Perovskite semiconductors promise highly efficient and inexpensive solar cells. However, the semi-organic material is very sensitive to temperature differences, which can quickly lead to fatigue damage in normal outdoor use. However, adding a dipolar polymer compound to the perovskite precursor solution greatly improves stability. This is now shown by an international team led by Antonio Abate, Helmholtz Zentrum Berlin (HZB), in the journal Science.
The solar cells produced in this way achieve efficiencies of well over 24 percent, which hardly drop even with dramatic temperature fluctuations between -60 and +80 degrees Celsius over a hundred cycles. This corresponds to about a year in outdoor use.
The material class of halide perovskites could make solar power even more efficient and at even lower costs. The materials are very cheap, can be processed into thin layers with minimal energy input and already achieve efficiencies that are significantly higher than those of conventional silicon solar cells.
“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, 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 explains.
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,” says Abate. Indeed 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 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 % efficiency in the end,” Abate emphasises. 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.
