How do polymer solar cells work

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Introduction to solar cells

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Technical University of Denmark (DTU)

Introduction to solar cells

494 notes

How do solar cells work, why do we need, and how can we measure their efficiency? These are just some of the questions Introduction to solar cells tackles. Whether you are looking for general insight in this green technology or your ambition is to pursue a career in solar, “Introduction to Solar Cells” is an excellent starting point. The course is a tour through the fundamental disciplines including solar cell history, why we need solar energy, how solar cells produce power, and how they work. During the course we cover mono- and multi-crystalline solar cells, thin film solar cells, and new emerging technologies. The course includes hands-on exercises using virtual instruments, interviews with field experts, and a comprehensive collection of material on solar cells. At the end of the course you will have gained a fundamental understanding of the field. This will allow you to identify the most interesting or relevant aspects to be pursued in your future studies or in your professional career.

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Polymer solar cells

We will now turn to an example of a third generation solar cell technology: namely polymer solar cells. Polymer solar cells promise to solve all the problems we see with other technologies. They are cheap and fast to produce, they are non-toxic and use non-scarce materials, and they exhibit the lowest energy payback time of any technology. In this module we will learn about pros and cons of this interesting technology.

Polymer solar cells4:52

How do polymer solar cells work3:29

Rencontrer les enseignants

Morten Vesterager Madsen
Researcher
DTU Energy
Martin Helgesen

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.

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