Solar cells made from perovskite have undergone rapid development in recent years. Perovskite is a mineral that has the same crystal structure as calcium-titanium-oxide (CaTiO3). The yield of this type of solar cell has risen to 22% in just a few years. A drawback for the moment, though, is the damaging effect of humidity: water vapor from the atmosphere reacts with the perovskite crystals ,causing a considerable reduction in the yield over time.
In the findings just published, the researchers have covered the sensitive layer of perovskite with a few atomic layers of aluminum oxide to protect against degradation caused by humidity. These layers are contained within the solar cell, between the layers of perovskite and electric contact. The researchers opted for aluminum oxide (Al2O3) since it can form immediately on any kind of surface.
“Although Al2O3 has electric insulating properties, it can still be used as a buffer layer between the semi-conductive perovskite and the conductive contacts by limiting the thickness of the layer to one nanometer or less,” said Dibyashree Koushik, FOM PhD student and first author. “Charge carriers can then tunnel electrically through the insular layer.”
The thin layers were applied by atomic layer deposition (ALD). Once manufactured, the researchers compared the stability of these cells with identical cells without this layer. The complete cells were not given any further protection against humidity with sealed packaging. Two months after being exposed to a humid atmosphere the cells containing the Al2O3 layer reached 60%-70% of their original yield while the reference cells managed only 12%.
Moreover, the layer boosted the yield from 15% to 18%. “That was an extra bonus for us,” Dr. Ruud Schropp., a professor at TU Eindhoven and the leader of the study, said. “We had not expected this and we cannot exactly explain why it is. But we will certainly be investigating.”