Under a scanning electron microscope, clear voids can be seen at the grain boundaries of the control perovskite film (left). These defects can lead to losses and reduce the efficiency of the film. With b-pV2F (right), the voids are greatly reduced. Image: G. Li/HZB.The photovoltaic (PV) materials known as halide perovskites are seen as a great hope for producing even more solar power at even lower costs. These materials are very cheap, can be processed into thin films with minimal energy input and already achieve solar-to-electrical-power efficiencies that are significantly higher than those of conventional silicon solar cells.
However, solar modules are expected to provide stable power for at least 20 years in outdoor conditions while exposed to large fluctuations in temperature. Silicon solar cells manage this easily, whereas the semi-organic halide perovskites quickly lose their performance.
"Sunlight can heat up the inside of a PV cell to 80°C; in the dark, the cell then cools down immediately to the outside temperature,” explains Antonio Abate, who heads a large group at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany. “This triggers large mechanical stresses in the thin layer of perovskite microcrystals, creating defects and even local phase transitions, so that the thin film loses its quality."
Together with his team and a number of international partners, Abate has now investigated a chemical modification that can significantly improve the stability of the perovskite thin film in different solar cell architectures. These include the p-i-n architecture, which is normally a little less efficient than the more commonly used n-i-p architecture. The researchers report their work in a paper in Science.
"We optimized the device structure and process parameters, building upon previous results, and finally could achieve a decisive improvement with b-poly(1,1-difluoroethylene) or b-pV2F for short," says Guixiang Li, a PhD student at HZB, who is supervised by Abate. b-pV2F is a polymer with molecules that resemble a zigzag chain occupied by alternating dipoles.
"This polymer seems to wrap around the individual perovskite microcrystals in the thin film like a soft shell, creating a kind of cushion against thermomechanical stress," says Abate.
Images from a scanning electron microscope show that the tiny crystal granules appear to nestle a little closer in cells with b-pV2F. "In addition, the dipole chain of b-pV2F improves the transport of charge carriers and thus increases the efficiency of the cell," says Abate. Using b-pV2F, Abate and his team were able to produce cells on a laboratory scale with efficiencies of up to 24.6%, a record for the p-i-n architecture.
They then subjected these solar cells to over a hundred cycles between 80°C and -60°C, and 1000 hours of continuous 1-sun equivalent illumination, corresponding to about one year of outdoor use. "Even under these extreme stresses, they still achieved 96% efficiency in the end," Abate says. If these losses can be reduced a little further, perovskite solar modules could still be producing most of their original output after 20 years. This goal is now coming within reach.
This story is adapted from material from Helmholtz-Zentrum Berlin für Materialien und Energie, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.