Solar Cell Basics: Selective Contacts


In the last video, I described this big
tent definition of how a solar cell works, and I was also trying to dispel that very
limited definition that you might have picked up from your semiconductor class,
or your favorite semiconductor teacher, that you
need a PN junction for this solar cell to work, instead what
I was trying to hammer in was this very simple but broad definition, which
requires this only two things for a solar cell to work. And yet it very, very broad. You can apply this definition to a
crystalline silicon based solar cell. You can also apply it to a thin film based
solar cell. such as based on CdTe or based on CIGS, or
you could even apply this to organic based photocells, and we will study all of these
in the subsequent videos. But the thing I was the two, the main
requirements that I want to emphasize again and again is the
very first requirement is this density of state bottleneck, and somebody
might say, you know, that he’s really using a fancy word for what he really
means is just this band gap, and this in fact true, if you’re
talking about crystalline silicon, and what you need is essentially this band gap and you have,
you need to have this conduction and valence band. People who are coming from organic
world, they can, you know they can identify this
as the lowest unoccupied molecular orbital and the
highest occupied molecular orbital and you need to have a forbidden gap between HOHO and LUMO states. And so this was no big deal we
saw that most of the materials, semiconductor materials that we know of
like silicon, germanium, gallium arsenide, they all form this band
gap very easily. So, the next thing that, you know, is a very important thing and I want to
emphasize that in this video, is that you need
selective contacts for for this solar action to happen.
So you know, what do I mean by this selective contact?
So let me draw this box for band diagram to explain this selective contact concept and let me also
put two contacts over here and I would say
this is my contact to collect electrons and this is
my contact to collect holes. So what is this selective contact mean? This selective contact means that when you have an incoming photon and it generates
this electrons and hole pair, the contacts
should be that your contact which is selective to holes, so essentially if your electrons comes over
here, this contact is selective to electrons so it
should be able to catch this electron. At the same time if a hole comes over here to this contact, this contact should
say hey, hole, you’re not welcome over here, and it should reflect this hole back.
Similarly this contact for holes, what it should do is that if a hole comes over here it says hey hole,
you’re welcome here. Please come in, and it should connect this
holes, while, at the same time, if if electrons come to this contact, it should say, you know, hey, get lost
electron. So, this is this is, you know, what I mean by this selective contact, so it’s a very
simple definition, but it’s very hard to implement in
reality. So another way to, you know, look at the
defined contacts is by this thing called a surface
recombination velocity. And it’s essentially given by the current
you get if, you have, you know. a difference, in the number of, carriers
in your contact and in the material to which it is contacting
to. So, what it means is that if you have more number of carriers just at this
interface, what what is the velocity at which, these
carriers would be collected, at this, at this interface?
And, what an ideal contact means that, you know, your surface recombination
velocity should be, infinite, for the carrier that your contact, for the
carrier you want to collect. That means that, you know, if you have if you have a hole coming in here, it should be collected immediately, and this
should be very high current, even if there’s, you know, a
small differential of hole concentration between this contact
and this semiconductor over here. So what this means is that, for this,
contact to be selective to holes, it means that my surface recombination
velocity for holes should be infinite. At the same time what it means is that, my
surface recombination velocity for electrons should be 0, so there should
be no electrons collected at this contact. Similarly over here, what it means is that
my surface recombination velocity for my
electrons should now be infinite. And my surface recombination velocity for
my holes should be zero for this contact. Another way to wrap my head around this
conundrum of selective contacts is to use this idea of
quasi-Fermi levels. So, assuming that, you know, this, my
semiconductor which is represented over here, it’s a
intrinsic semiconductor, so by default, its its Fermi energy
lies at, you know, the center of the band gap, and when you actually hit it, and shine
photons on top on, you know, on the semiconductor.
It, generates these, populations, excess populations of electrons, and this
excess population of holes. And one way to represent this excess population is by having two
quasi-Fermi levels. And you can have, you know, this is my
quasi-Fermi level for electrons, and this is my quasi-Fermi level for
holes. And, the way this then this selective contact concept can be applied, is that
now, my surface recombination velocity, at this
particular contact, since it’s selective to
electrons, my surface recombination velocity for electrons is
infinite. So all the electrons which, you know,
reach over here should essentially immediately combine over here and this
quasi-Fermi level for electrons should essentially match with this Fermi level of my
contact made over here. So this quasi-Fermi level for electrons
at this contact should essentially become like this. on the other hand, my my holes over here,
these guys if they reach over here, they don’t
recombine, they are reflected back. So my quasi-Fermi level for my holes should essentially remain constant near
this contact. Similarly, if I look at this other guy,
this other contact, which is selective to holes, so my quasi-Fermi levels for holes
should essentially match this contact which is which is selective and very good
at collecting holes. Similarly, the quasi-Fermi level for for
electrons should essentially remain the same at this contact, because this contact
does not want to collect any electrons. So this is a way if you, in an ideal world if you had ideal selective contact, this
is how your quasi-Fermi levels would look like. But now, you know, let this being you
know, the ideal theoretical world, . Let’s look at, you know, how do actual
contacts look like? So most actual contacts behave quite
opposite to, you know, what you actually want for a solar
junction. In fact, for a solar cell. So what, in fact most of these contacts,
they are not selective at all. If you, if you have a ideal or make
contact, if you have a ideal, say or make contact between a metal and
semiconductor, the typically the way the way it works is your surface recombination velocity for
your holes is infinite. At the same time, your surface
recombination velocity for your electron is infinite as
well. So it has essentially no selectivity at
all. And one way, you know, that you may
recall, one way to engineer or make make a low resistivity ohmic contact is by doping your
semiconductor very heavily. So, for example, you have this
semiconductor, and you wanted to make a, a, say a very good contact to a
n-type semiconductor. The way you would do it, you’ll dope it very
excessively n+ at this at this contact
region, and what that would do is essentially it would reduce this tunneling barrier for your
electrons. So now your electrons can tunnel in and
out very easily. But you know, if you look over here, so
you know, this is, this is how we made it a good contact
for the electrons, but, you know, if you had other
holes over here, these holes love to, you know, flow you know,
they’re, they’re like bubbles. They want to, you know, travel up like
bubbles in a vine. They want to go up, and they will essentially get collected at this contact,
as well. So, this contact is essentially, you know, it has a surface recombination velocity, which is
high for holes, as well as, it has a surface recombination velocity which is high for electrons, as
well. So, in reality, most of the contacts, if you want to make a contact selective to
electrons, you want to make this term over here, infinite, what it does is this term
over here becomes infinite as well. Similarly, if you wanted to make this term
infinite, it, you know there’s, it comes, it goes hand in hand that it becomes
selective to this electron as well. So, what happens, again, in that
quasi-Fermi level picture is that both my, both my electron and hole quasi-Fermi
level, they look like this. If I had, you know, just a intrinsic
semiconductor and had this excess population of electrons over here and my
holes in my valence band, they essentially if these, these contacts
you know, this, they’ll this contact will collect my holes as well as it will
collect my electrons, with you know equal efficiency, so I’ll get no
selectivity, and even though I generated these carriers, these electron-hole
pairs, I won’t be able to collect them. selectively these or get any current out of this semiconductor.
So, the next big question is, you know, how do I achieve
this selective contacts. So how do I make this selective contacts
to my to my to my semiconductor. And, so I want to describe three ways. The first way, which might be, again very
similar to you, and this is how most of you know, semiconductor classes
teach about solar cells, is to use a PN junction. And the way I view PN junction is that PN
junction is nothing but it’s a way to, you know, engineer selectivity in
these contacts to, selectively collect your,
electrons and holes. So shown here is this, PN, junction, and,
what this essentially does is that by creating this, depletion
region or this electric field, it engineers this selectivity into this
contact so now my now this it so this is how my quasi-Fermi levels look like over here.
So this would be my quasi-Fermi levels for electron and this
would be my quasi-Fermi levels for holes and let me
complete drawing. So what it does, it essentially separates, it breaks this symmetry into my intrinsic
semiconductor. And you know, it makes it causes diffusion
of these minority carriers, and it breaks my symmetry so that, you
know, I can now selectively collect my electrons here, and I can now selectively collect my holes over there, because, you
know. this is where, you know, my hole
concentration is higher, so I can collect it over here. This is where my this is where my hole concentration is higher, so I can collect
them here. This is where my electron concentration is
higher, and I can collect them here. And we’ll, you know, look into this p-n
junction in more detail, but it’s essentially, the way I look at it
is one way to, uh make, or you know, selectively collect these
electrons and holes. But, again, it’s a very limited
definition. So there are many other ways you could
make this selective contact. So another way, you know, you could selectively collect your electrons
and holes is you know, having utilizing this idea of having a heterostructure to you know,
engineer selectivity into your contact. So this might be, you know, for your uh
semiconductor material. Say, let’s say silicon, in which you know, you’re causing this photovoltaic action
and you are generating this electrons and holes,
and what you can do you can have this semiconductor
A, and this semiconductor B over here, so
that B has a band line-up so that it has this
conduction band offset. And A has this band line up so it that has this valence band offset with this
silicon. So now what happens is you know, if my electrons go over here, they are reflected
back but if my hold will go over here they
are, you know, collected very easily. Similarly, if my electrons now go over here, they’re selected very easily over
here. But my holes are reflected back. So this, you know, one way I’ve achieved
this ideal selective contact I wanted. And in fact, you know, we will see that
a lot of solar cells are, especially this design called HIT solar
cell or heterostructure based solar cell that is sold by Sanyo
which uses exactly this idea. Similarly your CdTe based solar cells,
they again, use this heterojunction between CdTe and CdS to,
again, engineer this heterostructure contact. And a lot of organic, in fact, well, all of organic photovoltaic
cell, they use this heterostructure between your
donor and acceptor organic layers to again achieve
this selectivity in your contact. But, so there’s another way you could
think about doing this. So another, yet another way you could
achieve selectivity in this contact is, you know, let’s say I have
this semi-conductor material and I have this… so shown here is a band diagram
but you know, in reality, it might be this
slab of semiconductor material. And, what I can do is, I can on one side, I can grow this or place this
oxide which has a lot of positive fixed
charge. And then, the other side I can you know
deposit on, on so on so one this slab over here, so over here I grow this positive oxide. And on the other side I grow this other
oxide which has a lot of negative fixed charge. And what it does to my band diagram is essentially this positive
charge over here, this will attract all my electrons so my electrons will get attracted and get
collector here. At the same time, it will repel these
holes back. Similarly, this negative charges over
here, it will attract my holes but it will repel my electrons back. So this is a, again one way I can
engineer selectivity into my [UNKNOWN]. So this is a, again a one way I can
engineer selectivity into my contact, and there’s a lot of research
or, going into developing films Such as aluminum oxide which has a lot of inherent charge, to essentially enhance
the selectivity of the contacts.

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