Improved photovoltaic devices, using transparent contacts

Transparent conducting coating is applied to narrow pn junction surface to provide ohmic contact for majority carrier flow. Coating can be made thick to prevent series resistance problem

Optimization of solar cells for air mass zero operation and a study of solar cells at high temperatures, phase 2

Crystal growth procedures, fabrication techniques, and theoretical analysis were developed in order to make GaAlAs-GaAs solar cell structures which exhibit high performance at air mass 0 illumination and high temperature conditions

The optimization of Ga (1-x)Al (x)As-GaAs solar cells for air mass zero operation and a study of Ga (1-x)Al (x)As-GaAs solar cells at high temperatures, phase 1

The three types of solar cells investigated were: (1) one consisting of a nGaAs substrate, a Zn doped pGaAs region, and a Zn doped Ga(1-x)Al(x)As layer, (2) one consisting of an nGaAs substrate, a Ge doped pGaAs region, and a pGa(1-x)Al(x)As upper layer, and (3) one consisting of an n+GaAs substrate, an nGa(1-x)Al(X)As region, a pGa(1-x)Bl(X) As region, and a pGa(1-y)Al(y)As upper layer. In all three cases, the upper alloy layer is thin and of high Al composition in order to obtain high spectral response over the widest possible range of photon energies. Spectral response, capacitance-voltage, current-voltage, diffusion length, sunlight (or the equivalent)-efficiency, and efficiency-temperature measurements were made as a function of device parameters in order to analyze and optimize the solar cell behavior

Optimization of solar cells for air mass zero operation and study of solar cells at high temperatures, phase 4

The Pd contact to GaAs was studied using backscattering, Auger analysis, and sheet resistance measurements. Several metallurgical phases were present at low temperatures, but PdGa was the dominant phase in samples annealed at 500 C. Ti/Pd/Ag contacts appeared to have the lowest contact resistance. Etchback epitaxy (EBE) was compared to saturated melt epitaxy (SME) method of growing liquid phase epitaxial layers. The SME method resulted in a lower density of Ga microdroplets in the grown layer, although the best solar cells were made by the EBE method. Photoluminescence was developed as a tool for contactless analysis of GaAs cells. Efficiencies of over 8 percent were measured at 250 C

The effect of seagrass habitat fragmentation on juvenile blue crab survival

Habitat fragmentation leads to small, isolated habitat patches in which ecological processes may differ substantially from those in larger, continuous habitats. Seagrass is a structurally complex but fragmented subtidal habitat that serves as a refuge from predation for juveniles of the blue crab, Callinectes sapidus Rathbun. I compared the effects of eelgrass ( Zostera marina L.) patch size and shoot density on juvenile blue crab survival both before (June) and after (September) shoot defoliation and cownose ray disturbance changed eelgrass habitat, and used artificial seagrass to determine the influence of eelgrass patch size on juvenile blue crab survival in the absence of covarying shoot density. Under natural conditions, eelgrass patch size, eelgrass shoot density and density-dependent cannibalism all influenced juvenile blue crab survival, but crab survival varied temporally. Crab survival was inversely correlated with eelgrass patch size in the absence of covarying shoot density, in contrast to patterns typically observed in fragmented terrestrial landscapes; this was likely due to low predator abundance in small patches. I tested the hypothesis that crab survival is maximized at an intermediate level of seagrass fragmentation due to the inverse relationship between crab survival and patch size by modeling the joint effects of patch size and proportional cover on juvenile blue crab survival. When I assumed predation on crabs to be independent of crab density, maximal crab survival (ca. 34%) occurred at intermediate values of seagrass fragmentation. Approximately 18% of crabs survived irrespective of the proportion of the landscape covered by seagrass when crab survival was assumed to be density-dependent. My findings indicate that (i) effects of habitat fragmentation on survival may differ between seagrass and terrestrial landscapes, (ii) seagrass habitat fragmentation has a significant but not overriding influence on faunal survival, and (iii) seagrass patch size, seagrass complexity, and blue crab density all influence juvenile blue crab survival, but their effects vary temporally. Habitat fragmentation studies should incorporate multiple scales of space and time, as well as potentially confounding environmental variables

Crime, Income Inequality, and Density at the Neighborhood Level

An economic model of crime gives policymakers a basis to understand how income inequality and population density relate to crime at the neighborhood level. This study reveals a negative and significant relationship between population density in Census tracts and both property and violent crime rates. It finds ambiguous results that vary by city for income inequality. This cross-sectional analysis of Census tracts in Chicago, Los Angeles, Houston, and Dallas uses crime and demographic data from the National Neighborhood Crime Study. This study also yields interesting results about the importance of residential stability for crime prevention and comments on possible urban design tools for crime reduction

Perceptions About Ethical Decision-Making Among North Dakota Elementary Principals

The perceptions of North Dakota elementary principals about ethics and ethical decisions in the workplace were examined in the light of a similar national study. The purpose of the study was to analyze the perceptions of elementary principals of North Dakota to develop a clearer view of what would be considered ethical behaviors and ethical concerns among North Dakota elementary principals. The cumulative perceptions helped clarify the general consensus regarding ethical standards for principals in North Dakota schools. A secondary purpose was to disseminate the results of this analysis in order to help elementary principals in North Dakota in their decision-making. The elementary principals of North Dakota were surveyed by means of both specific and open questions adapted from a national survey instrument (Keough 1992). Follow-up interviews were conducted by telephone with a randomly selected subset of respondents. Of 172 eligible elementary principals in North Dakota, 129 sent responses. The analysis, using Chi-square, compared responses of participants to fifty ethical questions about the amount of time spent as principal, length of time in their current position, rural-urban location, gender, age, and years of experience as principal. Findings of the survey indicated perceived ethical standards in matters covered by the survey. Strong ethical standards existed in the following areas: opposing employment of friends and relatives of school board members and principals, permitting students to participate in decision-making about those things in the school that affected them, and accepting gifts or permitting gifts to influence decision-making. Clear ethical standards did not exist in the following areas: permitting parents to choose the school they desire their child to attend and using VCR tapes with school classes in which the payment was for personal use but not for public (including school) use. In general, the findings of the survey were parallel to the findings from the national survey. Sometimes the weightings were a little different. Even in the two instances where the majority differed, the percentage differences were not substantial

Optimization of solar cells for air mass zero operation and a study of solar cells at high temperatures, phase 3

The etch-back epitaxy process is described for producing thin, graded composition GaAlAs layers. The palladium-aluminum contact system is discussed along with its associated problems. Recent solar cell results under simulated air mass zero light and at elevated temperatures are reported and the growth of thin polycrystalline GaAs films on foreign substrates is developed