Okay.

>> [INAUDIBLE].

>> There we go.

>> Okay, I'll ask again can you, can you hear us?

Can confirm in the chat box?

>> [INAUDIBLE].

>> Yes okay, so it seems we are, we are ready to start.

Okay. So first of all, I just introduce myself.

Let me just see if I can get this camera working.

I'm sitting over here.

I'm Morton.

I'll be relaying all your questions to Frederick and

me, me Frederick and Eva, sorry.

>> [LAUGH] Early in the morning.

>> It's early, it's early in the morning here.

And so, yeah.

We have Frederick and Eva here, and they're ready to answer any questions.

Okay, just can hear somewhat.

Is there, is the sound coming fine okay?

>> Can you hear me, and Eva [INAUDIBLE] now, yeah?

>> Can you hear us?

Morning.

Or afternoon, wherever you are.

[LAUGH].

>> Yeah, or mid-day, yeah.

>> [CROSSTALK] Okay, it seems to be okay.

>> Yes. >> Yeah, okay so

I think we'll just jump right into it.

I also prepared a few questions in advance so

there are a few already I did note them.

So we'll get to them at some point.

First of all we had a few questions about congregated polymers

in the in the quizzes and we talked quite a bit about it in the lectures.

But, but it seems it's probably worth doing some clarification about this.

So I guess Eva, you brought something up on the on the board here.

>> Yeah. Let's zoom in on it.

>> Let's see if we can zoom in and then you can.

>> I can type.

[NOISE].

Can you draw?

>> Okay I think they can see what you draw.

You on? Okay.

We also start with this.

So, I guess, the first question would probably be, we see both donor acceptors

as terms, when we talk about, or we see it in different contexts, so.

>> [INAUDIBLE].

>> [SOUND].

>> Sorry.

>> Okay. Yeah.

So we [CROSSTALK] just go.

>> Okay.

So the difference between the donor and acceptor has been a little bit confusing.

So we have in the polymer,

we've shown here a congregated polymer with side chains.

It's just an example.

And here we have the, the, this is my aerosol, which is on a sector unit.

And we have a donor, which is the [INAUDIBLE].

And these are all [INAUDIBLE] up.

[INAUDIBLE] So we have a donor and

a acceptor polymer system.

Now this whole polymer is the donor and

the [INAUDIBLE] is the acceptor which

accepts the electrons from the polymer.

So this was very simple, on the donor, and et cetera.

And I know there's a lot of confusion on this, but if you say,

in the polymer donor, acceptor, it's these two, and in the solar cell donor,

acceptor, it's the polymer, the whole polymer [INAUDIBLE].

And there's been some questions about congregation.

And as you can see here, the very very simple molecule

have the congregation here, that don't.

The double bond, and a single bond, double bond, single bond, double bond.

And it just continues like this.

And it's congregated,

because in this group is PyPy systems, we have the double bonds.

We can transfer the electrons through the polymer.

And here, down here, can we see those things?

>> Yep.

>> And we have shown a more complicated molecule.

You see we have another

unit here instead of the two five things we saw from the top.

We have one where we put a silane in here, which is attached to the two

[INAUDIBLE] and we have some side chains on this silane.

Now this is a congregated polymer backbone.

We have a, a double, single, double, single, double, single, double, single,

double, single bond system.

And we have the side chains.

And the reason why we add side chains to the polymers is

that we can have solubility, so we can, eh,

solubilize these polymers in ordinary organic

solvents together with piece at the end and

combine this to the active layer which we can [INAUDIBLE].

>> Okay thank you, Eva. [INAUDIBLE]. [SOUND] Okay. Just fiddling a bit

around with the controls here.

Okay, thank you Eva.

So, yeah, you just mentioned the side chains.

That's actually one of the other questions.

So, what is the purpose of side chains?

We just learn now, it's to, to increase the solubility.

A follow up question to this is,

does this have reverse effect on the solar cells in any way?

Both with regards to stability and efficiency.

Having the side chains in the final device.

>> The answers are a clear yes and also a clear no.

The side chains aren't sure.

And, you know, nature affects the stability.

We're talking, stability in terms of how the polymer will

react to light and, for instance, oxygen.

It's been shown that, that different side chains

are more prone to reaction with oxygen and light at the same time.

The side chain can also, can have a stabilizing effect, for

instance, on morphology.

So, as you learned already, a very important part of the function

of the organic solar cell is this find nanopathology,

where you have find face separation of pcpm and polymer.

[COUGH] Into discrete remains that helps [INAUDIBLE] association.

In such a, a can be very influential on, on, how that is stable over time.

So, in some cases, side chains will lead to find nanophology initially, and then

when you heat up, or as you do when you shine a light on the solar cell over time.

Pcpm crystals and the device will fail, not because the materials at such

a degree, but simply because, the face, domanes grow larger, and larger, and

therefore, the [INAUDIBLE] also becomes less efficient.

So there, you can say that there are two,.

Areas alone where the sightings can influence what we call stability or

operation stability.

Either it's a chemical degradation that is better or, or poorer,

or it's a morphological stability.

And I assume other examples such as adhesion to interfaces.

And in many other aspects.

In terms of the use of the sightings for, for solubilizing.

This is something unique and, and wouldn't want to prick the sauce.

Rosin from solution, one of [INAUDIBLE] active layer onto searches and

the other sightings are extremely important then.

If we then move on to this question of, of water solubility.

There are several ways to achieve water solubility.

One is, is where you make an emulsion.

So, you have in principle a hydrophobic polymer where the side chains are used

to solve the problem at hand.

Organic solvent.

And then you make an emulsion in water and remove the organic solids.

So, you in principle get like mayonnaise that is emulsion, but

between an oily component and water component.

We have the, the droplets in the mayonnaise, in that case is, if it's

an oil in water emulsion, which it would be here, is the, eh, organic polymer.

And in that case, you can process the water, and once you dry out the water,

you dissolve this polymer in water.

So in that case, eh, nothing happens, if you manage to trick another

emulsion-based, water-emulsion-based layer on top of the already printed layer.

If the side chains are made such that the eh, the polymer is water soluble,

then, of course, printing of water based ink on top of

an already water-based soluble polymer there can be a change.

Then you can, you can say goodbye to the fact that perhaps the dynamics of this

solution's slow.

So you manage to shrink the wet layer on top of the water soluble,

already printed water-soluble, soluble layer, and

you manage to dry it before it dissolves, they dissolve each other.

This can be achieved in some cases especially for

materials where the molecular weight is very large, and that it,

for those materials the solution is very slow.

But for small molecular weight materials is really a challenge that cannot

be solved.

And the way that we normally achieve it is by using solid between layers.

So, if we printed one layer for instance from water,

then we print the next one from an organic solid, assuring that the first printed

layer is not soluble in the subsequently processed layer sold.

A good example there is PETA which is a water-based dispersion.

So first you, for instance, coat PETA in water, and

then you coat the exit layer from an organic solid like in there.

There's no problem that, you can dip your PETA layer on an organic solid for

as long as you like, and it will never dissolve.

It's the same when we do it the other way around,

when we already have an organic layer printed on an organic solid,

and then we print PETA on top of a water or an alcohol-based solvent.

In that case, also the PETA can rest on top of the organic layer for

a long time without dissolution.

I hope this answers it but it, it's clear the,

the side chains are very influential at, at all levels in,

in an organic source from manufacturing to stability and operation.

Yeah? >> Okay, thank you Frederick.

I guess this-.

>> Maybe, maybe we can also add that with the side chains.

Since we accept that the, they affect the stability that we also have this step,

we can remove the side chains, but this you will learn more about in this weekend.

>> In this weekend.

>> [COUGH] I guess you can-.

>> because the stability is used to this, this week's lessons.

>> Yeah.

I guess if you want you can elaborate a little bit on, on this issue.

This is. A very interesting issue that we can do

that will clean up the side chains, and we do cross-linking in all of this.

If you can elaborate a little bit on this, that would be quite nice.

>> Yeah, you want to do it or [INAUDIBLE].

>> [INAUDIBLE] You can do it.

>> Okay.

Yeah, be, because the side chains are influential,

that we knew this already long ago.

That side chains do, influence the and, from many points of view,

once you've reached the morphology you want, once you've,

made your layer, then you don't need side chains any more.

And the, we explored, long ago, the first exploration of this,

where we would move the side chains, after you processed the flow.

The, is where you have a, a specific class of materials.

You have an ester, where the alcohol is.

Then you can ac, [CROSSTALK] you can actually, by heating it up,

it will eliminate.

And you can simply remove the side chains, so in,

in a case like P-frame, HOCT, for instance.

I don't like that is what the US told but you'll see in a minute.

There you can simply take a very soluble column, and once you heat it up,

it eliminates an alkyne and a carbon dioxide and you have the native conjugate.

Which is completely insoluble.

And that means also we, we made a series of materials of this kind,

and that means that once you make your morphology,

the, the one that you desire, if you're going to you freeze it.

Either you make it glass or you harden the polymer layer, so it's not only insoluble,

but the morphology will not evolve, evolve over time.

In addition, you could also, but once you form your film, the side chains

are passive in terms of light absorption and also [INAUDIBLE] carrier transport.

So you don't need them once you've achieved the morphology you want and

the layer you want.

That's what, what inspires.

And so, the polymer was the first generation in, in,

in achieving solubility.

And then, eh, the disadvantage is that which you needed relatively high

temperatures to move the [INAUDIBLE] and

high temperatures here is something like 300 degrees.

Most flexible substrates, they are only work until 140 or 150.

So, worked a lot on reducing the temperature and there, we have another.

We've made several other examples of, of the side chains.

And I think let's see what do here.

That's the P-frame HOC where you can see the side chains when you heat it up.

Goes away and you just have negative prototyping.

So you replace the ester group, and there's an oxygen missing at.

[CROSSTALK] You hadn't seen this [INAUDIBLE] yet.

And the ester group is simply replaced by a hydrogen atom.

And the carbon dioxide and the alkyl, they, they evaporate [INAUDIBLE] 300.

We then also developed another type, where we had a side chain.

In those, they can be removed at room temperature with acid treatment.

And, so that wood was cold.

And this also works, but then, of course, you need to use acid in there.

And this has other, in some cases at least, other implications.

And so there's nothing in this world that's free.

And the third option, for insoluble that has been explored is cross-linking.

This has been achieved using alkenes, using halides, asides, etc.

And this achieves.

The botany of morphological stabilization and

insolubalization with the photochemical stability is unaltered.

So, that means if you have side chains, a cross-linkable for

a chemical stable you hadn't solved before a chemical stability solvent.

Whereas this should do in the case of, of thermal cleaning and or acid removals.

So, I hope this sort of warms up for this week's.

>> Yep.

>> Many years now, drawing some examples of side chains on tolerance that we can

use for, for cross-linking.

Oops, just a little.

Okay, so we just lost one of the computers here but I.

We're still on.

So, no problem.

>> Yeah.

[INAUDIBLE]. >> Ever been.

>> Okay. >> I don't.

>> Let's see your drawing, Eva.

Maybe, can you all lean in or something, so we can see what's going on.

[SOUND] A little trapping.

Yes, you were right.

>> Yes, so here's the side chain with a bromine at the end, so

of an alkene, a side chain just on a schedule, a [INAUDIBLE].

And when we cross link, we add these two chains together and

we get inter network or network, and then the.

>> Three dimensional network.

>> Yeah. >> And then, it's no longer solid.

>> Yeah.

>> So that means that a cross link system like that is like and

we know this from brava.

Well, I, so, so, insoluble, it

is what's in or swell, so solvent can enter the network.

And that's also why you don't get system.

You only get the morphological stability in plastics,

which obviously, you can still access as it would normally between.

Okay.

I guess this was some really good answers.

Let me just see if I can find some quick questions that sort of relate to this.

One of the questions we had, I've seen for a few times is,

is the sort of the fundamental trade off between efficiency and stability?

>> Well, that's a good question.

In principle, I would think that it's, that what we've observed so

far is a clear anticorrelation.

I think it's because of the way the research has been carried out.

There's one school of people who have lots of materials that will give you

the highest efficiency no matter what, while not looking at stability at all.

And the people who have been looking at stability have not cared so

much about efficiency for they've been wanting stability.

And both groups of schools of people have been highly successful, except perhaps,

they didn't talk so well together, so and all are very high efficiency.

All the volunteers are not, does not present advisors that advisors.

And I think it's, I would expect or anticipate that, that once

that these groups start working together and

trying, unifying the stability and the efficiency, I think these two will meet.

I'm not saying that we will achieve ultimate stability for the ultimate

efficiency, I'm sure that compromise like all things in life will be made.

But at the moment, it's clear that high efficiency doesn't imply

highest ability [INAUDIBLE], but I'm sure it will come.

We have seen some examples that might have been fortuitous,

quality example [INAUDIBLE] a good example of relative but

based that it is not so high performing.

But ten years ago, there was a high performing formula extra question compared

to the other polymers explored at the time PPBs.

And there, you had a new polymer that was quite good, and

it was also quite steady, but of course, it's possible.

Yeah? Okay.

And I think the [INAUDIBLE] question is exactly along those lines.

Is there also, then, or how is it with fabrication and efficiency,

and I'd just like to read one question here from Mitch Wilson.

And his question is, basically, since you talk about [INAUDIBLE] and polymers, and

it seems that [INAUDIBLE] and polymers are the things that derive high efficiency.

When we see them in row to row coded devices they seem to be less efficient.

Is that true?

And is there some sort of production thing that makes it like this?

Or, why is this?

It's a good question, and I think already half answered.

The previous question was addressing wether stability and

efficiency went hand in hand.

And, of course, it doesn't yet because people haven't spoke [INAUDIBLE].

It's the same with [INAUDIBLE].

There's a big difference between a polymer that has

been optimized for high efficiency [INAUDIBLE].

And possibly, also processing in a specific environment such as a glovebox.

In a specific geometry such as a normal geometry

would evaporate it's reactive electrodes such as calcium.

And, it's clear that when you go to that processing, imaginary

processing of mine which is not inside a clock box, it's not using calcium.

It's in air, it's large, clumsy machinery timescales that you operate on along.

It's physical roles of file you have to scroll through a machine while you print

layer after layer.

So that means even if you work your very hardest,

you still have to go to lunch in these things.

So, some of these tricks that you can play in a glovebox where you can very rapidly

make your device and.

And, therefore, there's also other.

In addition to this time effect or

time latency of the crossing, there's the large area.

There is the possibilities for contamination which, personally,

I think, are not accreditation but it's been claimed that it's significant.

That's why people say you have been [INAUDIBLE] or not.

This I think remains to be seen.

The violation is that when you roll process you also.

[INAUDIBLE] fixed conditions such as drive conditions for some of the [INAUDIBLE].

[COUGH] That may [INAUDIBLE] especially when you have to process repeated on

top of active layers.

There, you normally have to drive for significant period of time

at a significant temperature, and many of those high performing materials simply

are not morphological state enough to endure the process.

So it's a question of the materials have not been developed for

enduring a process.

And quite often,

we've tried many times all the new hydroform framers we've tried them all.

And it's a great disappointment nearly every time.

We had a few that comes out of the process should we say, okay.

But certainly, not as anticipated but they are at least not destroyed, or

whereas, many of the cases we have dismally low performances for

these alleged hydroform materials.

Once they come through the >> Yeah.

Okay. Thank you.

Another question we have in here is, basically,

plastic versus glass substrates.

So, which one is better in terms degradation, processability, efficiency.

I think we can probably go through them one by one.

So, first of all, is it even possible to roll the process on glass?

>> Yeah. I mean, there that coin sells,

this makes it in glass.

It's a thin glass, that the thin, is flexible.

So it is truly glass.

And it is drawn in a very tall tower into a thin sheet, and

this makes it extra glassy and extra flexible.

It is still glass, so it means it can still shatter.

Unless, of course,

you put some requirement on your machinery with rollers diameters and these things.

But in principle, you can roll-to-roll print on glass.

I would say that this is hardly been explored yet

in the development process cause for or TV screens.

But in principle, flexiglass is possible.

We worked in the past with some glass that was point two millimeter thick on shop.

Also thin glass.

It's quite flexible.

But the disadvantage of glass is that once it breaks it shatters all over.

So it's not like plastic that will, you know, handle point stress without

defects propagating for, for long long cracks.

We make a hole, we punch a hole, but

nothing happens to the polymer fault right next to it.

So, to answer the question of whether you can process on flexible glass,

the answer is yes.

It remains to be seen whether it will prove useful in OPV.

Glass of course has many advantages, it's thermally extremely stable.

It does not stretch.

It does not compress.

If it is subjected to repet, repeated heating and

cooling cycles like [INAUDIBLE] process.

It enjoys high temperatures.

It has an ultra smooth surface.

It's chemically resistant.

It's quite abrasive resistant also.

So it's glass is a super material for sure.

Eh, in terms of permeability or

should we say packaging quality, is absolutely impervious to all permits.

Water, oxygen, carbon dioxide, all of these things, it is tight.

Air tight, as we know.

That's why we have windows in submarines and these things.

So, so glass of course are testing it really.

[INAUDIBLE] bodies a signficant amount of energy.

Except perhaps in it's very thin flexible glass form.

But if you want to make a glass slide it quickly becomes impractical.

And certainly, if you are operating with glass.

That's a stiff object, so rigid and glass plate.

They now take the role that the thin plastic foil will always win in

[INAUDIBLE].

So, that is the whole philosophy of OPV and the only reason that it's

interesting is that you can manufacture it, at horrendously high speed.

And what enables the horrendously high speed is the photovoltaic process.

So, to answer if glass works, yes, glass will work if it fits the process.

Otherwise glass is uninteresting.

Or, put differently, then, you may as well make a silicon solar cell on the glass.

>> Yep.

I guess the only part of the question you didn't really touch on.

And I guess it's probably because it's harder to answer.

Is there any sort of efficiency difference between

the different kinds of substrates that we have?

>> Glasses of course can be made of these, extremely transparent, [INAUDIBLE].

Whereas plastic foil.

It's can also be made equally transparent.

So you're basically limit [INAUDIBLE] high quality PET and glass.

In terms of optimum transmission,

they both transmit glass can of course be made to transmit as low as very high UV.

Whereas PET maybe cuts off where the [INAUDIBLE] salt.

So it can be made anywhere from maybe 260 and up and 360.

But the practical form of plastic foil is in the form of a barrier.

Where you have made a multi-layer structure, comprising plastic,

same plastic layers with some sort of inorganic, thin inorganic barrier layer.

This multi-layer may, gives us this, impermeability towards,

or relative impermeability towards water, water, oxygen and, and carbon dioxide.

But it also leads to transmission loss.

And therefore, the area foil used for

OPV is less transmissive optically than glass will ever, will always be.

Glass will always win in this state, for sure.

So, the compromise, you, that you make is that you want flexible plastic,

that doesn't shatter if you break it hard and break it.

This is another issue as you can imagine in the end.

You have to laminate your glass in some sort of plastic foil, because any point,

eh, stress only to, to catastrophic failure put to the device.

And this, this might be, in the end, prevented.

This we'll, we'll, we'll see.

So, it's, it's, seen many times,

in in history that when a technology fails because of something very simple.

And glass does shatter, period.

And if this is a problem then it will always be a problem,

an insoluble problem yeah?

>> Okay, let's stick to these substrate questions there's

another question here that's sort of revolves around the same thing.

So basically we use PT mostly as a substrate right now.

And the question is are there any renewable equivalents to PT as

a substrate something that's, that's.

>> Yup. >> More viable.

>> Yep, that's a very good question.

Certainly the answer is both yes and, and, and no.

There are several reusable versions, there's polylactic acids, micronite,

crystalline and cellulose.

Several of these polymers that, that come from, from numerous sources,

either like polylactic acid of course.

Lactic acid which a person can get out of sour milk.

Algae bacteria that store energy polyhydroxybutyrate.

This, these type of polymers can, can be, of those we explored,

probably lactic acid.

As a composite with clay, this polylactic acid has a very low melting point and

it becomes soft or floppy at about 65 degrees.

The reason that polylactic acid is so interesting is that it's biodegradable.

So if you can imagine just throwing polylactic acid based solar cell in

a communal compostation facility and it will simply rot away.

From a practical point of view if your solar cell can biodegrade,

normally will, no matter where you put it outside.

So there are of course also limitations to, to applicability of this.

PT is at the moment by far the most attractive polymer material used.

It's low cost, it has extremely high or can have an extremely high

optical quality it is, has a high temperature or thermostability.

And it is mechanically very low cost, so at that moment that is our best shot.

My crystalline solar cells also has these properties.

It has even better thermo-stability and possibly also can have,

at least it's claimed an optimal quality that's even higher.

The question is how how to comparable it is or how sustainable it is.

Because if it's costing too much.

It's actually quite demanding.

I think compared to PT,

which is a petrochemical product that can be combusted very clean.

And as you have heard a little bit about an ACA and

I think there might be a little bit in the end also.

Simply, if you take a typical polymer solar cell

that will make a package source.

So you can embody about 40 mega joules in a square meter.

The PT component composes more than 99% by weight, and

if you simply burn this component you get 5 mega joules.

And possibly, this is, is the best way of looking at it.

You can say we borrow the fossil fuel in the form of PT,

we extract solar energy for a while.

And then we're,

when we're done with that we simply decommission the PT by burning it.

And recovering the the thermal energy that we would have if we just burned the oil.

>> Yeah.

We have a, sort of a follow up question here.

>> [COUGH].

>> Mostly on the previous question about glass substrates.

Would it be possible to use existing glass, as a substrate?

This could be the,

the suggestion given here is, is windows on buildings for example.

>> Yeah, yeah, no problem.

Yeah.

Julius, yes, you can.

>> Okay.

>> Let's move on there's a question, specific question here.

About applying a fresh layer of chlorophyll to a solar cell.

And apparently, this should increase the efficiency.

Is this something you heard about, read about, tried out?

>> I don't know if I heard about that exactly.

There's been many examples of people trying to make,

to see optical up or down concentration by applying a dye.

Oh a, typically it's the organic, some of the rare

earth metal salts, or metals in, in salts.

That that, well say take light, I shortwave in nature length turn

it into a longer wavelength by fluorescent emission.

There's also some that have the ability to take two infrared photons and

convert them into visable photon.

And people have made all these experiments.

Generally, I would say that the cost of doing something is quite significant.

Whenever you make an operation that has, costs something for the environment.

And it benefits from these things are, I think to my knowledge not,

cannot be recouped.

The potential gain is maybe on the order of some percent.

And maybe you should be able in the lab experiments to prove,

in lab experiments prove a point.

But I think from a practical point of view increasing a polymer source that is

already see a 2% polymer source of.

And you add chlorophyll or whatever.

And sumerian or europium salt.

And then you boost the efficiency to 2.1 or 2.2%.

You still have to extract this material somehow, apply it, and

you have to assure that it is stable.

And I'm sure the chlorophyll would not be stable.

At least, not if exposed in, up to air and light at the same time.

It would quickly wear off.

So that means you are, you're not gaining anything.

You're just wasting time doing something.

You always have to think about will it last?

How much energy do I have to invest in doing it?

And what is the gain in efficiency of the source and

over the lifetime of the source?

I think that in the majority of cases, you would be better off simply not doing it.

And then label the slightly l, lower projection.

>> Okay, the next question we have here is actually about free OPV's.

And first of all I would like to say that we are working on the next batch so

if you didn't receive yours yet, we'll hopefully have it soon.

The, they have it ready for shipment quite soon.

But the question is, what sort of feedback would you like to have on the free OPVs,

once people have received it?

>> [COUGH] Well, I think, I forget how many orders we've had.

It's been over well 9,000 orders or so.

>> And the is, is about 2,000 sales.

And we of course, we had prepared ourselves, but not for 9,000 cells, so,

so so we have the cells.

They still need to be cut actually the making the cells is,

is the least of the problem.

The challenges that we need to cut them off on a laser cutter for this is not so

much of a problem.

Then we need to physically punch in these buttons.

So for every solar cell, we need to apply two buttons.

This is done by hand at the moment.

And then we also, by hand, have to print your efforts for

every word that is printed.

And stuck onto this solar cell by hand.

And then we have to apply the stamp as well.

So the biggest problem at the moment, the bottleneck is simply getting them posted.

And there is a scale you should be able to weren't as prepared for

as maybe we should have been.

I mean, we could have 9,000 cells and

then there would have been five people following this course.

And then you would, you know, had, it would have been the other situation.

But the feedback we would like.

I mean, anything.

We've seen some great videos already from on YouTube where people have,

have filmed themselves.

Take the cell outside.

And mash it with volt meter.

Bend it and.

Just, just do whatever you, you think you have the capacity to do.

I think this is the most important part for us.

That, that you receive it.

You work with it.

You learn something from it.

And you demonstrate to us through a video, through text, through rapport, whatever.

What you did.

But I think the easiest would be record a video.

It's all there.

Other people can see it.

Directly accessible.

So with your smartphone, whatever you film, you film with.

You can film voltages, currents, what you do the cell when you do it.

Conditions are evident from the, from the footage.

I think this is.

I've seen a few of those already on YouTube and I was,

I was smiling all the way.

Yeah, I think it's thank you very much.

Even though and all the time we don't even know if these cells make it to the,

to the end user.

It's great to see that they get there.

They still work.

I mean, we're not packaging.

We're not putting the in an envelope.

They're shipped like postcard and

it seems like they are remarkably resilient in that aspect.

They always work when they, they reach the, the recipient and I think this is.

>> At least in most cases.

>> In most cases, I mean I'm sure that some postman, they step on something or

it gets jammed somewhere.

And then, it's destroyed.

And then, I'm very sorry.

You, can contact us and we will for sure make, send another one, too.

I mean, it is free and everybody that followed this course deserved to have one.

And, and we'll get one.

We'll, we will [CROSSTALK]. >> Yeah.

Yeah.

Just make sure you send us a picture of the non-working device so

we can sort of see what went wrong.

>> Yeah, and we would like to learn.

It's also for us to learn and what happens when you ship like a letter, a servicer.

Maybe it's something we can do, we can made adhesion a little better.

We had it very early in the beginning.

Actually, you wouldn't notice this.

But the problem is con, is two buttons,

push buttons that you know from clothing that we use to make electrical contact.

They were close to the edge of the solar cell.

This actually in some cases led to the cell split, splitting apart.

And there we would, we change the design slightly.

So it's just a question of, you know, where we cut it and,

and where we put the, the button.

It sounds really simple and stupid, but of course, we didn't know.

And, and once we fixed this.

Then it seems like we don't get this in the post any more.

But there may be other things we get in the post.

So, problems we get in the post.

Also I should say that, the free OPV's we ship now, or

most of them at least, I think will be carbon-based.

Which is slightly different solar cell that we have relatively little experience

within the post.

So it's in the previous one was silver and this one is based fully on carbon.

And that is from an environmental point of view that we ship thousands of solar

cells into the world and we don't want to pollute the environment.

>> [COUGH]. >> When you dispose of it with,

with silver.

So on this carbon version, then, you can just throw in the bin if you want.

And I think, so it's maybe not as performing, but, but

I feel better at night.

By not having polluted the globe with solar cells for teaching purposes.

That people maybe don't know how to dispose correctly.

And maybe I don't even know how to dispose it correctly, but

at least we made an effort not to, to pollute unnecessarily.

>> Yep. Okay, there's a, follow up question.

And, so 303 how do I order?

Yeah, I think I'll just answer this.

Basically I can see somebody put in that you need to go to

plasticphotovoltaics.org and make the order.

And this is correct.

Although, we have put in a system that

does not allow anybody to order any more right now at least.

And this was done to ensure that, that all of you at Coursera will have first pick.

So what you need to do is,

you need to go to the Coursera page find the announcements.

And in there, there'll be I think it's probably.

Well it's three or four times since, well it's not the newest announcement.

Well there'll be a one-time use code.

So basically, there's a link you just click,

then it opens plasticphotovoltaics.org and you can do your order.

And that's it.

Okay there's a question about direct arylation here.

I think we'll just take that one now.

So basically it's mentioned in the lecture that

that using this method leads to lower efficiency.

I guess, is the question.

At lower bg and what is the cause of this?

Why is this method different than the other methods?

>> Well it's a rather new method, at least in this field.

And it uses some different conditions such, as you've seen in the videos.

And I believe that it is because the molecular weight it doesn't

get that high which is in direct arylation.

And maybe also due to some of the impurities we get a problem.

>> Yeah.

>> I, I think it's a, it's a paper by Barry Thompson.

Who compared preparation of P3HT

from stille cross coupling and direct arylation.

And he compared these two and

didn't get the same efficiency when he used two direct arylation.

And yeah so you can [INAUDIBLE] or we can post it on.

>> Yeah we can, we probably post the link somewhere.

And okay,

then there's another question here I think is, which is really interesting.

Basically about low bandgap polymers.

So, I think the problem is that, the person here is trying

to ask is is why is low bandgap polymers more efficient?

So he, he understands they increase the,

the, the current when you use low bandgap polymers.

But then at the same time you would, you would expect a decrease in, in voltage.

So why is the, the output power, then, higher?

>> Well it's a, I fully understand the logic behind this.

And I think the, eh, the answer is, eh, rest in the already

inefficient way to arm solar cells, make use of voltage.

So if you take an inorganic solar cell and you, [INAUDIBLE] voltage or

the voltage you can extract.

Quite closely follows the the bandgap of the material.

Whereas in the organic solar cell, the voltage is far lower than the bandgap.

So you could say in the no bandgap polymers we are not even close to

exploring compromise that, that the bandgap.

That give it, that bandgap puts on the theoretic voltage that can be achieved.

A good example is P3HT.

It has a bandgap of around two EV.

So two volts you should get out of p,

polymer P3HT cell if you we're very good or 1.9 volts.

And in practical terms, you get 0.5.

So that means you are throwing away three quarters of the voltage that that you,

in principle, could get.

And so the low bandgap polymers is more or less the same.

And that means we're still, we've managed to actually increase voltage for

the low bandgap polymers to maybe 0.8 volts or 0.7 volts.

Why harvesting more of it now?

So it's simply the she said the the lousiness of the organic solar cell.

Allows for the low band gap polymer to be better than,

than than higher bandgap polymers that are already there.

because we are not, we are not hitting the the, the bandgap limit.

And of course, once we've worked, that would be the case.

Then, then voltage would be lower as current went up and, eventually,

there's a maximum.

That is, of course, also why you want to explore tandem solar cells.

So you, in that way, use the lower bandgap solar cell or the,

the very infrared light.

Where you can only, where you cannot achieve a high voltage.

There you can just add this slight voltage with the high current or

the high current on top of a, another cell, maybe.

That, that has a high voltage and a, the similar current.

Yeah. >> But it's also about tuning the bandgap

of the polymer or suits the acceptor will use in the.

[CROSSTALK]. >> That's also true.

Yeah, that's also true.

>> You can do that, yeah.

>> Yeah.

>> Okay there's a, follow-up question.

Should, I'm not sure I fully understand it, but at least I will try and

rephrase it a little bit.

So when we use different polymers,

most of them have absorptions that stretches far than the IR.

And so, can we just use a thicker material.

And then harvest more light?

>> Mm-hm.

So, can you round that again, so, so, so.

>> Yes, indeed, if they had a tail extending into the infrared would it just

make the layer thicker?

>> Yeah. >> Yeah.

>> Yeah, so to increase the cross-section.

>> Yeah, in principle.

But of course, it's almost always a compromise that, that the carrier,

transport carrier extraction depends a lot on thickness.

And for the polymers we have, the majority of polymers,

they don't work well with active layers that are more than 250 nanometers thick.

We have a few that works at 400 nanometers, 500 and even less,

less polymers than we do have.

[INAUDIBLE] and it works to maybe a micron or more in thickness.

So it's one of those polymers yes, people compromise, fill factor and efficiency.

But to prove a point,

you would be able to extract light out there from the very edge of the bend.

But you have to remember that as you increase the thickness and

maybe the absorption in the infrared region.

You'll also become, or get an extreme absorption cross section in the visible.

Which means that you would have a lot of absorption at the front window,

if you like, of this thick layer.

So the very first part of the layer in the visible range will absorb,

absorb all the visible light.

And it will be totally dark, to say 500 nanometers of light further into

the device whereas the infrared will extend further.

And so the regions in this layer are to

generate charge carriers will be should we say perturbed.

Here you can generate charge carriers throughout the layer.

To enable efficient extraction.

And that means that if you have a, a layer where you're only at close to one of

the interfaces, generate all the charges.

You are forcing one of the charges to carry itself or

transport itself all the way through this very thick layer.

And this is not beneficial for performance.

So I can see the logic, but it wouldn't work in practical terms.

It's not, not called high efficiency.

It's not a scientific point of view to prove a point of.

[INAUDIBLE] for sure.

>> I guess this question also probably follow a little bit from how you make

traditional silicon solar cells.

Because they are typically extremely thick.

>> Yeah, but then in the silicon solar,

crystalline silicon solar cell has you know, an exceptional carrier lifetimes.

And enormously long you can have very very thick silicon.

And because the life, the carrier lifetime is so long of impurities.

You can transport carriers for millimeters and, and

you will have no appreciable loss.

So, so that is why it works so well.

And I should also add that silicon the absorption cross agent is more or

less constant across the spectrum.

Whereas if you took P3HT for instance and

explored just the infrared tail that would make it to 700 or something like that.

You would have in the visible range an enormous absorption.

So it's not a.

>> Yeah. >> One is a direct bandgap, and

the other was one is indirect.

I think this, this is an important distinction.

>> Yeah.

Exactly.

>> Okay, there is another question, here, I'm not sure how to phrase this exactly.

But I guess the question is will direct sunlight, or UV exposure,

generally lead to degradation of OPV.

And is this sort of a [LAUGH] fallacy of the technology?

>> Yeah, it could.

>> Good point.

I mean, anything organic will wear, weather in, in, in daily life,

and, and it is of course.

[INAUDIBLE] past.

We are very good at making filters for.

So, we use to make product for 400.

And, that's also what is [INAUDIBLE].

The car industry, lacquer on, on cars.

If we didn't have this protective lacquer on cars, you know, a lot of red and

green and blue cars, they wouldn't blue or green or red for very long.

And today, they can be standing in the sun for years and,

you know, and, and they won't be affected much by, by the light at least to the eye.

So, of course, I don't know if I would call it fallacy, if you,

we didn't have means of filtering out the UV, it would, for sure, but

we had the technical solution is there and we, we explore it.

So, so so it's a,

I think that is not the biggest issue that we that are interested in.

Yeah.

>> Okay. Then, there's another question here,

I don't know if you have a good answer for this.

But the question is directly, is there a potential for

using the active layer in OPV devices as a battery system?

>> As a battery system.

I mean, there are examples of people who I don't know why, or

if I would call it a real battery.

because when you have a battery then you normally accumulate charges at,

two electrodes, you do a chemical reaction that you can then reverse.

You can use resources as a competitor, and

there you can actually use especially in peta power and electric.

You can, in principle store some energy in this thing.

But, but not in anything you could sorry, nothing unusual for,

like would be like a nine volt battery for instance,

where you have plenty of power for a long period of time.

But you could imagine that you would

couple the solar cell you could maybe the battery on the back of the solar cell.

So once the printing technology becomes good enough,

you could print a battery first,and then the solar cell on top and, or

the solar cell first and the battery on the back.

And then, you could use this

stack where there's a solar cell in some of the layers.

And then, the some of the other layers that have been

connected is then the battery.

This would be impossible.

Yep.

Okay, there's a follow up question to what we just briefly discussed.

And, I think I'll try and elaborate a little bit on it.

The question goes back to the polymers.

And we can see that some of them have absorptions around 1000 nanometers.

So now we're into infrared light.

And, what the question is, I think is now we have,

are into, to quite low energy light situation.

And when it sort of the trade off, can we just keep harvesting longer and

longer wave length, and at some point, [INAUDIBLE] needs to be [INAUDIBLE].

>> I guess one of the-.

>> [CROSSTALK]. >> Easier to follow and

the other one can communicate.

>> Yeah.

He basically posted, [COUGH] this image in here.

So he's asking to, to this situation.

So basically, I don't think we can show it to the camera.

But what is posted is basically the spectrum of the sun.

And then, compared to the, the energy we can extract.

And then, we can see that a low band gap polymer is harvesting a lot more of

the energy basically.

But I guess,

yeah so his question is first of all a thousand nanometer is invisible light?

Yeah, it is invisible light.

>> Nope.

>> But I guess the question is, what is sort of the limit can we at some

point just extend and just say, we do a low blanket polymer that

absorbs at 2000 nanometers and will this be the end?

A good thing.

>> Then, you have to have an extension,

which is aligned with these [INAUDIBLE].

>> [INAUDIBLE] if we assume we would have this acceptor unit, let's assume.

And, of course, you can do it at 2000.

But then, you would only have half a volt.

That would be the band cap limit, and

my guess is that you have very low voltage for your device practically.

As i mentioned earlier there is this compromise in that where

we achieve practical voltages that are much, much lower that the band cap.

So, we're at 2,000 nanometers for instance,

you would harvest the majority of the light for sure.

But you would probably get a .1 volt or something.

Even I once made a no band cap polymer based solar cell.

It had a band cap of I think 2500 nanometers.

We had ridiculously low watts, like 150 milliwatts.

So, even though we harvest all the photons, we don't get the energy.

Because the energy is a product of the voltage and the current, and

that is why you need for there is a limit.

And this, the gardens will be there.

It's an inorganic or an organic source.

>> Yeah in the combination it's the most efficient.

>> Sort it a bit.

>> One point six one.

>> Okay. Thank you, I think.

Okay, I think we'll be rounding off now.

Let's see, I'll just go through, there are some follow-up questions.

Okay. So, one

of the questions was rephrased a little bit.

And so, are there difference between or

are there advantages of having OPBs when it comes to

direct sunlight as compared to diffused conditions or vice versa.

>> You haven't generally OPB performs well in diffused light.

>> Yeah.

>> because the discriminate less.

They they sold bio from all different directions.

So there the answers, of course, it is.

And also, on the surface of the sort of physical disposition of your sun so

if that has layers that reflect the light of oblique angles and

[INAUDIBLE] if you prepare well.

It will absorb light from the entire half space that, that it sees.

>> Yup. And then, the,

another follow up to this is how does this relate to degradation.

Are there any, will the sources last longer on the diffused light?

>> That's a good question.

I think it, since the.

When we talk failure of we assume it is not a catastrophic failure like,

a scratch, or a hole in the foil, or

a crack due to weathering, and then it becomes brittle.

We just assume that it is radiation induced failure, or

failure due to chemistry or

reorganization inside the otherwise [INAUDIBLE] normally operating device.

So a gradual thing, yeah.

Then, a light will for sure be, and that's, will have shown that it's more for

logically [INAUDIBLE] using infusion [INAUDIBLE].

And, of course, these polymer materials and layers they are not, since they

are polymer, since they were soluble once and since they are flexible chains.

They are not immobile.

It's not a rock.

It's not an igneous rock.

So, these polymer chains, they will move around inside the bodies, and

some of these movements can also be viewed as transport.

And, and some of these phenomena may lead to core degradation.

That is called a very gradual failure over time.

But in the majority of cases now, I'm alluding here to a very,

very long set of examples.

What I'm saying is, there are a lot of things that can happen slowly in

the device that leads to failure.

Generally, the overall failure rate in such a case correlates with the dose.

So the number of joules of light per square meter.

There is some dependence on the spectrum.

So the number of joules per nanometer per square meter.

So per nanometer of wavelength, of bandwidth in a certain wavelength.

And, whether this is diffuse or direct light, it doesn't really matter.

What I would normally expect is that in diffused light,

all intensity would be gone so the overall light would be lower.

Therefore I would, my answer would tend to be that

yes you would see a longer operation lifetime of the diffuse light.

But mostly due to the solar cell having experienced less light overall.

So what you really should look at is what is the, the,

the efficiency of life over time?

So you will integrate and

see how many joules do I extract over time per diffuse light or per direct light.

And there my guess is you would get exactly the same number or, or

thereabouts.

There is something with diffused light that you have less UV.

So therefore, that could also mean that, that you would have, you know,

either though you had a higher life dose.

You would have a seemingly better stability due to less UV,

less [INAUDIBLE] your high energy component in diffused light.

But, I, I think we.

It's got the, chances of you,

failing your device in some other ways are, are much larger.

So.

>> Okay, thank you Frederick for trying to answer a quite complicated question.

I've noted two more questions, and then we'll round off.

So the first one I think we can probably answer quite quickly, is.

Since we see degradation of the ozone layer.

Will this affect the, the OPV's?

because I guess we get more UV.

>> Yeah. My guess is that, with our humid barriers,

it won't be a problem.

But, but in principle you get more u, UV so there's more light in the hard area and

it is more, it is hard.

So, so in principle yeah.

>> And the last question is ICBA is another fullerenes material.

Have you tried using that in the room to room?

>> Yes, absolutely yes.

And, and ICBA is of course, brilliant material.

It was developed by Darin Laird of Plextronics.

And it gives higher voltage.

It efficiently addresses what Eager mentioned earlier with the following

[INAUDIBLE] polymer has to match also to better explore this.

The accessible voltage range for a given voltage range for a given [INAUDIBLE].

Now as a ICBA of course offers lot of millimoles

extra compared to an [INAUDIBLE] add on.

ICBA has, is, seemingly less morphalogical stable.

It's less stable to so we do get the high voltage.

This I think is,

is independent of even in morphologically revolution of these things.

It's where the electrons and holes end up in the materials.

And therefore, the voltage is fixed.

Fill factors are lower and operational stability for,

especially the ICBA, is lower.

We long ago that was also.

[INAUDIBLE].

And in principle the lower the energy levels the more for

chemically stable materials.

And the fullerenes are more stable, the more accepting they are,

electron accepting they are.

I mean in the ICBA case you explore the fact that it is less accepting.

And therefore, enabling higher open circuit boards.

But that also means that it is less.

So in all the case we, we've done ICBA,

proved less performing in terms of operations, so.

And also in terms of efficiency, but the voltage was there.

That would be also on the [INAUDIBLE].

>> The ICBA is actually-.

>> [CROSSTALK]. >> It's it's a little, I mean, basically,

we purchased it on, on, on, well, not quite, but nearly kilogram scale.

And then it's actually a reasonable price because it's relatively simple to make and

purify compared to PCDN.

>> Yeah. >> So I think.

In principle ICBA approach,

would enable a much better price structure then PCBM at the source.

>> Yeah.

>> Okay, thank you.

That was the last question, so I think we just have to say, thank you for

all the good questions here in the chat.

Thank you for participating so much, and, yeah, bye.

>> Bye. >> See you next week.

>> Bye.

Recorded Live Session III

Organic Solar Cells - Theory and Practice

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