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DOE OSTI2023

Interplay Between Strain and Thickness on the Effective Carrier Lifetime of Buffer-Mediated Epitaxial Germanium Probed by the Photoconductance Decay Technique

We report contactless effective minority carrier lifetime of epitaxially grown unstrained and in-plane <110> biaxially tensile-strained (001) germanium (..epsilon..-Ge) epilayers measured using microwave-reflectance photoconductance decay measurements. Strained Ge epilayers were grown using InxGa1-xAs linearly graded buffers on (001) GaAs substrates. Using homogeneous excitation of unstrained Ge epilayers, thickness-dependent separation of minority carrier lifetime components under low injection conditions yielded a bulk lifetime of 114 +/- 2 ns and low surface recombination velocity of 21.3 +/- 0.04 cm/s. More notably, an effective minority carrier lifetime of >100 ns obtained from sub-50 nm 1.6% tensile-strained Ge epilayers showed no degradation relative to the unstrained counterpart. Detailed material characterization using X-ray diffractometry revealed successful strain transfer of 0.61 and 0.89% to the Ge epilayers via InxGa1-xAs metamorphic buffers and confirms pseudomorphic growth. Lattice coherence observed at the e-Ge epilayer and InxGa1-xAs buffer heterointerfaces via transmission electron microscopy substantiates the prime material quality achieved. The relatively high carrier lifetimes achieved are an indicator of excellent material quality and provide a path forward to realize low-threshold Ge laser sources.

bulk lifetime↗

NASA NTRS1994

(abstract) Distribution of Unimolecular Lifetimes in Ion-Molecule Association Reactions

The distribution of unimolecular lifetimes of ion-molecule complexes formed in association reactions has been measured by ion cyclotron double-resonance. The mean unimolecular lifetimes of (H(sub 2)C(sub 6)N(sub 2)(sup +))(sup *) and (CH(sub 3)CN.CH(sub 3)(sup +))(sup *) were determined to be 180 (micro)s and 140 (micro)s respectively. A theoretical examination of the distribution of lifetimes of (CH(sub 3)CN.CH(sub 3)(sup +))(sup *) was conducted using a RRKM model. The RRKM distribution, when modified by experimental constraints, was found to be a good approximation of the experimentally determined lifetime distribution. The lifetimes for unimolecular dissociation and radiative relaxation, and the absolute efficiency of collisional relaxation are also reported.

unimolecular lifetimes ion-molecule complexes RRKM↗

DOE OSTI2015

Increased radiative lifetime of Tm{sup 3+}:{sup 3}F{sub 4} → {sup 3}H{sub 6} transition in oxyfluoride tellurite glasses

Highlights: • We prepare Tm{sup 3+}-doped tellurite-zinc glasses with F{sup −} substitution. • Thermal stability becomes better with increasing F{sup −} in present glasses. • Tm{sup 3+} 1.8 μm radiative lifetime increases with F{sup −} concentration. • The origin of the increased lifetime has been discussed. - Abstract: The 1.8 μm emission properties of Tm{sup 3+}-doped zinc tellurite glasses modified by the substitution of ZnF{sub 2} are investigated in this paper. The thermal stability, Raman and phonon sideband spectra, transmission and absorption spectra, emission spectra and decay curves are discussed. It is found that substitution of fluoride ions into the zinc tellurite matrix produces dramatic increase in the emission lifetime of Tm{sup 3+} 1.8 μm emission. Absorption, Raman and phonon sideband spectra are used to estimate the local structure of Tm{sup 3+} ions. These analyses indicate structural change around Tm{sup 3+} ions caused by substitution of fluoride ions monitors the increased intrinsic radiative lifetimes. An increase in the measured radiative lifetimes of the Tm{sup 3+}:{sup 3}F{sub 4} → {sup 3}H{sub 6} transition is observed. The origin has been discussed and the reduction of OH{sup −} absorption, decrease of maximum phonon energy and phonon density are considered to be dominant in all of the nonradiative relaxations.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Interface-Dependent Radiative Lifetimes of Yb3+, Er3+ Co-doped Single NaYF4 Upconversion Nanowires

The development of upconversion nanomaterials for many photonic applications requires a detailed understanding of their radiative lifetimes that in turn depend critically on local environmental conditions. In this work, hexagonal (ß-phase) sodium-yttrium-?uoride (NaYF4) nanowires (NWs) were synthesized and substitutionally co-doped with a luminescent solid solution of trivalent erbium and ytterbium ions. A single-beam laser trapping instrument was used in tandem with a piezo-controlled, variable-temperature stage to precisely vary the nanowire’s distance from the substrate. The spontaneous photoluminescence lifetime of the 4S3/2 ? 4I15/2 transition from Er3+ ions was observed to change by >60% depending on the ions’ separation distance from a planar (water/glass) dielectric interface. The 4S3/2 state lifetime is observed to increase by a factor of 1.62 ± 0.01 as the distance from the quartz coverslip increases from ~0 nm to~40 µm. Less signi?cant changes in the luminescence lifetime (=10%) were observed over a temperature range between 25 and 50 °C. The distance dependence of the lifetime is interpreted quantitatively in the context of classical electromagnetic coupling between Er3+ ions within the nanowire and the adjacent dielectric interface. We also demonstrate potential applications of the NaYF4 NWs for both controlling and probing temperatures at nanometer scales by integrating them within a poly(dimethylsiloxane) composite matrix.

upconversion, nanowires, radiative lifetime, optic↗

DOE OSTI2014

Effect of charge trapping on effective carrier lifetime in compound semiconductors: High resistivity CdZnTe

The dominant problem limiting the energy resolution of compound semiconductor based radiation detectors is the trapping of charge carriers. The charge trapping affects energy resolution through the carrier lifetime more than through the mobility. Conventionally, the effective carrier lifetime is determined using a 2-step process based on measurement of the mobility-lifetime product (μτ) and determining drift mobility using time-of-flight measurements. This approach requires fabrication of contacts on the sample. A new RF-based pulse rise-time method, which replaces this 2-step process with a single non-contact direct measurement, is discussed. The application of the RF method is illustrated with high-resistivity detector-grade CdZnTe crystals. The carrier lifetime in the measured CdZnTe, depending on the quality of the crystals, was between about 5 μs and 8 μs. These values are in good agreement with the results obtained using conventional 2-step approach. While the effective carrier lifetime determined from the initial portion of the photoresponse transient combines both recombination and trapping in a manner similar to the conventional 2-step approach, both the conventional and the non-contact RF methods offer only indirect evaluation of the effect of charge trapping in the semiconductors used in radiation detectors. Since degradation of detector resolution is associated not with trapping but essentially with detrapping of carriers, and, in particular, detrapping of holes in n-type semiconductors, it is concluded that evaluation of recombination and detrapping during photoresponse decay is better suited for evaluation of compound semiconductors used in radiation detectors. Furthermore, based on previously reported data, it is concluded that photoresponse decay in high resistivity CdZnTe at room temperature is dominated by detrapping of carriers from the states associated with one type of point defect and by recombination of carriers at one type of extended defects. The recombination at the extended defects produces long, logarithmic decay limiting substantially performance of CdZnTe detectors. This decay is associated with the “electrostatic trapping” of excess holes by the Schottky-type depletion space-charge regions formed around the defects.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2014

Minority carrier lifetime in mid-wavelength infrared InAs/InAsSb superlattices: Photon recycling and the role of radiative and Shockley-Read-Hall recombination mechanisms

The influence of radiative recombination on the minority carrier lifetime in mid-wavelength InAs/InAsSb superlattices was investigated. From the lifetime's dependence on temperature, photon recycling, and carrier concentration, it was demonstrated that radiative lifetime dominates for carrier concentrations >5 × 10{sup 14} cm{sup −3}, and Shockley-Read-Hall recombination starts to dominate the minority carrier lifetime for carrier concentrations <5 × 10{sup 14} cm{sup −3}. An observed increase of the minority carrier lifetime with increasing superlattice thickness was attributed to photon recycling, and good agreement between measured and theoretical values of the photon recycling factor was obtained.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Digitized detection of gamma-ray signals concentrated in narrow time windows for transient positron annihilation lifetime spectroscopy

A pulsed slow-positron beam generated by an electron linear accelerator was directly used for positron annihilation lifetime spectroscopy without any positron storage devices. A waveform digitizer was introduced to simultaneously capture multiple gamma-ray signals originating from positron annihilation events during a single accelerator pulse. The positron pulse was chopped and bunched with the chopper signals also sent to the waveform digitizer. Time differences between the annihilation gamma-ray and chopper peaks were calculated and accumulated as lifetime spectra in a computer. The developed technique indicated that positron annihilation lifetime spectroscopy can be performed in a 20 μs time window at a pulse repetition rate synchronous with the linear accelerator. Lifetime spectra of a Kapton sheet and a thermally grown SiO{sub 2} layer on Si were successfully measured. Synchronization of positron lifetime measurements with pulsed ion irradiation was demonstrated by this technique.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2015

Couplings between hierarchical conformational dynamics from multi-time correlation functions and two-dimensional lifetime spectra: Application to adenylate kinase

An analytical method based on a three-time correlation function and the corresponding two-dimensional (2D) lifetime spectrum is developed to elucidate the time-dependent couplings between the multi-timescale (i.e., hierarchical) conformational dynamics in heterogeneous systems such as proteins. In analogy with 2D NMR, IR, electronic, and fluorescence spectroscopies, the waiting-time dependence of the off-diagonal peaks in the 2D lifetime spectra can provide a quantitative description of the dynamical correlations between the conformational motions with different lifetimes. The present method is applied to intrinsic conformational changes of substrate-free adenylate kinase (AKE) using long-time coarse-grained molecular dynamics simulations. It is found that the hierarchical conformational dynamics arise from the intra-domain structural transitions among conformational substates of AKE by analyzing the one-time correlation functions and one-dimensional lifetime spectra for the donor-acceptor distances corresponding to single-molecule Förster resonance energy transfer experiments with the use of the principal component analysis. In addition, the complicated waiting-time dependence of the off-diagonal peaks in the 2D lifetime spectra for the donor-acceptor distances is attributed to the fact that the time evolution of the couplings between the conformational dynamics depends upon both the spatial and temporal characters of the system. The present method is expected to shed light on the biological relationship among the structure, dynamics, and function.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Lifetime experimental study of graphite cathode for relativistic backward wave oscillator

Graphite cathodes are widely used due to their good emission properties, especially their long lifetime. Some previous papers have researched their lifetime under certain conditions and uncovered some important phenomena. This paper is dedicated to research the lifetime of the graphite cathode under higher power. In the lifetime test, the voltage and current amplitudes are about 970 kV and 9.7 kA, respectively. The repetition rate is 20 Hz. An X-band relativistic backward wave oscillator is used to generate high power microwave by utilizing the electron beam energy. The experimental results demonstrate that the emission property of the graphite cathode remains quite stable during 10{sup 5} pulses, despite some slight deteriorations regarding the beam and microwave parameters. The macroscopic morphology change of the cathode blade due to material evaporation is observed by a laser microscope. The mass loss of the graphite cathode is about 60 μg/C. Meanwhile, the observation by a scanning electron microscope uncovers that the original numerous flaky micro-structures are totally replaced by a relatively smooth surface at the mid region of the cathode blade and a large number of new micro-protrusions at the blade edges during the lifetime test.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2021

Mechanisms for long carrier lifetime in Cd(Se)Te double heterostructures

II–VI semiconductors are used in numerous electro-optical applications. For example, CdTe-based solar technology is cost competitive with other electricity generation sources, yet there is still significant room to improve. Carrier lifetime has historically been well below the radiative recombination limit. Lifetimes reaching beyond 100 ns can significantly enhance performance and enable novel device structures. Here, double heterostructures (DHs) with passivated interfaces demonstrate lifetimes exceeding 1 µs, yet this appears only for CdSeTe and not for CdTe DHs. We compare the passivation mechanisms in CdTe and CdSeTe DHs. CdSeTe lifetimes on the order of 1 µs correspond to a combination of superior intragrain lifetime, extremely low grain boundary recombination and greater Te4+ interfacial presence compared to CdTe.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Origin of long lifetime of band-edge charge carriers in organic–inorganic lead iodide perovskites

Long carrier lifetime is what makes hybrid organic–inorganic perovskites high-performance photovoltaic materials. Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of large polarons, Rashba effect, ferroelectric domains, and photon recycling. Here, we show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system enters from a phase with lower rotational entropy to another phase with higher entropy. These results imply that the recombination of the photoexcited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Furthermore, searching for organic–inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.

14 SOLAR ENERGY↗

DOE OSTI2015

ALIGNMENTS OF BLACK HOLES WITH THEIR WARPED ACCRETION DISKS AND EPISODIC LIFETIMES OF ACTIVE GALACTIC NUCLEI

Warped accretion disks have attracted intense attention because of their critical role in shaping the spin of supermassive massive black holes (SMBHs) through the Bardeen–Petterson effect, a general relativistic effect that leads to final alignments or anti-alignments between black holes and warped accretion disks. We study such alignment processes by explicitly taking into account the finite sizes of accretion disks and the episodic lifetimes of active galactic nuclei (AGNs) that delineate the duration of gas fueling onto accretion disks. We employ an approximate global model to simulate the evolution of accretion disks, allowing us to determine the gravitomagnetic torque that drives the alignments in a simple way. We then track down the evolutionary paths for mass and spin of black holes both in a single activity episode and over a series of episodes. Given with randomly and isotropically oriented gas fueling over episodes, we calculate the spin evolution with different episodic lifetimes and find that it is quite sensitive to the lifetimes. We therefore propose that the spin distribution of SMBHs can place constraints on the episodic lifetimes of AGNs and vice versa. The applications of our results on the observed spin distributions of SMBHs and the observed episodic lifetimes of AGNs are discussed, although both measurements at present are too ambiguous for us to draw a firm conclusion. Our prescription can be easily incorporated into semi-analytic models for black hole growth and spin evolution.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

SHORT GMC LIFETIMES: AN OBSERVATIONAL ESTIMATE WITH THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS)

We describe and execute a novel approach to observationally estimate the lifetimes of giant molecular clouds (GMCs). We focus on the cloud population between the two main spiral arms in M51 (the inter-arm region) where cloud destruction via shear and star formation feedback dominates over formation processes. By monitoring the change in GMC number densities and properties across the inter-arm, we estimate the lifetime as a fraction of the inter-arm travel time. We find that GMC lifetimes in M51's inter-arm are finite and short, 20–30 Myr. Over most of the region under investigation shear appears to regulate the lifetime. As the shear timescale increases with galactocentric radius, we expect cloud destruction to switch primarily to feedback at larger radii. We identify a transition from shear- to feedback-dominated disruption, finding that shear is more efficient at dispersing clouds, whereas feedback transforms the population, e.g., by fragmenting high-mass clouds into lower mass pieces. Compared to the characteristic timescale for molecular hydrogen in M51, our short lifetimes suggest that gas can remain molecular while clouds disperse and reassemble. We propose that galaxy dynamics regulates the cycling of molecular material from diffuse to bound (and ultimately star-forming) objects, contributing to long observed molecular depletion times in normal disk galaxies. We also speculate that, in extreme environments like elliptical galaxies and concentrated galaxy centers, star formation can be suppressed when the shear timescale is short enough that some clouds will not survive to form stars.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Coatings for CSP Lifetime

The feasibility and performance of tower-technology-based concentrated solar power (CSP) is highly dependent on the efficiency of the energy transformation from sun to heat at the receiver. The higher the solar absorptivity of the receiver coating, the higher the efficiency of the plant as a whole. BrightSource Energy (BSE) has developed a series of High-Performance Coating (HPC) systems in order to achieve high absorptivity over the plant lifetime (25-35 years). This requires stable coatings that are easily applicable on the large receiver surface and will maintain their optical properties under intense solar flux and thousands of heating and cooling cycles in desert conditions. Different coating formulations are required according to the differing plant conditions: receiver materials, operating conditions (temperatures, daily cycles, etc.), and environmental conditions. BSE also developed a coating for the next generation of CSP receivers, such as those being under DOE’s CSP Gen3 program, which will be operated with high temperature heat transfer fluids at temperatures of up to 800°C, which is significantly hotter than the operating temperature of current systems. Once the coating is formulated, the next challenge is evaluating its lifetime properties. BSE has found several independent failure modes that impact HPC absorptivity degradation: • Decrease in HPC optical properties due to oxidation in the receiver tubes surface below the HPC; • HPC film deterioration due to cycling of temperatures and humidity due to daily operation startup and shutdown as well as changing ambient conditions; • Mechanical degradation due to erosion by sand and wind. Existing test methods examine various aspects independently, but do not provide a combined accelerated lifetime result. Creating such a combined test suite, with a way to interpret the results to predict the coating’s projected lifetime, was the ultimate goal of this project. The project was divided into three major workstreams: lab testing (individual failure mode tests and combined failure mode tests); developing a theoretical model for aging; and validation of the test apparatus via on-sun testing in near real-world conditions at CIEMAT-PSA. Developing a test apparatus that accurately controlled the temperature while also introducing the desired solar flux proved more challenging than expected. While in the end we did succeed in creating a test apparatus that can control temperature, solar flux, and humidity, the results did not appear to accelerate the lifetime of the samples as desired. We suspect that to properly accelerate the samples we must also subject the samples to increased amounts of oxygen. Similarly, while we successfully created a combined model that is publicly available, we were unable to validate it sufficiently to feel comfortable recommending it as a general guideline.

14 SOLAR ENERGY↗

DOE OSTI2023

Regiospecific Incorporation of Acetylene Linker in High‐Electron Mobility Dimerized Acceptors for Organic Solar Cells with High Efficiency (18.8%) and Long 1‐Sun Lifetime (> 5000 h)

Abstract The commercialization of organic solar cells (OSCs) requires both high power conversion efficiency (PCE) and long‐term stability. However, the lifetime of the OSCs containing small‐molecule acceptors (SMA) should be significantly enhanced. In this study, a series of planarity‐controlled is developed, high electron mobility dimerized SMAs (DSMAs) and realize OSCs with high‐performance (PCE = 18.8%) and high‐stability ( t 80% lifetime = 5380 h under 1‐Sun illumination). An acetylene linker with a planar triple bond is designed for dimerization of SMA units to afford the high backbone planarity necessary to achieve high crystallinity and electron mobility. To further engineer the molecular conformation and electron mobility of the DSMAs, different regioisomers of a Y‐based SMA are dimerized to yield three regioisomerically distinct DSMAs, denoted as DYA‐I, DYA‐IO, and DYA‐O, respectively. It is found that the crystallinity, electron mobility, and glass transition temperature of the DSMAs gradually increase in the order of DYA‐O, DYA‐IO, and DYA‐I, which, in turn, enhance the PCE and device stability of the resulting OSCs; DYA‐O (PCE = 16.45% and t 80% lifetime = 3337 h) < DYA‐IO (PCE = 17.54% and t 80% lifetime = 4255 h) < DYA‐I (PCE = 18.83% and t 80% lifetime = 5380 h).

Chemistry↗

DOE OSTI2025

Ambient Synthesis for Fe(II) Polypyridyl Complexes with an Order of Magnitude Increase in Charge-Transfer Excited-State Lifetimes over [Fe(bpy) 3 ] 2+

Replacing precious metals with abundant metals is an important research focus in photochemical energy conversion and storage to meet global energy demands. However, transition metal complexes (TMCs) based on abundant 3d metals typically possess photochemical disadvantages─such as short charge-transfer excited-state lifetimes─and molecular modifications have focused on optimizing the interplay of structure, dynamics, and energetics to overcome their limitations. One strategy to do so is the use of bespoke ligands that can extend the lifetimes of chemically useful excited states. Here, in this study, we report the synthesis and characterization of novel Fe(II) complexes featuring lengthy polypyridyl ligands that can be readily synthesized. Steady-state and transient absorption spectroscopies indicate that these complexes have desirable properties and their excited metal-to-ligand charge-transfer states live an order of magnitude longer than in the benchmark [Fe(bpy) 3 ] 2+ . This lifetime is largely preserved in the heteroleptic complexes, thereby enabling the preparation of asymmetric complexes. Additionally, we apply nonradiative transition theory to explain the long-time decay kinetics. In light of their ease of preparation and reasonable excited-state lifetimes, we suggest the use of these complexes in Fe(II) dye-sensitized solar cells, where the rate of charge injection would be competitive with increased lifetime.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Generalizing Reversible Triplet–Triplet Energy Transfer as a Strategy to Increase Excited-State Lifetime in Copper(I) Charge-Transfer Chromophores

In this work, we generalize reversible triplet–triplet energy transfer (TTET) as a strategy to achieve long (μs range) excited-state lifetimes in copper(I) photosensitizers of the general formula Cu(RNacNac)(CN-Ar) (RNacNac is a substituted β-diketiminate and CN-Ar is an aryl isocyanide). Six new complexes are presented where the isocyanide is substituted with a triplet–acceptor moiety─anthracene, naphthalene, or phenanthrene. In-depth photophysical analysis reveals that energetic matching of the charge-transfer ( 3 CT) and acceptor-localized 3 (π→π*) states is key to achieving long triplet excited-state lifetimes via reversible TTET. With 2-isocyanoanthracene as the acceptor ligand, 3 CT and 3(π→π*) are not energetically proximate, and irreversible quenching of 3 CT occurs. With a small 0.1–0.2 eV gap between the two triplet states, luminescence originates from the 3 CT state with a moderate excited-state lifetime of 250 or 283 ns observed. As key to the success of this approach, we demonstrate that altering the N-substituent on the RNacNac ligand modulates the energy of the 3 CT state, giving three complexes where the 3 CT and 3 (π→π*) energies are perfectly matched and the excited-state lifetimes are much longer, spanning 1.9–2.7 μs. As such, the key lesson of this work is that long triplet-state lifetimes in these compounds can be achieved by proper “mixing and matching” of the RNacNac ligand, which influences the 3 CT energy, and the triplet acceptor group which dictates the energy of the 3 (π→π*) state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Improving the Carrier Lifetime of Tin Sulfide via Prediction and Mitigation of Harmful Point Defects

Tin monosulfide (SnS) is an emerging thin-film absorber material for photovoltaics. An outstanding challenge is to improve carrier lifetimes to >1 ns, which should enable >10% device efficiencies. However, reported results to date have only demonstrated lifetimes at or below 100 ps. In this study, we employ defect modeling to identify the sulfur vacancy and defects from Fe, Co, and Mo as most recombination-active. We attempt to minimize these defects in crystalline samples through high-purity, sulfur-rich growth and experimentally improve lifetimes to >3 ns, thus achieving our 1 ns goal. This framework might prove effective for unlocking the lifetime potential in other emerging thin-film materials by rapidly identifying and mitigating lifetime-limiting point defects.

14 SOLAR ENERGY↗