectroscopy (XPS). The relationship between the structural and optical properties and the compositional variation were investigated. It was found that the optical bandgap decreases with increase in Sb content. The XPS core level spectra show a decrease in As₂S₃ percentage with increase in Sb content. This is confirmed from the shifting of the Raman peak from AsS₃ vibrational mode towards SbS₃ vibrational mode.     

Tuning the optical properties of alloyed CdSexS1 − x nanoparticles by changing the constituent stoichiometry

The spherical particles CdSe_xS_1 − x with 30-80 nm in radius have been successfully prepared by the hydrothermal reaction at 200 °C. The structure characterization which has been carried out using X-ray diffraction (XRD) shows hexagonal crystal structure. Novel properties have been observed via UV-visual absorption spectra and photoluminescence (PL) spectra. The absorption shoulder and the luminescence emission peaks have been tuned by changing the mole ratio of Se in the CdSe_xS_1 − x samples.     

Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.