tions with Dr. Richard Ahrenkiel's group at SERI in the area of minority-carrier ..... (8) K. Berfness, M. Ladle Ristow, and H.C. Hamaker,. Conference Record of the ...
PROJECTIONS
ACKNOWLEDGEMENTS
FOR THE FUTURE
This work has been supported by contracts from the Solar Energy Research Institute c/t XL-X-18063-1) and the Air Force Aero Propulsion and Power Laboratory (tF33615-88-C-2906). Critical solar cell performance measurements were Performed at SERI (Mr. Keith Emery) and Sandia National Laboratories (Or. Douglas Ruby).
Based on earlier loss analyses, we have shown that present cells achieve only about 80% of their theoretical efficiency, and have projected one-sun AM1.5 efficiencies of 26.5% with near-term improvements 151 This analysis is updated in Table 3 to reflect our most recent cell. We still believe that short-circuit current can be improved substantially by reducing extrinsic losses, particularly grid shadow (larger cell sizes), and by reduced emitter recombination. One promising approach in this direction is the use of GalnP2 in place of AlGaAs for the window layer. Very low interface recombination velocities have been demonstrated with this approach 1131 and one-sun efficiencies of 25.1% have been demonstrated [I“1 Table 3.
We gratefully acknowledge important collaborations with Dr. Richard Ahrenkiel’s group at SERI in the area of minority-carrier property measurements and Prof. Mark Lundstrom’s group at Purdue University in understanding and modeling cell loss mechanisms.
Comparison of preseni and projected GaAs cells to theoretical Conditions: One sun, AM 1.5 Global, 25’C.
Parameter
Theoretical Limit
Value
Present Cells (% of Limit)
“d”e
(94.0) (88.4)
1.029 27.89 86.43
Projected Cells (% of Limit)
1.050 29.0 87.0 26.5
24.8
limits.
(96.0) (91.9) (97.6) i86.Oj
REFERENCES I.
J.M. Woodall and H.J. Hovel, AppL Phys. Lett. a 492 (1977).
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S.P. Tobin, S.M. Vernon, C. Bajgar, S.J. Wojfczuk, M.R. Melloch A. Keshavarzi, T.B. Stellwag, S. Venkatensan, M.S. Lundstrom, and K.A. Emery, IEEE Trans. Electron Dev. II, 469-477
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M.E. Klausmeier-Brown, P.D. DeMoulin, H.L. Chuang, M.S. Lundstrom, M.R. ,Melloch and S.P. Tobin, Conf. Rec. 20th IEEE Photovoltaic Specialists Conf., 1988, pp. 503-507.
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6.
S.P. Tobin, S.M. Vernon, C. Bajgar, L.M. Geoffroy, C.J. Keavney, M.M. Sanfaco”, and “.E.Haven, Solar Cells3 103-115, (1988).
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S.M. this
1208-1210(1989L 13.
162
S.R. Kurtz, conference.
J.M. Olson, and A.E.
Kibbler,
this
High-Efficiency Grown
GaAs and Molecular
by
AlGaAs Solar Seam Epitaxy
Cells
M.R. Melloch,a S.P. Tobin,b C. Bajgar,b T.B. Stellwag,a A. Keshavarzi,a M.S. Lundstrom,a and K. Em@ a) School of Electrical Engineering,Purdue University,West Lafayette, IN 47907 b) Spire Corporation,Patriols Park,Bedford, MA 01730 c) Solar Energy Research Inslitute,Golden, CO 80401
ABSTRACT
deposited (MOCVD) material. The previously best reported solar cell fabricated from MBE material had a 1-sun AM1.5 efficiency of 15.7% when measured without an anlireflection coating (1); cells fabricated from MOCVD material have demonstrated efficiencies of 24.8% (2). (Table 1 is a summary of GaAs solar cell performance and Table II a summary of Alg.2Gag.eAs solar cell performance for cells fabricated from MOCVD and MBE material.) The previously best reported Alg.2Gag.9As solar cell fabricated from MBE material had a l-sun AM1 S efficiency of 12.9% (3), while Alg.2Gag.eAs cells fabricated from MOCVD material have achieved efficiencies of just wer 19% (4,5). In this paper we demonstrate that solar cells fabricated from GaAs and Al0.2Ga0.8As MBE material can have comparable performance to those fabricated from MOCVD material.
Previous to this work. solar cells fabricated from MBE material have been inferior in performance 10 those fabricated from MOCVD Material. We have obtained l-sun AM1.5 efficiencies of 23.8% for 0.25 cm2 area GaAs solar cells fabricated from MBE material which is comparable lo the performance obtained with MOCVD material. We have also obtained l-sun AM1.5 efficiencies of 16.1% for 0.25 Cm2 area Alo.22Gag.79As solar cells fabricated from MBE material. This efficiency is 3.2 percentage points higher than the previously best reported efficiency of 12.9% for an AIo.2Gag.eAs solar cell fabricated from MBE material.
INTRODUCTION
FABRICATION
Because molecular beam epitaxy (MBE) affords the capability to easily change film $tructw?s from growth 10 growth, it is a valuable tool for investigating how changes in solar cell film structure affect cell efficiencies. In this way optimized cell structures can be developed and then manufactured by whatever growth technique is the most economical. TO be a viable approach, solar cells fabricated from MBE material need to be comparable in performance to solar cells fabricated from metal organic chemical vapor
Table I. Summary
of GaAs
solar
performance
VOC 09
Growth
rechnique
cell
pectrum
The solar cells films used in this work were grown in a Varian GEN II MBE system. An extensive description of the conditioning and preparation of the MBE system and the sources has been reported elsewhere (6). The substrates used were two-inch diameter n+ GaAs liquid-encapsulatedCzochralski material. Before MBE growth, the substrates were degreased, etched in a 6OoC solution of 5:1:1 H2S04:H202:H20 for one minute, rinsed thoroughly in H20,
Area (cm2)
Jsc (mAlcm2)
27.28 27.12 27.35 27.89 27.56
86.8 84.5 84.9
1 .o
86.43 84.65
0.25 0.25
MOCVD MOCVD MCXXD IvKXVD MEE
AM15 AM1.5 AM15 AM15 AM15
24.0 23.7 24.8 23.8
1.033 1.046 1.019 1.029 1.018
MBE
AM1
16.0
0.92
23.0
76.0
0.095
MIX MBE ME
AM1 AMI AM15
17.5 17.1 15.8
0.867
29.2 23.9 22.07
69.3
0.25
76.3 76.5
0.05
0.94 0.939
1 .o
4.0
Group
Ref
Spire Varian Kopin Spire Purdue/ Spire Lincoln Labs Rockwell NTT CNRS
7 8 5 2 This work 9 10 11 1
163 0160-837119010000-01630 $1.00 1990 IEEE
Table II. Summary
of Alo.2Gao.sAs
solar
MOCVD MXVD MOCVD MXVD ME
AM1.5 AM1.5 AM2 AM2 AM15
18 19.2 16.3 16.1
-_ VCC -_(V) 1.21 1.19 1.18 1.19 1.22
f@E
AM1
12.9
t@E
AM1
11.6
ME
AM1.5
12.9
Growth rechnique
Spectrum
cell
performance
n GaAs
4x10” ad
2.10” Ed
“* AIo,,6Ga~~G&As/GaAAE Superlattiee
b
19 4.5 12 6.3
86 80 83 84 81
1 .o 0.25 0.25 0.5 0.25
1.10
4.6
76
0.1
1.08
14.4
75
0.5
14.2
79
0.25
Lincoln Labs Lincoln Labs NTT
ref 5 12 4 13 This work 14 14 3
400A
I) AlAs
I
0.5 pm p %~‘%7~~s
2x10” Cm’
“* c&As Bufler Layer
1x10” cm-’
rJ.25wn
3w
1
j.;:^.
I
Group Kopin Spire Varian Varian Purdue
A superlattice was used as a back surface field below the n-GaAs base region, es shown in Fig. la, for the GaAs solar cell. This superlattice consists of 20 periods of 26 .& barriers and 271\ wells. A superlattice was used es a beck surface field rather than a thick AlGaAs layer since, in MBE, a GaAs region grown on a superlattice exhibits a better electrical interface than when a GaAs region is grown on a thick AlGaAs layer. No back surface layer was incorporated below the base region in the A10.22GaO.76As solar cell. The processing of the MBE material into solar cells consisted of first coating the front side of the wafer with SiOz. An ohmic contact was then formed to the backside by evaporating and alloying AuGe followed by a Au confect evaporation. Next a pattern for the front grid mefallization wee formed using image-reversal phofolithography. After etching the SiOz in the grid openings, Cr and AU were evaporated for front contacts. The photoresist was dissolved lo lift off the excess metal and the contacts were sintered. A
and placed in a non-bonded Substrate holder. In the entry chamber of the MBE system, the sample was outgassed at 200°C for two hours and then moved into the buffer chamber of the MBE system where they were outgassed for 1 hour at 300°C. Film strwtures for both GaAs and Alg.22GaO.6As heteroface homojunction cells were grown and are displayed in Fig. 1. The GaAs solar cell film structure was grown at a substrate temperature of 600% and at a growth rate of 1 wmlhr. The AIg.22Gag.6As solar cell film was grown at a rate of lNm/hr and at a substrate temperature of 675oC except for the GaAs buffer and cap layers which were grown at a substrate temperature of 600°C. Solar cells of area 0.25 cm2 were then fabricated on half of each of the twoinch diameter substrates es will be described below. The other half of each wafer was used for diagnostic purposes (test devices such es varying area diodes, eiectrochemicaf capacitance-voltage profiling, etc.)
p ‘3x.&s
Area ‘cm2)
ISC (mA) 18.2
n*&ASSubsttate
I
“* GaAs Substrate
I ti\\w-
Fig. lb
Fig. 1a Cross-section of the GaAs solar cell.
164
- ix/i;
isi
IGaL-
- i\\\F
Cross~section of the Alg.22Gag.76As
solar cell.
““I ’ I i5;0.2 0.4 ;vMCCVD: MB6 --i i 0.0 1 \ I 300 500 700 900 ,000 WaYelengfh (ml)Of 2 .g 0.8 g w E 0.6 a 5
z s S
0.2
6
0.4 D--l 0.0 300
600
700
900
Wavelength (nm)
Fig. 2b Internal quantum efficiency for the AlO.22GaO.78As solar cell fabricated from MBE material.
Fig. 2a The dashed line is the internal quantum efficiency our GaAs solar cell fabricated from MBE material. The solid line is the internal quantum efficiency of the GaAs solar cell fabricated from MOCVD material from reference (2).
the wafer used for diagnostic purposes. These mesa-isolated diodes ranged in area from 2.5x10-5 cm2 to 0.25 cm.2 Typical IV characteristics for varying area GaAs and AlO.22GaO.78As diodes are displayed in Fig. 3. (Also displayed in Fig. 3 is a dashed line with a voltage dependence of eqV/2kT.) The small area GaAs and AlO.22GaO.78As diodes display the expected dependence of current on voltage Of,
phosphoric acid-based etchant was used to define the 0.5 cm by 0.5 cm cells. The remaining Si02 was removed just prior to a selective etch which removed the GaAs cap layer everywhere except under the grid lines and exposed the AlGaAs window layer. Finally, a double-layer antireflection coating of ZnS and MgF2 was thermally evaporated.
solar cells ELECTRICAL
CHARACTERIZATION
where JD2B is the bulk saturation current coefficient in A/cm,2 Jo2P is the perimeter coefficient in A/cm, A is the diode area in cm2 and P is the diode perimeter in cm. Some intermediate size diodes (0.01 cm2) display IV characteristics which are welt described by eq. (I) while others exhibit leakage currents at low bias (ideality factors > 2). All the large area diodes and solar cells exhibit IV characteristics with considerable leakage currents. It is apparent from the dependence of the leakage current on diode size that these leakage currents are due to isolated defects in our MBE-grown GaAs and A10.22GaO.78As films. We have not observed these leakage currents in GaAs diodes that were fabricated at the same time but from MOCVD material, suggesting a defect which is inherent to MBE material and not observed in MOCVD material. Such a defect is the “oval” defect and the oval defect density in the MBE GaAs film used in this work was 600 cm. -2 Displayed in Table Ill is a summary of the number of diodes exhibiting leakage out of the number tested for different area GaAs diodes; also displayed is the average number of oval defects expected for a given area diode. The data in Table III furiher suggests that the oval defects are a possible cause of the leakage currents. These leakage currents cause a slight reduction in the fill factor of our solar cells when operated under l-sun conditions. However, at high bias the leakage current in our cells is negligible and would not degrade the cells’ performance under concentration. As can be seen from equation (1). the part of the diode Current with an ideality factor of approximately 2 will have a component dependent on the diode’s area and a component dependent on the diode’s perimeter. The bulk and
The were measured under l-sun AM1.5 conditions at the Solar Energy Research Institute. The measured solar cell parameters for our most efficient GaAs and Alq.22GaO.78As cells are listed as the first MBE entry in Tables I and II respectively. For the GaAs solar cells a yield of 100% was obtained with the best solar cell demonstrating an efficiency of 23.8%. Excellent uniformity was also obtained for the GaAs solar cells with an average efficiency of 23.1% and a standard deviation of 0.4%. The most efficient A10.22GaO.78As cell demonstrated an efficiency of 16.1% which is 3.2 percentage points higher than the previously best reported efficiency of 12.9% for an AlO.2GaO.8As solar cell fabricated from MBE material(3). The internal quantum efficiency of our most efficient MBE-grown GaAs solar cell is displayed in Fig. 2a along with the internal quantum efficiency of the best reported MOCVDgrown GaAs solar cell (2). The significant difference in the film structures was that the aluminum mole fraction and thickness of the window layer were 0.8 and 300 A for the MOCVD-grown cell and 0.7 and 400 A for the MBE-grown cell. The internal quantum efficiencies of the cells in Fig. 2a are comparable, with the difference in the window layers clearly visible as a slight difference in response at short wavelengths. The internal quantum efficiency for our most efficient AlO.22GaO.78As cell is displayed in Fig. 2b and peaks at just over 0.9. Under l-sun operation, the major efficiency limiting mechanism in our GaAs and AlO.22GaO.78As solar cells is non-ideal leakage currents (ideality factors z 2) at low diode voltages. To investigate the source of these leakage currents, a series of mesa-isolated diodes was fabricated on the part of
165
10.2 10-a 104 3
0.25 cr
104
0.0
0.2
cl.4
0.6
0.8
1.0
1.2
Voltage (V) Fig. 3a
Voltage (V) Fig.
Comparison of IV characteristics for GaAs diodes fabricated from MBE material of area 0.25 cm,2 10m2 cm,2 and IO-5 cm.* IV characteristics for two different IO-* cm2 area diodes are shown, one which exhibits considerable leakage currents and one which does not.
Table
111. Correlation
diode area
between
0.01
2.5x10-3 1x10-4
diode
number of diodes tested
20 24 24 24
perimeter coefficients, Jo2a and Jo2p can be determined from our Series of varying area te?.t diodes. From equation (1) we can write,
Jm=Jo*B+Jo*&
Comparison
of IV characteristics for diodes fabricated from MBE material of area 0.25 cm.* 0.01 cm,2 and IO-4 cm.’ IV characteristics for two different 0.01 cm* area diodes are shown, one which exhibits considerable leakage currents and one- which does not.
AIo.zzGao.78A~
(cm?
0.25
3b
area and leakage
currents.
number of diodes exhibiting leakage currents
average number of oval defects
20 12 6 2
125 5 1.25 0.05
slope and intercept of Fig. 4b one obtains Jo2p=f .7x10-1 4 A/Cm and Jo2~=3.5XiO-‘~ A/Cm2 for the AlO.22GaO.78As diodes.
(2)
CONCLUSION
where Jo2 is the saturation current density (for a given size diode) of the current with an ideality factor of 2. By plotting Jo2 versus P/A for our series of diodes, the bulk and perimeter coefficients J02a and Jo2P can be determined from the intercept and slope of such a plot. Displayed in Fig, 4 are plots of Jo2 versus P/A for our series of GaAs and Alo.22Gao.78As diodes. When fitting experimental data such as in Fig. 4. if is important to note that small variations in the data will have a much larger effect on the intercept than the slope. Therefore. there will be a much larger uncertainty in the value Of Jo2a than in the value Of Jo2P determined from the data in Fig. 4. From the slope and intercept of Fig. 4a one obtains Jo2P=i .~xIO-‘~ A/cm and Jo2a=2.5~10-~~ A/Cm2 for the GaAs diodes. Frcm the
We have demonstrated a significant improvement in performance of GaAs and A10.22Ga0.78As solar cells fabricated from MBE material. We attribute the improvement lo the conditioning of the MBE system and sources, and the purity of the sources used. Further impro”eme”ts in cell Derformance is exoecfed with opbmization of the growth conditions and rediction in wal defect densities. ACKNOWLEDGEMENT This work was supported by the Solar Energy Research Institute for the U.S. Deoartment af Enwnv under subcontracts XL-5005018-I (Purbwe University) and XL8-18063 (Spire Corporation).
166
REFERENCES
K.A.
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P/A Ratio (l/cm) Fig. 4a
Plot of extracted Jo2 ver?.us perimeter-to-area ratio for GaAs diodes fabricated from MBE material.
,.,,
I.
,~rr,.
..,-.
_“,
-.--
\.”
--,.
sill”
I’
n
_ 1.0 2Lb 0.6 F2 -2 “5 0.0 LLLI 0 100
’
n
”
b
n
200
300
400
500
‘I
800
PIA Ratio (l/cm) Fig. 4b
Plot of extracted Jo2 versus perimeter-to-area ratio for A10.22GaO.78As diodes fabricated from MBE material.
167
