As a school child, Charlotte Platzer Björkman wanted to become a journalist. But her interest in physics also came early and ultimately proved a greater attraction. She was especially fascinated by the electrical properties of materials, and through her engineering studies and a thesis project at ABB in Västerås, she came to be guided into the research track.
“From the beginning, I wasn’t thinking of becoming a researcher, but at ABB Corporate Research, everyone had a PhD and was champions of the academic world. So I ended up in Uppsala and began working on solar cells.”
Shortage of raw materials
Right now, the market’s most viable thin-film solar cells are cadmium telluride and a material called CIGS. They have both demonstrated an efficiency of around 20 percent when they convert solar energy to electricity. But these solar cells contain tellurium and indium, both rare elements where indium is about as rare as silver and is very expensive. Consequently, they are not particularly suitable for mass production on the long term. The EU has even put indium on a critical list of 13 elements.
“A contributing cause of the high price of indium may be that China accounts for the majority of the mining, and a large part is now used in the production of flat screens. No immediate shortage is foreseen, but there is a risk of limitations if use continues to be scaled up at the same pace.”
And now, Platzer Björkman has a new solar cell material in the works, where rare and expensive metals are replaced by less expensive and more readily available elements: copper, zinc, tin and sulfur. The four-component mix is called CZTS.
“When I began as a doctoral student, there was talk about ‘if solar cells come’, but now, we have already passed that phase. Now, the costs have been pushed down so that we will see growing numbers of solar cells. And then the availability aspects will be very important.”
The mission is to find long-term sustainable solutions. But it is impossible to theoretically work out which materials work, rather direct experiments are necessary, explains Platzer Björkman.
It is indeed possible to find out which materials absorb sunlight with the right match against the solar spectrum or how efficient the energy transfer is in theory in the boundary layer between various materials. But when all parameters are weighed together, it gets harder to make theoretical predictions. The number of possible defects in the material and the boundary layers simply becomes too large in practice.
“After all, we want to get the bigger picture, like finding out which materials have a long lifetime and which materials can get the electrons on target without being lost along the way. Preferably in materials that also allow many defects, meaning polycrystalline materials that are built of many small grains and are less expensive and not as energy-intensive to produce.”
The new material CZTS has many of the desired properties. But so far, the efficiency only reaches just over 12 percent, and 20 percent is necessary for the material to be able to be useful in the future, says Charlotte Platzer Björkman.
“It needs to have a high efficiency because you have to remember that it is not only the active material that is costly. It also requires a front glass that should be stable for 20 years against e.g. hail, and there are converters in the large systems and large costs for the infrastructure. I actually don’t at all believe in the concept with a low efficiency except for niches such as certain toys, for example.”
Several drastic steps are probably necessary if CZTS will be able to achieve the desired level. One solution may be changing the solar cell’s front and back contacts; another may be replacing one of the materials. Tin could, for instance, be replaced by silicon, but it remains to be seen how the material quality is affected by this.
Multi solar cells might be the solution
By varying the mix, solar cells may be developed that capture light of various wavelengths. If different kinds of thin film cells are placed in layers on top of one another, the solar cell may match a broader part of the sun’s spectrum.
It can increase the efficiency dramatically; some stacked solar cells have an efficiency of more than 40 percent. Platzer Björkman will stack CZTS cells with cells that also have a little selenium in them.
“With the new and better equipment that we’ve been able to invest in thanks to the Wallenberg Academy Fellow grant, we have the opportunity to study the selenide and what effects it has on material quality and stability. It means a great deal to have access to the latest technology since there is a sharp competition in solar cell research.”
She is still fascinated by fundamental physics, by studying how the electrons move in semiconductor materials. However, she is also satisfied by working with something that benefits the environment.
“I think that one has a responsibility as a researcher to think about ethical issues. For me, this means working with an application that I feel is important, and to strive for sustainability in our processes and our material selections.”
Solar cells smaller than a strand of hair – The solar cells are built up of four thin layers. First a so-called back contact on a glass plate, which functions as a positive contact in the solar cell. On this, solar absorbing CIGS or CZTS layers are then placed. Thereafter, a thin so-called buffer layer is added and lastly a front contact (negative contact). The total thickness of the active layers in a thin film solar cell is around 3 micrometers, one twentieth of a strand of hair.
Text: Nils Johan Tjärnlund
Photo: Magnus Bergström