Category Energy ConversionTHE SOLAR CELL

Final Words

This work is primarily about solar cells and only secondarily concerned with the milieu in which the solar cells exist. Solar cells have been shown to be useful devices which convert sunlight directly to electricity. We have considered how they operate, and have constructed a realistic model which allows us to make predictions of both their efficiency and the power delivered by such devices. Such predictions are available for solar cells constructed from single crystal semiconductors. Chapter IX considers solar cells fabricated on polycrystalline semiconductors and with amorphous semiconductors...

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The System

As discussed in Chapter I, modem civilization consumes thousands of megawatts of electrical power. If solar cells are to supply a significant portion of this energy requirement, we must give some thought to overall system organization. Fundamentally, we require a source of heat energy and a source of electric energy to power our homes, factories and businesses. A single solar cell supplies direct current at approximately one volt. If this electrical energy is to be used efficiently we must electrically "stack" several solar cells in order to reach the 12 to 200 volts line voltages in common usage...

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Electrical Energy Storage

Classically, the storage of electrical energy has been achieved in two ways. First, utility companies have long used "pumped storage". In this technique excess electrical generating capacity has been utilized to supply energy to pump water into some reservoir behind a dam. During periods of high electrical demand, the stored water is released through conventional hydro-electric generating turbines converting the potential energy to electrical energy. The second conventional storage method stores electrical energy as chemical energy within a storage battery. The secondary, or rechargeable, battery has long been used in moving vehicles to supply starting power, and in the case of electric vehicles, motive power...

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Economics

As is true when we consider every other phase of this complex subject, it is not possible to conduct a discussion of solar cell system energy costs in isolation. Whether used as a "specialty" energy source to energize a wrist watch or power a pocket calculator, or as an energy source for an orbiting communications satellite, or to provide power to a remote desert site in North America, or as commercial of energy for general consumers, solar cells must compete with already existing energy sources and with other potential energy supplies. If solar-electric energy from solar cells is to be widely used it must be cost competitive...

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CONCLUDING THOUGHTS

Introduction

The initial chapter of this work is devoted to a discussion of the various factors involved in the "energy crisis"; surveying various sources of energy, considering the ecological consequences of their usage; and indicating the potential of a solar cell driven solar-electric energy system. Chapter П provides insight into the amount of energy available in sunlight as a function of the site location on the earth’s surface, the weather and the time of year. This second chapter also includes preliminary information on the ways and means of manipulating and concentrating sunlight. The third chapter is devoted to a brief review of solid state physics, semiconductor materials and those phenomena of importance when considering solar cells...

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Concluding Remarks

From the descriptions of the various polycrystalline and amorphous based solar cells, and die multitude of techniques for their fabrication, it is clear that we face complex problems. The discussions in this chapter are but a preliminary view into a complicated and fascinating field.

For the purposes of this work in reviewing the status of solar energy and solar cells, let us now proceed to a concluding chapter and consider a number of additional aspects concerning photovoltaics.

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Amorphous Silicon

In many ways, it is easier to prepare amorphous silicon (a-Si) samples than to fabricate polycrystalline samples [38][62]. Interestingly, elemental amorphous silicon does not appear to possess any commercially valuable photovoltaic properties. Solar cells constructed of elemental amorphous silicon exhibit conversion efficiencies of less than a percent and have electrical characteristics which vary with time. The reason for this is the existence of large numbers of vacancies, and other imperfec­tions within the semiconductor. When coupled with the nonperiodic arrangements of the silicon atoms, this creates vast numbers of allowed energy states...

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Amorphous Material Based Solar Cells

Amorphous materials are characterized by lack of long range order (long range, in this context, may be considered to be any distance in excess of one lattice constant) when the locations of the constituent atoms are considered. The resulting devices, in general, exhibit wider energy gaps than the crystalline varieties of the same material, higher

(often dramatically higher) absorption coefficients and, owing to a large number of non-symmetrical interatomic bonds, a significant number of energy states within the forbidden gap.

The wider energy gap for amorphous silicon facilitates a better match between the light absorbing semiconductor and the solar spectrum...

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