In this video, we will show how you make polymer tandem solar cells on the mini-world cooler. We've chosen here today to demonstrate how you both make them on a reflective substrate. Deshen here in the background, she's actually right now coating the first junction on a coated reflective substrate.

Later on, we'll see how she does it on a semi-transparent substrate. The tandem solar cell stack is of course a very complicated stack. In contrast to the single junction that may comprise five or six printed and coated layers, the tandem solar cell stack easily adds up to 12 or 14 layers.

There you need a good degree of control over your film forming method, the way you access your foil, the way you operate the machine. Here the mini-world cooler, the development of the mini-world cooler, really enables the realization of very complex multilayer, organic multilayer stacks.

Now it's obvious that when you make a multilayer stack like a tandem stack of say 12 or 13 layers, any error you introduce either in your processing method during one of the processing steps may accumulate and will eventually destroy your end result. The mini-world cooler has the advantage, specifically in this context,

that you are able to build up your complex stack without touching the surface of the foil. That means that once we have wrapped the foil around the drum on the mini-world cooler, we leave it there and we gradually coat one layer, next layer, next layer, next layer, etc.

until the stack is complete and the device is ready to be tested. In a scaling context, this is extremely useful because it allows you to eliminate the source of error that is inevitable if you have to touch the surface as you do in a large-scale machine where you will have transport of foil over mini-rollers. This is not to say that it's not possible.

We've certainly done it on several occasions, made successful tandem solar cell modules using full roll-to-roll processing, running over hundreds of rollers through such a process. But when you are either learning or developing or building a stack using new materials, new inks, the mini-world cooler really provides the environment that is needed

before you can step up to larger machinery. It's an advantage to have several mini-world coaters in cases where you want to build up different stacks, different architectures at the same time. And here we have, as I mentioned before, the reflective electrode, the reflective tandem, where the first printed electrode is non-transmissive and a coated silver electrode.

And then you start by processing the low band gap junction and followed by the wide band gap junction. And the illumination of this type of device is from the outside, from this side. So all illumination has to take through the coated and printed stack.

Here we have a different type of substrate. It's a flex electrode, so it's a printed, flex-printed silver grid with a rosary-screen-printed p-dot, 7-transparent p-dot electrode. And as you can see, there's a slight blue hue, so there is absorbance, obstacle absorbance from the p-dot. It is 7-transparent, but it certainly does absorb in the near-infrared,

which is not ideal for the tandem solar cells, or not as ideal as could be. This is why we have on the third mini-world coater here a silver nanowire electrode that has been printed, fully printed also. And this is, as you can see, much more transmissive. And in both those two cases, illumination will eventually be from the bottom side,

so from the side that is now facing the drum. So here, now Dusan finished on the reflective tandem, and she will now move on here and mount the slot I had on the next experiment. And we'll now see how she coats the wide band gap semiconductor on the flex electrode.

And for these two stacks here, where we shine light from what is the bottom side now, we will start by coating the wide band gap semiconductor, followed by the low band gap semiconductor. And in this case, the processing of the very back electrode is not very critical from an optical point of view, because we are not going to shine light through it.

Whereas in the first case, with the reflective tandem, the final electrode is extremely critical and has to be highly transparent. And this is, of course, the challenge in the reflective tandem. It certainly also has advantages. And once the first junction, whether it's a low band gap junction in the reflective tandem device, or the wide band gap junction in the transmissive tandem device,

we will need, after having slot decoded these active layers, we will need to apply the intermediate layer. This is a very critical layer. It requires several things. Firstly, it has to be optically very transmissive. It has to block carrier transport and enable the recombination of the holes and electrons

from the two respective junctions in the finished device. And one especially critical aspect from the processing point of view is that after we've realized or created the recombination layer, which comprises or is comprised of p dot and zinc oxide in this case,

we will need to apply a second junction. In the reflective case, this would be the wide band gap semiconductor. In those two transmissive cases, either with p dot or silver nanowires, it will be the low band gap semiconductor. And those are also coded for organic solvents. So the recombination layer here has to be impervious

to the solvents that are used to dissolve the second semiconductor. If it is not impervious, solvent will penetrate and will destroy the junction and will not observe tandem function. And typically, of course, you see this as a low voltage. So you will get a device that is operational. It's a solar cell for sure, but you will not get the voltage

that roughly corresponds to the sum of the individual voltages that you typically obtain for single junctions for those materials, less sum loss in the recombination layer, which typically comprises maybe 0.2 or 0.3 volts. And once the recombination layer has been made,

the last junction, the second junction, is applied, following exactly the same methodology as you saw in the beginning with the first junctions. In the final step, the back electrode is applied using slot decoding of a combination of P dots, and then the device is finished off with a printed silver electrode,

ready to be tested. I hope that you find that this shows the power of the mini roll coder, shows clearly that it can be used for single junctions and for very complex operations, such as the tandem junction shown here for three different architectures using a completely different set of materials.

So it's a versatile platform that enables a wide range of processing of both materials and device architectures.