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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Te/CdTe and Al/CdTe Interfacial Energy Band Alignment by Atomistic Modeling

A synergistic approach that incorporates first-principles atomistic modeling with numerical device simulations is used to systematically evaluate the role of heterointerfaces within metal-chalcogenide-based photovoltaic technologies. Two interfaces involving either a tellurium back contact or aluminum back electrode combined with a cadmium telluride absorber layer within cadmium-telluride-based thin-film solar cells are investigated on an atomic scale to determine the mechanisms contributing to variations in device performance. Electronic structures and predicted charge transport behavior with respect to cadmium and tellurium termination of the absorber layer are studied along the polar oriented CdTe{111} facets. The computational methodology reveals a noticeable contrast between the Schottky barrier forming Al/CdTe interface versus the Type I Te/CdTe heterojunction. Greater band bending features are exhibited by the cadmium termination as opposed to the tellurium termination for each interface case. Subsequent device modeling suggests that 3.6% higher photovoltaic conversion efficiency is achievable for the cadmium termination relative to the tellurium termination of the Te/CdTe interface. Based strictly on an idealistic representation, both interface models show the importance of atomic-scale interfacial properties for cadmium telluride solar cell device performance with their bulk properties being validated in comparison to published experimental data. Here, the synergistic approach offers a suitable method to analyze solar cell interfaces through a predictive computational framework for the engineering and optimization of metal-chalcogenide-based thin-film photovoltaic technologies.

Green’s function↗
DOE OSTI2020

Atomistic modeling of energy band alignment in CdTe(1 0 0) and CdTe(1 1 1) surfaces

An atomic-scale perspective of energy band alignment in CdTe surfaces has not been spatially studied despite the major role surfaces play in forming interfaces within CdTe-based thin film photovoltaic devices. Atomistic modeling based on density functional theory coupled with surface Green’s function is used for calculating energy band alignment of CdTe surfaces. The CdTe(1 0 0) ((1×1) and c(2×2) reconstruction) and CdTe(1 1 1) ((1×1) and (2×2) reconstruction) facets without and with surface relaxation provide insightful band bending characteristics that influence charge carrier transport. Results show that unrelaxed (1×1) CdTe(1 0 0) and CdTe(1 1 1) surfaces bend the valence band downward with surface polarity dictating the surface potential magnitude. The reconstructed CdTe(1 0 0) c(2×2) and CdTe(1 1 1) (2×2) surfaces result in favorable surface electronic features in relation to their unreconstructed variants. In addition, the structurally relaxed CdTe(1 1 1) surfaces develop an internal energy cusp potential that may enhance hole charge transport toward the back of CdTe solar cell devices. In conclusion, energy band alignments calculated within the study lead to a detailed understanding of how CdTe surfaces may affect CdTe-based thin film photovoltaic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Atomistic modeling of energy band alignment in CdSeTe surfaces

Atomistic modeling based on Desnity Functional Theory (DFT)- 1/2 method coupled with surface Green’s function has been employed to investigate the energy band alignment results in cadmium selenium telluride (CdSeTe) surfaces. The structural and electronic properties of the bulk ternary alloy were established before cleaving the CdSeTe low index surface facets. The dependency of atomic-scale electronic features on different plane orientations was explored by studying the energy band alignment diagrams in the unreconstructed and reconstructed surfaces. While the low index CdSeTe surface geometry reconstructions replicate the CdTe surface geometries, the energy band alignment features of the unreconstructed and reconstructed CdSeTe surfaces differ from those observed in CdTe low index surfaces. The structurally relaxed reconstructed CdSeTe surface results in purely flat bands as opposed to an internal cusp feature observed in CdTe surfaces. The presence of an internal energy cusp in CdTe(111) surfaces is expected to play a key role in enhancing the hole charge transport towards the back of CdTe-only solar cell device and the absence of such feature in CdSeTe surface may explain one of the reason for the lower performance of CdSeTe-only solar cell seen experimentally.

36 MATERIALS SCIENCE↗
DOE OSTI2022

A Many-Body Perturbation Theory Approach to Energy Band Alignment at the Crystalline Tetracene–Silicon Interface

Hybrid inorganic–organic semiconductor (HIOS) interfaces are of interest for new photovoltaic devices operating above the Shockley–Queisser limit. Predicting energy band alignment at the interfaces is crucial for their design, but represents a challenging problem due to the large scales of the system, the energy precision required and a wide range of physical phenomena that occur at the interface. To tackle this problem, many-body perturbation theory in the non-self-consistent GW approximation, orbital relaxation corrections for organic semiconductors, and line-up potential method for inorganic semiconductors which allows for tractable and accurate computing of energy band alignment in crystalline van-der-Waals hybrid inorganic–organic semiconductor interfaces are used. In this work, crystalline tetracene physisorbed on the clean hydrogen-passivated 1 × 2 reconstructed (100) silicon surface is studied. Using this computational approach, it is found that the energy band alignment is determined by an interplay of the mutual dynamic dielectric screening of two materials and the formation of a dipole layer due to a weak hybridization of atomic/molecular orbitals at the interface. The significant role of the exchange-correlation effects in predicting band offsets for the hybrid inorganic–organic semiconductor interfaces is also emphasized.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Low-Dimensional Contact Layers for Enhanced Perovskite Photodiodes

Controlling defects and energy-band alignments are of paramount importance to the development of high-performance perovskite-based photodiodes. Yet, concurrent improvements in interfacial contacts and defect reduction simply by tailoring bottom contacts have not been investigated. An effective strategy is reported that can simultaneously improve energy-band alignments and structural defects by introducing low-dimensional contact (LDC) layers at the bottom interface. It is found that LDC-based perovskites considerably suppress undesirable structural defects induced by microstrains, resulting in reduced nonradiative recombination centers and improved carrier lifetimes. Additionally, the resulting LDC-based interface structures help block minority carrier injection from the electrodes by forming built-in electric fields. As a consequence, LDC-based perovskite photodiodes showed improved light detection capabilities. Finally, the result opens an avenue to yield highly efficient photodiodes.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Triplet Management at Ligand-Perovskite Interface to Enhanced Photovoltaics Performance

Aromatic organic molecules are widely used as passivators in perovskite solar cells (PSCs). Efforts have been devoted to improving the energy alignments, band edge states, and defect passivation functionalities. However, potential energy loss through triplet states in such conjugated molecules has not been investigated in the past. Here, we report the observation of charge transfer from FAPbI 3 to the triplet state of a pyrene (Pyr)-based cationic ligand. Such a potential loss channel can be suppressed via delicate triplet management in polycyclic aromatic compounds. Interestingly, a subtle structural modification from Pyr to the triphenylene (TP) core gave rise to shallower triplets that prohibit electron transfer from perovskites. This strategy has led to TP-passivated PSCs achieving an enhanced power conversion efficiency (PCE) of 24.69% from 23.10% in Pyr-passivated counterparts. Moreover, the devices retained above 80% initial PCE after 4000 h of light soaking, which likely benefited from the reduced triplet electron trapping at the ligand-perovskite interface.

14 SOLAR ENERGY↗
DOE OSTI2022

Valence Disproportionation of GeS in the PbS Matrix Forms Pb 5 Ge 5 S 12 Inclusions with Conduction Band Alignment Leading to High n-Type Thermoelectric Performance

Converting waste heat into useful electricity using solid-state thermoelectrics has a potential for enormous global energy savings. Lead chalcogenides are among the most prominent thermoelectric materials, whose performance decreases with an increase in chalcogen amounts (e.g., PbTe>PbSe>PbS). Herein, we demonstrate the simultaneous optimization of the electrical and thermal transport properties of PbS-based compounds by alloying with GeS. The addition of GeS triggers a complex cascade of beneficial events as follows: Ge 2+ substitution in Pb 2+ and discordant off-center behavior; formation of Pb 5 Ge 5 S 12 as stable second phase inclusions through valence disproportionation of Ge 2+ to Ge0 and Ge 4+ . PbS and Pb 5 Ge 5 S 12 exhibit good conduction band energy alignment that preserves the high electron mobility; the formation of Pb 5 Ge 5 S 12 increases the electron carrier concentration by introducing S vacancies. Sb doping as the electron donor produces a large power factor and low lattice thermal conductivity (κ lat ) of ~0.61 Wm -1 K -1 . The highest performance was obtained for the 14% GeS-alloyed samples, which exhibited an increased room temperature electron mobility of ~121 cm 2 V -1 s -1 for 3 × 10 19 cm -3 carrier density, and a ZT, of 1.32 at 923 K. This is ~ 55% greater that the corresponding Sb-doped PbS sample and is one of the highest reported for the n-type PbS system. Moreover, the average ZT (ZT avg ) of ~0.76 from 400 to 923 K is the highest for PbS-based systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Fast and efficient Sb-based type-II phototransistors integrated on silicon

Increasing the energy efficiency and reducing the footprint of on-chip photodetectors enable dense optical interconnects for emerging computational and sensing applications. While heterojunction phototransistors (HPTs) exhibit high energy efficiency and negligible excess noise factor, their gain-bandwidth product (GBP) has been inferior to that of avalanche photodiodes at low optical powers. Here, we demonstrate that utilizing type-II energy band alignment in an Sb-based HPT results in six times smaller junction capacitance per unit area and a significantly higher GBP at low optical powers. These type-II HPTs were scaled down to 2 μm in diameter and fully integrated with photonic waveguides on silicon. Thanks to their extremely low dark current and high internal gain, these devices exhibit a GBP similar to the best avalanche devices (∼270 GHz) but with one order of magnitude better energy efficiency. Their energy consumption is about 5 fJ/bit at 3.2 Gbps, with an error rate below 10−9 at −25 dBm optical power at 1550 nm. These features suggest new opportunities for creating highly efficient and compact optical receivers based on phototransistors with type-II band alignment.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Interfacial Momentum Matching for Ohmic Van Der Waals Contact Construction

The difficulty of achieving ohmic contacts is a long-standing challenge for the development and integration of devices based on 2D materials, due to the large mismatch between their electronic properties and those of both traditional metal-based and van der Waals (vdWs) electrodes. Research has focused primarily on the electronic energy band alignment, while the effects of momentum mismatch on carrier transport across the vdWs gaps are largely neglected. Graphene-silicon junctions are utilized to demonstrate that electron momentum distribution can dominate the electronic properties of vdWs contacts. By judiciously introducing scattering centers at the interface that provide additional momentum to compensate the momentum mismatch, the junction conductivity is enhanced by more than three orders of magnitude, enabling the formation of high-quality ohmic contacts. The study establishes the framework for the design of high-performance ohmic vdWs contacts based on both energy and momentum matching, which can facilitate efficient heterogeneous integration of 2D–3D systems and the development of post-CMOS architectures.

2D-3D integration↗
DOE OSTI2022

Multimode Characterization Approach for Understanding Cell-Level PV Performance and Degradation

Cell-level degradation processes impact the economic viability and large-scale deployment prospects for both established and emerging photovoltaic (PV) technologies. This project addresses the need to develop experimental and device-modeling approaches for studying cell-level degradation processes in photovoltaic (PV) devices using a variety of characterization techniques that provide access to complementary material and device properties. Our results demonstrate that by coupling characterization results with device modeling it is possible to develop comprehensive understanding of processes leading to performance limitations and degradation. This project developed a suite of novel measurement techniques including pulsed-light-bias operando X-ray and photoelectron spectroscopy (popXPS), light-biased scanning microwave impedance microscopy (sMIM), and near-field transport imaging (TI). In addition, operando characterization methodologies and in situ stressing capabilities have been developed and applied for techniques including electron-beam-induced current (EBIC), cathodoluminescence (CL), and Kelvin probe force microscopy (KPFM). Device-physics models were developed and applied to simulate correlative, multi-mode measurements to extract material and device parameters that control performance degradation. These characterization and modeling techniques were applied in a multi-mode approach to probe cell-level degradation mechanisms in Cd(Se,Te) and hybrid perovskite PV devices. Together these efforts contribute to foundational PV degradation science by establishing a framework for understanding PV performance degradation at the cell level and benefit the U.S. PV industry by providing resources in the form of novel experimental capabilities, knowledge gained, and available expertise that can accelerate research and development of improved PV device materials and architectures. The project provided a comprehensive understanding of degradation in baseline Cd(Se,Te) solar cells provided by our collaborators at Colorado State University (CSU). EBIC and CL-based measurements and revealed unusual collection and recombination profiles in these devices, which underwent significant changes with during stressing. KPFM and operando XPS measurements showed that device stressing permanently alters energy-band alignments at the (Mg,Zn)O/Cd(Se,Te) interface, which in turn account for an observed loss in fill factor. Studies on hybrid perovskite devices were hampered to a significant extent by delays related to the pandemic. Nevertheless, a set of hybrid perovskite devices (supplied through an NREL-industry partnership) were stress tested and characterized with techniques including EBIC, sMIM, popXPS/popUPS and optically excited TI. Available results from these measurements informed the device modeling effort and suggest that defects and related band offsets at the C60/LiF/hybrid perovskite interface are the primary source of degradation in these devices.

14 SOLAR ENERGY↗
DOE OSTI2025

A 2D/3D Heterostructure Perovskite Solar Cell with a Phase‐Pure and Pristine 2D Layer

Abstract Interface engineering plays a critical role in advancing the performance of perovskite solar cells. As such, 2D/3D perovskite heterostructures are of particular interest due to their optoelectrical properties and their further potential improvements. However, for conventional solution‐processed 2D perovskites grown on an underlying 3D perovskite, the reaction stoichiometry is normally unbalanced with excess precursors. Moreover, the formed 2D perovskite is impure, leading to unfavorable energy band alignment at the interface. Here a simple method is presented that solves both issues simultaneously. The 2D formation reaction is taken first to completion, fully consuming excess PbI 2 . Then, isopropanol is utilized to remove excess organic ligands, control the 2D perovskite thickness, and obtain a phase‐pure, n = 2, 2D perovskite. The outcome is a pristine (without residual 2D precursors) and phase‐pure 2D perovskite heterostructure with improved surface passivation and charge carrier extraction compared to the conventional solution process. PSCs incorporating this treatment demonstrate a notable improvement in both stability and power conversion efficiency, with negligible hysteresis, compared to the conventional process.

14 SOLAR ENERGY↗
DOE OSTI2024

Robust Chemiresistive Behavior in Conductive Polymer/MOF Composites

Metal-organic frameworks (MOFs) are promising materials for gas sensing but are often limited to single-use detection. A hybridization strategy is demonstrated synergistically deploying conductive MOFs (cMOFs) and conductive polymers (cPs) as two complementary mixed ionic-electronic conductors in high-performing stand-alone chemiresistors. This work presents significant improvement in i) sensor recovery kinetics, ii) cycling stability, and iii) dynamic range at room temperature. The effect of hybridization across well-studied cMOFs is demonstrated based on 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 2,3,6,7,10,11-hexaiminotriphenylene (HITP) ligands with varied metal nodes (Co, Cu, Ni). A comprehensive mechanistic study is conducted to relate energy band alignments at the heterojunctions between the MOFs and the polymer with sensing thermodynamics and binding kinetics. The findings reveal that hole enrichment of the cMOF component upon hybridization leads to selective enhancement in desorption kinetics, enabling significantly improved sensor recovery at room temperature, and thus long-term response retention. This mechanism is further supported by density functional theory calculations on sorbate–analyte interactions. It is also found that alloying cPs and cMOFs enables facile thin film co-processing and device integration, potentially unlocking the use of these hybrid conductors in diverse electronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Spin Rectification and Electrically Controlled Spin Transport in Molecular-Ferroelectrics-Based Spin Valves

We report a spin-rectification effect in a spin-valve structure consisting of ferroelectric croconic acid (C 5 H 2 O 5 ) sandwiched between ferromagnetic electrodes La 0.7 Sr 0.3 MnO 3 and Co, which can be switched between a high-resistance (off ) and a low-resistance (on) state by a poling voltage. In the off state, the magnetoresistance (MR) sign reverses with the measurement voltage with a 0.1-V offset, suggesting a spin-rectification behavior, while in the on state the MR remains negative. These observations can be understood in terms of electrically controlled interfacial energy-band alignment either from the electrostatic effect or from the interfacial redox process. In conclusion, the observed spin-rectification effect suggests thepossibility of diodelike devices for spin-polarized current.

36 MATERIALS SCIENCE↗
NASA NTRS2004

Electron-Spin Filters Based on the Rashba Effect

Semiconductor electron-spin filters of a proposed type would be based on the Rashba effect, which is described briefly below. Electron-spin filters more precisely, sources of spin-polarized electron currents have been sought for research on, and development of, the emerging technological discipline of spintronics (spin-based electronics). There have been a number of successful demonstrations of injection of spin-polarized electrons from diluted magnetic semiconductors and from ferromagnetic metals into nonmagnetic semiconductors. In contrast, a device according to the proposal would be made from nonmagnetic semiconductor materials and would function without an applied magnetic field. The Rashba effect, named after one of its discoverers, is an energy splitting, of what would otherwise be degenerate quantum states, caused by a spin-orbit interaction in conjunction with a structural-inversion asymmetry in the presence of interfacial electric fields in a semiconductor heterostructure. The magnitude of the energy split is proportional to the electron wave number. The present proposal evolved from recent theoretical studies that suggested the possibility of devices in which electron energy states would be split by the Rashba effect and spin-polarized currents would be extracted by resonant quantum-mechanical tunneling. Accordingly, a device according to the proposal would be denoted an asymmetric resonant interband tunneling diode [a-RITD]. An a-RITD could be implemented in a variety of forms, the form favored in the proposal being a double-barrier heterostructure containing an asymmetric quantum well. It is envisioned that a-RITDs would be designed and fabricated in the InAs/GaSb/AlSb material system for several reasons: Heterostructures in this material system are strong candidates for pronounced Rashba spin splitting because InAs and GaSb exhibit large spin-orbit interactions and because both InAs and GaSb would be available for the construction of highly asymmetric quantum wells. This mate-rial system affords a variety of energy-band alignments that can be exploited to obtain resonant tunneling and other desired effects. The no-common-atom InAs/GaSb and InAs/AlSb interfaces would present opportunities for engineering interface potentials for optimizing Rashba spin splitting.

Ting, David Z.-Y.↗
DOE OSTI2024

Discovery of Stable Surfaces with Extreme Work Functions by High‐Throughput Density Functional Theory and Machine Learning

Abstract The work function is the key surface property that determines the energy required to extract an electron from the surface of a material. This property is crucial for thermionic energy conversion, band alignment in heterostructures, and electron emission devices. This work presents a high‐throughput workflow using density functional theory (DFT) to calculate the work function and cleavage energy of 33,631 slabs (58,332 work functions) that are created from 3,716 bulk materials. The number of calculated surface properties surpasses the previously largest database by a factor of ≈27. Several surfaces with an ultra‐low (<2 eV) and ultra‐high (>7 eV) work function are identified. Specifically, the (100)‐Ba‐O surface of BaMoO 3 and the (001)‐F surface of Ag 2 F have record‐low (1.25 eV) and record‐high (9.06 eV) steady‐state work functions. Based on this database a physics‐based approach to featurize surfaces is utilized to predict the work function. The random forest model achieves a test mean absolute error (MAE) of 0.09 eV, comparable to the accuracy of DFT. This surrogate model enables rapid predictions of the work function (≈ 10 5 faster than DFT) across a vast chemical space and facilitates the discovery of material surfaces with extreme work functions for energy conversion and electronic device applications.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Top Stack Optimization for Cu 2 BaSn(S, Se) 4 Photovoltaic Cell Leads to Improved Device Power Conversion Efficiency beyond 6%

Earth-abundant and air-stable Cu 2 BaSnS 4-x Se x (CBTSSe) and related thin-film absorbers are regarded as prospective options to meet the increasing demand for low-cost solar cell deployment. Devices based on vacuum-deposited CBTSSe absorbers have achieved record power conversion efficiency (PCE) of 5.2 % based on a conventional device structure using CdS buffer and i-ZnO/ITO window layers, with open-circuit voltage (V OC ) posing the major bottleneck for improving solar cell performance. The current study demonstrates a >20 % improvement in V OC (from 0.62 V to 0.75 V) and corresponding enhancement in PCE (from 5.1 % to 6.2 % without anti-reflection coating; to 6.5 % with MgF 2 anti-reflection coating) for solution-deposited CBTSSe solar cells. This performance improvement is realized by introducing an alternative successive ionic layer adsorption and reaction (SILAR)-deposited Zn 1-x Cd x S buffer combined with sputtered Zn 1-x Mg x O/Al-doped ZnO window/top contact layer, which offer lower electron affinities relative to the conventional CdS/i-ZnO/ITO stack and better matching with the low electron affinity of CBTSSe. A combined experimental (temperature- and light intensity-dependent V OC measurements) and device simulation (SCAPS-1D) evaluation points to the importance of addressing relative band offsets for both the buffer and window layers relative to the absorber in mitigating interfacial recombination and optimizing CBTSSe solar cell performance.

25 ENERGY STORAGE↗
DOE OSTI2021

Understanding the Role of CdTe in Polycrystalline CdSe x Te 1– x /CdTe-Graded Bilayer Photovoltaic Devices

Grading of bandgap by alloying CdTe with selenium to form a CdSe x Te 1–x /CdTe-graded bilayer device has led to a device efficiency over 19%. A CdSe x Te 1–x absorber would increase the short-circuit current due to its lower bandgap but at the expense of open-circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSe x Te 1–x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short-circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. In this report there is a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first-principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSe x Te 1–x .

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

Accelerating the development of new solar absorbers by photoemission characterization coupled with density functional theory

Abstract The expectation to progress towards Terawatts production by solar technologies requires continuous development of new materials to improve efficiency and lower the cost of devices beyond what is currently available at industrial level. At the same time, the turnaround time to make the investment worthwhile is progressively shrinking. Whereas traditional absorbers have developed in a timeframe spanning decades, there is an expectation that emerging materials will be converted into industrially relevant reality in a much shorter timeframe. Thus, it becomes necessary to develop new approaches and techniques that could accelerate decision-making steps on whether further research on a material is worth pursuing or not. In this review, we will provide an overview of the photoemission characterization methods and theoretical approaches that have been developed in the past decades to accelerate the transfer of emerging solar absorbers into efficient devices.

14 SOLAR ENERGY↗