Recent developments in radiation hardening silicon solar cells

Energy Conversion.

Vol. 11. pp. 75-90.

Pergamon Press, 1971.

Printed in Great Britain

Recent Developments in Radiation Hardening Silicon SolarCells Mm WOLF?

and 6, JI BRUCKER$

(Received 5 February 1971)

Introduction In the last few years, significant activity has taken place in two areas concerned with the radiation resistance of silicon solar cells, which will have come only peripherally to the attention of the space power systems engineer. At least a partial reason is that reports on this activity have been couched too much in the specialist’s language. This paper will therefore attempt to familiarize the power systems designer with the key results of this work so that he can incorporate them appropriately in his future plans. The mentioned activities fall into two areas: the continued research efforts on the lithium containing ‘p on n’ silicon solar cell; and study of the effects of radiation on silicon solar cells at low temperature as encountered on planetary missions reaching beyond 3 astronomical units. For the lithium containing solar cell, the mechanisms of radiation damage and of annealing will be explained with a simplified model. It will

situation differs greatly from that of near room temperature irradiation, and that considerable further effort will be needed to explore the magnitude of these differences and to understand their nature. The General Properties of Lithium in Silicon Shortly after the first findings had been published on the interaction of lithium with defects introduced into silicon by irradiation with nuclear particles (Ref. [l]), the possibility of attaining solar cells with improved radiation resistance by introducing lithium as the major impurity into the base region of ‘p on n’ silicon cells was recognized (Ref. [2]). This was rather quickly followed by preparation of first samples which showed remarkable spontaneous recovery of the diffusion length after this property had been reduced by nuclear irradiation (Refs. [2, 31). It may be noted that the base region diffusion length is that parameter which causes the observed changes in collection efficiency resulting

FZ CELLS

Fii. 1. Diiion length L as function of fluence 4. of electrons of 1 MeV energy for lithhuu-containing silicon sohu cells from Boatzone grown material and for standard ‘p on n’ solar cells. The irradiation was interrupted several tiuw for the duratious indicated. (Reproduced from Ref. [3] by courtesy of author.)

further be attempted to convey a feeling for the complexity of the problem, which has required a scientific effort of approximately 50 man-yr, spanned over the last 6 yr, to bring the understanding of the effects and the approach to potential solutions to the present level. In contrast, in the area of radiation effects at low temperature, it has only recently been recognized that the t Institute for Direct Energy Conversion, University of Pennsylvania,Philadelphia,Pennsylvania. # RCA Astro-ElectronicsDivision, Princeton, New Jersey. 75

from nuclear irradiation of solar cells. Figure 1 is a reproduction of the now classical curve (Ref. [3]) which presents the measured diffusion length as function of fluence (i.e. time-integrated radiation dose) of 1 MeV electrons for a standard ‘p on n’ silicon solar cell and a similar cell with lithium impurity introduced into the base region. The irradiations were interrupted at various fluence levels for very short time periods for the measurement of diffusion length, and for longer periods for lunch, overnight and weekend breaks. During the longer

M. WOLF and G. J. BRUCKER

76

breaks, when the cells were at room temperature, the lithium containing cell exhibited spontaneous recovery, while the normal cell showed no such behavior. This initial observation led to great hopes for considerable improvement in radiation resistance of silicon solar cells by the introduction of lithium, with a subsequent flurry of activity towards the preparation and exploration of such cells. Now, 5 yr later and after an expenditure of about 5 million dollars, an assessment of the results of this effort appears appropriate. The first question arising is essentially this: why has it taken such a large effort to reach the present, still incomplete development of a device having just one improved attribute? Interestingly, this effort considerably exceeds that spent on the original development of silicon solar cells to the time of fabrication of the first cells for space power use in the Vanguard satellite. The answer to this question lies clearly in the versatility of the behavior of lithium in silicon, which leads to a complex array of effects. Table 1 Properties of lithium in silicon 1. 2. 3. 4. 5. 6.

Forms donor (n-type impurity) Is mobile at room temperature, drifts in E field Complexes with acceptors (B, Ga), neutralizes them Complexes with oxygen, forms donor (LiO)+ Complexes with recombination centers, neutralizes them Complexes with vacancies, forms recombination centers

Effect of lithium in silicon solar cells (a) Useful only for n-type region (1 and 4) (b) Anneals radiation damage (2 and 5) [also participates in its creation (6)] (c) Uses itself up in the course of (b) (d) Versatility (l-6) leads to structure and defect sensitivity (tight process control)

Table 1 lists the properties of lithium in silicon. The first and most important is that of forming a donor, i.e. an n-type impurity in silicon. Second, lithium is mobile in the silicon crystal at room temperature. This means that in the presence of density gradients of lithium, it will diffuse to the region of lower concentra-

tion. Also, since it is found primarily as a positive ion, it has a tendency to drift in an electric field towards the region of greater negative potential. The third property is that it complexes with acceptors, i.e. p-type impurities like boron, gallium and indium. In so doing, the lithium neutralizes these acceptors, thus increasing the resistivity of p-type regions. As the fourth property, lithium has a tendency to also complex with other impurities, such as oxygen. In this particular case, the LiO complex is a donor, just as Li itself is. As the fifth item, lithium complexes with recombination centers and neutralizes them. This new complex does not act as a recombination center. However, as indicated in the sixth line, lithium also complexes with vacancies and, in so doing, forms recombination centers. Through this property, lithium participates in the original formation of radiation damage. As a result of these properties, the following important effects of lithium are observed in silicon solar cells (second section of Table 1). (a) Lithium, as a result of properties 1, 3 and 4, can be introduced only in n-type regions in sufficient quantity to enhance radiation resistance. Therefore, it can be practically used only in ‘p on n’ silicon solar cells which, however, have been found to be inherently more radiation-sensitive near room temperature than ‘n on p’ cells are, excepting the effects contributed by the lithium. (b) In accordance with properties 2 and 5, lithium is able to move at room temperature to radiation damage centers (recombination centers) and neutralize them. However, as a result of property 6, it also participates in the original creation of the radiation damage centers. (c) As a result of both the formation of recombination centers and their neutralization with the participation of lithium, lithium ions are used up as donors. Accordingly, the resistivity of the silicon is increased, which, if permitted to proceed too far, can have very undesirable effects on the device properties. (d) The versatility of lithium results in considerable sensitivity of the device structure and the material properties. This sensitivity leads in turn to the necessity of tight process control, if devices with a close range of

Table Z.Variables in Li containing Si solar cells (p/n) Base material Oxygen content

Dislocation density Impurity type (electrically active) Impurity density

Quartz crucible: 0 = 1017-101s cm-3 Float zone: 0 < 1016cm-3 Lopex 0 < 1015-1017 cm-3 Monex > k Sb

Li introduction Concentration (< 10X4-6. 1015 cm-s) Density gradient (lOia-4.1020 cm4) near junction Ancillary process effects

3.1013-2.1015cm-3 > Application Oil slurry Tin alloy Vapor transport Vat. evauoration Ion impiantation

Diffusion Temperature (325450°C) Time (20-480 min) Redistribution (N-120 min) Ambient

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M. WOLF and G. J. BRUciLER

78

energy. The time period of the plot extends over ap proximately 10 months, during which the cells were stored in air ambient at room temperature. The curves shown in Fig. 2 are average data for several cells taken from the same production lot and irradiated simultaneously, but are characteristic for the cell type, as observed on a number of similar production lots. The ‘n on p’ cells represented in Fig. 2 degraded during irradiation to approximately 67 per cent of their original power output, while the ‘p on n’ solar cell types degraded to about 48 per cent of original power output. In contrast to previous beliefs, the standard 10 Q cm ‘n on p’ cells showed a small degree of spontaneous recovery while stored at room temperature, as shown in Fig. 2. This recovery is considered real since unirradiated ‘n on p’ cells from the same batches did not show any change in power output. In contrast, the three types of lithium containing cells exhibited markedly different recovery characteristics. The cells prepared from floatzone refined silicon (solid curve) with a medium lithium concentration, i.e. one in the low 101s cm-3 range, exhibited very rapid recovery to about 92 per cent of original power output. This recovery occurred in a few hours so that it is not distinguishable in Fig. 2 due to the time scale used. In the following hundred days, however, the power output decreased to approximately 88 per cent of the original value, after which time period it remained constant. The dashed and the dash-dotted curves represent cells prepared from Czochralski grown silicon, with high and medium lithium concentrations, respectively. High lithium concentrations are in the middle to upper 101s cm-s range. Both of these cell types show a recovery behavior much slower than that of the cells prepared from floatzone material, with an extremely large recovery time constant exhibited by the cells with the medium lithium concentration. Even 290 days after irradiation, these cells had not reached a steady state value of power output. Figure 2 thus alludes to the fact that the ratio

of lithium to oxygen content is directly related to the speed of the recovery. It further indicates that higher values of relative power output after full recovery are attained by cells with larger lithium to oxygen concentration ratios. Figure 2 also shows another effect. The cells prepared from floatzone grown crystals exhibit a decrease in power output after the initial recovery. When this decrease was first observed a few years ago, it led to great alarm, the cells being considered unstable. The quartz crucible grown cells, with their slow recovery, appeared to be much more stable. It has been recognized only during the last year that the power output of the floatzone refined cells decreases only for a certain period of time and thereafter remains stable. Thus, the name ‘degradation’ for this decrease in power output after the initial rapid recovery does not appear to be quite appropriate. Rather, in analogy to the shape of the frequently observed response curve of electronic circuits to step function excitation, the observed phenomenon would be more suitably called a ‘recovery overshoot’, which then decays to a steady state recovery value. Figure 2 and the preceding two paragraphs convey a very favorable impression of the radiation properties of the lithium containing ‘p on n’ cells. This picture is not fair, since only the absolute power available, not the relative power output are of interest to the power systems designer. In terms of absolute power output, the situation is somewhat different. This is indicated by a re-plot of the data of Fig. 2 in absolute power output (Fig. 3). The original power output, before irradiation, was lower on the lithium containing cells than on the standard ‘n on p’ cells and lowest on the floatzone cells, with the very remarkable result, that after recovery, all cells showed approximately the same power output. The small advantage in absolute power output which the lithium containing cells would have had was counter balanced by the mentioned recovery of the ‘n on p’ cells. However, it cannot be overlooked that the cells

40 LEGEND: a

- --

-

-

10&m

??0

-

--

Q.C. p on n, high Li

?? -_BEFORE

n on p

F.Z. p on n. med. Li Q.C. p on n. med LI

IRRADIATION

RADIATION 3.1014cm-2

5

FLUENCE: ELECTRONS

(1 MeVl

i 0

1

0

50

100

150 -,

200

250

30(. DAYS

TIME

Fig. 3. Data of Pi. 2, replotted in absolute maximum power output. Power output values before and immediately after irradlation are indicated.

19

Recent Developmentsin Radiation Hardening Silicon Solar Cells

represented in Figs. 2 and 3 are approximately 2 yr old and do not represent the present state of the art in fabrication of lithium containing solar cells. It can therefore be argued that Fig. 2 may have some validity if cells of the various types can be prepared with original power output equal to or exceeding that of the present 10 Q cm ‘n on p’ cells. Cells with such high original power output are now available in small quantities for all three cell types represented in Figs. 2 and 3, but their irradiation and long term recovery behavior have not been sufficiently explored.

10-z

10-l

It should be noted, however, that not all lithium containing solar cells with low oxygen density have exhibited stability in the unirradiated case and an annealing overshoot after irradiation. Some cells have clearly exhibited a degradation of performance characteristics both before and after irradiation. A typical example is shown in Fig. 5, which displays the currentvoltage characteristic of such an irradiated cell shortly after manufacture (17 November 1967) and 11 months later (4 October 1968). For this cell, the short circuit current increased slightly, while both the open circuit

100 t = TIME

AFTER

10’ IRRADIATION

1112

103

(DAYS)

Fig. 4. Relative open circuit voltage V, short circuit current Z,_ and maximumpower output Pnp as function of time t after irradiation, for solar cells from tloatzone grown silicon of medium lithium couceutration, exhibiting the ‘recovery overshoot’ behavior. All data are related to the values before imdiation (t = O-). (Reproduced from Ref. [5] by courtesy of author.)

To further illustrate the question of ‘recovery overshoot’ and degradation, a plot of the change of solar cell performance parameters as function of time after irradiation is shown in Fig. 4 for the floatzone cells with medium lithium concentration. Here, the relative open circuit voltage and relative short circuit current are plotted together with the relative maximum power output. A difference to the similar graph of Fig. 2 is the abscissa which, in this case, presents time in the logarithmic scale, thus expanding the events during the period shortly after irradiation, and compressing the period 3-12 months later. The figure shows clearly that the mentioned annealing overshoot of the maximum power curve is caused by an even more pronounced overshoot in the short circuit current recovery curve, while the open circuit voltage increases monotonically with time after irradiation. This indicates that the annealing overshoot is caused by a change in the diffusion length, which is primarily influenced by the recombination center density. Further, a loss of charge carriers is not observed, which would result in a decrease of open circuit voltage during annealing, and which could lead to destruction of the cell characteristics as had been feared previously. These findings have been extensively corroborated through detailed measurements of the physical properties of irradiated silicon material.

voltage and the fill factor decreased considerably. Figure 6 shows the open circuit voltage of such a cell as function of time, covering nearly l+ yr (Ref. [5]). It is seen that the open circuit voltage decreased monotonically, continuing to do so even after nearly 13 yr. It should be noted that the differences in stability behavior of the cells prepared from low oxygen content material are related to differences in the cell fabrication processes. It has been found that certain cell production lots exhibited the overshoot, while others, prepared differently, showed the degradation. Of considerable concern for some time has been the common question of the validity of the results of accelerated testing. All experiments discussed so far have involved rather rapid irradiation to a relatively high fluence, with subsequent storage during which annealing occurred. In spacecraft service, the cells would experience a dose rate several orders of magnitude smaller, while annealing will occur simultaneous with the irradiation. At least during part of this time, the cells will be converting solar energy, and they may also undergo serious temperature cycling. To answer this question, experiments were carried out in a high-vacuum chamber with illumination while the cells were maintained at various constant temperatures (Ref. [8]). An isotope source provided long term irradiation at the

M. WOLF and G. J. BRUCRRR

0.2

I CELL o=

Tl-979

I - 2 x 1 cm

0

0.1

CURRENT,

mA

Fig. 5. Current voltage characteristic of an unstable unirradiated lithium containing solar cell prepared from floatzone grown silicon, before and after storage for an 11-month interval. (Reproduced from Ref. [7] by courtesy of author.)

460 0

100

300

200

400

500

d

_-TIME

Fig. 6. Open circuit voltage V,, as function of time in days for an unstable lithium contain@ solar cell of floatzone grown silicon.

equivalent rate of 1Ol2 cm-2 (day)-l electrons (1 MeV). These experiments, including their continuation to the present time (Ref. [9]), did not show any significant

differences in their results from those obtained in accelerated testing. Although most of the radiation damage experiments, Li-CELLS 0.8-

08 n” B

16 8 MeV

06

0

14 MeV

a0 h

p/cm*

o

NEUTRONS

I=4.5xlO’~,/c,~ 0.4

8 CELLS

04

<

0.6 -

PROTONS

I = 2 4 x IO’2

(Po=8.3%)

0

-

n/p

CELLS -

5 CELLS (PO = 10.3%)

FZ

EOR-cm

Po=33G-cm

I PO = lO.O%l 120 TIME

240 AFTER

360 BOMBARDMENT

480

600

720

(DAYS)

Fig. 7. Relative maximum power output of lithium containing solar cells of oxygen lean silicon as function of time after bombardment by 16.8 MeV protons to a fluence of 2.4.10*2 cm-*. Reference value is the power output before irradiation. Recovery to P/PO = 1 occurred in a few days after bombardment. (Reproduced from Ref. [7] by courtesy of author.)

/

0110

120

240 TIME

AFTER

I 360 BOMBARDMENT

(DAYS)

Fig. 8. Relative maximum power output of lithium co&bring solar cells of oxygen lean silicon as function of time after irradiation with 14 MeV neutrons to a fluence of 4.5.1012 cm-*. Reference value is the power output before irradiation. (Reproduced from Ref. [7] by courtesy of author.)

81

Recent Developments in Radiation Hardening Silicon Solar Cells

IZ-

l.O-

OBP/P,

‘-

OB-

04-

02-

Fig. 9. Relative maximum power output as function of fluence of reactor neutrons with energies greater than 0.01 MeV for commercial 3 P cm ‘n on p’, lS2 cm ‘p on n’ and lithium containing solar cells of oxygen lean silicon. The power output values before irradiition are indicated in the table insert. Recovered power output 30 days after bombardment to the highest fluence level is entered in the graph. (Reproduced from Ref. [2SJ.)

both on regular and on lithium containing silicon solar cells, have been performed with electrons as the damaging particles, the effects from energetic protons or neutrons are of equal or greater interest for some types of missions. It turns out that lithium containing solar cells which exhibit only a very marginal, if any, advantage over the standard 10 Q cm ‘12on p’ cells, show a very large improvement in radiation resistance under proton or neutron bombardment compared to these cells. Figures 7 and 8 give plots of relative power output as function of time after proton and neutron bombardment, respectively, similar to Fig. 2 for electron bombardment. Figure 7 shows the behavior of the maximum power output of lithium containing solar cells prepared from low oxygen content silicon over a period of nearly 2 yr after bombardment with 16.8 MeV protons to a fluence of 2.4 x 1Or2 cm-2. These cells which are some of the earliest lithium containing solar cells, exhibit the typical ‘annealing overshoot’ of the floatzone lithium cells. While the power output decreased during the first year after annealing, it remained constant during the second. In comparison, the standard 10 fi cm ‘n on p’ and 1 Q cm ‘p on n’ cells showed no recovery during this period, and provided relative power output of approximately 73 and 61 per cent, respectively, of that of the lithium containing cells at the end of 2 yr. Figure 8 gives a similar plot after irradiation with 14 MeV neutrons to a fluence of 4.5 x lOi cm-2. Here, the lithium containing cells of floatzone grown silicon show a gradual recovery of power output during a 30 day period, with constant power output during the subsequent 13 yr period. The absolute power output of the lithium containing cells after anneal is 65 per cent higher than that of the standard ‘~1on p’ cells, and 70 per cent higher than that of the standard ‘p on n’ cells. The great advantage of the lithium containing solar cells under heavy particle bombardment is also indicated in Fig. 9 where relative power output is plotted

as function of neutron fluence for lithium containing ‘p on IE’cells as well as for standard ‘n on p’ and ‘p on n’ cells. In this case, neutrons distributed over a broad energy spectrum as obtained from a swimming pool reactor were used. Although the initial power output of the lithium containing cells was approximately 20 per cent lower than that of the ‘11on p’ cell, it was as much as 100 per cent higher than the output of the ‘IZon p’ cells for fluences above 3 x lo12 neutrons cm-2. This means that the lithium containing cells are radiation resistant to approximately 30 times higher neutron fluences for equal power output compared to standard cells. Since a scarcity exists in heavy particle irradiation data, it was satisfying to find corroboration of these early measurements by recent results (Ref. [lo]). Figure 10 presents the absolute short circuit current values for lithium containing cells of floatzone silicon and for

INEUTRON

FLUENCE

(n/cm2>10KeV)

Fig. 10. Short circuit current of lithium containing solar cells of oxygen lean silicon as function of fluence of reactor neutrons with energies greater than 0.01 MeV, immediately after irradiation and 4800 hr later. Comparison data for typical 10 Q cm ‘n on p’ cells are indicated. (Reproduced from Ref. [lo] by courtesy of author.)

82

M. WOLF and G. J. BRUCKER

4%i I-__L-l!lIJ

I

0.1 TIME

I

I

I

I,

IO

(Years)

Fig. 11. Computed degradation of commercial 10 SL cm ‘n on p’ and of lithium containiog solar cells due to tbe combined effect of electron and proton bombardment (including solar Hares), integrated over time in orbit, for a synchronous orbit of 0” inclination. Tbe influence of 0.006 in. thick fused silicon covers is included. (Reproduced from Ref. [lo] by courtesy of author.)

30,w

II..,.. 10-e

t _._I 10-3 TIME

Fig.

i

,~ i

~I

-J-&d

I .o

,,,,s,‘~~

12. Computed plot, similar to Fig. 11. for a circular orbit of 2500 nautical miles altitude witb 0” inclination. (Reproduced from Ref. [lo] by courtesy of author.)

standard ‘n on p’ cells as function of neutron fluence and recovery time at 25°C storage temperature. Practically identical values have been obtained at 60°C storage temperature and for lithium containing cells of quartz crucible silicon at 60 and 100°C storage temperature, the only differences having been found in the degree of completeness of recovery at the highest fluence level at the end of the 4800 hr period. In another study of neutron irradiation effects (Ref. [ll]), the cells were maintained at a temperature of -50°C. At this temperature, the lithium annealing effect is inhibited, resulting in the observation that the cells behaved like regular ‘p on n’ cells. Differences in radiation tolerance by more than an order of magnitude can make the difference between practicality and impracticality of certain missions. Exposure to predominantly heavy particle (proton) irradiation is experienced in the central portion of the lower van Allen belt, that is, at altitudes of 1000-3000 nautical miles, as well as at synchronous altitude and on interplanetary missions. Also, certain military spacecraft designs have to consider the possibility of exposure to neutrons. Recent calculations of the effects of the integrated charged particle flux for two different orbits,

including solar flare protons for the synchronous orbit, and taking account of the annealing effect of lithium containing solar cells, indicate the considerable increase in mission life that can be expected by the use of these cells (Ref. [lo]). Figures 11 and 12 which present the results of these calculations as power profiles over the mission time, have equal validity for lithium containing cells from quartz crucible grown silicon annealing in the 60-100°C range, and for cells from floatzone grown silicon at approximately 25 “C. The Radiation Damage and Annealing Model The studies of the last few years which involved refined experimental techniques for the determination of the physical properties of silicon containing lithium, have led to a rather clear understanding for the causes of the differing behavior observed in the various cell types. Figure 13 presents a simplified model of the radiation damage and annealing mechanisms which occur in the lithium containing solar cells. The upper part of Fig. 13 shows the model for silicon with low oxygen content (often imprecisely called ‘oxygen free’). The mechanism depicted by this model will occur

Recent Developments in Radiation Hardening Silicon Solar Cells

I

DEFECT

ORIGINAL c

ANNEAL

I

OXYGEN

OXYGEN

OXYGEN

pip.

INTROOUCTION

83

FREE SILICON

CONTAINING

CONTAINING

WITH

LITHIUM

SILICON

SILICON

WITH

RICH

INL,*N,l

SOME

LlTHlUM

IN LITHIUM

(NOLAN,)

13. Radiation damage introduction aad annealing models for oxygen lean and oxygen rich silicon With various levels of lithium content. (Note: in the tlgure, the oxygen lean silicon is imprecisely labeled as oxygen free.)

only when the lithium concentration is significantly greater than the oxygen concentration as well as any residual donor concentration. Thus, this behavior will occur in floatzone refined crystals of originally high resistivity to which a medium or high lithium concentration (1015-1017 cm-s lithium) has been added, In the original state, before irradiation, the silicon atoms are located in their lattice positions in the crystal, which also contains interstitial ionized lithium atoms and free electrons. During irradiation, an energetic electron interacts with such a silicon atom, moving it from its lattice position into an interstitial position. Simultaneously, the now vacant lattice position captures an electron resulting in a ‘negatively charged vacancy’. This vacancy by itself does not act as a recombination center, but its introduction removes a majority carrier from the crystal. This negatively charged vacancy is extremely mobile, even at very low temperatures, such as that of liquid nitrogen, and moves through the lattice until it finds a positively charged lithium ion with which it forms a complex. At the same time, this complex absorbs another majority carrier (electron) from the crystal, resulting in a negatively charged vacancy-lithium complex. This complex acts as a recombination center, lowering the diffusion length and with it the short circuit current and to a small degree the open circuit voltage. Simultaneously, the loss of two majority carriers per defect introduced results in reduced open circuit voltage of the solar cell. During annealing, an additional lithium ion which, at temperatures above O”C, is also mobile in the silicon

lattice finds this recombination center and complexes with it. It may be noted that the mobility of the lithium ion is much lower than that of the vacancy, while the recombination center is essentially stationary. This neutral complex appears to have a tendency, however, to release the second lithium ion and revert back to the recombination center. This effect must occur at a rather slow rate, thus establishing a thermal equilibrium concentration ratio between the recombination center plus lithium ion and the annealed state containing two lithium ions. A second slow rate process has been observed which involves the further dissociation of the recombination center. In this case, the second lithium ion is ejected from the complex, together with an electron, resulting again in a highly mobile negatively charged vacancy, which now captures an interstitial silicon atom. This results in complete vanishing of the radiation induced defect, with a silicon atom again in an appropriate lattice position. Simultaneously, both lithium ions have returned to their mobile, interstitial status and both electrons are again free as majority carriers. The generation of the recombination centers proceeds extremely fast at room temperature, so that it is practically complete at the end of irradiation. The subsequent annealing process takes place, at room temperature, within hours after moderate radiation fluences, and within days after heavy irradiation. However, at temperatures below O”C, the lithium ions are hardly mobile, so that the first annealing step (lithium complexing) is essentially frozen in. It is not yet clear to what degree the negative slope portion of the annealing overshoot

84

M. WOLF and G. J. BRUCKER

may be related to the dissociation of the annealed complex. However, the slow and steady recovery of the open circuit voltage is clearly related to the dissociation of the recombination center with recovery of the majority carriers. In oxygen-rich (often imprecisely called oxygen containing) silicon, the situation is quite different, as is illustrated in the center and bottom portions of Fig. 13. In the center part of this figure, the case of silicon is depicted where the oxygen concentration substantially exceeds the lithium concentration. This is normally the case in quartz crucible grown silicon, except for the very highest lithium concentrations which may approach the case illustrated in the bottom portion of Fig. 13. With excess oxygen concentration, all lithium atoms are tied up with oxygen, forming LiO+ donors. Beyond this, free oxygen is available in the crystal in considerable concentrations. During irradiation, the situation is identical to that depicted in the upper part of Fig. 13 up to and including the introduction of the highly mobile vacancy. However, this vacancy now moves to one of the predominant oxygen sites and forms a recombination center which is a negatively charged vacancy-oxygen complex, called, in the jargon of the radiation physicist, an ‘A-center’. This ‘A-center’ can also anneal by absorption of a lithium ion. However, only a small fraction of the LiOf ions dissociate to form a thermal equilibrium with free oxygen and lithium ions. As before, these lithium ions are mobile and can move to the recombination center. After sufficient annealing time, two lithium ions and one majority carrier are observed to have complexed with each of the recombination centers neutralized. In this case, however, the generation of the free lithium ions by dissociation of LiO+ is the rate limiting step, rather than the lithium ion mobility, so that the annealing process proceeds at a much slower rate than in the upper model of Fig. 13, It has now been found that, in addition to the absorption of lithium ions for the neutralization of the ‘Acenter’, further lithium atoms can complex with the neutral center (Ref. [12]). In this process, majority carriers are used up, equal in number to that of the complexing lithium ions. Again, the dissociation of the LiOf is the rate limiting process. However, no saturation of this process has yet been observed, so that it has to be assumed that the neutral centers can absorb an indefinite number of lithium ions and majority carriers, in a manner similar to a precipitation process. Finally, in the bottom section of Fig. 13, the case of approximately equal lithium and oxygen concentrations is illustrated. This case can occur in quartz-crucible grown silicon with very high lithium concentrations (1017 cm-s), or in floatzone grown silicon with low lithium concentrations (lo15 cm-3). Here, there can be some free lithium not bound as LiO+, but little free oxygen is available. In this case the mobile, negatively charged vacancy complexes with the lithium-oxygen ion and a majority carrier to form a negatively charged recombination center which in this case contains the

vacancy, lithium and oxygen. Again, annealing takes place by absorption of two lithium ions (given sufficient time) and a majority carrier to form a neutral center. This neutral center then contains the vacancy, three lithium atoms and one oxygen atom. As in the oxygenrich case, further lithium atoms can complex with the neutral center, using up additional lithium ions and majority carriers. Since some free lithium ions are available, the annealing process is not completely tied to the dissociation of the lithium-oxygen ions. Therefore, the annealing proceeds at a somewhat faster rate than in the case of material with excess oxygen. It turns out that these three models of defect introduction and annealing quite satisfactorily describe most of the effects observed on lithium containing silicon solar cells. As mentioned earlier, additional impurities or defects in the silicon can complicate the situation further. However, they can be excluded from the devices with reasonable success, so that predictable characteristics are obtainable. Conclusions from the Efforts on Lithium Containing Solar Cells

The conclusions to be drawn from the results of the effort spent so far on lithium containing silicon solar cells can be enumerated as follows: 1. The speed of recovery after irradiation in lithium containing silicon solar cells depends directly on the concentration of free lithium ions in the silicon lattice. 2. The maximum power available from lithium containing silicon solar cells after irradiation with 1 MeV electrons and after recovery does not differ significantly from that available from the standard 10 R cm ‘n on p’ cells. If differences in power output after annealing should ultimately be established, they may be expected to remain small for electron irradiation, and to show the better results with the higher concentrations offree lithium. 3. Lithium containing solar cells exhibit a major advantage in radiation resistance after irradiation with heavy particles, that is protons or neutrons. For such particles, the lithium containing silicon solar cells can tolerate approximately 30 times larger radiation fluences for equal power output (after anneal) as the standard 10 Q cm ‘n on p’ cells. 4. Lithium containing cells of low oxygen content were previously considered unstable. Recent investigations have shown, however, that either a degradation or an annealing overshoot mechanism can exist in lithium containing silicon cells of low oxygen content, depending on the fabrication process used. Thus, properly prepared lithium containing solar cells from floatzone silicon are stable and exhibit a high level of performance after termination of the transient effect of the annealing overshoot. 5. The feared loss of majority carriers due to exhaustion of lithium in the annealing process and due to further complexing has been observed in the cells prepared from quartz crucible grown silicon. Such 10~s of majority carriers can ultimately lead to a decrease of open circuit voltage, an increased series resistance

Recent Developments ia Radiation Hardening Silicon Solar Cells

and finally, rectifying behavior of the contacts. In contrast, cells from floatzone crystals have shown recovery of majority carrier density as result of a slow ‘ideal’ annealing process. Also, recent evidence refutes the expected loss of free lithium to the p region (Refs. 17, 161). 6. At temperatures significantly above room temperature, the annealing process is accelerated. This is particularly interesting for cells prepared from oxygen rich material. 7. At temperatures below the ice point, the lithium is essentially immobile. Annealing can therefore not take place, and the damage introduction rate of the lithium containing solar cells corresponds to that of regular ‘p on n’ cells. 8. Long-term irradiation experiments at low dose rates, corresponding to those experienced on space vehicles, have shown that the behavior of lithium containing silicon solar cells during slow irradiation corresponds to simultaneous damage and annealing-as far as temperature permits the latter-and that its results are not significantly different from those determined from rapid irradiation and subsequent annealing. In general it can be stated that the physical mechanisms of radiation damage introduction and of annealing in lithium containing silicon solar cells are beginning to be reasonably well understood, and that the pilot line processes for these devices are starting to yield cells of controlled properties and satisfactory performance. In another year or two, the point should be reached, at which device qualification can be undertaken, full production started and the investigation effort relative to this device be directed towards clean-up of details, refinement of measurements and product improvement. The Environment on Outer Planet Missions The recent shift in emphasis in the planetary exploration portion of the U.S. space program towards Jupiter-

ASTRONOMICAL

85

and outer planet-missions, simultaneous with the space-demonstration of the feasability of electric propulsion, has opened a new problem complex of radiation effects on silicon solar cells. On these missions to greater distances from the sun, the incident light intensity, and with it the injection of charge carriers from photon absorption, will decrease by more than an order of magnitude. Also, as a result of the reduced light intensity, the operating temperature of the solar array will decrease from slightly above room temperature (300°K) near earth to 140°K at Jupiter (distance 5.2 astronomical units from the sun). At these low temperatures and light intensities, the efficiency of good silicon solar cells has been found to be approximately 50 per cent higher than that measured in near-earth conditions (Ref. [13]). The observed increase of efficiency is due to an increase of the open circuit voltage with decreasing temperature, while the short circuit current decreases slightly. As a result of this efficiency increase, the specific power output of the solar array (power per unit array area) does not decrease as rapidly as the incident light intensity. The temperature, efficiency and specific power output profiles for a typical solar array on a Jupiter mission are illustrated in Fig. 14, showing these three quantities as function of distance from the sun (earth = 1 AU) in astronomical units. The last point shown is the location of Jupiter. It should be noted that radiation effects have not been considered in the preparation of these curves. On these missions to the outer planets, the spacecraft may encounter significant effects from solar flares during the early part of the journey, and may later pass through the trapped radiation belts of Jupiter. For proper power system selection and design, the ability for prediction of performance changes during the mission is a necessity. For the solar cells, such changes were completely unexplored as recently as 2 yr ago for the environmental conditions expected on outer planet missions. An important aspect of such changes are the

UNITS

Fig. 14. Computed solar array temperature T, speci& power output P/A aud solar cell etliciency YJas function of distance from the sun (in astronomical units) for a Jupiter mission. Typical, present state of the art solar array characteristics assumed. 10

M.gWOLF andiG. J.[BRUCKER

86

radiation effects under the influence of the other environmental factors, particularly of low temperature and light intensity. At that time, the only knowledge about radiation effects in silicon solar cells at low temperature consisted in the observations that, for bombardment by high energy electrons, the introduction of defects decreases exponentially with decreasing bombardment temperature in n-type silicon, and decreases at a much slower rate (less than exponentially) in p-type silicon (Refs. [14, 151). The Effects of Solar Flare Protons Since the bombardment by solar flare protons de creases with the inverse-square distance from the sun, it causes most of its damage shortly after the spacecraft leaves the vicinity of earth, and its intensity reaches negligible levels at 3 astronomical units. In this early part of the journey, the solar array temperatures are not too greatly different from those encountered on near-earth missions, so that the radiation damage is still reasonably predictable. Thus, estimates predict a total power loss of approximately 15 per cent due to flare protons on ‘n on p’ cells for the mission. Considering the fact that the damage is caused exclusively by protons, it would be desirable to utilize the big advantage lithium containing silicon cells offer under this type of irradiation. However, to obtain these annealing properties, the panel temperature should be greater than approximately 280”K, since lithium is immobilized at lower temperatures. It is thus necessary to increase the panel operating temperature, e.g. by reduced emissivity of the panel backside. This will result in some power loss during the early part of the mission, but provide about 10 per cent more power from the solar array at distances beyond 3 astronomical units. Figure 15 presents the calculated temperature, efficiency and specific power output profiles (again without taking account of radiation effects) of a solar array with active temperature control to maintain the panel temperature

above 280°K out to distances of 2 astronomical units. As indicated by the dashed lines, the temperature of the array can be kept at lower values during the early part of the mission by adjustment of the incidence angle. Thus, annealing of flare proton damage can take place over most of the intense flare region. Beyond this distance of 2 astronomical units, the lithium containing solar cells would behave like ordinary ‘p on n’ cells. Effects from Electron Bombardment When a spacecraft reaches the vicinity of Jupiter, the solar cells will be bombarded by electrons and protons from the Jovian radiation belts which are similar to the Van Allen belts of earth. The nature and intensity of the radiation damage expected to occur on the solar cells in this environment are not yet adequately known. Although data from a few measurements of radiation effects on solar cells at low temperature are now available, they are not sufficient to provide a clear picture of the effects, nor are they statistically meaningful for quantitative predictions. In the following paragraphs, an attempt will be made to convey, in a simplified form, the presently existing tentative picture of the radiation effects from electron bombardment at low temperatures, and to present a few of the most interesting observations. As shown in Fig. 13, the first effect of a high energy electron is to dislodge a silicon atom, resulting in a negatively charged vacancy and an interstitial silicon atom. This vacancy is mobile at all temperatures considered here, and will complex with an impurity atom to form a recombination center similar to the process shown in the upper part of Fig. 13 for lithium or in the center part for oxygen. However, before this complexing occurs, there exists a probability for recombination of the vacancy-interstitial silicon atom pair, which decreases exponentially with increasing temperature. Thus, the generation of recombination centers increases exponentially with temperature, for constant bombard-

TPK)

ASTRONOMICAL

UNITS

Fig. 15. Solar array characteristics on a Jupiter mission, similar to Fii. 14, but with reduced solar array emissivity such as to maintain an array temperature above 280°K out to 2 astronomical units. The results of limitation of the temperature increases near earth by adjustment of the solar incidence angle are indicated by dashed lines.

Recent Developments in Radiation Hardening Silicon Solar Cells

ment fluence, between a low temperature limit near 100°K and a high temperature limit near 200°K. Although fewer defects are introduced through bombardment at lower temperature, the individual recombination centers are more effective in reducing diffusion length at this temperature. Thus, a solar cell irradiated at room temperature will show a lower diffusion length at about 140°K than at room temperature, but a similar cell irradiated at about 140°K will have a larger diffusion length at this temperature than the one irradiated at room temperature. This picture is very qualitative, and applies for n-type material (‘p on n’ cells) only. In p-type material (‘n on p’ cells), the difference in defect introduction with bombardment temperature is relatively small. This radiation damage model is corroborated by a few experimental data. Thus, a reduction of diffusion length by a factor of approximately 2 was measured upon reduction of temperature from 300 to 110°K on several cells, both in the irradiated and unirradiated condition, and on ‘n on p’ (Ref. [16]) as well as ‘p on n’ type (Ref. [17]). Also, significant differences in diffusion length were not observed on ‘n on p’ cells after bombardment at 300°K and at 110°K. Short circuit current measurements in airmass zero simulated sunlight provided similar results (Ref. [18]). cm+

‘O’m

WAVELENGTH Fig.

X -

16. Absorption coefficient a in silicon as function of wave length h for 300 and 77°K.

It may also be noted that, at irradiation below 14O”K, additional defects (called divacancies) are introduced, which lower the diffusion length further. These defects disappear, however, to an increasing degree upon heating above lOO”K, and 80 per cent of them vanish near 140°K. It would thus appear important to maintain

81

the solar array temperature at or above 140°K while traversing the Jovian radiation belts. A few additional effects vary with temperature so that they influence collection efficiency and increase the severity of the radiation effects at low temperature. Perhaps the most important of these is the shift in the absorption coefficient dependence on wavelength as illustrated in Fig. 16 (Refs. [19, 201). This shift occurs as a result of a change in the energy gap of the semiconductor. As a consequence, the long wavelength portion of the spectral response or collection efficiency curve shifts towards shorter wavelengths, as indicated in Fig. 17, curves 1 and 3, for 300 and I lOoK, respectively. Simultaneously, the minority carrier mobilities ,.&Dand PB for the diffused and the base regions, respectively, change with temperature, while the minority carrier lifetimes 70 and 'TB decrease towards lower temperature, as discussed before. Data for these quantities, derived from diffusion length measurements on typical 10 Q cm ‘n on p’ cells and from mobility calculations, are listed in Fig. 17 for temperatures T of 300 and 110°K (Ref. [16]). The changes of these material constants accentuate the effect of the absorption coefficient shift in the long wavelength part of the spectral response curve, while simultaneously narrowing the response from the short wavelength side also. It may be noted that the curves in Fig. 17 are the result of computer evaluations of the data contained in Fig. 16 and in the table of Fig. 17, using the dependence of collection efficiency on material and physical device parameters described in Ref. [21]. Upon irradiation to 1. lo15 cm-2 electrons of 1 MeV energy, the diffusion length decreased by a factor of nearly 3 at room temperature, but by more than 4 at llO”K, because of introduction of the mentioned divacancies. However, these were subsequently annealed out by warming the sample above 14O”K, resulting also in a 3 : 1 diffusion length ratio at 110°K. The change of spectral response with irradiation is indicated in Fig. 17 by arrows leading from curves 1 to 2 and 3 to 4 for room temperature and llO”K, respectively. The shift to shorter diffusion lengths which occurs with temperature reduction, brings the most severe part of the effect of diffusion length reduction due to irradiation into a wavelength range of larger absorption coefficients. Combining this with the larger degree of diffusion length reduction occuring upon irradiation, a much more severe effect on the spectral response curve should be expected than at room temperature. Furthermore, this change in the spectral response extends now into a range of larger photon flux in the airmass zero solar distribution curve, so that the overall collection efficiency, weighted by the airmass zero spectrum, or the short circuit current, show a much larger decrease (24 per cent) in the low temperature case, compared to the room temperature radiation degradation of 8 per cent. While the first value is in good agreement with other experimental data (Ref. [IS]) the latter ,value appears small compared to the commonly used value of 20 per cent for l-1015 cm-s electrons, or the value of 15 per cent for the correspond-

88

M. WOLF and G. J. BRUCKER

1.4

- 1.2

lrl-1.0

5

2 is

0.8 - 0.8 z

L L

z $06

-

0.4 -

0.2 -

o-

0.2

WAVELENGTH

$a!’

h

Fig. 17. Computed spectral response curves (co&&on efficiency qeou) for 300 and llO”K, using absorption coefficients from Fig. 16, measured material parameters (inserted table), and typical solar cell dimensions. Curves 1 and 3 are unirradiated, curves 2 and 4 after bombardment with 1 MeV electrons to a fluence of 1.1015 cm-*. The base region ditfusion lengths are: L(1) = 170 pm; L(3) = 65 pm; Z.(2) = 100 pm; I.(4) = 23 pm; and the overall cokction efficiencies, weighted by the airmass zero sunlight spectrum to 1.75 pm wavelength; q&l) = 0.733; q,))(3) = 0.675; qc0u(2) = 0.574; qcou(4) = 0.436. The numbers in parenthesis here refer to the curve numbers.

ing case in Ref. [18]. It may be noted, however, that the calculated short circuit current change at room temperature is, for an identical diffusion length change, in excellent agreement with experimental data on the short circuit current dependence on diffusion length, as measured in simulated airmass zero sunlight (Ref. [22]). A few further observations appear worth mentioning. It has been found that the differences in radiation damage with bombardment temperature disappear at fluences above l- lo15 cm-2 electrons. Further, the loss of open-circuit voltage is small, so that the power loss is primarily due to the loss of short-circuit current. Effects of Proton Bombardment Figure 18 presents a plot of output power versus fluence of protons of 3 MeV energy for ‘n on p’ cells bombarded and measured in vacuum at 140°K with 5 mW cm-s solar simulated radiation (‘in situ Jupiter conditions’) as well as under near-earth conditions (300°K and 140 mW cm-s solar simulated radiation). The graph shows clearly that the same type cell, irradiated in the Jupiter environment, exhibits considerably greater radiation damage. Upon bringing these cells to the near-earth environment after ptoton irradiation under Jupiter conditions, the cell performance increases to that normally found after bombardment in this environment (Ref. [23]). The converse of this behavior has been observed on the ‘p on n’ cells. Here, the power output is higher after

EARTH-ORBIT

N/P

CONDiTlONS

CELL

loncm 2x2in 0.013in THICK j Y

.3L 108 FLUENCE

I 10s OF 3 MeV

I 100 PROTONS per Cd,

1 IO” HITTING BARE

CELL

Fig.

18. Relative maximum power output of 10 fi cm ‘n onp’ solar cells as functions of fluence of 3 MeV protons, irradiated and measured under in situ Earth orbit and Jupiter flyby conditions. (Reproduced from Ref. [23] by courtesy of author.)

proton bombardment under ‘in situ Jupiter conditions’, and drops upon bringing the cells to near-earth conditions to the values normally determined after irradiation under these conditions (Ref. [23]). As a result, the ‘p on n’ cells exhibit better performance, on an absolute scale, than the ‘n on p’ cells do after proton bombardment to fluences above 1011 cm-2. This is illustrated in Fig. 19. It should be noted relative to this figure that the ‘n on p’ cells were typical, present production cells of 10 Q cm base resistivity, but that the ‘p on n’ cells

Recent Developments in Radiation Harden@ Silicon Solar Cells

cells, O.Ol3”thick

0.5

-I

t

0.0.----J108

I 10’0

IO9 FLUENCE

OF

3MeV

I IO”

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outer planet missions has barely been started. In this start, it has not only been found that conclusions cannot be drawn from earth-orbit type irradiation studies to the behavior of solar cells after bombardment under Jupiter environment conditions, but also, that this different behavior is such as to make solar arrays viable candidates for power systems for outer planet missions. As a consequence of this conclusion, the exploration of the behavior during and after bombardment under Jupiter conditions should not only be continued. but considerably accelerated. The objectives of this exploration are threefold : to understand the damage mechanisms and their results; to optimize cell design for maximum performance on the mission, based on the understanding thus gained; and to provide the needed design information for the power systems engineer.

PROTONS

Fig. 19. Maximum power output from 10 SL cm ‘n on p’ and 20 SL cm ‘p on fz’ solar cells, normalized for active cell area, as function of hence of 3 MeV protons under in situ Jupiter flyby conditions.

used were the only ones available to the experimenter and were non-typical, having 20 fi cm base resistivity and low power output before irradiation (Ref. [24]). Under proton bombardment, the damage introduced is of a different type than that described as resulting from electron bombardment. Protons produce large clusters of defects due to the heavy mass of the particles. In contrast to this behavior electrons produce pointdefects or isolated defects. These large clusters of defects give rise to a distribution of energy levels throughout the forbidden energy gap. Thus temperature and charge carrier injection critically determine how many of the energy levels are activated as recombination centers. Conclusions

89

about the Radiation Effects on Outer Planet Missions

The important and initial findings made in the investigation of solar cell radiation damage on Jupiter missions can be summarized as follows: 1. Only insufficient data on radiation damage effects under Jupiter conditions are yet available, and a picture of the mechanisms causing the observed effects is barely starting to emerge. 2. The available data clearly show that the radiation damage effects at low temperature and low solar intensity are quite different from those observed on near-earth missions. 3. ‘p on FZ’and ‘n on p’ cells behave differently after bombardment by electrons or protons under Jupiter conditions, with the ‘n on p’ cells seeming to lose their advantage under the latter type of irradiation. However, they appear to perform better after electron irradiation. 4. Lithium containing ‘p on n’ silicon solar cells may be used to substantially nullify the effects of solar flares which will be encountered during the early part of the mission. In conclusion, it may be observed that the exploration of the behavior of silicon solar cells for Juniter or other

Acknowledgements-The authors wish to express their deep appreciation for the wholehearted cooperation given by R. L. Debs, R. C. Downing, T. J. Faith and D. L. Reynard, who all contributed to this paper through provision of unpublished data or information and throuah discussions of various subject areas. The authors also want-to mention that they, in the face of a great wealth of published background material, and in the interest of brevity, consciously listed only those references from which thev dire& used information. The interested reader will find direction td the background literature in the publications listed herein, as well as in earlier review papers (Refs. [26-291).

References [l] V. S. Vavilov, I. V. Smimova and V. A. Chapnin,

Soviet Phys. solid St. 4, 830 (1962); V. S. Vavilov, Radiation Damage in Semiconductors, 7th Intern’1 Conf. on the Physics of Semiconductors, Royanumont, France, p. 115 (1964). [2] J. J. Wysocki, Record of the 5th Photovoltaic Specialists Conf., Vol. 2, D-7 (1966); J. J. Wysocki, P. Rappaport, E. Davison, R. Hand and J. J. Loferski, Appl. __ Phys. Lett. 9, 44 (1966). [3] J. J. Wysocki, Self-Healing Radiation-Resistant Silicon Solar Cells, Record of the 6th IEEE Photovoltaic Specialists Con& Vol. 3, pp. 96-109 (1967). [4] G. J. Brucker et al., First, Second and Third Quarterly Reports on a Study to Determine and Improve Design for Lithium-Dooed Solar Cells. Jet Prooulsion Laboratory Contract No. 952555: [S] T. J. Faith, J. P. Corra and A. G. Holmes-Siedle, Room Temperature Stability and Performance of Lithium-Containing Solar Cells-An Evaluation. 8th IEEE Photovoltaic Specialists Co& Seattle, Washington (1970). [6] Ref. 4 has been augmented by data provided in private communication by T. J. Faith, RCA Astro-Electronics Div. [7] T. J. Faith, G. J. Brucker, A. G. Holmes-Siedle and J. J. Wysocki, Long Term Stability of Lithium Doped Solar Cells Irradiated with Electrons, Protons and Neutrons, Record of the 7th IEEE Photovoltaic Specialists Conf., pp. 131-145 (1968). [8] D. L. Reynard and D. B. Orvis, Beta Irradiation of Lithium Doped Solar Cells, Record of the 8th IEEE Photovoltaic Specialists Conf., pp. 108-112 (1968). 191 D. L. Revnard. orivate communication (1970). [iOj R. G, Downing,’ private communication‘(l970). [ll] W. M. Morris, Effect of 14 MeV Neutrons on Silicon Solar Cells, Master Thesis, Dept of Physics, University of Utah (1970). [12] G. J. Brucker, Annealing of Electron Bombardment Damage in Lithium-Containing Silicon, presented at ZEEE Nuclear Eficts Co& San Diego, California (1970); IEEE Trans. to be published (1970). 1131 d. H. Liebert, Solar Cell Performance at Jupiter Temperature and Solar Intensitv. Record of the 7th ZEEE Photovoltaic Specialists Co&, ii. 92-96 (1968). 1141 G. J. Brucker, Phys. Rev. 183, 172, (1969). [15] R. _ L. Novak, .. __ unpublished Ph.D. Thesis, University of PennSylvania (1964).

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M. WOLF and G. J. BRUCRRR

[16] T. J. Faith, private communication. [17] G. J. Brucker et al.; Fourth Quarterly Report on a Study to Determine and Improve Design for Lithium Doped Solar Cells, Jet Propulsion Laboratory Contract No. 952555 (1970). [18] R. L. Statler et al., Semiannual Report for Solar Cell Research (Phase II), Jet Propulsion Laboratory Work Order 8056, Contract No. NAS7-100 (1970). 1191 _ _ W. C. Dash and R. Newman, Phvs. Rev. 99. 1151-1155 (1955). [20] H. R. Philipp and E. A. Taft, Phys. Rev. 120, 37-38 (1960). 1211 M. Wolf. Proc. ZnstnRadio Enms 48. 1246-1263 (1960). i22j W. C. dooley et al., Handbook of Space Environmental Effects on Solar Cell Power Systems, pp. IV-26IV-31, NASA Contract No. NASw-1345 (1968). [23] R. J. Debs and N. R. Hanes, Preliminary Results of Radiation and Jupiter Environment Tests on Solar Cells, 8th IEEE PhotovoltaicSpecialists Conf., Seattle, Washington (1970).

[24] R. J. Debs, private communication. [25] G. J. Brucker and B. Markow, Neutron Damage in Silicon Solar Cells, Record of the 6th IEEE Photovoltuic Specialists Co& pp. 53-63 (1967). [26] P. H. Fang, Present Status of Lithium Diffused Silicon Solar Cells, Record of the 6th ZEEE Photovoltaic Specialists Conf., pp. 110-121 (1967). [27] P. A. Berman, Status of Lithium Solar Cell Development, Record of the 7th IEEE Photovoltaic Specialists Conf., pp. 101-107 (1968). [28] P. A. Berman, Status of JPL-Sponsored Lithium Solar Cell Investigations, 8th IEEE PhotovoltaicSpecialists Co&, Seattle, Washington (1970); to be published in conference record. [29] J. Weingart, Defect Structure and Behavior in Electron Irradiated Lithium Diffused Silicon Solar Cells. 8th IEEE Photovoltaic Specialists Conf., Seattle, Washington (1970); to be published in conference record.