Comparing Environmental Impacts of Single-Junction Silicon and Silicon/Perovskite Tandem Photovoltaics-A Prospective Life Cycle Assessment.

Tandem photovoltaics applying perovskite on silicon are considered to be a possible route to sustaining continuous efficiency improvements and price reductions. A meaningful market share for such tandems is, however, at least a decade away. Herein, a comprehensive prospective life cycle assessment was conducted, comparing the full life cycle of monofacial and bifacial silicon/perovskite tandem panels with single-junction silicon panels produced up to 2050. The end-of-life included the recovery of silicon and silver. Climate change impacts per kilowatt hour were projected to decrease by two-thirds over time. Tandem panels are expected to reach impacts of 8-10 g CO2-eq/kWh in 2050, while single-junction panels may reach 11-13 g CO2-eq/kWh in 2050. Other midpoint impact categories with substantial contributions to damaging human health and ecosystem quality were toxicity, particulate matter formation, and acidification, with tandems having lower impacts in each category. Reductions in impacts over time are mainly the result of grid mix decarbonization and panel efficiency improvements. Balance-of-system and recycling were found to contribute substantially to these impact categories. To ensure that tandem panels provide environmental benefits, annual degradation rates should not exceed 1% for monofacial or 3% for bifacial tandems, and refurbishment of panels with advanced degradation is crucial.

A comparative analysis of sleep quality and time management skills between shift and non-shift police officers.

PURPOSE: The sleep quality of police officers working in shifts is negatively affected due to long working hours and sleeping less during the day. In our study, we aimed to examine the differences in sleep quality and time management skills in police officers working with shift and non-shift system. METHODS: 106 police officers, 46 of whom worked in the shift system, 60 of whom worked in the non-shift system, and whose mean age was 27.6 ± 0.9 [min 20, max 40] were included in the study. Sleep quality was assessed with the Pittsburgh Sleep Quality Index[PSQI] and time management skills with the Time Management Questionnaire[TMQ]. RESULTS: While the TMQ total score was 80.91 ± 12.61 for police officers working in shifts, it was 72.41 ± 12.62 for police officers working non-shift. The PSQI total score was 5 in both groups. There was a difference between the TMQ time planning, TMQ time attitudes sub-dimensions, and TMQ total scores of police officers working in the shift system and those working in the non-shift system[p < 0.05]. While poor sleep quality was observed between both groups, there was no difference in sleep quality[p > 0.05]. CONCLUSION: There was no difference in sleep quality between police officers working in shifts and non-shifts. However, the time management skills of police officers working in shifts were better than those of police officers working in non-shifts. We believe that training programs to improve sleep health and time management skills in police officers are needed for both the sleep quality and well-being of police officers and public safety.

Multi-chromatic silicon nanocrystals.

Silicon nanocrystals (SiNCs) have great potential to become environmental friendly alternatives to heavy-metal containing nanocrystals for applications including medical imaging, lighting and displays. SiNCs exhibit excellent photostability, non-toxicity and abundant resources, but their often reported inefficient and spectrally limited light emission seriously impair their applications. Here we demonstrate a new method that converts SiNCs into an efficient and robust multi-chromatic phosphor. Using ~15 keV electron-beam irradiation of oxide-capped SiNCs, we introduce several types of color centers into the nanocrystal's oxide shell with efficient blue, green and red emission bands, together yielding warm-white photoluminescence, even for a single SiNC. Introduced centers are not native to the original system and we relate them to known defects in silica. Unlike in the silica host, however, here the centers are efficiently optically excitable. Provided further optimization and up-scaling of this method, e-beam irradiated SiNCs can be of great interest as white phosphors for applications such as LEDs.

Analysis of temporal evolution of quantum dot surface chemistry by surface-enhanced Raman scattering.

Temporal evolution of surface chemistry during oxidation of silicon quantum dot (Si-QD) surfaces were probed using surface-enhanced Raman scattering (SERS). A monolayer of hydrogen and chlorine terminated plasma-synthesized Si-QDs were spin-coated on silver oxide thin films. A clearly enhanced signal of surface modes, including Si-Clx and Si-Hx modes were observed from as-synthesized Si-QDs as a result of the plasmonic enhancement of the Raman signal at Si-QD/silver oxide interface. Upon oxidation, a gradual decrease of Si-Clx and Si-Hx modes, and an emergence of Si-Ox and Si-O-Hx modes have been observed. In addition, first, second and third transverse optical modes of Si-QDs were also observed in the SERS spectra, revealing information on the crystalline morphology of Si-QDs. An absence of any of the abovementioned spectral features, but only the first transverse optical mode of Si-QDs from thick Si-QD films validated that the spectral features observed from Si-QDs on silver oxide thin films are originated from the SERS effect. These results indicate that real-time SERS is a powerful diagnostic tool and a novel approach to probe the dynamic surface/interface chemistry of quantum dots, especially when they involve in oxidative, catalytic, and electrochemical surface/interface reactions.

Characterization of Nanocrystal Size Distribution using Raman Spectroscopy with a Multi-particle Phonon Confinement Model.

Analysis of the size distribution of nanocrystals is a critical requirement for the processing and optimization of their size-dependent properties. The common techniques used for the size analysis are transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence spectroscopy (PL). These techniques, however, are not suitable for analyzing the nanocrystal size distribution in a fast, non-destructive and a reliable manner at the same time. Our aim in this work is to demonstrate that size distribution of semiconductor nanocrystals that are subject to size-dependent phonon confinement effects, can be quantitatively estimated in a non-destructive, fast and reliable manner using Raman spectroscopy. Moreover, mixed size distributions can be separately probed, and their respective volumetric ratios can be estimated using this technique. In order to analyze the size distribution, we have formulized an analytical expression of one-particle PCM and projected it onto a generic distribution function that will represent the size distribution of analyzed nanocrystal. As a model experiment, we have analyzed the size distribution of free-standing silicon nanocrystals (Si-NCs) with multi-modal size distributions. The estimated size distributions are in excellent agreement with TEM and PL results, revealing the reliability of our model.