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PROFESSOR: But today
we're going to be

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talking about crystalline
silicon solar cells.

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Now, for those of you who do not
work in crystalline silicon PV,

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the reason this
topic is important

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is because crystalline
silicon comprises about 90%

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of all solar cells
manufactured today.

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It's the dominant technology,
and the technologies

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that you're working on
are going to displace,

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or are aiming to displace
crystalline silicon,

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so it's good to know your enemy.

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For those who are working
on crystalline silicon,

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this is meant to be
a background of all

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of the different aspects--
the entire supply

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chain of crystalline
silicon-- so that you

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00:00:54,600 --> 00:00:56,840
gain insight into the areas
that you're not currently

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focused on.

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You're getting a perspective
of the bigger picture.

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Crystalline silicon PV has
been around since 1954.

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The original-- well, in
its current incarnation.

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00:01:08,160 --> 00:01:11,070
That was when Bell
Laboratories announced

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the development of the modern
crystalline silicon PV cell,

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and that was 6%
efficiency in 1954,

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published in general
applied physics,

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and the cell architecture,
it's obviously

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evolved over the
years but it's not

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00:01:24,940 --> 00:01:27,870
entirely dissimilar
from what we have today

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00:01:27,870 --> 00:01:32,740
as a cell architecture
for our modern PV cells.

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00:01:32,740 --> 00:01:35,160
So, over the course
of-- it's almost

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00:01:35,160 --> 00:01:37,680
been 60 years of development
of crystalline silicon

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00:01:37,680 --> 00:01:39,660
photovoltaic technology.

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00:01:39,660 --> 00:01:42,060
That means both the cell
itself, the materials

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00:01:42,060 --> 00:01:44,570
that go into it, and also the
manufacturing, or the methods

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00:01:44,570 --> 00:01:46,995
to produce said
materials and device,

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00:01:46,995 --> 00:01:48,370
over the course
of those, almost,

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60 years much
innovation has happened

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00:01:51,010 --> 00:01:54,780
both in terms of
manufacturing and technology.

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00:01:54,780 --> 00:01:59,100
So today, we'll be going
over kind of a status quo

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00:01:59,100 --> 00:02:02,980
snapshot of where crystalline
silicon stands and we brought

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00:02:02,980 --> 00:02:05,210
in a number of
show and tell items

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00:02:05,210 --> 00:02:09,660
so that you can see as we talk.

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So just for the
show and tell, we're

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00:02:14,780 --> 00:02:17,340
going to be moving from the
feedstock materials over here

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00:02:17,340 --> 00:02:20,615
finally into wafers
and cells on that side.

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00:02:20,615 --> 00:02:21,350
All right.

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00:02:23,890 --> 00:02:27,290
So, these lecture
notes are going

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00:02:27,290 --> 00:02:29,090
to be valid for both 10 and 11.

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We're going to split this up
over two classes to really

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00:02:32,140 --> 00:02:34,364
dive into some of the details.

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00:02:34,364 --> 00:02:35,780
The first question
is why silicon?

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Why did silicon evolve
as what is currently

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the dominant technology,
which is currently

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90 percent of the PV market,
and I think it boils down

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to a couple of reasons.

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00:02:47,770 --> 00:02:49,750
One is scalability.

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00:02:49,750 --> 00:02:53,680
If you look at the elemental
abundance, on the vertical axis

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it's abundance, atoms
of the element per 10

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to the 16 atoms of silicon.

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The reason that everything
is normalized to silicon

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is because there is,
well, quite a lot of it

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in the earth's crust.

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As you can see, it's the
second most abundant element

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on the Earth's crust.

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It just so happens that, out of
all the stardust that is here

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on the planet, we have a
high percentage of silicon

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like the moon and like
many other planets

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in our solar system--
at least the hard ones.

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You can see oxygen
is probably the,

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well, oxygen is the only element
with higher natural abundance

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in the earth's crust, the
upper crust, than silicon

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and we go down as
we go to higher

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and higher atomic number.

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The probability of formation due
to subsequent fusion reactions

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in stars decreases
and, hence, it

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follows this almost a
power law distribution

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as you can see there.

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So it's scalable.

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It is present in the Earth
in high enough capacity

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00:03:48,830 --> 00:03:50,210
to reach terawatt scales.

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It's nontoxic and, as
Don Sadoway likes to say,

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00:03:53,890 --> 00:03:55,700
if you want
batteries dirt-cheap,

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00:03:55,700 --> 00:03:57,400
you have to make
them out of dirt.

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00:03:57,400 --> 00:03:59,730
A similar expression is used
in the crystalline silicon

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00:03:59,730 --> 00:04:00,650
community.

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00:04:00,650 --> 00:04:03,630
I believe the quote
in 1366 is, "It's not

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00:04:03,630 --> 00:04:06,770
only good for the planet,
it is the planet."

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00:04:06,770 --> 00:04:09,250
A variety of riffs off
of this particular chart

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00:04:09,250 --> 00:04:11,550
right here, but from a
technological point of view,

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00:04:11,550 --> 00:04:14,530
why did silicon evolve to
the point where it is today?

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00:04:14,530 --> 00:04:17,589
It forms a very
tenacious surface oxide.

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00:04:17,589 --> 00:04:21,399
So, if you were to expose a
piece of pure silicon to air,

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00:04:21,399 --> 00:04:26,190
the surface oxide that forms
is very, very strong and very

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00:04:26,190 --> 00:04:28,410
resistant, and very dense.

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00:04:28,410 --> 00:04:30,920
So, unlike some
materials that corrode

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when exposed to
atmosphere, silicon

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00:04:33,240 --> 00:04:37,180
oxidizes maybe the first few 10s
of angstroms, 100 of angstroms,

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00:04:37,180 --> 00:04:42,530
and then it peters out so it's
diffusion-limited oxide growth

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00:04:42,530 --> 00:04:44,470
mechanism that
eventually stabilizes

105
00:04:44,470 --> 00:04:46,760
at a very thin but very
dense and very protective

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00:04:46,760 --> 00:04:48,000
oxide layer.

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00:04:48,000 --> 00:04:51,340
So the risk of having a
silicon wafer degrade inside

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00:04:51,340 --> 00:04:52,901
of a solar module is very low.

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00:04:52,901 --> 00:04:55,150
Furthermore, that oxide layer
from an electrical point

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00:04:55,150 --> 00:04:56,820
of view it's very passivating.

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00:04:56,820 --> 00:04:58,710
So as we studied
on, as we solved

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00:04:58,710 --> 00:05:01,800
in the exam, those interface
states or those surface states,

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00:05:01,800 --> 00:05:03,450
the surface of
semiconductor, those

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00:05:03,450 --> 00:05:05,480
can be reduced or
minimized by the presence

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00:05:05,480 --> 00:05:07,600
of certain passivating
layers, and it just so

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00:05:07,600 --> 00:05:11,610
happens that by the
benevolence of nature,

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00:05:11,610 --> 00:05:15,310
the silicon oxide, which is
shown in these red triangles

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00:05:15,310 --> 00:05:19,980
right here, has a very
low surface recombination

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00:05:19,980 --> 00:05:22,560
velocity, passivates
a surface very well,

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00:05:22,560 --> 00:05:24,880
and results in
high-performing devices.

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00:05:24,880 --> 00:05:27,570
In this particular case, they're
plotting emitter saturation

122
00:05:27,570 --> 00:05:30,150
current density in
femtoamps per centimeter

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00:05:30,150 --> 00:05:33,290
squared-- this is very, very
low-- versus sheet resistance.

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00:05:33,290 --> 00:05:36,030
This is essentially the dopant
concentration in the emitter,

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00:05:36,030 --> 00:05:38,830
so they're looking at how the
passivation quality changes

126
00:05:38,830 --> 00:05:41,560
as a function of dopant
density and silicon oxide

127
00:05:41,560 --> 00:05:44,340
works pretty well, and it's an
effective diffusion barrier.

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00:05:44,340 --> 00:05:46,020
And, probably most
significantly, those

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00:05:46,020 --> 00:05:49,220
are maybe one looking
forward rationale

130
00:05:49,220 --> 00:05:52,940
one technological or
scientific rationale

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00:05:52,940 --> 00:05:54,890
and as far as the
field is concerned,

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as far as engineering
community is concerned,

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00:05:57,320 --> 00:05:59,040
silicon has a lot of momentum.

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It's the most common
semiconductor material, silicon

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00:06:02,290 --> 00:06:05,280
and germanium were both
purified, more or less,

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00:06:05,280 --> 00:06:08,930
around the same decades but,
because silicon has a wider

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00:06:08,930 --> 00:06:12,210
band gap, you have a lower
thermal carrier concentration,

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00:06:12,210 --> 00:06:13,800
lower intrinsic
carrier concentration,

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00:06:13,800 --> 00:06:15,920
folks were able to make
transistors and devices

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00:06:15,920 --> 00:06:19,130
with lower noise out of
silicon as opposed to germanium

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00:06:19,130 --> 00:06:22,230
and silicon technology
really took off

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00:06:22,230 --> 00:06:25,330
in terms of the PV
industry benefited a lot

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00:06:25,330 --> 00:06:26,551
by that cross-pollination.

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00:06:26,551 --> 00:06:28,800
Many technologies came in
from the integrated circuits

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00:06:28,800 --> 00:06:32,336
industry to assist or give
a boost to the PV industry.

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00:06:32,336 --> 00:06:33,710
This number is a
little outdated,

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00:06:33,710 --> 00:06:34,876
it's now about $100 billion.

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00:06:34,876 --> 00:06:39,840
Hard to keep up with things
growing at 68% a year.

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00:06:39,840 --> 00:06:44,320
Technology acceptance results
in lower interest rates.

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So if you have a
technology that is

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00:06:47,080 --> 00:06:50,160
well-accepted by the market
then you go to a bank and say,

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00:06:50,160 --> 00:06:51,960
hey, I want to install
some of those things

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00:06:51,960 --> 00:06:53,751
and the bank says what
are those things you

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00:06:53,751 --> 00:06:55,330
say oh, hundreds of
thousands of them

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00:06:55,330 --> 00:06:56,180
have been installed already.

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It's OK.

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It's a proven technology.

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The bank says OK, I'll
lower your interest rates.

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00:06:59,651 --> 00:07:01,790
That means you pay
less money on interest.

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00:07:01,790 --> 00:07:04,030
Your capital is more cheap.

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00:07:04,030 --> 00:07:07,490
It works better in your
favor, and the opposite

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00:07:07,490 --> 00:07:10,170
is true with an entirely new
technology that's unproven.

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00:07:10,170 --> 00:07:13,170
So that's really
summing up why silicon.

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00:07:13,170 --> 00:07:15,490
Momentum, forward
motion if you will,

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00:07:15,490 --> 00:07:18,500
some inherent intrinsic
technological advantages,

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some of which are
listed here, and I'll

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00:07:21,030 --> 00:07:23,067
get to that in a
second, scalability.

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00:07:23,067 --> 00:07:24,900
To get back to the
technological advantages,

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00:07:24,900 --> 00:07:27,290
I think it's important to
recognize what they are so

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that when you're thinking
of a new material,

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00:07:30,350 --> 00:07:32,492
you can cross check and
say, gee, do I have these

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00:07:32,492 --> 00:07:33,450
or do I not have these.

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If I don't have them, it's
not the end of the world.

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00:07:35,040 --> 00:07:37,530
You might have other advantages
that overcome the ones

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00:07:37,530 --> 00:07:40,740
that silicon doesn't have.

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00:07:40,740 --> 00:07:43,360
Let's add some more
into this list.

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00:07:43,360 --> 00:07:45,940
Just stream of consciousness.

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00:07:45,940 --> 00:07:48,390
Silicon has a very
high refractive index

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00:07:48,390 --> 00:07:50,030
near the band gap edge.

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00:07:50,030 --> 00:07:52,330
So, near the band gap
edge, it's absorbing light

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00:07:52,330 --> 00:07:53,170
less efficiently.

182
00:07:53,170 --> 00:07:53,670
Right?

183
00:07:53,670 --> 00:07:56,660
It has a larger
attenuation length

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00:07:56,660 --> 00:07:59,350
of the light, a smaller optical
absorption coefficient right

185
00:07:59,350 --> 00:08:01,180
as you approach the band gap.

186
00:08:01,180 --> 00:08:04,200
So silicon absorbs poorly
in the infrared because it's

187
00:08:04,200 --> 00:08:05,840
an indirect band
gap semiconductor,

188
00:08:05,840 --> 00:08:08,730
but it also has a
very large optical,

189
00:08:08,730 --> 00:08:10,720
sorry, a very large
real component

190
00:08:10,720 --> 00:08:12,355
of the refractive index.

191
00:08:12,355 --> 00:08:15,020
Does anybody remember
what that refers to?

192
00:08:15,020 --> 00:08:17,360
Real component of
refractive index.

193
00:08:17,360 --> 00:08:18,540
Lesson number two.

194
00:08:18,540 --> 00:08:20,185
What does that dictate?

195
00:08:20,185 --> 00:08:21,060
AUDIENCE: Reflection.

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00:08:21,060 --> 00:08:22,390
PROFESSOR: Reflection, exactly.

197
00:08:22,390 --> 00:08:25,691
So, if I were to tailor and
index of refraction grading

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00:08:25,691 --> 00:08:27,190
on the front side
of my device, so I

199
00:08:27,190 --> 00:08:30,470
allow the light to be absorbed
efficiently, on the backside

200
00:08:30,470 --> 00:08:33,230
I can put a very large
index of refraction mismatch

201
00:08:33,230 --> 00:08:35,020
so that the light bounces back.

202
00:08:35,020 --> 00:08:36,929
In other words, the
light trapping silicon

203
00:08:36,929 --> 00:08:38,320
is benefited by
the fact that you

204
00:08:38,320 --> 00:08:41,289
have this awesome
reflection capability.

205
00:08:41,289 --> 00:08:43,319
The refractive
index is around 3.6,

206
00:08:43,319 --> 00:08:45,110
the real component of
the refractive index,

207
00:08:45,110 --> 00:08:47,600
in the infrared at
around 1070 nanometers.

208
00:08:47,600 --> 00:08:49,590
Which means that if you
design your cell right,

209
00:08:49,590 --> 00:08:51,714
you can get an extension
of the optical path length

210
00:08:51,714 --> 00:08:54,700
by a factor of 50
over the thickness.

211
00:08:54,700 --> 00:08:56,414
So if your thickness
of the device is d,

212
00:08:56,414 --> 00:08:58,330
the optical path length
can be increased up to

213
00:08:58,330 --> 00:09:01,700
about [? 51d. ?]
That's as a result

214
00:09:01,700 --> 00:09:03,880
of this great reflectance.

215
00:09:03,880 --> 00:09:07,090
Many other materials that are
being explored as PV materials

216
00:09:07,090 --> 00:09:08,760
have refractive
indices around two,

217
00:09:08,760 --> 00:09:10,676
which would mean that
your optical path length

218
00:09:10,676 --> 00:09:12,080
extension is around 16.

219
00:09:12,080 --> 00:09:13,580
So that's one thing
to keep in mind.

220
00:09:13,580 --> 00:09:15,663
even though it doesn't
absorb light quite as well,

221
00:09:15,663 --> 00:09:16,870
it traps light fairly well.

222
00:09:16,870 --> 00:09:19,310
Another advantage of
silicon is that it

223
00:09:19,310 --> 00:09:23,240
forms sp3 hybridized
orbitals, for chemists, it

224
00:09:23,240 --> 00:09:25,207
forms-- it's
tetrahedrally coordinated,

225
00:09:25,207 --> 00:09:27,040
in other words bond to
four other neighbors,

226
00:09:27,040 --> 00:09:30,000
and most 3D transition
metals don't do that.

227
00:09:30,000 --> 00:09:32,020
They don't bond in
that configuration.

228
00:09:32,020 --> 00:09:35,890
Some do but many don't
and, as a result,

229
00:09:35,890 --> 00:09:39,210
the solid's solubility in
other words, the ability

230
00:09:39,210 --> 00:09:42,280
to incorporate impurities into a
growing silicon crystal is low.

231
00:09:42,280 --> 00:09:45,790
It rejects the impurities
from the solid into the melt,

232
00:09:45,790 --> 00:09:48,552
and you're able to purify the
material very efficiently.

233
00:09:48,552 --> 00:09:50,510
That's not always the
case with most materials.

234
00:09:50,510 --> 00:09:52,670
Sometimes they incorporate
impurities very readily,

235
00:09:52,670 --> 00:09:54,296
up to a few atomic percents.

236
00:09:54,296 --> 00:09:56,170
The typical impurity
concentration of silicon

237
00:09:56,170 --> 00:09:58,870
is in the order of parts per
million, parts per billion,

238
00:09:58,870 --> 00:09:59,820
parts per trillion.

239
00:09:59,820 --> 00:10:01,660
Still can be enough, as you
learned during your homework

240
00:10:01,660 --> 00:10:03,785
assignments, still could
be enough to affect device

241
00:10:03,785 --> 00:10:06,220
performance but is very low.

242
00:10:06,220 --> 00:10:08,050
It would be a lot
worse if silicon

243
00:10:08,050 --> 00:10:10,710
were able to absorb more
impurities and so forth.

244
00:10:10,710 --> 00:10:14,120
So, there are a number of
reasons why the silicon PV

245
00:10:14,120 --> 00:10:18,150
technology has gained the
foothold that it has so

246
00:10:18,150 --> 00:10:21,000
to bump it out of its
leadership position,

247
00:10:21,000 --> 00:10:24,290
one really has to be clever
and the parameter of merit

248
00:10:24,290 --> 00:10:27,360
is performance per unit cost.

249
00:10:27,360 --> 00:10:31,340
Kilowatt hours per
dollar, if you will.

250
00:10:31,340 --> 00:10:33,940
So, we're going to talk about
the current manufacturing

251
00:10:33,940 --> 00:10:36,290
methods and materials
because this will give you

252
00:10:36,290 --> 00:10:39,240
an insight into the
dollars per kilowatt

253
00:10:39,240 --> 00:10:40,740
hour, the kilowatt
hours per dollar.

254
00:10:40,740 --> 00:10:44,110
Essentially, the cost
per unit energy produced.

255
00:10:44,110 --> 00:10:45,790
You can begin to
seize opportunities

256
00:10:45,790 --> 00:10:48,331
within the crystal silicon world
to improve the manufacturing

257
00:10:48,331 --> 00:10:50,560
process or you can begin
to say OK, you know what,

258
00:10:50,560 --> 00:10:51,982
this is way too complicated.

259
00:10:51,982 --> 00:10:53,690
Let me take a completely
different route.

260
00:10:53,690 --> 00:10:55,270
I'm going to develop a
new technology instead

261
00:10:55,270 --> 00:10:57,436
that will overcome these
manufacturing difficulties.

262
00:10:57,436 --> 00:10:59,090
So let's explore them in detail.

263
00:10:59,090 --> 00:11:00,950
First, the market.

264
00:11:00,950 --> 00:11:06,300
This is the evolution of market
share from 1980 to mid 2000s.

265
00:11:06,300 --> 00:11:10,620
After mid 2000s, the market just
continues growing at 68% a year

266
00:11:10,620 --> 00:11:12,570
and you really lose
resolution to this portion

267
00:11:12,570 --> 00:11:16,660
down here so it's to 2006 so
that we can actually see what's

268
00:11:16,660 --> 00:11:19,150
going on in the earlier days.

269
00:11:19,150 --> 00:11:22,110
In the earlier days,
1980, let's pick 1985,

270
00:11:22,110 --> 00:11:24,910
the market was split about
a third-third-third between

271
00:11:24,910 --> 00:11:27,730
thin films, amorphous
silicon namely,

272
00:11:27,730 --> 00:11:29,540
monocrystalline
silicon, and a material

273
00:11:29,540 --> 00:11:31,280
called multicrystalline silicon.

274
00:11:31,280 --> 00:11:32,630
Now let's go piece by piece.

275
00:11:32,630 --> 00:11:36,320
What is monocrystalline silicon,
multicrystalline silicon

276
00:11:36,320 --> 00:11:37,270
and thin films?

277
00:11:37,270 --> 00:11:40,230
Well thin films are
materials that are usually

278
00:11:40,230 --> 00:11:43,530
between a few hundred
nanometers up to about three,

279
00:11:43,530 --> 00:11:45,410
maybe five microns thick.

280
00:11:45,410 --> 00:11:47,070
To give you size
perspective, your hair

281
00:11:47,070 --> 00:11:48,850
is about 50 microns
in diameter, so we're

282
00:11:48,850 --> 00:11:51,480
talking about 1/50 the
width of your hair.

283
00:11:51,480 --> 00:11:53,590
That's the active absorber
layer and of course

284
00:11:53,590 --> 00:11:55,470
the plastics and encapsulates
and everything else that

285
00:11:55,470 --> 00:11:57,150
go around them make
it a bit thicker,

286
00:11:57,150 --> 00:11:59,130
but the absorber
layer is very thin

287
00:11:59,130 --> 00:12:02,434
and so you're not spending
much on your absorber layer.

288
00:12:02,434 --> 00:12:03,850
It absorbs light
very efficiently,

289
00:12:03,850 --> 00:12:07,240
has a very large
absorption coefficient,

290
00:12:07,240 --> 00:12:09,490
and is able to absorb
photons efficiently.

291
00:12:09,490 --> 00:12:11,751
Crystalline silicon,
on the other hand,

292
00:12:11,751 --> 00:12:14,125
does not absorb light as well
as many thin film materials

293
00:12:14,125 --> 00:12:15,930
so we need about an
order of magnitude

294
00:12:15,930 --> 00:12:19,380
to two orders of magnitude
thicker substrates,

295
00:12:19,380 --> 00:12:22,212
and the crystalline
silicon substrates today

296
00:12:22,212 --> 00:12:23,920
in commercial
manufacturing are typically

297
00:12:23,920 --> 00:12:30,150
between 160 to 190 microns,
with an average around 170, 180.

298
00:12:30,150 --> 00:12:32,870
So about four times the
thickness of your hair.

299
00:12:32,870 --> 00:12:34,887
Monocrystalline
silicon and multi.

300
00:12:34,887 --> 00:12:36,470
Let's talk about the
difference there.

301
00:12:36,470 --> 00:12:38,670
So, monocrystalline
silicon, folks

302
00:12:38,670 --> 00:12:41,390
are probably familiar seeing
pictures, at least something

303
00:12:41,390 --> 00:12:41,890
like this.

304
00:12:41,890 --> 00:12:42,730
Right?

305
00:12:42,730 --> 00:12:46,900
So this right here is an example
of a Cherkofsky silicon wafer.

306
00:12:49,690 --> 00:12:51,980
Appropriate for
integrated circuit work.

307
00:12:51,980 --> 00:12:54,780
I'll pass this around so
folks can get a sense.

308
00:12:54,780 --> 00:12:59,400
So this is an example of
a monocrystalline silicon

309
00:12:59,400 --> 00:13:01,730
wafer for the integrated
circuits industry.

310
00:13:01,730 --> 00:13:04,020
Let's analyze it in a
little bit more detail.

311
00:13:04,020 --> 00:13:07,757
So, the front surface is
polished, nicely polished.

312
00:13:07,757 --> 00:13:09,590
Polished to, I think,
somewhere in the order

313
00:13:09,590 --> 00:13:12,280
of a few nanometers
mean surface roughness.

314
00:13:12,280 --> 00:13:14,800
Using a chemical mechanical
polishing mechanism.

315
00:13:14,800 --> 00:13:18,840
The thickness is around
700-- or 675 microns.

316
00:13:18,840 --> 00:13:20,120
Somewhere in that range.

317
00:13:20,120 --> 00:13:21,710
So very, very thick wafer.

318
00:13:21,710 --> 00:13:23,690
The objective is not to break.

319
00:13:23,690 --> 00:13:24,260
Right?

320
00:13:24,260 --> 00:13:25,830
If you're making
integrated circuit,

321
00:13:25,830 --> 00:13:27,450
this entire wafer that
I'm holding right here

322
00:13:27,450 --> 00:13:29,580
could be worth a few 10s or
100s thousands of dollars

323
00:13:29,580 --> 00:13:31,220
by the end of the
processing sequence,

324
00:13:31,220 --> 00:13:33,594
so if one of these breaks,
that's an awful lot of revenue

325
00:13:33,594 --> 00:13:34,680
that the company's losing.

326
00:13:34,680 --> 00:13:36,812
So the substrate is
thick because they

327
00:13:36,812 --> 00:13:37,770
don't want it to break.

328
00:13:37,770 --> 00:13:40,450
Silicon is brittle
at room temperature.

329
00:13:40,450 --> 00:13:42,660
If you were to manufacture
solar cell out of this,

330
00:13:42,660 --> 00:13:44,210
you could but it would
be very expensive.

331
00:13:44,210 --> 00:13:45,584
The chemical
mechanical polishing

332
00:13:45,584 --> 00:13:47,260
that they use to
flatten the surface out

333
00:13:47,260 --> 00:13:49,720
costs a lot of money,
it's very time intensive,

334
00:13:49,720 --> 00:13:52,250
and the thickness of the
silicon is above and beyond

335
00:13:52,250 --> 00:13:54,290
what is necessary to
absorb light well.

336
00:13:54,290 --> 00:13:55,990
If anything, increasing
the thickness

337
00:13:55,990 --> 00:13:58,215
is just increasing your
emitter saturation current,

338
00:13:58,215 --> 00:14:00,340
since you have a higher
recombination current being

339
00:14:00,340 --> 00:14:01,822
driven by bulk recombination.

340
00:14:01,822 --> 00:14:03,280
You have more
recombination centers

341
00:14:03,280 --> 00:14:04,821
because you have a
greater thickness,

342
00:14:04,821 --> 00:14:06,640
and it's driving a
larger diffusion current

343
00:14:06,640 --> 00:14:08,740
from the emitter into the base.

344
00:14:08,740 --> 00:14:11,680
So making it this thick
really doesn't make sense.

345
00:14:11,680 --> 00:14:15,310
So I'll pass this around so
folks can kind of get a sense.

346
00:14:15,310 --> 00:14:17,390
Make sure this gets
the entire round.

347
00:14:17,390 --> 00:14:20,397
I'll be recycling those platens.

348
00:14:20,397 --> 00:14:22,230
Please hold, if you're
going to take it out,

349
00:14:22,230 --> 00:14:24,220
which you're welcome
to do, please hold it

350
00:14:24,220 --> 00:14:26,900
like a photograph.

351
00:14:26,900 --> 00:14:31,720
What I don't want to have happen
is folks put their fingerprints

352
00:14:31,720 --> 00:14:33,430
all over it.

353
00:14:33,430 --> 00:14:38,960
The wafers that are
used in the PV industry

354
00:14:38,960 --> 00:14:40,850
are cut from the
same ingot like that

355
00:14:40,850 --> 00:14:44,320
one, except that the
ingots, essentially,

356
00:14:44,320 --> 00:14:47,770
if you were to pack circular
wafers into a module,

357
00:14:47,770 --> 00:14:49,890
it would look
something like this.

358
00:14:49,890 --> 00:14:52,259
Here's your module
and, mind you,

359
00:14:52,259 --> 00:14:54,050
you're spending a lot
of money on the glass

360
00:14:54,050 --> 00:14:56,760
and the encapsulates
and the aluminum framing

361
00:14:56,760 --> 00:15:01,469
and so forth, and now your
solar cells look like that.

362
00:15:01,469 --> 00:15:03,760
There's probably more of them
that you can put in here,

363
00:15:03,760 --> 00:15:05,760
but what do you
notice about this?

364
00:15:05,760 --> 00:15:07,810
What is the packing density,
or packing fraction.

365
00:15:07,810 --> 00:15:08,830
It's very low, right?

366
00:15:08,830 --> 00:15:10,746
You're losing all of
this material in between.

367
00:15:10,746 --> 00:15:13,370
All that space is just
going to be blank space.

368
00:15:13,370 --> 00:15:16,690
Some of the earliest PV modules
actually use circular wafers,

369
00:15:16,690 --> 00:15:18,390
but the more modern
ones, what they do

370
00:15:18,390 --> 00:15:20,770
is a very complicated
cost analysis

371
00:15:20,770 --> 00:15:29,550
where they say, OK, if I were to
chop off the edges of my wafer

372
00:15:29,550 --> 00:15:32,330
and completely remove them,
I'd be losing a lot of silicon

373
00:15:32,330 --> 00:15:34,970
but I'd be increasing
the packing fraction.

374
00:15:34,970 --> 00:15:37,660
So in the limit that my
module materials, the glass,

375
00:15:37,660 --> 00:15:40,080
the encapsulant, the
framing materials

376
00:15:40,080 --> 00:15:43,070
are infinitely expensive and
my silicon costs nothing,

377
00:15:43,070 --> 00:15:44,740
I want to do this.

378
00:15:44,740 --> 00:15:47,300
In the limit that my
module materials are free

379
00:15:47,300 --> 00:15:51,010
and installation is free but
the silicon is super expensive,

380
00:15:51,010 --> 00:15:52,980
I want to keep
full round wafers,

381
00:15:52,980 --> 00:15:55,210
and the reality is that
we're somewhere in between.

382
00:15:55,210 --> 00:15:57,760
And so, instead of making
one or the other extreme,

383
00:15:57,760 --> 00:15:59,940
typically what you'll
see is something

384
00:15:59,940 --> 00:16:02,750
like this chopped
off, like that,

385
00:16:02,750 --> 00:16:06,480
where you have a pseudo-square.

386
00:16:06,480 --> 00:16:09,570
The wafer itself has
flat edges on the sides

387
00:16:09,570 --> 00:16:13,000
but it also has kind of
pseudo-rounded corners here,

388
00:16:13,000 --> 00:16:15,950
and Joe did we bring
any of those in?

389
00:16:15,950 --> 00:16:18,810
The psuedo-squares, the
monocrystalline psuedo-squares.

390
00:16:18,810 --> 00:16:20,190
These ones.

391
00:16:20,190 --> 00:16:20,690
OK.

392
00:16:20,690 --> 00:16:21,189
All right.

393
00:16:21,189 --> 00:16:21,776
No worries.

394
00:16:21,776 --> 00:16:23,150
I'll show them to
you next class.

395
00:16:23,150 --> 00:16:25,390
So, the idea is to
make-- cut it out

396
00:16:25,390 --> 00:16:27,200
of the same ingot
as that one right

397
00:16:27,200 --> 00:16:31,550
there, but make it thinner, on
the order of 170 microns thick,

398
00:16:31,550 --> 00:16:36,620
and to chop off part of the
edge, and how much you chop off

399
00:16:36,620 --> 00:16:40,060
depends on the dynamic pricing
of silicon versus module

400
00:16:40,060 --> 00:16:42,660
materials and installation
and whether or not

401
00:16:42,660 --> 00:16:44,230
you can sell the
module, if there's

402
00:16:44,230 --> 00:16:45,700
a certain threshold
of performance

403
00:16:45,700 --> 00:16:47,908
that it needs to reach
because obviously, if you have

404
00:16:47,908 --> 00:16:50,980
a bunch of dead space in here,
you're losing that to-- you're

405
00:16:50,980 --> 00:16:52,430
not producing power out of that.

406
00:16:52,430 --> 00:16:55,230
So if somebody wants a
module that's yay efficient,

407
00:16:55,230 --> 00:16:57,440
you might want to increase
the packing density.

408
00:16:57,440 --> 00:16:59,490
So that's
monocrystalline silicon.

409
00:16:59,490 --> 00:17:01,590
Multicrystalline silicon.

410
00:17:01,590 --> 00:17:03,110
Let's put it this way for now.

411
00:17:03,110 --> 00:17:05,026
We'll describe how
multicrystalline silicon is

412
00:17:05,026 --> 00:17:06,569
made, but for now
I'm going to say

413
00:17:06,569 --> 00:17:11,180
that multicrystalline silicon
is a crystalline silicon

414
00:17:11,180 --> 00:17:14,099
variety that is comprised
of many small grains.

415
00:17:14,099 --> 00:17:16,950
So if you look at a
multicrystalline silicon wafer,

416
00:17:16,950 --> 00:17:19,250
something like,
let's say, oh this

417
00:17:19,250 --> 00:17:21,430
is a perfect example
right in here.

418
00:17:21,430 --> 00:17:23,720
If you look at a
multicrystalline silicon wafer,

419
00:17:23,720 --> 00:17:30,050
you can see that it looks
nice and-- here maybe, that's

420
00:17:30,050 --> 00:17:31,670
probably an OK view of it.

421
00:17:31,670 --> 00:17:32,920
You can see individual grains.

422
00:17:32,920 --> 00:17:33,510
Right?

423
00:17:33,510 --> 00:17:35,600
If you look closely at it.

424
00:17:35,600 --> 00:17:38,520
And those are grains
of crystalline material

425
00:17:38,520 --> 00:17:40,387
that are joined by
grain boundaries.

426
00:17:40,387 --> 00:17:41,970
So the grain orientation
in one region

427
00:17:41,970 --> 00:17:43,780
might be pointing in
this direction, the grain

428
00:17:43,780 --> 00:17:45,240
orientation in the
neighboring region like that,

429
00:17:45,240 --> 00:17:47,220
and they come together
at a grain boundary

430
00:17:47,220 --> 00:17:50,220
and, when we have
polycrystalline materials

431
00:17:50,220 --> 00:17:55,310
like this, it's generally
indicative of some faster

432
00:17:55,310 --> 00:17:58,550
growth that didn't allow
for a nice homogeneous

433
00:17:58,550 --> 00:18:00,074
single crystal
material to evolve,

434
00:18:00,074 --> 00:18:02,490
and that's indeed what happens
during the multicrystalline

435
00:18:02,490 --> 00:18:03,680
silicon ingot growth.

436
00:18:03,680 --> 00:18:08,140
It's occurring under a slightly
modified growth condition then,

437
00:18:08,140 --> 00:18:11,140
say, that beautiful single
crystalline piece over there,

438
00:18:11,140 --> 00:18:14,160
and we'll explain how
they're made in a second.

439
00:18:14,160 --> 00:18:16,930
So those are the technologies
in general, the base absorber

440
00:18:16,930 --> 00:18:19,200
materials, and then there's
ribbon silicon which

441
00:18:19,200 --> 00:18:21,180
is a really, really
small fraction

442
00:18:21,180 --> 00:18:23,940
of the total production in
decreasing, but at one time,

443
00:18:23,940 --> 00:18:27,019
ribbon silicon was viewed as
the up and coming technology.

444
00:18:27,019 --> 00:18:29,060
Still today, there are
about 20 startup companies

445
00:18:29,060 --> 00:18:31,450
around the United States
working on some aspect of this

446
00:18:31,450 --> 00:18:34,569
and probably about a dozen
more around the world.

447
00:18:34,569 --> 00:18:35,069
Yeah.

448
00:18:35,069 --> 00:18:36,488
AUDIENCE: I had a
question about the multi.

449
00:18:36,488 --> 00:18:37,154
PROFESSOR: Yeah.

450
00:18:37,154 --> 00:18:39,892
AUDIENCE: So for
the multi and micro

451
00:18:39,892 --> 00:18:43,110
and poly, is that
different grain sizes?

452
00:18:43,110 --> 00:18:43,940
PROFESSOR: Sort of.

453
00:18:43,940 --> 00:18:47,140
So, multicrystalline silicon
is a polycrystalline silicon

454
00:18:47,140 --> 00:18:48,000
material.

455
00:18:48,000 --> 00:18:49,750
The definition of
multicrystalline silicon

456
00:18:49,750 --> 00:18:53,280
is that the average grain size
is about a centimeter squared,

457
00:18:53,280 --> 00:18:56,670
or larger, and that's where
multicrystalline came about.

458
00:18:56,670 --> 00:18:59,630
Polycrystalline silicon,
in the silicon community,

459
00:18:59,630 --> 00:19:01,290
has a very specific meaning.

460
00:19:01,290 --> 00:19:05,430
It means, usually a
plasma-enhanced chemical vapor

461
00:19:05,430 --> 00:19:08,670
deposited layer, so
PCVD-deposited layer

462
00:19:08,670 --> 00:19:12,520
of silicon, that has on the
order of one to five micron

463
00:19:12,520 --> 00:19:13,500
diameter grains.

464
00:19:13,500 --> 00:19:15,530
So very, very small
grain material.

465
00:19:15,530 --> 00:19:17,830
About 1/50 the
width of your hair.

466
00:19:17,830 --> 00:19:19,450
Maybe 1/10 the
width of your hair

467
00:19:19,450 --> 00:19:21,575
and, to distinguish it from
that really small grain

468
00:19:21,575 --> 00:19:23,570
material that will
perform very poorly,

469
00:19:23,570 --> 00:19:27,140
one calls this
multicrystalline silicon.

470
00:19:27,140 --> 00:19:29,309
AUDIENCE: And is there
microcrystalline silicon?

471
00:19:29,309 --> 00:19:31,350
PROFESSOR: There is also
microcrystalline silicon

472
00:19:31,350 --> 00:19:34,270
and microcrystalline
silicon is actually

473
00:19:34,270 --> 00:19:37,670
at the phase transition between
amorphous and polycrystalline

474
00:19:37,670 --> 00:19:38,530
silicon.

475
00:19:38,530 --> 00:19:40,880
So as you're going from
an amorphous material

476
00:19:40,880 --> 00:19:43,320
increasing the temperature,
let's say, of growth

477
00:19:43,320 --> 00:19:47,250
or increasing other parameters
during the deposition process,

478
00:19:47,250 --> 00:19:49,870
as you begin to evolve
from an amorphous material

479
00:19:49,870 --> 00:19:51,740
into a crystalline
material, you transition

480
00:19:51,740 --> 00:19:53,400
through this
microcrystalline regime

481
00:19:53,400 --> 00:19:54,937
which is a bit of a hybrid.

482
00:19:54,937 --> 00:19:56,520
It has some regions
that are amorphous

483
00:19:56,520 --> 00:19:58,720
and other regions
that are crystalline.

484
00:19:58,720 --> 00:20:02,190
In your assigned readings,
this book was assigned,

485
00:20:02,190 --> 00:20:05,820
and I believe in the syllabus
it says read chapter X.

486
00:20:05,820 --> 00:20:07,636
Unfortunately, there
is no chapter X.

487
00:20:07,636 --> 00:20:09,700
I guess you could
interpret it as 10,

488
00:20:09,700 --> 00:20:12,560
but the essence was that there
are two versions of the book.

489
00:20:12,560 --> 00:20:14,630
One is version three,
which was published

490
00:20:14,630 --> 00:20:16,840
about seven years ago, and
the newest version just

491
00:20:16,840 --> 00:20:18,330
came out last year.

492
00:20:18,330 --> 00:20:20,630
The newest addition
is addition three.

493
00:20:20,630 --> 00:20:24,450
So the chapters have rearranged
slightly, but what I'll do

494
00:20:24,450 --> 00:20:26,830
is I'll highlight crystalline
silicon solar cells

495
00:20:26,830 --> 00:20:28,660
and modules in here
so that you can

496
00:20:28,660 --> 00:20:31,430
get a sense of what
is in the chapter

497
00:20:31,430 --> 00:20:34,150
and you're welcome to
go back and have a look.

498
00:20:34,150 --> 00:20:36,840
So I'll go ahead and highlight
this chapter right here

499
00:20:36,840 --> 00:20:37,785
and pass it around.

500
00:20:37,785 --> 00:20:39,660
Feel free to glance
through the book as well.

501
00:20:39,660 --> 00:20:40,440
It's a great read.

502
00:20:40,440 --> 00:20:41,940
It dives into great
detail into each

503
00:20:41,940 --> 00:20:44,680
of the different technologies.

504
00:20:44,680 --> 00:20:45,180
OK.

505
00:20:45,180 --> 00:20:47,060
So, let's talk about
feedstock refining.

506
00:20:47,060 --> 00:20:49,390
We're going to start
the silicon value

507
00:20:49,390 --> 00:20:51,610
chain from the raw
materials and work our way

508
00:20:51,610 --> 00:20:55,050
all the way to the
final module at the end.

509
00:20:55,050 --> 00:20:57,620
So we'll start with the
feedstocks themselves.

510
00:20:57,620 --> 00:21:00,420
Down here is a rough
cost breakdown.

511
00:21:00,420 --> 00:21:03,170
Kind of think of
it as wafer, cell,

512
00:21:03,170 --> 00:21:05,060
module being like
a third-third-third

513
00:21:05,060 --> 00:21:07,410
of the total module cost
and then balance the system

514
00:21:07,410 --> 00:21:09,340
components beyond that.

515
00:21:09,340 --> 00:21:11,190
So we'll start
from our feedstocks

516
00:21:11,190 --> 00:21:12,690
and the raw materials
in the ground,

517
00:21:12,690 --> 00:21:14,340
we'll wind up with
systems on the roof,

518
00:21:14,340 --> 00:21:15,900
and we'll walk through
each of the different steps

519
00:21:15,900 --> 00:21:17,760
of current
manufacturing process.

520
00:21:17,760 --> 00:21:19,540
So raw materials.

521
00:21:19,540 --> 00:21:24,050
Shown here is quartz and
coal, for a very good reason.

522
00:21:24,050 --> 00:21:28,280
The way feedstock refining
occurs at the very first stage

523
00:21:28,280 --> 00:21:32,690
is to take oxidized silicon,
silicon dioxide, quartz

524
00:21:32,690 --> 00:21:37,400
and to reduce it to
silicon, say, silicon zero.

525
00:21:37,400 --> 00:21:41,440
Unoxidized silicon, which is
also called silicon metal.

526
00:21:41,440 --> 00:21:43,797
It's called a metal because
it is very low resistivity.

527
00:21:43,797 --> 00:21:46,005
It's very low resistivity
because there's a very high

528
00:21:46,005 --> 00:21:47,210
impurity content still.

529
00:21:47,210 --> 00:21:49,920
The purity of this material
coming out here is around 99,

530
00:21:49,920 --> 00:21:51,984
99.9% here.

531
00:21:51,984 --> 00:21:53,900
So, it sounds like a
high purity but, if we're

532
00:21:53,900 --> 00:21:56,010
talking about parts per
million of impurities,

533
00:21:56,010 --> 00:21:58,520
we have some further refining
steps to do after this.

534
00:21:58,520 --> 00:21:59,880
So let's walk through this.

535
00:21:59,880 --> 00:22:02,740
We start with the raw
materials in the upper left.

536
00:22:02,740 --> 00:22:04,360
It says raw material inputs.

537
00:22:04,360 --> 00:22:06,350
Carbon and SiO2.

538
00:22:06,350 --> 00:22:08,740
The SiO2 forms, usually, quartz.

539
00:22:08,740 --> 00:22:12,990
That can be some of high
purity pegmatite, it could be,

540
00:22:12,990 --> 00:22:16,277
for example, a hydrothermal
quartz, higher purity

541
00:22:16,277 --> 00:22:17,110
varieties of quartz.

542
00:22:17,110 --> 00:22:21,850
You could even use, maybe, a
metamorphic quartzite material.

543
00:22:21,850 --> 00:22:22,980
Let me explain.

544
00:22:22,980 --> 00:22:25,550
So, some of the highest
purity materials

545
00:22:25,550 --> 00:22:29,180
are coming from these veins
of magma that float up

546
00:22:29,180 --> 00:22:33,430
and then phase separated
during millennia.

547
00:22:33,430 --> 00:22:35,290
Some of the lowest
purity quartz is

548
00:22:35,290 --> 00:22:38,310
coming from sand,
essentially crushed rock that

549
00:22:38,310 --> 00:22:40,630
made its way into, say,
a beach-like environment

550
00:22:40,630 --> 00:22:43,010
and then rock was
deposited on top of that,

551
00:22:43,010 --> 00:22:46,510
pressure was increased,
and this whole mixture

552
00:22:46,510 --> 00:22:50,710
of mica, feldspar,
and of quartz got

553
00:22:50,710 --> 00:22:53,280
pushed together and
formed a solid block.

554
00:22:53,280 --> 00:22:57,084
That would be your
metamorphic quartz materials,

555
00:22:57,084 --> 00:22:58,750
and so you'd have a
much higher impurity

556
00:22:58,750 --> 00:23:00,750
content in the metamorphic
quartz than you would

557
00:23:00,750 --> 00:23:04,760
in, say, a high purity pegmatite
or hydrothermal quartz.

558
00:23:04,760 --> 00:23:07,280
Regardless, depending on the
feedstock source of the quartz,

559
00:23:07,280 --> 00:23:08,780
and there are people
who study this.

560
00:23:08,780 --> 00:23:10,738
Believe it or not, there
are entire departments

561
00:23:10,738 --> 00:23:13,111
dedicated to mining
quartz and figuring out

562
00:23:13,111 --> 00:23:15,360
where the different veins
of the highest purity quartz

563
00:23:15,360 --> 00:23:18,320
are, where you get them from.

564
00:23:18,320 --> 00:23:21,020
That's the SiO2 input
and the C input over here

565
00:23:21,020 --> 00:23:23,350
on the left hand side, Carbon.

566
00:23:23,350 --> 00:23:26,130
So, typically what is
used in the PV industry

567
00:23:26,130 --> 00:23:31,350
is either a fast-growing
wood source like eucalyptus

568
00:23:31,350 --> 00:23:34,240
or southern pine, right?

569
00:23:34,240 --> 00:23:36,460
Northern pine tends
to be slower growing,

570
00:23:36,460 --> 00:23:38,220
but eucalyptus and
southern pine both

571
00:23:38,220 --> 00:23:40,050
tend to be fairly fast-growing.

572
00:23:40,050 --> 00:23:43,240
You can tell by the
spacing in the rings,

573
00:23:43,240 --> 00:23:48,540
if you chop the tree down and
do a cross section, or coal.

574
00:23:48,540 --> 00:23:50,380
So carbon, essentially.

575
00:23:50,380 --> 00:23:54,910
And the two react inside
of this furnace right here

576
00:23:54,910 --> 00:23:57,860
and this furnace, just to
give you a sense of scale,

577
00:23:57,860 --> 00:23:59,420
here's a human being.

578
00:23:59,420 --> 00:24:00,510
This is the furnace.

579
00:24:00,510 --> 00:24:03,430
So it's about five stories
tall, 12 meters in diameter.

580
00:24:03,430 --> 00:24:05,180
It's a big, big, big creature.

581
00:24:05,180 --> 00:24:09,390
This furnace right here
is what is producing

582
00:24:09,390 --> 00:24:11,640
the reduced silicon
and what's happening

583
00:24:11,640 --> 00:24:14,770
is these feedstock chunks are
being thrown in at the top

584
00:24:14,770 --> 00:24:19,800
and there's an arc going
between the electrodes, usually

585
00:24:19,800 --> 00:24:23,950
some carbon-bearing
material, and a base contact,

586
00:24:23,950 --> 00:24:26,510
and so that arc creates
a very high temperature.

587
00:24:26,510 --> 00:24:28,610
Something in the order
of up to 2000 degrees

588
00:24:28,610 --> 00:24:30,850
Celsius, near the arc,
and the temperature

589
00:24:30,850 --> 00:24:33,050
decreases as you go
further and further away,

590
00:24:33,050 --> 00:24:34,852
so up near the top
here it might be even

591
00:24:34,852 --> 00:24:36,560
below the melting
temperature of silicon,

592
00:24:36,560 --> 00:24:38,140
somewhere around 1,200 degrees.

593
00:24:38,140 --> 00:24:41,020
So this is an extremely
inhomogeneous, messy system.

594
00:24:41,020 --> 00:24:44,750
This metallurgical grade silicon
refining furnace right here,

595
00:24:44,750 --> 00:24:49,510
this arc furnace, also called a
carbothermic reduction furnace,

596
00:24:49,510 --> 00:24:52,330
a very busy place.

597
00:24:52,330 --> 00:24:52,932
Lots going on.

598
00:24:52,932 --> 00:24:54,390
Extremely inhomogeneous
if you were

599
00:24:54,390 --> 00:24:56,882
to take a cross section
also in terms of temperature

600
00:24:56,882 --> 00:24:58,340
and in terms of
the chemical states

601
00:24:58,340 --> 00:25:00,040
of the different
constituents species,

602
00:25:00,040 --> 00:25:02,280
but the general
reaction that happens

603
00:25:02,280 --> 00:25:03,900
is the carbon would
much rather bond

604
00:25:03,900 --> 00:25:06,384
to the oxygen than silicon,
and so the carbon steals

605
00:25:06,384 --> 00:25:08,800
the oxygen from the silicon
reduces the silicon to silicon

606
00:25:08,800 --> 00:25:11,520
metal and CO2 is released.

607
00:25:11,520 --> 00:25:12,770
We'll get to that in a second.

608
00:25:12,770 --> 00:25:13,400
Flag that.

609
00:25:13,400 --> 00:25:14,525
Put an asterisk next to it.

610
00:25:14,525 --> 00:25:16,490
We'll come back to
that in a second.

611
00:25:16,490 --> 00:25:18,030
Other byproducts
of this reaction,

612
00:25:18,030 --> 00:25:20,220
so this is the liquid
silicon metal coming out here

613
00:25:20,220 --> 00:25:20,803
at the bottom.

614
00:25:20,803 --> 00:25:23,840
It's essentially liquid
molten silicon reduced,

615
00:25:23,840 --> 00:25:27,360
so silicon zero, not a silicon
oxide, reduced silicon metal,

616
00:25:27,360 --> 00:25:30,530
and then finally it's
poured into these buckets,

617
00:25:30,530 --> 00:25:34,660
also called ladles and
solidified, crushed up to size,

618
00:25:34,660 --> 00:25:36,477
and then distributed at the end.

619
00:25:36,477 --> 00:25:38,310
Other byproducts coming
out of this reaction

620
00:25:38,310 --> 00:25:40,761
include-- this is
liquid silicon up hear.

621
00:25:40,761 --> 00:25:42,260
It's very high
temperature and there

622
00:25:42,260 --> 00:25:45,870
are gases and a lot of oxygen
because of the reduction

623
00:25:45,870 --> 00:25:51,730
process, and so silica, or
SiO gas, can be produced

624
00:25:51,730 --> 00:25:55,470
and silica gas can begin
aggravating and forming

625
00:25:55,470 --> 00:25:59,710
very small particles,
almost like shards,

626
00:25:59,710 --> 00:26:03,030
of silicon oxide
material, and these

627
00:26:03,030 --> 00:26:05,910
can be on the order
of one to five microns

628
00:26:05,910 --> 00:26:08,320
and very rough and
jaggedy around the edges.

629
00:26:08,320 --> 00:26:11,020
Now, who here has studied public
health and knows anything about

630
00:26:11,020 --> 00:26:13,967
PM1 or PM1.5 denominations.

631
00:26:13,967 --> 00:26:14,800
Do they ring a bell?

632
00:26:14,800 --> 00:26:15,716
What are those Ashley?

633
00:26:15,716 --> 00:26:20,903
AUDIENCE: It's the
size of particles that

634
00:26:20,903 --> 00:26:22,200
can get stuck in your lungs.

635
00:26:22,200 --> 00:26:22,850
PROFESSOR: Exactly!

636
00:26:22,850 --> 00:26:23,349
Right?

637
00:26:23,349 --> 00:26:26,630
So PM1 or PM1.5 would refer
to the micron diameter,

638
00:26:26,630 --> 00:26:29,470
1 or 1.5 micron diameter
particle that would get stuck

639
00:26:29,470 --> 00:26:32,000
in the [INAUDIBLE] and
result, eventually,

640
00:26:32,000 --> 00:26:36,751
in edema or, probably, more of
water filling up in the lungs

641
00:26:36,751 --> 00:26:38,750
as a result of the body
trying to expunge these,

642
00:26:38,750 --> 00:26:40,150
and because they're
jaggedy and pointy,

643
00:26:40,150 --> 00:26:42,900
they get stuck in there and they
don't come out and eventually

644
00:26:42,900 --> 00:26:45,590
the people can even
affixate as a result.

645
00:26:45,590 --> 00:26:48,149
So, before in the past, when
we had these big smokestacks

646
00:26:48,149 --> 00:26:50,440
sitting on the top of these
metallurgical grade silicon

647
00:26:50,440 --> 00:26:53,530
refineries that would just spew
the silica dust into the air,

648
00:26:53,530 --> 00:26:55,650
the folks downstream
would be affected

649
00:26:55,650 --> 00:26:59,010
and this actually did happen, to
some degree, in, for, example,

650
00:26:59,010 --> 00:27:02,960
Kristiansand in Norway
and, as a result,

651
00:27:02,960 --> 00:27:06,810
the refineries began
putting in filters over here

652
00:27:06,810 --> 00:27:09,800
to prevent the silica
dust from getting thrown

653
00:27:09,800 --> 00:27:11,580
and spewed out
into the atmosphere

654
00:27:11,580 --> 00:27:14,430
and the filters are a very
interesting contraption.

655
00:27:14,430 --> 00:27:17,000
A lot of work went into
designing them just right

656
00:27:17,000 --> 00:27:19,900
to allow the air to go out
but the particulate matter

657
00:27:19,900 --> 00:27:22,480
to stay behind and once
every delta t, maybe

658
00:27:22,480 --> 00:27:26,360
in the order of an hour so,
the airflow direction inverse

659
00:27:26,360 --> 00:27:28,810
and all the dust comes
crashing down to the bottom

660
00:27:28,810 --> 00:27:30,569
and then gets collected
inside of here.

661
00:27:30,569 --> 00:27:33,110
It's kind of like pushing air
through the different direction

662
00:27:33,110 --> 00:27:35,970
through a sock, and all the
dust comes out to the bottom,

663
00:27:35,970 --> 00:27:39,050
you collect it, and
it's sold to the--?

664
00:27:39,050 --> 00:27:43,382
AUDIENCE: The footwear
industry for absorbing--

665
00:27:43,382 --> 00:27:44,340
PROFESSOR: It might be.

666
00:27:44,340 --> 00:27:46,400
I don't know, but I know
that the majority of it

667
00:27:46,400 --> 00:27:52,100
goes to the cement industry
and so, depending on the market

668
00:27:52,100 --> 00:27:54,830
rates of silicon, here at the
bottom metallurgical grade

669
00:27:54,830 --> 00:27:58,230
silicon, versus what the cement
industry is willing to pay,

670
00:27:58,230 --> 00:28:00,020
you might tune your
process to optimize

671
00:28:00,020 --> 00:28:01,790
for one industry or another.

672
00:28:01,790 --> 00:28:05,520
So, this is to say that early
on in refining processes,

673
00:28:05,520 --> 00:28:08,170
you're serving multiple
industries with one plant

674
00:28:08,170 --> 00:28:11,177
and volatility of
pricing is affected,

675
00:28:11,177 --> 00:28:13,260
in part, by what those
other industries are doing.

676
00:28:13,260 --> 00:28:14,600
What the demand there is.

677
00:28:14,600 --> 00:28:15,850
It's something to be aware of.

678
00:28:15,850 --> 00:28:18,399
Let's go back to the CO2 real
quick that's being emitted.

679
00:28:18,399 --> 00:28:20,440
So that is one of the
byproducts of the reaction.

680
00:28:20,440 --> 00:28:23,530
In terms of total CO2
content from the production

681
00:28:23,530 --> 00:28:27,680
of solar cells, the CO2 produced
during the reduction process

682
00:28:27,680 --> 00:28:30,777
is a small percentage, I think
something under 5% or 10%

683
00:28:30,777 --> 00:28:32,360
is the number I
pulled out of my head,

684
00:28:32,360 --> 00:28:35,260
it's a small percentage
of the total CO2 emitted

685
00:28:35,260 --> 00:28:38,850
during solar cell manufacturing
because the electricity that

686
00:28:38,850 --> 00:28:41,610
goes into producing the rest
of the solar cells coming

687
00:28:41,610 --> 00:28:43,350
from fossil fuel based
sources comprises

688
00:28:43,350 --> 00:28:46,270
the majority of CO2
emissions during fabrication

689
00:28:46,270 --> 00:28:47,380
of these devices.

690
00:28:47,380 --> 00:28:49,570
The electricity used to
run these electrodes,

691
00:28:49,570 --> 00:28:51,520
for instance, the
electricity used

692
00:28:51,520 --> 00:28:55,140
to melt this silicon
byproduct here,

693
00:28:55,140 --> 00:28:57,830
or to gasify it in the
subsequent reactions, that

694
00:28:57,830 --> 00:29:00,570
is the majority of the CO2
coming out of the process.

695
00:29:00,570 --> 00:29:02,740
Any questions so far about this?

696
00:29:02,740 --> 00:29:03,980
They're fun plants to see.

697
00:29:03,980 --> 00:29:07,460
We don't have too many
of them in the US.

698
00:29:07,460 --> 00:29:10,680
Majority of these carbothermic
reduction furnaces

699
00:29:10,680 --> 00:29:12,810
are either in China, Norway.

700
00:29:12,810 --> 00:29:15,460
Norway has a lot
of cheap hydropower

701
00:29:15,460 --> 00:29:19,030
so the hydroplant is usually
only a few 10s of kilometers

702
00:29:19,030 --> 00:29:21,690
away from the
refinery and if you

703
00:29:21,690 --> 00:29:24,380
go to, say, [INAUDIBLE]
in Norway, where they have

704
00:29:24,380 --> 00:29:25,980
a number of these
plants, you'll see

705
00:29:25,980 --> 00:29:27,479
not only silicon
being refined there

706
00:29:27,479 --> 00:29:31,080
but also magnesium,
other elements, aluminum

707
00:29:31,080 --> 00:29:33,579
being smelted in
the same peninsula--

708
00:29:33,579 --> 00:29:34,620
the same industrial park.

709
00:29:34,620 --> 00:29:40,847
AUDIENCE: When general mining
of silicon happens or silica,

710
00:29:40,847 --> 00:29:43,156
the Chinese have--

711
00:29:43,156 --> 00:29:44,530
PROFESSOR: The
reduction process,

712
00:29:44,530 --> 00:29:47,210
this carbothermic
reduction process here,

713
00:29:47,210 --> 00:29:49,340
the majority of it
happens at the same places

714
00:29:49,340 --> 00:29:53,140
like Norway or China-- places
that have cheap electricity.

715
00:29:53,140 --> 00:29:55,239
There's also a
feedstock refinery.

716
00:29:55,239 --> 00:29:57,030
I don't know if it
extends all the way back

717
00:29:57,030 --> 00:29:58,880
to the metallurgical
grade silicon refining,

718
00:29:58,880 --> 00:30:00,440
but there's a feedstock
refining facility

719
00:30:00,440 --> 00:30:01,981
going up in the
Middle East right now

720
00:30:01,981 --> 00:30:05,960
in Qatar, as a result of
the cheap natural gas.

721
00:30:05,960 --> 00:30:08,204
So, wherever you have
cheap access to energy,

722
00:30:08,204 --> 00:30:10,120
you can set one of these
plants up and get off

723
00:30:10,120 --> 00:30:11,910
and running and
your CO2 intensity

724
00:30:11,910 --> 00:30:14,650
will be dictated by the fuel
source that you're using.

725
00:30:14,650 --> 00:30:16,570
Hydro, in that case,
it might be low

726
00:30:16,570 --> 00:30:18,170
unless you take
methane into account

727
00:30:18,170 --> 00:30:20,760
that might be emitted
in the reservoir,

728
00:30:20,760 --> 00:30:24,250
if you have decaying biomass
underneath the water,

729
00:30:24,250 --> 00:30:26,497
but if you would
exclude that and if you

730
00:30:26,497 --> 00:30:28,580
look at the CO2 intensity
of the fossil fuels that

731
00:30:28,580 --> 00:30:30,177
are being burned,
it might be better

732
00:30:30,177 --> 00:30:32,760
to do it in, say, Norway, from
an environmental point of view,

733
00:30:32,760 --> 00:30:35,386
than to, say, manufacture
this stuff in China.

734
00:30:35,386 --> 00:30:35,886
Yeah.

735
00:30:35,886 --> 00:30:37,261
AUDIENCE: How many
kilowatt hours

736
00:30:37,261 --> 00:30:38,874
are we talking [INAUDIBLE]?

737
00:30:38,874 --> 00:30:40,640
PROFESSOR: Okay, so
what is the energy

738
00:30:40,640 --> 00:30:44,780
intensity of this process
right here, in other words.

739
00:30:44,780 --> 00:30:46,876
Well, why don't I
put a flag on that.

740
00:30:46,876 --> 00:30:48,750
Why don't we put a flag
on that and come back

741
00:30:48,750 --> 00:30:51,990
with specific numbers for
this process right here.

742
00:30:51,990 --> 00:30:54,232
I don't want to say something
and regret it later.

743
00:30:54,232 --> 00:30:56,065
AUDIENCE: Well, we know
the energy intensity

744
00:30:56,065 --> 00:30:57,350
of the solar panel itself.

745
00:30:57,350 --> 00:30:58,033
PROFESSOR: Yeah.

746
00:30:58,033 --> 00:30:59,116
AUDIENCE: But the energy--

747
00:30:59,116 --> 00:31:01,680
PROFESSOR: But
specifically what fraction

748
00:31:01,680 --> 00:31:04,760
comes from the MGSi
refining, I'd rather not

749
00:31:04,760 --> 00:31:07,200
pull something out of my head.

750
00:31:07,200 --> 00:31:09,200
Any other questions?

751
00:31:09,200 --> 00:31:11,340
OK.

752
00:31:11,340 --> 00:31:14,000
So somewhere in the order
of two million metric tons

753
00:31:14,000 --> 00:31:17,034
of metallurgical grade
silicon are produced annually.

754
00:31:17,034 --> 00:31:18,950
Probably somewhere in
the order of 10% of that

755
00:31:18,950 --> 00:31:21,600
is destined for the PV industry.

756
00:31:21,600 --> 00:31:23,950
The remainder gets
split among a variety

757
00:31:23,950 --> 00:31:24,970
of different industries.

758
00:31:24,970 --> 00:31:26,428
So what I'm talking
about here when

759
00:31:26,428 --> 00:31:29,080
I say metallurgical silicon, I'm
referring to this right here.

760
00:31:29,080 --> 00:31:30,250
This stuff coming out.

761
00:31:30,250 --> 00:31:34,670
It has about 99% or 99.9%
purity and it gets used

762
00:31:34,670 --> 00:31:36,030
in a variety of industries.

763
00:31:36,030 --> 00:31:38,174
So those industries
are: the PV industry,

764
00:31:38,174 --> 00:31:40,090
and we'll explain how
the rest of the refining

765
00:31:40,090 --> 00:31:43,370
happens, the integrated circuits
industry, that's the wafer that

766
00:31:43,370 --> 00:31:45,680
just went around that's
made its way back up here,

767
00:31:45,680 --> 00:31:51,510
and silicones those are--
so, a pet peeve of mine

768
00:31:51,510 --> 00:31:56,040
is hearing the word silicon and
silicone used interchangeably.

769
00:31:56,040 --> 00:31:59,710
Silicon is this element-- is an
element on the periodic table

770
00:31:59,710 --> 00:32:02,570
and it's the element that
comprises this wafer right

771
00:32:02,570 --> 00:32:03,250
here.

772
00:32:03,250 --> 00:32:06,642
Silicone, on the other
hand, is an organelle,

773
00:32:06,642 --> 00:32:09,100
I guess you could say, it's
not exactly organelle metallic,

774
00:32:09,100 --> 00:32:12,300
silicon isn't a
metal, but it would

775
00:32:12,300 --> 00:32:18,520
be a molecule that is comprised
of carbon atoms and silicon--

776
00:32:18,520 --> 00:32:21,300
silicon being in the middle and
the carbon being on the sides--

777
00:32:21,300 --> 00:32:25,440
and that is used as caulking or
sealing agent in your showers,

778
00:32:25,440 --> 00:32:29,110
for instance, or in
plumbing, round windows.

779
00:32:29,110 --> 00:32:32,320
It tends to be very flexible,
compliant but yet impermeable,

780
00:32:32,320 --> 00:32:34,940
preventing the inflow of gases.

781
00:32:34,940 --> 00:32:37,140
So silicones,
they're metal alloys

782
00:32:37,140 --> 00:32:39,430
including steel and aluminum.

783
00:32:39,430 --> 00:32:42,230
Why would you silicon there?

784
00:32:42,230 --> 00:32:45,085
What does it have to do
with steel or aluminum?

785
00:32:45,085 --> 00:32:45,910
Let me ask this.

786
00:32:45,910 --> 00:32:48,760
Has anyone ever played
with pure aluminum?

787
00:32:48,760 --> 00:32:52,300
Highly refined, ultra
high purity aluminum.

788
00:32:52,300 --> 00:32:54,030
Say five nines or six nines.

789
00:32:54,030 --> 00:32:54,850
Yes!

790
00:32:54,850 --> 00:32:56,470
What happens to
ultra-pure aluminum?

791
00:32:56,470 --> 00:32:57,941
AUDIENCE: It's really flexible.

792
00:32:57,941 --> 00:32:59,440
PROFESSOR: It's
really flexible, you

793
00:32:59,440 --> 00:33:01,127
can dent it with
your fingernail,

794
00:33:01,127 --> 00:33:02,710
and it wouldn't make
very great boxes.

795
00:33:02,710 --> 00:33:03,590
Right?

796
00:33:03,590 --> 00:33:07,650
So we need it to be stronger
and scratch-resistant and so

797
00:33:07,650 --> 00:33:09,990
we have these additives into
the aluminum, silicon being

798
00:33:09,990 --> 00:33:12,364
one of them, that increases
the strength of the aluminum,

799
00:33:12,364 --> 00:33:13,870
essentially
preventing plasticity

800
00:33:13,870 --> 00:33:16,780
or preventing a dislocation
flow into the material.

801
00:33:16,780 --> 00:33:20,630
So that's more or less how
silicon-- metallurgical grade

802
00:33:20,630 --> 00:33:25,170
silicon, also called MGSi as
shown up here at the very top--

803
00:33:25,170 --> 00:33:27,770
that's how MGSi gs is
distributed worldwide

804
00:33:27,770 --> 00:33:29,710
and that's the
current production.

805
00:33:29,710 --> 00:33:31,440
Now let me ask another question.

806
00:33:31,440 --> 00:33:36,170
Steel and aluminum, where
are those used the most?

807
00:33:36,170 --> 00:33:39,022
What industry uses
steel, aluminum the most?

808
00:33:39,022 --> 00:33:39,980
AUDIENCE: Construction.

809
00:33:39,980 --> 00:33:43,110
PROFESSOR: Constructive
industry, automotive industry.

810
00:33:43,110 --> 00:33:44,940
How fast are those
growing annually?

811
00:33:48,540 --> 00:33:50,630
Let's estimate it from GDP.

812
00:33:50,630 --> 00:33:51,880
Annual-- worldwide GDP.

813
00:33:51,880 --> 00:33:54,270
What's the worldwide
GDP growth look like.

814
00:33:54,270 --> 00:33:55,380
US is around 1%.

815
00:33:55,380 --> 00:33:55,942
China 8%.

816
00:33:55,942 --> 00:33:57,650
Let's pick a number
somewhere in between.

817
00:33:57,650 --> 00:33:58,550
Four, right?

818
00:33:58,550 --> 00:33:59,050
All right.

819
00:33:59,050 --> 00:34:01,630
So, let's say 4%, 5% worldwide.

820
00:34:01,630 --> 00:34:03,487
Silicone's probably
on that order.

821
00:34:03,487 --> 00:34:04,570
How about the PV industry.

822
00:34:04,570 --> 00:34:07,090
How fast is it going right now?

823
00:34:07,090 --> 00:34:09,615
Somewhere in the order of,
it's a volatile year right now,

824
00:34:09,615 --> 00:34:11,489
this one year, but in
the past, historically,

825
00:34:11,489 --> 00:34:14,510
it's been around
40% to 60% a year.

826
00:34:14,510 --> 00:34:16,560
So, where do you think
the price pressure

827
00:34:16,560 --> 00:34:18,810
for metallurgical grade
silicon is going to come from?

828
00:34:18,810 --> 00:34:19,585
What industry?

829
00:34:19,585 --> 00:34:20,710
It's going to come from PV.

830
00:34:20,710 --> 00:34:22,459
It's a small fraction
of the pie right now

831
00:34:22,459 --> 00:34:23,630
but it's growing fast.

832
00:34:23,630 --> 00:34:25,510
Something to keep in mind.

833
00:34:25,510 --> 00:34:29,420
So that's why, if you look at
pricing of metallurgical grade

834
00:34:29,420 --> 00:34:31,150
silicon, yes.

835
00:34:31,150 --> 00:34:33,870
Superimposed upon pricing
is a function of time.

836
00:34:33,870 --> 00:34:37,010
You have the global
macroeconomic situation.

837
00:34:37,010 --> 00:34:37,510
Right?

838
00:34:37,510 --> 00:34:40,020
So that's kind of the dampening
function on top of it all,

839
00:34:40,020 --> 00:34:43,850
but there's just this general
trend toward rising prices

840
00:34:43,850 --> 00:34:48,210
as you put increasing price
pressure on metallurgical grade

841
00:34:48,210 --> 00:34:48,811
silicon.

842
00:34:48,811 --> 00:34:50,310
So additional
refining capacity will

843
00:34:50,310 --> 00:34:54,070
be needed if the current growth
keeps up in this industry.

844
00:34:54,070 --> 00:34:56,630
So let me talk about going
from metallurgical grade

845
00:34:56,630 --> 00:35:00,400
silicon about two nines
to three nines pure.

846
00:35:00,400 --> 00:35:02,970
What I mean two nines
means 99%, three nines

847
00:35:02,970 --> 00:35:06,590
would be 99.9% pure,
to silicon that we

848
00:35:06,590 --> 00:35:08,670
can use for solar
cells, which typically

849
00:35:08,670 --> 00:35:11,730
has to be about six nines pure.

850
00:35:11,730 --> 00:35:15,010
And so this is called
the Siemens process which

851
00:35:15,010 --> 00:35:18,087
is purification through
gaseous distillation,

852
00:35:18,087 --> 00:35:19,920
and that's the method
that is currently used

853
00:35:19,920 --> 00:35:22,132
to make most of our silicon.

854
00:35:22,132 --> 00:35:23,590
So the way this
process works is we

855
00:35:23,590 --> 00:35:26,760
start with metallurgical
grade silicon at the top,

856
00:35:26,760 --> 00:35:29,400
represented by a little sack
of metallurgical grade silicon

857
00:35:29,400 --> 00:35:30,290
chunks.

858
00:35:30,290 --> 00:35:33,340
We produce silane gas out
of that metallurgical grade

859
00:35:33,340 --> 00:35:34,150
silicon.

860
00:35:34,150 --> 00:35:37,800
We essentially-
silane gas is SiH4.

861
00:35:37,800 --> 00:35:43,150
So it would essentially
be this right here.

862
00:35:43,150 --> 00:35:46,320
So you'd have a silicon atom
here, tetrahedrally coordinated

863
00:35:46,320 --> 00:35:50,720
with-- tetrahedrally meaning
four bonds with hydrogen atoms

864
00:35:50,720 --> 00:35:56,170
on the side-- and this is
silane gas-- well, silane--

865
00:35:56,170 --> 00:35:58,070
which, at room
temperature, is a gas

866
00:35:58,070 --> 00:36:01,660
and that's what happens
in this step right here.

867
00:36:01,660 --> 00:36:05,170
We're forming-- we're
gasifying the silicon.

868
00:36:05,170 --> 00:36:09,350
This process is the
distillation process.

869
00:36:09,350 --> 00:36:11,650
To extract the pure
silane gas, it's

870
00:36:11,650 --> 00:36:13,150
the distillation
process that's used

871
00:36:13,150 --> 00:36:16,689
in large towers similar to
fractional distillation where

872
00:36:16,689 --> 00:36:18,730
we might heat up the
material and then, depending

873
00:36:18,730 --> 00:36:23,490
on its mass, it settles down to
a certain height in that tower

874
00:36:23,490 --> 00:36:25,060
and we're able to extract it.

875
00:36:25,060 --> 00:36:27,210
The silane gas
here has been sold

876
00:36:27,210 --> 00:36:28,680
to the photovoltaics industry.

877
00:36:28,680 --> 00:36:30,050
LCD.

878
00:36:30,050 --> 00:36:31,790
Liquid crystal display.

879
00:36:31,790 --> 00:36:32,390
Right?

880
00:36:32,390 --> 00:36:34,914
Thin film industries as
well, they use silane.

881
00:36:34,914 --> 00:36:37,080
If you're depositing the
polycrystalline and silicon

882
00:36:37,080 --> 00:36:39,880
for your LCDs or if you're
making amorphous silicon

883
00:36:39,880 --> 00:36:41,954
solar cells, they
use silane as well.

884
00:36:41,954 --> 00:36:43,370
So this little
truck here might go

885
00:36:43,370 --> 00:36:44,995
to three different
companies, depending

886
00:36:44,995 --> 00:36:47,730
on who's willing to pay more.

887
00:36:47,730 --> 00:36:50,840
Most of the silane is
used for polysilicon.

888
00:36:50,840 --> 00:36:53,700
The gas has to be converted
back into a solid,

889
00:36:53,700 --> 00:36:56,600
and that's where this
particular process here,

890
00:36:56,600 --> 00:36:58,290
the Siemens process is used.

891
00:36:58,290 --> 00:37:00,370
Again, you have
a current passing

892
00:37:00,370 --> 00:37:02,970
through some seed
material and the gas

893
00:37:02,970 --> 00:37:04,910
is being cracked onto that seed.

894
00:37:04,910 --> 00:37:06,320
You form these rods.

895
00:37:06,320 --> 00:37:08,170
The rods are then
cracked into chunks

896
00:37:08,170 --> 00:37:10,980
and then the chunks are
loaded into ingot crucibles.

897
00:37:10,980 --> 00:37:11,480
Yes.

898
00:37:11,480 --> 00:37:17,560
AUDIENCE: So the silane gas is
shipped as a gas in the trucks.

899
00:37:17,560 --> 00:37:18,466
PROFESSOR: Sure.

900
00:37:18,466 --> 00:37:20,410
AUDIENCE: Or on rails?

901
00:37:20,410 --> 00:37:23,550
PROFESSOR: Well it's
pyrophoric, as you

902
00:37:23,550 --> 00:37:26,140
can guess from just glancing
at this chemical structure

903
00:37:26,140 --> 00:37:26,640
right here.

904
00:37:26,640 --> 00:37:27,890
It's highly reactive.

905
00:37:27,890 --> 00:37:31,690
Pyrophoric means that it can
combust at room temperature.

906
00:37:31,690 --> 00:37:34,650
It can catch on fire, meaning
there are more stable compounds

907
00:37:34,650 --> 00:37:38,370
than this that can form when
you react this gas with air

908
00:37:38,370 --> 00:37:42,170
and, during the early days
of silane development,

909
00:37:42,170 --> 00:37:44,145
folks really didn't
know much about it

910
00:37:44,145 --> 00:37:45,520
and there's some
early research--

911
00:37:45,520 --> 00:37:47,853
some of the earliest research
done here at MIT, in fact.

912
00:37:47,853 --> 00:37:51,050
They would fill up an evacuated
chamber with silane gas

913
00:37:51,050 --> 00:37:53,350
and spark and
nothing would happen.

914
00:37:53,350 --> 00:37:55,100
Spark a second time,
nothing would happen.

915
00:37:55,100 --> 00:37:56,500
Spark a third time, boom.

916
00:37:56,500 --> 00:37:57,110
OK.

917
00:37:57,110 --> 00:37:58,330
That's critical limit.

918
00:37:58,330 --> 00:37:59,220
Such and such amount.

919
00:37:59,220 --> 00:38:00,890
You know, they'd keep
increasing the amount

920
00:38:00,890 --> 00:38:02,000
and finally it would go boom.

921
00:38:02,000 --> 00:38:03,150
Tell you what, lets
repeat the experiment

922
00:38:03,150 --> 00:38:04,310
since we're good scientists.

923
00:38:04,310 --> 00:38:06,434
They'd repeat it and, at
low concentrations, click.

924
00:38:06,434 --> 00:38:07,305
Boom.

925
00:38:07,305 --> 00:38:07,930
That's strange.

926
00:38:07,930 --> 00:38:09,260
That was much lower this time.

927
00:38:09,260 --> 00:38:10,880
Let's repeat the
experiment one more time.

928
00:38:10,880 --> 00:38:11,210
Click.

929
00:38:11,210 --> 00:38:11,550
Click.

930
00:38:11,550 --> 00:38:11,880
Click.

931
00:38:11,880 --> 00:38:12,180
Click.

932
00:38:12,180 --> 00:38:12,470
Click.

933
00:38:12,470 --> 00:38:12,730
Click.

934
00:38:12,730 --> 00:38:12,960
Click.

935
00:38:12,960 --> 00:38:13,180
Click.

936
00:38:13,180 --> 00:38:13,390
Click.

937
00:38:13,390 --> 00:38:13,921
Boom.

938
00:38:13,921 --> 00:38:14,420
All right.

939
00:38:14,420 --> 00:38:16,090
I don't really
understand this gas,

940
00:38:16,090 --> 00:38:18,830
but I'm going to say it's
really dangerous so I'm

941
00:38:18,830 --> 00:38:21,200
going to have little
warning bells that

942
00:38:21,200 --> 00:38:23,170
will detect the silane
gas if it's leaking

943
00:38:23,170 --> 00:38:25,560
and tell people to get the
heck out of the building

944
00:38:25,560 --> 00:38:29,180
if it starts being leaked.

945
00:38:29,180 --> 00:38:31,540
It's also toxic for
humans, by the way.

946
00:38:31,540 --> 00:38:34,940
Very small dilute
concentrations can kill you

947
00:38:34,940 --> 00:38:37,920
and so three buildings
on campus, only three

948
00:38:37,920 --> 00:38:40,770
to my knowledge, are set up
with the proper safety equipment

949
00:38:40,770 --> 00:38:42,900
to use silane gas
in the laboratory.

950
00:38:42,900 --> 00:38:45,590
Building 13, which is the
material science building,

951
00:38:45,590 --> 00:38:48,340
and then-- MTL and related.

952
00:38:48,340 --> 00:38:52,189
So we have this gas right here.

953
00:38:52,189 --> 00:38:52,980
Extremely powerful.

954
00:38:52,980 --> 00:38:55,980
There are variants thereof.

955
00:38:55,980 --> 00:39:00,020
You can replace some
of the hydrogens

956
00:39:00,020 --> 00:39:10,280
with chlorine, like this and
now you have trichlorosilane.

957
00:39:10,280 --> 00:39:11,930
It's all one word.

958
00:39:11,930 --> 00:39:15,330
So tricholorsilane, I've just
replaced three of my silanes--

959
00:39:15,330 --> 00:39:17,310
my hydrogens with
chlorine and now I

960
00:39:17,310 --> 00:39:19,850
have a different molecule,
still silicon bearing,

961
00:39:19,850 --> 00:39:22,480
still very reactive,
but now reactive

962
00:39:22,480 --> 00:39:25,820
at different temperatures
and I can modify my process

963
00:39:25,820 --> 00:39:29,310
by substituting out some of
the hydrogens for chlorines.

964
00:39:29,310 --> 00:39:33,000
So we have the silane gas or
trichlorosilane or the variants

965
00:39:33,000 --> 00:39:36,370
thereof, loaded into some
transportation vehicle that

966
00:39:36,370 --> 00:39:40,030
is very safe, leak-proof
and preventing accidents

967
00:39:40,030 --> 00:39:45,020
on the road, to deliver
it to where it is going

968
00:39:45,020 --> 00:39:50,510
to be consumed, which are
these so-called polysilicon,

969
00:39:50,510 --> 00:39:54,300
or Siemens reactor
as shown here.

970
00:39:54,300 --> 00:39:56,350
Excuse me.

971
00:39:56,350 --> 00:39:59,240
What happens, or how the
process actually flows,

972
00:39:59,240 --> 00:40:00,362
let me go back one step.

973
00:40:00,362 --> 00:40:02,820
We're going to start from up
at the very top of the process

974
00:40:02,820 --> 00:40:04,830
and move all the way
down, showing you

975
00:40:04,830 --> 00:40:07,520
what the manufacturing equipment
looks like at each step.

976
00:40:07,520 --> 00:40:09,730
So, this is the
distillation process

977
00:40:09,730 --> 00:40:11,960
used to create the
silane and when

978
00:40:11,960 --> 00:40:15,740
you see one of these factories
just think of a refinery.

979
00:40:15,740 --> 00:40:20,770
In fact, the people who don't
like this particular process

980
00:40:20,770 --> 00:40:24,170
who aren't a fan of the
silane refining process

981
00:40:24,170 --> 00:40:26,330
and opt for other ways of
purifying their silicon,

982
00:40:26,330 --> 00:40:28,830
liken this to an oil refinery.

983
00:40:28,830 --> 00:40:32,190
The imagery is very stark there.

984
00:40:32,190 --> 00:40:36,040
The polysilicon production,
this is the Siemens reactor,

985
00:40:36,040 --> 00:40:38,540
it's much smaller in comparison
to the metallurgical grade

986
00:40:38,540 --> 00:40:39,620
silicon furnace.

987
00:40:39,620 --> 00:40:41,970
Much smaller than the
carbothermic reduction furnace.

988
00:40:41,970 --> 00:40:44,850
Here, we have a small
human or human next

989
00:40:44,850 --> 00:40:47,170
to the small contraption.

990
00:40:47,170 --> 00:40:49,500
Here are a series of
them lined, almost

991
00:40:49,500 --> 00:40:52,720
like little pods and,
out of this material,

992
00:40:52,720 --> 00:40:54,750
actually inside of
the furnace, you

993
00:40:54,750 --> 00:41:00,940
have these rods that are
passing current and heating up

994
00:41:00,940 --> 00:41:04,920
and the silicon is
cracking onto the rods.

995
00:41:04,920 --> 00:41:07,220
So we wind up with
six nines, usually

996
00:41:07,220 --> 00:41:11,390
called 6N solar grade silicon
as a result of this process.

997
00:41:11,390 --> 00:41:16,180
We could also go up to, even,
nine nines using the Siemens

998
00:41:16,180 --> 00:41:16,750
process.

999
00:41:16,750 --> 00:41:18,510
It could be very,
very pure depending

1000
00:41:18,510 --> 00:41:21,011
on how fast you grow, what the
purity of your silane gas is.

1001
00:41:21,011 --> 00:41:21,635
AUDIENCE: Yeah.

1002
00:41:21,635 --> 00:41:22,540
What is cracking.

1003
00:41:22,540 --> 00:41:23,654
What does that mean?

1004
00:41:23,654 --> 00:41:24,320
PROFESSOR: Sure.

1005
00:41:24,320 --> 00:41:27,330
So what it means is
this gas molecule comes

1006
00:41:27,330 --> 00:41:31,970
in, sees a solid surface,
the central atom right here,

1007
00:41:31,970 --> 00:41:34,740
the silicon atom, gets
deposited onto the surface,

1008
00:41:34,740 --> 00:41:37,550
becomes an adatom, which
means it's a surface atom,

1009
00:41:37,550 --> 00:41:41,370
it's scuttling around and
the remaining elements

1010
00:41:41,370 --> 00:41:44,215
within this molecule are then
free to move away as a gas.

1011
00:41:44,215 --> 00:41:45,840
AUDIENCE: So you've
broken those bonds.

1012
00:41:45,840 --> 00:41:46,464
PROFESSOR: Yes.

1013
00:41:46,464 --> 00:41:49,600
Effectively, you've added
the core constituent

1014
00:41:49,600 --> 00:41:53,740
of this molecule
onto the surface.

1015
00:41:53,740 --> 00:41:56,580
It's joined the collective if
you will and, in this matter,

1016
00:41:56,580 --> 00:41:59,530
the diameter of those
rods grows with time.

1017
00:41:59,530 --> 00:42:01,480
So what I'm going
to do is pass around

1018
00:42:01,480 --> 00:42:06,320
an example of a chunk coming
from this Siemens rod.

1019
00:42:06,320 --> 00:42:07,960
Be very gentle with it please.

1020
00:42:07,960 --> 00:42:10,400
On the outside you can
see a corrugated, rough,

1021
00:42:10,400 --> 00:42:12,520
cauliflower-like structure.

1022
00:42:12,520 --> 00:42:15,570
That's because you're
optimizing for deposition speed,

1023
00:42:15,570 --> 00:42:17,120
not for beauty of the surface.

1024
00:42:17,120 --> 00:42:18,770
You don't really
care how flat it

1025
00:42:18,770 --> 00:42:20,990
is, unless you're trying to
grow a very specific type

1026
00:42:20,990 --> 00:42:23,210
of material called flotsam,
which we get to the second,

1027
00:42:23,210 --> 00:42:25,180
but in general, if you're trying
to crack it up and break it

1028
00:42:25,180 --> 00:42:27,138
into a smaller piece and
into a chunk like this

1029
00:42:27,138 --> 00:42:28,970
and throw it into a
big ingot furnace,

1030
00:42:28,970 --> 00:42:31,178
it doesn't really matter
what the surface looks like.

1031
00:42:31,178 --> 00:42:33,590
On the inside, it's
pretty dense silicon

1032
00:42:33,590 --> 00:42:36,310
and, if look very carefully,
right in the middle there

1033
00:42:36,310 --> 00:42:37,410
you can see the rod.

1034
00:42:37,410 --> 00:42:38,910
The initial seeding rod.

1035
00:42:38,910 --> 00:42:40,812
It's a slightly different color.

1036
00:42:40,812 --> 00:42:42,770
So I'll pass these around
and please be gentle.

1037
00:42:42,770 --> 00:42:45,270
AUDIENCE: Is the seeding
rod just silicon?

1038
00:42:45,270 --> 00:42:46,900
PROFESSOR: It's
actually doped silicon,

1039
00:42:46,900 --> 00:42:49,030
so it's lower
resistivity so you can

1040
00:42:49,030 --> 00:42:52,330
pass more current through it.

1041
00:42:52,330 --> 00:42:57,600
This here is chunks, or smaller
chunks of the polysilicon

1042
00:42:57,600 --> 00:42:59,840
so, essentially, just
crushed polysilicon

1043
00:42:59,840 --> 00:43:02,610
and if you're trying to load
a crucible with big chunks

1044
00:43:02,610 --> 00:43:04,730
like this you'll leave
a lot of empty space

1045
00:43:04,730 --> 00:43:07,660
unless you crush some of this up
and make finer grains out of it

1046
00:43:07,660 --> 00:43:08,930
and fill in the gaps.

1047
00:43:08,930 --> 00:43:11,120
So I'll pass these
around right here

1048
00:43:11,120 --> 00:43:12,590
so you can have a look at them.

1049
00:43:12,590 --> 00:43:16,500
Those are examples of the
Siemens grade polysilicon.

1050
00:43:16,500 --> 00:43:19,940
This is a bigger rod.

1051
00:43:19,940 --> 00:43:22,150
Here is the seed coming
right through the middle.

1052
00:43:22,150 --> 00:43:23,650
Here's the surface
where you can see

1053
00:43:23,650 --> 00:43:29,060
it's kind of rough
and corrugated

1054
00:43:29,060 --> 00:43:32,570
and one of the biggest issues
with this feedstock refining

1055
00:43:32,570 --> 00:43:34,580
process is that there
are very large plants

1056
00:43:34,580 --> 00:43:36,070
and long lead times.

1057
00:43:36,070 --> 00:43:38,460
This is a plant construction
going on right now,

1058
00:43:38,460 --> 00:43:39,720
you can see.

1059
00:43:39,720 --> 00:43:44,070
Typical lead times are
between 18 and 24 months.

1060
00:43:44,070 --> 00:43:46,680
That's a long time between
when the board says yes, we

1061
00:43:46,680 --> 00:43:50,360
will create new silicon
refining capacity and product

1062
00:43:50,360 --> 00:43:52,040
starts to roll off
the production line

1063
00:43:52,040 --> 00:43:53,360
and into customers' hands.

1064
00:43:53,360 --> 00:43:55,660
It's a long time and
what this results in

1065
00:43:55,660 --> 00:43:58,090
are drastic oversupply
and undersupply

1066
00:43:58,090 --> 00:43:59,500
conditions in the market.

1067
00:43:59,500 --> 00:44:01,310
So the silicon
feedstock price goes

1068
00:44:01,310 --> 00:44:04,110
very high during periods
of undersupply and very low

1069
00:44:04,110 --> 00:44:06,370
in periods of oversupply
and we're in an oversupply

1070
00:44:06,370 --> 00:44:08,290
condition right now.

1071
00:44:08,290 --> 00:44:11,530
Five years ago, let
me quantify this.

1072
00:44:11,530 --> 00:44:13,830
Five years ago if you
went to the spot market--

1073
00:44:13,830 --> 00:44:16,080
maybe four years ago-- if
you went to the spot market,

1074
00:44:16,080 --> 00:44:20,480
you could pay $100 to $500
per kilogram of silicon.

1075
00:44:20,480 --> 00:44:23,070
That material that was just
right there I bet one you

1076
00:44:23,070 --> 00:44:25,900
would put it into your bag and
run away out the door right now

1077
00:44:25,900 --> 00:44:28,590
and be able to go to Mexico.

1078
00:44:28,590 --> 00:44:32,170
Now the polysilicon
prices are much,

1079
00:44:32,170 --> 00:44:33,520
much lower on the spot market.

1080
00:44:33,520 --> 00:44:36,830
Somewhere in the order of
$30 to $50 per kilogram.

1081
00:44:36,830 --> 00:44:39,680
About an order of
magnitude lower.

1082
00:44:39,680 --> 00:44:43,241
AUDIENCE: Isn't lower cost
silicon better for the PV

1083
00:44:43,241 --> 00:44:43,950
industry, though?

1084
00:44:43,950 --> 00:44:45,782
PROFESSOR: Is it better
for the PV industry?

1085
00:44:45,782 --> 00:44:47,920
As a customer most definitely,
it is good for you.

1086
00:44:47,920 --> 00:44:50,380
As an installer, it is most
definitely good for you.

1087
00:44:50,380 --> 00:44:52,440
As a polysilicon
producer who wants

1088
00:44:52,440 --> 00:44:54,440
to be a sustained
industry presence,

1089
00:44:54,440 --> 00:44:55,530
it's not good for you.

1090
00:44:55,530 --> 00:45:00,710
So this wide oscillation
between fat cat and scrawny

1091
00:45:00,710 --> 00:45:03,220
is not very good
for any industry.

1092
00:45:03,220 --> 00:45:06,450
It's unpredictable and it
causes some players to drop out.

1093
00:45:06,450 --> 00:45:06,992
AUDIENCE: OK.

1094
00:45:06,992 --> 00:45:09,241
PROFESSOR: And the investments
are very large as well.

1095
00:45:09,241 --> 00:45:11,570
As you go from the early
stage portions of the value

1096
00:45:11,570 --> 00:45:15,870
chain toward the module, the
investments generally decrease

1097
00:45:15,870 --> 00:45:19,650
and so this is an outlook
coming from last year--

1098
00:45:19,650 --> 00:45:22,040
the numbers are still a little
bit outdated-- polysilicon

1099
00:45:22,040 --> 00:45:25,500
production is buttressing
up against 200,000

1100
00:45:25,500 --> 00:45:28,900
metric tons per year at
this point in about 3/4

1101
00:45:28,900 --> 00:45:30,340
to the PV industry.

1102
00:45:30,340 --> 00:45:33,960
The cost of manufacturing
is between $20 and $25

1103
00:45:33,960 --> 00:45:38,450
per kilogram and 2010 prices
were around $50 to $70.

1104
00:45:38,450 --> 00:45:42,560
Now they're on
$30 to $50 in 2011

1105
00:45:42,560 --> 00:45:46,150
and the 2008 prices were around
$500 per kilogram in the spot

1106
00:45:46,150 --> 00:45:49,670
market and it really boils
down to the inability

1107
00:45:49,670 --> 00:45:51,200
to adapt to demand.

1108
00:45:51,200 --> 00:45:53,550
If you have a very
large contraption that

1109
00:45:53,550 --> 00:45:55,932
produces the feedstock materials
and it takes a long time

1110
00:45:55,932 --> 00:45:57,390
to build the
factories, you're just

1111
00:45:57,390 --> 00:46:00,870
not going to be able
to adjust fast enough.

1112
00:46:00,870 --> 00:46:03,700
Here's supply and demand,
demand being the red

1113
00:46:03,700 --> 00:46:05,210
and supply being the blue.

1114
00:46:05,210 --> 00:46:08,380
You can see how the oversupply--
the undersupply condition

1115
00:46:08,380 --> 00:46:13,740
of the mid 2000s really led
to our current condition.

1116
00:46:13,740 --> 00:46:17,300
So, alternatives to solar grade
silicon feedstock refining.

1117
00:46:17,300 --> 00:46:20,270
What are some people thinking
in terms of other processes

1118
00:46:20,270 --> 00:46:21,690
that they can use?

1119
00:46:21,690 --> 00:46:26,300
These are two processes
right here and, mind you,

1120
00:46:26,300 --> 00:46:30,180
when we were in this
situation with this price

1121
00:46:30,180 --> 00:46:32,215
for the silicon,
everybody and anybody

1122
00:46:32,215 --> 00:46:34,840
was coming up with new ideas of
how to manufacture the silicon.

1123
00:46:34,840 --> 00:46:39,630
Now that we're barely selling
at cost and in an oversupply

1124
00:46:39,630 --> 00:46:42,150
condition, many of these
ideas are having a struggle--

1125
00:46:42,150 --> 00:46:43,320
a hard time in the market.

1126
00:46:43,320 --> 00:46:44,936
They're struggling right now.

1127
00:46:44,936 --> 00:46:47,310
So fluidized bed reactor and
upgraded metallurgical grade

1128
00:46:47,310 --> 00:46:49,260
silicon.

1129
00:46:49,260 --> 00:46:52,050
Let's talk about each
of those in turn.

1130
00:46:52,050 --> 00:46:54,040
So what the fluidized
bed reactor folks

1131
00:46:54,040 --> 00:46:57,550
realized was, gee, if
we're depositing on a rod,

1132
00:46:57,550 --> 00:47:01,940
our surface area to volume
ratio is really large-- sorry,

1133
00:47:01,940 --> 00:47:03,120
is really small.

1134
00:47:03,120 --> 00:47:06,480
Our surface area to volume
ratio is going to be very small.

1135
00:47:06,480 --> 00:47:07,650
So think of it this way.

1136
00:47:07,650 --> 00:47:10,080
If we have a sphere,
a sphere would

1137
00:47:10,080 --> 00:47:11,910
be the quintessential
example where

1138
00:47:11,910 --> 00:47:15,880
we'd have a very large
surface area to volume ratio.

1139
00:47:15,880 --> 00:47:18,270
If we had a plate, we
would have, as well,

1140
00:47:18,270 --> 00:47:21,260
a very large surface
area to volume ratio

1141
00:47:21,260 --> 00:47:26,130
and in the case of the condition
prior, where you have this rod,

1142
00:47:26,130 --> 00:47:28,370
you really can't deposit
that quickly and so what

1143
00:47:28,370 --> 00:47:30,390
these folks decided was,
what we're going to do

1144
00:47:30,390 --> 00:47:35,290
is introduce small silicon
granules into this vessel,

1145
00:47:35,290 --> 00:47:38,114
into this evacuated
chamber, and-- here's

1146
00:47:38,114 --> 00:47:39,530
the evacuated
chamber right here--

1147
00:47:39,530 --> 00:47:45,482
and we're going to flow silane
gas into the system right here

1148
00:47:45,482 --> 00:47:46,940
and the smaller
particles are going

1149
00:47:46,940 --> 00:47:49,550
to go higher up because
of this flow of gas

1150
00:47:49,550 --> 00:47:52,820
coming in the bottom and
those will grow and eventually

1151
00:47:52,820 --> 00:47:56,230
settle down down here where
we can extract the bottom.

1152
00:47:56,230 --> 00:47:58,800
So we'll wind up with these
beautiful little silicon

1153
00:47:58,800 --> 00:47:59,585
granules.

1154
00:47:59,585 --> 00:48:01,710
These ones shown right
here, which I'll pass around

1155
00:48:01,710 --> 00:48:04,480
as well, those are coming
from a fluidized bed reactor,

1156
00:48:04,480 --> 00:48:08,020
and they're nice beautiful,
spherical granules that

1157
00:48:08,020 --> 00:48:10,540
are grown a lot faster, I
mean, a lot more silicon

1158
00:48:10,540 --> 00:48:14,240
is deposited per unit time than
through the Siemens process

1159
00:48:14,240 --> 00:48:15,640
as shown there in the back.

1160
00:48:15,640 --> 00:48:17,920
As a result, the energy
intensity is lower,

1161
00:48:17,920 --> 00:48:20,610
the cost is lower, there's
a very tricky process

1162
00:48:20,610 --> 00:48:22,586
to nail to get just
right, because you

1163
00:48:22,586 --> 00:48:24,085
have to get the gas
flows right, you

1164
00:48:24,085 --> 00:48:27,820
have to design the chamber
well, redo some purity contents.

1165
00:48:27,820 --> 00:48:29,520
It's a tricky
process, and so this

1166
00:48:29,520 --> 00:48:31,670
is being produced
right now, I believe,

1167
00:48:31,670 --> 00:48:32,980
by only a few companies.

1168
00:48:32,980 --> 00:48:35,510
REC has a capability
of doing it.

1169
00:48:35,510 --> 00:48:37,930
MEMC, as well,
has the capability

1170
00:48:37,930 --> 00:48:39,680
of doing this process.

1171
00:48:39,680 --> 00:48:41,370
By and large, most
silicon is coming

1172
00:48:41,370 --> 00:48:44,770
from the Siemens process.

1173
00:48:44,770 --> 00:48:45,270
Yup.

1174
00:48:45,270 --> 00:48:46,936
AUDIENCE: Sorry, both
of those companies

1175
00:48:46,936 --> 00:48:48,590
have the normal
refining process?

1176
00:48:48,590 --> 00:48:49,760
PROFESSOR: They have the
normal refining process.

1177
00:48:49,760 --> 00:48:50,801
AUDIENCE: The [INAUDIBLE]

1178
00:48:50,801 --> 00:48:53,640
PROFESSOR: Yup, and that's why
they developed this new one.

1179
00:48:53,640 --> 00:48:55,600
They had these smaller,
internal projects

1180
00:48:55,600 --> 00:48:57,220
that we're able to develop.

1181
00:49:00,750 --> 00:49:01,810
So, yeah.

1182
00:49:01,810 --> 00:49:03,734
I was just mentioning
the energy intensity.

1183
00:49:03,734 --> 00:49:05,400
This is the kilowatt
hours per kilogram,

1184
00:49:05,400 --> 00:49:07,690
going back to your question
about energy intensity.

1185
00:49:07,690 --> 00:49:10,660
This is trichlorosilane
based Siemens process,

1186
00:49:10,660 --> 00:49:12,600
silane based Siemens process.

1187
00:49:12,600 --> 00:49:15,370
They're more ore less comparable
in terms of energy intensity.

1188
00:49:15,370 --> 00:49:18,400
And the silance based
fluidized bed reactor process.

1189
00:49:18,400 --> 00:49:20,180
According to
internal REC numbers,

1190
00:49:20,180 --> 00:49:21,945
which are little rosy,
but never the less,

1191
00:49:21,945 --> 00:49:23,420
the trend is correct here.

1192
00:49:23,420 --> 00:49:25,560
It is lower somewhere
in the order

1193
00:49:25,560 --> 00:49:29,080
of an order of magnitude
energy intensity,

1194
00:49:29,080 --> 00:49:31,620
and cost is lower as well.

1195
00:49:31,620 --> 00:49:36,330
So let's move away from
the silicon refining

1196
00:49:36,330 --> 00:49:38,270
by distillation
process entirely.

1197
00:49:38,270 --> 00:49:41,817
Let's leave gaseous
distillation aside and say,

1198
00:49:41,817 --> 00:49:44,150
what if we were to take this
metallurgical-grade silicon

1199
00:49:44,150 --> 00:49:47,330
and, through liquid
purification routes,

1200
00:49:47,330 --> 00:49:49,450
result in high purity silicon.

1201
00:49:49,450 --> 00:49:50,440
How would we do that?

1202
00:49:50,440 --> 00:49:52,660
Well, if we turn to other
industries, the ones

1203
00:49:52,660 --> 00:49:56,770
that smelter aluminum or
refine manganese and so forth,

1204
00:49:56,770 --> 00:49:59,910
we would see a multitude
of different options

1205
00:49:59,910 --> 00:50:01,070
that we could borrow.

1206
00:50:01,070 --> 00:50:03,940
Slag refining, bleaching,
leaching solidification.

1207
00:50:03,940 --> 00:50:06,780
Let me walk through
them one by one.

1208
00:50:06,780 --> 00:50:09,430
Leaching-- that's
fairly straightforward.

1209
00:50:09,430 --> 00:50:14,370
So if we put in some
acid, for instance,

1210
00:50:14,370 --> 00:50:17,430
that dissolves the metals but
doesn't dissolve the silicon

1211
00:50:17,430 --> 00:50:19,827
we could leach the metals
out of the material,

1212
00:50:19,827 --> 00:50:21,410
and so that's the
essence of leaching.

1213
00:50:21,410 --> 00:50:23,210
You might crush
up your material,

1214
00:50:23,210 --> 00:50:27,270
in other ways other ways expose
the metals, or impurities,

1215
00:50:27,270 --> 00:50:29,830
to the acids inside
of your system.

1216
00:50:29,830 --> 00:50:31,820
Slag refining says,
gee, what if we

1217
00:50:31,820 --> 00:50:34,210
were to introduce some
material that could

1218
00:50:34,210 --> 00:50:36,240
absorb the metals into it?

1219
00:50:36,240 --> 00:50:37,920
The solubility of
the metals would

1220
00:50:37,920 --> 00:50:39,930
be higher inside
of the slag agent

1221
00:50:39,930 --> 00:50:41,490
than inside of the liquid.

1222
00:50:41,490 --> 00:50:45,210
Maybe we throw in calcium
oxide or yttrium oxide or some,

1223
00:50:45,210 --> 00:50:47,810
usually it's a metal oxide
that has a very high melting

1224
00:50:47,810 --> 00:50:51,910
temperature that remains a solid
or, at least a glassy solid,

1225
00:50:51,910 --> 00:50:54,300
and we pour it on
top of our silicon

1226
00:50:54,300 --> 00:50:57,730
and it's able to absorb, say,
the phosphorus or the boron

1227
00:50:57,730 --> 00:51:00,920
that's inside of our silicon so
that we reduce impurity content

1228
00:51:00,920 --> 00:51:02,480
and then we can add the
phosphorous and boron later

1229
00:51:02,480 --> 00:51:04,146
intentionally, but
to the concentrations

1230
00:51:04,146 --> 00:51:07,150
we want not to exuberantly
high concentrations that

1231
00:51:07,150 --> 00:51:08,800
might be found in nature.

1232
00:51:08,800 --> 00:51:11,580
Solidification-- during
this solidification process

1233
00:51:11,580 --> 00:51:13,455
you're taking your
molten silicon

1234
00:51:13,455 --> 00:51:15,080
and you're solidifying
it directionally

1235
00:51:15,080 --> 00:51:18,356
from the bottom up and, because
the solubility of impurities

1236
00:51:18,356 --> 00:51:20,730
tends to be larger in the
liquid than it is in the solid,

1237
00:51:20,730 --> 00:51:23,140
it's like dragging a comb
through the entire material

1238
00:51:23,140 --> 00:51:24,400
dragging out the impurities.

1239
00:51:24,400 --> 00:51:28,610
Concentrating them in the liquid
and leaving a more pure silicon

1240
00:51:28,610 --> 00:51:29,481
behind.

1241
00:51:29,481 --> 00:51:30,980
Obviously, at the
very, very end you

1242
00:51:30,980 --> 00:51:32,980
have this highly
concentrated region

1243
00:51:32,980 --> 00:51:35,680
of impurities which then you
have to slice off and remove,

1244
00:51:35,680 --> 00:51:37,350
so the solicitation
process doesn't

1245
00:51:37,350 --> 00:51:39,089
come without a yield penalty.

1246
00:51:39,089 --> 00:51:40,880
You still throw away
some of your material.

1247
00:51:40,880 --> 00:51:43,244
So you can't repeat the
solidification over and over

1248
00:51:43,244 --> 00:51:44,660
and over again, I
guess you could,

1249
00:51:44,660 --> 00:51:47,140
but you'd be losing
material every step.

1250
00:51:47,140 --> 00:51:49,720
So some combination of these
processes here, and others.

1251
00:51:49,720 --> 00:51:51,450
Other trickery.

1252
00:51:51,450 --> 00:51:55,910
Low temperature eutectic
formation with other elements,

1253
00:51:55,910 --> 00:51:57,230
for example.

1254
00:51:57,230 --> 00:52:00,260
Some combination of this is
used to refine the silicon

1255
00:52:00,260 --> 00:52:02,000
without creating
a gas out of it.

1256
00:52:02,000 --> 00:52:03,830
So wafer fabrication.

1257
00:52:03,830 --> 00:52:05,407
We're now going from
feedstock, we're

1258
00:52:05,407 --> 00:52:06,990
leaving feedstocking
behind, and we're

1259
00:52:06,990 --> 00:52:08,760
going to be talking about how
do you go from the feedstock

1260
00:52:08,760 --> 00:52:10,718
materials that are being
passed around the room

1261
00:52:10,718 --> 00:52:14,010
right now into a wafer that
you can then manufacture

1262
00:52:14,010 --> 00:52:15,330
a solar cell device out of?

1263
00:52:15,330 --> 00:52:16,890
One of these for instance.

1264
00:52:16,890 --> 00:52:20,280
So let's talk about wafer
fabrication right here.

1265
00:52:20,280 --> 00:52:21,980
So again, just to
situate ourselves,

1266
00:52:21,980 --> 00:52:24,730
we've gone from raw materials
to silicon feedstock

1267
00:52:24,730 --> 00:52:27,250
and now we're going to
feedstocks to wafers.

1268
00:52:27,250 --> 00:52:30,240
Any questions right now
before we dive into that?

1269
00:52:30,240 --> 00:52:30,740
Yeah.

1270
00:52:30,740 --> 00:52:32,470
AUDIENCE: Question about supply.

1271
00:52:32,470 --> 00:52:35,552
So silicon is very abundant
but the high purity

1272
00:52:35,552 --> 00:52:38,900
silica deposits-- are
they really abundant too?

1273
00:52:38,900 --> 00:52:40,180
PROFESSOR: Great question.

1274
00:52:40,180 --> 00:52:43,010
So the question was are the
high purity silica deposits

1275
00:52:43,010 --> 00:52:45,400
as abundant as, say, silicon.

1276
00:52:45,400 --> 00:52:45,900
Certainly.

1277
00:52:45,900 --> 00:52:49,120
If you bend over and rub your
fingers against the ground

1278
00:52:49,120 --> 00:52:52,080
you're probably going to
come up with, probably,

1279
00:52:52,080 --> 00:52:55,930
millions of trillions of silicon
atoms in your fingernails.

1280
00:52:55,930 --> 00:52:57,620
Those are not very purity.

1281
00:52:57,620 --> 00:53:01,260
So the highest security
quartz deposits are more rare

1282
00:53:01,260 --> 00:53:03,620
and they are sought after,
and so they're are known.

1283
00:53:03,620 --> 00:53:04,950
Their locations are known.

1284
00:53:04,950 --> 00:53:07,170
There's one specific
one in Norway,

1285
00:53:07,170 --> 00:53:09,310
one specific one
in North Carolina,

1286
00:53:09,310 --> 00:53:13,150
and so forth around the
world and there-- in a sense,

1287
00:53:13,150 --> 00:53:14,140
they go to places.

1288
00:53:14,140 --> 00:53:16,830
People have adjusted their
metallurgical-grade silicon

1289
00:53:16,830 --> 00:53:18,750
refineries and their
subsequent down process

1290
00:53:18,750 --> 00:53:19,922
for that particular ore.

1291
00:53:19,922 --> 00:53:22,130
Once you run out of it, it's
not that the world ends,

1292
00:53:22,130 --> 00:53:26,580
we just have to adjust for
the next feedstock source.

1293
00:53:26,580 --> 00:53:29,640
So, in principle, there
are people looking

1294
00:53:29,640 --> 00:53:31,080
at a variety of silicon inputs.

1295
00:53:31,080 --> 00:53:35,580
Anything from the dirtier,
compressed, metamorphic quartz

1296
00:53:35,580 --> 00:53:36,600
that I mentioned.

1297
00:53:36,600 --> 00:53:39,390
Some people looking
at rice husks,

1298
00:53:39,390 --> 00:53:41,206
which are silica rich as well.

1299
00:53:41,206 --> 00:53:42,580
Other people
looking at seashells

1300
00:53:42,580 --> 00:53:44,996
which, mostly calcium carbonate,
but other things as well.

1301
00:53:44,996 --> 00:53:47,860
I mean, there was a wide range.

1302
00:53:47,860 --> 00:53:50,690
When the price of silicon
was $500 per kilogram,

1303
00:53:50,690 --> 00:53:52,520
you got a multitude of ideas.

1304
00:53:52,520 --> 00:53:54,850
When the price comes
back down, people

1305
00:53:54,850 --> 00:53:57,744
tend to be more conservative.

1306
00:53:57,744 --> 00:54:02,100
AUDIENCE: Is silicon considered
a renewable resource?

1307
00:54:02,100 --> 00:54:04,389
PROFESSOR: Is silicon
considered a renewable resource.

1308
00:54:04,389 --> 00:54:06,180
It is not a renewable
resource in the sense

1309
00:54:06,180 --> 00:54:09,150
that, once you mine
it from the ground,

1310
00:54:09,150 --> 00:54:11,650
you've mined it from the ground
and you used in some way.

1311
00:54:11,650 --> 00:54:14,740
The reason it's
considered not an issue

1312
00:54:14,740 --> 00:54:16,920
is because there's
so much of it.

1313
00:54:16,920 --> 00:54:20,610
Not all of it, though, is
in the easy to access form.

1314
00:54:20,610 --> 00:54:21,110
Right?

1315
00:54:21,110 --> 00:54:22,651
Some of the silicon
might be bound up

1316
00:54:22,651 --> 00:54:26,200
within heavily
contaminated sources

1317
00:54:26,200 --> 00:54:30,800
and that's where the refining
ingenuity comes into play.

1318
00:54:30,800 --> 00:54:34,210
As long as prices remain low,
there's not too much interest,

1319
00:54:34,210 --> 00:54:36,100
say, for example,
that mine in Peru

1320
00:54:36,100 --> 00:54:42,780
that has titanium oxide needles
throughout their silicon

1321
00:54:42,780 --> 00:54:44,960
because why would
you want heavily

1322
00:54:44,960 --> 00:54:47,070
titanium contaminated silicon?

1323
00:54:47,070 --> 00:54:50,030
But as the price of
silicon, it probably will,

1324
00:54:50,030 --> 00:54:52,800
rise again then people might
take another look at that mine

1325
00:54:52,800 --> 00:54:55,380
and say gee, how can we
phase separate the rutile

1326
00:54:55,380 --> 00:54:57,240
and anatase from
the quartz early

1327
00:54:57,240 --> 00:54:59,110
on in the process by
crushing and etching

1328
00:54:59,110 --> 00:55:04,050
or something so we can access
this feedstock material.

1329
00:55:04,050 --> 00:55:06,090
We'll see.

1330
00:55:06,090 --> 00:55:08,950
It really depends on how the
market evolves, where people

1331
00:55:08,950 --> 00:55:11,385
go looking for their silicon,
but there's a lot of it

1332
00:55:11,385 --> 00:55:12,260
in the earth's crust.

1333
00:55:12,260 --> 00:55:14,936
AUDIENCE: You're not
concerned about silicon?

1334
00:55:14,936 --> 00:55:15,830
PROFESSOR: No.

1335
00:55:15,830 --> 00:55:16,850
Nope.

1336
00:55:16,850 --> 00:55:20,280
What is a bigger bottleneck
are are the refining steps

1337
00:55:20,280 --> 00:55:21,090
in between.

1338
00:55:21,090 --> 00:55:23,854
First it was the
reactors and soon it's

1339
00:55:23,854 --> 00:55:26,020
probably going to be the
metallurgical-grade silicon

1340
00:55:26,020 --> 00:55:28,950
reactors as well.

1341
00:55:28,950 --> 00:55:29,600
All right.

1342
00:55:29,600 --> 00:55:30,840
Wafers.

1343
00:55:30,840 --> 00:55:33,020
How do we get to these
from the raw feedstock

1344
00:55:33,020 --> 00:55:36,260
materials that are being passed
around the room right now?

1345
00:55:36,260 --> 00:55:38,741
So single crystalline
silicon ingot growth.

1346
00:55:38,741 --> 00:55:39,990
Let's walk through that first.

1347
00:55:39,990 --> 00:55:42,160
How do we get these
beautiful ingots?

1348
00:55:42,160 --> 00:55:45,910
They're about half of all
silicon market right now.

1349
00:55:45,910 --> 00:55:48,250
The biggest growth
method, by far,

1350
00:55:48,250 --> 00:55:50,530
is called Czochralski
growth and, named

1351
00:55:50,530 --> 00:55:54,920
after the Polish physicist
there Jan Czochralski.

1352
00:55:54,920 --> 00:55:58,320
What you do is you have
a bath of molten silicon.

1353
00:55:58,320 --> 00:55:59,840
A crucible, if you will.

1354
00:55:59,840 --> 00:56:01,480
This tends to be a
circular crucible,

1355
00:56:01,480 --> 00:56:03,150
rounded at the
bottom, usually made

1356
00:56:03,150 --> 00:56:05,195
of quartz with
heaters on the outside

1357
00:56:05,195 --> 00:56:07,070
to heat up the molten silicon.

1358
00:56:07,070 --> 00:56:09,180
To heat up the silicon
chunks in here.

1359
00:56:09,180 --> 00:56:10,640
Once everything
is molten, looking

1360
00:56:10,640 --> 00:56:12,370
like a big bathtub
of silicon, you

1361
00:56:12,370 --> 00:56:15,890
introduce a small
crystalline silicon seed

1362
00:56:15,890 --> 00:56:18,450
into that molten
silicon and then

1363
00:56:18,450 --> 00:56:20,880
you begin pulling while
rotating that seed.

1364
00:56:20,880 --> 00:56:22,992
So the seed is a
single crystal material

1365
00:56:22,992 --> 00:56:24,950
and what ends up happening
is, as you introduce

1366
00:56:24,950 --> 00:56:27,020
the seed into the material
and begin pulling,

1367
00:56:27,020 --> 00:56:29,000
you start pulling
out this crystal.

1368
00:56:29,000 --> 00:56:30,900
Single crystalline crystal.

1369
00:56:30,900 --> 00:56:33,850
It's a thing of beauty
and this seed is actually

1370
00:56:33,850 --> 00:56:35,790
very, very narrow in diameter.

1371
00:56:35,790 --> 00:56:38,370
It might be about that big
around so pretty narrow

1372
00:56:38,370 --> 00:56:40,830
in diameter and it's
being able to support

1373
00:56:40,830 --> 00:56:43,870
this ingot of a
few, usually a few,

1374
00:56:43,870 --> 00:56:46,310
tens to hundreds of kilograms
of mass underneath it

1375
00:56:46,310 --> 00:56:49,790
and that's because silicon is
very strong even though it's

1376
00:56:49,790 --> 00:56:51,260
brittle.

1377
00:56:51,260 --> 00:56:55,100
So if you weren't to apply,
say, for example, a shear

1378
00:56:55,100 --> 00:56:57,850
force on your silicon but
just to apply an axial load,

1379
00:56:57,850 --> 00:57:00,570
you could support a very,
large weight underneath it.

1380
00:57:00,570 --> 00:57:04,140
So the [INAUDIBLE] of silicon
is grown from the bottom

1381
00:57:04,140 --> 00:57:06,640
and eventually you wind
up with this nice ingot,

1382
00:57:06,640 --> 00:57:08,290
as shown right there.

1383
00:57:08,290 --> 00:57:12,690
The art that goes into growing
this properly is amazing.

1384
00:57:12,690 --> 00:57:15,770
I'll highlight it with one
small little example just

1385
00:57:15,770 --> 00:57:18,630
to illustrate the bigger
picture that a lot of effort

1386
00:57:18,630 --> 00:57:21,340
goes into making these
defect-free, quote unquote,

1387
00:57:21,340 --> 00:57:22,487
defect-free crystals.

1388
00:57:22,487 --> 00:57:24,820
They're called defect-free
because they contain no grain

1389
00:57:24,820 --> 00:57:27,080
boundaries and no dislocations.

1390
00:57:27,080 --> 00:57:30,490
They have impurities, they
have intrinsic point defects,

1391
00:57:30,490 --> 00:57:32,394
meaning vacancies or
interstitial atoms,

1392
00:57:32,394 --> 00:57:34,560
but they don't have grain
boundaries or dislocations

1393
00:57:34,560 --> 00:57:37,930
and so they're called
defect-free silicon.

1394
00:57:37,930 --> 00:57:40,380
You introduce that seed
down into the liquid melt.

1395
00:57:40,380 --> 00:57:41,370
Thermal stress happens.

1396
00:57:41,370 --> 00:57:41,870
Right?

1397
00:57:41,870 --> 00:57:47,690
Because you have the shock
between the solid silicon seed

1398
00:57:47,690 --> 00:57:49,550
encountering the liquid
for the first time.

1399
00:57:49,550 --> 00:57:51,040
So this locations [? form ?].

1400
00:57:51,040 --> 00:57:54,040
And you have to pull the
seed out in such a way,

1401
00:57:54,040 --> 00:57:57,040
you slowly rotate and
make this shoulder.

1402
00:57:57,040 --> 00:57:59,130
The shoulder has to be
as quick as possible

1403
00:57:59,130 --> 00:58:01,960
because you don't want
to waste material.

1404
00:58:01,960 --> 00:58:05,140
Everything inside this shoulder
right here gets thrown away.

1405
00:58:05,140 --> 00:58:07,920
So that little piece of material
right there gets tossed out.

1406
00:58:07,920 --> 00:58:10,800
So you want to make the
shoulders as narrow and as

1407
00:58:10,800 --> 00:58:12,450
quick as possible
so you can utilize

1408
00:58:12,450 --> 00:58:14,140
the majority of your ingot
but, at the same time,

1409
00:58:14,140 --> 00:58:15,690
you have to make
it thick enough so

1410
00:58:15,690 --> 00:58:17,850
that the dislocations
can move all the way

1411
00:58:17,850 --> 00:58:19,920
and propagate all the
way to the outside

1412
00:58:19,920 --> 00:58:21,960
and end and terminate
in the shoulder

1413
00:58:21,960 --> 00:58:24,420
before propagating
into the crystal.

1414
00:58:24,420 --> 00:58:26,849
So that's just one
example of the technology

1415
00:58:26,849 --> 00:58:28,140
that goes in the growing these.

1416
00:58:28,140 --> 00:58:30,660
Another might be, gee,
we're PV industry,

1417
00:58:30,660 --> 00:58:34,220
we want to make the stuff fast
whereas, in the IC industry

1418
00:58:34,220 --> 00:58:38,060
you can invest up to a few of
dollars per gram of silicon

1419
00:58:38,060 --> 00:58:40,110
and still make a profit
because you're selling

1420
00:58:40,110 --> 00:58:41,660
a computer at 1,000 bucks.

1421
00:58:41,660 --> 00:58:43,500
In the PV industry,
we can invest,

1422
00:58:43,500 --> 00:58:46,122
at most, a few tens of
cents per gram of silicon.

1423
00:58:46,122 --> 00:58:47,580
So we have to make
this stuff fast.

1424
00:58:47,580 --> 00:58:48,617
We can't dilly dally.

1425
00:58:48,617 --> 00:58:50,450
You might want to crank
up the growth speed,

1426
00:58:50,450 --> 00:58:52,480
then you run into issues
with defect concentrations,

1427
00:58:52,480 --> 00:58:54,080
intrinsic point
defect concentrations,

1428
00:58:54,080 --> 00:58:55,410
during the growth.

1429
00:58:55,410 --> 00:58:57,370
I'm illustrating this
just to highlight

1430
00:58:57,370 --> 00:59:01,820
the complexity of the growth
process of making these ingots,

1431
00:59:01,820 --> 00:59:04,560
and the latter example was
one that the PV industry

1432
00:59:04,560 --> 00:59:05,460
is facing today.

1433
00:59:05,460 --> 00:59:06,925
It's actually a
hot research topic.

1434
00:59:06,925 --> 00:59:07,425
Yeah.

1435
00:59:07,425 --> 00:59:08,091
And then Ashley.

1436
00:59:08,091 --> 00:59:11,074
AUDIENCE: The rotation speed
does that just affect time,

1437
00:59:11,074 --> 00:59:12,490
or does it affect other things?

1438
00:59:12,490 --> 00:59:15,036
PROFESSOR: So it affects
a multitude of things.

1439
00:59:15,036 --> 00:59:16,410
One of the things
that it affects

1440
00:59:16,410 --> 00:59:19,660
is the flow of, the
convective flow, of the melt.

1441
00:59:19,660 --> 00:59:21,540
So the liquid flow
inside of the melt

1442
00:59:21,540 --> 00:59:24,470
is, in part, determining
how much oxygen

1443
00:59:24,470 --> 00:59:26,430
gets transported
from this crucible

1444
00:59:26,430 --> 00:59:28,360
here into the growing crystal.

1445
00:59:28,360 --> 00:59:31,290
If you manage to suppress that
convective flow in the melt,

1446
00:59:31,290 --> 00:59:33,797
you will also suppress
oxygen transport

1447
00:59:33,797 --> 00:59:35,630
since the fusion is
going to be a lot slower

1448
00:59:35,630 --> 00:59:40,920
than turbulent transport
or [INAUDIBLE] transport

1449
00:59:40,920 --> 00:59:42,811
or convective transport.

1450
00:59:42,811 --> 00:59:43,310
Yeah.

1451
00:59:43,310 --> 00:59:43,600
Question?

1452
00:59:43,600 --> 00:59:44,860
AUDIENCE: I just
have two questions

1453
00:59:44,860 --> 00:59:46,510
so one is how fast
do you rotate it

1454
00:59:46,510 --> 00:59:49,430
and the other is what that
does control-- the diameter

1455
00:59:49,430 --> 00:59:51,286
because I've heard
of 12 inch wafers

1456
00:59:51,286 --> 00:59:53,150
versus like 18 inch wafers.

1457
00:59:53,150 --> 00:59:53,980
PROFESSOR: Sure.

1458
00:59:53,980 --> 00:59:56,210
So one of the things
that controls diameter

1459
00:59:56,210 --> 01:00:00,750
is the balance of
heat extraction.

1460
01:00:00,750 --> 01:00:03,160
So if you cool something down,
especially molten silicon,

1461
01:00:03,160 --> 01:00:04,560
it will freeze, it will grow.

1462
01:00:04,560 --> 01:00:06,330
If you heat it up,
it will shrink.

1463
01:00:06,330 --> 01:00:08,930
So that's one of the components
that controls the diameter.

1464
01:00:08,930 --> 01:00:12,850
The pull speed and how
you grow that shoulder,

1465
01:00:12,850 --> 01:00:14,700
essentially how you
heat up the material

1466
01:00:14,700 --> 01:00:17,020
and how fast you pull
at those initial stages,

1467
01:00:17,020 --> 01:00:18,440
also dictates the
diameter and you

1468
01:00:18,440 --> 01:00:20,110
can see in the
ingots themselves,

1469
01:00:20,110 --> 01:00:21,140
they're not perfect.

1470
01:00:21,140 --> 01:00:23,260
They have a little
bit of corregation

1471
01:00:23,260 --> 01:00:27,010
and that's the fluctuations of
the temperature of the melt,

1472
01:00:27,010 --> 01:00:28,710
fluctuations of
the heater output,

1473
01:00:28,710 --> 01:00:32,540
fluctuations of pull
speed, maybe what's pulling

1474
01:00:32,540 --> 01:00:35,760
this entire contraption
is kind of a stepper motor

1475
01:00:35,760 --> 01:00:38,500
that has a certain
granularity to it.

1476
01:00:38,500 --> 01:00:40,770
Results in corregated edges.

1477
01:00:40,770 --> 01:00:42,530
It's not perfect
and so there will

1478
01:00:42,530 --> 01:00:47,640
be some adjustment made to
the form factor of the edge

1479
01:00:47,640 --> 01:00:50,910
to get this nice round
wafer at the end of the day.

1480
01:00:50,910 --> 01:00:52,910
AUDIENCE: Does the
seed rod [INAUDIBLE]

1481
01:00:52,910 --> 01:00:54,965
all the way to the ingot
or just near the top?

1482
01:00:54,965 --> 01:00:55,840
PROFESSOR: All right.

1483
01:00:55,840 --> 01:00:57,540
So the entire ingot
becomes pattern

1484
01:00:57,540 --> 01:00:59,200
or templated by the seed rod.

1485
01:00:59,200 --> 01:01:01,750
So this entire ingot has the
same crystalline orientation

1486
01:01:01,750 --> 01:01:02,673
as a seed.

1487
01:01:02,673 --> 01:01:07,650
AUDIENCE: And is the seed doped
differently than the silicon?

1488
01:01:07,650 --> 01:01:09,490
PROFESSOR: It might
be but I'm not aware

1489
01:01:09,490 --> 01:01:11,600
that that affects
the overall process.

1490
01:01:11,600 --> 01:01:13,930
It could be that it's one
of the critical pieces

1491
01:01:13,930 --> 01:01:16,263
of the magic sauce that makes
it work but I'm not aware.

1492
01:01:19,730 --> 01:01:20,530
Rotation speed.

1493
01:01:20,530 --> 01:01:23,620
It's not rotating like
this it's a slow rotation

1494
01:01:23,620 --> 01:01:26,652
so I would-- let's see.

1495
01:01:26,652 --> 01:01:27,860
How many radians per second--

1496
01:01:27,860 --> 01:01:28,910
AUDIENCE: Can you see it?

1497
01:01:28,910 --> 01:01:30,185
PROFESSOR: You can
visually see it

1498
01:01:30,185 --> 01:01:31,518
if you looked at it long enough.

1499
01:01:31,518 --> 01:01:32,100
Yeah.

1500
01:01:32,100 --> 01:01:34,330
Yeah.

1501
01:01:34,330 --> 01:01:36,540
So one modification,
one variant,

1502
01:01:36,540 --> 01:01:38,760
of the single
crystalline growth method

1503
01:01:38,760 --> 01:01:40,100
is called float-zone growth.

1504
01:01:40,100 --> 01:01:44,100
You take a rod of poly, much
like that right over there

1505
01:01:44,100 --> 01:01:49,720
that's inside of here,
and you pass an RF coil,

1506
01:01:49,720 --> 01:01:52,220
radio frequency
coil, next to the rod

1507
01:01:52,220 --> 01:01:54,140
and what that does is,
essentially, heats up

1508
01:01:54,140 --> 01:01:56,870
the silicon, if it's
doped highly enough.

1509
01:01:56,870 --> 01:01:59,600
It will melt the
silicon locally.

1510
01:01:59,600 --> 01:02:03,230
Folks have probably heard
of fancy high-end stoves

1511
01:02:03,230 --> 01:02:07,130
that we can only probably hope
to afford in 10 or 15 years,

1512
01:02:07,130 --> 01:02:11,560
but these stoves that
are inductive heaters.

1513
01:02:11,560 --> 01:02:12,060
Right?

1514
01:02:12,060 --> 01:02:15,334
They're not resistive
heating elements,

1515
01:02:15,334 --> 01:02:17,250
they're inductive heating
elements and the way

1516
01:02:17,250 --> 01:02:21,860
that works is you have a radio
frequency source that then

1517
01:02:21,860 --> 01:02:25,870
is absorbed by, in the
case of the RF heater,

1518
01:02:25,870 --> 01:02:28,630
I believe it's a
specific type of iron

1519
01:02:28,630 --> 01:02:34,580
that the inductive
heating ovens need.

1520
01:02:34,580 --> 01:02:38,160
And so this RF coil here
is emitting energy, which

1521
01:02:38,160 --> 01:02:40,440
is absorbed by the
silicon and melting it,

1522
01:02:40,440 --> 01:02:44,100
and you start with the
polycrystalline rod coming

1523
01:02:44,100 --> 01:02:47,210
from the Siemens process
and in that case,

1524
01:02:47,210 --> 01:02:51,540
this rough, corrugated
material right here won't do.

1525
01:02:51,540 --> 01:02:52,070
Right?

1526
01:02:52,070 --> 01:02:54,360
This is too rough
for that RF coil

1527
01:02:54,360 --> 01:02:57,620
to pass over and be a
consistent distance away.

1528
01:02:57,620 --> 01:02:59,070
In the case of
float-zone growth,

1529
01:02:59,070 --> 01:03:02,687
you actually have to modify your
polysilicon production process.

1530
01:03:02,687 --> 01:03:04,270
You have to modify
the Siemens process

1531
01:03:04,270 --> 01:03:08,030
so that you get a
nice smooth rod, which

1532
01:03:08,030 --> 01:03:11,080
you can then pass the
RF coil next to and melt

1533
01:03:11,080 --> 01:03:13,360
and you again start with
a seed at the bottom,

1534
01:03:13,360 --> 01:03:16,600
your RF coil starts down here
and then the RF coil moves

1535
01:03:16,600 --> 01:03:18,950
through the material, almost
like a comb from the bottom

1536
01:03:18,950 --> 01:03:21,700
to the top, converting
the polysilicon

1537
01:03:21,700 --> 01:03:27,240
into nice single crystalline
material and, in the process,

1538
01:03:27,240 --> 01:03:29,560
it concentrates impurities
in this liquid region.

1539
01:03:29,560 --> 01:03:31,893
Since the liquids have a
higher solubility in the liquid

1540
01:03:31,893 --> 01:03:34,440
than they do in the solid,
the impurities are then

1541
01:03:34,440 --> 01:03:37,550
aggregated inside of the liquid
region and, again, like a comb,

1542
01:03:37,550 --> 01:03:40,050
they just get swept
out of the material.

1543
01:03:40,050 --> 01:03:41,810
Not all of them, but
a large percentage

1544
01:03:41,810 --> 01:03:43,510
of them, and so you
can make multiple

1545
01:03:43,510 --> 01:03:46,605
passes with this RF coil
to further concentrate

1546
01:03:46,605 --> 01:03:48,490
the impurities and
the extremities

1547
01:03:48,490 --> 01:03:50,990
and remove them
from the material.

1548
01:03:50,990 --> 01:03:52,240
So that's a float-zone method.

1549
01:03:52,240 --> 01:03:55,609
Very expensive material,
very high purity.

1550
01:03:55,609 --> 01:03:57,150
One of the reasons
it has high purity

1551
01:03:57,150 --> 01:04:00,310
is because you don't have
this quartz crucible nearby,

1552
01:04:00,310 --> 01:04:01,760
you don't have
this molten silicon

1553
01:04:01,760 --> 01:04:03,990
that's absorbing or
dissolving the quartz

1554
01:04:03,990 --> 01:04:06,920
and transporting the
oxygen into your crystal.

1555
01:04:06,920 --> 01:04:09,100
You have much lower carbon
and oxygen concentrations

1556
01:04:09,100 --> 01:04:10,590
to [INAUDIBLE]
float-zone material.

1557
01:04:10,590 --> 01:04:13,360
So if anybody is doing
experiments with silicon,

1558
01:04:13,360 --> 01:04:16,090
for whatever reason, using
it as a substrate material,

1559
01:04:16,090 --> 01:04:19,190
you want to think carefully
about what type of silicon you

1560
01:04:19,190 --> 01:04:21,490
source and from
where you source it.

1561
01:04:21,490 --> 01:04:23,792
You can find some very
poor quality silicon

1562
01:04:23,792 --> 01:04:25,250
out there in the
market, especially

1563
01:04:25,250 --> 01:04:26,930
if you going into
the aftersale market,

1564
01:04:26,930 --> 01:04:28,764
and we know this from some--

1565
01:04:28,764 --> 01:04:29,680
AUDIENCE: [INAUDIBLE].

1566
01:04:29,680 --> 01:04:32,600
PROFESSOR: --very
painful experiences.

1567
01:04:32,600 --> 01:04:35,055
And so there are
some better sources

1568
01:04:35,055 --> 01:04:36,680
from which to get
your wafers and we're

1569
01:04:36,680 --> 01:04:39,670
happy to talk
about that offline.

1570
01:04:39,670 --> 01:04:42,019
So, again, single
crystalline silicon.

1571
01:04:42,019 --> 01:04:43,560
We're going to
venture into the world

1572
01:04:43,560 --> 01:04:46,240
of multicrystalline silicon
ever so briefly here.

1573
01:04:46,240 --> 01:04:48,050
First, we'll start
about cast material

1574
01:04:48,050 --> 01:04:50,200
and, just to emphasize
here, we have

1575
01:04:50,200 --> 01:04:51,820
regions of crystalline
material that

1576
01:04:51,820 --> 01:04:54,737
have grain boundaries
separating the adjacent grains

1577
01:04:54,737 --> 01:04:56,820
and the reason we go into
multicrystalline silicon

1578
01:04:56,820 --> 01:05:00,110
is really oftentimes, it
is a lower cost method

1579
01:05:00,110 --> 01:05:03,221
of producing a silicon wafer
although you have the grain

1580
01:05:03,221 --> 01:05:03,720
boundaries.

1581
01:05:03,720 --> 01:05:06,980
So, again, single crystalline,
Czochralski and float-zone, you

1582
01:05:06,980 --> 01:05:10,030
wind up with round
wafers, typically

1583
01:05:10,030 --> 01:05:12,880
single crystalline variety, and
multicrystalline silicon wafers

1584
01:05:12,880 --> 01:05:19,680
tend to be more square-like
and more visibly multi-grained,

1585
01:05:19,680 --> 01:05:21,180
if you will.

1586
01:05:21,180 --> 01:05:22,860
So let's talk about
those for a minute.

1587
01:05:22,860 --> 01:05:26,880
How do you make a
multicrystalline silicon wafer?

1588
01:05:26,880 --> 01:05:31,000
Again, you would start with
the solar-grade silicon that

1589
01:05:31,000 --> 01:05:33,150
could either be coming
from the Siemens process,

1590
01:05:33,150 --> 01:05:35,233
it could be coming from
the fluidized bed reactor,

1591
01:05:35,233 --> 01:05:36,710
it could be coming
from an upgraded

1592
01:05:36,710 --> 01:05:38,876
metallurgical-grade silicon,
the liquid purification

1593
01:05:38,876 --> 01:05:41,870
route but, somehow, some way,
you get chunks of silicon,

1594
01:05:41,870 --> 01:05:44,292
or granules of silicon, that
have a high enough purity

1595
01:05:44,292 --> 01:05:45,750
for you to make
solar cells out of,

1596
01:05:45,750 --> 01:05:47,270
and high enough
purity is typically

1597
01:05:47,270 --> 01:05:50,510
in the order of one part per
million impurity content.

1598
01:05:50,510 --> 01:05:53,500
So you put your solar-grade
silicon into a crucible

1599
01:05:53,500 --> 01:05:55,750
and then you melt the
silicon inside of it.

1600
01:05:55,750 --> 01:05:58,522
Silicon melts at
1,414 degrees Celsius.

1601
01:05:58,522 --> 01:05:59,730
It's a very high temperature.

1602
01:05:59,730 --> 01:06:04,600
So 1,414 degrees Celsius is the
melting temperature of silicon.

1603
01:06:04,600 --> 01:06:05,940
And then it's cooled.

1604
01:06:05,940 --> 01:06:09,140
Not just randomly, but from
the bottom up and the reason

1605
01:06:09,140 --> 01:06:11,790
it's cooled from the
bottom up is because,

1606
01:06:11,790 --> 01:06:15,130
and here I guess you'll actually
have to come up and see this

1607
01:06:15,130 --> 01:06:18,200
after class, it's rather
difficult to see from here,

1608
01:06:18,200 --> 01:06:20,447
but this is a cross
section of a small ingot.

1609
01:06:20,447 --> 01:06:22,030
This is the outside
of the ingot where

1610
01:06:22,030 --> 01:06:25,210
it was contacting the
wall, these little pieces

1611
01:06:25,210 --> 01:06:27,050
of white stuff that
are flaking off,

1612
01:06:27,050 --> 01:06:32,290
this is the fused quartz silica
that forms the crucible wall,

1613
01:06:32,290 --> 01:06:34,770
and the silicon
nitride coating that

1614
01:06:34,770 --> 01:06:37,000
form the anti-stick
coating that prevented

1615
01:06:37,000 --> 01:06:39,484
the silicon from
sticking to the crucible,

1616
01:06:39,484 --> 01:06:41,150
and so it's kind of
rough and corrugated

1617
01:06:41,150 --> 01:06:44,210
but, if we were to rotate this
around and look at the inside,

1618
01:06:44,210 --> 01:06:47,269
this here is a cross
section of the actual ingot

1619
01:06:47,269 --> 01:06:49,060
from the inside and,
if you look carefully,

1620
01:06:49,060 --> 01:06:51,170
you'll see grains growing
from the bottom to the top.

1621
01:06:51,170 --> 01:06:52,760
You probably can't
see them from here,

1622
01:06:52,760 --> 01:06:54,450
you'll have to come up
after class and take a look,

1623
01:06:54,450 --> 01:06:57,070
but the grains are growing
from the bottom to the top

1624
01:06:57,070 --> 01:06:59,880
and that is called directional
solidification, or the result

1625
01:06:59,880 --> 01:07:01,130
of directional solidification.

1626
01:07:01,130 --> 01:07:02,130
Directional
solidification is when

1627
01:07:02,130 --> 01:07:03,870
you solidify from
the bottom to the top

1628
01:07:03,870 --> 01:07:05,230
and, typically, your
grain boundaries

1629
01:07:05,230 --> 01:07:06,771
are going to be
running perpendicular

1630
01:07:06,771 --> 01:07:09,300
to the solid-liquid interface,
so your grain boundaries

1631
01:07:09,300 --> 01:07:12,010
will be running up like this
as you grow your material

1632
01:07:12,010 --> 01:07:13,660
from the bottom to the top.

1633
01:07:13,660 --> 01:07:16,660
If you were to do
uncontrolled solidification

1634
01:07:16,660 --> 01:07:18,457
and all walls would
freeze the same time,

1635
01:07:18,457 --> 01:07:20,290
you'd have grains growing
in from the sides,

1636
01:07:20,290 --> 01:07:22,248
you'd have grains growing
in through the bottom

1637
01:07:22,248 --> 01:07:24,740
and then, when you slice
your wafer out horizontally,

1638
01:07:24,740 --> 01:07:26,990
the grain boundaries wouldn't
be running perpendicular

1639
01:07:26,990 --> 01:07:27,710
to the surface.

1640
01:07:27,710 --> 01:07:30,260
They might be running parallel
to the surface, in which case

1641
01:07:30,260 --> 01:07:32,610
they could wreck havoc on your
minority care diffusion length.

1642
01:07:32,610 --> 01:07:34,155
Imagine you being
an electron having

1643
01:07:34,155 --> 01:07:37,600
to travel across that grain
boundary that's between you

1644
01:07:37,600 --> 01:07:39,270
and the P-N junction.

1645
01:07:39,270 --> 01:07:41,720
Whereas, if the grain boundaries
are running perpendicular

1646
01:07:41,720 --> 01:07:43,390
to the surfaces,
now they're only

1647
01:07:43,390 --> 01:07:47,570
affecting very small areas of
the entire solar cell wafer.

1648
01:07:47,570 --> 01:07:49,170
So when I pick up
a wafer like this,

1649
01:07:49,170 --> 01:07:53,200
this wafer was chopped
from the ingot this way

1650
01:07:53,200 --> 01:07:55,090
or, to put it into
perspective here,

1651
01:07:55,090 --> 01:07:58,240
this wafer was sliced
out like that from this.

1652
01:07:58,240 --> 01:08:00,400
So the grain boundaries
were running perpendicular

1653
01:08:00,400 --> 01:08:04,270
to the surfaces and that way
they don't impede as much

1654
01:08:04,270 --> 01:08:06,690
with electron transport.

1655
01:08:06,690 --> 01:08:08,790
So the multicrystalline
silicon ingot is formed.

1656
01:08:08,790 --> 01:08:11,230
The ingot is then chopped
into these blocks,

1657
01:08:11,230 --> 01:08:17,330
usually between 16 and 24, that
means four bricks to an edge

1658
01:08:17,330 --> 01:08:19,270
or five bricks to an edge.

1659
01:08:19,270 --> 01:08:23,010
Some folks are exploring
six by six, so 36 bricks,

1660
01:08:23,010 --> 01:08:26,300
and then the bricks are
rotated on their side

1661
01:08:26,300 --> 01:08:30,020
and then sliced into wafers
and individual wafers come out.

1662
01:08:30,020 --> 01:08:34,689
So you can see the wafers
I've sliced from the bricks

1663
01:08:34,689 --> 01:08:35,960
as I showed you right here.

1664
01:08:35,960 --> 01:08:38,700
Is this diagram clear to folks?

1665
01:08:38,700 --> 01:08:40,300
In general since--
any confusions?

1666
01:08:40,300 --> 01:08:41,859
Any questions?

1667
01:08:41,859 --> 01:08:42,359
No.

1668
01:08:42,359 --> 01:08:43,760
AUDIENCE: How do
they cut the wafers?

1669
01:08:43,760 --> 01:08:45,176
PROFESSOR: How do
they cut wafers!

1670
01:08:45,176 --> 01:08:48,460
So this is a process called
wire sawing sign and this is

1671
01:08:48,460 --> 01:08:50,229
one of the most
beautiful technologies

1672
01:08:50,229 --> 01:08:52,939
because it was invented
in the PV industry

1673
01:08:52,939 --> 01:08:56,646
and transported back,
adopted by the IC industry.

1674
01:08:56,646 --> 01:08:58,520
So it's one of the few
examples of technology

1675
01:08:58,520 --> 01:08:59,760
that went the other way.

1676
01:08:59,760 --> 01:09:01,330
Let me get to that point.

1677
01:09:01,330 --> 01:09:02,890
AUDIENCE: How was
it done before?

1678
01:09:02,890 --> 01:09:04,939
PROFESSOR: It was
done by ID saws,

1679
01:09:04,939 --> 01:09:06,689
for instance, inner
diameter saws,

1680
01:09:06,689 --> 01:09:10,974
that would slice off wafers
like a wafer off of a salami.

1681
01:09:10,974 --> 01:09:13,604
AUDIENCE: So not a
wire but like a disk?

1682
01:09:13,604 --> 01:09:14,729
PROFESSOR: Like a disk saw.

1683
01:09:14,729 --> 01:09:15,229
Yeah.

1684
01:09:15,229 --> 01:09:16,229
Exactly.

1685
01:09:16,229 --> 01:09:19,450
Like the inner diameter meaning
your saw is like a rotating

1686
01:09:19,450 --> 01:09:22,351
blade and you're just
using, you know-- Yeah.

1687
01:09:22,351 --> 01:09:22,850
OK.

1688
01:09:22,850 --> 01:09:25,880
So directional solidification
of multicrystalline silicon.

1689
01:09:25,880 --> 01:09:29,569
This is a cross section of a
furnace that is solidifying

1690
01:09:29,569 --> 01:09:30,470
an ingot right here.

1691
01:09:30,470 --> 01:09:31,470
Here's your ingot.

1692
01:09:31,470 --> 01:09:34,109
This is a liquid silicon
and, essentially, it's

1693
01:09:34,109 --> 01:09:36,960
solidifying from the
bottom to the top

1694
01:09:36,960 --> 01:09:39,540
and, hopefully, we'll have
a tour of one of the world's

1695
01:09:39,540 --> 01:09:45,012
largest ingot solidification
furnace manufacturing

1696
01:09:45,012 --> 01:09:45,970
companies in the world.

1697
01:09:45,970 --> 01:09:47,594
So, they don't
manufacture the silicon,

1698
01:09:47,594 --> 01:09:50,619
they manufacture the furnace
that manufactures the silicon.

1699
01:09:50,619 --> 01:09:51,655
If that makes sense.

1700
01:09:51,655 --> 01:09:52,810
AUDIENCE: Do they also
make the crucible?

1701
01:09:52,810 --> 01:09:53,130
PROFESSOR: No.

1702
01:09:53,130 --> 01:09:54,180
That would be Vesuvius.

1703
01:09:54,180 --> 01:09:54,680
Yeah.

1704
01:09:54,680 --> 01:09:58,130
It would be other companies
that make the crucibles.

1705
01:09:58,130 --> 01:10:01,090
And these are some of
the furnaces right here.

1706
01:10:01,090 --> 01:10:02,230
The keyboard and monitor.

1707
01:10:02,230 --> 01:10:03,750
For size comparison, stairs.

1708
01:10:03,750 --> 01:10:05,630
So they're about
two stories tall.

1709
01:10:05,630 --> 01:10:07,890
You can go up here to the
top and look down into them.

1710
01:10:07,890 --> 01:10:09,077
It's pretty cool.

1711
01:10:09,077 --> 01:10:11,160
Using a little infrared
lens to block out the heat

1712
01:10:11,160 --> 01:10:14,460
so you don't get blinded
and the furnace itself--

1713
01:10:14,460 --> 01:10:16,310
all the action happens
inside of here.

1714
01:10:16,310 --> 01:10:19,910
The top can lift-- typically,
they're the bottom loaded.

1715
01:10:19,910 --> 01:10:22,370
You'll see this little
seal right here.

1716
01:10:22,370 --> 01:10:24,110
So this bottom part
typically comes down

1717
01:10:24,110 --> 01:10:26,026
because you want to trap
the heat inside of it

1718
01:10:26,026 --> 01:10:29,690
so you're not losing all that
and the bottom is removed,

1719
01:10:29,690 --> 01:10:33,540
the forklift comes in, picks
up this ingot and crucible

1720
01:10:33,540 --> 01:10:36,280
which could be a few of
kilograms in mass-- up

1721
01:10:36,280 --> 01:10:39,210
to about 600, maybe even
a ton-- and removes it

1722
01:10:39,210 --> 01:10:41,415
and places in the
proper location.

1723
01:10:41,415 --> 01:10:43,310
It's a pretty dirty environment.

1724
01:10:43,310 --> 01:10:46,030
The operator will typically
take a garden hose

1725
01:10:46,030 --> 01:10:48,240
and hose it down
inside afterward.

1726
01:10:48,240 --> 01:10:51,150
It's really an
antithesis of an IC

1727
01:10:51,150 --> 01:10:53,420
fab at this stage right here.

1728
01:10:53,420 --> 01:10:56,830
These are graphite
insulation materials

1729
01:10:56,830 --> 01:11:01,310
on the sides of the crucible
and this yellowish dust

1730
01:11:01,310 --> 01:11:03,830
that you see everywhere
is silica, again.

1731
01:11:03,830 --> 01:11:09,270
That nice fine grained dust
that's bad for you lungs.

1732
01:11:09,270 --> 01:11:12,350
The directional
solidification process

1733
01:11:12,350 --> 01:11:15,450
can be, to some degrees,
used interchangeably

1734
01:11:15,450 --> 01:11:18,030
with the so-called
Bridgeman process.

1735
01:11:18,030 --> 01:11:20,000
It's also a name
for a specific type

1736
01:11:20,000 --> 01:11:21,900
of directional solidification.

1737
01:11:21,900 --> 01:11:27,120
This is your ingot, this is
the ingot chopped into bricks,

1738
01:11:27,120 --> 01:11:30,539
and then the bricks-- here's an
ingot coming out of a furnace.

1739
01:11:30,539 --> 01:11:31,830
Those are the bricks over here.

1740
01:11:31,830 --> 01:11:32,970
This is a really tiny one.

1741
01:11:32,970 --> 01:11:34,330
It's like lab scale.

1742
01:11:34,330 --> 01:11:40,930
The big ones are about over
a meter along the long edge.

1743
01:11:40,930 --> 01:11:43,250
And then, to saw
them into wafers,

1744
01:11:43,250 --> 01:11:46,561
we use what's
called wire sawing.

1745
01:11:46,561 --> 01:11:49,610
These are several kilometers
of wires-- of wire.

1746
01:11:49,610 --> 01:11:51,270
One continuous wire,
several kilometers

1747
01:11:51,270 --> 01:11:56,880
long, typically of a
steel-based composite.

1748
01:11:56,880 --> 01:12:00,790
Running in these
bricks right here,

1749
01:12:00,790 --> 01:12:03,330
in the presence of a
glycol-based slurry, typically,

1750
01:12:03,330 --> 01:12:06,200
and silicon carbide
or diamond grit,

1751
01:12:06,200 --> 01:12:08,540
and the grit is being
pressured by the wire

1752
01:12:08,540 --> 01:12:09,850
against the silicon.

1753
01:12:09,850 --> 01:12:12,210
The grit is very small
in size-- micron size--

1754
01:12:12,210 --> 01:12:14,970
and it's, essentially, chipping
out small pieces of silicon

1755
01:12:14,970 --> 01:12:16,720
as this wire is
progressing through

1756
01:12:16,720 --> 01:12:19,080
and, over a period of
around 6 to 8 hours,

1757
01:12:19,080 --> 01:12:22,690
you saw through the entire
brick and you use, maybe,

1758
01:12:22,690 --> 01:12:24,650
four or eight of them at a time.

1759
01:12:24,650 --> 01:12:26,400
So if that wire were
to snap about halfway

1760
01:12:26,400 --> 01:12:29,440
through the process, all
those bricks are gone.

1761
01:12:29,440 --> 01:12:32,040
So it's very important
that the wire be

1762
01:12:32,040 --> 01:12:35,890
very robust and able to
support the sawing process

1763
01:12:35,890 --> 01:12:38,910
and, as I said, it's
several kilometers long

1764
01:12:38,910 --> 01:12:42,650
and moving at a speed of
a few meters per second.

1765
01:12:42,650 --> 01:12:44,850
So this is zinging along
through your material

1766
01:12:44,850 --> 01:12:47,915
in the presence of very
small grit and slurry,

1767
01:12:47,915 --> 01:12:50,540
and so the consumables that are
used in the wire sawing process

1768
01:12:50,540 --> 01:12:53,150
are enormous, and you lose
about half of your silicon

1769
01:12:53,150 --> 01:12:55,980
due to sawdust in this
process right here.

1770
01:12:55,980 --> 01:12:59,220
So this is a prime
candidate for replacement

1771
01:12:59,220 --> 01:13:01,910
of the manufacturing
process, even though it's

1772
01:13:01,910 --> 01:13:04,050
so commonly used today.

1773
01:13:04,050 --> 01:13:06,460
What I'm going to do is give
a quick pause right here

1774
01:13:06,460 --> 01:13:07,870
until our next
class, where we'll

1775
01:13:07,870 --> 01:13:10,200
pick up and talk about
ribbon growth, which

1776
01:13:10,200 --> 01:13:12,750
seeks to get around
all the complexities

1777
01:13:12,750 --> 01:13:14,770
of multicrystalline
silicon ingot growth

1778
01:13:14,770 --> 01:13:16,910
while still keeping
the cost advantage.

1779
01:13:16,910 --> 01:13:19,640
So with that, thank you.