Smart windows of yttrium hydride

Almost 40% of all energy consumed by humanity is used in cooling, heating, ventilating and lightning in buildings. Utilization of smart windows that automatically adjust the transmission and reflection of light and heat could drastically reduce this.

A few blogposts ago I wrote about our new synthesis method of making transparent yttrium hydride films. Yttrium and yttrium hydride films have earlier been used to make smart windows taking advantage of the hydrogen absorbtion in yttrium. It's a facinating reaction that was studied for many years, but due to a few drawbacks of this technology, it calmed off and never reached the market.

Now, just recently we made a discovery that might draw the attention to yttrium hydride based smart windows again. We found out that the transparent yttrium hydride films we had made had a very interesting reaction to light; they turned dark when they were illuminated! Under direct sunlight they gradually change colour, and the transparency is reduced by up to 50%. After the light exposure the material gradually turns back to its inital transparent state. This reaction is called photochromism, a very rare reaction which is observed very few existing materials.

The photochromic reaction in films of transparent yttrium hydride.

The reaction to light is indeed very fascinating. The video below gives an impression on how it works on a small sample deposited on glass, exposed to sunlight in our laboratory:

Imagine if a building in a hot and sunny area could reduce the energy consumption for cooling by up to 50% just by changing the windows! There are even more possible applications for this type of material, as sunglasses, displays, data storage etc. At the current stage we are working more on understanding what is really happening in this material and how to control the photochromic reaction.

Paper: "A new thin film photochromic material: Oxygen-containing yttrium hydride".
Published in: Solar energy materials and solar cells
DOI: 10.1016/j.solmat.2011.08.018

Solar cells with colors and high efficiency

Paper: Reduction of optical losses in colored solar cells with multilayer antireflection coatings
By: Josefine Selj, T.T. Mongstad, R. Søndenå and E.S. Marstein
Published in Solar Energy Materials and Solar Cells (2011)

You may have noticed that solar cells are normally dark blue or almost black in color. The reason for this is that you want to take as much as possible out of the light that hits the solar cell into the cell to generate electricity.

It is obviously best to have a totally black panel, but you can not just paint the panel black, because in that case you will not let any of the light into the cell, all of the light will be absorbed in the paint. The solution is to make an anti-reflective coating, a very thin layer of a transparent medium on top of the solar cell, that traps the light in the cell because of what we call destructive interference. The thickness of these layers are about 70 nm in normal solar cells, or about 1000 times thinner than a human hair. That thickness correponds to 1/4 of a wavelength of visible light, which means that the reflected light from the lower surface gets canceled out by the incoming light when it meets the surface and "wants to get back out". This is a well-known strategy to trap the light, which is applied in all solar cell concepts.

An anti-reflection coating made up by a single thin film reduces the reflection from a solar cell from around 30% to below 10%, as can be seen in the graph in Figure 1. If you are in doubt what the "wavelength of the light" means, it is a complicated way to say "color of the light" (See Color on Wikipedia).

Figure 1 - Reflection spectra as a function of the wavelength of the light, from a silicon wafer with and without single antireflection coating. 

This is all old news, and it is also old news that the color of the solar cell depends on the thickness of the anti-reflection coating (See graphic here). So, one can make solar cell with other colors also. That would of course be nice with artictecture in mind, that you can choose which color you want to have on your cells. The problem about that, is that as you change the thickness of the film, and thereby the color, you get quite much reflected light from the cell, so you loose a lot of efficiency. 

That's why we in a project we started approximately one year ago, decided to look at colored solar cells, and try to find a way to get nice colors without loosing so much light. We found that by using several layers of different very thin films we could both control the color of the solar cell, and keep the efficiency reasonably high. My colleague Josefine is an expert on porous silicon, which can be used to make very good anti-reflection coatings by making very many very thin layers with different optical properties, and she found that one could use porous silicon to make colored anti-reflection coatings without loosing more than 1% (absolute) of the efficiency (Figure 2). Also the more traditional coating silicon nitride in combination with silicon oxide turned out to give strong colors without loosing much of the efficiency (Figure 3).

Figure 2 - Colored reflection from spots on a silicon wafer on which a porous silicon anti-reflection coating has been applied.

Figure 3 - Green, red and blue color from a 3-layer stack of silicon nitride and silicon oxide. These layers are optimized to get as much efficiency as possible in combination with these nice colors.

In this work, we did not actually make any solar cells, we just made the antireflection coatings and calculated what the efficiency would be according to optical measurements on the samples. Indeed, colored solar cells are available on the market (see e. g. Lof solar), and I think we will see more of this in the future as building-integrated photovoltaics become more and more common.

Read the paper.

Paper: Transparent yttrium hydride films prepared by reactive sputtering

Last week the first scientific paper where I am the first author became available on the internet. That is of course a grand step for me on the way to obtaining my PhD degree.

It was a bit more than one year ago that I had just started working with a new material in my sputtering machine. I had an idea that I could make some nanometer-thick layers of a semiconducting material called yttrium hydride by putting layer by layer of yttrium atoms on a surface, while the whole process was going on under the presence of hydrogen gas. Yttrium is a somewhat rare metal (not so rare as to be extremely expensive), and by reacting with hydrogen it transforms into yttrium hydride. This yttrium hydride can appear metallic, black or yellow-transparent, depending on the amount of hydrogen absorbed. The transparent state was what interested me as a potential material for solar cells.

Thin layers of yttrium hydride had already been made by others, and was first reported in 1996 by a group in the Netherlands. However, as it is difficult to make yttrium take up hydrogen directly, the films were made by putting a thin layer of palladium on top of the yttrium film, and then exposing them to hydrogen gas. The palladium is helping to take up the hydrogen, but there are some problems. Firstly, it is an extremely expensive material, which has a similar price to gold. Secondly, in addition to help to take up hydrogen, it also helps the hydrogen to escape, so if you take the yttrium-palladium sample out of the hydrogen gas, it does not remain in the same state. These were my reasons to try this new method reactive sputtering, which had not been utilized before for this exact material.

Since this was a new method for this material, my expectations were not so high. First, I started just laying layers of yttrium atoms on some glass, and got some metallic-looking films that had the expected properties for yttrium metal films. Then I put some hydrogen on, and I was content to observe that I obtained films that were darker, similar to the black state of yttrium hydride. Then, increasing the hydrogen pressure a little bit more, I got a film with the transparent state of yttrium hydride! I was very happy to have been able to make this kind of material by a completely new method, and surprised by the facility with which I had done it.

Figure 1 - I was very happy to see how easy it was to form transparent, black and metallic (left to right on the photo), just by adjusting the hydrogen pressure in my process.

The last year, I have spent working on these samples. Unfortunately I have not been able to make any solar cells of this material, but I have discovered a lot of other interesting things. Some of them, like the finding that these films have a cubic crystal structure as opposed to hexagonal in other findings for the transparent yttrium hydride material, can be read about in the present paper. And for the people that are not metal hydride geeks, I can already tell you that some much more interesting reports will come out soon, subject of one or more papers I hope to wright in the near future.

Published in Journal of Alloys and Compounds, DOI: 10.1016/j.jallcom.2010.12.032

Get the paper (open access PDF).