The commercial application of Indium Phosphide solar cells in practical space missions is crucially dependent upon achieving a major cost reduction which could be offered by heteroepitaxy on cheaper, more rugged substrates. Furthermore, significant mass reduction, compatibility with mechanically stacked multijunction cells, and elimination of the current loss through glue discoloration, is possible in III-V solar cells by the development of ultrathin, directly glassed cells. The progress of a UK collaborative program to develop high efficiency, homojunction InP solar cells, grown by MOCVD on Si substrates, is described. Results of homoepitaxial cells (is greater than 17 percent 1 Sun AM0) are presented, together with progress in achieving low dislocation density heteroepitaxy. Also, progress in a UK program to develop ultrathin directly-glassed GaAs cells is described. Ultrathin (5 micron) GaAs cells, with 1 Sun AM0 efficiencies up to 19.1 percent, are presented, together with progress in achieving a direct (adhesive-less) bond between the cell and coverglass. Consequential development to, for example, cell grids, are also discussed

Cross, T. A.

Hardingham, C. M.

English

NASA Technical Reports Server

N9 -11 O6
HETEROEPITAXIAL InP, AND ULTRATHIN, DIRECTLY GLASSED,
GaAs III-V SOLAR CELLS.
C.M. Hardinqham, T.A. Cross
EEV Ltd
Chelmsford, Essex, England CM1 2QU.
Abstract:
The commercial application of Indium Phosphide solar cells in practical space missions is crucially
dependant upon achieving a major cost reduction which could be offered by heteroepitaxy on cheaper,
more rugged substrates. Furthermore, significant mass reduction, compatibility with mechanically stacked
multijunction cells, and elimination of the current loss through glue discoloration, is possible in III-V solar
cells by the development of ultrathin, directly glassed cells.
This paper describes the progress of a UK collaborative program to develop high efficiency, homojunction
lnP solar cells, grown by MOCVD on Si substrates. Results of homoepitaxial cells (>17% 1 Sun AM0) are
presented, together with progress in achieving low dislocation density heteroepitaxy.
Also, progress in a UK program to develop ultrathin directly-glassed GaAs cells is described. Ultrathin (5
micron) GaAs cells, with 1 Sun AM0 efficiencies up to 19.1%, are presented, together with progress in
achieving a direct (adhesive-less) bond between the cell and coverglass. Consequential development to,
for example, cell grids, are also discussed.
Keywords:
InP, GaAs, Heteroepitaxy, Ultrathin, Direct Glassing.
IIl-V solar cells vs Si.
It is well established that some IlI-V solar cell materials, such as GaAs or InP, offer substantial performance
improvements over conventional Si cells, due to three main reasons [ref 1]. The band-gap for both
materials is closer to the optimum for single-cell performance - in the case of GaAs (1.43eV) the maximum
predicted beginning-of-life (BOL) 1 sun AM0 efficiency is as high as 26% compared to a more modest
predicted maximum of around 22% for Silicon.
Secondly, it is well established that some IIi-V solar ceil materials offer much higher radiation resistance,
compared to conventional Si cells, which enhances their relative performance at end-of-life (EOL). This
is a very significant factor, particularly for InP which appears t0have higher radiation resistance than other
materials.
Thirdly, the degradation in performance at elevated temperatures, for the higher bandgap cell such as
GaAs, is much smaller than for Si cells.
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https://ntrs.nasa.gov/search.jsp?R=19940006934 2020-06-16T20:49:52+00:00Z
Furthermore, some III-V solar celt materials, including both GaAs and InP, have a direct band-gap. The
consequences of this are much high absorption coefficients than is the case for indirect band-gap materials
such as silicon. Thus all the light useful to the cell is absorbed within the first few microns, resulting in the
bulk of the cell material being redundant, leading to the possibility of ultrathin (<10 micron) cells.
InP Solar Cell types:
There are three main types of InP cell. These comprise the epitaxially grown cells, (by MOCVD, MBE or
related growth techniques), diffused junction cells (in which the junction is formed within the bulk substrate
by diffusion), and surface junction cells, where the junction is formed substantially at the surface, by
deposition of material (eg Schottky barrier type cells, or ITO cells).
The present program 'addresses both epitaxially grown cells (MOCVD growth, both homoepitaxial and
hetereepitaxial), and ITO/InP cells heteroface cells.
ITO/InP cells:
The Indium Tin Oxide/InP (ITO/InP) solar cells under consideration within the present program comprise
an RF sputter deposited layer of ITO on p-type InP (Figure i), and have been discussed more fully
previously [ref 2-4]. Analysis of eg CV measurements leads to the theory that a shallow homojunction cell
is formed, through the creation of a damaged layer just underneath the surface. The potential advantage
of such cells is that the requirement for expensive epitaxy processing is redundant, although to-date, BOL
efficiencies achieved with this type of cell have, in general, been lower than for epitaxial cells.
Epitaxial InP cells:
The epitaxial cells upon which the program has focused are shal!ow homojunction n'-p-p" cells, fabricated
with a lattice-matched InGaAs cap layer (Figure 2), with efficiencies approaching 18% 1 sun AM0 BOL
(Figures 3, 4). The program baseline is 2x2 cm cells. The baseline cell structure has been established
following pseudo-three dimensional modelling of the cell [ref 7]; this has incorporated lifetime values derived
from test structures closely emulating the cell conditions. Furthermore, series resistance effects have been
accurately taken into account using a distributed element approach, resulting in better optimisation of
collection grid design.
Irradiation Studies:
1 MeV electron irradiation studies have been carried out on both types of cell; further electron and proton
studies are scheduled for autumn '92, and spring '93. As expected, both types of cell stand up well to
electron irradiation, better than either Silicon or GaAs [refs 5,6], with over 75% power remaining after a
dose of 1E15 electrons (bare cells, in the dark) (Figure 5). The evidence suggests that still lower
degradation is experienced for cells under load [ref C]. Furthermore, significant recovery of the cell
parameters has been achieved at moderate temperatures (90°C).
' This work has been supported in part by the UK Department of Trade and Industry and the Science
and Engineering Research Council under the LINK Advanced Semiconductor Materials Programme, and
EEV acknowledge the contributions of the other participants in this program: Newcastle Photovoltaics
Applications Centre; Epitaxial Products International; University of Wales College Cardiff; and Pilkington
Group Research and Pilkington Space Technology.
25O
Commercial Applicability:
In order for Indium Phosphide solar cells to be commercially applicable for practical space missions their
cost must be significantly reduced below current levels. The (significant) cost of epitaxy can be eliminated
through use of ITO/InP cells as described above, or diffused junction cells. (For example, the only mission
to baseline InP cells for power production is the Japanese MUSES-A lunar orbiter using l x2cm diffused
junction InP cells [ref 8].) However, performance of the former type of cell has so far failed to match that
of epitaxial cells, the cells on MUSES-A averaging 16% (BOL).
The cost of InP cells is heavily dependant on material costs. Thus the greatest cost benefit will be through
heteroepitaxy on cheaper, more rugged, substrates. To this end, the program includes development of InP
grown heteroepitaxially on Si (with and without intermediate layers).
InP/Si Heteroepitaxy:
The critical problem to be addressed in heteroepitaxy is how to accommodate lattice mismatch between
the different materials. In the case of InP (5.869A) on Si (5.431A), this amounts to some 8%. The
approaches being considered, within the present program, to accommodate this include: (a) direct growth
of InP on Si via a "two-step" growth process; (b) growth of lnP/GaAs/Si via a double "two-step" process,
and (c), growth of InP/GalnAs/GaAs/Si, where the InP is grown on graded-composition InGaAs, (0-53%
to provide lattice matching at the InP interface).
Experiments have confirmed the critical nature of the mismatch, with dislocation densities 2 orders of
magnitude higher than acceptable being obtained for graded layers, and polycrystallinity evident on InP/Si.
However, for the case of InP/GaAs/Si, initial growths have produce films giving double crystal x-ray rocking
curves with FWHM of 500 arc sec, and FWHM of 320 arc secs has been achieved with post-growth
annealed samples, (although it is possible that the reduction seen on annealing is due to twin annihilation,
which gives rise to threading dislocations, and therefore wilt not provide "better" material quality than that
in the un-annealed state). Furthermore, growth of the intermediate GaAs layers on Si has produced x-ray
FWHM of 152 arc sec.
Ultrathin InP Cells:
One of the potential advantages of heteroepitaxial InP is the possibility of removing the bulk of the
heteroface material, leaving an ultrathin (5-10 micron) cell. This will facilitate mechanically stacked
multijunction cells, with little sub-bandgap absorption in the InP cell.
However, some manufacturing issues in ultrathin InP cells remain to be addressed, such as interconnection
techniques, supporting structure, etc. These issues are already being addressed for (related) ultrathin
GaAs cells.
Ultrathin GaAs cells:
Several workers have already reported ultra-thin GaAs solar cells [refs 9-13]. EEV have been developing
chemically etch-stopped GaAs cells grown on GaAs and Ge [ref 11], and have produced ultrathin (8
micron) cells up to 19.7% efficient, based on measured cell area of 3.7cm 2 (Figure 6). The technology is
readily transferable to heteroepitaxial InP cells.
251
Directly Glassed GaAs cells:
Critically important for their use, is the ability of ultrathin cells to withstand handling and integration
processes. Furthermore, one of the causes of current degradation over a cell's lifetime in space, is
discoloration of the coverglass adhesive. These two issues combine, to raise interest in adhesiveless
bonding of cells. One technique that has been tried is use of teflon bonding [refs 11,14]; however, of more
interest is the possibility of completely doing away with any bonding medium, and relying on a direct bond
between the coverglass and cell [ref 12].
This approach has been greatly facilitated by the advent of a coverglass material with expansion coefficient
matched to GaAs [ref 15], and EEV are involved a program Zto develop direct glassing of u!trathin GaAs
cells, following on from the adhesive-bonded ultrathin work. The bond is formed by ionic diffusion formed
by an electrostatic field applied during compression at elevated temperatures, requiring the cell to withstand
somewhat higher temperatures than standard for a short time (seconds/a few minutes).
Interconnecting ultrathin Cells:
The ease of interconnection of such cells is under consideration; when made in conjunction with direct
glassing, there is no adhesive-matrix to support the interconnect near the cell, and the problems associated
with the interconnect flexing and cracking the cell are significant. The option of providing interconnection
via metal attached to the coverglass rather than (primarily) to the cell is being considered (figure 7).
References:
1,
2.
3.
4.
5.
6.
7.
°
9.
10.
11.
12.
13.
14.
15.
T.A.Cross, Proc 2nd World Renewable Energy Conference, Reading, England, Sept.'92
N.M.Pearsalt, Proc 4th Int Conference on InP and Related Materials, Newport RI, April 92
C.Hardingham et al, Proc 2nd ESPC, Florence, Sept. '91.
NM.Pearsall et al, Proc 22nd IEEE PVSC, Las Vegas, Oct. '91.
K.A.BertneSS et al, Proc 21st IEEE PVSC, Kissimmee, May '90 -
K.Ando & M.Yamaguchi, Appl. Phys. Lett., 4"7 (8), pp846-8, '85.
J.E.Parrott & A.Potts, Proc 22nd IEEE PVSC, Las Vegas, Oct.'91.
K.Takahashi et al, Proc 2nd ESPC, Florence, Sept.'91.
R.W.McClelland et al, Proc 21st IEEE PVSC, Kissimmee, May '90.
N.P.Kim et al, Proc 22nd IEEE PVSC, Las Vegas, Oct.'91
C.Huggins et al, ibid
M.J.Nowlan et al, ibid
R.L.Bell et al, Sandia Report No 7460, '84.
P.White, Proc 2nd ESPC, Florence, Sept.'91, p551
P.White, ibid, p547
2 Part of this work has been carried out with the support of the procurement executive, Ministry of
Defence;
252
5um Au (Plated)
Contact Metal
<50nm n-type ITO
Shallow "dead" layer InP
InP
3-500urn p-type substrata
contact metal
5urn Au (Plated)
Figure 1: Schematic of ITO/InP Solar Cell
Sum
lure
<5onm n-type emitter structure
Au (Plated)
Contact Metal
n-type
InGaAs cap___ -
4urn p-type base
p-type buffer
3-500urn p-type substrate
contact metal,
5urn Au (Plated)
Figure 2: Schematic of Epitaxial InP Solar Cell
253
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UJ
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U
150
125
100
75-
80-
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o!
0
Coil "TriP 59-2 c'rc
]=¢ =141.5=R V0c =885=V
lap =133.6=R V=p=720=V
FF =0. 767 EFF=17.8Z
: : : t ; ; . ; : I
0.1 0.2 @.l 0.4 O.I 0.11 0.7 0.8 O.! LO :1.. JI,
VOLTS
Figure 3: 1 Sun AMO Photovoltaic measurement of 2x2 cm homoepitaxial n'-p-p* Solar Cell
100
C
UJ
,¢J
8O
6O
4O
2O
300 400 500 600 700 800 900
Wovelen9Lh nm
1000
Figure 4: Quantum Efficiency of 2x2 cm homoepitaxial n*-p-p" Solar Cell
254
Figure 5:
P,
/1
Po
IO0
75
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25
0
1012 10rr
.........- ..................
|
I
t
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v----V lec ITO/InP I
I
0 ...... -0 homo junction t
t
I
I
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V
10_3 10_4 10_s 10_
Electron fluence e/cm 2.
Percentage Power remaining for InP Solar Cells after 1 MeV electron irradiation.
l_lll.
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Figure 6:
i i m
Nomlnal Area = 4.00 c1"
Actual Area = ].75 cl"
Jsc = 31.84 mA/cm _
Voc = 1023 mV
rr = 0.s17
Elf = 19.7 %
Cell E2017-3 : EEV : 8pro Ultrathin GaAs
I, I ! | I I 4
O°l O 2 o.I 0.4 o.I o.I 0,1 O.i l.I l.o l,l
VOLTS
1 Sun AMO Photovoltaic measurement of 2x2cm ultrathin GaAs Solar Cell
FRONTINTERCONNECT
INTERCONNECTBONDS COVERGLASS
BACKINTERCONNECT
REAR
CONTACT
Figure 7: Interconnection to an ultrathin GaAs (or InP) Cell, with interconnect supported on
the coverglass.
255


Heteroepitaxial InP, and ultrathin, directly glassed, GaAs 3-5 solar cells

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