Search NASA⌕ Search

SEARCH · Search NASA

Results for “heterojunctions”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Carbon Nanotube Based Molecular Electronics

Carbon nanotubes and the nanotube heterojunctions have recently emerged as excellent candidates for nanoscale molecular electronic device components. Experimental measurements on the conductivity, rectifying behavior and conductivity-chirality correlation have also been made. While quasi-one dimensional simple heterojunctions between nanotubes with different electronic behavior can be generated by introduction of a pair of heptagon-pentagon defects in an otherwise all hexagon graphene sheet. Other complex 3- and 4-point junctions may require other mechanisms. Structural stability as well as local electronic density of states of various nanotube junctions are investigated using a generalized tight-binding molecular dynamics (GDBMD) scheme that incorporates non-orthogonality of the orbitals. The junctions investigated include straight and small angle heterojunctions of various chiralities and diameters; as well as more complex 'T' and 'Y' junctions which do not always obey the usual pentagon-heptagon pair rule. The study of local density of states (LDOS) reveal many interesting features, most prominent among them being the defect-induced states in the gap. The proposed three and four pointjunctions are one of the smallest possible tunnel junctions made entirely of carbon atoms. Furthermore the electronic behavior of the nanotube based device components can be taylored by doping with group III-V elements such as B and N, and BN nanotubes as a wide band gap semiconductor has also been realized in experiments. Structural properties of heteroatomic nanotubes comprising C, B and N will be discussed.

Srivastava, Deepak↗
DOE OSTI2021

Low voltage drop tunnel junctions grown monolithically by MOCVD

Tunnel junction devices grown monolithically by metal organic chemical vapor deposition were optimized for minimization of the tunnel junction voltage drop. Here, two device structures were studied: an all-GaN homojunction tunnel junction and a graded InGaN heterojunction-based tunnel junction. This work reports a record-low voltage drop in the graded-InGaN heterojunction based tunnel junction device structure achieving a de-embedded tunnel junction voltage drop of 0.17 V at 100 A/cm 2 . The experimental data were compared with a theoretical model developed through technology computer-aided design (TCAD) simulations that offer a physics-based approach to understanding the key components of the design space, which lead to a more efficient tunnel junction.

42 ENGINEERING↗
DOE OSTI2026

PV Lifetime Project - 2025 NLR Annual Report

DOE's PV Lifetime project was initiated in 2016 with the goal of accurately characterizing the early-life evolution of photovoltaic (PV) field performance. Different PV cell and module technologies result in different initial degradation rates due to effects like light-induced degradation (LID) and light and elevated temperature-induced degradation (LeTID). To accurately characterize the initial field degradation of maximum power (Pmp) requires the use of high-accuracy indoor IV curve measurements at standard test conditions. Therefore, PV modules involved in this study are removed from the field once or twice per year and brought indoors for measurement under constant temperature and irradiance conditions. Overall annual degradation rates are as follows: our first modules to be deployed (Jinko, Trina, QCells) have annual median degradation rate between -0.4%/yr and -0.5%/yr mainly concentrated in the first year. Mission Solar, LG and Panasonic modules are all displaying modest degradation, better than -0.3% / year. Indeed, Mission Solar fielded modules degraded less than their control modules which remain indoors and un-exposed. This is also true for the LONGi monofacial modules, which had some field degradation, but not as much as the degradation of the indoor control modules. The LONGi bifacial modules on the other hand have degraded more in the field than their monofacial counterparts, although still a modest amount (-0.4 %/yr). Of the four newest module types in the study, only one has had better than average degradation. REC360NP2 (N-type TOPCon) had a slight performance increase over the first year and a half of field deployment. For the other three new module types (plus one older module type), degradation was more rapid. In our study of 16 module types, four have demonstrated degradation faster than -1%/yr: two N-type Heterojunction, one PERC bifacial and one PERC shingled module. The two heterojunction modules in our study are degrading the most rapidly. Sunpreme n-HIT bifacial modules are showing a loss rate around -1.5%/yr, for over -10% total to date. This is largely attributed to loss in front-side Isc. This is distinct from the REC 405AA-Pure modules which have degraded -6.8% in only a year and a half, for an annualized decline of -3.9 %/yr. For this module type, the decline is roughly half in Voc, with the remaining split between FF and Isc. Of the remaining two module types, Prism Solar PERC bifacial has declined -5% total since 2019, although this loss appears to have stabilized in the most recent measurement. The Solaria PowerX-400R Shingled module type has also lost around -3.2% in the first 1.5 years of field deployment. It remains to be seen if these losses will continue with time.

14 SOLAR ENERGY↗
DOE OSTI2024

Screen-Printed Complex Ag Inks for Si HJT Metallization

Metallization using reactive metal inks has recently attracted significant research interest due to its advantages in cost of materials and manufacturing, while still achieving performance comparable to traditional fire-through particle pastes. Here, we present for the first time the use of reactive silver (Ag) inks via industrial screen-printing and inkjet printing methods for the metallization of different Si surfaces used in tunneling oxide passivating contacts (TOPCon) and Silicon heterojunction (SHJ) solar cells. Printed Ag lines exhibit a conductivity of ~5 Mu O·cm, which is approximately 3 times that of bulk Ag (1.59 µO·cm). The printed metal has a thickness of ~0.5-1.5 Mu m, an order of magnitude smaller than the current fire-through metal finger thickness (~15 Mu m). Contact resistivity measurements of the screen-printed and inkjet-printed samples on a transparent conducting oxide (TCO) surface show a very low value of ~0.2-12 mO·cm 2. Photoluminescence images of the metallized samples demonstrate minimal surface passivation degradation compared to the non-metallized areas (?iVoc <3.5 mV). Scanning electron microscopy images (SEM) reveal the structure of the printed metals on the Si surfaces as porous but much denser than fire-through Ag by nanoparticle paste. In the final presentation, we will showcase our results of printing these reactive Ag inks on high-efficiency heterojunction and TOPCon solar cells with full-area M6 wafers. Additionally, the adhesion of these reactive Ag inks on different Si surfaces according to ASTM D3359-17, as well as the performance of solar cells after standard IEC 61215 freeze/thaw and damp heat tests, will be presented. These new metal inks show promising potential as an alternative to the currently dominant particle-based pastes, offering lower Ag consumption and lower processing temperatures without compromising performance.

heterojunction solar cells↗
DOE OSTI2021

Templated Growth and Passivation of Vertically Oriented Antimony Selenide Thin Films for High-Efficiency Solar Cells in Substrate Configuration

Antimony selenide (Sb 2 Se 3 ) is a promising low-cost photovoltaic material with a 1D crystal structure. The grain orientation and defect passivation play a critical role in determining the performance of polycrystalline Sb 2 Se 3 thin-film solar cells. In this work, a seed layer is introduced on a molybdenum (Mo) substrate to template the growth of a vertically oriented, columnar Sb 2 Se 3 absorber layer by closed space sublimation. By controlling the grain orientation and compactness of the Sb 2 Se 3 seeds, obtain high-quality Sb 2 Se 3 absorber layers with passive Sb 2 Se 3 /Mo interfaces is obtained, which in turn improve the transport of photoexcited charge carriers through the absorber layer and its interfaces. Post-deposition annealing of absorber layers in ambient air is further utilized to passivate the defects in Sb 2 Se 3 and enhance the quality of the front heterojunction. As a result of systematic processing optimization, Sb 2 Se 3 planar heterojunction solar cells are fabricated in substrate configuration with a champion power conversion efficiency of 8.5%.

14 SOLAR ENERGY↗
DOE OSTI2020

Blocking Ion Migration Stabilizes the High Thermoelectric Performance in Cu 2 Se Composites

The applications of mixed ionic–electronic conductors are limited due to phase instability under a high direct current and large temperature difference. Here, it is shown that Cu 2 Se is stabilized through regulating the behaviors of Cu + ions and electrons in a Schottky heterojunction between the Cu 2 Se host matrix and in-situ-formed BiCuSeO nanoparticles. The accumulation of Cu + ions via an ionic capacitive effect at the Schottky junction under the direct current modifies the space-charge distribution in the electric double layer, which blocks the long-range migration of Cu + and produces a drastic reduction of Cu + ion migration by nearly two orders of magnitude. Moreover, this heterojunction impedes electrons transferring from BiCuSeO to Cu 2 Se, obstructing the reduction reaction of Cu + into Cu metal at the interface and hence stabilizes the β-Cu 2 Se phase. Furthermore, incorporation of BiCuSeO in Cu 2 Se optimizes the carrier concentration and intensifies phonon scattering, contributing to the peak figure of merit ZT value of ≈2.7 at 973 K and high average ZT value of ≈1.5 between 400 and 973 K for the Cu 2 Se/ BiCuSeO composites. Here, this discovery provides a new avenue for stabilizing mixed ionic–electronic conduction thermoelectrics, and gives fresh insights into controlling ion migration in these ionic-transport-dominated materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Baseplate Temperature–Dependent Vertical Composition Gradient in Pseudo–Bilayer Films for Printing Non–Fullerene Organic Solar Cells

Numerous previous reports on the sequential deposition (SD) technique have demonstrated that this approach can achieve a p–i–n active layer architecture with an ideal vertical composition gradient, which is one of the critical factors that can influence the physical processes that determine the photovoltaic performance of organic solar cells. Herein, a commonly used photovoltaic system comprised of PM6 as a donor and Y6 as an acceptor is investigated with respect to sequential blade–processing deposition to comprehensively explore the morphology characteristics as a function of baseplate temperature. A systematic study of the temperature–dependent blend morphology elucidates the SD–processed configuration merits and device physics behind temperature–controlled degree of vertical composition gradient, and constructs the temperature–microstructure–property relationship for the corresponding photovoltaic parameters. The result shows, as the temperature increases, the morphology of the active layer has undergone a distinct evolution from the pseudo–bulk heterojunction to a pseudo–planar heterojunction and then to a pseudo–planar bilayer, leading to a non–monotonic correlation between baseplate temperature and device performance. Further, this investigation not only reveals the importance of precisely controlling baseplate temperature for gaining vertical morphology control, but also provides a path toward rational optimization of device performance in the lab–to–fab transition.

14 SOLAR ENERGY↗
DOE OSTI2024

Identifying and Amplifying the Spontaneously Formed Photo‐Charge Contribution in Opaque and Semitransparent Organic Photovoltaics

Abstract Spontaneous photo‐charge (SP) generation within the non‐fullerene (NFA) bulk helps avoid exciton (EX) related loss as compared to the traditional heterojunction route in organic solar cells (OSCs). Unfortunately, the promising SP utilization attracts little attention at this time due to the lack of knowledge on its specific contribution to the photovoltaic performance and rational optimization in high‐efficiency devices. To fill the gap, the NFAs’ SP characteristics are related with their photocurrent loss and power conversion efficiency (PCE) in state‐of‐the‐art materials. Though higher SP population is good for efficient EX utilization, it simultaneously acts as an additional photo‐charge recombination channel. The former can be further enhanced by increasing the NFAs’ crystallinity with preferred lamellar growth, and the latter can be suppressed via electric doping. With the aid of the two‐step optimization strategy, the total photocurrent loss is reduced from 14.4% to 4.8%, accompanying with PCE increment from 16.9% to 18.5%. The SP utilization strategy is finally applied in the semitransparent OSCs (ST‐OSCs), due to its less heterojunction area sensitive photo‐charge generation characteristic. A light utilization efficiency (LUE) is then increased from 3.21% to 3.77%. The findings highlight the pursue of efficient SP utilization as a new design philosophy in future OSCs’ progresses.

Chemistry↗
DOE OSTI2024

Strong interlayer coupling and long-lived interlayer excitons in two-dimensional perovskite derivatives and transition metal dichalcogenides van der Waals heterostructures

Two-dimensional (2D) van der Waals (vdW) heterostructures offer new platforms for exploring novel physics and diverse applications ranging from electronics and photonics to optoelectronics at the nanoscale. The studies to date have largely focused on transition-metal dichalcogenides (TMDCs) based samples prepared by mechanical exfoliation method, therefore it is of signi ficant interests to study high-quality vdW heterostructures using novel materials prepared by a versatile method. Here, we report a two-step vapor phase growth process for the creation of high-quality vdW heterostructures based on perovskites and TMDCs, such as 2D Cs 3 Bi 2 I 9 /MoSe 2 , with a large lattice mismatch. Supported by experimental and theoretical investigations, we discover that the Cs 3 Bi 2 I 9 /MoSe 2 vdW heterostructure possesses hybrid band alignments consisting of type -I and type-II heterojunctions because of the existence of defect energy levels in Cs 3 Bi 2 I 9 . More importantly, we demonstrate that the type-II heterojunction in the Cs 3 Bi 2 I 9 /MoSe 2 vdW heterostructure not only shows a higher interlayer exciton density, but also exhibits a longer interlayer exciton lifetime than traditional 2D TMDCs based type-II heterostructures. We attribute this phenomenon to the reduced overlap of electron and hole wavefunctions caused by the large lattice mismatch. Finally, our work demonstrates that it is possible to directly grow high-quality vdW heterostructures based on entirely different materials which provide promising platforms for exploring novel physics and cutting-edge applications, such as optoelectronics, valleytronics, and high-temperature super fluidity.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Small Polarons and Surface Defects in Metal Oxide Photocatalysts Studied Using XUV Reflection–Absorption Spectroscopy

In photocatalytic transition metal oxides, the surface electronic structure and carrier kinetics are complicated by both charge trapping at defects and self-trapping by small polaron formation. These effects are specific to both the material and the type of defect or surface states involved. Here we review recent studies on ultrafast photoexcited charge carrier dynamics at the surfaces of alpha-Fe 2 O 3 (hematite) and NiO and at the interface of a alpha-Fe 2 O 3 /NiO heterojunction using extreme ultraviolet (XUV) reflection-absorption spectroscopy. We study dynamics of small polaron formation at hematite surfaces showing that the rate of carrier self-trapping is slower than in the bulk due to a greater lattice reorganization energy required for surface polaron formation. We also show that surface dynamics of hematite can be systematically tuned to control carrier transport properties through surface functionalization. To better understand the impact of defects on carrier events, we study charge carrier recombination of NiO. It is shown that O vacancies have no detrimental effect on carrier lifetime while grain boundaries cause fast recombination. Finally, we study a model heterojunction consisting of alpha-Fe 2 O 3 and NiO and find that interfacial charge transfer across these two materials occurs by a two-step mechanism where the interfacial electric field drives fast exciton dissociation followed by hole injection from alpha-Fe2O3 to NiO. Finally, these examples illustrate future opportunities to observe surface electron dynamics in photocatalytic materials with element and chemical state resolution using ultrafast XUV spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Stretchable Photodiodes with Polymer-Engineered Semiconductor Nanowires for Wearable Photoplethysmography

Healthcare systems worldwide have been stressed to provide sufficient resources to serve the increasing and aging population in our society. The situation became more challenging at the time of pandemic. Technology advancement, especially the adoption of wearable health monitoring devices, has provided an important supplement to current clinical equipment. Most health monitoring devices are rigid, however, human tissues are soft. Such a difference has prohibited intimate contact between the two and jeopardized wearing comfortableness, which hurdles measurement accuracy especially during longtime usage. Here, in this work, we report a soft and stretchable photodiode that can conformally adhere onto the human body without any pressure and measure cardiovascular variables for an extended period with higher reliability than commercial devices. The photodiode used a composite light absorber consisting of an organic bulk heterojunction embedded into an elastic polymer matrix. It is discovered that the elastic polymer matrix not only improves the morphology of the bulk heterojunction for obtaining the desired mechanical properties but also alters its electronic band structure and improves the electrical properties that lead to a reduced dark current and enhanced photovoltage in the stretchable photodiode. The work has demonstrated high fidelity measurements and longtime monitoring of heat rate variability and oxygen saturation, potentially enabling next-generation wearable photoplethysmography devices for point-of-care diagnosis of cardiovascular diseases in a more accessible and affordable way.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2026

Buried Interfaces in Organic Photocathodes for H 2 Evolution: Fermi-Level Pinning and Recombination

Herein, we demonstrate how Fermi-level pinning at buried contacts impacts solar fuel generation in all-polymer photocathodes by systematically comparing the effects of work function, hydroxyl coverage, and hydrogen evolution using chemically modified indium tin oxide (ITO) supports. Photovoltages and net photocathode performance are improved when the ITO is passivated using phosphonic acids, independent of work function, suggesting that the passivation reduces Fermi-level pinning at the buried interface arising from blended heterojunction interactions with surface metal hydroxyls. Transient photovoltage decay reveals differences in recombination mechanisms, supported by light intensity-dependent measurements. Briefly, nonpassivated, hydrophilic contacts exhibit trap-assisted recombination, while passivated, hydrophobic contacts follow bimolecular recombination. We then investigate changes in electroactivity of hole-transfer processes as a function of scan rate and repetitive cycling using a diffusion-controlled molecular redox probe, analogous to a hole-only device achieved via the electrolyte. The nonpassivated buried contacts exhibit higher overpotentials for oxidation, indicative of hole injection/extraction barriers. We observe irreversible electron transfer via the hole-transport level of the blended heterojunction and a strong cycle dependence, consistent with changes in the hole trap state density. Passivation results in more reversible redox behaviors, consistent with more Ohmic-like contacts. Collectively, these results provide context toward the realization of durable organic photoelectrodes with optimized photovoltages and net solar-to-hydrogen conversion efficiencies via fundamental understanding of the rates of carrier generation, recombination, and transport in high-dielectric aqueous environments and opportunities to characterize buried interfaces under device-relevant electric fields.

buried interfaces↗
DOE OSTI2020

Formation of Coherent 1H–1T Heterostructures in Single-Layer MoS 2 on Au(111)

Heterojunctions of semiconductors and metals are the fundamental building blocks of modern electronics. Coherent heterostructures between dissimilar materials can be achieved by composition, doping, or heteroepitaxy of chemically different elements. Here, we report the formation of coherent single-layer 1H–1T MoS 2 heterostructures by mechanical exfoliation on Au(111), which are chemically homogeneous with matched lattices but show electronically distinct semiconducting (1H phase) and metallic (1T phase) character, with the formation of these heterojunctions attributed to a combination of lattice strain and charge transfer. The exfoliation approach employed is free of tape residues usually found in many exfoliation methods and yields single-layer MoS 2 with millimeter (mm) size on the Au surface. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) have collectively been employed to elucidate the structural and electronic properties of MoS 2 monolayers on Au substrates. Bubbles in the MoS 2 formed by the trapping of ambient adsorbates beneath the single layer during deposition, have also been observed and characterized. Lastly, our work here provides a basis to produce two-dimensional heterostructures which represent potential candidates for future electronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C 60 . A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.

14 SOLAR ENERGY↗
DOE OSTI2023

Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe 2 -WS 2 -WSe 2 trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe 2 valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe 2 layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe 2 under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Metal-free photocatalysts for hydrogen evolution

This review focuses on the discussion of the latest progress and remaining challenges in selected metal-free photocatalysts for hydrogen production. The scope of this review is limited to the metal-free elemental photocatalysts (i.e. B, C, P, S, Si, Se etc.), binary photocatalysts (i.e. BC 3 , B 4 C, C x N y , h-BN etc.) and their heterojunction, ternary photocatalysts (i.e. BCN) and their heterojunction, and different types of organic photocatalysts (i.e. linear, covalent organic frameworks, microporous polymer, covalent triazine frameworks etc.) and their heterostructures. Following a succinct depiction of the latest progress in hydrogen evolution on these photocatalysts, discussion has been extended to the potential strategies that are deemed necessary to attain high quantum efficiency and high solar-to-hydrogen (STH) conversion efficiency. Issues with reproducibility and the disputes in reporting the hydrogen evolution rate have been also discussed with recommendations to overcome them. A few key factors are highlighted that may facilitate the scalability of the photocatalyst from microscale to macroscale production in meeting the targeted 10% STH. This review is concluded with additional perspectives regarding future research in fundamental materials aspects of high efficiency photocatalysts followed by six open questions that may need to be resolved by forming a global hydrogen taskforce in order to translate bench-top research into large-scale production of hydrogen.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗