So in the last few videos, I described to you
this cell design for a HIT cell, which was developed by Sanyo.
This was a big hit for them, this cell achieves one
of the highest Vocs that is possible for a crystalline silicon cell.
It also achieves efficiencies of more than 22% on large area cells.
But this cell design, it didn't just
happen by accident. So, I'm a history buff, I like to look at,
you know, how inventions, they get originated, and how they're developed.
So Sanyo, the company which back in the 1970s, late 70s and 80s was working on
these thin-film
silicon cells made out of amorphous silicon.
And then they started working on these crystalline silicon cells.
And the research for this, this, this
HIT cell started in the mid 90s, the
first commercial cell was I think developed in 1997.
But they were a company which had expertise in
both amorphous silicon and crystalline silicon, so they were very
appropriately placed to develop this HIT cell, which utilizes
both this crystalline silicon cell, and then this amorphous silicon cell
for passivation of the surface. So, you know, this, this Sanyo, at that
time, and at that place, was uniquely positioned to develop this technology.
So in this video I want to you know give you some more context and some unique
unique advantages which come along because of this HIT cell architecture.
So, one thing that I already
mentioned was that this cell can be processed at a low temperature.
This whole processing of this can be completed at, very low temperature as
compared to a conventional cell which requires a
temperature of, more than, 800 degrees centigrade.
So, as a result of that, there is, way less damaged caused in
in the crystalline silicon. So usually if you subject your
low-grade silicon to high temperature processes, there's defect
formation, cluster formation, which happens inside the cell.
But, this is the processing of the cell is completed at a very low temperature.
It benefits from the absence of these
these high temperature process, which cause defects.
So, in this video I want to mention some of the unique features, which this
HIT cell has.
So, one of the unique features which is often publicized by Sanyo or now,
Panasonic which acquired Sanyo, is that the
low temperature quotient of, for, this HIT cell.
So, as we, uh, in one of the previous videos
I described that the efficiencies of a the solar cell,
they fall as a function of temperature, so
when you increase the temperature, your efficiency typically falls.
And, the reason for that is your, is your is your current essentially,
your regeneration recombination current goes up, it's usually given by
this formula, where this temperature, power dependence of
temperature comes due to a density of state.
And this dependence comes due to the lowering
of the barrier height as you increase the temperature.
So usually you can express the current in your in your, in your solar cell
by this formula, where it's given by
the short circuit current minus this recombination current.
And this rises exponentially as a, as a function of voltage.
And one can equate the current to zero to find the Voc.
One can do that to find the open circuit voltage,
and then you could differentiate the open circuit voltage
with respect to temperature to find its temperature dependence.
So you get, if you do that, you will
get this formula where this temperature coefficient of Voc,
so this Voc as you see it has a negative temperature dependence.
So usually when you increase your temperature, your Voc decreases and
this what is a primary driver of your
efficiency decreasing as a, as a function of temperature.
So if you look at this, if you look
at this formula, this this temperature dependence of Voc,
it depends upon how, you know, it depends upon, if you look at just these two terms,
it depends upon the difference between the Voc and the band gap of the cell.
So the higher is more of Voc, or the higher is my open circuit voltage,
the closer it would be to the bandgap of the cell,
So that would mean that I'll have less temperature coefficient.
So the higher the Voc, the less is the difference between these two termsa
and that would give me a lower temperature dependence.
So, because of the low surface recombination velocity, in, achieved
in this HIT cell because of this good passivation between my
amorphous silicon and the crystalline silicon at the surface,
as a result of that my I01 is lower or my
Voc is higher. My Voc is, in fact in HIT cell, it's,
it's more than 0.7 eV, and that brings,
essentially, that reduces this term. As a result of that, the Voc decreases
as a function of temperature, it reduces.
So the HIT cell, essentially it has
a very good temperature coefficient for efficiency.
So, in a normal crystalline silicon-based cell your efficiency decreases
by approximately 0.45% for every degree increase
in the temperature.
So, if you have ten degree increase in the operating
temperature of the cell your efficiency would decrease by 4.5%.
In comparison to that, the HIT cell has
roughly a temperature dependence of half of that.
So, a
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So, for a HIT cell, the temperature dependence of efficiency is only
0.23 % decreased per degree increase in temperature.
So, if I, in this case, if I increase
the temperature by ten degrees centigrade, my efficiency only
decreases by 2.3 %.
So that makes these HIT cells very favorable for operation
in high temperature conditions where you are in deserts or
places where you are operating these cells are at high temperature.
So, they'll have much better efficiencies
as compared to these crystalline silicon cell.
Another very unique feature of this HIT design, which I find very interesting, is
that this design is essentially
symmetric as compared to whether you use a cell from the
top or whether you use from the bottom. Actually, I should not say it's symmetric.
But the cell could be functional whether you, you lit from the top.
If you lit from the top it would be essentially a n, if, let's say this, this
crystalline silicon is n-, so it would be an n ...
n- p cell.
Versus if you lit from the bottom It would be a, p n- n cell.
So one could optimize the cell such that it could be used both in the cases where
light falls from the top and light falls from the bottom.
And usually these bus bars or these fingers, these
contacts which are placed on the top and the bottom,
they're also placed you know, they're also placed symmetrically, so you could utilize
the cell both using it from the front or using it from the back.
That renders the cell to be very useful for you know, some unique applications,
where you have availability of light both from the top and from the bottom.
For example, given here are some some scenarios
in which this might be possible.
So for example, you're mounting these panels in this, at this slanted angle.
And you're mounting them on a roof, which is you know, which is
let's say which you had colored white, so it has very good reflectivity.
So you could be capturing this sunlight, which is coming directly from the
sun, but this roof, which is white in color, can also essentially reflect
this light from the surface.
And this could be incident from the back side of the cell.
Similarly, one, you could also place them vertically.
But a very useful scenario would be when these things are horizontally mounted.
But they're mounted at a height.
So instead of mounting, placing this directly on a surface.
You place it for example, at a particular height away from the surface.
And now when this surface is highly reflective, it could essentially reflect
this light which is coming again from the cell, and reflect it back to the cell.
So this makes for very interesting applications of this HIT cell.
And now it's, it's called a HIT double cell.
So it could double the HIT with some pun intended for this design.
And
I'm showing you one of the designs that I particularly find interesting.
So this is the Walt Disney studio, which is located in Burbank in California.
And if you visit them, they have this
solar panels installed on the roof of the studio.
But they also have this walkway on you
know, connecting two different buildings of the studio.
So this makes a very good sample case,
example case for illustrating for this HIT double design.
So you can see over here this, panels are mounted on top of this, walkway.
But this is, a small, you know this is a narrow walkway.
And these panels are mounted at quite some height.
So this makes a very good you know, a case where your
sunlight can hit these panels directly from the top,
but it could also come and hit this walkway,
and if this walkway is is highly reflective it can essentially reflect this
back to these solar panels and these solar panels can now essentially
absorb light from the bottom because they're using this HIT double design.
So this could,
depending upon how much light you can shine on this cell from the bottom, this
can again increase your efficiency, depending upon, and it's
highly subjective, you know, how these are mounted,
and how these are, what, what is the structure and is the reflection of all
these surface, but you could increase your efficiency
easily by 20 to 30% by using this
double kind of design under under some of these scenarios.