Carrier Drift-Mobilities and Solar Cell Models for Amorphous and Nanocrystalline Silicon

Hole drift mobilities in hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) are in the range of 10-3 to 1 cm2/Vs at room-temperature. These low drift mobilities establish corresponding hole mobility limits to the power generation and useful thicknesses of the solar cells. The properties of as-deposited a-Si:H nip solar cells are quite close to their hole mobility limit, but the corresponding limit has not been examined for nc-Si:H solar cells. We explore the predictions for nc-Si:H solar cells based on parameters and values estimated from hole drift-mobility and related measurements. The indicate that the hole mobility limit for nc-Si:H cells corresponds to an optimum intrinsic-layer thickness of 2-3 2m, whereas the best nc-Si:H solar cells (10% conversion efficiency) have thicknesses around 2 2m

Was T. S. Eliot\u27s tantalus Jar actually a Leyden Jar?

T. S. Eliot wrote the introduction to the volume of Ezra Pound’s Selected Poems that was published in 1928. In an important and oft-cited passage, he used the term “tantalus jar”. In the present paper, we show that this term was a coinage. It likely refers to the Leyden jar, which was an early device invented in the 1700s for storing electrical charge. Eliot may have become acquainted with it through The Golden Bough (1912), which he refers to in later work. We speculate as to whether Eliot’s coinage was intentional or not

Drift-Mobility Measurements and Mobility Edges in Disordered Silicons

Published electron and hole drift-mobility measurements in hydrogenated amorphous silicon (a-Si:H), amorphous silicon alloys (a-SiGe:H and a-SiC:H), and microcrystalline silicon (μc-Si:H) are analysed in terms of the exponential bandtail trapping model. A three-parameter model was employed using an exponential bandtail width E, the band mobility μ0, and the attempt-toescape frequency ν. Low-temperature measurements indicate a value around μ0 = 1 cm2 V−1 s−1 for both the conduction and valence bands over the entire range of materials. High temperature-measurements for electrons in a-Si:H suggest a larger value of 7 cm2 V−1 s−1. These properties and those of the frequency ν are discussed as possible attributes of a mobility edge

Hole Mobilities and the Physics of Amorphous Silicon Solar Cells

The effects of low hole mobilities in the intrinsic layer of pin solar cells are illustrated using general computer modeling; in these models electron mobilities are assumed to be much larger than hole values. The models reveal that a low hole mobility can be the most important photocarrier transport parameter in determining the output power of the cell, and that the effects of recombination parameters are much weaker. Recent hole drift-mobility measurements in a-Si:H are compared. While hole drift mobilities in intrinsic a-Si:H are now up to tenfold larger than two decades ago, even with recent materials a-Si:H cells are low-mobility cells. Computer modeling of solar cells with parameters that are consistent with drift-mobility measurements give a good account for the published initial power output of cells from United Solar Ovonic Corp.; deep levels (dangling bonds) in the intrinsic layer were not included in this calculation. Light-soaking creates a sufficient density of dangling bonds to lower the power from cells below the mobility limit, but in contemporary a-Si:H solar cells degradation is not large. We discuss the speculation that light-soaking is ‘self-limiting’ in such cells

Low-Mobility Solar Cells: a Device Physics Primer with Application to Amorphous Silicon

The properties of pin solar cells based on photogeneration of charge carriers into lowmobility materials were calculated for two models. Ideal p- and n-type electrode layers were assumed in both cases. The first, elementary case involves only band mobilities and direct electron–hole recombination. An analytical approximation indicates that the power in thick cells rises as the 1 4 power of the lower band mobility, which reflects the buildup of space-charge under illumination. The approximation agrees well with computer simulation. The second model includes exponential bandtail trapping, which is commonly invoked to account for very low hole drift mobilities in amorphous silicon and other amorphous semiconductors. The two models have similar qualitative behavior. Predictions for the solar conversion efficiency of amorphous silicon-based cells that are limited by valence bandtail trapping are presented. The predictions account adequately for the efficiencies of present a-Si :H cells in their ‘‘asprepared’’ state (without light-soaking), and indicate the improvement that may be expected if hole drift mobilities (and valence bandtail widths) can be improved

Polyaniline on Crystalline Silicon Heterojunction Solar Cells

Organic/inorganic heterojunction solar cells were fabricated on the (100) face of n-type silicon crystals using acid-doped polyaniline PANI with widely varying conductivities. For films with conductivities below 10−1 S/cm, the open-circuit voltage VOC increases with increasing film conductivity as expected when VOC is limited by the work function of the film. Extrapolation of these results to the higher conductivity films indicates that PANI could support VOC of 0.7 V or larger. VOC measurements for the cells with higher conductivity PANI saturated at 0.51 V. We speculate that uncontrolled surface states at the PANI/Si interface are reducing these values

How Green Lakes State Park Got Its Name: The optics and appearance of Fayetteville Green Lakes

The extraordinary and variable appearance of the Fayetteville Green Lakes in the spring, summer, and fall has been the subject of journalistic and scientific attention for more than 150 years. This article addresses the subject in two sections for differing readerships. The first section is a description of the essential science for a general readership. The second section is an abstract of the science for technically knowledgeable readers. The layout of the article is designed for a folded paper flier suitable for distribution to visitors to the lakes. The article describes the three key properties of the lakes’ waters that are responsible for the unusual optics. The first is the high concentration of dissolved calcium carbonate due to the limestone bed of the lakes. The second is the flourishing of a strain of cyanobacteria picoplankton in the lakes from the spring to the fall. The picoplankton absorb the red and blue components of sunlight to drive photosynthesis within the cells. The third is the precipitation of calcite nanocrystals in the water by the picoplankton, which causes seasonal “whiting” events. Rayleigh scattering by calcite aggregates then leads to the brilliant green and the opacity of the lakes when illuminated directly by sunlight

How Should We Set Pandemic Capacity Limits for Restaurants and Bars?

Restaurants and bars are places where airborne diseases like COVID-19 are easily transmitted from one patron to another. However, the connection between the capacity limits and the community infection rate has not been quantified and can appear arbitrary. This data slice describes calculations that could be used to help government officials determine restaurant and bar capacity limits to help limit risk

Thermionic Emission Model for Interface Effects on the Open-Circuit Voltage of Amorphous Silicon Based Solar Cells

We present computer modeling for effects of the p/i interface upon the open-circuit voltage VOC in amorphous silicon based pin solar cells. We show that the modeling is consistent with measurements on the intensitydependence for the interface effect, and we present an interpretation for the modeling based on thermionic emission of electrons over the electrostatic barrier at the p/i interface. We present additional modeling of the relation of VOC with the intrinsic layer bandgap EG. The experimental correlation for optimized cells is VOC = (EG/e)-0.79. The correlation is simply explained if VOC in these cells is determined by the intrinsic layer, and in particular by the (variable) bandgap and by a nonvarying valence bandtail width (about 48 meV) of this layer

Ventilation & Masks: Reducing Airborne Transmission of COVID-19 in a Classroom

We\u27ve calculated the number of COVID-19 infections that will be spread from a single COVID-19 superspreader to students and teachers in a classroom shared for 4 hours in different ventilation and masking conditions