A new, old material could transform solar cell technology

Image: Kári Sveinbjörnsson, 2017. Two perovskite solar cells from his research compared in size to a coin of one Swedish krona. CC BY-SA IGO 3.0.

In 2009 a Japanese researcher found out that a material called perovskite could convert sun light into electricity similar to silicon solar cells. Interestingly, this kind of material had already been known for quite some time. Perosvkite is the name of not only one particular material, but of different compositions sharing the same crystal structure, the perovskite structure. The first perovskites were discovered as early as 1839 in the Ural Mountains and were named after the Russian mineral expert Lev Perovski who first studied them. The new perovskite that has received a lot of attention for its possible use in solar cells is called methylammonium lead iodide, quite a tongue twister, which is why I will refer to it as perovskite.

I have talked to Kári Sveinbjörnsson, a PhD student in the Physical Chemistry Group at Uppsala University (Sweden), who is working on perovskite solar cells. He says that a great advantage of perovskite solar cells is the cheaper manufacturing processes. Producing silicon from sand (which consists of silicon dioxide) consumes large amounts of energy as temperatures over 2000 degrees Celsius are required. In addition, silicon in solar cells needs to be extremely pure which leads to a total of 14 manufacturing steps. Producing perovskite solar cells, on the other hand, requires lower number of  steps and much less energy. A solution of the perovskite is dropped on an electronically conductive glas substrate. It is then put into a rotating machine, called a spin coater, to evenly distribute the solution. This is carried out at 100 degrees Celsius to evaporate the liquid solvent.

One big drawback of perosvikite solar cells is their poor stability according to Sveinbjörnsson. Perovskite is especially sensitive to moisture in air. Therefore, it has a much shorter lifespan than silicon solar cells and can not yet compete with their lifetime of 20 to 30 years. However, a lot of research is being carried out to change that. Another disadvantage is the presence of the toxic metal lead in the structure (indicated by the full name methylammonium lead iodide). For this reason scientists are trying to substitute lead with tin or bismuth, two less harmful metals. However, this exchange comes at a cost as the substitution of lead decreases the efficiency of the solar cells.

The extremely fast development of their efficiency has resulted in great interest for perovskite solar cells. While only a few percent of sunlight energy were converted into electricity with the first perovskite cells in 2009, we are today at an efficiency of 22 %, not far from the 25 % of silicon cells. This is an incredibly rapid increase within only eight years. At the moment a lot of research is focussed on the upscaling of the production process. It can be seen in Sveinbjörnsons image above that perovskite cells for research are not much bigger than one Swedish krona coins. Therefore, upscaling is important to make perovskite cells large enough to cover big roof parts.

Perovskite solar cells need one transparent electric contact on the top, facing the sun, and one electric contact at the bottom (which does not have to be transparent) in order to create electricity. Normally, thin layers of gold or silver are used for this task, but they can easily disintegrate and start to move through the entire solar cell which lowers its efficiency and stability. The most important part of Sveinbjörnssons research has been carried out on this issue in a collaboration with Aalto University (Finland) and École Polytechnique Fédérale de Lausanne (EPFL, Switzerland). In this work so-called carbon nanotubes, in other words extremely thin threads made of carbon, were used as the bottom electronic contact instead of silver or gold which resulted in more stable cells. This approach could eventually lead to cheaper perovskite cells with longer lifespans.

To the question when we will be able to buy commercialized perovskite solar cells, Sveinbjörnsson answers that the falling prices of silicon solar cells make predictions difficult. But he believes that we could see the first large-scale perovskite modules with 15 to 20 % effeciency within the next five years. Some more research revealed that there are already a few companies working towards the commercialization of perovskite solar cells. One approach is for example to combine perovskite with silicon by putting a thin perovskite cell on top of a silicon cell. This way the total efficiency can be increased as silicon absorbs the red part of the sunlight and perovskite the blue-green part. It remains interesting to see when and in which form perovskite will be commercialized in solar cells in the next years.

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