Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
1992-09-21
2003-04-01
Weisstuch, Aaron (Department: 1109)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S265000, C257S431000, C257S613000, 43
Reexamination Certificate
active
06541695
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to photocells and the processes of their manufacture and relates more particularly to a class of materials, photocell structures compatible with these materials and associated manufacturing processes that produce high efficiency, inexpensive photocells.
CONVENTION REGARDING REFERENCE NUMERALS
In the figures, each element indicated by a reference numeral will be indicated by the same reference numeral in every figure in which that element appears. The first two digits of any 4 digit reference numerals and the first digit of any two or three digit reference numerals indicates the first figure in which its associated element is presented.
BACKGROUND OF THE INVENTION
The oil embargo of the late 1970's sensitized the world to the problem of limited petrochemicals in the world and the concentration of such chemicals in several regions around the world that are unstable economically and politically. This produced a step increase in the interest level for using renewable energy sources, such as solar power, wind power and tidal power. The recent war between the United States and Iraq has reconfirmed the need for a stable energy source that is not affected by political events around the world. In addition, the desire for clean air is so acute, that interest in nuclear power may be revived despite the well-known radiation dangers and lack of nuclear waste treatment methods. Unfortunately, the progress in developing alternate energy sources has been disappointing and has shown that the development of such technologies is very difficult.
Although there was initially a high level of hope that high efficiency, photovoltaic cells could be manufactured to produce directly from incident solar energy the large amounts of electricity utilized throughout the world, the photovoltaic cells produced up to now have been commercially viable only for special niche markets, such as: solar powered calculators for which consumers are willing to pay the additional cost to avoid the problems of battery replacement; solar powered telephones for use in areas that are remote from electrical power lines; and buildings located in regions of the country that are sunny and sufficiently remote from commercial power lines that solar power is a cost effective alternative. If solar energy is to provide a significant fraction of this country's or the world's power needs, the average cost per Watt for solar photovoltaic cells over the life of such cells must be reduced to a level that is competitive with the average cost per Watt of power from existing electrical utilities over the same period.
At the present time, the average cost per Watt of photovoltaic power, over the life of the photovoltaic cells, is more than five times the typical cost per Watt of electricity produced by present day electric power plants. It is therefore necessary to greatly reduce the cost of photovoltaic cells in order to reduce both the purchase price of a photocell array and the average cost of electricity produced by such cells over the useful life of that array. For solar electric energy to be practical for use by electrical power utilities or to be a practical alternative for use by electrical power consumers, a photocell design must be provided that: has a low material cost; has a highly efficient structure; and can be manufactured in large volumes by low-cost manufacturing processes. The design of this photocell requires an interactive analysis of materials, cell structure and fabrication processes. To produce low cost, efficient cells in the volumes needed to supply a significant fraction of the world's power needs, the manufacturing processes must provide high deposition rates and high layer uniformity over a large area photocell.
A first significant factor in the manufacturing cost of a solar photovoltaic cell is the cost of the materials needed to manufacture this cell. The cost of the material in the photosensitive, current-generating layer of the photovoltaic cell can be a significant fraction of the cost of manufacturing such photovoltaic cell. To efficiently convert incident radiation, this layer must convert most of the incident solar energy into electrical power. If the absorbance value of the photosensitive, current-generating material is small, then its thickness must be correspondingly large to absorb and convert most of the incident solar energy. Because many photosensitive, current-generating materials are relatively expensive, a significantly increased thickness of this layer can significantly increase the total cost of a photovoltaic cell utilizing that material.
Even when such low photosensitivity material is not expensive, it can still significantly increase the cost of the photovoltaic cell. The increased thickness of the photosensitive, current-generating layer increases the average pathlength that the photovoltaically generated charged species must travel to corresponding electrodes. This produces a concomitant increase in the electrical resistance of such layer, thereby decreasing power conversion efficiency. In order to avoid unduly degrading the amount of electrical power produced for a given flux of incident solar energy, the photosensitive, current-generating layer must have a high level of purity in order to have a high enough conductivity that resistive losses do not significantly degrade performance. Such increased material purity requirements can greatly increase the cost of such solar energy cells.
Much of the research and development of solar cells has been directed toward single crystal silicon photocells, because the tremendous amount of knowledge about solid-state circuits manufactured in a silicon substrate can then be applied to this problem. Silicon also has the advantage of being a non-toxic, readily available resource. However, crystalline silicon is a relatively poor solar absorber, because it is an indirect bandgap material. This means that a relatively thick crystalline layer must be utilized to absorb a significant fraction of the incident solar energy. Unfortunately, this increased thickness will degrade efficiency, because of the concomitant increase in the resistance across which the photogenerated charge carriers need to travel. This increase in resistance because of this increased thickness must be offset by a reduction in resistivity by the use of high purity, high perfection silicon layers. Such layers are very costly and therefore significantly increase the cost of single crystal silicon photocells.
These thick layers of silicon must be made by the expensive process of solidification from the melt in a single crystal boule that is then sliced to form the crystalline wafer. Approximately half of this crystal is lost during this slicing process, further increasing the cost. Even though the silicon photocells are durable and efficient, their cost is still prohibitively high for utility power. Although the conventional single crystal layer growth process can be modified to produce lower cost, polycrystalline photocells, this change in the material structure also reduces the efficiency of the resulting photocell, such that the resulting cost of electric power is still too high to compete with existing electrical utilities.
Amorphous silicon is attractive for use as the photosensitive, current-generating layer in photocells, because its high absorptivity for solar energy enables the photosensitive, current-generating layer to be extremely thin, thereby reducing the material cost of that layer and reducing the resistive losses of that layer. This amorphous silicon layer is also very insensitive to impurities. This results in a very inexpensive layer that, unfortunately, due to the nature of electricity transport in amorphous materials, has a very low efficiency.
Although the efficiency can be increased by producing several amorphous layers in a stacked arrangement, this also increases the cost enough that the resulting device is not commercially competitive. Amorphous silicon, which is actually an alloy of hydrogen
Frazzini John A
Weisstuch Aaron
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