Bandgap Profiling in CIGS Solar Cells Via Valence Electron Energy-Loss Spectroscopy

A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn1-xGaxSe2 solar cell that possesses intentional Ga/(In + Ga) composition variation. The EELS-determined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., 100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconductor materials and devices. Published by AIP Publishing

Evolution of the silicon bottom cell photovoltaic behavior during III-V on Si multi-junction solar cells production

The evolution of the Si bulk minority carrier lifetime during the heteroepitaxial growth of III-V on Si multi-junction solar cell structures via metal-organic vapor phase epitaxy has been analyzed. Initially, the emitter formation produces important lifetime degradation. Nevertheless, a progressive recovery was observed during the growth of the metamorphic GaAsP/Si structure. A step-wise mechanism has been proposed to explain the lifetime evolution observed during this process. The initial lifetime degradation is believed to be related to the formation of thermally-induced defects within the Si bulk. These defects are subsequently passivated by fast-diffusing atomic hydrogen -coming from precursor (i.e. PH3 and AsH3) pyrolysis- during the subsequent III-V growth. These results indicate that the MOVPE environment used to create the III-V/Si solar cell structures has a dynamic impact on the minority carrier lifetime. Consequently, designing processes that promote the recovery of the lifetime is a must to support the production of high-quality III-V/Si solar cells

A Current Mode Detector Array for Gamma-Ray Asymmetry Measurements

We have built a CsI(Tl) gamma-ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV gamma-rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion-nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires a high cold neutron flux, control of systematic errors at the ppb level, and the use of current mode gamma-ray detection with vacuum photo diodes and low-noise solid-state preamplifiers. The average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of gamma rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of $10^{-9}$ in a few hours. We report on the design, operating criteria, and the results of measurements performed to test the detector array.Comment: 33 pages, 20 figures, 2 table

Prospects of Scintillating Crystal Detector in Low-Energy Low-Background Experiments

Scintillating crystal detector offers potential advantages in low-energy (keV-MeV range) low-background experiments for particle physics and astrophysics. The merits are discussed using CsI(Tl) crystal as illustrations. The various physics topics which can be pursued with this detector technology are summarized. A conceptual design for a generic detector is presented.Comment: 20 pages, 1 tables, 7 figures, submitted to Astroparticle Physic

Identifying Individual Risk Factors and Documenting the Pattern of Heat-Related Illness through Analyses of Hospitalization and Patterns of Household Cooling

Background As climate change increases the frequency and intensity of extreme heat events researchers and public health officials must work towards understanding the causes and outcomes of heat-related morbidity and mortality. While there have been many studies on both heatrelated illness (HRI), there are fewer on heat-related morbidity than on heatrelated mortality. Objective To identify individual and environmental risk factors for hospitalizations and document patterns of household cooling. Methods We performed a pooled cross-sectional analysis of secondary U.S. data, the Nationwide Inpatient Sample. Risk ratios were calculated from multivariable models to identify risk factors for hospitalizations. Hierarchical modeling was also employed to identify relationships between individual and hospital level predictors of hospitalizations. Patterns of air conditioning use were analyzed among the vulnerable populations identified. Results Hospitalizations due to HRI increased over the study period compared to all other hospitalizations. Populations at elevated risk for HRI hospitalization were blacks, males and all age groups above the age of 40. Those living in zip-codes in the lowest income quartile and the uninsured were also at an increased risk. Hospitalizations for HRI in rural and small urban clusters were elevated, compared to urban areas. Conclusions Risk factors for HRI include age greater than 40, male gender and hospitalization in rural areas or small urban clusters. Our analysis also revealed an increasing pattern of HRI hospitalizations over time and decreased association between common comorbidities and heat illnesses which may be indicative of underreporting