Local Lifetimes Research Articles

Toward Highly Efficient Low‐Carbon Footprint Solar Cells: Impact of High‐Temperature Processing on Epitaxially Grown p‐Type Silicon Wafers

Conventional silicon (Si) wafers are produced by energy‐intensive ingot crystallization which is responsible for a major share of a solar cell's carbon footprint. This work explores Si epitaxially grown silicon wafers (EpiWafers) that are produced by direct epitaxial deposition of trichlorosilane on a reusable substrate. This approach requires less energy and material and hence offers a potential for reduced cost and carbon footprint. Solar cells made from EpiWafers usually suffer from efficiency losses due to recombination at structural crystal defects associated with epitaxial growth. The nature of these defects is investigated and defects at the EpiWafer's back surface are critical. Most of these defects are highly recombination‐active, pairwise‐connected misfit dislocations in the <110> direction. They originate from a lattice mismatch between the highly doped substrate and the less‐doped epitaxially grown layer. In this contribution, the detrimental impact of these defects can be mitigated using typical manufacturing processes of high‐efficiency solar cells, such as KOH etching, gettering, and oxidation. Local minority charge carrier lifetimes as high as 2.2 ms after industrially feasible processes are reported. Simulations using efficiency‐limiting bulk recombination analysis implies that the material would allow conversion efficiencies of up to 25.6% considering tunnel oxide‐passivated contact acting as rear emitter solar cell design.

Effect of Lockdown on Pollutant Levels in the Delhi Megacity: Role of Local Emission Sources and Chemical Lifetimes

The COVID-19 pandemic resulted in changed emission regimes all over the world. India also imposed complete lockdown on all modes of travel and industrial activities for about 2 months from 25-March-2020 and later unlocked these activities in a phased manner. Here, we study signatures of emissions changes on levels of atmospheric trace gases and aerosols contributing to air pollution over multiple sites in India’s capital Delhi covering various lockdown and unlock phases using satellite data and in-situ observations. The resulting changes in the levels of these species were compared with respect to their average of 2015–2019 to attribute for year to year and seasonal changes. A clear impact of lockdown was observed for AOD, PM, NO2, CO, and SO2 as a result of emission changes, while changed precursor levels led to a change in O3 chemical regimes impacting its concentrations. A detailed analysis of FLEXPART trajectories revealed increased PM levels over Delhi in north-westerly air masses sourced to Punjab region all the way up to Pakistan. Changes in aerosols and NO2 were not only restricted to the surface but transcended the total tropospheric column. The maximum decrease in PM, NO2, CO, and SO2 was observed during the month of total lockdown in April. The lockdown impact varied with species e.g., PM10 and PM2.5 as well as locations even within the periphery of Delhi. While surface level aerosols and NO2 showed significant and almost similar changes, AOD showed much lower decrease than tropospheric column NO2.

Self-Induced Heterogeneity in Deeply Supercooled Liquids.

A theoretical treatment of deeply supercooled liquids is difficult because their properties emerge from spatial inhomogeneities that are self-induced, transient, and nanoscopic. I use computer simulations to analyze self-induced static and dynamic heterogeneity in equilibrium systems approaching the experimental glass transition. I characterize the broad sample-to-sample fluctuations of salient dynamic and thermodynamic properties in elementary mesoscopic systems. Findings regarding local lifetimes and distributions of dynamic heterogeneity are in excellent agreement with recent single molecule studies. Surprisingly broad thermodynamic fluctuations are also found, which correlate well with dynamic fluctuations, thus providing a local test of the thermodynamic origin of slow dynamics.

Hydrogen Bonds between Ions of Opposite and Like Charge in Hydroxyl-Functionalized Ionic Liquids: an Exhaustive Examination of the Interplay between Global and Local Motions and Intermolecular Hydrogen Bond Lifetimes and Kinetics.

Hydroxyl-functionalized ionic liquids (ILs) represent a new interesting class of ILs where hydrogen bonds (HBs) play an important role: here, "typical" HBs between cations and anions (ca) are competing with "atypical" HBs connecting pairs of cations (cc). We study the equilibrium and kinetics of (cc) and (ca) HBs in 1-(n-hydroxyalkyl)-pyridinium bis(trifluoromethlysulfonyl)imide [HOCnPy][NTf2] ILs by means of molecular dynamics simulations. (cc) HBs are found to be between 0.96 and 3.76 kJ mol-1 stronger than their (ca) counterparts, depending on the alkyl chain length. HB lifetimes and kinetics are analyzed by means of HB population and reactive flux correlation functions. Essentially, four different HB lifetimes have to be considered, spanning about 3 orders of magnitude, each valid in its own right and each associated with different aspects of HB breaking and HB reformation. The long-time limiting behavior of the HB population correlation function is controlled by diffusion of the ions and can be quantitatively described by analytical expressions. The short-time HB behavior is tied to the localized dynamics of the hydroxyl group exploring its local solvation environment. A minimalist kinetic two-domain model is introduced to realistically describe the time evolution of the HB population correlation function for both (ca) and (cc) HBs over 5 orders of magnitude. By employing the reactive flux method, we determine the kinetics of HB breaking, unaffected by diffusion processes. We determine both, the ultrafast upper boundary and the average rate of HB breaking, allowing recrossing-events during the transient relaxation time period. For sufficiently long alkyl chains, all those computed HB lifetimes indicate a higher kinetic stability of (cc) HBs over (ca) HBs; for short chains, it is vice-versa.

Beyond Turnover Time: Constraining the Lifetime Distribution of Water Vapor from Simple and Complex Approaches

Abstract The time water vapor spends in the atmosphere from evaporation to precipitation, termed here the water vapor lifetime, is of fundamental relevance for characterizing the water cycle, for the turnover of mass and energy, causes of precipitation extremes, and the recycling of precipitation over land. While the global average lifetime of water vapor is commonly considered as about 8–10 days, recent work indicates that the distribution of water vapor lifetimes is highly skewed, and that a large part of the water vapor could have average lifetimes of about 4–5 days. Besides calling for scrutiny of these new estimates, these findings also prompt an investigation of the factors shaping the distribution of the lifetime of water vapor. Using idealized setups and reanalysis data, I explore the influence of heterogeneity and nonstationarity on water vapor age and lifetime. The combination of nonstationarity and heterogeneity allows for short and long local lifetimes and water vapor ages, while maintaining the global average mass balance and corresponding mean water vapor lifetime. A plausibility argument based on humidity-weighted winds suggests that median lifetimes of 4–5 days are more consistent with weather system patterns in the extratropics. I propose that the median of the lifetime is more representative, since its mean value is affected by uncertainty originating from a long, thin tail. To more comprehensively understand the water vapor lifetime, methods will need to report the full lifetime distribution. Simulations with artificial water tracers could thereby provide the framework to compare different methods consistently in the future, while stable water isotopes could serve as an observational constraint.

Characterization of transport regimes and the polar dome during Arctic spring and summer using in situ aircraft measurements

Abstract. The springtime composition of the Arctic lower troposphere is to a large extent controlled by the transport of midlatitude air masses into the Arctic. In contrast, precipitation and natural sources play the most important role during summer. Within the Arctic region sloping isentropes create a barrier to horizontal transport, known as the polar dome. The polar dome varies in space and time and exhibits a strong influence on the transport of air masses from midlatitudes, enhancing transport during winter and inhibiting transport during summer. We analyzed aircraft-based trace gas measurements in the Arctic from two NETCARE airborne field campaigns (July 2014 and April 2015) with the Alfred Wegener Institute Polar 6 aircraft, covering an area from Spitsbergen to Alaska (134 to 17∘ W and 68 to 83∘ N). Using these data we characterized the transport regimes of midlatitude air masses traveling to the high Arctic based on CO and CO2 measurements as well as kinematic 10 d back trajectories. We found that dynamical isolation of the high Arctic lower troposphere leads to gradients of chemical tracers reflecting different local chemical lifetimes, sources, and sinks. In particular, gradients of CO and CO2 allowed for a trace-gas-based definition of the polar dome boundary for the two measurement periods, which showed pronounced seasonal differences. Rather than a sharp boundary, we derived a transition zone from both campaigns. In July 2014 the polar dome boundary was at 73.5∘ N latitude and 299–303.5 K potential temperature. During April 2015 the polar dome boundary was on average located at 66–68.5∘ N and 283.5–287.5 K. Tracer–tracer scatter plots confirm different air mass properties inside and outside the polar dome in both spring and summer. Further, we explored the processes controlling the recent transport history of air masses within and outside the polar dome. Air masses within the springtime polar dome mainly experienced diabatic cooling while traveling over cold surfaces. In contrast, air masses in the summertime polar dome were diabatically heated due to insolation. During both seasons air masses outside the polar dome slowly descended into the Arctic lower troposphere from above through radiative cooling. Ascent to the middle and upper troposphere mainly took place outside the Arctic, followed by a northward motion. Air masses inside and outside the polar dome were also distinguished by different chemical compositions of both trace gases and aerosol particles. We found that the fraction of amine-containing particles, originating from Arctic marine biogenic sources, is enhanced inside the polar dome. In contrast, concentrations of refractory black carbon are highest outside the polar dome, indicating remote pollution sources. Synoptic-scale weather systems frequently disturb the transport barrier formed by the polar dome and foster exchange between air masses from midlatitudes and polar regions. During the second phase of the NETCARE 2014 measurements a pronounced low-pressure system south of Resolute Bay brought inflow from southern latitudes, which pushed the polar dome northward and significantly affected trace gas mixing ratios in the measurement region. Mean CO mixing ratios increased from 77.9±2.5 to 84.9±4.7 ppbv between these two regimes. At the same time CO2 mixing ratios significantly decreased from 398.16 ± 1.01 to 393.81 ± 2.25 ppmv. Our results demonstrate the utility of applying a tracer-based diagnostic to determine the polar dome boundary for interpreting observations of atmospheric composition in the context of transport history.

Optically coupled methods for microwave impedance microscopy.

Scanning Microwave Impedance Microscopy (MIM) measurement of photoconductivity with 50 nm resolution is demonstrated using a modulated optical source. The use of a modulated source allows for the measurement of photoconductivity in a single scan without a reference region on the sample, as well as removing most topographical artifacts and enhancing signal to noise as compared with unmodulated measurement. A broadband light source with a tunable monochrometer is then used to measure energy resolved photoconductivity with the same methodology. Finally, a pulsed optical source is used to measure local photo-carrier lifetimes via MIM, using the same 50 nm resolution tip.

Influence of local structure on photoluminescence properties of Eu3+ doped CeO2 red phosphors through induced oxygen vacancies by contrasting rare earth substitutions.

A new family of red phosphors, Ce0.9-xRExO2-δ:0.1Eu3+ (RE = Y and La; x = 0, 0.20, 0.40, 0.60, 0.80, and 0.90), was synthesized by a conventional solid-state route. The influence of contrasting rare earth substitutions (Y and La) in the system was investigated on the local structure and associated photoluminescence properties by various characterization techniques. Both trivalent ion substitutions lead to the same kind of variation during phase transformation from fluorite to the respective parent oxide structure (x ≥ 0.6). On the other hand, the substitutions have a distinct effect on local structure, absorption, luminescence and lifetimes. The smaller Y3+ ion substitution enables the ordering of oxygen vacancies in a lattice, inhibiting the defect formation of Ce3+ oxidation states. In contrast, the larger La3+ ion substitution liberates oxygen vacancies, allowing defect formation. Consequently, the concentration of Ce3+ is dependent on the ionic radius of the metal ions and it has a bearing on the band gap and luminescence properties of the system. Ce0.1Y0.8O2-δ:0.1Eu3+ phosphor exhibited maximum red emission intensity at 612 nm, which is 8 times higher than that of Ce0.9O2-δ:0.1Eu3+ and better than that of commercial Philips red phosphor, whereas La substitution yielded poor emission intensities with higher concentrations. The co-substitution of contrasting rare earth metals with Eu3+ allow the understanding of local structure and a smaller ion like Y3+ greatly functionalizes CeO2:Eu3+ phosphor.

Limiting Perovskite Solar Cell Performance by Heterogeneous Carrier Extraction

AbstractAlthough the power conversion efficiency of perovskite solar cells has improved rapidly, a rational path for further improvement remains unclear. The effect of large morphological heterogeneity of polycrystalline perovskite films on their device performance by photoluminescence (PL) microscopy has now been studied. Contrary to the common belief on the deleterious effect of morphological heterogeneity on carrier lifetimes and diffusivities, in neat CH3NH3PbI3(Cl) polycrystalline perovskite films, the local (intra‐grain) carrier diffusivities in different grains are all surprisingly high (1.5 to 3.3 cm2 s−1; comparable to bulk single‐crystals), and the local carrier lifetimes are long (ca. 200 ns) and surprisingly homogenous among grains, and uniform across grain boundary and interior. However, there is a large heterogeneity of carrier extraction efficiency at the perovskite grain–electrode interface. Improving homogeneity at perovskite grain–electrode contacts is thus a promising direction for improving the performance of perovskite thin‐film solar cells.

Limiting Perovskite Solar Cell Performance by Heterogeneous Carrier Extraction

Although the power conversion efficiency of perovskite solar cells has improved rapidly, a rational path for further improvement remains unclear. The effect of large morphological heterogeneity of polycrystalline perovskite films on their device performance by photoluminescence (PL) microscopy has now been studied. Contrary to the common belief on the deleterious effect of morphological heterogeneity on carrier lifetimes and diffusivities, in neat CH3 NH3 PbI3 (Cl) polycrystalline perovskite films, the local (intra-grain) carrier diffusivities in different grains are all surprisingly high (1.5 to 3.3 cm2 s-1 ; comparable to bulk single-crystals), and the local carrier lifetimes are long (ca. 200 ns) and surprisingly homogenous among grains, and uniform across grain boundary and interior. However, there is a large heterogeneity of carrier extraction efficiency at the perovskite grain-electrode interface. Improving homogeneity at perovskite grain-electrode contacts is thus a promising direction for improving the performance of perovskite thin-film solar cells.

Raman spectral probe on increased local vibrational modes and phonon lifetimes in Ho3+-doped Bi2O3micro-rods

We report the appearance and enhancement in intensity of impurity related local vibrational modes in Bi2O3 : Ho micro-rods along with normal modes. Pure and Ho-doped Bi2O3 micro-rods were synthesized by conventional co-precipitation method at 60 °C. The structural and morphological studies were carried out using powder X-ray diffraction technique and scanning electron microscopy, respectively. Raman spectroscopic studies reveal the existence of local phonon vibrational modes (LVM) due to the incorporation of Ho3+. Harmonic approximation method was employed to find the dopant-related peak in the Raman spectra. Variation in full width at half maximum for LVM with increase in Ho3+ was also investigated. This increase in FWHM indicates the decrease in crystallinity of the doped samples. The phonon lifetime calculation carried out for each samples and the decrease in phonon lifetime with doping concentration make this material a potential candidate for optical and electronic applications. Copyright © 2016 John Wiley & Sons, Ltd.

Quantitative Carrier Density Wave Imaging in Silicon Solar Cells Using Photocarrier Radiometry and Lock-in Carrierography

InGaAs camera-based low-frequency homodyne and high-frequency heterodyne lock-in carrierographies (LIC) are introduced for spatially resolved imaging of optoelectronic properties of Si solar cells. Based on the full theory of solar cell photocarrier radiometry (PCR), several simplification steps were performed aiming at the open circuit case, and a concise expression of the base minority carrier density depth profile was obtained. The model shows that solar cell PCR/LIC signals are mainly sensitive to the base minority carrier lifetime. Both homodyne and heterodyne frequency response data at selected locations on a mc-Si solar cell were used to extract the local base minority carrier lifetimes by best fitting local experimental data to theory.

Quantitative luminescence mapping of Cu(In, Ga)Se2 thin‐film solar cells

AbstractWe investigate photoluminescence and electroluminescence (PL and EL) emission images from Cu(In,Ga)Se2‐based solar cells by means of a Hyperspectral Imager. Using the generalized Planck's law, maps of the effective quasi‐Fermi level splitting Δμeff in absolute values are obtained. A good agreement is found between the spatially averaged splitting in PL and the global open‐circuit voltage. However, from a local carrier transport discussion, we conclude that the equality does not hold locally. The spatial variations are rather attributed to local depth variations of the quasi‐Fermi level splitting due to material properties spatial fluctuations. By comparing PL and EL emissions, we discuss qualitatively the local effective lifetimes and collection efficiencies. Copyright © 2014 John Wiley & Sons, Ltd.

OH + (E)- and (Z)-1-chloro-3,3,3-trifluoropropene-1 (CF3CH═CHCl) reaction rate coefficients: stereoisomer-dependent reactivity.

Rate coefficients for the gas-phase reaction of the OH radical with (E)- and (Z)-CF3CH═CHCl (1-chloro-3,3,3-trifluoropropene-1, HFO-1233zd) (k1(T) and k2(T), respectively) were measured under pseudo-first-order conditions in OH over the temperature range 213-376 K. OH was produced by pulsed laser photolysis, and its temporal profile was measured using laser-induced fluorescence. The obtained rate coefficients were independent of pressure between 25 and 100 Torr (He, N2) with k1(296 K) = (3.76 ± 0.35) × 10(-13) cm(3) molecule(-1) s(-1) and k2(296 K) = (9.46 ± 0.85) × 10(-13) cm(3) molecule(-1) s(-1) (quoted uncertainties are 2σ and include estimated systematic errors). k2(T) showed a weak non-Arrhenius behavior over this temperature range. The (E)- and (Z)- stereoisomer rate coefficients were found to have opposite temperature dependencies that are well represented by k1(T) = (1.14 ± 0.15) × 10(-12) exp[(-330 ± 10)/T] cm(3) molecule(-1) s(-1) and k2(T) = (7.22 ± 0.65) × 10(-19) × T(2) × exp[(800 ± 20)/T] cm(3) molecule(-1) s(-1). The present results are compared with a previous room temperature relative rate coefficient study of k1, and an explanation for the discrepancy is presented. CF3CHO, HC(O)Cl, and CF3CClO, were observed as stable end-products following the OH radical initiated degradation of (E)- and (Z)-CF3CH═CHCl in the presence of O2. In addition, chemically activated isomerization was also observed. Atmospheric local lifetimes of (E)- and (Z)-CF3CH═CHCl, due to OH reactive loss, were estimated to be ∼34 and ∼11 days, respectively. Infrared absorption spectra measured in this work were used to estimate radiative efficiencies and well-mixed global warming potentials of ∼10 and ∼3 for (E)- and (Z)-CF3CH═CHCl, respectively, on the 100-year time horizon.

The very short-lived ozone depleting substance CHBr<sub>3</sub> (bromoform): revised UV absorption spectrum, atmospheric lifetime and ozone depletion potential

Abstract. CHBr3 (bromoform) is a short-lived atmospheric trace compound that is primarily of natural origin and is a source of reactive bromine in both the troposphere and stratosphere. Estimating the overall atmospheric impact of CHBr3 and its transport to the stratosphere requires a thorough understanding of its atmospheric loss processes, which are primarily UV photolysis and reaction with the OH radical. In this study, UV absorption cross sections, σ (λ ,T), for CHBr3 were measured at wavelengths between 300 and 345 nm at temperatures between 260 and 330 K using cavity ring-down spectroscopy. The present results are compared with currently recommended values for use in atmospheric models, and the discrepancies are discussed. A parameterization of the CHBr3 UV spectrum for use in atmospheric models is developed, and illustrative photolysis rate calculations are presented to highlight the impact of the revised σ (λ, T) values on its calculated local lifetimes. For example, the photolysis rate in the tropical region obtained with the present spectral data is 10–15% lower (longer lifetime) than obtained using currently recommended cross section values. Seasonally dependent ozone depletion potentials (ODPs) for CHBr3 emitted in the Indian sub-continent were calculated to be 0.10, 0.34, 0.72, and 0.23 (winter, spring, summer, fall) using the semi-empirical relationship of Brioude et al. (2010).

Conformation and dynamics of the periplasmic membrane-protein–chaperone complexes OmpX–Skp and tOmpA–Skp

The biogenesis of integral outer-membrane proteins (OMPs) in Gram-negative bacteria requires molecular chaperones that prevent the aggregation of OMP polypeptides in the aqueous periplasmic space. How these energy-independent chaperones interact with their substrates is not well understood. We have used high-resolution NMR spectroscopy to examine the conformation and dynamics of the Escherichia coli periplasmic chaperone Skp and two of its complexes with OMPs. The Skp trimer constitutes a flexible architectural scaffold that becomes more rigid upon substrate binding. The OMP substrates populate a dynamic conformational ensemble with structural interconversion rates on the submillisecond timescale. The global lifetime of the chaperone-substrate complex is seven orders of magnitude longer, emerging from the short local lifetimes by avidity. The dynamic state allows for energy-independent substrate release and provides a general paradigm for the conformation of OMP polypeptides bound to energy-independent chaperones.

Spatio-Time-Resolved Cathodoluminescence Studies on Freestanding GaN Substrates Grown by Hydride Vapor Phase Epitaxy

A spatio-time-resolved cathodoluminescence (STRCL) system was constructed by replacing the electron beam (e-beam) gun of a conventional scanning electron microscope by the in-house manufactured pulsed e-beam gun, which is excited using femtosecond laser pulses. By using this system, STRCL measurements were carried out on low threading dislocation density freestanding GaN substrates grown by hydride vapor phase epitaxy. High-resolution cathodoluminescence imaging allows for visualization of nonradiative recombination channels in the vicinity of accidentally formed domain boundaries. Local cathodoluminescence lifetimes (τCL) for the near-band-edge (NBE) emission are shown to be sensitively position dependent. A linear relation between the equivalent internal quantum efficiency (ηint eq) and τCL for the NBE emission was observed at room temperature under a weak excitation condition, and spatially resolved excitation led to the observation of the highest ηint eq of 20 % with τCL of 3.3 ns.

Local lifetime and luminescence efficiency for the near-band-edge emission of freestanding GaN substrates determined using spatio-time-resolved cathodoluminescence

Spatio-time-resolved cathodoluminescence measurements were carried out on low threading dislocation density freestanding GaN substrates grown by hydride vapor phase epitaxy. High-resolution cathodoluminescence imaging allows for visualization of nonradiative recombination channels in the vicinity of accidentally formed inversion domain boundaries. Local cathodoluminescence lifetimes (τCL) for the near-band-edge (NBE) emission are shown to be sensitively position dependent. A linear relation between the equivalent internal quantum efficiency (ηinteq) and τCL for the NBE emission was observed at room temperature under a weak excitation condition, and spatially resolved excitation led to the observation of the highest ηinteq of 20% with τCL of 3.3 ns.

Utilizing Lifetimes to Suppress Random Coil Features in 2D IR Spectra of Peptides

We report that the waiting time delay in 2D IR pulse sequences can be used to suppress signals from structurally disordered regions of amyloid fibrils. At a waiting time delay of 1.0 ps, the random coil vibrational modes of amylin fibrils are no longer detectable, leaving only the sharp excitonic vibrational features of the fibril β-sheets. Isotope labeling with (13)C(18)O reveals that structurally disordered residues decay faster than residues protected from solvent. Since structural disorder is usually accompanied by hydration, we conclude that the shorter lifetimes of random-coil residues is due to solvent exposure. These results indicate that 2D IR pulse sequences can utilize the waiting time to better resolve solvent-protected regions of peptides and that local mode lifetimes should be included in simulations of 2D IR spectra.

Investigation of chlorine radical chemistry in the Eyjafjallajökull volcanic plume using observed depletions in non-methane hydrocarbons

As part of the effort to understand volcanic plume composition and chemistry during the eruption of the Icelandic volcano Eyjafjallajkull, the CARIBIC atmospheric observatory was deployed for three special science flights aboard a Lufthansa passenger aircraft. Measurements made during these flights included the collection of whole air samples, which were analyzed for non-methane hydrocarbons (NMHCs). Hydrocarbon concentrations in plume samples were found to be reduced to levels below background, with relative depletions characteristic of reaction with chlorine radicals (Cl). Recent observations of halogen oxides in volcanic plumes provide evidence for halogen radical chemistry, but quantitative data for free halogen radical concentrations in volcanic plumes were absent. Here we present the first observation-based calculations of Cl radical concentrations in volcanic plumes, estimated from observed NMHC depletions. Inferred Cl concentrations were between 1.3 × 10 and 6.6 × 10 Cl cm. The relationship between NMHC variability and local lifetimes was used to investigate the ratio between OH and Cl within the plume, with [OH]/[Cl] estimated to be ∼37. Copyright 2011 by the American Geophysical Union.