More stable solar cells and a three-layer ink

Solar cells made of perovskites are easy to produce, work in low light conditions, and are even able to function as a thin, flexible layer on your backpack, tarpaulin, or flexible display. Despite their many good qualities, perovskites are not ideal: they easily disintegrate when they come into contact with air or water and they currently contain toxic lead. Chemist Loredana Protesescu of the University of Groningen is working on more stable, lead-free perovskites and has recently received a grant to develop an ink that contains all components needed for a solar cell, such that it only needs to be applied to a surface before use.

FSE Science Newsroom | Charlotte Vlek

‘Perovskites are easy to manufacture, but they also easily disintegrate,’ Protesescu reports. ‘They easily dissolve, similar to salt in water.’ For that reason, the perovskites in solar panels that are currently on the market are sealed in a plastic covering. Their fragility has everything to do with the internal connections in the material, Protesescu explains. It’s a crystal structure that has only weak connections between the atoms of the structure.

Though the term perovskite originally refers to a mineral with a specific structure (see text box), we also use the term perovskite to refer to the family of materials with a similar structure. By searching for other atoms that could fit into the same structure, Protesescu and her colleagues are developing new perovskites that are more stable while maintaining the same positive properties.

Lead vs tin

A good candidate to make perovskites lead-free is tin. Recently, Maria Loi, professor at the University of Groningen, presented lead-tin perovskites that convert light into electricity with an efficiency of 25.9 per cent. ‘This is currently the state of the art in the field,’ says Protesescu, who works closely with Loi. Protesescu is working on tin-based perovskite nanostructures and her PhD student Julia Kraft was able to successfully replace all lead atoms by tin.

‘The next step is to go from a single crystal to an entire solar cell,’ Protesescu explains. Because her work focuses on the nanoscale, her group develops new perovskites in the lab and studies their properties, such as structure, stability, and its response to light. To see how these perovskites perform in real-world applications — for example LEDs, solar cells, or photodetectors — Protesescu turns them into thin films of nanocrystals. She then builds these films into working devices, putting their efficiency and potential to the test.

A chemist bakes a cake

Protesescu is also going to work on a type of three-layer ink that contains the entire solar cell and all its components. To do this, she recently received a grant from NWO, which allows her to work on such an ink for the next five years. ‘A solar cell has an active layer in which perovskites convert sunlight to electric charge,’ Protesescu describes. ‘This active layer is sandwiched between two outer layers that conduct the electric charges.’ In the three-layer ink that Protesescu intends to develop, all materials for such a sandwich are already present.

The idea is that the different components of the ink all take the right position as a result of external stimuli, such as a change of acidity or temperature. ‘Just like the “magic cake” that I like to bake with my kids. I make it with a single batter, but in the oven – that needs to be at exactly 150 °C – the heavier ingredients sink to the bottom and you get a creamy middle layer and an airy top layer. So you see: even with a simple process you can make something quite complex.’