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PROFESSOR: All right.

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Why don't we go ahead and
get this started here?

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We have a cornucopia of
different silicon materials

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out in front here in
display, and we'll

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walk through some
of them shortly.

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What I wanted to do right
at the beginning of class

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was to give a little bit of
an update on quiz number two.

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Some of you have probably
seen this already

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and are aware that
on Thursday we're

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expecting a short
little decision

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tree as to how to process your
solar cell to obtain the lowest

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dollars per watt peak.

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So this little exercise-- it
will last for about a month--

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is coincident with
our technology

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section of the class.

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So remember, we went
through the fundamentals.

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Now we're on the technologies.

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And then finally, in the
cross-cutting themes.

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So coincident with the
technologies portion

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is designing your own
solar cell and optimizing

00:01:12.370 --> 00:01:13.040
the dollars per watt.

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So this will entail actually
fabricating a solar cell, which

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is kind of fun.

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And Joe will be your guide
throughout this process,

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so you'll be able to actually
take a piece of bare silicon

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and finish up with a device,
a rudimentary device,

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but something to take a picture
yourself, post on Facebook,

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that sort of thing.

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You design your own solar cell.

00:01:34.260 --> 00:01:35.727
So the idea isn't
only to optimize

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for the performance
of the solar cell,

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but we decided to throw
in a little curve ball

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and design for
dollars per watt peak.

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Now this is a little bit
of a contrived exercise

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since we've arbitrarily chosen
what dollars are associated

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with each different
process step,

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but it's not too unlike what you
would face in actual industry

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if you had real data coming
off a production line

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and knew exactly what it
cost for each process step.

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So instead of having 30 plus
components in a more detailed

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cost model, we've
decided to simplify it

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to this little
diagram right here.

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So this is a flow chart
for the fabrication

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process of your solar cell.

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You'll start with a wafer.

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It has a certain cost
associated with it.

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You'll have some
decisions to make

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concerning light
management, whether you

00:02:17.580 --> 00:02:19.290
want to texture
your front surface

00:02:19.290 --> 00:02:21.600
or whether you want to
leave it bare and reflective

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like this right here.

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So whether you want a
reflective front service

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or you want to
texture it, there's

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a certain costs
associated with it.

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So you can probably go to some
online resource, like PVCDROM,

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and use their simulator or the
one you've already constructed

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for homework number
two, and calculate

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what the predicted efficiency
boost should be if you

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texture your front surface.

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Keep in mind on this very
simple solar cell here,

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we have no
anti-reflection coating.

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So the texturization
is pretty much

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all you've got for
light management.

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Next, on the emitter,
the choice is

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whether to make a deep
emitter or a shallow emitter.

00:02:57.590 --> 00:02:59.750
The text goes into
that in some detail.

00:02:59.750 --> 00:03:03.810
But your decision is basically
if you make a shallow emitter,

00:03:03.810 --> 00:03:05.810
you have less
Auger recombination

00:03:05.810 --> 00:03:07.410
in that front region.

00:03:07.410 --> 00:03:10.600
And so your blue response to
the device will be better.

00:03:10.600 --> 00:03:13.430
But you run the risk when you
do your contact metalization

00:03:13.430 --> 00:03:15.890
of firing through that
very shallow emitter

00:03:15.890 --> 00:03:17.860
and shunting your device.

00:03:17.860 --> 00:03:20.870
Whereas, if you decide
to go for a deep emitter,

00:03:20.870 --> 00:03:23.530
it stays longer inside
of the furnace because

00:03:23.530 --> 00:03:26.930
of the phosphorus will diffuse
deeper inside of the device.

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You blue response
will be poorer,

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but you'll have less
risk of shunting.

00:03:31.710 --> 00:03:33.500
So it's up to you to
use all of the tools

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that you've assembled so far
to make a value-based judgment

00:03:37.630 --> 00:03:40.930
whether or not it makes
sense to go with this or that

00:03:40.930 --> 00:03:42.590
as your selection choice.

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And finally, narrow
and wide fingers,

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this you can probably
guess already

00:03:46.610 --> 00:03:51.260
pertains to series resistance
and shading losses.

00:03:51.260 --> 00:03:54.470
So these are all
representative of trade-offs,

00:03:54.470 --> 00:03:56.330
trade-offs in terms
of the technology

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and trade-offs in terms of cost.

00:03:58.390 --> 00:04:00.710
And you have all the tools
necessary to calculate

00:04:00.710 --> 00:04:03.560
or estimate what these
outputs should be based

00:04:03.560 --> 00:04:05.780
on what you've learned so far.

00:04:05.780 --> 00:04:07.330
And so by Thursday,
what we've asked

00:04:07.330 --> 00:04:12.240
you to do is to make an
estimate of what technology

00:04:12.240 --> 00:04:15.290
pathway your company
is going to pursue.

00:04:15.290 --> 00:04:17.680
Remember, you want to
optimize the dollars per watt.

00:04:17.680 --> 00:04:19.404
You want to minimize
that quantity, which

00:04:19.404 --> 00:04:21.320
means you want to reduce
the number of dollars

00:04:21.320 --> 00:04:22.810
you invest in your solar cell.

00:04:22.810 --> 00:04:24.910
But you also want to
increase the watt peak

00:04:24.910 --> 00:04:26.440
that you get out of it.

00:04:26.440 --> 00:04:28.100
And so at the end
of the day, it'll

00:04:28.100 --> 00:04:30.630
be a performance/cost
trade-off in each

00:04:30.630 --> 00:04:32.780
of these different
process steps right here.

00:04:32.780 --> 00:04:34.890
And sometimes it won't
be entirely obvious

00:04:34.890 --> 00:04:38.280
which one to choose because
so many factors will converge.

00:04:38.280 --> 00:04:40.280
And so it'll be up to you
to make an engineering

00:04:40.280 --> 00:04:43.470
decision, a professional
judgment, as to which path

00:04:43.470 --> 00:04:45.140
you should pursue.

00:04:45.140 --> 00:04:47.290
Since it is kind of--
you know, there's

00:04:47.290 --> 00:04:49.020
a little element of
competition in here,

00:04:49.020 --> 00:04:51.977
so we decided the
dollars per watt peak

00:04:51.977 --> 00:04:53.810
shouldn't be completely
neglected at the end

00:04:53.810 --> 00:04:55.514
and we all get
certificates of merit

00:04:55.514 --> 00:04:56.930
and all feel good
about ourselves.

00:04:56.930 --> 00:04:59.471
We decided it should be worth
some part of the grade, but not

00:04:59.471 --> 00:05:00.952
such a large
portion of the grade

00:05:00.952 --> 00:05:03.160
that everybody's freaking
out and saying, oh my gosh,

00:05:03.160 --> 00:05:05.380
I don't have the right
tools to make this decision.

00:05:05.380 --> 00:05:07.980
I feel like I'm not
being graded fairly.

00:05:07.980 --> 00:05:10.220
So the portion of
dollars per watt

00:05:10.220 --> 00:05:12.040
is really only going
to be affecting

00:05:12.040 --> 00:05:15.579
10% of the final grade
of quiz number two.

00:05:15.579 --> 00:05:17.620
And so it will be based
on a ranking system where

00:05:17.620 --> 00:05:20.170
the highest one will
be 100% and so forth.

00:05:20.170 --> 00:05:21.560
But just 10% of your grade.

00:05:21.560 --> 00:05:24.040
So it's enough to,
I would say, create

00:05:24.040 --> 00:05:27.730
maybe a sting of the
pride if you don't happen

00:05:27.730 --> 00:05:30.652
to hit the highest
performance metric, but not

00:05:30.652 --> 00:05:33.110
enough to sting the actual
final grade of your class, which

00:05:33.110 --> 00:05:36.160
will be one lumped quiz,
quiz one, home works, final,

00:05:36.160 --> 00:05:36.920
and so forth.

00:05:36.920 --> 00:05:37.860
Right?

00:05:37.860 --> 00:05:39.430
Any questions about
quiz two so far?

00:05:39.430 --> 00:05:40.495
Yes, Jessica?

00:05:40.495 --> 00:05:43.465
AUDIENCE: I
completely understand,

00:05:43.465 --> 00:05:46.962
but there's even a
note in number three

00:05:46.962 --> 00:05:47.920
under the deep emitter.

00:05:47.920 --> 00:05:49.652
And you guys say,
any numbers you

00:05:49.652 --> 00:05:51.204
should give as
far as [INAUDIBLE]

00:05:51.204 --> 00:05:52.870
or are they responsible
for [INAUDIBLE].

00:05:52.870 --> 00:05:54.720
It seems like its
lacking some numbers.

00:05:54.720 --> 00:05:56.908
And I understand
optimization, but I'm

00:05:56.908 --> 00:06:00.468
having trouble putting
just how much better.

00:06:00.468 --> 00:06:02.884
And it say it'll be much more
effective if you do etching.

00:06:02.884 --> 00:06:04.468
Well, how much is
much more effective?

00:06:04.468 --> 00:06:05.967
PROFESSOR: Oh, the
etching for the--

00:06:05.967 --> 00:06:08.340
AUDIENCE: For the etching,
we gave a rough [INAUDIBLE].

00:06:08.340 --> 00:06:08.828
So you can look that up.

00:06:08.828 --> 00:06:09.804
AUDIENCE: I did look that up.

00:06:09.804 --> 00:06:11.268
And for the other ones, is there

00:06:11.268 --> 00:06:13.851
AUDIENCE: So that one, you can
get a pretty good estimate for.

00:06:13.851 --> 00:06:14.684
AUDIENCE: OK.

00:06:14.684 --> 00:06:17.159
For the other ones, is there
going to be a [INAUDIBLE]

00:06:17.159 --> 00:06:18.950
AUDIENCE: In terms of
shunting your device,

00:06:18.950 --> 00:06:20.991
it's really hard to predict
the shock resistance.

00:06:20.991 --> 00:06:24.100
But if you do shunt your
device, you essentially ruin it.

00:06:24.100 --> 00:06:26.945
So I would just take
that into account.

00:06:26.945 --> 00:06:29.852
You're not going to
get exact answers.

00:06:29.852 --> 00:06:32.169
But you can do your best
to estimate [INAUDIBLE]

00:06:32.169 --> 00:06:34.814
resistance from the [INAUDIBLE]
spacing and your emitter

00:06:34.814 --> 00:06:35.364
thinness.

00:06:35.364 --> 00:06:36.030
PROFESSOR: Yeah.

00:06:36.030 --> 00:06:37.330
Believe it or not,
you might feel

00:06:37.330 --> 00:06:38.954
like you don't have
the tools right now

00:06:38.954 --> 00:06:41.260
to get quantitative
answers, but you do.

00:06:41.260 --> 00:06:43.400
You have a number
of the tools here

00:06:43.400 --> 00:06:45.600
to get, say, 90% the way there.

00:06:45.600 --> 00:06:47.290
And in engineering,
90% of the way

00:06:47.290 --> 00:06:50.220
there is well beyond what you'll
actually face in the field.

00:06:50.220 --> 00:06:53.090
So that's pretty good.

00:06:53.090 --> 00:06:56.380
If you have specific
questions about what

00:06:56.380 --> 00:06:58.580
would be a good
resource to look up

00:06:58.580 --> 00:07:01.330
about this, what would be a good
resource to look up about that,

00:07:01.330 --> 00:07:02.310
send an email.

00:07:02.310 --> 00:07:05.740
And what I'll do, if I receive
something in that nature,

00:07:05.740 --> 00:07:08.384
I'll respond to the
class so that everybody

00:07:08.384 --> 00:07:10.425
has benefit to that
information and no one person

00:07:10.425 --> 00:07:12.050
is particularly advantaged.

00:07:12.050 --> 00:07:13.400
So it's worth a try.

00:07:13.400 --> 00:07:16.757
If it's something that was just
covered yesterday in lecture,

00:07:16.757 --> 00:07:18.340
I might be a little
bit more reticent.

00:07:18.340 --> 00:07:20.150
But if it is something
to the effect of,

00:07:20.150 --> 00:07:21.920
gee, how would the
lifetime improve

00:07:21.920 --> 00:07:25.020
with these different gettering
scenarios, sure, absolutely.

00:07:25.020 --> 00:07:27.221
We can give you a
little hand there.

00:07:27.221 --> 00:07:28.970
But everything else,
you should definitely

00:07:28.970 --> 00:07:30.840
have that information
available so far.

00:07:30.840 --> 00:07:33.840
This is meant to be a fun
exercise, but also one

00:07:33.840 --> 00:07:35.540
that illustrates the
trade-offs involved

00:07:35.540 --> 00:07:37.170
with designing solar cells.

00:07:37.170 --> 00:07:41.110
And trade-offs very
similar to this

00:07:41.110 --> 00:07:44.020
are evaluated on a
daily basis in industry,

00:07:44.020 --> 00:07:46.530
or perhaps not quite as often
as they should be in industry.

00:07:46.530 --> 00:07:47.680
But at some point, they were.

00:07:47.680 --> 00:07:49.430
And the designer of
the manufacturing line

00:07:49.430 --> 00:07:52.620
made those judgment calls.

00:07:52.620 --> 00:07:53.440
OK.

00:07:53.440 --> 00:07:58.640
So again, the pre-analysis,
what is due on Thursday

00:07:58.640 --> 00:08:00.487
is 20% of the grade.

00:08:00.487 --> 00:08:02.820
The dollars per watt peak
metric, at the end of the day,

00:08:02.820 --> 00:08:03.955
is only 10% of the grade.

00:08:03.955 --> 00:08:06.204
It's meant to really serve
as a stimulus, a little bit

00:08:06.204 --> 00:08:08.816
of competition, but not
meant to really harm you

00:08:08.816 --> 00:08:12.132
if you happen to not
achieve a good value there.

00:08:12.132 --> 00:08:14.090
And this is meant to be
an educational mission,

00:08:14.090 --> 00:08:16.920
so the solar cell efficiency
analysis at the end

00:08:16.920 --> 00:08:18.045
is really heavily weighted.

00:08:18.045 --> 00:08:20.253
We'll be walking through
some of the characterization

00:08:20.253 --> 00:08:21.690
tools in the
laboratory so you can

00:08:21.690 --> 00:08:24.040
determine what exactly went
wrong with your devices

00:08:24.040 --> 00:08:25.500
and quantify them.

00:08:25.500 --> 00:08:27.910
And that'll be a
real chance for you

00:08:27.910 --> 00:08:30.155
to get a tutorial of how
solar cells are not only

00:08:30.155 --> 00:08:32.530
made-- you'll be there when
they're actually fabricated--

00:08:32.530 --> 00:08:35.450
but also how they're analyzed
and how they're assessed.

00:08:35.450 --> 00:08:38.758
So it's up to you to really
grab this opportunity.

00:08:38.758 --> 00:08:40.549
Maybe if you're working
in your own devices

00:08:40.549 --> 00:08:42.190
and want to bring
some of them along,

00:08:42.190 --> 00:08:43.330
you're welcome to
do that as well.

00:08:43.330 --> 00:08:45.829
We won't take up the time when
everyone else is in the room,

00:08:45.829 --> 00:08:47.490
but we might stay
longer afterward

00:08:47.490 --> 00:08:50.430
and help you walk through
the analysis as well.

00:08:50.430 --> 00:08:52.816
And what we've done, just
to resituate ourselves,

00:08:52.816 --> 00:08:54.690
we talked about the
silicon feedstock, right?

00:08:54.690 --> 00:08:59.970
So we chatted about
how you go from quartz

00:08:59.970 --> 00:09:02.190
in the ground and the
carbon-baring feedstock

00:09:02.190 --> 00:09:07.940
material to the purified, highly
purified, silicon feedstock

00:09:07.940 --> 00:09:08.440
material.

00:09:08.440 --> 00:09:10.450
This right here is
probably on the order

00:09:10.450 --> 00:09:15.470
of somewhere between
8, 9, or 10 nines pure,

00:09:15.470 --> 00:09:19.540
very, very pure material,
this Siemens-grade polysilicon

00:09:19.540 --> 00:09:20.900
right here in my hand.

00:09:20.900 --> 00:09:23.660
And you've taken a look
at this during last class,

00:09:23.660 --> 00:09:27.010
so you have a sense of what
it is up close and personal.

00:09:27.010 --> 00:09:30.120
Silicon, in fact, has
been so well refined

00:09:30.120 --> 00:09:32.520
that, for a period of
time, NIST, the National

00:09:32.520 --> 00:09:34.260
Institutes of Standards
in Technologies,

00:09:34.260 --> 00:09:37.680
they were thinking about
redefining the unit of mass

00:09:37.680 --> 00:09:40.820
in terms of a silicon
boule, essentially a silicon

00:09:40.820 --> 00:09:43.870
sphere, that would be polished
down to about 4 nanometers

00:09:43.870 --> 00:09:46.090
mean surface roughness
with a very low defect

00:09:46.090 --> 00:09:48.870
density, isotopically
pure silicon

00:09:48.870 --> 00:09:50.920
to serve as a new
standard for mass

00:09:50.920 --> 00:09:53.470
because it could just
be purified so well

00:09:53.470 --> 00:09:56.750
and because their standard
reference units were beginning

00:09:56.750 --> 00:09:58.630
to shift relative
to all the others

00:09:58.630 --> 00:10:00.300
around the world,
the one in Paris

00:10:00.300 --> 00:10:03.080
relative to the ones that were
stored in Washington and Delhi

00:10:03.080 --> 00:10:04.400
and others around the world.

00:10:04.400 --> 00:10:07.230
The values of the mass were
shifting as a function of time

00:10:07.230 --> 00:10:08.980
when they would perform
these round-robin.

00:10:08.980 --> 00:10:11.110
So either the mass
in Paris was changing

00:10:11.110 --> 00:10:12.720
or everybody else was changing.

00:10:12.720 --> 00:10:15.530
This obviously was
unacceptable for an institute

00:10:15.530 --> 00:10:17.010
that was focused on standards.

00:10:17.010 --> 00:10:18.980
And so they decided
to reformulate

00:10:18.980 --> 00:10:20.840
the standard for mass.

00:10:20.840 --> 00:10:23.400
I'm not quite sure where that
project currently stands.

00:10:23.400 --> 00:10:25.210
So if anybody has
further information

00:10:25.210 --> 00:10:28.652
about the NIST unit of mass,
I'd be happy to hear it.

00:10:28.652 --> 00:10:31.110
But that gives you an idea of
how well-purified silicon can

00:10:31.110 --> 00:10:34.340
be and how well-controlled
it can be as well.

00:10:34.340 --> 00:10:36.790
During the integrated
circuit fabrication,

00:10:36.790 --> 00:10:41.210
which uses this ultra
purity silicon to produce

00:10:41.210 --> 00:10:44.680
very nice single crystal wafers
like this one right here,

00:10:44.680 --> 00:10:47.600
the investment per gram of
silicon can be on the order

00:10:47.600 --> 00:10:51.130
a few tens or even low hundreds
of dollars per gram of silicon

00:10:51.130 --> 00:10:52.600
and still turn a profit.

00:10:52.600 --> 00:10:54.910
But in the solar cell,
on the other hand,

00:10:54.910 --> 00:10:56.560
you can invest, at
most, a few tens

00:10:56.560 --> 00:10:58.280
of cents per gram of silicon.

00:10:58.280 --> 00:11:01.040
This is because
the solar cell has

00:11:01.040 --> 00:11:03.370
to compete against bulk power.

00:11:03.370 --> 00:11:04.800
That's its
competition coming out

00:11:04.800 --> 00:11:06.510
of the wall right over there.

00:11:06.510 --> 00:11:09.270
So the solar cell has to be
able to be produced much more

00:11:09.270 --> 00:11:10.000
cheaply.

00:11:10.000 --> 00:11:12.280
And as a result,
typically thinner wafers

00:11:12.280 --> 00:11:15.540
are used and less expensive
starting materials and faster

00:11:15.540 --> 00:11:18.610
growth methods, resulting in
more defect-rich materials.

00:11:18.610 --> 00:11:25.240
So one group decided, gee,
the embedded cost in the wafer

00:11:25.240 --> 00:11:27.480
is just so large,
it's just so large

00:11:27.480 --> 00:11:29.190
that we have to make it thinner.

00:11:29.190 --> 00:11:32.480
And we have to avoid using these
ingots, like this one right

00:11:32.480 --> 00:11:35.260
here, from which
these wafers are sawn.

00:11:35.260 --> 00:11:38.770
So your wafers are sawn
out of the ingot like this,

00:11:38.770 --> 00:11:40.870
like shown.

00:11:40.870 --> 00:11:44.710
During the process, about 50% of
the silicon is lost to sawdust.

00:11:44.710 --> 00:11:47.330
And they said, well, let's
develop a better way.

00:11:47.330 --> 00:11:49.340
Let's extrude the
wafers directly out

00:11:49.340 --> 00:11:52.610
of liquid molten silicon
and make ribbons of silicon

00:11:52.610 --> 00:11:53.440
instead.

00:11:53.440 --> 00:11:55.040
That way, we don't
have the sawdust,

00:11:55.040 --> 00:11:57.581
and we don't have to have this
expensive ingot solidification

00:11:57.581 --> 00:11:58.350
step.

00:11:58.350 --> 00:12:00.230
So ribbon growth
has been explored

00:12:00.230 --> 00:12:03.460
since the 1970s at least.

00:12:03.460 --> 00:12:05.870
And the advantages is that
you have no kerf loss,

00:12:05.870 --> 00:12:09.210
in other words, no sawdust, due
to wire sawing and, hence, more

00:12:09.210 --> 00:12:10.950
efficient silicon utilization.

00:12:10.950 --> 00:12:13.470
Immediately out of the gate,
if your wafer yields are

00:12:13.470 --> 00:12:16.060
comparable, you get
about a factor of 2 gain

00:12:16.060 --> 00:12:19.940
because this wafer right here
is about 170 microns thick.

00:12:19.940 --> 00:12:22.826
And the sawdust is around
170 microns as well.

00:12:22.826 --> 00:12:24.450
So that's about a
factor of 2 if you're

00:12:24.450 --> 00:12:26.190
able to produce a
ribbon of silicon

00:12:26.190 --> 00:12:28.120
directly out of the melt.

00:12:28.120 --> 00:12:30.450
The disadvantage is that
traditionally there's

00:12:30.450 --> 00:12:32.220
been lower material
quality and, hence,

00:12:32.220 --> 00:12:34.520
lower performance
because of the thermal

00:12:34.520 --> 00:12:36.260
stresses during
growth of a very, very

00:12:36.260 --> 00:12:38.680
thin foil or thin fin.

00:12:38.680 --> 00:12:40.310
The thermal stresses
can be larger,

00:12:40.310 --> 00:12:42.600
resulting in plasticity,
resulting in dislocations

00:12:42.600 --> 00:12:45.560
and other defects that can
reduce minority carrier

00:12:45.560 --> 00:12:46.580
lifetime.

00:12:46.580 --> 00:12:49.370
And traditionally, there has
been as well a higher capex.

00:12:49.370 --> 00:12:51.580
And a third disadvantage,
traditionally, in ribbon

00:12:51.580 --> 00:12:53.935
has been that the form factor
or the shape of the wafer

00:12:53.935 --> 00:12:56.390
has just been different
than the ingot material.

00:12:56.390 --> 00:12:57.919
Why is that important?

00:12:57.919 --> 00:13:00.210
Well, if you're trying to
displace the dominant design,

00:13:00.210 --> 00:13:05.140
the wafer, you would do well to
make your wafer the same size

00:13:05.140 --> 00:13:06.880
and shape as the
dominant design.

00:13:06.880 --> 00:13:07.730
Why is that?

00:13:07.730 --> 00:13:10.570
Well, if you want to make a cell
out of it or solar cell device,

00:13:10.570 --> 00:13:12.070
you'd want to make
sure that you can

00:13:12.070 --> 00:13:14.230
take advantage of the same
manufacturing equipment.

00:13:14.230 --> 00:13:15.688
And that's just a
plug-in-and-play,

00:13:15.688 --> 00:13:16.750
drop-in replacement.

00:13:16.750 --> 00:13:19.606
If you require customization
of the downstream components

00:13:19.606 --> 00:13:21.230
on the cell in the
module level, you'll

00:13:21.230 --> 00:13:24.100
wind up having to invest more
money in those processes, which

00:13:24.100 --> 00:13:26.510
might counteract the
advantage that you get out

00:13:26.510 --> 00:13:28.350
of using less silicon.

00:13:28.350 --> 00:13:29.272
Yes, Ashley?

00:13:29.272 --> 00:13:30.230
AUDIENCE: What's capex?

00:13:30.230 --> 00:13:30.380
Is it--

00:13:30.380 --> 00:13:31.255
PROFESSOR: Oh, capex.

00:13:31.255 --> 00:13:34.410
Capex stands for capital
expenditure, capital equipment

00:13:34.410 --> 00:13:35.270
expenditure.

00:13:35.270 --> 00:13:38.010
And that relates to the
cost of the equipment that

00:13:38.010 --> 00:13:42.400
is typically-- well, in the
business world, typically

00:13:42.400 --> 00:13:45.060
one undergoes what's called an
accelerated depreciation where

00:13:45.060 --> 00:13:47.640
you amortize the cost of the
equipment over five years

00:13:47.640 --> 00:13:50.230
but then assume that it runs
over a longer period, maybe

00:13:50.230 --> 00:13:55.720
7, 10 years or so
giving you profit back.

00:13:55.720 --> 00:13:59.150
So in layman's terms,
what this means is capex

00:13:59.150 --> 00:14:01.480
is the equipment
cost, in other words.

00:14:01.480 --> 00:14:03.980
And then you just take
the cost of the equipment

00:14:03.980 --> 00:14:05.247
and parse it out.

00:14:05.247 --> 00:14:07.330
For each wafer you produce,
you allocate a portion

00:14:07.330 --> 00:14:09.280
of equipment cost to that.

00:14:09.280 --> 00:14:11.750
So let's take a little
walk through history

00:14:11.750 --> 00:14:14.100
and go back to some
of the earliest

00:14:14.100 --> 00:14:17.270
methods of ribbon growth.

00:14:17.270 --> 00:14:20.620
So one of the earliest
forms of ribbon growth

00:14:20.620 --> 00:14:24.050
was the so-called edge
supported ribbon, also known

00:14:24.050 --> 00:14:25.270
as string ribbon.

00:14:25.270 --> 00:14:30.100
And there were developments
of this general technology

00:14:30.100 --> 00:14:31.420
in different places.

00:14:31.420 --> 00:14:34.720
Ely Sachs, former
professor here at MIT, now

00:14:34.720 --> 00:14:37.800
founder and CTO of 1366
Technologies just up

00:14:37.800 --> 00:14:41.510
the road in Lexington, developed
the string ribbon material

00:14:41.510 --> 00:14:45.830
here at MIT in the
early 1980s, late 1970s.

00:14:45.830 --> 00:14:48.350
And the general idea
was to use two filaments

00:14:48.350 --> 00:14:51.020
like so that would be
passed through a crucible.

00:14:51.020 --> 00:14:52.690
And then the silicon
would flow in

00:14:52.690 --> 00:14:57.060
between those two filaments
much like soapy water

00:14:57.060 --> 00:15:00.370
flows between the little
circle when you blow bubbles.

00:15:00.370 --> 00:15:03.450
So a meniscus would form here
and then eventually solidify

00:15:03.450 --> 00:15:05.140
into a solid piece
of silicon, and you'd

00:15:05.140 --> 00:15:08.709
have edge-supported ribbon,
otherwise known as string

00:15:08.709 --> 00:15:10.750
ribbon because you're
using the strings to define

00:15:10.750 --> 00:15:11.940
the edge of the ribbon.

00:15:11.940 --> 00:15:15.270
So I have a wafer here, an
example of a wafer here,

00:15:15.270 --> 00:15:18.480
a string ribbon sample.

00:15:18.480 --> 00:15:19.440
Oh, here it is.

00:15:19.440 --> 00:15:22.320
It's hiding from me.

00:15:22.320 --> 00:15:27.026
So this is an example of
one of those materials.

00:15:27.026 --> 00:15:28.230
Here we go.

00:15:28.230 --> 00:15:30.370
And like usual, it's good
to handle these wafers

00:15:30.370 --> 00:15:34.950
with some care almost
like a photograph.

00:15:34.950 --> 00:15:37.430
So here's an example
of a string ribbon

00:15:37.430 --> 00:15:41.510
wafer, one particular wafer
that was laser cut out

00:15:41.510 --> 00:15:42.700
of a growing ribbon.

00:15:42.700 --> 00:15:45.910
As you can see, this larger
ribbon right here-- these

00:15:45.910 --> 00:15:48.410
can grow up to be
a few meters long.

00:15:48.410 --> 00:15:49.670
They're rather long.

00:15:49.670 --> 00:15:53.220
You can pick them up if you
have gloves on your hands.

00:15:53.220 --> 00:15:55.890
And they're quite
flexible at that length.

00:15:55.890 --> 00:15:58.030
You could actually
even bend them

00:15:58.030 --> 00:16:01.320
with a radius of curvature
of about a couple of meters.

00:16:01.320 --> 00:16:04.880
So the reason you wear
gloves, obviously,

00:16:04.880 --> 00:16:07.904
is to prevent your fingers
some soiling the wafer.

00:16:07.904 --> 00:16:10.070
We talked about sodium
contamination and other forms

00:16:10.070 --> 00:16:11.050
of contamination.

00:16:11.050 --> 00:16:13.220
Silicon is nontoxic,
so it won't affect you.

00:16:13.220 --> 00:16:14.890
It's really you
affecting the wafer,

00:16:14.890 --> 00:16:17.000
much like putting
fingerprints all over

00:16:17.000 --> 00:16:19.680
a nice, clean photograph.

00:16:19.680 --> 00:16:22.840
So there were
similar technologies

00:16:22.840 --> 00:16:28.150
developed by Ted [INAUDIBLE]
at NREL out in Colorado.

00:16:28.150 --> 00:16:30.010
But the general idea
is shown right here.

00:16:30.010 --> 00:16:34.720
Now some of the earliest
edge-supported ribbon samples

00:16:34.720 --> 00:16:36.384
were developed back in 1970s.

00:16:36.384 --> 00:16:37.800
It really took a
while before they

00:16:37.800 --> 00:16:39.480
were commercialized in full.

00:16:39.480 --> 00:16:42.390
And that was done through
Evergreen Solar, which

00:16:42.390 --> 00:16:46.240
was founded in 1994 by Jack
Hanoka, Rich Chlebowski,

00:16:46.240 --> 00:16:49.590
and-- oh, goodness--
Mark Farber.

00:16:49.590 --> 00:16:52.790
So the three of them a
co-founded Evergreen Solar.

00:16:52.790 --> 00:16:55.530
And they developed the
string ribbon growth process

00:16:55.530 --> 00:16:56.640
shown right over here.

00:16:56.640 --> 00:17:00.350
Eventually two ribbons face
to face, and now four ribbons

00:17:00.350 --> 00:17:01.699
side by side.

00:17:01.699 --> 00:17:03.490
So this was called the
Gemini because there

00:17:03.490 --> 00:17:05.048
were two ribbons face to face.

00:17:05.048 --> 00:17:06.589
And then eventually,
the quad process

00:17:06.589 --> 00:17:09.180
were four ribbons edge to edge.

00:17:09.180 --> 00:17:11.510
And you can see the
conventional ingot

00:17:11.510 --> 00:17:12.810
multi-crystalline silicon.

00:17:12.810 --> 00:17:15.664
Here, the different
steps forming the ingot,

00:17:15.664 --> 00:17:18.020
eventually slicing, and so
forth and the string ribbon

00:17:18.020 --> 00:17:22.920
process here being much
simplified in correspondence.

00:17:22.920 --> 00:17:24.660
So not only was the
process simpler,

00:17:24.660 --> 00:17:26.910
but you'd use about
half as much silicon.

00:17:26.910 --> 00:17:30.810
And here's Rick Wallace,
the inventor and developer

00:17:30.810 --> 00:17:33.110
of the Gemini process,
up there showing

00:17:33.110 --> 00:17:37.300
one of these longer meter-length
ribbons with some flexibility.

00:17:37.300 --> 00:17:42.830
So the company had
a joint venture

00:17:42.830 --> 00:17:47.760
with REC and Q-Cells,
Norwegian and German companies

00:17:47.760 --> 00:17:50.450
respectively, to form
a factory in Germany.

00:17:50.450 --> 00:17:52.229
REC would supply the
silicon feedstock,

00:17:52.229 --> 00:17:53.770
Evergreen the growth
technology here,

00:17:53.770 --> 00:17:57.400
and Q-Cell some of the
cell fabrication expertise.

00:17:57.400 --> 00:18:00.592
And very recently,
Evergreen Solar

00:18:00.592 --> 00:18:02.300
encountered some
financial difficulties--

00:18:02.300 --> 00:18:04.716
we'll get into that during the
third section of the course

00:18:04.716 --> 00:18:08.000
when we talk about
cross-cutting themes-- and is

00:18:08.000 --> 00:18:10.160
in the process of
filing for bankruptcy.

00:18:10.160 --> 00:18:14.010
So this process-- so Sovello is
continuing as its own company,

00:18:14.010 --> 00:18:17.000
but the Evergreen plant here in
Massachusetts in Marlborough,

00:18:17.000 --> 00:18:20.340
about an hour west of here,
has effectively shut down.

00:18:20.340 --> 00:18:24.700
So that was the trajectory
of this particular technology

00:18:24.700 --> 00:18:29.460
through commercialization
and ultimately not making it.

00:18:29.460 --> 00:18:31.060
If you would like,
my personal opinion

00:18:31.060 --> 00:18:36.100
about why Evergreen
never quite took off,

00:18:36.100 --> 00:18:37.710
yes, there are some
technical factors,

00:18:37.710 --> 00:18:41.030
but as well it failed to
grow fast enough to keep up

00:18:41.030 --> 00:18:42.880
with the rest of the
industry and scale

00:18:42.880 --> 00:18:44.242
with the rest of the industry.

00:18:44.242 --> 00:18:45.700
And part of that
can be traced back

00:18:45.700 --> 00:18:48.210
to the mid 2000s when
silicon was scarce,

00:18:48.210 --> 00:18:51.450
the inability to source
the feedstock material.

00:18:51.450 --> 00:18:52.400
Yeah?

00:18:52.400 --> 00:18:53.350
AUDIENCE: Excuse me.

00:18:53.350 --> 00:18:55.534
Can you back one slide?

00:18:55.534 --> 00:18:56.200
PROFESSOR: Sure.

00:18:58.897 --> 00:18:59.605
It takes a while.

00:18:59.605 --> 00:19:00.624
It's a big file.

00:19:00.624 --> 00:19:01.124
OK.

00:19:01.124 --> 00:19:06.070
AUDIENCE: How do you seal the
space between the filaments

00:19:06.070 --> 00:19:07.606
and the bottom of the crucible?

00:19:07.606 --> 00:19:08.855
PROFESSOR: Right there, right?

00:19:08.855 --> 00:19:09.140
AUDIENCE: Yeah.

00:19:09.140 --> 00:19:11.556
PROFESSOR: Since this is your
graphite crucible right here

00:19:11.556 --> 00:19:13.400
and these are your
filaments popping up

00:19:13.400 --> 00:19:17.090
through the graphite, the beauty
is you don't have to seal that.

00:19:17.090 --> 00:19:20.960
The surface tension of silicon
is greater than that of water.

00:19:20.960 --> 00:19:24.320
So if you've ever filled up
water to the top of a glass

00:19:24.320 --> 00:19:26.290
and seen that
meniscus that forms,

00:19:26.290 --> 00:19:28.817
the silicon meniscus would
be even higher than that.

00:19:28.817 --> 00:19:29.900
AUDIENCE: Oh, that's cool.

00:19:29.900 --> 00:19:30.566
PROFESSOR: Yeah.

00:19:30.566 --> 00:19:31.798
It's pretty nifty.

00:19:31.798 --> 00:19:33.988
AUDIENCE: So I'm imagining
just like molten metal.

00:19:33.988 --> 00:19:34.700
You don't want that
spilling out the bottom.

00:19:34.700 --> 00:19:35.283
PROFESSOR: No.

00:19:35.283 --> 00:19:36.570
AUDIENCE: That's really cool.

00:19:36.570 --> 00:19:37.070
OK.

00:19:37.070 --> 00:19:37.570
Cool.

00:19:37.570 --> 00:19:38.966
PROFESSOR: Yeah.

00:19:38.966 --> 00:19:40.862
AUDIENCE: Are the
ribbons a single crystal?

00:19:40.862 --> 00:19:42.446
Or are there grain
boundaries in them?

00:19:42.446 --> 00:19:43.111
PROFESSOR: Yeah.

00:19:43.111 --> 00:19:45.160
So let me show you the
actual ribbon right here,

00:19:45.160 --> 00:19:47.520
and you can inspect
it first hand.

00:19:47.520 --> 00:19:50.629
These do indeed have
grain boundaries.

00:19:50.629 --> 00:19:52.920
So what I'll do is I'll place
the ribbon inside of here

00:19:52.920 --> 00:19:54.930
for ease of carrying around.

00:19:54.930 --> 00:19:56.640
If you'd like to take
it out, feel free.

00:19:56.640 --> 00:19:58.056
They're more where
this came from.

00:19:58.056 --> 00:20:00.390
So in case there was a little
accident along the way,

00:20:00.390 --> 00:20:01.860
don't feel too bad.

00:20:01.860 --> 00:20:04.400
The growth of an ingot
is about one to two days,

00:20:04.400 --> 00:20:06.630
but you get thousands
of wafers out.

00:20:06.630 --> 00:20:11.720
The growth of a wafer itself--
if the growth rate was around,

00:20:11.720 --> 00:20:14.670
say, let's pick a number
somewhere between 2

00:20:14.670 --> 00:20:17.309
and 5 centimeters
per minute, then

00:20:17.309 --> 00:20:18.850
it would take--
let's see, with this,

00:20:18.850 --> 00:20:20.770
you have a 15
centimeter wafer-- it

00:20:20.770 --> 00:20:23.210
would take somewhere on
the order of four minutes

00:20:23.210 --> 00:20:24.550
to grow wafer.

00:20:24.550 --> 00:20:29.680
And you'd have a faster
growth of single wafers

00:20:29.680 --> 00:20:32.965
from the ribbon process, of
course, lower throughput.

00:20:32.965 --> 00:20:35.090
The silicon utilization of
the wafer growth process

00:20:35.090 --> 00:20:40.190
was a lot higher than
that of the ingot growth.

00:20:40.190 --> 00:20:42.430
Some smart people
realized along the way

00:20:42.430 --> 00:20:46.070
that you could grow
these ribbons vertically,

00:20:46.070 --> 00:20:48.730
but you encountered
the following problem.

00:20:48.730 --> 00:20:52.640
During the growth
of-- here you go.

00:20:52.640 --> 00:20:54.730
During the growth of
a vertical ribbons,

00:20:54.730 --> 00:20:58.660
if this was the ribbon
growing vertically--

00:20:58.660 --> 00:21:00.160
it should be straight.

00:21:00.160 --> 00:21:00.935
Apologies.

00:21:00.935 --> 00:21:02.680
There we go.

00:21:02.680 --> 00:21:05.980
Let's make sure we're
good engineers here.

00:21:05.980 --> 00:21:11.180
And so this is meant to
represent a growing ribbon.

00:21:11.180 --> 00:21:15.580
This is the liquid, and this is
the solid silicon right here.

00:21:15.580 --> 00:21:20.544
The growth velocity would be
in this direction right here.

00:21:20.544 --> 00:21:23.210
So you're growing the ribbon out
of the melt. This is your melt.

00:21:23.210 --> 00:21:24.370
This is the ribbon
that's growing up.

00:21:24.370 --> 00:21:26.328
You're looking at the
cross section right here,

00:21:26.328 --> 00:21:28.430
so looking at the
ribbon edge on.

00:21:28.430 --> 00:21:30.090
So you're pulling it
in this direction,

00:21:30.090 --> 00:21:32.090
so the growth velocity is here.

00:21:32.090 --> 00:21:37.530
And the direction of
latent heat of fusion

00:21:37.530 --> 00:21:41.170
extraction-- so you have liquid
silicon solidifying here.

00:21:41.170 --> 00:21:43.990
During the solidification
process, there's heat released.

00:21:43.990 --> 00:21:45.680
And that heat has
to be conducted up

00:21:45.680 --> 00:21:48.320
the solid and then radiated
outward from the fin,

00:21:48.320 --> 00:21:50.240
from this thin ribbon.

00:21:50.240 --> 00:21:53.920
So the direction
of heat extraction

00:21:53.920 --> 00:21:57.240
is also parallel to the
direction of growth.

00:21:57.240 --> 00:21:59.970
What that means is
the growth velocity

00:21:59.970 --> 00:22:02.610
will be limited by the speed
at which you can extract heat

00:22:02.610 --> 00:22:04.850
up the ribbon and
then radiated outward.

00:22:04.850 --> 00:22:06.410
So there are many
ideas tossed around

00:22:06.410 --> 00:22:09.760
about potentially
growing in media

00:22:09.760 --> 00:22:13.140
that are able to extract
heat [INAUDIBLE] transport.

00:22:13.140 --> 00:22:14.550
You can use your imagination.

00:22:14.550 --> 00:22:17.350
But ultimately, growth
continues in air,

00:22:17.350 --> 00:22:20.950
and you're limited to, at
most, around 5 centimeters

00:22:20.950 --> 00:22:24.500
per minute growth velocity
because of the extraction

00:22:24.500 --> 00:22:25.530
of latent heat.

00:22:25.530 --> 00:22:27.320
If you try to grow
faster than that,

00:22:27.320 --> 00:22:31.240
you'll eventually just pull
the solid off of the liquid.

00:22:31.240 --> 00:22:36.070
It'll dissociate much like
pulling an ice cube off

00:22:36.070 --> 00:22:37.999
of a top of a glass of water.

00:22:37.999 --> 00:22:40.290
Surface tension won't be able
to hold the two together.

00:22:40.290 --> 00:22:44.040
So you have here a conundrum.

00:22:44.040 --> 00:22:45.500
How do you grow faster?

00:22:45.500 --> 00:22:47.160
If you want to
increase the throughput

00:22:47.160 --> 00:22:50.160
and instead of spending
minutes to grow wafer,

00:22:50.160 --> 00:22:53.130
you'd like to grow a wafer per
second, how do you do that?

00:22:53.130 --> 00:22:55.940
Well, one group of folks thought
about this a bit and said,

00:22:55.940 --> 00:22:58.360
well, what if we do this?

00:22:58.360 --> 00:23:01.980
If we take our growth velocity
and in some way, shape, or form

00:23:01.980 --> 00:23:03.790
now our growth
velocity is going to be

00:23:03.790 --> 00:23:06.300
perpendicular to the
direction of heat extraction,

00:23:06.300 --> 00:23:08.010
what would that
geometry look like?

00:23:08.010 --> 00:23:09.510
And they came up
with something that

00:23:09.510 --> 00:23:11.150
looked a bit like
this right here,

00:23:11.150 --> 00:23:13.210
a horizontal growth mechanism.

00:23:13.210 --> 00:23:16.280
So you see the [INAUDIBLE]
interface is now at an angle.

00:23:16.280 --> 00:23:19.680
It's almost vertical at
this point, a slight angle.

00:23:19.680 --> 00:23:23.820
And the pull velocity is
almost perpendicular to it.

00:23:23.820 --> 00:23:25.740
So now, you're able,
in theory at least,

00:23:25.740 --> 00:23:28.280
to grow much, much faster.

00:23:28.280 --> 00:23:29.770
This was a schematic
of the ribbon

00:23:29.770 --> 00:23:31.440
growth on silicon process.

00:23:31.440 --> 00:23:33.600
There's also another
company called AstroPower

00:23:33.600 --> 00:23:34.990
that developed silicon film.

00:23:34.990 --> 00:23:38.100
It was later purchased
by General Electric.

00:23:38.100 --> 00:23:42.430
So these technologies were
developed with the intent

00:23:42.430 --> 00:23:43.610
of pulling very, very fast.

00:23:43.610 --> 00:23:45.930
And indeed, you can
literally extrude the silicon

00:23:45.930 --> 00:23:48.720
at around 49 meters per second.

00:23:48.720 --> 00:23:53.160
But the problem about
this is that you wind up

00:23:53.160 --> 00:23:56.360
with very small grains and very
poor crystalline quality when

00:23:56.360 --> 00:23:58.420
you try to grow at the speeds.

00:23:58.420 --> 00:24:00.950
And so it winds up being a
metallurgical problem of how

00:24:00.950 --> 00:24:04.350
do you ensure the proper
grain size when you're growing

00:24:04.350 --> 00:24:06.140
using these technologies?

00:24:06.140 --> 00:24:08.660
So there is some
work in that regard,

00:24:08.660 --> 00:24:13.330
but never really took off
in commercial production.

00:24:13.330 --> 00:24:14.740
Yeah?

00:24:14.740 --> 00:24:17.485
AUDIENCE: So does
pulling at a lower

00:24:17.485 --> 00:24:20.160
speed with the horizontal
ribbon increase

00:24:20.160 --> 00:24:22.936
your quality by increasing
your grain size?

00:24:22.936 --> 00:24:23.810
Or is it not really--

00:24:23.810 --> 00:24:25.430
PROFESSOR: If you're able
to control the nucleation

00:24:25.430 --> 00:24:28.140
and growth process at the
very beginning, theoretically,

00:24:28.140 --> 00:24:29.399
that could be possible.

00:24:29.399 --> 00:24:29.940
AUDIENCE: OK.

00:24:32.277 --> 00:24:33.360
PROFESSOR: Yeah, question?

00:24:33.360 --> 00:24:36.080
AUDIENCE: You had mentioned form
factor for these wafers before.

00:24:36.080 --> 00:24:36.490
PROFESSOR: Yeah?

00:24:36.490 --> 00:24:38.531
AUDIENCE: So is there like
a standard form factor

00:24:38.531 --> 00:24:41.014
for solar cell manufacturing?

00:24:41.014 --> 00:24:41.860
PROFESSOR: Yep.

00:24:41.860 --> 00:24:45.350
So the standard form factor
today is akin to this one right

00:24:45.350 --> 00:24:45.850
here.

00:24:45.850 --> 00:24:50.510
It's about a 15.6
by 15.6 centimeter

00:24:50.510 --> 00:24:53.700
squared lateral dimension
form factor for the wafer.

00:24:53.700 --> 00:24:56.470
And I can pass this
one around as well.

00:24:56.470 --> 00:24:58.780
This right here is what's
called a "pseudo-square."

00:24:58.780 --> 00:25:01.500
You can see the edges
are kind of rounded off.

00:25:01.500 --> 00:25:05.301
And that's because it came
from a CZ wafer like this one.

00:25:05.301 --> 00:25:06.550
It was just chopped out of it.

00:25:06.550 --> 00:25:08.520
Let me see if these
two are coincidence.

00:25:08.520 --> 00:25:09.740
It would be a-- oh, yeah.

00:25:09.740 --> 00:25:12.040
Look at that.

00:25:12.040 --> 00:25:16.430
So you can see where the
solar cell actually came from.

00:25:16.430 --> 00:25:19.170
So that's the standard
diameter of a, say,

00:25:19.170 --> 00:25:22.400
linear dimension, usually
rectilinear shape, a square.

00:25:22.400 --> 00:25:25.110
And the multi-crystalline
silicon ingot material

00:25:25.110 --> 00:25:27.000
are typically of
this size as well.

00:25:27.000 --> 00:25:29.300
And you can already see
that these wafers that I

00:25:29.300 --> 00:25:31.960
have up here are a bit small.

00:25:31.960 --> 00:25:33.630
These were the previous
generation size.

00:25:33.630 --> 00:25:37.454
I believe these are 12.5
by 12.5 centimeter squared.

00:25:37.454 --> 00:25:39.620
Most laboratory devices
that you and your colleagues

00:25:39.620 --> 00:25:41.203
will manufacture are
on the order of 1

00:25:41.203 --> 00:25:43.843
by 1 centimeter or
smaller because-- well,

00:25:43.843 --> 00:25:45.490
because of a variety of factors.

00:25:45.490 --> 00:25:48.810
One is the transparent
conducting oxide

00:25:48.810 --> 00:25:50.490
as we saw in our
homework problem.

00:25:50.490 --> 00:25:53.340
We're limited in how big we can
make the device by the sheet

00:25:53.340 --> 00:25:55.761
resistance of that
transparent conducting oxide.

00:25:55.761 --> 00:25:57.510
Another problem that
we typically run into

00:25:57.510 --> 00:25:59.093
is just that we're
not able to deposit

00:25:59.093 --> 00:26:00.320
uniformly over a large area.

00:26:00.320 --> 00:26:02.770
We don't have a deposition
equipment for it in our labs.

00:26:02.770 --> 00:26:05.310
We're there trying to
optimize a new material.

00:26:05.310 --> 00:26:10.437
We don't necessarily worry about
making module-sized devices out

00:26:10.437 --> 00:26:10.936
of it.

00:26:10.936 --> 00:26:11.842
Yeah, question?

00:26:11.842 --> 00:26:13.508
AUDIENCE: Is there a
reason why the form

00:26:13.508 --> 00:26:18.814
factor is different than that
used for device manufacturing?

00:26:18.814 --> 00:26:19.480
PROFESSOR: Sure.

00:26:19.480 --> 00:26:22.714
AUDIENCE: Like [INAUDIBLE]
uses circular wafers.

00:26:22.714 --> 00:26:23.380
PROFESSOR: Yeah.

00:26:23.380 --> 00:26:26.560
So if we were to imagine a
bunch of circular wafers inside

00:26:26.560 --> 00:26:29.560
of this module over here, you
can imagine the circular wafers

00:26:29.560 --> 00:26:30.461
side by side.

00:26:30.461 --> 00:26:32.460
That was how they were
done at once upon a time.

00:26:32.460 --> 00:26:33.835
Obviously you
didn't have 8-inch.

00:26:33.835 --> 00:26:35.100
It was much smaller.

00:26:35.100 --> 00:26:37.100
Or a 6- or 8-inch.

00:26:37.100 --> 00:26:38.690
This would be a 6-inch wafer.

00:26:38.690 --> 00:26:40.697
But the wafers were
a little smaller,

00:26:40.697 --> 00:26:42.530
but you still have
circular wafers and a lot

00:26:42.530 --> 00:26:43.610
of dead space in between.

00:26:43.610 --> 00:26:45.610
So as you can see, because
of the rounded edges,

00:26:45.610 --> 00:26:47.500
the packing density is very low.

00:26:47.500 --> 00:26:49.100
The equivalent would
be, say, oranges

00:26:49.100 --> 00:26:50.850
at a market where they're
all stacked on top of another

00:26:50.850 --> 00:26:52.830
and you have all this
dead space in between.

00:26:52.830 --> 00:26:56.820
And so the idea was to optimize
between the cost of the silicon

00:26:56.820 --> 00:26:59.350
and the cost of the encapsulant
materials by shaving away

00:26:59.350 --> 00:27:02.810
a little bit of the
silicon and losing that--

00:27:02.810 --> 00:27:05.150
and perhaps recycling
it, to be honest--

00:27:05.150 --> 00:27:07.012
and the encapsulant
materials, where

00:27:07.012 --> 00:27:08.970
you have this dead space
in between the wafers,

00:27:08.970 --> 00:27:10.620
a small amount of
it, where you have

00:27:10.620 --> 00:27:13.090
glass and encapsulant but
no active device underneath.

00:27:17.020 --> 00:27:19.345
Another interesting
development, as you can see just

00:27:19.345 --> 00:27:20.970
from the device point
of view-- so this

00:27:20.970 --> 00:27:23.580
would be an Evergreen string
ribbon wafer right here, as you

00:27:23.580 --> 00:27:24.310
can see.

00:27:24.310 --> 00:27:27.530
And this right here,
a larger area device.

00:27:27.530 --> 00:27:30.900
Does anybody notice a
difference besides the shape?

00:27:30.900 --> 00:27:32.620
In particular, I lead
you to the busbars.

00:27:32.620 --> 00:27:35.760
How many of those
thick, vertical lines

00:27:35.760 --> 00:27:37.460
appear down the wafer?

00:27:37.460 --> 00:27:39.180
AUDIENCE: [INAUDIBLE]

00:27:39.180 --> 00:27:41.375
PROFESSOR: This has two,
and this has three, right?

00:27:41.375 --> 00:27:42.000
AUDIENCE: Yeah.

00:27:42.000 --> 00:27:43.833
PROFESSOR: So the
busbars-- the optimization

00:27:43.833 --> 00:27:46.360
of these busbars-- this one
has three-- that's really

00:27:46.360 --> 00:27:47.776
to minimize series resistance.

00:27:47.776 --> 00:27:49.400
Because now that I
have a larger wafer,

00:27:49.400 --> 00:27:52.147
you have so much current flowing
through it, being generated,

00:27:52.147 --> 00:27:54.480
that the series resistance
through those very thin metal

00:27:54.480 --> 00:27:57.990
wires would end up resulting
in large power losses,

00:27:57.990 --> 00:28:01.262
essentially heat
instead of electricity.

00:28:01.262 --> 00:28:02.720
And so they added
the third busbar,

00:28:02.720 --> 00:28:05.800
even though it increased the
shading, to reduce the series

00:28:05.800 --> 00:28:07.550
resistance losses.

00:28:07.550 --> 00:28:10.470
So you can see these
optimization problems are

00:28:10.470 --> 00:28:14.480
used quite frequently in solar.

00:28:14.480 --> 00:28:15.985
Let me go back one step.

00:28:15.985 --> 00:28:17.360
There was an
interesting question

00:28:17.360 --> 00:28:20.030
about could we grow
single crystals using

00:28:20.030 --> 00:28:22.900
the vertical ribbon growth.

00:28:22.900 --> 00:28:24.440
This is a technology.

00:28:24.440 --> 00:28:26.460
And I don't know if
there are actually

00:28:26.460 --> 00:28:29.120
any of these, many of these
samples left in the world.

00:28:29.120 --> 00:28:30.400
They're quite rare.

00:28:30.400 --> 00:28:31.950
So I do ask if you
want to come up

00:28:31.950 --> 00:28:33.640
here, take some care with it.

00:28:33.640 --> 00:28:36.447
This is a dendritic web sample.

00:28:36.447 --> 00:28:38.780
So this technology went out
of commercial manufacturing,

00:28:38.780 --> 00:28:41.526
I believe, in 2005.

00:28:41.526 --> 00:28:42.150
Must have been.

00:28:42.150 --> 00:28:43.820
Or 2004.

00:28:43.820 --> 00:28:45.620
It was developed
by Westinghouse,

00:28:45.620 --> 00:28:49.260
which is used to be one of the
powerhouses in solar located

00:28:49.260 --> 00:28:51.210
in Pittsburgh, Pennsylvania.

00:28:51.210 --> 00:28:56.490
They had a very
active solar activity.

00:28:56.490 --> 00:28:59.440
It was a kind of a crucible
out of which many solar experts

00:28:59.440 --> 00:29:01.520
then went into diaspora
around the United States

00:29:01.520 --> 00:29:03.940
and set up their own
activities elsewhere.

00:29:03.940 --> 00:29:06.374
And one of the technologies
that they developed

00:29:06.374 --> 00:29:08.790
was a single crystalline ribbon
technology like this right

00:29:08.790 --> 00:29:09.290
here.

00:29:09.290 --> 00:29:12.490
And if you look very closely,
it really is a single crystal.

00:29:12.490 --> 00:29:14.690
The growth methods
to make this, though,

00:29:14.690 --> 00:29:16.730
was extremely intricate.

00:29:16.730 --> 00:29:19.580
It involved, among other things,
control of the temperature,

00:29:19.580 --> 00:29:22.940
of the liquid silicon to within
1/100 of a degree Celsius

00:29:22.940 --> 00:29:27.230
at melting temperature, which is
an extreme feat of engineering.

00:29:27.230 --> 00:29:29.480
The uptime of these
pieces of equipment,

00:29:29.480 --> 00:29:31.760
meaning the growth
time, was around 50%.

00:29:31.760 --> 00:29:33.980
And the other 50% of
the time, the operators

00:29:33.980 --> 00:29:35.370
were trying to make it work.

00:29:35.370 --> 00:29:38.079
So it grew very,
very thin material.

00:29:38.079 --> 00:29:39.870
It wasn't able to scale
to the form factors

00:29:39.870 --> 00:29:41.170
that we see nowadays.

00:29:41.170 --> 00:29:42.810
The throughput was quite low.

00:29:42.810 --> 00:29:43.650
The cost was high.

00:29:43.650 --> 00:29:46.980
And so it didn't quite make
it, but from an engineering

00:29:46.980 --> 00:29:49.320
point of view, it was a
marvel in terms of what

00:29:49.320 --> 00:29:51.560
they were able to accomplish.

00:29:51.560 --> 00:29:54.960
So history of crystalline
silicon development

00:29:54.960 --> 00:29:57.440
is riddled with these
technologies that didn't quite

00:29:57.440 --> 00:29:59.850
make it with these
materials that

00:29:59.850 --> 00:30:02.050
were extremely inventive,
extremely ingenuitive.

00:30:02.050 --> 00:30:04.730
But at the end of the day,
the dollars per watt peak

00:30:04.730 --> 00:30:07.290
just couldn't continue to
justify their existence.

00:30:07.290 --> 00:30:09.130
And there were a number of
factors that could contribute

00:30:09.130 --> 00:30:10.046
to making that happen.

00:30:13.550 --> 00:30:18.260
So in terms of wafer
fabrication in general--

00:30:18.260 --> 00:30:21.390
this includes both the
wafers out of ingot materials

00:30:21.390 --> 00:30:24.050
but also ribbons--
where do I personally

00:30:24.050 --> 00:30:26.050
see this field going?

00:30:26.050 --> 00:30:29.610
These are some notes.

00:30:29.610 --> 00:30:32.640
So in terms of cost,
the cost per watt peak

00:30:32.640 --> 00:30:35.150
can be reduced by using
cheaper starting materials.

00:30:35.150 --> 00:30:39.080
That means instead of using this
expensive Siemens poly, perhaps

00:30:39.080 --> 00:30:42.670
an upgraded metallurgical
silicon process.

00:30:42.670 --> 00:30:45.120
Growing or sawing
thinner wafers.

00:30:45.120 --> 00:30:47.010
Growing, for example,
on a ribbon technique.

00:30:47.010 --> 00:30:51.770
Sawing, maybe making the saws
themselves thinner but more

00:30:51.770 --> 00:30:53.570
robust so that they
don't snap as they're

00:30:53.570 --> 00:30:55.970
pulling through the material
at about 5 meters per second

00:30:55.970 --> 00:30:59.040
in that slurry with the silicon
carbide or diamond grit.

00:30:59.040 --> 00:31:02.240
Very challenging
engineering as well.

00:31:02.240 --> 00:31:03.970
This second bullet
point right there

00:31:03.970 --> 00:31:07.820
can be encapsulated in a larger
team called improved silicon

00:31:07.820 --> 00:31:09.520
materials utilization.

00:31:09.520 --> 00:31:11.020
In other words, the
grams of silicon

00:31:11.020 --> 00:31:14.339
that you use to produce a
watt peak of a solar cell.

00:31:14.339 --> 00:31:15.880
So improving that
number right there.

00:31:18.400 --> 00:31:20.470
Increasing furnace
throughput-- that means

00:31:20.470 --> 00:31:22.837
increasing ingot size,
growth, speed, and so forth.

00:31:22.837 --> 00:31:24.420
There are many people
right now trying

00:31:24.420 --> 00:31:25.794
to grow these
ingot right here up

00:31:25.794 --> 00:31:29.090
to a ton, one metric
ton, so 1,000 kilograms.

00:31:29.090 --> 00:31:32.010
That would mean for
the full-sized wafers,

00:31:32.010 --> 00:31:37.730
you would have something on
the order of 6 by 6 bricks.

00:31:37.730 --> 00:31:39.430
It's pretty large, a
pretty large ingot.

00:31:39.430 --> 00:31:41.890
Maybe even 7 by 7.

00:31:41.890 --> 00:31:43.340
And improving the
material quality

00:31:43.340 --> 00:31:45.130
so that you can
improve efficiency,

00:31:45.130 --> 00:31:49.240
efficiency being a huge leverage
over the entire cost structure.

00:31:49.240 --> 00:31:51.960
Because if your solar cell is
able to produce more power,

00:31:51.960 --> 00:31:54.460
that means that you use less
encapsulant, and less material,

00:31:54.460 --> 00:31:56.960
and so forth per unit power
produced, and even less

00:31:56.960 --> 00:31:59.640
labor to install it and
less racking and framing

00:31:59.640 --> 00:32:01.310
materials downstream.

00:32:01.310 --> 00:32:04.654
The scaling issues, so
polysilicon production

00:32:04.654 --> 00:32:06.320
is currently-- well,
this is higher now.

00:32:06.320 --> 00:32:14.430
It's about 100,000
metric tons per year.

00:32:14.430 --> 00:32:17.750
And about half of that--
well, about a quarter of that,

00:32:17.750 --> 00:32:20.270
now, maybe a third is for
the semiconductor industry,

00:32:20.270 --> 00:32:23.580
about 3/4 for the PV industry.

00:32:23.580 --> 00:32:25.270
The slurry and the
silicon carbide grit

00:32:25.270 --> 00:32:27.389
needed for wire sawing
is, at some point,

00:32:27.389 --> 00:32:28.430
going to become an issue.

00:32:28.430 --> 00:32:30.705
These are huge
volumes of waste that

00:32:30.705 --> 00:32:33.350
need to be transported
through the factories.

00:32:33.350 --> 00:32:36.140
And of course, the silicon loss
due to wire sawing and ingot

00:32:36.140 --> 00:32:39.080
casting, resulting in
only 50% of the silicon

00:32:39.080 --> 00:32:41.340
here in this ingot being
used in the actual wafers

00:32:41.340 --> 00:32:42.790
to make solar cells.

00:32:42.790 --> 00:32:46.220
The technology
enablers-- using lower--

00:32:46.220 --> 00:32:48.560
let's put it this--
lower cost feedstocks.

00:32:48.560 --> 00:32:50.430
You can't compromise
on quality ultimately,

00:32:50.430 --> 00:32:53.350
so this is a little bit of
a false choice right here.

00:32:53.350 --> 00:32:55.740
Using lower cost
feedstocks produced

00:32:55.740 --> 00:33:00.880
by the upgraded metallurgical
route, for example.

00:33:00.880 --> 00:33:02.810
Producing and
handling thinner wafer

00:33:02.810 --> 00:33:06.070
and growing faster, larger,
higher quality ingots.

00:33:06.070 --> 00:33:08.960
And there's a lot
of innovation to be

00:33:08.960 --> 00:33:10.330
had in this space right here.

00:33:10.330 --> 00:33:14.150
I believe the numbers
in the last quarter,

00:33:14.150 --> 00:33:16.900
start-up companies raised
on the order of $250 million

00:33:16.900 --> 00:33:18.829
from venture capital.

00:33:18.829 --> 00:33:20.620
And that wasn't including
a new $50 million

00:33:20.620 --> 00:33:23.530
deal that was just announced
of a company attempting

00:33:23.530 --> 00:33:25.380
to produce upgraded
metallurgic grade

00:33:25.380 --> 00:33:29.340
silicon through liquid
routes, purification.

00:33:29.340 --> 00:33:30.970
This was just announced
this past week,

00:33:30.970 --> 00:33:32.300
if you go to Greentech Media.

00:33:32.300 --> 00:33:35.710
So there's still a lot of
active innovation in this area

00:33:35.710 --> 00:33:37.810
despite the current
market conditions.

00:33:37.810 --> 00:33:40.570
And those of you who are
looking for jobs right now,

00:33:40.570 --> 00:33:43.920
if you're clever, you'll
find them here in this space.

00:33:43.920 --> 00:33:45.700
Any questions so
far about wafers?

00:33:45.700 --> 00:33:46.497
Yes?

00:33:46.497 --> 00:33:50.789
AUDIENCE: Does laser
cutting cause as much dust?

00:33:50.789 --> 00:33:52.830
PROFESSOR: Does laser
cutting cause as much dust?

00:33:52.830 --> 00:33:55.750
So let's walk through that.

00:33:55.750 --> 00:34:00.830
If we're thinking about
the ribbon growing from,

00:34:00.830 --> 00:34:03.420
say-- from this
ribbon right here,

00:34:03.420 --> 00:34:05.580
I'm going to extract this wafer.

00:34:05.580 --> 00:34:08.350
So I need to make an
incision horizontally

00:34:08.350 --> 00:34:11.909
right around this
point right here.

00:34:11.909 --> 00:34:14.590
If you look at the total
height, the wafer's

00:34:14.590 --> 00:34:16.500
around 15 centimeters long.

00:34:16.500 --> 00:34:19.159
And the laser cut
itself is something

00:34:19.159 --> 00:34:22.510
on the order of
maybe, oh-- I'm going

00:34:22.510 --> 00:34:24.830
to guess-- a few tens
of microns, maybe

00:34:24.830 --> 00:34:26.610
100 microns in that order.

00:34:26.610 --> 00:34:29.530
And so that the amount of
kerf loss in that regard

00:34:29.530 --> 00:34:34.040
would be 100 microns over 15
centimeters, so a relatively

00:34:34.040 --> 00:34:35.570
insignificant fraction.

00:34:35.570 --> 00:34:38.600
If you're trying to chop up
this using a laser, yes, then

00:34:38.600 --> 00:34:40.154
you would have
significant losses.

00:34:40.154 --> 00:34:42.195
But since you're growing
that ribbon straight out

00:34:42.195 --> 00:34:43.949
of the melt, the
laser cuts themselves

00:34:43.949 --> 00:34:46.931
are a very small fraction
of the total silicon.

00:34:46.931 --> 00:34:47.430
Yep?

00:34:47.430 --> 00:34:50.210
AUDIENCE: Can the sawdust be
collected and remelted then?

00:34:50.210 --> 00:34:51.459
PROFESSOR: Wonderful question.

00:34:51.459 --> 00:34:53.510
Can the sawdust be collected
and remelted again?

00:34:53.510 --> 00:34:56.940
There was a lot of work done
to try to figure that out.

00:34:56.940 --> 00:34:58.780
At that point, the
sawdust is mixed

00:34:58.780 --> 00:35:01.880
with this glycol-based slurry,
and with the silicon carbide

00:35:01.880 --> 00:35:05.940
grit, and with fragments of iron
coming from the stainless steel

00:35:05.940 --> 00:35:08.410
wire, and nickel and
chromium and other impurities

00:35:08.410 --> 00:35:09.400
inside of the wire.

00:35:09.400 --> 00:35:10.960
And so a lot of the
work was focused

00:35:10.960 --> 00:35:15.850
on separation of those different
constituents, shall we say.

00:35:15.850 --> 00:35:21.090
And when the silicon prices were
very high, maybe in 2007, 2008,

00:35:21.090 --> 00:35:23.440
when the spot prices
were $500 a kilogram,

00:35:23.440 --> 00:35:25.610
there was a large
incentive to use

00:35:25.610 --> 00:35:29.400
every single drop of silicon
you had including separation.

00:35:29.400 --> 00:35:32.130
But in recent years,
the incentive to do that

00:35:32.130 --> 00:35:33.250
has really dropped.

00:35:33.250 --> 00:35:36.920
And the one company I knew
that had a very active slurry

00:35:36.920 --> 00:35:39.949
recycling program let it go.

00:35:39.949 --> 00:35:42.490
So there may be companies out
there that are looking into it,

00:35:42.490 --> 00:35:44.073
but I'm not aware
of their activities.

00:35:47.000 --> 00:35:47.870
OK.

00:35:47.870 --> 00:35:52.480
Let's hop forward into
cells and devices.

00:35:52.480 --> 00:35:55.034
So now we've talked
about the market shares

00:35:55.034 --> 00:35:57.200
of different technologies,
feedstock refining, wafer

00:35:57.200 --> 00:35:59.660
fabrication, how we make these
wonderful different pieces

00:35:59.660 --> 00:36:00.170
of silicon.

00:36:00.170 --> 00:36:02.170
Now we're going to talk
about going from a wafer

00:36:02.170 --> 00:36:04.480
into a solar cell device.

00:36:04.480 --> 00:36:06.660
So just to situate
ourselves, raw material,

00:36:06.660 --> 00:36:09.590
silicon feedstock, the module
in the system over here.

00:36:09.590 --> 00:36:11.650
In the middle, we have
the wafer to the cell.

00:36:11.650 --> 00:36:15.840
And this is the portion
of discussion forthwith.

00:36:15.840 --> 00:36:17.807
Cell processing.

00:36:17.807 --> 00:36:18.890
Let's have a look at this.

00:36:18.890 --> 00:36:20.660
Again, it's a very
different world

00:36:20.660 --> 00:36:24.660
now in a cell fab line then
it was in the crystallisation

00:36:24.660 --> 00:36:25.330
environment.

00:36:25.330 --> 00:36:27.754
So in wafer fab, which
means wafer fabrication

00:36:27.754 --> 00:36:29.420
and the section of
the company dedicated

00:36:29.420 --> 00:36:33.420
to producing wafers and ingots,
it was a little bit more dirty.

00:36:33.420 --> 00:36:36.090
You had forklifts moving
these big crucibles around

00:36:36.090 --> 00:36:37.962
with chunks of silicon
in it, operators

00:36:37.962 --> 00:36:40.170
coming by with garden hoses
and washing down furnaces

00:36:40.170 --> 00:36:41.810
after they're finished.

00:36:41.810 --> 00:36:43.750
Here in the cell
fab line, it looks

00:36:43.750 --> 00:36:45.390
almost more like a clean room.

00:36:45.390 --> 00:36:47.156
Almost, I say,
because these folks

00:36:47.156 --> 00:36:48.280
aren't in full bunny suits.

00:36:48.280 --> 00:36:50.789
They're usually just with
jackets with booties.

00:36:50.789 --> 00:36:52.330
Sometimes you see
them with hair nets

00:36:52.330 --> 00:36:57.030
as well to protect from hair
and other particulate matter

00:36:57.030 --> 00:36:58.540
from getting inside
of the tools.

00:36:58.540 --> 00:37:00.480
But by and large, the
wafers are brought in.

00:37:00.480 --> 00:37:03.230
And either in a series
of inline processes--

00:37:03.230 --> 00:37:05.040
this is a wafer,
wafer, wafer, wafer.

00:37:05.040 --> 00:37:06.810
So there are four
wafers across moving

00:37:06.810 --> 00:37:08.510
through what looks
like an etch tank

00:37:08.510 --> 00:37:12.840
to do the texturization
on the wafers.

00:37:12.840 --> 00:37:16.970
Whereas in wafer fab,
it was pretty dirty.

00:37:16.970 --> 00:37:18.511
In cell fab, it
looks pretty clean.

00:37:18.511 --> 00:37:20.510
You have a combination
of these inline processes

00:37:20.510 --> 00:37:21.551
like this one shown here.

00:37:21.551 --> 00:37:24.130
We have wafers on conveyor
belts moving through lines.

00:37:24.130 --> 00:37:26.760
And batch processes, where
little robots [? pick in ?]

00:37:26.760 --> 00:37:29.790
places, line wafers up
inside of crucibles or boats,

00:37:29.790 --> 00:37:33.190
and insert them into furnaces
for batch processing.

00:37:33.190 --> 00:37:36.430
So this is the crystalline
silicon cell fabrication.

00:37:36.430 --> 00:37:39.380
In on one side go
bare wafers like this,

00:37:39.380 --> 00:37:42.180
and out the other side come
fully processed solar cell

00:37:42.180 --> 00:37:44.910
devices.

00:37:44.910 --> 00:37:50.300
So the very first step
after wafer sawing

00:37:50.300 --> 00:37:52.060
is the saw damage etch.

00:37:52.060 --> 00:37:55.490
After the sawing process,
you have subsurface damage,

00:37:55.490 --> 00:37:57.440
something on the order
of 5 to 10 microns

00:37:57.440 --> 00:37:58.960
deep beneath the
wafer's surface.

00:37:58.960 --> 00:38:02.130
And keep in mind these are
only about 170 microns thick.

00:38:02.130 --> 00:38:05.520
So you have subsurface damage
that needs to be removed.

00:38:05.520 --> 00:38:08.270
And you can take advantage
of the subsurface damage

00:38:08.270 --> 00:38:11.470
by etching it in such a way
that you etch along the damage

00:38:11.470 --> 00:38:13.090
and form texturization.

00:38:13.090 --> 00:38:14.570
So it's a bit of
a two-in-one here.

00:38:14.570 --> 00:38:16.966
You clean the wafer, you
create your texturization,

00:38:16.966 --> 00:38:18.340
and you remove
your saw damage so

00:38:18.340 --> 00:38:20.840
that when you lift your
wafer, the wafer doesn't

00:38:20.840 --> 00:38:23.560
break because there's some
hairline fracture caused

00:38:23.560 --> 00:38:26.170
by the silicon carbide grit.

00:38:26.170 --> 00:38:27.962
After you have
your wafer-- so you

00:38:27.962 --> 00:38:29.170
start with your p-type wafer.

00:38:29.170 --> 00:38:31.070
And this represents
the cross section

00:38:31.070 --> 00:38:33.565
of the wafer from the
backside of the eventual cell

00:38:33.565 --> 00:38:35.190
to the front side of
the eventual cell,

00:38:35.190 --> 00:38:37.370
about 170 microns thick.

00:38:37.370 --> 00:38:42.190
Wide would be something on
the order of 15.6 centimeters

00:38:42.190 --> 00:38:43.400
in a real device.

00:38:43.400 --> 00:38:46.030
We're just looking at a
small section of it here.

00:38:46.030 --> 00:38:48.280
So as we walk through the
different steps of cell fab,

00:38:48.280 --> 00:38:50.680
we'll see them evolve over here.

00:38:50.680 --> 00:38:53.570
The first step after
the saw damage etch

00:38:53.570 --> 00:38:57.360
is to do what's called an
emitter diffusion, to create

00:38:57.360 --> 00:38:58.592
your p-n junction.

00:38:58.592 --> 00:39:00.300
Straight out of the
box, the p-n junction

00:39:00.300 --> 00:39:02.480
is created after
the saw damage etch.

00:39:02.480 --> 00:39:05.690
And typically, what we do is
deposit a lower resistance

00:39:05.690 --> 00:39:08.760
or more highly doped-- that's
why we have the 2 pluses here,

00:39:08.760 --> 00:39:11.960
that means very highly
doped-- emitter right

00:39:11.960 --> 00:39:14.560
underneath where the eventual
contact metalization will go.

00:39:14.560 --> 00:39:16.710
That's to reduce the
contact resistance.

00:39:16.710 --> 00:39:18.360
That's to create the
tunneling junction

00:39:18.360 --> 00:39:20.700
between the semiconductor
and the metal.

00:39:20.700 --> 00:39:21.320
OK.

00:39:21.320 --> 00:39:24.540
So we have the high
resistance emitter over here.

00:39:24.540 --> 00:39:27.425
This is representative
of a shallow emitter.

00:39:27.425 --> 00:39:28.800
You remember in
your quiz two you

00:39:28.800 --> 00:39:31.025
have this decision whether
to take a shallow or deep.

00:39:31.025 --> 00:39:32.900
This architecture, which
is used in industry,

00:39:32.900 --> 00:39:34.930
actually combines the
best of both worlds.

00:39:34.930 --> 00:39:38.330
It has a shallow emitter over
most of the solar cell device

00:39:38.330 --> 00:39:41.860
to improve the blue response,
minimize Auger recombination.

00:39:41.860 --> 00:39:44.940
But it also has a deeper emitter
right underneath the contact

00:39:44.940 --> 00:39:46.960
metalization to prevent
shunting and to reduce

00:39:46.960 --> 00:39:47.751
contact resistance.

00:39:49.080 --> 00:39:51.330
AUDIENCE: I assume we have
to choose one or the other.

00:39:51.330 --> 00:39:55.320
PROFESSOR: You have to choose
one or the other unfortunately.

00:39:55.320 --> 00:40:00.240
To create this-- it's really to
create the combination, what's

00:40:00.240 --> 00:40:01.990
called a selective emitter.

00:40:01.990 --> 00:40:06.480
It's an emitter because
it's the charge separation

00:40:06.480 --> 00:40:07.670
portion of the device.

00:40:07.670 --> 00:40:09.170
But it's also
selective in the sense

00:40:09.170 --> 00:40:12.250
that you selectively place
these low resistance portions

00:40:12.250 --> 00:40:15.310
across in a geometric fashion
underneath your eventual

00:40:15.310 --> 00:40:17.210
contact metalization.

00:40:17.210 --> 00:40:19.080
You have at the end
of this diffusion

00:40:19.080 --> 00:40:22.720
process what's called a
phosphorus silicate glass etch,

00:40:22.720 --> 00:40:25.380
PSG, Phosphorus
Silicate Glass etch.

00:40:25.380 --> 00:40:28.729
After defusing in the phosphorus
in the gaseous form, what

00:40:28.729 --> 00:40:30.270
you'll do-- or
actually, you'll watch

00:40:30.270 --> 00:40:33.370
it being done, since it's
happening inside of a furnace.

00:40:33.370 --> 00:40:39.427
This phosphorus-based gas will
deposit a thin glassy layer

00:40:39.427 --> 00:40:40.885
on the surface of
the sample, which

00:40:40.885 --> 00:40:43.080
then needs to be etched
off or removed before you

00:40:43.080 --> 00:40:44.440
can do further processing.

00:40:44.440 --> 00:40:47.290
So that's what the phosphorus
silicate glass etch is about.

00:40:47.290 --> 00:40:50.399
Then there's a nitride or a
silicon nitride anti-reflection

00:40:50.399 --> 00:40:52.190
coating that's placed
on the front surface.

00:40:52.190 --> 00:40:54.370
And as we calculated
in lecture number two,

00:40:54.370 --> 00:40:57.740
this silicon nitride
coating is only how thick?

00:40:57.740 --> 00:40:58.360
About?

00:40:58.360 --> 00:40:59.360
AUDIENCE: 70 nanometers.

00:40:59.360 --> 00:41:00.940
PROFESSOR: 70 nanometers, right?

00:41:00.940 --> 00:41:02.180
It's really, really thin.

00:41:02.180 --> 00:41:05.620
But yet, that's enough to create
that quarter wave interference

00:41:05.620 --> 00:41:09.125
effect that leads to a very
blue looking solar cell device.

00:41:09.125 --> 00:41:10.500
So the reason they
looked blue is

00:41:10.500 --> 00:41:12.750
because of that
anti-reflection coating.

00:41:12.750 --> 00:41:16.760
We are going to omit the ARC
coating in our design for quiz

00:41:16.760 --> 00:41:17.880
number two.

00:41:17.880 --> 00:41:20.569
It requires silane gas,
which we don't have access

00:41:20.569 --> 00:41:21.860
to down here in the laboratory.

00:41:21.860 --> 00:41:24.500
We'd have to go to either
[? NTL ?] or Harvard CNS

00:41:24.500 --> 00:41:25.860
to get that deposited.

00:41:25.860 --> 00:41:28.384
So because we want this to
be a hands-on experience,

00:41:28.384 --> 00:41:30.300
we don't to take the
wafers out of your hands,

00:41:30.300 --> 00:41:31.970
do some magic off to the
side, and bring them back

00:41:31.970 --> 00:41:32.890
and say, oh, here you go.

00:41:32.890 --> 00:41:34.380
Because your level of
ownership in the process

00:41:34.380 --> 00:41:36.630
just plummets in order of
magnitude in the process.

00:41:36.630 --> 00:41:38.921
We want you to be able to
see it every step of the way.

00:41:38.921 --> 00:41:42.330
So we omit the anti-reflection
coating in our quiz number two.

00:41:42.330 --> 00:41:44.390
But in commercial
production, that's done.

00:41:44.390 --> 00:41:47.750
And people pay a lot of
attention to that step.

00:41:47.750 --> 00:41:51.990
And finally, the metalization
is deposited on the sample

00:41:51.990 --> 00:41:53.040
and fired.

00:41:53.040 --> 00:41:55.050
Now, the metalization,
how is it deposited?

00:41:55.050 --> 00:41:59.170
We'll see in a few slides
what the screen printing

00:41:59.170 --> 00:42:00.082
process looks like.

00:42:00.082 --> 00:42:01.790
And then, we'll actually
do it ourselves.

00:42:01.790 --> 00:42:03.248
You'll press the
button on the tool

00:42:03.248 --> 00:42:06.720
and deposit your
metal on yourself.

00:42:06.720 --> 00:42:08.870
But the metalization
is typically

00:42:08.870 --> 00:42:13.122
deposited onto the devices on
the front side and on the back.

00:42:13.122 --> 00:42:16.250
The front side, you have to line
up-- in commercial production--

00:42:16.250 --> 00:42:18.500
line up with the low resistance
portion of the emitter

00:42:18.500 --> 00:42:22.740
so that you are able to
extract the full benefit

00:42:22.740 --> 00:42:24.665
from the selective
emitter design

00:42:24.665 --> 00:42:26.630
and not shunt your
device elsewhere.

00:42:26.630 --> 00:42:29.444
And on the back contact,
this is typically aluminum.

00:42:29.444 --> 00:42:30.860
The aluminum, some
of the aluminum

00:42:30.860 --> 00:42:34.770
will indiffuse into the silicon
and create a p-plus region

00:42:34.770 --> 00:42:38.750
in the back side here, which
is a minority carrier blockade

00:42:38.750 --> 00:42:39.400
layer.

00:42:39.400 --> 00:42:41.670
It pushes the electrons
away from the back junction

00:42:41.670 --> 00:42:43.220
and toward the emitter.

00:42:43.220 --> 00:42:45.650
And so it prevents back
surface recombination.

00:42:45.650 --> 00:42:47.690
So you see every
single little step

00:42:47.690 --> 00:42:49.196
of the solar cell fabrication.

00:42:49.196 --> 00:42:51.570
A lot of smart people spend
a lot of time thinking about,

00:42:51.570 --> 00:42:54.751
gee, how do I optimize two
or three things at once?

00:42:54.751 --> 00:42:55.500
Question up there?

00:42:55.500 --> 00:42:58.762
AUDIENCE: So both the front
side and backside metalization

00:42:58.762 --> 00:42:59.700
is [INAUDIBLE]

00:42:59.700 --> 00:43:01.400
PROFESSOR: No the front side
metalization in this case--

00:43:01.400 --> 00:43:03.130
thank you for that
clarification-- the front side

00:43:03.130 --> 00:43:04.546
metalization in
this case would be

00:43:04.546 --> 00:43:07.710
silver or silver-based paste.

00:43:07.710 --> 00:43:10.400
And in most
commercial production,

00:43:10.400 --> 00:43:15.440
this silver-based paste
includes metal oxides.

00:43:15.440 --> 00:43:17.580
It could be glassy frit.

00:43:17.580 --> 00:43:18.700
It could be lead oxide.

00:43:18.700 --> 00:43:20.980
It could be some
combination of elements.

00:43:20.980 --> 00:43:24.150
That is able to etch through the
silicon nitride anti-reflective

00:43:24.150 --> 00:43:25.060
coating.

00:43:25.060 --> 00:43:27.010
This is only 70
nanometers thick,

00:43:27.010 --> 00:43:29.860
but silicon nitride is
a very strong material.

00:43:29.860 --> 00:43:30.860
It's a ceramic material.

00:43:30.860 --> 00:43:33.600
So you have to be able to etch
through the silicon nitride

00:43:33.600 --> 00:43:36.750
and make electrical contact
with the silicon underneath.

00:43:36.750 --> 00:43:40.090
And some of the earliest screen
printed metalization cells that

00:43:40.090 --> 00:43:42.990
got in the range of 15%
or 16% efficiency only

00:43:42.990 --> 00:43:46.490
made electrical contact
about 10% of the silicon.

00:43:46.490 --> 00:43:49.410
But it was enough to have these
percolation paths for current

00:43:49.410 --> 00:43:51.430
to flow up into
the metalization.

00:43:51.430 --> 00:43:53.330
It's a miracle that
it works at all.

00:43:53.330 --> 00:43:57.384
But it's a very effective,
cheap manufacturing process

00:43:57.384 --> 00:43:58.800
that, nevertheless,
is still being

00:43:58.800 --> 00:44:00.512
used in commercial
production today, even

00:44:00.512 --> 00:44:02.845
among some of the highest
efficiency cell architectures.

00:44:06.010 --> 00:44:08.840
And so for each of these
different processing steps,

00:44:08.840 --> 00:44:10.750
somebody had to sit
there and think deeply

00:44:10.750 --> 00:44:14.180
about optimization of
different functions.

00:44:14.180 --> 00:44:16.156
The [? aluminium ?] on
the backside, somebody

00:44:16.156 --> 00:44:17.530
had to think about,
gee, how do I

00:44:17.530 --> 00:44:21.250
prevent the wafer from
bowing, bowing too much,

00:44:21.250 --> 00:44:23.590
due to coefficient
of thermal expansion

00:44:23.590 --> 00:44:26.250
mismatch between the
aluminium and the silicon?

00:44:26.250 --> 00:44:29.440
Somebody had to think about, how
do I create the right eutectic

00:44:29.440 --> 00:44:31.820
with the silicon-- the
aluminum silicon eutectic

00:44:31.820 --> 00:44:34.300
is around 577
degrees Celsius-- so

00:44:34.300 --> 00:44:37.010
that you create a good ohmic
contact on the backside?

00:44:37.010 --> 00:44:39.390
How do I diffuse in a certain
amount of the aluminum

00:44:39.390 --> 00:44:41.790
to create this back surface
field to prevent back

00:44:41.790 --> 00:44:42.830
surface recombination?

00:44:42.830 --> 00:44:45.050
How do I get the right
back surface reflectance

00:44:45.050 --> 00:44:46.750
of the light coming
off of here so

00:44:46.750 --> 00:44:49.070
that I have multiple optical
bounces through my device

00:44:49.070 --> 00:44:50.710
and so forth?

00:44:50.710 --> 00:44:54.260
So a lot of optimization goes
into making a solar cell device

00:44:54.260 --> 00:44:56.390
to get the Liebig's
law of the minimum,

00:44:56.390 --> 00:44:58.770
to get each plank Liebig's
law as high as you possibly

00:44:58.770 --> 00:45:01.320
can so you can achieve a
high device performance.

00:45:01.320 --> 00:45:02.990
So hopefully, this
walk through now,

00:45:02.990 --> 00:45:05.040
you can have an appreciation
for the difficulty

00:45:05.040 --> 00:45:07.600
that some of your colleagues
face at solar cell fabrication

00:45:07.600 --> 00:45:09.880
plants.

00:45:09.880 --> 00:45:11.450
Finally, as last
steps-- I mean, this

00:45:11.450 --> 00:45:13.740
is a real miniature
cross section

00:45:13.740 --> 00:45:15.240
in the lateral
dimension right here.

00:45:15.240 --> 00:45:17.510
We only have two contact
metalization fingers.

00:45:17.510 --> 00:45:19.551
If you look at this solar
cell device right here,

00:45:19.551 --> 00:45:20.970
we have several dozen, right?

00:45:20.970 --> 00:45:23.220
If I were to make a vertical
cross section through it,

00:45:23.220 --> 00:45:25.430
you'd see several
dozen contact fingers.

00:45:25.430 --> 00:45:27.590
But this is just meant
to be a caricature.

00:45:27.590 --> 00:45:31.870
So on the edge here,
we have edge isolation.

00:45:31.870 --> 00:45:34.990
And what this is doing is
preventing shunting pathways

00:45:34.990 --> 00:45:36.480
from going around to the back.

00:45:36.480 --> 00:45:38.340
So it's preventing
the emitter from being

00:45:38.340 --> 00:45:41.580
able to make electrical contact
to the backside of the device.

00:45:41.580 --> 00:45:43.670
And this is typically
done by inserting

00:45:43.670 --> 00:45:48.510
a trench, a laser-based
trench, just-- gosh,

00:45:48.510 --> 00:45:52.450
it must be on the order of half
a millimeter from the edge.

00:45:52.450 --> 00:45:55.550
I'm going to pass this around,
this solar cell device right

00:45:55.550 --> 00:45:56.150
here.

00:45:56.150 --> 00:45:57.910
And if you look
very, very carefully,

00:45:57.910 --> 00:46:03.270
it's literally a few hundred
microns from the edge at most.

00:46:03.270 --> 00:46:06.950
You may be able to see the
edge isolation, the trench that

00:46:06.950 --> 00:46:08.400
is formed by the laser.

00:46:08.400 --> 00:46:11.240
But it's very difficult to see.

00:46:11.240 --> 00:46:14.591
So I'll pass this finished
device around as well.

00:46:14.591 --> 00:46:16.090
And feel free to
pick it up and look

00:46:16.090 --> 00:46:17.850
at the backside
and the front side.

00:46:17.850 --> 00:46:20.440
On the back, you'll
see some silver paths

00:46:20.440 --> 00:46:22.220
in the middle of
all that aluminum.

00:46:22.220 --> 00:46:24.720
And if anybody has ever
tried to solder to aluminum,

00:46:24.720 --> 00:46:27.280
you know exactly why those
silver pads are there.

00:46:27.280 --> 00:46:30.120
It's so you can solder
to them and make contact

00:46:30.120 --> 00:46:32.720
to the back of one
device and contact it

00:46:32.720 --> 00:46:33.990
to the front of the next.

00:46:33.990 --> 00:46:35.830
And you'll notice
that they're aligned,

00:46:35.830 --> 00:46:38.120
so the back pads are
aligned with the front.

00:46:38.120 --> 00:46:39.820
So I'll pass this
around right here.

00:46:39.820 --> 00:46:41.392
Yes, Ashley?

00:46:41.392 --> 00:46:44.015
AUDIENCE: I assume
that in order to go

00:46:44.015 --> 00:46:47.540
in terms of where you want
to put the edge isolation,

00:46:47.540 --> 00:46:50.290
do you want it as
far out as possible

00:46:50.290 --> 00:46:51.790
so you're not losing
that edge part.

00:46:51.790 --> 00:46:53.790
But you also need to
make sure you're actually

00:46:53.790 --> 00:46:55.730
making a full [INAUDIBLE].

00:46:55.730 --> 00:46:56.929
There's some optimization--

00:46:56.929 --> 00:46:57.720
PROFESSOR: Exactly.

00:46:57.720 --> 00:46:58.890
AUDIENCE: [INAUDIBLE]

00:46:58.890 --> 00:46:59.730
PROFESSOR: Exactly.

00:46:59.730 --> 00:47:03.490
If your laser edge isolation
machine isn't well-calibrated,

00:47:03.490 --> 00:47:06.210
you're losing area, active
area, of your solar cell,

00:47:06.210 --> 00:47:08.620
hence your current output
is going to be lower.

00:47:08.620 --> 00:47:11.520
Because you know your solar cell
has a certain current density,

00:47:11.520 --> 00:47:15.070
a certain, say, milliamps
per square centimeter.

00:47:15.070 --> 00:47:17.920
But then if your area, if
you're square centimeter

00:47:17.920 --> 00:47:20.480
is smaller, because
you're cutting too far

00:47:20.480 --> 00:47:22.811
away from the edge, you're
throwing away good material.

00:47:22.811 --> 00:47:24.560
This isn't the trench
all the way through.

00:47:24.560 --> 00:47:26.040
It's just electrical isolation.

00:47:26.040 --> 00:47:28.290
So essentially, this material
over here still exists.

00:47:28.290 --> 00:47:29.789
It's still hanging
on to the device,

00:47:29.789 --> 00:47:31.210
but it's electrically isolated.

00:47:31.210 --> 00:47:35.210
This trench here is only about
a couple of microns deep.

00:47:35.210 --> 00:47:36.516
And you're losing area.

00:47:36.516 --> 00:47:38.140
This area over here
is not contributing

00:47:38.140 --> 00:47:39.770
to the photocurrent
of your device.

00:47:39.770 --> 00:47:42.170
Any electron making it up
into the emitter over here

00:47:42.170 --> 00:47:44.086
will just stay there and
recombine eventually.

00:47:44.086 --> 00:47:47.920
It won't be able to be
pulled out of the device.

00:47:47.920 --> 00:47:51.210
A funny, but true story--
there was a company

00:47:51.210 --> 00:47:53.644
once that I worked with
to solve a problem.

00:47:53.644 --> 00:47:55.310
And they were getting
lower efficiencies

00:47:55.310 --> 00:47:56.100
in their new process.

00:47:56.100 --> 00:47:57.780
And they couldn't figure
out for the life of them

00:47:57.780 --> 00:47:59.260
why they were getting
lower efficiencies.

00:47:59.260 --> 00:48:01.260
They checked everything,
everything, everything,

00:48:01.260 --> 00:48:02.020
everything.

00:48:02.020 --> 00:48:05.030
And it turned out that they
were cutting their wafers

00:48:05.030 --> 00:48:08.532
to a slightly larger size
than they were before.

00:48:08.532 --> 00:48:10.240
Actually, it was a
slightly smaller size,

00:48:10.240 --> 00:48:11.700
because it was a
lower efficiency.

00:48:11.700 --> 00:48:16.036
And their tester had embedded in
it a fixed number for the area.

00:48:16.036 --> 00:48:18.410
It wasn't measuring the area
of each wafer independently.

00:48:18.410 --> 00:48:20.862
It just had a fixed number
for the area of the cell.

00:48:20.862 --> 00:48:22.820
And so it was dividing
the total current output

00:48:22.820 --> 00:48:25.330
by a bigger area than
what was actually there.

00:48:25.330 --> 00:48:28.190
And so it was "measuring"
a lower efficiency

00:48:28.190 --> 00:48:31.190
than what actually the
cell was outputting.

00:48:31.190 --> 00:48:33.380
So again, these
geometric parameters

00:48:33.380 --> 00:48:34.920
can come up and
bite you if you're

00:48:34.920 --> 00:48:37.970
so fixated on the electrical
performance parameters.

00:48:37.970 --> 00:48:39.380
Testing and sorting.

00:48:39.380 --> 00:48:41.410
So after you create
your device, you

00:48:41.410 --> 00:48:43.140
have this beautiful solar cell.

00:48:43.140 --> 00:48:48.824
And just from simple electrical
engineering and maybe

00:48:48.824 --> 00:48:51.240
as an extreme example if you're
stringing Christmas lights

00:48:51.240 --> 00:48:53.324
together, you know that
if you have one bad apple,

00:48:53.324 --> 00:48:55.573
it can drag down the performance
of the entire string,

00:48:55.573 --> 00:48:57.470
right, if you're
connecting these in series.

00:48:57.470 --> 00:49:00.120
And so it makes sense to
test each of the cells

00:49:00.120 --> 00:49:02.670
individually and make sure
that you sort them together

00:49:02.670 --> 00:49:05.490
with their like cousins.

00:49:05.490 --> 00:49:07.140
So if you have high
performance cells,

00:49:07.140 --> 00:49:08.350
you bin them all together.

00:49:08.350 --> 00:49:10.620
And you make models of the
high performance cells.

00:49:10.620 --> 00:49:12.296
These will be high
performance modules.

00:49:12.296 --> 00:49:14.420
The bad apples you put
together with the bad apples

00:49:14.420 --> 00:49:15.460
and so forth.

00:49:15.460 --> 00:49:18.030
And that way you can extract
the maximum value out

00:49:18.030 --> 00:49:19.500
of the product you've created.

00:49:19.500 --> 00:49:21.180
You take your good
cells, you put them

00:49:21.180 --> 00:49:22.580
into a higher efficiency module.

00:49:22.580 --> 00:49:24.913
It looks exactly the same,
but its producing more power,

00:49:24.913 --> 00:49:27.510
so you can sell that module at
a higher price than you would,

00:49:27.510 --> 00:49:30.180
say, a lower power
output module.

00:49:30.180 --> 00:49:30.680
OK.

00:49:30.680 --> 00:49:33.490
So that's what the test
and sort is all about.

00:49:33.490 --> 00:49:36.930
Turnkey solar cell
fabrication lines,

00:49:36.930 --> 00:49:39.400
very common since the mid 2000s.

00:49:39.400 --> 00:49:43.850
There are companies--
Centrotherm, gosh, [INAUDIBLE],

00:49:43.850 --> 00:49:47.430
Roth & Rau, others that were
producing either turnkey

00:49:47.430 --> 00:49:50.710
equipment or even turnkey lines
for the entire fabrication

00:49:50.710 --> 00:49:51.400
line.

00:49:51.400 --> 00:49:53.550
Even a local company,
Spire, just up

00:49:53.550 --> 00:49:55.800
the road here in Massachusetts.

00:49:55.800 --> 00:49:58.560
These typically consisted
of wafer inspection systems

00:49:58.560 --> 00:49:59.720
on the input side.

00:49:59.720 --> 00:50:01.310
You don't want to invest
any money in a wafer that's

00:50:01.310 --> 00:50:02.800
ultimately going
to break, so you

00:50:02.800 --> 00:50:04.730
want to be able to inspect your
wafers coming in to make sure

00:50:04.730 --> 00:50:06.790
that they're high enough
quality to be worthy

00:50:06.790 --> 00:50:09.430
of your cell investment.

00:50:09.430 --> 00:50:11.989
Next, you have wet processing
to do the texturization.

00:50:11.989 --> 00:50:13.030
That's shown right there.

00:50:13.030 --> 00:50:15.800
Saw damage texturization.

00:50:15.800 --> 00:50:18.150
And these are
typically inline tools

00:50:18.150 --> 00:50:21.690
with little ceramic rollers,
some pretty nasty acids

00:50:21.690 --> 00:50:23.070
being use.

00:50:23.070 --> 00:50:24.346
Silicon is like a rock.

00:50:24.346 --> 00:50:25.720
And if you want
to etch the rock,

00:50:25.720 --> 00:50:30.650
you need to have some pretty
strong solutions, some very

00:50:30.650 --> 00:50:35.080
high or very low pH, very basic
or very acidic respectively.

00:50:35.080 --> 00:50:37.320
And most of the time, in
multi-crystalline silicon,

00:50:37.320 --> 00:50:39.150
we use an acidic solution.

00:50:39.150 --> 00:50:41.520
It textures the
wafer independent

00:50:41.520 --> 00:50:42.525
of grain orientation.

00:50:42.525 --> 00:50:43.900
For the single
crystal materials,

00:50:43.900 --> 00:50:47.665
we use a basic solution that
is isotropic or anisotropic

00:50:47.665 --> 00:50:48.165
in nature.

00:50:48.165 --> 00:50:49.940
It creates nice little pyramids.

00:50:49.940 --> 00:50:53.350
So here, you see the wafers
being drawn over an etch bath.

00:50:53.350 --> 00:50:55.660
And very small
quantities of liquid

00:50:55.660 --> 00:50:58.200
are used per wafer
in this arrangement.

00:50:58.200 --> 00:51:01.289
You just coat the wafer's
surface, and that's about it.

00:51:01.289 --> 00:51:02.830
If you were to do
it in a batch mode,

00:51:02.830 --> 00:51:05.347
you need a big bath
like a bathtub.

00:51:05.347 --> 00:51:06.930
And you dunk your
wafers inside of it.

00:51:06.930 --> 00:51:07.850
So that would be the bathtub.

00:51:07.850 --> 00:51:09.580
This would be the
shower equivalent.

00:51:09.580 --> 00:51:11.740
So more water efficient.

00:51:11.740 --> 00:51:14.226
In this case, acid efficient.

00:51:14.226 --> 00:51:16.100
And then the cells come
out on the other side

00:51:16.100 --> 00:51:19.020
and go into the emitter
diffusion process.

00:51:19.020 --> 00:51:20.540
And these are a
series of furnaces.

00:51:20.540 --> 00:51:23.050
We'll see one such furnace
over the course of quiz two

00:51:23.050 --> 00:51:24.790
when we make our solar cells.

00:51:24.790 --> 00:51:27.830
So this is the phosphorus
diffusion furnace right here.

00:51:27.830 --> 00:51:30.570
The wafers are typically loaded
into boats and then inserted

00:51:30.570 --> 00:51:35.610
into furnace where phosphorus
containing gas, POCL3,

00:51:35.610 --> 00:51:39.680
also called "pah-cul," is
flown into the chamber.

00:51:39.680 --> 00:51:43.119
The chlorine components
and the oxygen

00:51:43.119 --> 00:51:44.660
dissociate from the
phosphorus, which

00:51:44.660 --> 00:51:46.222
is then driven into the wafer.

00:51:46.222 --> 00:51:47.680
The oxygen reacts
with the silicon,

00:51:47.680 --> 00:51:50.250
creates that phosphorus
silicate glass on the surface.

00:51:50.250 --> 00:51:53.820
And the phosphorus is driven
into the solar cell creating

00:51:53.820 --> 00:51:57.410
the p-n junction,
creating your device.

00:51:57.410 --> 00:52:01.410
And here's an example of
Czochralski wafers being loaded

00:52:01.410 --> 00:52:04.324
into the phosphorus diffusion
furnace and then out again,

00:52:04.324 --> 00:52:05.865
just showing the
degree of automation

00:52:05.865 --> 00:52:07.330
of some of these furnaces.

00:52:07.330 --> 00:52:09.200
This showing a
stack not dissimilar

00:52:09.200 --> 00:52:10.980
from the one in the
laboratory downstairs

00:52:10.980 --> 00:52:14.850
in building 35 where we'll be
doing our phosphorus diffusion.

00:52:14.850 --> 00:52:18.270
So the next-- after we have our
p-n junction-- the next step

00:52:18.270 --> 00:52:21.080
would be to create the
anti-reflection coating.

00:52:21.080 --> 00:52:23.960
And this is done by a process
called Plasma Enhanced Chemical

00:52:23.960 --> 00:52:27.460
Vapor Deposition,
or PECVD for short.

00:52:27.460 --> 00:52:31.690
And in the PECVD process, you
flow in silane gas and ammonia.

00:52:31.690 --> 00:52:34.880
Silane is silicon with a bunch
of hydrogens, four of them.

00:52:34.880 --> 00:52:37.800
And ammonia is nitrogen
with a bunch of hydrogens.

00:52:37.800 --> 00:52:42.090
And the nitrogen and the silicon
react on the wafer's surface

00:52:42.090 --> 00:52:43.910
and create the silicon
nitride coating.

00:52:43.910 --> 00:52:47.640
The hydrogens, 90% of
it, evaporates off.

00:52:47.640 --> 00:52:50.080
But about 10% of
it hangs around.

00:52:50.080 --> 00:52:53.594
Between 1% and 10% go into the
wafer or stay at the interface

00:52:53.594 --> 00:52:56.010
there and eventually are driven
into the wafer passivating

00:52:56.010 --> 00:52:57.170
bulk defects.

00:52:57.170 --> 00:52:59.330
So again, a multitude
of different things

00:52:59.330 --> 00:53:01.770
going on at the same time.

00:53:01.770 --> 00:53:03.290
Eventually, the
visible effect is

00:53:03.290 --> 00:53:05.700
that you've created your
anti-reflection coating.

00:53:05.700 --> 00:53:09.340
The wafers go in looking shiny
and come out looking blue.

00:53:09.340 --> 00:53:12.250
But what's happening
underneath the surface

00:53:12.250 --> 00:53:14.960
is that some of the hydrogen
is going into the wafer.

00:53:14.960 --> 00:53:17.890
Hydrogen, the first element on
the periodic table, very tiny.

00:53:17.890 --> 00:53:20.840
And in the PECVD process,
where you have a plasma,

00:53:20.840 --> 00:53:24.110
you have hydrogen
ions, which basically

00:53:24.110 --> 00:53:26.550
means you have a proton
without its electron.

00:53:26.550 --> 00:53:29.667
And that proton is very fast,
moving through the lattice.

00:53:29.667 --> 00:53:32.000
There's lots of space for it
to move through the silicon

00:53:32.000 --> 00:53:32.557
lattice.

00:53:32.557 --> 00:53:34.140
And it's also very
reactive because it

00:53:34.140 --> 00:53:35.440
doesn't have that electron.

00:53:35.440 --> 00:53:37.890
So whenever it finds a
defect or a dangling bond,

00:53:37.890 --> 00:53:40.510
it'll usually lodge itself
there, and attach itself,

00:53:40.510 --> 00:53:42.270
and passivate that defect.

00:53:42.270 --> 00:53:43.820
And that's what
hydrogen passivation

00:53:43.820 --> 00:53:48.070
is all about during
the silicon nitride

00:53:48.070 --> 00:53:50.190
anti-reflective
coating deposition.

00:53:50.190 --> 00:53:57.495
These are examples of
inline processes for doing

00:53:57.495 --> 00:53:59.120
an anti-reflection coating.

00:53:59.120 --> 00:54:01.350
I believe there are a
few different variants

00:54:01.350 --> 00:54:03.310
of this inline
process, one of which

00:54:03.310 --> 00:54:05.120
is a sputtering
mechanism to deposit

00:54:05.120 --> 00:54:07.760
this anti-reflective coating.

00:54:07.760 --> 00:54:10.490
Of course, then during
sputtering process,

00:54:10.490 --> 00:54:12.530
you have to worry, as
well, about hydrogen.

00:54:12.530 --> 00:54:14.770
Do you have the benefit
of hydrogen passivation?

00:54:14.770 --> 00:54:16.960
Perhaps not as much, so
additional engineering

00:54:16.960 --> 00:54:17.770
is needed.

00:54:17.770 --> 00:54:20.250
But the inline process
could be potentially faster

00:54:20.250 --> 00:54:23.520
and higher throughput than the
batch process using the PECVD.

00:54:23.520 --> 00:54:26.710
So again, manufacturing
trade-offs.

00:54:26.710 --> 00:54:30.010
Next, we have the printing
line and screen printing.

00:54:30.010 --> 00:54:34.970
So this looks very similar to
screen printing for a t-shirt.

00:54:34.970 --> 00:54:38.390
Here is a t-shirt being
loaded into a screen printer.

00:54:38.390 --> 00:54:41.660
And here's a solar cell being
loaded into screen printer.

00:54:41.660 --> 00:54:43.780
This is a close up of
the screen, of what

00:54:43.780 --> 00:54:46.170
the screen actually looks like.

00:54:46.170 --> 00:54:48.530
Here, the screen,
which is comprised

00:54:48.530 --> 00:54:52.384
of this mesh of metal--
here the screen is bare.

00:54:52.384 --> 00:54:54.050
And so the metal
that's deposited on top

00:54:54.050 --> 00:54:56.030
can go through those
holes in the screen

00:54:56.030 --> 00:54:57.760
and onto the wafer
underneath it.

00:54:57.760 --> 00:55:01.302
And here, there's a coating, a
polymer coating of the screen,

00:55:01.302 --> 00:55:02.760
which prevents the
metal from going

00:55:02.760 --> 00:55:04.260
through the screen
at those places.

00:55:04.260 --> 00:55:06.750
So it shades the solar cell
underneath and prevents metals

00:55:06.750 --> 00:55:08.340
from being deposited there.

00:55:08.340 --> 00:55:10.230
And you have
fingers and busbars.

00:55:10.230 --> 00:55:13.530
And those are eventually
the thin little fingers

00:55:13.530 --> 00:55:16.090
that you see right
here going sideways

00:55:16.090 --> 00:55:17.720
and the vertical
busbars that you see

00:55:17.720 --> 00:55:20.012
going vertically right here.

00:55:20.012 --> 00:55:20.512
Question?

00:55:20.512 --> 00:55:21.137
AUDIENCE: Yeah.

00:55:21.137 --> 00:55:23.721
So when you do these [INAUDIBLE]
emitter [INAUDIBLE] where you

00:55:23.721 --> 00:55:25.970
have some areas of high
resistance emitters and others

00:55:25.970 --> 00:55:26.950
of low resistance--

00:55:26.950 --> 00:55:27.616
PROFESSOR: Yeah.

00:55:27.616 --> 00:55:29.430
AUDIENCE: Do you use
a screen to shield it.

00:55:29.430 --> 00:55:30.967
Or do you [INAUDIBLE].

00:55:30.967 --> 00:55:32.050
PROFESSOR: Great question.

00:55:32.050 --> 00:55:34.591
So some of the earliest designs
for the selective emitter--if

00:55:34.591 --> 00:55:36.580
we go back all the
way up to here.

00:55:36.580 --> 00:55:37.080
Yeah.

00:55:37.080 --> 00:55:38.880
To the selective
emitter portion.

00:55:38.880 --> 00:55:42.200
So the earliest designs
used photoresist process.

00:55:42.200 --> 00:55:45.260
But I would say nowadays,
there are a few technology

00:55:45.260 --> 00:55:47.840
options that are much
faster, one of which

00:55:47.840 --> 00:55:52.020
involves a creation of porous
silicon on certain regions

00:55:52.020 --> 00:55:54.410
of the wafer that you want
to etch back and create

00:55:54.410 --> 00:55:58.480
the shallow emitter leaving
the deeper region intact.

00:55:58.480 --> 00:56:00.500
And that, you could
use a form of shading.

00:56:00.500 --> 00:56:02.250
You could use a wax even for it.

00:56:02.250 --> 00:56:04.270
There are a variety
of technology options

00:56:04.270 --> 00:56:05.890
for achieving that goal.

00:56:05.890 --> 00:56:07.950
But in a sense, many of
the selective emitter

00:56:07.950 --> 00:56:10.860
designs involve a deep
diffusion first and then

00:56:10.860 --> 00:56:13.700
a partial etch back.

00:56:13.700 --> 00:56:15.780
For example, creation
of porous silicon

00:56:15.780 --> 00:56:18.110
and etching that material away.

00:56:18.110 --> 00:56:20.539
Ashley, you had a question?

00:56:20.539 --> 00:56:21.505
AUDIENCE: Oh, yeah.

00:56:21.505 --> 00:56:23.310
So what does "turnkey" refer to?

00:56:23.310 --> 00:56:24.240
PROFESSOR: Turnkey.

00:56:24.240 --> 00:56:25.230
Excellent question.

00:56:25.230 --> 00:56:28.170
So turnkey manufacturing
line-- what it refers to

00:56:28.170 --> 00:56:30.710
is that I'm the vendor
of the equipment.

00:56:30.710 --> 00:56:33.310
In one case, I
say, here, Ashley.

00:56:33.310 --> 00:56:34.520
Here's a piece of equipment.

00:56:34.520 --> 00:56:36.080
It's going to cost
you $1 million.

00:56:36.080 --> 00:56:37.930
And good luck getting
it set up and running.

00:56:37.930 --> 00:56:38.929
I'm out of here.

00:56:38.929 --> 00:56:39.470
I'll see you.

00:56:39.470 --> 00:56:40.030
AUDIENCE: Right.

00:56:40.030 --> 00:56:41.905
PROFESSOR: A smarter
company might come along

00:56:41.905 --> 00:56:44.120
and say, I'm going to
guarantee an output

00:56:44.120 --> 00:56:45.584
from my piece of equipment.

00:56:45.584 --> 00:56:47.000
I'm going to
guarantee that you'll

00:56:47.000 --> 00:56:51.840
be able to make 16.7% solar
cells, 16.7% efficiency.

00:56:51.840 --> 00:56:53.850
I will send my engineers
to your factory,

00:56:53.850 --> 00:56:56.350
and they will help you get the
equipment set up and running.

00:56:56.350 --> 00:56:57.930
And once it's
running up to spec,

00:56:57.930 --> 00:57:00.460
then they'll come back home,
and you'll be on your own.

00:57:00.460 --> 00:57:02.210
And you'll be able to
optimize it further.

00:57:02.210 --> 00:57:04.940
And so you walk in knowing
that you have this guarantee

00:57:04.940 --> 00:57:06.062
of a performance.

00:57:06.062 --> 00:57:07.770
Then you can go to
your financing agency.

00:57:07.770 --> 00:57:09.269
You can go to Joe
and say, hey, Joe,

00:57:09.269 --> 00:57:10.840
give me money for
my new factory.

00:57:10.840 --> 00:57:13.580
I have a guarantee that
I'm going to hit 16.7%

00:57:13.580 --> 00:57:15.860
and have a pathway
to get to 17.2.

00:57:15.860 --> 00:57:19.730
My CTO right here thinks--
she's a really small person,

00:57:19.730 --> 00:57:22.710
and she has a pathway to get
to another 0.5% out of it.

00:57:22.710 --> 00:57:24.790
And so you can go
to your financier

00:57:24.790 --> 00:57:26.740
and get money more
easily than, say,

00:57:26.740 --> 00:57:28.157
in the first
scenario where you're

00:57:28.157 --> 00:57:30.531
given a piece of equipment
and then the person high tails

00:57:30.531 --> 00:57:31.220
it out of there.

00:57:31.220 --> 00:57:31.650
AUDIENCE: Right.

00:57:31.650 --> 00:57:33.316
PROFESSOR: So turnkey
refers to the idea

00:57:33.316 --> 00:57:34.404
that you turn the line on.

00:57:34.404 --> 00:57:36.070
You essentially turn
the key, and you're

00:57:36.070 --> 00:57:37.389
getting high performance out.

00:57:37.389 --> 00:57:39.930
In reality, it takes a month or
two to ramp up to that point.

00:57:39.930 --> 00:57:40.330
AUDIENCE: Right.

00:57:40.330 --> 00:57:42.320
PROFESSOR: To get high yields
and to get high performance.

00:57:42.320 --> 00:57:43.950
But you have the
support of the company

00:57:43.950 --> 00:57:47.180
there on the ground
helping you achieve that.

00:57:47.180 --> 00:57:49.190
And the turnkey
lines were actually

00:57:49.190 --> 00:57:51.910
one of the real reasons why
technology flowed around

00:57:51.910 --> 00:57:53.890
the planet so quickly.

00:57:53.890 --> 00:57:56.440
Because up until
about the mid 2000s,

00:57:56.440 --> 00:58:00.830
high efficiency cell was
limited to a few laboratories

00:58:00.830 --> 00:58:02.640
and a few companies in the know.

00:58:02.640 --> 00:58:04.440
But once turnkey
equipment manufacturers

00:58:04.440 --> 00:58:06.230
got into to the
mix, they started

00:58:06.230 --> 00:58:08.550
creating these turnkey
lines and selling

00:58:08.550 --> 00:58:11.380
the equipment around the
world and the expertise of how

00:58:11.380 --> 00:58:13.810
to make high efficiency
devices, both the architecture

00:58:13.810 --> 00:58:15.350
and the processing know-how.

00:58:15.350 --> 00:58:18.210
And this is how, within
in the last 5 to 10 years,

00:58:18.210 --> 00:58:20.670
you've seen such an
explosion of companies

00:58:20.670 --> 00:58:23.500
around the globe in all sorts
of places that traditionally

00:58:23.500 --> 00:58:26.460
haven't been experts in solar
cell manufacturing suddenly

00:58:26.460 --> 00:58:29.050
knowing how to
manufacture solar cells.

00:58:29.050 --> 00:58:29.910
It flows.

00:58:29.910 --> 00:58:33.630
The know-how flows through
the equipment vendors.

00:58:33.630 --> 00:58:35.651
So finally, testing and sorting.

00:58:35.651 --> 00:58:37.900
This is the last stage of
the solar cell manufacturing

00:58:37.900 --> 00:58:38.610
process.

00:58:38.610 --> 00:58:40.640
Here, we see a little
pick-and-place.

00:58:40.640 --> 00:58:43.391
That means a little robot that
picks up wafers and deposits

00:58:43.391 --> 00:58:43.890
them.

00:58:43.890 --> 00:58:46.540
The simplest incarnation
is just suction cup.

00:58:46.540 --> 00:58:50.140
The more fancy ones involve
Bernoulli lifters, essentially

00:58:50.140 --> 00:58:52.710
pressure differentials
pulling wafers up.

00:58:52.710 --> 00:58:56.429
So you have wafers being
loaded onto a conveyor belt,

00:58:56.429 --> 00:58:58.470
coming off of one conveyor
belt onto another one.

00:58:58.470 --> 00:58:59.980
And they're moving forward.

00:58:59.980 --> 00:59:03.290
And what you see right
here in very low resolution

00:59:03.290 --> 00:59:05.740
are two probes coming down.

00:59:05.740 --> 00:59:09.550
This, evidently, is a two
busbar cell, not a three busbar

00:59:09.550 --> 00:59:10.520
cell like this one.

00:59:10.520 --> 00:59:14.020
The probes come down and make
contact with the busbars.

00:59:14.020 --> 00:59:16.260
And the probes have
multiple contact points,

00:59:16.260 --> 00:59:19.090
so the series resistance
along the busbars

00:59:19.090 --> 00:59:21.190
is not affecting
your measurement.

00:59:21.190 --> 00:59:23.177
Cell efficiency measurement
is always tricky

00:59:23.177 --> 00:59:25.010
because depending where
you put your probes,

00:59:25.010 --> 00:59:27.468
your measurements are going to
change because of the series

00:59:27.468 --> 00:59:28.270
resistance.

00:59:28.270 --> 00:59:30.702
So these probes
right here are long,

00:59:30.702 --> 00:59:32.410
and they contain
multiple contact points.

00:59:32.410 --> 00:59:34.284
And they're essentially
touching the busbars.

00:59:34.284 --> 00:59:38.320
And light flashes onto the
device simulating the sun, so

00:59:38.320 --> 00:59:40.730
simulating AM 1.5 conditions.

00:59:40.730 --> 00:59:44.596
And an IV curve is
measured, is swept.

00:59:44.596 --> 00:59:46.970
I can't really tell from the
photograph or from the movie

00:59:46.970 --> 00:59:50.402
right here whether the IV curve
is being swept at illumination,

00:59:50.402 --> 00:59:52.610
meaning you're sweeping your
voltage when the cell is

00:59:52.610 --> 00:59:55.260
illuminated, or whether
the illumination intensity

00:59:55.260 --> 00:59:57.320
itself is used to
vary the forward bias

00:59:57.320 --> 00:59:58.580
condition of the cell.

00:59:58.580 --> 01:00:01.097
They could be doing
it in one of two ways.

01:00:01.097 --> 01:00:02.680
But most likely,
what they're doing is

01:00:02.680 --> 01:00:06.650
they're flashing the lights,
measuring the IV characteristic

01:00:06.650 --> 01:00:08.910
of the device, and
then sorting the cell

01:00:08.910 --> 01:00:10.280
based on that performance.

01:00:10.280 --> 01:00:12.210
It goes into a computer.

01:00:12.210 --> 01:00:14.960
Efficiency is calculated, just
like you did on your homework.

01:00:14.960 --> 01:00:16.460
And just like that,
it's calculated.

01:00:16.460 --> 01:00:18.840
And then, as the cell
moves down the line,

01:00:18.840 --> 01:00:21.900
the robot knows, oh, that's
the cell that got 16.6.

01:00:21.900 --> 01:00:22.860
We put it over here.

01:00:22.860 --> 01:00:24.210
Oh, that next cell got 16.8.

01:00:24.210 --> 01:00:25.650
We put it over there.

01:00:25.650 --> 01:00:29.260
Some additional companies sort
their cells based on color

01:00:29.260 --> 01:00:32.090
because they want to have
the aesthetic appearance

01:00:32.090 --> 01:00:34.030
of homogeneity
within the module.

01:00:34.030 --> 01:00:38.540
They want every cell to be
of uniform aesthetic value

01:00:38.540 --> 01:00:42.924
inside of a module so that you
have a nice, uniform color.

01:00:42.924 --> 01:00:44.876
AUDIENCE: Is that
considered [INAUDIBLE].

01:00:47.770 --> 01:00:49.770
PROFESSOR: Whether or not
this module right here

01:00:49.770 --> 01:00:52.610
is considered uniform or
different would depend on you,

01:00:52.610 --> 01:00:53.440
Jessica.

01:00:53.440 --> 01:00:55.430
You're the customer,
and you decide

01:00:55.430 --> 01:00:57.900
whether this is good enough
for you or whether it's not.

01:00:57.900 --> 01:00:58.370
AUDIENCE: It's not.

01:00:58.370 --> 01:00:59.070
PROFESSOR: It's not?

01:00:59.070 --> 01:00:59.570
All right.

01:00:59.570 --> 01:01:01.830
Well, then we have
to work harder.

01:01:01.830 --> 01:01:06.120
So the customer requirements
really drive the industry.

01:01:06.120 --> 01:01:07.747
So some customers
are more discerning.

01:01:07.747 --> 01:01:10.080
Obviously, if this is going
to large field installation,

01:01:10.080 --> 01:01:11.520
we have big barbed
wire around it.

01:01:11.520 --> 01:01:13.311
Who cares as long as
the module's producing

01:01:13.311 --> 01:01:14.120
high performance?

01:01:14.120 --> 01:01:16.410
But if it's sitting on
the facade of the train

01:01:16.410 --> 01:01:18.309
station in downtown
Freiburg, Germany,

01:01:18.309 --> 01:01:20.100
where every single
person riding the train,

01:01:20.100 --> 01:01:22.020
entering the station,
sees the modules lining

01:01:22.020 --> 01:01:24.050
the side of Deutsche
Bahn's headquarters,

01:01:24.050 --> 01:01:26.060
you want to make sure
that those look nice.

01:01:26.060 --> 01:01:28.970
So there are differences
depending on where they go

01:01:28.970 --> 01:01:31.210
and where they're installed.

01:01:31.210 --> 01:01:33.250
High efficiency
cell architectures.

01:01:33.250 --> 01:01:35.620
So there are a plethora
of different architectures

01:01:35.620 --> 01:01:37.566
out there.

01:01:37.566 --> 01:01:40.480
There are some that, for
example, put all their contacts

01:01:40.480 --> 01:01:43.090
on the backside, so
there's no shading.

01:01:43.090 --> 01:01:45.670
And these are interdigitated
positive, negative, positive,

01:01:45.670 --> 01:01:48.480
negative, positive,
negative contacts here.

01:01:48.480 --> 01:01:50.630
So this is called an
interdigitated back contact

01:01:50.630 --> 01:01:51.720
structure.

01:01:51.720 --> 01:01:54.250
It's used by the company
called Sun Power.

01:01:54.250 --> 01:01:57.380
And so there's no metalization
loss on the front side.

01:01:57.380 --> 01:01:58.940
All your contacts
are on the back.

01:01:58.940 --> 01:02:00.352
Because lateral
carrier diffusion

01:02:00.352 --> 01:02:01.935
is involved, meaning
the carriers have

01:02:01.935 --> 01:02:03.476
to diffuse laterally,
they don't have

01:02:03.476 --> 01:02:05.330
to diffuse only
one dimensionally,

01:02:05.330 --> 01:02:07.979
you probably can't use
PC1D to model this cell.

01:02:07.979 --> 01:02:09.770
You'll have to use a
two-dimensional device

01:02:09.770 --> 01:02:11.880
simulation like Sentaurus.

01:02:11.880 --> 01:02:13.740
If anybody has any
two-dimensional device

01:02:13.740 --> 01:02:15.940
simulation questions, Ashley
right here in the front

01:02:15.940 --> 01:02:18.908
is our resident expert, so
you're welcome to ask her.

01:02:18.908 --> 01:02:19.844
AUDIENCE: [INAUDIBLE]

01:02:19.844 --> 01:02:20.510
PROFESSOR: Yeah.

01:02:22.907 --> 01:02:24.990
And then there are also
other device architectures

01:02:24.990 --> 01:02:28.780
which we'll get to during
our thin films discussions.

01:02:28.780 --> 01:02:32.690
A couple of ancillary
topics, barriers to scale.

01:02:32.690 --> 01:02:36.020
This is the size of a 1 gigawatt
peak plant manufacturing

01:02:36.020 --> 01:02:39.790
facility for wafers,
cells, and modules.

01:02:39.790 --> 01:02:41.390
This is a palm tree right here.

01:02:41.390 --> 01:02:42.300
These are roads.

01:02:42.300 --> 01:02:44.190
So you get a sense of scale.

01:02:44.190 --> 01:02:46.410
This is located in Singapore.

01:02:46.410 --> 01:02:49.570
It's a company called REC
that has this factory.

01:02:49.570 --> 01:02:51.310
These are 18-wheelers
right here that are

01:02:51.310 --> 01:02:55.760
taking the materials out and
selling them to customers.

01:02:55.760 --> 01:02:59.725
So you get a sense of the
scale of these facilities.

01:02:59.725 --> 01:03:01.180
They're rather big.

01:03:01.180 --> 01:03:03.810
And if you say, OK,
this is a gigawatt fab,

01:03:03.810 --> 01:03:06.417
but we need to be producing on
the scale of terawatts, which

01:03:06.417 --> 01:03:08.250
are three orders of
magnitude larger in area

01:03:08.250 --> 01:03:10.571
than this, how big is
that factory going to be?

01:03:10.571 --> 01:03:12.445
It's about half of the
state of Rhode Island.

01:03:15.124 --> 01:03:16.790
Granted, it'll be
distributed throughout

01:03:16.790 --> 01:03:19.370
many different regions, but
it's a big, big factory.

01:03:19.370 --> 01:03:21.410
So one of the
interesting questions is,

01:03:21.410 --> 01:03:23.430
can we produce the
silicon in a faster

01:03:23.430 --> 01:03:26.200
way that involves less area?

01:03:26.200 --> 01:03:28.010
Because area generally
relates to capital

01:03:28.010 --> 01:03:30.569
equipment costs, not
always, but quite typically.

01:03:30.569 --> 01:03:32.110
If you have a larger
area because you

01:03:32.110 --> 01:03:33.580
need more equipment in
there, for more equipment,

01:03:33.580 --> 01:03:34.850
it's a higher cost.

01:03:34.850 --> 01:03:38.240
So can the production costs
be reduced by a higher

01:03:38.240 --> 01:03:40.390
throughput growth mechanisms?

01:03:40.390 --> 01:03:43.345
So instead of using thin film
or crystalline technologies

01:03:43.345 --> 01:03:47.100
that are currently being used
today-- apologies for that.

01:03:47.100 --> 01:03:52.440
Instead, if we used, let's say,
a float glass-like process.

01:03:52.440 --> 01:03:55.249
So these would be extruded
pieces of silicon on some bed

01:03:55.249 --> 01:03:57.540
of--I don't know-- liquid
tin would be for float glass,

01:03:57.540 --> 01:03:59.240
an equivalent for silicon.

01:03:59.240 --> 01:04:03.610
You could reduce the area by
about two orders of magnitude.

01:04:03.610 --> 01:04:06.660
And if you envision instead
these high speed printers that

01:04:06.660 --> 01:04:10.020
print out your reports for
your exam or class notes,

01:04:10.020 --> 01:04:12.750
they're spitting out 55
pages per minute on 8 and 1/2

01:04:12.750 --> 01:04:15.210
by 11 inch squared sheets.

01:04:15.210 --> 01:04:18.100
If instead those were 15%
solar cells being printed,

01:04:18.100 --> 01:04:21.100
you could envision an area the
size of five football fields

01:04:21.100 --> 01:04:22.730
instead.

01:04:22.730 --> 01:04:25.860
So this starts opening
the mind that, wow,

01:04:25.860 --> 01:04:27.860
our way of manufacturing
these solar cells,

01:04:27.860 --> 01:04:30.240
this discrete process
where it's very

01:04:30.240 --> 01:04:32.760
segregated-- wafer,
cell, and module.

01:04:32.760 --> 01:04:36.060
Wafer manufacturing
almost like a commodity.

01:04:36.060 --> 01:04:38.210
Ingot of aluminum.

01:04:38.210 --> 01:04:40.320
The cell like a device.

01:04:40.320 --> 01:04:43.045
The module-- as we'll see in
a second-- like an automobile,

01:04:43.045 --> 01:04:44.480
an assembly process.

01:04:44.480 --> 01:04:47.110
If instead we managed to
blend these processes together

01:04:47.110 --> 01:04:49.400
and reduce the barriers,
the discrete barriers

01:04:49.400 --> 01:04:50.990
between these
different processes

01:04:50.990 --> 01:04:53.410
and reinvent the
manufacturing process thereof,

01:04:53.410 --> 01:04:56.920
we stand to make this a lot
cheaper, and a lot faster,

01:04:56.920 --> 01:04:59.300
and a lot smaller to produce.

01:04:59.300 --> 01:05:01.570
We might even have our own
solar cell manufacturing

01:05:01.570 --> 01:05:02.879
equipment mounted on our desk.

01:05:02.879 --> 01:05:05.420
When we need to print a solar
cell device or power something,

01:05:05.420 --> 01:05:07.100
we can produce it right there.

01:05:07.100 --> 01:05:08.850
So that's kind of the
vision of the future

01:05:08.850 --> 01:05:10.470
where this might
be going and why

01:05:10.470 --> 01:05:12.690
bright minds like
yourselves are needed.

01:05:12.690 --> 01:05:13.700
We talked about silver.

01:05:13.700 --> 01:05:15.210
We know there's a limit
for how much silver

01:05:15.210 --> 01:05:17.260
can be used in the front
contact metalization.

01:05:17.260 --> 01:05:20.374
We're using about
10% of it right now.

01:05:20.374 --> 01:05:22.290
And if you're looking
for environmental impact

01:05:22.290 --> 01:05:25.600
of crystalline
silicon technologies,

01:05:25.600 --> 01:05:27.160
I've included many
different sites

01:05:27.160 --> 01:05:29.285
right here that talk about
the environmental impact

01:05:29.285 --> 01:05:33.790
of solar cell manufacturing
since we have mentioned acids.

01:05:33.790 --> 01:05:36.080
We have mentioned
gases like silane.

01:05:36.080 --> 01:05:40.277
We've mentioned CO2 production
when we produce the wafers.

01:05:40.277 --> 01:05:42.110
We'll talk about this
later on in the class,

01:05:42.110 --> 01:05:47.000
but in essence, we're looking
at around 1/10 or 1/20 the CO2

01:05:47.000 --> 01:05:48.550
intensity of coal.

01:05:48.550 --> 01:05:51.140
So it's not a zero-emission
source to produce that module,

01:05:51.140 --> 01:05:55.510
but it certainly is a lot less
than, say, our fossil fuel

01:05:55.510 --> 01:05:57.700
sources.

01:05:57.700 --> 01:06:00.520
This declining US
market share has really

01:06:00.520 --> 01:06:02.500
captured the attention
of politicians

01:06:02.500 --> 01:06:07.030
lately, the fact that the US
used to comprise 75% of the PV

01:06:07.030 --> 01:06:08.200
production market.

01:06:08.200 --> 01:06:10.530
This is to produce and
manufacture the modules.

01:06:10.530 --> 01:06:13.070
And today, it's on the order 5%.

01:06:13.070 --> 01:06:17.040
This is a risen concern
within many in the DOE

01:06:17.040 --> 01:06:20.490
and today's DOE and government.

01:06:20.490 --> 01:06:22.700
Meanwhile, the market is
growing substantially.

01:06:22.700 --> 01:06:24.190
And so an open
question is, what is

01:06:24.190 --> 01:06:28.710
the future of US market share?

01:06:28.710 --> 01:06:31.270
If all goes well, we should
have a small Greentech Media

01:06:31.270 --> 01:06:34.500
article published on this topic
probably within about a week

01:06:34.500 --> 01:06:37.310
or so, so keep your eyes open.

01:06:37.310 --> 01:06:40.804
And let me briefly jump
into module manufacturing.

01:06:40.804 --> 01:06:41.720
Do we have a question?

01:06:41.720 --> 01:06:42.670
Oh, we're all set.

01:06:42.670 --> 01:06:43.170
OK.

01:06:43.170 --> 01:06:44.425
I'm going to hop into
module manufacturing.

01:06:44.425 --> 01:06:45.980
It'll be the last five minutes.

01:06:45.980 --> 01:06:48.770
Just to show you how you go
from the cell to the module,

01:06:48.770 --> 01:06:52.150
it's an assembly process,
very, very straightforward.

01:06:52.150 --> 01:06:56.376
We have coming in here sheets
of glass, encapsulate materials.

01:06:56.376 --> 01:06:58.500
And we'll be able to see
this up close and personal

01:06:58.500 --> 01:06:59.875
and feel the
materials when we go

01:06:59.875 --> 01:07:02.440
visit Fraunhofer CSE in
the first week of November.

01:07:02.440 --> 01:07:03.939
We have a field
trip going up there.

01:07:03.939 --> 01:07:05.090
That'll be a lot of fun.

01:07:05.090 --> 01:07:06.850
And the encapsulants
are a lot of fun.

01:07:06.850 --> 01:07:07.890
They're polymers.

01:07:07.890 --> 01:07:09.070
They're really tough.

01:07:09.070 --> 01:07:11.490
You can take the Tedlar
back skin, this white stuff

01:07:11.490 --> 01:07:15.260
here in the back of the device,
that white skin right there.

01:07:15.260 --> 01:07:17.170
That's called Tedlar.

01:07:17.170 --> 01:07:19.500
As the name would suggest,
it comes from DuPont.

01:07:19.500 --> 01:07:21.190
It's a polymer.

01:07:21.190 --> 01:07:22.189
Really, really tough.

01:07:22.189 --> 01:07:24.355
If try to take some in your
hand and try to tear it,

01:07:24.355 --> 01:07:26.813
it's nearly impossible, even
for the strongest people here.

01:07:26.813 --> 01:07:28.960
So it's impermeable,
very strong material.

01:07:28.960 --> 01:07:32.560
The ethyl vinyl
acetate, or EVA, is

01:07:32.560 --> 01:07:36.300
a polymer that infuses the
glass in the front side

01:07:36.300 --> 01:07:37.210
with the cell.

01:07:37.210 --> 01:07:39.001
And with the Tedlar in
the back, it kind of

01:07:39.001 --> 01:07:42.150
forms this sticky,
mushy material

01:07:42.150 --> 01:07:44.039
when you heat it up
above 150 degrees C.

01:07:44.039 --> 01:07:46.580
And it binds everything together
in what's called a laminate.

01:07:46.580 --> 01:07:48.572
So let's walk through
that real quick.

01:07:48.572 --> 01:07:50.030
To get to the point
of a module, we

01:07:50.030 --> 01:07:52.287
need to take our good
apples with our good apples

01:07:52.287 --> 01:07:54.370
or our bad apples with our
bad apples, essentially

01:07:54.370 --> 01:07:57.440
the like-binned cells, and
start stringing them together.

01:07:57.440 --> 01:07:59.290
That means contacting
the front side

01:07:59.290 --> 01:08:01.470
with the backside
of adjacent cells.

01:08:01.470 --> 01:08:03.021
So the front of one
cell is connected

01:08:03.021 --> 01:08:04.020
to the back of the next.

01:08:04.020 --> 01:08:05.936
The front of that one
is connected to the back

01:08:05.936 --> 01:08:07.140
to the next, and so forth.

01:08:07.140 --> 01:08:09.470
And they're connected in
series in a big, long string.

01:08:09.470 --> 01:08:12.160
And that's done at this tabbing,
stringing, and layup table.

01:08:12.160 --> 01:08:15.450
Typically, this is done by
an automated solder system.

01:08:15.450 --> 01:08:18.624
I just put the cells together,
and it wires them for you.

01:08:18.624 --> 01:08:21.040
But usually, there's a manual
inspection process afterward

01:08:21.040 --> 01:08:22.956
because sometimes the
soldering isn't perfect.

01:08:22.956 --> 01:08:25.720
A human being is typically there
fidgeting through, making sure

01:08:25.720 --> 01:08:27.970
that everything is primo.

01:08:27.970 --> 01:08:29.960
Then we have the
lamination process,

01:08:29.960 --> 01:08:31.262
which takes those strings.

01:08:31.262 --> 01:08:32.720
They're very fragile
at this point.

01:08:32.720 --> 01:08:34.590
They're just solar
cells connected

01:08:34.590 --> 01:08:39.090
with some solder-coated
wire, so they're

01:08:39.090 --> 01:08:40.470
very fragile at that point.

01:08:40.470 --> 01:08:44.310
And these are then laid
up on the top of sheets

01:08:44.310 --> 01:08:47.670
of the encapsulant
materials and the glass

01:08:47.670 --> 01:08:49.560
and eventually
laminated together

01:08:49.560 --> 01:08:53.120
to form that nice package.

01:08:53.120 --> 01:08:55.660
So at the lamination stage,
coming out of the lamination,

01:08:55.660 --> 01:08:58.160
we'd have the glass on one side,
the Tedlar in the the back,

01:08:58.160 --> 01:09:00.910
and the cells in between
encapsulated by the ethyl vinyl

01:09:00.910 --> 01:09:02.609
acetate, the EVA.

01:09:02.609 --> 01:09:05.210
And we wouldn't have
the frame yet around it.

01:09:05.210 --> 01:09:07.550
And so the put that
frame, we would

01:09:07.550 --> 01:09:11.560
need essentially a
large machine that

01:09:11.560 --> 01:09:13.510
takes those pieces
of extruded aluminum

01:09:13.510 --> 01:09:15.970
and pushes them together around
the edges of the laminate,

01:09:15.970 --> 01:09:17.310
fixing them on there.

01:09:17.310 --> 01:09:21.850
And this is the examples of
the tabbing and stringing

01:09:21.850 --> 01:09:22.840
right there.

01:09:22.840 --> 01:09:24.750
And let's see, OK.

01:09:24.750 --> 01:09:28.080
So the framing
materials right here

01:09:28.080 --> 01:09:30.970
are typically done by machines
in places with high labor

01:09:30.970 --> 01:09:31.550
costs.

01:09:31.550 --> 01:09:34.180
And they're done by human
beings pushing them together

01:09:34.180 --> 01:09:36.189
at regions of low labor cost.

01:09:36.189 --> 01:09:40.010
And finally, the junction
box is deposited at the end.

01:09:40.010 --> 01:09:41.700
And the junction
box, what it does

01:09:41.700 --> 01:09:45.520
is it collects the power
outputs from each of the cells

01:09:45.520 --> 01:09:49.200
and very conveniently
gives you two leads.

01:09:49.200 --> 01:09:52.180
So there could be some power
electronics inside of here

01:09:52.180 --> 01:09:56.960
that allows the current
to flow around this module

01:09:56.960 --> 01:09:59.290
if this module's under
performing, if it's broken,

01:09:59.290 --> 01:10:00.837
or if it's shaded.

01:10:00.837 --> 01:10:03.170
There would be a bypass diode
inside of the junction box

01:10:03.170 --> 01:10:05.128
that allows the power to
flow around the module

01:10:05.128 --> 01:10:06.710
and not get sunk into it.

01:10:06.710 --> 01:10:09.440
And it also works to
collect the power outputs

01:10:09.440 --> 01:10:12.557
from all the cells and
produces two leads,

01:10:12.557 --> 01:10:14.140
which can then be
conveniently plugged

01:10:14.140 --> 01:10:16.265
into either adjacent modules,
which would be strung

01:10:16.265 --> 01:10:19.600
in series, or into an inverter,
which would then take the DC

01:10:19.600 --> 01:10:24.330
power here and convert it into
AC power for your consumption.

01:10:24.330 --> 01:10:28.030
And that is how a
solar cell is made.

01:10:28.030 --> 01:10:31.026
So I welcome you to spend a
few minutes at the very end

01:10:31.026 --> 01:10:33.525
to come up and take a close
look at some of these materials.

01:10:36.070 --> 01:10:37.720
Ask some further questions.

01:10:37.720 --> 01:10:40.180
And on Thursday,
we'll start diving

01:10:40.180 --> 01:10:42.970
into thin film technologies
and talk about how

01:10:42.970 --> 01:10:45.090
those are made as well.