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En provenance du cours de Technical University of Denmark (DTU)

Organic Solar Cells - Theory and Practice

666 notes

Technical University of Denmark (DTU)

Organic Solar Cells - Theory and Practice

666 notes

The goal of the course is to give students awareness of the largest alternative form of energy and how organic / polymer solar cells can harvest this energy. The course provides an insight into the theory behind organic solar cells and describes the three main research areas within the field i.e. materials, stability and processing. NOTE: This course is a specialized course on organic solar cells. If you are looking for a more general solar cell course, we strongly recommend Introduction to solar cells (https://www.coursera.org/learn/solar-cells).

À partir de la leçon

Solar Cell Fabrication

By the end of this module you will be able to: -Describe the lab scale techniques and the pros and cons -Describe the difference between coating and printing -Identify the different coating and printing techniques.

Testing and Encapsulation8:26

Rencontrer les enseignants

Eva Bundgaard
PhD, Senior Scientist
DTU Energy
Frederik C. Krebs
PhD, Professor
DTU Energy
Mikkel Jørgensen
PhD, Senior Scientist
DTU Energy
Morten Vesterager Madsen
Researcher
DTU Energy

So, now, in the past few weeks, you've learned all about solar cells.

The materials for the solar cells.

And, we even discussed the film forming techniques.

The pros and cons.

Their differences.

And, how you would realize the organic or polymer solar

cell as a fully printed or coated multi layered structure.

Now there are two different ways to envisage this upscaling process.

Ideally, it's, of course, a printing machine.

A machine where you have a roll in one end,

and as you wind this roll through the machine, you apply

all the layers and at the very end of the machine,

you can roll it back off as a finished solar cell.

This sort of process, where everything is done in the

same passage through the machine is known as inline processing.

And it is the undisputedly, industrially, the

most adapted method for a completely finished product.

By this I mean that we know exactly what product we're going to make.

What it looks like.

Therefore, there's no problem in putting fresh roll in one

end, and extracting the finished solar cell in the other.

When we develop this process or put differently, before we

reach a stage, where we can start up a machine like that.

That goes ultra fast, and where all layers

are applied one after the other, without any error.

We need to develop the individual steps.

And therefore, discrete processing where we are still roll to roll

processing, but where we are doing one step at a time

on the roll, so we take a roll, we run it through a

machine, a printing or coating machine and apply one layer in the stack.

We take it from the from the rewinder at the end, put

it back on the unwinder on the machine, and apply the next layer.

And that way as scientists and developers, we can concentrate on the

application and performance of just one of the layers at a time

and we can go home and have dinner in the evening after

we've completed one layer, the second layer, the third layer, et cetera.

So having chosen this methodology where we start by

developing the inks, based on the materials that we have.

Then we establish how exactly to print, perhaps we redevelop the

ink so it works a little better for a given printing technique.

Before we can go all the way to the inline, we then have to pass this

discrete processing state where we do it for

real roll to roll, and it is truly scalable.

Or scaled and truly fast.

But we are just focusing on one layer at a time.

We need, of course, a development tool and, as you've seen

earlier, one of the best laboratory film forming methods is spin coating.

It's been used enormously in the past two decades actually.

This method is not scalable because it does not allow us to go

directly from the film form with spin coating, to the discretely

processed roll to roll step of each of the layers in the solar cell.

So we need another method, like spin coating, where we use little material, but

where the results can be transferred directly

onto the roll to roll processing platform.

For this purpose, we developed what's known as the mini roll coater.

So it's a drum, where we apply a foil.

It's, yeah the drum circumference is one meter so you have one meter of foil.

And it can be a piece of foil that

has been processed, roll to roll and then when we

reach a certain step it could be the active

layer for instance after having printed the first silver grid.

Printed the second PEDOT, coated the zinc oxide layer, and now we

want to focus on optimizing the ink for the active layer in this process.

So, you could say that the steps leading up to the active layer in

that case, are fully transferred, and fully scaled to a roll to roll process.

But the active layer is not.

So we can cut out meters of foil a meter at a time, put it on the

main roll coater and play with the active ink, and that way we use very little foil.

And we use very little active material when we develop the ink

so that it fits exactly on the larger roll to roll processing platform.

We can then, once we're confident or comfortable with it, we

can try and do, actually, on a full roll to roll machine.

And that way establish this transfer or the final scaling from something that

is nearly a roll to roll processing step to a full roll to roll processing step.

The mini roll coater has the advantage that, that it eliminates a lot of the

sources of error that you have on a, on a large roll to roll processing machine.

As you should, shall see in a while even a small pilot scale or

laboratory scale roll to roll processing machine easily has 30 or 40 rollers on it.

And whenever you roll foil through the machine, even once.

It has to pass all these rollers.

Some of the rollers are touching the back side of the

foil, some of them are touching the front side of the foil.

And the mini roll coater has the advantage that it doesn't touch the front side.

So if a source of error in your process is

that it doesn't handle the front surface, top surface is touched.

You won't see this on the mini roll coater.

So the mini roll coater enables you to do the full layers, or this particular layer

you're interested in, without touching it with anything

else than the application method that you're focusing on.

It could be slot die coating or flexographic printing or something else.

Now the mini roll coater then, allows us to

take it directly to the roll to roll processing platform.

And a roll to roll processing platform is of course a

much larger machine, and you need quite a long piece for it.

On the machine I show here, which

is small laboratory roll to roll processing machine.

There's 16 meters of foil from the unwinder to the rewinder.

That means, even to get started, you need, at

least 16 meters of foil just to thread the machine.

Now, this machine is, in spite of this 16 meters seeming like

a very long piece of foil, it's a very compact, laboratory machine.

It can fit in a small room, it's only a few

meters long, and it has many, many different printing stations on it.

There's a flexographic, a full roll of flexographic printing station.

Two slot dye coating stations, two ovens, and rotary screen printing station.

So this machine can do discreet processing.

You can thread it differently so you bypass some of the rollers, but still.

It enables you to do discrete processing of say, just

say flexographic printing or rotary screen printing, or slot dye coating.

You can also if you want, do experiments with in line processing.

Because the challenge, as I said, for with in

line processing is that the web speed is constant.

So all the printing and coating methods have to be operated at the same speed,

and it means when you make an error

it is transported through the entire machine.

And one error in one of the printing stations

will completely destroy all the foil that it comes

in contact with, and therefore also all the other

printing stations that it passes will each destroy foil.

So a natural step is of course, after you've done the discrete processing, you

start by inline processing by just taking two of the layers at a time.

And then in the very end, when you're sure that you have your product, you know what

it looks like, you know exactly how much

you're going to make, then you build a machine.

For the entire inline process and then you do only that on it.

Now on this machine here, for instance, if I wanted

to experiment with making slot dye coating followed by slot dye

coating, which could be an element of a standard roll to

roll process solar cell, I can do it in this machine.

I can also do flexographic printing and

slot dye coating or flexographic printing and

rotary screen printing, or I can do slot dye coating and rotary screen printing.

So, many combinations are possible since it has two ovens.

On this machine style, also some infrared

curing stations, so infrared lamps that heat.

So, it's a different kind of heat, from

the ovens, which are the blue boxes here, and they are basically hot air blowers.

So you blow hot air onto the surfaces of the foil and

thereby you dry whatever wet film you have and drive out the solvents.

In order to run such a machine, you need a lot of control equipment.

We have several things on it.

We have registration units.

So, small readers that look at the foil and

see where is the motif on the foil and then

it adjusts the coating of printing station, such that the

next print, or the print that you are actually applying.

Is in registry with the print or motif that's already present on your foil.

Now, registration is a critical issue.

And, fortunately for the organic solar cell, it

is critical, but it is not as critical,

for instance, as if you want to make a

very fine display for a TV or something like that.

So for the roll to roll process

organic solar cell, registration precision of

about 0.1 of a millimeter is more

than sufficient certainly for the laboratory scale experiments.

But I'm pretty convinced that even in industry processes, OPV would be

quite fine with an accuracy of about a 100 micron or 0.1 millimeter.

Now to aid the experiment on this, of course,

if you are running this very fast, at ten meters

or 20 meters a, a minute, it's so fast moving

the foil that you can see it with your eyes.

So you need those small optical readers

that can check the registration on the foil.

You also have something called a flash camera, and the flash

camera actually, it takes a snapshot of the print, and then

you, and then it leaves that on the screen for

a little while before it takes the next flash image.

And it, this allows you, as the operator to see,

am I registered, does the print look nice, is everything okay?

So it helps you, as the operator, to make sure everything is running.

And once it is, it can robustly run, even on,

on a laboratory machine like this for a kilometer or more.

Now in addition to all these pieces

of equipment that helps us both apply correctly.

But also to read out where already printed of the coated motif is.

We actually have to know each centimeter of foil on a roll.

And the rolls can be very long.

Typically, they are 1.2 to 1.7 kilometers long.

And this means that once you print it, if something happens somewhere

on the foil or you want to change a parameter along the way.

You have no possibility for remembering this.

Of course you could write with a pen on the foil but if you're

rolling very fast, you don't have the

possibility to write a detailed message to yourself.

So what we do here is on the very far end of the

machine, the right, far right hand side, we have a small ink jet printer.

And there we print for every motif, and every motif in

this case is a foot long, so, so 12 inches long.

We write a unique bar code, so it can contain information about the

day, what experiment it is, all sorts of things you want, and a number.

So with this bar code printed, it's like a birth

certificate for every foot of foil has this birth certificate.

When it runs at, through any piece of equipment in a machine

the flexo unit the oven, the slot dye coating, the rotary screen printer.

We read out this bar code, and that means we know at every

given point in time we know where every single piece of foil have been.

So if you stop the machine we know that solar cell number x2y was

stopped in the oven for half an hour while we had dinner or lunch.

Or here we changed the speed from ten meters a minute down to five and therefore

some of the layers had longer time in

the oven, and all this information is very important.

At the very end of the process, where we want perhaps, to understand why a certain

solar cell performs in a given way, and

the one right next to it performs quite differently.

So unless we have all this information, we don't know.

And in ordinary organic solar cell or polymer solar cells

we showed here, we had eight layers printed or coated.

And that means, it's quite a lot of times that this foil passes through the machine.

And there's plenty of possibilities for us to alter parameters.

And if we didn't have the machine recording

all this, there's no way we would remember.

So this is an absolutely necessary setup, to ensure you can progress scientifically

and technologically responsibly when you develop a new process.

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