Passage 18
The need for solar electricity is clear. It is safe, ecologically sound, efficient, continuously available, and it has no moving parts. The basic problem with the use of solar photovoltaic devices is economics, but until recently very little progress had been made toward the development of low-cost photovoltaic devices. The larger part of research funds has been devoted to the study of single-crystal silicon solar cells, despite the evidence that this technique holds little promise. The reason for this pattern is understandable and historical. Crystalline silicon, however, is particularly unsuitable to terrestrial solar cells.
Crystalline silicon solar cells work well and are successfully used in the space program, where cost is not an issue. While single crystal silicon has been proven in extraterrestrial use with efficiencies as high as 18 percent, and other more expensive and scarce materials can have ever higher efficiencies, costs must be reduced by a factor of more that 100 to make them practical for commercial uses. Beside the fact that the starting crystalline silicon is expensive, 95 percent of it is wasted and does not appear in the final device. Recently, there have been some imaginative attempts to make polycrystalline and ribbon silicon which are lower in cost than high-quality single crystals； but to date the efficiencies of these apparently lower-cost materials have been unacceptably small. Moreover, these materials are cheaper only because of the introduction of disordering in crystalline semiconductors, and disorder degrades the efficiency of crystalline solar cells.
This difficulty can be avoided by preparing completely disordered or amorphous materials. Amorphous materials have disordered atomic structure as compared to crystalline materials: that is, they have only short-range order rather that the long-range periodicity of crystals. The advantages of amorphous solar cells are impressive. Whereas crystalline silicon must be made 200 microns thick to absorb a sufficient amount of sunlight for efficient energy conversion, only 1 micron of the proper amorphous materials is necessary. Crystalline silicon solar cells cost in excess of 100 per square foot, but amorphous films can be created at a cost of about 50 cents per square foot.
Although many scientists were aware of the very low cost of amorphous solar cells, they felt that they could never be manufactured with the efficiencies necessary to contribute significantly to the demand for electric power. This was based on a misconception about the feature which determines efficiency. For example, it is not the conductivity of the material in the dark which is relevant, but only the photoconductivity, that is, the conductivity in the presence of sunlight. Already, solar cells with efficiencies well above 6 percent have been developed using amorphous materials, and further research will doubtless find even less costly amorphous materials with higher efficiencies.
65.The author is primarily concerned with _______.
(A) discussing the importance of solar energy
(B) explaining the functioning of solar cells
(C) presenting a history of research on energy sources
(D) describing a possible solution to the problem of the cost of photovoltaic cells
66.According to the passage, which of the following encouraged use of silicon solar cells in the space program?
I.the higher cost of materials such as gallium arsenide
II.the fairly high extraterrestrial efficiency of the cells
III.the relative lack of cost limitations in the space program
(A)I only (B)II only (C)I and II only (D)II and III only
67.In the second paragraph, he author mentions recent attempts to make polycrystalline and ribbon silicon primarily in order to ______.
(A) minimize the importance of recent improvements in silicon solar cells
(B) demonstrate the superiority of amorphous materials over crystalline silicon
(C) explain why silicon solar cells have been the center of research
(D) contrast crystalline silicon with polycrystalline and ribbon silicon
68.Which of the following pairs of terms does the author regard as most nearly synonymous?
(A)solar and extraterrestrial
(B)photovoltaic devices and solar cells
(C)crystalline silicon and amorphous materials
(D)amorphous materials and higher efficiencies
69.The material in the passage could best be used in an argument for _______.
(A) discontinuing the space program
(B) increased funding for research on amorphous materials
(C) further study of the history of silicon crystals
(D) increased reliance on solar energy
70.The tone of the passage can best be described as _______.
(A) analytical and optimistic
(B) biased and unprofessional
(C) critical and discouraged
(D) hesitating and inconclusive