2. 考研
  3. Sunlight is free, but that is no reason to waste it. Yet even the best silicon solar cells—by far the most【C1】________ sort—conv

Sunlight is free, but that is no reason to waste it. Yet even the best silicon solar cells—by far the most【C1】________ sort—conv

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问题     Sunlight is free, but that is no reason to waste it. Yet even the best silicon solar cells—by far the most【C1】________ sort—convert only a quarter of the light that falls on them. Silicon has the【C2】________of being cheap: manufacturing improvements have brought its price to a point where it is snapping at the heels of fossil fuels.【C3】________many scientists would like to replace it【C4】________something fundamentally better.
    John Rogers, of the University of Illinois, Urbana-Champaign, is one. The cells he has【C5】________can convert 42.5％ of sunlight.【C6】________improved, Dr Rogers reckons, their efficiency could rise to 50％. Their【C7】________is that they are actually not one cell, but four, stacked one on top of another.
    Solar cells are made of semiconductors, and every type of semiconductor has a【C8】________called a band gap that is different from that of other semiconductors. The band gap【C9】________the longest wavelength of light a semiconductor can absorb (it is transparent to longer wavelengths). It also fixes the【C10】________amount of energy that can be【C11】________ from shorter wavelength. The result is that long-wavelength photons are lost and short-wave ones incompletely utilised.
    Dr Rogers【C12】________this by using a different material for each layer of the stack. He chooses his materials【C13】________ the bottom of the band gap of the top layer matches the top of the band gap of the one underneath, and so on【C14】________ the stack. Each layer thus【C15】________off part of the spectrum, converts it efficiently into electrical energy and passes the rest on.
    The problem is that the materials needed to make these semiconductors are【C16】________. But Dr Rogers has found a way to overcome this.【C17】________solar-cell modules are completely covered by semiconductor, but in his only 0.1％ of the surface is so covered. The semiconducting stacks, each half a millimeter square, are【C18】________over that surface many dots. Each stack then has a pair of cheap glass lenses【C19】________over it. These focus the sun’s light onto the stack, meaning that all【C20】________light meets a semiconductor.
【C14】
选项 A、along
B、down
C、to
D、across
答案B
解析 从空格前面的句子“上层底的能隙同下一层顶的能隙相匹配”及前面提到的“电池有四块叠加”可推出，作者对电池的讨论由上及下，故选B项down。
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