Leuven researchers stabilize perovskites for better solar panels
Researchers from KU Leuven, among others, have discovered a way to stabilize well-performing perovskites at room temperature. This could eventually lead to a new generation of solar panels.
Researcher Julian Steele and an international team of scientists discovered that the perovskites can take on the desired black state if a layer of the crystals is attached to a glass plate. This is done by heating the perovskites to 330 degrees Celsius; this causes them to expand and adhere to the glass. After heating, the crystals are immediately cooled to room temperature. This method fixes the atoms in the crystals, restricting their movement and keeping them black. This color guarantees optimal absorption of sunlight.
Perovskites are soft, malleable, artificial crystals that can convert sunlight into electricity. They absorb a lot of light, can transport electrons well and are cheap to make. Pure perovskite solar cells are much more efficient than silicon crystals, but the most promising type of perovskite, cesium lead triiodide, is unstable at room temperature. According to Steele, the material manages to stabilize under various lab conditions, but normally the crystals tend to rearrange themselves at room temperature. In this yellow phase, the structure has changed and the crystals are very soft, comparable to a plate of jam, according to the researchers. This, for example, in contrast to silicon crystals, which are very strong.
Cesium lead triiodide was chosen because these perovskites perform very well, but it is also one of the most unstable perovskites. “That means the method we described should work for other unstable perovskites,” Steele says.
It has been known for some time that perovskites can maintain their blackness after heating, but even after the invention of the Leuven researchers, there are still a number of questions. For example, it is still a mystery how exactly the binding happens. “Understanding how this mechanism works is needed to be able to develop solar panels that use pure perovskite crystals,” Steele says. According to him, price, stability and performance determine the quality of solar cells. He says the entry level to make solar panels with perovskites is relatively low, so they could be beneficial for people in developing countries, among other things.
It will take some time before perovskite solar cells become commercially interesting. This is not only related to the instability of, for example, the cesium lead triiodide, but also to the fact that the crystals degrade relatively quickly. To combat this, TU/e researchers presented a study in May in which they propose adding a protective layer of fluoride to the perovskite production process. In the solar energy sector, a gold standard is assumed in the form of an efficiency retention of 85 percent after 10 to 15 years. The addition of fluoride is a step in the right direction, according to the Eindhoven researchers, but the efficiency of the perovskite cells after this protective layer is still nowhere near this standard.
The KU Leuven research was presented in the scientific journal Science, under the title Thermal unequilibrium of strained black CsPbI3 thin films.
Schematic representation of cesium lead triiodide in the yellow state. Below that is a representation of the rearrangement in the black state.