FINAL REPORT

PHY-93R004

• Ir _ /

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LDEF SOLAR CELL RADIATION

EFFECTS ANALYSIS

Prepared by

Carol J. Rives

Joseph L. Azarewicz

Lloyd Massengill

Physitron, Inc.

3304 Westmill Drive

Huntsville, AL 35805

Prepared for

National Aeronautics and Space Administration

George C. Marshall Space Flight Center

Attn: EH15

Marshall Space Flight Center, AL 35812

Prepared under

Order No. H-13099D

January 20, 1993

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TABLE OF CONTENTS

Section

1.0

2.0

2.1

2.1.1

2.1.2

2.1.3

2.2

2.3

2.3.1

2.4

3.0

4.0

5.0

INTRODUCTION .................................................................................. 1

ANALYSIS .............................................................................................. 1

Background ............................................................................................. 1

Diode Current, Id .................................................................................... 2

Photogeneration Current, Ig ................................................................. 2

Open Circuit Voltage, Voc ..................................................................... 3

Data Collection ....................................................................................... 3

Radiation Effects ..................................................................................... 3

Short Circuit Current, Iso Radiation Degradation ............................. 5

8

Analysis ...................................................................................................

RESULTS ................................................................................................. 12

CONCLUSIONS ..................................................................................... 13

RECOMMENDATIONS ........................................................................ 14

REFERENCES ......................................................................................... 15

ii

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LIST OF FIGURES

1

2

3

4

5

6

Title

Circuit Model of Solar Cell ...................................................................

Minority Carrier Diffusion Length vs. Short Circuit

Current Density of Conventional n-on-p Silicon Solar Cells .......

Minority Diffusion Length in p Material ............................................

Short Circuit Current Density (Absolute and Relative) ...................

Short Circuit Current Density (Absolute and Relative) ...................

Short Circuit Current Density (Absolute and Relative) ...................

6

8

11

11

12

LIST OF TABLES

Table

1

2

3

Title

Solar Cell Descriptions ..........................................................................

Dose LDEF Mission Caused by Trapped Protons and

Electrons Behind an Aluminum Slab ..............................................

Lifetime Damage Constants for Neutrons, Electrons,

and Protons .........................................................................................

4

5

7

iii
1.0 INTRODUCTION

Because of the extended time that the Long Duration Exposure Facility (LDEF)

mission stayed in space, the solar cells on the satellite experienced greater environments

than originally planned. The cells showed an overall degradation in performance that is

due to the combined effects of the various space environments. The purpose of this

analysis is to calculate the effect of the accumulated radiation on the solar cells, thereby

helping Marshall Space Flight Center (MSFC) to unravel the relative power degradation

from the different environments.

2.0 ANALYSIS

2.1 BACKGROUND

Solar cell damage depends on the radiation environment and the construction of

the solar cell itself. The solar cell is a bipolar junction, where the illuminated side of the

junction is heavily doped, while the unexposed side is lightly doped. The heavily

doped region is called the emitter, while the lightly doped side is called the base.

Incident photons excite hole electron pairs in the cell material. Because of carrier

density gradients near the junction, electrons diffuse toward the junction in the p-type

material, while holes diffuse to the junction in the n-type region. The high electric field

in the depletion layer accelerates the carriers to the opposite side of the junction. The

carriers crossing the junction result in the photon generated cell current.

The photo generated current can be modeled by using the circuit shown in

Figure I where 18 is the photogeneration current, Id is the diode current, Rsh is the cell

Incident

Photons

Figure 1. Circuit Model of Solar Cell

1
shunt resistance (usually ~10 4 ohms), Rsr is the series resistance (small for a high quality

cell), and RL is the load resistance. This circuit is useful to understand how radiation

affects the material parameters and therefore the solar cell electrical performance.

Various dements of the circuit model are described below.

2.1.1 Diode Current, Ia

The diode current, Id, represents the ordinary diode conduction current for the pn diode forming the solar cell. This current is given by the equation:

Id=Io [eXP_nKTJ t vo)- lj

(1)

where Io is the diode saturation current, Vo is the voltage across the diode, e is the

charge on an electron (1.6 x 10 -10 coulombs), k is Boltzman's constant, T is the

temperature in kelvins, and n is an idealization factor which relates to the quality of the

junction (References 1, 2). The diode saturation current is related to the diode material

parameters. This current is given in standard textbooks (Reference 2) as follows:

Io-i ,_" _AA _ ND ] (2)

where Dw Dp are the diffusion constants, xw xp are the carrier lifetimes, NA, ND are the

impurity carrier concentrations in each region, and Aj is the junction area.

2.1.2 Photogeneration Current, I.

The photogeneration current can be found by analyzing the above circuit.

Applying Kirchoff's laws, the load current J can be written as:

i = is- i0 [exp (e (V + IRsr)).l] (V + I_r) (3)

nkT Rsh

where all quantities are as defined above.

The photogeneration current is related to the short circuit current of the

illuminated cell. The short circuit current can be found by setting the output voltage V

in the circuit to zero (V = 0). The result of this calculation is

(4)

2
where Isc is the short circuit current.

For the usual case of well-constructed solar cells, Rsr is approximately 0.5 ohms,

and Rsh is approximately 10 4 ohms (References 3 and 4). Under these conditions, Isc is a

good approximation of Ig.

2.1.3 Open Circuit Voltage. V_

The open circuit voltage is the output voltage as the load resistance approaches

infinity. To find a usable expression for the open circuit voltage, consider Eq. (3), which

describes the output current for the solar cell equivalent circuit. The open circuit

voltage is found by setting the load current, I, of Eq. (3) equal to zero.

Under the condition that the shunt current, Ish, is small with respect to the diode

short circuit current, the form for the open circuit voltage is given as

(s)

2.2 DATA COLLECTION

Physitron was provided a description of the solar cells to be analyzed. Both

types are silicon, n-on-p, manufactured by Applied Solar Energy Corporation. The

thick type (8 rail) was tested with a variety of covers. The thin type (2 rail) was tested

with a single 2 mil thick microsheet cover. The solar cell descriptions are shown in

Table 1.

The LDEF environment included cosmic rays, protons, neutrons, and electrons.

The principal contributors to the effects in the solar cells were electrons and protons. A

tabulation of those environments is shown in Table 2. The proton fluence was on the

order of 109 per cI_ 2 with energies ranging from 0.5 to 200 MeV. The electron fluence

ranged from 108 to 1012 per ¢m 2 with energies between 0.5 to 3 MeV.

One notes from the table that the electron dose dominates at the surface, with a

value of approximately 2.53 x 106 rads at the surface. The LDEF mission had one

uncovered solar cell, which therefore would experience the maximum damage from the

electron fluence at the surface. The surface fluence can be estimated from this

equivalent 1 MeV electron dose to be approximately 5 x 1014 electrons/cm 2. This

fluence can be used to estimate the solar cell degradation.

2.3 RADIATION EFFECTS

The effects of ionizing radiation on solar ceils is primarily due to degradation in

minority carrier lifetimes resulting from lattice damage and recombination center

creation.

3
Table 1. Solar Cell Descriptions

I. Thick Solar Ceils

rI. Thin Solar Cells

Manufacturer: Applied Solar Energy Corporation

Size: 2 x 4 cm, 8 mil thick

Type: Silicon, n-on-p, 2 ohrn-cm, junction depth about 0.25 micron

Contacts: Chemical vapor deposited dielectric wraparound '_q"pad at back center of

cell, Ti-Pd-Ag material.

Dual anti-reflectance coatings

Aluminum back surface reflector contact thickness -6 microns minimum

No back surface field

a. No cover

b. 6 rail thick 0211 microsheet

Manufactured by OCLI

Anti-reflectance coating only

DC 93-500 cover adhesive ~ 2 mils thick

c. 6 rail thick 0211 microsheet

Manufactured by OCLI

Anti-reflectance coating, ultraviolet filter - 350 nm

DC 93-500 cover adhesive

d. 6 rnil thick frosted (etched) fused silica cover

Manufactured by OCLI

Anti-reflectance coating, ultraviolet filter - 350 nm

DC 93-500 cover adhesive

e. 6 mil thick fused silica cover

Manufactured by OCLI

Anti-reflectance coating, ultraviolet filter - 350 nm

DC 93-500 cover adhesive

f. 6 rail thick microsheet (12 cell module back side exposed)

Manufactured by OCLI

Anti-reflectance coating, ultraviolet filter - 350 nm

DC 93-500 cover adhesive

Mounted with solar ceil modules face down

Exposed surface is 1 oz. rolled annealed copper printed circuit with antitarnish coating encapsulated between 2 sheets of I mil Kapton H film, 0.5

rail high temperature polyester adhesive. The solar cells are bonded to the

copper interconnect using parallel gap electric resistance welding. The

solar cells are bonded to the Kapton substrate using two strips of double

back 3M Isotac acrylic adhesive.

Manufacturer: Applied Solar Energy Corporation

Size: 2 x 4 cm, 2 rail thick

Type: Silicon, n-on-p, 10 ohm-cm

Contacts: Junction wraparound, Ti-Pd-Ag materials (_ 0.25 micron thick)

Dual anti-reflectance coatings

No back surface reflector

Back surface field

2 rail thick microsheet cover

Manufactured by Pilkington P.E. Limited

No coatings or filters

DC 93-500 cover adhesive

4
Table 2. Dose LDEF Mission Caused by Trapped Protons and Electrons

Behind an Aluminum Slab

Thickness

( rn/cm2)

Electrons

(rads)

2.53 x 106

Protons

(rads)

1340

0.05 3680 560

0.1 1150 488

0.2 310 418

0.3 130 380

0.4 69 355

0.5 40.9 335

1.0 4.13 274

2.0 0.99 212

5.0 0.0583 130

Total

(rads)

2.5 x 106

4240

1640

728

511

424

376

278

212

130

10.0 0.0296 75.3 75.3

20.0 35.0 35.0

30.0 19.6 19.6

40.0 12 12

2.3.1 Short Circuit Current. I_ Radiation Degrada_ign

First, one notes that the short circuit current for the solar ceU can be

approximated by a log-linear curve fit. As shown in Figure 2, the form for Isc can be

found experimentaUy to fit the equation:

Isc = a in(L) + b (6)

where L can be identified as the cell diffusion length and a, b are experimentally

determined constants. One notes that the diffusion length is given by:

L= _ (7)

where D is the diffusion constant and x is the minority carrier lifetime.

To determine the radiation response, one can use the usual lifetime degradation

response given by:

L = i + • (8)

5
Figure 2.

5

t

I O0 ¢

E 5

._o

_ 10

" 5 _

Cohvenlmi'_ _ , .... n-on-p_lmon| cell,

10ohm-cm,18mils_Jck

1.0

5 10 15 20 25 30 35 40

Short circuit current density, rnA/cm 2

Minority Carrier Diffusion Length vs. Short Circuit Current Density

of Conventional n-on-p Silicon Solar Cells

where Kz is a lifetime damage constant and • is a fluence of particles. • can represent

proton fluence or electron fluence.

To interpret this in terms of solar cell response, we combine the lifetime damage

equation with the definition of diffusion length. The resulting equation,

1_ _. 1___+ K_ (9)

L 2 L_

provides an expression for the degradation of the diffusion length. The factor, K, is the

diffusion length damage constant. Values for the damage coefficients can be

determined from the literature. Table 3 gives some typical values (Reference 2).
Table 3. Lifetime Damage Constants for Neutrons, Electrons and Protons

(References 2 and 5)

Substrate

Resistivity

(ohm-cm)

n-type

1

10

p-type

1

1 MeV Neutron Damage Constants

(cm2/s)

I x 10 -3

5 x 10-6

3 x 10 -6

2 x 10-6

In ection Level

I x 10 -1

2 x 10-6

5 x 10 "7

5 x 10 "7

10 2 x 10-6 5 x 10 -7

20 MeV Proton Damage Constants

Kz (crn2/s)

n-ty W

1 -5 x 10..6 ~2 x 10 -6

10 2.5 x 10 -6

p-type

1 -2 x 10 -6 --1 x 10 -6

10 -5 x 10-6

3 MeV Electron Damage Constants

Kz (¢m2/s)

10

p-type

1

10

-2 x 10 -7

-5 x 10-8

-3 x 10-8

~I x 10-8

-5 x 10 -8

~I x 10 -8

-5 x 10 -9

-3 x 10 -9

A typical constant for 1 MeV electron on an n-on-p solar cell structure is 1.7 x

10 -10. Using a typical starting diffusion length for electrons in p material of 100 _trn

gives the curve shown in Figure 3.

7
E

o

ee-

.--i

e"

r-t

e2

LU

0.012

0.01

0.008

0.006

0.004

0.002

0 i ! I I I I I I

le+011 le+012 le+013 le+014 le+015 le+016 le+017

Figure 3.

Electron Fluence (cm 2)

Minority Diffusion Length in p Material

2.4 ANALYSIS

At low photon energies (wavelengths longer than the junction depth of the solar

cell), most of the solar cell current originates from diffusion charge collection from the

back-side material. For an n-on-p solar cell, the device of study here, this collection will

be electrons (minority carriers) diffusing from the p-type substrate material to the

junction region. For these devices, the junction depth is approximately 0.25 Ixm.

The peak of the natural solar energy spectrum occurs in the blue light region, i.e.,

wavelengths around 0.5 p.m. Since this is where most of the solar cell energy originates,

we will use 0.5 p.m as the wavelength of interest here. Since this wavelength is longer

than the 0.25 I_m junction depth, we assume long wavelength conditions apply. The

generated photocurrent density due to the electron diffusion in the p-type region is

given by (Reference 5):

8
Jn = qF (l-R) O_Ln

o_2L_-1

e×p +w)]

SnLn. cosh H _ exp (-(zH) + sinh H___+ (zLn t

_o f 1 ox l- ,

_Ln {SnLnt sinh'l H / + cosh [HI

(:o)

where

= the optical absorption coefficient which can range from 103 to 104 CIn"Ifor

wavelengths of 0.9 to 0.5 _ra. Since we are most concerned with the blue

solar energy peak at a wavelength of 0.5 B-m, we win use 104 cm -1 for c_,

but this report presents results for this value as well as 5 x 103 and 103 cmfor comparison;

F = the photon flux striking the solar cell. At the one solar constant level of

radiation at the earth (1353 W/m 2) with an air mass factor of zero (no

atmospheric attenuation), the solar flux can be approximated as 2.5 x 1017

photons/cm2/sec in the blue light region;

R = the fraction of photons reflected from the surface, we will arbitrarily

assume about 20% reflection;

q = the electronic charge;

Ln = the diffusion length of electrons in the p material;

Xj = the n-p junction depth in cm;

W = the depletion region width in cm;

Sn = the surface recombination velocity at the back surface, usually a large

number approaching infinity. Here we used a value of I x 108 with good

results. The results shown below do not depend heavily on this number;

Dn = the diffusion constant of electrons in the p region; and

H = the p region depth.

In order to gain best estimate approximations for some of these parameters, we

used the solar cell information provided for the thick cells. Based on the resistivity of 2

ohm-cm, we can approximate the acceptor doping level of the p region as 6 x 1015 cm -3.

Using this doping level and assuming an abrupt junction at the n-p interface, the

depletion region width will be 0.4 _m. Also, the diffusion coefficient will be

approximately 1,500 cm2/sec. The 8 rnil thick layer corresponds to 203.2 t_n. The

9
junction depth is 0.25 _tm. These numbers give us a starting point for rough

photocurrent calculations using Eq. (10).

Note the {exp(-rxH)} terms in Eq. (10). For the values given above, these terms are

approximately 1.5 x 10 -9. Compared to the other terms in the equation, these terms are

negligibly small and can be ignored. Thus, Eq. (10) can be reduced to:

i_a a cosh(_)+sinh _ (11)

qF (I-R)0_La exp [-,Xj + W )] 0_Cn [_n_ ) sinh (_) cosh (_) Jn --- o_2L2 -1 - +

Using the values given above, the results of Figure 3 and Eq. (11), we can plot the

short-circuit current density for the solar cell as a function of electron fluence for several

a values. Since the solar cells are exposed to an entire spectrum of radiation, a single

value of 0c is probably not an ideal model for the exposure. However, the c¢ value of 104

cm-1 approximates the blue light peak of the spectrum and can be used as a good

comparative parameter in the investigation of degradation with electron exposure.

These plots for a values of 103, 5 x 103, and 104 are shown in Figures 4 through 6.

The photon flux impinging on the solar cell (F) and the reflection ratio (R) in Eq.

(11) are highly variable and dependent on the orbit altitude, energy spectrum reaching

the cell, as well as the physical construction of the solar cell itself. However, the

degradation of the cell output with electron exposure can be determined without

relying on these parameters by defining a relative (or normalized) short-circuit current

density. If Eq. (11) at the electron exposure level of interest is normalized by the current

density at zero electron exposure, the parameter F, R, and q are normalized out. The

resulting equation is given by:

Jx: _ 2T2 "_nO -1

Jo Lno o_2L2 -1

(XLnX -

(KLno -

Dn

S,-,LaODnc°sh(_oo)+sinh(_-O-O)

(12)

10
0.05 1

• o0.03 0.6

:::::::::::::::::::::"

O.Ol .. 0.2 ,_

0 0

le+011 1e+012 le+013 le+014 le+015 le+016 1e+017

Electron Fluence (cm"2)

(k ==0.9 l_m)

Figure 4. Short-circuit Current Density (Absolute and Relative)

0.O4 " 0.8 "_

t'-

_' 0.03 ' 0.6

0.02

0.01

=-.

r_

C)

m

m

............................................ _-._. 04

m

I I I 0

le+011 le+012 le+013 le+014 le+015 le+016 le+017

L)

N

¢1

O

z

Electron Fluence (cm"2)

(X = 0.6 ImP)

Figure 5. Short-circuit Current Density (Absolute and Relative)

11
.a,

ra

"a

o

0.05

0.04

0.03

0.02

0.01

I

)

I I I I I , I I

1e+011

0.8

0.6

0.4

0.2

0

le+012 le+013 le+014 le+015 le+016 le+017

¢,0

r--

(D

E3

"E

L=.

o

"0

N

E

0

z

Electron Ruence (cm"2)

(X = 0.5 I_m)

Figure 6. Short-circuit Current Density (Absolute and Relative)

where

Jx =

J0 =

Ln0 =

L x=

the current density at the electron exposure level of interest,

the current density at zero radiation exposure,

the diffusion length at zero electron exposure, and

the diffusion length at the electron fluence of interest, as given in

Figure 3.

Using Eq. (12), a percentage degradation in the output short-circuit current density can

be estimated without relying on the highly unpredictable parameters of F and tL These

results are shown in Figures 4 through 6 by the right-hand axis.

3.0 RESULTS

Using the approximation of a blue-light peak ((_ = 104), it can be seen from

Figure 6 that electron degradation does not begin until an approximate fluence of 1014

cm -2. At the fluence of interest here, i.e., 5 x 1014 cm-2, it can be seen from Figure 6 that a

degradation of approximately 10% is expected in the short-circuit density output of the

solar cell of this study. That is, the generated photocurrent is reduced to 0.9 of its

original, pre-radiation value. In terms of power output, this would be expected to

12
roughly correspond to a power output of (0.9) 2, or 0.81, with respect to the original

power output. Thus, these rough calculations predict an approximate 19% degradation

in power output for this cell at an electron fluence of 5 x 1014 cm -2. The results show

that at higher electron fluences, degradation is expected to increase exponentially. For

example, at an electron fluence of 1 x 1017 cm -2, the relative current density given by

Figure 6 is approximately 0.7. This would correspond to a power degradation of

approximately 50%. Even worse degradations are evident in the curves for lower

absorption parameters, i.e., longer wavelengths.

This analysis has been based on the 1 MeV electron equivalency environment.

Variations in the solar cell base resistivity and thickness can cause variations in

response to the radiation. In addition, protons with energies less than 5 MeV may

deposit entirely in the solar cell (especially the thicker ones) and cause significant

damage. The good news is that cover glasses can, and usually do, stop a significant

portion of the particles with energies that are the most damaging.

In general, for maximum radiation hardness, solar cells with low base resistivity

should be used. In addition, a large initial diffusion length minimizes the degradation

in overall performance due to a radiation-induced decrease in that diffusion length.

4.0 CONCLUSIONS

The solar cells on LDEF were exposed to much higher fluences than originally

planned, providing a unique opportunity to study long-term environmental effects on

their operability. In order to bound the effects due to the radiation environment

(electrons and protons), Physitron has predicted the degradation in a typical thick solar

cell from Applied Solar Energy Corporation due to the radiation on an uncovered cell.

The analysis shows that a power degradation on the order of 20% is what would be

expected from the environment of 5 x 1014 cm 2. Although no explicit calculation was

performed for the thin cell, one might predict a somewhat greater degradation due to

their higher base resistivity and the back surface field, which can be very sensitive to

radiation effects. All cells with cover sheets should experience minimal radiation

degradation due to the filtering effect. The same would be true of the cells that had

their back surface exposed.

In conclusion, this analysis does not reveal any unexpected effects in the solar

cell performance due to the radiation exposure. By using cover glass and selecting ceils

with low base resistivity and large diffusion lengths, any long-term degradation effects

due to the radiation environment should be minimized.

13
5.0 RECOMMENDATIONS

The foregoing analysis is useful from the perspective of drawing a direct

comparison between theoretical and empirical behavior. The fairly close agreement

between the observed and predicted behavior of the LDEF solar ceils enhances

confidence with which such calculations can be made in the future. Clearly, highly

reliable behavior models and predictive techniques enable greater precision during the

parts selection process. Ultimately, this is important when making trade-off decisions

between cost, system requirements, and performance margin.

Three recommendations are made relative to this effort and possible follow-on

activities.

(1) Similar calculations cart be run for a broader range of technologies for the

purpose of developing enhancements to the model provided in this report.

(2) An evaluation cart be performed on currently available solar cell technologies

and designs with respect to ionizing dose response. As appropriate, predicted behavior

could be supported with existing mission data and laboratory testing. Physitron has the

capability of conducting low dose-rate laboratory testing of this type. Findings from

this effort would be provided to NASA in the form of an automated database and/or in

written form.

(3) A first-principles, automated predictive model could be developed for use by

NASA scientists and engineers. Such a model could be in a desktop (PC/Macintosh)

environment and would enable the user to perform "quick-look" predictions for specific

technologies of interest.

Physitron is pleased to provide this report and looks forward to supporting

NASA in the future as may be warranted.

14
REFERENCES

1. Carter, J. R., and Tada, H. Y. "Solar Cell Radiation Handbook," TRW Systems,

Report No. 2145-6001-RU-00, June 1973.

2. Messenger, G. C., and Ash, M. S. The Effects of Radiation on Electronic System_,

New Yorlc Van Nostrand Reinhold, 1992.

3. Tada, H. Y., Carter, J. R., Anspaugh, B. E., and Downing, R. G. "Solar Cell

Radiation Handbook," Jet Propulsion Laboratory, Pub. 82-69, November 1982.

4. Wolf, M., and Rauschenbach, H. "Series Resistance Effects on Solar Cell

Measurements," Advanced Energy Conversions, Vol. 3, 1963.

5. Sze, S. M. Physics of Semiconductor Devices, New York: Wiley and Sons, 1969.

15
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

January 20T 1993 Final Report

4. TITLE AND SUBTITLE

Final Report, LDEF Solar Cell Radiation Effects Analysis 506-41-11

6. AUTHOR(S)

Carol J. Rives

Joseph L. Azarewicz

Lloyd Massenaill

7. PERFORMING ORGANIZATION NAME(S) AND AODRESS(ES)

Physitron, Inc.

3304 Westmill Drive

Huntsville, AL 35805

9. SPONSORING/MONITORINGAGENCYNAME(S)ANO ADDRESS(ES)

NASA/George C. Marshall Space Flight Center

Attn: EH15

Marshall Space Flight Center, AL 35812

May 20_ 1992-Jan. 20,1993

5. FUNDING NUMBERS

8. PERFORMING ORGANIZATION

REPORT NUMBER

PHY-93RO04

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION / AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

An analysis to determine the effects of the LDEF ionizing radiation

environment on solar cell performance was done. For unprotected cells, power

loss of approximately 20% is estimated. Cells with cover glass are expected to

see much smaller degradation.

14. SUBJECTTERMS

LDEF, radiation, solar cells

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