Uncovering temperature-dependent exciton-polariton relaxation mechanisms in hybrid organic-inorganic perovskites.

Hybrid perovskites have emerged as a promising material candidate for exciton-polariton (polariton) optoelectronics. Thermodynamically, low-threshold Bose-Einstein condensation requires efficient scattering to the polariton energy dispersion minimum, and many applications demand precise control of polariton interactions. Thus far, the primary mechanisms by which polaritons relax in perovskites remains unclear. In this work, we perform temperature-dependent measurements of polaritons in low-dimensional perovskite wedged microcavities achieving a Rabi splitting of [Formula: see text] = 260 ± 5 meV. We change the Hopfield coefficients by moving the optical excitation along the cavity wedge and thus tune the strength of the primary polariton relaxation mechanisms in this material. We observe the polariton bottleneck regime and show that it can be overcome by harnessing the interplay between the different excitonic species whose corresponding dynamics are modified by strong coupling. This work provides an understanding of polariton relaxation in perovskites benefiting from efficient, material-specific relaxation pathways and intracavity pumping schemes from thermally brightened excitonic species.

Suspended Graphene Membranes to Control Au Nucleation and Growth.

Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays may show emergent photonic or electronic behavior, and once the nuclei coalesce into thin films, the nucleation density influences parameters such as surface roughness, stress, and grain boundary structure. Tailoring substrate properties to control nucleation is therefore a powerful tool for designing functional thin films and nanomaterials. Here, we examine nucleation control for metals deposited on two-dimensional materials in a situation where substrate effects are absent and heterogeneous nucleation sites are minimized. Through quantification of faceted, epitaxial Au island nucleation on graphene, we show that ultralow nucleation densities with nuclei several micrometers apart can be achieved on suspended graphene under conditions where we measure 2-3 orders of magnitude higher nucleation density on the adjacent supported substrate. We estimate diffusion distances using nucleation theory and find a strong sensitivity of nucleation and diffusion to suspended graphene thickness. Finally, we discuss the role of surface roughness as the main factor determining nucleation density on clean free-standing graphene.

The influence of knee bolster on lumbar spinal stenosis parameters on MR images.

OBJECTIVE: To estimate the effect of the knee bolster use during an MRI on lumbar spinal stenosis parameters and low back pain-related disability. METHODS: A repeated-measurement study of 27 males and 19 females with mean age 55.78 ± 14.36, referred for an MRI of the lumbar spine due to low back pain, performed with and without standard knee bolster. A radiologist evaluated the lumbar lordosis Cobb's angle, the cross-sectional area of the right and left intervertebral foramina and spinal canal at L1-L2, L2-L3, L3-L4 spinal levels. Spinal symptoms were evaluated by the Oswestry Disability Questionnaire. RESULTS: The Cobb angle of lumbar lordosis was found significantly greater on an MRI performed without knee bolster than with bolster (47.30 ± 9.90 vs. 42.57 ± 10.62, p < 0.001). The cross-sectional area of the intervertebral foramina and spinal canal at all evaluated levels was smaller when performed without knee bolster than with bolster. However, differences were significant only at the L1-L2 level and in the spinal canal at all evaluated levels. The Cobb angle, measured with and without knee bolster, showed significant positive correlations with back pain while standing and walking. The spinal canal area without knee bolster showed greater correlations with the Oswestry score parameters than with knee bolster. CONCLUSIONS: MR images at evaluated spinal levels taken without knee bolster showed greater correlations of the spinal canal cross-sectional area with the Oswestry score than ones with knee bolster. Thus, one may prefer MR images acquired without a bolster below the knee compared to an MRI with a knee bolster.

High-Speed Vapor Transport Deposition of Perovskite Thin Films.

Intensive research of hybrid metal-halide perovskite materials for use as photoactive materials has resulted in an unmatched increase in the power conversion efficiency of perovskite photovoltaics (PVs) over the last couple of years. Now that lab-fabricated perovskite devices rival the efficiency of silicon PVs, the next challenge of scalable mass manufacturing of large perovskite PV panels remains to be solved. For that purpose, it is still unclear which manufacturing method will provide the lowest processing cost and highest quality solar cells. Vapor deposition has been proven to work well for perovskites as a controllable and repeatable thin-film deposition technique but with processing speeds currently too slow to adequately lower the production costs. Addressing this challenge, in the present work, we demonstrate a high-speed vapor transport processing technique in a custom-built reactor that produces high-quality perovskite films with unprecedented deposition speed exceeding 1 nm/s, over 10× faster than previous vapor deposition demonstrations. We show that the semiconducting perovskite films produced with this method have excellent crystallinity and optoelectronic properties with 10 ns charge carrier lifetime, enabling us to fabricate the first photovoltaic devices made by perovskite vapor transport deposition. Our experiments are guided by computational fluid dynamics simulations that also predict that this technique could lead to deposition rates on the order of micrometers per second. This, in turn, could enable cost-effective scalable manufacturing of the perovskite-based solar technologies.

High Tolerance to Iron Contamination in Lead Halide Perovskite Solar Cells.

The relationship between charge-carrier lifetime and the tolerance of lead halide perovskite (LHP) solar cells to intrinsic point defects has drawn much attention by helping to explain rapid improvements in device efficiencies. However, little is known about how charge-carrier lifetime and solar cell performance in LHPs are affected by extrinsic defects (i.e., impurities), including those that are common in manufacturing environments and known to introduce deep levels in other semiconductors. Here, we evaluate the tolerance of LHP solar cells to iron introduced via intentional contamination of the feedstock and examine the root causes of the resulting efficiency losses. We find that comparable efficiency losses occur in LHPs at feedstock iron concentrations approximately 100 times higher than those in p-type silicon devices. Photoluminescence measurements correlate iron concentration with nonradiative recombination, which we attribute to the presence of deep-level iron interstitials, as calculated from first-principles, as well as iron-rich particles detected by synchrotron-based X-ray fluorescence microscopy. At moderate contamination levels, we witness prominent recovery of device efficiencies to near-baseline values after biasing at 1.4 V for 60 s in the dark. We theorize that this temporary effect arises from improved charge-carrier collection enhanced by electric fields strengthened from ion migration toward interfaces. Our results demonstrate that extrinsic defect tolerance contributes to high efficiencies in LHP solar cells, which inspires further investigation into potential large-scale manufacturing cost savings as well as the degree of overlap between intrinsic and extrinsic defect tolerance in LHPs and "perovskite-inspired" lead-free stable alternatives.

Direct-indirect character of the bandgap in methylammonium lead iodide perovskite.

Metal halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high-efficiency solar cells and light-emission devices. However, there is currently debate over what drives the second-order electron-hole recombination in these materials. Here, we propose that the bandgap in CH3NH3PbI3 has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect bandgap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect bandgap. Interestingly, in the low-temperature orthorhombic phase, fast quenching of mobile charges occurs independent of the temperature and photon excitation energy. Our work provides a new framework to understand the optoelectronic properties of metal halide perovskites and analyse spectroscopic data.

The Impact of Phase Retention on the Structural and Optoelectronic Properties of Metal Halide Perovskites.

The extent to which the soft structural properties of metal halide perovskites affect their optoelectronic properties is unclear. X-ray diffraction and micro-photoluminescence measurements are used to show that there is a coexistence of both tetragonal and orthorhombic phases through the low-temperature phase transition, and that cycling through this transition can lead to structural changes and enhanced optoelectronic properties.

Photo-induced halide redistribution in organic-inorganic perovskite films.

Organic-inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced 'brightening' of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance.

Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Tunability of the optical band edge in thin PbS films chemically deposited on GaAs(100).

The optical properties of chemical-solution-deposited thin films of lead sulfide (PbS) were investigated using infrared transmission and photoluminescence spectroscopies. The synthesized films are characterized by a wide range of microstructures, from 15 nm nanocrystals up to monocrystals. Energy bandgap values for bulk and nanostructured layers varied from 0.41 eV up to 0.48 eV, respectively. Blueshifts in both absorbance and emission peaks of the nanostructured layers were obtained due to quantum size effects. The optical properties of the films are shown to be size-dependent, with the band edge increasing with temperature.

Silicon photonic crystals doped with colloidally synthesized lead salt semiconductors nanocrystals.

The fabrication of two-dimensional and three-dimensional silicon photonic crystals doped with lead salt nanocrystals is reported. The silicon based photonic crystals of macro-porous silicon are fabricated by electro-chemical etching via masked silicon wafers with the periodicity along the third dimension is achieved by modulating the anodization current and voltage. The chemical solution deposition technique has been utilized to deposit thin layers of lead salts (PbS and PbSe) nanocrystals into the pores. Infrared transmission measurements revealed a considerable red-shift of the photonic band gap in a good agreement with numerical calculations.