So how do polymer solar cells work?

There are some key differences between the way polymer solar cells

work and the way traditional silicon solar cells work.

So with the traditional silicon solar cell,

we have the p-n junction, as you remember.

And this is quite comparable to what we call a bilayer structure for polymer solar cell.

So we have an electron donor,

an electron acceptor, the two on top of each other.

So basically forming the p-n junction.

The problem with the structure is that unlike silicon,

the diffusion length for the carriers is really really short.

So the lifetime of the exciton,

is not long enough that this really makes sense.

So, either we need to make really, really thin materials;

and when we talk about thin,

we talk about 10 nanometers this is the diffusion length for the carriers.

So typically we'd end up having

so thin materials that we'd not really be able to absorb enough light.

And even still, this will not be enough that would get high efficiency.

So for polymer solar cells,

we need to scrap the thinking of a bilayer structure.

This is not how we make most polymer solar cells today.

What we do instead, is we create what is called a bulk heterojunction;

and the bulk heterojunction is a junction in which we

basically mix up the p and the n-type material.

So instead of having just two materials stack on top of each other,

we sort of scramble this layer,

and make sure that inside this layer we have junctions going all over.

The important thing is, that we need interconnectedness,

between the donor and the acceptor material.

So basically, we need to make sure that

either phase can reach their respective electrodes.

So this puts high requirements on this bulk heterojunction.

The formation of this layer is really critical to polymer solar cells,

and this is one of the key areas that needs to be

optimized in order to have an efficient solar cell.

So let's take a look at the layer stack that is required for polymer solar cell.

So first of all, we of course have a substrate.

The substrate can be glass,

but more typically it's a flexible plastic material.

We also of course have the active layer.

So this is where the absorption takes place,

this is where we have our p-n junction,

in the form of a bulkhead heterojunction.

And this material will typically be a blend of

a conjugated polymer and some sort of C60 derivative, typically PCBM.

On either side of the active layer,

we have transport layers.

The transport layers ensure that we have

a selective transport of electrons and holes going to either electrode.

So one layer is an electron transport layer,

while the other's a whole transport layer.

On top and bottom,

we of course have electrodes to carry away the current being generated by the solar cell.

An important thing to notice with polymer solar cells is that typically,

the bottom electrode can be made of ITO,

so indium tin oxide.

As we learned from the thin film module,

indium is quite scarce and this is a problem.

So, we really want to avoid using indium in polymer solar cells,

and during the recent years,

there's been a lot of progress on this,

and the polymer solar cells I can show you here,

are actually made without indium.

So there's no indium tin oxide here.

Instead we have a middle grid electrode.

How do polymer solar cells work

Introduction to solar cells

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