Vacuum process pushes perovskite solar cell efficiency to 20%
Michael Graetzel and his team at the Ecole Polyechnique Federale de Lausanne (EPFL) in Switzerland have found news ways to grow perovsite materals for larger-size perovskite solar cells, reaching over 20% efficiency and matching the performance of conventional thin-film solar cells of similar sizes. Briefly reducing the pressure while fabricating the perovskite crystals allows the higher efficiencies with a low cost manufacturing process.
Layering perovskites on top of silicon to make hybrid solar panels may actually boost the silicon solar-cell industry, as shown by researchers at imec in Belgium last week. Graetzel and his team see the efficiency of these structures exceeding 30%, with the theoretical limit being around 44%. The improved performance would come from harnessing more solar energy as the higher energy light would be absorbed by the perovskite top layer while lower energy sunlight passing through the perovskite would be absorbed by the silicon layer.
Graetzel is known for his transparent dye-sensitized solar cells, and the first perovskite solar cells were dye-sensitized cells where the dye was replaced by small perovskite particles. The latest prototype, roughly the size of an SD card, looks like a piece of glass that is darkened on one side by a thin film of perovskite and uses a slightly different technique.
The team first dissolve a selection of compounds in a liquid to make some “ink”. They then place the ink on a special type of glass that can conduct electricity. The ink dries, leaving behind a thin film that crystallizes on top of the glass when mild heat is applied. The end result is a thin layer of perovskite crystals, but the tricky part is growing the thin film with smooth and regular layers of perovskite with large crystal grain size in order to increase photovoltaic yields.
For instance, spinning the cell when the ink is still wet flattens the ink and wicks off some of the excess liquid, leading to more regular films. A new vacuum flash technique used by Graetzel and his team also selectively removes the volatile component of this excess liquid. At the same time, the burst of vacuum flash creates seeds for crystal formation, leading to very regular and shiny perovskite crystals of higher quality with the higher efficiency.