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O'1960^,A n Vrr a^ e y L ^ y ^jroA reliable electrical system is necessary to pro-

vide on-board power for a spacecraft in orbit, or in alocation, such as on the surface of the moon, whereexternal power is not availahle. At present, themost common source of such power is the solarcell, a silicelz crystal which converts sunightdirectly into electrical energy.

Solar cells have provided primary electrical powerfor the majority of unmanned space missions con-ductad by the National Aeronautics and SpaceAdministration. Among spacecraft using solar cellsfor on-board electrical power have been photo-graphic missions to the moor of Ranger, Surveyor,and Lunar Orbiter; the Mariner flights to Venus and

Mars; and numerous weather, communications, andscientific satellites.

Thousands of solar cells are required to generateeven small amounts of electrical power. Mariner IV,which took and returned to earth the first photos ofMars, carried 28,224 solar cells. Their maximumpower output near earth was 670 watts. The poweroutput decreases when the intensity of illumination:decreases. Therefore at Mars, much further fromthe sun, this output diminished to 32.0 watts.

All solar cells in use today are made from silicon,although there are a few other materials whichcould be used. But pure silicon, properly treated,makes an excellent solar cell. Oddly enough, the

sa p c o n

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EXPLORER

treatment involves the addition of impurities suchas phosphorous and boron.

A solar cell is composed of a solid piece of siliconconsisting of two electrically different regions, or"hattery plates." Just as the plates of ordinary bat-teries behave in accordance with the materials andimpurities of which they are composed, the regionsof the solar cell behave differently, since one con-tains phosphorous and the other boron.

The first step in the manufacture of solar cellsis to prepare silicon crystals containing traceamounts of boron. The boron may constitute as littleas 0.00002 percent of the crystal. silicon has fourvalence electrons. Each atom forms covalent bondswith each of four adjacent atoms, forming fourelectron pairs. Boron has only three valence elec-trons. Therefore when a boron atom, added as animpurity, substitutes for a silicon atom, ita three

electrons can pair with the electrons of three neigh-boring silicon atoms. But the electron of the fourthsilicon neighbor is paired with ahole. The absenceof an electron is called a hole because it does infact act like a vacancy. It an electron moves :nto thehole to complete this electron pair, a hole is leftbehind, into which another electron may move.leaving another hole. Thus a hole can trzvelthrough the crystal and be in effect a conductor ofelectric current.

Because 3 hole accepts a moving electron, itacts like a positive charge. Since the boron-contain-ing silicon contains many positively chargedholes, it is called P-type silicon. (P comes from"accePtor," but it is convenient to think of P for"positive.")

The P-type silicon crystals are cut into very thinsquare or rectangular slices about 0.008 inch to

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0.015 inch islices are then heated in anoven into w.horous vapor is introduced.The phosphor.. ,etrates the silicon to a depth ofa few millionths ut an inch. The phosphorous is thenremoved from one surface and the sides, leavingone surface coated. The concentration of phos-phorous is about 100 times that of boron.

Phosphorous has five valence electrons. The fifthelectron is not needed to complete the four covalenthnnds with adjacent silicon atoms. This fifth elec-Lrun is donated to the crystal and is free to movethrough it. Thus the phosphorous-boron-containingsilicon is electron rich, with many free negativeelectrons. It is called N-type. (N comes from "do-Nor," but it's handy to think of N for "negative.")

The boundary between the P and N regions iscalled a RN junction. The solar cell itself is referredto as an N-on-P cell, because it has an N region ontop (toward the light) of a P region. When thisboundary is first formed, there is a flow of electronsi, m the N region into the holes of the P region,with a corresponding movement of holes from theP region to the N region. Thi , 'esults in an internalvoi;age barrier, which makes it difficult for addi-tional electrons to flow from the N region to the Pregi, % n, since the holes that were there ha ve ineffect been filled.

When the solar cell is exposed to sunlight, aphotan of light penetrating the silicon in the Nregion will ionize an atom, which releases an elec-tron, or, more properly, an electron - hole pair.This in effect charges the "plates" of the solar cell"battery." Sincr most of these light-generated elec-trons are repelled by the barrier at the N-P inter,acE,they can more easily flow through a circu i t outsidethe cell. Metallic contacts that have been made tothe two regions of the solar cell carry the electronsthrough the external circuit, so that they constitutean electrical current to do useful work. When the cellis illuminated, electrons flow internally from f toN, and holes from N to P. The N region is nega-tive with respect to the P region in an illuminatedcell, as indicated in the accompanying drawing.

TIROS IV

SOLAR... . CELLS

Under the sunlight of outer space near earth, asolar cell measuring only one centimeter by twocentimeters can produce a current of 60 milliam-peres at four-tenths of a volt, delivering up to 25milliwatts of power. Thus it is apparent that manythousands of the small cells are required to developadequate electricity for spacecraft power systems.The cells are wired together in series, to increasevoltage and in paraliel to increase current. As anadded bonus, connecting them in parai!el assuresthat if a few are damaged or malfunction in space,the remaining cells are able to continue their workwithout a critical loss in power.

The solar cells are mounted on a firm backing, orpaddle, and will operate only when exposed to thesun. Several disadvantages of solar cells are ap-parent. Since the cells do not generate electricitywhen they are shaded from the sun, they are usual;y

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accompanied by rechargeable batteries, which fur-nish power during dark periods. When in sunlight,the solar cells recharge the batteries in addition tofurnishing operating power for the spacecraft. As-sembling the cells takes much meticulous handlabor. The cells are fragile, and the paddle device onwhich they are usually mounted adds costly weightto a spacecraft. Mariner IV's four separate solar-cellpaddles each weighed 18.7 pounds about 13 per-cent of the spacecraft's total weight of 575 pounds.

An important problem facing the designer ofsolar-cell power systems intended to operate inspace is radiation damaPe to the cells. This problemhas been attacked in two ways. The past procedurewas to protect the cells with a thin window of quartzor sapphire. But such a procedure made the powersystem extremely heavy. A better solution is to usemore radiation- resistant cells. 1 ne new N-on-P cellsare superior in this respect to the older P-on-N cells.

Although large silicon solar-cell arrays can pro-vide ample power for more ambitious space mis-sions, they are heavy, complex, and costly. Lighter ,simpler, and cheaper solar cells are needed. Anarray of flexible cells, for instance, which could berolled up like a window shade during launch andthen unrolled for use in space might be a solution.

Considerable progress on flexible cells, calledthin-film solar cells, has been made. These are pro-duced by evaporating a thin film of semiconductormaterial, such as cadmium sulfide, onto a metallizedplastic. Flexible film cells develop rnore power perpound than presently-used silicon cells. Unfortu-nately, the thin-film cells are less efficient than

silicon cells, since they require more area and sufferdegradation under cycles of temperature change.They have not been adequately tested for durabilityin the hos t i le space environment.There are, of course, other power systems such asfuel cells, batteries, and various nuclear systemsin use or under study for applications where solarcells cannot be used effectively. Ho-!ever solar cells,which are extremely reliable, will continue to provideprimary on-board electric power for most unmannedsatellites.ACTIVITY:

Many hobby shops sell silicon solar cells atreasonable prices. These were originally manufac-tured for space applications but failed to meet therigorous efficiency requirements. Obtain such a celland test its voltage output in the dark, in sunlight,and under artificial light, using a high-impedancevoltmeter. Connect the cell to a 10-ohm load andobserve the change in voltage as you vary lightintensity.QUESTIONS:1. Silicon is a member of what group in the chem-ist's periodic table?2. Elements from what group are used as P-typeadditives? A s N-type additives?3. If a one centimeter by two centimeter cell has anefficiency of 9 percent and delivers 25 milliwatts atthe earth's distance from the sun, what is the in-tensity of sunlight in watts per square centimeter?In watts per square foot?

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