题目内容
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 photo-voltaic devices is economic, but until recently very little progress has been made toward the development of low-cost photo-voltaic devices. The larger part of the research funding has been devoted to study of single-crystal silicon solar cells, despite the evidence, including that of the leading manufacturers of crystalline silicon, that the technique holds little promise. The reason for this pattern is understandable and historical. Crystalline silicon is the active element in the very successful semiconductor industry, and virtually all of the solid state devices contain silicon transistors and diodes. 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 had been proven in extraterrestrial use with efficiencies as high as 18 percent, and other more expensive and scarce materials such as gallium arsenide can have even higher efficiencies, costs must be reduced by a factor of more than 100 to make them practical for commercial use. Besides 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 arrays have been unacceptably small. Moreover, these materials are cheaper only because of the introduction of disordering in crystalline semiconductors, and disorder degrades the efficiencies of crystalline solar cells.
This dilemma 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 than the long-range periodicity of crystals. The advantages of amorphous solar ceils are impressive. Whereas crystals can be grown as wafers about four inches in diameter, amorphous materials can be grown over large areas in a single process, whereas crystalline silicon must be made 200 micron of the proper amorphous materials in 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 photo-conductivity, 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 doubtlessly find even less costly amorphous materials with higher efficiencies. (499)
The author is primarily concerned with ______.
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听力原文： GONAIVES, Haiti--Trucks dumped scores of bodies into a mass grave in this flood-ravaged city still littered with corpses, as officials said the death toll from Tropical Storm Jeanne rose to more than 1,070 and could double again.
There was no funeral ceremony when the bodies were dumped into a 14 foot-deep hole at sunset Wednesday. Dozens of bystanders shrieked, held their noses against the stench and demanded officials collect bodies in nearby waterlogged fields.
The confirmed death toll rose to 1,072, with 1,013 bodies recovered in Gonaives alone, according m Dieufort Deslorges, spokesman for the government's civil protection agency.
He said the number of people missing in the floods rose to 1,250. Only a couple dozen bodies have been identified, and nobody was taking count at the site of the mass grave.
"We're demanding they come and take the bodies from our fields. Dogs are eating then," said bystander Jean Lebrun, listing demands made by residents in the neighborhood whose opposition to mass graves had delayed burials.
"We can only drink the water people died in," the 35-year-old farmer said, citing a lack of potable water in this city of 250,000, with parts still knee-deep in water five days after the storm's passage.
Hurricane experts said Wednesday that Jeanne--now over the open Atlantic as a hurricane--could loop around and head toward the Bahamas then threaten the storm-weary southeastern United States as early as this weekend.
Haiti has lost large numbers of people to a serious ______.
In Britain, the Conservative Party and the Labor Party are the major political powers. In
Mrs. Mirla used to work for the government. Why did Mrs. Mirla come back to work in the field？
The captive, Kenneth Bigley, appealed to British Prime Minister Tony Blair to intervene. "I think this is possibly my last chance,'" he said. "I don't want to die."
Bigley was being held by a militant group led by Jordanian-born terror mastermind Abu Musabal Zarqawi. The group has already beheaded Americans Eugene Armstrong and Jack Hensley, whom it abducted along with Bigley from the Westerners' Baghdad home last week.
On Wednesday, the group also posted a video of Hensley's killing on the Internet, as it had two days earlier of Armstrong's beheading. Henaley's decapitated body was found Wednesday in Baghdad.
More than 130 foreigners have been kidnapped in Iraq, and at least 26 of them have been killed. Many more Iraqis have also been seized in the chaos since Saddam was ousted last year, in many eases for ransom.
In the video, the British hostage ______.
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