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The halide perovskite material class is a major opportunity for even greater solar electricity at much lower costs. The materials are very inexpensive, can be manufactured into thin films with little energy input, and already attain much greater efficiencies than those of traditional silicon solar cells.
However, solar modules are expected to provide stable output for at least 20 years in outdoor conditions while exposed to large temperature fluctuations. Silicon PV manages this efficiently, whereas the semi-organic perovskites lose performance rather fast. "Sunlight can heat up the inside of a PV cell to 80 Celsius; in the dark, the cell then cools down immediately to the outside temperature. 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," said Prof. Antonio Abate, who heads a large group at HZB. Together with his team and a number of international partners, he has investigated a chemical variation that significantly improves the stability of the perovskite thin film in different solar cell architectures, among them the p-i-n architecture, which normally is a little less efficient than the more often used n-i-p architecture. "We optimised 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, who is doing his PhD supervised by Prof. Abate. b-pV2F molecules 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," Abate added. In fact, scanning electron microscope images show that in the cells with b-pV2F, the tiny granules nestle a little closer. "In addition, the dipole chain of b-pV2F improves the transport of charge carriers and thus increases the efficiency of the cell," said Abate. They produced cells on a laboratory scale with efficiencies of up to 24.6%, which is a record for the p-i-n architecture. The newly produced solar cells were subjected to over a hundred cycles between +80 Celsius and -60 Celsius and 1000 hours of continuous 1-sun equivalent illumination. That corresponds to about one year of outdoor use. "Even under these extreme stresses, they still achieved 96 per cent efficiency in the end," Abate said. That is already in the right order of magnitude. If it is now feasible to reduce the losses a little further, perovskite solar modules could still produce most of their original output after 20 years -- this goal is now coming within reach. (ANI)
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