I won't apologize
for for whether or not this turns out to be a haphazard presentation.
Because I pulled slides from a lot of different places that we have slides.
And some of them are kind of technical and some of them kind of aren't, right.
so I don't know.
We'll just kind of see how it goes, but Oner is here.
He's a manager of our advanced
device technologies so, you know, whenever a
customer or grant proposal needs some crazy
ass new thing Oner is, Oner is the tip of the spear on that.
so after, afterwards if, I mean, if we hang
around for a few minutes he's also a Stanford PhD.
And came out about the same time as me, right?
I mean anyway so the company that, that myself
and, and and Homan Yuen and Vijit Sabnis, two other
PhD's here, started in 2007 is called Solar Junction.
And we make multi-junction 3-5 material based solar cells.
For two applications.
One of them for terrestrial based concentrators, and
I'll talk a little bit about what that is.
And this class of technology also powers basically all the space
satellites that we fly as the human race, you know, today.
So we're
trying to break into that market.
you know, it takes a long time to fly
billion dollar satellites, because they want a lot of credibility.
But one day we will have it.
So let's see. So let's just get started.
So where do we fit?
you know, there's all these different types of solar
Let's see if I can get this to run here.
Here you got your nice flat plate here.
And you've got nano solar and thin film and what not over here.
And you've got solar thermal, where you heat up heat
up basically a fluid in these tubes.
And then you've got concentrating photo voltaics.
And this is where we play over here. and if you look at what that is.
Basically it's all the same idea.
You take some type of a concentrating element and the light
shines through this and focuses down to a very small cell.
And concentrations today in our customer systems are around a 1000 X
to 1500 X levels.
So, it's a very high concentration kind of system.
And you basically concentrate you, you're basically imaging the
sun almost although it's non imaging optics and what
not, but you're kind of like imaging the sun
down onto the chip with a, with a giant magnification.
And there are many differnt companies around
the world that are pursuing a variety
of different approaches of implementing some kind
of concentrating element shining down onto a cell.
and so you've got, you know, giant dishes that focus
onto a central receiver array, and you've got, you know,
mirrors that focus down onto a single cell, or you've
got fresnel lenses, kind of fresnel lens approach over here.
you know, you just take a sheet of plastic and stamp
a fresnel lens, and focus the light down onto a cell.
so there, there's a whole variety of different different companies out there.
We do the solar cells. We don't do
the rest of the systems. Our customers do those systems.
Now because you need to essentially image the
sun down onto the cell, our technology is applicable.
In this region here and in Australia and South Africa.
So places it's not very good for Maine. There isn't enough sun there.
you want uncloudy days because you really want to capture
all the direct normal light, focus it down onto itself.
So cloudy days aren't so good so you're,
you're really looking at, you know, North Africa and.
The Southwest and Mexico and a variety of places.
So to kind of recap here, what, what happens is in
concentrated PV you focus the light down on to the cell.
We make the cells. We also package those cells.
Right now we're on a triple junction cell format.
So we've got three junctions, we'll talk a little bit about that.
Our technology platform and road map allows us
to add more junctions and continue to improve efficiency.
And our target over the next few years is to get on up to 50%.
we have a materials break through that really
allows us to blow past the previous kind
of device paradigm in this area and, and
get well above 40% which, which we're doing.
our 44% is a world record. It's been measured at
NREL, it is a certifying agency.
We've, our previous world record was measured at
both Fraunhofer and NREL at 43.5% and we are
enabling customers to make you know, capture more than
a third of the sunlight at a module level.
So after you account for system losses and what not.
You know, a, a, a 40% cell turns into a mid-30s percent module.
so these are the highest conversion efficiencies
for PV that I'm aware of.
and certainly at the cell level, we hold that, that record there.
in CPV, basically the cell efficiency matters a
lot, because the cell is a relatively small portion.
Of the overall system cost.
So if you can improve cell efficiency, you don't have to deploy
as many panels, and it saves you a whole lot of money.
So, unlike some other forms of solar where it's
all about getting the cost out of the solar cell,
in our technology set You can, because of the concentration, you can actually pay
for pouring more technology into the solar
cell, provided it gives you the efficiency benefit.
Which offsets other costs elsewhere in the system.
And as a company we've really grown, we've, we have we, we, we've
proven out the technology, our market is growing, our customers want our product.
We have a pilot line in San Jose. we do everything.
So bare wafers come in, and product goes
out the door. we've been sold out for 2012.
We've signed up a manufacturing partner in IQE.
They're an international epi depositionl company.
They're handling our epi deposition expansion.
We have a, a program from the Department of Energy to expand our
wafer fab, which we're doing in Sunnyvale, so we have a second building now.
With a six inch fab over there.
So, you know, the, the arc of our company's been pretty awesome really.
We, we started an
executive office with just a few people and a dream.
And, and and now we have two buildings
and 70,000 square feet, or some God awful thing.
And you know, a couple of fabs and we're off to the races.
And and as I said before, our core technology is good for CPV
but it can also power space satellites and we're looking at that market.
So company history just briefly, we were founded in 2007.
and we have these two buildings here, you know
we've got a fab of about, about 50 people.
and right now we do bare substrates in
and solar cell product comes out of our buildings.
and. yea, it's it hasn't been boring.
I'll tell you, start up life is it's full of ups and downs.
If you ever think about doing it, you will feel awesome on some days.
You're like oh, how could this possibly fail.
And then on other days
you're going to be like how could I possibly have been so
stupid, as to spend any of my effort on this thing.
This is never going to work, or it's going to die, or any number of things.
So.
You know, going through all those phases, you kind of
learn to get a steady hand at the ups and downs.
because there are ups and downs. So here's the chart.
So the chart is up here, you know, we're up there.
We hold the previous two world records, and our competitors,
you know, took us took us up to this point,
and, and we've kind of, we like to think we've
kind of taken over for, taken over from here guys,
thanks for all the hard work but, we've got it now.
and we'll talk about how we get up here into these efficiency numbers
Yea. So, there you go.
Okay, so a little bit about how we do all of that.
A bit about three five multijunction cells.
So, we're all familiar with the sun.
It puts out light. black body kind of radiation more or less.
And this is the spectrum that you see if you're out in space.
But, once you come down through the atmosphere you've got all this
absorption in the atmosphere so you've got these absorption lines and what not.
So the spectrum that you're really
capturing is either the red or the green spectrum.
And and different times of the year and, and when
the sun's in different part, parts of, of the horizon.
You know, this spectrum can actually change significantly.
So, this is the standard that we all design against.
But, if you're designing a device where the spectral content of
the light matters, you gotta pay attention to how that spectrum changes.
So, keep that in mind as we go through this.
So, one of the major losses in solar cells is the thermalization loss.
If a photon in this very broad spectrum from the sun comes in
at the same band gap as the material that's absorbing it, then you
create an electron hole, and they're
basically sitting at the conduction, and the
valance span and everything's happening and you
haven't lost a whole lot of energy.
You collect, say, a blue, a blue photon
and you create this electron and hole pair way
up here, and they rattled on down to
the conduction and valence band, and you lose some
amount of energy.
And this is one of the things that
we're able to battle in multi-junction solar cells.
So, if I again look at the solar spectrum here and I look at silicon, silicon
absorbs this yellow part of the spectrum and
the rest of the infra red passes on through.
So if I look at the energy conversion efficiency of a photon
at any one of these wavelengths here right around the band gap.
Really high energy conversion ratio.
But out here in the blue you're not going to be converting
the energy in that photon nearly as efficiently.
In multi-junction solar cells what we do is
we break up the light, into different junctions.
Hey look at that.
And the blue is absorbed on top, the red is absorbed on bottom.
And what this allows us to do is it allows us to beat the thermalization loss.
at least, sort of, right?
Another whole bunch of other losses in any of these systems
but this is the main bit that gets you to higher efficiencies.
now what do these cells look like? Well they look like something like that.
This is obviously larger than life.
you know, this might be a millimeter on the side.
And they, we build them on wafers.
So here's a four inch wafer with a bunch of solar cells on them.
and if you forward bias them, they all light up
because they're basically LEDs, and we just use them in reverse.
and the stack might look something like this.
This is more of a competitor's stack.
I'll get into what our stack looks like.
But you know, it, it A lot of
different layers in this thing, it's kind of complicated.
you've got a top cell, you've got a tunnel diode that
connects the top cell, top P injunction and the middle P injunction.
Another tunnel diode, connecting the bottom P injunction.
You've got front surface fields and back surface fields and
you've got graded doping in, in in emitters and bases
to reduce losses. And, So, there's.
A whole lot of optimization and engineering of the materials
stack and the materials properties goes into making good solar cells.
But what you can ultimately think about is you
can think about a circuit model that looks like this.
You've basically got three diodes.
And each of these diodes is connected by a tunnel junction.
And the thing to remember
here is What I write right here, that the subcell with the lowest
current is going to limit the overall current out of this entire device.
All right.
So let's just keep that in mind.
So if we go back to this idea of splitting
up the spectrum into different,
into different junctions, you're going to want to
create a simulator or something that says gee, if, if
I were to create these three band gaps and absorb light
in these three chunks How are, you know, how much
current is that going to generate in each of the three subcells?
And am I current matched or misbalanced?
How do I optimize this this, this triple junction absorber?
And if you look at what people have done, not Solar Junction but
others, to date is they've done a materials platform very much like this.
You've got an end gap solar cell with a band gap about here.
Basically a
gallium arsenide solar cell with a, with a
band gap here and then they used germanium.
and if you look at what the reason they do that
is because all of those materials are all going to be lattice matched.
So you can take a germanium substrate and you can grow
gallium arsenide on it and you can grow end gap on it.
Everything is lattice matched, everybody's happy, all the atoms line up.
so that's great.
Here are the band gaps here for those three materials.
But you go and you do your spectral
analysis and your current analysis and you find
that the top cell generates about 14 milliamps
per centimeter squared at the 1, at 1-sun.
the middle is 14.
The bottom, germanium salt, produces a whole lot of
extra current, so you're absorbing a bunch of photons.
And you're generating a potential current but you're not
actually harvesting any energy out of all of that.
So there's something non-optimal about this way of slicing up the spectrum.
Now, you could try moving the band gaps of the top to around, to
re-balancing to maybe around 15 to 16 milliamps in each of the three junctions.
you could do that.
And you can think about doing that.
But the problem of course is, what materials are you going to use to do that?
You can still use germanium down here.
You can still find a
gallium arsenide-based lattice-match material here.
But there's no material in
this middle region to give you that band gap.
and so what some of our competitors have done to break past The core
node of a germanium based triple junction
is they've gone to non lattice match growth.
So what they've done is, you know, if this
is one crystal lattice, this is another crystal lattice.
They basically said, hey, we don't need to match.
We'll just grow crappy material.
And that doesn't really work for solar cells.
It's very hard. It's a very
hard problem.
If we could easily grow lattice-mismatched 3- to
5-base materials, we'd grow them all on silicon.
And, you know, things would be very different.
You'd all be studying that.
Some of you are. But it would be mainstream, right?
So that's a very challenging class of problem.
So you really need some new materials in order to pull off re-engineered band gaps
in this kind of a multi-junction paradigm.
You need a material where you can tune the band
gap and maintain lattice constant maintain a lattice constant so
that you can always, so whatever the optimal triple junction
device is for a certain spectrum, well you can build that.
When you go to four junctions, you can ask your
computer, what are the optimal four slices of the spectrum?
Gives you four band gaps, you can build that.
That's what you want.
so another way of rebalancing
these currents is to keep the top two cells
exactly the same, Gallium arsenide-based cell and InGaP-based cell.
But instead of using germanium which has a band gap down out, out, out, out here.
You know, can you find another material that has, a much higher bandgap that
gives you kind of a current matched,
current matched stack here that re-optimizes the device.
And if you look.
If you look, if you look around here at one of 1EV.
There really isn't a lattice matched material that sits here.
So, you have to go non-lattice matched.
and we do see, competitors out there who are trying to do
this, so they, they try to grow, they try to grow these materials
with these big, huge step-graded buffers to grade the lattice constant from
gallium arsenide to something else that they can get 1 eV out of.
and again
that's a very challenging kind of, of problem.
So our solution at Solar Junction is to use a dilute
nitride material system, which many of you probably know was developed in
[INAUDIBLE].
Kind of invented in Japan and came here, and then Jim Harris got a hold of it, and.
People in his group worked on it for a good decade or so.
and and we took that and perfected it even further.
So what we do is we grow our one eV bottom cell with
a dilute nitride material which maintains
lattice matching across all of these cells.
and the way we do that is, if you took gallium arsenide and
you added indium down to indium arsenide, you go this way with lattice constant.
Turns out if you add nitrogen you have this big bowing parameter.
So you can add nitrogen and reduce the band gap.
And hey, if you just balance them just right here's what you can do.
You can maintain lattice constant with gas, with gallium arsenide
but you have access to a wide range of band
gaps here.
So when I use existing materials that are lattice-matched to gas which give
me access to these band gaps up here and I add to that
the dilute nitrite material I basically have a suite of materials that I
can use to put band gap anywhere I'd like in the relevant solar spectrum.
Anywhere I'd like.
And they'll all be lattice-matched.
So, now when you do a, when you, you know, try to
optimize your triple junction cell, you can put bandgaps wherever you like.
>> And, they'll all be lattice matched.
When you, when we go to four junctions, we'll ask our computer,
you know, computer what are the best four bandgaps you can use?
We'll actually be able to build that device.
Five junctions and so on.
So, all of a sudden, you've opened up materials.
Has opened up technology road map that allows you to
go from where we were, where our competitors were limited
just be, just below 40%, now on up to 44,
which is maxing out what is basically possible in a triple
junction cell.
And then, we'll be able to go to
four junctions and five junctions and even six junctions.
And continue to push the efficiency ever higher.
So, kind of here's, you know here's our pitch for lattice matching.
and, you know, against lattice matching. You don't like defects.
okay, so, in a nutshell that's that.
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