A.DrRogersgetsroundthisbyusingadifferentmaterialforeach...

问题详情：

A. Dr Rogers gets round this by using a different material for each layer of the stack.

B. The technology continues to pop up in new devices all the time, from sunglasses to electric vehicle charging stations.

C. But many scientists would like to replace it with something fundamentally better.

D. Suitably improved, Dr Rogers reckons, their efficiency could rise to 50%.

E. The hope for a “solar revolution” has been floating around for decades.

F. The band gap defines the longest wavelength of light a semiconductor can absorb (it is transparent to longer wavelengths).

Sunlight is free, but that is no reason to waste it. Yet even the best silicon solar cells – by far the most common sort – convert only a quarter of the light that falls on them. Silicon has the merit of being cheap: manufacturing improvements have brought its price to a point where it is snapping at the heels of fossil fuels.  67. ______

John Rogers, of the University of Illinois, Urbana-Champaign, is one. The cells he has devised can convert 42.5% of sunlight.  68. ______  Their secret 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 property called a bank gap that is different from that of other semiconductors.  69. ______  It also fixes the maximum amount of energy that can be captured from shorter wavelength. The result is that long-wavelength photons are lost and short-wave ones incompletely utilized.

70. ______ He chooses his materials so that the bottom of the band gap of the top layer matches the top of the band gap of the one underneath, and so on down the stack. Each layer thus chops 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 costly. But Dr Rogers has found a way to overcome this. Normal 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 scattered over that surface many dots. Each stack then has a pair of cheap glass lenses mounted over it. These focus the sun’s light onto the stack, meaning that all incident light meets a semiconductor.

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