Electrical Engineering

Insights into the growth processes and evolution of microstructure in intrinsic hydrogenated silicon (Si:H) films obtained from real-time spectroscopic ellipsometry (RTSE) are extended to the characterization of the optoelectronic properties of the corresponding solar cells. To assess the effects of transition regions from the amorphous to mixed microcrystalline phases, cell structures with and without such regions at different depths in the i-layer from the p-contact have been investigated. Experimental results are presented that clearly demonstrate changes in the mobility gap, Eµ, of the materials as their microstructure evolves with thickness, further supporting the important effect of the hydrogen dilution ratio R (R[H2]/[SiH4]) on the transition between the amorphous and microcrystalline phases. Light J-V characteristics at room temperature and dark J-V characteristics at different temperatures were measured on p(a-SiC:H:B)-i(Si:H)-n(µc-Si:H:P) solar cell structures with i-layers of different thicknesses and R values. The mobility gaps of both the amorphous and microcrystalline intrinsic-layer materials as well as those of the transition layers are obtained from dark J(V,T) measurements. Using numerical simulation, both the light and the dark J-V characteristics are self-consistently modeled assuming sharp changes in the mobility gaps at the intrinsic layer transition thicknesses determined by RTSE.

In order to obtain more insight into the nature of the recovery in the light induced changes at room temperature in hydrogenated amorphous silicon (a-Si:H) solar cells the relaxation of the photocurrents in the light induced changes in protocrystalline a-Si:H thin films were investigated. Immediately upon the removal of 1 sun illumination recoveries in the photocurrents are found like those present in the currents in the dark current- voltage characteristics in corresponding p-i-n solar cells. The striking similarity between the results on thin films and the corresponding dark forward bias current-voltage characteristics of solar cells suggest that the recoveries obtained with low generation rates (5x10 cm s ) in the films are a measure of annealing kinetics of the defect states around midgap in the bulk of the films. The rates of recoveries decrease with higher carrier generation rates and the length of the light induced degradation. Results are presented which indicate that the history of creation and annealing of light induced defect states is important in determining subsequent creation and annealing kinetics.

Generally the dark forward bias current voltage (JD-V) characteristics of a-Si:H solar cells are analyzed without clearly separating their contributions due to carrier recombination in the bulk from that at the p/i interface regions nor those imposed by carrier injection from the p and n contacts. Furthermore their exponential regimes are interpreted and fitted with constant diode quality factor n with modeling which is based on many fitting parameters that have not been reliably established. A new approach has been developed for the analysis of JD-V characteristics for the entire voltage range relevant to the operation of solar cells, which has allowed the three contributions to be identified and characterized. It is based on the analysis of their bias dependent differential diode quality factors, n(V), from which important information on the energy distribution of the defect states in the i-layers has been obtained. Results are presented and discussed here for p-i-n a-Si:H solar cells having sufficiently low p/i interface recombination so that the limitations by the bulk recombination on open circuit voltage, Voc, can be identified. These results are then correlated with the defect state distributions previously obtained from the analysis on differential diode quality factor n(V) characteristics for hydrogen diluted and undiluted intrinsic layers both in the annealed state as well as after introducing light induced defects.

For the first time direct correlations are obtained between the light induced changes under 1 sun illumination in the properties of a-Si:H and those in the characteristics of p-i-n cells incorporating identically-prepared i-layers. These correlations were obtained after account was taken of the effects that the location of the electron and hole quasi-Fermi levels have on the carrier recombination that occurs through the different gap states. The changes in midgap state density, as measured on the films and reflected in the subgap absorption at 1.2 eV, are directly correlated with changes in the dark I-V characteristics under low forward bias. In this case small quasi-Fermi level splitting is present so the recombination of the injected carriers is determined by the midgap states in the bulk of the i-layer. In addition, the changes in the electron mobility-lifetime products as measured on the films are correlated with changes in the fill factor measured on cells under the same conditions as long as large quasi-Fermi level splitting is present and recombination occurs through states spanning a wide region of the gap, such as occurs under 1 sun illumination. The results explain (i) the failure of numerous attempts to correlate the degradation of solar cells reliably with the creation of dangling bond defects and (ii) the inadequacy of the large number of modeling results that assume such a correlation.

A study was carded out with the goal of obtaining high open circuit voltages (Voc) in a-Si:H n-i-p solar cells, taking into account the evolutionary nature of the microstructure of the p-layers during growth. It is found that cells with players in the protocrystalline Si:H growth regime give the highest values of Voc not those with microcrystalline Si:H p-layers. Evidence for this conclusion is presented whereby Voc is related directly to the microstructure of the p-layers, as characterized using spectroscopic ellipsometry, atomic force microscopy, and electrical measurements. The results clarify the origins of (i) inconsistencies associated with attributing high Voc in n-i-p cells to the microcrystallinity of the p-layers, as well as (ii) the inability to obtain similarly high values in p-i-n superstrate cells. Because the microstructure of p-type protocrystalline Si:H depends on that of the underlying i-layer, it is not possible to optimize the cell parameters based on an understanding of the process unless detailed characterization of the p-layer in the actual cell configuration is performed.

Studies have been carried out on high quality hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells with protocrystalline i-layers to establish the nature of p/i interfaces and to quantify their contributions to various solar cell characteristics. The p-a-SiC:H,B/i-a-Si:H/n-μcSi:H,P cell structures used had the a-Si:H i-layers deposited from hydrogen diluted silane with R≡[H2 ]/[SiH4]=10. The high quality p/i interface regions obtained with R=10, indicated by the high and stable open circuit voltage (Voc) values, were further improved by increasing R in the 200 Å of a-Si:H adjacent to the a-SiC:H layer. From the systematic improvement and ability to obtain p/i interface regions with outstanding quality, it was possible to track their contributions to cell characteristics relative to those from the bulk. Results of dark current voltage (JD-V) and short circuit current-open circuit voltage (Jsc-Voc) characteristics are presented which clearly demonstrate that even high quality interface regions in p-i-n cells can mask some contributions of protocrystalline bulk layers. Results are also presented and discussed on how the relative contributions of bulk and p/i interfaces can be isolated and quantified so they can be used as inputs for reliable analysis of solar cell characteristics

A systematic study has been carried out to quantify the effect of microcrystallite nucleation in the intrinsic layer of protocrystalline Si:H p-i-n solar cells prepared by rf plasma enhanced chemical vapor deposition (PECVD). Real-time spectroscopic ellipsometry (RTSE) results that have previously identified the transitions from amorphous to microcrystalline phase were confirmed with atomic force microscopy (AFM) images. The effects of the phase transitions in the bulk intrinsic layer, as well as near the p/i interface of p-i-n cells, have been evaluated as a function of film thickness and H2-dilution ratio (R=[H2]/[SiH4]). Illuminated and dark J-V characteristics are correlated with the microstructural evolution of the Si:H films yielding insight into the “sharpness” of the transition. Evidence of abrupt changes in the mobility gap is obtained from the selfconsistent numerical modeling of cell characteristics.

The ability to characterize the phase of the intrinsic (i) layers incorporated into amorphous silicon [a-Si:H] and microcrystalline silicon [μc-Si:H] thin film solar cells is critically important for cell optimization. In this research, a new method has been developed to extract the thickness evolution of the μc-Si:H volume fraction in mixed phase amorphous + microcrystalline silicon [(a+μc)-Si:H] i-layers. This method is based on real time spectroscopic ellipsometry measurements performed during plasma enhanced chemical vapor deposition of the films. In the analysis, the thickness at which crystallites first nucleate from the a-Si:H phase can be estimated, as well as the nucleation density and microcrystallite cone angle. The results show very good correlations with structural and electronic device measurements.

Despite enormous efforts over the course of several decades, sustainable development on the African continent remains incompletely realized. Top-down methodologies have largely failed and many sustainable development organizations now support small-scale direct approaches, which provide the tools and knowledge necessary for people to help themselves. The new approach to sustainable development of open source appropriate technology (OSAT) builds on these successes. OSAT is a development method for traditional appropriate technologies (following Schumacher). The fundamental concept behind OSAT is that anyone can learn how to make and use AT free of intellectual property concerns, and can simultaneously add to the collective open source knowledge base by contributing ideas, observations, and experimental data. This approach to drive sustainable development is promising because the continuous peer review should result in better quality, higher reliability, more flexibility and of course less expensive (free) AT than conventional design/patenting. As OSAT is being developed largely in English, Africa’s many languages present a fundamental challenge to OSAT’s use for sustainable development on this continent. This paper provides potential solutions to this challenge by reviewing i) the ongoing merger between Appropedia (English) and Ekopedia (French), ii) systematic translations at the NGO Catalytic Communities, and iii) virtual service learning in languages. Despite enormous efforts over the course of several decades, sustainable development on the African continent remains incompletely realized. Top-down methodologies have largely failed and many sustainable development organizations now support small-scale direct approaches, which provide the tools and knowledge necessary for people to help themselves. The new approach to sustainable development of open source appropriate technology (OSAT) builds on these successes. OSAT is a development method for traditional appropriate technologies (following Schumacher). The fundamental concept behind OSAT is that anyone can learn how to make and use AT free of intellectual property concerns, and can simultaneously add to the collective open source knowledge base by contributing ideas, observations, and experimental data. This approach to drive sustainable development is promising because the continuous peer review should result in better quality, higher reliability, more flexibility and of course less expensive (free) AT than conventional design/patenting. As OSAT is being developed largely in English, Africa’s many languages present a fundamental challenge to OSAT’s use for sustainable development on this continent. This paper provides potential solutions to this challenge by reviewing i) the ongoing merger between Appropedia (English) and Ekopedia (French), ii) systematic translations at the NGO Catalytic Communities, and iii) virtual service learning in languages.

To effectively teach science in the elementary classroom, pre-service K-8 teachers need a basic understanding of the underlying concepts of physics, which demand a strong foundation in mathematics. Unfortunately, the depth of mathematics understanding of prospective elementary teachers has been a growing and serious concern for several decades. To overcome this challenge, a two-pronged attack was used in this study. First, students in mathematics courses were coupled with physical science courses by linking registration to ensure co-requisites were taken. This alone improved passing rates. Secondly, an energy conservation project was introduced in both classes that intimately tied the theoretical mathematics base knowledge to problems in physical science, energy efficiency, and household economics. These connections made the mathematics highly relevant to the students and improved both their theoretical understanding and their grades. Together, the two approaches of tying mathematics to physical science and applying mathematical skills to solving energy efficiency problems have shown to be extremely effective at improving student performance. This five-year study not only exhibited record improvements in student performance, but also can be easily replicated at other institutions experiencing similar challenges in training pre-service elementary school teachers.

The appropriate technology (AT) movement is being driven by inventors and innovators who are interested in designing technologies that are culturally, environmentally, and economically appropriate, and feasible to construct and use for people anywhere in the world. This paper examines how open sharing of designs, specifications, and technical information can enhance effectiveness, widespread use, and innovation of AT. This commons-based open design method has been highly successful for software development (i.e., open source), and has also begun to be used in other fields through unique partnerships and new information-sharing tools on the internet. This paper critically demonstrates key examples of open design successes that can be applied to development of AT. It also identifies potential barriers to open-sourcing AT designs, analyzes business models for open design in the context of AT, and outlines practical solutions with examples currently underway.

Currently, those working for a sustainable development in a vast array of contexts all over the world are often duplicating efforts. In an era where a rapid transition towards sustainability is needed, such wasted effort is no longer tolerable. This paper will discuss current work to overcome this challenge by creating an Open Sustainability Network (OSN) that links relevant individuals, programs, courses, projects, and organizations aimed at just sustainable development. The paper will build an understanding of, and collaboration between, relevant online tools. The paper concludes that the OSN can: 1) develop partnerships with sites with online tools to alleviate some of the technological overhead; 2) help inform entrepreneurs and expanding businesses about the challenges and opportunities presented in social entrepreneurship; and 3) benefit service learning implementation by acting as a repository for appropriate technologies, systems, and policies, while also acting as a clearinghouse for international and local collaborations.

Many people in less developed countries drink water with microbial contamination, which leads to the annual death of 5 million children. Although some people currently boil water, all microbes that cause disease in humans do not survive at temperatures >65oC, which solar water pasteurizers can easily produce. These pasteurizers are similar to box solar cookers, and typically have a small rectangular reflector. The objective of this work is to calculate the increase in output due to compound parabolic concentrators (CPCs) using numerical simulation. A CPC concentrates the maximum amount of radiation on a planar receiver consistent with the laws of thermodynamics. Since the average water temperature is nearly independent of the solar radiation, the heat losses are nearly constant. Therefore, higher concentration factors yield significantly higher efficiencies. Depending on climate, CPCs increase output by 1,000%-4,000%, while the additional reflector would only ~double the cost and necessitate weekly tilting.

As both population and energy use per capita increase, modern society is approaching physical limits to its continued fossil fuel consumption. The immediate limits are set by the planet’s ability to adapt to a changing atmospheric chemical composition, not the availability of resources. In order for a future society to be sustainable while operating at or above our current standard of living a shift away from carbon based energy sources must occur. An overview of the current state of active solar (photovoltaic) energy technology is provided here to outline a partial solution for the environmental problems caused by accelerating global energy expenditure. The technical, social, and economic benefits and limitations of photovoltaic technologies to provide electricity in both off-grid and on-grid applications is critically analyzed in the context of this shift in energy sources. It is shown that photovoltaic electrical production is a technologically feasible, economically viable, environmentally benign, sustainable, and socially equitable solution to society’s future energy requirements