77 Search Results

The performance of conventional p-type CdTe solar cells has plateaued in recent years, motivating exploration of n-type absorbers. Here, we evaluate a homojunction solar cell employing iodine-doped CdTe (CdTe:I) as the absorber in a Ti₃C₂T_x MXene/p-CdTe:As/n-CdTe:I/indium structure through SCAPS-1D simulations and prototype devices. Optimized simulations predict efficiencies above 25% for thin CdTe:I absorbers (~0.7 μm). In contrast, the first prototype achieved only ~1.36% efficiency with V_OC = 0.48 V, J_SC = 6.45 mA/cm² and FF = 43.8%. When the simulation is adjusted to match the actual device structure, and the effective illumination is reduced to account for front-side light loss,more » the predicted V_OC (0.48 V) and JSC (6.74 mA/cm²) closely reproduce the experimental values. This suggests that the device performance is primarily limited by reduced front-side photon transmission and other material non-idealities. These results highlight the promise of iodine-doped n-type CdTe and identify clear pathways for further efficiency improvement.« less

Chromium (Cr) is a common impurity in β-Ga₂O₃ crystals, where its characteristic R1 and R2 luminescence lines are susceptible to both the host crystal field and externally applied fields. Here, in this work, we demonstrate that the Cr cathodoluminescence (CL) quenches toward the bulk of the crystal but enhances with applied reverse bias, reflecting the effect of free carrier depletion with increasing electric field. Furthermore, we illustrate that the R1/R2 CL intensity ratio, measured as the integrated area ratio of R1 to R2, can be used as a direct probe of the electric field in a Ni-β-Ga₂O₃: Cr Schottky diode.more » This optical calibration method provides a complementary approach to conventional C–V and I–V measurements for determining electric field strength in the depletion region of β-Ga₂O₃-based Schottky diodes and can be extended to other semiconductors and multilayer device structures.« less

Monoclinic gallium oxide (β-Ga₂O₃) single crystals have a Raman mode at ∼250 cm⁻¹ that is strongly correlated with free-electron density. Prior work attributed this peak to an electronic excitation of a shallow donor impurity band. However, heavily n-type thin films grown by metalorganic chemical vapor deposition or molecular beam epitaxy do not have the peak. In the present work, an alternate model is proposed: the 250 cm⁻¹ Raman peak arises from Ga clusters, defined as two or more Ga atoms that form Ga–Ga bonds. Raman mapping reveals variations in the frequency that are consistent with a distribution of cluster sizes.more » The intensity of the peak decreases as the temperature is raised, attributed to melting of the Ga clusters. First-principles calculations indicate that the 250 cm⁻¹ mode is due to Ga–Ga bond-stretching vibrations. As the Fermi energy is raised, the formation of Ga–Ga dimers becomes energetically favorable, explaining the correlation between n-type conductivity and the appearance of the Raman peak.« less

Only a handful of high-halide salt waste forms have been demonstrated for vitrification-based immobilization strategies for halide-salt nuclear waste streams (e.g., pyroprocessing wastes, molten salt reactor wastes) and they all have low waste loading potential and most have low chemical durabilities for high-alkali streams. An alternative approach to direct salt immobilization is salt partitioning prior to waste form fabrication and one option for partitioning is halide removal (called dehalogenation). Removing the halogen fraction through dehalogenation can significantly reduce the waste volume required for disposal in the primary waste form. Furthermore, when dehalogenation is performed using organic acids, the dehalogenation reagentmore » can decompose during high-temperature vitrification, reducing waste loading limitations in the waste form. In the current work, different organic acids (i.e., oxalic, formic, acetic, oxamic, and citric) were evaluated for dehalogenation efficiency of a simple chloride salt simulant (7.19% LaCl₃, 53.77% LiCl, and 39.04% KCl, by mole) and a more complex chloride salt simulant called ERV3 (electrorefiner version 3) at 150 °C–300 °C and using H⁺/Cl^–molar ratios of 1:1, 2:1, and 3:1. Additionally, a borosilicate glass waste form called TARS (or the average of refined specifications) was formulated, produced, and characterized for dehalogenated ERV3.« less

This study systematically documents defect levels in n-type iodine-doped cadmium telluride (CdTe:I) crystals as measured by thermoelectric effect spectroscopy, Hall-effect measurements, and photoluminescence and calculated by density functional theory. The primary donor, I_Te, was identified with an activation energy of ∼0.05 eV (measured)/∼0.13 eV (calculated). Deep acceptor states, V_Cd and I_Te–V_Cd complex, exhibited activation energies of ∼0.1 eV (measured)/∼0.19 eV (calculated) and ∼0.12 eV (measured)/∼0.24 eV (calculated), respectively. Self-compensation and Fermi level pinning were observed at ∼0.7 eV (measured)/∼1.08 eV (calculated), contributing to high resistivity in as-grown samples. Post-growth Cd annealing effectively removes compensation centers, releases iodine donors, and significantlymore » reduces resistivity, resulting in an ideal n-type solar cell material.« less

A new technique, termed the stirred‐reactor coupon analysis (SRCA) method, has been developed to measure the rate of glass dissolution in forward‐rate conditions. Monolithic glass coupons are partially masked with an inert material before placement in a large volume of well‐mixed solution with known chemistry and temperature for a predetermined duration. After the test, the mask is removed, and the difference in step height between the protected area and the exposed corroded portions of the sample coupon is measured to determine the extent of glass dissolution. The step height is converted to a rate measurement using the test duration andmore » glass density. Test parameters such as sample surface preparation and test duration were evaluated to determine their effects on the measured rates. Additionally, results from an interlaboratory study (ILS) consisting of 12 laboratories from 11 different institutions are presented, where each laboratory performed 12 independent tests. When removing experimental outlier data, the 95% reproducibility limits for the SRCA method has no statistical difference with previously published standardized test methods used to determine the forward rate of glass dissolution. Overall, this paper describes steps necessary to perform the test method and provides the statistical calculations to evaluate test accuracy.« less

The intensity of red C⁢r³⁺ photoluminescence (PL) in monoclinic gallium oxide (𝛽⁢−G⁢a₂⁢O₃) is suppressed by 𝑛-type conductivity, an effect that has been attributed to a Cr deep acceptor level in the bandgap. In 𝑛-type material, such an acceptor level would be occupied, resulting in the C⁢r²⁺ oxidation state and the absence of C⁢r³⁺ PL. To test this model, 𝑛-type 𝛽⁢−G⁢a₂⁢O₃ crystals co-doped with Cr and Zr (a donor) were grown from the melt. The samples show C⁢r³⁺ optical absorption bands and a high free-electron concentration of 4 × 10¹⁸ c⁢m⁻³. If Cr were an acceptor, then it would be fullymore » compensated and therefore would not exhibit the C⁢r³⁺ optical signature. Hybrid functional calculations indicate that Cr occupies the substitutional octahedral Ga(II) site and that the C⁢r²⁺ state is energetically unfavorable, i.e., Cr is not an acceptor. Weak C⁢r³⁺ PL was observed in the Cr/Zr co-doped samples. In conclusion, the quenching of PL may be caused by a transfer of energy to free electrons, a nonradiative process that would reduce the emission intensity.« less

One of the primary advantages of β-Ga₂O₃ over incumbent wide bandgap semiconductors is the ability to grow directly from the melt. Melt growth, using Czochralski or similar methods, results in impurities in the crystal which originate from the crucible, such as iridium and other transition metals like chromium. These impurities exhibit optoelectronic signatures useful for their identification and sensitive to the Fermi energy of a given crystal (i.e., signatures vary with the electrical conductivity of the matrix). In this work, we describe how laser Raman systems can be used to map and spatially correlate Cr³⁺ photoluminescence, electronic-coupled Raman scattering frommore » Ir⁴⁺ d–d internal transitions, and the Raman line attributed to hydrogenic shallow donors. Laser ablation inductively coupled plasma mass spectrometry directly measured spatially dependent relative metal concentrations and confirmed spectroscopic signals resulting from heterogeneities in impurity concentrations in β-Ga₂O₃ boules. Mapping of photoluminescence and Raman-related signatures is, thus, demonstrated as an effective and facile method for spatial measurement of chemical heterogeneities in both insulating and conductive melt-grown β-Ga₂O₃ crystals.« less

Here, waste forms containing glassy and crystalline phosphate and silicate phases were produced to immobilize salt waste simulants from pyroprocessing and characterized by using Raman spectroscopy, Mössbauer spectroscopy, X-ray diffraction, scanning electron microscopy, heat capacity, and chemical durability measurements. In this work, a phosphosilicate waste form is presented to leverage the benefits of both borosilicate glasses and iron phosphate glasses. To improve waste loading, prior to immobilization, salt simulants were successfully dechlorinated using ammonium dihydrogen phosphate, mixed with a borosilicate frit (5–30 wt %) and Fe₂O₃, and vitrified. Additions of 2.5–15 wt % borosilicate glass (NBS3) improved normalized release ratesmore » for Cs relative to iron-phosphates without NBS3, resulting in chemical durabilities similar to high-level waste borosilicate glass reference materials. The release rates of the alkalis (i.e., Li, Na, K, Cs) were the lowest with the addition of 5 wt % NBS3. Although Sr was not specifically targeted in this study, evidence exists that it preferentially partitioned with Si to form an amorphous droplet phase within the iron phosphate glass matrix.« less

Relationship between thermophysical properties and phonon mean free path. Heat capacity, viscosity, and thermal conductivity in ionic liquids decrease as mean free path decreases and dynamics become less “solid-like” and collect motion diminishes.