The emergence of a pandemic affecting the respiratory system can result in a significant demand for face masks. This includes the use of cloth masks by large sections of the public, as can be seen during the current global spread of COVID-19. However, there is limited knowledge available on the performance of various commonly available fabrics used in cloth masks. Importantly, there is a need to evaluate filtration efficiencies as a function of aerosol particulate sizes in the 10 nm to 10 μm range, which is particularly relevant for respiratory virus transmission. We have carried out these studies for several common fabrics including cotton, silk, chiffon, flannel, various synthetics, and their combinations. Although the filtration efficiencies for various fabrics when a single layer was used ranged from 5 to 80% and 5 to 95% for particle sizes of <300 nm and >300 nm, respectively, the efficiencies improved when multiple layers were used and when using a specific combination of different fabrics. Filtration efficiencies of the hybrids (such as cotton−silk, cotton−chiffon, cotton−flannel) was >80% (for particles <300 nm) and >90% (for particles >300 nm). We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration. Cotton, the most widely used material for cloth masks performs better at higher weave densities (i.e., thread count) and can make a significant difference in filtration efficiencies. Our studies also imply that gaps (as caused by an improper fit of the mask) can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of "fit" and leakage, while allowing the exhaled air to vent efficiently. Overall, we find that combinations of various commonly available fabrics used in cloth masks can potentially provide significant protection against the transmission of aerosol particles.

Using vacuum process, we fabricated Cu 2 ZnSnS 4 solar cells with 8.4% efficiency, a number independently certified by an external, accredited laboratory. This is the highest efficiency reported for pure sulfide Cu 2 ZnSnS 4 prepared by any method. Consistent with literature, the optimal composition is Cu-poor and Zn-rich despite the precipitation of secondary phases (e.g., ZnS). Despite a very thin absorber thickness (~600 nm), a reasonably good short-circuit current was obtained. Time-resolved photoluminescence measurements suggest a minority carrier-diffusion length on the order of several hundreds of nanometers and relatively good collection of photo-carriers across the entire absorber thickness.

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High efficiency Cu2ZnSnS4 solar cells have been fabricated on glass substrates by thermal evaporation of Cu, Zn, Sn, and S. Solar cells with up to 6.8% efficiency were obtained with absorber layer thicknesses less than 1 μm and annealing times in the minutes. Detailed electrical analysis of the devices indicate that the performance of the devices is limited by high series resistance, a “double diode” behavior of the current voltage characteristics, and an open circuit voltage that is limited by a carrier recombination process with an activation energy below the band gap of the material.