10. Wafer Silicon-Based Solar Cells, Part I

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visit MIT OpenCourseWare at ocw.mit.edu. PROFESSOR: But today
we're going to be talking about crystalline
silicon solar cells. Now, for those of you who do not
work in crystalline silicon PV, the reason this
topic is important is because crystalline
silicon comprises about 90% of all solar cells
manufactured today. It's the dominant technology,
and the technologies that you're working on
are going to displace, or are aiming to displace
crystalline silicon, so it's good to know your enemy. For those who are working
on crystalline silicon, this is meant to be
a background of all of the different aspects–
the entire supply chain of crystalline
silicon– so that you gain insight into the areas
that you're not currently focused on. You're getting a perspective
of the bigger picture. Crystalline silicon PV has
been around since 1954. The original– well, in
its current incarnation. That was when Bell
Laboratories announced the development of the modern
crystalline silicon PV cell, and that was 6%
efficiency in 1954, published in general
applied physics, and the cell architecture,
it's obviously evolved over the
years but it's not entirely dissimilar
from what we have today as a cell architecture
for our modern PV cells.

So, over the course
of– it's almost been 60 years of development
of crystalline silicon photovoltaic technology. That means both the cell
itself, the materials that go into it, and also the
manufacturing, or the methods to produce said
materials and device, over the course
of those, almost, 60 years much
innovation has happened both in terms of
manufacturing and technology. So today, we'll be going
over kind of a status quo snapshot of where crystalline
silicon stands and we brought in a number of
show and tell items so that you can see as we talk. So just for the
show and tell, we're going to be moving from the
feedstock materials over here finally into wafers
and cells on that side. All right. So, these lecture
notes are going to be valid for both 10 and 11.

We're going to split this up
over two classes to really dive into some of the details. The first question
is why silicon? Why did silicon evolve
as what is currently the dominant technology,
which is currently 90 percent of the PV market,
and I think it boils down to a couple of reasons. One is scalability. If you look at the elemental
abundance, on the vertical axis it's abundance, atoms
of the element per 10 to the 16 atoms of silicon. The reason that everything
is normalized to silicon is because there is,
well, quite a lot of it in the earth's crust.

As you can see, it's the
second most abundant element on the Earth's crust. It just so happens that, out of
all the stardust that is here on the planet, we have a
high percentage of silicon like the moon and like
many other planets in our solar system–
at least the hard ones. You can see oxygen
is probably the, well, oxygen is the only element
with higher natural abundance in the earth's crust, the
upper crust, than silicon and we go down as
we go to higher and higher atomic number.

The probability of formation due
to subsequent fusion reactions in stars decreases
and, hence, it follows this almost a
power law distribution as you can see there. So it's scalable. It is present in the Earth
in high enough capacity to reach terawatt scales. It's nontoxic and, as
Don Sadoway likes to say, if you want
batteries dirt-cheap, you have to make
them out of dirt. A similar expression is used
in the crystalline silicon community. I believe the quote
in 1366 is, "It's not only good for the planet,
it is the planet." A variety of riffs off
of this particular chart right here, but from a
technological point of view, why did silicon evolve to
the point where it is today? It forms a very
tenacious surface oxide.

So, if you were to expose a
piece of pure silicon to air, the surface oxide that forms
is very, very strong and very resistant, and very dense. So, unlike some
materials that corrode when exposed to
atmosphere, silicon oxidizes maybe the first few 10s
of angstroms, 100 of angstroms, and then it peters out so it's
diffusion-limited oxide growth mechanism that
eventually stabilizes at a very thin but very
dense and very protective oxide layer.

So the risk of having a
silicon wafer degrade inside of a solar module is very low. Furthermore, that oxide layer
from an electrical point of view it's very passivating. So as we studied
on, as we solved in the exam, those interface
states or those surface states, the surface of
semiconductor, those can be reduced or
minimized by the presence of certain passivating
layers, and it just so happens that by the
benevolence of nature, the silicon oxide, which is
shown in these red triangles right here, has a very
low surface recombination velocity, passivates
a surface very well, and results in
high-performing devices. In this particular case, they're
plotting emitter saturation current density in
femtoamps per centimeter squared– this is very, very
low– versus sheet resistance. This is essentially the dopant
concentration in the emitter, so they're looking at how the
passivation quality changes as a function of dopant
density and silicon oxide works pretty well, and it's an
effective diffusion barrier. And, probably most
significantly, those are maybe one looking
forward rationale one technological or
scientific rationale and as far as the
field is concerned, as far as engineering
community is concerned, silicon has a lot of momentum.

It's the most common
semiconductor material, silicon and germanium were both
purified, more or less, around the same decades but,
because silicon has a wider band gap, you have a lower
thermal carrier concentration, lower intrinsic
carrier concentration, folks were able to make
transistors and devices with lower noise out of
silicon as opposed to germanium and silicon technology
really took off in terms of the PV
industry benefited a lot by that cross-pollination. Many technologies came in
from the integrated circuits industry to assist or give
a boost to the PV industry. This number is a
little outdated, it's now about $100 billion. Hard to keep up with things
growing at 68% a year. Technology acceptance results
in lower interest rates. So if you have a
technology that is well-accepted by the market
then you go to a bank and say, hey, I want to install
some of those things and the bank says what
are those things you say oh, hundreds of
thousands of them have been installed already. It's OK. It's a proven technology. The bank says OK, I'll
lower your interest rates. That means you pay
less money on interest.

Your capital is more cheap. It works better in your
favor, and the opposite is true with an entirely new
technology that's unproven. So that's really
summing up why silicon. Momentum, forward
motion if you will, some inherent intrinsic
technological advantages, some of which are
listed here, and I'll get to that in a
second, scalability. To get back to the
technological advantages, I think it's important to
recognize what they are so that when you're thinking
of a new material, you can cross check and
say, gee, do I have these or do I not have these.

If I don't have them, it's
not the end of the world. You might have other advantages
that overcome the ones that silicon doesn't have. Let's add some more
into this list. Just stream of consciousness. Silicon has a very
high refractive index near the band gap edge. So, near the band gap
edge, it's absorbing light less efficiently. Right? It has a larger
attenuation length of the light, a smaller optical
absorption coefficient right as you approach the band gap. So silicon absorbs poorly
in the infrared because it's an indirect band
gap semiconductor, but it also has a
very large optical, sorry, a very large
real component of the refractive index.

Does anybody remember
what that refers to? Real component of
refractive index. Lesson number two. What does that dictate? AUDIENCE: Reflection. PROFESSOR: Reflection, exactly. So, if I were to tailor and
index of refraction grading on the front side
of my device, so I allow the light to be absorbed
efficiently, on the backside I can put a very large
index of refraction mismatch so that the light bounces back. In other words, the
light trapping silicon is benefited by
the fact that you have this awesome
reflection capability. The refractive
index is around 3.6, the real component of
the refractive index, in the infrared at
around 1070 nanometers.

Which means that if you
design your cell right, you can get an extension
of the optical path length by a factor of 50
over the thickness. So if your thickness
of the device is d, the optical path length
can be increased up to about [? 51d. ?]
That's as a result of this great reflectance. Many other materials that are
being explored as PV materials have refractive
indices around two, which would mean that
your optical path length extension is around 16. So that's one thing
to keep in mind. even though it doesn't
absorb light quite as well, it traps light fairly well. Another advantage of
silicon is that it forms sp3 hybridized
orbitals, for chemists, it forms– it's
tetrahedrally coordinated, in other words bond to
four other neighbors, and most 3D transition
metals don't do that.

They don't bond in
that configuration. Some do but many don't
and, as a result, the solid's solubility in
other words, the ability to incorporate impurities into a
growing silicon crystal is low. It rejects the impurities
from the solid into the melt, and you're able to purify the
material very efficiently. That's not always the
case with most materials. Sometimes they incorporate
impurities very readily, up to a few atomic percents. The typical impurity
concentration of silicon is in the order of parts per
million, parts per billion, parts per trillion. Still can be enough, as you
learned during your homework assignments, still could
be enough to affect device performance but is very low. It would be a lot
worse if silicon were able to absorb more
impurities and so forth. So, there are a number of
reasons why the silicon PV technology has gained the
foothold that it has so to bump it out of its
leadership position, one really has to be clever
and the parameter of merit is performance per unit cost.

Kilowatt hours per
dollar, if you will. So, we're going to talk about
the current manufacturing methods and materials
because this will give you an insight into the
dollars per kilowatt hour, the kilowatt
hours per dollar. Essentially, the cost
per unit energy produced. You can begin to
seize opportunities within the crystal silicon world
to improve the manufacturing process or you can begin
to say OK, you know what, this is way too complicated. Let me take a completely
different route. I'm going to develop a
new technology instead that will overcome these
manufacturing difficulties. So let's explore them in detail. First, the market. This is the evolution of market
share from 1980 to mid 2000s. After mid 2000s, the market just
continues growing at 68% a year and you really lose
resolution to this portion down here so it's to 2006 so
that we can actually see what's going on in the earlier days. In the earlier days,
1980, let's pick 1985, the market was split about
a third-third-third between thin films, amorphous
silicon namely, monocrystalline
silicon, and a material called multicrystalline silicon.

Now let's go piece by piece. What is monocrystalline silicon,
multicrystalline silicon and thin films? Well thin films are
materials that are usually between a few hundred
nanometers up to about three, maybe five microns thick. To give you size
perspective, your hair is about 50 microns
in diameter, so we're talking about 1/50 the
width of your hair. That's the active absorber
layer and of course the plastics and encapsulates
and everything else that go around them make
it a bit thicker, but the absorber
layer is very thin and so you're not spending
much on your absorber layer. It absorbs light
very efficiently, has a very large
absorption coefficient, and is able to absorb
photons efficiently. Crystalline silicon,
on the other hand, does not absorb light as well
as many thin film materials so we need about an
order of magnitude to two orders of magnitude
thicker substrates, and the crystalline
silicon substrates today in commercial
manufacturing are typically between 160 to 190 microns,
with an average around 170, 180. So about four times the
thickness of your hair.

Monocrystalline
silicon and multi. Let's talk about the
difference there. So, monocrystalline
silicon, folks are probably familiar seeing
pictures, at least something like this. Right? So this right here is an example
of a Cherkofsky silicon wafer. Appropriate for
integrated circuit work. I'll pass this around so
folks can get a sense. So this is an example of
a monocrystalline silicon wafer for the integrated
circuits industry. Let's analyze it in a
little bit more detail. So, the front surface is
polished, nicely polished. Polished to, I think,
somewhere in the order of a few nanometers
mean surface roughness. Using a chemical mechanical
polishing mechanism. The thickness is around
700– or 675 microns. Somewhere in that range. So very, very thick wafer. The objective is not to break. Right? If you're making
integrated circuit, this entire wafer that
I'm holding right here could be worth a few 10s or
100s thousands of dollars by the end of the
processing sequence, so if one of these breaks,
that's an awful lot of revenue that the company's losing.

So the substrate is
thick because they don't want it to break. Silicon is brittle
at room temperature. If you were to manufacture
solar cell out of this, you could but it would
be very expensive. The chemical
mechanical polishing that they use to
flatten the surface out costs a lot of money,
it's very time intensive, and the thickness of the
silicon is above and beyond what is necessary to
absorb light well. If anything, increasing
the thickness is just increasing your
emitter saturation current, since you have a higher
recombination current being driven by bulk recombination. You have more
recombination centers because you have a
greater thickness, and it's driving a
larger diffusion current from the emitter into the base.

So making it this thick
really doesn't make sense. So I'll pass this around so
folks can kind of get a sense. Make sure this gets
the entire round. I'll be recycling those platens. Please hold, if you're
going to take it out, which you're welcome
to do, please hold it like a photograph. What I don't want to have happen
is folks put their fingerprints all over it. The wafers that are
used in the PV industry are cut from the
same ingot like that one, except that the
ingots, essentially, if you were to pack circular
wafers into a module, it would look
something like this. Here's your module
and, mind you, you're spending a lot
of money on the glass and the encapsulates
and the aluminum framing and so forth, and now your
solar cells look like that.

There's probably more of them
that you can put in here, but what do you
notice about this? What is the packing density,
or packing fraction. It's very low, right? You're losing all of
this material in between. All that space is just
going to be blank space. Some of the earliest PV modules
actually use circular wafers, but the more modern
ones, what they do is a very complicated
cost analysis where they say, OK, if I were to
chop off the edges of my wafer and completely remove them,
I'd be losing a lot of silicon but I'd be increasing
the packing fraction.

So in the limit that my
module materials, the glass, the encapsulant, the
framing materials are infinitely expensive and
my silicon costs nothing, I want to do this. In the limit that my
module materials are free and installation is free but
the silicon is super expensive, I want to keep
full round wafers, and the reality is that
we're somewhere in between. And so, instead of making
one or the other extreme, typically what you'll
see is something like this chopped
off, like that, where you have a pseudo-square. The wafer itself has
flat edges on the sides but it also has kind of
pseudo-rounded corners here, and Joe did we bring
any of those in? The psuedo-squares, the
monocrystalline psuedo-squares.

These ones. OK. All right. No worries. I'll show them to
you next class. So, the idea is to
make– cut it out of the same ingot
as that one right there, but make it thinner, on
the order of 170 microns thick, and to chop off part of the
edge, and how much you chop off depends on the dynamic pricing
of silicon versus module materials and installation
and whether or not you can sell the
module, if there's a certain threshold
of performance that it needs to reach
because obviously, if you have a bunch of dead space in here,
you're losing that to– you're not producing power out of that. So if somebody wants a
module that's yay efficient, you might want to increase
the packing density. So that's
monocrystalline silicon. Multicrystalline silicon. Let's put it this way for now. We'll describe how
multicrystalline silicon is made, but for now
I'm going to say that multicrystalline silicon
is a crystalline silicon variety that is comprised
of many small grains.

So if you look at a
multicrystalline silicon wafer, something like,
let's say, oh this is a perfect example
right in here. If you look at a
multicrystalline silicon wafer, you can see that it looks
nice and– here maybe, that's probably an OK view of it. You can see individual grains. Right? If you look closely at it. And those are grains
of crystalline material that are joined by
grain boundaries. So the grain orientation
in one region might be pointing in
this direction, the grain orientation in the
neighboring region like that, and they come together
at a grain boundary and, when we have
polycrystalline materials like this, it's generally
indicative of some faster growth that didn't allow
for a nice homogeneous single crystal
material to evolve, and that's indeed what happens
during the multicrystalline silicon ingot growth.

It's occurring under a slightly
modified growth condition then, say, that beautiful single
crystalline piece over there, and we'll explain how
they're made in a second. So those are the technologies
in general, the base absorber materials, and then there's
ribbon silicon which is a really, really
small fraction of the total production in
decreasing, but at one time, ribbon silicon was viewed as
the up and coming technology. Still today, there are
about 20 startup companies around the United States
working on some aspect of this and probably about a dozen
more around the world. Yeah. AUDIENCE: I had a
question about the multi. PROFESSOR: Yeah. AUDIENCE: So for
the multi and micro and poly, is that
different grain sizes? PROFESSOR: Sort of. So, multicrystalline silicon
is a polycrystalline silicon material. The definition of
multicrystalline silicon is that the average grain size
is about a centimeter squared, or larger, and that's where
multicrystalline came about.

Polycrystalline silicon,
in the silicon community, has a very specific meaning. It means, usually a
plasma-enhanced chemical vapor deposited layer, so
PCVD-deposited layer of silicon, that has on the
order of one to five micron diameter grains. So very, very small
grain material. About 1/50 the
width of your hair. Maybe 1/10 the
width of your hair and, to distinguish it from
that really small grain material that will
perform very poorly, one calls this
multicrystalline silicon. AUDIENCE: And is there
microcrystalline silicon? PROFESSOR: There is also
microcrystalline silicon and microcrystalline
silicon is actually at the phase transition between
amorphous and polycrystalline silicon. So as you're going from
an amorphous material increasing the temperature,
let's say, of growth or increasing other parameters
during the deposition process, as you begin to evolve
from an amorphous material into a crystalline
material, you transition through this
microcrystalline regime which is a bit of a hybrid. It has some regions
that are amorphous and other regions
that are crystalline. In your assigned readings,
this book was assigned, and I believe in the syllabus
it says read chapter X. Unfortunately, there
is no chapter X.

I guess you could
interpret it as 10, but the essence was that there
are two versions of the book. One is version three,
which was published about seven years ago, and
the newest version just came out last year. The newest addition
is addition three. So the chapters have rearranged
slightly, but what I'll do is I'll highlight crystalline
silicon solar cells and modules in here
so that you can get a sense of what
is in the chapter and you're welcome to
go back and have a look.

So I'll go ahead and highlight
this chapter right here and pass it around. Feel free to glance
through the book as well. It's a great read. It dives into great
detail into each of the different technologies. OK. So, let's talk about
feedstock refining. We're going to start
the silicon value chain from the raw
materials and work our way all the way to the
final module at the end. So we'll start with the
feedstocks themselves. Down here is a rough
cost breakdown. Kind of think of
it as wafer, cell, module being like
a third-third-third of the total module cost
and then balance the system components beyond that. So we'll start
from our feedstocks and the raw materials
in the ground, we'll wind up with
systems on the roof, and we'll walk through
each of the different steps of current
manufacturing process.

So raw materials. Shown here is quartz and
coal, for a very good reason. The way feedstock refining
occurs at the very first stage is to take oxidized silicon,
silicon dioxide, quartz and to reduce it to
silicon, say, silicon zero. Unoxidized silicon, which is
also called silicon metal. It's called a metal because
it is very low resistivity. It's very low resistivity
because there's a very high impurity content still.

The purity of this material
coming out here is around 99, 99.9% here. So, it sounds like a
high purity but, if we're talking about parts per
million of impurities, we have some further refining
steps to do after this. So let's walk through this. We start with the raw
materials in the upper left. It says raw material inputs. Carbon and SiO2. The SiO2 forms, usually, quartz. That can be some of high
purity pegmatite, it could be, for example, a hydrothermal
quartz, higher purity varieties of quartz. You could even use, maybe, a
metamorphic quartzite material. Let me explain. So, some of the highest
purity materials are coming from these veins
of magma that float up and then phase separated
during millennia. Some of the lowest
purity quartz is coming from sand,
essentially crushed rock that made its way into, say,
a beach-like environment and then rock was
deposited on top of that, pressure was increased,
and this whole mixture of mica, feldspar,
and of quartz got pushed together and
formed a solid block.

That would be your
metamorphic quartz materials, and so you'd have a
much higher impurity content in the metamorphic
quartz than you would in, say, a high purity pegmatite
or hydrothermal quartz. Regardless, depending on the
feedstock source of the quartz, and there are people
who study this. Believe it or not, there
are entire departments dedicated to mining
quartz and figuring out where the different veins
of the highest purity quartz are, where you get them from. That's the SiO2 input
and the C input over here on the left hand side, Carbon. So, typically what is
used in the PV industry is either a fast-growing
wood source like eucalyptus or southern pine, right? Northern pine tends
to be slower growing, but eucalyptus and
southern pine both tend to be fairly fast-growing.

You can tell by the
spacing in the rings, if you chop the tree down and
do a cross section, or coal. So carbon, essentially. And the two react inside
of this furnace right here and this furnace, just to
give you a sense of scale, here's a human being. This is the furnace. So it's about five stories
tall, 12 meters in diameter. It's a big, big, big creature. This furnace right here
is what is producing the reduced silicon
and what's happening is these feedstock chunks are
being thrown in at the top and there's an arc going
between the electrodes, usually some carbon-bearing
material, and a base contact, and so that arc creates
a very high temperature.

Something in the order
of up to 2000 degrees Celsius, near the arc,
and the temperature decreases as you go
further and further away, so up near the top
here it might be even below the melting
temperature of silicon, somewhere around 1,200 degrees. So this is an extremely
inhomogeneous, messy system. This metallurgical grade silicon
refining furnace right here, this arc furnace, also called a
carbothermic reduction furnace, a very busy place. Lots going on. Extremely inhomogeneous
if you were to take a cross section
also in terms of temperature and in terms of
the chemical states of the different
constituents species, but the general
reaction that happens is the carbon would
much rather bond to the oxygen than silicon,
and so the carbon steals the oxygen from the silicon
reduces the silicon to silicon metal and CO2 is released. We'll get to that in a second. Flag that. Put an asterisk next to it. We'll come back to
that in a second. Other byproducts
of this reaction, so this is the liquid
silicon metal coming out here at the bottom.

It's essentially liquid
molten silicon reduced, so silicon zero, not a silicon
oxide, reduced silicon metal, and then finally it's
poured into these buckets, also called ladles and
solidified, crushed up to size, and then distributed at the end. Other byproducts coming
out of this reaction include– this is
liquid silicon up hear. It's very high
temperature and there are gases and a lot of oxygen
because of the reduction process, and so silica, or
SiO gas, can be produced and silica gas can begin
aggravating and forming very small particles,
almost like shards, of silicon oxide
material, and these can be on the order
of one to five microns and very rough and
jaggedy around the edges.

Now, who here has studied public
health and knows anything about PM1 or PM1.5 denominations. Do they ring a bell? What are those Ashley? AUDIENCE: It's the
size of particles that can get stuck in your lungs. PROFESSOR: Exactly! Right? So PM1 or PM1.5 would refer
to the micron diameter, 1 or 1.5 micron diameter
particle that would get stuck in the [INAUDIBLE] and
result, eventually, in edema or, probably, more of
water filling up in the lungs as a result of the body
trying to expunge these, and because they're
jaggedy and pointy, they get stuck in there and they
don't come out and eventually the people can even
affixate as a result. So, before in the past, when
we had these big smokestacks sitting on the top of these
metallurgical grade silicon refineries that would just spew
the silica dust into the air, the folks downstream
would be affected and this actually did happen, to
some degree, in, for, example, Kristiansand in Norway
and, as a result, the refineries began
putting in filters over here to prevent the silica
dust from getting thrown and spewed out
into the atmosphere and the filters are a very
interesting contraption.

A lot of work went into
designing them just right to allow the air to go out
but the particulate matter to stay behind and once
every delta t, maybe in the order of an hour so,
the airflow direction inverse and all the dust comes
crashing down to the bottom and then gets collected
inside of here. It's kind of like pushing air
through the different direction through a sock, and all the
dust comes out to the bottom, you collect it, and
it's sold to the–? AUDIENCE: The footwear
industry for absorbing– PROFESSOR: It might be.

I don't know, but I know
that the majority of it goes to the cement industry
and so, depending on the market rates of silicon, here at the
bottom metallurgical grade silicon, versus what the cement
industry is willing to pay, you might tune your
process to optimize for one industry or another. So, this is to say that early
on in refining processes, you're serving multiple
industries with one plant and volatility of
pricing is affected, in part, by what those
other industries are doing. What the demand there is. It's something to be aware of. Let's go back to the CO2 real
quick that's being emitted. So that is one of the
byproducts of the reaction. In terms of total CO2
content from the production of solar cells, the CO2 produced
during the reduction process is a small percentage, I think
something under 5% or 10% is the number I
pulled out of my head, it's a small percentage
of the total CO2 emitted during solar cell manufacturing
because the electricity that goes into producing the rest
of the solar cells coming from fossil fuel based
sources comprises the majority of CO2
emissions during fabrication of these devices.

The electricity used to
run these electrodes, for instance, the
electricity used to melt this silicon
byproduct here, or to gasify it in the
subsequent reactions, that is the majority of the CO2
coming out of the process. Any questions so far about this? They're fun plants to see. We don't have too many
of them in the US. Majority of these carbothermic
reduction furnaces are either in China, Norway. Norway has a lot
of cheap hydropower so the hydroplant is usually
only a few 10s of kilometers away from the
refinery and if you go to, say, [INAUDIBLE]
in Norway, where they have a number of these
plants, you'll see not only silicon
being refined there but also magnesium,
other elements, aluminum being smelted in
the same peninsula– the same industrial park. AUDIENCE: When general mining
of silicon happens or silica, the Chinese have– PROFESSOR: The
reduction process, this carbothermic
reduction process here, the majority of it
happens at the same places like Norway or China– places
that have cheap electricity. There's also a
feedstock refinery. I don't know if it
extends all the way back to the metallurgical
grade silicon refining, but there's a feedstock
refining facility going up in the
Middle East right now in Qatar, as a result of
the cheap natural gas.

So, wherever you have
cheap access to energy, you can set one of these
plants up and get off and running and
your CO2 intensity will be dictated by the fuel
source that you're using. Hydro, in that case,
it might be low unless you take
methane into account that might be emitted
in the reservoir, if you have decaying biomass
underneath the water, but if you would
exclude that and if you look at the CO2 intensity
of the fossil fuels that are being burned,
it might be better to do it in, say, Norway, from
an environmental point of view, than to, say, manufacture
this stuff in China.

Yeah. AUDIENCE: How many
kilowatt hours are we talking [INAUDIBLE]? PROFESSOR: Okay, so
what is the energy intensity of this process
right here, in other words. Well, why don't I
put a flag on that. Why don't we put a flag
on that and come back with specific numbers for
this process right here. I don't want to say something
and regret it later. AUDIENCE: Well, we know
the energy intensity of the solar panel itself. PROFESSOR: Yeah. AUDIENCE: But the energy– PROFESSOR: But
specifically what fraction comes from the MGSi
refining, I'd rather not pull something out of my head. Any other questions? OK. So somewhere in the order
of two million metric tons of metallurgical grade
silicon are produced annually. Probably somewhere in
the order of 10% of that is destined for the PV industry. The remainder gets
split among a variety of different industries. So what I'm talking
about here when I say metallurgical silicon, I'm
referring to this right here. This stuff coming out. It has about 99% or 99.9%
purity and it gets used in a variety of industries. So those industries
are: the PV industry, and we'll explain how
the rest of the refining happens, the integrated circuits
industry, that's the wafer that just went around that's
made its way back up here, and silicones those are–
so, a pet peeve of mine is hearing the word silicon and
silicone used interchangeably.

Silicon is this element– is an
element on the periodic table and it's the element that
comprises this wafer right here. Silicone, on the other
hand, is an organelle, I guess you could say, it's
not exactly organelle metallic, silicon isn't a
metal, but it would be a molecule that is comprised
of carbon atoms and silicon– silicon being in the middle and
the carbon being on the sides– and that is used as caulking or
sealing agent in your showers, for instance, or in
plumbing, round windows. It tends to be very flexible,
compliant but yet impermeable, preventing the inflow of gases. So silicones,
they're metal alloys including steel and aluminum. Why would you silicon there? What does it have to do
with steel or aluminum? Let me ask this.

Has anyone ever played
with pure aluminum? Highly refined, ultra
high purity aluminum. Say five nines or six nines. Yes! What happens to
ultra-pure aluminum? AUDIENCE: It's really flexible. PROFESSOR: It's
really flexible, you can dent it with
your fingernail, and it wouldn't make
very great boxes. Right? So we need it to be stronger
and scratch-resistant and so we have these additives into
the aluminum, silicon being one of them, that increases
the strength of the aluminum, essentially
preventing plasticity or preventing a dislocation
flow into the material. So that's more or less how
silicon– metallurgical grade silicon, also called MGSi as
shown up here at the very top– that's how MGSi gs is
distributed worldwide and that's the
current production.

Now let me ask another question. Steel and aluminum, where
are those used the most? What industry uses
steel, aluminum the most? AUDIENCE: Construction. PROFESSOR: Constructive
industry, automotive industry. How fast are those
growing annually? Let's estimate it from GDP. Annual– worldwide GDP. What's the worldwide
GDP growth look like. US is around 1%. China 8%. Let's pick a number
somewhere in between. Four, right? All right. So, let's say 4%, 5% worldwide. Silicone's probably
on that order. How about the PV industry. How fast is it going right now? Somewhere in the order of,
it's a volatile year right now, this one year, but in
the past, historically, it's been around
40% to 60% a year. So, where do you think
the price pressure for metallurgical grade
silicon is going to come from? What industry? It's going to come from PV.

It's a small fraction
of the pie right now but it's growing fast. Something to keep in mind. So that's why, if you look at
pricing of metallurgical grade silicon, yes. Superimposed upon pricing
is a function of time. You have the global
macroeconomic situation. Right? So that's kind of the dampening
function on top of it all, but there's just this general
trend toward rising prices as you put increasing price
pressure on metallurgical grade silicon. So additional
refining capacity will be needed if the current growth
keeps up in this industry. So let me talk about going
from metallurgical grade silicon about two nines
to three nines pure. What I mean two nines
means 99%, three nines would be 99.9% pure,
to silicon that we can use for solar
cells, which typically has to be about six nines pure.

And so this is called
the Siemens process which is purification through
gaseous distillation, and that's the method
that is currently used to make most of our silicon. So the way this
process works is we start with metallurgical
grade silicon at the top, represented by a little sack
of metallurgical grade silicon chunks. We produce silane gas out
of that metallurgical grade silicon. We essentially-
silane gas is SiH4. So it would essentially
be this right here. So you'd have a silicon atom
here, tetrahedrally coordinated with– tetrahedrally meaning
four bonds with hydrogen atoms on the side– and this is
silane gas– well, silane– which, at room
temperature, is a gas and that's what happens
in this step right here. We're forming– we're
gasifying the silicon. This process is the
distillation process. To extract the pure
silane gas, it's the distillation
process that's used in large towers similar to
fractional distillation where we might heat up the
material and then, depending on its mass, it settles down to
a certain height in that tower and we're able to extract it.

The silane gas
here has been sold to the photovoltaics industry. LCD. Liquid crystal display. Right? Thin film industries as
well, they use silane. If you're depositing the
polycrystalline and silicon for your LCDs or if you're
making amorphous silicon solar cells, they
use silane as well. So this little
truck here might go to three different
companies, depending on who's willing to pay more. Most of the silane is
used for polysilicon. The gas has to be converted
back into a solid, and that's where this
particular process here, the Siemens process is used. Again, you have
a current passing through some seed
material and the gas is being cracked onto that seed. You form these rods. The rods are then
cracked into chunks and then the chunks are
loaded into ingot crucibles. Yes. AUDIENCE: So the silane gas is
shipped as a gas in the trucks. PROFESSOR: Sure. AUDIENCE: Or on rails? PROFESSOR: Well it's
pyrophoric, as you can guess from just glancing
at this chemical structure right here.

It's highly reactive. Pyrophoric means that it can
combust at room temperature. It can catch on fire, meaning
there are more stable compounds than this that can form when
you react this gas with air and, during the early days
of silane development, folks really didn't
know much about it and there's some
early research– some of the earliest research
done here at MIT, in fact. They would fill up an evacuated
chamber with silane gas and spark and
nothing would happen. Spark a second time,
nothing would happen. Spark a third time, boom. OK. That's critical limit. Such and such amount. You know, they'd keep
increasing the amount and finally it would go boom. Tell you what, lets
repeat the experiment since we're good scientists.

They'd repeat it and, at
low concentrations, click. Boom. That's strange. That was much lower this time. Let's repeat the
experiment one more time. Click. Click. Click. Click. Click. Click. Click. Click. Click. Boom. All right. I don't really
understand this gas, but I'm going to say it's
really dangerous so I'm going to have little
warning bells that will detect the silane
gas if it's leaking and tell people to get the
heck out of the building if it starts being leaked. It's also toxic for
humans, by the way. Very small dilute
concentrations can kill you and so three buildings
on campus, only three to my knowledge, are set up
with the proper safety equipment to use silane gas
in the laboratory. Building 13, which is the
material science building, and then– MTL and related. So we have this gas right here. Extremely powerful. There are variants thereof. You can replace some
of the hydrogens with chlorine, like this and
now you have trichlorosilane.

It's all one word. So tricholorsilane, I've just
replaced three of my silanes– my hydrogens with
chlorine and now I have a different molecule,
still silicon bearing, still very reactive,
but now reactive at different temperatures
and I can modify my process by substituting out some of
the hydrogens for chlorines. So we have the silane gas or
trichlorosilane or the variants thereof, loaded into some
transportation vehicle that is very safe, leak-proof
and preventing accidents on the road, to deliver
it to where it is going to be consumed, which are
these so-called polysilicon, or Siemens reactor
as shown here. Excuse me. What happens, or how the
process actually flows, let me go back one step. We're going to start from up
at the very top of the process and move all the way
down, showing you what the manufacturing equipment
looks like at each step.

So, this is the
distillation process used to create the
silane and when you see one of these factories
just think of a refinery. In fact, the people who don't
like this particular process who aren't a fan of the
silane refining process and opt for other ways of
purifying their silicon, liken this to an oil refinery. The imagery is very stark there. The polysilicon production,
this is the Siemens reactor, it's much smaller in comparison
to the metallurgical grade silicon furnace. Much smaller than the
carbothermic reduction furnace. Here, we have a small
human or human next to the small contraption. Here are a series of
them lined, almost like little pods and,
out of this material, actually inside of
the furnace, you have these rods that are
passing current and heating up and the silicon is
cracking onto the rods.

So we wind up with
six nines, usually called 6N solar grade silicon
as a result of this process. We could also go up to, even,
nine nines using the Siemens process. It could be very,
very pure depending on how fast you grow, what the
purity of your silane gas is. AUDIENCE: Yeah. What is cracking. What does that mean? PROFESSOR: Sure. So what it means is
this gas molecule comes in, sees a solid surface,
the central atom right here, the silicon atom, gets
deposited onto the surface, becomes an adatom, which
means it's a surface atom, it's scuttling around and
the remaining elements within this molecule are then
free to move away as a gas.

AUDIENCE: So you've
broken those bonds. PROFESSOR: Yes. Effectively, you've added
the core constituent of this molecule
onto the surface. It's joined the collective if
you will and, in this matter, the diameter of those
rods grows with time. So what I'm going
to do is pass around an example of a chunk coming
from this Siemens rod. Be very gentle with it please. On the outside you can
see a corrugated, rough, cauliflower-like structure. That's because you're
optimizing for deposition speed, not for beauty of the surface. You don't really
care how flat it is, unless you're trying to
grow a very specific type of material called flotsam,
which we get to the second, but in general, if you're trying
to crack it up and break it into a smaller piece and
into a chunk like this and throw it into a
big ingot furnace, it doesn't really matter
what the surface looks like.

On the inside, it's
pretty dense silicon and, if look very carefully,
right in the middle there you can see the rod. The initial seeding rod. It's a slightly different color. So I'll pass these around
and please be gentle. AUDIENCE: Is the seeding
rod just silicon? PROFESSOR: It's
actually doped silicon, so it's lower
resistivity so you can pass more current through it. This here is chunks, or smaller
chunks of the polysilicon so, essentially, just
crushed polysilicon and if you're trying to load
a crucible with big chunks like this you'll leave
a lot of empty space unless you crush some of this up
and make finer grains out of it and fill in the gaps.

So I'll pass these
around right here so you can have a look at them. Those are examples of the
Siemens grade polysilicon. This is a bigger rod. Here is the seed coming
right through the middle. Here's the surface
where you can see it's kind of rough
and corrugated and one of the biggest issues
with this feedstock refining process is that there
are very large plants and long lead times. This is a plant construction
going on right now, you can see.

Typical lead times are
between 18 and 24 months. That's a long time between
when the board says yes, we will create new silicon
refining capacity and product starts to roll off
the production line and into customers' hands. It's a long time and
what this results in are drastic oversupply
and undersupply conditions in the market. So the silicon
feedstock price goes very high during periods
of undersupply and very low in periods of oversupply
and we're in an oversupply condition right now. Five years ago, let
me quantify this. Five years ago if you
went to the spot market– maybe four years ago– if
you went to the spot market, you could pay $100 to $500
per kilogram of silicon. That material that was just
right there I bet one you would put it into your bag and
run away out the door right now and be able to go to Mexico. Now the polysilicon
prices are much, much lower on the spot market. Somewhere in the order of
$30 to $50 per kilogram. About an order of
magnitude lower. AUDIENCE: Isn't lower cost
silicon better for the PV industry, though? PROFESSOR: Is it better
for the PV industry? As a customer most definitely,
it is good for you.

As an installer, it is most
definitely good for you. As a polysilicon
producer who wants to be a sustained
industry presence, it's not good for you. So this wide oscillation
between fat cat and scrawny is not very good
for any industry. It's unpredictable and it
causes some players to drop out. AUDIENCE: OK. PROFESSOR: And the investments
are very large as well. As you go from the early
stage portions of the value chain toward the module, the
investments generally decrease and so this is an outlook
coming from last year– the numbers are still a little
bit outdated– polysilicon production is buttressing
up against 200,000 metric tons per year at
this point in about 3/4 to the PV industry. The cost of manufacturing
is between $20 and $25 per kilogram and 2010 prices
were around $50 to $70. Now they're on
$30 to $50 in 2011 and the 2008 prices were around
$500 per kilogram in the spot market and it really boils
down to the inability to adapt to demand. If you have a very
large contraption that produces the feedstock materials
and it takes a long time to build the
factories, you're just not going to be able
to adjust fast enough.

Here's supply and demand,
demand being the red and supply being the blue. You can see how the oversupply–
the undersupply condition of the mid 2000s really led
to our current condition. So, alternatives to solar grade
silicon feedstock refining. What are some people thinking
in terms of other processes that they can use? These are two processes
right here and, mind you, when we were in this
situation with this price for the silicon,
everybody and anybody was coming up with new ideas of
how to manufacture the silicon. Now that we're barely selling
at cost and in an oversupply condition, many of these
ideas are having a struggle– a hard time in the market. They're struggling right now. So fluidized bed reactor and
upgraded metallurgical grade silicon. Let's talk about each
of those in turn. So what the fluidized
bed reactor folks realized was, gee, if
we're depositing on a rod, our surface area to volume
ratio is really large– sorry, is really small. Our surface area to volume
ratio is going to be very small. So think of it this way. If we have a sphere,
a sphere would be the quintessential
example where we'd have a very large
surface area to volume ratio.

If we had a plate, we
would have, as well, a very large surface
area to volume ratio and in the case of the condition
prior, where you have this rod, you really can't deposit
that quickly and so what these folks decided was,
what we're going to do is introduce small silicon
granules into this vessel, into this evacuated
chamber, and– here's the evacuated
chamber right here– and we're going to flow silane
gas into the system right here and the smaller
particles are going to go higher up because
of this flow of gas coming in the bottom and
those will grow and eventually settle down down here where
we can extract the bottom. So we'll wind up with these
beautiful little silicon granules. These ones shown right
here, which I'll pass around as well, those are coming
from a fluidized bed reactor, and they're nice beautiful,
spherical granules that are grown a lot faster, I
mean, a lot more silicon is deposited per unit time than
through the Siemens process as shown there in the back.

As a result, the energy
intensity is lower, the cost is lower, there's
a very tricky process to nail to get just
right, because you have to get the gas
flows right, you have to design the chamber
well, redo some purity contents. It's a tricky
process, and so this is being produced
right now, I believe, by only a few companies. REC has a capability
of doing it. MEMC, as well,
has the capability of doing this process. By and large, most
silicon is coming from the Siemens process. Yup. AUDIENCE: Sorry, both
of those companies have the normal
refining process? PROFESSOR: They have the
normal refining process. AUDIENCE: The [INAUDIBLE] PROFESSOR: Yup, and that's why
they developed this new one. They had these smaller,
internal projects that we're able to develop. So, yeah. I was just mentioning
the energy intensity. This is the kilowatt
hours per kilogram, going back to your question
about energy intensity. This is trichlorosilane
based Siemens process, silane based Siemens process. They're more ore less comparable
in terms of energy intensity.

And the silance based
fluidized bed reactor process. According to
internal REC numbers, which are little rosy,
but never the less, the trend is correct here. It is lower somewhere
in the order of an order of magnitude
energy intensity, and cost is lower as well. So let's move away from
the silicon refining by distillation
process entirely. Let's leave gaseous
distillation aside and say, what if we were to take this
metallurgical-grade silicon and, through liquid
purification routes, result in high purity silicon. How would we do that? Well, if we turn to other
industries, the ones that smelter aluminum or
refine manganese and so forth, we would see a multitude
of different options that we could borrow. Slag refining, bleaching,
leaching solidification. Let me walk through
them one by one. Leaching– that's
fairly straightforward. So if we put in some
acid, for instance, that dissolves the metals but
doesn't dissolve the silicon we could leach the metals
out of the material, and so that's the
essence of leaching. You might crush
up your material, in other ways other ways expose
the metals, or impurities, to the acids inside
of your system.

Slag refining says,
gee, what if we were to introduce some
material that could absorb the metals into it? The solubility of
the metals would be higher inside
of the slag agent than inside of the liquid. Maybe we throw in calcium
oxide or yttrium oxide or some, usually it's a metal oxide
that has a very high melting temperature that remains a solid
or, at least a glassy solid, and we pour it on
top of our silicon and it's able to absorb, say,
the phosphorus or the boron that's inside of our silicon so
that we reduce impurity content and then we can add the
phosphorous and boron later intentionally, but
to the concentrations we want not to exuberantly
high concentrations that might be found in nature. Solidification– during
this solidification process you're taking your
molten silicon and you're solidifying
it directionally from the bottom up and, because
the solubility of impurities tends to be larger in the
liquid than it is in the solid, it's like dragging a comb
through the entire material dragging out the impurities. Concentrating them in the liquid
and leaving a more pure silicon behind. Obviously, at the
very, very end you have this highly
concentrated region of impurities which then you
have to slice off and remove, so the solicitation
process doesn't come without a yield penalty.

You still throw away
some of your material. So you can't repeat the
solidification over and over and over again, I
guess you could, but you'd be losing
material every step. So some combination of these
processes here, and others. Other trickery. Low temperature eutectic
formation with other elements, for example. Some combination of this is
used to refine the silicon without creating
a gas out of it. So wafer fabrication. We're now going from
feedstock, we're leaving feedstocking
behind, and we're going to be talking about how
do you go from the feedstock materials that are being
passed around the room right now into a wafer that
you can then manufacture a solar cell device out of? One of these for instance. So let's talk about wafer
fabrication right here. So again, just to
situate ourselves, we've gone from raw materials
to silicon feedstock and now we're going to
feedstocks to wafers. Any questions right now
before we dive into that? Yeah. AUDIENCE: Question about supply. So silicon is very abundant
but the high purity silica deposits– are
they really abundant too? PROFESSOR: Great question.

So the question was are the
high purity silica deposits as abundant as, say, silicon. Certainly. If you bend over and rub your
fingers against the ground you're probably going to
come up with, probably, millions of trillions of silicon
atoms in your fingernails. Those are not very purity. So the highest security
quartz deposits are more rare and they are sought after,
and so they're are known. Their locations are known. There's one specific
one in Norway, one specific one
in North Carolina, and so forth around the
world and there– in a sense, they go to places. People have adjusted their
metallurgical-grade silicon refineries and their
subsequent down process for that particular ore. Once you run out of it, it's
not that the world ends, we just have to adjust for
the next feedstock source. So, in principle, there
are people looking at a variety of silicon inputs. Anything from the dirtier,
compressed, metamorphic quartz that I mentioned. Some people looking
at rice husks, which are silica rich as well. Other people
looking at seashells which, mostly calcium carbonate,
but other things as well.

I mean, there was a wide range. When the price of silicon
was $500 per kilogram, you got a multitude of ideas. When the price comes
back down, people tend to be more conservative. AUDIENCE: Is silicon considered
a renewable resource? PROFESSOR: Is silicon
considered a renewable resource. It is not a renewable
resource in the sense that, once you mine
it from the ground, you've mined it from the ground
and you used in some way. The reason it's
considered not an issue is because there's
so much of it. Not all of it, though, is
in the easy to access form. Right? Some of the silicon
might be bound up within heavily
contaminated sources and that's where the refining
ingenuity comes into play. As long as prices remain low,
there's not too much interest, say, for example,
that mine in Peru that has titanium oxide needles
throughout their silicon because why would
you want heavily titanium contaminated silicon? But as the price of
silicon, it probably will, rise again then people might
take another look at that mine and say gee, how can we
phase separate the rutile and anatase from
the quartz early on in the process by
crushing and etching or something so we can access
this feedstock material.

We'll see. It really depends on how the
market evolves, where people go looking for their silicon,
but there's a lot of it in the earth's crust. AUDIENCE: You're not
concerned about silicon? PROFESSOR: No. Nope. What is a bigger bottleneck
are are the refining steps in between. First it was the
reactors and soon it's probably going to be the
metallurgical-grade silicon reactors as well.

All right. Wafers. How do we get to these
from the raw feedstock materials that are being passed
around the room right now? So single crystalline
silicon ingot growth. Let's walk through that first. How do we get these
beautiful ingots? They're about half of all
silicon market right now. The biggest growth
method, by far, is called Czochralski
growth and, named after the Polish physicist
there Jan Czochralski. What you do is you have
a bath of molten silicon. A crucible, if you will. This tends to be a
circular crucible, rounded at the
bottom, usually made of quartz with
heaters on the outside to heat up the molten silicon. To heat up the silicon
chunks in here. Once everything
is molten, looking like a big bathtub
of silicon, you introduce a small
crystalline silicon seed into that molten
silicon and then you begin pulling while
rotating that seed. So the seed is a
single crystal material and what ends up happening
is, as you introduce the seed into the material
and begin pulling, you start pulling
out this crystal.

Single crystalline crystal. It's a thing of beauty
and this seed is actually very, very narrow in diameter. It might be about that big
around so pretty narrow in diameter and it's
being able to support this ingot of a
few, usually a few, tens to hundreds of kilograms
of mass underneath it and that's because silicon is
very strong even though it's brittle. So if you weren't to apply,
say, for example, a shear force on your silicon but
just to apply an axial load, you could support a very,
large weight underneath it.

So the [INAUDIBLE] of silicon
is grown from the bottom and eventually you wind
up with this nice ingot, as shown right there. The art that goes into growing
this properly is amazing. I'll highlight it with one
small little example just to illustrate the bigger
picture that a lot of effort goes into making these
defect-free, quote unquote, defect-free crystals. They're called defect-free
because they contain no grain boundaries and no dislocations. They have impurities, they
have intrinsic point defects, meaning vacancies or
interstitial atoms, but they don't have grain
boundaries or dislocations and so they're called
defect-free silicon. You introduce that seed
down into the liquid melt. Thermal stress happens. Right? Because you have the shock
between the solid silicon seed encountering the liquid
for the first time.

So this locations [? form ?]. And you have to pull the
seed out in such a way, you slowly rotate and
make this shoulder. The shoulder has to be
as quick as possible because you don't want
to waste material. Everything inside this shoulder
right here gets thrown away. So that little piece of material
right there gets tossed out. So you want to make the
shoulders as narrow and as quick as possible
so you can utilize the majority of your ingot
but, at the same time, you have to make
it thick enough so that the dislocations
can move all the way and propagate all the
way to the outside and end and terminate
in the shoulder before propagating
into the crystal. So that's just one
example of the technology that goes in the growing these. Another might be, gee,
we're PV industry, we want to make the stuff fast
whereas, in the IC industry you can invest up to a few of
dollars per gram of silicon and still make a profit
because you're selling a computer at 1,000 bucks.

In the PV industry,
we can invest, at most, a few tens of
cents per gram of silicon. So we have to make
this stuff fast. We can't dilly dally. You might want to crank
up the growth speed, then you run into issues
with defect concentrations, intrinsic point
defect concentrations, during the growth. I'm illustrating this
just to highlight the complexity of the growth
process of making these ingots, and the latter example was
one that the PV industry is facing today. It's actually a
hot research topic. Yeah. And then Ashley. AUDIENCE: The rotation speed
does that just affect time, or does it affect other things? PROFESSOR: So it affects
a multitude of things. One of the things
that it affects is the flow of, the
convective flow, of the melt.

So the liquid flow
inside of the melt is, in part, determining
how much oxygen gets transported
from this crucible here into the growing crystal. If you manage to suppress that
convective flow in the melt, you will also suppress
oxygen transport since the fusion is
going to be a lot slower than turbulent transport
or [INAUDIBLE] transport or convective transport. Yeah. Question? AUDIENCE: I just
have two questions so one is how fast
do you rotate it and the other is what that
does control– the diameter because I've heard
of 12 inch wafers versus like 18 inch wafers. PROFESSOR: Sure. So one of the things
that controls diameter is the balance of
heat extraction. So if you cool something down,
especially molten silicon, it will freeze, it will grow. If you heat it up,
it will shrink.

So that's one of the components
that controls the diameter. The pull speed and how
you grow that shoulder, essentially how you
heat up the material and how fast you pull
at those initial stages, also dictates the
diameter and you can see in the
ingots themselves, they're not perfect. They have a little
bit of corregation and that's the fluctuations of
the temperature of the melt, fluctuations of
the heater output, fluctuations of pull
speed, maybe what's pulling this entire contraption
is kind of a stepper motor that has a certain
granularity to it. Results in corregated edges. It's not perfect
and so there will be some adjustment made to
the form factor of the edge to get this nice round
wafer at the end of the day.

AUDIENCE: Does the
seed rod [INAUDIBLE] all the way to the ingot
or just near the top? PROFESSOR: All right. So the entire ingot
becomes pattern or templated by the seed rod. So this entire ingot has the
same crystalline orientation as a seed. AUDIENCE: And is the seed doped
differently than the silicon? PROFESSOR: It might
be but I'm not aware that that affects
the overall process. It could be that it's one
of the critical pieces of the magic sauce that makes
it work but I'm not aware. Rotation speed. It's not rotating like
this it's a slow rotation so I would– let's see.

How many radians per second– AUDIENCE: Can you see it? PROFESSOR: You can
visually see it if you looked at it long enough. Yeah. Yeah. So one modification,
one variant, of the single
crystalline growth method is called float-zone growth. You take a rod of poly, much
like that right over there that's inside of here,
and you pass an RF coil, radio frequency
coil, next to the rod and what that does is,
essentially, heats up the silicon, if it's
doped highly enough. It will melt the
silicon locally. Folks have probably heard
of fancy high-end stoves that we can only probably hope
to afford in 10 or 15 years, but these stoves that
are inductive heaters. Right? They're not resistive
heating elements, they're inductive heating
elements and the way that works is you have a radio
frequency source that then is absorbed by, in the
case of the RF heater, I believe it's a
specific type of iron that the inductive
heating ovens need.

And so this RF coil here
is emitting energy, which is absorbed by the
silicon and melting it, and you start with the
polycrystalline rod coming from the Siemens process
and in that case, this rough, corrugated
material right here won't do. Right? This is too rough
for that RF coil to pass over and be a
consistent distance away. In the case of
float-zone growth, you actually have to modify your
polysilicon production process. You have to modify
the Siemens process so that you get a
nice smooth rod, which you can then pass the
RF coil next to and melt and you again start with
a seed at the bottom, your RF coil starts down here
and then the RF coil moves through the material, almost
like a comb from the bottom to the top, converting
the polysilicon into nice single crystalline
material and, in the process, it concentrates impurities
in this liquid region.

Since the liquids have a
higher solubility in the liquid than they do in the solid,
the impurities are then aggregated inside of the liquid
region and, again, like a comb, they just get swept
out of the material. Not all of them, but
a large percentage of them, and so you
can make multiple passes with this RF coil
to further concentrate the impurities and
the extremities and remove them
from the material. So that's a float-zone method. Very expensive material,
very high purity. One of the reasons
it has high purity is because you don't have
this quartz crucible nearby, you don't have
this molten silicon that's absorbing or
dissolving the quartz and transporting the
oxygen into your crystal.

You have much lower carbon
and oxygen concentrations to [INAUDIBLE]
float-zone material. So if anybody is doing
experiments with silicon, for whatever reason, using
it as a substrate material, you want to think carefully
about what type of silicon you source and from
where you source it. You can find some very
poor quality silicon out there in the
market, especially if you going into
the aftersale market, and we know this from some– AUDIENCE: [INAUDIBLE].

PROFESSOR: –very
painful experiences. And so there are
some better sources from which to get
your wafers and we're happy to talk
about that offline. So, again, single
crystalline silicon. We're going to
venture into the world of multicrystalline silicon
ever so briefly here. First, we'll start
about cast material and, just to emphasize
here, we have regions of crystalline
material that have grain boundaries
separating the adjacent grains and the reason we go into
multicrystalline silicon is really oftentimes, it
is a lower cost method of producing a silicon wafer
although you have the grain boundaries. So, again, single crystalline,
Czochralski and float-zone, you wind up with round
wafers, typically single crystalline variety, and
multicrystalline silicon wafers tend to be more square-like
and more visibly multi-grained, if you will.

So let's talk about
those for a minute. How do you make a
multicrystalline silicon wafer? Again, you would start with
the solar-grade silicon that could either be coming
from the Siemens process, it could be coming from
the fluidized bed reactor, it could be coming
from an upgraded metallurgical-grade silicon,
the liquid purification route but, somehow, some way,
you get chunks of silicon, or granules of silicon, that
have a high enough purity for you to make
solar cells out of, and high enough
purity is typically in the order of one part per
million impurity content. So you put your solar-grade
silicon into a crucible and then you melt the
silicon inside of it. Silicon melts at
1,414 degrees Celsius.

It's a very high temperature. So 1,414 degrees Celsius is the
melting temperature of silicon. And then it's cooled. Not just randomly, but from
the bottom up and the reason it's cooled from the
bottom up is because, and here I guess you'll actually
have to come up and see this after class, it's rather
difficult to see from here, but this is a cross
section of a small ingot. This is the outside
of the ingot where it was contacting the
wall, these little pieces of white stuff that
are flaking off, this is the fused quartz silica
that forms the crucible wall, and the silicon
nitride coating that form the anti-stick
coating that prevented the silicon from
sticking to the crucible, and so it's kind of
rough and corrugated but, if we were to rotate this
around and look at the inside, this here is a cross
section of the actual ingot from the inside and,
if you look carefully, you'll see grains growing
from the bottom to the top.

You probably can't
see them from here, you'll have to come up
after class and take a look, but the grains are growing
from the bottom to the top and that is called directional
solidification, or the result of directional solidification. Directional
solidification is when you solidify from
the bottom to the top and, typically, your
grain boundaries are going to be
running perpendicular to the solid-liquid interface,
so your grain boundaries will be running up like this
as you grow your material from the bottom to the top. If you were to do
uncontrolled solidification and all walls would
freeze the same time, you'd have grains growing
in from the sides, you'd have grains growing
in through the bottom and then, when you slice
your wafer out horizontally, the grain boundaries wouldn't
be running perpendicular to the surface. They might be running parallel
to the surface, in which case they could wreck havoc on your
minority care diffusion length. Imagine you being
an electron having to travel across that grain
boundary that's between you and the P-N junction. Whereas, if the grain boundaries
are running perpendicular to the surfaces,
now they're only affecting very small areas of
the entire solar cell wafer.

So when I pick up
a wafer like this, this wafer was chopped
from the ingot this way or, to put it into
perspective here, this wafer was sliced
out like that from this. So the grain boundaries
were running perpendicular to the surfaces and that way
they don't impede as much with electron transport. So the multicrystalline
silicon ingot is formed. The ingot is then chopped
into these blocks, usually between 16 and 24, that
means four bricks to an edge or five bricks to an edge. Some folks are exploring
six by six, so 36 bricks, and then the bricks are
rotated on their side and then sliced into wafers
and individual wafers come out. So you can see the wafers
I've sliced from the bricks as I showed you right here. Is this diagram clear to folks? In general since–
any confusions? Any questions? No. AUDIENCE: How do
they cut the wafers? PROFESSOR: How do
they cut wafers! So this is a process called
wire sawing sign and this is one of the most
beautiful technologies because it was invented
in the PV industry and transported back,
adopted by the IC industry. So it's one of the few
examples of technology that went the other way.

Let me get to that point. AUDIENCE: How was
it done before? PROFESSOR: It was
done by ID saws, for instance, inner
diameter saws, that would slice off wafers
like a wafer off of a salami. AUDIENCE: So not a
wire but like a disk? PROFESSOR: Like a disk saw. Yeah. Exactly. Like the inner diameter meaning
your saw is like a rotating blade and you're just
using, you know– Yeah. OK. So directional solidification
of multicrystalline silicon. This is a cross section of a
furnace that is solidifying an ingot right here. Here's your ingot. This is a liquid silicon
and, essentially, it's solidifying from the
bottom to the top and, hopefully, we'll have
a tour of one of the world's largest ingot solidification
furnace manufacturing companies in the world. So, they don't
manufacture the silicon, they manufacture the furnace
that manufactures the silicon. If that makes sense. AUDIENCE: Do they also
make the crucible? PROFESSOR: No. That would be Vesuvius. Yeah. It would be other companies
that make the crucibles. And these are some of
the furnaces right here.

The keyboard and monitor. For size comparison, stairs. So they're about
two stories tall. You can go up here to the
top and look down into them. It's pretty cool. Using a little infrared
lens to block out the heat so you don't get blinded
and the furnace itself– all the action happens
inside of here. The top can lift– typically,
they're the bottom loaded. You'll see this little
seal right here. So this bottom part
typically comes down because you want to trap
the heat inside of it so you're not losing all that
and the bottom is removed, the forklift comes in, picks
up this ingot and crucible which could be a few of
kilograms in mass– up to about 600, maybe even
a ton– and removes it and places in the
proper location. It's a pretty dirty environment. The operator will typically
take a garden hose and hose it down
inside afterward. It's really an
antithesis of an IC fab at this stage right here. These are graphite
insulation materials on the sides of the crucible
and this yellowish dust that you see everywhere
is silica, again.

That nice fine grained dust
that's bad for you lungs. The directional
solidification process can be, to some degrees,
used interchangeably with the so-called
Bridgeman process. It's also a name
for a specific type of directional solidification. This is your ingot, this is
the ingot chopped into bricks, and then the bricks– here's an
ingot coming out of a furnace. Those are the bricks over here. This is a really tiny one. It's like lab scale.

The big ones are about over
a meter along the long edge. And then, to saw
them into wafers, we use what's
called wire sawing. These are several kilometers
of wires– of wire. One continuous wire,
several kilometers long, typically of a
steel-based composite. Running in these
bricks right here, in the presence of a
glycol-based slurry, typically, and silicon carbide
or diamond grit, and the grit is being
pressured by the wire against the silicon. The grit is very small
in size– micron size– and it's, essentially, chipping
out small pieces of silicon as this wire is
progressing through and, over a period of
around 6 to 8 hours, you saw through the entire
brick and you use, maybe, four or eight of them at a time.

So if that wire were
to snap about halfway through the process, all
those bricks are gone. So it's very important
that the wire be very robust and able to
support the sawing process and, as I said, it's
several kilometers long and moving at a speed of
a few meters per second. So this is zinging along
through your material in the presence of very
small grit and slurry, and so the consumables that are
used in the wire sawing process are enormous, and you lose
about half of your silicon due to sawdust in this
process right here. So this is a prime
candidate for replacement of the manufacturing
process, even though it's so commonly used today. What I'm going to do is give
a quick pause right here until our next
class, where we'll pick up and talk about
ribbon growth, which seeks to get around
all the complexities of multicrystalline
silicon ingot growth while still keeping
the cost advantage. So with that, thank you.

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