In this video I want to introduce the concept of a back
surface field, which is many a times also abbreviated as BSF, and
how this BSF can essentially reduce the surface recombination, so it
can reduce the surface recombination, at the back surface of the solar cell.
So it can reduce
the surface recombination at the back surface.
So we want to understand what this BSF is and
how the heck you, it does reduce the surface recombination at the back surface.
So first I want to illustrate where this you know,
problem of surface recombination comes in the solar cell.
So I have
drawn this solar cell, which is made on this p-type silicon wafer.
So there's an n+ emitter layer on the top of, towards the top of the solar cell.
And there are these
metallic fingers which are connecting to these emitter.
And connecting to the base is this contact at the back, which could be aluminum.
And what I've done further is that I've taken
this cross-section of this of this solar cell,
and I've taken the liberty of rotating it by
90 degree, and I've I've drawn it over here.
So, the light would be shining to, from the top.
And in this picture, it's toward from this direction.
And that essentially produces these electrons
and hole pairs throughout the solar cell.
So to illustrate, to, you know, understand any semiconductor
device, it's always best to draw the band diagram.
So let me draw the band diagram
of of this solar cell. So I have this n+ emitter,
and then I have this p base, so I have this p base towards the back.
And then I'll connect these bands, and that would give
me the band diagram of my, of my solar cell.
So we'll collect my electrons on the emitter side,
on the n+ side over here.
And I want to collect my holes on this
p side
if I want to use this, solar cell to produce electricity.
But on the other hand, simultaneously, I am generating
these holes in this n+ region as well.
And I am generating these electrons in this p+ region as well.
So now these, these, holes which are present in the emitter region,
or these electrons which are present in the base region,
they can now reach the surface,
and the surface has this larger density of surface traps
that we talked about in one of the previous videos.
And if this minority carrier, this electron
reaches the surface, it can now essentially recombine.
It can use this trap state [INAUDIBLE] on the surface,
and it can annihilate the electron and hole pair.
It can recombine with the
hole using this surface state. Similarly, this hole which reaches the
surface can again now annihilate or it can get recombined.
So usually on the, on the front side at least, on the emitter side, people put a
very good passivation using, using silicon oxide, or a combination of
silicon oxide nitride, silicon nitride, and which also serves as a good
which also serves as a good anti-reflection coating.
But this is especially a big problem at the,
at the back contact.
And not only will this you know this
electron will recombine with a hole pair as
well, but it could have been a electron
which could have been collected on the emitter side.
So, all the electrons which are generated in the depletion
region, so let's say this, this would be my depletion region,
plus a vicinity of one or two diffusion length away from this
.
So let's say this is my diffusion length in my p-type semiconductor,
and so all these electrons which were generated
in this depletion region, plus this distance of
one diffusion length away from the depletion region,
could have been collected on the emitter side.
And if this diffusion length was greater
than this distance which my contact is, so if this
diffusion length was large, then this is again the
problem is even worse because I could have easily
collected this electron on the, on the emitter side.
So
there is need to repel this electron away from
the surface or keep it away from this back surface.
And the idea
which is used is is a very simple idea, and it's even simpler to realize.
And let me illustrate the way this idea is
realized by drawing this by drawing this p-n junction again.
So again I have my
my n+ emitter. And I have this p base.
What I do on the back is I add this p+ layer, this heavily doped p+ layer
at the back.
And let me again you know again, to understand it, I need a band diagram.
I, you know, pretty much to understand
any semiconductor device, I need a band diagram.
So I need to illustrate what happens using this band diagram.
So again I have this n+ layer over here, and I have this p layer this p base layer.
And this is my band diagram. And let me draw
it for the case of no applied voltage and no light shining.
So, this would be my Fermi level, which is in equilibrium.
And now what happens at the back?
So, now at the back I've added this p+ layer.
So, my band towards the back is essentially would bend
something like this, so you know, I have this p+ layer, so my valence band will
move even closer to the Fermi level, so
my band diagram would look something like this.
So now let me think of those electron and hole pairs
again, and let me draw the surface states which exist as well.
So now these electrons and hole pairs generated over here,
So, I have
this electron and hole pairs that generated over here.
Electrons love to travel down the hill,
so all these electrons which are present in this depletion width, in this depletion
width, plus some diffusion length away from
this depletion width would all be collected.
So this is my depletion width.
And then a distance of diffusion length away from this depletion width.
So,
all these electrons over here, they would be collected, they would have to go down
the hill and and they will be collected on my, on my, on my emitter contact.
Similarly, my holes which are generated in this region
would be again collected on this base contact.
So my problem was that this electron was
instead, you know, it could go to the surface
in this previous case,
this electron could go on the surface and it can get recombined.
So, now let's, you know, let's understand what I've
done by introducing this, p+ layer over here.
So, if I've introduce this p+ layer
over here, my band diagram is of this shape.
Where I've created this you know, this upward-facing slope towards the surface.
So now this electron is essentially, it's repelled
from going near the surface, by this field which is
created as a result of having this p+ layer.
And also so this p+ layer, it helps
in collecting these holes towards my base contact.
Because this essentially you know, they, they the holes have to
travel up the hill and electrons love to travel down the hill.
So by creating this upward-facing hill over here,
I'm repelling these electrons away from the surface.
And this is what my back surface field is, so this is my back surface field.
So now by creating this p+ region over
here, I've created this field towards the back surface.
And this is now, this field let me, you
know, zoom, if I zoom in the band over here.
The presence of this field over here, this is repelling
these electrons from reaching the back surface.
And there are several ways I can realize this back surface field.
The most common and the most easiest way to realize
that this p+ doping to create this back surface field,
So I want to create this p+ doping in this region.
So the easiest way to do that is to allow this aluminum from the back and this
aluminum act as a decent p-type dopant in silicon.
And it creates this p+ layer towards the back
which repels the repels the electrons from reaching the back surface.
Another way
to create this back surface field would be again to you know, introduce boron to do a
implant or a diffusion of boron from the back
surface, to create this back surface field as well.
So in summary, this presence of this p+ layer at the back, it creates this surface
field, and that surface field, it repels minority carriers away from the back
surface, and it helps in reducing
the surface recombination towards the back surface.
So hopefully you're able to understand this back surface field,
and whenever you see this term, BSF, you'll understand what's happening.