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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Impact of the isotope effect on divertor detachment and pedestal structure in DIII-D

Dedicated H-mode hydrogen experiments have been conducted at DIII-D and are compared to similar deuterium plasmas under a range of divertor conditions, revealing significant effects of the isotope mass on divertor and pedestal conditions. H-mode confinement is found to be systematically lower in hydrogen and experiences a worse degradation with increased fueling. This coincides with a higher ne,sep in hydrogen for similar fueling rates. When ne,ped is matched in hydrogen and deuterium as are the ne and Te pedestal structures, the target conditions deviate strongly from each other. An analysis of two major factors in the pedestal structure, pedestal stability and neutral fueling, shows they are not sufficient to explain either the pedestal differences between isotopes or the hydrogen pedestal structure respectively. An analysis of hydrogen and deuterium density ramps demonstrates a 20% higher ion current and a 30% higher upstream density at detachment onset in hydrogen. The characteristics of target heat flux and divertor radiation are shown to change significantly with the isotope mass in each divertor operational regime. The results shown here indicate divertor conditions trend with ion mass in a manner beneficial for divertor performance in future devices.

Mattes, Ray [Univ. of Tennessee, Knoxville, TN (Un↗
DOE OSTI2020

Initial Results From the Newly Upgraded LTX- β

LTX-β, the upgraded Lithium Tokamak Experiment, recently began its first campaign with a goal to study the transport properties of gradient-free electron temperature profile equilibria with increased toroidal field and neutral beam injection. The temperature-gradient-free equilibria in LTX were enabled by the lithium plasma-facing surface. To make the lithium surface conditions more consistent for LTX-β, a new, faster lithium evaporator system has been developed. The toroidal magnetic field has been nearly doubled to ≥0.3 T. Auxiliary heating and fueling with a high flux (35 A, 20 kV) neutral beam has begun. The beam provides core fueling without cold edge neutrals, to test whether flat temperature profiles can be sustained, and makes possible core ion temperature measurements via charge exchange recombination spectroscopy. Neutral beam fueling has been demonstrated with increases in line-integrated density. The coupling of the beam matches well with NUBEAM predictions for shine through. Improved performance has been observed following lithium evaporative coatings and an increase in field strength.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Monte Carlo event generator for initial conditions of conserved charges in nuclear geometry

At top collider energies where baryon stopping is negligible, the initial state of heavy-ion collisions is overall charge neutral and predominantly composed of gluons. Nevertheless, there can also be significant local fluctuations of the baryon number, strangeness, and electric charge densities about zero, perturbatively corresponding to the production of quark/antiquark pairs. These previously ignored local charge fluctuations can permit the study of charge diffusion in the quark-gluon plasma (QGP), even at top collider energies. In this paper we present a new model denoted ICCING (initial conserved charges in nuclear geometry) which can reconstruct the initial conditions of conserved charges in the QGP by sampling a (g→q¯q) splitting probability over the initial energy density. We find that the new charge distributions generally differ from the bulk energy density; in particular, the strangeness distribution is significantly more eccentric than standard bulk observables and appears to be associated with the geometry of hot spots in the initial state. The new information provided by these conserved charges opens the door to studying a wealth of new charge- and flavor-dependent correlations in the initial state and ultimately the charge transport parameters of the QGP.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Interactive effects of elevated CO 2 , warming, reduced rainfall, and nitrogen on leaf gas exchange in five perennial grassland species

Abstract Global changes can interact to affect photosynthesis and thus ecosystem carbon capture, yet few multi‐factor field studies exist to examine such interactions. Here, we evaluate leaf gas exchange responses of five perennial grassland species from four functional groups to individual and interactive global changes in an open‐air experiment in Minnesota, USA, including elevated CO 2 (eCO 2 ), warming, reduced rainfall and increased soil nitrogen supply. All four factors influenced leaf net photosynthesis and/or stomatal conductance, but almost all effects were context‐dependent, i.e. they differed among species, varied with levels of other treatments and/or depended on environmental conditions. Firstly, the response of photosynthesis to eCO 2 depended on species and nitrogen, became more positive as vapour pressure deficit increased and, for a C 4 grass and a legume, was more positive under reduced rainfall. Secondly, reduced rainfall increased photosynthesis in three functionally distinct species, potentially via acclimation to low soil moisture. Thirdly, warming had positive, neutral or negative effects on photosynthesis depending on species and rainfall. Overall, our results show that interactions among global changes and environmental conditions may complicate predictions based on simple theoretical expectations of main effects, and that the factors and interactions influencing photosynthesis vary among herbaceous species.

Pastore, Melissa A.↗
DOE OSTI2024

Stable Pentagonal Layered Palladium Diselenide Enables Rapid Electrosynthesis of Hydrogen Peroxide

Electrosynthesis of hydrogen peroxide (H 2 O 2 ) via the two-electron oxygen reduction reaction (2e – ORR) is promising for various practical applications, such as wastewater treatment. However, few electrocatalysts are active and selective for 2e – ORR yet are also resistant to catalyst leaching under realistic operating conditions. Here, a joint experimental and computational study reveals active and stable 2e – ORR catalysis in neutral media over layered PdSe 2 with a unique pentagonal puckered ring structure type. Computations predict active and selective 2e – ORR on the basal plane and edge of PdSe 2 , but with distinct kinetic behaviors. Further, electrochemical measurements of hydrothermally synthesized PdSe 2 nanoplates show a higher 2e – ORR activity than other Pd–Se compounds (Pd 4 Se and Pd 17 Se 15 ). PdSe 2 on a gas diffusion electrode can rapidly accumulate H 2 O 2 in buffered neutral solution under a high current density. The electrochemical stability of PdSe 2 is further confirmed by long device operational stability, elemental analysis of the catalyst and electrolyte, and synchrotron X-ray absorption spectroscopy. This work establishes a new efficient and stable 2e – ORR catalyst at practical current densities and opens catalyst designs utilizing the unique layered pentagonal structure motif.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Performance prediction applying different reduced turbulence models to the SMART tokamak

The SMall Aspect Ratio Tokamak (SMART) is currently being commissioned at the University of Seville and will be able to compare the performance of positive and negative triangularity plasmas at low aspect ratio. Predictive simulations have been performed for different machine scenarios and heating schemes using the TRANSP code. The objectives of these simulations are to predict the parameters expected in positive triangularity plasmas, to guide diagnostic development, and to validate transport models. Several reduced turbulence models have been used to predict electron and ion temperatures for the operational phase 2. All models provide similar results from approximately mid-radius to the separatrix but important discrepancies are found in the core region. These positive triangularity results are compared with experiments from a similar size machine like GLOBUS-M2. The multi-mode model (MMM) shows the best agreement. Simulations with different boundary conditions have been performed and no strong differences have been observed between them. The impact of neutral beam injection (NBI) on the predicted profiles has also been addressed. Rotation reduces turbulence levels so higher temperatures are achieved when included in the simulations. Studying the different contributions to the thermal diffusivities, it is observed that electron temperature gradient (ETG) turbulence dominates at the plasma core while micro-tearing modes (MTM) dominate at the edge in the electron channel. In the ion channel, the neoclassical contribution is dominant at the core and at the very edge while the Weiland component, which includes ion temperature gradient mode (ITG), trapped electron mode (TEM), kinetic ballooning mode (KBM), peeling mode (PM) and collisionless and collision dominated magnetohydrodynamic (MHD) modes governs the mid-radius region. For phase 3, two plasmas with different electron densities have been studied. The case with lower density matches well a specific discharge of GLOBUS-M2. The higher density plasma shows high performance with β N ≈ 3.8.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

CO 2 electrolysis to multicarbon products in strong acid

Carbon dioxide electroreduction (CO 2 R) is being actively studied as a promising route to convert carbon emissions to valuable chemicals and fuels. However, the fraction of input CO 2 that is productively reduced has typically been very low, <2% for multicarbon products; the balance reacts with hydroxide to form carbonate in both alkaline and neutral reactors. Acidic electrolytes would overcome this limitation, but hydrogen evolution has hitherto dominated under those conditions. We report that concentrating potassium cations in the vicinity of electrochemically active sites accelerates CO 2 activation to enable efficient CO 2 R in acid. We achieve CO 2 R on copper at pH <1 with a single-pass CO 2 utilization of 77%, including a conversion efficiency of 50% toward multicarbon products (ethylene, ethanol, and 1-propanol) at a current density of 1.2 amperes per square centimeter and a full-cell voltage of 4.2 volts.

Huang, Jianan Erick↗
DOE OSTI2024

Sentiment analysis of the United States public support of nuclear power on social media using large language models

This study utilized large language models (LLMs) to analyze public sentiment in the United States (US) regarding nuclear power on social media, focusing on X/Twitter, considering climate change challenges and advancements in nuclear power technology. Approximately, 1.26 million nuclear tweets from 2008–2023 were examined to fine-tune LLMs for sentiment classification. We found the crucial role of accurate data labeling for model performance, with potential implications for a 15% improvement, achieved through high-confidence labels. LLMs demonstrated better performance compared to traditional machine learning classifiers, with reduced susceptibility to overfitting and up to 96% classification accuracy. LLMs are used to segment the US public tweets into policy and energy-related categories, revealing that 68% are politically themed. Policy tweets tended to convey negative sentiment, often reflecting opposing political perspectives and focusing on nuclear deals and international relations. Energy-related tweets covered diverse topics with predominantly neutral to positive sentiment, indicating broad support for nuclear power in 48 out of 50 US states. The US public positive sentiments toward nuclear power stemmed from its high power density, reliability regardless of weather conditions, environmental benefits, application versatility, and recent innovations and advancements in both fission and fusion technologies. Negative sentiments primarily focused on waste management, high capital costs, and safety concerns. The neutral campaign highlighted global nuclear facts and advancements, with varying tones leaning towards positivity or negativity. An interesting neutral theme was the advocacy for the combined use of renewable and nuclear energy to attain net-zero goals.

Energy & Fuels↗
DOE OSTI2026

Accessibility and Reactivity of Pentavalent Plutonium under Alkaline Conditions

Plutonium exhibits a particularly complex and rich aqueous chemistry due to a dynamic redox equilibrium that can result in the coexistence of four different oxidation states in aqueous solution. Within this equilibrium, pentavalent plutonium is among the most soluble and dominant oxidation states of Pu in neutral to alkaline aqueous solutions of relevance to environmental chemistries and the extreme chemical environments encountered in high-level radioactive wastes. Despite this, the fundamental chemistry of Pu­(V) under such conditions remains poorly understood. Here, in this study, we demonstrate the redox accessibility of Pu­(V) in alkaline media based on the observations of an electrochemically reversible Pu­(VI/V) couple at E 1/2 = 0.19 V vs Hg/HgO (0.29 V vs NHE), allowing us to electrolytically generate Pu­(V) in alkaline solution. These Pu­(V) solutions remained stable under ambient conditions for at least 9 days before precipitation of crystalline Na 2 [Pu V O 2 (OH) 3 ]·2H 2 O, the first structurally characterized Pu­(V) hydroxide. The modest potential of the Pu­(VI/V) redox couple allowed us to reproduce Na 2 [Pu V O 2 (OH) 3 ]·2H 2 O synthetically as well as isolate two additional novel Pu­(V)-bearing hydroxide phases Na­[Pu V O 2 (OH) 2 (H 2 O)]·2.5H 2 O and K 2 [Pu V O 2 (OH) 3 ]·2H 2 O as single crystals from alkaline reactions under mild hydrothermal conditions (<200 °C) using Na 2 O 2 and KO 2 as reductants. This reactivity is rationalized and informed by our electrochemical measurements. The properties of the Pu­(V) solids were further characterized by using vibrational and optical spectroscopy.

Kravchuk, Dmytro V. [Argonne National Laboratory (↗
DOE OSTI2021

Finite-size correction for slab supercell calculations of materials with spontaneous polarization

Abstract The repeated slab approach has become a de facto standard to accurately describe surface properties of materials by density functional theory calculations with periodic boundary conditions. For materials exhibiting spontaneous polarization, we show that the conventional scheme of passivation with pseudo hydrogen is unable to realize a charge-neutral surface. The presence of a net surface charge induces via Gauss’s law a macroscopic electric field through the slab and results in poor size convergence with respect to the thickness of the slab. We propose a modified passivation method that accounts for the effect of spontaneous polarization, describes the correct bulk limits and boosts convergence with respect to slab thickness. The robustness, reliability, and superior convergence of energetics and electronic structure achieved by the proposed method are demonstrated using the example of polar ZnO surfaces.

Chemistry↗
DOE OSTI2026

Overview of Wendelstein 7-X high-performance operation

The Wendelstein 7-X (W7-X) stellarator has completed two consecutive experimental campaigns OP 2.2 (Sep.-Dec. 2024) and OP 2.3 (Feb.-May 2025) under a new operational strategy enabling more than one year of uninterrupted device availability. This approach, supported by exceptionally high subsystem reliability, allowed sustained high-efficiency plasma operations with up to 80–100 discharges per day across a broad range of magnetic configurations. Several key technical upgrades-most notably the first operation of a 1.5 MW class steady-state gyrotron, a new steady-state pellet injector, and advanced real-time feedback control systems significantly enhanced heating, fueling, and plasma control capabilities. Together, these improvements enabled major advances in long-pulse performance, high-β operation, and confinement optimization. Long-pulse discharges achieved 1.8 GJ of injected energy under fully detached divertor conditions, while reduced-field scenarios facilitated record volume-averaged β values approaching 3%. High-performance plasmas with centrally peaked density profiles, created via neutral beam injection (NBI) or sustained pellet fueling, demonstrated strongly reduced turbulent transport and stellarator-record fusion triple products. Complementary studies of power exhaust and divertor heat loads revealed the role of scrape-off-layer drift physics in shaping strike-line patterns under attached conditions. Together, the results from OP 2.2 and OP 2.3 significantly expand the operational space of W7-X and strengthen its role as a leading platform for steady-state stellarator research and reactor-relevant plasma scenarios.

Wendelstein 7-X↗
DOE OSTI2025

Synthesis and Characterization of [Ni(H 2 O)(7-P Ph 2 N ArSO3 ) 2 ](NaBF 4 ) for Light-Driven Quantum Dot-Catalyst Hydrogen Evolution

Light-driven hydrogen generation from water is a route to carbon-neutral fuels. However, the integrated light absorbers and catalysts must be compatible and functional under the same conditions. A sulfate-functionalized complex, [Ni(H 2 O)(7-P Ph 2 N ArSO3 ) 2 ](NaBF 4 ), where P Ph 2 N ArSO3 = 4-(3,6-diphenyl-1,3,6-azadiphosphepan-1-yl)benzenesulfonate), was synthesized and characterized. A solid-state structure from single-crystal X-ray crystallography is also reported. The electrocatalytic activity for hydrogen evolution of this complex in 7:3 H 2 O:CH 3 CN was characterized by cyclic voltammetry and controlled potential electrolysis, with a maximum observed rate of 83 s –1 . Density functional theory was used to characterize the favorable binding of the sulfate functionalities to cadmium sulfide quantum dots. Experimental assembly of these catalysts onto cadmium sulfide quantum dots was successful. In conclusion, both fast electron transfer and light-driven pH-dependent hydrogen evolution were observed.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Benchmarking of hydrodynamic plasma waveguides for multi-GeV laser-driven electron acceleration

Hydrodynamic plasma waveguides initiated by optical field ionization have recently become a key component of multi-GeV laser wakefield accelerators. Here, we present the most complete and accurate experimental and simulation-based characterization to date, applicable to current multi-GeV experiments and future 100 GeV-scale laser plasma accelerators. Crucial to the simulations is the correct modeling of intense Bessel beam interaction with meter-scale gas targets, the results of which are used as initial conditions for hydrodynamic simulations. The simulations are in good agreement with our experiments measuring evolving plasma and neutral hydrogen density profiles using two-color short pulse interferometry, enabling realistic determination of the guided mode structure for application to laser-driven plasma accelerator design. Published by the American Physical Society 2024

Physics↗
DOE OSTI2024

Ion transport and ultra-efficient osmotic power generation in boron nitride nanotube porins

Nanotube porins form transmembrane nanomaterial-derived scaffolds that mimic the geometry and functionality of biological membrane channels. We report synthesis, transport properties, and osmotic energy harvesting performance of another member of the nanotube porin family: boron nitride nanotube porins (BNNTPs). Cryo–transmission electron microscopy imaging, liposome transport assays, and DNA translocation experiments show that BNNTPs reconstitute into lipid membranes to form functional channels of ~2-nm diameter. Ion transport studies reveal ion conductance characteristics of individual BNNTPs, which show an unusual C 1/4 scaling with ion concentration and pronounced pH sensitivity. Reversal potential measurements indicate that BNNTPs have strong cation selectivity at neutral pH, attributable to the high negative charge on the channel. BNNTPs also deliver very large power density up to 12 kW/m 2 in the osmotic gradient transport experiments at neutral pH, surpassing that of other BNNT-based devices by two orders of magnitude under similar conditions. Our results suggest that BNNTPs are a promising platform for mass transport and osmotic power generation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Hydrogenous Content in Residues from Aqueous Chloride Pu and Am Recovery Operations

Byproduct residues generated from Aqueous Chloride Pu and Am recovery operations at Los Alamos National Laboratory have been investigated and determined to contain high weight % chloride salts and minimal hydrogen content. Hydrogen content is the result of limited surface area adsorption of ambient moisture. The assumption used in technical analysis NCS-TECH-19-029 to model potentially hydrogenous items as mixtures of polyethylene is shown to be overly conservative. Our results and literature precedent indicate that water adsorption of 1-3 % by weight is expected for hydroxide cakes and Pu oxides, and a conservative bounding assumption for these materials is 5% water by weight. LOH results for a dissolution heel stored long term showed 6.25% weight loss, which was conservatively assumed to be water. A conservative bounding assumption for dissolution heels is 8 wt.% water content. This equates to 0.55 % hydrogen by weight for hydroxide cakes, and 0.88% hydrogen by weight for dissolution heels. The bulk composition of dissolution heels reflects the composition of the original byproduct feed material, chloride salts of potassium, sodium, magnesium and calcium. The bulk composition of calcined hydroxide precipitation residues is composed of potassium chloride as a result of potassium hydroxide neutralization of hydrochloric acid solutions. Metal hydroxides generated during the hydroxide precipitation process are decomposed under the conditions of calcination to metal oxide forms.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2023

Interaction of Inner Heliosheath Ions with Electromagnetic Ion Cyclotron Waves

Abstract Inner heliosheath (IHS) ions are expected to be exposed to various waves, shocks, and turbulence, which can affect ion distributions and thus their charge-exchange rates with interstellar neutral atoms. This work addresses the potential significance of electromagnetic ion cyclotron (EMIC) waves under expected IHS conditions. From a kinetic dispersion relation, we find the possibility of frequent triggering of EMIC instability in the IHS. The threshold anisotropy of proton temperatures required for the instability is small, ( T ⊥ – T ∣∣ )/ T ∣∣ ≈ 0.1 or less, mainly due to high plasma β (ratio of the plasma to magnetic pressures). Numerical calculations on the scattering of ions (protons, He + , and He 2+ with energy of 0.01–50 keV) based on two models for EMIC waves with a moderate intensity indicate significant scattering in the pitch angle (mostly a few tens of degrees) and energy (mostly a few tens of percent) although details depend on the energy and pitch angle of each species and adopted EMIC wave models. This occurs on a short timescale (<100 times the gyro-period of each ion species). Resonant scattering in a few to a few tens of keV (corresponding to the expected pickup ion energy in the IHS) is easily expected unless the wavenumber is too large. The scattering effect is distinguished among different species such that for lower gyrofrequency ions (He + versus He 2+ and He 2+ versus protons), the main scattering effect moves toward a lower energy domain. All these results imply continuous disturbance of ion distributions by possibly prevailing EMIC waves in the IHS.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Plasma-Coupled Flow Reactor Studies of Low-Temperature Plasma Assisted Kinetics of Methanol Blended with CO2

Ignition technologies based on low-temperature plasmas (LTP) have the potential to operate next generation engines at elevated pressures and increased dilution limits promoting higher efficiencies. From a practical standpoint, research on LTP igniters has shown to enhance combustion and ignition, improve flame stability, and extend the dilution limits of combustion. All of the aforementioned gains are complemented with higher ignition efficiencies. However, the biggest challenge with incorporating this technology into engines is the knowledge gap of how exactly plasma chemistry effects can enhance the basic combustion phenomena. This coupled with the lack of validated kinetic mechanisms for plasma-combustion chemistry is the biggest obstruction to recognize efficient ignition, especially for application relevant fuels and biofuels. In order to comprehensively evaluate the effects of LTP on an oxygenated fuel specific system, this present study examines the kinetics of methanol plasma-assisted pyrolysis and oxidation using a custom-built plasma flow reactor (PFR). Experimental regimes are also further extended to understand the effects of adding CO2 to the mixture and its consequence on reaction kinetics. The PFR is installed with a dielectric-barrier discharge (DBD) configuration to induce LTP into the fuel mixture. Non-equilibrium plasmas are generated by high-voltage pulses (nearing 20 KV) administered by a plasma pulser at high-pulse repetition rates (up to 10 kHz). In order to better understand and isolate plasma chemistry and its effect on neutral chemistry, the experiments were carried out by heavily diluting reactive mixtures in nitrogen at near isothermal conditions. This suppresses the effect of exothermic reactions on chemistry allowing stable intermediates and products to be detected and quantified using ex-situ GC/MS diagnostic methods. Experiments were carried out at 0.5 atm pressure and over a wide range of temperatures from 523 K to 1203 K. Experimental results depicted the enhancement in intermediates production as well as overall lower temperatures required for complete fuel consumption in the plasma specific cases as opposed to their pure thermal counterpart. Formation of oxygenated and nitrile compounds specific to plasma assisted pyrolysis cases illustrated the efficacy of LTP to introduce new reaction pathways accelerating fuel decomposition. Increase in reactivity at lower temperatures is sought to be an effect induced by plasma chemistry postulating the acceleration in intermediates production. The onset of thermal ignition in plasma assisted oxidation is seen 200 K earlier than thermal oxidation highlighting efficient fuel conversion to final byproducts. Enhanced collisional processes afforded by LTP is seen to perturb reaction pathways between oxygenated fuel radicals and N-atoms to alter overall chemical reactivity. New insights into kinetics are gained from this study highlighting governing plasma assisted combustion (PAC) pathways for oxygenated fuel reaction chemistry. The results from this study can be used to develop future mechanisms specific to plasma chemistry which can bridge the mechanistic knowledge gap for LTP ignition so that future engines can adopt LTPs in their design for efficient combustion.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2023

Degradation behavior of CIGS solar Cells: A parametric analysis

A parametric study of the effect of bulk and interface properties on device characteristics as a function of heat and light stress under open-circuit and short-circuit conditions is developed for Cu(In,Ga)(S,Se) 2 solar cells using SCAPS-1D simulator. The variables and interdependencies modeled include: (1) conduction band offset (CBO) between buffer and absorber; (2) buffer ionized donor density; (3) CIGS shallow acceptor density; (4) CIGS deep acceptor density; (5) CIGS shallow donor density; (6) ionized acceptor concentration in the ordered vacancy compound (OVC) CIGS/buffer interface; and (7) back contact work-function. An increase in absorber shallow acceptor or a decrease in shallow donor density results in an increase in net carrier density and open-circuit voltage (V OC ). Further, a decrease in V OC is observed with an increase in the deep acceptors or neutral midgap defects in the CIGS due to higher charge carrier recombination. A change of CBO from spike to cliff condition is a dominant mechanism of decreasing VOC with an interface defect density of 10 12 cm -2 . The minimum depletion width is found to be specifically sensitive to CBO and interface defect density. An inflection in the capacitance–voltage curves (in reverse bias) is observed with simultaneous increase in bulk deep acceptor density and shallow donor density near the back contact.

14 SOLAR ENERGY↗