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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Analysis of Back-Contact Interface Recombination in Thin-Film Solar Cells

The open-circuit voltage (VOC) in a generic TCO/buffer/absorber/back-contact thin-film solar cell device is a key parameter in the recombination analysis. In particular, VOC is sensitively influenced by the interface recombination at the buffer/absorber front interface and at the absorber/back-contact interface. This paper reports the temperature, excitation light intensity, and wavelength-dependent open-circuit voltage analysis to separate and quantify recombination rates in solar cells at the front and back interfaces, in the depletion regions, and in the quasi-neutral region. The wavelength-dependent VOC analysis is exploited to extract the absorber/back-contact recombination coefficient. The experimentally observed results are verified using SCAPS-1D (one dimensional-a solar cell capacitance simulator) simulation.

14 SOLAR ENERGY↗
DOE OSTI2018

Internal Resistive Barriers Related to Zinc Diffusion During the Growth of Inverted Metamorphic Multijunction Solar Cells

Majority carrier barriers at heterointerfaces are a common source of non-linear resistance that hinders concentrator solar cell performance. The source of a particular barrier is often unclear in a multijunction device with numerous heterointerfaces. In this work, we demonstrate Zn-dopant diffusion during inverted metamorphic multijunction (IMM) device growth to be one key cause of internal barrier formation. Using an inverted GaAs/GaAs tandem solar cell with a high temperature annealing layer grown in between each subcell, we simulate the annealing conditions of a multijunction growth in a simplified structure. Through analysis of the device by secondary ion mass spectrometry (SIMS) and electrochemical capacitance-voltage profiling, we show that annealing causes Zn to diffuse out of the top cell Ga 0.5 In 0.5 P back surface field (BSF) and accumulate in the GaAs base. Through equilibrium band modeling, we show that the resultant doping profile forms an energetic barrier to hole flow in the valence band, which correlates with fill factor losses in the current-voltage curves measured under concentration. When we, instead, employ a C-doped Al 0.2 Ga 0.8 As BSF layer in the top cell, we do not observe evidence of a heterojunction barrier. We attribute this difference to the reduced diffusivity of carbon, confirmed by SIMS, as well as more favorable valence band offsets between GaAs and Al 0.2 Ga 0.8 As. Finally, we compare 5-junction IMM cells with Al 0.2 Ga 0.8 As:C and Ga 0.5 In 0.5 P:Zn BSF layers in the GaAs third junction, respectively, and show a significantly improved concentrator device performance when Al 0.2 Ga 0.8 As:C is employed. Lastly, we demonstrate the importance of designing annealing tolerance into multijunction structures that are subjected to extended annealing during growth.

14 SOLAR ENERGY↗
DOE OSTI2019

Enhanced p-Type Doping in Polycrystalline CdTe Films: Deposition and Activation

An in situ nonequilibrium method to increase hole density in polycrystalline CdTe thin films to 10 16 cm -3 using group V substitution on Te is presented. Single-phase CdTe films doped with P, As, and Sb were deposited at 550 degrees C at 100-200 nm/s onto moving cadmium sulfide/high resistance transparent buffer layer/transparent conductive oxide /glass superstrates by vapor transport deposition in Cd overpressure from high purity compound sources. Doping levels before and after activation were determined by capacitance-voltage analysis of diagnostic devices. Secondary ion mass spectrometry depth profiling confirmed dopant incorporation levels of 10 17 -10 18 atoms/cm 3 in as-deposited films. Electronic activation was carried out by post-deposition annealing in Cd or CdCl 2 vapor with fast cooling, increasing acceptor concentrations to >10 15 cm -3 for P and >10 16 cm -3 for As and Sb, compared with mid -10 14 cm -3 acceptor levels for undoped CdTe films. The activation methods are compatible with post-deposition processing presently used for high-efficiency CdTe solar cells. For the dopants As and Sb, the acceptor concentration increased by substitutional As Te and Sb Te formation, respectively, which was validated by cathodoluminescence spectroscopy.

14 SOLAR ENERGY↗
DOE OSTI2019

Performance Analysis of 0.4–1.2-μm CdTe Solar Cells

Thin-film CdTe solar cells with an absorber thickness range of 0.4 to 1.2 μm were fabricated by close-space sublimation on a transparent MgZnO window layer. Microscopy cross sections showed that a significant fraction of the CdTe crystallites extended throughout the absorber layer, and capacitance measurements demonstrated that the absorbers were depleted into forward bias. Current and fill factor loss analyses were used to identify dominant loss mechanisms in the devices as a function of absorber thickness, and photoluminescence measurements were increasingly influenced by back-surface recombination as the absorber was made thinner. Furthermore, the thinner CdTe devices showed modest current and fill factor reduction but overall good diode quality, and the highest efficiency of 15% was achieved with 1.2 μm CdTe.

14 SOLAR ENERGY↗
DOE OSTI2022

Evaluating Recombination Mechanisms in RbF Treated Cu(In${}_\mathrm{x}$Ga$_\mathrm{1-x}$)Se$_{2}$ Solar Cells

Rubidium fluoride (RbF) postdeposition treatment (PDT) has been shown to improve the performance of Cu(In x Ga 1-x )Se 2 (CIGS) photovoltaic devices. Here, in this study, temperature-dependent current voltage (JVT) and time-resolved photoluminescence (TRPL) experiments were combined with modeling using the solar cell capacitance simulator (SCAPS) computer code to investigate the effect of the RbF PDT. Two devices, one as-deposited and one with RbF PDT, were deposited by a three stage coevaporation process. JVT measurements suggest the dominant recombination mechanism may be tunneling-enhanced recombination via bandtail states, but that defect states in the bandgap can also be important. RbF PDT is shown to decrease the characteristic energy of the bandtails. TRPL data show an increase in the minority carrier lifetime after RbF PDT, leading to an improved open-circuit voltage. SCAPS modeling indicates that the dominant recombination mechanism is dependent on the specific defect makeup of a device, suggesting that small changes in processing conditions can impact device behavior. This explains the observation that, for some devices, defect states in the gap dominate while others, as is the case here, appear to be dominated by bandtails.

14 SOLAR ENERGY↗
DOE OSTI2023

A Reconfigurable Planar Capacitor Sensor for Thru the Jacket Dielectric Constant Measurement of Cable Insulation

Planar capacitor sensors have been proposed to measure the aging-related dielectric constant variation of the jacket or insulation materials of cables. Although capacitor sensors have been used to monitor the aging of cable insulation by removing the jacket, through the jacket measurement of cable insulation dielectric constant variation is difficult because once the sensor is placed on the jacket it would either measure the aging of the jacket or that of the jacket and insulation together. A reconfigurable sensor along with innovative cable-specific models could be used to determine the jacket and insulation aging using the same sensor. For the proposed novel reconfigurable sensor, the electric field penetration depth is changed by activating and deactivating PIN diode switches. In one instance, the sensor measures the dielectric constant variation of the jacket, while in the next, it measures the dielectric constant variation of both the jacket and the insulation. Here, by leveraging previously developed capacitance estimation models from large-scale finite element simulations for a specific cable, these two sets of measurement data are used to evaluate the aging-related dielectric constant variation of the insulation. Experimental data from aged cable specimens are presented to demonstrate the operation of the sensor.

47 OTHER INSTRUMENTATION↗
DOE OSTI2025

Impact of Geomagnetic Induced Current Neutral Blocking Devices on Distance Relays in Sub-transmission Networks with IBR

Geomagnetically induced currents (GICs) can flow through transmission lines during geomagnetic disturbances, such as solar flares or coronal mass ejections. These currents can cause problems like transformer saturation and equipment damage. The most common method of mitigating GICs involves installing GIC neutral blocking devices (NBDs) in transformer neutrals. However, the wide application of capacitive GIC blocking devices may have unintended adverse effects on other devices, such as distance protection relays. As the number of inverter-based resources being connected to the transmission and sub-transmission systems increases, the likelihood of a sub-transmission line protected by a distance relay connected to a transformer with NBDs is increasing. Therefore, distance relays fed by IBRs and transmission lines with GIC-NBDs must be studied. This paper studies the effect of GIC-NBDs on a 69kV sub-transmission line of various lengths fed by a 25 MVA IBR and synchronous source. This work focused on the behavior of the GIC-NBDs using the measured apparent phase-to-ground and phase-to-phase impedance calculated by the relay and the source impedance ratio (SIR) during various electrical faults.

Patel, Trupal R [Sandia National Laboratories (SNL↗
DOE OSTI2021

Giant Polarization in Nanodielectrics

The relative permittivity of a material is a scaling factor for capacitors and the devices based upon them; the larger the relative permittivity, the greater the degree of miniaturization, or potential for energy storage. Materials with a relative permittivity than that of silicon nitride (approximately 7) are referred to as high-dielectric constant materials. Values of about 100 are typical in titanium dioxide rutile. Values of about 10,000 are observed in barium titanate in the region of the ferroelectric transition, which while impressive, is not very useful due to the strong temperature dependence. The observation of a relative permittivity of over 100,000 in the calcium copper titanate material sparked considerable interest because it showed little temperature dependence between 100 and 600 K over most of the radio-frequency range. Further investigation revealed that this material appears to be naturally nanotextured and that the colossal permittivity was likely due to the surface and/or internal barrier layer capacitance effect, although the issue is not settled. Unfortunately, the dielectric losses in this class materials are relatively high. A new strategy to achieve high dielectric permittivity with low loss involves using localized lattice defect states through ambipolar co-doping; these intrinsic defect complexes give rise to strong dipoles that are responsible for a relative permittivity of 10,000 with exceptionally low dielectric losses over most of the radio frequency range and excellent thermal stability.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Group-V Doping Impact on Cd-Rich CdTe Single Crystals Grown by Traveling-Heater Method

Cd-rich CdTe single crystals were grown from Cd solvent in the traveling-heater method (THM) rather than the more common Te solvent used for high-resistivity CdTe. The growth process from Cd solution in terms of the solid-liquid interface shape and group-V doping under Cd-rich condition were investigated. Stable, flat solid-liquid interfaces can be obtained by optimizing THM furnace temperature profile. An apparent doping limit in the 10^16 to 10^17 cm^ -3 range which scales weakly with increasing group-V element concentrations in the Cd-rich CdTe samples were indicated from capacitance-voltage measurement. The two-photon excitation time-resolved photoluminescence revealed a bulk lifetime of 20-40 ns.

14 SOLAR ENERGY↗
DOE OSTI2018

Using Module Leakage Current Modeling to Understand Corrosion Chemistry

When fielded, modules are exposed to high voltages in the presence of moisture and/or elevated temperature this allows significant leakage currents to flow between the cell and ground. When current changes from an electronic to an ionic form, electrochemical reactions are promoted creating the potential for a wide variety of degradation reactions. This work investigates the nature of this current flow to gain a better understanding of the effect. At the cell surface, current preferentially flows to the metallization and under negative bias will split water to form OH - ions. It is found that transient effects occur in the leakage current as a result of some capacitive and electrochemical effects. This work seeks to provide important insight into the nature of the charge transfer species and the factors that most strongly affect leakage rates to provide better correlation to module performance.

14 SOLAR ENERGY↗
DOE OSTI2020

Nanometer-Scale Imaging of Inhomogeneous Active Charge Carriers in Arsenic-Doped CdTe Thin Films

We report nanometer-scale imaging of active carrier distribution of As-doped CdTe films by scanning capacitance microscopy (SCM). We developed SCM sample preparation for CdTe by ion-milling followed by thermal processing. The nanometer-resolution carrier delineation for CdTe was validated by imaging on a CdTe cross-section sample made by a molecular beam epitaxy layer stack with As-doping concentrations of 10^15 ~10^18 /cm 3 . We found that the carrier distribution in As-doped films was significantly nonuniform, with inhomogeneity ranging from sub-um to a few um and concentration variation of one order of magnitude (low 10^16 to low 10^17 /cm 3 ). This nonuniformity is distributed randomly, independent of grain structure and grain boundary (GB). We used Kelvin probe force microscopy (KPFM) and cathodoluminescence (CL) to further map the surface potential and radiative illumination on the same area as the SCM image. Higher potential and lower CL intensity were found on GBs but not on SCM contrast, illustrating positive GB charging and GB recombination but not GB-distinguished doping. The overall KPFM potential image is in rough agreement with the SCM carrier distribution, in terms of Fermi-level position relative to the bandgap edge - thus resulting in the band-edge potential fluctuation. Nonuniform carrier concentration, potential fluctuation, and defect recombination can all together cause the Voc deficit of the As-doped CdTe device.

14 SOLAR ENERGY↗
DOE OSTI2022

Effect of Near-Interface Compensation of CdSeTe Absorber Layers on Solar Cell Performance

Arsenic has been shown to be an effective p-type dopant of CdTe. However, challenges remain in the fabrication of high efficiency CdTe solar cells using As. As-doped CdTe is prone to self-compensation and observed accumulation of dopant atoms in CdTe near the interface with MgZnO (MZO) suggests that this could be occurring. In this study, we use SCAPS 1-D modeling software to investigate the effect of near-interface compensation. We consider three cases: shallow donors, deep recombination centers, and a thin layer of excess positive charge accumulation. All of these cases are shown to have significant effects on the current-voltage characteristics, while the thin charge layer also affects capacitance-voltage measurements.

arsenic↗
DOE OSTI2019

Seamless Transition Between Dual Operating Modes of A Single-Phase Quasi-Z-Source Inverter

A model predictive control (MPC) based seamless transition between standalone and grid-connected operations is proposed for the active power filter (APF) integrated single-phase quasi-Z-source inverter (qZSI) power system. The predictive models of AC output, qZS network, and APF branch are established. The qZSI operating modes are analyzed to form corresponding cost functions. With the predictions, references, and switching states, the minimum cost function is determined to find the optimal switching state for qZSI power devices. The APF branch compensates completely the dc-side double-line-frequency (2ω) ripple under MPC, so low qZS inductance and capacitance are achieved without loss of performance. Simulation results demonstrate the proposed solution to have an unseamed transition.

Li, Xiao↗
DOE OSTI2024

Cryogenic Front-End ASICs for Low-Noise Readout of Charge Signals

This paper presents design details and measurement results of LArASIC, a front-end application specific integrated circuit (ASIC) designed for low-noise readout of charge signals generated in neutrino study experiments within liquid argon time projection chambers. LArASIC comprises of 16-channels of programmable charge amplification and pulse shaping stages that provide a voltage readout proportional to the input charge and was optimized for operation at liquid argon temperature, i.e., 89 K. The chip was fabricated in a 180 nm CMOS process. Measurements at liquid nitrogen temperature, i.e., 77 K, indicate that the channel outputs have high linearity (INL < 0.1%) within the operating range, an equivalent noise charge of 534 electrons for a peaking time of 1 µs and a detector capacitance of 150 pF, and a worst-case inter-channel cross-talk of 0.35%. The paper also presents design choices made in the process of migrating LArASIC to CHARMS, an ASIC to be fabricated in a 65 nm process that includes all features provided by LArASIC, along with additional digital programmability for improved robustness and flexibility. CHARMS is intended for use in future high-energy physics experiments that require high-resolution charge or light readout with shorter pulse peaking times.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

Simulation of Fast-Rise Transients in a Large-Power Transformer Winding

A high-altitude electromagnetic pulse (HEMP) may pose a significant threat to the electrical power grid. In this context, the present study considers here the potential impact of the HEMP early-time component (E1) on the winding of a typical transmission power line transformer. A time-domain, fully electromagnetic, three-dimensional, finite-element model was used to simulate the propagation of an E1-induced voltage surge into the transformer winding. The analysis is focused on the first few turns of the winding, the ones that are impacted by the largest electric field resulting from the applied pulse. The model follows the transient dynamics, while the pulse is traveling along the conductor, and the electric field is established across the insulation gaps through capacitive coupling. The simulations also considered the effect of an electrically floating, “shield” winding conductor. This is used as a countermeasure to improve the voltage uniformity across the winding during fast-rise transients. In this case, the propagation of the E1-induced pulse is compared with that from a slower rising, lightning standard waveform. The simulation results show that the presence of the shield conductor can effectively reduce the peak electric field between winding turns, in the presence of either E1 or lightning waveforms.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2024

Nanosecond Transient Validation of Surge Arrester Models to Predict Electromagnetic Pulse Response

The impact of high-altitude electromagnetic pulse events on the electric grid is not fully understood, and validated modeling of mitigations, such as lightning surge arresters (LSAs) is necessary to predict the propagation of very fast transients on the grid. Experimental validation of high frequency models for surge arresters is an active area of research. Further, this article serves to experimentally validate a previously defined ZnO LSA model using four metal-oxide varistor pucks and nanosecond scale pulses to measure voltage and current responses. The SPICE circuit models of the pucks showed good predictability when compared to the measured arrester response when accounting for a testbed inductance of approximately 100 nH. Additionally, the comparatively high capacitance of low-profile arresters show a favorable response to high-speed transients that indicates the potential for effective electromagnetic pulse mitigation with future materials design.

42 ENGINEERING↗