The AstroPower self-supporting, transparent AlGaAs top solar cell can be stacked upon any well-developed bottom solar cell for improved system performance. This is an approach to improve the performance and scale of space photovoltaic power systems. Mechanically stacked tandem solar cell concentrator systems based on the AlGaAs top concentrator solar cell can provide near term efficiencies of 36 percent (AMO, 100x). Possible tandem stack efficiencies greater than 38 percent (100x, AMO) are feasible with a careful selection of materials. In a three solar cell stack, system efficiencies exceed 41 percent (100x, AMO). These device results demonstrate a practical solution for a state-of-the-art top solar cell for attachment to an existing, well-developed solar cell

Barnett, A. M.

Dinetta, L. C.

Hannon, M. H.

Mcneely, J. B.

English

NASA Technical Reports Server

N91-19187 li
A1GaAs Top Solar Cell for Mechanical Attachment in a
Multi-Junction Tandem Concentrator Solar Cell Stack*
L.C. DiNetta, M.H. Hannon, and J.B. McNeely
AstroPower, bw.
Newark, DE
A.M. Barnett
University of Delaware
Newark, DE
Introduction
The theoretical limits for the A1GaAs/Si tandem stack were determined using
a solar cell model by Nell [ref. 1] based on tabulated standard spectra, the fit of
experimentally achieved open-circuit voltages, and the assumption of unit quantum
efficiency. More recent work [ref. 2] indicates that Nell's model underestimates open-
circuit voltages. Incorporating this change, a theoretical AM0 efficiency of 42.9%
at 100 suns is predicted for the tandem stack. The 100x values were calculated by
assuming a linear increase in short-circuit current values and scaling the remainder of
the cell operating parameters in accord with the diode equation. The result of device
modeling shown in Table I assumes a subbandgap transparency of 100%.
Shown in Figure 1 are iso-efficiency curves for various bandgap combinations.
The curves indicate that there are a wide range of acceptable bandgap combinations
within a given performance window.
A value of 95% transparency to sub-bandgap photons was used in the prediction
of the "best case" performance. For direct-bandgap materials, predictions were de-
termined by reducing the open-circuit voltage, short-circuit current, and fill factor to
96%, 91%, and 96%, respectively, of their theoretical limits. Indirect bandgap ma-
terial reductions are correspondingly 91%, 96%, and 96%, respec(ively [ref. 3]. This
permits stack performance predictions for the various combinations of top and bot-
tom solar cell bandgaps. Performance of the "best case" AIGaAs top solar cells with
1.93 and 1.80 eV bandgaps on silicon bottom solar cells are listed in Table II. The
"best case" performance of the individual solar cells assumes no reflective losses or
grid obscuration. However, the current generated by the bottom solar cell is limited
by the bandgap of the top solar cell and reduced further by the transparency of the
top solar cell to sub-bandgap photons.
*The work was supported in part by NASA Lewis Research Center under contract
NAS3-25570.
6O
https://ntrs.nasa.gov/search.jsp?R=19910009874 2020-03-19T18:49:03+00:00Z
Four-terminal MSMJ tandem stack assembliesappear to be the best short term
method to a 30%commerciallyavailablesolarcell asdemonstratedby recent resultsat
AstroPowerand elsewhere.Modelingof a four terminal stackof A1GaAson InGaAsP
indicates that it will be possibleto achieveefficienciesof greaterthan 35%. InGaAsP
alsohas the addedadvantageof greater radiation hardnesswhencomparedto silicon.
Predictions of this stack appearin Table III.
The next logical step in the developmentof high efficiency solar converter systems
is the goal of 40% overall conversion efficiency. Table IV demonstrates the results
of our modeling utilizing materials which are being developed at this facility. The
predicted efficiencies include losses and are "best case" examples, not theoretical
maximum values.
Results
The top solar cell is fabricated on a thick A1GaAs substrate that is i) highly
transparent, ii) sufficiently rugged so as not to require additional support during
subsequent processing, and iii) utilizes existing process technology. Liquid phase
epitaxial growth techniques are employed for materials fabrication. When compared
to MBE and MOCVD, the LPE technique has demonstrated higher quality A1GaAs
material. Li [ref. 4] has reported that A1GaAs layers prepared by LPE have lower
defect density than those prepared by other techniques. Minority carrier lifetimes
of AIGaAs for LPE grown material are larger than for MOCVD grown malerial as
reported by Ahrenkiel and Dunlavy [ref. 5]. Both Saletes [ref. 6] and Mayet [ref. 7]
have reported material problems with compositions near the 1.93 eV direct-indirect
energy bandgap cross-over region. Lewis and coworkers [ref. 8], on the other hand,
have reported excellent devices at this bandgap. Regardless, our modeling indicates
that the tandem stack efficiency is virtually unchanged with a top solar cell bandgap
between 1.93 and 1.80 eV.
Utilizing an A1GaAs substrate that has a bandgap equal to or greater than the
active device region of the solar cell provides the required high transparency. The
reflection plus transmission curves of the substrate layer with a wider bandgap than
the active layer and the active layer grown on the substrate layer appear in Figure 2.
The base and emitter can be tuned to any desirable energy bandgap between 1.43
eV and 1.93 eV, the direct-indirect crossover point for the A1-Ga-As mixed crystal
system. Typically, a 10-micron base and a 0.1 micron emitter are used with a graded
bandgap window layer to reduce surface recombination. The graded window layer also
improves the current collection since i) a field exists to attract the minority carriers
to the junction, and ii) a gradual change in index of refraction minimizes reflection
at the A1GaAs window/emitter interface [ref. 7].
The devices fabricated are p/n structures. The solar cell is a 4 mm diameter
active area design based on a 5 mm x 5 mm die. Ohmic contacts are accomplished
61
through the deposition of conducting alloys to the top and bottom surfaces. Presently,
tile contact scheme consists of Au-Ge compounds for the n+ contacts and Au-Zn
compounds for the p+ contacts.
An "A1GaAs filter" was fabricated to study the transparency of the material
to sub-bandgap photons. The filter consisted of a 1.9 eV active layer on a self-
supporting A10.7Ga0.3As layer of approximately 120 microns thickness. Quantum
efficiency curves of a silicon solar cell with and without the filter in place are shown
in Figure 3. Calculating the transparency of the "A1GaAs filter" is straight forward.
First, a ratio of the filtered bottom cell light generated current to the unfiltered light
generated current is calculated.
Ratiojl = Jlfiltere d/Jlunfiltered
Then, the same rationale is applied to the photon flux assuming unit quantum
efficiency of the bottom cell.
Ratio¢ = _sub_bandgap/_Silicon
The transparency is calculated as:
Transparency = Ratiojl / Ratio¢
Results of this calculation yield a sub-bandgap transparency of 91% for the AM0
solar spectrum.
Detailed analysis of the R+T measurement of the A1GaAs filter revealed that
free-carrier absorption in tile A1GaAs substrate was less than 2%, and the remaining
loss was purely reflective.
Quantum efficiency and current-voltage curves were employed to evaluate the
quality of the A1GaAs solar cells. Growth conditions were changed to optimize the
operation of the solar cell based on the results of these measurements. Parameters
included junction depth and emitter doping, window thickness, base layer thickness
and doping, and contact alloy time and temperature.
Characteristics of a wide bandgap solar cell are presented in Figures 4 and .5.
As can be seen from the external quantum efficiency measurement, there is a sharp
band-edge demonstrating excellent material quality. This is supported further by the
high fill factors yielded by this device.
A decrease in the quantum efficiency corresponding to an increase in aluminum
content of the AI-Ga-As mixed crystal system has been reported [ref. 8]. With greater
than 32% A1, there is a dramatic reduction in quantum efficiency and therefore a
decrease in the solar cell performance. The operating characteristics of the 1.80 eV
A1GaAs concentrator solar cell are presented in Figures 6 and 7.
62
The internal quantum efficiency of the device is over 90% for much of the absorbed
spectrum when allowance is made for the grid obscuration.
Results of the two best AstroPower A1GaAs solar cells to date and the predicted
stack efficiencies are demonstrated in Table V. Silicon performance is projected based
on 95% top solar cell transparency.
Performance of this stack configuration would be enhanced with a few basic re-
finements in fabrication. Substitution of an anti-obscuration, prismatic cover with a
matched grid structure will reduce contact shading to zero. With these improvements
the A1GaAs concentrator solar cells will yield the predicted efficiencies shown in Table
VI.
The approach used to fabricate a free-standing A1GaAs top concentrator cell,
useful for concentrator systems, was to grow a 50 micron thick graded base layer
on the GaAs substrate. After contacts are made to the A1GaAs device on the GaAs
substrate, the GaAs is etched away under the illuminated region of the A1GaAs device.
This device structure is shown in Figure 8.
The method for substrate removal used to fabricate the above devices is an etch
bath consisting of H202, NH4OH, and EDTA added as a complexing agent. This etch
exhibits a temperature dependent rate as can be seen in Figure 9. Excellent control
can be exercised over the rate of etching by controlling the temperature: making it
possible to remove the GaAs substrate quickly and at the same time continuing to
maintain the selectivity of the etch.
A family of current-voltage curves was measured during each step in the fabrica-
tion process. No degradation of the solar cell operating characteristics, exhibited by
device G-156 4/:2, was observed during the processing of the device. Most A1GaAs
solar cells possess excellent process survivability as shown in Figure 10. After this
particular device was processed, the glass slide onto which it had been bonded with
RTV silicone was found to have cracked and consequently the device was removed.
During the removal the lead attachment which had been made to the front and back
of the solar cell was inadvertently removed. The solar cell was retested and no cell
degradation was exhibited by this device even after the abusive handling.
Conclusion
The AstroPower self-supporting, transparent A1GaAs top solar cell can be stacked
upon any well-developed bottom solar cell for improved system performance. This is
a unique approach to improve the performance and scale of space photovoltaic power
systems. Mechanically stacked tandem solar cell concentrator systems based on the
A1GaAs top concentrator solar cell can provide near term efficiencies of 36% (AM0,
100x). Possible tandem stack efficiencies greater than 38% (100x, AM0) are feasible
with a careful selection of materials. In a three solar cell stack, system efficiencies
63
exceed 41% (100x, AM0) . These device results demonstrate a practical solution for
a state-of-the-art top solar cell for attachment to an existing, well-developed solar
cell.
References
[ 1. ] M. E. Nell, and A. M. Barnett, IEEE Trans. Electron Devices, bf ED-34,
257.
[ 2. ] P. H. Mauk, N. E. Terranova, and A. M. Barnett, To be published.
[ 3. ] A. M. Barnett, and J. S. Culik, 19th IEEE Photovoltaic Specialists Confer-
ence, 931, 1987.
[ 4. ] Sheng S. Li, 15th IEEE Photovoltaic Specialists Conference, 1283, 1981.
[ 5. ] R. K. Ahrenkiel, and D. J. Dunlavy, Journal of Vacuum Science and Tech-
nology, 7, 822, 1989.
[ 6. ] A. Saletes, A. Rudra, P. Basmaji, J. F. Carlin, M. Leroux, J. P. Contour, P.
Gibard and C. Verie, 19th IEEE Photovoltaic Specialists Conference, 124, 1987.
[ 7.] L. Mayet, M. Gavand, B. Montegu, and A. Laugier, 20th IEEE Photovoltaic
Specialists Conference, 597, 1988.
[ 8.] C. R. Lewis, H. C. Hamaker and R. T. Green, Journal of Electronic Materials,
16, 1987.
9.] A. W. Blakers, A. Wang, A. M. Milne, J. Zhao, X. Dai and M. A. Green, 4th
International Photovoltaic Science gJ Engineerinq Conference, Sydney, Australia,
1989.
64
Figure I.
2`50
Mazlmura: 3S.8%
_'. (1.97eV, 1.13oV)
o _tflclenc'y > 34%
2.00 /jEtflclency > 32%
oE_
o
o..
>.
1._
• • ! • , • , ! • ! • ,
ft.20 130 2.29 2.70 " 3.20
Energy Gap Top Solar Call (eV_
Iso-efficiency curves for various bandgap combinations
(AMO,lX) 12].
Figure 2.
+
n-
AIGaAs _ _
Substrate / / A t,vAJe_ er
i _ i i i i i i '1 i [ i i i i i i i '11 _1 i i i _1 i t i i i 1 I i i i i i i i i I
450 550 650 750
Wavelengt.h(Nanometers)
Reflection + transmission of active layer on substrate
layer.
65
Figure 3.
100-
U
C
Q)
U
o_
LM
E
-I
C
0
-1
0
80
6O
443
20
0
380
/_Unfiltered Silicon Solar Cell
Itered Silicon Solar Cel,I , , t
Tll_ll_'_ll, =11 i ,1 fl|f ill j ii_llll IZ=II i, i,_la I =ll=ll= Jl, ,,1 ,I, I,Jlll',480 280 o80 7_o _8o 980 1o6o
Wavelength(Nanometers)
AIGaAs top solar cell transparency demonstrated by
placement over an existing cell.
°--
u
°--
U_
E
:Io
G - 113 #t, 1.93 eV AK_As
Ext.emol OuQntum Effk:lency
Two Layer Antl-reflect_on Coatlng
_ao =a ",_, ...... _'_o
Wavelength(Nanometers)
1.40 "1-_
1.20 1
0 0.40 .
G - 113 #1, 1.g3 eV AIGcV_
V. = 1.352 v
J= = 11.2. rnA/cm z, AklO. 1X
FF = 0.851
n== 91:_
0.00
Current (mA/cm2)
Figure 4. External quantum
efficiency of 1.93 eV AIGaAs
solar cell with grid obscuration.
Figure 5. Operating
characteristics of 1.93 eV AIGaAs
solar cell with grid obscuration.
66
G - 121 _1, 1.80 eV AIGaAs
Ex_emol Quantum Efficiency
cu>,eo Two Layer Anti-reflection Caating
0
Wovelength(Nanometers)
1.20
1.OO
O OJ_O
O.SO
0.40
O.2O
0.00
G - 12-I ,_I, 1.80 eV AICaAs
= 1.295 v
jr== = 1S.8 rr'.A/cm _, AI_O. I X
FF = 0.84
n = 15.2.1
Current (rnA/cm2)
Figure 6. External quantum
efficiency of a 1.80 eV AIGaAs
solar cell with grid obscuration.
Figure 7.Operating
characteristics of a 1.80 eV
AIGaAs solar cell with grid
obscuration.
Figure 8.
Front Contoc ts
/ J "-.,,
Re=oved 6aAs Substrate
_RR Coating/
_AI Cai_ AS W;ndo. Lay,r
_p - type Rl= Gel_= As:Be--n type AI, Cat_ , Rs;Sn
_n - type GaAs:Si Substrat,
_'"---Sack Contac Is
AIGaAs/AIGaAs on GaAs contact for concentrator systems.
67
1.20
Figure 9.
_I.00
C
o--
E
°_
E
_-'0.60
O
C_ O.4O
L.,.I0.20
)...... _,g._..... _._ ..........................4s oo 5o oo '_._' .... '_80.00
Temperoture (C)
Etch-Rate vs. Temperature for a selective GaAs etch.
68
Or-_
O
>
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Current
OJRVE# V_ _" 2 FF Eff"
(voTts) (mAFcm ) (_)
1 1.331 6.93 0.85 5.81
2 1.329 6.93 0.85 5.81
3 1.341 9.08 0.85 7.67
4 1.333 10.03 0.85 8.41
5 1.333 10.03 0._ 8.41
Figure i0. Operating characteristics of AIGaAs
during processing.
CC_NY
Initial
Front lead attac_
AR coating & ITIVbond to
glass suite
GaAs substr_te removed &
bad_ lead attach
removal of RIV and lead
attachment
top solar cell
69
Table I
Predicted Theoretical Maximum Efficiency for
a Four-Terminal AiGaAs/Si Mechanical Stack
(AM0, 135 mW/cm 2)
Material
e_ Vo___cc Jsc F F Eff(IX) Eff(100X)( ) (volts) (_7_m 2) (%) (%)
AIGaAs 1.93 1.588 21.55 0.91 23.0 24.7
Silicon 1.13 0.788 31.88 0.85 15.8 18.2
Four Terminal Stack Efficiency ---> 38.8 42.9
Table II
"Best Case" Prediction for Four-Terminal
Configuration of AiGaAs/Si
Mechanical Stack
(AM0, 135 mW/cm 2)
Material E Vo___cc Jsc F F Eff(iX) Eff (100X)
(_) (volts) (_m 2) (%) (%)
AIGaAs 1.93 1.52 19.6 0.87 19.2 20.7
silicon 1.13 0.71 29.0 0.82 12.6 14.7
Four Terminal Stack Efficiency ---> 31.8 35.4
_ate_ia! Voc Jsc F_FF(_) (volts) (_7_m 2)
Eff(IX) Eff(100X)
(%) (%)
AIGaAs 1.80 1.45 23.1 0.87 21.6 23.4
Silicon 1.13 0.71 25.5 0.82 .II.0 12.9
Four Terminal Stack Efficiency ---> 32.6 36.3
7O
Table III
"Best Case" Prediction for Four-Terminal
Configuration of AIGaAs/InGaAsP
Mechanical Stack
(AM0, 135 mW/cm 2)
Material
AIGaAs
InGaAsP
(E__)e_ Voc Jsc F_FF Eff(IX)(volts) (_-_m 2) (%) (%)
1.80 1.450 23.1 0.87 21.6 23.4
1.08 0.763 27.7 0.81 .12.7 14._____7
Four Terminal Stack Efficiency ---> 34.3 38.1
Table IV
"Best Case" Prediction for Six-Terminal
Configuration of AIGaAs/InP/InGaAsP
Mechanical Stack
(AM0, 135 mW/cm 2)
Material
(e_) Vo__cc Jsc(volts) (mAim _)
AIGaAs 1.93 1.520 19.6
InP 1.35 1.009 18.2
InGaAsP 0.89 0.519 22.4
FF ff___ _ff(100X l
(%) (%)
0.87 19.2 20.7
0.84 11.4 12.8
0.77 6.6 8.1
Six Terminal Stack Efficiency ---> 37.2 41.6
71
Table V
AstroPower AIGaAs Top Solar Cell Data
with 95% Transparency on State of the Art Bottom Solar Cell
(AM0, lOOX)
Material E Vo___c Jsc F F Eff
(e-5) (volts) (_2) (_)
AiGaAs 1.93 1.47 1.12 0.85 10.4
Silicon[9] 1.13 0.813 2.90 0.814 14.2
Four Terminal Stack Efficiency ---> 24.6
Material
AIGaAs
Silicon[9]
E_ Vo___qc Jsc F F Eff
(e_) (volts) (A/_m 2) (%)
1.80 1.4!4 1.88 o.84 16.5
1.13 0.813 2.59 0.814 12.7
Four Terminal Stack Efficiency ---> 29.2
Table VI
AstroPower AIGaAs Solar Cell Data Used for
Predicted Performance of AIGaAs/Si Mechanical Stack
with Anti-Obscuration Prismatic Cover and 95% Transparency
(AMO, 100X)
Material E Voc Jsc F F Ef___ff
(e-%) (volts) (Ap-_2) (_)
AIGaAs 1.93 1.47 1.27 0.85 i1.8
Silicon[9] 1.13 0.813 2.90 0.814 14.____2
Four Terminal Stack Efficiency ---> 26.0
Material
(e_) Voc Jsc F F Ef__f(volts) (A_ 2) (%)
AIGaAs 1.80 1.414 2.14 0.84 18.8
Silicon[9] 1.13 0.813 2.59 0.814 12.____7
Four Terminal Stack Efficiency ---> 31.5
72


AlGaAs top solar cell for mechanical attachment in a multi-junction tandem concentrator solar cell stack

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