A comparison is made between indium phosphide solar cells whose p-n junctions were processed by open tube capped diffusion, and closed tube uncapped diffusion, of sulfur into Czochralski grown p-type substrates. Air mass zero, total area, efficiencies ranged from 10 to 14.2 percent, the latter value attributed to cells processed by capped diffusion. The radiation resistance of these latter cells was slightly better, under 1 MeV electron irradiation. However, rather than being process dependent, the difference in radiation resistance could be attributed to the effects of increased base dopant concentration. In agreement with previous results, both cells exhibited radiation resistance superior to that of gallium arsenide. The lowest temperature dependency of maximum power was exhibited by the cells prepared by open tube capped diffusion. Contrary to previous results, no correlation was found between open circuit voltage and the temperature dependency of Pmax. It was concluded that additional process optimization was necessary before concluding that one process was better than another

Borrego, J. M.

Ghandhi, S. K.

Hart, R. E., Jr.

Parat, K. K.

Swartz, C. K.

Weinberg, I.

English

NASA Technical Reports Server

N87-26441
COMPARAIIVE PERFORMANCE OF DIFFUSED JUNCIION
INDIUM PHOSPHIDE SOLAR CELLS
I. Welnberg, C.K. Swartz, and R.E. Hart, Jr.
NASA Lewis Research Center
Cleveland, Ohio
and
S.K. Ghandhl, J.M. Borrego, and K.K. Parat
Rensselaer Polytechnic Institute
Troy, New York
A comparison was made between indium phosphide solar cells whose p-n junctions were
processed by open tube capped diffusion, and closed tube uncapped diffusion, of
sulphur into Czochralski grown p-type substrates. Air mass zero, total area, effi-
ciencies ranged from I0 to 14.2%, the latter value attributed to cells processed by
capped diffusion. The radiation resistance of these latter cells was slightly bet-
ter, under 1 MeV electron irradiation. However, rather than being process depen-
dent, the difference in radiation resistance could be attributed to the effects of
increased base dopant concentration. In agreement with previous results, both
cells exhibited radiation resistance superior to that of gallium arsenide. The
lowest temperature dependency of maximum power was exhibited by the cells prepared
by open tube capped diffusion. The average value of dPm/dT, including cells of
both types, was found to be -(5.3± 1.2)xlO -2 mW/cm2 OK at 60oc. Calcu-
lated values if dVoc/dT were in reasonable agreement with experimental values.
However, contrary to previous results, no correlation was found between open cir-
cuit voltage and the temperature dependency of Pmax. It was concluded that addi-
tional process optimization was necessary before concluding that one process was
superior to the other.
INTRODUCTION
It has been demonstrated that n/p nu_uju,_ion'-".... _ :-_'um,,,u, _^_"<v,,u_v,,,desolar cells h_._:,,_
properties which make them excellent candidates for use in the space radiation
environment.l,2,3, 4 This follows from their excellent radiation resistance,
annealibility at relatively low temperatures and under the influence of light and
their potentially high efficiencies.l,5, 6 These desirable properties have served
as a stimulus for renewed InP solar cell research both in the USA and abroad.3, 4
However, due to the different ways in which results are reported, and the various
standards and light sources used in measuring cell performance it is difficult to
meaningfully compare the results emanating from different laboratories. 3 For
example, results appear in the literature quoting active area efficiencies and
under AM1.5, AM1 and AMO light intensities. For space use, the latter spectrum
with parameters reported in terms of total area has long been the sole accepted
mode for reporting cell performance measurements. In the present case we compare
the performance of indium phosphide solar cells processed by two different tech-
niques, in two separate laboratories. Performance data are obtained, in the same
simulator, under air mass zero conditions with efficiencies and current densities
261
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reported in terms of total rather than active area. Our interest lies in comparing
cell performance parameters of unirradiated cells and after exposure to 1 MeV
electrons and the variations in performance under varying temperature conditions.
EXPERIMENT
The diffused junction InP cells differed principally in the method of p/n junction
formation. Referring to Fig. I, the cells whose junction were formed by closed
tube diffusion (labeled J cells) were processed at the Nippon Telephone and Tele-
graph, Electrical Communication Laboratories in Ibaraki, Japan. / On the other
hand, the cells processed by open tube capped diffusion (labeled R cells) were pro-
cessed at the Rensselaer Polytechnic Institute. 8 In both cases, the starting
material was p-type, zinc-doped Czochralski grown indium phosphide, sulphur being
the n-dopant. Major cell fabrication details are shown in the figure. Additional
processing details can be found in references 7 and 8. All performance measure-
ments were carried out at NASA Lewis using an air mass zero X-25 zenon arc solar
simulator with a gallium arsenide solar cell used as a standard. The standard cell
was calibrated at air mass zero using the Lewis high altitude aircraft technique. 9
The cells were irradiated by 1 MeV electrons in the Naval Research Laboratories Van
De Graaf accelerator. Temperature dependency measurements on unirradiated cells
were performed in a nitrogen atmosphere using a variable temperature chamber into
which the X-25 simulator beam was introduced through a glass port built into the
side of the simulator.
RESULTS
The performance parameters of several R and J cells, determined at 25oc, are
shown in Table I. The efficiencies of 14.2% for R cells and 13.6% for the J cells
are the highest AMO, total area efficiencies, measured at 25oc, at NASA Lewis,
for these cell types. However, these are not the highest efficiencies obtained for
InP cells. For example, we have measured an AMO total area efficiency of 15.9%, at
25oc for a p+n InP cell whose junction was formed by OMCVD.I0 On the other
hand, AMO active area efficiencies of 18%, at 20oc, have been reported for a
p+-l-n cell also fabricated by OMCVD.II Since this latter cell had 10% front
grid coverage, the total area AMO efficiency at 25oc is calculated to be 16%. To
obtain this latter value at 25oc, we used the temperature correction factor
dPm/dT = -5.3 x 10 -2 mW/cm2 OK where Pm is cell maximum power output. This
numerical value is obtained in a following section of the present work.
Results of the 1 MeV electron irradiations are shown in Figs. 2, 3, and 4 for the
InP cells listed in Table II. Also shown are results for state of the art GaAs
cells, obtained from Varian. 12 Preirradiation cell parameters for the GaAs cells
are shown in Table III. Since these latter cells had efficiencies close to 20%,
they produce more output power over the present fluence range than do the InP
cells. However, when plotted on a normalized basis, in terms of preirradiation
output power, the InP cells exhibit greater radiation resistance. This latter
result is in agreement with previous results.l, 2
262
DISCUSSION
From Fig. 2, the cells processed by open tube capped diffusion have slightly higher
radiation resistance, at the lower fluences, than the cells processed by closed
tube diffusion. However, one cannot state from these results that one specific
process inherently results in superior radiation resistance. This follows from the
observation that, in the present concentration range, radiation resistance in-
creases with base dopant concentration. 13
Considering the remaining cell performance parameters it is seen that, for irrad-
iated InP,the greatest drop occurs in Isc while for GaAs, the greatest drop occurs
in Voc as a result of the irradiation. This would tend to indicate that a decrease
in diffusion length with irradiation is a major factor in decreased output for the
InP cells while for GaAs, the change in dark current with irradiation appears to be
the major factor in decreased cell output.
Decreased cell output power with increasing temperature is an additional signifi-
cant loss factor. An example of the variation in cell output power with tempera-
ture shown in Fig. 5, while Fig. 6 graphically summarizes the variation in cell
maximum power, Pmax, with temperature determined for a number of R and J cells at
60oc. This temperature was chosen as representative of that experienced by solar
cells in low earth and geosynchronous orbit. Cell performance parameters for these
cells are listed in Table IV. From the table, the average value of dPm/dT for
these cells is -(5.3+1.2) x 10-2 mW/cm2 OK.
To consider temperature effects in greater detail it is convenient to express the
temperature dependencies of the cell performance parameters as relative variations,
using the expression 14
1 dPm = 1 dlsc + 1 dVoc + 1 dFF (I)
Pm dT Isc dT Voc dT FF dT
The temperature variation terms in the right hand side of Equation 1 are listed in
Table V In the past, the term in dVoc/dT was_found to be a major factor in the
variation of Pmax with temperature.14, 15 Hence, we examine this term in some
detail using the expression 14
dVoc = Voc-Eg(T) - 3k - _,T (T+28) + kT dlsc (2)
dT T q-- (T+O)2 I_c°_',dT
where k is Boltzmann's constant and q is the electronic charge with,
Eg(T) = the band gap at temperature T in electron volts
Eg(T) = Eg(O) - _T 2
(T+_)
Eg(O) = 1.421 electron volts
= 6.63 x lO-4 V/OK
(3)
= 552 OK
263
It is noted here that, in reference 16, the value of _ is given as 162OK. How-
ever, this latter quantity yields the value 1.292 eV for Eg at 300OK rather than
the more correct value 1.351 eV. 16 The present value of _ used in Eq. 3 yields
the more correct bandgap value at 300OK.
Using the preceding set of equations, calculated values of dVoc/dT are found to be
in reasonable agreement with experiment (Table VI). The success of equation 2 in
predicting experimental values has led to the prediction of an inverse relation be-
tween Voc and the absolute magnitude of (I/Voc)(dVoc/dT). 14 From Fig. 7, it is
seen that the present data is in rough agreement with the preceding statement.
Also, if the term in dVoc/dT were dominant in Eq. I, then there should be an in-
verse relation between Voc and the magnitude of (I/Pm) (dPm/dT). 14 However, as
seen from Fig. 8, this is not the case for the present data. In fact, from the
data of Table V, one cannot generalize and state that for InP, the temperature
variation of any specific quantity is dominant in determing the temperature varia-
tion of Pmax. Noting however that the temperature dependencies in Table V are both
positive and negative, it is desirable that dlsc/dT be as large as possible while
the absolute magnitude of the negative terms in Voc and FF be as small as possible.
In general, the temperature dependence of Isc arises from the fact that diffusion
length increases with temperature while band gap decreases with increasing tempera-
ture, the net result being an increase in current. With respect to the temperature
dependence of Voc, from Eq. 2, it is seen that at fixed temperature, higher values
of Voc lead to the desired lower values in the absolute magnitude of dVoc/dT. In
this respect it is noted that, in Eq. 2, the term in dlsc/dT yields a relatively
small contribution to dVoc/dT the principal contribution arising from the first
term. The fill factor term in Eq. 1 is perhaps the most process dependent. Here
one would expect shunt (Rsh) and series resistance (Rs) to play significant roles.
However, a theoretical expression for dFF/dT is needed which includes the effects
of Rs and Rsh. 14 Aside from this, the factors operative in determining the tem-
perature dependencies of Voc and Isc are apparent from the preceding discussion and
warrant further consideration in attempts to reduce the temperature dependency of
Pmax.
CONCLUSION
In comparing the performance of these diffused junction cells one cannot conclude
that either process is preferable in terms of increased radiation resistance. Al-
though the cells prepared by open tube capped diffusion have slightly higher effi-
ciencies, as measured at Lewis, it is possible that neither process was optimized
at the time the present cells were processed. Hence a choice, based on efficiency,
would be premature. The greatest difference appears in the temperature dependency,
which favors the R cells. However the data is inconsistent in the sense that the
present results do not show the dependency of dPm/dT on open circuit voltage which
was observed in previous work. 14,15 In addition, the exact dependencies of the
temperature variation of fill factor on Rs and Rsh is unclear at present. Thus, in
summation, it can be stated that further efforts in both theory and experiment are
required before concluding that one process is preferable to the other.
264
REFERENCES
I. Yamaguchl, M.; Uemura, C.; and Yamamoto, A.: Radiation Damage in InP Single
Crystals and Solar Ceils. J. Appl. Phys., vol. 55, no. 6, Mar. 15, 1984,
pp. 1429-1436.
2. Yamamoto, A.; Yamaguchl, M.; and Uemura, C.: High Conversion Efficiency and
High Radiation Resistance InP Homojunctlon Solar Cells. Appl. Phys. Left.,
vol. 44, no. 6, Mar. 15, 1984, pp. 611-613.
3. Welnberg, I.; and Brlnker, D.J.: Indium Phosphide Solar Cells - Status and
Prospects for Use in Space. Advancing Toward Technology Breakout in Energy
Conversion (21st IECEC), Vol. 3, American Chemical Society, 1986,
pp. 1431-1435.
4. Welnberg, I.; Swartz, C.K.; and Hart, R.E. Jr.: Potential for Use of InP
Solar Ceils in the Space Radiation Environment. Proceedings of the 18th
Photovoltalc Specialists Conference, IEEE, 1985, pp. 1722-1724.
5. Gordla, C.; Geler, J.V.; and Welnberg, I.: Near Optimum Design of InP
HomoJunctlon Solar Cells. Space Photovoltalc Research and Technology
Conference, NASA CP-2475, 1986, to be published.
6. Ando, K.; and Yamaguchi, M.: Radiation Resistance of InP Solar Cells Under
Light Illumination. Appl. Phys. Lett., vol. 47, no. 8, Oct. 15, 1985,
pp. 846-848.
7. Yamamoto, A.; Yamaguchl, M.; and Uemura, C.: High Efficiency Homojunctlon
InP Solar Cells. Appl. Phys. Lett., vol. 47, no. 9, Nov. l, 1985, pp. 975-977.
8. Parat, K.K., et al.: Solar Ceils in Bulk InP, Made by an Open Tube Diffusion
Process. Solid State Electron., vol. 30, no. 3, Mar. 1987, pp. 283-287.
9. Brandhorst, H.W. Jr.; and Boyer, E.O.: Calibration of Solar Cells Using
High-Altltude Aircraft. NASA 1N D-2508, 1965.
lO. Shen, C.C.: Arizona State University, Private Communication, 1986.
II. Yamaguchl, M., et al.: 22% Efficient and High-Radlatlon Reslstant InP Solar
Ceils. Proceedings 2nd International Photovoltalc Science and Engineering
Conference, Kobe, Japan, 1986, pp. 573-576.
12. Werthen, J.G., et al.: 21% (One Sun, Air Mass Zero) 4 cm2 GaAs Space
Solar Cells. Appl. Phys. Lett., vol. 48, no. l, Jan. 6, 1986, pp. 74-75.
13. Yamamoto, A.; Yamaguchi, M.; and Uemura, C.: Fabrication of High-Efflclency
n÷-p Junction InP Solar Cells by Using Group Vlb Element Diffusion into
p-Type InP. IEEE Irans. Electron Devices, vol. 32, no. 12, Dec. 1985,
pp. 2780-2786.
14. Fan, J.C.C.: Theoretical Temperature Dependence of Solar Cell Parameters.
Solar Cells, vol. 17, no. 2-3, Apr.-May 1986, pp. 309-315.
265
15.
16.
Weinberg, I; Swartz, C.K.; and Hart, R.E. Jr.: Performance and lemperature
Dependencies of Proton Irradiated n/p and p/n GaAs and n/p Silicon Cells.
Eighteenth IEEE Photovoltalc Specialists Conference, 1985, pp. 344-349.
Bachmann, K.J.: Properties, Preparation, and Device Applications of Indium
Phosphide. Annual Reviews of Materials Science, Vol. II, Annual Reviews,
Inc., Palo Alto, CA., 1981, pp. 441-484.
TABLE I
AMO PERFORMANCE PARAMETERS OF InP CELLS
T=25Oc
CELL CARRIER EFFICIENC? Voc Jsc FF
_(cm -_) (%) (mV) (ma/cm 2 ) (%)_
R-I 4.5XI 016 14.2 814 30.5 78.5
R-2 9.0XIO 16 1 3.7 825 28.8 79.1
R-3 4.5XI016 12.9 815 26.3 82.6
J-I 5.0Xl 01 5 13.6 826 27.6 81 . 8
J-2 5.0XI 01 5 1 3.3 823 28.0 79.0
J-3 1 .OXIO 17 10.05 812 22.6 75.1
TABLE II
PRE-IRRADIATION InP SOLAR CELL PARAMETERS
T:25°C
CELL CARRIER EFFIC!ENCY Voc Jsc FF
CONC
( cm---m-_-3--) (%) (mV) (ma/cm 2 ) (%)
R-3 4.5XI016 12.9 815 26.3 82.6
R-4 4.5XI016 12.7 814 26.0 82.6
J-2 5.0XI 01 5 1 3.3 823 28.0 79.0
J-4 5.0XI ol 5 12.1 81 3 27.8 73.5
266
TABLE III
PRE-IRRADIATION AMO PARAMETERS OF GaAs CELLS
T=25°C
EFFI Cl ENCY Voc Jsc FF
(mv) (ma/cm 2 ) (%)
19.6 1041 31.8 81.2
19.8 1044 32.1 81 .I
TABLE IV
PERFORMANCE PARAMETERS OF InP
T=60Oc
CELLS
CELL
R-5
R-6
j-5
J-6
J-7
J-8
CARRIER
CONC
c- 3)
45XlO 16
45XlO 16
50XlO 15
50XIO 15
I OXlO 16
1 OXlO 16
Pm
(mw/cm 2 )
1495
1553
16 Ol
1564
i245
1344
Voc Jsc
(mv) (ma/cm 2 )
741 25.62
749 27
751 29.6
767 25.9
725 24. "31
731 25.6
FF dPm/dT
(mw/cm 2 OK)
788 -3.13XI0 -2
-2
768 -4.86XI0
711
-2
-6.23XI 0
-2
789 -6.25XI0
69 5 SVl n-2
-- AI _J
71 2 -6.3X10 -2
267
TABLE V
TEMPERATURE VARIATION TERMS
T=60°C
CELL
1 dlsc 1 dVoc 1 dFF
Is----c tIT Voc dT FF dT
(°K-I)
R-5 12.6XI0 -4
R-6 6.25X10 -4
-4J-5 7.67Xio
-4J-6 5.56XI 0
J-7 9.66X10 -4
-4
J-8 6.33XI0
(OK-I) (OK-I)
-3 12XIO "3 -3 71XlO -4
-3 IIXIO -3 -I 12XIO -3
-3 09XIO -3 -I 63XI0 -3
-2 88X10 -3 -I 24X10 -3
-3 28X10 -3 -I 35X10 -3
-3 37X10 -3 -2 39X10 -3
TABLE VI
CALCULATED AND MEASURED TEMPERATURE VARIATIONS OF Voc
T=60oc
CELL MEASURED
R-5
R-6
J-5
J-6
J-7
J-8
-2 31
-2 33
-2 32
-2 21
-2 38
-2 46
dVoc
dT
(mv/°K)
CALCULATED
-2 38
-2 39
-2 32
-2 34
-2 46
-2 45
268
J-CELLS
CLOSED TUBE DIFFUSION
In2S 3 p-lnP PIIOSPItORUS
R-CELLS
OPEN TUBE GAPPED DIFFUSION
SlO 2
J-CELLS R-CELLS
DIFFUSION TEMP. (°K) 626 ZOO
DIFFUSION TIME (rain) tO0 25
AR COATING SIO SlO
FRONT CONTACT Au Au
BACK CONTACT Au/Zn A.IZn
CELL AREA (cm 2 ) 0.273 0.313
FIGURE 1.- JIIllCliort FORf_TION PROCESSES l/Ill1 APfflTIONAI INP CELL DETAILS,
I.O
o.g
O.S
NORMALIZED
EFFICIENCY
0.7
0.6
0.5
1013
_ R-I.P
..... j.h_O
-_ GaAs (p/n)
\
"\
1 I
1014 1016
1 MeV ELECTRONFLUENCE (cm-2;
I
1016
FIGURE 2.- NIJI'JIALli_ED EFFIEIEIICY VS J IkV I=LECII{Off FLUErlEE.
269
|.U_
0.9 4
0.8 u
NORMALIZED
OPEN
CIRCUIT
VOLTAGE 0.7
0.8--
0.5
1013
R-InP: J-lnP
GaAe (pin}
I !
1014 1015
1 MeV ELECTRONFLUENCE (cm -2)
FIGURE 3.- tIORIIALIZEDOPEN CIRCII]IVOLIAGE VS i f_V ELECIROH FLIIE;ICE,
I
1016
1.0
0.9
NORMALIZED
SHORT
CIRCUIT
CURRENT 0.7-
0.64
0.5
1013
_ "---_--'-__.__ __
_ _" "_---Z._
R-InP
...... J-InP
GaAs (p/n)
I I
1014 1015
1 MeV ELECTRON FLUENCE [cm -2)
FIGURE _.- _IORMALIZEDSHORT CIRCUIT CURREIITVS i fIEVELECTROIIFLUEIICE,
I
1010
270
32--
24--
Pmax
(mwlcm 2)
16--
(R- CELL)
dP._. -- -4.86X 10 -2 mw/cm2°K
dT
T>2OO°K
8 I f I
120 200 280 360
TEMPERATURE (°K)
FIGURE 5, - VARIATION OF CELL MAXIMUM POWER WITH
TEMPERATURE FOR A CELL PROCESSED BY OPEN TUBE
DIFFUSION,
dT
(xlo 2)
(mwlcm 2 oR)
10-
J-CELLS R-CELLS
o0_
0 _
[]
GaAs
FIGURE F,,- TEfIPERATURE DEPEPIDENCY OF CELL PIAXIMUM
POWER, GAAs At_D SJ DATA SHOWH FOR COP'IPARISION,
Voc
(MV)
800
750
7OO
r--i.-.
I r I
2.8 3.0 3.2 3.4X10 -2
MAGNITUDE OF (1/Voc)(DVoc/OT) (OK-l)
FIGURE 7,- Voc VS MAGNITUDE OF (I/Voc)(DVoc/UT),
Voc
(MV)
800
750
7OO
[]
[]
[]
[]
D
I
1,0 2,0
r'_G?IITUDE OF (I/PM)(UPM/DT) (°K-l)
FIGURE 8,- Voc VS MAGr£1TUDE OF (IIPM)(DP,,.JDT)
[]
I
3,0XlO "3
2/l


Comparative performance of diffused junction indium phosphide solar cells

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