The main points of discussion, conclusions and recommendations of a workshop on InP materials and cell fabrication are given. The importance of assessing the quality of p-Inp crystals supplied by different vendors, back contacts to solar cells, junction formation, energy conversion efficiency, testing for radiation resistance, and future develpments were among the topics discussed

Coutts, T. J.

English

NASA Technical Reports Server

N87-26454
InP MATERIALS/CELL FABRICATION
T.J. Courts (Workshop Chairman)
Solar Energy Research Institute
Golden, Colorado
This report describes the main points of discussion, conclusions, and
recommendations of the Workshop on InP held at the NASA SPRAT conference. An
unusual level of interest in this workshop was shown with 37 delegates being
in attendance. The names, affiliations and telephone numbers of these persons
are shown in Appendix I.
The meeting commenced with a presentation by the chairman (T.J. Coutts),
of issues he believed are central to the development of InP as a solar cell
absorber. The object of this was to stimulate subsequent discussion in the
anticipation that the topic may be relatively unfamiliar to most of the
attendees. A copy of all slides used in the chairman's presentation is given
in Appendix II. The key points emerging from the workshop were summarized on
the final morning of the conference. The viewgraphs used for this
presentation are also appended.
I. Chairman's Presentation
It is considered that it will become important to assess the quality of
p-InP crytals supplied by different vendors, whether they are to be used as an
active part of the device or as a substrate for the growth of epi-layers. At
present very few manufacturers will deliver moderately doped (1015-1017 cm -3)
p-type material. Only five have been found so far (slide 4). At SERI, the
analyses shown in slide 5 can now be routinely performed and the capabilities
are being extended systematically. Examples of photoluminescence (PL)
measurement of minority carrier lifetime and its variation with impurity
concentration are shown in (slides 6 and 7). PL has also been used to assess
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the effect of low power rf plasma exposure of substrates and typical spectra
are shown in slide 8. Cathodoluminescence (CL) has also been used to examine
the surfaces of freshly prepared substrates. Slide 9 shows the presence of
many non-radiative defects. In future, attempts will be made to profile the
depth distribution of the defects and to minimize their density using
different preparation/conditions. It is also planned to use high resolution
cross-sectional transmission electron microscopy using a newly acquired ion-
milling machine.
The issue of back contacts to solar cells is one frequently ignored by
researchers, but in reality it is of great importance. In slide 10 the
obvious parameters influencing the contact resistance are shown and in slide
11 the areas which, it is considered, should be studied. Such investigations
are now well underway at SERI and initial results are beginning to clarify the
changes shown in slide 12. The nature of the junction formation was then
discussed and a review of substrate cleaning techniques was presented; see
slides 13, 14, 15. Generally, the effectiveness of cleaning techniques is not
quantified, but the group at SERI (under NASA support) is currently using
monochromatic ellipsometry (in-house) and spectroscopic ellipsometry (in
conjunction with D. Aspnes of Bell Labs) to investigate native oxides and
surface roughness. An example of native oxide growth after cleaning is shown
in slide 16. Using techniques developed more recently, residual oxide
thicknesses have been reduced below 10 A. Slide 17 shows the Raman spectrum
(performed by F. Pollack's group at Brooklyn College) of a freshly polished
substrate. The llne width indicates minimal surface damage and residual
strain. Hence, is seems possible that substrate preparation techniques are
adequate to produce reasonable devices, but the CL study shows that there is
room for improvement in substrate quality.
Techniques for junction formation were then discussed and it was pointed
out that the largest values of Voc had been achieved by the group at Arizona
State University, using a p- on n-substrate; slides 18, 19. A photoreflec-
tance spectrum of an ITO/InP junction prepared by (ion-beam) sputtering is
shown in slide 20 (obtained at Brooklyn College), and the sharpness of the
features again indicates that reasonable quality devices are being made.
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L_
Techniques like this should be applied by other workers using different
fabrication methods.
An estimate of the maximum probable AMI.5, 100 mW cm -2 efficiency was
then made (slide 2]). This was 23%; i.e., approximately equal to that of GaAs
devices. It was pointed out that further improvements will probably depend on
the development of cells grown by OMCVD. SERI and several other groups are
now investigating this possibility. Technological aspects of optimized grid-
ding and AR coatings were then discussed (slides 23, 24, 25, 26). SERI has
put considerable effort into these topics over the duration of the NASA con-
tract and it is expected that this ground work will be very valuable when good
quality junctions are being produced. Slide 28 shows a summary of the highest
efficiencies reported in the literature for InP based cells.
Finally, in slide 29 other issues were raised. Not least of these is the
question of reporting efficiencies accurately. It was emphasized that there
is a great need to report AM0 data for cells of realistic area and under well
defined experimental conditions; and to report only total area efficiencies.
Issues were also raised on the reliability of radiation resistance data; on
the cost of InP (this is falling, but needs to fall much further), and on the
chemical stability of significant interfaces. Under the category "Others",
issues related to increasing the power/mass ratio, utilizing concentrator
systems, the use of non-InP substrates for epi-films etc were raised. Discus-
sion of these will be left until the end of the report on the main points
emerging from the workshop conversations, subsequent to these opening remarks
by the chairman.
II. Workshop Discussions
2.1 Need to Increase Efficiency of InP Based Cells
At present, the highest efficiency reported for an InP based cell is
around 15% (AM0). This is based on a calculation performed by I. Weinberg of
NASA Lewis Research Center on a result reported by the Japanese group at
N.T.T. Although 15% is very encouraging, the device cannot really be taken
seriously until substantial improvements have been made; despite its excellent
radiation resistance. There is scope for improvement in Jsc, Voc and fill-
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factor, but particularly in the first two of these. The largest value of
AMI.5, 100 mW cm -2 reported is 27.7 mA cm -2 (ITO/InP discussed by Coutts at
this meeting). From the spectral response of this cell it was calculated that
Jsc at AM0 would be 33.5 mA cm -2 which for an 8% grid coverage, translates to
36.5 mA cm-2 active area. With improved gridding and AR coating, an upper
limit of 37 mA cm -2 would seem realistic, for the total area. This is equiva-
lent to 90% x the maximum theoretical value which is comparable to that
achieved for more mature technologies.
There was extensive discussion about the maximum achievable Voc. Finally
a value of 0.7 x Eg = 945 mV was agreed upon, again by comparison with GaAs.
The highest Voc reported to date is 845 mV (by Shen et al, at this meeting)
so, as with Jsc' there is an estimated improvement of about 10-12% to be made.
If such an improvement in Voc were made there would also be a significant
gain in FF, and an estimate of 85% was made for the practicable maximum.
Hence, the participants of the workshop believed that a well-funded program on
InP cells could ultimately lead to an AM0 efficiency of 21.7%. This rather
startling figure caused a reconsideration, based on intuition, and one or two
participants felt that 20.5% was perhaps a more realistic target. Earlier in
the meeting, Goradia had made a similar prediction.
2.2 Means of Improving Efficiency
Although InP is usually believed to have a low surface recombination
velocity, it was suggested that passivation would nevertheless, lead to
further significant gains in Voc. A lattice matched ternary window layer to
InP, analogous to AIGaAs/GaAs, is a possibility, and one which may be
appropriate is AiAsSb. The disadvantage of this material is that it may be
difficult to grow an abrupt AiAsSb/InP interface. Assuming that the devices
are ultimately based on epitaxially grown InP, the transition to AIAsSb will
of necessity be more demanding than that from GaAs to AiGaAs. Considerable
effort has been made to passivate the surface of InP with various insulators
for MISFET's. Varying degrees of success have been achieved and this is
another area worth studying in the present context. It may, for example, be
possible to fabricate devices based on the PESC design used for Si cells.
368
Advances could also be made using deeper junctions but this would neces-
sitate a substantial improvement in the materials properties and, in parti-
cular, in the minority carrier lifetime. Improvements in material quality of
substrates were also believed to be important and the need to characterize
crystals supplied by different vendors was emphasized.
Finally, there are substantial improvements to be made in efficiency
merely through better gridding and A.R. coatings. The internal quantum effi-
ciency of many devices is already high, but substantial losses result from
lack of attention to processing aspects.
2.3 Efficiency Measurements
At present, efficiencies quoted in the literature tend to lack any stan-
dardized form. Worse, it is not always clear whether a particular measurement
refers to AM0, AMI, AMI.5 or some other air mass number, what simulator was
used, whether total or active area was used, and what the measurement tempera-
ture was. Hence, workers in the field are encouraged, at least, to specify
their measurement conditions accurately. A further preference was expressed
that measurements should all refer to AM0, since these cells are more likely
to be used for space, rather than terrestrial applications.
Finally, the view was expressed that data would be much more significant
if larger area devices were made. Clearly, much greater inaccuracies result
for small area cells. An acceptable area was, however, not defined, but areas
greater than 0.3 cm 2 are to be preferred.
2.4 Radiation Testing
The present high level of interest in InP based cells results from their
excellent radiation resistance. It is necessary, therefore, that these
results (mainly of the Japanese group) be confirmed. It is also necessary to
perform the radiation tests on devices of as high efficiency as are available.
This is particularly relevant since there are indications that radiation
resistance may be a function of impurity concentration, which in turn
influences efficiency. Hence, it may be that high efficiency and high radia-
tion resistance are mutually exclusive.
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In this context, radiation damage models should be developed to identify
the defects leading to degradation, and to account for the apparently high
radiation resistance.
This has also raised the issue that there may be materials with even
greater radiation resistance than InP. Might it be possible to predict these
in advance rather than devoting years of study to a less than optimum
material?
2.5 Future Developments
At some point it will be necessary to increase the power/mass ratio of
these cells. To date, only cursory studies have been made and it may be
advisable to initiate more thorough systems studies at an early date.
Since cost is of pre-eminent importance (as stressed by J. Scott-Monck in
the final workshop presentation of this conference) a reduction in the cost of
InP is necessary. Early consultations with substrate suppliers could there-
fore be advantageous and could lead to forecasts of cost/supply. Alterna-
tively, studies of InP growth on other low cost substrates would seem to be a
promising area, even if technically complicate_. CLEFT studies could also
lead to cost reductions and, funding permitting, should be pursued.
Finally, concentrator studies should be initiated with a view to deter-
minign cost and system design considerations. This could include determining
the optimum concentration ratio, the reduction in radiation damage by the
concentrator optics etc.
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