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Hysteresis and Its Correlation to Ionic Defects in Perovskite Solar Cells
Full text linkIon migration has been reported to be one of the main reasons for hysteresis in the current-voltage (J-V) characteristics of perovskite solar cells. We investigate the interplay between ionic conduction and hysteresis types by studying Cs0.05(FA0.83MA0.17)0.95Pb(I0.9Br0.1)3 triple-cation perovskite solar cells through a combination of impedance spectroscopy (IS) and sweep-rate-dependent J-V curves. By comparing polycrystalline devices to single-crystal MAPbI3 devices, we separate two defects, β and γ, both originating from long-range ionic conduction in the bulk. Defect β is associated with a dielectric relaxation, while the migration of γ is influenced by the perovskite/hole transport layer interface. These conduction types are the causes of different types of hysteresis in J-V curves. The accumulation of ionic defects at the transport layer is the dominant cause for observing tunnel-diode-like characteristics in the J-V curves. By comparing devices with interface modifications at the electron and hole transport layers, we discuss the species and polarity of involved defects
Structural basis of antimicrobial membrane coat assembly by human GBP1
Full text linkGuanylate-binding proteins (GBPs) are interferon-inducible guanosine triphosphate hydrolases (GTPases) mediating host defense against intracellular pathogens. Their antimicrobial activity hinges on their ability to self-associate and coat pathogen-associated compartments or cytosolic bacteria. Coat formation depends on GTPase activity but how nucleotide binding and hydrolysis prime coat formation remains unclear. Here, we report the cryo-electron microscopy structure of the full-length human GBP1 dimer in its guanine nucleotide-bound state and describe the molecular ultrastructure of the GBP1 coat on liposomes and bacterial lipopolysaccharide membranes. Conformational changes of the middle and GTPase effector domains expose the isoprenylated C terminus for membrane association. The α-helical middle domains form a parallel, crossover arrangement essential for coat formation and position the extended effector domain for intercalation into the lipopolysaccharide layer of gram-negative membranes. Nucleotide binding and hydrolysis create oligomeric scaffolds with contractile abilities that promote membrane extrusion and fragmentation. Our data offer a structural and mechanistic framework for understanding GBP1 effector functions in intracellular immunity
Flexibility in noisy cell-to-cell information dynamics
Full text linkThe exchange of molecules allows cells to exchange information. How robust is the information to changes in cell parameters? We use a mapping between the stochastic dynamics of two cells sharing a stimulatory molecule and parameters akin to an extension of Landau's equilibrium phase transition theory. We show that different single-cell dynamics lead to the same dynamical response—a flexibility that cells can use. The companion equilibrium Landau model behaves similarly, thereby describing the dynamics of information in a broad class of models with coupled order parameters
Silicon-Inspired Analysis of Interfacial Recombination in Perovskite Photovoltaics
Full text linkPerovskite solar cells have reached an impressive certified efficiency of 26.1%, with a considerable fraction of the remaining losses attributed to carrier recombination at perovskite interfaces. This work demonstrates how time-resolved photoluminescence spectroscopy (TRPL) can be utilized to locate and quantify remaining recombination losses in perovskite solar cells, analogous to methods established to improve silicon solar cell passivation and contact layers. It is shown how TRPL analysis can be extended to determine the bulk and surface lifetimes, surface recombination velocity, the recombination parameter, J0, and the implied open-circuit voltage (iVoc) of any perovskite device configuration. This framework is used to compare 18 carrier-selective and passivating contacts commonly used or emerging for perovskite photovoltaics. Furthermore, the iVoc values calculated from the TRPL-based framework are directly compared to those calculated from photoluminescence quantum yields and the measured solar cell Voc. This simple technique serves as a practical guide for screening and selecting multifunctional, passivating perovskite contact layers. As with silicon solar cells, most of the material and interface analysis can be done without fabricating full devices or measuring efficiency. These purely optical measurements are even preferable when studying bulk and interfacial passivation approaches, since they remove complicating effects from poor carrier extraction
The potential of strigolactones to shift competitive dynamics among two Rhizophagus irregularis strains
Full text linkStrigolactones are phytohormones that influence arbuscular mycorrhizal fungal (AMF) spore germination, pre-symbiotic hyphal branching, and metabolic rates. Historically, strigolactone effects have been tested on single AMF strains. An open question is whether intraspecific variation in strigolactone effects and intraspecific interactions can influence AMF competition. Using the Rhizophagus irregularis strains A5 and C2, we tested for intraspecific variation in the response of germination and pre-symbiotic growth (i.e., hyphal length and branching) to the strigolactones GR24 and 5-deoxystrigol. We also tested if interactions between these strains modified their germination rates and pre-symbiotic growth. Spore germination rates were consistently high (> 90%) for C2 spores, regardless of treatment and the presence of the other strain. For A5 spores, germination was increased by strigolactone presence from approximately 30 to 70% but reduced when grown in mixed culture. When growing together, branching increased for both strains compared to monocultures. In mixed cultures, strigolactones increased the branching for both strains but led to an increase in hyphal length only for the strain A5. These strain-specific responses suggest that strigolactones may have the potential to shift competitive dynamics among AMF species with direct implications for the establishment of the AMF community
Detecting single nanoparticles using fiber-tip nanophotonics
Full text linkSensing nano-objects, from nanoparticles to molecules, has become a crucial need in environmental monitoring, medical diagnostics, and drug development. Detection of single particles and molecules is highly desirable, as it provides specific information on size, dynamics, and interactions. Current nanophotonic implementations rely on complex optical readout schemes, limiting their application in the field. Here we demonstrate a nanophotonic fiber-tip sensor with a compact sensor footprint and a simple readout scheme. We leverage advanced design methods to simultaneously achieve a small mode volume Vm = 0.74 (λ/n)3, narrow linewidth 1λ = 0.4 nm, and a large modulation 1R ≈ 20% in reflection from the fiber. This unique combination of properties opens the way to sensing weak nanoscale perturbations in the vicinity of the fiber tip. In particular, we experimentally demonstrate the real-time detection of single 50 nm nanoparticles. This opens a route towards real-time sensing of single nanoparticles, and potentially single molecules, in environmental monitoring and diagnostics
The effect of In(Ga)As/GaAs quantum dots on the optical loss of photonic crystal cavities
Full text linkWe present a systematic investigation of the optical losses in GaAs photonic crystal cavities with and without embedded self-assembled In(Ga)As quantum dots (QDs) to shed light on additional loss mechanisms related to the presence of the QDs. To clarify the role of the measurement method, we propose an experimental configuration where the optical properties can be evaluated simultaneously through reflection and photoluminescence measurements. Independently of the measurement method, we observe a reduced quality (Q) factor in cavities with embedded QDs when compared to the passive counterparts. Our analysis indicates that these additional losses—about 7 GHz—are unrelated to direct excitonic absorption for the investigated areal QD densities of 175 μ m − 2 . We analyze several mechanisms which could explain our observations and suggest that a possible origin could be unsaturable absorption from midgap defects introduced by the QD growth
Interlaboratory Comparison on Absolute Photoluminescence Quantum Yield Measurements of Solid Light Converting Phosphors with Three Commercial Integrating Sphere Setups
Full text linkScattering luminescent materials dispersed in liquid and solid matrices and luminescent powders are increasingly relevant for fundamental research and industry. Examples are luminescent nano- and microparticles and phosphors of different compositions in various matrices or incorporated into ceramics with applications in energy conversion, solid-state lighting, medical diagnostics, and security barcoding. The key parameter to characterize the performance of these materials is the photoluminescence/fluorescence quantum yield (Φf), i.e., the number of emitted photons per number of absorbed photons. To identify and quantify the sources of uncertainty of absolute measurements of Φf of scattering samples, the first interlaboratory comparison (ILC) of three laboratories from academia and industry was performed by following identical measurement protocols. Thereby, two types of commercial stand-alone integrating sphere setups with different illumination and detection geometries were utilized for measuring the Φf of transparent and scattering dye solutions and solid phosphors, namely, YAG:Ce optoceramics of varying surface roughness, used as converter materials for blue light emitting diodes. Special emphasis was dedicated to the influence of the measurement geometry, the optical properties of the blank utilized to determine the number of photons of the incident excitation light absorbed by the sample, and the sample-specific surface roughness. While the Φf values of the liquid samples matched between instruments, Φf measurements of the optoceramics with different blanks revealed substantial differences. The ILC results underline the importance of the measurement geometry, sample position, and blank for reliable Φf data of scattering the YAG:Ce optoceramics, with the blank’s optical properties accounting for uncertainties exceeding 20%
Nanocavities for Molecular Optomechanics: their fundamental description and application
Full text linkVibrational Raman scattering─a process where light exchanges energy with a molecular vibration through inelastic scattering─is most fundamentally described in a quantum framework where both light and vibration are quantized. When the Raman scatterer is embedded inside a plasmonic nanocavity, as in some sufficiently controlled implementations of surface-enhanced Raman scattering (SERS), the coupled system realizes an optomechanical cavity where coherent and parametrically amplified light–vibration interaction becomes a resource for vibrational state engineering and nanoscale nonlinear optics. The purpose of this Perspective is to clarify the connection between the languages and parameters used in the fields of molecular cavity optomechanics (McOM) versus its conventional, “macroscopic” counterpart and to summarize the main results achieved so far in McOM and the most pressing experimental and theoretical challenges. We aim to make the theoretical framework of molecular cavity optomechanics practically usable for the SERS and nanoplasmonics community at large. While quality factors (Q) and mode volumes (V) essentially describe the performance of a nanocavity in enhancing light-matter interaction, we point to the light-cavity coupling efficiencies (η) and optomechanical cooperativities (
) as the key parameters for molecular optomechanics. As an illustration of the significance of these quantities, we investigate the feasibility of observing optomechanically induced transparency with a molecular vibration─a measurement that would allow for a direct estimate of the optomechanical cooperativity
Light-controlled morphological development of self-organizing bioinspired nanocomposites
Full text linkNature's intricate biominerals inspire fundamental questions on self-organization and guide innovations towards functional materials. While advances in synthetic self-organization have enabled many levels of control, generating complex shapes remains difficult. Specifically, controlling morphologies during formation at the single micro/nanostructure level is the key challenge. Here, we steer the self-organization of barium carbonate nanocrystals and amorphous silica into complex nanocomposite morphologies by photogeneration of carbon dioxide (CO2) under ultraviolet (UV) light. Using modulations in the UV light intensity, we select the growth mode of the self-organization process inwards or outwards to form helical and coral-like morphologies respectively. The spatiotemporal control over CO2 photogeneration allows formation of different morphologies on pre-assigned locations, switching between different growth modes—to form for instance a coral on top of a helix or vice versa, and subtle sculpting and patterning of the nanocomposites during formation. These findings advance the understanding of these versatile self-organization processes and offer new prospects for tailored designs of functional materials using photochemically driven self-organization