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PROFESSOR: So folks,
we're going to get started

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00:00:28,690 --> 00:00:31,960
into thin films for a moment,
but I saw two of you, at least,

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00:00:31,960 --> 00:00:37,080
in the class at Eli Yablonovitch
talk on, gosh, what was it,

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00:00:37,080 --> 00:00:37,810
Tuesday?

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00:00:37,810 --> 00:00:39,110
Tuesday is was.

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How many attended the
talk-- show of hands?

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00:00:41,821 --> 00:00:42,320
Three?

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00:00:42,320 --> 00:00:43,710
OK, three, awesome.

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00:00:43,710 --> 00:00:46,960
I must have missed one of
you-- very interesting talk.

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00:00:46,960 --> 00:00:48,580
This was a talk
about solar cells

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00:00:48,580 --> 00:00:50,430
given from the
perspective of somebody

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00:00:50,430 --> 00:00:52,050
who does light management.

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00:00:52,050 --> 00:00:55,760
And so I wanted to share with
you a book that is essentially

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00:00:55,760 --> 00:00:59,260
from where he takes his
efficiency calculations, which

22
00:00:59,260 --> 00:01:01,570
are based largely
on thermal dynamics

23
00:01:01,570 --> 00:01:04,349
and less on the
continuity equations--

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00:01:04,349 --> 00:01:07,150
Peter Wurfel's book
Physics of Solar Cells,

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00:01:07,150 --> 00:01:08,660
a brilliant, brilliant book.

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00:01:08,660 --> 00:01:10,110
I'm going to pass it around.

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00:01:10,110 --> 00:01:14,740
On page 33, very easy number
to remember-- 2 times 3, 3.

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00:01:14,740 --> 00:01:17,570
On page 33, he starts
delving into the derivation

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00:01:17,570 --> 00:01:20,860
that Eli Yablonovitch
presented during his talk,

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00:01:20,860 --> 00:01:24,010
so folks can follow along from
a thermodynamics point of view

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00:01:24,010 --> 00:01:26,260
and maybe read up a
little more and understand

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00:01:26,260 --> 00:01:27,620
that perspective.

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00:01:27,620 --> 00:01:31,260
But he very, very
briefly touched

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00:01:31,260 --> 00:01:35,017
upon essentially
the same physics

35
00:01:35,017 --> 00:01:36,600
but from the perspective
of what we've

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00:01:36,600 --> 00:01:38,570
been talking about a
class in terms of carrier

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00:01:38,570 --> 00:01:40,140
densities and current flows.

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00:01:40,140 --> 00:01:41,840
He had it on the
bottom of a slide,

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00:01:41,840 --> 00:01:43,537
perhaps halfway
through the talk,

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00:01:43,537 --> 00:01:44,870
on four different bullet points.

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00:01:44,870 --> 00:01:47,580
Does anybody remember
what those were?

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00:01:47,580 --> 00:01:50,100
Why did he achieve such a
high efficiency conversion

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00:01:50,100 --> 00:01:53,650
efficiency with the
gallium arsenide cell?

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00:01:53,650 --> 00:01:54,825
Anybody remember that one?

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00:01:54,825 --> 00:02:00,192
He had a thin device, so by
thinning the device down,

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00:02:00,192 --> 00:02:01,900
if he's able to
concentrate the carriers,

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00:02:01,900 --> 00:02:03,750
in other words, if he's able
to collect all of the charge

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00:02:03,750 --> 00:02:05,760
carriers inside of
that very thin layer,

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00:02:05,760 --> 00:02:08,889
he'll have a higher
charge carrier density.

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00:02:08,889 --> 00:02:10,610
And the charge
carrier density is

51
00:02:10,610 --> 00:02:13,190
what influences the
separation of the quasi Fermi

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00:02:13,190 --> 00:02:16,040
energies, which is what
influences the voltage output

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00:02:16,040 --> 00:02:17,270
of the device.

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00:02:17,270 --> 00:02:19,050
So he was able to
obtain a higher voltage

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00:02:19,050 --> 00:02:21,380
output because he had
a thinner solar cell.

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00:02:21,380 --> 00:02:24,490
He was able to concentrate the
carriers in that thinner region

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00:02:24,490 --> 00:02:27,440
by light trapping,
by light management.

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00:02:27,440 --> 00:02:30,104
And so as a result of having a
higher carrier concentration,

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00:02:30,104 --> 00:02:32,270
he had a higher separation
of the quasi Fermi levels

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00:02:32,270 --> 00:02:35,130
and hence a higher voltage
output of his device.

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00:02:35,130 --> 00:02:37,740
So in reality, it was very
simple from the perspective

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00:02:37,740 --> 00:02:39,490
of what we've been
learning in class here,

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how he was able to obtain
the very high efficiencies

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00:02:41,656 --> 00:02:43,040
of gallium arsenide.

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00:02:43,040 --> 00:02:45,100
The physics is well known;
it's not new physics.

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00:02:45,100 --> 00:02:48,560
It's actually quite old physics,
and that that approach has

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been used within the crystalline
silicon solar cell community

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00:02:51,530 --> 00:02:52,970
for some time as well.

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00:02:52,970 --> 00:02:55,080
The back surface reflectors
off of the devices

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00:02:55,080 --> 00:02:57,610
are highly optimized
and the texture,

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00:02:57,610 --> 00:02:59,560
as well, to scatter the light.

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00:02:59,560 --> 00:03:01,710
So I would invited
you to take a look,

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00:03:01,710 --> 00:03:03,240
and this is another
example of how

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00:03:03,240 --> 00:03:06,980
technologies can flow from
one photovoltaic system

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00:03:06,980 --> 00:03:07,970
into another.

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So you can learn a lot
from material systems

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00:03:10,530 --> 00:03:13,350
that you aren't working
on necessarily yourself.

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00:03:13,350 --> 00:03:15,940
That's another take-home
message from the talk, at least

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00:03:15,940 --> 00:03:16,940
what I walked away with.

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00:03:16,940 --> 00:03:18,523
Any other impressions
that folks would

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00:03:18,523 --> 00:03:20,952
like to share before we
dive into the lecture?

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00:03:20,952 --> 00:03:21,452
Yeah.

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AUDIENCE: I just have question
about carrier collection.

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How is it possible
to extract any energy

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00:03:27,575 --> 00:03:30,300
from carriers
which are generated

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00:03:30,300 --> 00:03:31,934
in front of the junction?

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00:03:31,934 --> 00:03:33,850
Because even if they
diffuse another junction,

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00:03:33,850 --> 00:03:35,705
they have nothing to fall down?

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00:03:35,705 --> 00:03:37,830
PROFESSOR: OK, so you have
to think about it always

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00:03:37,830 --> 00:03:39,370
from the perspective of
the minority carrier.

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00:03:39,370 --> 00:03:41,250
So if you generate an
electron-hole pair,

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00:03:41,250 --> 00:03:44,039
your minority carrier is now
a hole, in the n plus region.

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00:03:44,039 --> 00:03:45,830
And that hole diffuses
across the junction.

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00:03:45,830 --> 00:03:46,990
The electron stays.

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00:03:46,990 --> 00:03:47,823
AUDIENCE: I see, OK.

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00:03:49,950 --> 00:03:51,550
PROFESSOR: So did
anybody else pick up

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00:03:51,550 --> 00:03:54,100
on the point at the very
beginning of his presentation?

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00:03:54,100 --> 00:03:57,980
He said a P-N junction isn't
necessary to separate charge.

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00:03:57,980 --> 00:03:58,652
OK, that's fine.

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00:03:58,652 --> 00:04:00,110
We've talked about
heterojunctions.

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00:04:00,110 --> 00:04:02,840
We all agree there are other
ways to separate charge.

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00:04:02,840 --> 00:04:05,220
And he said an
electric field is not

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00:04:05,220 --> 00:04:07,580
necessary to separate
charge, but then

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00:04:07,580 --> 00:04:11,140
he immediately went into
discussing how the chemical

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00:04:11,140 --> 00:04:13,030
potential was slightly
lower in the contact

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00:04:13,030 --> 00:04:14,750
than it was in the
semiconductor, which

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00:04:14,750 --> 00:04:16,260
would result in a
charge imbalance, which

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00:04:16,260 --> 00:04:17,260
would result in a field.

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00:04:17,260 --> 00:04:19,339
And I think Gene Fitzgerald
from material science

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00:04:19,339 --> 00:04:21,380
and engineering department--
Professor Fitzgerald

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00:04:21,380 --> 00:04:23,766
called him out on it and
said, isn't there a field

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00:04:23,766 --> 00:04:24,890
there at the metal contact.

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00:04:24,890 --> 00:04:27,570
He said, quiet, wise guy,
we'll get back to you later.

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00:04:27,570 --> 00:04:30,910
But essentially his point was
a very small electric field

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00:04:30,910 --> 00:04:32,330
is necessary.

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00:04:32,330 --> 00:04:35,650
So his point was a
matter of degrees,

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00:04:35,650 --> 00:04:37,810
that you don't need a
massive electric field.

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A very slight
field is all that's

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00:04:39,510 --> 00:04:44,232
necessary to start driving a
current through your system.

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00:04:44,232 --> 00:04:46,790
I just wanted to
make sure we didn't

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leave that talk thoroughly
confused with our head

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00:04:50,120 --> 00:04:52,570
on backwards.

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00:04:52,570 --> 00:04:54,980
We're going to talk about
thin film materials today.

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00:04:54,980 --> 00:04:56,510
Why thin film solar cells?

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00:04:56,510 --> 00:04:59,260
Well, we've been talking about
crystalline silicon solar cells

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00:04:59,260 --> 00:05:02,100
that have a lower optical
absorption coefficient, so you

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00:05:02,100 --> 00:05:03,660
need a larger
amount of material,

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00:05:03,660 --> 00:05:05,620
or a larger optical
path length, to absorb

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00:05:05,620 --> 00:05:07,340
a significant
fraction of the light.

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00:05:07,340 --> 00:05:09,380
Already, in lecture
number two, we

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00:05:09,380 --> 00:05:11,230
saw how other
material systems that

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00:05:11,230 --> 00:05:13,260
have higher optical
absorption coefficients

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00:05:13,260 --> 00:05:15,690
are able to absorb this
equivalent amount of light

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in a thinner amount of
material and less material.

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00:05:19,060 --> 00:05:20,770
So to put this in
perspective, what

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00:05:20,770 --> 00:05:23,270
we're talking about on one hand
with the crystalline silicon

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00:05:23,270 --> 00:05:25,330
devices is we might have
a device that's maybe

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00:05:25,330 --> 00:05:27,680
three or four times the
thickness of your hair

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00:05:27,680 --> 00:05:30,810
in crystalline silicon, and
for the other materials, so

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00:05:30,810 --> 00:05:32,310
these thin film
materials, you might

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00:05:32,310 --> 00:05:35,650
be talking about a material
absorber that has maybe

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00:05:35,650 --> 00:05:37,410
100th the thickness,
so something

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00:05:37,410 --> 00:05:40,900
under a micron or a 50th
of the width of your hair.

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00:05:40,900 --> 00:05:42,857
So that's the
perspective of scale

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00:05:42,857 --> 00:05:44,065
that we want to have in mind.

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00:05:44,065 --> 00:05:45,564
When we're talking
about thin films,

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00:05:45,564 --> 00:05:48,260
we're talking about thin
materials, really on the order

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00:05:48,260 --> 00:05:49,480
of one micron or so.

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00:05:49,480 --> 00:05:51,890
And even brittle materials,
at one micron thickness,

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00:05:51,890 --> 00:05:57,710
if deposited on compliant
substrates, can be flexible.

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00:05:57,710 --> 00:06:00,090
Another thing to keep in
mind in thin film technology

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is that the scale of
the thin films industry

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00:06:02,520 --> 00:06:05,590
is about 1/10 that of
silicon industry right now.

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00:06:05,590 --> 00:06:07,450
So the crystalline
silicon industry

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00:06:07,450 --> 00:06:11,440
is going full force,
gangbusters right now,

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00:06:11,440 --> 00:06:14,920
and the thin film is
a growing fraction,

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00:06:14,920 --> 00:06:18,070
but it's on the order of 10%
of the total world market.

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00:06:18,070 --> 00:06:21,740
And many, many, many startup
companies, which are young,

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00:06:21,740 --> 00:06:25,090
dynamic, fun-- and that's why
today I'm not wearing a tie;

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I'm in startup mode.

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00:06:26,230 --> 00:06:27,451
I'm a lot more relaxed.

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00:06:27,451 --> 00:06:29,700
We're going to be talking
about thin film technologies

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00:06:29,700 --> 00:06:32,420
and diving into some fun work.

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00:06:32,420 --> 00:06:36,590
So we'll talk about these
specific technologies

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00:06:36,590 --> 00:06:39,390
of thin film materials, and
before we get into those,

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00:06:39,390 --> 00:06:42,020
I'm going to address some
general topics about deposition

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00:06:42,020 --> 00:06:45,090
and, of course, general
parameters that affect

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00:06:45,090 --> 00:06:48,010
all thin film material systems.

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00:06:48,010 --> 00:06:50,750
We have to appreciate the sheer
diversity of technologies that

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00:06:50,750 --> 00:06:51,916
are out there on the market.

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00:06:51,916 --> 00:06:54,890
We have a variety of different
solar cell materials that

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00:06:54,890 --> 00:06:57,140
are available, some of
which are thin films,

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00:06:57,140 --> 00:07:00,120
other ones, wafer-based
crystalline silicon.

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00:07:00,120 --> 00:07:03,220
And all of these
technologies have

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00:07:03,220 --> 00:07:06,360
to consider cost resource
availability and, eventually,

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00:07:06,360 --> 00:07:08,000
environmental impact as well.

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00:07:08,000 --> 00:07:09,416
So these or some
of the things I'd

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00:07:09,416 --> 00:07:12,364
like you to keep on the
forefront of your mind as we

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00:07:12,364 --> 00:07:14,030
talk about these
different technologies.

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00:07:14,030 --> 00:07:16,720
Think about the broader picture,
and ultimately, this cost,

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00:07:16,720 --> 00:07:21,840
or the amount of money
per unit energy produced,

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00:07:21,840 --> 00:07:24,900
is really paramount in
determining marketability

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00:07:24,900 --> 00:07:28,370
and determining the
scales to which they'll

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00:07:28,370 --> 00:07:30,150
penetrate the market.

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00:07:30,150 --> 00:07:32,410
This is the one slide
that you have printed out.

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00:07:32,410 --> 00:07:35,250
You have one per pair of
students, or you should.

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00:07:35,250 --> 00:07:40,000
So if you don't have access
to that particular slide,

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00:07:40,000 --> 00:07:43,370
feel free to share it with
the person next to you.

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00:07:43,370 --> 00:07:45,700
This is representing
as a function of time,

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00:07:45,700 --> 00:07:49,800
going back to the 1970s, the
record solar cell conversion

191
00:07:49,800 --> 00:07:51,210
efficiency.

192
00:07:51,210 --> 00:07:54,221
The chart is maintained by
a certain Larry Kazmerski

193
00:07:54,221 --> 00:07:54,720
at NREL.

194
00:07:54,720 --> 00:08:00,580
Actually, he used to be the
head of NREL's solar program.

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00:08:00,580 --> 00:08:03,810
He stepped down a few years ago
after a very successful run--

196
00:08:03,810 --> 00:08:04,730
many years.

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00:08:04,730 --> 00:08:08,190
And he's a bit of a father
of the US PV industry.

198
00:08:08,190 --> 00:08:10,310
He's been around for
a long time and has

199
00:08:10,310 --> 00:08:12,310
been tracking the growth
of the PV industry

200
00:08:12,310 --> 00:08:16,540
and, of course, the improvement
of performance over time.

201
00:08:16,540 --> 00:08:19,410
Many of these devices-- many
of these record efficiency

202
00:08:19,410 --> 00:08:23,490
devices-- are very small
area, and many of them

203
00:08:23,490 --> 00:08:26,430
were actually grown for the
intent, the explicit purpose,

204
00:08:26,430 --> 00:08:27,792
of getting onto this chart.

205
00:08:27,792 --> 00:08:30,000
And so when you're trying
to make a record efficiency

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00:08:30,000 --> 00:08:32,464
device, you do things
a little differently.

207
00:08:32,464 --> 00:08:33,880
Let me give you
an example; you'll

208
00:08:33,880 --> 00:08:37,045
optimize your anti-reflection
coding for air not for glass,

209
00:08:37,045 --> 00:08:38,150
right?

210
00:08:38,150 --> 00:08:41,549
So if you want to
minimize the reflectance

211
00:08:41,549 --> 00:08:43,690
off the front surface,
you'll be optimizing it

212
00:08:43,690 --> 00:08:45,990
for air, which has a
refractive index of one, as

213
00:08:45,990 --> 00:08:49,250
opposed to glass, which has
a refractive index of 1.5.

214
00:08:49,250 --> 00:08:52,510
So there are some tricks
that one does and engages in,

215
00:08:52,510 --> 00:08:55,400
some are a little bit
under the table, too.

216
00:08:55,400 --> 00:08:57,410
It has been done in
the past that people

217
00:08:57,410 --> 00:09:00,350
would do an HF dip of their
silicon-based solar cells

218
00:09:00,350 --> 00:09:02,690
right before measuring
the efficiency.

219
00:09:02,690 --> 00:09:06,690
The hydrofluoric acid would
result in surface pacification,

220
00:09:06,690 --> 00:09:09,800
but, of course, it would
result in a very low surface

221
00:09:09,800 --> 00:09:13,020
adhesion of the metal, and so
the metal flake off afterwards.

222
00:09:13,020 --> 00:09:16,250
It wouldn't pass the tensile,
but you would nevertheless

223
00:09:16,250 --> 00:09:18,460
achieve instantaneously
higher efficiency.

224
00:09:18,460 --> 00:09:20,674
Those practices have
largely been weeded out.

225
00:09:20,674 --> 00:09:22,090
These were the
early days, when it

226
00:09:22,090 --> 00:09:24,460
was a wild west of solar cells.

227
00:09:24,460 --> 00:09:27,790
In more recent times, there
are some very strict standards,

228
00:09:27,790 --> 00:09:30,280
and there are only a few
laboratories around the world

229
00:09:30,280 --> 00:09:33,590
where you can take these
standard measurements.

230
00:09:33,590 --> 00:09:35,790
The one at NREL
is extremely well

231
00:09:35,790 --> 00:09:38,590
staffed in terms of the
quality of the people.

232
00:09:38,590 --> 00:09:40,450
They're notoriously
under resourced,

233
00:09:40,450 --> 00:09:41,940
but that's another issue.

234
00:09:41,940 --> 00:09:44,340
But in terms of the quality
of the people there,

235
00:09:44,340 --> 00:09:47,970
very, very good, very
thorough, very pedantic

236
00:09:47,970 --> 00:09:49,940
and careful about taking
their measurements.

237
00:09:49,940 --> 00:09:51,720
And if you ever have
a question about how

238
00:09:51,720 --> 00:09:53,740
to perform a solar cell
efficiency measurement,

239
00:09:53,740 --> 00:09:55,030
they're a very good resource.

240
00:09:55,030 --> 00:09:57,810
Their website would be
an excellent place to go.

241
00:09:57,810 --> 00:10:02,320
So these data points here versus
time represent the record cell

242
00:10:02,320 --> 00:10:03,390
efficiencies.

243
00:10:03,390 --> 00:10:06,020
They may be on very, very
small pieces of material.

244
00:10:06,020 --> 00:10:09,220
They may be on a centimeter
squared, perhaps even smaller,

245
00:10:09,220 --> 00:10:11,540
so they're not necessarily
representative of what is

246
00:10:11,540 --> 00:10:13,420
in commercial production today.

247
00:10:13,420 --> 00:10:16,950
Let me give you one example;
the record crystalline silicon

248
00:10:16,950 --> 00:10:22,060
cells, which are in blue here,
has been around 25% for about

249
00:10:22,060 --> 00:10:24,130
a decade-- actually,
a little more

250
00:10:24,130 --> 00:10:27,430
than a decade-- and
the record efficiency

251
00:10:27,430 --> 00:10:33,900
crystalline silicon device
has, in essence, not

252
00:10:33,900 --> 00:10:36,440
been so planted for
many, many years.

253
00:10:36,440 --> 00:10:38,740
There are a number
reasons for that.

254
00:10:38,740 --> 00:10:42,440
It's very much approaching its
theoretical efficiency limit.

255
00:10:42,440 --> 00:10:44,830
People haven't necessarily
tried specifically

256
00:10:44,830 --> 00:10:47,450
to get a record efficiency
crystalline silicon device.

257
00:10:47,450 --> 00:10:49,480
They're more intent on
making lower cost silicon

258
00:10:49,480 --> 00:10:51,560
devices than record
efficiency ones,

259
00:10:51,560 --> 00:10:54,720
and the average
module efficiencies

260
00:10:54,720 --> 00:10:56,310
are somewhere down
at around here--

261
00:10:56,310 --> 00:11:00,530
actually, somewhere
in the 13% to 15%

262
00:11:00,530 --> 00:11:02,960
range for module, average
module efficiency.

263
00:11:02,960 --> 00:11:06,960
You have some modules that
are in the 18%, 19% 20% range,

264
00:11:06,960 --> 00:11:08,900
but most of them are
significantly lower.

265
00:11:08,900 --> 00:11:10,941
And the record cell
efficiencies, as you can see,

266
00:11:10,941 --> 00:11:12,119
is 25%.

267
00:11:12,119 --> 00:11:13,910
So there's a significant
delta between what

268
00:11:13,910 --> 00:11:15,993
is commercially available
and what the record cell

269
00:11:15,993 --> 00:11:16,970
efficiency is.

270
00:11:16,970 --> 00:11:18,820
There are several
reasons for this.

271
00:11:18,820 --> 00:11:20,500
To make a record
efficiency cell,

272
00:11:20,500 --> 00:11:23,110
you have to throw everything
at Liebig's law of the minimum.

273
00:11:23,110 --> 00:11:25,693
You have to make sure that every
plank is really, really high.

274
00:11:25,693 --> 00:11:27,370
That costs a lot
of money typically,

275
00:11:27,370 --> 00:11:30,320
and so doing that cheaply
is a big challenge.

276
00:11:30,320 --> 00:11:32,090
Some companies,
like First Solar,

277
00:11:32,090 --> 00:11:34,110
for instance, has some
of the lowest cost

278
00:11:34,110 --> 00:11:35,290
models in the market.

279
00:11:35,290 --> 00:11:37,680
We'll describe how they're
made in a few slides.

280
00:11:37,680 --> 00:11:40,590
First Solar forwent the
anti-reflective coating

281
00:11:40,590 --> 00:11:43,220
on their glass for many
years, because it just

282
00:11:43,220 --> 00:11:45,080
didn't make cost sense.

283
00:11:45,080 --> 00:11:48,550
It didn't help optimize
this function right here.

284
00:11:48,550 --> 00:11:50,240
Although, you'd get
more energy out,

285
00:11:50,240 --> 00:11:52,690
the dollars that it took
to add that component just

286
00:11:52,690 --> 00:11:54,740
didn't make sense for them.

287
00:11:54,740 --> 00:11:57,900
So you have to think about a
few different perspectives.

288
00:11:57,900 --> 00:11:59,980
You have to think
both in terms of cost,

289
00:11:59,980 --> 00:12:01,520
and in terms of performance.

290
00:12:01,520 --> 00:12:04,130
The performance, what it
does or what it tells you

291
00:12:04,130 --> 00:12:06,850
is that this material
system has potential.

292
00:12:06,850 --> 00:12:10,490
It has been demonstrated we
can get the high performance.

293
00:12:10,490 --> 00:12:12,120
It's a proof of concept.

294
00:12:12,120 --> 00:12:15,390
The trick now is to
get there at low cost,

295
00:12:15,390 --> 00:12:17,570
and that's pretty much
what you should walk away

296
00:12:17,570 --> 00:12:19,114
from this chart having seen.

297
00:12:19,114 --> 00:12:21,530
Another thing to keep in mind
is that it takes a long time

298
00:12:21,530 --> 00:12:23,540
to improve the performance
of a new material.

299
00:12:23,540 --> 00:12:25,700
If you're starting out
somewhere down around here,

300
00:12:25,700 --> 00:12:28,580
it's going to take you a while
to reach higher efficiencies.

301
00:12:28,580 --> 00:12:31,800
Granted we can learn a lot from
the previous material systems.

302
00:12:31,800 --> 00:12:36,010
We could learn a lot by
reading those old NREL project

303
00:12:36,010 --> 00:12:38,480
reports that are available
online of all the people

304
00:12:38,480 --> 00:12:40,310
who were working towards these
record efficiencies-- what they

305
00:12:40,310 --> 00:12:41,910
did differently,
how they advanced,

306
00:12:41,910 --> 00:12:43,731
and how they improved
cell performance--

307
00:12:43,731 --> 00:12:45,230
and leverage that
information as you

308
00:12:45,230 --> 00:12:46,999
try to develop your material.

309
00:12:46,999 --> 00:12:49,540
But the fact of the matter is,
it'll still take a bit of time

310
00:12:49,540 --> 00:12:52,772
to develop new technologies,
and you can see that

311
00:12:52,772 --> 00:12:54,230
by some of the
newer materials that

312
00:12:54,230 --> 00:12:55,961
are coming along down
here, for example,

313
00:12:55,961 --> 00:12:57,210
the organic-based solar cells.

314
00:13:00,600 --> 00:13:03,700
So thin films,
general issues-- so we

315
00:13:03,700 --> 00:13:05,420
talked about the
advantages here,

316
00:13:05,420 --> 00:13:09,800
that we're squeezing the
cost of the absorber layer

317
00:13:09,800 --> 00:13:11,940
out of the module, which
is excellent from a cost

318
00:13:11,940 --> 00:13:13,170
point of view.

319
00:13:13,170 --> 00:13:15,650
But obviously, there
are trade-offs involved.

320
00:13:15,650 --> 00:13:17,470
If it was all a
walk in the park,

321
00:13:17,470 --> 00:13:21,057
we would be 100% thin films and
have abandoned silicon by now.

322
00:13:21,057 --> 00:13:22,890
There are both advantages
in this advantages

323
00:13:22,890 --> 00:13:24,440
with thin films.

324
00:13:24,440 --> 00:13:26,590
Instead of disadvantages,
perhaps a happier way

325
00:13:26,590 --> 00:13:28,800
of looking at this is
challenges and opportunities

326
00:13:28,800 --> 00:13:31,850
for getting PhDs and
other advanced degrees.

327
00:13:31,850 --> 00:13:33,920
So let's go up into advantages.

328
00:13:33,920 --> 00:13:35,720
The advantages of thin
films, quite simply,

329
00:13:35,720 --> 00:13:39,300
is that you're using a very thin
amount of material, so thin,

330
00:13:39,300 --> 00:13:42,170
in fact, that it's virtually
insignificant in terms

331
00:13:42,170 --> 00:13:44,510
of the total cost
structure of your module.

332
00:13:44,510 --> 00:13:47,930
One perspective is that if
you're depositing a thin film

333
00:13:47,930 --> 00:13:51,350
using a fairly
low-cost technique,

334
00:13:51,350 --> 00:13:55,940
like a c spaced
sublimation type process,

335
00:13:55,940 --> 00:13:59,760
you may be able to deposit
the material for as much cost

336
00:13:59,760 --> 00:14:01,550
as it takes the
cardboard that separates

337
00:14:01,550 --> 00:14:04,230
the modulus from each other in
the stack that's being loaded

338
00:14:04,230 --> 00:14:06,082
onto the 18-wheeler
out of the factory,

339
00:14:06,082 --> 00:14:07,290
to put things in perspective.

340
00:14:07,290 --> 00:14:10,000
It's very cheap to
deposit these thin layers.

341
00:14:10,000 --> 00:14:12,995
Now let's hop down to the
disadvantages real quick.

342
00:14:12,995 --> 00:14:14,620
If you're depositing
a very thin layer,

343
00:14:14,620 --> 00:14:17,300
and it's not high
efficiency, then you

344
00:14:17,300 --> 00:14:19,410
need more glass, more
encapsulants, more framing

345
00:14:19,410 --> 00:14:21,190
materials, more labor, and
everything for the same amount

346
00:14:21,190 --> 00:14:22,070
of power out.

347
00:14:22,070 --> 00:14:25,306
If your efficiency is low,
your costs will be higher.

348
00:14:25,306 --> 00:14:27,180
Even if you have a dirt
cheap absorber layer,

349
00:14:27,180 --> 00:14:29,180
you might as well get
the absorber for free.

350
00:14:29,180 --> 00:14:31,850
If your efficiency is too
low, all the other commodity

351
00:14:31,850 --> 00:14:34,860
materials are going to
outweigh that cost advantage

352
00:14:34,860 --> 00:14:37,220
because the commodity
materials scale with area.

353
00:14:37,220 --> 00:14:39,650
If you have low efficiency,
you need a larger area module

354
00:14:39,650 --> 00:14:42,080
to make the same
amount of power.

355
00:14:42,080 --> 00:14:44,290
So, at some point, if
you look at the cost

356
00:14:44,290 --> 00:14:46,080
of the material
versus efficiency,

357
00:14:46,080 --> 00:14:47,940
you start entering
negative territory.

358
00:14:47,940 --> 00:14:49,920
You actually have to be paid.

359
00:14:49,920 --> 00:14:52,410
If you're producing like
an 8% or a 7% module,

360
00:14:52,410 --> 00:14:55,620
typically, you would have to
pay your customer for them

361
00:14:55,620 --> 00:14:57,690
to accept your module.

362
00:14:57,690 --> 00:15:01,180
So you really have to
achieve a minimum efficiency

363
00:15:01,180 --> 00:15:04,230
target to be cost competitive,
and as a rule of thumb,

364
00:15:04,230 --> 00:15:08,390
that's typically 10% to 12%
for today's cost of glass

365
00:15:08,390 --> 00:15:12,690
encapsulance framing materials
and labor and installation

366
00:15:12,690 --> 00:15:14,650
and so forth.

367
00:15:14,650 --> 00:15:16,330
So back up to the
advantages-- there's

368
00:15:16,330 --> 00:15:19,030
a potential here for a
very low thermal budget.

369
00:15:19,030 --> 00:15:22,160
If we're able to print,
say, a micron-thick layer

370
00:15:22,160 --> 00:15:25,040
onto a substrate,
remember, we go back

371
00:15:25,040 --> 00:15:27,300
to that the high
speed printer analogy,

372
00:15:27,300 --> 00:15:29,480
there's a potential for a
low thermal budget, which

373
00:15:29,480 --> 00:15:32,260
means thermal budget
is the amount of heat

374
00:15:32,260 --> 00:15:35,440
that you're introducing
during the processing.

375
00:15:35,440 --> 00:15:37,680
As a result of a very
low thermal budget,

376
00:15:37,680 --> 00:15:39,530
you have a potential
cost decrease.

377
00:15:39,530 --> 00:15:41,540
Instead of heating things
up to 1,400 degrees

378
00:15:41,540 --> 00:15:45,789
C over several hours, having
all that massive amounts

379
00:15:45,789 --> 00:15:47,330
of electricity that
go into producing

380
00:15:47,330 --> 00:15:50,090
the crystalline silicon
wafers, here, potentially, we

381
00:15:50,090 --> 00:15:53,470
could be printing stuff
on flexible substrates.

382
00:15:53,470 --> 00:15:55,430
So that's the thermal
budget argument.

383
00:15:55,430 --> 00:15:59,540
In terms of conformal deposition
and flexible substrates,

384
00:15:59,540 --> 00:16:02,320
there's a potential here
for roll-to-roll deposition.

385
00:16:02,320 --> 00:16:05,080
Picture a newspaper plant,
where you have one roll of paper

386
00:16:05,080 --> 00:16:08,030
on one side being pulled on
to another spool in the other,

387
00:16:08,030 --> 00:16:10,520
with some deposition process
happening in between.

388
00:16:10,520 --> 00:16:14,140
If you can deposit on a flexible
substrate, this is the vision.

389
00:16:14,140 --> 00:16:16,855
And if you're not depositing
onto a flexible substrate

390
00:16:16,855 --> 00:16:19,390
but onto hard substrate
like this one right here--

391
00:16:19,390 --> 00:16:21,060
this is glass, a
thin film material

392
00:16:21,060 --> 00:16:23,180
deposited on glass right
here, a very small one.

393
00:16:23,180 --> 00:16:24,140
Oops, some tape on the front.

394
00:16:24,140 --> 00:16:25,431
Let me get rid of that for you.

395
00:16:27,768 --> 00:16:28,665
Here we go.

396
00:16:33,980 --> 00:16:38,320
It's in a nice little
protective coating here,

397
00:16:38,320 --> 00:16:41,370
so you can have a look at it
without worrying about getting

398
00:16:41,370 --> 00:16:43,790
your fingerprints all over it.

399
00:16:43,790 --> 00:16:48,610
And the company name is
fully removed-- check.

400
00:16:48,610 --> 00:16:52,340
This is an example of a
thin film material deposited

401
00:16:52,340 --> 00:16:55,310
on glass without any
anti-reflective coating, just

402
00:16:55,310 --> 00:16:57,520
the absorber material,
so you can get a sense.

403
00:16:57,520 --> 00:16:58,630
It looks great.

404
00:16:58,630 --> 00:17:00,270
It's about a micron thick.

405
00:17:00,270 --> 00:17:04,270
It's about 170 times
thinner than those wafers

406
00:17:04,270 --> 00:17:06,555
that you saw on Tuesday.

407
00:17:06,555 --> 00:17:08,430
So that's an example of
a thin film material.

408
00:17:08,430 --> 00:17:10,025
It will be making its rounds.

409
00:17:10,025 --> 00:17:11,900
There's a large amount
of technology transfer

410
00:17:11,900 --> 00:17:14,849
with a thin film display, the
flat panel display industry,

411
00:17:14,849 --> 00:17:18,359
with deposition on glass
like that one right there.

412
00:17:18,359 --> 00:17:20,950
And there's a potential it'll
be very nice for building

413
00:17:20,950 --> 00:17:22,477
integrated PV applications.

414
00:17:22,477 --> 00:17:24,060
If you're able to
get rid of the glass

415
00:17:24,060 --> 00:17:25,670
and deposit on a
conformal substrate,

416
00:17:25,670 --> 00:17:30,540
you could envision roof shingles
or other flexible substrates

417
00:17:30,540 --> 00:17:33,120
that would allow you conformal
coverage on undulating roof

418
00:17:33,120 --> 00:17:35,950
tops and so forth.

419
00:17:35,950 --> 00:17:38,550
Radiation hardness-- this
is just a small aside,

420
00:17:38,550 --> 00:17:41,270
but there are some materials
that have better radiation

421
00:17:41,270 --> 00:17:42,420
hardness than silicon.

422
00:17:42,420 --> 00:17:43,860
What does radiation
hardness mean?

423
00:17:43,860 --> 00:17:46,151
It means that if I send
something to outer space, where

424
00:17:46,151 --> 00:17:48,190
we don't benefit
from the radiation

425
00:17:48,190 --> 00:17:51,170
shield of our own atmosphere in
the Van Allen belts on earth,

426
00:17:51,170 --> 00:17:56,740
and we have proton bombardment
and other forms of radiation

427
00:17:56,740 --> 00:17:59,600
striking are module and creating
damage within the absorber

428
00:17:59,600 --> 00:18:03,410
layer, some compounds are
naturally better at resisting

429
00:18:03,410 --> 00:18:05,860
degradation of
performance than others,

430
00:18:05,860 --> 00:18:07,789
and that's what
radiation hardness means.

431
00:18:07,789 --> 00:18:09,330
So there are some
thin film materials

432
00:18:09,330 --> 00:18:13,200
that are exceptional
for space applications.

433
00:18:13,200 --> 00:18:15,240
The challenges and--
oh, go ahead, Ashley.

434
00:18:15,240 --> 00:18:16,990
ASHLEY: Is gallium
arsenide one of them?

435
00:18:16,990 --> 00:18:19,114
PROFESSOR: We're going to
show you in a few slides.

436
00:18:19,114 --> 00:18:22,260
We'll compare them all as a
function of radiation exposure

437
00:18:22,260 --> 00:18:23,160
time.

438
00:18:23,160 --> 00:18:25,210
The disadvantages, or
shall we say challenges

439
00:18:25,210 --> 00:18:28,072
and opportunities for PhD
and master's students,

440
00:18:28,072 --> 00:18:30,280
lower efficiencies in
crystalline silicon potentially

441
00:18:30,280 --> 00:18:31,591
larger module costs.

442
00:18:31,591 --> 00:18:34,090
If you're able to improve the
performance of these thin film

443
00:18:34,090 --> 00:18:38,530
materials, wow, you have
now equivalent performance

444
00:18:38,530 --> 00:18:40,950
of crystalline silicon
but at much lower cost.

445
00:18:40,950 --> 00:18:43,460
Good for you-- you have
a marketable product.

446
00:18:43,460 --> 00:18:47,110
Potential for capital intensive
production equipment-- not all

447
00:18:47,110 --> 00:18:50,080
of the production equipment
is as low cost and as low

448
00:18:50,080 --> 00:18:52,900
thermal budget as simply
printing on a piece of paper.

449
00:18:52,900 --> 00:18:56,510
As a matter of fact, that's one
of the more avant garde and R&D

450
00:18:56,510 --> 00:18:58,730
type of deposition processes.

451
00:18:58,730 --> 00:19:01,280
Most deposition processes
and the vast majority

452
00:19:01,280 --> 00:19:04,590
of companies used are actually
quite capital intensive,

453
00:19:04,590 --> 00:19:07,300
and the cost of the
equipment can add up.

454
00:19:07,300 --> 00:19:09,930
Sometimes, not always,
but sometimes scarce

455
00:19:09,930 --> 00:19:11,760
elements are used.

456
00:19:11,760 --> 00:19:14,790
We're going to have a debate
about that on next class,

457
00:19:14,790 --> 00:19:16,822
on Tuesday.

458
00:19:16,822 --> 00:19:18,030
Put an asterisk next to that.

459
00:19:18,030 --> 00:19:20,238
I'll get back to those as
soon as this slide is over.

460
00:19:20,238 --> 00:19:23,580
And spatial uniformity is a
challenge during deposition.

461
00:19:23,580 --> 00:19:28,210
Imagine trying to deposit a
film one-micron thick over glass

462
00:19:28,210 --> 00:19:31,120
that is one meter in size.

463
00:19:31,120 --> 00:19:34,570
You're talking about a six order
of magnitude aspect ratio here.

464
00:19:34,570 --> 00:19:38,730
So we have to somehow deposit
a film a micron thick in layers

465
00:19:38,730 --> 00:19:40,170
that are even
thinner, that might

466
00:19:40,170 --> 00:19:43,320
be only a few tens or hundreds
of nanometers on top of that

467
00:19:43,320 --> 00:19:46,290
and below that absorber
layer to separate charge,

468
00:19:46,290 --> 00:19:48,910
for instance, and that's really
challenging to do on a very

469
00:19:48,910 --> 00:19:50,869
large scale, and that
is an engineering

470
00:19:50,869 --> 00:19:52,910
challenge or a process
engineering challenge that

471
00:19:52,910 --> 00:19:56,870
had many startup companies
flailing for a long time.

472
00:19:56,870 --> 00:19:59,232
Think of spatial homogeneity
in the following manner;

473
00:19:59,232 --> 00:20:00,940
if you have one region
of your solar cell

474
00:20:00,940 --> 00:20:03,356
that's producing a lot of
power, and the region next to it

475
00:20:03,356 --> 00:20:07,550
is not, and they're connected in
parallel through the contacts,

476
00:20:07,550 --> 00:20:10,160
power will flow from the good
region into the bad region.

477
00:20:10,160 --> 00:20:14,420
So you have internal current
loops inside of your module.

478
00:20:14,420 --> 00:20:16,610
That is essentially
decreasing the power output

479
00:20:16,610 --> 00:20:18,420
of your module itself.

480
00:20:18,420 --> 00:20:21,950
So that's why
homogeneity is important.

481
00:20:21,950 --> 00:20:24,109
This is just to
represent the vision

482
00:20:24,109 --> 00:20:26,650
of a roll-to-roll process in
the upper right-hand side there.

483
00:20:26,650 --> 00:20:32,500
Kind of a visionary cartoon
that is being enacted

484
00:20:32,500 --> 00:20:34,630
by one company, in
particular, Uni-Solar,

485
00:20:34,630 --> 00:20:35,660
based out of Michigan.

486
00:20:35,660 --> 00:20:37,750
They do have a
roll-to-roll process

487
00:20:37,750 --> 00:20:39,720
and PCBD-- we'll
describe what that

488
00:20:39,720 --> 00:20:42,830
is in a second-- deposition
of this material,

489
00:20:42,830 --> 00:20:44,381
so-called amorphous silicon.

490
00:20:44,381 --> 00:20:46,380
And here are some building
integrated solutions,

491
00:20:46,380 --> 00:20:47,950
just showing you what
you can accomplish

492
00:20:47,950 --> 00:20:49,190
or what the vision would be.

493
00:20:49,190 --> 00:20:53,170
If had have this really flexible
substrate that you could

494
00:20:53,170 --> 00:20:57,020
literally take it as a
roll from Home Depot,

495
00:20:57,020 --> 00:20:59,490
bring up to your rooftop,
splay it out on your roof,

496
00:20:59,490 --> 00:21:02,400
much like you'd lay down a
piece of tarp or plastic,

497
00:21:02,400 --> 00:21:07,030
and take a staple gun or a nail
gun and drill it into location,

498
00:21:07,030 --> 00:21:11,072
that would be an example of a
much reduced installation cost.

499
00:21:11,072 --> 00:21:13,530
So you have the potential here
of reducing the installation

500
00:21:13,530 --> 00:21:18,484
cost of solar as a result of
the form factor of your module.

501
00:21:18,484 --> 00:21:20,400
And this here is another
example of a building

502
00:21:20,400 --> 00:21:22,820
integrated photovoltaic
solution within films.

503
00:21:22,820 --> 00:21:25,040
The fact that it looks
really nice, is really sleek,

504
00:21:25,040 --> 00:21:27,430
you'd never guess that those
are solar panels there,

505
00:21:27,430 --> 00:21:29,860
and that's, of course, from
an aesthetic point of view,

506
00:21:29,860 --> 00:21:32,300
a huge benefit.

507
00:21:32,300 --> 00:21:34,040
Common growth
methods-- how do we

508
00:21:34,040 --> 00:21:37,980
make that sample of
copper indium gallium

509
00:21:37,980 --> 00:21:40,210
diselenide, that thin
film material that happens

510
00:21:40,210 --> 00:21:42,460
to be making its way around
the classroom right now,

511
00:21:42,460 --> 00:21:44,220
how do we actually make it?

512
00:21:44,220 --> 00:21:46,750
Well, not only the
material I just described,

513
00:21:46,750 --> 00:21:48,190
there are other
materials as well.

514
00:21:48,190 --> 00:21:51,460
We'll talk about the general
classes of growth method.

515
00:21:51,460 --> 00:21:55,840
So this is the material science
processing class condensed

516
00:21:55,840 --> 00:21:57,010
into a few slides.

517
00:21:57,010 --> 00:21:59,280
Bear with me; this
very high level,

518
00:21:59,280 --> 00:22:02,150
but it aims to highlight
the techniques that are

519
00:22:02,150 --> 00:22:04,500
most commonly used in PV today.

520
00:22:04,500 --> 00:22:05,950
We're going to
start with what are

521
00:22:05,950 --> 00:22:08,910
called vacuum-based thin-film
deposition technologies.

522
00:22:08,910 --> 00:22:11,960
And the reason I'm separating
vacuum from non-vacuum

523
00:22:11,960 --> 00:22:13,894
is because if you
have a system that

524
00:22:13,894 --> 00:22:16,060
is comprised of these large
stainless steel chambers

525
00:22:16,060 --> 00:22:18,435
that you typically see when
you go walking in the physics

526
00:22:18,435 --> 00:22:21,410
building, if you have
a vacuum chambers,

527
00:22:21,410 --> 00:22:22,930
those are typically
quite costly,

528
00:22:22,930 --> 00:22:24,355
at least the large
scale ones that

529
00:22:24,355 --> 00:22:26,470
are in commercial production.

530
00:22:26,470 --> 00:22:28,010
As the name would
suggest, you need

531
00:22:28,010 --> 00:22:32,970
to have pumps to suck out the
air inside of the chamber,

532
00:22:32,970 --> 00:22:34,760
and that's how you
create the vacuum.

533
00:22:34,760 --> 00:22:37,370
The vacuum is necessary
because typically you're

534
00:22:37,370 --> 00:22:40,230
transporting atoms from
some sort of source,

535
00:22:40,230 --> 00:22:43,785
either gas or a solid
target, onto the substrate.

536
00:22:43,785 --> 00:22:45,910
So you're transferring
individual atoms or clusters

537
00:22:45,910 --> 00:22:49,700
of atoms from some
source onto the substrate

538
00:22:49,700 --> 00:22:52,380
that will ultimately hold
your thin film device.

539
00:22:52,380 --> 00:22:55,640
And that process
requires a limited number

540
00:22:55,640 --> 00:22:58,970
of interactions of those
atoms or clusters of atoms,

541
00:22:58,970 --> 00:23:01,300
in other words, a
large mean-free path,

542
00:23:01,300 --> 00:23:04,140
as these make their
way to your substrate.

543
00:23:04,140 --> 00:23:06,250
And that's why the vacuum
is typically required

544
00:23:06,250 --> 00:23:07,730
in these deposition systems.

545
00:23:07,730 --> 00:23:11,090
There are a variety of ways
to accomplish this goal.

546
00:23:11,090 --> 00:23:13,890
One class of
techniques is called

547
00:23:13,890 --> 00:23:17,960
Chemical Vapor Deposition,
often referred to as CVD.

548
00:23:17,960 --> 00:23:21,250
This typically involves
flowing in some form of gas

549
00:23:21,250 --> 00:23:25,350
into your chamber and
then allowing that gas

550
00:23:25,350 --> 00:23:29,070
to react on the
surface of your sample

551
00:23:29,070 --> 00:23:31,070
or above the surface
of your sample

552
00:23:31,070 --> 00:23:33,870
and ultimately depositing
on the surface.

553
00:23:33,870 --> 00:23:38,240
The chemistries involved in CVD
processes can be quite complex,

554
00:23:38,240 --> 00:23:40,610
and the reaction
process itself can

555
00:23:40,610 --> 00:23:42,550
be very difficult to master.

556
00:23:42,550 --> 00:23:44,100
So you might have
some friends who

557
00:23:44,100 --> 00:23:46,852
are involved in spectroscopy
shining lasers at their system

558
00:23:46,852 --> 00:23:48,310
and looking at the
absorption lines

559
00:23:48,310 --> 00:23:50,880
and trying to figure out how
these molecules are evolving

560
00:23:50,880 --> 00:23:52,530
between when they're
inserted into the chamber

561
00:23:52,530 --> 00:23:54,390
and when they actually
wind up as your film,

562
00:23:54,390 --> 00:23:55,810
because understanding
the reaction,

563
00:23:55,810 --> 00:23:58,018
the chemical reactions, that
take place is essential,

564
00:23:58,018 --> 00:24:01,104
is key, to really
controlling the CVD process.

565
00:24:01,104 --> 00:24:02,770
The other class of
technologies involved

566
00:24:02,770 --> 00:24:05,440
is called PVD, or
Physical Vapor Deposition,

567
00:24:05,440 --> 00:24:07,740
and this tends to be a
bit more straightforward.

568
00:24:07,740 --> 00:24:11,750
We tend to have atoms
of a specific type.

569
00:24:11,750 --> 00:24:13,880
They may be ionized, or
they may be charge neutral,

570
00:24:13,880 --> 00:24:15,838
and they're making their
way to your substrate.

571
00:24:15,838 --> 00:24:17,860
And the chemistry tends
to be much more simple,

572
00:24:17,860 --> 00:24:21,850
but the apparatus around
it to give the incentive

573
00:24:21,850 --> 00:24:23,460
for the atoms to
leave the target

574
00:24:23,460 --> 00:24:26,980
and deposit on your substrate,
that tends to be more complex.

575
00:24:26,980 --> 00:24:29,960
And so some of these
tools, especially

576
00:24:29,960 --> 00:24:33,460
molecular-beam epitaxy can
be very expensive, very slow,

577
00:24:33,460 --> 00:24:36,640
but very high quality, but
very expensive as a result.

578
00:24:36,640 --> 00:24:39,290
And so a very
simple way to think

579
00:24:39,290 --> 00:24:42,900
about the vacuum-based
deposition technologies

580
00:24:42,900 --> 00:24:47,052
is a compromise-- this is an
oversimplification indeed,

581
00:24:47,052 --> 00:24:48,510
but it's an easy
way to get started

582
00:24:48,510 --> 00:24:51,130
about thinking of the parameter
space of all these techniques.

583
00:24:51,130 --> 00:24:54,860
It's a compromise between
speed and quality.

584
00:24:54,860 --> 00:24:56,980
Some of the techniques
that are fastest

585
00:24:56,980 --> 00:24:58,924
also tend to be the
lowest quality materials,

586
00:24:58,924 --> 00:25:00,840
and the other ones that
tend to be the slowest

587
00:25:00,840 --> 00:25:02,756
tend to produce the
highest quality materials.

588
00:25:02,756 --> 00:25:06,020
How do you optimize somewhere
in between, somewhere

589
00:25:06,020 --> 00:25:08,810
in that parameter space, to
get reasonably high material,

590
00:25:08,810 --> 00:25:11,430
just enough that you can produce
a high efficiency device--

591
00:25:11,430 --> 00:25:13,760
remember that saturation
of device performance

592
00:25:13,760 --> 00:25:14,980
versus diffusion length.

593
00:25:14,980 --> 00:25:16,400
At some point, it just
doesn't make sense

594
00:25:16,400 --> 00:25:17,710
to keep optimizing
your material.

595
00:25:17,710 --> 00:25:18,820
You've got it good enough.

596
00:25:18,820 --> 00:25:21,070
You're good to go.

597
00:25:21,070 --> 00:25:25,460
So that's one of the things to
consider when you're choosing

598
00:25:25,460 --> 00:25:26,970
your deposition system.

599
00:25:26,970 --> 00:25:30,530
So let's go into a few
examples of these vacuum-based

600
00:25:30,530 --> 00:25:31,860
deposition systems.

601
00:25:31,860 --> 00:25:34,150
Within the PVD techniques,
within the Physical Vapor

602
00:25:34,150 --> 00:25:37,120
Deposition techniques, one
of the most commonly used

603
00:25:37,120 --> 00:25:39,820
in manufacturing, at
least in some startups--

604
00:25:39,820 --> 00:25:44,160
you have examples like
MiaSole-- is sputtering.

605
00:25:44,160 --> 00:25:46,686
And this sputtering
process is essentially

606
00:25:46,686 --> 00:25:47,810
very, very straightforward.

607
00:25:47,810 --> 00:25:49,490
You have a plasma.

608
00:25:49,490 --> 00:25:53,730
The plasma consists of
atoms that are charged.

609
00:25:53,730 --> 00:25:57,340
These are accelerated
toward your target, which

610
00:25:57,340 --> 00:25:59,170
is comprised of the
elements that you want

611
00:25:59,170 --> 00:26:01,610
to deposit onto your substrate.

612
00:26:01,610 --> 00:26:03,190
Your substrate is
sitting up top.

613
00:26:03,190 --> 00:26:07,310
And this target material is
sputtered off and eventually

614
00:26:07,310 --> 00:26:10,350
makes its way up and sticks
to and eventually grows

615
00:26:10,350 --> 00:26:13,620
the film on that orange
platen up here at the top.

616
00:26:13,620 --> 00:26:15,370
That is your substrate.

617
00:26:15,370 --> 00:26:17,300
The substrate is facing down.

618
00:26:17,300 --> 00:26:19,000
Why is the substrate
looking down?

619
00:26:19,000 --> 00:26:21,230
Why wouldn't you invert
this and put the target

620
00:26:21,230 --> 00:26:23,270
on top and in the
substrate in the bottom?

621
00:26:23,270 --> 00:26:25,730
What could happened
then in terms of purity

622
00:26:25,730 --> 00:26:26,920
of the deposition process?

623
00:26:26,920 --> 00:26:28,070
Let's go to Kristy.

624
00:26:28,070 --> 00:26:30,084
AUDIENCE: Things
could fall onto it.

625
00:26:30,084 --> 00:26:32,500
PROFESSOR: So stuff, gunk,
could fall onto your substrate.

626
00:26:32,500 --> 00:26:34,870
You're trying to grow a
thin film a micron thick,

627
00:26:34,870 --> 00:26:36,770
and you're trying to
avoid any imperfection,

628
00:26:36,770 --> 00:26:39,080
and now gravity is working
against you in that case.

629
00:26:39,080 --> 00:26:40,910
Because, if you
were to invert this,

630
00:26:40,910 --> 00:26:42,220
your target would be on top.

631
00:26:42,220 --> 00:26:44,980
You could have stuff raining
down onto your substrate.

632
00:26:44,980 --> 00:26:46,930
There are a few people
who sputter down.

633
00:26:46,930 --> 00:26:47,722
It's very tricky.

634
00:26:47,722 --> 00:26:49,930
You have to be able to
control your process very well

635
00:26:49,930 --> 00:26:51,560
and avoid flakes
from coming off.

636
00:26:51,560 --> 00:26:54,050
There are folks who
sputter sideways,

637
00:26:54,050 --> 00:26:56,100
saves some ground
space in their factory.

638
00:26:56,100 --> 00:26:59,480
They might load things
vertically, put them in.

639
00:26:59,480 --> 00:27:03,240
And many people, at
least in R&D, sputter up.

640
00:27:03,240 --> 00:27:06,790
So again, you're
creating this plasma.

641
00:27:06,790 --> 00:27:10,040
The charged species are
accelerated toward the target.

642
00:27:10,040 --> 00:27:13,540
They sputter off
atoms, which are then

643
00:27:13,540 --> 00:27:16,330
deposited on to your substrate,
which is there at the top.

644
00:27:16,330 --> 00:27:19,380
And the film that was
just being passed around

645
00:27:19,380 --> 00:27:21,600
is an example of
a sputtered film.

646
00:27:21,600 --> 00:27:23,910
The spatial uniformity
of sputtering

647
00:27:23,910 --> 00:27:25,860
over large area
depositions can be

648
00:27:25,860 --> 00:27:27,810
in the order of a few percent.

649
00:27:27,810 --> 00:27:29,970
So the ability to
control this process

650
00:27:29,970 --> 00:27:34,970
in terms of spatial
uniformity is fairly good.

651
00:27:34,970 --> 00:27:37,060
You could also employ
radio frequency modulations

652
00:27:37,060 --> 00:27:38,080
to the bias voltage.

653
00:27:38,080 --> 00:27:41,060
That's called RF sputtering
for Radio Frequency.

654
00:27:41,060 --> 00:27:45,530
Industrial applications usually
involve large rotating targets.

655
00:27:45,530 --> 00:27:47,320
So for those of
you-- how many people

656
00:27:47,320 --> 00:27:48,770
actually work with some
sputtering materials

657
00:27:48,770 --> 00:27:49,936
or have done it in the past?

658
00:27:49,936 --> 00:27:52,716
One, two, three,
four, five, six, OK.

659
00:27:52,716 --> 00:27:54,590
So you know that, at
least in the laboratory,

660
00:27:54,590 --> 00:27:57,131
if you have a fixed target, you
wind up with that race track,

661
00:27:57,131 --> 00:27:57,750
right?

662
00:27:57,750 --> 00:27:59,210
So if you have a fixed
target in the lab,

663
00:27:59,210 --> 00:28:00,876
and you're trying to
deposit your films,

664
00:28:00,876 --> 00:28:03,490
if you wear it
down several hours,

665
00:28:03,490 --> 00:28:05,880
eventually the metal that
you're trying to deposit,

666
00:28:05,880 --> 00:28:07,980
or the ceramic that
you're trying to deposit,

667
00:28:07,980 --> 00:28:10,710
will usually wind up having
a bit of shape to it.

668
00:28:10,710 --> 00:28:12,620
Instead of being
flat on the surface,

669
00:28:12,620 --> 00:28:14,280
you'll have what's
called a race track;

670
00:28:14,280 --> 00:28:16,160
it'll dipped down
near the edges,

671
00:28:16,160 --> 00:28:19,440
and that can result in a
change of the deposition

672
00:28:19,440 --> 00:28:23,359
rate of the species that
you're trying to deposit.

673
00:28:23,359 --> 00:28:24,900
And from a homogeneity
point of view,

674
00:28:24,900 --> 00:28:26,960
that might be disastrous
in the company,

675
00:28:26,960 --> 00:28:30,440
and so there are methods
to move your target

676
00:28:30,440 --> 00:28:33,650
to avoid that sort of
effect from happening.

677
00:28:33,650 --> 00:28:36,040
And when we talk
about large targets,

678
00:28:36,040 --> 00:28:38,140
we're really talking about
large targets, right?

679
00:28:38,140 --> 00:28:40,900
These aren't your lab scale
two-inch or three-inch,

680
00:28:40,900 --> 00:28:45,240
these are much, much bigger
in commercial production.

681
00:28:45,240 --> 00:28:50,210
So in terms of comparing
sputtering against other growth

682
00:28:50,210 --> 00:28:54,750
technologies, there
are technologies

683
00:28:54,750 --> 00:28:56,330
that are more conformal.

684
00:28:56,330 --> 00:28:58,900
Because this is more of a
line-of-sight deposition

685
00:28:58,900 --> 00:29:02,440
technique, the atoms are
moving toward your substrates.

686
00:29:02,440 --> 00:29:07,370
But if you have some
shape to your substrate,

687
00:29:07,370 --> 00:29:10,380
maybe you have a ledge
or a ridge, in that case,

688
00:29:10,380 --> 00:29:13,090
you won't necessarily
coat that uniformly.

689
00:29:13,090 --> 00:29:16,780
You might have less being
deposited on that edge rather

690
00:29:16,780 --> 00:29:18,140
than the flat sections.

691
00:29:18,140 --> 00:29:21,727
And so conformality of coverage,
or conformal surface coverage,

692
00:29:21,727 --> 00:29:23,060
can be an issue with sputtering.

693
00:29:25,710 --> 00:29:28,190
Let's talk about
the next technique

694
00:29:28,190 --> 00:29:34,310
that is commonly used in
inorganic thin-film deposition.

695
00:29:34,310 --> 00:29:35,830
Excuse me.

696
00:29:35,830 --> 00:29:39,780
This is called metalorganic
chemical vapor deposition.

697
00:29:39,780 --> 00:29:41,780
So again we notice the
CVD appearing at the end.

698
00:29:41,780 --> 00:29:44,030
We know it's a Chemical
Vapor Deposition process.

699
00:29:44,030 --> 00:29:47,260
MO in this case, standing
for Metalorganic.

700
00:29:47,260 --> 00:29:50,070
The reason metalorganic
is because we typically

701
00:29:50,070 --> 00:29:53,170
have a metal, like this
representing the indium right

702
00:29:53,170 --> 00:29:57,700
here, and then little organic
compounds on the outside.

703
00:29:57,700 --> 00:29:58,700
Those are methyl groups.

704
00:29:58,700 --> 00:30:01,510
The little gray and the
two white dots, those

705
00:30:01,510 --> 00:30:05,060
represent three methyl
groups around the indium, so

706
00:30:05,060 --> 00:30:06,060
trimethylindium.

707
00:30:06,060 --> 00:30:10,710
And what we do is we flow these
molecules into our reaction

708
00:30:10,710 --> 00:30:13,835
chamber and control the
temperature gradients inside

709
00:30:13,835 --> 00:30:15,970
in such a way to
have those molecules

710
00:30:15,970 --> 00:30:19,610
deposit on the surface, leaving
the indium behind, or the metal

711
00:30:19,610 --> 00:30:22,130
behind, and the
reaction products flow

712
00:30:22,130 --> 00:30:27,057
away out the back, and that
is represented chemically

713
00:30:27,057 --> 00:30:27,890
here on the surface.

714
00:30:27,890 --> 00:30:31,090
This is zooming in right at
the surface of our sample

715
00:30:31,090 --> 00:30:33,690
so right where the gas interacts
with the thin film material

716
00:30:33,690 --> 00:30:35,090
that you're depositing.

717
00:30:35,090 --> 00:30:39,110
This is representing the
incoming metalorganic molecule

718
00:30:39,110 --> 00:30:40,450
reaching the surface.

719
00:30:40,450 --> 00:30:44,800
This represents, right
here, the separation where

720
00:30:44,800 --> 00:30:49,160
we have the indium shown
in black right here,

721
00:30:49,160 --> 00:30:52,420
and then the methyl
groups are moving off,

722
00:30:52,420 --> 00:30:54,290
and essentially, those
will be sucked out

723
00:30:54,290 --> 00:30:57,340
of the chamber, leaving behind,
in this particular case,

724
00:30:57,340 --> 00:31:00,560
you have a layer of indium
forming, probably another layer

725
00:31:00,560 --> 00:31:01,940
of material underneath.

726
00:31:01,940 --> 00:31:05,270
Say, for example, your
other species comprising

727
00:31:05,270 --> 00:31:08,740
the thin film may be phosphorus,
so it would be indium phosphide

728
00:31:08,740 --> 00:31:10,230
growth.

729
00:31:10,230 --> 00:31:13,500
This metalorganic
chemical vapor deposition

730
00:31:13,500 --> 00:31:15,120
is very nice from
the point of view

731
00:31:15,120 --> 00:31:18,590
that you tend to form
homogeneous films-- very

732
00:31:18,590 --> 00:31:20,690
good surface coverage.

733
00:31:20,690 --> 00:31:24,860
The disadvantages would be that
many of the inputs and outputs

734
00:31:24,860 --> 00:31:27,540
are toxix-- not always,
but many of them are.

735
00:31:27,540 --> 00:31:29,130
They have to be
volatile and reactive

736
00:31:29,130 --> 00:31:31,850
so that you can crack
the metal on your surface

737
00:31:31,850 --> 00:31:34,509
and create the thin film.

738
00:31:34,509 --> 00:31:36,050
If it wasn't reactive,
you would just

739
00:31:36,050 --> 00:31:38,380
have it flowing through
and leaving, not having

740
00:31:38,380 --> 00:31:40,060
a reactant with your substrate.

741
00:31:40,060 --> 00:31:42,180
But because of the reactivity
involved, oftentimes

742
00:31:42,180 --> 00:31:44,640
these are not very
friendly for human beings

743
00:31:44,640 --> 00:31:46,540
or for other organisms.

744
00:31:46,540 --> 00:31:48,997
It was not uncommon in the
early days of MOCVD reactor

745
00:31:48,997 --> 00:31:51,330
development where they'd have
this little stack going up

746
00:31:51,330 --> 00:31:52,630
to the roof, and then
when they'd do maintenance

747
00:31:52,630 --> 00:31:55,088
on the roof, they'd find all
these dead birds lying around.

748
00:31:55,088 --> 00:31:58,070
That obviously has improved
since people have put up

749
00:31:58,070 --> 00:32:02,030
the appropriate filtration
on the output of their growth

750
00:32:02,030 --> 00:32:05,950
system, so-called scrubbers, to
prevent toxic gases from being

751
00:32:05,950 --> 00:32:07,420
released into the atmosphere.

752
00:32:07,420 --> 00:32:11,450
But you do have
some old stories.

753
00:32:11,450 --> 00:32:14,300
So the proper design of
metalorganic precursors

754
00:32:14,300 --> 00:32:14,970
is essential.

755
00:32:14,970 --> 00:32:18,280
You can easily see how if you
change the molecule that you're

756
00:32:18,280 --> 00:32:20,790
bringing in, all of a sudden
now, your reaction temperatures

757
00:32:20,790 --> 00:32:21,331
are changing.

758
00:32:21,331 --> 00:32:23,870
The rate of deposition
is changing,

759
00:32:23,870 --> 00:32:26,850
and you have to optimize your
growth process all over again.

760
00:32:26,850 --> 00:32:29,010
So part of the trick
of doing good MOCVD

761
00:32:29,010 --> 00:32:32,360
is knowing your chemistry, being
able to design or synthesize

762
00:32:32,360 --> 00:32:35,410
these metalorganic precursors.

763
00:32:35,410 --> 00:32:38,720
And the deposition process is
very sensitive to temperature,

764
00:32:38,720 --> 00:32:42,830
pressure, the precise surface
orientation, and preparation,

765
00:32:42,830 --> 00:32:46,010
what carrier gases, as well, are
mixed in with the metalorganic

766
00:32:46,010 --> 00:32:47,790
precursor that
you're putting in,

767
00:32:47,790 --> 00:32:50,382
and the byproducts obviously
need to be managed.

768
00:32:50,382 --> 00:32:51,920
So that's MOCVD in a nutshell.

769
00:32:51,920 --> 00:32:52,420
Yes?

770
00:32:52,420 --> 00:32:55,724
AUDIENCE: And pure quality
is much better with MOCVD

771
00:32:55,724 --> 00:32:57,620
than it is for
sputtering, right?

772
00:32:57,620 --> 00:32:59,530
PROFESSOR: It depends
on a lot of factors.

773
00:32:59,530 --> 00:33:01,110
So the reason the
purity of MOCVD

774
00:33:01,110 --> 00:33:02,750
is generally better
than sputtering

775
00:33:02,750 --> 00:33:04,620
is because the mass
flow controllers

776
00:33:04,620 --> 00:33:08,500
necessary to control
the gas flow specific

777
00:33:08,500 --> 00:33:10,594
for particular types of gases.

778
00:33:10,594 --> 00:33:12,510
Now, in sputtering,
because of the versatility

779
00:33:12,510 --> 00:33:15,700
of the sputtering chamber,
you could take this target out

780
00:33:15,700 --> 00:33:18,020
and put-- maybe
Ashley comes along

781
00:33:18,020 --> 00:33:19,630
into your sputtering
chamber, and she

782
00:33:19,630 --> 00:33:21,470
puts in another target
of another metal.

783
00:33:21,470 --> 00:33:22,870
And now you're depositing
two different metals

784
00:33:22,870 --> 00:33:23,840
in the same sputtering chamber.

785
00:33:23,840 --> 00:33:25,507
You're going to get
cross contamination.

786
00:33:25,507 --> 00:33:28,006
There are things you can do to
minimize cross contamination.

787
00:33:28,006 --> 00:33:30,577
You can have a chimney around
your target to prevent flakes

788
00:33:30,577 --> 00:33:31,510
from coming down.

789
00:33:31,510 --> 00:33:34,940
You could sandblast
the sidewall coating

790
00:33:34,940 --> 00:33:37,246
and so forth to prevent
stuff, gunk, from building up

791
00:33:37,246 --> 00:33:38,620
around the side,
but you're still

792
00:33:38,620 --> 00:33:40,578
going to get a lot of
cross contamination here.

793
00:33:40,578 --> 00:33:42,920
And furthermore, the
purity of your film

794
00:33:42,920 --> 00:33:45,020
is dictated by the
purity of your re-target.

795
00:33:45,020 --> 00:33:47,830
And if you go online and look
at [INAUDIBLE] or CERAC or some

796
00:33:47,830 --> 00:33:51,770
of the big metal selling
firms, which are essentially

797
00:33:51,770 --> 00:33:55,340
from where the target
manufacturers are purchasing

798
00:33:55,340 --> 00:33:57,230
their precursors and
they compact them

799
00:33:57,230 --> 00:34:01,520
and make their targets, the
target purity, or the metal

800
00:34:01,520 --> 00:34:05,880
purity, is only on the order
of maybe 2/9 to 6/9 pure,

801
00:34:05,880 --> 00:34:07,660
typically within that range.

802
00:34:07,660 --> 00:34:10,370
So from an MOCVD
point of view, you

803
00:34:10,370 --> 00:34:12,210
could do a distillation
process and increase

804
00:34:12,210 --> 00:34:15,810
the purity of your precursor
gas and avoid that.

805
00:34:15,810 --> 00:34:20,820
So I think two big reasons why
MOCVD can produce higher purity

806
00:34:20,820 --> 00:34:23,199
films in the
sputtered system, one

807
00:34:23,199 --> 00:34:26,230
is the quality of the
target, and the other,

808
00:34:26,230 --> 00:34:29,110
I think, bigger parameter, at
least in our growth system,

809
00:34:29,110 --> 00:34:31,199
is cross contamination.

810
00:34:31,199 --> 00:34:32,964
And whenever you
deposit, say, an EML--

811
00:34:32,964 --> 00:34:35,172
they have a sputtering system
there from AJA, or over

812
00:34:35,172 --> 00:34:37,904
at Harvard CNS, there's another
AJA sputtering system there--

813
00:34:37,904 --> 00:34:39,570
you're going to get
cross contamination.

814
00:34:39,570 --> 00:34:41,080
Just look to the log
book and see what

815
00:34:41,080 --> 00:34:42,288
people have tried to deposit.

816
00:34:42,288 --> 00:34:44,260
It gets kind of scary.

817
00:34:44,260 --> 00:34:47,370
PECVD, Plasma Enhanced
Chemical Vapor Deposition--

818
00:34:47,370 --> 00:34:51,020
so similar to the previous
variety right here,

819
00:34:51,020 --> 00:34:53,370
but instead of saying,
OK, we're going

820
00:34:53,370 --> 00:34:56,889
to put the burden of the
design, the scientific design,

821
00:34:56,889 --> 00:34:59,620
onto this interface right here
and on to the chemist, who

822
00:34:59,620 --> 00:35:02,320
has to design this molecule
that reaches a surface

823
00:35:02,320 --> 00:35:05,740
and breaks up in just the right
way in an orderly fashion,

824
00:35:05,740 --> 00:35:08,740
leaving behind the metal and
letting the other gases go

825
00:35:08,740 --> 00:35:10,290
away, what we're
going to do here

826
00:35:10,290 --> 00:35:14,360
instead is to shift the burden
of separation onto the plasma.

827
00:35:14,360 --> 00:35:16,370
So the centers around
the physicists.

828
00:35:16,370 --> 00:35:17,580
We can flow in gases.

829
00:35:17,580 --> 00:35:19,570
We can break them up
inside of a plasma,

830
00:35:19,570 --> 00:35:22,460
atomize them or, at least,
create radicalized versions

831
00:35:22,460 --> 00:35:24,940
of them and then
allow them to it

832
00:35:24,940 --> 00:35:27,710
on to the substrate--
very simple in theory.

833
00:35:27,710 --> 00:35:29,700
In practice, what happens
inside that plasma,

834
00:35:29,700 --> 00:35:31,325
depending on the
temperature, depending

835
00:35:31,325 --> 00:35:33,255
on the frequency
and other factors,

836
00:35:33,255 --> 00:35:35,630
you'll get different types--
and the pressure, especially

837
00:35:35,630 --> 00:35:39,834
the pressure-- you'll get
different types of molecules

838
00:35:39,834 --> 00:35:40,750
forming in the plasma.

839
00:35:40,750 --> 00:35:42,790
They may be charged, and
they'll be accelerated

840
00:35:42,790 --> 00:35:44,290
toward your substrate
and eventually

841
00:35:44,290 --> 00:35:46,740
grow and form a thin film.

842
00:35:46,740 --> 00:35:49,910
But depending on what
species you have up there

843
00:35:49,910 --> 00:35:51,960
that is being deposited
on your surface,

844
00:35:51,960 --> 00:35:54,780
you'll get different
types of thin films

845
00:35:54,780 --> 00:35:56,920
growing-- different
quality material.

846
00:35:56,920 --> 00:35:59,840
And so, again, this
shifts the burden back

847
00:35:59,840 --> 00:36:03,270
to the spectroscopist to
measure what is exactly

848
00:36:03,270 --> 00:36:04,930
the composition of that plasma.

849
00:36:04,930 --> 00:36:08,122
What is the active molecule
that's being accelerated

850
00:36:08,122 --> 00:36:09,330
and deposited on the surface?

851
00:36:09,330 --> 00:36:12,080
And usually it's some
probability distribution

852
00:36:12,080 --> 00:36:15,330
function of varied species.

853
00:36:15,330 --> 00:36:18,355
The plasma is created
by this radio frequency.

854
00:36:21,460 --> 00:36:23,056
Let's put it this
way; usually you

855
00:36:23,056 --> 00:36:27,530
have a plasma frequency
of around 13.56 megahertz.

856
00:36:27,530 --> 00:36:30,162
Does anybody know why
this 13.56 keeps on coming

857
00:36:30,162 --> 00:36:31,120
up over and over again?

858
00:36:31,120 --> 00:36:31,619
Yeah?

859
00:36:31,619 --> 00:36:34,270
AUDIENCE: [INAUDIBLE] energy
to the ionized hydrogen, right?

860
00:36:34,270 --> 00:36:39,486
PROFESSOR: Well, if
we're thinking about eV,

861
00:36:39,486 --> 00:36:42,300
that would certainly be the
energy necessary to remove

862
00:36:42,300 --> 00:36:45,990
the electron from
the hydrogen atom,

863
00:36:45,990 --> 00:36:47,686
but this is another reason.

864
00:36:47,686 --> 00:36:48,186
Yeah?

865
00:36:48,186 --> 00:36:49,910
AUDIENCE: It's a
special bend that's

866
00:36:49,910 --> 00:36:54,037
dedicated for these crazy
noise-emitting medical and

867
00:36:54,037 --> 00:36:54,870
industrial purposes.

868
00:36:54,870 --> 00:36:57,100
PROFESSOR: Exactly,
so this is falling

869
00:36:57,100 --> 00:36:59,570
within the radio
frequency regime, which

870
00:36:59,570 --> 00:37:01,080
would affect communications.

871
00:37:01,080 --> 00:37:06,200
And if everybody was allowed
to run rough shod around,

872
00:37:06,200 --> 00:37:08,920
creating these very
high intensity emission

873
00:37:08,920 --> 00:37:11,020
sources of radio
frequency waves,

874
00:37:11,020 --> 00:37:15,320
we would very likely have
interruptions to our police

875
00:37:15,320 --> 00:37:19,370
communications or maybe even
our radios or cell phones.

876
00:37:19,370 --> 00:37:22,340
And so, at some point,
they had to say, look,

877
00:37:22,340 --> 00:37:25,860
we have to assign definite
bands within the radio frequency

878
00:37:25,860 --> 00:37:29,490
space and allocate them
to specific purposes.

879
00:37:29,490 --> 00:37:31,650
In one band, they allocated
to all the scientists

880
00:37:31,650 --> 00:37:33,200
and medical personnel
and said, you

881
00:37:33,200 --> 00:37:37,025
have to operate your equipment
in these specific bands,

882
00:37:37,025 --> 00:37:38,400
and we'll give
you a few of them,

883
00:37:38,400 --> 00:37:40,769
because we know that one
frequency doesn't work for all

884
00:37:40,769 --> 00:37:42,060
the things you're trying to do.

885
00:37:42,060 --> 00:37:45,820
But for medical equipment,
for scientific equipment,

886
00:37:45,820 --> 00:37:49,380
and I believe even some home
electronics, like microwaves,

887
00:37:49,380 --> 00:37:52,320
there are specific
bands dedicated to them.

888
00:37:52,320 --> 00:37:56,060
And that's why we have
this 13.56 number popping

889
00:37:56,060 --> 00:37:57,640
up over and over again.

890
00:37:57,640 --> 00:38:00,100
The reality is that if
you change the frequency,

891
00:38:00,100 --> 00:38:01,800
you'll change the
nature of your plasma.

892
00:38:01,800 --> 00:38:04,950
You may change the deposition
rates and the quality

893
00:38:04,950 --> 00:38:06,220
of your film as well.

894
00:38:06,220 --> 00:38:08,640
And so there are people
who get special permits

895
00:38:08,640 --> 00:38:12,040
and have these radio
frequency shielded

896
00:38:12,040 --> 00:38:14,880
rooms, where they do experiments
outside-- or excursions

897
00:38:14,880 --> 00:38:17,880
outside-- of the
13.56 megahertz range.

898
00:38:17,880 --> 00:38:21,580
So this is PCBD-- excellent
conformal surface coverage

899
00:38:21,580 --> 00:38:22,230
again.

900
00:38:22,230 --> 00:38:24,480
Because you're biasing
your substrate,

901
00:38:24,480 --> 00:38:26,321
you're able to conform.

902
00:38:26,321 --> 00:38:27,820
The electric field
is usually always

903
00:38:27,820 --> 00:38:30,820
perpendicular to the
surface, and so the angle

904
00:38:30,820 --> 00:38:35,660
of entry of those atoms or
molecules, the ionized species,

905
00:38:35,660 --> 00:38:38,150
entering the surface is going
to be normal to that surface.

906
00:38:38,150 --> 00:38:44,160
And you can get good coverage
around rough textured surfaces.

907
00:38:44,160 --> 00:38:47,060
The deposition is very sensitive
to temperature, pressure,

908
00:38:47,060 --> 00:38:48,570
power, carrier gases.

909
00:38:48,570 --> 00:38:51,470
Power of the-- here,
as well, shown.

910
00:38:51,470 --> 00:38:53,014
And the byproducts,
as well, need

911
00:38:53,014 --> 00:38:55,180
to be managed because
sometimes you're sucking out--

912
00:38:55,180 --> 00:38:58,936
in this particular case, you
could be pulling out silane,

913
00:38:58,936 --> 00:39:00,810
as shown right there,
and we talked about all

914
00:39:00,810 --> 00:39:05,580
of the risks involved a
silane in our last class.

915
00:39:05,580 --> 00:39:08,440
So, as you could guess, each
of those different deposition

916
00:39:08,440 --> 00:39:13,140
techniques is used or is favored
for specific material systems.

917
00:39:13,140 --> 00:39:16,269
And we shouldn't
forget, as we talk

918
00:39:16,269 --> 00:39:18,060
about all these fancy
vacuum equipment that

919
00:39:18,060 --> 00:39:20,140
look nice and cool as you
walk through the labs,

920
00:39:20,140 --> 00:39:22,490
and you see these big
stainless steel chambers,

921
00:39:22,490 --> 00:39:25,830
we shouldn't forget about the
simpler, lower cost, lower

922
00:39:25,830 --> 00:39:28,170
thermal budget, lower
capital equipment cost

923
00:39:28,170 --> 00:39:31,170
techniques-- the solution-based
deposition methods.

924
00:39:31,170 --> 00:39:33,480
And these involve printing.

925
00:39:33,480 --> 00:39:35,560
They involve a
electrodeposition,

926
00:39:35,560 --> 00:39:38,080
spin casting,
colloidal synthesis,

927
00:39:38,080 --> 00:39:40,530
layer-by-layer deposition--
developed here at MIT--

928
00:39:40,530 --> 00:39:44,080
and other technologies as well.

929
00:39:44,080 --> 00:39:46,600
I want to point out
two technologies,

930
00:39:46,600 --> 00:39:50,310
in general, the first of which
is still under some development

931
00:39:50,310 --> 00:39:50,890
printing.

932
00:39:50,890 --> 00:39:53,150
Obviously, we have
inkjet printers.

933
00:39:53,150 --> 00:39:54,400
That's pretty straightforward.

934
00:39:54,400 --> 00:39:56,690
But printing
fractional solar cells

935
00:39:56,690 --> 00:39:58,410
is something being
commercialized

936
00:39:58,410 --> 00:40:01,910
by only a handful of companies,
Nanosolar being one of them.

937
00:40:01,910 --> 00:40:10,260
And I would say there aren't
any authoritative textbooks that

938
00:40:10,260 --> 00:40:12,130
will describe for
you their technology,

939
00:40:12,130 --> 00:40:13,540
because it's
largely under wraps.

940
00:40:13,540 --> 00:40:16,562
They're a startup company, and
it's not publicly available.

941
00:40:16,562 --> 00:40:19,020
Electrodeposition, on the other
hand, is fairly well known.

942
00:40:19,020 --> 00:40:21,790
You're, again, applying
a voltage difference

943
00:40:21,790 --> 00:40:24,820
between two electrodes, one of
which will be your substrate,

944
00:40:24,820 --> 00:40:27,070
and depositing a
species contained

945
00:40:27,070 --> 00:40:29,850
within your electrolytic
solution onto that substrate,

946
00:40:29,850 --> 00:40:30,940
growing your layers.

947
00:40:30,940 --> 00:40:33,020
Because you're growing
it at room temperature,

948
00:40:33,020 --> 00:40:38,754
these films, I would say,
tend to have rough surfaces.

949
00:40:38,754 --> 00:40:40,670
That could be a downside
of electrodeposition.

950
00:40:40,670 --> 00:40:42,809
They might have some
pinholes as well.

951
00:40:42,809 --> 00:40:44,725
But you do get fairly
large grained materials.

952
00:40:44,725 --> 00:40:47,445
It can be a very gentle growth
process, and, of course,

953
00:40:47,445 --> 00:40:49,940
the advantage is
lower temperature.

954
00:40:49,940 --> 00:40:52,240
So you have a variety of
different growth techniques.

955
00:40:52,240 --> 00:40:55,770
Let's talk about the general
issues involved with thin films

956
00:40:55,770 --> 00:40:58,800
in general, and then we'll dive
into the specific materials.

957
00:40:58,800 --> 00:41:01,400
So taking the same tact as
we've taken the full class,

958
00:41:01,400 --> 00:41:03,530
going from fundamentals
toward the technologies.

959
00:41:03,530 --> 00:41:04,066
Yes?

960
00:41:04,066 --> 00:41:05,482
AUDIENCE: I'm just
curious; do you

961
00:41:05,482 --> 00:41:08,145
know of any companies that
actually use electrodeposition?

962
00:41:08,145 --> 00:41:09,645
PROFESSOR: I know
of some companies.

963
00:41:09,645 --> 00:41:11,730
Let me think which I
can talk publicly about.

964
00:41:15,270 --> 00:41:18,800
So IBM, they presented at the
Electrochemical Society meeting

965
00:41:18,800 --> 00:41:21,230
last Monday here in Boston.

966
00:41:21,230 --> 00:41:22,920
They're an example
of a company that

967
00:41:22,920 --> 00:41:25,180
is developing
electrochemical deposition

968
00:41:25,180 --> 00:41:28,800
processes for material systems,
including copper zinc tin

969
00:41:28,800 --> 00:41:31,260
sulfide and copper indium
gallium diselenide.

970
00:41:31,260 --> 00:41:33,580
We'll talk about the
latter in a few slides,

971
00:41:33,580 --> 00:41:35,200
but that's one
example of a company.

972
00:41:38,240 --> 00:41:42,250
So general issues in thin
films-- thin film compounds

973
00:41:42,250 --> 00:41:44,020
are typically, not
always, but typically,

974
00:41:44,020 --> 00:41:47,600
binary, ternary, quaternary,
or multinary semiconductors.

975
00:41:47,600 --> 00:41:50,450
Meaning you don't have
just one element comprising

976
00:41:50,450 --> 00:41:51,770
the semiconductor species.

977
00:41:51,770 --> 00:41:55,010
You might have several, and
they form a crystal structure

978
00:41:55,010 --> 00:41:58,920
with repeating structure but
alternating atoms typically.

979
00:41:58,920 --> 00:42:03,420
And so, if you have multiple
atoms in one compound,

980
00:42:03,420 --> 00:42:05,730
a couple of issues
could arise and need

981
00:42:05,730 --> 00:42:08,360
to be controlled
to grow good films.

982
00:42:08,360 --> 00:42:10,990
The first involves
phase stability.

983
00:42:10,990 --> 00:42:14,067
What is shown right here in
mulitnary parameter space,

984
00:42:14,067 --> 00:42:15,900
this is the chemical
potential zinc, copper,

985
00:42:15,900 --> 00:42:19,720
and tin in a so-called zinc
copper tin sulfide material

986
00:42:19,720 --> 00:42:20,600
system.

987
00:42:20,600 --> 00:42:24,650
This red fin right here is
showing you the parameter space

988
00:42:24,650 --> 00:42:27,500
within which this
compound is stable.

989
00:42:27,500 --> 00:42:31,630
If your stoichiometry takes an
excursion from that red fin,

990
00:42:31,630 --> 00:42:35,180
you could wind up in
a bi-phase regime.

991
00:42:35,180 --> 00:42:37,800
Meaning you have CZTS
and something else,

992
00:42:37,800 --> 00:42:40,110
a copper tin sulphide,
a zinc sulfide,

993
00:42:40,110 --> 00:42:41,710
or some other
species that happens

994
00:42:41,710 --> 00:42:44,190
to be nearby in phase space.

995
00:42:44,190 --> 00:42:46,280
One way to think
about this is it's

996
00:42:46,280 --> 00:42:48,660
just you have a
homogeneous material.

997
00:42:48,660 --> 00:42:52,250
If you exceed a solubility limit
in one direction or another,

998
00:42:52,250 --> 00:42:55,270
you'll have precipitation
of a secondary phase.

999
00:42:55,270 --> 00:42:58,760
So you have to make sure
that in a gross perspective,

1000
00:42:58,760 --> 00:43:02,060
on a percents basis,
you're in the right regime

1001
00:43:02,060 --> 00:43:02,840
of stoichiometry.

1002
00:43:02,840 --> 00:43:05,210
Stoichiometry being the
ratio of different elements

1003
00:43:05,210 --> 00:43:06,280
in your system.

1004
00:43:06,280 --> 00:43:08,780
So it's like cooking; you need
the right set of ingredients

1005
00:43:08,780 --> 00:43:11,540
to make the right material.

1006
00:43:11,540 --> 00:43:15,965
Now, that has to do with-- large
excursions from stoichiometry

1007
00:43:15,965 --> 00:43:17,690
can result in phase
decomposition.

1008
00:43:17,690 --> 00:43:19,990
Small excursions from
stoichiometry, a much more

1009
00:43:19,990 --> 00:43:22,130
subtle effect can occur.

1010
00:43:22,130 --> 00:43:25,890
Let's imagine for a moment that
we have two species comprising

1011
00:43:25,890 --> 00:43:28,150
are binary material.

1012
00:43:28,150 --> 00:43:30,870
One species has three
valence electrons.

1013
00:43:30,870 --> 00:43:34,500
The other species has
five valence electrons.

1014
00:43:34,500 --> 00:43:37,330
Now, because of a small
error in stoichiometry,

1015
00:43:37,330 --> 00:43:39,700
maybe something in the
order of a few tens

1016
00:43:39,700 --> 00:43:42,620
or hundreds of parts per
million in stoichiometry,

1017
00:43:42,620 --> 00:43:44,170
we didn't get the
ratio just right.

1018
00:43:44,170 --> 00:43:45,670
We were off by a little bit.

1019
00:43:45,670 --> 00:43:48,540
Now we have one of our
compounds in excess

1020
00:43:48,540 --> 00:43:51,390
and the other one in deficiency.

1021
00:43:51,390 --> 00:43:55,540
If we have a different number
of electrons surrounding

1022
00:43:55,540 --> 00:44:00,020
the atoms, we could wind up with
an excess free carrier density.

1023
00:44:00,020 --> 00:44:02,250
In other words, you could
self-dope your material

1024
00:44:02,250 --> 00:44:03,666
if you're unlucky,
in other words,

1025
00:44:03,666 --> 00:44:06,170
if the material system
has a propensity for this.

1026
00:44:06,170 --> 00:44:08,254
And you can change the
free carrier concentration,

1027
00:44:08,254 --> 00:44:10,545
and because the free carrier
concentration is changing,

1028
00:44:10,545 --> 00:44:12,460
you might even
change your mobility.

1029
00:44:12,460 --> 00:44:14,460
So there are some
effects that can

1030
00:44:14,460 --> 00:44:20,810
occur as a result of small
excursions from stoichiometry.

1031
00:44:20,810 --> 00:44:22,470
As a result of the
self-doping, you're

1032
00:44:22,470 --> 00:44:26,020
shifting the Fermi energy
inside your semiconductor.

1033
00:44:26,020 --> 00:44:28,140
And as a result of
shifting the Fermi energy,

1034
00:44:28,140 --> 00:44:32,325
it might lead to a
cascade series of events.

1035
00:44:32,325 --> 00:44:33,700
There could be
other defects that

1036
00:44:33,700 --> 00:44:36,950
form as a result of the
Fermi energy change.

1037
00:44:36,950 --> 00:44:40,270
You could have other
so-called antisite defects.

1038
00:44:40,270 --> 00:44:42,790
Atoms could switch positions
inside of your lattice,

1039
00:44:42,790 --> 00:44:46,920
and as a result of that,
have very low minority

1040
00:44:46,920 --> 00:44:49,570
carrier lifetime in
certain materials.

1041
00:44:49,570 --> 00:44:53,490
So nailing the stoichiometry
both from a very large sense,

1042
00:44:53,490 --> 00:44:55,141
to avoid phase
decomposition, instead

1043
00:44:55,141 --> 00:44:56,890
of having a dalmatian
film, you have phase

1044
00:44:56,890 --> 00:45:03,010
pure film, and from
a local perspective,

1045
00:45:03,010 --> 00:45:05,340
once you get on to
this phase space

1046
00:45:05,340 --> 00:45:07,850
where you can grow
your film well,

1047
00:45:07,850 --> 00:45:10,510
you want to make sure that your
stoichiometry is controlled

1048
00:45:10,510 --> 00:45:12,610
to avoid self-doping
and to prevent

1049
00:45:12,610 --> 00:45:14,790
certain types of
intrinsic point defects

1050
00:45:14,790 --> 00:45:17,550
from forming that might
lower minority carrier

1051
00:45:17,550 --> 00:45:20,450
lifetime or change
carrier concentration,

1052
00:45:20,450 --> 00:45:22,280
change other properties
of your film.

1053
00:45:22,280 --> 00:45:24,530
For those who are working
on these sorts of materials,

1054
00:45:24,530 --> 00:45:28,060
I'm happy to talk ad
nauseam about these topics,

1055
00:45:28,060 --> 00:45:32,590
maybe after class, since this
is a more detailed subject.

1056
00:45:32,590 --> 00:45:37,120
Another topic of interest
in thin films is grain size.

1057
00:45:37,120 --> 00:45:39,800
At some point, grains
don't matter anymore.

1058
00:45:39,800 --> 00:45:42,620
The grain size,
typically if you exceed

1059
00:45:42,620 --> 00:45:45,240
the thickness of your film
by about a factor five.

1060
00:45:45,240 --> 00:45:46,740
In other words, the
grain diameter's

1061
00:45:46,740 --> 00:45:50,040
about five times wider than
the thickness of your film,

1062
00:45:50,040 --> 00:45:52,050
grain size is not
as much of an issue.

1063
00:45:52,050 --> 00:45:54,290
But if you do have
very small grains,

1064
00:45:54,290 --> 00:45:57,120
they can impact performance,
because carriers will interact

1065
00:45:57,120 --> 00:45:58,360
with those grain boundaries.

1066
00:45:58,360 --> 00:46:00,580
And depending how
recombination active they are

1067
00:46:00,580 --> 00:46:03,110
or where the grain boundary
is pinning the Fermi energy,

1068
00:46:03,110 --> 00:46:05,310
the density of state at
that, at the grain boundary,

1069
00:46:05,310 --> 00:46:08,130
will dictate the effect
on device performance.

1070
00:46:08,130 --> 00:46:11,060
So these are some
very rough plots

1071
00:46:11,060 --> 00:46:15,010
in crystalline silicon
for thin film devices

1072
00:46:15,010 --> 00:46:16,640
and for some thicker
ones as well.

1073
00:46:16,640 --> 00:46:19,046
So performance is a
function the grain size.

1074
00:46:19,046 --> 00:46:21,420
And I show crystalline silicon
because the data is really

1075
00:46:21,420 --> 00:46:23,750
well developed for it,
but you see similar types

1076
00:46:23,750 --> 00:46:28,080
of plots for organic materials,
for some inorganic thin film

1077
00:46:28,080 --> 00:46:30,450
materials, like
CIGS and so forth.

1078
00:46:30,450 --> 00:46:33,760
And this convolutes a
few different parameters.

1079
00:46:33,760 --> 00:46:35,970
You have to take into account
that the recombination

1080
00:46:35,970 --> 00:46:37,969
activity of the grain
boundary is also a factor.

1081
00:46:40,840 --> 00:46:43,790
The next topic, general
topic of interest,

1082
00:46:43,790 --> 00:46:46,630
another tool that we'll want
to have an our material science

1083
00:46:46,630 --> 00:46:48,820
toolkit as we start
designing these materials,

1084
00:46:48,820 --> 00:46:50,470
we have to think
about the interfaces

1085
00:46:50,470 --> 00:46:52,440
between the different materials.

1086
00:46:52,440 --> 00:46:54,530
Especially in thin
films, interfaces

1087
00:46:54,530 --> 00:46:57,940
are so important because
we don't much bulk anymore.

1088
00:46:57,940 --> 00:46:59,860
So the device could
really be affected

1089
00:46:59,860 --> 00:47:02,090
or device performance
really reduced

1090
00:47:02,090 --> 00:47:05,410
if we don't pay proper
attention to our interfaces.

1091
00:47:05,410 --> 00:47:07,490
What are these plots over here?

1092
00:47:07,490 --> 00:47:11,040
These plots are used to grow
some very high efficiency

1093
00:47:11,040 --> 00:47:15,100
materials, for example, by
MOCVD or molecular-beam epitaxy.

1094
00:47:15,100 --> 00:47:18,080
And what is represented
on the horizontal axis

1095
00:47:18,080 --> 00:47:19,780
is lattice constant.

1096
00:47:19,780 --> 00:47:22,750
Lattice constant refers to the
equilibrium spacing of atoms

1097
00:47:22,750 --> 00:47:24,070
inside of your material.

1098
00:47:24,070 --> 00:47:27,940
So this regular repeating unit
cell that defines a crystal

1099
00:47:27,940 --> 00:47:30,450
has a certain lattice
constant, a certain distance--

1100
00:47:30,450 --> 00:47:33,810
physical distance--
shown here in angstroms.

1101
00:47:33,810 --> 00:47:37,350
The energy of the gap is
shown on the vertical axis.

1102
00:47:37,350 --> 00:47:39,920
And if we want to select two
or three of these materials

1103
00:47:39,920 --> 00:47:42,600
to stack on top of one
another to absorb well

1104
00:47:42,600 --> 00:47:44,820
at different portions
of the solar spectrum,

1105
00:47:44,820 --> 00:47:48,610
we'll be choosing, for example,
one band gap at around 1.9 eV,

1106
00:47:48,610 --> 00:47:50,386
another band gap
of 1 eV, or maybe

1107
00:47:50,386 --> 00:47:51,760
if we want three
materials, we'll

1108
00:47:51,760 --> 00:47:54,524
go even higher at the top
end and lower at the low end.

1109
00:47:54,524 --> 00:47:55,940
So we'll stack
different materials

1110
00:47:55,940 --> 00:47:58,050
on top of each other to
absorb preferentially

1111
00:47:58,050 --> 00:48:00,420
in different regions
of the solar spectrum

1112
00:48:00,420 --> 00:48:02,600
and hence exceed the
Shockley-Queisser efficiency

1113
00:48:02,600 --> 00:48:05,600
limit, because now we're
absorbing well in two or three

1114
00:48:05,600 --> 00:48:08,310
different colors as
opposed to just one.

1115
00:48:08,310 --> 00:48:10,500
And the energy gap
here is important

1116
00:48:10,500 --> 00:48:12,810
because you want
maybe one material

1117
00:48:12,810 --> 00:48:14,914
at 1 eV, one material
at about 1.9 eV.

1118
00:48:14,914 --> 00:48:16,580
But you also want to
make sure that they

1119
00:48:16,580 --> 00:48:18,630
can grow on top of each
other, that you're not

1120
00:48:18,630 --> 00:48:20,380
going to get a mismatch
of that interface,

1121
00:48:20,380 --> 00:48:22,560
that the lattice constants
aren't so different that you

1122
00:48:22,560 --> 00:48:24,680
wind up with these dangling
bonds at the interface, where

1123
00:48:24,680 --> 00:48:26,380
you have an atom
coming down and nothing

1124
00:48:26,380 --> 00:48:28,450
on the other side
for it to bond to.

1125
00:48:28,450 --> 00:48:31,270
And so you need to make sure
that the materials that you

1126
00:48:31,270 --> 00:48:34,770
grow are matched in lattice
constant but varying

1127
00:48:34,770 --> 00:48:38,542
in band gap, if you're trying
to grow a multi-junction device,

1128
00:48:38,542 --> 00:48:40,500
if you're trying to grow
a very high efficiency

1129
00:48:40,500 --> 00:48:41,820
solar cell device.

1130
00:48:41,820 --> 00:48:46,490
And so the growth or matching of
materials one on top of another

1131
00:48:46,490 --> 00:48:49,300
is important, especially
for the multi-junctions,

1132
00:48:49,300 --> 00:48:51,770
also for some of the
single junction materials

1133
00:48:51,770 --> 00:48:54,730
if you really want to minimize
the interface recombination.

1134
00:48:54,730 --> 00:48:56,460
So let's look at
this growth system

1135
00:48:56,460 --> 00:48:59,810
up here, the one
that is typically

1136
00:48:59,810 --> 00:49:02,100
used in high efficiency
solar cell materials.

1137
00:49:02,100 --> 00:49:07,440
We have germanium right here,
gallium arsenide, and indium

1138
00:49:07,440 --> 00:49:10,430
gallium phosphide,
which is essentially

1139
00:49:10,430 --> 00:49:14,029
a mixture between
gallium phosphide up here

1140
00:49:14,029 --> 00:49:15,320
and indium phosphide down here.

1141
00:49:15,320 --> 00:49:17,940
You can alloy the two together
and get an indium gallium

1142
00:49:17,940 --> 00:49:21,024
phosphide mixture and stack
these three materials on top

1143
00:49:21,024 --> 00:49:22,940
of one another-- germanium,
gallium, arsenide,

1144
00:49:22,940 --> 00:49:24,240
and indium gallium phosphide.

1145
00:49:24,240 --> 00:49:25,190
They have three
different band gaps.

1146
00:49:25,190 --> 00:49:27,550
The absorb in three different
regions of the solar spectrum.

1147
00:49:27,550 --> 00:49:29,440
But they have a very
similar lattice constant,

1148
00:49:29,440 --> 00:49:31,564
and so the interfaces will
be very well maintained.

1149
00:49:31,564 --> 00:49:35,210
That's an example of using
a chart like this to design

1150
00:49:35,210 --> 00:49:36,580
your solar cell materials.

1151
00:49:39,760 --> 00:49:42,860
Next topic is
material abundances.

1152
00:49:42,860 --> 00:49:46,917
If we're trying to engineer
all of these other parameters

1153
00:49:46,917 --> 00:49:49,500
that we've been talking about--
the lattice constant, the band

1154
00:49:49,500 --> 00:49:53,000
gap, the grain size
that also is a function

1155
00:49:53,000 --> 00:49:55,510
of how the material grows,
the ability to self-dope.

1156
00:49:55,510 --> 00:49:58,190
We have all of these material
issues that we have first

1157
00:49:58,190 --> 00:49:59,800
and foremost in our minds.

1158
00:49:59,800 --> 00:50:00,990
We go to the periodic table.

1159
00:50:00,990 --> 00:50:02,364
We find some
compounds that work.

1160
00:50:02,364 --> 00:50:03,890
We're really happy about it.

1161
00:50:03,890 --> 00:50:06,250
But then, all of a sudden,
life comes along and slaps us

1162
00:50:06,250 --> 00:50:07,860
the face and says,
well, we don't

1163
00:50:07,860 --> 00:50:09,934
have enough of this
material to really scale

1164
00:50:09,934 --> 00:50:12,100
to get all the way to the
terawatt cell [INAUDIBLE].

1165
00:50:12,100 --> 00:50:14,530
Oh, I wish I had known about
this before when I first

1166
00:50:14,530 --> 00:50:16,450
got started.

1167
00:50:16,450 --> 00:50:20,180
So we're presenting to you
upfront the state-of-the-art

1168
00:50:20,180 --> 00:50:22,870
of what is known about
material abundances.

1169
00:50:22,870 --> 00:50:27,220
And these last two studies
right here, APS Energy Critical

1170
00:50:27,220 --> 00:50:30,670
Elements and the DOE Critical
Material Strategy, both of them

1171
00:50:30,670 --> 00:50:33,200
represent a synthesis of
the information, essentially

1172
00:50:33,200 --> 00:50:35,440
the equivalent to the
IPCC reports in climate

1173
00:50:35,440 --> 00:50:38,170
change, but the best
synthesis that we

1174
00:50:38,170 --> 00:50:40,730
have right now
about the abundances

1175
00:50:40,730 --> 00:50:42,065
of different elements out there.

1176
00:50:42,065 --> 00:50:44,275
There are as well a
variety of different papers

1177
00:50:44,275 --> 00:50:46,775
that have been published in the
subject over the last couple

1178
00:50:46,775 --> 00:50:50,340
of decades or even earlier.

1179
00:50:50,340 --> 00:50:54,020
So what we have to keep in mind
is that our stardust out there

1180
00:50:54,020 --> 00:50:56,060
is not in infinite supply.

1181
00:50:56,060 --> 00:50:58,530
Every element we have on
the planet that we know of

1182
00:50:58,530 --> 00:51:00,699
came from fusion
reactions in stars,

1183
00:51:00,699 --> 00:51:02,740
and there was a probability
distribution function

1184
00:51:02,740 --> 00:51:06,280
of the appearance of those
elements as a function of z

1185
00:51:06,280 --> 00:51:09,651
on the planet as a result biased
toward the lighter elements.

1186
00:51:09,651 --> 00:51:11,150
And some of the
heavier elements are

1187
00:51:11,150 --> 00:51:15,860
in lesser supply, that we
know of, on the Earth's crust.

1188
00:51:15,860 --> 00:51:18,250
Not to say that the
deposits don't exist.

1189
00:51:18,250 --> 00:51:20,470
Not to say that these
studies right here

1190
00:51:20,470 --> 00:51:23,330
are the authoritative
end-all and be-all.

1191
00:51:23,330 --> 00:51:25,370
We might discover next
year or next month

1192
00:51:25,370 --> 00:51:28,680
for tomorrow huge deposit
of a particular element

1193
00:51:28,680 --> 00:51:31,780
at a specific spot, let's
say, under the Arctic.

1194
00:51:31,780 --> 00:51:34,790
But from what we know right
now, that's the stardust

1195
00:51:34,790 --> 00:51:36,320
that we have to work with.

1196
00:51:36,320 --> 00:51:38,070
These are our abundances.

1197
00:51:38,070 --> 00:51:40,000
So if you'd like to
design around it,

1198
00:51:40,000 --> 00:51:43,445
I'd advise looking into
those reports as well.

1199
00:51:43,445 --> 00:51:45,320
And finally, radiation
hardness, getting back

1200
00:51:45,320 --> 00:51:47,570
to Ashley's question, gee,
what are the most radiation

1201
00:51:47,570 --> 00:51:49,040
hard species?

1202
00:51:49,040 --> 00:51:53,950
This is the efficiency of solar
cell performance normalized

1203
00:51:53,950 --> 00:51:56,130
at the very start
of a test, and this

1204
00:51:56,130 --> 00:52:00,430
is the equivalent
radiation damage.

1205
00:52:03,220 --> 00:52:06,460
You could also think about
this as the amount of momentum

1206
00:52:06,460 --> 00:52:09,456
or energy depending
transferred to the atoms

1207
00:52:09,456 --> 00:52:10,830
inside of your
semiconductor that

1208
00:52:10,830 --> 00:52:13,300
would result in lattice
damage that would result

1209
00:52:13,300 --> 00:52:16,604
in a decrease of minority
carrier lifetime or mobility,

1210
00:52:16,604 --> 00:52:18,520
which ultimately would
impact cell performance

1211
00:52:18,520 --> 00:52:19,544
and efficiency.

1212
00:52:19,544 --> 00:52:20,960
We can see that
different material

1213
00:52:20,960 --> 00:52:24,120
systems have different
degrees of radiation hardness.

1214
00:52:24,120 --> 00:52:26,000
Some maintain their
high efficiency

1215
00:52:26,000 --> 00:52:28,970
until very high radiation
dose, and others

1216
00:52:28,970 --> 00:52:30,820
degrade much quicker.

1217
00:52:30,820 --> 00:52:32,590
And look at this.

1218
00:52:32,590 --> 00:52:35,490
This is a dose in orbit per
year, right around there.

1219
00:52:35,490 --> 00:52:38,070
And you can already begin
to see that some of our most

1220
00:52:38,070 --> 00:52:42,500
common compounds are not doing
too well out there-- not doing

1221
00:52:42,500 --> 00:52:45,840
too well in outer space.

1222
00:52:45,840 --> 00:52:48,910
So we have the
radiation hardness

1223
00:52:48,910 --> 00:52:51,540
to take into account if we're
putting these solar panels

1224
00:52:51,540 --> 00:52:53,550
out there into outer space.

1225
00:52:53,550 --> 00:52:55,924
This is one older study
I would definitely

1226
00:52:55,924 --> 00:52:56,840
encourage you to look.

1227
00:52:56,840 --> 00:52:58,840
There may be some newer studies.

1228
00:52:58,840 --> 00:53:01,340
As the solar cell
efficiency improves,

1229
00:53:01,340 --> 00:53:03,540
they become more
sensitive because you

1230
00:53:03,540 --> 00:53:06,580
begin decreasing your
efficiency with smaller

1231
00:53:06,580 --> 00:53:09,550
variations in the minority
carrier diffusion length.

1232
00:53:09,550 --> 00:53:12,070
So those charts make look a
different as time goes by.

1233
00:53:12,070 --> 00:53:15,130
AUDIENCE: Is the effect of
the radiation cumulative?

1234
00:53:15,130 --> 00:53:18,111
So for example, gallium
arsenide or any of these

1235
00:53:18,111 --> 00:53:21,780
would just continue to degrade
as they're out in space?

1236
00:53:21,780 --> 00:53:25,130
PROFESSOR: So is effective
radiation dose cumulative?

1237
00:53:25,130 --> 00:53:28,090
I am not the expert on
this particular topic.

1238
00:53:28,090 --> 00:53:30,940
But from what I know about
radiation exposure of detectors

1239
00:53:30,940 --> 00:53:35,460
at synchrotrons, which
is a little similar,

1240
00:53:35,460 --> 00:53:40,490
not quite the same, the
mechanisms involved with this

1241
00:53:40,490 --> 00:53:42,790
essentially involve
atomic displacements

1242
00:53:42,790 --> 00:53:43,630
within the lattice.

1243
00:53:43,630 --> 00:53:45,710
You have atoms physically
being displaced

1244
00:53:45,710 --> 00:53:47,760
from their equilibrium
positions as they interact

1245
00:53:47,760 --> 00:53:49,220
with this incoming radiation.

1246
00:53:49,220 --> 00:53:52,000
And the probability that it
occurs is a function of time,

1247
00:53:52,000 --> 00:53:55,330
will increase per unit
volume, and hence it

1248
00:53:55,330 --> 00:53:57,860
can be thought of as
accumulative exposure effect.

1249
00:53:57,860 --> 00:54:00,610
The first order impact would be
on minority carrier diffusion

1250
00:54:00,610 --> 00:54:04,440
length, impacting both
lifetime and mobility.

1251
00:54:04,440 --> 00:54:08,230
And to the effect that you have
a relationship between exposure

1252
00:54:08,230 --> 00:54:11,007
time-lattice displacement,
lattice displacement-minority

1253
00:54:11,007 --> 00:54:13,173
carrier diffusion length,
minority carrier diffusion

1254
00:54:13,173 --> 00:54:15,900
length and efficiency, you might
be able to model this effect.

1255
00:54:15,900 --> 00:54:18,600
That would be my
uninformed answer.

1256
00:54:18,600 --> 00:54:20,800
Again, you might want to
look this up yourself.

1257
00:54:20,800 --> 00:54:21,345
Question?

1258
00:54:21,345 --> 00:54:23,720
AUDIENCE: Do you know why the
cadmium telluride improves?

1259
00:54:23,720 --> 00:54:26,620
PROFESSOR: Oh, why does it
improve with efficiency?

1260
00:54:26,620 --> 00:54:28,680
I don't know
specifically about why

1261
00:54:28,680 --> 00:54:30,214
that is for this
particular case,

1262
00:54:30,214 --> 00:54:31,880
but do know that some
materials are what

1263
00:54:31,880 --> 00:54:33,780
are called defect tolerant.

1264
00:54:33,780 --> 00:54:37,840
Some are more naturally able
to withstand antisite defects

1265
00:54:37,840 --> 00:54:41,360
or a certain concentration
of damage, internal surfaces,

1266
00:54:41,360 --> 00:54:43,670
voids, grain boundaries.

1267
00:54:43,670 --> 00:54:48,940
Cad-tel, cadmium telluride,
is fairly defect tolerant.

1268
00:54:48,940 --> 00:54:54,190
It's one of nature's gifts
to humanity in that regard.

1269
00:54:54,190 --> 00:54:57,250
The degree to which a material
can be defect tolerant

1270
00:54:57,250 --> 00:54:58,927
depends partly on
the carrier density.

1271
00:54:58,927 --> 00:55:00,510
If you have very
high carrier density,

1272
00:55:00,510 --> 00:55:03,950
you tend to screen defects.

1273
00:55:03,950 --> 00:55:07,430
Another reason why they could
be defect tolerant-- all

1274
00:55:07,430 --> 00:55:10,305
of these compounds are
somewhere between an ionic

1275
00:55:10,305 --> 00:55:12,350
and a covalent semiconductor.

1276
00:55:12,350 --> 00:55:15,370
In a covalent semiconductor,
those materials

1277
00:55:15,370 --> 00:55:23,060
tend to be very defect
intolerant because there's

1278
00:55:23,060 --> 00:55:25,930
the conduction band and valence
band as a function of position,

1279
00:55:25,930 --> 00:55:26,981
tend to be very flat.

1280
00:55:26,981 --> 00:55:29,230
The material tends to be
fairly homogeneous throughout

1281
00:55:29,230 --> 00:55:31,370
the electron densities,
fairly well distributed

1282
00:55:31,370 --> 00:55:33,000
in a covalently bonded material.

1283
00:55:33,000 --> 00:55:35,650
In an ionic material, you tend
to have charge localization.

1284
00:55:35,650 --> 00:55:38,090
So energy as a
function of position

1285
00:55:38,090 --> 00:55:40,205
might look like this
on an atomic scale

1286
00:55:40,205 --> 00:55:42,580
as you go from one atom to
the next atom on your lattice,

1287
00:55:42,580 --> 00:55:44,170
to the next one, to the
next one, to the next one,

1288
00:55:44,170 --> 00:55:46,070
and that reflects the
localization of charge

1289
00:55:46,070 --> 00:55:47,050
in your material.

1290
00:55:47,050 --> 00:55:50,590
Those materials can be
more defect intolerant,

1291
00:55:50,590 --> 00:55:53,470
because conduction can happen
more from a hopping mechanism

1292
00:55:53,470 --> 00:55:57,300
than from a band
conduction mechanism,

1293
00:55:57,300 --> 00:55:59,010
and this is really
a gradient between,

1294
00:55:59,010 --> 00:56:01,850
and most materials, they tend
to be partially ionic partially

1295
00:56:01,850 --> 00:56:04,550
covalent going
down the list here.

1296
00:56:04,550 --> 00:56:07,170
And there's a bit of a shift
between cadmium sulfide

1297
00:56:07,170 --> 00:56:10,680
in zinc sulfide in terms
of the ionicity covalence.

1298
00:56:10,680 --> 00:56:14,510
So cad-tel would be
with the chalcogen

1299
00:56:14,510 --> 00:56:16,530
two lower than sulfur
on the periodic table.

1300
00:56:16,530 --> 00:56:18,821
I would imagine it would be
at this transition as well,

1301
00:56:18,821 --> 00:56:21,020
but I'd have to look that up.

1302
00:56:21,020 --> 00:56:24,030
Gives a place to get started
and read more about it.

1303
00:56:24,030 --> 00:56:25,910
And then reliability
and degradation--

1304
00:56:25,910 --> 00:56:27,310
this is important.

1305
00:56:27,310 --> 00:56:28,810
This is a crystalline
silicon module

1306
00:56:28,810 --> 00:56:30,880
being loaded into
environmental testing chamber.

1307
00:56:30,880 --> 00:56:32,296
Inside of that
chamber, the module

1308
00:56:32,296 --> 00:56:34,300
is going to be put
through hell and back.

1309
00:56:34,300 --> 00:56:36,117
The temperature is
going to be raised.

1310
00:56:36,117 --> 00:56:37,700
The humidity is going
to be pumped in.

1311
00:56:37,700 --> 00:56:39,140
Sometimes ultraviolet
light is even

1312
00:56:39,140 --> 00:56:41,400
put in there in some of the
more modern advanced ones.

1313
00:56:41,400 --> 00:56:44,430
And then the temperature
can drop down

1314
00:56:44,430 --> 00:56:47,564
to temperatures as low
as minus 40 degrees C,

1315
00:56:47,564 --> 00:56:49,480
depending on what the
environmental chamber is

1316
00:56:49,480 --> 00:56:51,890
designed to do, exactly
how it's designed

1317
00:56:51,890 --> 00:56:53,820
to stress or test your module.

1318
00:56:53,820 --> 00:56:56,770
And the idea is to promote
an accelerated degradation

1319
00:56:56,770 --> 00:57:00,215
of the module on purpose to test
what its failure modes will be,

1320
00:57:00,215 --> 00:57:02,340
and we'll see these we go
take a tour of Fraunhofer

1321
00:57:02,340 --> 00:57:04,310
CSE in a couple weeks.

1322
00:57:04,310 --> 00:57:06,540
This is a crystalline silicon
module being loaded in.

1323
00:57:06,540 --> 00:57:09,665
If you were to put a
thin film material--

1324
00:57:09,665 --> 00:57:12,440
and crystalline silicon are
materials very, very thick,

1325
00:57:12,440 --> 00:57:16,030
and we said at the native
oxide was very tenacious.

1326
00:57:16,030 --> 00:57:19,430
It was only a few tens or maybe
hundreds of angstroms thick,

1327
00:57:19,430 --> 00:57:23,604
and the junction depth was
about on the order of a micron.

1328
00:57:23,604 --> 00:57:25,520
If you have water attacking
the surface of you

1329
00:57:25,520 --> 00:57:27,960
silicon wafer, water
vapor, really not too

1330
00:57:27,960 --> 00:57:30,500
much of an issue, and silicon's
is fairly inert anyway.

1331
00:57:30,500 --> 00:57:34,150
But now if you take a fairly
reactive material, a thin film

1332
00:57:34,150 --> 00:57:37,110
material that might react with
air or might react with water,

1333
00:57:37,110 --> 00:57:43,770
and it's so thin that the these
rusting modes, or reaction

1334
00:57:43,770 --> 00:57:45,630
modes, the weathering
modes, can really

1335
00:57:45,630 --> 00:57:49,437
impact a large fraction of
the thickness of your device.

1336
00:57:49,437 --> 00:57:51,020
Now you've become a
lot more sensitive

1337
00:57:51,020 --> 00:57:52,730
to accelerated degradation.

1338
00:57:52,730 --> 00:57:55,290
Now you've become a lot more
sensitive to the elements,

1339
00:57:55,290 --> 00:57:57,700
and this includes
both oxygen and water.

1340
00:57:57,700 --> 00:58:03,400
So if the ambient
is able to penetrate

1341
00:58:03,400 --> 00:58:05,770
through the encapsulate and
get to the active absorber

1342
00:58:05,770 --> 00:58:08,220
material, you may have
accelerated degradation

1343
00:58:08,220 --> 00:58:10,200
of module performance
as a result.

1344
00:58:10,200 --> 00:58:12,229
And so there also
some unique failure

1345
00:58:12,229 --> 00:58:13,520
modes within thin film modules.

1346
00:58:13,520 --> 00:58:17,040
If you have two different
species comprising

1347
00:58:17,040 --> 00:58:18,990
your compound, one
of them might be

1348
00:58:18,990 --> 00:58:20,980
prone to move in electric field.

1349
00:58:20,980 --> 00:58:24,250
For example, copper is
notorious for zipping along

1350
00:58:24,250 --> 00:58:26,566
in electric field,
in electromigrating.

1351
00:58:26,566 --> 00:58:27,940
And so that's a
failure mode that

1352
00:58:27,940 --> 00:58:31,920
doesn't exist in large thick
crystalline silicon modules

1353
00:58:31,920 --> 00:58:33,430
but could existent
in thin films.

1354
00:58:33,430 --> 00:58:35,960
And so because of all of this,
and because of the growing

1355
00:58:35,960 --> 00:58:38,860
realization that the way we
test crystalline silicon modules

1356
00:58:38,860 --> 00:58:42,370
and drive them failure
is not the same

1357
00:58:42,370 --> 00:58:45,610
that we might be able to achieve
failure in a thin film module.

1358
00:58:45,610 --> 00:58:47,350
There are newer
testing protocols,

1359
00:58:47,350 --> 00:58:50,750
such as this IEC 61853,
that have been introduced

1360
00:58:50,750 --> 00:58:55,070
in an attempt to do test
appropriately thin film

1361
00:58:55,070 --> 00:58:57,340
modules for their
respective failure modes.

1362
00:58:57,340 --> 00:59:00,660
And this is, I would say,
still work in progress.

1363
00:59:00,660 --> 00:59:02,520
So much so, that we
have a group project

1364
00:59:02,520 --> 00:59:04,780
focused in part on this.

1365
00:59:04,780 --> 00:59:07,499
It's still a work
in process to try

1366
00:59:07,499 --> 00:59:09,040
to figure out how
do we appropriately

1367
00:59:09,040 --> 00:59:11,930
test these thin film
modules toward the point

1368
00:59:11,930 --> 00:59:14,180
where they can fail.

1369
00:59:14,180 --> 00:59:16,670
Any questions so
far on these topics?

1370
00:59:16,670 --> 00:59:21,100
Because these are general issues
that will affect all thin film

1371
00:59:21,100 --> 00:59:23,020
materials, I wanted
to make sure that we

1372
00:59:23,020 --> 00:59:24,920
were comfortable with
these general topics

1373
00:59:24,920 --> 00:59:27,850
before we dove in any
detail into the technologies

1374
00:59:27,850 --> 00:59:28,430
themselves?

1375
00:59:28,430 --> 00:59:28,929
Yes?

1376
00:59:28,929 --> 00:59:30,982
AUDIENCE: A question
about lattice matching--

1377
00:59:30,982 --> 00:59:33,065
is it important to lattice
match the semiconductor

1378
00:59:33,065 --> 00:59:34,520
to the contact as well?

1379
00:59:34,520 --> 00:59:36,950
Or is that not as important

1380
00:59:36,950 --> 00:59:38,640
PROFESSOR: So the
question was is it

1381
00:59:38,640 --> 00:59:42,180
important to lattice match the
semiconductor to the contact?

1382
00:59:42,180 --> 00:59:46,900
So let me emphasize that in
many semiconductor contact

1383
00:59:46,900 --> 00:59:50,260
combinations you would have
a highly doped semiconductor

1384
00:59:50,260 --> 00:59:52,840
right before the contact, a
very localized region of highly

1385
00:59:52,840 --> 00:59:55,410
doped semiconductor that would
create a tunneling injunction.

1386
00:59:55,410 --> 00:59:58,432
In that case, the density
of states at the interface

1387
00:59:58,432 --> 01:00:00,640
doesn't matter because you
have a tunneling junction.

1388
01:00:00,640 --> 01:00:02,973
You're be able to tunnel
straight from the semiconductor

1389
01:00:02,973 --> 01:00:03,790
into the contact.

1390
01:00:03,790 --> 01:00:05,564
The lattice matching
would matter, though,

1391
01:00:05,564 --> 01:00:07,230
if you didn't have a
tunneling junction.

1392
01:00:07,230 --> 01:00:09,954
If you had a regular
Schottky ohmic contact,

1393
01:00:09,954 --> 01:00:11,870
then you might have to
worry about the density

1394
01:00:11,870 --> 01:00:13,745
of interface states,
which would be regulated

1395
01:00:13,745 --> 01:00:15,820
by the number of dangling
bonds, and then you

1396
01:00:15,820 --> 01:00:17,830
might want every
single atom pairing up

1397
01:00:17,830 --> 01:00:19,690
with a neighbor
on the other side.

1398
01:00:19,690 --> 01:00:21,260
So lattice matching would be
important for the contacts

1399
01:00:21,260 --> 01:00:21,759
there.

1400
01:00:24,180 --> 01:00:26,640
All right, fun
stuff-- wow, we've

1401
01:00:26,640 --> 01:00:29,770
had a good dose of material
science of the day.

1402
01:00:29,770 --> 01:00:31,510
Thin film cost
structure-- I just wanted

1403
01:00:31,510 --> 01:00:33,010
to highlight one quick thing.

1404
01:00:33,010 --> 01:00:35,827
This material right here, that's
not the absorber material.

1405
01:00:35,827 --> 01:00:37,785
The absorber material is
a really tiny fraction

1406
01:00:37,785 --> 01:00:39,120
of the material.

1407
01:00:39,120 --> 01:00:42,300
This comprises the other
materials within the module

1408
01:00:42,300 --> 01:00:43,380
as well.

1409
01:00:43,380 --> 01:00:46,530
So the encapsulates,
the glass, and so fort,

1410
01:00:46,530 --> 01:00:49,470
just keep that in mind
as a kind of asterisks.

1411
01:00:49,470 --> 01:00:51,295
So it's typical thin
film cost structure.

1412
01:00:51,295 --> 01:00:52,680
It evolves with time.

1413
01:00:52,680 --> 01:00:54,380
This is a few years
old, this slide,

1414
01:00:54,380 --> 01:00:57,690
but it gives you a
sense, a feeling.

1415
01:00:57,690 --> 01:01:02,740
In terms of global production,
this is a year-old data now

1416
01:01:02,740 --> 01:01:05,360
from 2010.

1417
01:01:05,360 --> 01:01:10,080
During this past year-- so this
was 2009 data shown in 2010.

1418
01:01:10,080 --> 01:01:17,000
The 2010 market was very harsh
for the thin film producers,

1419
01:01:17,000 --> 01:01:20,550
many of which tend to be in the
United States and in Europe.

1420
01:01:20,550 --> 01:01:23,780
In 2010, prices
dropped quite a bit,

1421
01:01:23,780 --> 01:01:25,785
and that favored the
low-cost Chinese solar cell

1422
01:01:25,785 --> 01:01:27,160
manufacturers,
many of which were

1423
01:01:27,160 --> 01:01:29,170
invested in crystalline
silicon technology.

1424
01:01:29,170 --> 01:01:34,370
So by no detriment
to the technology

1425
01:01:34,370 --> 01:01:37,590
itself, market
dynamics worldwide,

1426
01:01:37,590 --> 01:01:40,320
due to other factors, tended
to favor crystalline silicon

1427
01:01:40,320 --> 01:01:41,104
in the past year.

1428
01:01:41,104 --> 01:01:42,520
And the market
share of thin films

1429
01:01:42,520 --> 01:01:45,550
contracted a bit so it's now
about 90% crystalline silicon,

1430
01:01:45,550 --> 01:01:49,410
10% then films
worldwide in 2010.

1431
01:01:49,410 --> 01:01:52,290
But the break down between
the different thin film

1432
01:01:52,290 --> 01:01:55,210
technologies, we had the
so-called cadmium telluride,

1433
01:01:55,210 --> 01:01:57,660
CIGS, so that's Copper
Indium Gallium Diselenide,

1434
01:01:57,660 --> 01:01:59,200
and amorphous silicon.

1435
01:01:59,200 --> 01:02:02,280
And the dynamic
between 2009-2010

1436
01:02:02,280 --> 01:02:04,800
was that the amorphous
silicon shrank a bit.

1437
01:02:04,800 --> 01:02:08,160
CIGS grew, and cad-tel continued
growing, but more marginally

1438
01:02:08,160 --> 01:02:10,240
because it was already
big to begin with.

1439
01:02:10,240 --> 01:02:13,124
So you could think of
this red portion growing

1440
01:02:13,124 --> 01:02:15,540
at the expense of the green,
if you want to translate this

1441
01:02:15,540 --> 01:02:18,970
into 2010 numbers.

1442
01:02:18,970 --> 01:02:22,230
So what is CIGS, cad-tel,
amorphous silicon--

1443
01:02:22,230 --> 01:02:25,130
what are those materials?

1444
01:02:25,130 --> 01:02:26,540
Well, we'll get into that.

1445
01:02:26,540 --> 01:02:30,330
I think the best thing to do is
to leave this for next class.

1446
01:02:30,330 --> 01:02:32,930
I'll briefly go
over cad-tel just

1447
01:02:32,930 --> 01:02:34,710
because it is so important.

1448
01:02:34,710 --> 01:02:38,470
It is the biggest-- the
single biggest US solar cell

1449
01:02:38,470 --> 01:02:41,420
manufacturer is producing
cadmium telluride solar cells,

1450
01:02:41,420 --> 01:02:42,860
or cad-tel for short.

1451
01:02:42,860 --> 01:02:46,980
And to make just a description
of what you're solar panel

1452
01:02:46,980 --> 01:02:48,620
would look like
in cross section,

1453
01:02:48,620 --> 01:02:51,520
this is your glass
on the backside here.

1454
01:02:51,520 --> 01:02:54,000
This ITO Indium Tin Oxide.

1455
01:02:54,000 --> 01:02:55,620
It is a what?

1456
01:02:55,620 --> 01:02:57,930
A transparent conducting
oxide, very good.

1457
01:02:57,930 --> 01:03:00,082
So the ITO is a transparent
connecting oxide.

1458
01:03:00,082 --> 01:03:02,290
Your light, your sunlight,
is coming in through here.

1459
01:03:02,290 --> 01:03:04,200
So this is electrically
conductive layer,

1460
01:03:04,200 --> 01:03:08,160
but it's transparent, so it's
allowing the sunlight through.

1461
01:03:08,160 --> 01:03:10,330
Tin oxide, we'll get
to that in a second.

1462
01:03:10,330 --> 01:03:12,700
Cadmium sulfide and
cadmium telluride--

1463
01:03:12,700 --> 01:03:15,030
so the cadmium
telluride layer is

1464
01:03:15,030 --> 01:03:17,020
a layer that's absorbing
most of the sunlight

1465
01:03:17,020 --> 01:03:19,070
and producing
electron hole pairs.

1466
01:03:19,070 --> 01:03:22,610
The cadmium sulfide is
providing the header junction

1467
01:03:22,610 --> 01:03:25,520
separating charge at the header
junction between the cad-tel

1468
01:03:25,520 --> 01:03:27,110
and the cad-sulfide.

1469
01:03:27,110 --> 01:03:30,260
This tin oxide is generally
an intrinsic layer.

1470
01:03:30,260 --> 01:03:35,237
It's assisting here with the
ITO on the front contact,

1471
01:03:35,237 --> 01:03:37,570
and then you have your back
contact that extracts charge

1472
01:03:37,570 --> 01:03:38,910
from the back.

1473
01:03:38,910 --> 01:03:40,960
There are a couple
more tricks to creating

1474
01:03:40,960 --> 01:03:44,710
a good cad-tel device that
involve chlorine treatment

1475
01:03:44,710 --> 01:03:46,590
and passivation of defects.

1476
01:03:46,590 --> 01:03:49,420
That's where some of
the activation comes in.

1477
01:03:49,420 --> 01:03:51,450
This is another view
of the cad-tel device

1478
01:03:51,450 --> 01:03:54,100
in cross section,
another example.

1479
01:03:54,100 --> 01:03:56,100
This transparent conducting
oxide, in this case,

1480
01:03:56,100 --> 01:04:02,130
is fluorine doped tin
oxide, another TCO material.

1481
01:04:02,130 --> 01:04:04,480
But very similar structure
here, the cad-tel

1482
01:04:04,480 --> 01:04:06,390
being the p-type
material, and cad-sulfide

1483
01:04:06,390 --> 01:04:07,901
the n-type material.

1484
01:04:07,901 --> 01:04:10,150
The thicknesses of these
different layers you can see.

1485
01:04:10,150 --> 01:04:12,740
The cad-tel is only
a few microns thick,

1486
01:04:12,740 --> 01:04:15,960
and the cad-sulfide
this is even thinner.

1487
01:04:15,960 --> 01:04:18,710
It's a very thin layer just
serving to separate the charge.

1488
01:04:18,710 --> 01:04:25,260
The band diagram of a cad-tel
solar cell is shown right here.

1489
01:04:25,260 --> 01:04:28,530
We have the cad-tel here and
the cad-sulphide right here.

1490
01:04:28,530 --> 01:04:29,930
So you can see the junction.

1491
01:04:29,930 --> 01:04:34,890
Notice, because of the
thickness of this layer,

1492
01:04:34,890 --> 01:04:36,650
the band bending
at these interfaces

1493
01:04:36,650 --> 01:04:38,950
extends quite an
appreciable percentage

1494
01:04:38,950 --> 01:04:40,550
of the total thickness
of your device.

1495
01:04:40,550 --> 01:04:41,966
Whereas in crystalline
silicon, we

1496
01:04:41,966 --> 01:04:44,750
have the band bending right near
the junction region, so right

1497
01:04:44,750 --> 01:04:47,580
within a few hundreds of
nanometers, maybe a micron away

1498
01:04:47,580 --> 01:04:50,710
from the junction, and the
device is 100 microns thick.

1499
01:04:50,710 --> 01:04:54,200
So we had 100 microns
approximately of this flat band

1500
01:04:54,200 --> 01:04:56,360
condition, at least in the dark.

1501
01:04:56,360 --> 01:04:59,889
Here we have bending extending
an appreciable percentage

1502
01:04:59,889 --> 01:05:01,430
of the total thickness
of our device,

1503
01:05:01,430 --> 01:05:03,805
just by virtue of the fact
that we have such a thin film.

1504
01:05:06,460 --> 01:05:12,920
And the characteristics,
the deposition technology

1505
01:05:12,920 --> 01:05:16,180
of cad-tel, as I said, it's
nature's gift to humankind.

1506
01:05:16,180 --> 01:05:18,160
If you put cadmium and
tellurium in together

1507
01:05:18,160 --> 01:05:22,530
in a pot and start heating it
up, what evaporates is cad-tel.

1508
01:05:22,530 --> 01:05:24,950
It congruently evaporates.

1509
01:05:24,950 --> 01:05:28,320
So you could use a process
called close space sublimation,

1510
01:05:28,320 --> 01:05:31,016
where you essentially
sublime your cad-tel,

1511
01:05:31,016 --> 01:05:32,640
and you deposit it
onto your substrate.

1512
01:05:32,640 --> 01:05:34,410
If you try to do this
with most other compounds

1513
01:05:34,410 --> 01:05:35,784
in the periodic
table, you'll get

1514
01:05:35,784 --> 01:05:38,679
either one element or the other
element evaporating first.

1515
01:05:38,679 --> 01:05:39,970
They'll create an overpressure.

1516
01:05:39,970 --> 01:05:40,960
They'll evaporate
off, and you won't

1517
01:05:40,960 --> 01:05:42,626
get your compound
depositing, but you'll

1518
01:05:42,626 --> 01:05:45,200
get one element depositing on
your substrate preferentially.

1519
01:05:45,200 --> 01:05:48,120
cad-tel, again, nature's
gift to humankind.

1520
01:05:48,120 --> 01:05:51,260
The two come off together and
form a cad-tel layer and so

1521
01:05:51,260 --> 01:05:54,130
that congruent evaporation makes
it very nice from a deposition

1522
01:05:54,130 --> 01:05:57,050
point of view-- very low cost,
high throughput deposition

1523
01:05:57,050 --> 01:06:00,240
process for solar benefits from.

1524
01:06:00,240 --> 01:06:03,290
Environmental
concerns-- cadmium has

1525
01:06:03,290 --> 01:06:08,120
raised quite a bunch of
concerns amongst folks

1526
01:06:08,120 --> 01:06:12,650
in environmental groups because
it's a known carcinogen.

1527
01:06:12,650 --> 01:06:17,740
It is responsible
in Japan for, I

1528
01:06:17,740 --> 01:06:20,300
believe it was called
the itai-itai ban, which

1529
01:06:20,300 --> 01:06:22,320
means "the ouch-ouch disease."

1530
01:06:22,320 --> 01:06:25,610
It was a disease
that was acquired

1531
01:06:25,610 --> 01:06:28,950
by folks exposed to cadmium
during manufacturing.

1532
01:06:28,950 --> 01:06:31,510
And as a result,
cad-tel solar panels

1533
01:06:31,510 --> 01:06:34,070
are not allowed to be
installed in Japan.

1534
01:06:34,070 --> 01:06:37,830
So First Solar cannot sell
its cad-tel products in Japan.

1535
01:06:37,830 --> 01:06:41,650
There are very tightly
regulated emissions laws

1536
01:06:41,650 --> 01:06:44,460
in the EU and the United States,
especially in the EU, where

1537
01:06:44,460 --> 01:06:48,560
cradle-to-grave recycling
of cad-tel solar panels

1538
01:06:48,560 --> 01:06:49,479
are necessary.

1539
01:06:49,479 --> 01:06:51,270
So you'll see a lot of
cad-tel solar panels

1540
01:06:51,270 --> 01:06:54,360
in large field installations or
in commercial buildings where

1541
01:06:54,360 --> 01:06:56,955
it's very easy to collect
them all after their 20-

1542
01:06:56,955 --> 01:06:59,600
or 25-year life span and bring
them back to the factory,

1543
01:06:59,600 --> 01:07:01,141
as opposed to having
them distributed

1544
01:07:01,141 --> 01:07:03,160
amongst hundreds of
thousands of smaller systems

1545
01:07:03,160 --> 01:07:05,330
deposited on rooftops.

1546
01:07:05,330 --> 01:07:09,780
The arguments in favor of having
cadmium inside of solar panels

1547
01:07:09,780 --> 01:07:10,960
is the following.

1548
01:07:10,960 --> 01:07:12,810
It's better to tie
up cadmium inside

1549
01:07:12,810 --> 01:07:16,580
of a relatively inert compound,
cad-tel, then to have it go off

1550
01:07:16,580 --> 01:07:19,240
and cause problems on its own.

1551
01:07:19,240 --> 01:07:23,880
If you heat it up, the cad-tel
would evaporate congruently.

1552
01:07:23,880 --> 01:07:27,010
Typically cadmium is
so-called "negligible" amounts

1553
01:07:27,010 --> 01:07:29,510
are released during fires, and
they put it in between quotes

1554
01:07:29,510 --> 01:07:32,290
because these are studies,
very good studies,

1555
01:07:32,290 --> 01:07:37,570
and I trust the work coming
out of the [INAUDIBLE] group

1556
01:07:37,570 --> 01:07:38,410
very much.

1557
01:07:38,410 --> 01:07:41,470
His critics would argue
that, well, the studies

1558
01:07:41,470 --> 01:07:43,820
were paid for, in
part, by First Solar,

1559
01:07:43,820 --> 01:07:46,140
so how do you trust
studies like that?

1560
01:07:46,140 --> 01:07:48,570
I would counter and say,
this is a pretty good group.

1561
01:07:48,570 --> 01:07:50,730
Out of all the people
to life cycle analysis,

1562
01:07:50,730 --> 01:07:54,410
he's within the top tier.

1563
01:07:54,410 --> 01:07:57,430
So it's some
question as to that.

1564
01:07:57,430 --> 01:07:59,554
People do question were
the temperatures used

1565
01:07:59,554 --> 01:08:01,970
in these studies representative
of what you would actually

1566
01:08:01,970 --> 01:08:04,030
get in the hot zone of
a fire and so forth.

1567
01:08:04,030 --> 01:08:05,830
There's a public fear
and perception issue.

1568
01:08:05,830 --> 01:08:06,810
It's a big deal.

1569
01:08:06,810 --> 01:08:14,950
And the folks would argue that
much less cadmium is released

1570
01:08:14,950 --> 01:08:16,955
per kilowatt hour than,
say, in a battery, where

1571
01:08:16,955 --> 01:08:20,850
we would use a nickel cadmium
battery, for instance.

1572
01:08:20,850 --> 01:08:24,080
And safe production methods
now-- fully automated,

1573
01:08:24,080 --> 01:08:25,670
and recycling is
guaranteed by law.

1574
01:08:25,670 --> 01:08:28,699
So have arguments in
favor and against.

1575
01:08:28,699 --> 01:08:30,240
I'm going to stop
right here, and I'm

1576
01:08:30,240 --> 01:08:33,020
going to pull aside the
teams during the last 15

1577
01:08:33,020 --> 01:08:34,170
minutes of class.

1578
01:08:34,170 --> 01:08:35,470
I emailed to you.

1579
01:08:35,470 --> 01:08:38,810
If those of you had checked
your email before last night

1580
01:08:38,810 --> 01:08:41,260
at 5 o'clock, should have
received an email saying you're

1581
01:08:41,260 --> 01:08:43,260
on a particular project team.

1582
01:08:43,260 --> 01:08:45,344
Find your partners,
cluster together.

1583
01:08:45,344 --> 01:08:46,760
Joe and I are going
to come around

1584
01:08:46,760 --> 01:08:48,399
to spend a few minutes
with each of you

1585
01:08:48,399 --> 01:08:51,520
just to make sure that
our first steps are clear

1586
01:08:51,520 --> 01:08:54,620
and that we have a forward
path and we gain some momentum.

1587
01:08:54,620 --> 01:08:57,609
So self-assemble and don't
leave the room before the chance

1588
01:08:57,609 --> 01:08:59,690
to come talk to you.