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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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00:00:45,880 --> 00:00:47,380
and are aware that
on Thursday we're

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00:00:47,380 --> 00:00:50,460
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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00:00:54,340 --> 00:00:55,940
dollars per watt peak.

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00:00:55,940 --> 00:01:00,440
So this little exercise-- it
will last for about a month--

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00:01:00,440 --> 00:01:02,610
is coincident with
our technology

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00:01:02,610 --> 00:01:03,610
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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00:01:10,380 --> 00:01:12,370
is designing your own
solar cell and optimizing

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00:01:12,370 --> 00:01:13,040
the dollars per watt.

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00:01:13,040 --> 00:01:15,539
So this will entail actually
fabricating a solar cell, which

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is kind of fun.

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00:01:16,550 --> 00:01:20,980
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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00:01:24,210 --> 00:01:26,460
and finish up with a device,
a rudimentary device,

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00:01:26,460 --> 00:01:31,590
but something to take a picture
yourself, post on Facebook,

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00:01:31,590 --> 00:01:32,870
that sort of thing.

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00:01:32,870 --> 00:01:34,260
You design your own solar cell.

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00:01:34,260 --> 00:01:35,727
So the idea isn't
only to optimize

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00:01:35,727 --> 00:01:37,310
for the performance
of the solar cell,

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00:01:37,310 --> 00:01:39,490
but we decided to throw
in a little curve ball

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00:01:39,490 --> 00:01:41,720
and design for
dollars per watt peak.

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00:01:41,720 --> 00:01:43,720
Now this is a little bit
of a contrived exercise

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00:01:43,720 --> 00:01:46,795
since we've arbitrarily chosen
what dollars are associated

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00:01:46,795 --> 00:01:48,170
with each different
process step,

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00:01:48,170 --> 00:01:51,170
but it's not too unlike what you
would face in actual industry

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00:01:51,170 --> 00:01:53,310
if you had real data coming
off a production line

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00:01:53,310 --> 00:01:56,110
and knew exactly what it
cost for each process step.

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00:01:56,110 --> 00:02:00,400
So instead of having 30 plus
components in a more detailed

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00:02:00,400 --> 00:02:04,070
cost model, we've
decided to simplify it

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00:02:04,070 --> 00:02:06,069
to this little
diagram right here.

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00:02:06,069 --> 00:02:07,860
So this is a flow chart
for the fabrication

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00:02:07,860 --> 00:02:09,600
process of your solar cell.

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00:02:09,600 --> 00:02:11,366
You'll start with a wafer.

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00:02:11,366 --> 00:02:14,100
It has a certain cost
associated with it.

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00:02:14,100 --> 00:02:15,680
You'll have some
decisions to make

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concerning light
management, whether you

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want to texture
your front surface

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00:02:19,290 --> 00:02:21,600
or whether you want to
leave it bare and reflective

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00:02:21,600 --> 00:02:22,540
like this right here.

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00:02:25,164 --> 00:02:27,080
So whether you want a
reflective front service

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or you want to
texture it, there's

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00:02:28,496 --> 00:02:30,250
a certain costs
associated with it.

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00:02:30,250 --> 00:02:34,819
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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00:02:42,700 --> 00:02:44,380
texture your front surface.

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00:02:44,380 --> 00:02:46,540
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.

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00:02:57,590 --> 00:02:59,750
The text goes into
that in some detail.

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00:02:59,750 --> 00:03:03,810
But your decision is basically
if you make a shallow emitter,

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00:03:03,810 --> 00:03:05,810
you have less
Auger recombination

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in that front region.

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00:03:07,410 --> 00:03:10,600
And so your blue response to
the device will be better.

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But you run the risk when you
do your contact metalization

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00:03:13,430 --> 00:03:15,890
of firing through that
very shallow emitter

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00:03:15,890 --> 00:03:17,860
and shunting your device.

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00:03:17,860 --> 00:03:20,870
Whereas, if you decide
to go for a deep emitter,

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00:03:20,870 --> 00:03:23,530
it stays longer inside
of the furnace because

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00:03:23,530 --> 00:03:26,930
of the phosphorus will diffuse
deeper inside of the device.

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00:03:26,930 --> 00:03:29,000
You blue response
will be poorer,

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00:03:29,000 --> 00:03:31,710
but you'll have less
risk of shunting.

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So it's up to you to
use all of the tools

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00:03:33,500 --> 00:03:37,630
that you've assembled so far
to make a value-based judgment

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00:03:37,630 --> 00:03:40,930
whether or not it makes
sense to go with this or that

89
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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00:03:45,152 --> 00:03:46,610
this you can probably
guess already

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00:03:46,610 --> 00:03:51,260
pertains to series resistance
and shading losses.

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00:03:51,260 --> 00:03:54,470
So these are all
representative of trade-offs,

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00:03:54,470 --> 00:03:56,330
trade-offs in terms
of the technology

95
00:03:56,330 --> 00:03:58,390
and trade-offs in terms of cost.

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00:03:58,390 --> 00:04:00,710
And you have all the tools
necessary to calculate

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00:04:00,710 --> 00:04:03,560
or estimate what these
outputs should be based

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00:04:03,560 --> 00:04:05,780
on what you've learned so far.

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00:04:05,780 --> 00:04:07,330
And so by Thursday,
what we've asked

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00:04:07,330 --> 00:04:12,240
you to do is to make an
estimate of what technology

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00:04:12,240 --> 00:04:15,290
pathway your company
is going to pursue.

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00:04:15,290 --> 00:04:17,680
Remember, you want to
optimize the dollars per watt.

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00:04:17,680 --> 00:04:19,404
You want to minimize
that quantity, which

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00:04:19,404 --> 00:04:21,320
means you want to reduce
the number of dollars

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00:04:21,320 --> 00:04:22,810
you invest in your solar cell.

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00:04:22,810 --> 00:04:24,910
But you also want to
increase the watt peak

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00:04:24,910 --> 00:04:26,440
that you get out of it.

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00:04:26,440 --> 00:04:28,100
And so at the end
of the day, it'll

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00:04:28,100 --> 00:04:30,630
be a performance/cost
trade-off in each

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00:04:30,630 --> 00:04:32,780
of these different
process steps right here.

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00:04:32,780 --> 00:04:34,890
And sometimes it won't
be entirely obvious

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00:04:34,890 --> 00:04:38,280
which one to choose because
so many factors will converge.

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00:04:38,280 --> 00:04:40,280
And so it'll be up to you
to make an engineering

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00:04:40,280 --> 00:04:43,470
decision, a professional
judgment, as to which path

115
00:04:43,470 --> 00:04:45,140
you should pursue.

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00:04:45,140 --> 00:04:47,290
Since it is kind of--
you know, there's

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00:04:47,290 --> 00:04:49,020
a little element of
competition in here,

118
00:04:49,020 --> 00:04:51,977
so we decided the
dollars per watt peak

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00:04:51,977 --> 00:04:53,810
shouldn't be completely
neglected at the end

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00:04:53,810 --> 00:04:55,514
and we all get
certificates of merit

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00:04:55,514 --> 00:04:56,930
and all feel good
about ourselves.

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00:04:56,930 --> 00:04:59,471
We decided it should be worth
some part of the grade, but not

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00:04:59,471 --> 00:05:00,952
such a large
portion of the grade

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00:05:00,952 --> 00:05:03,160
that everybody's freaking
out and saying, oh my gosh,

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00:05:03,160 --> 00:05:05,380
I don't have the right
tools to make this decision.

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00:05:05,380 --> 00:05:07,980
I feel like I'm not
being graded fairly.

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00:05:07,980 --> 00:05:10,220
So the portion of
dollars per watt

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00:05:10,220 --> 00:05:12,040
is really only going
to be affecting

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00:05:12,040 --> 00:05:15,579
10% of the final grade
of quiz number two.

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00:05:15,579 --> 00:05:17,620
And so it will be based
on a ranking system where

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00:05:17,620 --> 00:05:20,170
the highest one will
be 100% and so forth.

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00:05:20,170 --> 00:05:21,560
But just 10% of your grade.

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00:05:21,560 --> 00:05:24,040
So it's enough to,
I would say, create

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00:05:24,040 --> 00:05:27,730
maybe a sting of the
pride if you don't happen

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00:05:27,730 --> 00:05:30,652
to hit the highest
performance metric, but not

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00:05:30,652 --> 00:05:33,110
enough to sting the actual
final grade of your class, which

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00:05:33,110 --> 00:05:36,160
will be one lumped quiz,
quiz one, home works, final,

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00:05:36,160 --> 00:05:36,920
and so forth.

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00:05:36,920 --> 00:05:37,860
Right?

140
00:05:37,860 --> 00:05:39,430
Any questions about
quiz two so far?

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00:05:39,430 --> 00:05:40,495
Yes, Jessica?

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00:05:40,495 --> 00:05:43,465
AUDIENCE: I
completely understand,

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00:05:43,465 --> 00:05:46,962
but there's even a
note in number three

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00:05:46,962 --> 00:05:47,920
under the deep emitter.

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00:05:47,920 --> 00:05:49,652
And you guys say,
any numbers you

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00:05:49,652 --> 00:05:51,204
should give as
far as [INAUDIBLE]

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00:05:51,204 --> 00:05:52,870
or are they responsible
for [INAUDIBLE].

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00:05:52,870 --> 00:05:54,720
It seems like its
lacking some numbers.

149
00:05:54,720 --> 00:05:56,908
And I understand
optimization, but I'm

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00:05:56,908 --> 00:06:00,468
having trouble putting
just how much better.

151
00:06:00,468 --> 00:06:02,884
And it say it'll be much more
effective if you do etching.

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00:06:02,884 --> 00:06:04,468
Well, how much is
much more effective?

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00:06:04,468 --> 00:06:05,967
PROFESSOR: Oh, the
etching for the--

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00:06:05,967 --> 00:06:08,340
AUDIENCE: For the etching,
we gave a rough [INAUDIBLE].

155
00:06:08,340 --> 00:06:08,828
So you can look that up.

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00:06:08,828 --> 00:06:09,804
AUDIENCE: I did look that up.

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00:06:09,804 --> 00:06:11,268
And for the other ones, is there

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00:06:11,268 --> 00:06:13,851
AUDIENCE: So that one, you can
get a pretty good estimate for.

159
00:06:13,851 --> 00:06:14,684
AUDIENCE: OK.

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00:06:14,684 --> 00:06:17,159
For the other ones, is there
going to be a [INAUDIBLE]

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00:06:17,159 --> 00:06:18,950
AUDIENCE: In terms of
shunting your device,

162
00:06:18,950 --> 00:06:20,991
it's really hard to predict
the shock resistance.

163
00:06:20,991 --> 00:06:24,100
But if you do shunt your
device, you essentially ruin it.

164
00:06:24,100 --> 00:06:26,945
So I would just take
that into account.

165
00:06:26,945 --> 00:06:29,852
You're not going to
get exact answers.

166
00:06:29,852 --> 00:06:32,169
But you can do your best
to estimate [INAUDIBLE]

167
00:06:32,169 --> 00:06:34,814
resistance from the [INAUDIBLE]
spacing and your emitter

168
00:06:34,814 --> 00:06:35,364
thinness.

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00:06:35,364 --> 00:06:36,030
PROFESSOR: Yeah.

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00:06:36,030 --> 00:06:37,330
Believe it or not,
you might feel

171
00:06:37,330 --> 00:06:38,954
like you don't have
the tools right now

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00:06:38,954 --> 00:06:41,260
to get quantitative
answers, but you do.

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00:06:41,260 --> 00:06:43,400
You have a number
of the tools here

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00:06:43,400 --> 00:06:45,600
to get, say, 90% the way there.

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00:06:45,600 --> 00:06:47,290
And in engineering,
90% of the way

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00:06:47,290 --> 00:06:50,220
there is well beyond what you'll
actually face in the field.

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00:06:50,220 --> 00:06:53,090
So that's pretty good.

178
00:06:53,090 --> 00:06:56,380
If you have specific
questions about what

179
00:06:56,380 --> 00:06:58,580
would be a good
resource to look up

180
00:06:58,580 --> 00:07:01,330
about this, what would be a good
resource to look up about that,

181
00:07:01,330 --> 00:07:02,310
send an email.

182
00:07:02,310 --> 00:07:05,740
And what I'll do, if I receive
something in that nature,

183
00:07:05,740 --> 00:07:08,384
I'll respond to the
class so that everybody

184
00:07:08,384 --> 00:07:10,425
has benefit to that
information and no one person

185
00:07:10,425 --> 00:07:12,050
is particularly advantaged.

186
00:07:12,050 --> 00:07:13,400
So it's worth a try.

187
00:07:13,400 --> 00:07:16,757
If it's something that was just
covered yesterday in lecture,

188
00:07:16,757 --> 00:07:18,340
I might be a little
bit more reticent.

189
00:07:18,340 --> 00:07:20,150
But if it is something
to the effect of,

190
00:07:20,150 --> 00:07:21,920
gee, how would the
lifetime improve

191
00:07:21,920 --> 00:07:25,020
with these different gettering
scenarios, sure, absolutely.

192
00:07:25,020 --> 00:07:27,221
We can give you a
little hand there.

193
00:07:27,221 --> 00:07:28,970
But everything else,
you should definitely

194
00:07:28,970 --> 00:07:30,840
have that information
available so far.

195
00:07:30,840 --> 00:07:33,840
This is meant to be a fun
exercise, but also one

196
00:07:33,840 --> 00:07:35,540
that illustrates the
trade-offs involved

197
00:07:35,540 --> 00:07:37,170
with designing solar cells.

198
00:07:37,170 --> 00:07:41,110
And trade-offs very
similar to this

199
00:07:41,110 --> 00:07:44,020
are evaluated on a
daily basis in industry,

200
00:07:44,020 --> 00:07:46,530
or perhaps not quite as often
as they should be in industry.

201
00:07:46,530 --> 00:07:47,680
But at some point, they were.

202
00:07:47,680 --> 00:07:49,430
And the designer of
the manufacturing line

203
00:07:49,430 --> 00:07:52,620
made those judgment calls.

204
00:07:52,620 --> 00:07:53,440
OK.

205
00:07:53,440 --> 00:07:58,640
So again, the pre-analysis,
what is due on Thursday

206
00:07:58,640 --> 00:08:00,487
is 20% of the grade.

207
00:08:00,487 --> 00:08:02,820
The dollars per watt peak
metric, at the end of the day,

208
00:08:02,820 --> 00:08:03,955
is only 10% of the grade.

209
00:08:03,955 --> 00:08:06,204
It's meant to really serve
as a stimulus, a little bit

210
00:08:06,204 --> 00:08:08,816
of competition, but not
meant to really harm you

211
00:08:08,816 --> 00:08:12,132
if you happen to not
achieve a good value there.

212
00:08:12,132 --> 00:08:14,090
And this is meant to be
an educational mission,

213
00:08:14,090 --> 00:08:16,920
so the solar cell efficiency
analysis at the end

214
00:08:16,920 --> 00:08:18,045
is really heavily weighted.

215
00:08:18,045 --> 00:08:20,253
We'll be walking through
some of the characterization

216
00:08:20,253 --> 00:08:21,690
tools in the
laboratory so you can

217
00:08:21,690 --> 00:08:24,040
determine what exactly went
wrong with your devices

218
00:08:24,040 --> 00:08:25,500
and quantify them.

219
00:08:25,500 --> 00:08:27,910
And that'll be a
real chance for you

220
00:08:27,910 --> 00:08:30,155
to get a tutorial of how
solar cells are not only

221
00:08:30,155 --> 00:08:32,530
made-- you'll be there when
they're actually fabricated--

222
00:08:32,530 --> 00:08:35,450
but also how they're analyzed
and how they're assessed.

223
00:08:35,450 --> 00:08:38,758
So it's up to you to really
grab this opportunity.

224
00:08:38,758 --> 00:08:40,549
Maybe if you're working
in your own devices

225
00:08:40,549 --> 00:08:42,190
and want to bring
some of them along,

226
00:08:42,190 --> 00:08:43,330
you're welcome to
do that as well.

227
00:08:43,330 --> 00:08:45,829
We won't take up the time when
everyone else is in the room,

228
00:08:45,829 --> 00:08:47,490
but we might stay
longer afterward

229
00:08:47,490 --> 00:08:50,430
and help you walk through
the analysis as well.

230
00:08:50,430 --> 00:08:52,816
And what we've done, just
to resituate ourselves,

231
00:08:52,816 --> 00:08:54,690
we talked about the
silicon feedstock, right?

232
00:08:54,690 --> 00:08:59,970
So we chatted about
how you go from quartz

233
00:08:59,970 --> 00:09:02,190
in the ground and the
carbon-baring feedstock

234
00:09:02,190 --> 00:09:07,940
material to the purified, highly
purified, silicon feedstock

235
00:09:07,940 --> 00:09:08,440
material.

236
00:09:08,440 --> 00:09:10,450
This right here is
probably on the order

237
00:09:10,450 --> 00:09:15,470
of somewhere between
8, 9, or 10 nines pure,

238
00:09:15,470 --> 00:09:19,540
very, very pure material,
this Siemens-grade polysilicon

239
00:09:19,540 --> 00:09:20,900
right here in my hand.

240
00:09:20,900 --> 00:09:23,660
And you've taken a look
at this during last class,

241
00:09:23,660 --> 00:09:27,010
so you have a sense of what
it is up close and personal.

242
00:09:27,010 --> 00:09:30,120
Silicon, in fact, has
been so well refined

243
00:09:30,120 --> 00:09:32,520
that, for a period of
time, NIST, the National

244
00:09:32,520 --> 00:09:34,260
Institutes of Standards
in Technologies,

245
00:09:34,260 --> 00:09:37,680
they were thinking about
redefining the unit of mass

246
00:09:37,680 --> 00:09:40,820
in terms of a silicon
boule, essentially a silicon

247
00:09:40,820 --> 00:09:43,870
sphere, that would be polished
down to about 4 nanometers

248
00:09:43,870 --> 00:09:46,090
mean surface roughness
with a very low defect

249
00:09:46,090 --> 00:09:48,870
density, isotopically
pure silicon

250
00:09:48,870 --> 00:09:50,920
to serve as a new
standard for mass

251
00:09:50,920 --> 00:09:53,470
because it could just
be purified so well

252
00:09:53,470 --> 00:09:56,750
and because their standard
reference units were beginning

253
00:09:56,750 --> 00:09:58,630
to shift relative
to all the others

254
00:09:58,630 --> 00:10:00,300
around the world,
the one in Paris

255
00:10:00,300 --> 00:10:03,080
relative to the ones that were
stored in Washington and Delhi

256
00:10:03,080 --> 00:10:04,400
and others around the world.

257
00:10:04,400 --> 00:10:07,230
The values of the mass were
shifting as a function of time

258
00:10:07,230 --> 00:10:08,980
when they would perform
these round-robin.

259
00:10:08,980 --> 00:10:11,110
So either the mass
in Paris was changing

260
00:10:11,110 --> 00:10:12,720
or everybody else was changing.

261
00:10:12,720 --> 00:10:15,530
This obviously was
unacceptable for an institute

262
00:10:15,530 --> 00:10:17,010
that was focused on standards.

263
00:10:17,010 --> 00:10:18,980
And so they decided
to reformulate

264
00:10:18,980 --> 00:10:20,840
the standard for mass.

265
00:10:20,840 --> 00:10:23,400
I'm not quite sure where that
project currently stands.

266
00:10:23,400 --> 00:10:25,210
So if anybody has
further information

267
00:10:25,210 --> 00:10:28,652
about the NIST unit of mass,
I'd be happy to hear it.

268
00:10:28,652 --> 00:10:31,110
But that gives you an idea of
how well-purified silicon can

269
00:10:31,110 --> 00:10:34,340
be and how well-controlled
it can be as well.

270
00:10:34,340 --> 00:10:36,790
During the integrated
circuit fabrication,

271
00:10:36,790 --> 00:10:41,210
which uses this ultra
purity silicon to produce

272
00:10:41,210 --> 00:10:44,680
very nice single crystal wafers
like this one right here,

273
00:10:44,680 --> 00:10:47,600
the investment per gram of
silicon can be on the order

274
00:10:47,600 --> 00:10:51,130
a few tens or even low hundreds
of dollars per gram of silicon

275
00:10:51,130 --> 00:10:52,600
and still turn a profit.

276
00:10:52,600 --> 00:10:54,910
But in the solar cell,
on the other hand,

277
00:10:54,910 --> 00:10:56,560
you can invest, at
most, a few tens

278
00:10:56,560 --> 00:10:58,280
of cents per gram of silicon.

279
00:10:58,280 --> 00:11:01,040
This is because
the solar cell has

280
00:11:01,040 --> 00:11:03,370
to compete against bulk power.

281
00:11:03,370 --> 00:11:04,800
That's its
competition coming out

282
00:11:04,800 --> 00:11:06,510
of the wall right over there.

283
00:11:06,510 --> 00:11:09,270
So the solar cell has to be
able to be produced much more

284
00:11:09,270 --> 00:11:10,000
cheaply.

285
00:11:10,000 --> 00:11:12,280
And as a result,
typically thinner wafers

286
00:11:12,280 --> 00:11:15,540
are used and less expensive
starting materials and faster

287
00:11:15,540 --> 00:11:18,610
growth methods, resulting in
more defect-rich materials.

288
00:11:18,610 --> 00:11:25,240
So one group decided, gee,
the embedded cost in the wafer

289
00:11:25,240 --> 00:11:27,480
is just so large,
it's just so large

290
00:11:27,480 --> 00:11:29,190
that we have to make it thinner.

291
00:11:29,190 --> 00:11:32,480
And we have to avoid using these
ingots, like this one right

292
00:11:32,480 --> 00:11:35,260
here, from which
these wafers are sawn.

293
00:11:35,260 --> 00:11:38,770
So your wafers are sawn
out of the ingot like this,

294
00:11:38,770 --> 00:11:40,870
like shown.

295
00:11:40,870 --> 00:11:44,710
During the process, about 50% of
the silicon is lost to sawdust.

296
00:11:44,710 --> 00:11:47,330
And they said, well, let's
develop a better way.

297
00:11:47,330 --> 00:11:49,340
Let's extrude the
wafers directly out

298
00:11:49,340 --> 00:11:52,610
of liquid molten silicon
and make ribbons of silicon

299
00:11:52,610 --> 00:11:53,440
instead.

300
00:11:53,440 --> 00:11:55,040
That way, we don't
have the sawdust,

301
00:11:55,040 --> 00:11:57,581
and we don't have to have this
expensive ingot solidification

302
00:11:57,581 --> 00:11:58,350
step.

303
00:11:58,350 --> 00:12:00,230
So ribbon growth
has been explored

304
00:12:00,230 --> 00:12:03,460
since the 1970s at least.

305
00:12:03,460 --> 00:12:05,870
And the advantages is that
you have no kerf loss,

306
00:12:05,870 --> 00:12:09,210
in other words, no sawdust, due
to wire sawing and, hence, more

307
00:12:09,210 --> 00:12:10,950
efficient silicon utilization.

308
00:12:10,950 --> 00:12:13,470
Immediately out of the gate,
if your wafer yields are

309
00:12:13,470 --> 00:12:16,060
comparable, you get
about a factor of 2 gain

310
00:12:16,060 --> 00:12:19,940
because this wafer right here
is about 170 microns thick.

311
00:12:19,940 --> 00:12:22,826
And the sawdust is around
170 microns as well.

312
00:12:22,826 --> 00:12:24,450
So that's about a
factor of 2 if you're

313
00:12:24,450 --> 00:12:26,190
able to produce a
ribbon of silicon

314
00:12:26,190 --> 00:12:28,120
directly out of the melt.

315
00:12:28,120 --> 00:12:30,450
The disadvantage is that
traditionally there's

316
00:12:30,450 --> 00:12:32,220
been lower material
quality and, hence,

317
00:12:32,220 --> 00:12:34,520
lower performance
because of the thermal

318
00:12:34,520 --> 00:12:36,260
stresses during
growth of a very, very

319
00:12:36,260 --> 00:12:38,680
thin foil or thin fin.

320
00:12:38,680 --> 00:12:40,310
The thermal stresses
can be larger,

321
00:12:40,310 --> 00:12:42,600
resulting in plasticity,
resulting in dislocations

322
00:12:42,600 --> 00:12:45,560
and other defects that can
reduce minority carrier

323
00:12:45,560 --> 00:12:46,580
lifetime.

324
00:12:46,580 --> 00:12:49,370
And traditionally, there has
been as well a higher capex.

325
00:12:49,370 --> 00:12:51,580
And a third disadvantage,
traditionally, in ribbon

326
00:12:51,580 --> 00:12:53,935
has been that the form factor
or the shape of the wafer

327
00:12:53,935 --> 00:12:56,390
has just been different
than the ingot material.

328
00:12:56,390 --> 00:12:57,919
Why is that important?

329
00:12:57,919 --> 00:13:00,210
Well, if you're trying to
displace the dominant design,

330
00:13:00,210 --> 00:13:05,140
the wafer, you would do well to
make your wafer the same size

331
00:13:05,140 --> 00:13:06,880
and shape as the
dominant design.

332
00:13:06,880 --> 00:13:07,730
Why is that?

333
00:13:07,730 --> 00:13:10,570
Well, if you want to make a cell
out of it or solar cell device,

334
00:13:10,570 --> 00:13:12,070
you'd want to make
sure that you can

335
00:13:12,070 --> 00:13:14,230
take advantage of the same
manufacturing equipment.

336
00:13:14,230 --> 00:13:15,688
And that's just a
plug-in-and-play,

337
00:13:15,688 --> 00:13:16,750
drop-in replacement.

338
00:13:16,750 --> 00:13:19,606
If you require customization
of the downstream components

339
00:13:19,606 --> 00:13:21,230
on the cell in the
module level, you'll

340
00:13:21,230 --> 00:13:24,100
wind up having to invest more
money in those processes, which

341
00:13:24,100 --> 00:13:26,510
might counteract the
advantage that you get out

342
00:13:26,510 --> 00:13:28,350
of using less silicon.

343
00:13:28,350 --> 00:13:29,272
Yes, Ashley?

344
00:13:29,272 --> 00:13:30,230
AUDIENCE: What's capex?

345
00:13:30,230 --> 00:13:30,380
Is it--

346
00:13:30,380 --> 00:13:31,255
PROFESSOR: Oh, capex.

347
00:13:31,255 --> 00:13:34,410
Capex stands for capital
expenditure, capital equipment

348
00:13:34,410 --> 00:13:35,270
expenditure.

349
00:13:35,270 --> 00:13:38,010
And that relates to the
cost of the equipment that

350
00:13:38,010 --> 00:13:42,400
is typically-- well, in the
business world, typically

351
00:13:42,400 --> 00:13:45,060
one undergoes what's called an
accelerated depreciation where

352
00:13:45,060 --> 00:13:47,640
you amortize the cost of the
equipment over five years

353
00:13:47,640 --> 00:13:50,230
but then assume that it runs
over a longer period, maybe

354
00:13:50,230 --> 00:13:55,720
7, 10 years or so
giving you profit back.

355
00:13:55,720 --> 00:13:59,150
So in layman's terms,
what this means is capex

356
00:13:59,150 --> 00:14:01,480
is the equipment
cost, in other words.

357
00:14:01,480 --> 00:14:03,980
And then you just take
the cost of the equipment

358
00:14:03,980 --> 00:14:05,247
and parse it out.

359
00:14:05,247 --> 00:14:07,330
For each wafer you produce,
you allocate a portion

360
00:14:07,330 --> 00:14:09,280
of equipment cost to that.

361
00:14:09,280 --> 00:14:11,750
So let's take a little
walk through history

362
00:14:11,750 --> 00:14:14,100
and go back to some
of the earliest

363
00:14:14,100 --> 00:14:17,270
methods of ribbon growth.

364
00:14:17,270 --> 00:14:20,620
So one of the earliest
forms of ribbon growth

365
00:14:20,620 --> 00:14:24,050
was the so-called edge
supported ribbon, also known

366
00:14:24,050 --> 00:14:25,270
as string ribbon.

367
00:14:25,270 --> 00:14:30,100
And there were developments
of this general technology

368
00:14:30,100 --> 00:14:31,420
in different places.

369
00:14:31,420 --> 00:14:34,720
Ely Sachs, former
professor here at MIT, now

370
00:14:34,720 --> 00:14:37,800
founder and CTO of 1366
Technologies just up

371
00:14:37,800 --> 00:14:41,510
the road in Lexington, developed
the string ribbon material

372
00:14:41,510 --> 00:14:45,830
here at MIT in the
early 1980s, late 1970s.

373
00:14:45,830 --> 00:14:48,350
And the general idea
was to use two filaments

374
00:14:48,350 --> 00:14:51,020
like so that would be
passed through a crucible.

375
00:14:51,020 --> 00:14:52,690
And then the silicon
would flow in

376
00:14:52,690 --> 00:14:57,060
between those two filaments
much like soapy water

377
00:14:57,060 --> 00:15:00,370
flows between the little
circle when you blow bubbles.

378
00:15:00,370 --> 00:15:03,450
So a meniscus would form here
and then eventually solidify

379
00:15:03,450 --> 00:15:05,140
into a solid piece
of silicon, and you'd

380
00:15:05,140 --> 00:15:08,709
have edge-supported ribbon,
otherwise known as string

381
00:15:08,709 --> 00:15:10,750
ribbon because you're
using the strings to define

382
00:15:10,750 --> 00:15:11,940
the edge of the ribbon.

383
00:15:11,940 --> 00:15:15,270
So I have a wafer here, an
example of a wafer here,

384
00:15:15,270 --> 00:15:18,480
a string ribbon sample.

385
00:15:18,480 --> 00:15:19,440
Oh, here it is.

386
00:15:19,440 --> 00:15:22,320
It's hiding from me.

387
00:15:22,320 --> 00:15:27,026
So this is an example of
one of those materials.

388
00:15:27,026 --> 00:15:28,230
Here we go.

389
00:15:28,230 --> 00:15:30,370
And like usual, it's good
to handle these wafers

390
00:15:30,370 --> 00:15:34,950
with some care almost
like a photograph.

391
00:15:34,950 --> 00:15:37,430
So here's an example
of a string ribbon

392
00:15:37,430 --> 00:15:41,510
wafer, one particular wafer
that was laser cut out

393
00:15:41,510 --> 00:15:42,700
of a growing ribbon.

394
00:15:42,700 --> 00:15:45,910
As you can see, this larger
ribbon right here-- these

395
00:15:45,910 --> 00:15:48,410
can grow up to be
a few meters long.

396
00:15:48,410 --> 00:15:49,670
They're rather long.

397
00:15:49,670 --> 00:15:53,220
You can pick them up if you
have gloves on your hands.

398
00:15:53,220 --> 00:15:55,890
And they're quite
flexible at that length.

399
00:15:55,890 --> 00:15:58,030
You could actually
even bend them

400
00:15:58,030 --> 00:16:01,320
with a radius of curvature
of about a couple of meters.

401
00:16:01,320 --> 00:16:04,880
So the reason you wear
gloves, obviously,

402
00:16:04,880 --> 00:16:07,904
is to prevent your fingers
some soiling the wafer.

403
00:16:07,904 --> 00:16:10,070
We talked about sodium
contamination and other forms

404
00:16:10,070 --> 00:16:11,050
of contamination.

405
00:16:11,050 --> 00:16:13,220
Silicon is nontoxic,
so it won't affect you.

406
00:16:13,220 --> 00:16:14,890
It's really you
affecting the wafer,

407
00:16:14,890 --> 00:16:17,000
much like putting
fingerprints all over

408
00:16:17,000 --> 00:16:19,680
a nice, clean photograph.

409
00:16:19,680 --> 00:16:22,840
So there were
similar technologies

410
00:16:22,840 --> 00:16:28,150
developed by Ted [INAUDIBLE]
at NREL out in Colorado.

411
00:16:28,150 --> 00:16:30,010
But the general idea
is shown right here.

412
00:16:30,010 --> 00:16:34,720
Now some of the earliest
edge-supported ribbon samples

413
00:16:34,720 --> 00:16:36,384
were developed back in 1970s.

414
00:16:36,384 --> 00:16:37,800
It really took a
while before they

415
00:16:37,800 --> 00:16:39,480
were commercialized in full.

416
00:16:39,480 --> 00:16:42,390
And that was done through
Evergreen Solar, which

417
00:16:42,390 --> 00:16:46,240
was founded in 1994 by Jack
Hanoka, Rich Chlebowski,

418
00:16:46,240 --> 00:16:49,590
and-- oh, goodness--
Mark Farber.

419
00:16:49,590 --> 00:16:52,790
So the three of them a
co-founded Evergreen Solar.

420
00:16:52,790 --> 00:16:55,530
And they developed the
string ribbon growth process

421
00:16:55,530 --> 00:16:56,640
shown right over here.

422
00:16:56,640 --> 00:17:00,350
Eventually two ribbons face
to face, and now four ribbons

423
00:17:00,350 --> 00:17:01,699
side by side.

424
00:17:01,699 --> 00:17:03,490
So this was called the
Gemini because there

425
00:17:03,490 --> 00:17:05,048
were two ribbons face to face.

426
00:17:05,048 --> 00:17:06,589
And then eventually,
the quad process

427
00:17:06,589 --> 00:17:09,180
were four ribbons edge to edge.

428
00:17:09,180 --> 00:17:11,510
And you can see the
conventional ingot

429
00:17:11,510 --> 00:17:12,810
multi-crystalline silicon.

430
00:17:12,810 --> 00:17:15,664
Here, the different
steps forming the ingot,

431
00:17:15,664 --> 00:17:18,020
eventually slicing, and so
forth and the string ribbon

432
00:17:18,020 --> 00:17:22,920
process here being much
simplified in correspondence.

433
00:17:22,920 --> 00:17:24,660
So not only was the
process simpler,

434
00:17:24,660 --> 00:17:26,910
but you'd use about
half as much silicon.

435
00:17:26,910 --> 00:17:30,810
And here's Rick Wallace,
the inventor and developer

436
00:17:30,810 --> 00:17:33,110
of the Gemini process,
up there showing

437
00:17:33,110 --> 00:17:37,300
one of these longer meter-length
ribbons with some flexibility.

438
00:17:37,300 --> 00:17:42,830
So the company had
a joint venture

439
00:17:42,830 --> 00:17:47,760
with REC and Q-Cells,
Norwegian and German companies

440
00:17:47,760 --> 00:17:50,450
respectively, to form
a factory in Germany.

441
00:17:50,450 --> 00:17:52,229
REC would supply the
silicon feedstock,

442
00:17:52,229 --> 00:17:53,770
Evergreen the growth
technology here,

443
00:17:53,770 --> 00:17:57,400
and Q-Cell some of the
cell fabrication expertise.

444
00:17:57,400 --> 00:18:00,592
And very recently,
Evergreen Solar

445
00:18:00,592 --> 00:18:02,300
encountered some
financial difficulties--

446
00:18:02,300 --> 00:18:04,716
we'll get into that during the
third section of the course

447
00:18:04,716 --> 00:18:08,000
when we talk about
cross-cutting themes-- and is

448
00:18:08,000 --> 00:18:10,160
in the process of
filing for bankruptcy.

449
00:18:10,160 --> 00:18:14,010
So this process-- so Sovello is
continuing as its own company,

450
00:18:14,010 --> 00:18:17,000
but the Evergreen plant here in
Massachusetts in Marlborough,

451
00:18:17,000 --> 00:18:20,340
about an hour west of here,
has effectively shut down.

452
00:18:20,340 --> 00:18:24,700
So that was the trajectory
of this particular technology

453
00:18:24,700 --> 00:18:29,460
through commercialization
and ultimately not making it.

454
00:18:29,460 --> 00:18:31,060
If you would like,
my personal opinion

455
00:18:31,060 --> 00:18:36,100
about why Evergreen
never quite took off,

456
00:18:36,100 --> 00:18:37,710
yes, there are some
technical factors,

457
00:18:37,710 --> 00:18:41,030
but as well it failed to
grow fast enough to keep up

458
00:18:41,030 --> 00:18:42,880
with the rest of the
industry and scale

459
00:18:42,880 --> 00:18:44,242
with the rest of the industry.

460
00:18:44,242 --> 00:18:45,700
And part of that
can be traced back

461
00:18:45,700 --> 00:18:48,210
to the mid 2000s when
silicon was scarce,

462
00:18:48,210 --> 00:18:51,450
the inability to source
the feedstock material.

463
00:18:51,450 --> 00:18:52,400
Yeah?

464
00:18:52,400 --> 00:18:53,350
AUDIENCE: Excuse me.

465
00:18:53,350 --> 00:18:55,534
Can you back one slide?

466
00:18:55,534 --> 00:18:56,200
PROFESSOR: Sure.

467
00:18:58,897 --> 00:18:59,605
It takes a while.

468
00:18:59,605 --> 00:19:00,624
It's a big file.

469
00:19:00,624 --> 00:19:01,124
OK.

470
00:19:01,124 --> 00:19:06,070
AUDIENCE: How do you seal the
space between the filaments

471
00:19:06,070 --> 00:19:07,606
and the bottom of the crucible?

472
00:19:07,606 --> 00:19:08,855
PROFESSOR: Right there, right?

473
00:19:08,855 --> 00:19:09,140
AUDIENCE: Yeah.

474
00:19:09,140 --> 00:19:11,556
PROFESSOR: Since this is your
graphite crucible right here

475
00:19:11,556 --> 00:19:13,400
and these are your
filaments popping up

476
00:19:13,400 --> 00:19:17,090
through the graphite, the beauty
is you don't have to seal that.

477
00:19:17,090 --> 00:19:20,960
The surface tension of silicon
is greater than that of water.

478
00:19:20,960 --> 00:19:24,320
So if you've ever filled up
water to the top of a glass

479
00:19:24,320 --> 00:19:26,290
and seen that
meniscus that forms,

480
00:19:26,290 --> 00:19:28,817
the silicon meniscus would
be even higher than that.

481
00:19:28,817 --> 00:19:29,900
AUDIENCE: Oh, that's cool.

482
00:19:29,900 --> 00:19:30,566
PROFESSOR: Yeah.

483
00:19:30,566 --> 00:19:31,798
It's pretty nifty.

484
00:19:31,798 --> 00:19:33,988
AUDIENCE: So I'm imagining
just like molten metal.

485
00:19:33,988 --> 00:19:34,700
You don't want that
spilling out the bottom.

486
00:19:34,700 --> 00:19:35,283
PROFESSOR: No.

487
00:19:35,283 --> 00:19:36,570
AUDIENCE: That's really cool.

488
00:19:36,570 --> 00:19:37,070
OK.

489
00:19:37,070 --> 00:19:37,570
Cool.

490
00:19:37,570 --> 00:19:38,966
PROFESSOR: Yeah.

491
00:19:38,966 --> 00:19:40,862
AUDIENCE: Are the
ribbons a single crystal?

492
00:19:40,862 --> 00:19:42,446
Or are there grain
boundaries in them?

493
00:19:42,446 --> 00:19:43,111
PROFESSOR: Yeah.

494
00:19:43,111 --> 00:19:45,160
So let me show you the
actual ribbon right here,

495
00:19:45,160 --> 00:19:47,520
and you can inspect
it first hand.

496
00:19:47,520 --> 00:19:50,629
These do indeed have
grain boundaries.

497
00:19:50,629 --> 00:19:52,920
So what I'll do is I'll place
the ribbon inside of here

498
00:19:52,920 --> 00:19:54,930
for ease of carrying around.

499
00:19:54,930 --> 00:19:56,640
If you'd like to take
it out, feel free.

500
00:19:56,640 --> 00:19:58,056
They're more where
this came from.

501
00:19:58,056 --> 00:20:00,390
So in case there was a little
accident along the way,

502
00:20:00,390 --> 00:20:01,860
don't feel too bad.

503
00:20:01,860 --> 00:20:04,400
The growth of an ingot
is about one to two days,

504
00:20:04,400 --> 00:20:06,630
but you get thousands
of wafers out.

505
00:20:06,630 --> 00:20:11,720
The growth of a wafer itself--
if the growth rate was around,

506
00:20:11,720 --> 00:20:14,670
say, let's pick a number
somewhere between 2

507
00:20:14,670 --> 00:20:17,309
and 5 centimeters
per minute, then

508
00:20:17,309 --> 00:20:18,850
it would take--
let's see, with this,

509
00:20:18,850 --> 00:20:20,770
you have a 15
centimeter wafer-- it

510
00:20:20,770 --> 00:20:23,210
would take somewhere on
the order of four minutes

511
00:20:23,210 --> 00:20:24,550
to grow wafer.

512
00:20:24,550 --> 00:20:29,680
And you'd have a faster
growth of single wafers

513
00:20:29,680 --> 00:20:32,965
from the ribbon process, of
course, lower throughput.

514
00:20:32,965 --> 00:20:35,090
The silicon utilization of
the wafer growth process

515
00:20:35,090 --> 00:20:40,190
was a lot higher than
that of the ingot growth.

516
00:20:40,190 --> 00:20:42,430
Some smart people
realized along the way

517
00:20:42,430 --> 00:20:46,070
that you could grow
these ribbons vertically,

518
00:20:46,070 --> 00:20:48,730
but you encountered
the following problem.

519
00:20:48,730 --> 00:20:52,640
During the growth
of-- here you go.

520
00:20:52,640 --> 00:20:54,730
During the growth of
a vertical ribbons,

521
00:20:54,730 --> 00:20:58,660
if this was the ribbon
growing vertically--

522
00:20:58,660 --> 00:21:00,160
it should be straight.

523
00:21:00,160 --> 00:21:00,935
Apologies.

524
00:21:00,935 --> 00:21:02,680
There we go.

525
00:21:02,680 --> 00:21:05,980
Let's make sure we're
good engineers here.

526
00:21:05,980 --> 00:21:11,180
And so this is meant to
represent a growing ribbon.

527
00:21:11,180 --> 00:21:15,580
This is the liquid, and this is
the solid silicon right here.

528
00:21:15,580 --> 00:21:20,544
The growth velocity would be
in this direction right here.

529
00:21:20,544 --> 00:21:23,210
So you're growing the ribbon out
of the melt. This is your melt.

530
00:21:23,210 --> 00:21:24,370
This is the ribbon
that's growing up.

531
00:21:24,370 --> 00:21:26,328
You're looking at the
cross section right here,

532
00:21:26,328 --> 00:21:28,430
so looking at the
ribbon edge on.

533
00:21:28,430 --> 00:21:30,090
So you're pulling it
in this direction,

534
00:21:30,090 --> 00:21:32,090
so the growth velocity is here.

535
00:21:32,090 --> 00:21:37,530
And the direction of
latent heat of fusion

536
00:21:37,530 --> 00:21:41,170
extraction-- so you have liquid
silicon solidifying here.

537
00:21:41,170 --> 00:21:43,990
During the solidification
process, there's heat released.

538
00:21:43,990 --> 00:21:45,680
And that heat has
to be conducted up

539
00:21:45,680 --> 00:21:48,320
the solid and then radiated
outward from the fin,

540
00:21:48,320 --> 00:21:50,240
from this thin ribbon.

541
00:21:50,240 --> 00:21:53,920
So the direction
of heat extraction

542
00:21:53,920 --> 00:21:57,240
is also parallel to the
direction of growth.

543
00:21:57,240 --> 00:21:59,970
What that means is
the growth velocity

544
00:21:59,970 --> 00:22:02,610
will be limited by the speed
at which you can extract heat

545
00:22:02,610 --> 00:22:04,850
up the ribbon and
then radiated outward.

546
00:22:04,850 --> 00:22:06,410
So there are many
ideas tossed around

547
00:22:06,410 --> 00:22:09,760
about potentially
growing in media

548
00:22:09,760 --> 00:22:13,140
that are able to extract
heat [INAUDIBLE] transport.

549
00:22:13,140 --> 00:22:14,550
You can use your imagination.

550
00:22:14,550 --> 00:22:17,350
But ultimately, growth
continues in air,

551
00:22:17,350 --> 00:22:20,950
and you're limited to, at
most, around 5 centimeters

552
00:22:20,950 --> 00:22:24,500
per minute growth velocity
because of the extraction

553
00:22:24,500 --> 00:22:25,530
of latent heat.

554
00:22:25,530 --> 00:22:27,320
If you try to grow
faster than that,

555
00:22:27,320 --> 00:22:31,240
you'll eventually just pull
the solid off of the liquid.

556
00:22:31,240 --> 00:22:36,070
It'll dissociate much like
pulling an ice cube off

557
00:22:36,070 --> 00:22:37,999
of a top of a glass of water.

558
00:22:37,999 --> 00:22:40,290
Surface tension won't be able
to hold the two together.

559
00:22:40,290 --> 00:22:44,040
So you have here a conundrum.

560
00:22:44,040 --> 00:22:45,500
How do you grow faster?

561
00:22:45,500 --> 00:22:47,160
If you want to
increase the throughput

562
00:22:47,160 --> 00:22:50,160
and instead of spending
minutes to grow wafer,

563
00:22:50,160 --> 00:22:53,130
you'd like to grow a wafer per
second, how do you do that?

564
00:22:53,130 --> 00:22:55,940
Well, one group of folks thought
about this a bit and said,

565
00:22:55,940 --> 00:22:58,360
well, what if we do this?

566
00:22:58,360 --> 00:23:01,980
If we take our growth velocity
and in some way, shape, or form

567
00:23:01,980 --> 00:23:03,790
now our growth
velocity is going to be

568
00:23:03,790 --> 00:23:06,300
perpendicular to the
direction of heat extraction,

569
00:23:06,300 --> 00:23:08,010
what would that
geometry look like?

570
00:23:08,010 --> 00:23:09,510
And they came up
with something that

571
00:23:09,510 --> 00:23:11,150
looked a bit like
this right here,

572
00:23:11,150 --> 00:23:13,210
a horizontal growth mechanism.

573
00:23:13,210 --> 00:23:16,280
So you see the [INAUDIBLE]
interface is now at an angle.

574
00:23:16,280 --> 00:23:19,680
It's almost vertical at
this point, a slight angle.

575
00:23:19,680 --> 00:23:23,820
And the pull velocity is
almost perpendicular to it.

576
00:23:23,820 --> 00:23:25,740
So now, you're able,
in theory at least,

577
00:23:25,740 --> 00:23:28,280
to grow much, much faster.

578
00:23:28,280 --> 00:23:29,770
This was a schematic
of the ribbon

579
00:23:29,770 --> 00:23:31,440
growth on silicon process.

580
00:23:31,440 --> 00:23:33,600
There's also another
company called AstroPower

581
00:23:33,600 --> 00:23:34,990
that developed silicon film.

582
00:23:34,990 --> 00:23:38,100
It was later purchased
by General Electric.

583
00:23:38,100 --> 00:23:42,430
So these technologies were
developed with the intent

584
00:23:42,430 --> 00:23:43,610
of pulling very, very fast.

585
00:23:43,610 --> 00:23:45,930
And indeed, you can
literally extrude the silicon

586
00:23:45,930 --> 00:23:48,720
at around 49 meters per second.

587
00:23:48,720 --> 00:23:53,160
But the problem about
this is that you wind up

588
00:23:53,160 --> 00:23:56,360
with very small grains and very
poor crystalline quality when

589
00:23:56,360 --> 00:23:58,420
you try to grow at the speeds.

590
00:23:58,420 --> 00:24:00,950
And so it winds up being a
metallurgical problem of how

591
00:24:00,950 --> 00:24:04,350
do you ensure the proper
grain size when you're growing

592
00:24:04,350 --> 00:24:06,140
using these technologies?

593
00:24:06,140 --> 00:24:08,660
So there is some
work in that regard,

594
00:24:08,660 --> 00:24:13,330
but never really took off
in commercial production.

595
00:24:13,330 --> 00:24:14,740
Yeah?

596
00:24:14,740 --> 00:24:17,485
AUDIENCE: So does
pulling at a lower

597
00:24:17,485 --> 00:24:20,160
speed with the horizontal
ribbon increase

598
00:24:20,160 --> 00:24:22,936
your quality by increasing
your grain size?

599
00:24:22,936 --> 00:24:23,810
Or is it not really--

600
00:24:23,810 --> 00:24:25,430
PROFESSOR: If you're able
to control the nucleation

601
00:24:25,430 --> 00:24:28,140
and growth process at the
very beginning, theoretically,

602
00:24:28,140 --> 00:24:29,399
that could be possible.

603
00:24:29,399 --> 00:24:29,940
AUDIENCE: OK.

604
00:24:32,277 --> 00:24:33,360
PROFESSOR: Yeah, question?

605
00:24:33,360 --> 00:24:36,080
AUDIENCE: You had mentioned form
factor for these wafers before.

606
00:24:36,080 --> 00:24:36,490
PROFESSOR: Yeah?

607
00:24:36,490 --> 00:24:38,531
AUDIENCE: So is there like
a standard form factor

608
00:24:38,531 --> 00:24:41,014
for solar cell manufacturing?

609
00:24:41,014 --> 00:24:41,860
PROFESSOR: Yep.

610
00:24:41,860 --> 00:24:45,350
So the standard form factor
today is akin to this one right

611
00:24:45,350 --> 00:24:45,850
here.

612
00:24:45,850 --> 00:24:50,510
It's about a 15.6
by 15.6 centimeter

613
00:24:50,510 --> 00:24:53,700
squared lateral dimension
form factor for the wafer.

614
00:24:53,700 --> 00:24:56,470
And I can pass this
one around as well.

615
00:24:56,470 --> 00:24:58,780
This right here is what's
called a "pseudo-square."

616
00:24:58,780 --> 00:25:01,500
You can see the edges
are kind of rounded off.

617
00:25:01,500 --> 00:25:05,301
And that's because it came
from a CZ wafer like this one.

618
00:25:05,301 --> 00:25:06,550
It was just chopped out of it.

619
00:25:06,550 --> 00:25:08,520
Let me see if these
two are coincidence.

620
00:25:08,520 --> 00:25:09,740
It would be a-- oh, yeah.

621
00:25:09,740 --> 00:25:12,040
Look at that.

622
00:25:12,040 --> 00:25:16,430
So you can see where the
solar cell actually came from.

623
00:25:16,430 --> 00:25:19,170
So that's the standard
diameter of a, say,

624
00:25:19,170 --> 00:25:22,400
linear dimension, usually
rectilinear shape, a square.

625
00:25:22,400 --> 00:25:25,110
And the multi-crystalline
silicon ingot material

626
00:25:25,110 --> 00:25:27,000
are typically of
this size as well.

627
00:25:27,000 --> 00:25:29,300
And you can already see
that these wafers that I

628
00:25:29,300 --> 00:25:31,960
have up here are a bit small.

629
00:25:31,960 --> 00:25:33,630
These were the previous
generation size.

630
00:25:33,630 --> 00:25:37,454
I believe these are 12.5
by 12.5 centimeter squared.

631
00:25:37,454 --> 00:25:39,620
Most laboratory devices
that you and your colleagues

632
00:25:39,620 --> 00:25:41,203
will manufacture are
on the order of 1

633
00:25:41,203 --> 00:25:43,843
by 1 centimeter or
smaller because-- well,

634
00:25:43,843 --> 00:25:45,490
because of a variety of factors.

635
00:25:45,490 --> 00:25:48,810
One is the transparent
conducting oxide

636
00:25:48,810 --> 00:25:50,490
as we saw in our
homework problem.

637
00:25:50,490 --> 00:25:53,340
We're limited in how big we can
make the device by the sheet

638
00:25:53,340 --> 00:25:55,761
resistance of that
transparent conducting oxide.

639
00:25:55,761 --> 00:25:57,510
Another problem that
we typically run into

640
00:25:57,510 --> 00:25:59,093
is just that we're
not able to deposit

641
00:25:59,093 --> 00:26:00,320
uniformly over a large area.

642
00:26:00,320 --> 00:26:02,770
We don't have a deposition
equipment for it in our labs.

643
00:26:02,770 --> 00:26:05,310
We're there trying to
optimize a new material.

644
00:26:05,310 --> 00:26:10,437
We don't necessarily worry about
making module-sized devices out

645
00:26:10,437 --> 00:26:10,936
of it.

646
00:26:10,936 --> 00:26:11,842
Yeah, question?

647
00:26:11,842 --> 00:26:13,508
AUDIENCE: Is there a
reason why the form

648
00:26:13,508 --> 00:26:18,814
factor is different than that
used for device manufacturing?

649
00:26:18,814 --> 00:26:19,480
PROFESSOR: Sure.

650
00:26:19,480 --> 00:26:22,714
AUDIENCE: Like [INAUDIBLE]
uses circular wafers.

651
00:26:22,714 --> 00:26:23,380
PROFESSOR: Yeah.

652
00:26:23,380 --> 00:26:26,560
So if we were to imagine a
bunch of circular wafers inside

653
00:26:26,560 --> 00:26:29,560
of this module over here, you
can imagine the circular wafers

654
00:26:29,560 --> 00:26:30,461
side by side.

655
00:26:30,461 --> 00:26:32,460
That was how they were
done at once upon a time.

656
00:26:32,460 --> 00:26:33,835
Obviously you
didn't have 8-inch.

657
00:26:33,835 --> 00:26:35,100
It was much smaller.

658
00:26:35,100 --> 00:26:37,100
Or a 6- or 8-inch.

659
00:26:37,100 --> 00:26:38,690
This would be a 6-inch wafer.

660
00:26:38,690 --> 00:26:40,697
But the wafers were
a little smaller,

661
00:26:40,697 --> 00:26:42,530
but you still have
circular wafers and a lot

662
00:26:42,530 --> 00:26:43,610
of dead space in between.

663
00:26:43,610 --> 00:26:45,610
So as you can see, because
of the rounded edges,

664
00:26:45,610 --> 00:26:47,500
the packing density is very low.

665
00:26:47,500 --> 00:26:49,100
The equivalent would
be, say, oranges

666
00:26:49,100 --> 00:26:50,850
at a market where they're
all stacked on top of another

667
00:26:50,850 --> 00:26:52,830
and you have all this
dead space in between.

668
00:26:52,830 --> 00:26:56,820
And so the idea was to optimize
between the cost of the silicon

669
00:26:56,820 --> 00:26:59,350
and the cost of the encapsulant
materials by shaving away

670
00:26:59,350 --> 00:27:02,810
a little bit of the
silicon and losing that--

671
00:27:02,810 --> 00:27:05,150
and perhaps recycling
it, to be honest--

672
00:27:05,150 --> 00:27:07,012
and the encapsulant
materials, where

673
00:27:07,012 --> 00:27:08,970
you have this dead space
in between the wafers,

674
00:27:08,970 --> 00:27:10,620
a small amount of
it, where you have

675
00:27:10,620 --> 00:27:13,090
glass and encapsulant but
no active device underneath.

676
00:27:17,020 --> 00:27:19,345
Another interesting
development, as you can see just

677
00:27:19,345 --> 00:27:20,970
from the device point
of view-- so this

678
00:27:20,970 --> 00:27:23,580
would be an Evergreen string
ribbon wafer right here, as you

679
00:27:23,580 --> 00:27:24,310
can see.

680
00:27:24,310 --> 00:27:27,530
And this right here,
a larger area device.

681
00:27:27,530 --> 00:27:30,900
Does anybody notice a
difference besides the shape?

682
00:27:30,900 --> 00:27:32,620
In particular, I lead
you to the busbars.

683
00:27:32,620 --> 00:27:35,760
How many of those
thick, vertical lines

684
00:27:35,760 --> 00:27:37,460
appear down the wafer?

685
00:27:37,460 --> 00:27:39,180
AUDIENCE: [INAUDIBLE]

686
00:27:39,180 --> 00:27:41,375
PROFESSOR: This has two,
and this has three, right?

687
00:27:41,375 --> 00:27:42,000
AUDIENCE: Yeah.

688
00:27:42,000 --> 00:27:43,833
PROFESSOR: So the
busbars-- the optimization

689
00:27:43,833 --> 00:27:46,360
of these busbars-- this one
has three-- that's really

690
00:27:46,360 --> 00:27:47,776
to minimize series resistance.

691
00:27:47,776 --> 00:27:49,400
Because now that I
have a larger wafer,

692
00:27:49,400 --> 00:27:52,147
you have so much current flowing
through it, being generated,

693
00:27:52,147 --> 00:27:54,480
that the series resistance
through those very thin metal

694
00:27:54,480 --> 00:27:57,990
wires would end up resulting
in large power losses,

695
00:27:57,990 --> 00:28:01,262
essentially heat
instead of electricity.

696
00:28:01,262 --> 00:28:02,720
And so they added
the third busbar,

697
00:28:02,720 --> 00:28:05,800
even though it increased the
shading, to reduce the series

698
00:28:05,800 --> 00:28:07,550
resistance losses.

699
00:28:07,550 --> 00:28:10,470
So you can see these
optimization problems are

700
00:28:10,470 --> 00:28:14,480
used quite frequently in solar.

701
00:28:14,480 --> 00:28:15,985
Let me go back one step.

702
00:28:15,985 --> 00:28:17,360
There was an
interesting question

703
00:28:17,360 --> 00:28:20,030
about could we grow
single crystals using

704
00:28:20,030 --> 00:28:22,900
the vertical ribbon growth.

705
00:28:22,900 --> 00:28:24,440
This is a technology.

706
00:28:24,440 --> 00:28:26,460
And I don't know if
there are actually

707
00:28:26,460 --> 00:28:29,120
any of these, many of these
samples left in the world.

708
00:28:29,120 --> 00:28:30,400
They're quite rare.

709
00:28:30,400 --> 00:28:31,950
So I do ask if you
want to come up

710
00:28:31,950 --> 00:28:33,640
here, take some care with it.

711
00:28:33,640 --> 00:28:36,447
This is a dendritic web sample.

712
00:28:36,447 --> 00:28:38,780
So this technology went out
of commercial manufacturing,

713
00:28:38,780 --> 00:28:41,526
I believe, in 2005.

714
00:28:41,526 --> 00:28:42,150
Must have been.

715
00:28:42,150 --> 00:28:43,820
Or 2004.

716
00:28:43,820 --> 00:28:45,620
It was developed
by Westinghouse,

717
00:28:45,620 --> 00:28:49,260
which is used to be one of the
powerhouses in solar located

718
00:28:49,260 --> 00:28:51,210
in Pittsburgh, Pennsylvania.

719
00:28:51,210 --> 00:28:56,490
They had a very
active solar activity.

720
00:28:56,490 --> 00:28:59,440
It was a kind of a crucible
out of which many solar experts

721
00:28:59,440 --> 00:29:01,520
then went into diaspora
around the United States

722
00:29:01,520 --> 00:29:03,940
and set up their own
activities elsewhere.

723
00:29:03,940 --> 00:29:06,374
And one of the technologies
that they developed

724
00:29:06,374 --> 00:29:08,790
was a single crystalline ribbon
technology like this right

725
00:29:08,790 --> 00:29:09,290
here.

726
00:29:09,290 --> 00:29:12,490
And if you look very closely,
it really is a single crystal.

727
00:29:12,490 --> 00:29:14,690
The growth methods
to make this, though,

728
00:29:14,690 --> 00:29:16,730
was extremely intricate.

729
00:29:16,730 --> 00:29:19,580
It involved, among other things,
control of the temperature,

730
00:29:19,580 --> 00:29:22,940
of the liquid silicon to within
1/100 of a degree Celsius

731
00:29:22,940 --> 00:29:27,230
at melting temperature, which is
an extreme feat of engineering.

732
00:29:27,230 --> 00:29:29,480
The uptime of these
pieces of equipment,

733
00:29:29,480 --> 00:29:31,760
meaning the growth
time, was around 50%.

734
00:29:31,760 --> 00:29:33,980
And the other 50% of
the time, the operators

735
00:29:33,980 --> 00:29:35,370
were trying to make it work.

736
00:29:35,370 --> 00:29:38,079
So it grew very,
very thin material.

737
00:29:38,079 --> 00:29:39,870
It wasn't able to scale
to the form factors

738
00:29:39,870 --> 00:29:41,170
that we see nowadays.

739
00:29:41,170 --> 00:29:42,810
The throughput was quite low.

740
00:29:42,810 --> 00:29:43,650
The cost was high.

741
00:29:43,650 --> 00:29:46,980
And so it didn't quite make
it, but from an engineering

742
00:29:46,980 --> 00:29:49,320
point of view, it was a
marvel in terms of what

743
00:29:49,320 --> 00:29:51,560
they were able to accomplish.

744
00:29:51,560 --> 00:29:54,960
So history of crystalline
silicon development

745
00:29:54,960 --> 00:29:57,440
is riddled with these
technologies that didn't quite

746
00:29:57,440 --> 00:29:59,850
make it with these
materials that

747
00:29:59,850 --> 00:30:02,050
were extremely inventive,
extremely ingenuitive.

748
00:30:02,050 --> 00:30:04,730
But at the end of the day,
the dollars per watt peak

749
00:30:04,730 --> 00:30:07,290
just couldn't continue to
justify their existence.

750
00:30:07,290 --> 00:30:09,130
And there were a number of
factors that could contribute

751
00:30:09,130 --> 00:30:10,046
to making that happen.

752
00:30:13,550 --> 00:30:18,260
So in terms of wafer
fabrication in general--

753
00:30:18,260 --> 00:30:21,390
this includes both the
wafers out of ingot materials

754
00:30:21,390 --> 00:30:24,050
but also ribbons--
where do I personally

755
00:30:24,050 --> 00:30:26,050
see this field going?

756
00:30:26,050 --> 00:30:29,610
These are some notes.

757
00:30:29,610 --> 00:30:32,640
So in terms of cost,
the cost per watt peak

758
00:30:32,640 --> 00:30:35,150
can be reduced by using
cheaper starting materials.

759
00:30:35,150 --> 00:30:39,080
That means instead of using this
expensive Siemens poly, perhaps

760
00:30:39,080 --> 00:30:42,670
an upgraded metallurgical
silicon process.

761
00:30:42,670 --> 00:30:45,120
Growing or sawing
thinner wafers.

762
00:30:45,120 --> 00:30:47,010
Growing, for example,
on a ribbon technique.

763
00:30:47,010 --> 00:30:51,770
Sawing, maybe making the saws
themselves thinner but more

764
00:30:51,770 --> 00:30:53,570
robust so that they
don't snap as they're

765
00:30:53,570 --> 00:30:55,970
pulling through the material
at about 5 meters per second

766
00:30:55,970 --> 00:30:59,040
in that slurry with the silicon
carbide or diamond grit.

767
00:30:59,040 --> 00:31:02,240
Very challenging
engineering as well.

768
00:31:02,240 --> 00:31:03,970
This second bullet
point right there

769
00:31:03,970 --> 00:31:07,820
can be encapsulated in a larger
team called improved silicon

770
00:31:07,820 --> 00:31:09,520
materials utilization.

771
00:31:09,520 --> 00:31:11,020
In other words, the
grams of silicon

772
00:31:11,020 --> 00:31:14,339
that you use to produce a
watt peak of a solar cell.

773
00:31:14,339 --> 00:31:15,880
So improving that
number right there.

774
00:31:18,400 --> 00:31:20,470
Increasing furnace
throughput-- that means

775
00:31:20,470 --> 00:31:22,837
increasing ingot size,
growth, speed, and so forth.

776
00:31:22,837 --> 00:31:24,420
There are many people
right now trying

777
00:31:24,420 --> 00:31:25,794
to grow these
ingot right here up

778
00:31:25,794 --> 00:31:29,090
to a ton, one metric
ton, so 1,000 kilograms.

779
00:31:29,090 --> 00:31:32,010
That would mean for
the full-sized wafers,

780
00:31:32,010 --> 00:31:37,730
you would have something on
the order of 6 by 6 bricks.

781
00:31:37,730 --> 00:31:39,430
It's pretty large, a
pretty large ingot.

782
00:31:39,430 --> 00:31:41,890
Maybe even 7 by 7.

783
00:31:41,890 --> 00:31:43,340
And improving the
material quality

784
00:31:43,340 --> 00:31:45,130
so that you can
improve efficiency,

785
00:31:45,130 --> 00:31:49,240
efficiency being a huge leverage
over the entire cost structure.

786
00:31:49,240 --> 00:31:51,960
Because if your solar cell is
able to produce more power,

787
00:31:51,960 --> 00:31:54,460
that means that you use less
encapsulant, and less material,

788
00:31:54,460 --> 00:31:56,960
and so forth per unit power
produced, and even less

789
00:31:56,960 --> 00:31:59,640
labor to install it and
less racking and framing

790
00:31:59,640 --> 00:32:01,310
materials downstream.

791
00:32:01,310 --> 00:32:04,654
The scaling issues, so
polysilicon production

792
00:32:04,654 --> 00:32:06,320
is currently-- well,
this is higher now.

793
00:32:06,320 --> 00:32:14,430
It's about 100,000
metric tons per year.

794
00:32:14,430 --> 00:32:17,750
And about half of that--
well, about a quarter of that,

795
00:32:17,750 --> 00:32:20,270
now, maybe a third is for
the semiconductor industry,

796
00:32:20,270 --> 00:32:23,580
about 3/4 for the PV industry.

797
00:32:23,580 --> 00:32:25,270
The slurry and the
silicon carbide grit

798
00:32:25,270 --> 00:32:27,389
needed for wire sawing
is, at some point,

799
00:32:27,389 --> 00:32:28,430
going to become an issue.

800
00:32:28,430 --> 00:32:30,705
These are huge
volumes of waste that

801
00:32:30,705 --> 00:32:33,350
need to be transported
through the factories.

802
00:32:33,350 --> 00:32:36,140
And of course, the silicon loss
due to wire sawing and ingot

803
00:32:36,140 --> 00:32:39,080
casting, resulting in
only 50% of the silicon

804
00:32:39,080 --> 00:32:41,340
here in this ingot being
used in the actual wafers

805
00:32:41,340 --> 00:32:42,790
to make solar cells.

806
00:32:42,790 --> 00:32:46,220
The technology
enablers-- using lower--

807
00:32:46,220 --> 00:32:48,560
let's put it this--
lower cost feedstocks.

808
00:32:48,560 --> 00:32:50,430
You can't compromise
on quality ultimately,

809
00:32:50,430 --> 00:32:53,350
so this is a little bit of
a false choice right here.

810
00:32:53,350 --> 00:32:55,740
Using lower cost
feedstocks produced

811
00:32:55,740 --> 00:33:00,880
by the upgraded metallurgical
route, for example.

812
00:33:00,880 --> 00:33:02,810
Producing and
handling thinner wafer

813
00:33:02,810 --> 00:33:06,070
and growing faster, larger,
higher quality ingots.

814
00:33:06,070 --> 00:33:08,960
And there's a lot
of innovation to be

815
00:33:08,960 --> 00:33:10,330
had in this space right here.

816
00:33:10,330 --> 00:33:14,150
I believe the numbers
in the last quarter,

817
00:33:14,150 --> 00:33:16,900
start-up companies raised
on the order of $250 million

818
00:33:16,900 --> 00:33:18,829
from venture capital.

819
00:33:18,829 --> 00:33:20,620
And that wasn't including
a new $50 million

820
00:33:20,620 --> 00:33:23,530
deal that was just announced
of a company attempting

821
00:33:23,530 --> 00:33:25,380
to produce upgraded
metallurgic grade

822
00:33:25,380 --> 00:33:29,340
silicon through liquid
routes, purification.

823
00:33:29,340 --> 00:33:30,970
This was just announced
this past week,

824
00:33:30,970 --> 00:33:32,300
if you go to Greentech Media.

825
00:33:32,300 --> 00:33:35,710
So there's still a lot of
active innovation in this area

826
00:33:35,710 --> 00:33:37,810
despite the current
market conditions.

827
00:33:37,810 --> 00:33:40,570
And those of you who are
looking for jobs right now,

828
00:33:40,570 --> 00:33:43,920
if you're clever, you'll
find them here in this space.

829
00:33:43,920 --> 00:33:45,700
Any questions so
far about wafers?

830
00:33:45,700 --> 00:33:46,497
Yes?

831
00:33:46,497 --> 00:33:50,789
AUDIENCE: Does laser
cutting cause as much dust?

832
00:33:50,789 --> 00:33:52,830
PROFESSOR: Does laser
cutting cause as much dust?

833
00:33:52,830 --> 00:33:55,750
So let's walk through that.

834
00:33:55,750 --> 00:34:00,830
If we're thinking about
the ribbon growing from,

835
00:34:00,830 --> 00:34:03,420
say-- from this
ribbon right here,

836
00:34:03,420 --> 00:34:05,580
I'm going to extract this wafer.

837
00:34:05,580 --> 00:34:08,350
So I need to make an
incision horizontally

838
00:34:08,350 --> 00:34:11,909
right around this
point right here.

839
00:34:11,909 --> 00:34:14,590
If you look at the total
height, the wafer's

840
00:34:14,590 --> 00:34:16,500
around 15 centimeters long.

841
00:34:16,500 --> 00:34:19,159
And the laser cut
itself is something

842
00:34:19,159 --> 00:34:22,510
on the order of
maybe, oh-- I'm going

843
00:34:22,510 --> 00:34:24,830
to guess-- a few tens
of microns, maybe

844
00:34:24,830 --> 00:34:26,610
100 microns in that order.

845
00:34:26,610 --> 00:34:29,530
And so that the amount of
kerf loss in that regard

846
00:34:29,530 --> 00:34:34,040
would be 100 microns over 15
centimeters, so a relatively

847
00:34:34,040 --> 00:34:35,570
insignificant fraction.

848
00:34:35,570 --> 00:34:38,600
If you're trying to chop up
this using a laser, yes, then

849
00:34:38,600 --> 00:34:40,154
you would have
significant losses.

850
00:34:40,154 --> 00:34:42,195
But since you're growing
that ribbon straight out

851
00:34:42,195 --> 00:34:43,949
of the melt, the
laser cuts themselves

852
00:34:43,949 --> 00:34:46,931
are a very small fraction
of the total silicon.

853
00:34:46,931 --> 00:34:47,430
Yep?

854
00:34:47,430 --> 00:34:50,210
AUDIENCE: Can the sawdust be
collected and remelted then?

855
00:34:50,210 --> 00:34:51,459
PROFESSOR: Wonderful question.

856
00:34:51,459 --> 00:34:53,510
Can the sawdust be collected
and remelted again?

857
00:34:53,510 --> 00:34:56,940
There was a lot of work done
to try to figure that out.

858
00:34:56,940 --> 00:34:58,780
At that point, the
sawdust is mixed

859
00:34:58,780 --> 00:35:01,880
with this glycol-based slurry,
and with the silicon carbide

860
00:35:01,880 --> 00:35:05,940
grit, and with fragments of iron
coming from the stainless steel

861
00:35:05,940 --> 00:35:08,410
wire, and nickel and
chromium and other impurities

862
00:35:08,410 --> 00:35:09,400
inside of the wire.

863
00:35:09,400 --> 00:35:10,960
And so a lot of the
work was focused

864
00:35:10,960 --> 00:35:15,850
on separation of those different
constituents, shall we say.

865
00:35:15,850 --> 00:35:21,090
And when the silicon prices were
very high, maybe in 2007, 2008,

866
00:35:21,090 --> 00:35:23,440
when the spot prices
were $500 a kilogram,

867
00:35:23,440 --> 00:35:25,610
there was a large
incentive to use

868
00:35:25,610 --> 00:35:29,400
every single drop of silicon
you had including separation.

869
00:35:29,400 --> 00:35:32,130
But in recent years,
the incentive to do that

870
00:35:32,130 --> 00:35:33,250
has really dropped.

871
00:35:33,250 --> 00:35:36,920
And the one company I knew
that had a very active slurry

872
00:35:36,920 --> 00:35:39,949
recycling program let it go.

873
00:35:39,949 --> 00:35:42,490
So there may be companies out
there that are looking into it,

874
00:35:42,490 --> 00:35:44,073
but I'm not aware
of their activities.

875
00:35:47,000 --> 00:35:47,870
OK.

876
00:35:47,870 --> 00:35:52,480
Let's hop forward into
cells and devices.

877
00:35:52,480 --> 00:35:55,034
So now we've talked
about the market shares

878
00:35:55,034 --> 00:35:57,200
of different technologies,
feedstock refining, wafer

879
00:35:57,200 --> 00:35:59,660
fabrication, how we make these
wonderful different pieces

880
00:35:59,660 --> 00:36:00,170
of silicon.

881
00:36:00,170 --> 00:36:02,170
Now we're going to talk
about going from a wafer

882
00:36:02,170 --> 00:36:04,480
into a solar cell device.

883
00:36:04,480 --> 00:36:06,660
So just to situate
ourselves, raw material,

884
00:36:06,660 --> 00:36:09,590
silicon feedstock, the module
in the system over here.

885
00:36:09,590 --> 00:36:11,650
In the middle, we have
the wafer to the cell.

886
00:36:11,650 --> 00:36:15,840
And this is the portion
of discussion forthwith.

887
00:36:15,840 --> 00:36:17,807
Cell processing.

888
00:36:17,807 --> 00:36:18,890
Let's have a look at this.

889
00:36:18,890 --> 00:36:20,660
Again, it's a very
different world

890
00:36:20,660 --> 00:36:24,660
now in a cell fab line then
it was in the crystallisation

891
00:36:24,660 --> 00:36:25,330
environment.

892
00:36:25,330 --> 00:36:27,754
So in wafer fab, which
means wafer fabrication

893
00:36:27,754 --> 00:36:29,420
and the section of
the company dedicated

894
00:36:29,420 --> 00:36:33,420
to producing wafers and ingots,
it was a little bit more dirty.

895
00:36:33,420 --> 00:36:36,090
You had forklifts moving
these big crucibles around

896
00:36:36,090 --> 00:36:37,962
with chunks of silicon
in it, operators

897
00:36:37,962 --> 00:36:40,170
coming by with garden hoses
and washing down furnaces

898
00:36:40,170 --> 00:36:41,810
after they're finished.

899
00:36:41,810 --> 00:36:43,750
Here in the cell
fab line, it looks

900
00:36:43,750 --> 00:36:45,390
almost more like a clean room.

901
00:36:45,390 --> 00:36:47,156
Almost, I say,
because these folks

902
00:36:47,156 --> 00:36:48,280
aren't in full bunny suits.

903
00:36:48,280 --> 00:36:50,789
They're usually just with
jackets with booties.

904
00:36:50,789 --> 00:36:52,330
Sometimes you see
them with hair nets

905
00:36:52,330 --> 00:36:57,030
as well to protect from hair
and other particulate matter

906
00:36:57,030 --> 00:36:58,540
from getting inside
of the tools.

907
00:36:58,540 --> 00:37:00,480
But by and large, the
wafers are brought in.

908
00:37:00,480 --> 00:37:03,230
And either in a series
of inline processes--

909
00:37:03,230 --> 00:37:05,040
this is a wafer,
wafer, wafer, wafer.

910
00:37:05,040 --> 00:37:06,810
So there are four
wafers across moving

911
00:37:06,810 --> 00:37:08,510
through what looks
like an etch tank

912
00:37:08,510 --> 00:37:12,840
to do the texturization
on the wafers.

913
00:37:12,840 --> 00:37:16,970
Whereas in wafer fab,
it was pretty dirty.

914
00:37:16,970 --> 00:37:18,511
In cell fab, it
looks pretty clean.

915
00:37:18,511 --> 00:37:20,510
You have a combination
of these inline processes

916
00:37:20,510 --> 00:37:21,551
like this one shown here.

917
00:37:21,551 --> 00:37:24,130
We have wafers on conveyor
belts moving through lines.

918
00:37:24,130 --> 00:37:26,760
And batch processes, where
little robots [? pick in ?]

919
00:37:26,760 --> 00:37:29,790
places, line wafers up
inside of crucibles or boats,

920
00:37:29,790 --> 00:37:33,190
and insert them into furnaces
for batch processing.

921
00:37:33,190 --> 00:37:36,430
So this is the crystalline
silicon cell fabrication.

922
00:37:36,430 --> 00:37:39,380
In on one side go
bare wafers like this,

923
00:37:39,380 --> 00:37:42,180
and out the other side come
fully processed solar cell

924
00:37:42,180 --> 00:37:44,910
devices.

925
00:37:44,910 --> 00:37:50,300
So the very first step
after wafer sawing

926
00:37:50,300 --> 00:37:52,060
is the saw damage etch.

927
00:37:52,060 --> 00:37:55,490
After the sawing process,
you have subsurface damage,

928
00:37:55,490 --> 00:37:57,440
something on the order
of 5 to 10 microns

929
00:37:57,440 --> 00:37:58,960
deep beneath the
wafer's surface.

930
00:37:58,960 --> 00:38:02,130
And keep in mind these are
only about 170 microns thick.

931
00:38:02,130 --> 00:38:05,520
So you have subsurface damage
that needs to be removed.

932
00:38:05,520 --> 00:38:08,270
And you can take advantage
of the subsurface damage

933
00:38:08,270 --> 00:38:11,470
by etching it in such a way
that you etch along the damage

934
00:38:11,470 --> 00:38:13,090
and form texturization.

935
00:38:13,090 --> 00:38:14,570
So it's a bit of
a two-in-one here.

936
00:38:14,570 --> 00:38:16,966
You clean the wafer, you
create your texturization,

937
00:38:16,966 --> 00:38:18,340
and you remove
your saw damage so

938
00:38:18,340 --> 00:38:20,840
that when you lift your
wafer, the wafer doesn't

939
00:38:20,840 --> 00:38:23,560
break because there's some
hairline fracture caused

940
00:38:23,560 --> 00:38:26,170
by the silicon carbide grit.

941
00:38:26,170 --> 00:38:27,962
After you have
your wafer-- so you

942
00:38:27,962 --> 00:38:29,170
start with your p-type wafer.

943
00:38:29,170 --> 00:38:31,070
And this represents
the cross section

944
00:38:31,070 --> 00:38:33,565
of the wafer from the
backside of the eventual cell

945
00:38:33,565 --> 00:38:35,190
to the front side of
the eventual cell,

946
00:38:35,190 --> 00:38:37,370
about 170 microns thick.

947
00:38:37,370 --> 00:38:42,190
Wide would be something on
the order of 15.6 centimeters

948
00:38:42,190 --> 00:38:43,400
in a real device.

949
00:38:43,400 --> 00:38:46,030
We're just looking at a
small section of it here.

950
00:38:46,030 --> 00:38:48,280
So as we walk through the
different steps of cell fab,

951
00:38:48,280 --> 00:38:50,680
we'll see them evolve over here.

952
00:38:50,680 --> 00:38:53,570
The first step after
the saw damage etch

953
00:38:53,570 --> 00:38:57,360
is to do what's called an
emitter diffusion, to create

954
00:38:57,360 --> 00:38:58,592
your p-n junction.

955
00:38:58,592 --> 00:39:00,300
Straight out of the
box, the p-n junction

956
00:39:00,300 --> 00:39:02,480
is created after
the saw damage etch.

957
00:39:02,480 --> 00:39:05,690
And typically, what we do is
deposit a lower resistance

958
00:39:05,690 --> 00:39:08,760
or more highly doped-- that's
why we have the 2 pluses here,

959
00:39:08,760 --> 00:39:11,960
that means very highly
doped-- emitter right

960
00:39:11,960 --> 00:39:14,560
underneath where the eventual
contact metalization will go.

961
00:39:14,560 --> 00:39:16,710
That's to reduce the
contact resistance.

962
00:39:16,710 --> 00:39:18,360
That's to create the
tunneling junction

963
00:39:18,360 --> 00:39:20,700
between the semiconductor
and the metal.

964
00:39:20,700 --> 00:39:21,320
OK.

965
00:39:21,320 --> 00:39:24,540
So we have the high
resistance emitter over here.

966
00:39:24,540 --> 00:39:27,425
This is representative
of a shallow emitter.

967
00:39:27,425 --> 00:39:28,800
You remember in
your quiz two you

968
00:39:28,800 --> 00:39:31,025
have this decision whether
to take a shallow or deep.

969
00:39:31,025 --> 00:39:32,900
This architecture, which
is used in industry,

970
00:39:32,900 --> 00:39:34,930
actually combines the
best of both worlds.

971
00:39:34,930 --> 00:39:38,330
It has a shallow emitter over
most of the solar cell device

972
00:39:38,330 --> 00:39:41,860
to improve the blue response,
minimize Auger recombination.

973
00:39:41,860 --> 00:39:44,940
But it also has a deeper emitter
right underneath the contact

974
00:39:44,940 --> 00:39:46,960
metalization to prevent
shunting and to reduce

975
00:39:46,960 --> 00:39:47,751
contact resistance.

976
00:39:49,080 --> 00:39:51,330
AUDIENCE: I assume we have
to choose one or the other.

977
00:39:51,330 --> 00:39:55,320
PROFESSOR: You have to choose
one or the other unfortunately.

978
00:39:55,320 --> 00:40:00,240
To create this-- it's really to
create the combination, what's

979
00:40:00,240 --> 00:40:01,990
called a selective emitter.

980
00:40:01,990 --> 00:40:06,480
It's an emitter because
it's the charge separation

981
00:40:06,480 --> 00:40:07,670
portion of the device.

982
00:40:07,670 --> 00:40:09,170
But it's also
selective in the sense

983
00:40:09,170 --> 00:40:12,250
that you selectively place
these low resistance portions

984
00:40:12,250 --> 00:40:15,310
across in a geometric fashion
underneath your eventual

985
00:40:15,310 --> 00:40:17,210
contact metalization.

986
00:40:17,210 --> 00:40:19,080
You have at the end
of this diffusion

987
00:40:19,080 --> 00:40:22,720
process what's called a
phosphorus silicate glass etch,

988
00:40:22,720 --> 00:40:25,380
PSG, Phosphorus
Silicate Glass etch.

989
00:40:25,380 --> 00:40:28,729
After defusing in the phosphorus
in the gaseous form, what

990
00:40:28,729 --> 00:40:30,270
you'll do-- or
actually, you'll watch

991
00:40:30,270 --> 00:40:33,370
it being done, since it's
happening inside of a furnace.

992
00:40:33,370 --> 00:40:39,427
This phosphorus-based gas will
deposit a thin glassy layer

993
00:40:39,427 --> 00:40:40,885
on the surface of
the sample, which

994
00:40:40,885 --> 00:40:43,080
then needs to be etched
off or removed before you

995
00:40:43,080 --> 00:40:44,440
can do further processing.

996
00:40:44,440 --> 00:40:47,290
So that's what the phosphorus
silicate glass etch is about.

997
00:40:47,290 --> 00:40:50,399
Then there's a nitride or a
silicon nitride anti-reflection

998
00:40:50,399 --> 00:40:52,190
coating that's placed
on the front surface.

999
00:40:52,190 --> 00:40:54,370
And as we calculated
in lecture number two,

1000
00:40:54,370 --> 00:40:57,740
this silicon nitride
coating is only how thick?

1001
00:40:57,740 --> 00:40:58,360
About?

1002
00:40:58,360 --> 00:40:59,360
AUDIENCE: 70 nanometers.

1003
00:40:59,360 --> 00:41:00,940
PROFESSOR: 70 nanometers, right?

1004
00:41:00,940 --> 00:41:02,180
It's really, really thin.

1005
00:41:02,180 --> 00:41:05,620
But yet, that's enough to create
that quarter wave interference

1006
00:41:05,620 --> 00:41:09,125
effect that leads to a very
blue looking solar cell device.

1007
00:41:09,125 --> 00:41:10,500
So the reason they
looked blue is

1008
00:41:10,500 --> 00:41:12,750
because of that
anti-reflection coating.

1009
00:41:12,750 --> 00:41:16,760
We are going to omit the ARC
coating in our design for quiz

1010
00:41:16,760 --> 00:41:17,880
number two.

1011
00:41:17,880 --> 00:41:20,569
It requires silane gas,
which we don't have access

1012
00:41:20,569 --> 00:41:21,860
to down here in the laboratory.

1013
00:41:21,860 --> 00:41:24,500
We'd have to go to either
[? NTL ?] or Harvard CNS

1014
00:41:24,500 --> 00:41:25,860
to get that deposited.

1015
00:41:25,860 --> 00:41:28,384
So because we want this to
be a hands-on experience,

1016
00:41:28,384 --> 00:41:30,300
we don't to take the
wafers out of your hands,

1017
00:41:30,300 --> 00:41:31,970
do some magic off to the
side, and bring them back

1018
00:41:31,970 --> 00:41:32,890
and say, oh, here you go.

1019
00:41:32,890 --> 00:41:34,380
Because your level of
ownership in the process

1020
00:41:34,380 --> 00:41:36,630
just plummets in order of
magnitude in the process.

1021
00:41:36,630 --> 00:41:38,921
We want you to be able to
see it every step of the way.

1022
00:41:38,921 --> 00:41:42,330
So we omit the anti-reflection
coating in our quiz number two.

1023
00:41:42,330 --> 00:41:44,390
But in commercial
production, that's done.

1024
00:41:44,390 --> 00:41:47,750
And people pay a lot of
attention to that step.

1025
00:41:47,750 --> 00:41:51,990
And finally, the metalization
is deposited on the sample

1026
00:41:51,990 --> 00:41:53,040
and fired.

1027
00:41:53,040 --> 00:41:55,050
Now, the metalization,
how is it deposited?

1028
00:41:55,050 --> 00:41:59,170
We'll see in a few slides
what the screen printing

1029
00:41:59,170 --> 00:42:00,082
process looks like.

1030
00:42:00,082 --> 00:42:01,790
And then, we'll actually
do it ourselves.

1031
00:42:01,790 --> 00:42:03,248
You'll press the
button on the tool

1032
00:42:03,248 --> 00:42:06,720
and deposit your
metal on yourself.

1033
00:42:06,720 --> 00:42:08,870
But the metalization
is typically

1034
00:42:08,870 --> 00:42:13,122
deposited onto the devices on
the front side and on the back.

1035
00:42:13,122 --> 00:42:16,250
The front side, you have to line
up-- in commercial production--

1036
00:42:16,250 --> 00:42:18,500
line up with the low resistance
portion of the emitter

1037
00:42:18,500 --> 00:42:22,740
so that you are able to
extract the full benefit

1038
00:42:22,740 --> 00:42:24,665
from the selective
emitter design

1039
00:42:24,665 --> 00:42:26,630
and not shunt your
device elsewhere.

1040
00:42:26,630 --> 00:42:29,444
And on the back contact,
this is typically aluminum.

1041
00:42:29,444 --> 00:42:30,860
The aluminum, some
of the aluminum

1042
00:42:30,860 --> 00:42:34,770
will indiffuse into the silicon
and create a p-plus region

1043
00:42:34,770 --> 00:42:38,750
in the back side here, which
is a minority carrier blockade

1044
00:42:38,750 --> 00:42:39,400
layer.

1045
00:42:39,400 --> 00:42:41,670
It pushes the electrons
away from the back junction

1046
00:42:41,670 --> 00:42:43,220
and toward the emitter.

1047
00:42:43,220 --> 00:42:45,650
And so it prevents back
surface recombination.

1048
00:42:45,650 --> 00:42:47,690
So you see every
single little step

1049
00:42:47,690 --> 00:42:49,196
of the solar cell fabrication.

1050
00:42:49,196 --> 00:42:51,570
A lot of smart people spend
a lot of time thinking about,

1051
00:42:51,570 --> 00:42:54,751
gee, how do I optimize two
or three things at once?

1052
00:42:54,751 --> 00:42:55,500
Question up there?

1053
00:42:55,500 --> 00:42:58,762
AUDIENCE: So both the front
side and backside metalization

1054
00:42:58,762 --> 00:42:59,700
is [INAUDIBLE]

1055
00:42:59,700 --> 00:43:01,400
PROFESSOR: No the front side
metalization in this case--

1056
00:43:01,400 --> 00:43:03,130
thank you for that
clarification-- the front side

1057
00:43:03,130 --> 00:43:04,546
metalization in
this case would be

1058
00:43:04,546 --> 00:43:07,710
silver or silver-based paste.

1059
00:43:07,710 --> 00:43:10,400
And in most
commercial production,

1060
00:43:10,400 --> 00:43:15,440
this silver-based paste
includes metal oxides.

1061
00:43:15,440 --> 00:43:17,580
It could be glassy frit.

1062
00:43:17,580 --> 00:43:18,700
It could be lead oxide.

1063
00:43:18,700 --> 00:43:20,980
It could be some
combination of elements.

1064
00:43:20,980 --> 00:43:24,150
That is able to etch through the
silicon nitride anti-reflective

1065
00:43:24,150 --> 00:43:25,060
coating.

1066
00:43:25,060 --> 00:43:27,010
This is only 70
nanometers thick,

1067
00:43:27,010 --> 00:43:29,860
but silicon nitride is
a very strong material.

1068
00:43:29,860 --> 00:43:30,860
It's a ceramic material.

1069
00:43:30,860 --> 00:43:33,600
So you have to be able to etch
through the silicon nitride

1070
00:43:33,600 --> 00:43:36,750
and make electrical contact
with the silicon underneath.

1071
00:43:36,750 --> 00:43:40,090
And some of the earliest screen
printed metalization cells that

1072
00:43:40,090 --> 00:43:42,990
got in the range of 15%
or 16% efficiency only

1073
00:43:42,990 --> 00:43:46,490
made electrical contact
about 10% of the silicon.

1074
00:43:46,490 --> 00:43:49,410
But it was enough to have these
percolation paths for current

1075
00:43:49,410 --> 00:43:51,430
to flow up into
the metalization.

1076
00:43:51,430 --> 00:43:53,330
It's a miracle that
it works at all.

1077
00:43:53,330 --> 00:43:57,384
But it's a very effective,
cheap manufacturing process

1078
00:43:57,384 --> 00:43:58,800
that, nevertheless,
is still being

1079
00:43:58,800 --> 00:44:00,512
used in commercial
production today, even

1080
00:44:00,512 --> 00:44:02,845
among some of the highest
efficiency cell architectures.

1081
00:44:06,010 --> 00:44:08,840
And so for each of these
different processing steps,

1082
00:44:08,840 --> 00:44:10,750
somebody had to sit
there and think deeply

1083
00:44:10,750 --> 00:44:14,180
about optimization of
different functions.

1084
00:44:14,180 --> 00:44:16,156
The [? aluminium ?] on
the backside, somebody

1085
00:44:16,156 --> 00:44:17,530
had to think about,
gee, how do I

1086
00:44:17,530 --> 00:44:21,250
prevent the wafer from
bowing, bowing too much,

1087
00:44:21,250 --> 00:44:23,590
due to coefficient
of thermal expansion

1088
00:44:23,590 --> 00:44:26,250
mismatch between the
aluminium and the silicon?

1089
00:44:26,250 --> 00:44:29,440
Somebody had to think about, how
do I create the right eutectic

1090
00:44:29,440 --> 00:44:31,820
with the silicon-- the
aluminum silicon eutectic

1091
00:44:31,820 --> 00:44:34,300
is around 577
degrees Celsius-- so

1092
00:44:34,300 --> 00:44:37,010
that you create a good ohmic
contact on the backside?

1093
00:44:37,010 --> 00:44:39,390
How do I diffuse in a certain
amount of the aluminum

1094
00:44:39,390 --> 00:44:41,790
to create this back surface
field to prevent back

1095
00:44:41,790 --> 00:44:42,830
surface recombination?

1096
00:44:42,830 --> 00:44:45,050
How do I get the right
back surface reflectance

1097
00:44:45,050 --> 00:44:46,750
of the light coming
off of here so

1098
00:44:46,750 --> 00:44:49,070
that I have multiple optical
bounces through my device

1099
00:44:49,070 --> 00:44:50,710
and so forth?

1100
00:44:50,710 --> 00:44:54,260
So a lot of optimization goes
into making a solar cell device

1101
00:44:54,260 --> 00:44:56,390
to get the Liebig's
law of the minimum,

1102
00:44:56,390 --> 00:44:58,770
to get each plank Liebig's
law as high as you possibly

1103
00:44:58,770 --> 00:45:01,320
can so you can achieve a
high device performance.

1104
00:45:01,320 --> 00:45:02,990
So hopefully, this
walk through now,

1105
00:45:02,990 --> 00:45:05,040
you can have an appreciation
for the difficulty

1106
00:45:05,040 --> 00:45:07,600
that some of your colleagues
face at solar cell fabrication

1107
00:45:07,600 --> 00:45:09,880
plants.

1108
00:45:09,880 --> 00:45:11,450
Finally, as last
steps-- I mean, this

1109
00:45:11,450 --> 00:45:13,740
is a real miniature
cross section

1110
00:45:13,740 --> 00:45:15,240
in the lateral
dimension right here.

1111
00:45:15,240 --> 00:45:17,510
We only have two contact
metalization fingers.

1112
00:45:17,510 --> 00:45:19,551
If you look at this solar
cell device right here,

1113
00:45:19,551 --> 00:45:20,970
we have several dozen, right?

1114
00:45:20,970 --> 00:45:23,220
If I were to make a vertical
cross section through it,

1115
00:45:23,220 --> 00:45:25,430
you'd see several
dozen contact fingers.

1116
00:45:25,430 --> 00:45:27,590
But this is just meant
to be a caricature.

1117
00:45:27,590 --> 00:45:31,870
So on the edge here,
we have edge isolation.

1118
00:45:31,870 --> 00:45:34,990
And what this is doing is
preventing shunting pathways

1119
00:45:34,990 --> 00:45:36,480
from going around to the back.

1120
00:45:36,480 --> 00:45:38,340
So it's preventing
the emitter from being

1121
00:45:38,340 --> 00:45:41,580
able to make electrical contact
to the backside of the device.

1122
00:45:41,580 --> 00:45:43,670
And this is typically
done by inserting

1123
00:45:43,670 --> 00:45:48,510
a trench, a laser-based
trench, just-- gosh,

1124
00:45:48,510 --> 00:45:52,450
it must be on the order of half
a millimeter from the edge.

1125
00:45:52,450 --> 00:45:55,550
I'm going to pass this around,
this solar cell device right

1126
00:45:55,550 --> 00:45:56,150
here.

1127
00:45:56,150 --> 00:45:57,910
And if you look
very, very carefully,

1128
00:45:57,910 --> 00:46:03,270
it's literally a few hundred
microns from the edge at most.

1129
00:46:03,270 --> 00:46:06,950
You may be able to see the
edge isolation, the trench that

1130
00:46:06,950 --> 00:46:08,400
is formed by the laser.

1131
00:46:08,400 --> 00:46:11,240
But it's very difficult to see.

1132
00:46:11,240 --> 00:46:14,591
So I'll pass this finished
device around as well.

1133
00:46:14,591 --> 00:46:16,090
And feel free to
pick it up and look

1134
00:46:16,090 --> 00:46:17,850
at the backside
and the front side.

1135
00:46:17,850 --> 00:46:20,440
On the back, you'll
see some silver paths

1136
00:46:20,440 --> 00:46:22,220
in the middle of
all that aluminum.

1137
00:46:22,220 --> 00:46:24,720
And if anybody has ever
tried to solder to aluminum,

1138
00:46:24,720 --> 00:46:27,280
you know exactly why those
silver pads are there.

1139
00:46:27,280 --> 00:46:30,120
It's so you can solder
to them and make contact

1140
00:46:30,120 --> 00:46:32,720
to the back of one
device and contact it

1141
00:46:32,720 --> 00:46:33,990
to the front of the next.

1142
00:46:33,990 --> 00:46:35,830
And you'll notice
that they're aligned,

1143
00:46:35,830 --> 00:46:38,120
so the back pads are
aligned with the front.

1144
00:46:38,120 --> 00:46:39,820
So I'll pass this
around right here.

1145
00:46:39,820 --> 00:46:41,392
Yes, Ashley?

1146
00:46:41,392 --> 00:46:44,015
AUDIENCE: I assume
that in order to go

1147
00:46:44,015 --> 00:46:47,540
in terms of where you want
to put the edge isolation,

1148
00:46:47,540 --> 00:46:50,290
do you want it as
far out as possible

1149
00:46:50,290 --> 00:46:51,790
so you're not losing
that edge part.

1150
00:46:51,790 --> 00:46:53,790
But you also need to
make sure you're actually

1151
00:46:53,790 --> 00:46:55,730
making a full [INAUDIBLE].

1152
00:46:55,730 --> 00:46:56,929
There's some optimization--

1153
00:46:56,929 --> 00:46:57,720
PROFESSOR: Exactly.

1154
00:46:57,720 --> 00:46:58,890
AUDIENCE: [INAUDIBLE]

1155
00:46:58,890 --> 00:46:59,730
PROFESSOR: Exactly.

1156
00:46:59,730 --> 00:47:03,490
If your laser edge isolation
machine isn't well-calibrated,

1157
00:47:03,490 --> 00:47:06,210
you're losing area, active
area, of your solar cell,

1158
00:47:06,210 --> 00:47:08,620
hence your current output
is going to be lower.

1159
00:47:08,620 --> 00:47:11,520
Because you know your solar cell
has a certain current density,

1160
00:47:11,520 --> 00:47:15,070
a certain, say, milliamps
per square centimeter.

1161
00:47:15,070 --> 00:47:17,920
But then if your area, if
you're square centimeter

1162
00:47:17,920 --> 00:47:20,480
is smaller, because
you're cutting too far

1163
00:47:20,480 --> 00:47:22,811
away from the edge, you're
throwing away good material.

1164
00:47:22,811 --> 00:47:24,560
This isn't the trench
all the way through.

1165
00:47:24,560 --> 00:47:26,040
It's just electrical isolation.

1166
00:47:26,040 --> 00:47:28,290
So essentially, this material
over here still exists.

1167
00:47:28,290 --> 00:47:29,789
It's still hanging
on to the device,

1168
00:47:29,789 --> 00:47:31,210
but it's electrically isolated.

1169
00:47:31,210 --> 00:47:35,210
This trench here is only about
a couple of microns deep.

1170
00:47:35,210 --> 00:47:36,516
And you're losing area.

1171
00:47:36,516 --> 00:47:38,140
This area over here
is not contributing

1172
00:47:38,140 --> 00:47:39,770
to the photocurrent
of your device.

1173
00:47:39,770 --> 00:47:42,170
Any electron making it up
into the emitter over here

1174
00:47:42,170 --> 00:47:44,086
will just stay there and
recombine eventually.

1175
00:47:44,086 --> 00:47:47,920
It won't be able to be
pulled out of the device.

1176
00:47:47,920 --> 00:47:51,210
A funny, but true story--
there was a company

1177
00:47:51,210 --> 00:47:53,644
once that I worked with
to solve a problem.

1178
00:47:53,644 --> 00:47:55,310
And they were getting
lower efficiencies

1179
00:47:55,310 --> 00:47:56,100
in their new process.

1180
00:47:56,100 --> 00:47:57,780
And they couldn't figure
out for the life of them

1181
00:47:57,780 --> 00:47:59,260
why they were getting
lower efficiencies.

1182
00:47:59,260 --> 00:48:01,260
They checked everything,
everything, everything,

1183
00:48:01,260 --> 00:48:02,020
everything.

1184
00:48:02,020 --> 00:48:05,030
And it turned out that they
were cutting their wafers

1185
00:48:05,030 --> 00:48:08,532
to a slightly larger size
than they were before.

1186
00:48:08,532 --> 00:48:10,240
Actually, it was a
slightly smaller size,

1187
00:48:10,240 --> 00:48:11,700
because it was a
lower efficiency.

1188
00:48:11,700 --> 00:48:16,036
And their tester had embedded in
it a fixed number for the area.

1189
00:48:16,036 --> 00:48:18,410
It wasn't measuring the area
of each wafer independently.

1190
00:48:18,410 --> 00:48:20,862
It just had a fixed number
for the area of the cell.

1191
00:48:20,862 --> 00:48:22,820
And so it was dividing
the total current output

1192
00:48:22,820 --> 00:48:25,330
by a bigger area than
what was actually there.

1193
00:48:25,330 --> 00:48:28,190
And so it was "measuring"
a lower efficiency

1194
00:48:28,190 --> 00:48:31,190
than what actually the
cell was outputting.

1195
00:48:31,190 --> 00:48:33,380
So again, these
geometric parameters

1196
00:48:33,380 --> 00:48:34,920
can come up and
bite you if you're

1197
00:48:34,920 --> 00:48:37,970
so fixated on the electrical
performance parameters.

1198
00:48:37,970 --> 00:48:39,380
Testing and sorting.

1199
00:48:39,380 --> 00:48:41,410
So after you create
your device, you

1200
00:48:41,410 --> 00:48:43,140
have this beautiful solar cell.

1201
00:48:43,140 --> 00:48:48,824
And just from simple electrical
engineering and maybe

1202
00:48:48,824 --> 00:48:51,240
as an extreme example if you're
stringing Christmas lights

1203
00:48:51,240 --> 00:48:53,324
together, you know that
if you have one bad apple,

1204
00:48:53,324 --> 00:48:55,573
it can drag down the performance
of the entire string,

1205
00:48:55,573 --> 00:48:57,470
right, if you're
connecting these in series.

1206
00:48:57,470 --> 00:49:00,120
And so it makes sense to
test each of the cells

1207
00:49:00,120 --> 00:49:02,670
individually and make sure
that you sort them together

1208
00:49:02,670 --> 00:49:05,490
with their like cousins.

1209
00:49:05,490 --> 00:49:07,140
So if you have high
performance cells,

1210
00:49:07,140 --> 00:49:08,350
you bin them all together.

1211
00:49:08,350 --> 00:49:10,620
And you make models of the
high performance cells.

1212
00:49:10,620 --> 00:49:12,296
These will be high
performance modules.

1213
00:49:12,296 --> 00:49:14,420
The bad apples you put
together with the bad apples

1214
00:49:14,420 --> 00:49:15,460
and so forth.

1215
00:49:15,460 --> 00:49:18,030
And that way you can extract
the maximum value out

1216
00:49:18,030 --> 00:49:19,500
of the product you've created.

1217
00:49:19,500 --> 00:49:21,180
You take your good
cells, you put them

1218
00:49:21,180 --> 00:49:22,580
into a higher efficiency module.

1219
00:49:22,580 --> 00:49:24,913
It looks exactly the same,
but its producing more power,

1220
00:49:24,913 --> 00:49:27,510
so you can sell that module at
a higher price than you would,

1221
00:49:27,510 --> 00:49:30,180
say, a lower power
output module.

1222
00:49:30,180 --> 00:49:30,680
OK.

1223
00:49:30,680 --> 00:49:33,490
So that's what the test
and sort is all about.

1224
00:49:33,490 --> 00:49:36,930
Turnkey solar cell
fabrication lines,

1225
00:49:36,930 --> 00:49:39,400
very common since the mid 2000s.

1226
00:49:39,400 --> 00:49:43,850
There are companies--
Centrotherm, gosh, [INAUDIBLE],

1227
00:49:43,850 --> 00:49:47,430
Roth & Rau, others that were
producing either turnkey

1228
00:49:47,430 --> 00:49:50,710
equipment or even turnkey lines
for the entire fabrication

1229
00:49:50,710 --> 00:49:51,400
line.

1230
00:49:51,400 --> 00:49:53,550
Even a local company,
Spire, just up

1231
00:49:53,550 --> 00:49:55,800
the road here in Massachusetts.

1232
00:49:55,800 --> 00:49:58,560
These typically consisted
of wafer inspection systems

1233
00:49:58,560 --> 00:49:59,720
on the input side.

1234
00:49:59,720 --> 00:50:01,310
You don't want to invest
any money in a wafer that's

1235
00:50:01,310 --> 00:50:02,800
ultimately going
to break, so you

1236
00:50:02,800 --> 00:50:04,730
want to be able to inspect your
wafers coming in to make sure

1237
00:50:04,730 --> 00:50:06,790
that they're high enough
quality to be worthy

1238
00:50:06,790 --> 00:50:09,430
of your cell investment.

1239
00:50:09,430 --> 00:50:11,989
Next, you have wet processing
to do the texturization.

1240
00:50:11,989 --> 00:50:13,030
That's shown right there.

1241
00:50:13,030 --> 00:50:15,800
Saw damage texturization.

1242
00:50:15,800 --> 00:50:18,150
And these are
typically inline tools

1243
00:50:18,150 --> 00:50:21,690
with little ceramic rollers,
some pretty nasty acids

1244
00:50:21,690 --> 00:50:23,070
being use.

1245
00:50:23,070 --> 00:50:24,346
Silicon is like a rock.

1246
00:50:24,346 --> 00:50:25,720
And if you want
to etch the rock,

1247
00:50:25,720 --> 00:50:30,650
you need to have some pretty
strong solutions, some very

1248
00:50:30,650 --> 00:50:35,080
high or very low pH, very basic
or very acidic respectively.

1249
00:50:35,080 --> 00:50:37,320
And most of the time, in
multi-crystalline silicon,

1250
00:50:37,320 --> 00:50:39,150
we use an acidic solution.

1251
00:50:39,150 --> 00:50:41,520
It textures the
wafer independent

1252
00:50:41,520 --> 00:50:42,525
of grain orientation.

1253
00:50:42,525 --> 00:50:43,900
For the single
crystal materials,

1254
00:50:43,900 --> 00:50:47,665
we use a basic solution that
is isotropic or anisotropic

1255
00:50:47,665 --> 00:50:48,165
in nature.

1256
00:50:48,165 --> 00:50:49,940
It creates nice little pyramids.

1257
00:50:49,940 --> 00:50:53,350
So here, you see the wafers
being drawn over an etch bath.

1258
00:50:53,350 --> 00:50:55,660
And very small
quantities of liquid

1259
00:50:55,660 --> 00:50:58,200
are used per wafer
in this arrangement.

1260
00:50:58,200 --> 00:51:01,289
You just coat the wafer's
surface, and that's about it.

1261
00:51:01,289 --> 00:51:02,830
If you were to do
it in a batch mode,

1262
00:51:02,830 --> 00:51:05,347
you need a big bath
like a bathtub.

1263
00:51:05,347 --> 00:51:06,930
And you dunk your
wafers inside of it.

1264
00:51:06,930 --> 00:51:07,850
So that would be the bathtub.

1265
00:51:07,850 --> 00:51:09,580
This would be the
shower equivalent.

1266
00:51:09,580 --> 00:51:11,740
So more water efficient.

1267
00:51:11,740 --> 00:51:14,226
In this case, acid efficient.

1268
00:51:14,226 --> 00:51:16,100
And then the cells come
out on the other side

1269
00:51:16,100 --> 00:51:19,020
and go into the emitter
diffusion process.

1270
00:51:19,020 --> 00:51:20,540
And these are a
series of furnaces.

1271
00:51:20,540 --> 00:51:23,050
We'll see one such furnace
over the course of quiz two

1272
00:51:23,050 --> 00:51:24,790
when we make our solar cells.

1273
00:51:24,790 --> 00:51:27,830
So this is the phosphorus
diffusion furnace right here.

1274
00:51:27,830 --> 00:51:30,570
The wafers are typically loaded
into boats and then inserted

1275
00:51:30,570 --> 00:51:35,610
into furnace where phosphorus
containing gas, POCL3,

1276
00:51:35,610 --> 00:51:39,680
also called "pah-cul," is
flown into the chamber.

1277
00:51:39,680 --> 00:51:43,119
The chlorine components
and the oxygen

1278
00:51:43,119 --> 00:51:44,660
dissociate from the
phosphorus, which

1279
00:51:44,660 --> 00:51:46,222
is then driven into the wafer.

1280
00:51:46,222 --> 00:51:47,680
The oxygen reacts
with the silicon,

1281
00:51:47,680 --> 00:51:50,250
creates that phosphorus
silicate glass on the surface.

1282
00:51:50,250 --> 00:51:53,820
And the phosphorus is driven
into the solar cell creating

1283
00:51:53,820 --> 00:51:57,410
the p-n junction,
creating your device.

1284
00:51:57,410 --> 00:52:01,410
And here's an example of
Czochralski wafers being loaded

1285
00:52:01,410 --> 00:52:04,324
into the phosphorus diffusion
furnace and then out again,

1286
00:52:04,324 --> 00:52:05,865
just showing the
degree of automation

1287
00:52:05,865 --> 00:52:07,330
of some of these furnaces.

1288
00:52:07,330 --> 00:52:09,200
This showing a
stack not dissimilar

1289
00:52:09,200 --> 00:52:10,980
from the one in the
laboratory downstairs

1290
00:52:10,980 --> 00:52:14,850
in building 35 where we'll be
doing our phosphorus diffusion.

1291
00:52:14,850 --> 00:52:18,270
So the next-- after we have our
p-n junction-- the next step

1292
00:52:18,270 --> 00:52:21,080
would be to create the
anti-reflection coating.

1293
00:52:21,080 --> 00:52:23,960
And this is done by a process
called Plasma Enhanced Chemical

1294
00:52:23,960 --> 00:52:27,460
Vapor Deposition,
or PECVD for short.

1295
00:52:27,460 --> 00:52:31,690
And in the PECVD process, you
flow in silane gas and ammonia.

1296
00:52:31,690 --> 00:52:34,880
Silane is silicon with a bunch
of hydrogens, four of them.

1297
00:52:34,880 --> 00:52:37,800
And ammonia is nitrogen
with a bunch of hydrogens.

1298
00:52:37,800 --> 00:52:42,090
And the nitrogen and the silicon
react on the wafer's surface

1299
00:52:42,090 --> 00:52:43,910
and create the silicon
nitride coating.

1300
00:52:43,910 --> 00:52:47,640
The hydrogens, 90% of
it, evaporates off.

1301
00:52:47,640 --> 00:52:50,080
But about 10% of
it hangs around.

1302
00:52:50,080 --> 00:52:53,594
Between 1% and 10% go into the
wafer or stay at the interface

1303
00:52:53,594 --> 00:52:56,010
there and eventually are driven
into the wafer passivating

1304
00:52:56,010 --> 00:52:57,170
bulk defects.

1305
00:52:57,170 --> 00:52:59,330
So again, a multitude
of different things

1306
00:52:59,330 --> 00:53:01,770
going on at the same time.

1307
00:53:01,770 --> 00:53:03,290
Eventually, the
visible effect is

1308
00:53:03,290 --> 00:53:05,700
that you've created your
anti-reflection coating.

1309
00:53:05,700 --> 00:53:09,340
The wafers go in looking shiny
and come out looking blue.

1310
00:53:09,340 --> 00:53:12,250
But what's happening
underneath the surface

1311
00:53:12,250 --> 00:53:14,960
is that some of the hydrogen
is going into the wafer.

1312
00:53:14,960 --> 00:53:17,890
Hydrogen, the first element on
the periodic table, very tiny.

1313
00:53:17,890 --> 00:53:20,840
And in the PECVD process,
where you have a plasma,

1314
00:53:20,840 --> 00:53:24,110
you have hydrogen
ions, which basically

1315
00:53:24,110 --> 00:53:26,550
means you have a proton
without its electron.

1316
00:53:26,550 --> 00:53:29,667
And that proton is very fast,
moving through the lattice.

1317
00:53:29,667 --> 00:53:32,000
There's lots of space for it
to move through the silicon

1318
00:53:32,000 --> 00:53:32,557
lattice.

1319
00:53:32,557 --> 00:53:34,140
And it's also very
reactive because it

1320
00:53:34,140 --> 00:53:35,440
doesn't have that electron.

1321
00:53:35,440 --> 00:53:37,890
So whenever it finds a
defect or a dangling bond,

1322
00:53:37,890 --> 00:53:40,510
it'll usually lodge itself
there, and attach itself,

1323
00:53:40,510 --> 00:53:42,270
and passivate that defect.

1324
00:53:42,270 --> 00:53:43,820
And that's what
hydrogen passivation

1325
00:53:43,820 --> 00:53:48,070
is all about during
the silicon nitride

1326
00:53:48,070 --> 00:53:50,190
anti-reflective
coating deposition.

1327
00:53:50,190 --> 00:53:57,495
These are examples of
inline processes for doing

1328
00:53:57,495 --> 00:53:59,120
an anti-reflection coating.

1329
00:53:59,120 --> 00:54:01,350
I believe there are a
few different variants

1330
00:54:01,350 --> 00:54:03,310
of this inline
process, one of which

1331
00:54:03,310 --> 00:54:05,120
is a sputtering
mechanism to deposit

1332
00:54:05,120 --> 00:54:07,760
this anti-reflective coating.

1333
00:54:07,760 --> 00:54:10,490
Of course, then during
sputtering process,

1334
00:54:10,490 --> 00:54:12,530
you have to worry, as
well, about hydrogen.

1335
00:54:12,530 --> 00:54:14,770
Do you have the benefit
of hydrogen passivation?

1336
00:54:14,770 --> 00:54:16,960
Perhaps not as much, so
additional engineering

1337
00:54:16,960 --> 00:54:17,770
is needed.

1338
00:54:17,770 --> 00:54:20,250
But the inline process
could be potentially faster

1339
00:54:20,250 --> 00:54:23,520
and higher throughput than the
batch process using the PECVD.

1340
00:54:23,520 --> 00:54:26,710
So again, manufacturing
trade-offs.

1341
00:54:26,710 --> 00:54:30,010
Next, we have the printing
line and screen printing.

1342
00:54:30,010 --> 00:54:34,970
So this looks very similar to
screen printing for a t-shirt.

1343
00:54:34,970 --> 00:54:38,390
Here is a t-shirt being
loaded into a screen printer.

1344
00:54:38,390 --> 00:54:41,660
And here's a solar cell being
loaded into screen printer.

1345
00:54:41,660 --> 00:54:43,780
This is a close up of
the screen, of what

1346
00:54:43,780 --> 00:54:46,170
the screen actually looks like.

1347
00:54:46,170 --> 00:54:48,530
Here, the screen,
which is comprised

1348
00:54:48,530 --> 00:54:52,384
of this mesh of metal--
here the screen is bare.

1349
00:54:52,384 --> 00:54:54,050
And so the metal
that's deposited on top

1350
00:54:54,050 --> 00:54:56,030
can go through those
holes in the screen

1351
00:54:56,030 --> 00:54:57,760
and onto the wafer
underneath it.

1352
00:54:57,760 --> 00:55:01,302
And here, there's a coating, a
polymer coating of the screen,

1353
00:55:01,302 --> 00:55:02,760
which prevents the
metal from going

1354
00:55:02,760 --> 00:55:04,260
through the screen
at those places.

1355
00:55:04,260 --> 00:55:06,750
So it shades the solar cell
underneath and prevents metals

1356
00:55:06,750 --> 00:55:08,340
from being deposited there.

1357
00:55:08,340 --> 00:55:10,230
And you have
fingers and busbars.

1358
00:55:10,230 --> 00:55:13,530
And those are eventually
the thin little fingers

1359
00:55:13,530 --> 00:55:16,090
that you see right
here going sideways

1360
00:55:16,090 --> 00:55:17,720
and the vertical
busbars that you see

1361
00:55:17,720 --> 00:55:20,012
going vertically right here.

1362
00:55:20,012 --> 00:55:20,512
Question?

1363
00:55:20,512 --> 00:55:21,137
AUDIENCE: Yeah.

1364
00:55:21,137 --> 00:55:23,721
So when you do these [INAUDIBLE]
emitter [INAUDIBLE] where you

1365
00:55:23,721 --> 00:55:25,970
have some areas of high
resistance emitters and others

1366
00:55:25,970 --> 00:55:26,950
of low resistance--

1367
00:55:26,950 --> 00:55:27,616
PROFESSOR: Yeah.

1368
00:55:27,616 --> 00:55:29,430
AUDIENCE: Do you use
a screen to shield it.

1369
00:55:29,430 --> 00:55:30,967
Or do you [INAUDIBLE].

1370
00:55:30,967 --> 00:55:32,050
PROFESSOR: Great question.

1371
00:55:32,050 --> 00:55:34,591
So some of the earliest designs
for the selective emitter--if

1372
00:55:34,591 --> 00:55:36,580
we go back all the
way up to here.

1373
00:55:36,580 --> 00:55:37,080
Yeah.

1374
00:55:37,080 --> 00:55:38,880
To the selective
emitter portion.

1375
00:55:38,880 --> 00:55:42,200
So the earliest designs
used photoresist process.

1376
00:55:42,200 --> 00:55:45,260
But I would say nowadays,
there are a few technology

1377
00:55:45,260 --> 00:55:47,840
options that are much
faster, one of which

1378
00:55:47,840 --> 00:55:52,020
involves a creation of porous
silicon on certain regions

1379
00:55:52,020 --> 00:55:54,410
of the wafer that you want
to etch back and create

1380
00:55:54,410 --> 00:55:58,480
the shallow emitter leaving
the deeper region intact.

1381
00:55:58,480 --> 00:56:00,500
And that, you could
use a form of shading.

1382
00:56:00,500 --> 00:56:02,250
You could use a wax even for it.

1383
00:56:02,250 --> 00:56:04,270
There are a variety
of technology options

1384
00:56:04,270 --> 00:56:05,890
for achieving that goal.

1385
00:56:05,890 --> 00:56:07,950
But in a sense, many of
the selective emitter

1386
00:56:07,950 --> 00:56:10,860
designs involve a deep
diffusion first and then

1387
00:56:10,860 --> 00:56:13,700
a partial etch back.

1388
00:56:13,700 --> 00:56:15,780
For example, creation
of porous silicon

1389
00:56:15,780 --> 00:56:18,110
and etching that material away.

1390
00:56:18,110 --> 00:56:20,539
Ashley, you had a question?

1391
00:56:20,539 --> 00:56:21,505
AUDIENCE: Oh, yeah.

1392
00:56:21,505 --> 00:56:23,310
So what does "turnkey" refer to?

1393
00:56:23,310 --> 00:56:24,240
PROFESSOR: Turnkey.

1394
00:56:24,240 --> 00:56:25,230
Excellent question.

1395
00:56:25,230 --> 00:56:28,170
So turnkey manufacturing
line-- what it refers to

1396
00:56:28,170 --> 00:56:30,710
is that I'm the vendor
of the equipment.

1397
00:56:30,710 --> 00:56:33,310
In one case, I
say, here, Ashley.

1398
00:56:33,310 --> 00:56:34,520
Here's a piece of equipment.

1399
00:56:34,520 --> 00:56:36,080
It's going to cost
you $1 million.

1400
00:56:36,080 --> 00:56:37,930
And good luck getting
it set up and running.

1401
00:56:37,930 --> 00:56:38,929
I'm out of here.

1402
00:56:38,929 --> 00:56:39,470
I'll see you.

1403
00:56:39,470 --> 00:56:40,030
AUDIENCE: Right.

1404
00:56:40,030 --> 00:56:41,905
PROFESSOR: A smarter
company might come along

1405
00:56:41,905 --> 00:56:44,120
and say, I'm going to
guarantee an output

1406
00:56:44,120 --> 00:56:45,584
from my piece of equipment.

1407
00:56:45,584 --> 00:56:47,000
I'm going to
guarantee that you'll

1408
00:56:47,000 --> 00:56:51,840
be able to make 16.7% solar
cells, 16.7% efficiency.

1409
00:56:51,840 --> 00:56:53,850
I will send my engineers
to your factory,

1410
00:56:53,850 --> 00:56:56,350
and they will help you get the
equipment set up and running.

1411
00:56:56,350 --> 00:56:57,930
And once it's
running up to spec,

1412
00:56:57,930 --> 00:57:00,460
then they'll come back home,
and you'll be on your own.

1413
00:57:00,460 --> 00:57:02,210
And you'll be able to
optimize it further.

1414
00:57:02,210 --> 00:57:04,940
And so you walk in knowing
that you have this guarantee

1415
00:57:04,940 --> 00:57:06,062
of a performance.

1416
00:57:06,062 --> 00:57:07,770
Then you can go to
your financing agency.

1417
00:57:07,770 --> 00:57:09,269
You can go to Joe
and say, hey, Joe,

1418
00:57:09,269 --> 00:57:10,840
give me money for
my new factory.

1419
00:57:10,840 --> 00:57:13,580
I have a guarantee that
I'm going to hit 16.7%

1420
00:57:13,580 --> 00:57:15,860
and have a pathway
to get to 17.2.

1421
00:57:15,860 --> 00:57:19,730
My CTO right here thinks--
she's a really small person,

1422
00:57:19,730 --> 00:57:22,710
and she has a pathway to get
to another 0.5% out of it.

1423
00:57:22,710 --> 00:57:24,790
And so you can go
to your financier

1424
00:57:24,790 --> 00:57:26,740
and get money more
easily than, say,

1425
00:57:26,740 --> 00:57:28,157
in the first
scenario where you're

1426
00:57:28,157 --> 00:57:30,531
given a piece of equipment
and then the person high tails

1427
00:57:30,531 --> 00:57:31,220
it out of there.

1428
00:57:31,220 --> 00:57:31,650
AUDIENCE: Right.

1429
00:57:31,650 --> 00:57:33,316
PROFESSOR: So turnkey
refers to the idea

1430
00:57:33,316 --> 00:57:34,404
that you turn the line on.

1431
00:57:34,404 --> 00:57:36,070
You essentially turn
the key, and you're

1432
00:57:36,070 --> 00:57:37,389
getting high performance out.

1433
00:57:37,389 --> 00:57:39,930
In reality, it takes a month or
two to ramp up to that point.

1434
00:57:39,930 --> 00:57:40,330
AUDIENCE: Right.

1435
00:57:40,330 --> 00:57:42,320
PROFESSOR: To get high yields
and to get high performance.

1436
00:57:42,320 --> 00:57:43,950
But you have the
support of the company

1437
00:57:43,950 --> 00:57:47,180
there on the ground
helping you achieve that.

1438
00:57:47,180 --> 00:57:49,190
And the turnkey
lines were actually

1439
00:57:49,190 --> 00:57:51,910
one of the real reasons why
technology flowed around

1440
00:57:51,910 --> 00:57:53,890
the planet so quickly.

1441
00:57:53,890 --> 00:57:56,440
Because up until
about the mid 2000s,

1442
00:57:56,440 --> 00:58:00,830
high efficiency cell was
limited to a few laboratories

1443
00:58:00,830 --> 00:58:02,640
and a few companies in the know.

1444
00:58:02,640 --> 00:58:04,440
But once turnkey
equipment manufacturers

1445
00:58:04,440 --> 00:58:06,230
got into to the
mix, they started

1446
00:58:06,230 --> 00:58:08,550
creating these turnkey
lines and selling

1447
00:58:08,550 --> 00:58:11,380
the equipment around the
world and the expertise of how

1448
00:58:11,380 --> 00:58:13,810
to make high efficiency
devices, both the architecture

1449
00:58:13,810 --> 00:58:15,350
and the processing know-how.

1450
00:58:15,350 --> 00:58:18,210
And this is how, within
in the last 5 to 10 years,

1451
00:58:18,210 --> 00:58:20,670
you've seen such an
explosion of companies

1452
00:58:20,670 --> 00:58:23,500
around the globe in all sorts
of places that traditionally

1453
00:58:23,500 --> 00:58:26,460
haven't been experts in solar
cell manufacturing suddenly

1454
00:58:26,460 --> 00:58:29,050
knowing how to
manufacture solar cells.

1455
00:58:29,050 --> 00:58:29,910
It flows.

1456
00:58:29,910 --> 00:58:33,630
The know-how flows through
the equipment vendors.

1457
00:58:33,630 --> 00:58:35,651
So finally, testing and sorting.

1458
00:58:35,651 --> 00:58:37,900
This is the last stage of
the solar cell manufacturing

1459
00:58:37,900 --> 00:58:38,610
process.

1460
00:58:38,610 --> 00:58:40,640
Here, we see a little
pick-and-place.

1461
00:58:40,640 --> 00:58:43,391
That means a little robot that
picks up wafers and deposits

1462
00:58:43,391 --> 00:58:43,890
them.

1463
00:58:43,890 --> 00:58:46,540
The simplest incarnation
is just suction cup.

1464
00:58:46,540 --> 00:58:50,140
The more fancy ones involve
Bernoulli lifters, essentially

1465
00:58:50,140 --> 00:58:52,710
pressure differentials
pulling wafers up.

1466
00:58:52,710 --> 00:58:56,429
So you have wafers being
loaded onto a conveyor belt,

1467
00:58:56,429 --> 00:58:58,470
coming off of one conveyor
belt onto another one.

1468
00:58:58,470 --> 00:58:59,980
And they're moving forward.

1469
00:58:59,980 --> 00:59:03,290
And what you see right
here in very low resolution

1470
00:59:03,290 --> 00:59:05,740
are two probes coming down.

1471
00:59:05,740 --> 00:59:09,550
This, evidently, is a two
busbar cell, not a three busbar

1472
00:59:09,550 --> 00:59:10,520
cell like this one.

1473
00:59:10,520 --> 00:59:14,020
The probes come down and make
contact with the busbars.

1474
00:59:14,020 --> 00:59:16,260
And the probes have
multiple contact points,

1475
00:59:16,260 --> 00:59:19,090
so the series resistance
along the busbars

1476
00:59:19,090 --> 00:59:21,190
is not affecting
your measurement.

1477
00:59:21,190 --> 00:59:23,177
Cell efficiency measurement
is always tricky

1478
00:59:23,177 --> 00:59:25,010
because depending where
you put your probes,

1479
00:59:25,010 --> 00:59:27,468
your measurements are going to
change because of the series

1480
00:59:27,468 --> 00:59:28,270
resistance.

1481
00:59:28,270 --> 00:59:30,702
So these probes
right here are long,

1482
00:59:30,702 --> 00:59:32,410
and they contain
multiple contact points.

1483
00:59:32,410 --> 00:59:34,284
And they're essentially
touching the busbars.

1484
00:59:34,284 --> 00:59:38,320
And light flashes onto the
device simulating the sun, so

1485
00:59:38,320 --> 00:59:40,730
simulating AM 1.5 conditions.

1486
00:59:40,730 --> 00:59:44,596
And an IV curve is
measured, is swept.

1487
00:59:44,596 --> 00:59:46,970
I can't really tell from the
photograph or from the movie

1488
00:59:46,970 --> 00:59:50,402
right here whether the IV curve
is being swept at illumination,

1489
00:59:50,402 --> 00:59:52,610
meaning you're sweeping your
voltage when the cell is

1490
00:59:52,610 --> 00:59:55,260
illuminated, or whether
the illumination intensity

1491
00:59:55,260 --> 00:59:57,320
itself is used to
vary the forward bias

1492
00:59:57,320 --> 00:59:58,580
condition of the cell.

1493
00:59:58,580 --> 01:00:01,097
They could be doing
it in one of two ways.

1494
01:00:01,097 --> 01:00:02,680
But most likely,
what they're doing is

1495
01:00:02,680 --> 01:00:06,650
they're flashing the lights,
measuring the IV characteristic

1496
01:00:06,650 --> 01:00:08,910
of the device, and
then sorting the cell

1497
01:00:08,910 --> 01:00:10,280
based on that performance.

1498
01:00:10,280 --> 01:00:12,210
It goes into a computer.

1499
01:00:12,210 --> 01:00:14,960
Efficiency is calculated, just
like you did on your homework.

1500
01:00:14,960 --> 01:00:16,460
And just like that,
it's calculated.

1501
01:00:16,460 --> 01:00:18,840
And then, as the cell
moves down the line,

1502
01:00:18,840 --> 01:00:21,900
the robot knows, oh, that's
the cell that got 16.6.

1503
01:00:21,900 --> 01:00:22,860
We put it over here.

1504
01:00:22,860 --> 01:00:24,210
Oh, that next cell got 16.8.

1505
01:00:24,210 --> 01:00:25,650
We put it over there.

1506
01:00:25,650 --> 01:00:29,260
Some additional companies sort
their cells based on color

1507
01:00:29,260 --> 01:00:32,090
because they want to have
the aesthetic appearance

1508
01:00:32,090 --> 01:00:34,030
of homogeneity
within the module.

1509
01:00:34,030 --> 01:00:38,540
They want every cell to be
of uniform aesthetic value

1510
01:00:38,540 --> 01:00:42,924
inside of a module so that you
have a nice, uniform color.

1511
01:00:42,924 --> 01:00:44,876
AUDIENCE: Is that
considered [INAUDIBLE].

1512
01:00:47,770 --> 01:00:49,770
PROFESSOR: Whether or not
this module right here

1513
01:00:49,770 --> 01:00:52,610
is considered uniform or
different would depend on you,

1514
01:00:52,610 --> 01:00:53,440
Jessica.

1515
01:00:53,440 --> 01:00:55,430
You're the customer,
and you decide

1516
01:00:55,430 --> 01:00:57,900
whether this is good enough
for you or whether it's not.

1517
01:00:57,900 --> 01:00:58,370
AUDIENCE: It's not.

1518
01:00:58,370 --> 01:00:59,070
PROFESSOR: It's not?

1519
01:00:59,070 --> 01:00:59,570
All right.

1520
01:00:59,570 --> 01:01:01,830
Well, then we have
to work harder.

1521
01:01:01,830 --> 01:01:06,120
So the customer requirements
really drive the industry.

1522
01:01:06,120 --> 01:01:07,747
So some customers
are more discerning.

1523
01:01:07,747 --> 01:01:10,080
Obviously, if this is going
to large field installation,

1524
01:01:10,080 --> 01:01:11,520
we have big barbed
wire around it.

1525
01:01:11,520 --> 01:01:13,311
Who cares as long as
the module's producing

1526
01:01:13,311 --> 01:01:14,120
high performance?

1527
01:01:14,120 --> 01:01:16,410
But if it's sitting on
the facade of the train

1528
01:01:16,410 --> 01:01:18,309
station in downtown
Freiburg, Germany,

1529
01:01:18,309 --> 01:01:20,100
where every single
person riding the train,

1530
01:01:20,100 --> 01:01:22,020
entering the station,
sees the modules lining

1531
01:01:22,020 --> 01:01:24,050
the side of Deutsche
Bahn's headquarters,

1532
01:01:24,050 --> 01:01:26,060
you want to make sure
that those look nice.

1533
01:01:26,060 --> 01:01:28,970
So there are differences
depending on where they go

1534
01:01:28,970 --> 01:01:31,210
and where they're installed.

1535
01:01:31,210 --> 01:01:33,250
High efficiency
cell architectures.

1536
01:01:33,250 --> 01:01:35,620
So there are a plethora
of different architectures

1537
01:01:35,620 --> 01:01:37,566
out there.

1538
01:01:37,566 --> 01:01:40,480
There are some that, for
example, put all their contacts

1539
01:01:40,480 --> 01:01:43,090
on the backside, so
there's no shading.

1540
01:01:43,090 --> 01:01:45,670
And these are interdigitated
positive, negative, positive,

1541
01:01:45,670 --> 01:01:48,480
negative, positive,
negative contacts here.

1542
01:01:48,480 --> 01:01:50,630
So this is called an
interdigitated back contact

1543
01:01:50,630 --> 01:01:51,720
structure.

1544
01:01:51,720 --> 01:01:54,250
It's used by the company
called Sun Power.

1545
01:01:54,250 --> 01:01:57,380
And so there's no metalization
loss on the front side.

1546
01:01:57,380 --> 01:01:58,940
All your contacts
are on the back.

1547
01:01:58,940 --> 01:02:00,352
Because lateral
carrier diffusion

1548
01:02:00,352 --> 01:02:01,935
is involved, meaning
the carriers have

1549
01:02:01,935 --> 01:02:03,476
to diffuse laterally,
they don't have

1550
01:02:03,476 --> 01:02:05,330
to diffuse only
one dimensionally,

1551
01:02:05,330 --> 01:02:07,979
you probably can't use
PC1D to model this cell.

1552
01:02:07,979 --> 01:02:09,770
You'll have to use a
two-dimensional device

1553
01:02:09,770 --> 01:02:11,880
simulation like Sentaurus.

1554
01:02:11,880 --> 01:02:13,740
If anybody has any
two-dimensional device

1555
01:02:13,740 --> 01:02:15,940
simulation questions, Ashley
right here in the front

1556
01:02:15,940 --> 01:02:18,908
is our resident expert, so
you're welcome to ask her.

1557
01:02:18,908 --> 01:02:19,844
AUDIENCE: [INAUDIBLE]

1558
01:02:19,844 --> 01:02:20,510
PROFESSOR: Yeah.

1559
01:02:22,907 --> 01:02:24,990
And then there are also
other device architectures

1560
01:02:24,990 --> 01:02:28,780
which we'll get to during
our thin films discussions.

1561
01:02:28,780 --> 01:02:32,690
A couple of ancillary
topics, barriers to scale.

1562
01:02:32,690 --> 01:02:36,020
This is the size of a 1 gigawatt
peak plant manufacturing

1563
01:02:36,020 --> 01:02:39,790
facility for wafers,
cells, and modules.

1564
01:02:39,790 --> 01:02:41,390
This is a palm tree right here.

1565
01:02:41,390 --> 01:02:42,300
These are roads.

1566
01:02:42,300 --> 01:02:44,190
So you get a sense of scale.

1567
01:02:44,190 --> 01:02:46,410
This is located in Singapore.

1568
01:02:46,410 --> 01:02:49,570
It's a company called REC
that has this factory.

1569
01:02:49,570 --> 01:02:51,310
These are 18-wheelers
right here that are

1570
01:02:51,310 --> 01:02:55,760
taking the materials out and
selling them to customers.

1571
01:02:55,760 --> 01:02:59,725
So you get a sense of the
scale of these facilities.

1572
01:02:59,725 --> 01:03:01,180
They're rather big.

1573
01:03:01,180 --> 01:03:03,810
And if you say, OK,
this is a gigawatt fab,

1574
01:03:03,810 --> 01:03:06,417
but we need to be producing on
the scale of terawatts, which

1575
01:03:06,417 --> 01:03:08,250
are three orders of
magnitude larger in area

1576
01:03:08,250 --> 01:03:10,571
than this, how big is
that factory going to be?

1577
01:03:10,571 --> 01:03:12,445
It's about half of the
state of Rhode Island.

1578
01:03:15,124 --> 01:03:16,790
Granted, it'll be
distributed throughout

1579
01:03:16,790 --> 01:03:19,370
many different regions, but
it's a big, big factory.

1580
01:03:19,370 --> 01:03:21,410
So one of the
interesting questions is,

1581
01:03:21,410 --> 01:03:23,430
can we produce the
silicon in a faster

1582
01:03:23,430 --> 01:03:26,200
way that involves less area?

1583
01:03:26,200 --> 01:03:28,010
Because area generally
relates to capital

1584
01:03:28,010 --> 01:03:30,569
equipment costs, not
always, but quite typically.

1585
01:03:30,569 --> 01:03:32,110
If you have a larger
area because you

1586
01:03:32,110 --> 01:03:33,580
need more equipment in
there, for more equipment,

1587
01:03:33,580 --> 01:03:34,850
it's a higher cost.

1588
01:03:34,850 --> 01:03:38,240
So can the production costs
be reduced by a higher

1589
01:03:38,240 --> 01:03:40,390
throughput growth mechanisms?

1590
01:03:40,390 --> 01:03:43,345
So instead of using thin film
or crystalline technologies

1591
01:03:43,345 --> 01:03:47,100
that are currently being used
today-- apologies for that.

1592
01:03:47,100 --> 01:03:52,440
Instead, if we used, let's say,
a float glass-like process.

1593
01:03:52,440 --> 01:03:55,249
So these would be extruded
pieces of silicon on some bed

1594
01:03:55,249 --> 01:03:57,540
of--I don't know-- liquid
tin would be for float glass,

1595
01:03:57,540 --> 01:03:59,240
an equivalent for silicon.

1596
01:03:59,240 --> 01:04:03,610
You could reduce the area by
about two orders of magnitude.

1597
01:04:03,610 --> 01:04:06,660
And if you envision instead
these high speed printers that

1598
01:04:06,660 --> 01:04:10,020
print out your reports for
your exam or class notes,

1599
01:04:10,020 --> 01:04:12,750
they're spitting out 55
pages per minute on 8 and 1/2

1600
01:04:12,750 --> 01:04:15,210
by 11 inch squared sheets.

1601
01:04:15,210 --> 01:04:18,100
If instead those were 15%
solar cells being printed,

1602
01:04:18,100 --> 01:04:21,100
you could envision an area the
size of five football fields

1603
01:04:21,100 --> 01:04:22,730
instead.

1604
01:04:22,730 --> 01:04:25,860
So this starts opening
the mind that, wow,

1605
01:04:25,860 --> 01:04:27,860
our way of manufacturing
these solar cells,

1606
01:04:27,860 --> 01:04:30,240
this discrete process
where it's very

1607
01:04:30,240 --> 01:04:32,760
segregated-- wafer,
cell, and module.

1608
01:04:32,760 --> 01:04:36,060
Wafer manufacturing
almost like a commodity.

1609
01:04:36,060 --> 01:04:38,210
Ingot of aluminum.

1610
01:04:38,210 --> 01:04:40,320
The cell like a device.

1611
01:04:40,320 --> 01:04:43,045
The module-- as we'll see in
a second-- like an automobile,

1612
01:04:43,045 --> 01:04:44,480
an assembly process.

1613
01:04:44,480 --> 01:04:47,110
If instead we managed to
blend these processes together

1614
01:04:47,110 --> 01:04:49,400
and reduce the barriers,
the discrete barriers

1615
01:04:49,400 --> 01:04:50,990
between these
different processes

1616
01:04:50,990 --> 01:04:53,410
and reinvent the
manufacturing process thereof,

1617
01:04:53,410 --> 01:04:56,920
we stand to make this a lot
cheaper, and a lot faster,

1618
01:04:56,920 --> 01:04:59,300
and a lot smaller to produce.

1619
01:04:59,300 --> 01:05:01,570
We might even have our own
solar cell manufacturing

1620
01:05:01,570 --> 01:05:02,879
equipment mounted on our desk.

1621
01:05:02,879 --> 01:05:05,420
When we need to print a solar
cell device or power something,

1622
01:05:05,420 --> 01:05:07,100
we can produce it right there.

1623
01:05:07,100 --> 01:05:08,850
So that's kind of the
vision of the future

1624
01:05:08,850 --> 01:05:10,470
where this might
be going and why

1625
01:05:10,470 --> 01:05:12,690
bright minds like
yourselves are needed.

1626
01:05:12,690 --> 01:05:13,700
We talked about silver.

1627
01:05:13,700 --> 01:05:15,210
We know there's a limit
for how much silver

1628
01:05:15,210 --> 01:05:17,260
can be used in the front
contact metalization.

1629
01:05:17,260 --> 01:05:20,374
We're using about
10% of it right now.

1630
01:05:20,374 --> 01:05:22,290
And if you're looking
for environmental impact

1631
01:05:22,290 --> 01:05:25,600
of crystalline
silicon technologies,

1632
01:05:25,600 --> 01:05:27,160
I've included many
different sites

1633
01:05:27,160 --> 01:05:29,285
right here that talk about
the environmental impact

1634
01:05:29,285 --> 01:05:33,790
of solar cell manufacturing
since we have mentioned acids.

1635
01:05:33,790 --> 01:05:36,080
We have mentioned
gases like silane.

1636
01:05:36,080 --> 01:05:40,277
We've mentioned CO2 production
when we produce the wafers.

1637
01:05:40,277 --> 01:05:42,110
We'll talk about this
later on in the class,

1638
01:05:42,110 --> 01:05:47,000
but in essence, we're looking
at around 1/10 or 1/20 the CO2

1639
01:05:47,000 --> 01:05:48,550
intensity of coal.

1640
01:05:48,550 --> 01:05:51,140
So it's not a zero-emission
source to produce that module,

1641
01:05:51,140 --> 01:05:55,510
but it certainly is a lot less
than, say, our fossil fuel

1642
01:05:55,510 --> 01:05:57,700
sources.

1643
01:05:57,700 --> 01:06:00,520
This declining US
market share has really

1644
01:06:00,520 --> 01:06:02,500
captured the attention
of politicians

1645
01:06:02,500 --> 01:06:07,030
lately, the fact that the US
used to comprise 75% of the PV

1646
01:06:07,030 --> 01:06:08,200
production market.

1647
01:06:08,200 --> 01:06:10,530
This is to produce and
manufacture the modules.

1648
01:06:10,530 --> 01:06:13,070
And today, it's on the order 5%.

1649
01:06:13,070 --> 01:06:17,040
This is a risen concern
within many in the DOE

1650
01:06:17,040 --> 01:06:20,490
and today's DOE and government.

1651
01:06:20,490 --> 01:06:22,700
Meanwhile, the market is
growing substantially.

1652
01:06:22,700 --> 01:06:24,190
And so an open
question is, what is

1653
01:06:24,190 --> 01:06:28,710
the future of US market share?

1654
01:06:28,710 --> 01:06:31,270
If all goes well, we should
have a small Greentech Media

1655
01:06:31,270 --> 01:06:34,500
article published on this topic
probably within about a week

1656
01:06:34,500 --> 01:06:37,310
or so, so keep your eyes open.

1657
01:06:37,310 --> 01:06:40,804
And let me briefly jump
into module manufacturing.

1658
01:06:40,804 --> 01:06:41,720
Do we have a question?

1659
01:06:41,720 --> 01:06:42,670
Oh, we're all set.

1660
01:06:42,670 --> 01:06:43,170
OK.

1661
01:06:43,170 --> 01:06:44,425
I'm going to hop into
module manufacturing.

1662
01:06:44,425 --> 01:06:45,980
It'll be the last five minutes.

1663
01:06:45,980 --> 01:06:48,770
Just to show you how you go
from the cell to the module,

1664
01:06:48,770 --> 01:06:52,150
it's an assembly process,
very, very straightforward.

1665
01:06:52,150 --> 01:06:56,376
We have coming in here sheets
of glass, encapsulate materials.

1666
01:06:56,376 --> 01:06:58,500
And we'll be able to see
this up close and personal

1667
01:06:58,500 --> 01:06:59,875
and feel the
materials when we go

1668
01:06:59,875 --> 01:07:02,440
visit Fraunhofer CSE in
the first week of November.

1669
01:07:02,440 --> 01:07:03,939
We have a field
trip going up there.

1670
01:07:03,939 --> 01:07:05,090
That'll be a lot of fun.

1671
01:07:05,090 --> 01:07:06,850
And the encapsulants
are a lot of fun.

1672
01:07:06,850 --> 01:07:07,890
They're polymers.

1673
01:07:07,890 --> 01:07:09,070
They're really tough.

1674
01:07:09,070 --> 01:07:11,490
You can take the Tedlar
back skin, this white stuff

1675
01:07:11,490 --> 01:07:15,260
here in the back of the device,
that white skin right there.

1676
01:07:15,260 --> 01:07:17,170
That's called Tedlar.

1677
01:07:17,170 --> 01:07:19,500
As the name would suggest,
it comes from DuPont.

1678
01:07:19,500 --> 01:07:21,190
It's a polymer.

1679
01:07:21,190 --> 01:07:22,189
Really, really tough.

1680
01:07:22,189 --> 01:07:24,355
If try to take some in your
hand and try to tear it,

1681
01:07:24,355 --> 01:07:26,813
it's nearly impossible, even
for the strongest people here.

1682
01:07:26,813 --> 01:07:28,960
So it's impermeable,
very strong material.

1683
01:07:28,960 --> 01:07:32,560
The ethyl vinyl
acetate, or EVA, is

1684
01:07:32,560 --> 01:07:36,300
a polymer that infuses the
glass in the front side

1685
01:07:36,300 --> 01:07:37,210
with the cell.

1686
01:07:37,210 --> 01:07:39,001
And with the Tedlar in
the back, it kind of

1687
01:07:39,001 --> 01:07:42,150
forms this sticky,
mushy material

1688
01:07:42,150 --> 01:07:44,039
when you heat it up
above 150 degrees C.

1689
01:07:44,039 --> 01:07:46,580
And it binds everything together
in what's called a laminate.

1690
01:07:46,580 --> 01:07:48,572
So let's walk through
that real quick.

1691
01:07:48,572 --> 01:07:50,030
To get to the point
of a module, we

1692
01:07:50,030 --> 01:07:52,287
need to take our good
apples with our good apples

1693
01:07:52,287 --> 01:07:54,370
or our bad apples with our
bad apples, essentially

1694
01:07:54,370 --> 01:07:57,440
the like-binned cells, and
start stringing them together.

1695
01:07:57,440 --> 01:07:59,290
That means contacting
the front side

1696
01:07:59,290 --> 01:08:01,470
with the backside
of adjacent cells.

1697
01:08:01,470 --> 01:08:03,021
So the front of one
cell is connected

1698
01:08:03,021 --> 01:08:04,020
to the back of the next.

1699
01:08:04,020 --> 01:08:05,936
The front of that one
is connected to the back

1700
01:08:05,936 --> 01:08:07,140
to the next, and so forth.

1701
01:08:07,140 --> 01:08:09,470
And they're connected in
series in a big, long string.

1702
01:08:09,470 --> 01:08:12,160
And that's done at this tabbing,
stringing, and layup table.

1703
01:08:12,160 --> 01:08:15,450
Typically, this is done by
an automated solder system.

1704
01:08:15,450 --> 01:08:18,624
I just put the cells together,
and it wires them for you.

1705
01:08:18,624 --> 01:08:21,040
But usually, there's a manual
inspection process afterward

1706
01:08:21,040 --> 01:08:22,956
because sometimes the
soldering isn't perfect.

1707
01:08:22,956 --> 01:08:25,720
A human being is typically there
fidgeting through, making sure

1708
01:08:25,720 --> 01:08:27,970
that everything is primo.

1709
01:08:27,970 --> 01:08:29,960
Then we have the
lamination process,

1710
01:08:29,960 --> 01:08:31,262
which takes those strings.

1711
01:08:31,262 --> 01:08:32,720
They're very fragile
at this point.

1712
01:08:32,720 --> 01:08:34,590
They're just solar
cells connected

1713
01:08:34,590 --> 01:08:39,090
with some solder-coated
wire, so they're

1714
01:08:39,090 --> 01:08:40,470
very fragile at that point.

1715
01:08:40,470 --> 01:08:44,310
And these are then laid
up on the top of sheets

1716
01:08:44,310 --> 01:08:47,670
of the encapsulant
materials and the glass

1717
01:08:47,670 --> 01:08:49,560
and eventually
laminated together

1718
01:08:49,560 --> 01:08:53,120
to form that nice package.

1719
01:08:53,120 --> 01:08:55,660
So at the lamination stage,
coming out of the lamination,

1720
01:08:55,660 --> 01:08:58,160
we'd have the glass on one side,
the Tedlar in the the back,

1721
01:08:58,160 --> 01:09:00,910
and the cells in between
encapsulated by the ethyl vinyl

1722
01:09:00,910 --> 01:09:02,609
acetate, the EVA.

1723
01:09:02,609 --> 01:09:05,210
And we wouldn't have
the frame yet around it.

1724
01:09:05,210 --> 01:09:07,550
And so the put that
frame, we would

1725
01:09:07,550 --> 01:09:11,560
need essentially a
large machine that

1726
01:09:11,560 --> 01:09:13,510
takes those pieces
of extruded aluminum

1727
01:09:13,510 --> 01:09:15,970
and pushes them together around
the edges of the laminate,

1728
01:09:15,970 --> 01:09:17,310
fixing them on there.

1729
01:09:17,310 --> 01:09:21,850
And this is the examples of
the tabbing and stringing

1730
01:09:21,850 --> 01:09:22,840
right there.

1731
01:09:22,840 --> 01:09:24,750
And let's see, OK.

1732
01:09:24,750 --> 01:09:28,080
So the framing
materials right here

1733
01:09:28,080 --> 01:09:30,970
are typically done by machines
in places with high labor

1734
01:09:30,970 --> 01:09:31,550
costs.

1735
01:09:31,550 --> 01:09:34,180
And they're done by human
beings pushing them together

1736
01:09:34,180 --> 01:09:36,189
at regions of low labor cost.

1737
01:09:36,189 --> 01:09:40,010
And finally, the junction
box is deposited at the end.

1738
01:09:40,010 --> 01:09:41,700
And the junction
box, what it does

1739
01:09:41,700 --> 01:09:45,520
is it collects the power
outputs from each of the cells

1740
01:09:45,520 --> 01:09:49,200
and very conveniently
gives you two leads.

1741
01:09:49,200 --> 01:09:52,180
So there could be some power
electronics inside of here

1742
01:09:52,180 --> 01:09:56,960
that allows the current
to flow around this module

1743
01:09:56,960 --> 01:09:59,290
if this module's under
performing, if it's broken,

1744
01:09:59,290 --> 01:10:00,837
or if it's shaded.

1745
01:10:00,837 --> 01:10:03,170
There would be a bypass diode
inside of the junction box

1746
01:10:03,170 --> 01:10:05,128
that allows the power to
flow around the module

1747
01:10:05,128 --> 01:10:06,710
and not get sunk into it.

1748
01:10:06,710 --> 01:10:09,440
And it also works to
collect the power outputs

1749
01:10:09,440 --> 01:10:12,557
from all the cells and
produces two leads,

1750
01:10:12,557 --> 01:10:14,140
which can then be
conveniently plugged

1751
01:10:14,140 --> 01:10:16,265
into either adjacent modules,
which would be strung

1752
01:10:16,265 --> 01:10:19,600
in series, or into an inverter,
which would then take the DC

1753
01:10:19,600 --> 01:10:24,330
power here and convert it into
AC power for your consumption.

1754
01:10:24,330 --> 01:10:28,030
And that is how a
solar cell is made.

1755
01:10:28,030 --> 01:10:31,026
So I welcome you to spend a
few minutes at the very end

1756
01:10:31,026 --> 01:10:33,525
to come up and take a close
look at some of these materials.

1757
01:10:36,070 --> 01:10:37,720
Ask some further questions.

1758
01:10:37,720 --> 01:10:40,180
And on Thursday,
we'll start diving

1759
01:10:40,180 --> 01:10:42,970
into thin film technologies
and talk about how

1760
01:10:42,970 --> 01:10:45,090
those are made as well.