High efficiency, two terminal, multijunction AlGaAs/GaAs solar cells were reproducibly made with areas of 0.5 sq cm. The multiple layers in the cells were grown by Organo Metallic Vapor Phase Epitaxy (OMVPE) on GaAs substrates in the n-p configuration. The upper AlGaAs cell has a bandgap of 1.93 eV and is connected in series to the lower GaAs cell (1.4 eV) via a metal interconnect deposited during post-growth processing. A prismatic coverglass is installed on top of the cell to reduce obscuration caused by the gridlines. The best 0.5 sq cm cell has a two terminal efficiency of 23.0 pct. at 1 sun, air mass zero (AM0) and 25 C. To date, over 300 of these cells were grown and processed for a manufacturing demonstration. Yield and efficiency data for this demonstration are presented. As a first step toward the goal of a 30 pct. efficient cell, a mechanical stack of the 0.5 sq cm cells described above, and InGaAsP (0.95 eV) solar cells was made. The best two terminal measurement to date yields an efficiency of 25.2 pct. AM0. This is the highest reported efficiency of any two terminal, 1 sun space solar cell

Chung, B.-C.

Klausmeier-Brown, M.

Kuryla, M. S.

Ristow, M. Ladle

Virshup, G. F.

Werthen, J. G.

Wickham, K. R.

English

NASA Technical Reports Server

N91-30220
RECENT ADVANCEMENTS IN MONOLITHIC AIGaAs/GaAs SOLAR CELLS
FOR SPACE APPLICATIONS
K. R. Wickham, B-C. Chung, M. Klausmeier-Brown, M. S. Kuryla, M. Ladle Ristow, G. F. Virshup, J. G. Werthen
VS Corporation and Varian Associates
3075 Hansen Way, Palo Alto, CA 94303
ABSTRACT
High efficiency, two-terminal, multijunction AIGaAs/GaAs solar cells have been reproducibly made with
areas of 0.5 cm2. The multiple layers in the cells were grown by OMVPE on GaAs substrates in the n-p
configuration. The upper AIGaAs cell has a bandgap of 1.93 eV and is connected in series to the lower GaAs cell
(1.4 eV) via a metal interconnect deposited during post-growth processing. A prismatic coverglass is installed on
top of the cell to reduce obscuration caused by the gridlines. The best 0.5 cm 2 cell has a two-terminal efficiency
of 23.0 % at 1 sun, air mass zero (AM0) and 25 °C. To date, over 300 of these cells have been grown and
processed for a manufacturing demonstration. Yield and efficiency data for this demonstration will be presented.
As a first step toward the goal of a 30 % efficient cell, a mechanical stack of the 0.5 cm2 cells described above,
and InGaAsP (0.95 eV) solar cells was made. The best two-terminal measurement to date yields an efficiency of
25.2 % AM0. This is the highest reported efficiency of any two-terminal, 1 sun space solar cell.
INTRODUCTION
In terms of space qualified photovoltaic cells, the most important variables to control are: weight, cost,
efficiency, operating lifetime, and radiation resistance. This paper will address the efficiency of space
photovoltaic cells. To get to very high efficiencies (i.e. 25 - 30+ %) without the additional complexity of
concentrating optics, one must utilize two or three junctions in tandem. The additional junctions increase the
energy range that is usefully absorbed by the cell, and therefore, increase the efficiency. In order to reduce the
weight and complexity of a panel of multijunction cells, a monolithic structure is ideal. Up until now, two junction
monolithic solar cells have been in the research phase, but in this paper we report the first manufacturing
demonstration of these devices. A monolithic three junction cell has never been made. However, attempts at
mechanically stacking three junctions have been successful, and will be discussed below.
A computer model, described in detail in reference 1, has been used to determine the optimal design
parameters for IlI-V materials based single and multijunction photovoltaic cells. The program can optimize such
structural parameters as doping levels, concentration gradients, layer thicknesses, indices of refraction for anti-
reflective (AR) coatings, and gridline aspect ratio and spacing to achieve the highest efficiency for the given
solar concentration, air mass and operating temperature. Predictions of efficiency for one, two, and three
junction cells as a function of temperature are shown in Fig. 1. At 25 °C, the predicted efficiency for a single
junction GaAs cell is approximately 22 %. This value has been achieved with our single junction GaAs cell, and a
pilot line for manufacturing has been installed. The expected efficiency for a 1.93 eV AIGaAs on 1.42 eV GaAs
two junction cell is approximately 25 % . Currently we have achieved 23.0 % efficiency using a metal
interconnect cascade cell (MICC) ( ref. 2). There are other two junction cells which have higher predicted
efficiencies than the line shown, but they involve lattice mismatched material structures (i.e. AIGaAs/InGaAs), or
quaternary materials which are quite difficult to grow repeatably and uniformly. The three junction line shows the
optimized efficiency for the same AIGaAs/GaAs cell as the two junction line, but includes a low bandgap (1.0 eV)
InGaAs lower cell. This structure has the potential to achieve efficiencies of up to 31% at 25 °C (1 sun, AM0).
DEVICE GROWTH AND PROCESSING
The metal interconnect cascade cell (MICC) consists of an upper 1.93 eV AIGaAs cell and a lower 1.42 eV
GaAs cell (both in the n on p configuration) which are connected in series by a metal interconnect deposited
during post-growth processing. These cells are grown by atmospheric pressure OMVPE on p-type GaAs
substrates, as described in reference 3. The schematic cross section of an MICC is shown in Fig. 2. The base
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https://ntrs.nasa.gov/search.jsp?R=19910020906 2020-03-19T17:29:39+00:00Z
andemitteroftheGaAslowercellaregrownfirst.A highlydoped AIGaAs window is grown on top of the emitter
to reduce the surface recombination velocity at this interface. Next, p+ and n+ contact layers are grown which will
be shorted together during later processing to eliminate the intrinsic p-n junction formed by the two solar cells,
and to connect the two cells in series. The AIGaAs base and emitter of the upper cell are grown next,followed by
another highly doped AIGaAs window layer. Finally, a GaAs cap layer is added to ensure good ohmic contact to
the top metal grid, and to protect the device during processing.
Fig. 3 shows the processing sequence used to fabricate the MICC devices. During processing, the front
and back metalizations are deposited by evaporation. The metalization contact to n-type material includes
Au/Ge/Ni/Au layers with additional Pt/Au layers on the top metal grid for easy soldering or welding. The p-type
metalization contacts are Au/Zn/Au. A wide and a subsequent narrow trench are etched in the upper cell to
expose the p+ and n+ contact layers of the device. The metal interconnect consists of two evaporated metal
layers deposited in the trench areas which form good ohmic contacts to the p+ and n+ contact layers. Contact
resistance after a 400 °C anneal is typically very good, with values below lx10 -5 D-cm2. An isolation etch is
performed to allow electrical testing of the devices before they are scribed and separated into individual cells.
The n+ GaAs cap layer is selectively etched away between the gridlines prior to the deposition of the AR
coating. The two-layer AR coating which includes TiO2 and MgO is sputtered onto the exposed AIGaAs window
layer. The indices of refraction of the two layers are designed to minimize reflection when a prismatic coverglass
is applied to the surface. This coverglass, supplied by Entech, is a series of lenses which bends light in toward
the active area of the cell and away from the gridlines. This effectively eliminates the obscuration caused by the
gridlines.
DEVICE TESTING
The testing of the devices always involves a spectral response measurement of both the upper and lower
cell. A separate integrated current density is obtained for the upper and lower cell based on the AM0 spectrum.
Current-voltage (I-V) measurements are obtained using a two-color simulator which includes two light sources as
shown in Fig. 4. The use of two light sources allows the current in each junction to be controlled independently.
The light source for the AIGaAs junction has a short-pass filter, and that for the GaAs junction has a long-pass
filter. The intensity of each light source is set so that the current density out of each junction matches the
integrated current density obtained from the spectral response measurement of the corresponding junction.
After the lamps are set to the appropriate intensity, computer controlled I-V measurements are made. The
reason for using this test method is that any AM0 simulator introduces errors because of "spikes" at various
wavelengths. By using empirical data for the AM0 spectrum and obtaining the integrated current density from
the spectral response, the I-V measurements will be more accurate and the current densities of the two
junctions can be realistically compared.
RESULTS AND DISCUSSION
Our best MICC cell has an efficiency of 23.0 %. An I-V curve for this device is shown in Fig. 5. In addition to
this best result, a manufacturing demonstration has been completed for 0.5 cm2 AIGaAs/GaAs MICCs. Twenty-
two wafers were grown with the device structure of an MICC. Each wafer contains either fourteen or sixteen 0.5
cm2 cells (depending on whether a D-shaped or a square wafer is used. These 22 wafers resulted in 342 cells
grown. During processing and scribing, 45 of the cells were broken. We hope to reduce this breakage rate of 13
% by using a more pointed scribing tip. All of the cells were put through a screening test in wafer form to get an
estimate of their performance. The test method was simply to choose one cell on the wafer, set the lamp current
so that 15.5 mA/cm2 was obtained out of each junction at short circuit, and then test all the devices on the wafer
at that lamp intensity. By doing this we get a wafer map of the expected efficiencies for each cell. After the wafers
were scribed, 297 cells were available for individual efficiency measurements.
115 cells which had screening "efficiencies" of at least 20 % were tested with an "electronic" coverglass.
When a prismatic coverglass is applied to an MICC the measured increase in device current is 21% for the upper
AIGaAs cell and 9 % for the GaAs cell. We therefore simulated the coverglass effect during testing by multiplying
the corresponding integrated current densities by 21% (AIGaAs) and 9 % (GaAs). This simulation of the
coverglass saves installation time, and allows the MICCs to undergo subsequent radiation testing. Seventy-four
of the MICCs had actual efficiencies of at least 20 %. Therefore, the yield for good cells is 25 % as compared to
the 297 testable cells, and 21.6 % as compared to the 342 cells grown. A histogram showing the efficiencies of
these 115 cells is shown in Fig. 6. These results do not show a large number of cells approaching our best MICC
efficiency of 23 %. However, this is the first manufacturing demonstration of monolithic two junction cells, and
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improvement in both efficiency and yield is to be expected with further process refinements and manufacturing
experience.
In addition to the 0.5 cm2 devices we have made, a scale-up to 2x2 cm2 MICC devices is in progress. Our
very first attempt at fabricating the devices indicated that our material growth quality was very good, but a
problem with one of the metalization steps prevented them from operating properly. Several additional wafers
have been grown and the devices are currently being processed using a method which will prevent the
metalization problem encountered earlier. If successful, these will be the first monolithic two junction solar cells
to be made on an area as large as 4 cm2. They will then me made into a panel of six cells to be flown in a space
based experiment.
As a proof of concept project for the near term, a method was developed for stacking a MICC above a low
bandgap third junction. Several low bandgap solar cells were used, but the best result was obtained from a
InGaAsP cell (ref. 4) which was processed at Varian. The mechanical fixture allowed electrical access to the third
junction so that it could be measured independently, and the MICC has the capability to be measured in either a
two or three terminal configuration. Table I shows the device parameters of each cell in the stack and the results
of a two terminal measurement of the entire stack. The efficiency obtained at 1 sun, AM0 is 25.2 %. This
represents the highest 1 sun, AM0 efficiency ever reported. The sum of the efficiencies of each component cell
is 25.48 %. This is higher than the two-terminal measurement of the entire stack because the GaAs cell is current
limiting the other two cells, thus bringing their efficiencies down slightly. In general, the GaAs component cell
has a lower current density than the AIGaAs cell. To remedy this we suggest increasing the bandgap of the
AIGaAs cell slightly to allow more light into the GaAs cell. With better current matching, the efficiency of the two
terminal configuration will rise. In addition, the MICC cell used had a two terminal efficiency of only about 21%;
this is 2% lower than our best MICC cell so further efficiency gains can be expected. Fig. 7 shows the external
quantum efficiency of the three junctions in the stack.
CONCLUSIONS
Because both component cells can be measured independently, the MICCs have an advantage over
tunnel junction based devices in terms of testing and diagnostics. With tunnel junction devices, only two
terminal measurements can be made. However, with the MICC one can measure the efficiency of each
component cell, determine which cell degrades the most during radiation experiments, and find the current
limiting cell. Ultimately, the tunnel junction concept is preferred because it reduces post-growth processing
complexity and does not involve reducing the active area of the upper junction. For this reason, tunnel junction
development is currently proceeding in parallel with the MICC work.
It is clear that a monolithic integration of three junctions is superior to mechanical stacking if the devices are
to actually be used in space. Efficiencies higher than 30 % have been predicted for monolithic three junction
devices. Future work in this area should concentrate on monolithic integration of three junctions using grading
layers or a superlattice to compensate for the lattice mismatch between GaAs and InGaAs or other low bandgap
materials.
REFERENCES
1). H.C. Hamaker; "Computer modeling study of the effects of inhomogeneous doping and/or composition
in GaAs solar-cell devices"; J. Appl. Phys., .58, p. 2344; (1985).
2). B-C. Chung, G. F. Virshup, S. Hikido, and N. R. Kaminar; "27.6% efficiency (1 sun, air mass 1.5) monolithic
AIo.37Gao.63As/GaAs two-junction cascade solar cell with prismatic cover glass"; Appl. Phys. Lett. 55, p. 1741;
(1989).
3). B-C. Chung, G. F. Virshup, and J. G. Werthen, High-efficiency, one-sun (22.3% at air mass 0; 23.9% at air
mass 1.5) monolithic two-junction cascade solar cell grown by metalorganic vapor phase epitaxy"; Appl. Phys.
Lett. 52, p. 1889; (1988).
4). InGaAsP material grown by M. Wanlass at the Solar Energy Research Institute.
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TableI. Device parameters for the three junction mechanical stack of an MICC on an InGaAsP cell.
i
Efficiency, I°/,ol V_x_ (V) Fill Factor J§_; (mA/cm 2)
AIGaAs 12.00 1.398 0.75 15.77
GaAs 9.09 0.994 0.78 15.63
InGaAsP 4.39 0.521 0.72 15.98
Efficiency Sum 25.48
Stack 25.20 2.882 0.77 15.63
All measurements are two terminal, and made at 25 °C under 1 sun, AM0 illumination.
Device area 0.5 cm2
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Figure 1. - Predicted efficiency vs. temperature for selected one-, two-, and three-junction solar cells.
17-4
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17-5
Filtering
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Figure 4. - Testing configuration for I-V measurements of two junction MICCs.
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Figure 5. - An I-V curve of the best 0.5 cm2 MICC, showing the measured device parameters.
17-6
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Figure 6. - Efficiency Histogram for 115 of the best 0.5 cm2 MICC cells produced in the manufacturing
demonstration.
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Figure 7. - Spectral Response of the three junction mechanical stack. The stack includes a 0.5 cm2 MICC
above a 0.5 cm2 InGaAsP solar cell.
17-7



Recent advancements in monolithic AlGaAs/GaAs solar cells for space applications

Recent advancements in monolithic AlGaAs/GaAs solar cells for space applications

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