- -

NASA TECHNICAL NOTE

EFFECT OF RADIATION ON CERIUM-DOPED

/ +SOLAR-CELL COVER GLASS I "

J ,

b.

by Gilbert A. Haynes

Langley Research Center Hampton, Va. 23365

N A T I O N A L AERONAUTICS A N D SPACE A D M I N I S T R A T I O N W A S H I N G T O N , D. C. DECEMBER 1970

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TECH LIBRARY KAFB, NM

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~~

1. Report No. 2. Government Accession No.

NASA TN D-6024 I 4. Title and Subtitle

E F F E C T OF RADIATION ON CERIUM-DOPED SOLAR-CELL COVER GLASS

7. Author(s)

Gilbert A. Haynes

9. Performing Organization Name and Address

NASA Langley Research Center Hampton, Va. 23365

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration Washington, D.C. 20546

15. Supplementary Notes

~- - . .

16. Abstract

0132785 3. Recipient's Catalog No.

5. Report Date December 1970

6. Performing Organization Code

8. Performing Organization Report No.

L-7323 10. Work Unit No.

120-33-17-01 11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Note 14. Sponsoring Agency Code

The resu l t s of an investigation to determine the feasibility of using an inexpensive, radiation-resistant solar-cel l cover g lass t o replace synthetic fused quartz a r e reported. Several samples of a frequently used solar-cel l cover g lass were doped with various amounts of cerium. These samples were i r radiated with 1 MeV electrons to a maximum fluence of 5 X 1014 electrons pe r square centimeter and with 22 MeV protons to a maxi­mum fluence of 1013 protons pe r square centimeter. The effects of the electrons and protons used in this investigation a r e representat ive of those of the corpuscular radia­tion found in space.

Cerium doping of the cover g lass improves its radiation resis tance. The optimum level of cer ium doping appears to be between 1 and 2 percent by weight.

17. Key Words (Suggested by Authorls) ) 18. Distribution Statement

Solar -cell cover g lass Unclassified - Unlimited Cerium-doped g lass

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. NO. of Pages 22. Rice*

Unclassified Unclassified 13 $3.00

EFFECT OF RADIATION ON CERIUM-DOPED

SOLAR-CELL COVER GLASS

By Gilbert A. Haynes Langley Research Center

SUMMARY

The resul ts of an investigation to determine the feasibility of using an inexpensive, radiation-resistant solar-cell cover glass to replace synthetic fused quartz are reported. Several samples of a frequently used solar-cell cover glass were doped with various amounts of cerium. These samples were irradiated with 1 MeV electrons to a maximum fluence of 5 X 1014 electrons per square centimeter and with 22 MeV protons to a maxi­mum fluence of 1013 protons per square centimeter. The effects of the electrons and protons used in this investigation are representative of those of the corpuscular radia­tion found in space.

Cerium doping of the cover glass improves its radiation resistance. The optimum level of cerium doping appears to be between 1 and 2 percent by weight.

INTRODUCTION

Transparent materials a r e used to cover solar cells for thermal control and for shielding in a radiation environment. These cover materials must not discolor in a radi­ation environment. Synthetic fused quartz is the most frequently used material because of its radiation resistance (see ref. 1). Fused-quartz covers are somewhat expensive to produce because they have to be cut to s ize and ground and polished to the desired thick­ness (usually between 0.076 and 0.76 mm) and surface quality. For thicknesses less than 0.254 mm, the breakage rate of fused quartz during panel fabrication is high. Annealed sapphire, which is even more radiation resistant than fused quartz, has also been used as solar -cell cover material; however, sapphire is a very expensive material.

The other frequently used cover material is Corning #0211 glass (Micro-Sheet). This glass has a thermal coefficient of l inear expansion that more closely matches the silicon solar-cell material than does fused quartz. Micro-Sheet is drawn to thickness and fire polished during manufacture. Consequently, only cutting to s ize and coating with blue reflective and antireflective coatings, commonly used on cell covers, are required as postproduction operations. The finished product is not as fragile as fused quartz, and the breakage rate is lower. The cost of a finished solar-cell cover of

Micro-Sheet is much l e s s than that of a finished fused-quartz cover; however, Micro-Sheet is not as radiation resis tant (see ref. 1).

The purpose of this investigation was to determine the feasibility of using Micro-Sheet doped with cerium as a radiation-resistant solar-cell cover glass. The use of Micro-Sheet instead of fused quartz would substantially reduce the cost of many solar-cell power systems. It is known that cerium, when used as a doping element in glass , can inhibit radiation-induced discoloration. (See refs. 1, 2 , 3, 4, and 5.)

EXPERIMENTAL PROCEDURE

Samples containing various amounts of cerium were tested. Laboratory procedures required that the samples be ground and polished instead of drawn and fire polished as with production lots. The samples were doped with 1-, 3-, and !%percent cerium oxide by weight, and the control samples were undoped. Each sample was 25.4 X 12.7 X 3.18 mm in size. The samples were much thicker than solar-cell covers, but the absorption effects due to cerium doping were more easily observed. The thickness was more than sufficient to stop the 1 MeV electron and 22 MeV proton radiation employed in the experiment.

The sample parameters measured included the spectral transmission and the wide-band transmission. The spectral transmission was measured with a Cary Recording Spectrophotometer, Model 14. The wide-band transmission was measured with an N on P silicon solar cell and a xenon-arc solar simulator. The solar-cell response (0.4 to 1.2 pm) determined the bandwidth and spectral weighting. (See ref. 6.) These parameters were measured before irradiation and after each exposure of each sample to radiation.

The samples were irradiated with 22 MeV protons at the Oak Ridge 86-inch cyclotron. One sample at a time was irradiated. Four samples f rom each of the four lots were exposed. Two samples from each lot were exposed at a flux of approx­imately 109 protons per square centimeter per second (p/cmz-sec) to a fluence of

8.7 x 10l1 p/cm2, and the remaining two samples were exposed to a fluence of 1013 p/cm2. These fluences were sufficient to produce measurable damage. The samples were separated in this manner because all parametr ic tes t s were performed at the Langley Research Center, and data f rom samples at no fewer than two exposures were desired. The irradiation was in air with the samples near room temperature.

The electron irradiation was performed with the 1.0 MeV cascade-rectifier elec­t ron accelerator at the Langley Research Center. All samples were irradiated simulta­neously with a swept beam. The irradiation was done in vacuum, and the sample temper­a tures were less than 311' K. Parametr ic measurements were made on all samples before irradiation and after exposure to fluences of 1013 to 5 X 1014 electrons per square centimeter (e/cm2) at fluxes of 1O1O e/cmz-sec for the lowest fluence and 10l1 e/cmZ-sec for the highest fluence.

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The effects of the 1 MeV electrons and 22 MeV protons used in this experiment are representative of those of the corpuscular radiation found in the near-earth orbi­tal environment.

RESULTS AND DISCUSSION

When Micro-Sheet is doped with cerium, it is affected spectrally as shown in fig­u r e 1. The ultraviolet absorption is increased, and a definite absorption band develops at 0.575 p m as the level of the doping element is increased. The visual effect is a yel­lowish coloration. To reduce this coloration, the amount of doping element should be held to the minimum required for optimum radiation resistance. The spectral response of a typical solar cell is between 0.4 and 1.2 pm. The total effect of cerium doping of the cover glass is shown in figure 2 , where the normalized wide-band transmission repre­sents the wide-band transmission of doped glass relative to that of undoped glass. It should be emphasized that the samples tested are 4 to 40 t imes thicker than most solar-cell covers. Consequently, the transmission losses shown in figure 2 would be much less for optimized covers.

Proton Irradiation Tests

The effects of 22 MeV protons on the spectral transmission of cerium-doped Micro-Sheet are shown in figure 3. Figure 3(a) shows that undoped Micro-Sheet is very suscep­tible to radiation damage. Spectral-transmission changes appear visually as a brownish discoloration in the glass. Figures 3(a) to 3(d) indicate substantial improvements in the radiation resistance of the glass as cerium is added.

Figure 4 shows the effects of proton irradiation on the wide-band transmission of the glass with various amounts of cerium doping. The curve for undoped glass confirms the radiation sensitivity indicated in figure 3(a). This curve also i l lustrates the nonlinear degradation of transmission as a function of fluence with most of the damage occurring at low fluences. The curve for 1-percent cerium-doped glass i l lustrates the improvement in proton-radiation resistance resulting from the addition of cerium. The remaining two curves indicate a further improvement in radiation resistance as the amount of cerium doping is increased. Figure 5 shows the effect of proton irradiation on the wide-band transmission as a function of the level of cerium doping. It appears that the optimum level of cerium doping is between 1 and 2 percent for the exposure levels discussed.

Electron Irradiation Tests

The effects of 1 MeV electrons on the spectral transmission of cerium-doped Micro-Sheet are shown in figure 6. The effects due to electron irradiation are similar to the effects due to proton irradiation. Figure 7 shows the effects of the electron irradiation

3

on the wide-band transmission of doped and undoped glass. The curve for undoped glass confirms the radiation sensitivity indicated in figure 6(a); it a lso i l lustrates the nonlinear degradation of transmission as a function of fluence with most damage occurring at low fluences. The curve for l -percent cerium il lustrates the improvement in electron-radiation resistance resulting f rom the addition of cerium. Figure 8 shows the effects of irradiation on the wide-band transmission of glass as a function of the level of cerium doping. As in figure 5, it appears that the optimum level of cerium doping is between 1 and 2 percent. The discoloration associated with electron irradiation is shown in fig­u r e 9. Proton-induced discoloration is similar.

During the electron-irradiation tes ts , electron-discharge patterns, f i s sures in the glass, developed in all samples at all fluences. Figure 9 i l lustrates the discharge effect in undoped glass (see arrows). Sample GOX 1 (manufacture's sample designation) was unirradiated, sample GOX 7 was exposed to 9 x 1013 e/cm2, and sample GOX 5 was exposed to 5 x 1014 e/cm2. The resul ts of these tests and previous experiments (refs. 1, 4, 7, and 8) suggest that several factors affect the probability of occurrence of the electron-discharge patterns. Factors which appear to reduce this probability are low dielectric constant, low dissipation factor, method of fabrication, low irradiation ra te , and samples thin enough so that the irradiating particles are not absorbed. There has been no occurrence of electron discharges in previous ground or flight experiments using thin, commercially produced Micro-Sheet solar-cell covers. Hence, it is not expected that such a problem would develop as a result of adding cerium.

CONCLUSIONS

The experiment on the effects of radiation on cerium-doped Corning #0211 glass (Micro-Sheet) has shown the following:

1. Substantial degradation in the transmission properties of undoped g lass is caused by 1 MeV electrons at fluences of 1013 electrons per square centimeter and greater and by 22 MeV protons at fluences of 8.7 X 10l1 protons per square centimeter and greater. These effects are representative of those of the corpuscular radiation found in space.

2. The induced degradation of the glass is a nonlinear function of the total fluence of the radiation. Most of the damage occurs at low fluences for undoped glass.

3. Micro-Sheet doped with 1 percent or more of cerium shows an improvement in resistance to 1 MeV electron and 22 MeV proton radiation. The optimum amount of doping element appears to be between 1 and 2 percent by weight.

4. Irradiation with electrons caused electron discharges in all samples tested. The discharges, which resul t in f i ssures in the glass, are not expected to develop in optimized solar-cell covers. The covers will be thin enough so that the irradiating particles are not completely absorbed.

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5. Based on the above conclusions, it appears feasible to use cerium-doped Micro-Sheet as an inexpensive, radiation-resistant solar-cell cover.

Langley Research Center, National Aeronautics and Space Administration,

Hampton, Va., September 4, 1970.

REFERENCES

1. Haynes, Gilbert A.; and Miller, William E.: Effects of 1.2 and 0.30 MeV Electrons on the Optical Transmission Properties of Several Transparent Materials. NASA TN D-2620, 1965.

2. Soga, Naohiro; and Tashiro, Megumi: Role of Cerium Ions in Preventing the y-Ray Induced Coloration of Glass. J. Ceram. Assoc. Japan, vol. 70, no. 5, 1962, pp. 143-147.

3. a r c h e r , John F.; and Bowman, Richard E., eds.: Effects of Radiation on Materials and Components. Reinhold Pub. Corp., c. 1964.

4. Kreidl, N. J.; and Hensler, J. R.: Gamma Radiation Insensitive Optical Glasses. J. Opt. SOC.Amer., vol. 47, no. 1, Jan. 1957, pp. 73-75.

5. Kreidl, N. J.; and Hensler, J. R.: Formation of Color Centers in Glasses Exposed to Gamma Radiation. J. Amer. Ceram. SOC.,vol. 38, no. 12, Dec. 1955, pp. 423-432.

6. Haynes, Gilbert A.; and Ellis , Walter E.: Effects of 22 MeV Proton and 2.4 MeV Electron Radiation on Boron- and Aluminum-Doped Silicon Solar Cells. NASA T N D-4407, 1968.

7. Gross, Bernhard: Irradiation Effects in Borosilicate Glass. Phys. Rev., Second ser., vol. 107, no. 2, July 15, 1957, pp. 368-373.

8. Rosebury, Fred: Handbook of Electron Tube and Vacuum Techniques. Addison-Wesley Pub. Co., Inc., c.1965.

5

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IT

100

80

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20

0 3.0

Wavelength, i i m

Figure 1.-Effect of cerium doping on the spectral transmission of Micro-Sheet.

e a,

$ n

90 1 \

\

I 2 3 4 5 6 7

Level of c e r i u m doping, percent

Figure 2.- Effect of cerium doping on the wide-band transmission of Micro-Sheet.

6

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1

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f.4 .5 .6 7 2.0 3.0

Wavelength, pm

(b) 1-percent cerium doped.

Figure 3 . - Effec t of 22 MeV proton i r r a d i a t i o n on t h e s p e c t r a l transmission of Micro-Sheet.

7

c

100

80

c al g 60 C .-0 VI VI.­5 40 c e l­

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(e) 3-percent cerium doped

100

80

c c 0)

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E 40 VI cmL c

2 0

O . 3 Wavelength, pm

(d) 5-percent cerium doped.

Figure 3.- Concluded.

8

.. ..... ...,,....,, .., . .

0 .2 .4 .6 .8 I.o 1.2 x 1013

Fluence, p/cm2

Figure 4.- Effect of 22 MeV proton irradiation on the wide-band transmission of. cerium-doped Micro-Sheet.

0 I 2 3 4 5 6 7 Level of c e r i u m doping, percent

Figure 5.- Effect of cerium on the wide-band transmission of proton-irradiated Micro-Sheet.

9

L

I 3.O

Wavelength, y m

(a) Undopea.

100

80

c r= a,u a,

a 60 t .-0 VI VI._ E v)

m 40 L+­

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7C J n irradiated

L x e/cm2

L x 1014 e/cm2 j x 1014 e/cm2

1

O.3 4 .; Wavelength, wm

(b) 1-percent cerium doped.

Figure 6 . - Effec t of 1MeV e lec t ron i r r a d i a t i o n on t h e s p e c t r a l transmission of Micro-Sheet.

10

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80 =+U n i rradiated

c S I 1x 1013 e/cm2alL

60 1 x 1 0 1 ~e/cm2

S 5 x 1014e&0._ VI cn._

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( c ) ? -percent cerium doped.

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F igu re 6.- Concluded.

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Wa 1 percent 1 c

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2 3 4 5 6 x IOi4

Fluence, e lcm2

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Figure 7.-Effect of 1 MeV electron irradiation on the wide-band transmission of cerium-doped Micro-Sheet.

M0 Unirradiated

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Level of c e r i u m doping, percent

Figure 8.-Effect of cerium on the wide-band transmission of electron-irradiated Micro-Sheet.

12

t

14Unir radiated 9 XIO 13 e/cm2 5 ~ 1 0e/cm 2

i

I

L-68-6750 Figure 9.- The discoloration and electron-discharge patterns of Micro-Sheet resulting

from electron irradiation. I O

NASA-Langley, 1970 -9 L-7323 13

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